a UJ a a a ru -a F ELEMENTARY TEXT-BOOK OP ZOOLOGY. GENERAL PART AND SPECIAL PART: PROTOZOA TO INSECT A. BY DR. C. GLAUS. Professor of Zoology and Comparative Anatomy in flif l T nir, i:iify of }'if>ia; Director of the Zoological Station at Trieste. TRANSLATED AND EDITED BY ADAM SEDGWICK, M.A., Fellow und Lecturer of Trinity College, Cambri>{ /<-, WITH THK ASSISTANCE OF F. G. HEATHCOTE, B.A., Trinity College, CiimbriJyr.. WITH 706 WOODCUTS. NEW YORK: MACMILLAN & CO. J J *t PREFACE TO THE ENGLISH TRANSLATION, I UNDERTOOK the translation of Professor Glaus' excellent "Lehrbuch der Zoologie " with a view of supplying- the want, which has long been felt by teachers, as well as students in this country, of a good elementary text-book of Zoology. Professor Clans' works on zoology are already well known in this country ; and I think it will be generally admitted that they take the first place amongst the zoological text-books of the present day. It has been decided to publish the English translation in two volumes. The second volume, which begins with Mollusca, is in the press, and will, I trust, appear early in the autumn. The German has been, with one or two unimportant exceptions, closely followed throughout. These exceptions, and the few additions which I have thought it necessary to make, have in all cases been indicated by enclosure within brackets. I must ask the indulgence of the reader towards the errors and deficiencies of this translation. I trust that they will be found to be neither numerous nor important. I have to thank Mr. Heathcote for the assistance he has given me in the laborious work of translation. I am also indebted to Professors Newton and Foster, Dr. Gadow, and Mr. W. Heape for advice and assistance. ADAM SEDGWICK. TRINITY COLLEGE. CAMBRIDGE, 1884. TABLE OF CONTENTS. GENERAL PART. CHAPTER I. Page ORGANIZED AND UNORGANIZED SUBSTANCES . . . 914 CHAPTER II. ANIMALS AND PLANTS 1524 CHAPTER III. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL 24131 INDIVIDUAL, ORGAN, STOCK 24 Repetition of organs and parts of the body 25 CELLS AND CELL TISSUES 29 Nucleus and Nucleolus ......... 29 Cell-membrane 29 Reproduction of Cells and division of Nucleus . . . .30 1. Cd.U and Cell Aggregate* 32 Isolated cells, e.g., blood corpuscles, ova, etc. ..... 32 Epithelium 34 Epidermal exoskeleton ......... 34 Glandular tissue .......... 36 2. Tissues of the connective substance ...... 37 Cellular or vesicular 37 Mucous or gelatinous 37 Reticular, adenoid .......... 38 Fibrillar 38 Elastic 39 Cartilage. 39 Osseous tissue 40 3. Jlwscular tissue 43 4. ferrous tissue .......... 45 INCREASE IN SIZE AND PROGRESSIVE DIFFERENTIATION, ETC. . 47 Unicellular stage . . .48 Multicellular stage . . . . . . . .... 49 CORRELATION AND CONNECTION OF ORGANS 50 Doctrine of Final Causes ........ 51 'Type" Scope of Morphology 52 STRUCTURE AND FUNCTION OF THE COMPOUND ORGANS . . 52 Digestive organs 53 Salivary glands, liver, pancreas ....... 58 TABLE OF CONTENT-. 5 Page Organs of circulation 51 Heart i;i .Arteries and veins .......... 62 Eeart and vessels of vertebrates . . .... 64 Ori/unx nj- rcxjii riitinii ........... 67 Branchiae 69 Lungs, trachea? 69 Trachea! gills 71 Renewal of external medium 72 Venous and arterial blood . . ... ... . .73 Animal lu-at. 73 ' Organs of secretion .......... 74 Kidneys 75 Water-vascular vessels and segmental organs 75 Vertebrate kidneys 76 Cutaneous glands 77 ORGANS OF ANIMAL LIFE 78 ShrlrtaJ structure.-* .......... 79 Xi'rroiix xijxti-m 79 Si-tisc organ* S3 Tactile organs 84 Auditory organs 85 Visual organs 85 Facetted eye 88 Simple eye .89 Olfactory organs 91 INTELLIGENCE AND INSTINCT 93 REPRODUCTIVE ORGANS 95 Biogenesis 96 Asexual reproduction 96 Sexual reproduction 97 Hermaphroditism 99 Separation of the sexes 100 Sexual differences 101 Sexual dimorphism 104 Parthenogenesis 105 DEVELOPMENT .... .107 Fertilisation of the ovum 108 Segmentation of the ovum 110 Food-yolk Ill Blastosphere 113 Formation of gastmLa 114 Primitive streak 115 Germinal layers .116 Theory of Gastrrea 117 DIRECT DEVELOPMENT AND METAMORPHOSIS ]19 Effect of food-yolk on development 120 Explanation of Metamorphosis 121 ALTERNATION OF GENERATIONS. POLYMORPHISM AND HETEROGAMY 123 Metagenesis 123 Explanation of Metagenesis 124 Polymorphism 126 Heterogamy 127 Predogenesis 128 Hetcrogamv of Trematoda . 129 6 TABLE OF CONTENTS. CHAPTER IV. Page HISTORICAL REVIEW 181-139 Aristotle ....... Pliny ..... Renaissance of Sciences in Sixteenth century . Ray i ......... I?" 4 Linnaeus . Cuvier ...... .135 St. Hilaire, Goethe, Oken . 137 Classification of the present day ... . .138 CHAPTER V. MEANING OF THE SYSTEM .... 139179 Species Varieties ......... Sterility of hybrids .... Fertility of hybrids ..... Sterility and fertility of mongrels . .143 Lamark ........ Lyell's influence on Geology ..... THEORY OF DESCENT BASED ON NATURAL SELECTION (DARWINISM) . Darwin ............ Natural selection ...... Origin of vaiieties, races and species ... .148 Progressing divergence of characters, and disappearance of inter- mediate forms ...... Species according to the theory of evolution . . . . 1 50 Natural system ........ .150 EVIDENCE IN FAVOUR OF THE THEORY OF DESCENT . .151 Evi built up of different parts Por tissues (organs of a lower order), and these a ,,: : , ; b again are composed of the FIG. 1. it, young ova of a Medusa ; I, mother-cells ultimate unit of cell, the of spermatozoa of a Vertebrate ; one of them pre- c ^ The cell last of all sented amoeboid movement. is to be traced back to the germ cell (ovum, spermoblast) (fig 1.) The cell by its properties stands in direct contrast to the crystal, and potentially possesses the properties of the living organism. It consists of a small lump of a semifluid albuminous substance (proto- plasm\ containing, as a rule, a dense or vesicular structure, the nucleus, and is frequently surrounded by a peripheral structureless membrane. If the latter is not developed, the presence of life is indicated by a more or less pronounced amoeboid movement, the fluid protoplasm sending out and drawing in processes of a continually changing form. In this organised fundamental structure, from which all tissues and organs of animals and plants are developed, lie latent all the characters of the organism. The cell is, therefore, in a certain sense the first form of the organism, and indeed the simplest organism. While its origin points to the pre-existence of cells of a similar kind, its maintenance is rendered possible by metabolism. The cell has its ORGANISED AND UNORGANISED SUBSTANCES. 13 nourishment and excretion, its growth, movement, change of form, and reproduction. With participation of the nucleus it begets by division or endogenous cell formation new units like itself, and furnishes the material for the construction of tissues, for the for- mation, growth and change of the body. With justice, therefore, is the cell recognised as the special embodiment of life, and life as the activity of the cell. Fie. 2. AmoebaXProtogenes) porrecta (after Max Schultze). Nor is this conception of the significance of the cell as the criterion of organisation and as the simplest form of life contradicted by the facts that the nucleus also sometimes fails (so-called cytodes of H?eckel), and that bodies undoubtedly manifesting vital phenomena are known which are structureless under the highest power of the microscope. Many Schizomycetes (Micrococcus) are so small that it is difficult to distinguish them in some cases from the granules of precipitates, especially when they show only molecular motion [Brownean movements] (fig. 3). Consequently, the living protoplasm) with its unknoivn molecular arrangement, is the only absolute test of the cell and organism in general. While appreciating the essential differences which have been 14 GENEBAL PART. expressed in the above discussion of the properties of living things and unorganised bodies, we must not in our criticism of the relations between them lose sight of the fact, that in numerous lower forms of life, metabolism, and all the activities of life can be completely suppressed by the removal of warmth and water, without there- by injuring the capacity of the organism for continuing to live ; and further, that in the smallest organism's, which are proved to be such by their capacity of repro- ducing themselves by their meta- bolism, and it is impossible, by means of the very strongest powers of the microscope, to detect any organization. Since, moreover, the organic matter composing such forms consist of combinations which can be produced by synthe- 3,-Schizomycetes (after F. Cohn). sis > independently of organization, , Micrococcus; 6, Bacterium termo, we must allow that hypothesis a Bacteria found in putrefying bodies . . .,, . . both in motile and Zoogiaa form. certain justification which asserts that the simplest forms of life have been developed from unorganised matter, in which the same chemical elements occur as are found in organisms. Since no fundamental difference has been shown to hold between the matter and force of crystals and those of organized beings, we might look upon the first appearance of life as essentially only the" solution of a difficult mechanical problem (with Du Bois Reymond), were we not obliged to conclude that there is present even in the simplest and most primitive organisms the germs of sensation and consciousness, attributes which we cannot regard as simply the results of the movement of matter. ANIMALS AND PLANTS. 15 CHAPTER II. ANIMALS AND PLANTS. THE division of living bodies into animals and plant* rests on a series of ideas early impressed on our minds. In animals we observe free movements and independent manifestations of life, arising from internal states of the organism, which point to the existence of consciousness and sensation. In the majority of plants, which pass their lives fixed in the earth, we miss locomotion and independent activities indicative of sensation. Therefore we ascribe to animals voluntary movement and sensation, and also a mind which is the seat of these. Nevertheless these conceptions apply only to a proportionately narrow circle of organisms, viz., to the highest animals and plants. With the progress of experience, the conviction is forced upon us that the traditional conception of animals and plants needs, so far as science is concerned, to be modified. For although we find no difficulty in distinguishing a vertebrate animal from a phanero- gamous plant, still our conceptions do not suffice when we come to the simpler and lower forms of life. There are numerous instances amongst the lower animals in which power of locomotion and distinct signs of sensation and consciousness are absent ; while, on the other 1 land, there are plants which possess irritability and the power of free movement. Accordingly the properties of animals and plants have to be compared more closely, and at the same time the question has to be discussed, whether there are any absolute distinctive characters which sharply separate the one kingdom from the other. 1 . In their entire form and organization there seems to be an. essential contrast between animals and plants. Animals possess a number of internal organs of complicated structure, lodged within a compact outline ; while in plants the nutritive and excretory organs are spread out as external appendages, with a considerable superficial extension. In the one case there is found an inner, and in the other an outer position for the absorbent surface. Animals have a mouth for the entry of solid and fluid nutritive matters, which are digested and absorbed in the interior of an alimentary canal, into which open, various glands, (salivary glands, liver, pancreas, etc). The useless solid remains of the food pass out through the anus as faeces. The nitrogenous waste material is excreted by a special urinary 16 GE1STKEAL PABT. organ (kidney), mostly in a fluid form.' For the movement and circulation of the fluid carrying the absorbed nutriment, there is a pulsatory pump (heart) and a system of blood vessels, while respira- tion is usually carried on in terrestrial animals by lungs, and in aquatic animals by gills. Finally, animals possess internally placed generative organs, and a nervous system, and sense organs for the production of sensation. In plants, on the contrary, the vegetative organs have a much simpler form. Roots serve to absorb fluid nutriment, while the leaves act as respiratory and assimilating organs, taking in and giv- ing out gas. The complicated systems of organs found in animals are absent, and a more uniform parenchyma of cells and vessels, in which the sap moves, composes the body of plants. The gener- ative organs also are placed in external appendages, and there are no nervous and sense organs. Nevertheless, the above mentioned differences are not universally found, but rather hold only for the higher animals and plants, and gradually disappear with the simplification of the organization. Even among vertebrates, and still more is it the case amongst mollusca, and the lower segmented animals, the respiratory and vascular organs are considerably simplified. The lungs or gills may fail as special organs, and be replaced by the whole outer surface of the body. The blood vessels are simplified, and sometimes they and the heart are absent, the blood being moved in more irregular streams in the body cavity and- in the wall-less spaces in the organs. Similarly, the digestive organs are simplified ; salivary glands and liver may no longer be found as glandular appen- dages of the alimentary canal. The alimentary canal may become a blind, branched, or simple sac (Trematoda), or a central cavity, the walls of which are in contact with the body wall (Ceelenterata). The mouth and alimentary canal may also fail (Cestodes), nourish- ment being taken in by osmosis through the outer walls of the body as in plants. Finally, nerves Fie. 4. Branch of a Polyparium of Corallhim rubrum (after Lacaze Duthiers). P, Polyp. ANIMAL AND VEGETABLE TISSUES. 17 -Pa and sense organs are totally absent in many organisms, which are looked upon as animals, e.g., in the whole of the Protozoa. With such reduction of the internal organs it is easy to understand that the simpler lower animals, such as colonies of polyps and the Sipho- nophora, should often in their outer appearance and the manner of their growth resemble plants, with which they were formerly con- founded, especially when they at the same time lacked the power of free locomotion (Polyps, Hy- droids, figs. 4, 5). In these cases it is as difficult to apply the idea of " indi- viduality" as it is in the vegetable kingdom. 2. Bettveen animal and vegetable tissues there exists also generally an important difference. While in the vegetable tissues the cells preserve their original form and independence, in the animal tissues they undergo very various modifications at the expense of their independence. Accordingly vegetable tissues consist of uniform cell - aggregates, the individual cells of which have retained FlG ' 5 ' ~ p ^opbora hydrostatic*. P,,, Pneuma- tophor ; S, Swimming-bells ; T, Dactylozooid ; sharply -, marked bounda- P, polypite or stomach with the tentacles, Sf. ; ries; while in animal tis- ^' terminal swdli^s on the latter provided with thread-cells ; G, Clusters ot gonophores sues the cells give rise to extremely different structures, in which the cells as such do not always remain recognisable. The reason for this unlike condition of the tissues must apparently be sought in the different structure of 18 ANIMALS AND PLANTS. the cell itself; the vegetable cell being surrounded outside its pri- mordial utricle by a thick non-nitrogenous cuticle, the cellulose capsule ; while the animal cell possesses a very delicate nitrogenous membrane, or instead of this only a more viscous boundary layer of of its own semi-fluid contents. Nevertheless, there are also vegetable cells provided only with a simple naked primordial utricle ; and, on the other hand, animal tissues which resemble vegetable tissues in the fact that the cells remain independent and develop a capsule (Chorda dorsalis, cartilage, supporting cells in the tentacles of hydroids, fig. 6). FIG. 6. a, Vegetable parenchyma (after Sachs). I, Axial-cells from the tentacles of Cam- panularia. Neither can we, as has been done by many investigators, regard the multicellular composition of the body as a necessary sign of animal life. For not only are there many iinicellular algae and fungi, but also animal organisms which are composed of one simple or complexly differentiated cell (Protozoa). Finally, it is not possible to see any reason why unicellular animals should not exist, especially when we consider that the cell forms the starting-point for the development of the animal body. 3. Least of all can a test be found in the reproductive processes. In plants indeed we find a predominance of the asexual method of increase by spores and buds, but similar methods of increase are widely present amongst the lower and more simply organised ani- mals. Sexual reproduction is effected both in animals and plants by processes which are essentially similar ; consisting in both of the fusion of the male element (spermatozoon] with the female element (ovum] ; and the form of these elements presents in both kingdoms a great agreement, at any rate they are in every case derived from cells. The structure and position of the generative organs inside the body, or as outer appendages of it, cannot be regarded as a distin- guishing mark, inasmuch as in both kingdoms the greatest difference prevails in this respect. METABOLISM IN ANIMALS AND PLANTS. 19 4. The chemical constituents and the metabolic processes in animals and plants present, on the whole, important features of difference. Formerly great importance was attached to the fact that plants consist chiefly of ternary (non-nitrogenous) compounds, while animals consist of quaternary nitrogenous compounds ; and a greater impor- tance was attached in the former to the carbon, in the latter to the nitrogen. But ternary compounds are found to be largely present in the animal body, e.g., fats, carbohydrates ; while, on the other hand, quaternary proteids play an important part in those parts of a plant which are especially active in growth. Protoplasm found in the living vegetable cell is richly nitrogenous, and of an albuminous nature ; and it agrees in its micro-chemical reactions with sarcode, the contractile substance of the lower animals. In addition, the modifications of egg albumen, known as fibrin, albumen, and casein, are also found in vegetable cells. Finally, it is not possible to mention any substance which is universally and exclusively found either in animals or in plants. Chlorophyll (green colouring matter of leaves) occurs in the lower animals (Stentor, Hydra, Bonellia), while, on the other hand, it is totally absent in Fungi. Cellulose, a peculiar non-nitrogenous substance found in the outer membranes of vegetable cells, occurs in the mantle of Ascidians. Cholesterin, and certain substances especially characteristic of nervous tissue-, are also found in plants (Leguminosse). Of far greater importance is the difference in the nourishment and metabolic processes. Plants take up with certain salts (phosphates and sulphates of the alkalies and earths) more especially water, carbonic dioxide (carbonic acid), and nitrates or ammonia compound.-, and build up organic compounds of a higher grade from these binary inorganic substances. Animals, in addition to taking up water and salts, require organic food, especially carbon compounds (fat) and nitrogenous, albuminous substances; which, in the cycle of metabo- lism, break down to nitrogenous waste products (amides and acids), kreatin, tyrosin, lecucin, urea, etc.; uric acid, hippuricacid, etc. Plants exhale oxygen, whilst they are decomposing carbon dioxide by means of their chlorophyll under the influence of light, and are forming in their chlorophyll corpuscles organic substances from carbon dioxide and solutions containing combined nitrogen. Animals take up oxygen through their respiratory organs for the maintenance of their meta- bolism. The processes of metabolism and of respiration, therefore, in the two kingdoms are indeed mutually determinant, but have an exactly opposite result. The life of animals depends on the analysis 20 ANIMALS AND PLANTS. of complex compounds, and is essentially an oxidation process, by which potential energy is converted into kinetic (movement, produc- tion of heat, light). The vital activity of plants, on the contrary, is based, so far as it relates to assimilation, on synthesis, and is essentially a process of reduction ; under the influence of which the energy of warmth and light is stored up, kinetic energy being converted into potential. Nevertheless, this difference also is not applicable as a test in all cases. Recently the attention of investigators has been turned, especially by Hooker and Darwin,* to the remarkable nutri- tive and digestive processes in a group of plants which were first observed a hundred years ago (Ellis). The plants in question catch, after the manner of animals, small organisms, especially in- Dmsera rotundif oiia, sects, and absorb from them^ through with partially contracted tentacles the glandular surface of their leaves the organic matter after a chemical process resembling animal digestion (leaves of the Sun-dew, Drosera rotundifolia, and the fly-catcher, Dioncea muscipula. Figs. 7 & 8). Many parasitic plants and almost all fungi have not, however, in general, the power of making organic substances from inorganic, but suck up organic juices ; and in taking tip oxygen and giving out carbonic acid, they present a respi- ratory process resembling that found in animals. It was established by Saussure's observations that all plants require oxygen at certain intervals; that in those parts of plants which are not green, not possessing chlorophyll, and also in the green parts in the absence of sunlight, i.e. at night, a consumption of oxygen and exhalation * Compare "specially Ch. Darwin, " Insectivorous Plants." London. 1875. FIG.'S. Leaf of Dionjea muscipula in expanded condition (after Darwin). MOVEMENT AND SENSATION AS TEST OF ANIMALS. 2L of carbonic acid goes on. In plants, therefore, together with the characteristic deoxidation process, there is always found a process of oxidation analogous to that occurring in animal me- tabolism ; by which a part of the assimilated substances is again destroyed. The growth of plants is impossible without the con- sumption of oxygen and the production of carbonic acid. The more energetic the growth, the more oxygen is consumed, as indeed the germinating seed or the quickly unfolding leaf arid flower buds rapidly consume oxygen and excrete carbonic acid. In this con- nection should be mentioned the fact that the movements of proto- plasm depend upon the inspiration of oxygen. The production of heat (in germination), also of light (Agaricus olearius) is accompanied by an active consumption of oxygen. Finally, there are organisms (yeast cells, Schizomycetes) which indeed manufacture both nitro- genous and albuminous compounds, but do not assimilate the carbon of carbonic acid, but rather derive the necessary carbon from pre- pared carbohydrates (Pasteur, Cohn). 5. Voluntary movement and sensation, according to the common view, is the chief characteristic of animal life. Formerly, the power of free locomotion was looked upon as a necessary property of animals ; and as a consequence of this the fixed colonies of Polyps were considered to be plants, until Peyssonnel brought forward proof of their animal nature, a view which by the influence of the great naturalists of the last century has gained general recognition. More recently, on the discovery of the existence of motile spores of alga?, it was first recog- nised that plants also, especially at certain stages of their development (fig. 9), possessed the power of free locomotion, so that we are compelled to direct our attention to the signs by which the voluntary J J FIG. 9. Zoospores, a, of Ph.ysa.rum ; b, of Monostroma ; nature of the movement <. <>f uiotkrix-, d, of Bedogomum -, e, of rac-/ /<'<* T . , , ,. , . (after Reinke). can be decided for a dis- tinction between the respective movements of animals and plants. As such for a long time was regarded the contractile nature of the movement as opposed to the uniform movements of plants carried out with rigid bodies. In the place of muscles, which as a special tissue are absent in the 22 ANIMALS AND PLANTS. FIG. 10. Zoospores of AetliaUum sept it-it M :ifier de Bary. a, in condition of hatching ; b, as mastigopods ; c, in the amoeboid stage; d, a piece of plasmodium. lower animals, there is present an undifferentiated albuminous substance known as sarcode, the contractile matrix of the body. The viscous contents of vegetable cells, known as protoplasm, possesses likewise the power of contractility, and re- sembles sarcode in its most essential properties. Both present the same chemical reactions and agree in the fre- quent presence of cilia, vacuoles, and streams of granules. Pulsating spaces, the contractile vacuoles, are not ex- clusively a possession of sarcode, but may also occur in the protoplasm of vegetable cells (Gonium, Chlamydo- monas, Chcetopkora). The contractility of the protoplasm of vegetable cells is, as a rule, limited by the cellulose membrane, but in the naked cells of Volvocina and Saprolegnia, and in the amoeba-like forms occurring in the development of Myxomycetes, the contractile power is as intense as in the sarcode of Infusoria and Rhizopoda. The amoeboid move- ments of the plasmodium of Myxomycetes (fig. 10) are not inferior in intensity to those of a genuine Amoeba belonging to the Rhizo- poda, e.g., Amceba polypodia (prin- ceps], (fig. 11). In these similar phenomena of movement of the lower animals and plants we seek in vain for any test of volition, the interpretation of which will depend upon the individual judgment of the observer. The faculty of sensation, which is inconceivable as a function of matter and which must be always FIG. 11. Amoeba Dtiftyiofpjicera polypodia. pre-supposed wherever we have to do with voluntary movement, can by no means be affirmed with certainty in all animal organisms. Many of the lower animals entirely lack a nervous system and sense organs, and, on stimulation, exhibit *<\ *. ' ' " .' \\ //// w- //vx ,.V;ilil^^ X, nucleus. Pr, contractile vacuole (after Fr E. Schultze). IRRITABILITY OF PLANTS. 23 but slight movements not more intense than those of plants. This irritability, however, appears widely present among the higher plants. The sensitive plants move their leaves on the application of mechani- cal stimuli (Jfimosece), or bend like the sundew (Drosera, fig. 7) small knobbed processes of the leaf surface which are comparable to the tentacles of polyps. The fly-catcher (Dioncea, fig. 8) brings the two halves of the leaf together in a valve-like manner when touched by insects. The stamens of the Centaurea contract along their whole length on mechanical and electrical stimulation, and according to the same laws as do the muscle of the higher animals. Many flowers open and shut under the influence of light at cerbain times of the day. Accordingly irritability as well as contractility appears to be a property both of vegetable tissue and of the protoplasm of vegetable cells ; and it is not possible to determine whether volition and sensation, which we exclude from these phenomena in plants, play a part in the similar sensory and motor phenomena of the lower animals. In none of the above-mentioned characteristics of animal and vegetable life, then, do we find any absolute test, and we are not in a position to indicate the presence of a sharp line between the two kingdoms. From the common starting-point of the contractile substance* animals and plants are developed in different directions ; at the beginning of their development they present many kinds of resem- blance, and it is only on their attaining a more complete organization that the full opposition between them is apparent. In this sense, without wishing to draw a sharp line between the two series of organization, we can define our conception of an animal by putting together all the characteristics distinguishing the direction of animal development. An animal, therefore, is to be defined as an organism provided with the power of free and voluntary movement, and with sensation ; whose organs are internal, and are derived from a development of the internal surfaces of the body ; which needs organic food, inspires oxygen, changes potential energy into kinetic under the influence of oxidation processes in metabolism, and excretes carbonic acid and nitrogenous waste productr. * The formation of an intermediate kingdom for the simplest forms of life is neither scientifically justified, nor from practical considerations desirable. On the contrary, the acceptance of the Protista would only double the difficulty of determining the limit. 24 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. Zoology is the science which has animals for its subject, and which seeks to examine the phenomena of their structure and life, as well as their relations to one another and to the outer world. CHAPTER III. THE ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. In the foregoing comparison of animals and plants for the establishment of a correct idea of the meaning of the word "animal," the great variety and the numerous grades of animal structure have been hinted at. Just as the complex organism is built up from the ovum by a process of gradual differentiation, and often during its free life passes through conditions which lead in ascending series to an ever higher development of the parts and to a more complete performance of functions; so, if the animal kingdom be examined as a whole, there is apparent a similar law of gradually progressing development, of an ascent from the simple to the complex, manifest both in the form of the body and in the compositicn of its parts as well as in the completeness of the phenomena of life. It is true that the grades of animal structure do not, like those of the developing individual, follow the one upon the other in a single continuous series ; and the parallel between the developmental gradation of types in the animal kingdom as a whole and the suc- cessive conditions of an individual animal breaks down in so far as we distinguish in the former, as opposed to the latter, a number of types of animal structure often overlapping, but still, in their higher development, essentially different from each other. These we regard as the highest divisions of the system. INDIVIDUAL ORGAN STOCK. The animal organism, when viewed from a physiological and mor- phological stand-point, presents itself as an independent and indivisible unit, as a " complete individual." Amputated limbs or excised parts of the body do not develop into new animals ; in fact we cannot usually remove a single piece of the body without thereby endanger- ing the life of the organism, for it is only as a complex of all its parts that the body can retain its full vital energy. With reference to the property of the indivisibility of the individual, we understand INDIVIDUAL. 25 by the terra organ every part of the body which as a unit subordi- nate to the higher unit of the organism presents a definite form and structure, and performs a corresponding function ; that is to say, an organ is one of those numerous instruments on the combined work- ing of which the life of the individual depends. There are certainly among the simpler animals many instances in which the term individual in its usual sense cannot be rightly applied. In such cases we have to do with structures which from their development must be termed individuals, and represent indi- viduals, accordingly, in a morphological sense. A great many of them are, however, fused to a common stock, forming what is known as a colony, and are related physiologically to this, as organs are to an organism. They are accordingly incomplete or morphological indivi- duals, which are usually incapable of leading a separate existence ; and, when they differ from each other in form and function, dividing amongst themselves the labours, the performance of which is neces- sary for the maintenance of the whole colony, they always perish if separated from it. Such polymorphous 1 '' stocks of animals present the properties of individuals although they are morphologically aggregations of indi- viduals which behave physiologically as organs (fig. 5). On the other hand, groups of organs can acquire individual independence. In the animal body organs do not always remain single, but the same organ may be often repeated. The manner of the repetition is dependent on the kind of symmetry, which may be radiate or bilateral. In animals with radiate symmetry, the Radiata, it is possible to connect two opposite points of the body by an axis, which may be called the chief axis, and to divide the body by sections passing through this axis into a number of equivalent and symmetrical parts known as antimeres. The organs which are not repeated are situated in the chief axis of the body, while the other organs, which are uniformly repeated in each autimere, are situated peripherally. Each antimere contains, therefore, a definite group of organs and represents a secondary unit, which, together with its fellows arid the central organs, constitutes the primary unit, i.e., the perfect animal. In a radiate animal a number of lines can be drawn at right angles to the chief axis, corresponding in number to the antimeres, and each passing along the middle of an antimere; such lines are known as radial. Similarly, a corresponding number of inter-radial lines * Vitli- K. Leuckart, " Ueber den Polymorphismus der Individueu und die Erscheiuung der Arbeitstheilung in der Natur." Giessen, 1851. 26 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEKAL. can be drawn, passing between the antimeres. A vertical section through a radial line divides the corresponding antimere into two FIG. 12/i. Sea-urchin (diagrammatic). J, inter-radius with the double row of interambulacral plates and the genital organs G ; R, radii with the double row of arnbulacral-plates perforated by the ambulacral pores* A, anus. FIG. 125. Shell of a Sea-urchin seen from above. R, radius with the per- forated plates ; J, inter-radius with the corresponding generative organs and their pores. equal parts, while a similar section through an inter-radial line divides one antimere from its neighbour. Radiate animals may have two, three, etc., radii ; and in animals which possess an uneven number of radii, one radius and one inter-radius always fall in the same vertical plane (fig. 12, b, and fig. 13). In animals with an even number of radii, on the con- trary, each vertical plane passes through two radii or two inter- radii. A vertical section passing through one radius would, if pro- longed, pass through the radius of the opposite antimere (fig. 14). For example, an animal with four radii possesses" four antimeres, each of which will be divided into two, by two radial vertical sections passing at right angles to each other through the chief axis ; while they will all be separated from each other by two similarly directed inter-radial sections. Biradiate forms (the Ctenophora) possess, on the contrary, only two radii, which lie in a common vertical plane. A second vertical plane crossing the first at right angles passes through the inter-radii, and FIG. 13. Star-fish (diagrammatic). G, genera' ive organ in inter-radius ; Af, position of the ambulacral feet in the radii. BILATERAL STMMETBY. 27 divides the antimeres from each other. The first, in which the greater number of organs are repeated, may be designated the fnmsverse plane, while the second, corresponding to the median plain' of bilateral animals, is known as the sagittal plane (fig. 147;). Gf i: FIG. 14a. Acalepha larva (Ephyra). Jtk, marginal body ; Gf, gastric fila- ment. Re, radial-canal ; O, mouth. FIG. 14 b. Ctenopheran seen from above. S, sagittal plane ; T, trans verse plane ; R, vibratile plates ; Gf, gastric canals. In the bilateral arrangement, which is found also in each individual antimere of the Radiata, only one plane, the median plane, can be imagined, which passes through the chief axis and divides the body into two exactly similar parts (right and left). These two halves, as opposed to antimeres, may be termed parameres. In bilateral animals we distinguish an anterior and posterior end, a right and a left side, and a dorsal and a ventral surface. The unpaired organs are placed in the middle line, on each side of which, in the two halves of the body, are placed the paired organs. The plane which is placed at right angles to the median plane (passing from right to left) and separates the unlike dorsal and ventral halves of the body, is known as the lateral plane. The anti- meres of the Radiata also consist of two parameres, and are therefore bilateral, in that the vertical plane passing through the radius like the median plane divides them into two similar parts. The same groups of organs or similar parts of the same organ may also be repeated in a longitu- FIG. is. segmented dinal direction. This occurs especially frequently "' ( p iychaete). . r l P>>. pharynx ; D, ali- in bilateral, less frequently in radiate animals mentary canal; c, (strobUa). The body thus obtains a segmentation, cirms; F ' teutacle - and is divisible into successive sections, the segments or metameres i> 28 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. which are placed one behind the other, and more or less completely resemble each other in structure (Annelids, fig. 15). The successive segments may in structure and function appear completely equiva- lent, and represent, like the antimeres of the Radiata, individuals of a lower order, which on the severance of their mutual connec- tion can acquire independence and remain alive for a shorter or longer period (proglottis of Cestodes). In animals of higher organization the segments are much more intimately connected, and are mutually dependent, but they lose at the same time their complete hornonorny. In the same degree as the metameres acquire an unlike structure, and corresponding to this a varying importance in the life of the organ- ism, they lose their individual independence, and sink more and more to the value of organs. The metameres in the polymorphous colonies are quite analogous to the segments of the individual. In them there follow, one behind the other, similar groups of different individuals, each of which fulfils singly the conditions necessary for existence, and there- fore can continue to live as a colony of a lower order when separated from the stock (Eudoxia, Diphyes, fig. 16). The distinction into a higher and lower order also holds for organs. There are organs which are reducible to a single cell, or to an aggregation of equivalent cells (simple organs), and others in the formation of w T hich various cells and tissues (compound organs) partici- pate, and which frequently, in their turn, may be divided into parts different in structure and function. The compound organs of higher order are composed of different parts which function as organs of a lower order. These, again, are composed of various kinds of cell sand cell derivates, which are organs of a still lower order. Finally, in the last analysis, we come to the cell or the area of protoplasm corresponding to it, which is the simplest and ultimate organ. On the other hand, we group together organs of different order, which are intimately connected so far as their chief function is concerned, under the name of system (vascular system, nervous system) or apparatus (digestive apparatus), although we cannot clearly distinguish them from compound organs. FIG. 10.- Portion of Diphyes fter R. Leuckart). D, hydropliylliura ; Grs, gono- phore ; P, Polyp with tentacle. The groups of individuals separate them- selves as Eudoxia. CELL NUCLEUS. 29 CELLS AND CELL TISSUES. The constituent parts of which an organ is made up are known ;is ii->ues. They possess a definite structure, visible with the help of a microscope, and have either the form of cells or of structures derived from cells. Tissues have a function corresponding to their special structure, and this function determines the whole function of the organ. They may, therefore, be regarded as organs of a lower order. The ultimate unit, the organ of the lowest order, or ele- mentary organ,* from which all tissues are derived, is the cell. The essential part of a cell is not, as we have already seen, the membrane or the nucleus, but the protoplasm, with its special molecular arrangement, in which reside the functions of independent movement, of metabolism and of reproduction (fig. 1). The nucleus of a cell is either a solid mass of protoplasm or a more fluid structure enclosed by a firm membrane, and may con- tain one or more solid bodies (nucleolus). Different as are the forms which the nucleus may take, it always contains a fluid sub- stance, the nuclear fluid, and a pro- toplasmic substance, the nuclear substance of a special importance for the functions of the nucleus (fig. 17). An important and very general property of protoplasm is its power of contractility. The living mass presents, in connection with metabolism, phenomena of move- ment. These movements are not merely confined to the currents of solid particles suspended in the viscous contents of the cell, but are shown also in the change of form of the whole cell. If the outer part of the protoplasm has condensed so as to give rise to a cell membrane, i.e., if the cell has acquired a distinct wall, the changes in its form are very much restricted. In other cases the movement shows itself in a quick or slow change in the outer form. The cell in this case manifests * Th. Schwann, " Microscopische Untersuchungen liber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere uncl Pflanzen." Berlin, IMI'.I. Fr. Leydig, " Lehrbuch der Histologie des menschen und der Thiere." Frank- furt a. M. 1857. FIG 17. Different forms of nuclei (after R. Hertwisi). a, nucleus from a cell of a Malpig-hian tubule of a caterpil- lar, b, nucleus of a Heliozoon with a cortical layer and nucleolus in the nuclear fluid. c, nucleus from the egg of a Sea-urchin. Nucleolus im- bedded in a protoplasmic fibrous net- work surrounded by nuclear fluid. 30 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEEAL. the so-called amoeboid motion ; it sends out processes, draws them in again, and is able by such means to change its position. This capacity of change of form is especially possessed by young undif- ferentiated cells, which have not developed an outer membrane. Such cells in their later growth usually develop a cell membrane., which accordingly is not, as was formerly supposed, a necessary constituent of the cell, but is merely an indication that the cell has undergone a certain amount of differentiation from its early indifferent condition. It has been already pointed out that the fundamental properties which distinguish the life of organisms manifest themselves also in the life of their constituent cells. According to our present knowledge, cells always originate from pre-existing cells ; a process of free cell formation, as conceived by Schwann and Schleiden, indicated by the precedent origin of nuclei in a formative organic material, has never been proved. Such a process may, however, take place when the formative matter is the plasma of a cell, or of several cells fused together (plasmoclium). In such cases we have a process of free cell forma- tion (e.g., spore formation in Myxomycetes) which certainly is not clearly marked off from a process of new formation within the mother cell, and is to be looked upon as a modification of the so-called endogenous cell formation. This leads us to a consideration of the very widely distributed method of cell increase by division. When the cell has reached a certain size by the absorption and assimilation of nutrient matter, the protoplasm separates itself into two nearly equal portions, this process being usually preceded by the division of the nucleus. Each portion receives half of the original nucleus. During its division the nucleus undergoes, as has been recently shown in many instances, peculiar differentiations and changes (fig. 18). It becomes spindle-shaped ; its contents take on the form of longitudinally arranged strise, running from pole to pole of the spindle ; the centre of each of the striae becomes thickened, giving rise to a cross equatorial zone of granular matter, the nuclear plate (thickened zone). The central thickenings constituting the nuclear plate divide. Each half travels towards the poles of the spindle, and becomes there enclosed in a clear fluid mass, which appears in the protoplasm. From these two structures the new nuclei are formed at the poles of the now dumb-bell shaped nuclear spindle, the $true of which vanish during the constriction of the protoplasm, which has already commenced and quickly progresses. The division CELL DIVISION. 31 is completed when the young nuclei, proceeding from the two poles of the nuclear spindle and the surrounding clear protoplasm, have attained their definite size, and the remains of the fibres have been absorbed. During these processes the protoplasm of the cell has gradually become more and more constricted by a furrow which is directed transversely to the long axis of the nuclear spindle, and which after the completion of the division of the nucleus brings about a separa- tion of the cell contents into two masses the daughter cells (% 18). If the products of the division are unequal, so that the smaller portion may be looked upon as a production of the larger, we give the name " budding " to this form of reproduction. FIG. 18. Processes of cell division in an embryonic blood corpuscle of a chick (after Biitscbli). K, nuclear spindle. Kp, nuclear plate or equatorial thickening-. Finally, the term endogenous cell formation is applied to that method of increase in which the cells originate within the mother- cell. In this case the protoplasm does not divide by a progressive constriction and separation into two or more parts, but differentiates itself round the neAvly formed nuclei, with which the original nucleus may persist. The ovum which we have to contemplate as the starting-point of the development of the organism produces by these various methods of cell multiplication the material of cells which serves for the for- mation of the tissues. Groups of originally indifferent and similar cells break up and assume severally a changed appearance. The constituent elements undergo various differentiations, and from them and their derivates is produced a definite form of tissue, endowed with a function corresponding to the peculiarity of its structure. The separation of groups of different cells leading to the establish- ment of various tissues prepares the way for the physiological division of labour between the organs, which, like the tissues compos- ing them, can, according to the functions which they perform, be divided into organs of vegetative life and organs of animal life. The former have to do with the nutrition and maintenance of 32 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. the body; the latter, on the contrary, serve for movement and sensation, functions which are exclusively the property of animals (as opposed to plants). For the sake of clearness we will divide the vegetative tissues into two groups, into cells and cell-aggregates (epithelium), and into tissues 'of connective substance. In the tissues of animal life we distinguish muscular and nervous tissues. This classification of the tissues has no other aim than to facilitate a general review of the different forms of tissue, and to render possible a criticism, of their relationships; it lays no claim to establish an absolutely sharp line between the various groups. 1. Cells and cell-aggregates. Cells may either be free and isolated from each other, floating in a fluid medium, or they may be placed near one another forming part of an aggregation of cells spread out superficially. To the former belong the cells of the blood,- chyle, and lymph. The blood of invertebrates, which is generally colourless, and if FIG. 10. Blood-corirascles (after Ecker). a, colourless blood corpuscles from the heart of the fresh- water mussel ( Anodonta) . b, from the caterpillar of Sphinx, c, red corpuscles from Proteus, d, from the smooth adder, d', lymph corpuscles of the same, e, red corpuscles of the frog, f, of the pigeon. /', lymph corpuscles of the same, g, red blood corpuscles of man. the blood of vertebrates, which is with few exceptions red, consists of a fluid albuminous plasma containing numerous blood-corpuscles in suspension. These corpuscles are in invertebrates irregular often spindle-shaped cells, endowed with the capacity of amoeboid move- ment. In the blood of vertebrates, in addition to such colourless amoeboid corpuscles there are found red corpuscles (discovered by Swammerdarn in the frog) ; and these are so numerous as to give the blood a uniformly red appearance to the unaided eye. They are thin discs with an oval, nearly elliptical or circular (Mammalia Petromyzon) contour, with nuclei in the first case, and without nuclei in the second (except in the embryo) (fig 19). They contain ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 33 the red colouring matter of the blood, haemoglobin, which plays so important a part in respiration. They arise in all probability from the colourless corpuscles which are always far less numerous in normal blood. The colourless corpuscles are genuine cells of variable form, and have the power of amoeboid motion (migration into tissues, regeneration of tissues, etc.) ; they come from the lymphatic glands, in which they arise as lymph corpuscles, and eventually pass with the lymph stream into the blood. The ova and spermospores, after 'Fie. 20. Spermatozoa, a, of Medusa, b, of a Nematode. c, of a Crab, d, of Torpedo. e, of Salamander (with undulating membrane). /, of Frog, g, of a Monkey (Cerco- pitbecus). they have separated from the epithelial layer in the wall of the ovary and testis, as well as the spermatozoa produced from the spermospores, respectively belong to the category of free cells. The form and size of the spermatozoa present great variations. They always consist of a modified cell, frequently of a very small cell with a long nagellum, nucleus, and remains of protoplasm. In many cases the head is elongated into a fibre-like structure, or is twisted like a corkscrew (Birds, Selachians). Sometimes a distinct head is absent, and the spermatozoon is thread-like (Insects). In the Nematodes the sperm- 34 GENERAL PABT. atozoon is hat-shaped ; while in Crustacea it has the form of a cell, with long radiating processes (fig. 20). Epithelial tissues consist of aggregations of cells which as simple or stratified layers cover the external and internal surfaces of the body, and which line its closed spaces (Endothelium). According to the different shape of the cells composing it, we distinguish cylin- drical, ciliated, and pavement epithelium. In the first case the cells, in consequence of the elongation of the long axis, are cylindrical (fig. 21, c) ; in the second, the free surface of the cells is beset with vibratite cilia or flagella (fig. 21, d), which are continuous with the living protoplasm of the cell. If only one flagellum projects from the cell (sometimes a flat cell fig. 21, 6-) then the name flagellate cell is applied (collared cell of sponges, fig. 21, e). Finally, in the case of pavement epithelium (fig. 21, a) the cells are flattened; and if there FIG-. 21. Various kinds of epithelial cells, a, Flat cell*. I, flat cells with flagella (from a Medusa), c, cylindrical cells. , c). The cells occupy spaces in the coni- FIG. 30. Incrusted cartilage, or cartilage bone. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 41 pact intercellular substance, which is also traversed by numerous canals, known as Haversinn canals. These contain the nutritive blood-vessels and correspond exactly in their course and branchings to the latter. The intercellular substance consists of lamellae, which are arranged concentrically round the canals. The Haversian canals begin on the surface of the bone, which is covered by a vascular and nervous connective tissue layer, known as periosteum, and open into larger spaces (marrow spaces), which in the long bones occupy the axis of the bone, but in the FIG. 31. Longitudinal section through a , i long bone (after Kolliker). }. In their further develop- ment the greatest part of the cell protoplasm appears to give rise to contractile muscle- substance ; and sometimes the whole cell becomes elongated into a muscle fibre. Two kinds of muscles, which are morphologically and physiologically different, are to be distinguished, viz., the smooth muscles, or con- tractile fibre-cells ; and the cross-striped muscle-substance. '' These cells have been called neuro-musrular cells ; a misleading term, since it cannot be shown that they have had anything to do with the origin of ganglion cells. FIG. 344. Muscle-epithelium of a Medusa ( Anrelia) . 44 GENERAL PAET. In the first case we have to do with flat, spindle-shaped, or band- shaped elongated cells, and with layers of such cells. They react slowly to nervous stimuli : they enter the condition of contraction gradually, and remain, contracted for some time. The contractile substance appears for the most part to be homogeneous, but it is sometimes longitudinally striated. The smooth muscles have the widest distribution amongst the Invertebrata ; but they are also found in vertebrates, in the walls of numerous organs (vessels, ducts of glands, intestinal wall) (fig. 35). Cross-striped muscle consists of cells, more frequently of multi- nucleated so-called primitive bun- dles. It is characterised by the partial or complete transforma- tion of its protoplasm into a cross- FIG. 35. a, smooth muscle fibres isolated. I, piece of an artery (after Frey) ; 1, outer connective tissue layer ; 2, the middle layer formed of smooth muscle fibres ; 3, non-nu- cleated inner layer. Cf - warn iraiHumiiamra HUB',, .' , .. . ! FiG.3G. a, Primitive fibre. i,cross-striped muscle fibre (primitive muscle bundle) of La cert;i with nerve termination. striped substance, consisting of special doubly refracting elements (sarcous elements) connected to- gether by a simply refracting inter- mediate substance (fig. 36, a, b~). Physiologically, this form of mus- cular tissue is characterised by the energetic and considerable contraction which immediately follows its excitation, a property which renders it especially suitable for the carrying out of powerful movements (muscles of vertebrate skeleton). In the simplest cases the cross-striped fibrilhe are produced by the deeper parts of the myoblasts, which form a continuous flat surface epithelium (muscle epithelium) above the layer of delicate fibres (Medusa? and Siphonophora) (fig. 34 b). In the higher animals they ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 4-J arise from the transformation of a greater quantity of protoplasm, and almost the whole contents of the cell are concerned in their production. Rarely the cells remain single, and never acquire more than one nucleus, .so that the muscle is composed of only a single cell (eye muscles of Daphna). Sometimes the cells become elongated into long fibres, the primitive bundles ; the nuclei at the same time increase in number, and a membrane, the sarcolemma, becomes developed on the outer surface of each fibre. More frequently, however, the primitive bundles arise by the fusion of several cells placed in a row. Either the nuclei come to lie close to the sarco- lemma in a peripherally-placed layer of finely granular protoplasm, or they are arranged in a row in the axis of the fibre in some finely granular non- contractile protoplasm. The finer and coarser muscular bundles are composed of many primitive bundles (fibres) placed close together and held together by connective tissue. The fibrillation of the muscular bundles corresponds to the direction of the primitive bundles (muscles of Vertebrata). Finally, both the simple cells, and the multi-nucleated muscles which arise from them, may be branched (heart of Vertebrata, intestine of Arthropods, etc). 4. Nervous tissue. As a rule, nervous tissue is found with mus- cular tissue, and is the means by which stimuli are conveyed to the latter; but above all, it is the seat of sensation and the will. With regard to this important function it would appear probable that in phylogeny the elements of nervous tissue have not arisen in con- nection with muscular tissue, but in connection with the sense cells found in the skin, i.e., differentiated Ectoderm cells, and that then, still remaining connected with the sense-cells, they have travelled inwards into the subjacent tissue ; while the connection with the muscle-cells, which at first possessed an independent irritability, is only secondary. Nerve-tissue contains two distinct structural elements, nerve cells or ganglion cells, and nerve fibres ; both possess a distinct minute structure and molecular arrangement, as well as chemical compo- sition. The ganglion cells act as centres for nerve-stimuli, and are found especially in the central organs which are known as brain, spinal cord, or simply ganglia. They usually possess a finely granular contents, with a large nucleus and nucleolus and one or more pro- cesses (unipolar, bipolar, multipolar, ganglion cells), one of which is the root of a nerve fibre (fig. 37, a, b). Frequently the ganglion cells are enclosed in connective tissue 46 GENERAL PART. sheaths, which are prolonged over their processes and so over the nerve fibres. Very generally several ganglion cells are enclosed in a common sheath. Nerve fibres are either centrifugal, i.e., they carry nervous impulses from the central organ to the peripheral organs (motor, secretory nerves) : or they are centripe- tal, i.e., they carry them from the periphery to the central organs (sensory nerves). They are prolongations of ganglion cells, and, like them, are fre- FIG. 38. Nerve fibres (partly after M. Schultze). a, non-medullated sympa- thetic fibre. I, medullated fibres, one of them commencing with coagulation of the axis, cylinder, c, medullated nerve fibre with the sheath of Schwann. FIG. 37. a bipolar ganglion cell. 4, nerve cell, from the human spinal cord (anterior cormi), (after Gerlach). P, pigment body. queutly enclosed in a nucleated sheath. The larger and smaller nerves are composed of a number of such fibres bound together. According to the minute structure of the nervous sub- stance we distinguish two kinds of nerve fibres (1) the so-called medullated nerves, with a double contour: (2) the non-medullated or naked axis cylinders (fig. 38, a, b, c). The former are distinguished by the fact that, 011 the death of the nerve and as the result of coagulation, a strongly refractile fatty substance which forms a sheath for the nerve fibre comes into view. This sheath is known as the medullary sheath, and the central fibre as the axis cylinder. The medullary sheath disappears near the ganglion cell, the axis cylinder only entering the protoplasm ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 47 of the latter. They possess in addition an outer sheath, known as the sheath of Schwann (cerebro-spinal nerves of most vertebrates). In the second form, i.e., in the non-niedullated nerve fibres, the me- dullary sheath is absent, the axis cylinder being either naked or sur- rounded by a connective tissue sheath. The axis cylinder here also is connected with a ganglion cell (sympathetic nerves, nerves of Cyclostornata and Invertebrates). Very often, however, and this is especially the case with sense nerves, we find that the axis cylinder may break up into very fine nerve fibrilla?, and be, so to speak, resolved into its elements. Finally, the nerves of In- vertebrates very often appear as finely striated bundles of tibrilhe, in which, on account of the absence of a sheath, it is not possible to recognise the limits of the individual axis cylinders. Peripherally the sensory nerves become connected with accessory structures (end-or- gans), derived usually from epithelial cells and their cuti- cular products, or rarely from connective tissue substance (Tactile organs). The eiid- organs are therefore for the most part derived from modi- fied epithelial cells (sensory epithelium). Ganglion cells are frequently found inserted in the course of the nerve fibres close to their termination (fig. 39, , I, c.} FIG. 39. Rod-shaped sense cells from the olfac- tory organ (after Max Schultze). a, from the frog -. Sz, supporting cell between two ciliated rod-cells, b, from man. c, from pike. Pro- bable connection between the nerve fibrilla? and the sense cells. INCREASE IN SIZE AND PROGRESSIVE DIFFERENTIATION, DIVISION OF LABOUR AND PERFECTION. The lowest organisms possess neither tissues nor organs formed from cells. The whole oi-ganism consists of a single cell. The bo.ly of such an animal is composed of protoplasm, and it.- skin of the 48 GENERAL PART. cell membrane. The latter is often without an opening for the entrance of solid bodies ; the entrance of food being entirely effected by endosrnosis. In such cases, e.y., in the Gregarines and parasitic Opalines, the outer body-wall suffices, like the membrane of the cell, for the performance of such vegetative functions as the absorption of food and the removal of the excretory products. The protoplasm (Sarcode) constitutes the body parenchyma, and is the seat of the animal and vegetative vital activities. Accordingly there results a definite connection between the functions of the peripheral layer and of the included mass, in which the processes of animal and vegetative life are carried on. This connection pre-supposes a definite relation between the superficial area of the surface and the size of the mass, and this relation changes as growth proceeds. For while the surface increases by squares, the mass increases by cubes ; while the mass increases in three dimensions, the surface only increases in two, and therefore as growth proceeds the relation changes- .to the disadvantage of the latter. In other words, with increase of size the superficial area becomes relatively smaller. Finally it becomes relatively so small that the vegetative processes cannot be carried on, and it is necessary for the mainte- nance of life that for a given energy of life it should be increased by the production of new surfaces. This holds not only for the simple unicellular organisms, which resemble cells in their nutritive processes, but also for cells them- selves whose size never exceeds certain fixed limits. Further, as the organism increases in size, not only does it divide into several cells, but these cells arrange themselves in such a way as to give the largest possible extent of surface. The cellular organism accord- ingly acquires not only an outer but also an inner surface on which the cells are arranged in a regular layer. With the appearance of an inner surface, a division of labour is established. The outer layer carries on the animal functions and such vegetative processes as those of respiration and excretion, while the inner (digestive cavity} serves for the reception and digestion of food. We thus not only see that increase in size must be accompanied by an increase in the complexity of organization, but also bring out at the same time the essential characteristics of animal organization. The numerous cells developed from the original simple organism were at first equivalent to one another, and all endeavoured to take up a peripheral position (colonies of Protozoa Volvox Blastospheere) (fig. 40, a, b.) Then, in consequence of the needs of the growing organism, THE OASTEULA. 49 it became necessary that they should be divided, so as to bound two surfaces, into an external and an internal layer ; the one forming the outer wall of the body and known as ectoderm, and the other lining the central cavity (digestive ^ cavity) known as endoderm ; these ,,,ILI/JULIII, two layers being continuous with one another at the opening of the central digestive cavity, or mouth opening (fig. 40 c). The cells of the two layers, in correspondence with the difference in their function, possess a different structure. Those of the outer layer, which carry on the animal functions, are usually cylindrical ciliated cells containing a pale albuminous substance ; those of the inner layer are more rounded and of a darkly granular aspect ; they may also bear cilia for the movement of the contents of the cavity which they line. In actual fact we find this form, which from a physiological standpoint is the simplest organism with cellular dif- ferentiation that we can conceive of, realised in the two-layered " gas- trula," which appears in the de- velopment of almost all groups of the animal kingdom as a free- swimming larva, and to which the adult sexually mature Crelenterate closely approximates. As the organism increases in size, additional complications ensue. These result partly from a still fur- Fu . 40 _ U; Cell colony of young ,-,,., x ther increase of surface brought <;i<>i'tito,- (after stein), i, Biastosphcre sluice of an Acalepha larva (Aureliu about by secondary imaginations tili . ifa) _ c , Gastnila stage of 6 ; EC, and partly from the appearance of Ectoderm; En, En.ioderm; o, Blasto some intermediate tissue placed be- P ve < mouth of <*astruia). tween the two primary layers. The secondary invaginations perform special functions and give rise to glands; while the intermediate 50 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. tissue, developed from one or both of the primary layers, primitively serves as a support for the body and forms the skeleton ; and it also gives rise to muscles which increase the organism's power of move- ment and apply themselves, on the one hand, to the ectoderm (soma/tic muscles), and on the other, to the endoderm (splanchnic muscles). Between the primary layers of the body there is primi- tively present a space, the primary body cavity.* Subsequently a second space, developed as a split in the intermediate tissue may appear, giving rise to the secondary body cavity. t From the latter the vascular system is developed. Contemporaneously with the appearance of muscles a nervous system is usually differentiated from modified cells of the outer layer. Outgrowths from the body also are developed, which may have either a radiate or a bilateral arrangement. They take the form either of organs of nutrition (gills) originating from the need for an increase of surface, or of organs of prehension and movement (tentacles, limbs). The increasing complexity of organization depends, therefore, not only upon the extension of the surfaces endowed with vegetative functions, and on the appearance of the organs of animal life, but also on a progressing process of division of labour ; which results in a clearer and more definite localization of the various functions, necessary for the maintenance of life, in special organs. The greater this specialization the more completely will each organ be able to discharge its special functions, and supposing a proper co-ordination between the working of all the organs, a great advantage accrues to the organism, which is thereby rendered capable of a higher and more complete life. Therefore we find, as a general rule, that the larger the body and the more complex the organization, the higher and more perfect is the life. In this relation, however, the form and arrangement of the organs which characterize the various group-; (types), as well as the special conditions of life which are limited by them, must be taken into account as compensating factors. CORRELATION AND CONNECTION OF ORGANS. The organs of the animal body stand in a mutually limiting rela- tion to one another, not only in their form, size, and position, but also in their actions ; for since the existence of an organism depends upon the blending of the individual performances of all its organs to a united manifestation, the various parts and organs must all, in * Usually known as segmentation cavity. ED. f Usually known as " body cavity." or ' crelom." El). DOCTEINE OF FINAL CAUSES. 51 a definite and regular manner, be adjusted and subordinated to one another. This relation of dependence, necessarily resulting from the conception of the organism, has been very suitably termed " Corre- lation" of organs; and many years ago served for the establishment of several principles, the cautious application of which has been of great service to the comparative method. Each organ, in order that it may properly discharge the functions which are requisite for the maintenance of the entire machine, must comprise a certain number of working units, and consequently must have a certain size and possess a form dependent partly on its func- tions and partly on its relation with other organs. If an organ becomes abnormally enlarged it increases at the expense of the sur- rounding organs, and the form, size, and function of the latter become injuriously modified. It, in fact, led Geoffroy St. Hilaire to enunciate the " principe du balancement des organes," a principle which was not at first generally accepted, but which enabled that investigator to establish the doctrine of " Abnornialites " (Teratology). The organs which are physiologically similar, i.e., organs which per- form in general the same function, as, for instance, the teeth or the alimentary canal or the organs of movement, undergo great and various modifications; and the particular methods of nutrition and habits of life, as well as the external conditions which must be ful- filled if the life of any particular genus is to continue, depend upon the special arrangement and action of the individual organs. Given therefore the special form and arrangement of a particular organ or part of an organ, it is possible to arrive at conclusions concerning the special structure, not only of many other organs, but even of the entire organism, and to reconstruct to a certain extent the whole animal so far as its essential featxires are concerned. This was first done by Cuvier for many extinct Mammalia, with the aid of scanty fragments of fossil bones and teeth, in a masterly manner. If we regard the life of the animal and its maintenance, not as the result, but as the end sought, as the aim of all the special arrange- ments and actions of the individual organs and parts, we are led to the" principe des causes finales " (des conditions d 'existence] of Cuvier, and consequently to the so-called teleological doctrine by which we certainly do not attain to a mechanico-physical explanation. However that may be, this theory, if it be regarded merely as an expression of the reciprocal relations which necessarily exist between the form and function of the parts and of the whole, and not in the Cuvierian sense as implying the existence of design, renders important and 52 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. indispensable service to the understanding of the complicated corre- lations and the harmonious adjustments in the organic world. The same plan of structure and arrangement of the organs is not found, as Geoffroy St. Hilaire asserted in his theory of analogies, in the whole animal kingdom ; but, on the contrary, there are, as Cuvier stated, several plans of organization or types. The term "Type" was applied by Cuvier to the chief, i.e., the most compre- hensive and general divisions of his system ; and each type was distinguished by the sum of the characters of its form and structure. In the essential characteristics of their structure, the higher and lower members of the same type agree, while in the unimportant details they present the most marked differences. The different types themselves do not represent absolutely isolated groups, nor groups which are exactly equivalent to one another, but in a greater or less degree they are related to one another ; this is evident after an examination of the lower forms and a careful comparison of the developmental histories. To morphology belongs the task of pointing out the identity of plan under the most diverse conditions of organization and habits of life, not only among animals of the same group but also between those of different groups. This science has for its object the determination of homologies, as opposed to analogies which concern the similarity of function, i.e., the physiological equivalence of organs found in different groups, e.g., the wing of a bird and that of a butterfly. That is to say, it has to trace back to the same primitive structure parts of organisms belonging to the same or different groups, which with a different structure and under deviating conditions of life discharge different functions ; as, for example, the wing of a bird and the fore-limb of a mammal ; and so to show their morphological equivalence. In the same way the organs of similar structure which are repeated in the body of the same animal, e.g., the fore and hind limbs, are designated as homologous. THE STRUCTURE AND FUNCTION OF THE COMPOUND ORGANS. The vegetative organs comprise the organs of nourishment which are necessary for all living organisms, whether animal or vegetable. In the former, however, they gradually and in the most intimate connection with the progressive development of the animal functions, attain a higher and more complicated structure. In animals, the reception of food is followed by its digestion. The substances to be assimilated, which have been made soluble by digestion, enter a DIGESTIVE ORGANS. 06 nutrient fluid (blood) which permeates the body, and is carried in more or less definite tracts to all the organs. To the latter the blood yields its ingredients, and receives from them such decom- position products as have become useless, and carries them away to be excreted in definite organs. The organs which serve for the performance of the different functions of nutrition and excretion Ill/ > III / t i i i l ' ! / FIG. 41. Rotalia veneta (after M. Schultze) with a diatom caught in the pseurtopodial network. consist of the apparatus for the reception of food and for its dii/rx- fion, and for blood formation ; and of the organs of circulation, respiration, and of excretion. Digestive organs. Even animals which have only the value of a single cell (Protozoa) swallow solid particles of food. This is effected 54 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. in the simplest cases, as in the Amoebae and Rhizopoda, by prolon- gations of the sarcode (pseudopodia) surrounding the foreign body (tig. 41). In the Infusoria, which are covered by a firm cuticle, there is a central semi-fluid mass of sarcode (endoplasrn), which is distinct from the more compact peripheral layer of sarcode (ecto- plasm), and which receives the nutrient substances through the mouth and digests them. Rows of larger cilia are present, which serve the purpose of procuring food (adoral ciliated zone of the Ciliata) (fig. 42). FIG. 42. Stylonychia mytilus (after Steiu) viewed from the ventral surface ; We, adoral zone of cilia ; C, contractile vacunle; N, nucleus; A r ',nucle olns (paranucleus) ; A, anus. a FIG. 43. Longitudinal section through the body of an Anthozooid (Octactinia). 31, stomachic tube with the mouth open- ing in the centre of the feather-like tenta- cles ; Mf, mesenteric folds ; G, genital organs. Among the animals with cellular differentiation (Jletazoa), the internal cavity of the body in the Coelenterata (morphologically identical with the alimentary cavity and not with the body cavity of other animals) functions as a digestive cavity, and its peripheral radially arranged portions as a system of vascular canals (gastro- ALIMEXTARY CANAL. 55 vascular canals). In the larger Polyps (Anthozoa) a tube derived from an invagination of the oral disc projects into the central part of the digestive cavity. This is known as the stomach of the polyp, although it serves entirely for the introduction of food, and should be called rather the buccal or cesophageal tube (fig. 43). Organs for the prehension of food are found even with this simple digestive system. For near the mouth are placed radially or bilate- rally arranged appendages or processes of the body, which set up --EG RK FIG. 41. Aurelia aurita seen from the oral surface. MA, the four oral tentacles with the mouth in the centre ; Gk, genital folds ; GH, opening of the genital pouches ; Jii-, mar- ginal bodies ; A'G, radial canals ; T, tentacles at the margin of tho disc. currents to convey small particles of food, or as tentacles seize foreign bodies and convey them to the mouth (Polyps, Medusae) (tig. 44). Such appendages serving for the capture of prey may also be placed further from the mouth (tentacles of Medussj, Siphonophora, Ctenophora). When tho digestive cavity acquires a wall distinct from the body wall, and usually separated from the latter by the body cavity (ex- cepting the parenchymatous worms), it appears in the simplest cases as a blind tube, which may be either simple, bifurcated, or branched 56 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. (fig. 45), with sharply marked off pharyngeal structures (Treinatoda, Turbellaria), or as a tube communicating with the exterior by an anus (fig. 46). In the last case it becomes divided so as to lead to the distinction of three parts (1) of the fore-gut (oesophagus) for the reception of the food, (2) of the mid-gut for the digestion of the food, and (3) of the hind-gut for the expulsion of the undigested remains of the food. Sometimes the alimentary canal aborts ; and, as in the mouthless Protozoa (Opalina), the mouth opening may be absent (Acanthocephala, Cestoda, Rhizoce- phala). In the higher animals, usually, not only is the number of the divisions greater, but their shape and structure becomes more com- plicated. The organs for the seizure of food also become more complicated, and the appendages placed nearest the mouth often become modified to subserve this func- tion. A special chamber, the buccal cavity, becomes marked off from the fore-gut, in front of or within which hard structures, such as -jaws and teeth, for the seizure and mastication of the food are placed (Vertebrata, Gastropoda); and into which secretions (salivary) having a digestive function are poured. The masticatory organs are sometimes placed completely outside the body in front of the mouth, and consist of modi- fied limbs ( Arthropoda), which in the parasites are metamorphosed into structures for piercing and sucking ; or they may have shifted so as to lie entirely within the pharynx (Rotifera, errant Annelids) or in a muscular dilatation of the posterior end of this organ. At this place there is usually developed a widened chamber, the stomach, which by A FIG. 45. Alimentary canal of Distomum kepaticum (after R. Leuckart) ; D, alimen- tary canal ; O, mouth. FIG. 46. Alimentary canal of a young uematode. O, mouth ; Oe, fore-gut (oesophagus) with pharyngeal dilatation, Ph ; D, mid-gut ; A, anus. INTESTINE. JIJJ repeated mechanical action (masticatory stomach of Cray-fish) or by the secretion of digestive fluids (pepsin) furthers digestion ; or it may, as in birds, subserve both these functions. From the stomach the food passes into the mid-gut. Dilatations and out-growths of the buccal cavity give rise to cheek and throat pouches, of the ceosphagus to the ci'op, of the stomach to blind sacs which serve as reservoirs for the food (stomach of Ruminants) (figs. 47 & 4(8). In the middle section of the alimentary ca- R nal,or intestine, the digestive processes, al- ready c o m - menced in the mouth by the action of the salivary secre- tion and con- tinued in the stomach by the action of the pepsin of the gastric juice (upon albumins in an acid solution), is completed. The food constituents which have been so far unacted upon (chyme) are in the intestine submitted to the action of the secretions of the liver, pancreas, and intestinal glands, and by them converted into the chyle, which is absorbed by the intestinal walls ; the albumins being converted, as in the stomach, into soluble FIG. 48. Alimentary canal of modifications by the action of trypsin a butterfly.* proboscis (ma- /. lctin _ however, only in alkaline solutions). xilla 1 ) ; Sp, salivary glands: x Oe, oesophagus; s, sucking The intestine often attains a great length, stomach ; .%, Maipighian ^ becomes divided into regions possessing tubules ; Ad, rectum. a different structure; e.g., in the intestine of mammals three regions can be distinguished duodenum, jejunum, and ileum. Its surface is, as a rule, increased by the develop- ment of folds and villi, and sometimes of outgrowths. Amongst PIG. 47. Alimentary canal and ac- cessory glands of a caterpillar. O, mouth ; Oe, oesophagus ; Sp 1>, salivary glands ; Se, spinning irlaiuls; MD, intestine (mid-gut); .l/i, rectum (hind gut) ; M G, Ma!- pighian tubes. 53 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. the Invertebrata it is often possible to distinguish an anterior especially widened portion of the intestine, which receives the hepatic secretion and is called stomach from the posterior, narrower, and longer .section, which is known as intestine. The hindermost section of the alimentary canal or hind gut, which is not always sharply marked off from the intestine, is especially concerned with the collection and repulsion of the undigested remains of the food, or fpeces. It may also possess csecal appendages attached to its anterior part, and possessing a digestive function. In the lower animals it is a small structure, but in the higher animals it at- tains a much more considerable length, and receives anteriorly one (Mammalia) or two (Birds) c?eca, and it may be sub-divided into two pai'ts, known as large intestine and rectum ; in the Vertebrata its hind end receives the ducts of various glands (kid- ney, generative organ*, anal glands). It may in addition dis- charge other functions, e.g., a respiratory (larvae of LibelluHdse) or a secretory function (larva of Ant Lion). The salivary glands, liver, and pancreas are to be regarded as outgrowths of the alimentary canal which have become diffe- rentiated into glands. The secretion of the salivary glands is poured into the buccal cavity, and there performs two functions (1) it dilutes the food, (2) it has a chemical action upon it, converting the starch into sugar : they are absent in many aquatic animals and are especially developed in herbivorous animals. FIG. 49. Alimentary caual of a bird. Of, oesophagus; A', crop ; Din, proventriculus ; A"..<, gizzard; D, small intestine; P, pan- creas placed in the loop of the duodenum ; //, liver; 0, the two casca; U t ureter ; Or, oviduct; Ai 5"rf -.0 FIG. 52. Male of Branchipus stagnalis with mnny- chambered heart or dorsal vessel fig, the lateral openings in which are repeated in every seg- ment. D, intestine ; 3f, mandible ; Sd, shell gland; Er, branchial appendage of the llth pair of legs; T, te^ti.-. A r FIG. 53. Heart of a Copepod (Calanella) with an ante- rior artery, A. Os, ostia ; V, valves at the arterial ostium ; M, muscle. maintains a continuous circulation of the blood. The heart is either sac-shaped, with two lateral or one anterior slit-like opening (Daphnia, Calanus) (fig. 51), or elongated and divided into successive chambers and perforated by many pairs of slit-like openings (Insects, Apus) (fig. 52). As a rule, each chamber possesses a pair of laterally placed <)-} OBGANJZATIOX AND DLVELOP.MEXT OF ANIMALS IX GENEBAL. ostia, provided with lip-like valves, which act so as to allow the blood only to enter the organ. From the heart, as central organ of the circulation, well defined canals, the blood vessels, are then developed, which in the Invertebrata may alternate with lacunae not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart Avhich are provided with independent walls, and developed into blood vessels (marine Copepoda, Calanella, fig. 53). At a higher stage of development not only do these efferent vessels acquire a more complicated structure, but a part of the lacuna-system, especially in the neighbourhood of the heart, acquires a membranous invest- ment, and gives rise to vessels which carry the blood back to the ,P* A.al FIG. 54. Heart and blood vessels nnd gills of the crayfish. C, heart, in a blood sinus ; with Pa several pairs of ostia; Ac, cephalic aorta; Aab, abdominal aorta; An, sternal artery. pericardial sinus, from which it passes through the venous ostia into the heart (Scorpions, Decapods) (fig. 54). In other cases (Molluscs) the blood flows directly from the afferent vessels into the heart, the walls of the vessel being directly continuous with the walls of the heart. The heart in such cases consists of two chambers, the one known as auricle serves for the reception of the returning blood, the other known as ventricle for its propulsion (fig. 55). The vessels passing from the ventricle and carrying the blood from the heart are called arteries ; those returning the blood to it are called veins, and, in the higher animals, are distinguished from the arteries by their thinner walls. Between the ends of the arteries and the beginning of the veins the body cavity intervenes either as HEART OF VERTEBRATES. <".', a blood sinus or as a system of blood-lacunae; or the arteries ami veins are connected by a network of delicate vessels, the capillaries. If the connection between arteries and veins is effected by capillaries in all parts of the vascular system, and the body cavity, as in the Vertebra ta, no longer functions as a blood sinus, the vascular system is spoken of as being completely closed. In the Vertebrates and segmented worms the vascular system ob- tains a considerable development before a true heart is differential e4 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. \ from the ventral vessel. The anterior pair, placed behind the mouth, unite beneath the notochord to form the root of the median body artery (descending or dorsal aorta) which receives the hinder succes- sive pairs of lateral vessels. This dorsal artery gives off branches to the muscles of the body wall and the viscera, from which the venovis blood in part is returned to the ventral pharyn- geal vessel; part of it, however, before reaching the latter, traverses a capillary network in the liver. From the hinder part of the ventral pha- ryngeal vessel there is developed, in the higher Vertebrata, the heart, which at first has the shape of an S-shaped tube, but later acquires a conical form and becomes divided into auricle and ventricle. The former receives the blood returning from the body and passes it on into the more powerful ventricle, from which arises an anterior vessel, the ascending or cardiac aorta, presenting a swelling at its root, known as the aortic bulb. This vessel leads, by means of lateral vascular arches, the arterial arches, into the dorsal aorta, which passes backwards beneath the vertebral column, and supplies the body. Valves placed at the two ostia of the ventricles regulate the direction of the blood stream ; and they are so arranged as to prevent } ackward fl f blood f th car( }i ac J aorta into the ventricle in diastole, and from . , . . , . the ventricle into the auricle in systole. j n consequence of the insertion of the respi- ratory organs on to the system ot the arterial arcnes the latter, and at the same time the structure of the heart, assumes various degrees f comp }i ca ti O n. In fishes (fig. 57), four or five pairs of gills are inserted in the course of the arterial arches, which break up into a respiratory capillary net- work in the branchial leaflets. From this network the arterialised blood is collected into efferent branchial arches, the branchial veins, corresponding each to a branchial artery ; and these unite to form the dorsal aorta. In such cases the heart remains simple, and receives venous blood. FIG. 56.-Anterior part of the vascular system ofanOHgochseteworm (Ssenuris) (after Ge- genbaur). in the dor- sal vessel the Hood moves from behind forward ; in the ven- trai vessel from before backwards (see ar- rows). //, heart-like dilated transverse lateral vessels. PULMONARY CIRCULATION. 65 With the appearance of lungs as respiratory organs (Dipnoi, Perennibranchiate Amphibia, larvae of Salamanders and Batra- chians) (fig. 58), the heart obtains a more complicated structure, in that the auricle becomes divided into a right and left division, the latter of which receives the arte- rialised blood, returning from the lungs by the pulmonary veins. The septum between the two divisions of the auricle may, how- ever, remain incomplete (Dipnoi, Proteus). The advehent pulmon- ary vessels, the pulmonary arte- ries, always proceed from the FIG. 57. Diagram of the circulator y organs of an osseous fish. V, ventricle ; Sa, aortic bulb with the arterial arches which carry the venous blood to the gills ; Ao, dorsal aorta into which open the vessels from the gills or branchial veins Ab. N, kidney ; D, alimen- tary canal ; L/C, portal circulation. FIG. 58. Gills (Sr) and pulmonary sacs (P) of a perennibranchiate amphibian. Ap, pulmonary artery proceeding from the posterior of the four aortic arches. The other three lead to the three pairs of gills ; D, alimentary tract ; A, aorta. posterior vascular arch, which, as a rule, loses its relation to the branchial respiration. On the disappearance of the gills, which is completed during the metamorphosis in the Salamandrina and Batrachia, the pulmonary 5 66 OKGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. arteries obtain a much more considerable size and become the direct continuation of the hindermost pair of vascular arches, while the remaining and primitively most important portions of the latter, i.e. the portions leading to the dorsal aorta, are reduced to rudimentary ducts (Ductus Botalli) or completely obliterated. Contemporaneously with these changes there appears a fold in the lumen of the ventral or cardiac aorta, leading to a separation of the posterior vascular arch (pulmonary artery), which now receives through the ventricle venous blood from the right auricle, from the system of anterior arches which give origin to the cephalic vessels and dor- sal aorta and receive arterial blood from the left auricle (mixed, how- ever, with venous blood in the ventricle) (fig. 59). In Reptiles the sepa- ration of the arterial from the venous blood is more complete, in that there is an incomplete ventricular septum which foreshadows the later division of the ventricle into a right and a left half. From the left division arises the right aortic arch, which gives origin in its further course, to the arteries to the head (carotid arteries). A vessel to the lungs and a left aortic arch may also be distinguished. The left aortic arch and pulmonary artery receive only venous blood, while the right aortic arch, and therefore the carotids which proceed from it, receive principally arterial blood from the left side of the ventricle (fig. 60). The ventricular septum, and consequently the separation of the right from the left ventricle, is found complete for the first time FIG. 59. Circulatory organs of the frog. P, left lung, right lung is removed ; Ap, pulmonary artery ; Vp, pulmonary vein ; Vc, vena cava inferior ; Ao, dorsal aorta ; N, kidney ; D, alimentary canal ; Lk, portal circulation. LYMPHATIC SYSTEM. 67 in the Crocodilia, and in these animals the right aortic arch arises from the left ventricle. But the separation of the arterial and venous blood is even now not quite complete, for at the point where the two aortic arches cross one another there is a passage (foramen Panizza?) leading from one into the other, and through which a communication may take place. It is only in Birds and Mammals, in which, as in the Crocodilia, the right and left ventricle are completely separated, that a separation between the two kinds of blood is completely effected (fig. 61). In Birds the right aortic arch persists, and the left entirely disappears ; while in Mammalia the opposite obtains, the left arch per- sisting and giving rise to the dorsal aorta. In these animals the blood is essentially diffe- rent from the chyle both in colour and composition, and there is present a special system of chyle and lymph vessels. This system origi- nates in simple tissue spaces, which are without walls, and its main trunks open into the vascular system. The con- tents are derived from the nutrient material absorbed from the intestine (chyle), and from the fluids which have transuded into the tissues from the capillaries (lymph), and they serve to renovate the blood. In the actual course of the lymph and chyle, i.e., in the lymphatic vessels themselves, are placed peculiar glandular organs, known as lymphatic glands (blood glands), in which the lymph receives its form elements (lymph corpuscles = white blood corpuscles). Organs of Respiration. The blood needs for the retention of its properties not only this continued renovation by the addition of nutrient fluids, but also the constant introduction of oxygen, with the reception of which is clofely connected the excretion of cai'bonic A Fir:. GO. Heart and great vessels of a Cheloniim. Ad, right auricle ; As, left auricle; Ao.it. right aortic arch ; An.s, left aortic arch ; Ao, aorni ; C, carotids ; Ap, pulmonary arteries. 68 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. acid (and water). The exchange of these two gases between the blood and the external medium is the essential part of the respiratory process, and is effected through organs which are suited for carrying on this process either in air or in water. In the simplest cases the exchange of these two gases takes place through the general surface of the body ; and in all cases, even when special respira- tory organs are present, the outer skin also takes part in respiration. FIG. 62. Diagram of the great arteries of a mammal with reference to the fira-embry- oiiic arterial arches (after Rathke). c, common carotids ; c', external carotid ; c", inter- nal carotid; A, aorta. Ap, pulmonary artery ; Aa, aortic arch. Inner surfaces also may be con- cerned in this exchange, especially those of the digestive cavity and intestine, or, as in the Echi- noderms, in which a separate vascular system is developed, the surface of the whole body cavity. Respiration in water obviously takes place under far more un- favourable conditions for the introduction of oxygen than does the direct respiration in air, because it is only the small quantity of FIG. 61. Diagram of the circulation in an animal with a completely separated right and left ventricle, and a double circulation (after Huxley). Ad, right auricle receiv- ing the superior and inferior venas cavae, Yes, and I'd ; Dth, thoracic duct, the main trunk of the lymphatic system ; Ad, right auricle ; Vd, right ventricle ; Ap, pulmonary artery ; P, lung ; Vp, pulmon- ary vein ; As, left auricle ; Vs, left ven- tricle ; Ao, aorta ; D, intestine ; L, liver ; !>', portal vein; Lr, hepatic vein- RESPIRATORY ORGANS. 69 Ct oxygen dissolved in water which is available. Hence this form of respiration is found in animals low in the scale of life in which the metabolic processes are less energetic (worms, molluscs, and fishes). Organs of aquatic respiration, or gills, have the form of external appendages possessing as large a surface extension as possible. They consist of simple or aiitler- shaped or dendritically branched processes (fig. 63 a, b), or of FIG. 63cr. Head and anterior body segments of a Eunice, viewed from the dorsal sur- face. T, tentacles. Ct, tentacular cirrus. C, parapodial cirrus. Br, parapodial gill. lancet-shaped closely-packed leaves with a large surface extension (fig. 64). Br FIG. 64. Transverse section through the gill of a Teleostean fish, b, branchial leaf- let with capillaries ; c, branchial artery con- taining venous blood ; d, branchial vein con- taining arterial blood. a, branchial bar. FIG. 63i. Transverse section through the body of Eu- nice. Br, gill ; C, cirrus ; P, parapodiuni with a bundle of seta? ; D, alimentary canal ; N, nervous system The organs of aerial respiration, on thecontrary, are internal. They present likewise the condi- tion favourable for an exchange of gases between the air and the bloou, viz., a large extent of surface. They have the form, either of lungs or air-bearing tubes. In the first case (Spiders, Vertebrates) they consist of spacious sacs with alveolar or spongy Thus instead -Mh 70 OHGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. walls, traversed by numerous septa and folds which bear an extremely rich network of capillaries. The air tubes or tracheae (fig. 65) consti- tute a branched system of canals which extend throughout the whole body, and carry the air to all the organs, of the respi- ratory pro- cess being localised, as it is in ani- ' mals with lungs, it is carried on in all tissues and organs of the body, which are surrounded by a fine trachea! network. Nevertheless, the air tubes in the case of the modification known as fan- tracheoi present an approximation in their structures to lungs, in that the main stems, without further hollow leaves. FIG. 65. Traehese with fine branches (after Leydig). Z, cellular, outer wall ; Sp, spiral thread. Tr- branching, give rise to flat T 'St FIG. 666. Lateral view of head and body of an Acridium. St, stigmata ; T, Tympanum. Openings in the body wall are present, placing the organs of aerial respiration in communica- tion with the exterior. These openings may be numerous, and paired, placed symmetrically on the sides FIG. 6G. Tracteal .^y* tern of a Dipterous hu-vsi. Tr, Longitudi- nal stem of the right side with tufts of trn- chese; St', and St", anterior and posterior stigmata; Mh, or;il hooks. TBACIIE.E. 71 of the body (fig. 66 a, b) (stigmata of Insects, Spiders), or they rnay be more restricted in number, and communicate also with cavities of complicated structure which are used for other functions (nasal cavities of Vertebrates). In the aquatic larvse of certain FIG. 67a. Larva of an Ephemeral fly with seven pairs of tracheal gills A7, slightly magnified ; Tk, isolated tracheal gill strongly magnified. FIG. 67i. Tracheal sys- tem at the sides of the alimentary canal of an Agrion larva (after L. Dufour). ZV, main tracheal trunk ; JT/, tracheal gills ; If a, the three simple eyes. Insects (Epherneridfe, Libellulidse) the tracheae may be without any external openings. In such cases processes of the body filled with a close network of tracheae, which take up oxygen from the water, and are known as tracheal gills, are developed (fig. 67 a, b). In rare instances tracheal gills are developed on the wall of the rectum, and 7Z ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. thus acquire a protected position (rectal respiration of Aeschna, Libellula). In other respects the branchial and pulmonary respiratory pro- cesses are essentially the same. In the pulmonate snails (Lymnseus), the pulmonary cavity may be filled with water, and yet continue to function as a respiratory organ (in the young state and also under special conditions in the adult, the animal remaining permanently in deep water). With this fact before us of an air-breathing surface functioning as a gill, it will not surprise us to find that gills and branching folds of skin, which under normal circumstances serve for breathing in water, can, provided they be protected from shrivelling up and desiccation either by their position in a damp space or by their copious blood supply, function as lungs, and allow their pos- sessors to live and breathe on land (Crabs, Birgus latro, labyrintho- branchiate Fishes). A rapid renewal of the medium which carries the oxygen and surrounds the respiratory surfaces is of the greatest importance for the gaseous exchanges. We find, therefore, very often special arrangements, by which the removal of that part of the respiratory medium which has been deprived of oxygen and saturated with carbonic acid and the introduction of another portion con- taining oxygen and free of carbonic acid, is effected. In the simplest cases this renewal can, although not very efficiently, be brought about by the movements of the body, or by a continuous oscillation of the respiratory surfaces themselves ; a method which is especially common when the gills are placed in the region of the mouth and function also as organs of food prehension, e.g., the tentacles of many attached animals (Polyzoa, Brachiopoda, tubi- colous Worms, etc.) Very frequently the gills appear as appendages of the organs of locomotion, e.g., of the swimming or ambulatory feet (Crustacea, Annelids), the movement of which brings about a renewal of the respiratory medium around the gills. The move- ments become more complicated when the gills are enclosed in special chambers (Decapoda, Pisces), or when the respiratory organs are placed within the body, as happens in the case of tracheae and lungs, in which case also a renewal of the air is effected either by a more or less regular movement of neighbouring parts, or by rhyth- mical contractions and dilatations of the air-chamber, constituting the so-called respiratory movements. The term respiration is now not only applied to these movements so obvious to the eye in air- breathing animals, but also to the osmotic processes, secondarily AHIMAL HEAT. 73 dependent upon the entrance and exit of air, which effect the gaseous exchanges. Taken strictly in this sense it is an incorrect term, inasmuch as in the respiratory movements of animals pro- vided with branchial cavities we have to do with the entrance and exit of water. In the higher animals provided with red blood, the difference in the condition of the blood before and after its passage through the respiratory organs is so striking that it is possible to distinguish blood rich in oxygen from blood rich in carbonic acid, by the colour. The latter is dark red, and is known as venous blood ; the former, i.e., blood which has just left the gills or lungs, on the contrary, has a bi-ight red colour, and is known as arterial blood. While the terms venous and arterial are used in an anatomical sense to express the nature of the blood-vessel, those carrying the blood to the heart being called venous, and those carrying it from the heart arterial, they are also used in a physiological sense as an expression for the two conditions of the blood before and after its passage through the respiratory organs, i.e., to express the quality of the blood. Since, however, the respiratory organs may be inserted in the course of either the venous or arterial vessels, it is obvious that, in the first case, there must be venous vessels carrying arterial blood, (Molluscs and some Vertebrates), and, in the latter, arterial vessels carrying venous blood (Vertebrates). Animal heat. The intensity of respiration stands in direct relation to the energy of the metabolism. Animals which breathe by gills and absorb but little oxygen are not in a position to oxidise a large quantity of organic constituents, and can only transform a small quantity of potential into actual energy. They perform, therefore, not only a proportionately smaller amount of muscular and nervous work, but also produce in only a small degree the peculiar molecular movements known as heat. The source of this heat is to be sought, not, as was formerly ei-roneously supposed, in the respiratory organs, but in the active tissues. Animals in which thermogenic activities are small have no power of keeping independently their own internal heat when exposed to the temperature influences of the surrounding medium. This is also true of those air-breathing animals in which the metabolic and thermogenic activities are great, but which, in consequence of their small size, offer a relatively very large surface for the loss of heat by radiation (Insects). On account of the ex- changes of heat which are continually taking place between the animal body and the surrounding medium, the temperature of the 74 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. former must in such animals be largely dependent on that of the latter, falling and rising with it. Hence, most of the lower animals are poikUothermic,* or, as they have less appropriately been called, cold-blooded. The higher animals, on the contrary, in which, on account of their highly developed respiratory organs and energetic metabolism, the thermogenic activity is great, and which are protected from a rapid loss of heat by radiation by the size of their bodies and by the possession of a covering of hairs or feathers, possess the power of maintaining a constant temperature, which is independent of the rising and falling of the temperature of the surrounding medium. Such animals are designated homothermic, or warm-blooded. Since they require a high internal temperature, varying only within small limits, as a necessary condition for the normal course of the vital processes, or one may say for the maintenance of life itself, they must possess within themselves a series of regulators whose function is to keep the body temperature within its proper limits, when the temperature of the surrounding medium is high. This may be effected either by diminishing the production of internal heat (diminishing the metabolism) or by increasing the loss of heat from the surfaces of the body (by radiation, evaporation of secretions, cooling in water) ; and, on the contrary, when the temperature of the outer medium is too low, by increasing the production of internal heat (increasing the metabolic activity by more plentiful food supply, more vigorous movements), or by diminishing the loss of heat by the development of better protective coverings. When the conditions necessary for the action of these regulators are absent (want of food, small and unprotected bodies), we find either the phenomenon of winter sleep, in which life is preserved with a temporary lowering of the metabolic processes ; or, when the metabolic processes of the organism do not enter into abeyance, the remarkable phenomena of migration (migration of birds). Organs of Secretion. The respiratory organs stand to a certain extent intermediate between the organs of nutrition and those of excretion, in that they take in oxygen and excrete carbonic acid. In addition to this gas a number of excrementitious substances, mostly in a fluid form, which have entered the blood from the tissues, pass out by the lungs. The function, however, of excretion * Col. Bergmann, " Ueber die Verhaltnisse der Warmeokonomie der Thiere 7.\\ ihrer Grosse," Gottinger Stiulien, 1847; also Bergmann und Leuckart, Anatomisch-physiologische Uebersicht des Tkierreichs," Stuttgart, 1852. URINARY ORGANS. 75 is mainly discharged by the special secretory organs. These have the form of glands of a simple or complex structure which originate from imaginations of the outer skin or of the intestinal wall, and consist essentially of simple or branched tubes, or of racemose and lobulated glands. Among the various substances which by the aid of the epithelial lining of the walls of glands are removed from the blood and some- times utilised further for the performance of various functions, the nitrogenous excretory substances are especially important. The organs by which the excretion of these ultimate products of meta- bolism are effected are the kidneys. In the Protozoa they are represented by the contractile vacuoles ; in the Worms they appear as the so-called water- vascular vessels, and are constituted of a system of branched canals which take their origin in delicate internal ciliated funnels, which open into the spaces in the parenchymatous tissues or the body cavity. In the latter case the ciliated funnels have a wide opening. In the Platyelminthes (flat worms) the efferent ducts of the system consist of two main lateral trunks (fig. 68, Ex.), which frequently open together at the hind end of the body by means of a medium terminal contractile vesicle (fig. 68, ep). In the segmented worms the paired kidneys are repeated in every segment, and are known as segmented organs (figs. 69 and 70). The shell-glands of Crustacea are in all probability to be traced back to these segmental organs : as are also the paired kidney (organ of Bojanus) of mussels, and the unpaired renal sac of Snails, both of which communicate by means of an internal opening with the pericardial division of the body cavity. In the air-breathing Arthropods and some Crustacea (Orchestia) the urinary organs are tubular appendages (Malpighian vessels) of the hind gut. In the Vertebrata the urinary organs or kidneys obtain a greater independence, and open to the exterior by special FIG. G8. Yoirag Distomum (after La Valette). Ex, main stems of the excretory system ; Ep, ex- cretory pore ; O, month, syith sucker ; S, sucker in the middle of the ventral surface ; P, pha- rynx ; D, alimentary canal. 70 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. openings which are usually common to the generative organs ; they consist essentially of a number of coiled tubes, which in the more pi'irnitive types of Vertebrates have a ciliated funnel-shaped opening into the body cavity (Dogfish embryo, fig. 71). The individual tubules of which the verte- Wtr FIG. 69. Longitudinal section through the medicinal Leech (after R. Leuckart). D, ali- mentary canal ; G, brain ; Ok, ventral chain cf ganglia ; Ex, excretory canals (seg- mental organs, water- vascular system) . FIG. 70. Diagrammatic representation of the segmental organs of a segmented worm (after C. Semper). Ds, dissepi- ment ; Wtr, ciliated funnels which lead into the coiled tubes. brate kidney is composed do not open directly to the exterior, as do the segmental organs of Annelids, but there is present on. each side of the body a duct, the kidney duct, which receives the tubules of its own side and opens posteriorly into the cloaca. They also possess an important structure peculiar to the kidney of the Vertebrata known as the " Malpighian body," which consists of a capsular widening of the lumen of each CUTANEOUS GLANDS. i t Wtr tubule, into which projects a coil of arterial blood vessels known as the glomerulus (fig. 72). Very generally the outer body surface is the seat of special secre- tions which frequently play an impor- tant part in the economy of the animal, and are used especially as a means of protection and defence. The same is true also of the secretions of the accessory glands opening into the anterior or posterior end of the ali- mentary canal (salivary glands, poison glands, anal glands) (fig. 73). To the class of cutaneous glands belong, in the first place, the sweat- glands and the sebaceous glands of Mammalia. The fluid secretion of the former, on account of the ease with which it is evaporated, is of special use in keeping the body cool, while that of the latter keeps the integument and FIG. 71. Diagrammatic represen- tation of the kidney (segmental organs) of a dog-fish embryo (after C. Semper). Wtr, ciliated funnels ; Ug, kidney duct. Tr its special covering soft and supple. The coccygeal glands of water- birds are derived from an aggre- gation of sebaceous glands ; their secretion by keeping the feathers oiled preserves them from becom- ing saturated with water during swimming. The unicellular and multicell- ular integumentary glands, which are found so widely present in Insects, belong, for the most part, to the category of oil and fat- glands. Aggregations of cells whose function is to secrete calcareous matters and pigment are especially widely present in the integu- ment of the Mollusca, and serve for the building up of the beautifully FIG. 72. Ciliated funnel and Malpighian body from the anterior part of the kidney of Proteus (after Spengel). Nr, kidney tubule ; Tr, ciliated funnel ; J/^, Malpig- hian body. 78 OKGANIZATION AND DEVELOPMENT OP ANIMALS IN QENEKAL. coloured and variously shaped shells of these animals. Integumen- tary glands and aggregations of glands may also acquire a relation to the acquisition of food (spinning glands of Spiders). Finally, mucous glands are very widely present in the skin of animals which live in damp localities (Amphibia, Snails) and in water (Fishes, Annelids, Medusas). ORGANS OF ANIMAL LIFE. Of the so-called animal functions, that of locomotion is the most conspicuous. Animals perform movements for the purpose of procuring food and escaping from their enemies. The muscles used for locomotion are, as a rule, and especially in the simpler forms, intimately united with the skin, and give rise to a muscular body wall (Worms), the alternate shorten- ing and elongation of which brings about a movement of the body. The muscles may also be especially concentrated in parts of the body wall, e.g., in the subum- brellar surface of Medusae beneath the supporting gelatinous tissue, or in the ventral surface of the body giving rise to a foot-like organ (Molluscs), or they may be broken up into a series of successive and similar segments (Annelids, Arthropods, Vertebrates). The latter arrangement prepares the way for the rapid and more complete form of movement found in animals in which the hard parts also, whether exoskeletal (Arthropods) or endoskeletal (Vertebrata), have become divided into a series of longitudinally arranged segments or rings, which offer a firm attach- ment to and are moved by the segments of the muscular system. By this arrangement more powerful muscular actions are rendered possible. Thus it becomes indispensable that hard parts should be developed to act as a skeletal support for the soft parts, and also to protect them. The skeletal structures may be external, in which case they have the form either of external shells, tubes or successive rings, and are FIG. 73. Alimentary canal with its accessory glands of a beetle (Carabus) (after LiJonDuf our). Oe, oesophagus ; Jn, crop ; PC, proventriculus ; Chd, chylific ventricle ; Mg, Malpighian tu- bules ; -ff, rectum ; Ad, anal glands with bladder. ENDO SKELETON. 79 usually products of the external skin (chititi), or they may be internal (cartilage, bone) and give rise to vertebrce (fig. 74 a, b). In either case the body becomes divided at right angles to its long axis into a series of segments, which, in the simpler cases of locomotion, are homonomous (Annelids, Myriapods, Snakes). As development progresses some of the muscles required for locomotion gradually lose their relation to the long axis of the body, and acquire a relation to secondary axes; and in this way conditions are acquired for the accomplishment of more difficult and complete forms of locomotion. The hard parts in the long axis of the body then lose their primitive FIG. 74 a Diagram of the vertebral column of aTeleostean fish with verte- bral constriction of the notochorcl. Ch, notochord; Wk, bony vertebral bodies ; J, membranous intervertebral section. FIG. 74 b Vertebra of a fish. 5", ver- tebral body. Ob, neural arch (neum- pophysis) ; Ub, haemal arch (hfeinapo- physis) ; D. neural spine ; D', hwmal spine ; K, rib. uniform segmentation and partially fuse with one another to form several successive regions, the parts of which are capable of a greater or less amount of movement upon one another (head, neck, thorax, lumbar region, etc.) In this case, however, the parts of the skeleton of the chief axis are usually less movable upon one another, while, on the contrary, a much more perfect locomotion is effected by the extensive movements of the paired extremities or limbs. The limbs likewise possess a solid skeleton, to which the muscles are attached, and which is usually elongated and may be external or internal, and is attached more or less closely to the axial skeleton. The most essential property of animals is that of sensation. This 80 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. property, like that of movement, resides in definite tissues and organs which constitute the nervous system. For those cases in which a nervous system has not separated from the common contractile basis (sarcode) or from the uniform cell parenchyma of the body, we may suppose that the organism possesses the first beginnings of an irritability serving for perception. This, however, can scarcely be called sensation, for sensation pre-supposes the presence of conscious- ness of the unity of the body, and this we can scarcely attribute to the simplest animals without a nervous system. The appearance of muscles is coincident with that of the nervous tissues, which are developed in connection with the sense epithe- lium of the surface (Polyps, Medusae, Echinoderms). In such cases the nerve fibres and ganglion cells which all lie mingled together keep their ectodermal position and their connection with the sense epithe- lium. The view that the first diffe- rentiation of the nervous and mus- cular tissues is to be sought in the so-called neuromuscular cells of the fresh-water polyps and Medusae has been shown by later researches to be untenable. The arrangements of the nervous system can be traced back to three distinct types (1) the radial ar- rangement found in the radiate animals; (2) the bilateral arrange- ment found in segmented Worms, Arthropods, and Molluscs; (3) the bilateral arrangement of the Vertebrata. In the first case the central organs are radially repeated ; in the Echinoderms as the so- called ambulacral brains or nerves, which are found in the arms and are connected together by a circumoral nervous commissure contain- ing ganglion cells (fig. 75). In the second type the nervous system, in the simplest cases, consists of an unpaired or paired gangiionic mass placed in the anterior part of the body above the pharynx, and known as the supra-O3Sophageal ganglion or brain. From this centre radiate in the simplest cases (Turbellaria) nerves which have a bilaterally sym- metrical distribution, and of which two are larger than the others, and take a lateral course (fig. 76). FIG. 75. Diagram of the nervous sys- tem of a star-fish. N, nerve ring which connects together the five am- bulacral centres. NERVOUS SYSTEM. 81 At a higher stage of development a circum-pharyngeal nerve ring is developed. With the commencing segmentation of the body the number of ganglia increases, and in addition to the brain there is present a ventral nervous system consisting either of ventral cord D FIG. 76. Alimentary canal and nsrvous system of Mesosto- mum Ehrenbergi (after Graff). G, the paired cerebral ganglia with two eye-spots ; St. one of the two main lateral nerves ; Z>,alirnentary canalwithmonth and pharynx. C rt G 3 - G FIG. 77. Nervous system of FIG. 78. Nervous system the larva of Coccinella (after Ed. Brandt). G, su- pra-oesophageal ganglion or brain ; Gfr, frontal ganglion; Sy, subosso- phageal ganglion ; G',-G", the eleven ganglia of the ventral chain of thorax and abdomen. of adult Coccinella (after Ed. Brandt). A(j, optic gansrlion. The other let- ters as in fig. 77. (Gephyrea) or of a ventral chain of ganglia, which may have a homonomous (Annelids) or heteronomous (Arthropods) arrangement (figs. 77 and 78). The concentration of the nervous system begun 6 82 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. in the latter case may, by the fusion of the brain and ventral cord, be carried to a still further extent, so that in many cases (numerous Arthropods) only a sub-oesophageal ganglion is present. In Molluscs, animals in which segments are not de- veloped, the subcesophageal ganglion is represented by the pedal ganglion, and there is in addition a third pair of ganglia constituting the visceral ganglia (fig. 55). In Vertebrates, the nervous centres are arranged as a cord, lying on the dorsal side of the skeletal axis, and known as the spinal cord, the segmentation of which is indicated by the regular repetition of the spinal nerves. This cord, which is traversed by a central canal, is anteriorly widened and (except in Amphioxus) differentiated into a complicated ganglionic apparatus, the brain (fig. 79). The so-called sympathetic or visceral nervous system appears in the higher animals (Vertebrata, Arthropoda, Hiru- dinea, etc.) as a comparatively indepen- dent part of the nervous system. It consists of ganglia and plexuses of nerves which stand in connection with the central nervous system, but are not under the direct control of the will of the animal. They innervate the organs of digestion, circulation, respiration, and generation, and they can carry on their functions for a longer or shorter time after destruction of the sensory and motor centres. In the Vertebrata (fig. 80), the system of visceral nerves consists of a double chain of ganglia, placed on each side of the vertebral column and con- nected with the spinal nerves and the spinal-like cranial nerves, by connecting branches, the rami communicantes. The ganglia correspond in number with the above- mentioned spinal and cranial nerves, and they send nerves to the FIG. 79. Brain and spinal cord of a pigeon. //, cerebral hemispheres ; Cb, optic lobes ; C, cerebellum ; Mo, medulla oblongata. Sp, spinal nerves. SENSE OEGANS. 83 blood vessels and viscera, which there form a complicated network of nervous fibres containing here and there ganglion cells. The nervous sys- tem possesses further peripheral apparatus, the sense organs, the function of which is to bring about the perception of certain conditions of the outer world as im- pressions of a definite mode of sensation (specific energy of nerves* Joh. Miiller). These peripheral organs usually have the form of peculiarly arranged aggrega- tions of hair-shaped or rod-shaped nerve terminations (hair- cells, rod-cells of sen- sory epithelium) con- nected by fibrilhie with ganglion cells, through which under the action of external influences a move- ment of the nervous substance is set up, which travels to the central organ and there affects con- : In opposition to the differences in the quali- ties of the sensations produced by each indi- vidual sense organ (colour, tone). FIG. 80. Nervous system of the frog (after Ecker). Ol olfactory nerves ; O, eye ; Op, optic nerve ; Vy, Gasseriau ganglion ; Xg, ganglion of vagus ; Spn 1, first spinal nerve ; Sr, brachial nerve ; Sql-W, the ten ganglia of the sym- pathetic system. Ji, ischial nerve. 84 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. sciousness as a specific .sensation. To these end-cells there are often added cuticular structures, whose function is to communicate the external movement to the nervous substance (retinal rods). The special sensations have quite gradually been developed from the general sensations (comfort, discomfort, pleasure, pain), i.e., nerves of special sense have been derived from sensory nerves which have acquired a special form of peripheral termination, and so become accessible to a special stimulus with which the special sensation is always associated. But it is not till a higher stage of development is reached that the sense-perceptions can be compared according to the nature of the sensations with those of our own body. We can estimate the sense energies of the lower animals exceedingly vaguely, and only by the insufficient method of com- paring them with our own sensations; and it is certain that among the lower ani- mals there are many forms of sensation of which we, in consequence of the spe- cialised nature of our own senses, can have no concep- tion. Probably of all the senses, that of touch is the most widely distributed, and with this we certainly often see a number of special sensations united. It is generally distributed , over the whole surface of the body ; frequently, however, it is con- centrated on processes and appendages of it. Probably the tentacular appendages of the Coelenterata and Echinodermata have this signifi- cance. In the Bilateralia with a differentiated head there are contractile or stiff segmented processes on the head, the antennce or feelers which in the worms are repeated as paired cirri on every segment of the body. It is often possible to trace special nerves to the skin and to find touch organs containing their endings. In the Arthropoda the ganglionic end-swelling of a tactile nerve usually lies beneath a cuticular appendage, such as a bristle, which transmits the mechanical pressure on its point to the nerve (fig. 81). FI&. 81. Nerves with ganglion cells (G) beneath a tactile bristle (TV) from the skin of Corethra larva. AUDITORT AND VISUAL ORGANS. 85 In the Primates amongst the Mammalia there are present papilla; in the skin (especially on the volar surface) in which the structures known as touch-bodies, containing the termination of tactile nerves, are placed (fig. 82). In addition to the general sensibility and the tactile sensations, the higher animals possess, as a special form of sensibility, the capacity of distinguishing different temperatures. The sensations of sound are produced through an organ, the auditory organ, which is, in a certain measure, a special modification of a tactile organ. The auditory organ in its simplest form appears as a closed vesicle filled with fluid (Endolymph) and one or more calcareous concretions (otoliths) ; and containing in its walls rod or hair cells in which the nerve fibrillse end (fig. 83). Sometimes the vesicle lies on a ganglion of the central ner- vous system (Worms), sometimes at the end of a shorter or longer nerve, the auditory nerve (Molluscs, Decapoda). In many aqua- tic animals the vesicle may be open and its contents communicate directly with the exter- nal medium, in which case the otoliths may be represented by small particles such as sand- grains which have entered it from the exterior (Decapod Crustaceans). In Molluscs a deli- cate sensory epithelium (macula acustica, fig. 83 Cz, Hz.}, marks the percipient portion of the inner wall of the vesicle ; while in Crus- tacea the fibres of the auditory nerve end in cuticular rods or hairs which project from the wall of the vesicle, and, like the olfactory hairs of the antenna?, bring about the nervous excitations. In the Yertebrata not only does the auditory vesicle obtain a more complicated form (mem- branous labyrinth), but there are also added to it apparatuses for conducting and magnifying the sound (fig. 84). . The tympanum of Acrideidse and Locustidse, which is generally looked upon as an auditory organ, is built upon quite a different type, since here, instead of a vesicle filled with fluid, air cavities serve for the action of the sound waves on the nerve-endings. The visual organs or eyes * are, after the tactile organs, the most widely distributed, and indeed are found in all possible stages * Cf. E. Leuckart, ' Orpranolusrie cles Auges," Graefe and S'amisch, Hand- burh der Oplitbalniologie. Bd. II. FIG. 82. Tactile papilla from the volar surface with the touch corpuscle and its nerve N. 86 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. of perfection. In the simplest cases they are known as eye-spots, and consist of irritable protoplasm, i.e., nervous substance, containing pig- ment granules ; and in this form they are perhaps scarcely capable of distinguishing light from darkness, but are only susceptible to the warm rays. It is hardly possible to conceive that pigment is indis- pensable for the sensation of light, because there are many eyes of complicated structure from which pigment may be altogether absent. The view, however, according to which the pigment itself is sensitive to light, i.e., is chemically changed by the light waves and transmits the excitation produced by these movements to the protoplasm or fz Ot Wz FIG. 83 Auditory vesicle of a Heteropod (Pterotrachea). Jf, acoustic nerve ; Of, otolith the fluid of the vesicle ; Wz, ciliated cells on the inner wall of the vesicle ; Hz, auditory cells ; Cz, central cell. the adjacent nervous substance cannot in itself be contradicted, but it is by no means clear that such changes are produced by the light rays as opposed to the heat rays. Of greater importance in this relation appears the special nature of the nerve endings, through which certain movements, progressing in regular waves, the so-called ether waves, are transmitted to the nerve fibres and give rise to a stimulus which travels to the central organ and is by it perceived as light. In all cases in which in the lower animals specific nerve endings cannot be made out, we have probably only to do with a forerunner of the eye, consisting merely of the pigmented termina- BEFRACTILE MEDIA AND PIGMENT. 87 tion of a cutaneous nerve which is sensitive only to gradations of temperature. Although the sensation of light is the function of the nerve centre, the rods and cones at the end of the optic nerve fibres are the elements which convert the external movement of the ether waves into an excitation of the optic nerve fibres adequate for the production of the sensation of light. For the perception of an image refractile apparatuses in front of the terminal expansion of the optic nerve (retina) are necessary ; and further, the elements of the latter imist be sufficiently isolated to admit of the stimuli set up in them being carried as separate movements to the nerve centre. Instead of a general sensation of light a complex sensation made up of many separate perceptions is produced, which corre- spond in position and quality with the parts of the exciting source. For the refraction of the light convex and often lens- shaped thickenings of the body covering (cor- nea, corneal lens) through which the rays pass into the eye, are developed ; refractile bodies are also found behind the cornea (lens, crystalline cone). The rays diverging from the various parts of the source of the light are, by means of the refractile media, collected and brought to corresponding foci on the retina or peripheral expansion of the optic nerve, which consists of the rod-shaped ends of the nerve fibres and some more or less complicated ganglionic structures. Lately, in consequence of the discovery of the visual purple * in the outer segments of the rods, it has been attempted to reduce the excitation of the end apparatus of the optic nerve to a photo-chemical process taking place in the retina. The fact that the diffuse pigment (visual purple) of the outer segments of the rods is bleached by the f In addition to the older works of Krohn, H. Miiller, M. Schultze, cf. Boll Sit zungsberi elite cler Akad. Berlin, 1876 and 1877, also E \vald and Kiihne. FIG. 84. Diagram of the auditory labyrinth. .1. of a fish. II. of a bird. III. of a mammal (after Wal- deyer). V, utricle with the three semicircular canals ; S, saccule ; US, alveus communis ; C, cochlea ; L, la- gena ; R, aqueductus vestibuli ; Cr, canalis reuniens. 88 OKGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEKAL. action of light is of the highest interest, but it cannot be taken as proving a direct participation of the visual purple in the visual process, inasmuch as the visual purple is not present in those parts of the eye in which alone a distinct image is formed, viz., the macula lutea and, generally, the outer segments of the cones. The pigment of the eye seems to be of importance for absorbing the superfluous rays of light which would be injurious to the per- ception of an image. It is distributed partly immediately outside the retina, forming the choroid coat of the eye, which extends also inwards between the individual retinal elements ; and partly in front of the lens, giving rise to a transversely placed curtain, the iris which is pierced by an opening, \h& pupil, capable of contrac- ting and dilating. In the higher grades of development the whole eye is, as a rule, enclosed in a hard, connective tis- sue coat, the sclerotic, and thus marked off as an eye bulb. The arrangements by which the shining points of an object act in regular ar- rangement on corre- sponding points of the optic nerve and so render possible the perception of an image vary, and are closely dependent upon the whole structure of the eye. Leaving out of consideration the simplest eyes, such as we find in Worms and the lower Crustacea, two types of eye are to be distin- guished. 1. The first form occurs in the so-called facetted eyes* (figs. 85 & 86) of Arthropods (Crustacea and Insects). The retina of such eyes has a hemispherical form, the convex surface being directed out- wards, and consists of large compound nerve rods, the retinulaj * See Job. Mailer, "Zur vergleichenden Physiologie des Gesichtssinnes," Leipzig, ls2i>. H. Grenadier, ' : Untersuclumgen iiber das Sehorgan der Arthro- podeu," Gottiiigen, 1879. A FIG 8F. Diagrammatic representation of the compound eye of aLibellula. C, cornea ; K, crystalline cone ; P, pigment ; .ff, nerve rods of retina ; Fb, layer of fibres : Gz, layer of ganglion cells ; Rf, retinal fibres ; Fk, crossing of fibres. UXICORNEAL EYE. S'.l -K (figs. 85 & 86 Rf & It), which are separated from one another by pigment sheaths. In front of these rods are placed the strongly refractile crystalline cones (&), and in front of these again the lens- shaped corneal facets (C < f). The eye is enclosed by a firm, chitinous layer, which, following the sheath of the entering optic nerve, surrounds its soft parts and reaches as far as the cornea. That part of the eye which is known as optic nerve corresponds in a great measure to the retina itself, and contains a layer of ganglion cells and of nerve fibres. A reversed and reduced picture of the object is thrown behind each convex corneal facet (lying- far from the sensitive layer of nervous rods), and only the perpendicular rays can be perceived since all the others are absorbed by the pigment. Ac- cordingly the light impressions caused by these axial rays, whose number corresponds with the separate nerve rods, form a mosaic on the retina which repeats the arrangement of the parts of the external object emitting light. The picture which is here formed lacks, however, brilliancy and dis- tinctness. 2. The second form of eye, which is widely distri- buted in the animal kingdom (the simple eye, Annelids, Insects, Arachnida, Molluscs, Verte- brates) corresponds to a globular camera obscura with collecting lenses (cornea, lens) on its exposed anterior wall on which the light falls and usually with additional dioptric media filling the optic chamber (vitreous humour.) The simple eye of Insects seems to have originated from the simple metamorphosis of part of the integument, beneath which are placed the end organs of the optic nerve (fig. 87). The cuticular covering (CL) projects as a lens- shaped thickening into the subjacent layer of transparent, elongated, hypodermis cells (Gk), within which are placed elongated rod-like nerve- cells with refractile cuticular portions, closely aggregated to form a retina (fig. 87 Hz). The hypodermis cells surrounding the edge of the lens are filled with pigment, and form an iris-like dark ring FIG. 86. Three fa- cets with retinulse from the com- pound eye of a cockchafer (after Grenadier) . The pi u'ment has been dissolved a\\;i\ from two of them . F, corneal facet. E, crystalline cone. P, pigment sheath. P-, chief pigment cell. P", pigment cells of the second order. S, retinulae. 90 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. through the opening in which the rays of light enter the eye to fall on the terminal segments of the retinal cells (fig. 87). In the more highly developed forms of this type of eye, especially in the Vertebrate eye, the peripheral portion of the optic nerve spreads out so as to form a cup-shaped nervous membrane, the retina, placed immediately behind the refractile media and surrounded by a vascular pigmented membrane, the choroid. The choroid, again, is surrounded by a tough supporting membrane composed of fibrous connective tissue, and known as the sclerotic, which is continued over the anterior part of the eye, i.e., that part through which the light passes, as a thinner transparent membrane. Of the refractile media which are placed behind the cornea and fill the cavity of the optic bulb, viz., the aque- ous humour, the lens (fig. 88 L\ the vitre- ous humour (6-7), the lens is the most powerful. Grasped by the thickened muscular anterior part of the choroid (the ciliary body (Cc) and ciliary processes), the peripheral part of its anterior face is covered by a forward continuation of the choroid, the iris (Jr\ which, as a ring-like contractile border, forms a kind of diaphragm perforated by a central contractile opening, the pupil, through which the light enters the eye (fig. 88). The reversed image which is formed in the hinder part of the Vertebrate eye on the cup-shaped retina has a very considerable brilliancy and definition. The eyes of many Cephalopods may be looked upon as a modifica- tion of this type of eye. In the eye of Nautilus the lens is absent, and the light enters through a small opening. In this case a reversed, but not brilliant, image is formed on the retina placed on the hinder wall of the eye. To enable the eye to see clearly objects in different directions and Rz FIG. 87. Transverse section through the simple eye of a beetle larva (.partly after Grenacher). CL, corneal lens ; Gk, the subjacent hypoclermis cells, the vitreous humour of Authors ; P, pigment in the peripheral cells of the lat- i ter ; Jfo, retinal cells. St, cuticular rods of the latter. OLFACTORY ORGAN. 91 Cr. at different distances, special apparatuses for its movement and accommodation are necessary. They are represented by muscles which can in the former case move the optic bulb and modify the direction of sight in obedience to the will of the animal, and in the latter act upon the refractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the head on which the eye is placed is prolonged so as to give rise to a movable stalk-like process, which bears the eye at its extremity. The eyes of Vertebrata possess in addition special protective arrangements, e.g., eyelids, lacrymal glands. The position and number of the eyes present very great variations amongst the lower animals. The paired arrangement on the head appears to be the general rule among the higher animals ; nevertheless visual organs sometimes occur on parts of the body far removed from the brain, as for instance, in Euphausia, Pecten, Spondy- lus, and certain Annelids (Sabellidse). In the Radiata the eyes are repeated at the periphery of the body in each radius. In the star fishes they lie at the extreme end of the ambulacral furrow at the tip of the arms, in the Acalephse as the marginal bodies on the edge of the umbrella. The sense of smell appears to be less widely distributed. Its func- tion is to test the quality of gaseous matters and to produce in consciousness the special form of sensation known as " Smell." This sense in aquatic animals which breathe through gills cannot be sharply marked off from that of taste. The small pits, standing in connec- tion with nerves and provided with an epithelial lining of hair-bearing sense cells are to be looked upon as the simplest form of olfactory organ (Medusa?, Heteropoda, Cephalopoda). Nevertheless scattered hair cells (Lamellibranchiata) may also have to do with .the same sensation. In the Arthropocla the cuticular appendages of the FIG. 88. Transverse section through the human eye (after Arlt). C, cornea ; L, lens ; J>, iris with pupil ; Cc, ciliary body ; 61, vitreous humour ; -ffi, retina ; Sc sclerotic ; Ch, choroirt. Ml, macula lutea ; Po, papilla optica ; -A'o, optic nerve. 92 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. antenna? in which the gangliated swollen extremities of nerves occur are to be explained as olfactory fibres. In the Vertebrata the olfactory organ usually has the form of a paired pit or cavity placed on the under surface of the head (nasal cavity), on the walls of which the ends of the olfactory nerve are distributed. The higher air- breathing Vertebrata are distinguished by the fact that in them this cavity communicates with the pharynx, and by the great surface extension (in a confined area) of the much-folded olfactory mucous membrane. The fibres of the olfactory nerve terminate in delicate elongated cells, bearing * rods or hairs and placed between the epithelial cells of this mucous membrane. The special sense of taste is confined to the mouth and pharynx. Its function, from what we know of the higher organisms, is to test the quality of fluid sub- stances, and to bring about the special sensation of taste. The presence of this sense can be demonstrated with certainty in the Ver- tebrata, and it is connected with the distribution of a special nerve of taste, the glossopharyngeal, which in man supplies the tip, edges, and root of the tongue and also parts of the soft palate, making these parts capable of the taste sensation. The so-called taste-buds found in special papilla? (papilla? circum- vallata 1 ), with their central fibre-like cells, are explained as the percipient organs of this sense (fig. 89 a, b, c). Taste is, as a rule, connected with the tactile and temperature sensations of the buccal cavity, and also with the olfactory sensations. Finally, special organs of taste appear to be present also in the Mollusc* and Arthropods as a specific sensory epithelium at the entrance to the buccal cavity. In the lower animals the taste and olfactory organs are still less FIG. 89. a Transverse section through a circum- vnllate papilla of a calf (after Th. W.Engelmann). N, nerve ; Gk, taste buds in the side-wall of the papilla, PC. b, isolated taste bud from the lateral taste organs of a rabbit, c, isolated supporting cells (D~) and sense cells (S:) from the same. PSYCHICAL LIFE AND INSTINCT. 93 clearly distinguishable than in the higher, and there are numerous senses of an intermediate character for the purpose of testing the surrounding medium. The sense-organs of the lateral line of Fishes and Salamanders, and the organs resembling taste-buds of the Hirudinea and Chsetopoda have been described as organs of a sixth sense. They probably bring about certain sensations referring to the quality of the water. PSYCHICAL LIFE* AND INSTINCT. The higher animals are not only rendered conscious of the unity of their organization by their feelings of comfort and discomfort, pleasure and pain, but also possess the power of retaining residua of the impressions of the outer world conveyed through the senses, and of combining them with simultaneously perceived conditions of their bodily state. In what manner the irritability of the lower pro- toplasmic organisms leads by gradual transitions and intermediate steps to the first affection of sensation and consciousness is as Completely hidden from us as are the nature and essence of the psychical pi-ocesses which we know are dependent on the movement of matter. We are,' however, justified in supposing that a nervous system is indispensable for the development of these internal conditions which may be compared with that condition of our own organization called consciousness. Again, as animals have sense-organs capable of receiving impressions of definite quality from external causes, together with a capacity for retaining in their memory residua of their perceptions, and the power of connecting them with present and with the recollection of past states of bodily sensation so as to form judgments and conclusions, they possess all the conditions essential for the operation of the intelligence; and, as a matter of fact, they do manifest in an elementary form nearly all the phenomena which distinguish human intelligence. The actions of animals are not only voluntary, the result of experi- ence and intellectual activity, but are also largely determined by internal impulses which work independently of consciousness, and .cause numerous, often very complicated, actions useful to the organism. Such impulses tending to the preservation of the individual and the * W. Wundt, " Vorlesungen iiber die Menschen mid Thierseelc." 2 Bde. Leipzig, 18U3. W. Wundt, " Grundzlige der physiologischen P Leipzig, 1874. 94 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. species are called instincts;* and they are usually regarded as a special property of the lower animals, and contrasted with the conscious reason, of Man. But just as the latter must be looked upon as a higher form of the understanding and intellect, and not as something essentially distinct from them, so a closer examination shows that instinct and the conscious understanding do not stand in absolute contrast, but rather in a complex relation, and cannot be sharply marked off from one another. For if, according to the general view, we recognise the essence of instinct in the unconscious and the innate, still we find that actions which were at first performed under the direction of conscious intelligence become, by constant practice, completely instinctive and are performed unconsciously; and that, in accordance with the theory of descent, which the whole connection of natural phenomena renders so probable, instincts have been developed from small beginnings, and have only been able to reach the high and complicated forms which we admire in many of the more highly organised animals (Hymenoptera), when assisted by a certain amount, however small, of intellectual activity. Instinct accordingly may be rightly defined as a mechanism which works unconsciously, and is inherited with the organization, and which, when set in motion by external or internal stimuli, leads to the performance of appropriate actions, which apparently are directed by a conscious purpose. We must not, however, forget that while the intellectual activities are the direct means whereby higher and more complicated instincts arise from simple ones, they themselves depend upon mechanical processes. We may well suppose that the simplest form of instinct is identical with the definite reaction of living matter following a stimulus, or, in other words, with that special form of molecular change which is caused by an external action (as, for instance, the contraction of an Amoeba when broue-ht O into contact with a foreign body). By the theory of partly instinctive, partly intellectual processes, we may explain the phenomena of association in societies so often found among the higher animals,f i.e., the association of numerous " Compare H. S. Reimarius, "Allgemeine Betrachtungen iiber die Triebe cler Thiere," Hamburg, 1773. P.Flourens, " De 1'instinct et de 1'intelligence des animaux," Paris, 1851. t The origin of the so-called animal stocks with incomplete or confined individuality among the lower animals is quite different, and merely determined by processes of growth ; at the same time the advantage for the preservation of the species gained by the fusion is the same. Cf. the animal stocks of the Vorticellid;e, Polyps, and Siphonophora, Bryozoa and Tunicata. EEPRODrCTITE ORGANS. 95 individuals into communities the so-called animal-polities which may be complicated by the division of labour (Bees, Wasps, Ants, Termites). In fact here the combined action appears to be mutually assisting or mutually limiting as we find in those forms the so-called animal stocks, the individuals of which are bound together by continuity of body. The advantages to be gained by this mutual rendering of service are not merely limited to the greater facilities for nourish- ment and defence, and therefore for the preservation of the in- dividual : but, above all, tend to the maintenance of the offspring, and hence to the preservation of the species. It is for this reason that the simplest and commonest associations, from which the more complicated communities, subdivided by partition of labour, are derived, are generally communities of both sexes of the same species. REPRODUCTIVE ORGAXS. On account of the limit set to the duration of the life of every organ- ism, it appears absolutely necessary for the preservation of the animal and vegetable kingdoms that new life should originate. The forma- tion of new organisms might be due to spontaneous generation (generatio equivoca) ; and formerly this was supposed to take place, not only in the simpler and lower organisms, but also in the more complicated and higher. Aristotle thought that Frogs and Eels arose spontaneously from slime ; and the appearance of maggots in putre- fying meat was, till Redi's time, explained in the same manner. With the progress of science the limits within which this supposition could be applied became ever narrower, so that they soon came to include only the Entozoa and small animals found in infusions. Finally it has been shown by the researches of late years that these organisms also must, for the most part, be withdrawn from the region of the generatio equivoca ; so that at present, when the question of spontaneous generation is discussed, it is only the lowest organisms, those found in putrefying infusions, that are considered. The greater number of investigators,* supported by the results of * Cf. especially Pasteur, " Memoire sur les corpuscules organises qui existent dans I'atmosphere " (Ann. des. Sc. Nat.), 1861 ; also " Experiences relatives aux generations dites spontanees '' (Compt. rend, de 1'Acad. des Sciences, tome 50). 96 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. numerous experiments, have rejected, even for the latter animals, the idea of spontaneous generation, which, however, still finds in Ponchet* a prominent and zealous supporter. Biogenesis, as opposed to abiogenesis, or spontaneous generation, must be regarded as the usual and normal form of reproduction. Fundamentally it is nothing else than a growth of the organism beyond the sphere of its own individuality, and can be always reduced to a separation of a part of the body, which develops into an indi- vidual resembling the parent organism. Nevertheless the nature and method of this process differ extraordinarily ; and various kinds of reproduction can be distinguished, viz., fission, budding (spore- formation), sexual reproduction.-^ Reproduction by fission, which, with that by budding and spore- formation, is included under the term monogenous asexual reproduc- tion, is found widely scattered in the lowest animals, and is also of special importance for the reproduction of the cell. It consists simply of a division of the organism into two parts by means of a constriction which gradually becomes deeper, and eventually leads to the separation of the whole body of the organism into two individuals of the same kind. If the division remains permanently incomplete, and its products do not completely sepa.rate from each other, con- pound colonies of animals arise. The number of individuals in such colonies increases by a continuation of the process of incomplete and often dichotomous division of the newly-formed individuals (Yorti- cella, Polyp stocks). The di vision may take place in various direc- tions longitudinal, transverse, or diagonal. Budding differs from fission by a precedent disproportionate and asymmetrical . growth of the body, giving rise to a structure not absolutely necessary to the parent organism which is developed to a new individual, and by a process of constriction and division becomes independent. If the buds remain permanently attached to the parent, we have here also the conditions necessary for the formation of a colony (Polyp colonies). Sometimes the budding takes place at various parts of the outer surface of the body, irregularly or obeying definite laws (Ascidians, Polyps) ; sometimes it is localised to a definite part of the body, separated off as a Germ- stock (Salpa, stolo prolifer). The cell-layers distinguished as germinal * Pouchet, " Nouvelles experiences sur la generation spontanee et la resist- ance vitale," Paris, 1864. f Of. R. Leuckart's article, " Zeugung " in R. Wagner's " Handworterbuch der Physiologic." REPRODUCTION" BY SPORES. SEXUAL REPRODUCTION. layers are repeated in the commencing buds, and from them the organs are differentiated. The reproduction by spores is characterised by the production within the organism of cells, which develop into new individuals in situ or after leaving the organism. But this conception of spores, which is taken from the vegetable kingdom, can only be applied to the Protozoa and coincides with endogenous cell-division. The cases of so-called spore-formation amongst the Metazoa (germinal sacs of Trematodes) are probably identical with egg formation, and are to be reduced to a precocious maturation and spontaneous development of ova (Parthenogenesis, Pagdogenesis). The digenous or sexual reproduction depends upon the production of two kinds of germinal cells, the combined action of which is necessary for the de- ^- T 8v I'd velopment of a new or- ganism. The one form of germ cells contains the material from which the new individual arises, and is known as the egg-cell, or merely egg (ovum}. The second form, the sperm-cell (spermato- zoon), contains the ferti- lising material, semen or sperm, which fuses with the contents of the egg- cell, and in a way which is not understood gives the impetus to the de- velopment of the egg. The cell structures from which the eggs and sperm arise are called sexual organs, for reasons which will be evi- dent in the sequel ; the eggs being produced in the female organ or ovary, and the semen in the male organ or testis. The egg is the female, and the semen the male product. The structure of the sexual organs presents extraordinary diffe- rences and numerous grades of progressive complication. In the simplest cases, both products arise in the body wall, the cells of which give rise at determined places to ova or spermatozoa (Coelenterata). Sometimes they arise in the ectoderm (Hydroid-Meduase), sometimes in the entoderm (Acalepha, Authozoa). A 7 FIG. 90. Generative organs of a Heteropod (Pterotra- chea) after R. Leuckart. a, Male-organs ; T, testis- Vd, vas deferens. I, female organs ; OP, ovary ; J1J, albumeu gland; Ss, receptaculum seminis ; Fu, va- gina. similar arrangement 98 OKGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. obtains in the marine Polychpeta, in which the ova and spermatozoa are developed from the epithelium of the body-cavity (mesoderm), and dehisced into the body cavity. Usually, however, special glands, the ovaries and testes, are developed, which perform no other function than that of secreting ova and spermatozoa (Echinoderms). As a rule, however, there are found associated with the male and female generative glands accessory structures and a more or less com- plicated arrangement of ducts, which discharge definite functions in connection with the development of the generative products subse- quent to their separation from the glands, and ensure a suitable meeting between the male and female elements (fig 90). The ovaries are provided with ducts, the oviducts, which are not rarely derived a Jls FIG. 91, a. The female organs of Pulex (after Stein). Ov, ovarian tubes ; 22s, receptaculum semiuis ; V, vagina ; Gl, accessory gland, b, The male generative organs of a water-bug (Nepa) (after Stein). T, testis ; Vd, vasa defereutia ; Gl, accessory glands ; D, ductus ejacu- latorius. from structures serving quite another purpose (segmental organs). The oviducts, in their course, may receive glandular appendages of various kinds which furnish yolk for the nourishment of the ovum, or albumen to surround it, or material for the formation of a hard egg-shell (chorion). These functions may be sometimes discharged by the ovarian wall (Insects), so that the egg when it enters the oviduct has taken up its accessory yolk and acquired its firm egg- shell. Very often the ducts also discharge these various functions, and are divided into corresponding regions ; they are often dilated at part of their course to form a reservoir for the retention of the HEEMAPHEODITISH. 9t) Zd^AW^M eggs or of the developing embryos (uterus). Their terminal section presents differentiations subserving fertilization (receptaculum seminis, vagina, copulatory pouch, external generative organs). The efferent ducts of the testis, the vasa deferentia, likewise frequently give rise to reservoirs (vesiculse seminales) and receive glands (pros- tate), the secretion of which mixes with the sperm fluid or surrounds aggregations of the spermatozoa with a firm sheath (spermatophors). The terminal section of the vas deferens becomes exceedingly muscular, and gives rise to a ductus ejaculatorius, which, as a rule, is accompanied by an external organ of copulation to facilitate the conveyance of the semen into the female generative organs. The generative organs present either a radial (Coelenterata, Echinodermata) or a bilate- rally symmetrical arrangement (fig. 91), a contrast which is visible in the typical arrange- ment of all the systems of organs. The simplest and most primitive condition of the generative organs is the her- maphrodite. Ova and sper- matozoa are produced in the body of one and the same individual, which thus unites in itself all the conditions necessary for the preservation of the species, and alone represents the species. Instances of hermaphroditism are found in every group of the animal kingdom. But they are especially nume- rous in the lower groups, and also in animals in which the movements are slow (Land-snails, Flat-worms, Hirudinea, Oligochceta), or which live singly (Cestoda, Trematoda), or in attached animals which are without power of changing their position (Cirripedia, Tunicata, Bryozoa, Oysters). The hermaphrodite arrangement of the gene- rative organs presents great variation, which, to a certain extent, forms a gradual series tending towards the separation of the sexes. In the simplest cases, the points of origin of the two kinds of generative products lie close to one another, so that the spermatozoa and ova meet directly in the parent body (Ctenophora, Chrysaoni). FIG. 92. Sexual organs of a Pteropod (Cymbulia) (after Gegenbaur.) a, Zd, hermaphrodite gland with common duct ; Us, receptaculum seminis ; U, uterus. I, Acinus of the hermaphrodite gland of the same. 0, ova ; S, spermatozoa. 100 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-vascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common tq the ova and spermatozoa (fig. 92), but which, as in Helix (fig. 93), may partially separate into vas deferens and oviduct. In other cases the ovaries and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trematoda, rhabdoccele Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appears to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertilization is sufficient for the production of off- spring. But this original condition of self-fertiliza- tion appears to be the ex- ception in almost all hermaphrodites. In those fio'Jtl animals in which the ovary and testis are not com- pletely separated from one another cross-fertilization is rendered necessary, and self -fertilization prevented by the fact that the male and female elements are matured at different times (Snails, Salps). From *this form of complete hermaphroditism the generative organs pass through a stage of incomplete hermaphroditism, in which, though the organs of both sexes are present, one of them is rudi- mentary, to reach the dioecious condition in which the sexes are completely separated (Distomumfillicolle and hcematobiuiu.). Animals in which the sexes are distinct not unfrequently present traces of an FIG. 93. Sexual organs of the Roman Snail (Helix pomatia). Zd, hermaphrodite gland ; Zg, its duct ; Ed, albumen gland ; Od, oviduct and seminal groove ; Vd, vas defereus ; P, protrusible penis ; Fl, flagellum ; Us, receptaculum seminis ; D, finger-shaped gland ; L, Spiculum amoris ; Go, common genital opening. SEPARATION OF THE SEXES. 101 hermaphrodite arrangement ; such, for instance, as may be seen in the arrangement of the generative ducts of the Vertebra ta. In the Amphibia both male and female generative ducts, which are secondarily derived from the urinary ducts, are developed in each individual. The oviduct (Miillerian duct) in the male atrophies, and is only repre- sented by a small rudiment (fig. 96&, My] ; while, on the contrary, in the female, the vas deferens (Wolifian duct) is rudimentary, or, as in Amphibia, functions as the efferent duct for the kidney secre- tion (fig. 96a, hg). With the separation of the male female gene- N FIG. 94. Generative appara- tus of a rhabdoccee Tur- bellarian (vortex viridis) (after M. Schultze). T, tes- tis ; Vd, vas deferens ; Vs, seminal vesicle ; P, pro- trusible penis ; OB, ovary ; Va, vagina ; M, uterus ; D, yolk gland ; Rs, recep- tacnlmn serninis. and rative organs in different indivi- duals the most complete form of sexual reproduc- tion, so far as con- cerns division of labour, is reached; but at the same time a progressing dimorphism of the male and female individuals be- comes apparent. This is due to the fact that the or- ganization in bi- sexual animals is more and more influenced by the deviating func- tions of the sexual organs, and with FIG. 95. Generative appa- ratus of the medicinal leech. T, testis ; Vd, vas deferens ; Nh, vesicula seminalis ; Pr, prostate ; C, penis ; OB, ovaries with vagina and female generative opening. the increasing complication of sexual life becomes modified for the performance of special accessory functions connected with the production of ova and spermatozoa. In the first place, the modification of the generative ducts of the two sexes in accordance with the function they have to perform determines the development of secondary sexual characters and of sexual dimorphism. Other organs as well as the generative appa- 102 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. ratus present differences in the two sexes, being modified for the Fis. 96a. Left urinary and generative or- gans of a female Salamander without the cloaca. Ov, ovary ; N, kidney ; lig, urin- ary duct corresponding to the Wolffiaa duct ; My, Miillerian duct as oviduct. FIG. 96J, Left urinary and generative organs of a male Salamander, more diagrammatic. T, testis ; Ve, vasa efferentia ; N, kidney with its collecting tubules ; Mg, Miille- rian duct as a rudiment; Wg, Wolffian duct or vas deferens ; Kl, cloaca with ac- cessory glaiids Dr, of the left side. performance of special functions in the sexual life. The female is FUNCTIONS OF MALE AND FEMALE. 103 the passive agent in copulation, merely receiving the semen of the male ; the female possesses material from which the offspring p I Pis. 97a.- Male of Aphis platanoides. oc, ocelli ; Hr, honey tubes ; P, copulatory organ. develop, and accordingly takes care of the development of the fertilised egg and of the later fate of the offspring. Hence the female usually possesses a less active body and numerous arrangements for the protection and nourishment of her offspring, which develop either from eggs laid by the mother and sometimes carried about with her, or in the maternal body and are born alive. The function of the male is to seek, to excite, and to hold the female during copulation ; hence, as a rule, he possesses greater vigour and power of movement, higher development of the senses, various means of exciting sexual Hr FIG. 976, Apterous oviparous female of the same. feeling, such as brighter colour- ing, louder and richer voice, pre- hensile organs, and external organs for copulation (fig. 97, a, b). In exceptional cases, the functions relating to the maintenance of 104 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. the offspring may be discharged by the male, e.g., Alytes and the Lophobranchia. Male birds also often share with the female the labour of building the nest, of bringing up and protecting the young. But it is a rare exception to find, as in Cottus and the Stickleback (Gasterosteus), that the care and protection of the young fall exclusively upon the male, that he only bears the brood pouch and alone builds the nest, an exception which bears strong witness to the fact that the sexual differences both in form and function were first acquired by adaptation. In extreme cases, the sexual dimorphism may lead to so great a difference in the sexes that without a knowledge of their development Sp a FIG. 98. Chondracanthus gibbosus, magnified about 6 times, a, female from the side. 6, female from the ventral surface with the male (F) attached, c, male isolated, under strong magnification. An', anterior antenna ; An", clasping antennas ; F' and F", the two pairs of feet ; A, eye ; OD, egg sacs ; Oe, oesophagus ; D, intestine ; M, mouth parts ; T, testis ; Vd, vas deferens ; Sp, spermatophore. and sexual relations, the one sex would be placed in a different family and genus to the other. Such extremes are found in the Rotifera and parasitic Copepoda (Chondracanthus, Lernreopoda, fig. 98, , b, c), and are to be explained as the result of a parasitic mode of life. The difference in the two kinds of individuals representing and maintaining the species, whose copulation and mutual action was known long before it was possible to give a correct account of the roal nature of reproduction, has led to the designation " sexes," from which tho term sexual has been taken to apply to the organs and manner of reproduction. PARTHENOGENESIS. 105 In reality sexual reproduction is nothing else than a special form of growth. The ova and spermatoblasts represent the two forms of germinal cells which have become free, and which, after a mutual interaction in the process of fertilization, develop into a new organism. Nevertheless under certain conditions the egg can, like the simple germ cell, undergo spontaneous development ; numerous instances of this mode of development, which is known as partheno- genesis, are found in Insects. The necessity of fertilization therefore \ Hr FIG. 99. Viviparous form of Aphis platanoides. Oc, ocelli ; Hr, honey tubes. no longer enters into our conception of the egg-cell, and no absolute physiological test is left to enable us to distinguish it from the germ- cell. It is usual to regard the place of origin in the sexual organ and in the female body as a feature distinguishing the ovum from a germ cell, but even with this morphological test we do not in each individual case arrive at the desired result (Bees, Bark-lice, Psyckidce). We have already given prominence to the fact that ovaries and testes, in the simplest cases, consist of nothing more than groups of cells of the epithelium of the body cavity or of the outer skin. These, however, do not acquire the character of sexual organs until, at a higher stage of differentiation, the contrast between the 'two 106 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEKAL. sexual elements has made its appearance. When the male elements, and with them the necessity of fertilization, are absent, and when, at the same time, the organ which produces the germ cells possesses, in its full development, a structure similar to that of an ovary, it becomes very difficult to distinguish whether we have to do with a pseudovary (germ-gland), and with an animal which reproduces asexually ; or with an ovary and a true female, whose eggs possess the capacity of developing spontaneously. It is only a comparison with the sexual form of the animal which makes the distinction possible. To take the case of the Plant-lice or Aphides ; in these animals we find a generation of viviparous individuals, easily distinguishable from the true oviparous females, which copulate and lay eggs. They resemble the latter in the fact that they are provided with a similar reproductive gland, constructed upon the ovarian type ; but they differ fi-om them in this important peculiarity, that they are without organs for copulation and ferti- lization (in correspondence with the absence of the male animal) (fig. 99). The reproductive cells of the organs known as pseudovaries ha.ve an origin precisely similar to that of eggs in the ova- ries, and only differ from ova in the very early commencement of the embryonic development. The viviparous individuals will therefore be more correctly regarded as agamic females peculiarly modified in the absence of organs for copulation and fertilization ; and the reproductive cells are by no means to be relegated to the category of germ-cells (as formerly was done by Steenstrup). We must therefore speak of the reproductive pro- cesses in the Aphides as being sexual and partheno- genetic and not sexual and asexual. A comparison of the mode of reproduction of the Bark-lice with that of the Aphides, especially of the species Pem- phigus terebinthi, puts the correctness of this supposition beyond the sphere of doubt. A similar condition is found in the viviparous larva of Cecidomyia. Here the rudiment of the generative glands very early assumes a structure resembling that of the ovary, and produces a number of \-Tl FIG. 100. Vivipa- rous Cecidomyia (Miastor) larva (after Al. Pagen- stecher). Tl, Daughter larvae developed from the rudimentary ovary. DEVELOPMENT. 107 reproductive cells which resemble ova in their method of origin, and at once develop into larvze. The pseudovary is clearly derived from the rudiment of the sexual gland, but without ever reaching complete development (fig. 100). The ovary acquires to a certain extent the signification of an organ for producing germ-cells, and it is not improbable that many products (Redia, Sporocyst) regarded as spores or germ-cells correspond to embryonic ovaries which produce ova capable of spontaneous development. Fro. 101. Ovum of Neplielis (after O. Hertwig). a, the ovum half-an-hour after deposition. a projection of the protoplasm indicates the commencing- formation of the first polar body ; the nuclear spindle is visible. 6, The same an hour later, with polar body extruded, and after entrance of the spermatozoon. Sk, male pronucleus. c, The same another hour later without egg membrane, and with two polar bodies and male pronucleus (Sk) ; d, the same an hour later with approximated female and male pronuclei ; Ek, polar bodies. DEVELOPMENT. It follows from the facts of sexual reproduction that the simple cell must be regarded as the starting-point for the development of the organism. The contents of the ovum spontaneously or under the influence of fertilization enter upon a series of changes, the final result of which is the rudiment of the body of the embryo. These changes consist essentially in a process of cell division which implicates the whole protoplasm of the ovum, and is known, as segmentation. 103 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. For a long time the behaviour of the germinal vesicle at the commencement of segmentation and its relation to the nuclei of the first formed segments were obscure, and the knowledge of the changes and fate of the spermatozoa which enter the ovum in the process of fertilization was, in like manner, in a very unsatisfactory state. Of late years, numerous investigations, especially those of Biitschli, 0. Hertwig, Fol, etc., have thrown some light on these hitherto completely obscure processes. It was supposed that in a ripe ovum preparing itself for segmentation the germinal vesicle disappeared, JE* FIG. 102, a, b. Parts of the ovum of Asterias glacialis with spermatozoa, embedded in the mucilaginous coat (after H. Fol.) c, upper part of the ovum of Petromyzon (after Calberla). Am, micropyle ; Sp, spermatozoa; Jm, path of the speiinatozoon ; Ek, female pronucleus ; Eh, membrane of ovum ; Ehz, prominences of the same. and a new nucleus was formed quite independently of it ; and that the persistence and the participation of the germinal vesicle in the for- mation of the nuclei of the first segmentation spheres were exceptional (Siphonophora, Entoconcha, etc.) Thorough investigations carried out on the eggs of numerous -animals have, however, shown that as a matter of fact the germinal vesicle of the ripe ovum only experi- ences changes in which the greater part of it, together with some of FERTILIZATION. the protoplasm of the ovum, is thrown out of the egg as the so-called directive bodies or polar cells (tig. 101). The part of it, however, which remains in the ovum retains its significance as a nucleus, and is known as the female pronucleus. This fuses with the single spermatozoon (male pronucleus) which has forced its way into the ovum (fig. 102); and the compound structure so formed constitutes the nucleus of the fertilized ovum, or as it is generally called, the first segmentation nucleus. <~y / N. : ii -jj / FIG. 103. Development of a Star-fish, Asteracanthion berylinus (after Alex. Agass'z). 1, Commencing segmentation of the flattened egg at one pole are seen the polar bodies ; 2, stage with two segments ; 3, with four ; 4, with eight ; 5, with thirty-two segments ; G, later stage ; 7, blastosphere with commencing imagination ; 8 and 9, more advanced stages of iuvagination. The opening of the gastrula cavity becomes the anus. This new nucleus, which divides to give rise to the nuclei of the first segmentation spheres, would appear therefore to be the product of the fusion or conjugation of the part of the germinal vesicle, which remains behind in the ovum, with the male pronucleus, which is a derivative of the spermatozoon which has entered the ovum. Fertilization u-oidd appear, therefore, to depend upon the addition 110 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. of a new dement bringing about the regeneration of the primary nucleus of the ovum or germinal vesicle, and would have impressed its influence on the constitution of the conjugated nucleus. The regenerated ovum is therefore the starting-point of the subsequent generations of cells which build up the embryonic body. Both the origin of the polar bodies which takes place in the ripe ovum independently of fertilization, and the division of the segmen- tation nucleus are accompanied by the appearance of the nuclear spindle and star-shaped figures at the poles of the spindle which are so characteristic of the division of nuclei. The male pronucleus, before it fuses with the female pronucleus, also becomes surrounded by a layer of clear protoplasm, around which a star-shaped figure appears (fig. 101). In those cases in which segmentation takes place without a precedent fertilization (parthenogenesis), the female pronucleus appears to possess within itself the properties of the first segmentation nucleus. The fertilization is followed by the process known as segmentation, in which the ovum gradually divides into a greater and greater number of smaller cells. Segmentation may be total, i.e., the whole ovum segments (fig. 103), or it may be partial, in which case only a portion segments (fig. 105). Total segmentation may be regular and equal, the resulting seg- ments being of equal size (fig. 103) ; or it may sooner or later become irregular, the resulting segments being of two kinds the one smaller and containing a preponderating amount of protoplasm, the other larger and containing more fatty matter. In these cases the seg- mentation is said to be unequal. The process of division proceeds much more quickly in the smaller segments, while in the larger and more fatty segments it is much slower, and may eventually come to a complete standstill. The development of the frog's egg will serve as an example of unequal segmentation, of which there are various degrees (fig. 104). In this egg a dark pigmented and protoplasmic portion can be distinguished from a lighter portion containing much fatty matter or food yolk. The former is always turned uppermost in the water, and is therefore called the upper pole of the egg. The axis which connects the upper pole with the lower- is known as the chief axis. The planes of the two first segmentation furrows pass through the chief axis and are at right angles to each other. They divide the egg into four equal parts. The third furrow (fig. 104, 4) is equatorial, taking place in a horizontal plane, and cutting the chief axis at right angles. It lies, however, nearer IIOLOBLASTIC AND MEEOBLASTIC SEGMENTATION. Ill the upper pole than the lower, and marks the line of division between the upper and smaller portion of the egg from the lower FIG. 104. Unequal segmentation of the Frog's egg (after Ecker) in ten successive stages. and larger portion, in which the segmentation proceeds much more slowly than in the former. In partial segmentation we find a sharply marked contrast between the formative and nutritive parts of the -" egg, inasmuch as the latter does not seg- ment. The terms holoblastic and me- roblastic therefore have been applied to total and partial seg- mentation respec- tively. Nevertheless, in total segmentation also, either groups of segments of a definite quality, or, at any rate, a fluid yolk material may be used for the nourishment of the developing embryo. In fact, the contents of every egg consists of two parts (1) of a viscous albu- minous protoplasm; and (2) of a fatty granular matter, the deutoplasm, or food yolk. The first is derived from the protoplasm FIG. 105. Segmentation of the germinal disc of a Fowl's egg, surface view (after Kolliker). A, germinal disc with the first vertical furrow ; B, the same with two vertical furrows crossing one another at right angles ; C and I), more ad- vanced stages with small central segments. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. of the original germinal cell, while the yolk is only secondarily developed with the gradual growth of the first ; and not unfrequently it is derived from the secretion of special glands (yolk glands, Trema- todes) ; it may even be added in the form of cells. In the Ctenophora and other Crelenterata we see already in the first-formed segments the separation of the formative matter or peripheral ectoplasm from the nutritive matter or central endoplasm. In eggs undergoing a partial segmentation the formative matter usually lies on one side of the large unsegmenting food yolk. In accordance with this, the segments of such eggs, known as telolecithal, arrange themselves in the form of a flat disc (germinal disc) hence this kind of segmentation has been called discoidal (eggs of Aves, Reptilia, Pisces) (fig. 105). The food yolk may, however, have a central position. In such centrolecithal eggs the segmentation is FIG. 106. Unequal segmentation of the centrolecithal egg of Gamrnarus locusta (in part after Ed. van Beneden). The central yolk mass does not appear till a late stage and undergoes later an " after-segmentation." confined to the periphery, and is sometimes equal (Palsenion) and sometimes unequal (fig. 106). The central yolk mass may at first remain unsegmented, but later it may undergo a kind of after- segmentation and break up into a number of cells (fig. 106). Again, in other cases the food yolk, at the commencement of segmentation, has a peripheral position, so that the cleavage process is at first confined to the inner parts of the egg, and only in later stages, when the food yolk has gradually shifted into the centre of the egg, appears as a peripheral layer on the surface. This is found especially in the eggs of Spiders (fig. 107). The first processes of segmentation in these at first ectolecitlial ova are withdrawn from observation, since they take place in the centre of an egg covered by a superficial layer of food yolk, until the nuclei with their protoplasmic invest- BLASTOSPIIERE. 113 ment reach the periphery, and the fatty and often darkly-granular food yolk conies to constitute the central mass of the egg (Insects). As various as the forms of segmentation are the methods by which the segments are applied to the building up of the embryo. Fre- quently in cases of equal segmentation the segments arrange them- selves in the form of a one-layered vesicle, the blastosphere, the central cavity of which not rarely contains fluid elements of the food yolk ; or they are at once divided into two layers around a central cavity containing fluid; or they form a solid mass of cells without FIG. 107. Six stages in the segmentation of a spider's egg(Philodromus limbatus) after Hub. Ludwig. A, egg with two deutoplasmic rosette-like masses (segmentation spheres) ; B, the rosette-like masses with their centrally placed nucleated protoplasm without egg membrane ; C, egg with a great number of rosette-like masses ; D, the rosette-like masses have the form of polyhedral deutoplasmic columns, each of which has a cell of the blas- toderm lying immediately superficial to it ; E, stage with blastoderm completely formed ; F, optical section through the same. The yolk columns form within the blastoderm a closed investment to the central space. any central cavity. In numerous cases, especially when the food yolk is relatively abundant (unequal and partial segmentation) or the food supply continuous, the embryonic development is longer ami more complicated. The embryonic rudiment in such cases has at first the form of a disc of cells lying on the yolk ; it soon divides into two layers, and then grows round the yolk. 8 114 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The two-layered gastrula is, as a rule, developed from the blasto- sphere by imagination (embolic invagination). In this process the one luilf (sometimes distinguished by the larger size and more granular nature of its cells) of the cell wall of the blastosphere is pushed in upon the other half (fig. 108), and on the narrowing of the FIG. 108. A, Blastosphere of Amphioxus ; B, invagination of the same; C, gastrula, invagi- nation completed ; O, blastopore (after B. Hatschek). aperture of invagination (Uastof>ore, mouth of gastrula) becomes the endodernial layer (hypoblasf) lining the gastrula cavity. The outer layer of cells constitutes the ectoderm or epiblast. This mode of formation of the gastrula, which is very common, is found, e.g., in Ascidians, and amongst the Vertebrata in Amphioxus (fig. 108). More rarely the gastrula arises by delamination. This process consists of a concentric splitting of the cells of the blastosphere into an outer layer (epiblast), and an inner (hypoblast) (fig. 109). A FIG. 109. Transverse sections through three stages in the segmentation of Geryonia (after H. Fol.) A, stage with thirty-two segments, each segment is divided into an external finely granular protoplasm (ectoplasm) and an inner clearer layer (endoplasm) ; B, later stage ; C, embryo after delaminatinn; with ectoderm slightly separated from the endoderm, which is composed of large cells surrounding the segmentation cavity. The central cavity of the gastrula in this case is derived from the original segmentation cavity, and the gastrula mouth is only secondarily formed by perforation. This method of development PKIMITIVE STREAK. 115 of the gastrula has hitherto only been observed in some hydroid Medusse (Geryonia). Finally, when the inequality of the segmentation is very pro- nounced, the gastrula is formed by a process known as epibole. In this process of development the epiblast cells, which are early distin- guishable from the much larger hypoblast cells, spread themselves over the latter as a thin layer (fig. 110); and in this, as in the second method of development of the gastrula, the cavity of the gastrula is, as a rule, a secondary formation in the centre of the closely-packed mass of hypoblast cells. The blastopore is usually found at the point where the complete enclosure of the hypoblast is effected. It sometimes happens that a part of the primary blastosphere is developed more rapidly than the remainder, and gives rise to a A PIG. 110. A, Unequal segmentation of the egg of Bonellia ; , epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streak is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an evolutio ex una parte, and the other an evolutio ex omnibus partihus. It is not, however, possible to draw a sharp line between these two methods of development, nor have they the significance which was formerly ascribed to them, for closely allied forms may present great differences in this respect according to the amount of food yolk and the duration of the embryonic development. The Coelenterata, the Echinoderms, the lower Worms and Mol- luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) present us with examples of regular development of all parts of the 116 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. body of the embryo which, if the yolk membrane fails, has no need of a special protective envelope. In this latter group, however, the formation of the germinal streak, which is in close relation with the formation of the nervous system, is accomplished later, during the post- embryonic development, when the larva is free-swimming and can procure its own food. In like manner many Polychaetes and Arthropods (Branchipus) only acquire a germinal streak in the course of their later growth as larvse. In all cases in which the embryonic development begins by the formation of a germinal streak, the embryo only becomes definitely limited after the yolk has been gradually surrounded, as a result of processes which are connected with the complete entry of the yolk into the body cavity (Frogs, Insects), or with the origin of a yolk sac from which the yolk passes gradually into the body of the embryo (Birds, Mammals). The progressive organization of this latter, up to its exit from the egg membranes, presents in each group such extraordinary variations that it is not possible to give a general account of them. Of primary importance is the fact that in the rudiment of the germ two cell layers first make their appearance one the ectoderm, which gives rise to the outer integument; and the other the endoderm, from which arises the lining membrane of the digestive cavity and of the glands opening into ib. Between these two layers there is formed, either from the outer or the inner layer, or from both layers, an intermediate layer, known as the mesoderm. From the mesoderm ame the muscular system and the connective tissues, the corpuscles of the lymph and blood, and the vascular system. The body cavity may either be derived from the persisting segmentation cavity, i.e., the primitive space between the ectoderm and endoderm (primary body cavity), or it may be developed secondarily as a split in the mesoderm (coelom), or as outgrowths from the rudiment of the alimentary canal (archenteron), in which, case it is known as an enteroccele body .cavity. The nervous system and organs of sense are probably in all cases derived from the ectoderm, very frequently as pit- or groove-like imaginations which are subsequently constricted off. On the other hand, the urinary and generative organs arise both from the outer and inner layers as well as from the middle layer, which is itself derived from one of the primary layers or from the walls of the primary single -layered blastosphere. Accordingly, as a rule the rudiments of the skin and glandular HOMOLO&Y OF THE GERMINAL LAYERS. 117 lining of the alimentary canal are the first differentiations in the embryo ; and many embryos, the so-called Planuhe and Gastrulse, on leaving the egg, have only these two layers and an internal cavity, the archenteron. Then follows the development of the nervous and muscular systems, the latter taking place sometimes contem- poraneously with or after the first appearance of the skeleton, especially in cases in which a germinal streak is developed. The urinary organs and various accessory glands, the blood-vessels and respiratory organs do not appear till later. The degree of difference between the offspring on attaining the free condition (i.e., at birth or hatching) and the sexually mature adults, both as regards form and size as well as organization, varies considerably throughout the animal kingdom. It is a very striking fact that an embryo provided with a central cavity and a body wall composed of only two layers of cells appears in different groups of animals as a freely moveable larva capable of leading an independent life. Having recognized this fact, it was not a great step, especially as Huxley* some time ago had compared the two membranes of the body wall of the Medusa? (called later by Allman ectoderm and endoderni) with the outer and inner layers of the vertebrate blastoderm. (epiblast and hypoblast), to arrive at the conclusion that there was a similar phylogenetic origin for the similar larvse of very different animal types, and to trace back the origin of organs functionally resembling each other to the same primitive structure. It was A. Kowalewskit who, by the results of his numerous researches on the development of the lower animals, first gave this conception the groundwork of fact. He not only proved the occui- rence of a two-layered larya in the development of the Ccelenteratn, Echinoderrns, Worms, Ascidians, and in Arnphioxus amongst Verte- brates, but also on the ground of the great agreement in the later developmental stages of the larvae of Ascidians and Amphioxus and in the mode of origin of equivalent organs in the embryos of Worms, Insects, and Vertebrata, protested against the hitherto universally received view implied in Cuvier's conception of types, that the organs of different types could not be homologous with one another. * Thomas Huxley. " On the Anatomy and Affinities of the family of Medusas." Philosophical Transactions. London, 1849. f Of. A. Kowalewski's various papers in the " Meinoires de 1'Acad. de Peters- bourg," on Cteiiophora, Phoronis, Holothurians, Ascidians, and Amphioxus, 1866 and 1867. 113 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. Inasmuch as Kowalewski,* from the results of his embryological work, drew the conclusion that the nervous layer and embryonic skin of Insects and Vertebrates are homologous, and that the germinal layers of Amphioxus and Vertebrates correspond with those of Molluscs (Tunicata) or worms, he was in agreement with the long recognised fact that anatomical transitional forms and intermediate links between the different groups or types of animals exist, and that these latter do not represent absolutely isolated planes of organization, but the highest divisions in the system, and he only gave in reality an embryological expression to the claims of the descent theory. In fact, the conclusion which Kowalewski reached was completely correct viz., that the homologies of the germinal layers in the different types afford a scientific basis for comparative anatomy and embryology, and must be recognised as the starting-point for the proper understanding of the relationships of the types. For this position we find amongst the vertebrata proofs at every step. But while his own comprehensive embryological experiences inspired Kowalewski, the founder of the theory of the germinal layers, with a prudent reserve, other investigators, inclined to bold generalization, appeared at once with ready theories, in which the results of embryo- logical investigations were interpreted in accordance with the theory of descent. Among these E. Haeckel's gastrsea theory is especially prominent, which raises no less a claim " than to substitute, in the place of the classification hitherto received, a new system based on phylogeny, of which the main principle is homology of the germinal layers and of the archenteron, and secondarily on the differentiation of the axes (bilateral and radial symmetry) and of the ccelorn." E. Haeckelf designated the larval form used as the point of depar- ture the Gastrula, and believed to have found in it the repetition in embryonic development of a common primitive form, to which the origin of all Metazoa may be traced back. To this hypothetical prototype, which is supposed to have lived in very early times during the Laurentian period, he gave the name of Gastrcea, and called the ancient group, supposed to be widely scattered and to consist of many families and genera, by the name Gastrceadce. From this sup- position was deduced the complete homology of the outer and inner * A. Kowalewski, "Embryologische Stuclien an Wiirmern und Arthropoden." Petersburg, 1871, p. 58-60. f E. Haeckel, " Gastraeatheorie," Jen. nat, Zeitschrift, 1874.'' For criticism see C. Clans, " Die Typeulehre und Haeckel's sogenannte Gastrseatheorie," Vienna, 1874. DIRECT DEVELOPMENT AND METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced bu-k to the ectoderm and the other to the endoderm of the hypothe- tical Gastnea ; while for the middle layer, which is only secondarily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this theory, which is essentially an extension of the Baer-Remak theory of the germinal layers from the Yertebrata to the whole group of Metazoa, with its pretentious and hasty speculation has created a basis for comparative embryology ; such a basis can only be obtained as the result of comprehensive investigations. DIRECT DEVELOPMENT AND METAMORPHOSIS. The more complete the agreement between the just born young and the adult sexual animal, so much the greater, especially in the higher animals, will be the du- ration of the embryonic development and the more complicated the developmental processes of the embryo. The post-embryonic develop- ment will, in this case, be confined to simple processes of growth and perfection of the sexual organs. When, how- ever, embryonic life has, relatively to the height of the organization, a quick and simple course ; FlG m ._ Larval stages of the Frog (after Ecker) when, ill Other Words, embryo some time before hatching-, with wart-like gill , , , . -I papillae on the visceral arches. I, Larva some time tne embryo IS born in after hatching, with external branchite. c, Older larva, an immature condition with norn y beak and small branchial clefts beneath the integumentary operculum, with internal branchia? and at a relatively low N, nasal pit; S, sucker; E, branchise; A, eye; II:, stage of organization, horny teeth. the post-embryonic development will be more complicated, and the young animal, in addition to its increase in size, will present various processes of transformation and change of form. In such cases, the just hatched young, as opposed to the adult animal, is called a Larva, and develops gradually to the form of the adult 120 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. sexual animal. The development of larvje, however, is by no means direct and uniform, but is complicated by the necessity for special contrivances to enable them to procure food and to protect them- selves ; sometimes taking place in an entirely different medium, under different conditions of life. This kind of post-embryonic development is known as metamorphosis. Well-known examples of metamorphosis are afforded by the deve- lopmental histories of the Insecta and Amphibia. From the eggs of Frogs and Toads proceed larvse provided with tails, but without limbs, the so-called Tadpoles (fig 111). These, with their laterally compressed tails and their gills, remind one of fishes, and they possess organs of attachment in the form of two small cervical suckers by which they can anchor themselves to plants. The mouth is provided with horny plates ; the spirally coiled intestine is surprisingly long ; the heart is simple; and the vascular arches have the piscine relations. Later, as development proceeds, the external branchiae abort, and are replaced by new branchiae covered by folds of the integument, the caudal fin is enlarged, and the fore and hind limbs sprout out ; the fore limbs remain for some time covered by the integument, and only subsequently break through it to appear on the surface. Meanwhile the lungs have developed as appendages of the anterior part of the alimentary canal, and supplant the gills as respiratory organs, a double circulation is developed, and the horny beak is cast off. Finally the tail gradually shrinks and atrophies ; on the completion of which the metamorphosis of the aquatic tadpole into the frog or toad suited for life on land is accomplished (fig. 112). We have then to consider two kinds of development, viz., develop- ment with a metamorphosis and direct development, which in extreme cases are distinctly opposed to each other, but are connected by inter- mediate methods. The size of the egg, or, in other words, the amount of food yolk available for the use of the embryo in proportion to the size of the adult animal appears to be a factor of primary importance in any explanation of these two distinct processes (R. Leuckart). Animals with a direct development require generally in pro- portion to the height of their organization and the size of their bodies that their eggs should be provided with a rich endowment of food yolk, or that the developing embryo should possess a special accessory source of nutriment ; they arise therefore either from relatively large eggs (Birds), or they are developed inside, and in close connection with the maternal body, by which arrangement they have a continual supply of food material (Mammals). Animals, BELA.TIOK OF METAMORPHOSIS TO FERTILITY. 121 on the contrary, which pass through a metamorphosis always arise from eggs of relatively small size, are hatched in an immature con- dition as larvse, and obtain independently, by their own activity, the materials which have been withheld from them while in the egg, but which are necessary for their full development. The number of embryos produced in the case of a direct development is, in proportion to the total weight of the material applied by the mother for reproductive purposes, far smaller than in the case of a develop- ment with metamorphosis. The fertility of animals whose young a FIG. 112, Later stages in the development of Pelobates fuscus. a, larva without limbs with well developed tail ; b, older larva with hind limbs ; a, larva with two pairs of limbs ; d, young frog with caudal stump ; e, young frog after complete atrophy of tail. undergo a metamorphosis, or, in other words, the number of offspring produced from a given mass of formative material, is increased to an extraordinary degree, and has, in the complicated relations of organic life, a great physiological significance, though systematically it is of little importance. Some time ago it was attempted to explain these indirect meta- morphoses, in which both processes of reduction and new development take place, as the result of the necessity which the simply organized 122 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. larva, hatched at an early stage of development, laboured under of acquiring special arrangements for its protection and nourishment (R. Leuckart). The proof that such relations do exist between the special larval organs and the peculiar methods of nutrition and protection is an important factor for the full understanding of these remarkable processes, but still is by no means an explanation of them. It is only by aid of the Darwinian principles and the theory of descent that we can get nearer to an explanation. According to this theory, the form and structure of larvse are to be considered in relation to the development of the race, i.e. phylogeny, and are to be derived from the various phases of structure through which the latter has passed in its evolution, and in such a way that the younger larval stages would correspond to the primitive, and the older, on the other hand, to the more advanced and more highly organized animals, which have appeared later in the history of the race. In this sense the developmental processes of the individual constitute a more or less complete recapitulation of the developmental history of the species, complicated, however, by secondary variations due to adaptation, which have been acquired in the struggle for existence * (Fritz M tiller's fundamental principle, called by Haeckel the funda- mental law of biogenesis). The greater the number of stages, therefore, through which the larva passes, the more completely is the ancestral history of the species preserved in the developmental history of the individual ; and it is the more truly preserved the fewer the peculiarities of the larva, whether independently acquired, or shifted back from the later to the earlier periods of life (Copepoda.) On the other hand, there are certain larval forms without any phylogenetic meaning which are to be explained as having been secondarily acquired by adaptation (many Insect larvse). The historical record preserved in the developmental history becomes, however, gradually defaced by simplification and shortening of the free development ; for the successive phases of development are gradually more and more shifted back in the life of the embryo, and run their course more rapidly and in an abbreviated form, under the protection of the egg membranes, and at the cost of a rich supply of nutrient material (yolk, albumen, placenta). In animals with a direct development, therefore, the complicated deve- lopment within the egg membranes is a compressed and simplified * Fritz Miiller, " Fur Darwin," Leipzig, 1863, p. 7581. ALTERNATION OF GENERATIONS. 123 metamorphosis, and hence the direct development, as opposed to the metamorphosis, is a secondary form of development. . ALTERNATION OF GENERATIONS, POLYMORPHISM AND HETEROGAMY. Both in direct development and indirect development by means of a metamorphosis, the successive stages take place in the life- history of the same individual. There are, however, instances of free development, in which the individual only passes through a part of the developmental changes, while the offspring produced by it accomplishes the remaining part. In this case the life-history of the species is represented by two or more generations of indivi- duals, which possess different forms and organization, exist under different conditions of life, and reproduce in different ways. Such a manner of development is known as alternation of genera- tions (metagenesis), and consists of the regular alternation of a sexually differentiated generation with one or more generations reproducing asexually. This phenomenon was first discovered by the poet Charnisso* in the Salpidae ; but the observation remained for more than twenty years unnoticed. It was rediscovered by J. Steenstrup, t and discussed in the reproduction of a series of animals (Medusas, Trematoda) as a law of development. The essence of the process consists in this, that the sexual animals produce offspring, which through their whole life remain different from their parents, but can give rise by an asexual process of reproduction to a gener- ation of animals which resemble in their organization and habits of life the sexual form, or again produce themselves asexually, their offspring assuming the characters of the original sexual animal. So that in the last case the life of the species is composed of three different generations proceeding from one another, viz., sexual form, first asexual form, and second asexual form. The development of the two, three, or more generations may be direct, or may take place by a more or less complicated metamorphosis ; similarly the asexual and the sexual generations sometimes differ but little from each other (e.g. Salpa), and sometimes present relations analogous to those which exist between a larva and the adult animal (e,. (6) Vermes. With white blood, simple heart, and unsegmented antennae Mollusca, Intestina, Testacea, Zoophyta, Infusoria. While the followers of Linnaeus developed still further this barren and one-sided zoographical treatment and erroneously looked upon the framework of this system as an exact and complete expression of the whole of nature, Cuvier, by combining Comparative Anatomy with Zoology, laid the foundations of a natural system. George Cuvier, born at Mompelgard 1762, and educated at the Karlsakademie at Stuttgart, later Professor of Comparative Anatomy at the Jardin des Pla-ntes in Paris, published his comprehensive in- 136 HISTOBICAL REVIEW. vestigations in numerous works, especially in his " Lemons d'Anatomie comparee" (1805). In his celebrated treatise * published in 1812, on the arrangement of animals according to their organization, he established a new and essentially changed classification, which was the first serious attempt to build up a natural system. Cuvier did not, as most zootomists had done, look upon anatomical discoveries and facts as in themselves the aim of his researches, but he contemplated them from a comparative point of view, which led him to the establishment of general principles. By considering the peculiarities in the ar- rangements of the organs in relation with the life and unity of the organism, he recognised the reciprocal dependence of the individual organs, and appreciating fully the idea of the " correlation " of parts already discussed by Aristotle, he developed his principle of the con- ditions of existence without which an animal cannot live (principe des conditions d'existence ou causes finales). " The organism con- sists of a single and complete whole, in which single parts cannot be changed without causing changes in all the other parts." By com- paring the organizations of many different animals, he found that the important organs are the most constant, and that the less important vary most in their form and development, and even are not univer- sally present. He was thus led to the principle so important for the systematist of the subordination of characters (principe de la subordination des caracteres). Without being ruled by the pre-conceived idea of the unity of all animal organization, he became convinced, from a conside- ration of the differences in the nervous system and in the arrangement of the more important systems of organs, that there were in the animal kingdom four main types (embranchements], " general plans of structure on which the respective animals appear to be modelled, and whose individual subdivisions, as they may be called, are only slight modifications based on the development or the addition of some parts, without the plan of the organization being thereby essentially changed." These four groups (embranchements Cuvier, Typen Blainville) were the Vertebrata, Mollusca, Articulata, and Radiata. The views of Cuvier, who in knowledge of anatomical and zoologi- cal detail stood far above all his contemporaries, were, however, in opposition to the theories of men of note (the so-called School of * " Sur un nouveau rapprochement a etablir entre les classes qui composent le regne animal." Ann. des Museum d'Hist. Nat., Tom XIX., 1812. ST. HILAIEE, OKEN, VON BAEE. 137 Natural Philosophy). In France, Etienne Geoffrey St. Hilaire pre- eminently defended the idea, which had been already expressed by Buffon, of the unity of the plan of animal structure, according to which the animal kingdom consisted of an unbroken gradation of animals. Convinced that nature always worked with the same materials, he put forward his theory of analogies, according to which the same parts, though differing in their form and the degree of their development, should be found in all animals ; and, further, his theory of connections (principe des connexions), according to which the same parts always appear in the same mutual position. A third funck- mental principle was that of the equivalence of organs, an increase in the size of one organ being accompanied by the diminution of another organ. The application of this principle had important results, and led to the scientific foundation of Teratology. His generalizations were, however, in the main hasty, in that they were founded on facts taken only from the Vertebrates ; and if applied outside that group must lead to many rash conclusions, e.g., that Insects are Vertebrates turned on to their backs. In Germany, Goethe and the natural philosophers Oken and Schelling pronounced in favour of the unity of animal organization, but it must be confessed without taking account in a comprehensive manner of the actual facts. The result of this controversy which in France was carried on with considerable vehemence was, that Cuvier's view was victorious, and his principles met with the more undivided assent since it appeared that they were confirmed by C. E. v. Baer's ernbryological work. Many gaps and errors were certainly discovered by later investigators in Cuvier's classification, and in detail it was much changed, but the establishment of his animal types as the chief groups of the system was retained, and was supported by the results of the developing Science of Embryology. The most essential of the modifications which it has become neces- sary to make in Cuvier's system relate chiefly to the increase in the number of types. The Infusoria were some time ago removed from the Radiata, and as Protozoa arranged by the side of the four other groups. Lately the number of groups has been increased by the division of the Radiata into Coelenterata and Echinodermata, and of the Articulata into Arthropoda and Verrnes, and of the Mollusca into three groups. In our times, however, Cuvier's view has experienced an essential modification in favour of the Natural Philosophers, and the idea of 138 HISTORICAL REVIEW. the absolute independence and isolation of each group must be given up. With a more complete study it has been shown that inter- mediate forms exist connecting the various types, and that conse- quently no sharp line of demarcation can be drawn between them. But just as the transitional forms between animals and plants cannot abolish the distinction between these two most important conceptions of the organic kingdom, so the existence of such transitional forms does not in any way affect the value of the idea of groups and types as the chief divisions of the animal system, but only renders it probable that the different groups have developed from a similar or common starting-point. And to this corresponds the fact, which has become recognised with the progress of embryological knowledge, that similar larval stages and tissue-layers (germinal layers) are found in the develop- mental history of the different types. This fact points to a genetic connection. Likewise the results of anatomical and embryological comparison have rendered it probable that the types are by no means absolutely independent, but are subordinated to one another in more or less close relation, that especially the higher groups are genetically to be derived from the Worms, a group which certainly includes extremely dissimilar forms, and eventually will, without doubt, be broken up into several types. We consider it, under such circumstances, con- venient, in the present state of science, to distinguish nine types as the chief divisions, and to characterise them in the following manner : (1) Protozoa. Of small size, with differentiations within the sar- code, without cellular organs, with predominating asexual repro- duction. (2) Ccelenterata. Radiate animals segmented in terms of 2, 4, or 6 ; mesoderm of connective tissue, often gelatinous ; and a central body cavity common to digestion and circulation (gastro-vascular space). (3) Echinodermata. Radiating animals, for the most part of pen- tamerous arrangement ; with calcareous dermal skeleton, often bear- ing spines ; with separate alimentary and vascular systems ; and with nervous system and ambulacral feet. (4) Vermes. Bilateral animals with unsegmented or uniformly (hornonornous) segmented body, without jointed appendages (limbs), with paired excretory canals sometimes called water- vascular system. (5) Arthropoda. Bilateral animals with heteronomously segmented MEANING OF THE SYSTEM. 139 bodies and jointed appendages, with brain and ventral chain of ganglia, ((>) Molluscoidea. Bilateral, unsegmented animals with ciliated circlet of tentacles or spirally rolled buccal arms ; either polyp-like and provided with a hard shell case, or mussel-like with a bivalve shell, the valves being anterior and posterior ; with one or more ganglia connected together by a perioesophageal ring. (7) Mollusca. Bilateral animals with soft, unsegmented body, without a skeleton serving for purposes of locomotion ; usually enclosed in a single or bivalve shell, which is excreted by a fold of the skin (mantle) ; with brain, pedal-ganglion and mantle-ganglion. (8) Tunicata. Bilateral unsegmented animals with sac-shaped or barrel-shaped bodies, and a large mantle cavity perforated by two openings ; simple nervous ganglion, heart and gills. (9) Vertebrata. Bilateral animals with an internal cartilaginous or osseous segmented skeleton (vertebral column), which gives off dorsal processes (the neural arches) to surround a cavity for the reception of the spinal cord and brain ; and ventral processes (the ribs) which bound a cavity for the reception of the vegetative organs ; never with more than two pairs of limbs. CHAPTER V. MEANING OF THE SYSTEM. VERY different opinions have been held in different places and at different times as to the value of the system. In the last century the French Zoologist Buffon held the system to be a pure invention of the human mind ; while more recently L. Agassiz thought that a real meaning could be attributed to all the divisions of the system. He explained the natural system founded on relationship of organiza- tion as a translation of the thoughts of the Creator into human language, by the investigation of which we become unconsciously interpreters of his ideas. But it is clear that we cannot call that arrangement, which is derived from the relations of organization founded in nature, an invention of man. Similarly it is preposterous to deny the sub- jective participation of our intellectual activity, since in every system there is expressed a relation of the facts of nature to our comprehen- 14:0 MEANING OF THE SYSTEM. sion and to the state of scientific knowledge. In this sense Goethe appropriately calls a natural system a contradictory expression. In establishing systems, that which comes into contemplation consists of the individual forms which are the objects of observation. Every systematic conception, from that of the species to that of the type, depends upon the bringing together of siruiliar properties, and is an abstraction of the human mind. Species. The great majority of investigators, till very recently, Avere agreed in looking upon the species as an independently created unit with special properties which were retained in propagation, and were really contented with the fundamental idea in Linnaeus' defini- tion of species : Tot numeramus species quot ab initio creavit in- finitum ens." This view also accorded with a dogma prevalent in Geology, according to which the flora and fauna of the successive periods of the earth's history were completely isolated, being created afresh at the beginning and destroyed by a vast catastrophe at the end of each period. It was supposed that no living thing could be preserved through one of these catastrophes from one period into the next ; that every species of animal and plant was specially created with definite characters, which it retained unchanged until it was destroyed. This idea was confirmed by the difference between the fossil remains of Vertebrates (Cuvier) and Molluscs (Lamarck), and the living forms of these types. As a matter of fact, however, neither in the animal nor in the vegetable kingdom do offspring resemble exactly the parent forms from which they have originated, but present differences more or less considerable, so that the idea of absolute identity must be removed from our definition of species, and agreement in the most essential particulars introduced in its place. The species would ac- cordingly, in close agreement with Cuvier's definition, include all living forms which have the most essential properties in common, are descended from one another, and produce fruitful descendants. All the facts of natural life, however, can by no means be arranged agreeably to this conception, which has for a fundamental principle that all essential peculiarities must be preserved unaltered by repro- duction through all time. The great difficulties in defining species which occur in practice, and which prevent a sharp line being drawn between species and variety, indicate the insufficiency of the conception. Varieties. Individuals belonging to the same species do not resemble each other in all particulars, but present differences which, SPECIES AND TAETETT. 141 on closer investigation, suffice to distinguish the individual forms. Combinations of modified characters are often present in the same species, and occasion important variations (varieties) which can be inherited by the descendants. The more important of such variations which are maintained by reproduction are called constant varieties or subspecies, or races, and are divided into natural races and artificial or domesticated races. The former are found in free natural life, and are usually confined to definite localities. They have arisen in course of time in conse- quence of conditions of climate, and under the influence of variations in manner of life and nourishment. The domesticated races, on the other hand, owe their origin to the care and cultivation of man. They comprise only domestic animals whose origin is still unknown. Varieties, however, which have arisen from one species may differ very surprisingly from one another ; in fact, they may present more important features of difference than do distinct natural species. An example of this is found in the domesticated race of pigeons, whose common descent from Columba livia (the rock pigeon) was shown by Darwin to be very probable. They are capable of such striking alterations, that their varieties, known as tumbler pigeons, fantail pigeons, etc., were held by ornithologists, who were without knowledge of their origin, to be real species, and were even divided into different genera. In the natural state, too, it often happens that varieties cannot be distinguished from species by the quality of their characteristics. It is customary to consider that the essential of a character is to be found in the constancy of its occurrence, and to recognise varieties by the fact that their characteristics are more variable than those of species. If forms which are widely different can be connected by a continuous series of intermediate forms, they are held to be varieties of the same species. But if such intermediate forms are absent, they are held to be distinct species, even when the differences between them (so long as they are constant) are less. Under such circumstances we can understand that in the absence of a positive test, the individual judgment and the natural tact of the observer decides between species and variety ; * and how it is that the opinions of different observers have differed so widely in * The establishment of the conception of sub-species is completely at variance with the common conception of species, and is the most striking proof that systematists themselves recognize that the distinction between species and sul >-species is a relative one. 142 MEANING OF THE SYSTEM. practice. This relation has been excellently and thoroughly discussed by Darwin and Hooker. As an example of the difference of opinion on this subject, Nageli * divided the Ilieracice found in Germany into three hundred species, Fries into one hundred and six, Koch into fifty-two, while other authors recognise hardly more than twenty. Nageli indeed says, " There is no genus of more than four species on which all botanists are agreed, and many examples may be cited in which, since Linnaeus' time, the same species have been repeatedly divided up and re-united." We are therefore driven, in order to determine the essential pro- perty distinguishing species and variety, to consider the most impor- tant characteristic for the conception of species, a characteristic which has hardly ever been used in practice, i.e., the community of descent and the capacity for fruitful interbreeding. This means of determination, however, is also insufficient. It is a commonly known fact that animals which belong to different species pair with one another and produce hybrids, e.g., horse and ass, wolf and dog, fox and dog. Widely differing species, which are placed in different genera, have even been known to cross with one another, and to produce progeny, such as the he-goat and sheep, and the she- goat and ibex. The hybrids however are, as a rule, sterile. They are intermediate forms with imperfect generative system, with- out the power of propagation ; and even in those cases where there is a power of reproduction (such cases are most frequently met with amongst female hybrids), there is a tendency to revert to the paternal or maternal species. There are, however, exceptions to the sterility of the hybrid which appear to afford weighty proof against immutability of species. The experiments in breeding between the hare and rabbit, made on a large scale in Angoulerne by Roux, have shown that their progeny, the hare-rabbit, is perfectly fertile. Half-bred hybrids of the rabbit and hare have been bred, and have been reproductive through many generations of pure in-breeding. In like manner careful inquiries into the hybridism of plants, especially the investigations of W. Herbert, lead to the conclusion that many hybrids are as perfectly productive among themselves as genuine species. In a state of nature, too, hybrids of various kinds are found. Such hybrids have frequently been taken for independent specie?, and have been described as such (Tetrao medius, hybrid of Tetrao * C. Nageli, " Entstehung uncl Begriff der naturhistorischen Art." Munich, 1865. FERTILITY OF HYBRIDS. 143 urogallus and Tetrao tetrix ; Abramidopsis Leuckarti, Bliccopsis (ibrtniiorutilus, and others are, according to von Siebold, hybrids.) Sterility of hybrids is not the rule here, for a great number of wild plants have been recognised as hybrid species (Kolreuter, Gartner, Niigeli Cirsium, Cytisus, Rubus). This seems to render it the less doubtful that amongst animals which have been domesticated by man, persistent transitional forms can be obtained from originally different species, by gradual alteration brought about by cross breeding. Pallas, adopting this view, gave it as his opinion that closely allied species, though at first they may refuse to breed together, or may produce sterile offspring, will, after long domestication, produce fertile progeny. And in fact, it has been shown to be probable that some of our domestic animals have originated in prehistoric times as the result of the unintentional crossing of different species. Riitimeyer especially endeavoured to prove this mode of origin for the domestic ox (Bos taurus), which he regarded as a new race resulting from the crossing of at least two ancestral forms (Bos primigenius, brachyceros). It may be looked upon as certain that the domestic pig and cat, and the numerous breeds of dogs, have originated from several wild species. In connection with the exceptional cases which have just been discussed, we may lay great stress upon the perfect reproductive capabilities of mongrels, that is, of the progeny produced by the crossing of different varieties of the same species ; though here also we meet with exceptions. Disregarding those cases in which me- chanical causes render the interbreeding of different varieties im- possible, it seems, according to the observations of breeders whose word may be depended upon, that certain varieties have difficulty in crossing with one another ; and further that certain forms which have been bred by selection from a common stock are altogether in- capable of fertile intercourse. The domestic cat introduced into Paraguay from Europe has, according to Rengger, become essentially altered in process of time, and has acquired a marked aversion to the European ancestral form. The European guinea pig does not breed with the Brazilian form, from which it is probably descended. The Porto-Santo rabbit, which was exported from Europe to Porti - Santo near Madeira in the fifteenth century, is so much altered that it can no longer breed with the European race of rabbits. The evident difficulty of precisely defining the conception of species, in presence of the exi-tence of a gradual, almost uninterrupted series 144 MEANING OF THE SYSTEM. of animal forms, and of the results of artificial selection, had already, in the beginning of this century, induced illustrious and highly esteemed naturalists to dispute the dominant views on the immuta- bility of species. In the year 1809, Lamark, in his " Philosopkie Zoologique" broached the theory of the descent of species from one another. He referred the gradual alterations in some degree to changing conditions of life, but mainly to use and disuse of organs. Geoffrey St. Hilaire, too, the advocate of the idea of unity of organization of all animals and the opponent of Cuvier, expressed his conviction that species had not existed unaltered from the be- ginning. While agreeing essentially with Lanark's theory of the origin and transmutation of species, he ascribed a less influence to the inherent activity of the organism, and believed that he could explain the alterations through the direct operation of changes in the environment (monde ambiant). The change in the fundamental views of Geology which took place at a later period must be ascribed to the opinions of these investigators. Lyell endeavoured (Principles of Geology) to explain geological alterations by means of the forces in operation at the present day, working gradually and without interruption through extended periods of time, and rejected the Cuvierian theory of mighty revolutions and catastrophes which destroyed all life. When the geologists with Lyell had given up the hypothesis of periodic disturbance of the course of natural events, they were obliged to assume the continuity of organic life during the successive periods of the formation of the earth, and to endeavour to account for the immense alterations of the organic world by slight influences operat- ing gradually and without interruption throughout long periods of time. The variability of species, the origin of new species from previous ancestral forms in the course of ages, has become, accord- ingly, since the time of Lyell, a necessary postulate of geology in order to explain naturally the differences of animals and plants in successive periods without the supposition of repeated acts of creation. THE TRANSMUTATION THEORY, OR THEORY OF DESCENT, BASED ON THE PRINCIPLE OF NATURAL SELECTION (DARWINISM). Nevertheless a more securely grounded theory based upon a firmer standpoint was needed in order to give more force to the Trans- mutation hypothesis which had remained disregarded ; and this NATTJBAL SELECTION. 145 service was effected by the English naturalist, Charles Darwin, who employed a mass of scientific material to found a theory of the origin and mutation of species. This theory, which is closely con- nected with the views of Lamark and Geoffroy and in harmony with Lyell's doctrines, has received an almost universal recognition, not only on account of the simplicity of its principle, but also because of the objective and convincing way in which his genius expounded it. Darwin * starts from the principle of heredity, according to which the characteristics of parents are transmitted to their off- spring. But side by side with this, there is an adaptation determined by the peculiar conditions of nourishment, a limited variability of form, without which individuals of like descent would be identical. While heredity tends to reproduce identical characteristics, individual variations appear in the descendants of the same species, and in this way modifications arise, which in their turn are submitted to the law of heredity. Cultivated plants and domestic animals, the individual forms of which vary far more than do those living in a state of nature, are especially disposed to alteration ; and capability of domestication is in reality nothing else than the capability of an organism to siibordinate and adapt itself to altered conditions of nourishment and way of life. The so-called artificial breeding, by which man succeeds by judicious choice in cultivating in plants and animals definite properties cor- responding to his requirements, depends on the interaction of heredity and individual variation ; and it is probable that the numerous races of domestic animals were in this way bred unconsciously by man, just as in our own days large numbers of new varieties are bred by judi- cious choice of the male and female parents. Similar processes are also at work in natural life, calling into existence new alterations and varieties. Here also we find a selection which is occasioned by the struggle of organisms for existence, and may be called a natural selection. All plants and animals are engaged, as Decandolle and Lyell had asserted ten years previously, in a mutual struggle for existence among themselves and with external conditions. A plant has to fight against circumstances of climate, season, and soil ; and has also to compete for existence with other plants which, by their superabundant increase, endanger the possibility of it> preservation. Plants serve as food for animals, which themselves are engaged in a mutual struggle with each other ; the carnivorous * Ch. Darwin. ' ; On the Origin of Species by means of Natural Selection." London, 1859. 10 146 MEANING OF THE SYSTEM. feeding very largely upon herbivorous animals. Then again, all are struggling to multiply in great numbers. Each organism produces far more descendants than can in general be preserved. With a definite degree of fertility, a corresponding amount of destruction must take place ; for in the absence of the latter the number of individuals would so increase in geometrical progression that no locality would suffice for the sustenance of their progeny. If, on the contrary, the protection afforded by fertility, size, special organiza- tion, colour, etc., were removed, the species would soon vanish from the earth. Amongst the complex conditions and interactions of life, even the most remotely connected organisms struggle with each other for existence (e.g., the clover and the mice); but the most violent strife is waged between individuals of the same species, which seek the same food and are exposed to the same dangers. In this strife it necessarily happens that those individuals which are placed in the most favourable situation, through their special properties, have the greatest chance of maintaining themselves and of multiplying, and, in consequence, of reproducing the modifications useful to the species and of preserving them in their descendants, or even sometimes of increasing them. Just as in the breeding of domesticated animals, an artificial selec- tion is made by man to perpetuate and increase advantageous variations; so in the natural breeding of wild animals, in conse- quence of the struggle for existence, a selection is made by nature which leads to the preservation of modifications useful to the species. Since, however, the struggle for existence in closely related forms must be the more violent the more nearly they resemble one another, the most divergent types will have the greatest chance of enduring and of producing descendants. Hence a divergence of characters and an extinction of intermediate forms is the necessary consequence. Thus by the combination of useful properties and by the accumula- tion of hereditary peculiarities, which were primitively of little im- portance, varieties gradually arise which ever diverge more and more ; and this is what Darwin sought to prove by happily chosen examples. We can now comprehend why everything in the organism is directed towards one end, which is to ensure existence in the most perfect way. The great series of phenomena which could hitherto only receive a teleological explanation are thus brought into causal relation, and can be explained as the inevitable result of efficient causes, and their natural connection is thus rendered intelligible. CAUSE OF VARIATIONS. 147 This principle of natural selection, which is the basis of the Dar- winian theory, rests, on the one hand, on the interaction of adaptation and heredity, and on the other, on the struggle for existence which can be shown to occur everywhere in nature. In its fundamental idea the natural selection theory is essentially an application of the doctrine of Malthus to plants and animals. Developed simultaneously by Darwin and Wallace, * it received from the former a most comprehensive scientific foundation. We must certainly admit that Darwin's selection theory, although supported by what we know of biological processes and of the opera- tion of the laws of nature, is very far from discovering the final causes and physical connection of the phenomena of adaptation and heredity, since it is unable to explain why such or such a variation should appear as the necessary consequence of a change in the vital conditions, and how it is that the manifold and wonderful phenomena of heredity are a function of organised matter. It is clearly a great exaggeration when enthusiastic supporters of the Darwinian theory t say that it ranks as equal to the gravitation theory of Newton, because "it is founded upon a single law, a single effective cause, namely, upon the interaction of adaptation and heredity." They overlook the fact that we have here only to do with the proof of a mechanical and causal connection between series of biological phenomena, and not in the remotest degree with a physical explanation. Even if we are justified in connecting the phenomena of adaptation with the processes of nourishment, and in conceiving heredity as a physiological function of the organism, we still stand and regard these phenomena as "the savage who sees a ship for the first time." While the complicated phenomena of heredity \ remain completely unintelligible, we are only in a position to explain in general terms certain modifications of organs, on physical grounds, by the altered conditions of metabolism. It is only rarely as in the case of the operation of use and disuse that we are able more directly to relate the development or the atrophy of organs to the increase or decrease in their nutritive activity, i.e., to give a chemico-physical explanation. Darwin has been unjustly reproached with having left chance to : Compare also A. R. Wallace. "Contributions to the Theory of Natural Selection." t Compare E. Haeckel, " Natiirliche Schopfungsgeschichte. 4. Auflage. Berlin, 1873. % It is clearly a misuse of the word ; 'Law" to represent the numerous partially opposed and limiting phenomena of heredity as so many ' laws of heredity," as E. Haeckel does. 148 MEANING OF THE SYSTEM. play a considerable part in his attempt to account for the origin of varieties, with having accounted for everything by the struggle for existence, and with having given too little prominence to the direct influence of physical action on the mutation of forms. This reproach seems to arise from a misapprehension. Darwin says himself that the expression " chance," which he often uses to explain the presence of any small alteration, is a totally incorrect expression, and is only used to express our complete ignorance of the physical reasons for such particular variation. If Darwin has by a series of considerations arrived at the conclu- sion that the conditions of life, such as climate, nourishment, etc., exercise but a small direct influence upon variability, since, for in- stance, the same varieties have arisen under the most different conditions, and different varieties under the same conditions, and that the complex adaptation of organism to organism cannot be produced by such influences, still he recognises in the alteration of the vital conditions and the mode of nourishment the primary cause of slight modifications of structure. But it is only natural selection o * which accumulates those alterations, so that they become appreciable to us and constitute a variation which is evident to our senses. It is exactly upon the intimate connection of direct physical action with the consequences of natural selection that the strength of the Dar- winian theory rests. The origin of varieties and races would appear, however, to consti- tute only the first stage in the processes of the continual changes of organisms. However slowly the process of selection may work, yet there is no limit to the extent and magnitude of the changes, or to the endless combinations of reciprocal adaptations of living beings if we allow a very long period of time for its operation. With the aid of this new factor of duration of time, which, according to geo- logical facts, cannot be rejected, but stands to an unlimited extent at our service, the gap between variety and species disappears. Since the former are continually diverging with the lapse of time and the more they do so and become differentiated in their organization so much the better will they be fitted to fill different places in the eco- nomy of nature and to increase in number they at length attain the value of species, which in a state of nature do not interbreed, or, at any rate, only exceptionally produce progeny. Thus, according to Damvin, a variety is a species in process of format ion. Variety and species are connected by continuous series of transitions, and are not absolutely distinct from one another ; but are only relatively separated PROGRESSING DIVERGENCE OF CHARACTERS. 149 by the amount of difference in their morphological and physiological characteristics. This conclusion of Darwin's, which extends the result of natural selection from the production of variety to that of species, is ob- stinately and often bitterly opposed by those who subordinate the phenomena of nature to traditional ideas. Even if they do not deny the facts of variability, and even admit the inriuence of natural selection on the formation of natural varieties, they yet continue true to the belief that there is an absolute separa- tion between species and race-variety. As a matter of fact, however, we are not in a position to draw such a line of separation. Neither the quality of the distinctive characteristics nor the results of cross- ing afford us a distinctive criterion between species and variety. The fact, however, that we are not able to give any satisfactory defini- tion of the conception of species, precisely because we, are unable clearly to distinguish between species and variety, adds so much the more weight to Darwin's argument, since neither the variability of the organism and the struggle for existence nor the great antiquity of life upon the globe can be contested. The variability of forms is a firmly established fact ; so, too, is the .struggle for existence. Now if we add the operations of natural selection to these two factors, we are able to understand the origin of varieties. If we imagine the same process which has led to the formation of varieties continued through a greater number of genera- tions and effective through a longer period of time and we are the more justified in making use of these long periods of time, since with their help astronomy and geology have been enabled to explain many phenomena the diverging characteristics will become more and more marked, and will at last gain the value of distinctive species- characters. In still greater periods of time the species will become so far separated from one another by the simultaneous disappearance of the intermediate forms that they will represent different genera. Accordingly the greater differences of organization which are ex- pressed in the higher divisions of the system, such as orders and sub-orders, etc., require a longer interval of time for their accom- plishment, and an extinction of a greater number of intermediate forms. Finally, the different ancestral forms of the classes of a group may be referred to a common starting-point ; and since the different groups of animals are connected by many intermediate forms, the number of the ancestral forms becomes much reduced. 150 MEANING OF THE SYSTEM. The undifferentiated contractile substance, sarcode or protoplasm, was probably the starting-point of all organic life. If these suppositions are correct, species no longer retain the signifi- cation of independent and immutable units, and appear, according to the great law of evolution, only as transient groups of forms, capable of change, and confined to longer or shorter periods of time, to definite conditions of life, and preserving, as long as these conditions do not vary, a constancy in their essential characters. The different categories of the system show the closer or more remote degree of relationship ; and the system is the expression of genealogical relationship founded upon descent. All systems, however, must be imperfect and full of gaps, since the extinct ancestors of organisms living at the present time can only be very imperfectly supplied by the geological record ; numerous intermediate forms are wanting and finally no traces of organic remains from the most ancient periods are preserved. Only the ultimate twigs of the enormously ramified ancestral tree are accessible to us in sxifiicieiit number. Only the extreme points of the twigs are completely preserved ; while of the numerous rami- fications of the branches only the existence of a stump here and there has been demonstrated. Hence it appears quite impossible, in the present state of our knowledge, to attain to a sufficiently sure representation of this natural genealogical tree of organisms ; and while we admire the bold speculations of E. Haeckel's genealogical attempts, it must be admitted that at present there is room for innumerable possibilities in detail, and that subjective judgment holds a more conspicuous place than objective certainty of fact. Hence we must be contented for the present with an incomplete and more or less artificial arrangement ; although the conception of the natural system theoretically is established. When the fundamental arguments of the Darwinian theory of selection and the transmutation theory founded upon it are submitted to criticism, it is soon apparent that direct proof by investigation is now, and perhaps always will be, impossible ; for the theory is founded upon postulates which cannot be submitted to direct inquiry. Periods of time which cannot be brought within the limits of human observation are required for the alteration of forms under natural conditions of life ; and the extremely complicated interactions, which in the natural state under the form of natural selection are tending to change plants and animals, can only be grasped in a general sense, while in their details they are practically unknown to us. Further, plants and animals which are under the influence of EVIDENCE FROM MORPHOLOGY. 151 natural selection are entirely inaccessible to the experiments of man, and the relatively few forms which man has, in a greater or less space of time, brought completely within his power, have been and are being altered and modified by the so-called artificial selection. The action of the natural selection, in Darwin's sense, is therefore in general incapable of direct proof, and even for the origin of varieties can only be illustrated and rendered probable by hypothe- tical examples. Against this we must, however, set the fact that there is a great probability in favour of the correctness of the theories of descent and transmutation of species, which have never received better support than from the natural selection theory of Darwin ; and that this probability is supported, not only by the whole weight of morphological evidence, but also by the testimony of Palaeontology and of geographical distribution. EVIDENCE IN FAVOUR OF THE THEORY OF DESCENT. If the transmutation of species is to be regarded as an hypothesis, because it is incapable of being demonstrated by direct observation, then its value depends upon its correspondence with the facts and phenomena of nature. Evidence from Morphology. The whole of Morphology tends to show the correctness of the theory of transmutation of species. The degrees of resemblance between species which was for a long time expressed by the metaphorical term " relations/tip," and which rested upon an agreement in more or less important characteristics, led to the establishment of systematic groups, of which the highest, the kingdom or type, was founded upon a similarity in the most general features of organization and development. The agreement of numerous animals in the general plan of their organization, e.g., the common possession by fishes, reptiles, birds, and mammals of a rigid column forming the axis of the body, and the dorsal position in regard to this of the central nervous system and the ventral position of the organs of nourishment and reproduction, are very well explained, according to the theories of selection and descent, by the derivation of all Vertebrates from a common ancestor possessing the characteristics of the type, while the supposition of a plan of the Creator renounces all explanation. In like manner is explained that similarity of characteristics by which the remaining groups and sub-groups, from class to genus, are distinguished, as well as the possibility of dividing all organized beings into groups subordinated the one to the other. 152 MEANING OF THE SYSTEM. The impossibility of a sharply defined classification is also rendered comprehensible by the theory of descent. The theory requires the existence of forms transitional between intimately and remotely allied groups ; and explains, as a result of the disappearance, in course of time, of numerous types which have been worsted in the struggle for existence, the fact that groups of equal value are of such various extent, and are often only represented by single forms. It is not only systematic characters, but also the innumerable facts brought to light by the science of Comparative Anatomy which point to a nearer or more remote relationship between the different groups. For example, if we examine the structure of the extremities or the brain of Vertebrates, we find, in spite of considerable differ- ences (sometimes bridged over by intermediate forms) in the various groups, that in all they are built upon a common type of struc- ture. This type is found very variously modified and more or less differentiated in each secondary group, according to the different functions which the organ has to fulfil and according to the exigencies of the mode of life to which each species is subjected. In the fin of the whale, in the wing of the bird, in the anterior limb of the quadruped, and in the human arm it can be shown that there are present the same bones, here short and broad and irnrnoveably con- nected, there elongated and jointed in different ways to allow of corresponding movements, sometimes with every part fully developed, sometimes simplified in one way or another, and partly or entirely rudimentary. Evidence from the facts of Dimorphism and Polymorphism. The phenomena of dimorphism and polymorphism in the same species, and the sexual differences which have been developed in animals originally hermaphrodite, may be quoted as important evi- dence of the extensive influence of adaptation. Male and female forms differ not only in the fact that the former produce spermatozoa and the latter ova, but they exhibit numerous secondary sexual characteristics connected with the different func- tions which the male and female respectively have to perform. The existence of these secondary characteristics can in all cases be satisfactorily explained by means of natural selection. We may therefore, in a certain sense, speak of a sexual selection * by means of which the two sexes have been, in course of time, gradually sepa- rated from one another, not only in peculiarities of form and organiza- * Ch. Darwin, "The Descent of Man, and. Selection in Relation to Sex," Vol. I. and II. London 1871. EVIDENCE FROM DIMORPHISM. 153 tion, but also in habits of life, in such a way as to favour the preservation of the race. Since the male sex generally has to take a more active part in the acts of copulation and fertilization it is comprehensible that the male form should differ more from the young than the female which supplies material for the formation and nourishment of the embryo and is charged with the care of the progeny. Very frequently the male sex is capable of quicker and more facile movements ; in many Insects the male alone has the power of flight, while the female remains without wings (fig. 97). In the strife which the males of similar species have to wage for the possession of the females, those individuals which are most favoured by their organization (in respect of strength, capability for motion, prehensile organs, beauty, organs for production of sound, etc.) will prove the conquerors ; while those females which possess properties especially favourable to the prosperity of the offspring will best fulfil their task. At the same time variations in the duration of development, in the mode of growth and structure, may in a more passive way be favourable under the special conditions of life of the species. The secondary sexual characters may sometimes acquire such importance as to lead to essential and deeply engrained modification of the organism, and to a true sexual dimorphism (males of Botifera with no digestive tube, dwarfed males of Bonellia, Trichosomum crassicauda). It is a significant fact that dimorphism of sex reaches its highest extreme in parasites. In many parasitic Crustacea (Siphonostoma) such extreme cases, in which the large shapeless females have lost the organs of sense and locomotion, and even segmentation, while the males are small and dwarfed, are connected by numerous inter- mediate forms ; and the circumstances which have operated as the cause of this sexual dimorphism are not far to seek. The influence of favourable conditions of nourishment which parasites enjoy does away with the necessity of rapid and frequent locomotion, increases in the female the capacity of producing reproductive material, and brings about such an alteration of form that the power of locomotion is diminished and the organs of movement atrophy and may com- pletely vanish. The body acquires an unwieldy, shapeless character in consequence of the enormous size of the ovary which is filled with eggs, and throws out outgrowths and processes into which the ovaries project, or else acquires an unsymmetrical saclike form. The seg- mentation is lost and the limbs degenerate ; the slender moveable abdomen which, when the animal was free-swimming, was an essen- 154 MEANING OF THE SYSTEM. tial aid to locomotion, is reduced more and more till it becomes a short, unsegmented stump. The appearance of such a parasite is so strange that one can easily comprehend how it was that formerly one of these abnormal groups, the Lerncece, was placed among the endoparasitic Worms, or even among the Mollusca. The more the female remains behind the type of its fully-developed, free-living allies, so much the more do the two sexes become morpho- logically remote from one another, for the form and organization of the male also are affected by the changed conditions of life, but in a different manner.* In the male sex the more favourable and abundant nourishment may not affect the necessity of locomotion and the development of the locomotive organs in so direct a manner, since the sexual activity of the male and the necessity for locomotion in order to select a female remain unaltered. Even when locomo- tion is reduced and rendered difficult, parasitism does not, in the case of the male, lead either to a complete loss of segmentation or to such unsymmetrical growths as we observe in many female parasitic Crus- tacea. The large quantity of generative material produced, which in the female is of the greatest importance for the preservation of the species, and which therefore favours the development of a large, shapeless, unwieldy body, is the less conspicuous in the male because a very small quantity of sperm serves for the fertilization of an enormous number of ova. Thus, then, the extreme degree of parasitism in the male, even when accompanied by a confined and more creeping mode of loco- motion, does not lead to an excessive increase in size nor produce an unsegmented and strange form of body, but, on the contrary, gives rise to the symmetrically formed, dwarfed pigm?ean males. This extreme state is, however, connected with the normal state by numerous intermediate steps. Thus we find in the Lernaeopocls that the size of the male Adheres is only slightly reduced, while the true dwarfed males of the Lernceopoda and Chondracanthidce are attached, like small parasites (fig. 98), to the posterior end of the female body, which is relatively enormously large. The preparation of a large amount of sperm which implies the possession of a large body, would only be a useless expenditure of material and time in the life of the species, and this must have been avoided by the influence of natural selection. In addition to this sexual dimorphism we find in various groups of animals especially in the insects which live together in great * Compare C. Clans. " Die freilebenden Copepoden." 1863. EYIDEXCE FROM MIMICRY. 155 societies, the so-called animal communities a third group of indi- viduals (sometimes even divided into several series of forms) which are without generative organs and are incapable of reproduction, but which assume the functions of protecting, of providing nourish- ment for the community, and of caring for the young. Adaptive peculiarities suitable for the discharge of these functions are apparent in their structure and organization. These sterile indivi- duals are in the Hymenoptera aborted females. Among the ants they are divided into workers and soldiers. Amongst the Termites they are derived from both males and females, in which the genera- tive organs are reduced. Sterile individuals are also found amongst animals (Fishes) which do not form communities, and were formerly taken for particular species and described as such. Polymorphism is most highly developed in the Hydroids which are united in stocks the Siphonophora. The numerous cases of dimorphism and polymorphism in either sex of the same species, should be regarded from the same point of view. Dimorphic females among insects have been observed, e.g., in the Malayan Papilionidce (P. Memnon, Pamnon, Ormenus), in cer- tain species of Hydroporus and Dytiscus, as also in the Neurotemis, a genus of the Neuroptera. In these cases, as a rule, one of the female forms is more nearly related in form, and colour to the male orrn whose peculiarities it has assumed. In other cases the differences are more connected with climate and season (seasonal dimorphism of butterflies), and also affect the male animal. They may be connected with the different forms of reproduction (parthen- ogenesis), and lead to the phenomenon of heterogamy (Chermes Phylloxera, Aphis). Much more rarely we find two kinds of males with dissimilar secondary sexual characters connected with copula- tion, as in the case of the " smellers " and " claspers " :|: described by Fritz Miiller in the Isopoda (Tanais dubius). Evidence from Mimicry. Another series of phenomena which may probably be referred to useful adaptation is the so-called mimicry. Certain animal forms come to resemble other widely- distributed species, which are protected by any peculiarity of form and colour, so closely that they seem to have copied them. The cases of mimicry which have been principally made known by Bates and Wallace are directly connected with the protective resemblances mentioned above ; that is, the resemblance of many animals in colour and body shape to the objects amongst which they * Fritz Miiller, " Facts for Darwin," p. 22. 156 MEANING OF THE SYSTEM. live. For example, amongst the butterflies certain Leptalidce resemble in outward appearance and in mode of flight a species of the family Heliconius (fig. 116), which appears to be protected from the pursuit of birds and lizards by a yellow disagreeable-smelling fluid, and share the same locality with the above-mentioned species. The most perfect instances of mimicry are found in the Tropics of the Old World, where the Danaidce and Acrceidce are imitated by the Papilionidse (Danais niavius, Pajnlio liippocoon Danais echeria, Papilio cenea Acrcea yea y Panopcea hirce). Cases of mimicry fre- quently occur between insects of different orders ; butterflies imitate the form of Hymenoptera, which are protected by the possession of spines (Sesia bombyliformis Bombus hortorum, etc.) In the same way certain beetles resemble bees and wasps (Charis melipona, Odontocera odyneroides), and the Orthopteran genus Con- dylodera tricondyloides from the Philippines is like a genus of Cicindelff (Tricondyla\ Numerous Diptera have the form and colour of stinging tiphegidw and Wasps. Also among Vertebrates (Serpents and Birds) some examples of mimicry are known. Evidence from Rudimen- tary Organs, Rudimentary organs, too, which are so common, are satisfactorily ex- plained by the theory of selec- tion as the result of non- employment of such organs. Organs which were formerly functional have gradually or even suddenly become functionless as a result of adaptation to special conditions of life, and, through want of exercise, have, after the lapse of generations, become weaker and finally aborted or degraded (Parasites). We cannot, however, assert that rudimentary organs -are in all cases useless. They have, on the contrary, often gained secondary functions, though this may be difficult to demon- strate. We find, for instance, in certain snakes (Pt/thonidcv) that there are small processes armed with claws at the sides of the anus (anal FIG. 116. u, Leptalis Theonoe, var. Leitconoi; (Pieris). I, Ithoiiiiu Ilerdina (the mimicked Heliconius). (After Bates.) EVIDENCE FROM EMBRYOLOGY. 157 These are the hind limbs which have become rudimentary, and which do not subserve locomotion but, in the male at least, assist in copulation. The blind worms possess a rudimentary shoulder girdle and breast bone, although the anterior extremities are want- ing : these bones may be connected with the need of protecting the heart, or may aid in respiration. When we see that the upper incisor teeth are developed in the foetus of many ruminants, and that these teeth are never cut, and that the embryos of the whalebone whales have the rudiments of teeth in their jaws, which they soon lose and never make use of in mastication, it is much more rational to ascribe to these .structures a part-in the growth of the jaw than to hold them for wholly useless. The rudimentary wings of the penguin are employed as oars, those of the ostrich as aids to running and as weapons for protection. The rudimentary stumps of the kiwi, on the contrary, appear valueless. In many cases we are not in a position to assign any function or value to rudimentary organs. Evidence from Embryology. The results of embryology too, i.e., the individual development from the ovum to the fully developed form, are in complete agreement with the Darwinian theories of selection and descent. The fact that the animals belonging to one type have, as a rule, embryos which are much alike and undergo a similar developmental process, and that the closer the relationship between the adult forms the greater the similarity in their develop- ment (with some remarkable exceptions), supports the conception of a common ancestry and the hypothesis of differing gradations of blood- relationship. If the groups of different value which correspond to the divisions and subdivisions of our classification are genetically derived from more or less remote ancestral forms, then the individual develop- ment will present >o many the more common features the closer the forms stand to their common ancestor. The fact that animals which differ much from one another and exist under very different conditions of life show an unusual agree- ment in their post-embryonic development up to a more or less late period (the free Copepoda, parasitic Crustacea, Cirripedia), is in no wise opposed to the theory, but may be explained by the influence which adaptation has exerted not only during the period of sexual life, but also during each developmental period, causing changes which have been inherited in corresponding periods of life. The phenomena of metamorphosis afford numerous proofs of the fact that the adaptation of the embryonic form is as complete as 158 MEANING OF THE SYSTEM. that of the adult and we can thus understand how larvae of many insects belonging to different orders can present great resemblances to one another and be unlike the larvae t)f insects of the same order. While as a general rule the development of the individual is an advance from a simpler and lower organization to one more complex which has become more perfect by a continued division of labour among its parts and we shall later find a parallel to this law of perfection of the individual in the great law of progressive perfection in the development of groups yet the course of development may, in particular -cases, lead to numerous retrogressions, so that we may find the adult animal to be of lower organization than the larva. This phenomenon, which is known as retrogressive metamorphosis (Cirripedia and parasitic Crustacea), corresponds to the demands of the selection theory, since under more simple conditions of life, where nourishment is more easily obtained (parasitism), degradation and even the loss of parts may be of advantage to the organism. Again, the facts of embryonic development, when considered in relation to the gradations expressed in the .system are in complete accord with the theory of evolution. Numerous examples may be cited to prove that features, not only of the simple and more primitive, but also of the more perfectly organised groups of the same type, are reflected in the successive phases of foetal life. In the case of a complicated free development by metamorphosis, which is usually correlated with an unusual simplification of the foetal development within the egg- membranes, the relation of the successive larval stages to the allied smaller groups of the system, to the genera, families and orders, is more direct and striking. For example, in the early stages of the embryonic development of mammals certain structures occur, which in the lower fishes endure throughout life. Later stages show peculiarities which correspond to the characters of amphibia. The metamorphosis of the frog begins with a stage which in form and organization and mode of locomotion agrees with the fish type ; and this stage is succeeded by numerous other larval stages in which the characters of the other orders of Amphibia (Perenni- branchiata, Salamandrinidfe) and of individual families and genera of the same are repeated. This undeniable likeness between the successive stages of individual development and between allied groups of the system allows us to institute a parallel between the former and the evolution of the species. The evolution of the species finds, it is true, a most imper- fect expression in the relationship of the systematic groups, and can GEOGRAPHICAL DISTRIBUTION. 159 only be inferred from the history of the past for which palaeon- tology affords us but slight material. This parallel, which naturally presents numerous greater or smaller variations in detail, is explained by the theory of evolution, according to which the developmental history of the individual appears to be a short and simplified repetition, or in a certain sense a recapitulation, of the course of development of the species.' 1 ' The historical record preserved in the developmental history of the individual must often be more or less blurred and obscure on account of the many adaptations which have occurred during the embryonic development, or during larval life. Especially in those cases where the peculiar conditions of the struggle for existence demand a simplification, the development will take a more direct course from the ovum to the perfect animal, will be thrown back into an earlier period of life, and finally will be completed before the animal is hatched, until, in absence of a metamorphosis, the historical record is completely suppressed. On the contrary, in the cases of progres- sive transformation where the larval states are gradually modified and live under similar conditions of life, the history of the species will be less imperfectly reproduced in that of the individual. Evidence from the Facts of Geographical Distribution. Unlike the facts of morphology, those of geographical distribution raise great difficulties for the theory principally because the phenomena are very complicated and our experiences are still too limited to permit of our establishing general laws. The present distribution of plants and animals over the surface of the earth is clearly the combined result of the earlier distribution of their ancestors and of the geologi- cal changes which have since taken place, the modifications in the extent and position of land and water, which must have had an influence on the fauna and flora. Accordingly the geographical distribution of plants and animals f appears intimately connected with that part of geology which has for its aim the investigation of the most recent occurrences in the formation of the earth's crust and its contents. It cannot, therefore, be confined to an examination of the areas of distribution of the animals and plants of the present day, but must take cogni- zance of the distribution of the remains, enclosed in the most recent formations, of the nearest relations and ancestors of living forms, in * Fr. Miiller, " Filr Darwin,'' Leipzig, 1864. f A.R.Wallace, " The Geographical Distribution of Animals," London, 1876. P. L. Sclater. " Address to the Biological Section of the Brit. Association." 1875. 160 MEANING OF THE SYSTEM. order to find an historical explanation of the known facts of distribu- tion. Although in this sense the science of animal geography is still in its infancy, yet numerous and important phenomena of geographical distribution receive a satisfactory explanation according to the theory of transmutation of species on the supposition of migrations and gradual changes brought about by natural selection. It is a most important fact that neither the resemblance nor the want of resemblance of the animals inhabiting different localities can be completely explained as the result of climatic and physical conditions. Closely allied species of plants and animals often appear under very different natural conditions, while a completely different fauna and flora can exist in a similar climate and on a similar soil. On the other hand, the extent of the difference between two fauna is closely connected with the limitations of space and the barriers and hindrances to free migration. The Old and New Worlds, which, leaving out of consideration the polar connection, are completely separated, have in part a very different fauna and flora, although with regard to the climatic and physical conditions of existence there are innumerable parallels which would equally favour the prosperity of the same species. In particular if we compare the districts of South America with regions situated in the same latitude and possessing the same climate in South Africa and Australia, we find three fauna and flora which differ considerably, while the natural productions from different latitudes of South America with entirely different climates are closely allied. Here the northern animals are indeed specifically different from the southern, but belong to similar or nearly allied genera with the peculiar stamp characteristic of South America. Zoological Provinces. The surface of the earth can be divided into from six to eight regions according to the general features of the terrestrial and fresh-water fauna. These regions can indeed only be considered as a relative expression for large natural districts of distribution, since they cannot be applied to all groups of animals in the same manner, and it is impossible that they should differ in like degree and in the same direction. There must also be inter- mediate regions combining the characteristics of the neighbouring regions with peculiarities of their own ; and the question must arise whether these should not be taken as independent regions. The merit of having first established a natural division of the earth into zoological regions and sub-regions belongs to Sclater. This naturalist founded his system on the distribution of birds, and dis- ZOOLOGICAL PROVINCES. 161 tinguished six regions, the limits of which agreed fairly well with the distribution of Mammalia and Eeptilia. These regions are (1) The Pahmrctic Region Europe, the temperate part of Asia, and North Africa as far as Mount Atlas. (2) Nearctic Region -Greenland and North America as far as North Mexico. (3) Tli e Ethiopian Region Africa, south of Atlas, Madagascar, and the Mascarenes with South Arabia. (4) The Indian Region India south of the Himalayas, to South China, Borneo and Java. (5) The Australian Region Celebes and Lombok eastward to Australia, and the South Sea Islands. (6) The Neotropical Region South America, the Antilles, and South Mexico. Other naturalists (Huxley) have since shown that the four first of these regions have a much greater resemblance to one another than any one of them has to the Australian or South American regions ; that New Zealand is entitled by the peculiarities of its fauna to be considered as forming a region by itself ; finally, that a circumpolar* province should be formed equal in value to the Palse- arctic and Nearctic. Wallace objects to the establishment either of a New Zealand or of a circumpolar region, and advocates the adoption of the six regions of Sclater on practical grounds, but suggests the modification that since the South American and Australian are much more isolated, the regions should not be of equal value. These regions are bounded by extended seas, lofty mountain ranges, or vast sandy deserts, and obviously such boundaries do not constitute effective barriers to the migration of all animals, but allow certain groups to pass from one region to another. The obstacles to immigration and emigration appear in certain places, at all events in the present time, to be insurmountable; * Andrew Murray, on the contrary, in his work on the geographical dis- tribution of Mammalia in 1866, distinguishes only four divisions the Palrearctic, Indo-African, the Australian, and the American. Riitimeyer. recognises in addi- tion to the six provinces of Sclater a Mediterranean and Circumpolar province. J. A. Allen ("Bulletin of the Museum of Comparative Zoology, Cambridge," vol. ii.) proposes to distinguish eight regions, in connection with " the law of circumpolar distribution of life in zones : " (1) Arctic realm ; (2) North Temper- ate realm ; (3) Tropical American realm : (4) Indo-African Tropical realm ; (5) Tropical South American realm ; ( ( >) Temperate African realm ; (7) Ant- arctic realm : (8) Australian realm. 11 162 MEANING OF THE SYSTEM. but in past ages, when the divisions of land and water were different, they must have been, for many forms of life, easily surmountable. The expression "centre of creation," which has long been used in the sense of a tolerably denned district of dis- tribution or better still, Riitimeyer's word, " centre of distribution" has as a fundamental idea the endemic appearance of definite groups of typical species and their gradual extension "' towards the boundaries of the said region, a conception which harmonizes well with the theory of the origin of species through gradual alterations. The same laws apply also to the distribution of the inhabitants of the sea. Great seas studded with islands which serve to confine the land animals may favour the migration of marine species, while extended continents, which allow their inhabitants to wander freely over them, confine the sea animals within limits which cannot be passed. A great number of sea animals live only in the shallow water round the coast, and their distribution thus often coincides with that of the land animals ; whereas the animals found on the opposite coasts of great continents are very different. For example, the sea animals of the east and west coasts of South and Central America differ to such a degree that, with the exception of a series of fishes, which, according to Giinther, are found on both sides of the Isthmus of Panama, only a few forms are common to the two coasts. In the same way we find that the marine inhabitants of the east insular district of the Pacific differ completely from those of the west coast of South America. If, however, we advance to the west of this part of the Pacific till we come to the coast of Africa in the other hemisphere, we find that the fauna of this extensive district cannot be so sharply distinguished. Many species of fish are found from the Pacific to the Indian Ocean. Numerous Mollusca of the South Sea Islands live also on the east coast of Africa, almost beneath the opposite meridian. In this case the limits of distribu- tion are not impassable, as numerous islands and coasts afford a rest- ing place to wandering inhabitants of the sea. In respect of the different haunts of the inhabitants of the sea, we must make a dis- tinction between the littoral animals, which are distributed along the coasts, and live under different conditions and at different depths on the bottom of the sea, and the pelagic animals, which swim on the surface. * Compare Riitimeyer's Essay, "Ueber die Herkunft unserer Thierwelt." Basel and Genf. 1867." EVIDENCE FKOM PAL.EONTOLOGT. IBIi But there also exists, at considerable depths and on the bottom of the sea, a rich and varied animal life. This has only lately been brought to our knowledge principally by the deep-sea explorations from North America, Scandinavia, and England. In place of that want of animal life which we should on <> priori grounds expect to find, we see that numerous lowly organised animals of the most different groups are able to exist even at the greatest depths. Besides the lowest sarcode animals of the Foraminifera (Globigerina ooze), we find especially silicious sponges, certain corals. Echinoderms, and Crustacea.* The representatives of the latter are in part of low type, but gigantic, and many of them blind. It is also a fact of more than ordinary interest, as showing the continuity of living creatures from successive geological forma- tions up to the present time, that the deep sea animals are allied to ancient types which occur in Mesozoic formations, especially in chalk. Evidence from Palaeontology. The results of geological and palivontoloyical inquiry give us a third great series of facts in support of the theory of slow alterations of species and the gradual development of genera, families, orders, etc. The firm crust of our earth is formed of numerous and enormous rock strata, which have been deposited in a definite series by water in course of time, and also of the so-called volcanic or plutonic rocks, masses which have been forcibly ejected from the molten interior of the earth. The former or sedimentary deposits, which have under- gone numerous alterations in the originally horizontal arrangement of their strata as well as in the petrographical condition of their rocks, contain a quantity of the fossilized remains of former plants and animals which have become buried in them, and thus afford an historical record of a rich fauna and flora which existed during the earlier periods of the earth's development. Although these so-called fossils have made us acquainted with a very considerable number of ancient organisms presenting great diversity of form, yet they only constitute a very small portion of the enormous quantity of living beings which have at all times existed upon the earth. They suffice, however, to teach us that a different fauna and flora existed at the time when each individual deposit was being formed, and that * Compare Wyville Thomson. " The depths of the sea. An account of the general results of the dredging- cruizes of the Porcupine and Lightning, during- the summer months of 1868, 1869, 1870." London. 1873. Also the results of thc- f 'hall t' nij': i- expedition 1874-1 87<>. 1(34 MEANING OF THE SYSTEM. the deeper a stratum comes in the series, that is, the earlier it appeal's in the history of the earth, so much the more its fauna and flora differ from those of the present time. The more nearly one stratum follows another in the series, the closer the relationship between their respective fossils. Every sedimentary formation possesses characteristic fossils which appeal- very frequently ; and from these, taking into account the succession of strata and the petrographic characters of the rocks, the place occupied by the stratum in the geological system can be denned with tolerable accuracy. Without doubt the characters of the fossils and the relative posi- tions of the strata are the most important aids to the determination of the geological age of the deposit ; at any rate they furnish a more reliable criterion than does the structure of the rocks. The idea entertained in earlier times that rocks of the same period always possessed a similar, and rocks of a different period a dissimilar structure, has lately been given up as erroneous. Stratified or sedimentary deposits have arisen in every period under similar condi- tions. In past times, as at the present time, they were caused by the deposition of clay, of fine or coarse sand, of fine and coarse debris, by chemical precipitation of carbonates and sulphates of lime and magnesia, of silica and oxide of iron, and by accumulation of solid animal and vegetable remains. These have become transformed only in course of time into such hard rocks as argillaceous and calcareous schists, limestone, sandstone, dolomite, and conglomerates of many kinds ; as the result of many causes, such as mechanical pressure of superincumbent masses, increase of temperature, internal chemical processes, and so forth. Even though the peculiar structure of rocks may in many cases afford good ground for conjecture as to the relative age, yet it is certain that deposits of similar age may show an entirely different petrographical character ; and, on the other hand, that deposits of very different ages may have given rise to rock forma- tions that can be scarcely or not at all distinguished from one another. The old idea that deposits of the same age must everywhere contain the same fossils, could only be maintained as long as geological inves- tigations were confined to small districts. Similarly the idea, closely connected with the former, that the various geological formations, characterised by a series of definite strata, are entirely independent of one another, no longer obtains credit. The various forma- GEOLOGICAL PERIODS. 105 tions,* as the group of strata of one district of distribution and belong- ing to one period are named, cannot be divided petrographically or * The following table may serve for a bird's-eye view of the geological period* and their most important formations : [liecent Periods (alluvium, marine and fresh-wator QUARTIARY PERIOD J formations) . (DilwoialandAllweial

. 0. C. Marsh, "Principal Characters of the Dinocerata." Anicr. Journal oj Si-ii-nrr and Art. Vol. xi., 1876. 0. C. Marsh. " Principal Characters of the Brontotherid.se." Anii'r. Journal of Sfh'iu-i- and Art. Vol. xi., 187l'>. f Compare H. Filhol, ' Recherches sur les Phosphorites du Quercy, fitude des fossils qu'on y rencontre et specialement des Mammiferes." Ann. g&ologiqncs, Vol. vii., 187n.sist> as a rule of a tougher exoplasm and a fluid granular endoplasm. The pseudopodia are lobed or finger-shaped processes of considerable 186 PROTOZOA. size, occasionally tougher slender processes without granule streams (figs. 125 and 126). Ama'lxt j>ri//<-rj>x Ehrbg.. A. tcrrii-nln Greef., Petf/lopnx il itHm/ims Clap. Lachm. Here should also be placed the famrms Butlii/litu* llnt-clu-ll Huxl.. which is found in the' deep sea mud of the Atlantic Ocean, if it is indeed a living organism (and not simply a deposit of Gypsum). At'ci'lln mli/in-ix Ehrbg.. JJiffft/i/ia jM'ott'ifni'Hii* Ehrbg.. Euylypha ylobom Cart, have shells and tough, pointed, dichotomously branching pseudopodia (fig. 125). FIG. 124. -Nummulitic Limestone, with horizontal section of 1$. J intuits (after Zittell). FIG. 126. oblonffa(fdter Stein). FIG. 125. TSuglypha gJobotu (after Hertwig and Lesser). FIG. 127. Acervulina globosa (after M. Schnltze). 2. Sub-order : Reticularia (Thalamophora). Principally marine Bliizopods with extremely slender anastomosing pseudopodia, with granule streams in the latter, rarely naked (Protoaenes, Lieber- kiihnia), usually with membranous or calcareous shell, which is single-chambered (Monolludamia} or many-chambered (Polythalamia) (fig- 127). HELIOZOA. 187 ing 1. IinjH'i-fiiratn. With membranous or calcareous shell, which is without fine purrs, hut possesses, in one place, an opening, either simple or sieve-like, through which the p-mdupodia project. To these belong the GromitJt/; with a mem- branous chitinous shell : Gi'umi/i in'ifoniiix Duj.. and Mil'mlnlfr. with a poroellanous shell : Comuspira planorbis M. Sch.. Milioln ryrloxfinit/i. M. Sch., from the Miliolite chalk. 2. Perfui'ittti. The shell, which is usually calcareous, is invariably pierced with innumerable fine pores as well as by one larger opening, and has complicated passages in the partition walls of its chambers. The Luijcnida- have a hard shell, with a large opening surrounded by a toothed lip : Ln/jcna culf/arix Williamson. The Grlotigerinidce on the contrary have a hyaline shell pierced by large pores, and a simple slit-like open- Orln/liitii intirrrxit D'Orb.. 'nni IniUnnli'x D'Orb.. Rotnlln. D'Orb., Tt-.rtnl>-hi. D'Orb. The greatest size is attained by the -Y>/niii(fht/rI(e. which possess a firm shell and an in- ternal skeleton, which last is pierced by a complicated canal system : Polystomella Lam.. Nwrnmulvna D'Orb. Order 2. HELIOZOA. ' Fresh-water lihizopods usually tvit/t pulsating vacu- ole, and one or more nuclei. A radial silicious skeleton sometimes present. The sarcode body sends out in all directions tough radiating pseudopodia (fig. 128). When a skeleton is secreted, it consists either of radially arranged silicious spines (Acanthocystis) or of latticed silicious shells (ClatJirulinci), and so closely resembles the skeleton of the Radiolaria that the Heliozoa have been actually described as fresh-water Radiolaria. They differ from the Radiolaria in the absence of the complicated * L. Cienkowski, " Ueber Clathrullna." Archie, fur miJtmsl:. Anatinnir, Tom III., 18t;7. R. Greeff, "Ueber Kadiolarien und radiolariena'lmliche Rhizopoden des stissen \Vassers." Tom V. & XT. R. Hertwig und Lesser. ' Uber Rhizopoden und denselben nahe stehende Organismen." Suppl. Tom X.. 1874. Also Archer and F. E. Schultze, etc. FIG. 128. Young Actinosphmrium, still witli a single nucleus (after F. E. Schultze). -A', Nucleus. 188 PROTOZOA. differentiations of the sarcode, particularly of the central capsule. One or more nuclei may be present in the central mass. An im- portant distinguishing mark is afforded by the presence of the pulsating vacuoles, which have not been observed in any marine Hadiolarian. The reproduction very frequently takes place by fission, occasionally FIG. 129. Thalassicolla pelagica, with central capsule and single largenucleus, also numerous alveoli in the protoplasm (after E. Haeckel). after previous conjugation of one or more individuals, also during encystment. Multiplication by spores has also been observed (Clathrulina). in the Actinopliryidfs there is no skeleton secreted : ActinosphcEriv/m Eidilnn'n'tl Ehrbg. The central matter contains numerous nuclei. ActinopJirys sol Ehrbg. of small size, with a single central nucleus. In the A<'antlicnxti(l(e slender silicious spikes are found : Ai-tintJtoci/xtix Sjthtifera Greet!, with silicious spikes and needles. In Clatfii'iilhta there is a latticed silicious shell, and the body has a stalk : Chitliniliini eleg&ns Cienk. RADIOLARIA. 189 Order 3. RADIOLARIA.* Marine Rhizopoda with complicated differentiation of the sarcode body, with central capsule and radial silicious skeleton. The sarcode body contains a membranous porous capsule (the central capsule}, in which is contained a tough slimy protoplasm with vacuoles and granules (intracapsular sarcode}, fat and oil globules, and albuminous bodies, and more rarely crystals and con- cretions. The intracapsular mass contains also a single large nucleus or several small nuclei. The sarcode which surrounds the capsule and which emits on all sides simple or anastomosing pseudopodia, contains numerous yellow cells, sometimes pigment masses : and in some cases delicate trans- parent vesicles, or alveoli, are found in the peripheral layer between the radia- ting pseudopodia (Thal-as- sicolla pelagica, fig. 129). Many Radio! aria form colonies, and are composed 130.AefliiJiiiff,-ii Miilleri (after E. Haeckel). of -numerous individuals. In such colonies the al- veoli are placed in the common protoplasm, which contains in it:- elf. not as in the monozoic Radiolaria a single cen- tral capsule, but a number of capsules. Only a few species remain naked and without tirm deposits ; as a rule, the soft body possesses a silicious skeleton, which either lies entirely outside the central capsule (Ectolithia) or is partially within it (Entolithia). In the most simple cases the skeleton consists of small, simple, or toothed silicious needles (spicula) united together, which sometimes give rise to a fine sponge work round the periphery of the proto- plasm, e.y., Pliijseuiatium. In a higher grade we find stronger hollow silicious spicules, which radiate from the middle point of the body to the periphery in regular number and order, e.g., Acanthometra * Job. Miiller. ' Uel>er die Thalassicolleu, Polyeystincu und Acanthometivn.' AbJt. ill- 1- Bcrl. Aliuil. 1858. E. Haeckel. il Die Radiolarien." Eine Monographic Berlin. 1862. 190 PROTOZOA. (fig. 130). A fine peripheral framework of spicules may be added to these. In other cases simple or compound lattice-works, and pierced shells of various external form (like helmets, bird-cages, shells, etc.) are found, and on the periphery of these, spicules and needles, and even external concentric shells of similar shape may be formed, e.g., Polycystina (figs. 131 and 132). Up to the present time but little has been made out about the reproduction of these animals. Besides fission (Polycyttaria), the formation of germs has been observed. These are formed from the contents of the central capsule, and, after the bursting of the latter, become free-swimming mastigopods. Radiolaria are inhabitants of the sea, and swim at the surface, but are also able to sink to deeper levels. Fossil remains of Ra- diolaria have been made know r n in great numbers by Ehrenberg, e.g. from the chalky marl and polishing slate found at certain parts of the coast of the Mediterranean (Caltanisetta in Sicily, Zante and ^Egina in Greece), and in particu- lar from the rocks of Barbados and Nikobar, where the Radiolaria have given rise to widely extended rock formations. Samples of sand also from very con- siderable depths have shown themselves rich in Radiolarian shells. I. Radiolaria monozoa. Radiolaria which remain solitary. 1. Fam. Thalassicollidae. Skeleton absent or consisting of single spicules not joined together. Tlialaxsicolla (without skeleton) undent ti Huxl.. Plnjste- mat-ium Mullen Schn. 2. Fam. Polycystinidoe. The skeleton consists of a simple or divided latticed shell, the long axis of which is bounded by two poles of different structure. Ifdioapliffrn. Euryrtidiuin ijnlrrl/Hi-it!i/ Joll. Miill. II. Puhjci/ttaria. Radiolaria which form colonies with several central capsul-'-i Amongst the Sphaerozoa a skeleton is wanting or consists of single pieces not joined together. CvUozmnii 'incrmc, K. Haeck. X/iIttrrnzomit jiii/trfiifmn, Joh. Miill. In C'<>lloxj)!ie/->-tr the skeleton consists of simple latticed spheres, each <>f which encloses a central capsule, Colloxjth (/!<( Hu-rleiji Joh. Miill. FIG. 132. Eucyrtidium cranoides (after E. Haeckel) CLASS II. INFUSORIA.* Protozoa with a definite form, and provided with on external membrane, bearing either flag etta or cilia. Mouth and anus usually, contractile vacuole and one or more nuclei ahuays present. Infusoria were discovered towards the end of the 17th century * Ehrenberg, " Die Infusionsthierchen als vollkommene Organismen." 1838. Balbiani, "Etudes sur la Reproduction des Protozoaires," J/nini. dr In Phyx.. Torn. III. Balbiani, '-Recherches sur les phenomenes sexuels des Infusoires," 192 PEOTOZOA. in a vessel of stagnant water by A. voii Leeuwenhoek, who made use of a magnifying glass for the examination of small organisms. The name Infusoria, which was at first used to denote all animalculse which appear in infusions and are only visible with the aid of a microscope, was first brought into use by Ledermuller and Wrisberg in the last century. Later on the Danish naturalist O. Fr. Miiller made valuable additions to our knowledge of Infusoria. He observed their conjugation and their reproduction by fission and gemmation, and wrote the first systematic woi'k on the subject. O Fr. Miiller included a much larger number of forms than we do now-a-days, for he placed among the Infusoria all invertebrate water animal- cule without jointed organs of locomotion and of microscopical size. The knowledge of Infusoria received a new impulse from the comprehensive researches of Ehrenberg. The principal work of this investigator, " Die Infusionsthierchen als vollkommene Organismen," discovered a kingdom of organisms hardly thought of. These were observed and portrayed under the highest microscopic powers. Many of Ehrenberg's drawings may even yet be taken as patterns, and are hardly surpassed by later representations, but the significance of the facts observed has been essentially corrected by more recent investi- gations. Ehrenberg also conceded too great an extent to the group of Infusoria, including not only the lowest plants such as Diatomacece, Desmidiacpffi, under the name of Polyyastrica anentera, but also the much more highly organised Kotifera. As he chose the organization of the last-named for the basis of his explanations, he was led into numerous errors. Ehrenberg ascribed to the Infusoria mouth and anus, stomach and intestines, testis and ovary, kidneys, sense-organs, and a vascular system, without being able to give reliable statement of the nature of these organs. There very soon came a, reaction in the way of regarding the Inf usorian structure ; for the discoverer of the Rhizojioda, Dujarclin, as well as Von Siebold and Kolliker (the latter taking into consideration the so-called Xucleus and Nucleolus), referred the Inf usorian body to the simple cell. In the subsequent works of Stein, Claparede, Lachmann, and Balbiani numerous differentiations were certainly shown to exist, which, however, can all be referred to differentiation of the body of the cell. This view is supported by Jotini. ili' In PJit/x.. Tom. IV. Claparede und Lachmann. '-Etudes sur les Inf usi lives et les Khizopodes," 2 vol. Geneve, 1858 ISC. 1. E. Haeckel. " Znv Movpholugie dev Infusovien" Ji-n Zritxchrift. Tom. VII.. 1873. Biitschli. " Studien iitaev die ersten Entwickelungvnrgan.tre des Eizelle. die Zelltheilung und die Conjugation des Infusorien," Frankfurt. 187(>. FLAGELLATA. 193 the more recent work of Biitschli, who has shown that the repro- duction of these animals is essentially similar to that of the cell. The outer boundary of the body is usually formed by a cuticle, a delicate, transparent membrane, the surface of which is beset with vibratile and moving appendages of various kinds arranged in regular order. In the smallest Infusoria, the Flagellata, we find only one or two long whip-like cilia ; while the more highly differentiated Ciliata are usually richly provided with cilia. According to the varying thickness of the external membrane, which cannot in all cases be isolated, and according to the different condition of the peripheral parenchyma of the body, we get forms which change their shape, forms which have a fixed shape and armoured forms. If the simply organized Flagellata, which present numerous affinities and transitional forms to the Algae and Fungi, are not entirely removed from the region of the Infusoria, the two principal groups to be distinguished are the Ciliata and Flagellata. Order 1. FLAGELLATA. * Infusoria of small size, characterised by j)ossessing one or more long whip-like cilia, usually placed at one end of the oval body. A row of cilia sometimes and a nucleus always present. The Flagellata are Infusoria the locomotive organs of which consist of one or more whip-like cilia, rarely with an accessory row of cilia. They pass through an inactive stage, and in their develop- ment as well as in their mode of nourishment are allied to certain Fungi. The reasons for regarding the Flagellata as Protozoa are the perfect contractility of the body, which is not surpassed by Myxomycetes in the mastigopod stage ; also the contractility of the cilia, the apparently purposed and voluntary movements, the occurrence of contractile vacuoles, and, as has been established in many cases, the reception of solid substances into the body through an opening at the base of the flagellum. Nevertheless these phenomena are by no means a test of animal organization. The Monadince are a large group of Flagellata, found for the most part in putrefying infusions, and are hard to distinguish from the monads usually regarded as fungi. They reproduce themselves by ' Besides Ehrenberg, Claparede, and Lachmami, loc. cit., compare Stein, Organismus der Infusionsthiere," Tom. III., 1878. Biitschli, ' Beitrage zur Kenntniss der Flagellaten." Zeitsi'lir.fiir Wixx. ZooL, Tom. XXX. Dallinger and Drysdale, " Researches on the Life-history of the Monads," Monthly ). Journal, Tom. X. XIII. 33 194 PBOTOZOA. transverse fission, and also by spore formation in an encysted condition; the latter method seems in many forms to be preceded by conju- gation. The best known species are Cercomonas Duj. and Trichomonas Donne, of which the first is characterised by the possession of a caudal filament, while Trichomonas has an undulating row of cilia close to the flagella, which are usually two in number (fig. 133). They live principally in the intestines of Vertebrates, but are also found in Invertebrates. Cercomonas intestinalis Lainbl. and Trichomonas vaginalis Donne, are found in Man. The Monads,* which cannot be sharply separated from the Monadince, are simple cells free from chlorophyll, the swarm, spores of which usually pass into an amoeboid stage, and after receiving nourish- ment enter upon a motionless stage characterised by the possession of a firm cell-membrane. A number of them (Monas, Pseudospora, Colpodella), the so-called Zoospores, are mastigopods resembling the mastigopocls (swarm spores) of Myxo- mycetes, and, with the exception of Colpodella, grow up to creeping Amoebae which protrude pointed pseudopodia. In this stage they may also be simply regarded as small plasmodia, especially when, as in Monas amyli, several masti- i33.-, ctrcomona* intern,, gopods f use together to form the amoeba. b, Trichomonas vaginalis after E. They then take in Colpodella without first entering the amoeba stage a globu- lar form, their surface develops a membrane, and in this cyst they break up by division of protoplasm into a number of segments which pass out as swarm spores and repeat the course of development (Colpodella pugnax to Chlamydomonas, Pseudospora volvods). Other Monads, the so-called Tetraplasta (Vampyrella, Nuclearia), do not pass through the mastigopod (swarm spore) stage. Their pro- toplasm during the inactive encysted stage gives rise by division into two or four, to the same number of Actinophrys-like Amoeba?, of which some, like Golpoddla, suck their nourishment from alga cells (Spirogyra, Oedogonia Diatomacea, etc.), and some envelope ex- traneous bodies. In mode of nourishment and locomotion the monads are allied to the Rhizopods, but also to lower fungus forms like Chytridium. * L. Cienkowski, " Beitrage zur Kentniss dcr Mnnaden," Archiv fur Microxk. Anatomic. Tom. I., 1865. L. Cienkowski, " Uber Palmellaceen und einige Flagellaten,'' Tom. VI., 1870. VOLVOCINID^ ASTASIAD,*;. 195 In their whole developmental cycle they agree very closely with uni. cellular algte and fungi ; still the analogy to the developmental processes of many Infusoria, Atnpliileptus, is not to be passed over. Spwmella vulgaris (termo Ehrbg.) of Cienkowski shows a somewhat different development and cyst formation ; it receives solid food (by aid of the food vacuoles) and is fixed by a fibre, as also Ckromulina nebulosa Cnk., and Ochracea Ehrbg. A second group nearly allied to the Algre (Protococcacea) is that of the Volvocinidce. These organisms consist of colonies of cells united by a common gelatinous substance, and the following characteristics indicate their close relationship to the Algse : (1) in the inactive stage they possess a cellulose membrane ; (2) they exhale oxygen ; (3) they possess an abundance of chlorophyll and of vegetable red or brown coloured oils. PIG. 134. Ettfflena viritHs. ; Ueber Noctiluca miUaris," Archir. fur microslt. Ana- tonne, 1871 and 1872. NOCTILTTCA. 197 of the sea, and possesses a peach-shaped body which is surrounded by a cuticular envelope, and bears a tentacle-like appendage. A furrow- like invagination is situate at the base of this appendage, at one end of which is the mouth close to a tooth-like prominence and a slender vibratile flagellum. The soft body consists of a central mass of contractile protoplasm, connected by fine and anastomosing threads with a layer of the same substance which lines the cuticular envelope of the body. In the central protoplasm lies a clear body, the nucleus, and the spaces between the radiating processes, which exhibit the phenomena of granule currents, are filled with fluid. The contractile substance extends into the appendage, and there assumes a cross- striped appearance (fig. 136). FIG. 136.Noctiluca miliaris (partly after CienkowsW). N, Nu- cleus, a, Single animal. I, conjugation of two individuals. c and d, swarm spores. The reproduction takes place by means of fission (Brightwell), pre- ceded by division of the nucleus ; or by spore formation (Zoospores). In the latter case, the flagellum is absorbed or thrown off, and the Noctiluca assumes a spheroidal shape.- After the disappearance of the nucleus, the sarcode contents accumulate on the inner side of one region of the cuticle, divide into from two to four masses which are not sharply separated from one another, and the cuticular envelope is thrust out into a corresponding number of protuberances. These Lmds increase and form numerous wart-like prominences, the future spores. They arise, therefore, at the expense of the protoplasmic contents of the disc, which is gradually exhausted in their for- 198 PKOTOZOA. mation. The buds separate themselves from the membrane and become free as small spores, with nucleus and cylindrical appendage, to assume the Noctiluca form under circumstances which have as yet not been closely observed. According to Cienkowski, conjugation may take place between normal forms as well as between encysted forms. The Noctiluca owe their name to their power of producing light, a power which they share with numerous sea animals, such as Medusae, Pyrosoma, etc. The light proceeds from the peripheral layer of protoplasm. Under certain conditions they rise from the depths of the sea to the surface in such enor- mous numbers as to cause wide tracts of the sea to give out a reddish light. It is after sunset, and especially in the evening, when the sky is overcast, that we get the beautiful phenomenon of the phosphorescent sea. The species distributed in the North Sea and in the Atlantic Ocean is Noctiluca miliaris. Nearly allied is the Mediterranean Leptodiscus medusoides R. Hertwig. vi Order 2. CILIATA.* Ciliated Infusoria with mouth and anus, sarcode body of complicated structure (with endoplasm and exoplasm), ivith nucleus and paranucleus (nucleolus). . iyr.-&yioschia my tu, The locomotive cuticular appendages that (after stein), (seen from we most frequently meet with are slender ventral side). Wz, Adoral .,. , . , , zone of cilia ; c, contractile Cllia ' wni ch often cover the whole surface of vacuole ; N, nucleus ; N>, the body in close rows, and give it a striped paranucleus ; A, anus. appearance. The cilia are usually stronger in the region of the mouth, and are here grouped so as to form an adoral zone of large cilia, which, during swimming, causes a whirl- pool, and conducts the matter which serves as nourishment into the mouth (fig. 137). This adoral zone is more highly developed in fixed Infusoria such as the bell animalcule, the surface of which has no regular arrangement of cilia. In these animals there are * Besides Ehrenberg, Claparede, Lachmann, Biitschli, 1. c.. compare especially Fr. Stein, " Der Organismus der Infusionsthiere." I. and II., Leipzig, 1859 and 1867 CILIATA. ID!) one or more rings of large cilia round the edge of a raised lid- like flap which is capable of being shut down. There is also an in- ferior row of cilia upon this flap running to the mouth. The free-swimming Infusoria often possess in addition to these delicate cilia and zones of cilia, thicker hairs and stiff bristles, and more or less bent hooks, which are em- ployed in locomotion and for attachment. Certain fixed Infusoria as Stentor (fig. 138) and Cothumia secrete external coverings or shells, into which they retract themselves. Nourishment is taken in in a few cases by endosmosis through the whole surface of the body, e.y., the parasitic Opalina. The Acineta feed themselves by sucking the body of their prey. They are without a mouth, and are incapable of taking in solid food. But they possess a number of long, narrow, contractile tentacles, which radiate from the surface of their bodies, and have the form of delicate tubes, presenting a structureless external wall and a FIS. 138. Stentor semi-fluid granular axis. The Acineta applies one or more of these organs to the body of an extraneous organism, when the substance of the latter travels down the interior tentacle into the body of the Acineta (fig. 139). By far the greatest num- ber of Infusoria possess an oral aperture, usually near the anterior pole of the body, and a second aperture which acts as anus, and which can be seen in a definite part of the body as a slit during the exit of the excreta. The body parenchyma, which is bounded by the external membrane, is divided into a viscid exoplasni and a more fluid endoplasm, into gullet ; P I', pulsating vacuole ; N, nucleus. of the granular axis of the FIG. 139. Acineta femimequinum Ehrbg., which is sucking the body of a small Infusorian (Enchelys) (after Lachmann). T, sucking tentacle ; I', vacuole ; jV, nucleus. 200 PROTOZOA. which a slender oesophagus, rarely supported by firm rods (Chilodon, Nassulci), often projects (fig. 140). Through this the food stuff passes into the endoplasm, in which it gives rise to food vacuoles. The latter undergo a slow rotating movement round the body in the endoplasm, which is caused by the contractility of the sarcode. During this process the food is digested, and finally the solid, useless remainder is ejected through the anal aperture. A digestive canal, bounded by distinct walls, exists no more than do the numerous stomachs which Ehrenberg, who was deceived by the food vacuoles, ascribed to his Infusoria polygastrica. In all cases where a digestive canal has been described, we have to do with peculiar strings and trabeculae of the internal parenchyma which enclose in their inter- stices spaces filled with a clear fluid. The more viscid exoplasm is pre-eminently to be regarded as the motor and sensory layer of the body. In it we find differentiations resembling muscles (Stentor, the stalk of Vorti- cell(t). Sometimes small rod-shaped bodies are present (e.g., Bursaria leucas, Nassulci), which are comparable to the thread cells of Turbellaria and Ccelenterata. The contractile vacuoles appear as further differentiations of the external layer, structures which to the number of one or more are found in quite definite portions of the body. They are clear, mostly spherical spaces filled with a fluid ; they diminish suddenly and then vanish, but gradually reappear and increase to their original size. These pulsating vacuoles are usually connected with one or more vessel- like lacume, which swell considerably during the contraction of the vacuole. These structures have been compared to the water vascular system of Botifera and Turbellaria, and have been explained as excretory an interpretation which has in its favour the fact that the contractile vacuoles in certain cases open to the exterior through a fine pore at the surface, through which granules pass to the exterior. The nucleus and nudeolus lie in the exoplasm of the infusorian body. The nucleus, which ten years ago was compared to the nucleus of the simple cell, is a structure of variable shape but with a definite position in the body. One, or more than one, may be present. It is sometimes round or oval, sometimes elongated, being drawn out FIG. 140. Chilodon citctd- lus (after Stein), with gullet resembling a fish-basket. N, nucleus with nucleolus, excreta are passing out of the anus. KEPEODUCTJON OF CILIATA. 201 to the shape of a horse-shoe or a band, and may be broken up into a number of fragments. It contains a granular viscid substance, is bounded by a delicate membrane, and, b according to the erroneous views of Stein and Balbiani, gives rise to ova or to germi- nal spores. The nucleolus or paranucleus also varies in form, position, and number in different species. It is always much smaller than the nucleus, and is strongly refractile ; it usually lies close to the nucleus, or even sunk in a cavity of the latter. Both play an important part in the reproduction of the Infusoria. The most usual method of reproduction in the Infusoria is by fission. When the forms reproduced remain together and connected with the parent, a colony of Infusoria is formed, e.g., the stocks of Epistylis and Carchesium. Fission usually takes place by a trans- verse division (at right angles to the long axis), as in the Oxytrichiche, N [(J. 141. a, Axiiidiyca lyncaater (after Steiii) . l>, Avjiitlisca poly Sty - la, during fission (after Stein). FIG. 142. PodopJirya geniinipin-a (after R. Hertwig). w.with extended suction-tubes and pre- hensile tentacles, with two contractile vacuoles. 6, the same with ripe buds, in which processes of the branched nucleus N enter, c, free young form. Stentoridce, etc., and, obeying definite laws, follows conjugation and division on the one hand of the nuclei, and on the other of the nucleoli (fig. 141). Less frequently (Vorticella) the fission takes place through the long axis (fig. 143, a, It), and far more rarely in a diagonal direction. The asexual reproduction is often preceded by encystment, which appears to be of great importance for the 202 PROTOZOA. preservation of the Infusoria from desiccation. The animal retracts its cilia, contracts its body to a globular mass, and then secretes a transparent cyst, which hardens and protects the animal, thus en- abling it to survive in damp air. In the water, the contents of the cyst divide into a number of parts, which attain freedom by the bursting of the cyst, each one becoming a young animal. Moreover, many Infusoria (Acinetce) produce with participation of the nucleus a number of buds asexually, which separate them- selves from the walls of the parent body (fig. 142). The broods of Sphserophrya make their way into the interior of other Infusoria, s Parama?cium and Stylonychia, nourish themselves at the cost of the enlarged nu- cleus, and form em- bryos by fission. These embryos swarm out, and were for a long time taken by Stein for the embryo broods of Stylonychia (tig. 144, b). The process of con- jugation observed by Leeuwenhoek and 0. Fr. Miiller is very general, and is con- nected with changes of the nucleus and nucleolus. These changes, which gave rise to the erroneous interpretation of the two structures as ovary and testis, are in reality simply preparatory to a process of regeneration of the nucleus by parts of the paranu- cleus, a process comparable to the phenomena of the fertilization of the ovum in sexual reproduction. The conjugation of two Infusoria occurs in very different ways, and leads to a more or less complete fusion, which, after regeneration of the nucleus, is followed by an increase in the frequency of fission. Paramcecium, Stentor, Spirostoma, during conjugation, become con- FIG. 143. TorticeUa microsfoma (after Stein), a, In process of fission ; If, nucleus ; the mouth apparatus in each por- tion is formed afresh, oe, gullet, b, Fission is completed, the smaller product is set free after the formation of a posterior ring of cilia ; w, adoral zone of cilia, c, Vorti- cella in process of bud-like conjugation ; i", the bud-like individuals attached. CONJUGATION OF CILIATA. 203 nected by their ventral surfaces ; other Infusoria with a flat body like Oxytrichina, Chilodon, by their sides ; while Enchelys, Halteria, Coleps, join together the anterior extremi- ties of their bodies, giving the appear- ance of transverse fission. A lateral conjugation also takes place not un- frequently in Vorti- cella, Trichodina, etc., between individuals of unequal size, the smaller one having the appearance of a bud (bud-like conju- gation) (tig. 143, c). The alterations which the nucleus and paranudeus un- dergo during and after conjugation have been especially worked out in Paramcecium and Stylonychia (fig 144 , 145). When several nuclei are present they a FIG. 144. a, Sfylonychia mytilus, in process of conjugation. The nucleus is depicted c nriug division (Balbiani's so- called ova); the nucleoli have divided into four spheres (sup- posed spermcapsules) . 6, Stylonychia filled with parasitic Sphcerophrya (after Balbiani). Nb FIG. 145. Stylonychia mytihtg in process of conjugation, slightly magnified, (treated with acetic acid), (after Biitschli). , Stage of conjugation with two nucleoli (paranuclei) ; Nb, the four pieces into which the nucleus has divided in each individual. 4, Stage of conjuga- tion with four nucleoli, of these JV' becomes the nucleus, and ' the two nucleoli ; JV4, the four remaining pieces of the old nucleus, c, Stylonychia on the sixth day after conjuga- tion with nucleus and two nucleoli. fuse together to form a single oval body (Bulbiani), the substance of which takes a finely fibrous structure previous to further fission, 204 PROTOZOA. 7! PF like the substance of a true cell nucleus, when undergoing division. The paranucleus too increases in size and becomes striated, and divides into a number of bodies by a single or re- peated division. Some of these bodies produced by the division of the nucleus and paranucleus disappear or are cast out, and others are employed in the formation of the new nucleus and paranucleus. The processes of regene- ration are for the most part not com- pleted until the conjugating animals ' have separated. Conjugation is probably followed by a repeated division (fig. 146). The mode of life of the Infusoria, which principally inhabit fresh water, is very various. Most of them lead an independent life, and take up larger or smaller bodies, even Rotifera, as nourishment. Some, as Amphilej)tus, select fixed Infusoria, as Epistylis and Carchesium, for their prey, and swallow them down as far as the origin of the stalk they then, while fixed on the stalk, secrete a capsule, and divide up into two or more individuals, which pass out. Certain Infu- soria, as the mouthless Opalina, and many Bursa- ridse, are parasitic in the intestine and bladder of Vertebrates. To these belongs the 'Balantidium coli from the large intestine of Man (fig. 147). 1. Sub-order : Holotricha. - - Body uniformly covered with cilia, which are arranged in longitu- dinal rows, and are shorter than the body. Longer cilia are sometimes found in the region of the FIG. 146. ParamtBcium Bursaria about one hour after conjugation (after Biitschli) . , nucleolus ; N, nucleus ; P V, contractile vacuole. Two of the nucleoli have become clear spheres. FiG.U7.-~aiantid;/im coKwith twopulsa- mouth tmg vacuoles (after nuc^us U Ues r ^a Besides the Parasitic Opalinfe (Ojjalina! ranarum), with- starcT-V-anule tha^ out moutl1 or anus, the following families belong to this has been eaten, a group : ball of excrement is Fam. Trachelidae. Body of changeable shape prolonged passing out of the j n t o an anterior neck-like process. Mouth ventral, without longer cilia. TracJtclius orum Ehrbg., Ampliilcptus fasci- cola Ehrbg. Fam. Colpodidse. Form of body definite. Mouth ventral, in a depression, CILTATA. 205 always furnished with long cilia or undulating membranes. Paratixrriinn Aiirella Fr. Miiller. P. linrxaria Focke, Colpoda cwullus Ehrbg., Glaucoma scintilla us Ehrbg. 2. Sub-order : Heterotricha. Body uniformly covered with fine cilia, which are arranged in longitudinal rows, with a distinct adoral zone of cilia. Fam. Bursaridae. The adoral zone of ciliais on the edge usually of the left half of the body. B/trmria tnincntcUn 0. Fr. Mull.. Bnlantldhint coll Malmst., parasitic in the colon of man ; Spirostonvum iniibii/in/m Ehrbg. Fam. Stentoridae. At the anterior end of the body is a peristomial space with a funnel-shaped depression, without any distinct gullet. Stcntor polymorplms, 0. Fr. Mull., St. cceruhu* Ehrbg. 3. Sub-order : Hypotricha. Body with sharply defined dorsal and ventral surface. The convex dorsal surface is usually naked, the ventral covered with cilia and beset with styles and processes, mouth on the ventral surface. Fam. Oxytrichidae. Body elongated to an oval. On the left half of the ventral surface is a peristomial region, with an adoral zone of cilia. The ventral surface is beset at either edge by a marginal row of cilia, and also with bristles and hooks. Stylongcliia jmstulata Ehrbg., with eight anterior styles, five ventral, and five anal cilia. O.rytrirlui cj'Ma, O. Fr. Miiller. Fam. Chilodontidae. Body usually armoured, with gullet in the form of a fish-basket. Chilodon cucullus Ehrbg. 4. Sub-order : Peritricha. Infusoria with cylindrical or bell-shaped partially ciliated body. The cilia are placed on an adoral ciliated disc, and frequently on a ring-like zone. Fam. Vorticellidae. Peritricha with adoral spiral of cilia, without a shell, attached by a style, usually forms colonies. Vorticella micruxtoma Ehrbg.. Epistylls plicatilix Ehrbg., Zoothamnium arbuscula Ehrbg., Carchesiv/m polyp-ilium Ehrbg. Fam. Trichodinidae. Peritricha with adoral spiral of cilia and circle of cilia as well as an apparatus for attachment at the posterior end. Trichodinapediculus Ehrbg. Fam. Halteriidse. Near the adoral spiral of cilia is an equatorial zone of longer cilia. Haltrria, volt ox Clap. Lachm. 5. Sub-order : Suctoria. Body usually without cilia, with knobbed tentacle-like processes which serve as sucking tubes. Fam. Acinetidae. Acineta mystacina Ehrbg., Podoplwrya cijclopnm Clap. Lachm., Splieeroplwya Clap. Lachm. As an appendix to the Protozoa we will now proceed to consider the Srlt<:<>- , which approach more nearly to the Fungi, and the Gn-yariniiJtr. 206 PBOTOZOA. 1. The SchizomycetidcB* (Bacteria) are small globular or rod-shaped bodies which are found in decaying matter, and are especially numerous on the surface of putrefying fluids, where they give rise to a slimy film (fig. 148). They are most nearly allied to the fungus of yeast, with which they also agree in their manner of nourishment, in that they make use of ammonia and organic com- pounds containing carbon. Like the yeast fungus they excite and maintain the fermentation or, as may happen, putrefaction of organic matter by with- drawing its oxygen or by attracting oxygen from the air (reduction or oxyda- tioii ferments). But they are clearly separated from the fungi by their deve- lopment, for they increase by dicidiitr/ into tn-o Italrcs, while the yeast fungus (SuccJiaromyccs, Hurmisciuiii) forms buds which separate off as spores. The transverse division takes place, after the cell has become elongated, by a con- striction of the protoplasm and by the secretion of a cross partition wall. The daughter-cells either divide at once, or remain united and produce chains of Bacteria (filiform Bacteria) by afresh fission. Sometimes the successive genera- tions of cells remain connected by a gelatinous substance, and so produce irre- gular shaped gelatinous masses (zoof/loea). Sometimes they become free and are dispersed in swarms. They may also settle on the bottom in the form of a a / r, ^ S\\ . .- 'j WW,(-t > \* i s ft'^'/J v ft e 't! . . . i . v*y ff' FIG. 148. Scliizomycetes (after F. Cohn). a, Micrococcus. I, Bacterium termo, bacteria of putrefying fluids, both in the motile and zoogloea form. granular precipitate, as soon as the nourishment in the fluid is exhausted. The greater number have a motile and a motionless stage ; in the first they rotate themselves about their long axis, but are also able to bend and extend, but never to serpentine. Their activity seems to be connected with the presence of oxygen. Owing to the absence of sexual reproduction, the division of Bacteria into genera and species is beset with such difficulty that we must content ourselves with establishing, in an artificial fashion, form species and physiological species and varieties without always being able to demonstrate their independence. F. Cohn distinguishes four groups : (1) Globular Bacteria, Mirrococciix (Jforta* and Mi/codcr-ma) ; (2) Rod Bacteria (Bacterium} ; (3) Filiform Bacteria (Bacillus and Vibrio') ; (4) Spiral Bacteria (Spirillum and Spirocheeta). The Globular Bacteria are the smallest forms, and only exhibit molecular movements. They cause various forms of decomposition, but not putrefaction. * F. Cohn, "Beitrage zur Biologic der Pflanzen." Heft 2 and 3, 1872 and 1875. ' ; Untersuchungen iiber Bakterien," 1, 2, and 3 (Bacterium termo). Com- pare further the works of Eberth and Klebs. BACIERIA GREGARINIDvE. 207 They can only be divided, according to their various methods of development, into chromogenous (pigment), zymogenous (fermentation), and pathogenous (contagion) divisions. The first appear in coloured gelatinous masses and vegetate in the Zoogloeaform, e:;/.. M. prodigiosus Ehbrg. in potatoes, etc., To the Zymogenous belong M. uretc. urine ferment ; to the Pathogenous M. nii-rlnif. the Pox Bacteria, M. xepticux of py;emia, M. dlplithcricHS of diphtheritis. The Rod Bacteria form small chains or threads, and exhibit spontaneous motions, especially in the presence of abundant nourishment and oxygen. Here belongs Bacterium termo Ehrbg. distributed in all animal and vegetable infusions and the necessary ferment in putrefaction, just as yeast is in alcohol fermentation ; also B. Lineula Ehrbg. of considerable size, which exists in spring water and in standing water, in which there are no products of putrefaction, and, as well as the former, has a zoogloea jelly. Another Bacterium form acts as ferment of lactic acid. ^ according to Hoffman. Of the Filiform Bacteria the motile Bacillus (vibrio) Kiibtilix Ehrbg. occasions butyric acid fermentation, but is also found in infusions together with B. term a. Very nearly allied and hardly to be distinguished is the motionless Bacillux (inthracis of inflammation of the spleen. Vibrio ruaula and si'rpeit* are charac- terised by constant undula- tions of the chain. Finally these lead to the spiral forms of which Spirocluctxj>ra , etc.) sickle-shaped bodies arise in the spores, which, without passing through an amoeboid stage. give rise to young Gregarines. Mono- ci/xtix iiffilift from the testis of the earth-worm. Gn-garina L. Duf. (Clepsidrina Hammersch.) Body with flat partition wall and wart-like head at anterior end. In the young stage the anterior end of Gr. blat- tariim v. Sieb. is fixed in the cells of the intestinal epithelium of Blatta. Gr. polymorpha Hammersch. in the meal- worm. [The Gregarines are found mainly in Invertebrata. They may be divided into two main groups, the Pol ijnjst Idea and the Monoeystidea. In the former, whicli are found for the most part in Arthropods, there is a partition dividing the body into two parts ; in the latter, which are found chiefly in Vermes, there is no such partition.] The structures long known as Pxorospcrm* from the liver of the rabbit, the slime of the intestine, the gills of fishes, and the muscles of many Mammalia, etc., present a great resemblance to the PseudonavicellEe ; but we are not yet fully enlightened as to their nature. The case is the same with the structures known as Rainey'sor Mischer's corpuscles from the muscles of, e.g., the pig ; and PIG. 150. Rainey's corpuscles from the flesh of a pig. a, An animal inside a mus- cle fibre. I, Posterior end of the same, strongly magnified ; C, cnticular layer ; B, spores. CCELENTERATA. the parasitic animals from various wood-lice and Crustacea, which were assigned by Cienkowski to the fungi, under the name of Amalldium jjarnxitlciim. remind us by their reproduction no less of the Gregarinae and abed their cysts. The fucriffiK which we meet with in the cells of the epithelium of the intes- tine as well as in the bile- ducts of Mammalia should also be regarded as Grega- i-hiff (fig. 151). They trans- form themselves into egg- shaped zoosperms by the formation of a capsule and the production of several spores from their granular contents. In Ooccidium oviforme from the liver of man and of the rabbit there are only four spores formed, which become sickle- shaped rods. FIG. 151. Coceidum oni forme from the liver of the rabbit, magnified 550 diarn. (after R. Leuckart). c, il, Stages of spore formation which have only been observed outside the cells. CHAPTER VII. GCELENTERATA (ZOOPHYTES).* Radially symmetrical animals ivith a body composed of cells. They have a body-cavity which serves alike for circulation and digestion (yastrovascidar space). Amongst the C tclenterata we meet for the first time with organs and tissues composed of cells. In addition to the external and internal epithelium, cuticular, calcareous, and silicious structures, as well as muscles, nerves, and sense-organs are very generally present. On the other hand, the internal' surface of the body is not differ- entiated into organs of circulation and digestion distinct from each other. The vegetative processes are performed by the internal sur- face of the gastric cavity, the gastrovascular space, of which the central part functions as stomach and intestine, the peripheral as vascular system. Tl. Leuckart was the first to recognise the importance of these characters, and made use of them to separate the Polyps and the Meduscc from the Echinoderms, thus resolving Cuvier's type of Radiata into the types of Ccelenterata and Echinodermata. It is only in more recent years that Naturalists have been con- vinced of the close relationship between the Porifera and the Polyps ' R. Leuckart. " Ueber die Morphologic und Verwandschaft.sverhaltnisse niederer Thiere,'' Braunschweig. 1S4S. 14 210 CCELENTERATA. and Medusa:, and have included the former in the group of the Ccelenterata. The Porifera were for a long time taken for plants, and more recently for Protozoon-stocks. While, however, the Polyps and Medusae are distinguished as Cnidaria and are characterised by the possession of nematocysts and by the higher differentiation of their tissues, the Porifera or Spongiaria present more simple forms of tissue in the spongy structure of their body, and are without nemato- cysts. The entire structure of the body may, generally speaking, be described as radial, although the radial symmetry does not appear in most sponges, and among the Cnidaria transitions towards lateral symmetry are ap- parent. Similar organs are usually repeated round the body axis four or six times or in multiples of these numbers. Four distinct types of body form are met with in the group Ccelente- rata, viz., that of the Sponge ; of the Polyp ; of the Me- dusa ; and of the FIG. 152. Young Si/con (after Fr. B. Schulze). O, Osculum or exhalent pore ; P, pore in the wall. The Sponge type. The sim- plest form of Sponge is represented by a fixed cylindrical tube, with an exhalent opening, the Oscidmn, at the free end (fig. 152). The contractile wall is supported by skeletal spicules, and is pierced by numerous inhalent pores, through which water and small food particles pass into the ciliated internal s^pace. By the fusion of separate indi- viduals, and by reproduction by gemination, the latter being the more frequent mode, widely different Sponge stocks with compli- cated canal systems are formed. The polyzooid nature of these is made apparent by the presence of many oscula. The Polyp type. The Polyp has the form of a cylindrical or club-shaped tube, of which the posterior end is fixed and the opposed MEDUSA CTENOPHO R. 211 FIG. 153. Sagartia nicea (after Gosse). free pole pierced by an oral opening situated on a flat or conical prominence, the oral cone. The mouth is surrounded by one or more circles of tentacles, and leads into a simple cylindrical body cavity (Hydroidpolyps), or through an cesophageal tube into a compli- cated gastrovascular cavity (Anthozoa, fig. 153). The disappearance of the tentacles gives rise to the so-called polypoid form, which consists of a simple hollow tube fur- nished with a mouth. The Medusa type. The free-swim- ming Medusa consists of a flattened disc or arched bell of gelatinous or cartilaginous consistence, from the under surface of which hangs a central stalk, the manubrium, bearing at its free end the mouth. This inanubrium is frequently prolonged in the neighbourhood of the mouth into numerous lobes and tentacles, while from the edge of the disc arise a varying number of thread-like tentacles. The central cavity of the body, into which the hollow manubrium leads, is called the gastric cavity, and from it peripheral pouches or radial canals, the so-called vessels, run to the edge of the disc, where, as a rule, they are con- nected by a circular vessel. The movements of the Me- dusa are effected by the alter- nate contraction and dilatation of the muscular under surface of the bell, i.e. of the subum- brella. Rudimentary Medusae, in which the manubrium and marginal tentacles are absent, are found. They are called Medusoids, and do not acquire individual independence, but remain attached to the body of the Medusa or Polyp from which they are budded. The Medusae and Polyps, in spite of the important differences between them, are but modifications of the same plan of structure. A Medusa may be compared to a free, flattened Polyp, possessing a large gastric cavity and a muscular and enlarged oral disc. The Ctenophor type has fundamentally the form of a sphere, FIG. 154. Medusa of the Podocoryne came* with four tentacles at the edge of the disc, ovaries and manubrium, immediately after separa- tion from the stock. 212 CCELENTEEATA. beset with eight meridional rows of vibratile plates, which, working like oars, serve for locomotion (fig. 155). The body parenchyma in the Sponges consists principally of arno3ba-like cells, which frequently bear flagella, but which never produce stinging threads. In the Cnidaria (Polyps and Medusse), in certain cells the peculiar struc- tures known as thread cells (fig. 156 )are developed . They consist of small capsules filled with fluid, and containing a sharp-pointed, spi- rally coiled thread; they are developed in cells which may be called cnido- blasts. Under cer- tain mechanical conditions, e . g . under influence of the pressure pro- duced by contact with a foreign body, these cap- sules burst, and the thread is sud- denly protruded, and either fastens on to the cause of disturbance or FIG. 155. Ct/dippr (Hnrmij plumosa (after Chnn). mouth. o into it a part of the fluid contents pierces it, carrying FIG. ise. Nematocysts and cuidoblasts of Siphonophora. ( , and 6j with the cn i c i oc ii of the cel1 - c to e > Nemato- cysts with evaginated thread. or the capsule. In many parts of the body, and especially on the tentacles, which serve for the capture of prey, these small microscopic weapons are collected in masses, and are often united in a peculiar arrangement to form batteries of thread cells. DEVELOPMENT. 213 The tissues (which are composed of cells) are generally arranged in two or three layers, of which the external layer is known as ectoderm and forms the outer skin, while the internal layer, the endoderm, line-; the gastric cavity. Between the two there is developed a delicate homogeneous sup- porting membrane or a stronger layer of connective tissue, in which the skeletal elements are developed. This intermediate layer is known as the mesoderm. The skeletal formations present great variations in structure and arrangement. Muscles are formed in the deeper part of the ectoderm as cell- processes (the so-called neuromuscular fibres), but often penetrate within the rnesoblast as independent cell structures. Sense epi- thelium, nerve fibrilla?, and ganglion cells also appear as differentia- tions of the ectoderm. The endoderm cells, on the other hand, often bear cilia, and are principally concerned in the processes of digestion and secretion. Where the tissues are upon the whole of homogeneous structure, we find a preponderance of asexual reproduction by fission and gemma- tion. If the individual forms so produced remain united, they give rise to the colonies which are so widely distributed amongst the Polyps and Sponges, and which, by the continual multiplication of their individuals, may in course of time attain a very considerable size. But we also meet everywhere with the sexual reproduction, in that ova or spermatozoa are produced in the tissues, usually in the region of the gastrovascular cavity, in a definite portion of the body. As a rule, the ova come in contact with the spermatozoa away from the place where they are produced ; either within the body cavity or outside the parent body, in the sea-water. In a few cases only do both the sexual elements originate in the body of the same indivi- dual, as, for example, in many of the Spongiaria, some Anthozoa, and in the hermaphrodite Ctenophora. As a rule, in the colonies of Aiitfiozoa the monoecious arrangement of sexes obtains, the indivi- duals of the same stock being partly male, partly female. Some are dioecious, e.g. Veretittum, Dipliyes, Apolemia. The development of the Coelenterata for the most part consists of a metamorphosis. The just hatched young differ from the sexual animal in the form and structure of the body, and pass through larval stages. The greater number of them leave the egg as ciliated larvae, which resemble somewhat an Infusorian in external appearance. They acquire a mouth, body cavity, and organs for obtaining food, either dining their existence as free larvae, or after 214 CCELENTEEATA. attachment to solid surrounding objects in the sea. If the young forms, which differ from the sexual animal, gain the power of re- producing by budding, the development leads to various forms of alternation of generation. SUB-GROUPS. I. SPONGIARIA*=:PORIFERA. The body has a spongy consistence and is composed of masses of cells capable, of amueboid movements and supported by a solid, calcareous, silicious, or horny skeleton. There are external pores, an internal canal system, and one or many exhalent openings (oscula). The sponges are at present universally regarded as Coelenterata, and in this group they are distinguished from the Gnidaria (Polyps and Medusae). They are composed of a contractile tissue, which is usually supported by a f ramework composed of spicules and fibres ; the whole being arranged in such a manner that there exists on the external wall of the body larger and smaller openings ; and in the interior a system of canals and spaces in which a continuous stream of water is maintained by the vibratile motion of cilia. Amoeba-like cells, net-like membranes of sarcode, flagellated cells, spindle cells, ova, spermatozoa, and tissues derived as excretions from cells are present as the histological elements of the Sponge body. The chief mass of the contractile parenchyma is composed of the arnceba- like cells. These are granular cells, which, FIG. 157. - Amoeba-like cell of like Anwebfe, have no external membrane, Spongilla. can protrude and retract processes, and take into their interior foreign substances (fig. 157). The framework or skeleton, which we find wanting only in the soft * Literature : Nardo G. D., " System cler Schwamme," Isis, 1833 and 1834. Grant, " Observations and Experiments on the .Struct, and Funct. of Sponges," Edin. Phil Journal. 18251827. Bowerbank, " On the Anatomy and Physio- logy of the Spongiadae," Pliilox. Trait*.. 1858 and 181)2. Lieberkiihn. " Beitrage zur Entwickelungsgeschichte der Spongillen," Matter's Arcfi/i:, 1856. Lieber- kiilm, " Zur Anatomic der Spongien." Mailer's Arcliir., 1857. 1859. 1863. 1865, 1867. 0. Schmidt, " Die Spongien des adriatischen Mceres," Leipzig, W. Eng- elmann. 1862, as 'well as Supplement. Leipzig, W. Engelmanu. 1864, 1866, 1S68. E. Haeckel, "Die Kalkschwamme," 3 Bde, .Berlin. 1872. Fr. E. Schulze, " Untersuchungeii iibcr den Bau und die Entwickelung der Spongien," Zeitxelirift.fiir wiss. Zuol., 18761880. POHIFEEA. 215 gelatinous Sponges or Myxospongia, is composed of horny fibres or silic-ious or calcareous spicule.s. The horny fibres form, without exception, anastomosing networks of varying degi-ees of thickness, and present a lamellated structure (fig. 158), which indicates that they are formed of a number of layers. They are formed by excretion as hardening portions of sarcode. The calcareous needles (fig. 159) are simple or three- and four- rayed spicules, and take their origin, as do the silicious structures, in the interior of cells. The silicious spicules present, however, an extraordinary variety of form : some of them constitute a connected frame- work of silicious fibres, and others are free silicious bodies with simple or branched central canals (fig. 160). The latter are found in the form of needles, spindles, cylinders, hooks, anchors, wheels, and crosses, and arise in nucleated cells, pro- bably as deposits round a hardening of FIG. 158. Piece of network of organic matter (central fibre). In order to understand the morphology of the Spongiaria we must begin by examining the structure of a young Sponge, which proceeds from the fixed larva. The young Sponge, after the formation of a ciliated gastric cavity and an ex- halent opening or osculum, has the form of a simple hollow tube, the walls of which are pierced by pores for the passage of small food particles suspended in the water (fig. 152). horny fibres from Etispongia equina. In this stage we can distinguish three layers - ( 1 ) an entoderm, formed of elongated flagellated cells; (2) a mesoderni, the skeleto- genous cell layer, the structure of which recalls connective tissue; and (3) an ectoderm, which forms the outer layer of the Sponge, and consists of a flat epithelium. The cylindrical cells of the endo- derin possess at their free ends surrounding the flagellum a delicate FIG. 159. Calcareous Spicules of Sycon. 216 CCELENTEBATA. hyaline marginal membrane, which, derived from a prolongation of the hyaline plasma, projects as a hollow cylinder resembling the protoplasmic collar of certain Flagettata* (Gylicomastiges). [This FIG. 160. Silex bodies from different silicious Sponges, a, Silex needle from Spongillu, inside the cell. I, Amphidisc of a gemmule of Spongilla. c, Anchor from Ancorina. d, Hook from Esperia. e, Star from Chondrilla. f, Anchor from EuplecMla aspergillnm. g, h, needle rays from the same. /, Six-rayed needle from the same, with central canal. structure is commonly known as the collar, and the cells as the collared cells.] The thick layer in which the skeletal spicules are produced consists of a hyaline matrix, in which irregularly branched or spindle-shaped amoeboid cells are embedded, and may be regarded, like the gelatinous svibstance of the Acalepha, as mesoderm, while the external, clearly defined, flat epi- thelium (also in the Asconia, Leucosolenici) is to be considered as ectoderm. The pores or inhalent openings so cha- racteristic of the Sponge body are in reality only intercellular spaces, and are able to close themselves, vanish and be replaced by new pores, which arise by the separation of one cell from another (fig. 161). * Upon this ground Clark declared the Sponges to be nearly allied to the , and regarded them as great colonies of the latter. FIG. 101. Portion of the exter- nal layer of Sjiongilla with the pores, P (after Luberkuhn) . PORIFEBA. 21' Amongst the calcareous Sponges, the simple Sponge with inhalent pores and terminal osculum (Olt/nthns-iovm) is represented by the stock-forming Leucosolenia (Grant ia), which is composed of numerous hollow cylinders. The structure of this sponge has been described by Lieberkiilm. In the Syconidce, the body cavity has a more complicated form. The central space opens into secondary peripheral spaces or radial tubes, which are lined by ciliated cells, and open externally through the inhalent pores (fig. 162). In other calcareous Sponges (Leuconidce^) the radial canals have the form of irregular parietal canals, giving off branches to the periphery and possessing dilated, ciliated chambers. This form of internal canal system is also found in most of the stock-forming, silicious Sponges (fig. 163). Sponge forms may become more complicated by the formation of stocks ; the originally simple Sponge, which has developed from a single cili- ated larva, gives rise by budding and incomplete fission to a polyzoid sponge body ; or several originally separate individu - als, each of which has origi- nated from a single larva, fuse together to form a com- pound sponge stock. Both these methods of growth are repeated in a similar manner in the formation of the stocks of Polyps (fig. 164). In the same way that the fan-like FIG. 163. Section of Cortieiimi cancteluli-un, (after nets of the Fan Coral (RMpi- Fr. B. Schulze). at, Ciliated chamber of the rf jl M lwil) are formed parietal canal. by the repeated fusion of its branches, the gastrovascular cavities of which anastomose, so also in the case of the branching sponges, as a result of the same pro- cess, reticulate, or coiled or even massive stocks are formed (fig. 165). FIG. 162. Longitudinal section through Si/con raphanut, slightly mag- nified. 0, Osculum with collar of spicules ; Rt, radial tubes which open into the central cavity. 218 CCELENTEBATA. In this case the canal system, in which the modifications before described for each individual Sponge are repeated, becomes more complex, partly through the formation of anastomoses, and partly because irregular gaps and winding passages make their appearance between the fused branches of the stock and form spaces which lead into the ciliated cavities. Reproduction takes* place mainly asexually by fission and the production of germs or gemmules, but also by the formation of ova and sperm capsules. The gemmules are in the fresh-water Spongilla masses of cells which are surrounded by a firm, shell composed of silicious structures (amphidiscs), and, like encysted Protozoa, pass through a long period of rest and inac- tivity. After the expiration of the cold and sterile season of the year, the contents pass out of the opening of the capsule and gene- rally surround the latter, and with increasing growth become differentiated into amoeboid cells and all the essential parts of a new small sponge body. Multiplication by means of gemmules is also common among the marine Sponges. The gemmules take their origin under certain conditions as small globules surrounded by a membrane. The contents are essentially formed of sponge cells and spicules, and, after a longer or shorter period of inactivity, reach the exterior by the rupture of the membrane. Sexual reproduction was first demonstrated with certainty by Lieberkiihn for Spongilla, but more recently has been shown to exist in almost every group of Sponges. In most fa Qya ^ spevma tozoa seem to reach maturity at different times in the same Sponge. The spermatozoa are needle-shaped, and lie in small spaces lined with cells. The ova, like the mother cells of the spermatozoa, are modified cells of the parenchyma, and are derived from cells of the same tissue layer (mesoderm) in which the needles and skeletal structures take their origin. The ova are naked amoeboid cells, and pass into the canal system, while in the viviparous Sycons they PIG. i^.- (after O. Schmidt). PORIFERA. remain in the niesoderni, and there undergo their development. It is only later that the ciliated embryos or larvae fall into the canal system, pass out, and attach themselves, to de- velop into a young sponge. The embryonic de- velopment among the calcareous sponges is most accurately FIG. 165. Euapoiigia qfficinalis titlriuficu, with a numuer of i r j.i oscula, O (after Fr. E. Schulze). known tor the Syconidce from the investigations of Fr. Schulze and Barrois. a FIG. 100. Development of Si/con nijihaiiiit (after Fr. E. Schulze). , Ripe ovum. 4, Stage with four segmentation cells, c, Stage of segmentation with sixteen cells, d, Blastosphere with large dark granular cells at the open pole, e, Free-swimming larva, one-half of the body (entodermal) being formed of long ciliated cells, the other (ectodermal) of large granular cells. 220 C(ELENTEEATA. After the completion of the tolerably regular segmentation (fig. 1G6, a c), /S'i/con (Syca/ndroi) ra/phanus passes through a. blastosphere stage, during which the greater half of the ovum consists of clear cylindrical cells, and the smaller half at the still open pole of large dark granular cells (fig. 166, d). The cylindrical cells of the larger half develop cilia, and the embyro passes out of the body cavity and becomes a free-swimming larva, which attaches itself and alters its shape in such a manner that the dark cells grow over the ciliated portion of the globe, which is meanwhile invaginating. The ecto- derm and mesoderm are derived from the dark granular cells, a.nd the ciliated cells give rise to the entoderm of the gastric cavity. Later on the body of the sponge be- comes cylindrical, the osculum makes its appearance, and calcareous needles appear in the wall, which becomes pierced by pores (fig. 167). With the excep- tion of Spongilla, the sponges are FIG. 167. Young Si/con (after Fr. E. Schulze). O, Osculum or exhalent aperture ; P, pores of the wall. marine, and are met with under very different con- ditions, and covering a wide area of distribution. The horny .sponges live in shallow seas, as also the Myxos^>ongice and ChalinecK, or siliciceratous Sponges ; while the Hexactinellidce inhabit very considerable depths. Petrified remains of sponges are also found preserved in various formations, for instance in the chalk ; and these remains differ much from the greater number of those living. On the other hand, the glassy sponges of the deep sea agree so fully with the ancient forms that they seem to be the direct descendants of the latter. Finally, many of the principal groups extend back into the palaeozoic age, in which Lithistidce and Hexactinellidtu especially are met with in the most ancient Silurian PORIFERA. 221 strata. Hence palaeontology affords us no facts for determining- the phylogenetic development of the Porifera, CLASS I. SPONGIA. (With the characteristics of the Group). Order 1 . MYXOSPONGIA (gelatinous sponges). Soft, fleshy sponges, without any skeleton, with a hyaline gelatinous mesoderru, often containing fibrous cords. The ectoderm cells are fairly elongated, and bear flagella. Fam. Halisarcidae. Jfiilixni-ca Duj. H. lolndurixO. S., colour dark violet; encrusts stones; Sebenico. H. Diijurdinil Johnst., forms a white encrustmeut on the Laminaria of the North Sea. Order 1. CERAOSPONGIA (horny sponges). For the most part branched or massive sponge stocks, with a framework of horny fibres, in which grains of silex and sand are present as foreign bodies. Fam. Spongiadae. Enxponi/ia O. S., with very elastic fibrous framework, of equal strength throughout, mostly capable of being used for bath and washing- sponges. E. (idriitttfa 0. S., ryuhi/i O. S., zimocca 0. S., in the Greek Archi- pelago, nuilixxinM O. S.. Levantine sponge, cup-shaped. $p)ujclia i-h-ijtinx Nardo. Order 3. HALICHONDRI.E (siliciceratous sponges). Sponges of very various shapes, with usually uniaxal silicious needles, simple silicious spicula, which are connected by delicate or firmer plasmatic structures, disposed in networks or enclosed in sponge fibres. Of the numerous families the following may be mentioned : Fam. Chrondrosidae. {(rHmmmere'), Coriaceous sponges. Chrondrosia rcni- for-mix Nardo. Fam. Suberitidae. Sponges of massive form, with knobbed silex spicules, which, as a rule, are arranged in network. Suberlti's Mardo. Sense cells in the ectoderm ; Gz, ganglion cells ; Nf, nerve . , . , -.-, . -. , , fibres ; StJ, supporting lamella ; E, entoderm cells. pro- cesses of the sense cells. Amongst many Medusae (Craspedota and Charybdea) we find a single or double nerve ring near the edge of the disc, while in the Polyps (Actinia], the nerve fibres have a more irregular distribution (fig. 169). through CLASS 1. ANTHOZOA* = ACTINOZOA (Coral polyps). Polyps ivith cesophageal tube and mesenteric folds, with internal generative organs (no medusoid sexual generation), usually with solid mesodermal calcareous skeleton. The polyps of the Actinozoa are distinguished from the polyps * Ehrenberg, " Beitrage zur pbysiologischen Kenntniss der Korallenthiere im Allgemcinen und besonders des rothcn Meeres." Ehrenberg. " Uber die Nati^r und Bildung der Korallenbanke," AM. :>'> are met with. In rare cases all the individuals are hermaphrodite, e.y., Ceria'iitJius. The embryos produced from the fertilised ovum, which undergo a complete segmentation, are frequently born alive as cilated larvae, and possess an internal gastric cavity, and an oral aperture .situated at the pole, which is directed backwards during movement. They then rix themselves by the pole opposed to the oral aperture and protrude in the region of the mouth first two, then four, eight, twelve, etc., tentacles; in the Octactinia eight tentacles at once. In the Poll/actinia, the tentacles and mesenteric pouches of which are arranged in multiples of six, it was till recently erroneously believed with M. Edwards that six primary mesenteries were first developed, then six secondary between them : then twelve were formed, then twenty-four, etc., so that mesenteries of equal size were of equal age and belonged to a cycle formed at one time. Lacaze Duthiers however produced proofs that the increase of mesenteries and of tentacles follows an entirely different law of growth, and that these structures in the first phases of development show a bilateral symmetry; and it is only later that the six radial symmetry appears by the equalization of the alternating elements of unequal age. A remnant of the primitive bilateral symmetry is moreover often preserved in the elongated mouth slit, which falls in the plane of the two primary tentacles. Amongst the Polyactinw. the very young larva? of the Actinia (A. Hiese'iiiltryant/temuin, Sagartia, B anodes) have been most accu- rately investigated. They are small ciliated planula?, one pole of which is somewhat drawn out and bears a tuft of longer cilia. The opposite end of the body is flattened and pierced with a mouth. This leads by a short cesophageal tube, which arises by invagination. into the narrow gastric cavity. The first differentiation consists in the appearance of two folds placed opposite each other, which divide the gastric cavity into two unequal chambers. The mouth is drawn out in the form of a longitudinal slit symmetrical with and at right angles to these primary mesenteric folds : so that by means of them the position of the median plane can be determined. Two new folds soon arise in the larger chamber, which we will call the anterior ; these lie opposite to one another and symmetrically with the median plane ; so that four chambers are now present, an anterior, a posterior, and two smaller lateral ones. A third pair of folds are then developed in the posterior space, and a fourth pair follow- quickly in the lateral chambers : the fourth pair are slightly smaller 15 226 CCELENTERATA. than the preceding ones. After an interval four new folds appear, one on each side of the two primary mesenteries (tig. 170). The twelve gastrovascular chambers thus formed gradually become equal in size, and can be separated into two unpaired chambers situated in the median plane, and into five pairs placed symmetrically on either side of it. FIG. 170. From tlie history of the development of Actinia mesembryamthermim (after Lacaze Duthiers). a, Larva with eight mesenteries and two coiled bands; O, mouth. I, Slightly more advanced larva with the commencement of eight* tentacles, c, d, Young Actinia with twenty -four tentacles, two longitudinal sections at right angles to one anothei. e, Mouth and tentacles seen from the oral surface. The tentacles begin to develop before the appearance of the fifth and sixth pairs of mesenteries. They appear at the oral end of the gastrovascular chambers, and the tentacle of the anterior unpaired ACTINOZOA. 227 FIG. m.Blaatotrockus n/itrix (after C. Sem- per). LK, Lateral bud. chamber* appears first, surpassing in size those which follow it. The opposite (posterior) unpaired tentacle and the other paired tentacles then make their first appearance as small wart-like prominences. When the twelve tentacles have been formed, they become alter- nately equalised, so that six larger tentacles, amongst which are reckoned the unpaired ten- tacles of the long axis, alternate with the same number of smaller ones, and we have two circles of six tentacles of the first and the same number of the second order. The asexual reproduction by gemmation and fission is of great significance. Buds can be formed in various positions, even at the oral end, in which case a strobila-like form appears. In BlastotrocJius the buds appear at right angles to the axis of the parent animal (fig. 171). , If the individuals so produced remain connected with one another, a polyp-stock is formed, which may attain very various forms and great size. As a rule the individuals are imbedded in a common body mass, the ccenenchym, and their gastric cavities communicate more or less directly, so that the juices acquired in the in- dividual polyps penetrate into the collective stock. This stock affords us an excellent example of an animal community built up out of similar members. The formation of the generative products alone is distributed, as a rule, to different individuals, which, however, unite in dis- charging all animal and vege- tative functions together (fig. 172). The skeletal formations of the FIG. 172. Branch of a Polyparium of CoraUium polyps are specially worthy of rulr>u " (after Lacaze Du ^ers). P, Polyp, remark (Polyparia). In almost every case, with the exception of Ac- tinia, there is a deposit of solid calcareous. matter in the mesoderm, and Like the first tentacle of the young Scyphtstoma polyp among the Hytl ro- Medusee. 228 CCELENTEEATA. according to the density of this deposit, there is produced a leathery, chalky, or even stony framework. If isolated needles or toothed rods (fig. 173) of calcareous substance are distributed beneath the epidermis and the ccenenchyma, the polyp-stock has a fleshy, leathery nature (Alcyonaria) ; but if, on the contrary, the calcareous structures are fused together or are cemented together in a larger mass, a solid, more or less firm, often stony cal- careous skeleton is developed (Madareporaria). In the individual animals the formation of this sub-epidermic skeleton begins on the foot surface, and advances thence in such a manner that near the calcareous foot-plate there is formed in the under part of the polyp body a more or less cup-shaped theca, from which numerous perpendicular plates, the septa, radiate in- wards. In the cup-shaped cal- careous framework of the individual poly}), the structure of the gastrovascular cavity is repeated, with the exception that the calcareous septa cor- respond to the interspaces of the mesenteries (fig 174). The number of the septa in- creases as does that of the mesenteries and tentacles with the age of the polyp according to the same laws. At the same time a great number of systematically important modifications of the skeleton are effected by further differentiation. A column-like, calcareous mass sometimes arises in the axis of the cup (columella^, and in its neighbourhood a circle of calcareous rods (pali), which are separate from the septa (fig. 175). There may further be formed between the lateral surfaces of the septa processes of calcareous substance as iiiterseptal rods or horizontal shelves (dissejrimenta) : also on the outer side of the wall of the theca ribs (costc^) projecting beyond its external surface, and similar dissepi- ments may be produced between these. FIG. 173. Calcareous bodies (Sderodfrmifrx) of Alcyonaria (after Kolliker). a, of Plexaiiretlu. l>, of Gorgonia. c, of Alcyonivm. ACIINOZOA. 229 The important diversities of form in the polyp stocks are not only occasioned by the differences of structure of the skeleton of the FIG. 175. Vertical section through the cup of Cyathi- na Cyathus (after Milne Edwards). 8, Septa ; P, pali ; C, columella. polyp, but are also the resultant of varying methods of growth by gemmation and imper- FIG. 17-1. -Vertical section through a polyp <'r jwrnianentltj attached to hi/droid forms. This class includes the small polyps and polyp stocks, and the Medusce which form the sexual generation. The Polypomedusce have always a simpler structure than the Anthozoa to which they are also usually infe- rior in size. They lack oesophagus, septa, and gastrovas- cular pouches. Only the polyps of the a- sexual generation of the Scyphomedusa? [Acraspeda], known as Scyphistoma, pos- sess a remnant of the gastric folds as four gastric ridges from which filaments are developed. The polyp stocks develop in rare cases (Mille- poridce) a compact calcareous framework comparable to the polyparium. When skeletal formations are present they con- sist as a rule of more or less horny secre- FIG. Iso it. Branch of an Olielia-stnek lO, fieluflnnxa). O, Mouth of a nutritive polyp with extended tentacles. J/, Medusa buds on the body of a proliferous polyp (Vjla.-to- style) ; Th, bell-shaped cup (theca) of a nutritive polyp. tions of the ectoderm, which as delicate tubes surround the stem and its ramifications, and sometimes form small cup-like structures surrounding the polyp, and known as * Eseholtz, "System cler Acalephen,'' Berlin, 1829. Th. Huxley, "Memoir on the Anatomy and Affinities of the Medusae," Phil. Trans., London, 1849. 234 C(ELENTEEATA. hydrothecae (fig. 180 ). A more or less stiff mesoderm lamella is also developed in the interior of the body wall, between the ectoderm and the endoderni. This serves to support the soft parts of the animal, and, in the Medusce, is in. part represented by the gelatinous connective tissue of the disc. The Medusa (fig. 180 b) is without doubt morphologically higher than the Polyp, since it represents the mature sexual individual, while the Polyp performs the nutritive and vegetative functions. The Medusa, in correspondence with its power of free locomotion, possesses an ectodernml nervous system and sense organs. The nervous system consists of nerve fibres and ganglion cells, and is usually specially concentrated round the edge of the disc, where it forms a double ring of fibres running parallel to the circular vessel. The sense organs are the so-called marginal bodies. The generative pro- ducts of the Medusae either have their origin in the ectoderm, in which case they may be developed on the under surface of the disc (sub- umbrella) in the ecto- derm immediately un- derlying the r a dial canals (Eucopidce), or in the ectoderm of the manubrium (Oceanidce) ; or they may arise from the endoderni of the under surface of the umbrella (Scyphomedusw) . Both Polyps and Medusa? frequently remain at a lower grade of morphological differentiation, the former becoming polypoid appen- dages, the latter medusoid buds enclosing the generative products. In either case they are situated on the stem or on some part of the Polyp. The individuality of such appendages appears limited ; the medusoid or polypoid animal sinks, physiologically speaking, to the value of a portion of the body or of an organ, while the entire stock L. Agassiz, " Contribution to the [Natural History of the United States, Aca- lephte," vol. iii., 1860, vol. iv., 1862. E. Haeck'el, "System der Medusen," Tom. I. and II.. Jena, 1880 and 1881. FIG. 180 b. Free Medusa of Obelia gelatinosa, as yet without generative organs ; g, auditory vesicles. HYDEOZOA. 235 approaches more nearly to a single organism. The more completely polymorphism and division of labour are impressed upon the polypoid and medusoid appendages, so much higher becomes the unity of the whole which is morphologically a colony of animals. In these cases it is often difficult to distinguish between budding and simple growth. For a long time it was considered as a remarkable circumstance, hardly admitting of a satisfactory explanation, that organisms which differed so widely as Polyps and Medusae they had, indeed, been systematically separated as different classes should only form dif ferent stages in the life-history of a single cycle of development and thus be united in the closest genetic connection. The theory of " Alternation of Generations " contained only a description of the matter, and offered no explanation. The discovery of the mode of origin of the Medusa as a bud on the body of the Polyp rirst clearly demonstrated the direct relation of the two forms, for it proved that the Medusa is a flattened, disc-shaped Polyp with a shallow but wide gastric cavity, the peripheral part of which has, by the fusion of its iipper and lower walls along four, six, or eight radiating areas, become divided into the vascular pouches (gastric pouches), or, as they are called, radial canals, which correspond to the gastrovascular pouches of the Anthozoa. The differences consist, in connection with the discoidal form, mainly in the position of the gastric tube as an external appendage, the mami- brium, and in the great reduction in height of the radially extended septa (mesenteries), which are traversed by a layer of endoderm cells. the vascular or endoderm lamella. This layer is derived from the fusion mentioned above of the aboral with the oral layer of the endoderm of the peripheral part of the gastro-vascular cavity. At the same time the oral disc becomes enlarged and concave to form the cavity of the bell, the ectodermal lining of which gives rise to the muscles of the subumbrella. The supporting substance of the arched (after it is freed from its attachment) aboral surface of the disc becomes very much thickened and gives rise to the gelatinous substance (mesoclermic), which sometimes contains cells; while that of the oral surface keeps the character of a thin but tirm lamella, and serves as a support for the muscles on the under surface of the disc. The tentacles accordingly arise near the edge of the disc, and become the marginal tentacles of the Medusa. In addition to these, four simple or branched oral appendages appear as outgrowths from the manubrium. In addition to the sexual reproduction, asexual multiplication is 236 CCELENTERATA. widely distributed, especially amongst the polypoid forms, in which it leads to the formation of polymorphous animal stocks. The two forms of reproduction alternate for the most part in regular order, so as to produce different generations. There are, however, Meduscu (Aeginopsis, Pelayia] which proceed without alternation of genera- tions and develop directly from the ovum by continuous development with metamorphosis ; but, as a general rule, the egg of the Medusa (phanero-codonic gonophore) or the medusoid generative bud (adelo- codonic gonophore) produces a Polyp, and this Polyp either at once, by transverse fission (8cy2Jhomedusce), or later, after a longer period of growth, in which a sessile or free-swimming polyp stock is pro- duced, gives rise to a generation of free-swimming Medusa?, or of medusoid buds which never become separate from the polyp stock. The Hydromeduste feed entirely on animal substances, and for the most part are inhabitants of the warmer seas. The free-moving Medusce and Siphonophora are phosphorescent. Order 1. HYDROJIEDUS/E.* Colonial forms, the individual Polyps of which are without oesophageal tube or mesenteric folds. The. sexual generation has the form either of small free-swimming Medusw provided with a velum (Craspedote Jfedusce) or of medusoid generative buds (rudimentary Medusce) which remain attached to the hydroid colony. The Polyps and polypoid forms are the asexual individuals. They form small moss- or tree-like stocks which are frequently surrounded by chitmous or horny tubes (cuticular skeleton). These exoskeletal structures may become extended into cup-like hydrothecse surrounding the individual Polyps. The stem and ramified branches [ccenosark] contain a central canal which communicates with the gastric space of each individual Polyp and polypoid appendage and contains the common nourishing fiuid. The Polyps have no eesophageal tube, and the ciliated gastric cavity is undivided by mesenteries. As a rule, the ectoderm and entoderrn remain simple, and are only separated by a thin interposed supporting lamella which does not contain cells. The presence of elongated muscle fibres as processes of the ectodermal epithelial cells is very general (Hydra, Podocoryne). These muscles may, however, * L. Agassiz, Contributions to the Natural History of the United States of America.'' vol. ii. iv.. 1860 1862. G. J. Allman. -A Monograph of the Gymnoblastic or Tubularian Hydroids," vol. i. and ii., London, 1871 aud 1872. N. Kleinenberg, "Hydra," Leipzig. 1872. 0. and E. Hertwig. "Das Nerven- system uncl die Sinnesorgane der Medusen,' 1 Leipzig, 1878. IfTDEO/OA. 237 be separated as an independent layer of nucleated fibre cells below the epithelium. The Polyps are not invariably alike, proliferous Polyps (or Blastostyles) being frequently found as well as the nutritive ones. The proliferous Polyps develop generative buds 011 their walls. The sterile Polyps may differ from one another in the number of tentacles and in their entire form, so that different kinds of individuals may be found on a single stock. Thus we rind the polymorphism of the- Siphonophora foreshadowed amongst the Hydroidea (Podocoryne, Plumularia). The generative products are only exceptionally developed in the Polyp body itself, in which case they are produced in the ecto- derm (Hydra}. This exception is probably to be looked upon as an extreme case of degeneration of a medusoid bud. As a rule the generative products are de- veloped in special medusoid buds [gono- phores] formed from both cell -layers. In the most simple cases the budding in- dividuals of the sexual generation contain a PIG. 181. Podocoryne cariiea (after C. Grobben). P, Polyp ; -I/, Medusa bud on the proliferating polyp ; &', spiral- zooid ; Sk, skeleton Polyp (compare the free Medusa, fio-. 154). diverticulum of the gastric "cavity of the polyp-shaped parent or of the axial cavity of the hydroid stock. The generative products become accumulated around 'this diverticulum (Hydractirtia echinata, Claim squaxiata}. In a more advanced stage we rind a mantle-like envelope enclosing the bud, and con- stituting the rudiment of the umbrella, with a continuous vascular lamella or with more or less developed radial vessels (Tubularia coronata, Eudendrium ramosum, Van Ben.) Finally, at the highest stage, the buds develop into small Medusa? (Campanularia yelatinosa van Ben., tfarsia tubutosa), which become free, and sooner or later, 238 CCELENTEKATA. often only after a long period of free life, in which they become much larger and undergo a metamorphosis, reach sexual maturity. The Medusae belonging to the order Hydromedusa? are, with but few exceptions, distinguished from the Acalephce (Scyphomedusag) by their smaller size although certain forms, for example Aequorea, may attain such a size as to have a diameter of more than a foot and by their simpler organization. The number of their radial vessels is smaller (4, 6, or 8), their sense organs (marginal bodies) are not covered by folds of membrane (hence Gymnophthalmata Forbes), and they have a muscular velum (hence Craspedota Gegenbaur) (fig. 182). The generative products are always formed from the ectoderm, and originate on the walls of the radial canals or of the rnanubrium, but never, as in the Acalepha, in diverticula of theerastric cavity. The hyaline gelatinous substance of our Medusae is, as a rule, structureless, and contains no cellular elements ; there may, how- ever, be fibres running per- pendicularly through it (Liriojje). These fibres are probably derived from cell processes of the ecto- derm and entoderm, and have arisen contemporane- ously with the gelatinous disc, which is itself to be looked upon as an excretion product of the adjoining ectoderm and entoderm epithelium. The nerve-ring is placed at the edge of the disc at the point of insertion of the velum. It is covered by a sense epithelium com- posed of small cells bearing sense hairs, and has the form of a double fibrous cord containing ganglion cells. The larger upper nerve-ring runs above the velum, while the weaker nerve-ring, on the other hand, is placed below it. The lower nerve-ring is composed of larger fibres and larger ganglion cells ; bundles of fibrilla? pass off from it to supply the muscles of the velum and subumbrella, where they form a sub-epithelial plexus interspersed with ganglion cells, between FIG. 182. Plduhiduun ntriubile represented from the underside of the umbrella. T', Velum ; O, mouth ; Ov, ovary ; Ob, auditory vesicle ; Sf, tentacles on the margin of the disc ; Sw, marginal swellings. HTDEOZOA. 239 the muscular epithelium and the fibrous layer. The ganglion cells in the upper nerve-ring are smaller, and the fibrillpe given off from it pass to the tentacles. The fibrilhf of the sense nerves may be derived from both rings. The marginal bodies have long been recognised as sense organs, and are either eye spots (ocelli) or auditory vesicles ; hence the ffydromeditsce may be divided into two groups, the Ocellata or Vesiculata. In the Vesiculata the auditory vesicles are situated at the edge of the under side of the umbrella, and contain one or more concretions (otolitJt) which are formed in the interior of cells. Peculiar sense cells surround each vesicle-like cell containing a concretion. The curved hairs of these sense cells (auditory hairs) are in contact with the con- cretion vesicle. A nerve fibrilla enters the basis of the auditory cells (fig. 183). FIG. 183. Sense organ on the nerve-ring and circular vessel of Octorchia (after O. and R. Hertwig). Rb, Sense organ ; O, O', two otoliths; Hh, audi- tory cilia ; Hz, auditory cells ; Nv, upper nerve-ring ; Kr/, cir- cular vessel. (Type of the audi- tory organ of the Vesiculalu.) The audi- tory organs of the Tra- chymedusce are placed above the velum, and are in con- nection with the upper nerve ring ; they have the form of FIG. 184. Auditory vesicle of Get-y- oiii.t (Caruiai-hiii), seen from the surface (after O. and R. Hertwig-) . JV and N', The auditory nerves ; Ot, otolith ; Hz, auditory cells ; Jilt, auditory cilia (type of the auditory organ of the Trachy- medusas). small projecting tentacles furnished with otoliths and auditory hairs. The tentacle may either project freely on the surface (Trachynema\ or, as in G&ryonia, it may be placed in a vesicle (fig. 184) which lies in the gelatinous substance of the disc and close to the edge of the latter. Separate sexes are almost invariably the rule, but it is rare to find that the colonies are dkecious, i.e., that male and female medusoids are developed in different colonies (Tubularia). Gemma- tion has occasionally been observed among the Medusa (Sarsia prolifera) and division (Sfomobrachium mirabile). The larva? of Cunina, which are parasitic on the Geryonidw, may also there give rise to a cluster of buds. 240 C(KLKNTEBATA. The development of the ovum, which is, as a rule, naked (i.e., with- out a vitelline membrane), has hitherto only been completely followed out in a few cases. In every case the segmentation seems to be com- plete, and leads to the formation of a segmentation cavity and a single-layered blastoderm [a single-layered blastosphere]. The latter gives rise to a second endodermal layer of cells, which lines the segmentation cavity. The segmentation cavity thus becomes converted into the gastric cavity of the future polyp. The spherical or oval larva now either attaches itself and gives rise by budding to a small hydroid stock, or swims freely and develops directly into a small Medusa (Trachytnedusfe). The Medusa, after becoming free, usually undergoes a more or less fundamental change of form, which concerns not only the alteration caused by the enlargement of the umbrella and manubrium, but also the increase, according to definite laws, of the marginal tentacle.-, sense organs (Tima), and the radial canals (Aequorea). We must remark, however, that the sexually complete Medusa? exhibit very considerable variations in size, number of sense organs and tentacles (Phyalidiv/m var labile, Clythia volubilis]. The difficulty of systematic arrangement is augmented by the fact that closely allied Polyp stocks can produce different sexual forms. Thus, for example, Monocaulus gives rise to sessile generative buds and ( 'orymorpha to free Medusce (Steenstrupia). Medusa? of identical structure also, which one would place in the same genus, may form the sexual generations of hydroid stocks belonging to different families (isogonism). There are also cases in which we find Meduso> of closely allied genera, some developed from hydroid stocks by an alternation of generations, and others developed directly. Hence it appears just as little satisfactory to found a classification entirely upon the sexual generations as to pay attention to the asexual generation alone. (1) Sub-order: Eleutheroblastece. Simple bydroid Polyps without medusoid buds ; both generative products are developed in the body- Avall of the Polyp. Fam. Hydroidae. Ifi/dnt, the fresh-water Polyp. H. ririrfix L.. Il.fuscn L.. remarkable for great powers of reproduction. (2) Sub-order: Hydrocorallice. Coral-like hydroid stocks with cal- careous coenenchyuia and tubular hydrothec* opening to the exterior by pores. Some of these contain the larger nutritive animals, while others contain animals without a mouth and beset with tentacles. HTBROZOA HYDROMEDUSJ:. 241 The latter are arranged usually in the form of a circle round each of the nutritive animals. The polyparia are found in the fossil state. Fain. Milleporidae. Millrjiord I,. M. nlrii-nrnix L. Fain. Stylasteridae. (3) Sub-order: Tubulai'ice (Ocellata). Polyp stocks which are either naked or clothed by a chitinous periderni without cup-shaped hydrothecse surrounding the polyp head. The generative buds arise on the body of the Polyp or on the stock. The Medusce which are set free belong to the genera Oceania, Sarsia, etc., and have ocelli. Fam. Clavidae. Polyp stocks with a chitinous periderni. Polyp club-shaped. with scattered, simple, filiform tentacles. The generative buds arise on the Polyp body and for the most part remain sessile. Cordylopliiim Allm. The stock is branched ; there are stolons which grow over external objects. Oval gonophores covered by the perisarc. The animals are dioecious. In fresh water C. l/ia/xfrix Allm. Alb'n-oln Kirchp.. Elbe, Schleswig. The following are marine genera Clara 0. Fr. Mailer. Allied are the Eiidcnrfrirfte with Eudcinl r'nnii i-ii nnixtim. L. Fam. Hydractinidae. Polyp stocks with flat extended coenenchyma and firm encrusted skeletal excretions. The Polyps are club-shaped, with a circle of simple tentacles. In addition to the latter there are large tentacle-shaped Polypoids (Spiralzooids). IIi/lyp heads ; the generative buds are sessile. T. (Thamnocnidia Ag.) cm-omit,, Abilg. dioecious. Corymorplia Sars. The stalk of the solitary polyp is clothed with a gelatinous; pL-ridenn. attaches itself by root-like processes, and con- tains radial canals which lead into the wide digestive cavity of the Polyp- head. The freed Mnlnxn is bell-shaped, with one marginal tentacle, and bulbous swellings at the end of the other radial canals. C. nut mix Sars.. C. mi n a Alder. (4) Sub-order: Campamdarue (Vesiculata). The chitinous skeletal tubes widen out round the Polyp-head to form cup-like hydrothecae. The Polyp-head, the oral cone (proboscis), and tentacles can be in most cases completely retracted into these hydrothecse. The generative buds arise almost regularly on the walls of the proliferous individuals, which have neither mouth nor tentacles. The buds are sometimes sessile, and sometimes become separated off 16 242 C, hydrophyllium ; G, gono- phore ; T, dactylozooid ; Sf, tentacle ; P, polyp ; O, mouth of the latter ; Nk, battery of nematocysts. ' Besides Kolliker, C. Vogt. Huxley and others, compare C. Gegenbaur. " Beobachtungen iiber Siphonoiihoren," Z< -itxrlirift fiir w/.v.v. Znol., 1853. C. 244 CCELENTEBATA. D nophora physiologically as an organism and its appendages a? organs. In this connection we may mention that the sexual medu- soid generation is so little independent that it only exceptionally (Velellidce) reaches the morphological grade of the free-swimming Medusa. In place of the attached and ramified hydroid-stocks we find in the Siphonophora a free-swimming con- tractile unbranched stem (hydrosoma),. which is rarely provided with simple lateral branches. The upper end of the hydro- soma is frequently dilated to the form of a flask (pneumatophore), and contains an air chamber [pneumatocyst] (fig. 185). In every case there is a central space in the axis of the stem in which the nutritive fluids are kept in constant motion by the contractility of the walls and by the move- ments of the cilia. The air sac or pneu- matocyst at the apex of the hydrosoma is connected to the chamber which contains it by radial septa, and in many cases attains a considerable size (Physalicb). It func- tions as a hydrostatic apparatus, and in those forms, which have a long spiral hydrosoma (Physopkoridce}, serves to keep the body in an upright position. In some cases the gaseous contents can escape freely by one or more openings. The appendages which are attached to the spirally twisted bilaterally symmetrical stem and whose cavities communicate with that of the stem are of at least two kinds (1) The polypoid nutritive animals with their tentacles ; (2) the medusoid sexual buds. The nutritive Polyps (hydranths) are simple tubes provided with a mouth, and never FIG. 186. A portion of the stem and appendages of Halhtemma tergestlnum. St, Stem ; D, hy- drophyllmm ; T, dactylozooid; Sf, tentacle of the latter ; Wg, female, Mg, male, gonophores. Gegenbanr. -'None Beitriige zur Kenntniss tier Siphonophoren." Nova Tom. XXVII., 1859. R. Leuckart, Zoologische Untersuchungen," I.. Giessen. 185S. R. Leuckart. Zur naheren Kenntniss tier Siphonophoren von Nizza." . I rr/i/r. filr Ntifi/rt/exeli , 1854. C. Clans, " Ueber Halistcnnna tergestiuum n. s. nebst Bemerkungen iiber den feinercn Bau der Physophoriden. ' Arliritt'/t mix firm Ziii/ixi-lirii Inxtitiit. drr Univ. Wioi. rfr.. Tom. I.. 187S. E. Met- schnikoff. " Stndien iiber die Entwickeluug der Mecluscn und Siphonophoren," Zi'itsrli.fiir /m*. ZnoL, Tom. XXIV., 1874.' HTDROZOA SIPHONOPHOBA . 245 possess a circle of tentacles. They always, however, have a long tentacle arising from their base. This tentacle can be extended to a considerable length, and be retracted into a spiral coil. It rarely has a simple form, but, as a rule, it bears a number of unbranched lateral twigs, which are also very contrac- tile. These tentacles are invariably beset with a great number of nema- tocysts, which in many places are closely packed and have a regular arrangement. These aggregations of thread-cells are especially found on the lateral branches of the tentacles, and give rise to large, brightly-coloured swellings, the batteries of nematocysts. The batteries show considerable variations a I FIG. 187. Group of buds of a Physophor at the bottom of the pneumatophore. C, Central cavity ; S/c, nectocalyx bud with the ectodermal ingrowth. FIG. 188. Development of Agnl mojiix Siirxil (after Metschnikoff). a, Ciliated larva, b, Stage with developing hydrophyllium (D). c, Stage with cap-shaped hydrophyllium (D) and developing pneumatophore (/). d, Stage with three hydrophyllia, (D, D', D"), polyp (P), and tentacle. in form in the various species, genera, and families, and such varia- tions afford valuable characters for systematic classification. 246 CffiLEKTEEATA. D The second form of appendage, the gonophores, usually possess a bell-shaped mantle containing circular and radial vessels, and surround- ing the central stalk or clapper (nianubriurn), which is tilled with ova or spermatozoa. They usually arise in clusters at the base of the tentacles, more rarely from the nutritive Polyps themselves (e.g. in Velella}. The male and female generative products always arise separately in differently shaped buds, but are usually found closely approximated on the same stock (fig. 186). There are, however, also dioecious 812^0- nopliora, or if the niedusoid buds or gonophores be regarded as generative organs, 8i2)hono- pkora of distinct sexes, e.g., Apolemia uvaria and Dipliyes acuminata. The ripe sexual Medusoids frequently become separated from the stock, i.e. after the development of the generative products, and only rarely become liberated as small Medusce (Chri/somitra in the relellidffi), which produce generative products during their free life. Besides the constant nutri- tive Polyps and medusoid gonophores, there are incon- stant appendages, which are also modified Polypoids or Medusoids. These are the mouthless worm-like dactylo- zoids (fig. 186), which, like the Polyps, are provided with a tentacle, which is, however, shorter and simpler, and has no lateral branches or aggregations of nematocysts ; also the leaf -shaped hard cartilaginous liydropliyllia, which serve to protect the polyps, dactylozoids, and gonophores ; and finally the appendages known as nectoccdyces, which are placed beneath the pneumatophore. The nectocalyces have a structure similar to that of the Medusa?, though their bilateral symmetry is apparent ; FIG. 189. Small larval stock of Agalmopsis after the type of Atlwrybia. Lf, Pneumatophore ; D, hydrophyllhun ; Nlc, groups of nemato- cysts ; P, polyp. HYDEOZOA SIPHONOPIIORA. 247 they are, however, without nianubrium, mouth, tentacles, and sense organs. The deeply concave sub-umbrella surface of the nectocalyx is largely developed and has a very powerful muscular covering in rela- tion to its exclusively locomotive function. All the appendages are developed as buds formed of ectoderm, entoderm, and containing a central cavity which communi- cates with the central space of the stem. In the nectocalyces and gonophores an ecto- dermal ingrowth gives rise to the covering of the sub-umbrella and to the generative product.* respectively (fig. 1ST). The ova, of which there is often only one in each female gono- phore, are large, and have no vitelline mem- after im- undergo a regular brane, and, pregnation, complete and segmentation. A nectocalyx (Diphyes) is the first structure formed in the free-swim- ming larva, or the upper part of the body of the larva gives rise to a cap- shaped protective cover or hydrophy Ilium as well as a pneumato- phore, and the under part becomes the primary nutritive polyp (Ayalmopsis, tig. 188). Since new buds give rise to leaf -shaped hydrophyllia, a small stock with FIG. 190. Phi/sopJiora hyilruxfatica. Pn, Pneumatophore ; 5, nectocalyces arranged in double rows on the swim- ming column ; T, dactylozoid ; P, polyp (nutritive individual) with tentacles, Sf; Kb, groups of nemato- cysts on the latter ; G, clusters of generative buds. 248 Pn provisional appendages is formed which allows us to regard the develop- ment of the >S i phono- pliora as a metamorphosis (fig. 188 and 189). The crown of hydro- phyllia, which is com- pleted by the addition of fresh hydrophyllia after the appearance of a tentacle with provisional groups of neraatocysts, persists only in Athory- bia, where a swimming column with nectocalyces is never foimed. In Agalmop&is and Physophora the primary hydrophyllia of the larva fall off' as the stem be- comes larger, and are replaced by nectocalyces. (1) Sub-order: Physor phoridw. Stem short, extended in the form of a sac (fig. 190), or elongated spirally (fig. 191), with a pneumato- phore, usually nectocaly- ces, which are arranged in two or more rows on a swimming column below the piieumatophore. Hydrophyllia and dacty- lozooids are usually present, and alternate with the polyps and gonophores in regular order. The body of the larva usually develops FIG. Wl.Haliftemmatergeftmum. Pn, pneumatophore S. Nectocalyx ; f, polyp ; D, hyclrophyllium ; JV7-, groups of neraatocysts HTDEOZOA. SIPHOXOPIIORA. 249 first a polyp with pneumatophore and tentacle beneath an apical hydrophy Ilium. The female gonophore has only one egg. Fain. Athorybiadae. With a bunch of hydrophyllia in place of the swim- ming column ; resembling a persistent larval stage. Athori/lia ri>xpux Forsk., Mediterranean. Fam. Diphyidae. With two very large nectocalyces at the upper end of the stem and opposite to each other. Diplnjfx iicuminatti Lkt., dioecious ; with Eiido,r}u cKmjianiilatii. Aliyln pmtuijona Esch., with EiuJo.rid mlio'idex, Mediterranean. Spliceroneetes Huxl. = J/^r//;////r.v ('Is., ty. i/n/cili* Cls. with D\i>lo- j>li>/.-( invrmix. Mediterranean. (4) Sub-order : Discoidece. Stem compressed to a flat disc, with a system of canal-like spaces (central cavity). Above lies the pneumatocyst in the form of a disc-shaped reservoir of car- tilaginous consistence composed of concentric canals opening to the exterior. The polypoid and medusoid appendages are situate on the under side of the disc. In the centre is a large nutritive Polyp, around which are a number of smaller ones. To the base of these small Polyps are attached the gonophores. The dactylozooids are not far from the edge of the disc. The gonophores are set free as small Medusw (Chry- somitra], which do not produce the generative material till long after separation. Fam. Velellidae. Vrlrlln xj>irhaped Calycozoa and Charybdeidce differs from the types above described, and re- sembles that of the more primitive Scyphistoma stage, in that the gastric cavity presents only four peripheral vascular pouches, which are very wide, and separated by extremely thin septa. The worm-like movable tentacles of the gastric cavity, the gastric filaments, which are not found in any Hydromedusce afford an im- portant distinctive mark. They correspond to the so-called mesenteric filaments of the Anthozoa, and afford the same aid to digestion through the secretion of their glandular entodermal covering. In every case they are attached to the sub-umbrella wall of the stomach, and fall in the four radii of the generative organs (radii of the second order), which alternate with the radii of the angles of the mouth, or radii of the first order. They visually follow the inner edge of the generative organs in a simple or convoluted curved line. The existence of the nervous system of the Acalepha has only recently been demonstrated with certainty. It has been proved that the centres of the nervous system are contained in the ectoderm of, the stalk and base of the marginal bodies, and consist of a considerable layer of nerve fibrillse deep in the ciliated ectodermal epithelium, the nerve cells of which are elongated in the form of a rod, and bend round at their basal extremities to be continued directly into the nerve fibrillre (fig. 196). There is in addition a widely distributed and important peripheral nerve plexus in the muscles of the sub-umbrella. Up to the present time no investigations have completely elucidated the manner in which this nerve plexus is related to the nerve centres of the marginal bodies, and how the latter are connected with one another. The existence of a nerve ring on the sub-umbrella surface has been proved only for the Charybdeidce, in which the edge of the disc is not notched (fig. 169). The antimeres of the Acaleplm show in all cases a great degree of individuality, and, when cut off, are able to live for a considerable time. The marginal bodies, as well as the pit-like depressions on the dorsal side of the excavations in which the marginal bodies are placed (olfactory pits), must be considered as sense-organs. The marginal bodies are morphologically the remnants of reduced tentacles. They may be seen on the under side of the umbrella in the stage of the Ephyra, and are overgrown by portions of the edge of the umbrella (Steganophthalmata). [They contain a central canal lined by endoderm and continuous with the gastro-vascular system of the disc, fig. 196]. They appear in all cases to unite the functions HTDROZOA SCYPHOMEDUS.i;. 255 of ocular and auditory apparatus. The auditory function is provided for by a large sac containing crystals, which originates from the cells of the eiitoderm ; while the eye consists of a mass of pigment lying on the dorsal or ventral face, and nearer the end of the stalk. In some exceptional cases (Nausithoe) it is provided with a refractile cuticular lens. But it is in the Charybdeida- that the sense body reaches the highest development ; for in them, in addition to the terminal :-ac of otoliths, there is also present, in the wall of the dilated vascular space of the papilla, an extremely complicated visual organ, formed of four small paired and two large unpaired eyes, in which lens, vitreous body, and retina can be distinguished. The four generative organs of the Acalepha can be easily dis- tinguished in consecpaence of their size and their bright colouring. In some cases, at any rate in the Discopkora, they protrude as folded bands into special cavities in the umbrella, the so-called sub-genital pits (hence the term Pkanerocarpce Esch.) In all cases these bands lie on the lower (sub-umbrella) wall of the digestive cavity (figs. 194, 195), from which they originate as leaf -like prominences. The upper surface is covered with gastric epithelium ; the under, which is turned towards the sub-umbrella, with germinal epithelium, the elements of which, in the process of development, pass into the gelatinous substance of the band. The formation of the cavities in the sub-umbrella of the Discophora is due to a local growth of the gelatinous substance of the sub -umbrella; in some cases, however, they may be completely absent (Discomedusa, Nausithoe). The mature generative products are dehisced into the gastric cavity, and pass out through the mouth; but in many cases the ova undergo their embryonic development either in the ovary (Chrysaora) or in the oral tentacles (Aurelia). Separate sexes are the rule. Male and female individuals, however, apart from the colour of their generative organs, have only slight sexual differences, as, for instance, the form and length of the tentacles (Aurelia), Chrysaora is hermaphrodite. In the Discophora the development is generally accompanied by an alternation of generations ; the asexual generations being repre- sented by the Scyphistoma and Strubila; but in exceptional cases it is direct (Pelagia). In all cases a complete segmentation leads to the formation of a ciliated larva, the so-called planula, which attaches itself by the pole which is directed forwards in swimming. This pole is, however, opposite to the gastrula mouth, which in the meantime becomes closed, while round the mouth, which is 256 CCELEXTERATA. formed as a perforation at the free end, the tentacles appear. As in the embryo Actinia, two opposite tentacles first make their appearance : not, however, simultaneously, the one appearing after the other, so that the young larva about to develop into the Scyphis- toma presents a bilaterally symmtrical structure. Subsequently the second pair appear in a plane at right angles to the plane of the first tentacles. These four tentacles mark the radii of the first order. Then alternating with these, but in a less regular suc- cession, the third and fourth pairs appear ; and soon after in the plane of these latter four longitudinal folds of the gastric cavity are developed (radii of the second order or of the gastric filaments and genital organs). The eight-armed ticyphistoma soon produces eight fresh tentacles, which succeed one another in irregular succession, and alternate with the tentacles already present. Their position determines the inter- mediate radii of the fiiture young Discophor or Ephyra, After the formation of the circle of tentacles and the secretion of a clear basal periderm (Chrysaora), the Scyphistoma is capable of reproduction by fission and gemmation. At first the Scyphistoma appears to multiply only by budding ; the second mode of reproduction, the process of strobilization, begins later. This consists essentially in the fission and division of the anterior half of the body into a number of segments, thus changing the Scyphistoma to a Strobila. The separation of the segments progresses continuously from the anterior end to the base of the Strobila, so that after the disappearance of the tentacles, first the terminal segment, then the second, and so forth, attain independent existence. Each segment becomes an Ephyra, developing eight pairs of elongated marginal lobes, with a marginal body in the notch which separates the two lobes of the same pair. It is these marginal lobes which give to the edge of the umbrella of the Ephyra its characteristic appearance. The young Ephyra gradually acquires the special peculiarities of form and organization of the sexually mature animal (vide figs. 113 A). The number of nematocysts accumulated on the upper surface of the disc and on the tentacles of many Medusas enable them to cause a perceptible stinging sensation on contact. Many, e.y. Pelagia, are phosphorescent. According to Panceri, this phenomena originates in the fat-like contents of certain epithelial cells on the surface. In spite of the delicacy of their tissues, certain large Medusa have left impressions in the lithographic slate of Sohlenhofen (Mfldusites circularis, etc.) SCI'PIIOME-DUS.E CALYCOZOA. (1) Sub-order: Calycozoa (( 'ylico/na). Cup-shaped Acalepha attached !>// their aboral pole. '/'/''// four wide, vascular pouches separate*! !>;/ narrow walls, and eiylit unn,- like 2'ocesses beset with tentacles on the edge of the umbrella. The Caltjcozoa are best considered in their relation to the >SV/////.v- finiia. They may be looked upon as Scyphistonia deprived of their tentacles, which indeed are only transitory structures, and elongated so as to assume the form of a cup, and changed in several particulars which are characteristic of the medusa stage. The four septa arise by the fusion of the four gastric folds with the wide oral disc, which becomes drawn in and concave like a sub- umbrella. These four septa separate the same mimber of gas- rt b FIG. 107. (, A Culycozim,!. (Litcrrwiriinfrwu the oral surf ace magnified about 8 diameters. S, Septa of the four gastric pouches; L, longitudinal muscle fibres with the genital band; Ht, marginal tentacles. I, The Calycoznon seen from the side ; G, Genital organs ; Gin, gastric fold in the stalk ; at the base is the foot gland. trovascular pouches : while the margin of the cup is drawn out into eight arm-like processes, from which groups of short, knobbed tentacles arise (rig. 197). The genital organs extend on the oral wall of the umbrella into the arms as eight band-shaped, plicated ridges. They run along in pairs at the lower part of each septum in the gastric cavity. The ovum, according to Fol, undergoes a complete segmentation, which results in a single-layered blastosphere. This becomes an oval, two- layered larva, which becomes ciliated, swims freely about, and finally attaches itself. The further development probably takes place directly without alternation of generations. 17 258 CCELENTERATA. Fam. Lucernaridae. Lucernaria 0. Fr. M tiller, Calycozoa with four radial chambers ; without genital pouches, and without the accessory chambers of the digestive cavity alternating with these. L. qun/J ricunt/.t 0. Fr. Miiller. campanulata Lmx. Crater olo^lnix Clark, with genital pouches and four chambers of the gastric cavity alterna- ting with them. Cr. Li-ni-ltarti Tschb. lielgolandica Lkt. , Heligoland. The Lucaniar'ni are without exception marine animals, and are remarkable for their great reproductive power. Accord- ing to A. Meyer, if the stalk be cut off, the cup reproduces a new one. and injured individuals, and even excised pieces, can become perfect animals. (2) Sub-order : Marsupialida (Lobophora}. Tetra-radiate Acalepha having a four-sided pouch-like form. The velum has a smooth margin, and contains vessels prolongations of the gastro-vascular systeni\. On the margin of the disc there are four vertically placed lobe-like appen- dages. There are four covered sense organs, and the same number of vascular pouches separated by nar- roio partition ivalls. The Charybdei? are distinguished by the deep bell shape of their body, and were formerly reckoned as " Craspedota " among the Hydro- medusce, with which they certainly have some characteristics in com- mon. Amongst these character- istics the most striking is the possession of a smooth-edged velum, which, however, contains vessels. FIG. \w.-d,aryide. H. Fol, ' Bin Beitrag /,ur Anatomic und Entwicklungsgeschichte einiger Rip- p.-n.|iiallcn,'' Inaugural dissertation. Jena. 1809. A. Agassiz, "Embryology of the Ctenophone," Cambridge. U.S.. 1874. C. Chun, "Die Ctenophoren des Golfes von Neapel," Leipzig. 1880. 262 C(ELE>*TEEATA. Gf R 2)ossess an wsopli.ayeal tube and a yastro-vascular canal system. Two lateral tentacles, which can be retracted into pouches, are often present. The Ctenopliora, possess a shape which can in all cases be reduced to a sphere. They are radially symmetrical free-swim- ming Coelenterata of gelatinous consistence. The body is often bilaterally compressed, so that it is possible to distinguish two planes passing through the long axis at right angles to one an- FIG. 201. Cydippe, seen from the apical pole, s, Sagittal plane ; T, other : these transverse plane; X, swimming are the*W/^- plates ; Of, gastro-vascular system. tal plane and the transverse plane, and are analogous to the median (longitudinal vertical), and lateral (longitudinal horizontal) planes of bilaterally symmetrical animals (tig. 201). The arrangement of the internal organs bears a relation to these two planes. All parts of the body which occur in pairs, as the two tentacles, the gastric canals, the hepatic bands of the stomach, and the vessels which give origin to the eight lateral canals, all lie in the transverse plane, while the sagittal plane coincides with the longer axis of the oesophageal (gastric) tube (whence also called the gastric plane), the two so- called polar-fields, and the terminal vessels of the infundibuluui. - The inf undibulum is so compressed that FlG zaz.Ct/iHp its longest diameter falls in the lateral ;'''"'-' 0^ Chnn )- Mouth. plane, which on this account is sometimes called the infundibular plane. Since these two planes divide the body into halves, which correspond with one another, and since there is no division into dorsal and ventral surfaces, the arrangement of the body may be said to be bi-radially symmetrical, but cannot be called CTENOPHORA. 263 bilaterally symmetrical, although each half possesses this property. The body is divided by these two perpendicular planes into four similar quadrants. Locomotion is principally effected by the regular vibration of the hyaline swimming plates, which are disposed over the surface of the body in eight meridional rows, in such a way that each quadrant possesses two rows of plates, a transverse and a sagittal (fig. 202). Locomotion is also assisted by the contractility of the muscle fibre* of the gelatinous tissue ; this contractility in the band-shaped Cestidce causes an undulating motion of the whole body. The mouth, which is sometimes surrounded by umbrella-shaped lobed processes of the gelatinous tissue, leads into a wide (Beroe) or narrow oesophageal tube, which in the latter case soon becomes flattened and broad. The cesophageal tube is furnished with two hepatic bands, and com- municates posteriorly, by an opening capa- ble of being closed by muscles, with the gastric cavity, or, as it is com- monly called, the in- fundibulum. The long cesophageal tube projects and opens freely into the infundibulum, and is completely surrounded by the gelatinous sub- stance, as far as the level of the two longitudinal vessels which accompany the two lateral surface-; in the transverse plane. The infundibulum, which is in all cases compressed in a direction at right angles to the oesophageal tube, gives oft' eight vessels to the swimming-plates. These vessels have a bi-radial symmetry. It also gives off two vessels, which are dilated into two terminal sacs ; the latter surround the sense-organ at the aboral pole, which is known as the.otolith vesicle, and each of them opens to the exterior by an orifice which is placed in a diagonal plane and is capable of being closed. Two tentacular vessels may arise from the bottom of the infundibulum. The internal surface both of the resophageal tube and of the infundibulum and its vessels seem to be completely clothed with cilia. FIG. 203. Aboral end of Callitmirn lialata (after E. Hei'twig) . a 1 , The two polar spaces ; , the beginning of the eight rows of swimming plates, between which the otolith vesicle and the nerve plate are seen. 264 CffiLENTERATA. Up to the present time, the nervous system of the Ctenophora (fig. 203) is but imperfectly known. There is no doubt that the large vesicle found at the aboral pole, with its clear fluid and vibratile otoliths, is a sense-organ ; it is also exceedingly probable, taking into consideration the organization of the Acalepha, that the central nervous system of the CteiwpJtwa is contained in the thickened base of the vesicle, t/te Otolith j>/), from the lateral filaments of the tentacle of E/i/ilo- camig stationis (after R. Hertwig). i-f, Prolonga- tion of the contractile thread of a prehensile cell. CTENOPIIORA. the stage with four segments, the segments are so disposed that two perpendicular planes placed between them would correspond to the two principal planes of the fully developed animal. Each of the four spheres gives rise to one of the four quadrants of the adult animal (Fol.) The whole mass of the finely granular exoplasm now becomes collected at the upper end of the segmentation spheres, where it is separated off and gives rise to eight ne\v small spheres. These, by continued division, break up into a great number of small nucleated cells, which increase rapidly and gvow round the eight large seg- mentation spheres or the cells produced from them. The young Ctenophorn sooner or later leave the egg membranes, and at this period differ more or less from the sexually mature animal in the simpler and usually more spherical form of the body, in the small size of the tentacles and swimming plates, and in the differ- ence in the relative size of the oesophageal tube, infuiidibulum, and vascular canals. The differences are most striking in the lobed Gtmoplwra (with the exception of Cestuiii), the embrj'os of which have a great similarity to the young of Ci/<>, and have no traces of bi-radial structure. It is only after a, longer period of larval life that the completely mature form is attained by the unequal growth of the swimming plates and their canals, the out- growth of the tentacle-like processes, and the formation of two lobe-like projections round the mouth from those halves of the body which correspond to the longer rows of swimming plates. The phenomenon remarked by Chun is worthy of notice, that the young of Eii-ch radial arrangement (r<>yi>hir Echinoderms). It is, however, easy to show that this regular radial symmetry never occurs in its perfect form. Since one organ or another, e.g., the rnadreporic plate, the stone canal, heart, etc., always remains single, and does not fall in the axis of the body, it will be only those planes, in the radius or inter-radius of which the unpaired organs fall, which can fulfil the 268 ECHIXODERMA.TA. conditions which admit of the body being divided into two exactly symmetrical halves. Even these planes do not exactly fulfil these con- ditions, since the re- maining organs are not strictly symmetrical in regard to such a plane. Very frequently one of the rays differs in size from the others, and then we have an in-eyu,- !>///'(!/ in the external form of the Echinoderm, which renders the bi- lateral symmetry visible even from the exterior. The pentamerous body of the Echinoderm may become bilateral, the plane of the unpaired ray forming a median plane, on each side of which two pairs of equal rays are repeated. We can distinguish an upper sur- face (apical pole) and an under (oral pole), a right and left side (the two paired rays and their inter-radii), an anterior end (un- paired radius) and a posterior (unpaired inter-radius). In the irregular Sea-urchins, the bilate- rally symmetrical form is still more strongly marked. Not only is the unpaired radius of abnormal size and form, and not only are the angles at which the principal ray and the accessory rays cut each other equal only in pairs, but FIG. ws.Sciiizaster also in the Clypeastridea (fig. 207), ventril1 *! de - ,> , Jl pores of the ambulacra! feet. the anus is removed from the dorsal pole to the ventral half of the body in the unpaired inter-radius, FIG. 20". Clypeaster rosaceus, from the dorsal side. The madreporic plate is situate in the centre and is sur- rounded by five genital pores and by the five-leaved rosette. The unpaired radius is directed forwards. At the side is the median portion of the ventral sur- face. O, mouth ; A, anus. ) , from the ISIV1UM TRIVIl'M. 269 wliile, in Spatangidce, both poles, or only the oral pole, are shifted in the direction of the unpaired radius, and become eccentric (tig. 208). Only a few of the regular EcJiiiiviln<- rians), or ends in a porous calca- reous plate, the madreporic plate, which is inserted in the external covering of the body, and through the pores of which the sea water percolates into the lumen of the canal system. The position of the madreporic plate varies con- siderably. In the Clypeastridea it is at the apical pole ; in the Cidaridea and Spatangidea it is interradial, and falls in the an- terior right interradius near the apex ; in the Asteridea it is also interradial and dorsal ; in the Eurycdidce and the Ophiuridce it lies on one of the five buccal plates. Some Echinoderms, e.g., species of Ophidiaster and Eclii- naster echinites, possess several stone canals and madreporic plates. On the lateral branches of the five or more radial trunks are found the appendages known as the ambulacral feet (fig. 216). These are extensible tubes or sacs, which pass through pores and openings in the dermal skeleton and project on the surface of the body. They are capable of being swollen out, and are frequently r. 210. Diagrammatic section through one of the arms of Asteracanthion (after W. Lange). JV, Nervous system ; P, ambulacralfeet ; A, calcareous portions of integument ; T, dermal FIG. 215. Diagramatic representation of the water-vascular system of a Star-fish. He, Circular vessel ; Ap, ampulla 3 or Polian vesicles ; Stc, stone canal ; M, madreporic plate ; P, ambulacral feet connected with the side twigs of the radial canals ; Aj>', the ampulla; of the same. vided with a sucking disc at their free extremity. Con- tractile ampulla? are placed at the point of junction of the tube feet with the side branch of the radial vessel ; they force the 18 bninchia. 274 ECHINODEEMATA. fluid into the feet and cause them to swell, and hence to project. A number of feet so distended affix themselves by means of their sucking discs ; they then contract and draw the body slowly in the direction of the radii. The number and distribution of these appendages are subject to numerous modifications. Sometimes FIG. 217. Sea-urchin divided along the equatorial line (after Tiedemann). D, Digestive canal fixed to the shell by mesentery ; G, generative organs ; J, inter-radial plates. they are arranged in rows along the whole length of the meridian from the oral region to the periproct (Cidaridea and Pentacta}. Sometimes they are scattered irregularly over the whole surface of the body, or only over the foot-like ventral surface, as in the PIG. 218. Longitudinal section through the arm and disc of Solust-er endeca (somewhat altered after G. O. Sars). 0, mouth leading into the wide stomach ; A, anus ; L, radia- hepatic diverticulum of the stomach ; (?, genital organs ; Md, madreporic plate ; Js, inter- radial diverticulum of the rectum ; Af, ambulacral feet Holothurians. . In some cases they are confined to the oral surface, as in all the Asteroidea. We are able therefore to distinguish an ambulacra! and an antambulacral zone the first coinciding with the oral and ventral surfaces, the latter with the dorsal surface. Never- theless the ambulacral feet are variously constructed, and do not in AMBULACRAL APPENDAGES. ALIMENTARY CANAL. 275 M. all cases serve for locomotion. In addition to the ambulacral feet, great tentacle-like tubes may be present as appendages of the water- vascular system; the circle of tentacles round the mouth of ffolo- t tlmrians (fig. 209) is composed of such appendages. We also find leaf-like appendages arranged over four or five-leaved rosette- shaped areas, forming the ambulacral gills of the Spatangidea and Clypeastridea (figs. 207 and 208). The irregular Sea-urchins all possess in addition ambulacral feet upon the ventral surface. These are in the Cly- j)eastridea ; almost mi- croscopic in size ; they are very numerous, and are arranged in branched rows or are irregularly distributed over the surface. The Echinodermata possess an alimentary canal distinct from the body cavity ; it can be divided into three parts oesopha- gus, stomach, and rectum. The anus is placed usually at the Wl &^ Cl centre of the apical pole, rarely in an inter- radius on the ventral side. The intestine may, however, end blindly, as for example in all the Ophiuridce and EuryaHln-', also in the genera Aster o- Ctenodiscus, and Liddia, which have no anus. The mouth FIG. 219. Holothuria tululwa, opened longitudinally (after M. Edwards). O, Month in the midst of the tentacles (T) ; D. digestive canal ; Sc, stone canal ; P, Polian vesicle ; Kg, circular vessel of the water-vascular system ; Ov, ovaries ; Ay, ambulacral vessel ; J/, longitudinal muscles; Gf, vessel to the intestine; Cl, cloaca; If'/, respiratory trees. 276 1CHINODERMATA. is often surrounded by projecting skeletal plates armed with spicules. There may even be developed, as in the Cidaridea and Clypeastridea, pointed teeth covered with enamel, constituting a powerful masticatory apparatus, which again is supported around the oesophagus by a system of plates and rods. This apparatus is known as Aristotle's Lantern (fig. 214). In the Holothurians, on the other hand, there is a calcareous ring round the oesophagus. It is formed of ten pieces, and serves for the attachment of the longi- tudinal bundles of the dermal muscles. In the Star-fishes the digestive canal is invariably short, sac-like, and beset with branched blind appendages, some of which lie in the disc, while some project a long way into the arms. In the Asteroidea we find five pairs of strongly developed multilobed diverticula of the middle division of the alimentary canal (fig. 218). The five diverticula of the short rectum which fall in the interradii are much shorter, and perhaps perform the function of kidneys, while the longer diverticula increase the digesting surface. In the other Echinoderms the narrow intestine is much increased in length, and is either, as in Comatula, coiled round a spindle in the axis of the disc, or, as in the Sea-urchins, describes some convolutions (fig. 217), and is attached to the inner surface of the shell by fibres and membranes. In the Holothurians also the intestine is, as a rule, much longer than the body, and is usually folded upon itself three times and attached by a sort of mesentery (tig. 219). The true vascular system is very difficult to trace. It consists in most Echinoderms of a ring-like vascular plexus surrounding the oesophagus. From this circular vessel radial vessels pass off one to each ray, and these trunks again give off other branches. There is in addition on the dorsal surface a second circular vessel, which sends off branches to the stomach and generative organs. These two vascular rings are connected by a supposed heart, which, according to Ludwig, consists of a close plexus of contractile vessels. In the Holothurians, besides the vascular ring round the oesophagus, only two trunks with their branches to the intestine are known. The blood is a clear, slightly coloured fluid, in which numerous colourless blood corpuscles are suspended. Special organs of respiration are by no means universally found. The entire surface of the external appendages, as well as the surface of the organs suspended in the body cavity, and especially of the intestine, appear to play a part in the exchange of the gases of the blood. The sea-water very likely enters by the pores in the niadre- RESPIRATION. NERVOUS SYSTEM. 277 poric plate into the body cavity, and is kept in active movement by the cilia which line the body cavity and the pevihreinal canals ; in this way the surface of the internal organs is continually bathed by water. The leaf -like and pinnate ambulacral appendages (ambulacral branchice) of the irregular Sea-urchins are regarded as special organs of respiration, as also are the crecal tubes (dermal braiichia-), which project from the surface of the integument and communicate with o the body cavity in some regular Sea-urchins and in the Asteridea. These dermal branchiae are dis- tributed in the Asteridea over the whole dorsal surface as simple tubes, and in the Echini they surround the mouth as five pairs of branched tubes. Lastly there are the so-called respiratory trees of Holothurians ; these are two large tree-like branched tubes which open by a common stem into the cloaca. The water which is taken into these organs can be again ejected with great force (fig. 219). The nervous system (fig. 220) consists of five principal nerves running down the five rays. These nerves in the Asteridea lie imme- diately beneath the epidermis of the ambulacral groove, external to the radial blood vessel and water vascular trunk : they send off numerous fibres to the ambulacral feet, the muscles of the spines, pedicellarise, etc. These ectodermal bands may be looked upon as the central part of the nervous system ("ambulacral brains" of J. Miiller). Near the mouth they divide into two parts, which unite with corresponding branches from the other radial trunks to form a nervous ring containing ganglion cells. The tentacle-like arnbulacral feet which in the Asteridea and Ophiuridea are present in simple number at the end of the arms are supposed to have the value of tactile organs. The same significance has been attributed to the tentacles of the Holothurids and to the pencil-like tactile feet of the Spatangidw. Organs resembling eyes FIG. 220. Diagram of the nervous sys- tem of a Star-fish. N, The nerve ring connecting the five ambulacral cen- tres. Oc FIG. 221. Astropccten nitnia- fiacus, end of ray with the eye (Oc) surrounded by spicules (after E. Haeckel). 278 ECHINODEEMA.TA. (I have been found in the Echinoidea and Asteridea. In the former (Cidaridea) there are, on special plates (ocular plates], at the apical pole, five tentacle-like prominences, in each of which a nerve ends. The eyes of the Asteridea are most accurately known. According to Ehrenberg's discovery, they have the form of red pigment spots, and lie on the ventral side of the rays at the distal end of the ambulacra! groove. They are spherical pedunculated prominences, and the convex surface is covered by a simple membrane, which hides a number of conical simple eyes (fig. 221). The simple eyes appear to have their axes directed towards a common point. They each con- sist of a red ma.ss of pigment surrounding a refractive body, and a nervous apparatus. Reproduction is mainly sexual, and separate sexes are the rule. Only Sy- napta and A mp h iura are hermaphrodite. The organs of reproduction of the two sexes are ex- tremely alike, so that if it were not that the colour of the generative products is different, the seminal fluid is mostly white and the ova red or yellow, a microscopical examina- tion of the contents of FIG. 222. Genital organs of Echinus. Ad, Rectum; G, genital glands lying on the interambulacral plates ; a, rows of amptilla?. the generative glands would be the only means of distinguishing between them. Sexual differences of the external form or of definite parts of the body are only very rarely present, since as there is no copulation the sexual functions are usually confined to the secretion and preparation of the generative material. Ova and spermatozoa, with some rare excep- tions, first come in contact in the sea water outside the body of the mother. Internal fertilization, which is very rare, occurs in several viviparous species of Ampliiurn and Phyllophorus. The number and position of the generative organs are generally in strict correspondence with the radial structure ; nevertheless there are numerous excep- tions to this. In the regular Echinoidea, five-lobed ovaries or testes, which are composed of branched blind tubes, lie in the interradii on the internal surface of the dorsal part of the shell (fig. 222). The DEVELOPMENT. 279 ducts of these glands are live in number, and open to the exterior through five openings in the skeletal plates (genital plates) around the apical pole (figs. 200, 222). In the irregular Spatanyidce the generative organ of the posterior interradius is always absent, and the number of glands may be three or two. In the Asteridea the five pairs of genital glands have the same interradial arrangement : sometimes however', they project into the arms : the apertures for the exit of the generative products lie on the dorsal side, and in each interradius two places may be found, each of which is pierced in a sieve-like manner by a number of such openings (fig. 223). In the Ophiuridce ten lobed generative glands composed of a number of blind tubes are developed around the stomach ; their products pass through special passages into pouches, and from thence to the exterior through paired slits on the ventral side between the arms. The generative glands of the Crinoidea are concealed in the arms and pinnules. In the Holothurians, the generative organs are reduced to one branched gland, the duct of which opens to the exterior not far from the anterior pole of the body on the dorsal side (fig. 219). The development of the Echinodermata presents as a rule a complicated meta- morphosis, and is characterised by the possession of bilateral larval stages. Many Holothurians are developed with- out passing through these larval stages, as also are certain Sea-urchins, as Anochanus, Hemiaster, and some Aste- roidea, which are either viviparous (Amphiura squa/mata) or lay only a small number of eggs, and protect them during their development in a brood pouch. In these cases also the first stage is a ciliated embryo, which is either developed directly or passes through a much simplified metamorphosis. In the cases of a complicated metamorphosis, the ovum, after under- going a nearly equal segmentation, gives rise to a spherical embryo, the cellular wall of which is ciliated and encloses a central gelatinous substance (fig. 103). A pitlike depression of the cellular wall gives rise to the first rudiment of the alimentary canal, and the opening of this depression (gastrula mouth) to the anus. The ciliated embryo becomes elongated and gradually takes the form of a long, oval, more FIG. 223. Part of the inter-radius of a star-fish (Sohister) with the gen- erative glands (G) and the groups of pores (sieve plates) on the dor- sal skin (after J. Miiller and Tros- chel). 280 ECHINODBIIMATA. or less pear-shaped larva, in which a slightly arched dorsal, two symmetrical lateral, and a saddle-shaped ventral surface can be dis- tinguished. The cilia which are concentrated upon the raised edge of the ventral depression give rise to a continuous ciliated band which serves as a locomotive apparatus. [This band first appears as two separate ciliated ridges placed transversely, one in front of, and the other behind the mouth (fig. 224, 3). These soon become con- nected laterally.] The alimentary canal, which has now acquired an anterior opening, the mouth, consists of three portions, the oeso- phagus, the stomach, and the intestine. The wide mouth leading into the oesophagus is situated within the band of cilia on the ventral surface ; the anus is also ventral, but external to the ciliated band in the region of the posterior pole. Before the appearance FIG. 224. Larval development of Anteracanthiun \berylinug (after A. Agassiz) (for earlier stages see fig. 103). 1, stage where the mouth (O) has just appeared, represented in profile ; A, blastopore (anus) ; D, intestine ; Vp, vaso-peritoneal sac. 2, Somewhat older stage in surface view with two separated vaso-peritoneal sacs. 3, Later stage, from the ventral side, with two transverse ciliated ridges (W) ; the sac on the left side has an excretory pore. 4, Young Bipinnaria with double band of cilia (W). of the mouth, another organ is separated from the alimentary canal : this is a sac-like ciliated tube, which opens to the exterior by a pore on the dorsal surface, and represents the first commencement of the ambulacra! system. A second organ, which also has its origin from the rudimentary digestive canal, consists of the disc-shaped lateral sacs (fig. 224), from the walls of which the peritoneal lining of the body cavity is produced. With their progressive development the larvse of the Sea-urchin, the Starfish, and the Holothurian diverge more and more widely from one another. The raised edge of the depression just mentioned, with its band of cilia, becomes bent and prolonged into processes (fig. TYPES OF LARA T .E. 281 225) of different form. These processes are arranged with a strict regard to bilateral symmetry, and their number, position, and size essentially determine the special shape of the body. An anterior and a posterior ventral region of the band of cilia can be distinguished from the lateral parts which form the dorsal portions ; the latter curve round and pass into the former at the anterior and posterior ends of the body (tig. 225, b). The dorso-lateral parts may, however, unite anteriorly with one another without passing into the anterior ventral band ; in this case the anterior continuations of the latter pass directly into one another so as to form an independent prajoral ring, while the dorso-lateral and posterior ventral portions of the origin- ally continuous band form a longitudinally directed post-oral ring. This arrangement is characteristic of the larvte of the Asteridea (Bipinnaria, Brachiolaria']. In all other forms a single longitudinal band of cilia only is pre- sent. In the larva? of Holo- thurians, the Auricularia (tig. 225), the processes re- main short and soft ; they are found on the dorso- lateral edges and on the posterior dorso-ventral arch of the band of cilia ; they also appear on the posterior ventral (umbrella) and the anterior ventral (oral shield) parts of the band. The processes have a similar disposition in Bipinnaria, where, however, they are often much longer, but are in this case also not provided with calcareous rods. The Bracltiolaria are distinguished from the Bipinnaria by the possession of three anterior arms, which are placed between the anterior portions of the two rings of cilia, and serve as a fixing apparatus. The bilateral larvaj of the Opliiurids and Sea- Urchins, the so-called Pluteus forms, are distinguished by their lai-ge rod- shaped processes, which are supported by a system of calcareous rods. FIG. ZZo.Aitricttlci.riti larva- (after J. Muller). , from the dorsal side ; I, from the ventral side. 0, mouth beneath the oral shield ; Of, oeso- phagus ; M, stomach ; T), intestine with anus (A) P, peritoneal sac ; T", Water-vascular rosette with pore ; R, calcareous wheel-like bodies. 282 ECHINODERMATA. The Pluteus larv?e of the OpMiirids possess long lateral arms on the anterior dorso- ventral arch of the band, on the dorso- lateral edge, and 011 the edge of the pos- terior ventral hood. The Pluteus larva of the Sea-urchin has no lateral arms, but pro- cesses are developed on the edge of the anterior ventral hood (fig. 226). The larv* of the Spatangidce are characterised by an unpaired apical rod, and those of Echinus and Echinocidaris by thepresence of ciliated epaulettes (fig. 227). The transformation of the laterally symmetrical larva with its bilateral processes and com- plicated organization into the body of the adult Echino- derrn is not in all cases effected in the same manner. In the Sea-urchins and Star- fishes the young animal is developed by a process of new formation within the body of the larva, the stomach, intestine, and dorsal sac alone persisting ; while the transformation of the FIG. 226. Pluteus of a Spatangus with so-called apical rod (St) (after J. Miiller). Auricularia into Synapta takes place without the loss of so many parts of the larva, the young passing through a pupa-like intermediate stage. In the first case a We '&' mass of FIG 227. Platen* larva of EcJiintm lirittiis with four ciliated epaulettes (We) (after E. Metschuikofi) from the ventral side. O, Mouth ; A, anus. METAMORPHOSIS. 283 interstitial tissue tilled with round cells is formed external to the lateral discs, and with participation of the thickening skin. This tissue becomes the seat of calcareous deposits, and forms the dermal skeleton of the adult Echinoderm (fig. 228 a, b). The canal of the dorsal pore has in the meantime changed its simple form and developed into the circular vessel with divert i- cula, which are destined to become the ambulacra! trunks. As development progresses, the young animal appears as a more or less spherical or pentagonal body, or as a star with short arms, in propor- FIG. 228. Sijihimii-iu from Triest forming 1 a stage in the development of the Star-fish (St) (after .1. Miiller). a. Earlier stage. M, stomach; A, anus; I", ambulacral rosette with ciliated tube opening by the dorsal pore ; S, stone canal. I, Older stage. tion as it predominates over the larva. Finally, after the sprouting out of the ambulacral feet, the young Echinoderm becomes separated from the larval body, which not unfrequently remains attached to the former, like the remnants of a broken-down framework. The stomach, which is taken into the interior of the body of the Echinoderm, is torn from the oesophagus of the larva (JBipinnaria), and acquires a new oesophagus and mouth. The dorsal pore becomes the pore of the madreporic plate. The Si/npti "US Stalk is O, mouth ; A, anus, of the disc, which is represented from only present ill the the oral side. r m , , . young form. The body with the contained viscera appears, therefore, as the calyx at the upper end of the stalk, and only in exceptional cases is directly * J. S. Miller, " A Natural History of the Crinoidea or Lily-shaped Animals," Bristol, 1821. J. V. Thompson, " Sur le Pentacrinus Europseus, Petat de jeunesse du genre Comatula," L'institut, 1835. J. Miiller, " Ueber den Ban von Pentacriuus caput Medusas," AbJuutdl. drr fieri. Akad., 1841. J. Miiller, "Ueber die Gattnng Comatula mid ihre Arten," Alliainll. drr fieri. AJtad.\ 1847. Leop. v. Much, "Ueber Cystideen," Abhandl. drr fieri. AA-ad., 1S44. Ferd. Eomer. " Monographic der fossilen Crinoideeu familie dor Blastoidecn," CRINOIDEA. 287 attached by its dorsal apex. The segments of the stalk, which are mostly pentagonal, are connected by bands of tissue, and are pierced by a central canal, which serves for nutrition, and contains a central and five peripheral blood vessels ; at certain distances they bear hollow and segmented cirri, which are arranged in whorls. The dorsal surface of the calyx is covered externally by regularly arranged calcareous plates, while the upper (ventral) surface, on which the mouth and anus are situate, is clothed with a leathery FIG. 232.Comiitnl(i mnlifrrranea represented from the ventral side. O, mouth ; A, anus. The pinnulae are filled with the generative products. skin. At the margin of the cup there arise movable, simple or forked, and often branched arms, which are supported by a solid framework consisting of dorsally placed calcareous plates, which are movable upon one another by special muscles. In almost Arch, fur Xatxrgescli, 1S.">1. \V. Thompson, "On the Embryology of the Antedon rosaceus," Phil. Trans. Bo;/. Sat-., Tom 155, lSi;r>. \V. 1!. Carpenter, "Researches on the Structure. Physiology and Development of Antedon rosaceus," Ibid., Tom 15(i. A. Gott'e, " Vergl. Bntwickelungsgeschichte cler Comatula Mediterranea," A whir, fiir nilrkriiali. Anntmnic. Tom XII. 11. Ludwig, " Morphol. Studien an Echinodermen," Leipzig, 177. 288 ECHINODERMATA. every case the arms bear, either on their main stems or on their branches, lateral appendages, the pinnules, which have an alternate arrangement on each side, one being attached to each segment of the arms. Essentially the pinnules represent the ultimate ramifications of the arms. The mouth, as a rule, lies in the centre of the cup. From it certain furrows, the ambulacral grooves, traverse the disc (fig. 231) FIG. 233. Developmental stages of Comntnla (Anfedon), much enlarged, a, free-swimming larva with tuft and rings of cilia (Wr), also with rudimentary calcareous plates. 6, At- tached Pentacrinoid form of the same animal. O, Oralia ; S, Radialia ; S, Basalia ; Cd, Centrodorsal plate, c, Older stage described as Pentad-inns europaeus with arms and cirri (after Thomson) . and pass on to the arms, and their branches and pinnules; they are lined by soft skin, and carry the tentacle-like ambulacral appendages. The anus, when it is present, lies excentrically on the ambulacral (ventral) surface of the disc. The development of the living genus Comatula, which begins with a barrel-shaped larva with four bands of cilia and leads to the fixed stage of the Pen- C'RHfOIDEA. 289 tacrinus form (P. Europwiis) (tig. 233), consists of a complicated metamorphosis. The greater number of Crinoids belong to the oldest periods of the history of the earth (the Cambrian, Silurian, Devonian, and the Carboniferous formations). Existing forms live mostly at considerable depths. We distinguish two orders, the Tesse/dta and the A rticulata. The latter is represented by numerous fossil forms, but by only a few living genera as Penta- erinus, Holopus, and Cuiiitnln (rig. 234). The cup is always less completely provided with plates than in the fossil Tesselata. AETICULATA. Fam. Pentacrinidae. Crinoids with ten arms, several times bifurcated. There is a pentagonal stalk with whorled cirri. Pentaerinus cujmt Mi'dvste, Mill, from the Antilles. P. MiilU-ri Oerst., West Indian Ocean. The fossil forms are : Encrinus lllliforinix Schl. (fig. 23-1) from the Muschelkalk ; also Ajtiocrinu*. allied to the existing Khizocrim/us lofotcnxlx Sars. and to Bathycrinus gradl/x, and aldricliiainix W. Th., from the deep sea. Allied to this group is the third existing genus Holupvs, from the West Indies, with calyx attached by a short Fi G .294.EncnniifU!li- unjointed prolongation of its apex. H. Itatif/ii d'Orb. Fam. Comatulidae. Stalked only in the young state. The adult animal is free. There are usually ten arms at the margin of the flattened body ; mouth and anus are present. The Coma- tulirfce possess the power of striking their arms towards the ventral surface and so of propelling themselves amidst the sea-weeds. The vermiform larva, with its four ciliated girdles, makes its appearance within the egg-membranes. It acquires a mouth and anus, also a tuft of cilia at the posterior end of the body, and swims about freely. It passes later, by the formation of cal- careous rings and rows of plates, into the stage of the stalked Pentaerinus^ from which the ('oniatula is produced by the separation of the cup from the stalk. Comatulti mediterranea Lam.. Antcdon ruxacea Link., known in the young attached stage as Pentaerinus EurujHifux. Actinometra J. Miill. ' To the Crlito'ulx are allied the fossil Ci/xtidra and lilaxtuidca. The Cyxtidcu were provided with short stalks and slightly developed arms. Their generative organs were enclosed in the calyx, whence their products escaped through a genital opening capable of being closed by movable valves. They are found as fossils in the Cambrian, Silurian, and Devonian formations and the Carboni- ferous limestone. To this group belong the genera Spha.eromtes, ('nri/oi-rintix. Apiocystites. The Blastoidea have no arms, and only possess ambulacral areas on the calyx, which is attached by a segmented column. Pentatrematites. 30 for mis from the Mus- chelkalk. 290 ECHINODERMATA. CLASS II. ASTEROIDEA (STARFISHES).* Echinoderms with dorso-ventrally compressed pentagonal or star- s/taped body. Tlie ambulacra! feet are confined to the ventral surface. Internal skeletal pieces in the ambulacra articulated together like vertebra;. The Star-fishes are chiefly characterised by the predominating pentagonal or star-like discoidal shape of the body, to the ventral FIG. 235. Echtiaffer xenfun, from the oral surface (after A. Agassiz). O, mouth ; Af, amtaulacral feet. surface of which the ambulacral feet are confined (fig. 235). The radii are long in comparison with the inter-radii, which are very short in consequence of the divergence of the interambulacral rows of plates; they constitute more or less projecting movable arms, with movable skeletal structures. These latter consist of transversely arranged, paired calcareous plates (ambulacral ossicles), * J. Miiller and Troschel. " System cler Astcriden." Brunswick, 1841. Com- pare besides the numerous papers of Krohn. Sars. Liitken, L. Agassiz. etc. ASTEROIDEA. which reach from the mouth to the end of the arms, and are articulated together like vertebrae. The skeleton of the Asteroidea is distinguished from the globular or flattened shell of the Echinoidea by the fact that the ambulacra! and interambulacral plates are confined to the ventral surface, and that on the outer side of the former there is a deep ambidacral yroove, which contains, outside the ossicles and beneath the soft skin (which in Ophiurids possesses special calcareous plates), the nerve trunks, the peri- haemal canals with the blood-vessels and the ambulacral trunks. In the Ophiuridea the ambulacral groove is covered by the dermal plates so that the ambulacral feet project at the sides of the arms. Upon the dorsal surface the dermal skeleton appears leathery; it is, however, as a rule, filled with small calcareous plates, on which are placed spines, protuberances, and papilla?, constituting a covering of a most varied kind. At the mar- gin in the dorsal integu- ment there is usually a row of larger cal- careous plates (superior mar- ginal plates) (i* .T O f? \ ' ' FIG. 236. Skeletal plates of Astropecten Hemprichtii (after J. Miiller). Upon the veil- DR, Dorsal marginal plates ; VR, ventral marginal plates, Ap, am- ,i _ s bulacral ossicles ; Jp, intermediate interambulacral plates ; Adp, anterior adambulacral plates forming an angle of the mouth. we can distin- guish, in addition to the internally placed ambulacral ossicles, inferior marginal ossicles (fig. 236, VR), also the adambulacral plates (A dp), and the intermediate interambulacral plates (Jp). The two last corre- spond to the interambulacral plates of the Echinoidea, where they occur as two or more rows, which are united along the whole length of the inter-radius : in the Asteroidea, however, they separate from one another at an angle, and are disposed along the opposed sides of ad- jacent arms. The ambulacral ossicles are calcareous bodies articulated together like vertebrae, with spaces between their lateral processes for the passage of the vessel connecting the ampulla? with the radial vessel and the tube feet. The right and left pieces of each double row are either (Ophiuridea) immovably connected by a suture, or are 292 ECHINODEBMATA. (Stelleridea) movably articulated by teeth, which tit into one another at the bottom of the anibulacral groove ; the latter only (Stelleridea) possess transverse muscles 011 the anibulacral ossicles, and are able to bend their arms together towards the ventral surface. The Ophiuridea are provided with longitudinal muscles only, by means of which they are able to bend their arms to the right and left in a horizontal plane with a serpentine movement. The mouth is always placed in the centre of the ventral surface in a pentagonal or star-shaped depression, the edges of which are usually beset with stiff papillae. The inter-radial angles are marked by the junction of two adambulacral plates, and frequently function as organs of mastication. The anus may be wanting ; when present, it invariably lies at the apical pole. The madreporic plate, of which there may be one or more, is situated inter-radially and dorsally (Stelleridea), or on the inner surface of one of the buccal plates (Ophiuridea), on the exterior of which a pore may be present. Development in certain cases takes place without the interposition of a bilateral larval phase with bands of cilia. When such larvae are developed, they have the form of a Pluteus (Ophiurid) or Bipin- naria and BracJtiolaria (Stellerid). The great power of regeneration possessed by Starfishes is not confined to the reproduction of lost arms, but may lead to the new formation of portions of the disc, or even of the entire disc from a single separated arm. This process thus amounts to a species of asexual reproduction by fission, and has been especially observed in forms with six (Ophiactis) or more than six (Linckia) arms. Fossil star-fishes are found as far back as the lower Silurian strata (Palceaster), where intermediate forms between Stelleridea and Ophite-idea (Protaster) make their appearance. Sub-Class 1. STELLERIDBA (Aster idea) STARFISHES. Asteroidea ^vhose arms are, prolongations of the disc, and contain the hepatic appendages of the alimentary canal, and also the, generative organs. They possess a deep, uncovered anibulacral groove running along the ventral surface, in which groove the ambulacral feet are disposed in roivs. The Stelleridea usually possess broad arms, and are characterised by the fact that the ambulacral ossicles of the two sides are connected by transverse muscles and are movable upon one another. The anus lies at the aboral pole, but may be wanting in certain genera (Astropecten). The madreporic plate and the genital pores are STELLERIDEA. 293 situate inter-radially and upon the dorsal surface. The multilobed branched diverticula of the stomach extend into the cavities of the arms (fig. 218). On the ventral sin-face of the latter, two or four rows of ambulacral feet project from the deep ambulacral groove, the edge of which is beset with papilla? (fig. 235). Pedicdlarice are also found, and dermal gills projecting through the tentacular pores of the dorsal surface. They feed principally upon Mottusca, and, by means of their ambulacral feet, crawl slowly upon the bottom of the sea. Some few of them are developed by a very simple process of metamorphosis within the brood-pouch of the mother : but the greater number of them pass through the free larval stages of Bipinna/ria and Brachiolaria (figs. 224 and 228). Fam. Asteriadae. The- cylin- drical ambulacral feet end in broad suctorial discs, and are usually arranged in four rows along each ambulacral groove. Astcriax L. (AgteracantTiiori), A. fflacialia 0. F. Miiller., Ile- liatrr lirJianthnx Gray. Fam. Solasteridse. The cylin- drical ambulacral feet are dis- posed in two rows. Rays long, often more than rive Snltixfrr j)ft2)poxn>i Retz., EcJi.innstcr sepositias Retz., Ophidiagt&r Ag., Linckia Nardo. Fam. Astropectinidae. Am- bulacral feet conical, and with- out suctorial disc, arranged in two rows. There is no anus. aiirtintiiicnx Thil. Lmditi Forb.. CtenmUxrits Miill. Tr. Fam. Brisingidae. l>udy shaped like an Ophiurid. Rays distinct from the disc with only a narrow internal cavity. Jirixini/a comnntti Sars. FIG. 237. Asteriscu* verrnculatus, with the dorsal skin removed. Ld, Ralial appendages or hepatic tubes of the stomach ; G, generative glands. Axtropectcn, Sub-Class 2. OPHIURIDEA (Brittle Stars'). Asteroidea characterised by the absence of an anus, and by the pos- session of Ion;/ cylindrical arms which are sharpf-ij distinct from the ilixi.-, and do not contain appendages of the alimentary canal. The ambulacral groove is covered by the dermal plates so that the ambulacral feet project at the sides of the arms. The Ophiuridea can be at once distinguished by the flexible cylindrical arms, which are sharply distinct from the disc, and enclose 294 ECHINODERMATA. wanting, as are pedicellarice. genital pouches (bursse), *//, ind no diverticula of the alimentary canal. The movements of the arms are principally in the horizontal plane, and in many cases permit of a creeping locomotion amongst marine plants. The ambulacra! groove is always covered by special dermal plates, and the ambulacral feet project laterally between the spicules and plates on the upper surface (fig. 238). In a few cases the arms are branched, and can be rolled up in the direction of the mouth. In such cases the ventral groove is closed by a soft skin (Astrophyton). The anus is always The generative products pass into from these directly to the exterior through inter-radial paired slits. The madreporic plate lies upon the ventral sur- face in one of the buccal plates. Some few Ophiu- rids are viviparous, e.g., Amphiura squamata; these do not undergo metamor- phosis. Most pass through the Platens larval stage, e.g.. Ophioglypha Lym., (Ophiolepis] ciliata with larval stage Pluteus 2)aradoxus. FIG. 238. OpJiiothrixfragilis. The ends of the rays have been removed. GS, Slits of the genital pouches ; K, masticatory ossicles. Fam. Ophiuridae. With simple unbranched arms, and with ventral plates to the ambulacral groove. They are divided into special genera according to the peculiar character of the dermal covering and of the buccal armature. Opliiotltrir Mull. Tr. The back is provided with granules, hairs, or spicules. The lateral plates of the arms bear spicules. Opli. fnujills O. Fr. Miiller. Opliiura Lam. (OjihiofJernia). Two pairs of genital slits in each interbrachial space. 0. lon/jicathi Link., Ujrftwh-jtis Liitk., Amphlura Forb. Fam. Euryalidse. Mostly with branched arms which can be curved towards the mouth and are without plates ; the ventral groove closed with soft skin. AstrojjJii/to/t, verrticiixum Lam.. Indian Ocean. .-1. arlwrescenx Rond.. Mediter- ranean. Asteron/yx Loveni Mull. Tr. CLASS III. ECHINOIDEA,* SEA-URCHINS. Spherical, heart-shewed, or disc-shaped Echinoderms with immovable skeleton composed of calcareous plates. The skeleton encloses the body * Besides the works of J. Th. Klein, compare E. Desor, "Synopsis des Echinirk'S fossiles," 1854 to 1858. S. Loven, "Etudes sur les Echinoidees." Stockholm 1874. Al. Agassiz, "Revision of the Echini," Cambridge, 1872- 1874. ECHINOIDEA. 2! to after the manner of a shell, and carries movable spines. There is invariably a mouth and anus, and locomotive and often respiratory a mbnlacral appendages. The dermal skeletal plates are connected together so as to form a linn immovable shell, which has no arm-like prolongations in the direction of the rays, and is sometimes regularly radial, sometimes irregular or symmetrical. With some rare exceptions among the fossil Perischcechinidos, as Lepidocentrus, the calcareous plates are firmly connected with one another by sutures, and are usually arranged in twenty meridional rows. These rows (fig. 206) are disposed in pairs, and correspond alternately with the radii and the inter-radii. The five radial pairs are the ambulacra! plates, and are pierced by rows of fine pores for the exit of tube feet (fig. 212, P), and bear, as do the broad interambulacral plates, spherical promi- nences and tubercles to which the differently shaped spines are movably articulated. The body form of the Sea-urchins, as con- trasted with that of the Star-fish, depends upon the meridional arrangement of the rows of plates, and, at the same time, on the continuity of the interambulacral rows. The position of the nerves and ambulacral vascular trunks beneath the skeleton is the special characteristic of the internal organization. Pedicellarice are found between the spicules, and are especially numerous in the region of the mouth. Some Cidaridea are provided with branched respiratory tubes. The genital pores are disposed inter-radially on the genital plates near the apical pole. One of these genital plates is, as a rule, also the madreporic plate. The ocular plates, which are radial in position, are also pierced (figs. 206, 212). The regular Sea-urchins are often symmetrical, one radius being longer or shorter than the others, which are equal to each other. So we find long oval forms which are laterally symmetrical, having the mouth and anus central, and an anterior unpaired radius (Acrocladia, Eckinometra). In the irregular Sea-urchins the anus is thrust away from the apical pole into the unpaired radius (Clypea- strida}. The mouth also often has an eccentric position in front (SpatangidoB, fig. 208), in which case the masticatory apparatus is always wanting. In many regular forms all the ambulacral feet have the same shape, and are provided with a suctorial disc supported by calcareous bodies ; in others the dorsal feet are unprovided with a disc, and are pointed and often have an indented edge. In addition to the ambulacral feet, the irregular Sea-urchins almost all possess ambu- 296 ECHINODERMATA. lacral branchiae upon a rosette formed of large pores on the dorsal surface (fig. 239). The locomotive feet are very small in Clypeastridce, and are distributed either over the whole surface of the ambulacra, or are confined to branching rows upon the ventral surface. In the Spatangidce there are peculiar bands upon the upper surface, the fascioles or semitce (fig. 239), upon which, in place of the spicules, knobbed bristles with active cilia (davulce) are distributed. Develop- ment takes place with a Pluteus larval stage, in which the larva is provided with ciliated epaulettes or with an apical rod. The Sea-urchins live, as a rule, near the coast, and feed on molluscs, small marine animals, and Fuci. Some species of Echinus have the power of boring holes in the rocks in which they live. We find many fossil shells, especially in the chalk. FIG. 239. Sringopsin lynfera with the fascioles or Semites surround- ing the rosette. A, anus. Order 1. CIDARIDEA= REGULAR SEA-URCHINS. Echinoidea with central mouth and equal band-like ambulacra ; with teeth and masticatory apparatus ; with sub-central anus in the apical space. Fam. Cidaridae. With, very narrow ambulacra! and broad interambulacral areas, on both of which are large perforated tubercles and club-shaped spines. There arc no oral branchise. Cidaris mi'tularia Lain., Phylla fan-thus imperlal'ns Lam.. East Indies. Fam. Echinidae. Sea-urchins. The pores are grouped in transverse rows ; there is a round, thin shell, broad ambulacra! spaces bearing tubercles and spines, which are mostly short and pear-shaped. Oral branching arc present. To.rop- nc-ustcs varlegat-Hx, Lam.. Echinus mclo Lam.. Strongylocentrotus llvidvs Brit. sa.ratilis Lin., Mediterranean. Fam. Echinometridae. With long oval shell, imperforate tubercles and oral branchiae. Ecliinometra oblong a Blainv., Podoplwra atrata Brdt.. Acrocladia trigimaria Ag., Pacific. Order 2. CLYPEASTRIDEA. Irregular Echinoidea compressed into the form of a shield. Mouth central and furnished with masticatory apparatus. Very broad ambu- lacra, five-leaved ambulacral rosette round the a,pical pole, and very HOLOTHUROIDEA. 297 small tube feet. Five genital pores in the region of the madreporic plate. Fa 111. Clypeastridae. The edge of the disc without indentations, roxiicfiix I, am. (tig. 2o7), Echinoeyam/us pusillus 0. F. MUller, Mediterranean. Fain. Scutellidae. Flattened Erlihuiiilai with a shell often lobed or per- f orated, with rows of pores for the amlmlacr.il feet. Lolioplmra liifont Ag., Rotula Jf.iiiujjJiii Klein, Africa. Order 3. SPATANGIDEA. Irregular Echinoldea of a more or less heart-shaped form, with eccentric mouth and anus. There are no teeth or masticator i/ apparatus, and there is usually a four-leaved ambulacral rosette and four genital plates. As a rule there are semita? and four genital pores, but the number of the latter may be reduced to three and two. Fani. Spatangidae. Spatanf/vx purjiurt'iis 0. Fr. Miill., Mediterranean : Schiznxter cantilifrrux Ag., Jii'isxHit Klein. CLASS IV. HOLOTHUROIDEA.* Wormlike elongated Echinoderms with a leathery body wall, with contractile tentacles surrounding the mouth ; anus terminal. The Holothuria approach the worms in possessing an elongated cylindrical shape and a bilateral symmetry, which is expressed in many ways. In particular they possess so striking a resemblance, so far as their exterior is concerned, to many Gephyrea that formerly they were included in the same group. The body-covering never consists of a firm calcareous shell like that which we find in other Echinoderms, but always remains soft and leathery, the calcareous matter being confined to a few isolated particles of definite form. In rare cases (Cuvieria], scales are present in the dorsal skin. These are arranged like the slates on a roof, and may even take the form of spicule-like appendages (Echinocucumis). The bilateral symmetry results not only from the existence of un- paired organs, but from the contrast which is often very distinctly expressed between the dorsal and ventral surfaces. The ambulacral feet are not in all cases regularly arranged in the five meridional * Cr. J. Jaeger, " De Holothuriis." Dissert, inaug. Turici, 1833. J. F. Brandt. " Prodromus descriptionis animalium ab H. Mertensio in orbis ten-arum circumnavigatione observatorum," Fasc. I. Petropoli, 1835. J. MUller, " Ueber Synapta digitata und liber die Erzeugung von Schnecken in Holothurien," Berlin. 1852. A. B.iur, ' Beitrage zur Naturgeschichte der Synapta digitata." Dresden, 18(U. C. Semper, (i Reisen im Archipel der Philippinen," Tom 1.. Leipzig, 1868. 298 ECHINODERMATA. rows from the oral to the anal pole, but may be principally or altogether confined to the three rays of the so-called trivium. In this latter case the Holothurid moves upon a more or less foot-like ventral surface. The ambulacral feet may also be distributed uni- formly over the surface of the body, especially on the ventral side. As a rule, the tube-feet have a cylindrical shape, and terminate with a suctorial disc : in other cases they are conical, and the suctorial disc is absent. The tentacles, which are in communication with the water-vascular system, and represent specially modified ambulacral appendages, are simple or pennate, or even dendritic (Dendrochirota) or shield-shaped (Aspidochirota), that is, provided with a disc, which is often divided into many parts. In certain genera (Synaptct), the ambulacral feet are altogether wanting, and the tentacles re- main as the sole appendages of the ambulacral system (fig. 240). Locomotion is effected by the strongly developed dermal mus- cular system, the longitudinal fibres of which are attached to the calcareous ring surrounding the oesophagus. It is charac- teristic of the water- vascular system that the stone canal, which is usually simple, hangs freely in the body cavity, ending in a calcareous framework com- parable to the madreporic plate. The respiratory trees at the end of the intestine perform the func- tion of respiration, while certain glandular appendages (organs of Cuvier), which open into the rectum, may be regarded as excretory organs: these, as well as the respiratory trees, may be wanting. The generative organs consist of a bundle of branched tubes, the duct of which opens on the dorsal surface in the region of the mouth. The genus Xt/itapta is hermaphrodite. The development is in many Holothurians direct (as e.g. in Holothuria tremula according to Koren and Danielssen) ; where there is a com- plicated metamorphosis, the larva? have the Auricular ia form, and pass through a barrel-shaped pupa stage. FIB. 240. Syiiiijitu inhcerens (after Quatre- fages). 0, Mouth ; A, anus : the intestine can be seen through the skin. HOLOTHUROIDEA. 299 The Holothurians are partly nocturnal in their habits, and live at the bottom of the sea, for the most part in shallow places near the coast, Avhere they crawl slowly upon the bottom. The Synaptidce, which have no feet, burrow in the sand. They feed on the smaller marine animals, which, in the Dendrocliirota, are carried to the mouth by means of the branched, tree-like tentacles. The Aspido- chirota fill their intestine with sand, which they eject from the anus by means of the current of water from the respiratory trees. * It is worthy of notice that they (especially the Aspidochirota] can eject through the anal opening the intestine, which breaks off easily behind the vascular ring, and are able to renew it. The Synapta, when irritated, are able to break their body into several pieces by violent muscular contractions. Order 1. PEDATA. Numerous ambulacral feet, which are sometimes arranged regularly in the meridians, and sometimes districted over the whole surface. Fam. Aspidochirotae. With shield-shaped tentacles. Holothvria L. With scattered ambulacral feet, which are conical on the dorsal side, and are without suckers. H. tulndosta Gmel.. Adriatic and Mediterranean : //. edidix Less., the edible Trepan.? of the East Indian seas. Fam. Dendroehirotae. With tree-like branched tentacles. Cucumarla Blainv. Ambulacral feet arranged in regular rows. C. frondosa Gr. Psoliix Oken. Ambulacral feet confined to the foot-like ventral surface of the trivium. Ps. pliant apus. (-Jr. Order '2. APOD A. No ambulacral feet ; as a rule without respiratory trees ; the tentacles are usually branched or pinnate. Fam. Synaptidae. Hermaphrodite and without respiratory trees. In the skin there are wheel-shaped calcareous bodies or projecting masses shaped like anchors, and affixed to calcareous plates. Si/nujitu iJii/itnta Mntg. with anchor- shaped calcareous bodies. J. Miiller discovered in their bodies parasitic cylin- drical animals with spermatozoa and ova. which latter develop into small shell-bearing Gastropods (Entucnnrhu mii-abilitf). Chlrodota Esch. Skin beset with rows of small tubercles bearing calcareous wheel-shaped bodies. The Guv. is furnished with respiratory trees. ENTEROPNEUSTA. The remarkable genus Balanoglossus must be placed here. It is the representative of a class, Enteropneusta Gegenb., :|: allied to the Echinodermata, but usually classed with the Vermes, and presenting * A. Kowalevski. ' Anatomie des Balanoglossus Delle Chiaje," Mi'ituures dc, VAcad. iinjjer. /, with cerebral gan- i/ftiiii. The,]) are often provided with suckers and hooks, and are usually hermaphrodite. The series of forms included under this class are mostly Entozoa, or else live in the mud and beneath stones in the water. In their organization they occupy the lowest place among the worms. Their body is more or less flattened, and is either unsegmented or is divided by transverse constrictions into a number of successive divisions, which, although forming parts of one animal, yet have a strong tendency towards individualisation, and frequently attain to separa- tion and lead an independent life. These segments are products of growth in the direction of the long axis of the body, and stand in a special relation to reproduction. They are by no means to be con- sidered as necessarily indicating a high grade of organization, as does the segmentation of the Annelida. The alimentary canal may be altogether wanting (Cestoda), or, if present, may be without an anus (Trematoda, Turbellaria). The nervous system is usually composed of a double ganglion above the oesophagus, giving off small nerves anteriorly and laterally, and two sterns backwards. In many Platyhelminthes simple eye-spots occur, either with or without refractive bodies, and more rarely there is an auditory vesicle. Blood-vessels and organs of respiration are found only in the Nemertinea. The excretory (water vascular) system is everywhere developed. With the excep- tion of the Microstomidce and Nemertinea, hermaphroditism is the rule. The female generative glands consist of distinct yolk-glands and ovaries. The development very frequently takes place by a very complicated process of metamorphosis connected with alternation of generations. Order 1. TURBELLARIA.* Free living Platyhelminthes with oval or leaf-shaped body, with soft skin covered with cilia. They possess a mouth and aproctous * Duges, - Rccherches sur Porganisation et les moenrs (To Planaires." Ann. flex Sc. Nat., Ser. I., Tom XV. A. S. Oerstedt. "Entwurf einer systcmatischen KinthciluiiLL- imd speciellen Beschreibuntj; der Plattwiirmern," Copenhagen, 1844. l>e Quatrefages. Memoire suv quelques PTanariees marines," AHH. th-x Sc. Nut.. 1845. M. Schultze. " Beitriige zur ^aturgeschiehte der Turbellarien." (ircifswald, 1.S51. L. (JrafE, -'Zur Keuntniss der Turbellarien," Zntsclirifi fur Wis*. Zonl.. Tom XXIV. L. Graff, ' Neue Mittheihingen iibor Turbellarien.'' Zeitsch. f. w/x.v. ZooL. xxv., 1875. P. Hallez. "Contributions a 1'histoire naturelle des Turbellaries," Lille, 1879. 310 PLA.TTHELMINTHES. alimentary canal. Hooks and suckers are absent. A cerebral ganglion is present. The Turbellaria usually possess an oval flattened body, and reach only a small size. The uniform ciliation of the body is connected with their existence in fresh and salt water, beneath stones, in mud, and even in damp earth. Only in exceptional cases do we meet with apparatuses for adhering, viz., small hooks and suckers. The skin consists of a single layer of cells, or of a finely granular layer containing nuclei, which is sup- ported by a stratified basal membrane, and covered externally by a special homogeneous membrane bearing cilia and comparable to a cuticula. Peculiar integumentary structures, which have the form of rods or spindles, and, like the nematocysts in Coelenterata, take their origin, in cells, are not unfre- quently present. Various pigments are also often found embedded in the epi- dermis, and of these pigments the green- coloured vesicles, in Vortex viridis for example, which are identical with chlo- rophyl corpuscles, are specially worthy of remark. Pear-shaped mucous glands are also present. Beneath the conspicu- ous basement membrane which supports the epidermis lies the dermis. It con- tains the strongly developed derma] muscular system embedded in a connec- tive tissue layer formed of round, often branched cells. A body cavity between FIG. 247. Alimentary canal and ner- , , , n -, ,. vous system of af->rfwa &,*. the bodv wal1 and the alimentary canal, rgii (after Graff). G, the two i s? as a ru i e) absent : it may, however, cerebral ganglia with two eye spots; st, the two lateral nerve in many cases be recognised as a system trunks; L '.alimentary canal with of lacunse or as a continuous cavity mouth and pharynx. surrounding the alimentary canal. The nervous system consists of two ganglia connected by a com- missure, and giving off nerve fibres in various directions ; of these, two especially large lateral trunks run backwards, one on either side (fig. 247). The latter are connected at regular intervals by delicate transverse trunks. In a number of dendrocoelous Turbellarians a TUKBELLARIA. 311 diverticulum of the stomach runs forward alcove the transverse commissure in a groove between the two cerebral lobes (L&ptopla/na). In some genera of Planarians, a ring-shaped double commissure has been shown to exist in the brain (Polycelis), and ganglion-like swellings, from which nerves are given oft', have been observed on. the lateral nerve-trunks (Sphyrocephalus, Polycladus). With regard to sense organs, eye spots are tolerably widely distri- buted among the Turbellaria. They either lie in pairs upon the cerebral ganglia or are connected with short nerves given off from the latter. More frequently two larger eyes with refractive struc- tures are developed. Otocysts are but rarely present, e.g., in Monocelis among the Bhabdocoda a single one is present lying upon the cerebral ganglion. The integument is without doubt en. dowed with a highly developed tactile sense ; the large hairs and stiff bristles which project between the cilia may possibly be of importance in this relation. Lateral ciliated pits, which may also be explained as sense organs, are in rare cases present at the anterior end of the body (compare the Nemertinea\ Mouth and digestive apparatus are never wanting but the former is frequently removed from, the ventral surface of the anterior end of the body to the middle or, indeed, even to the posterior region. According to Metschnikoff and Ulianin, a stomach may in some cases be absent (Convoluta, Schizo- 2>rora), and be replaced, as in Infusoria, by a soft internal parenchyma. The mouth leads into a muscular pharynx, which can usually be protruded after the mariner of a proboscis. The alimentary canal, of which the internal wall is frequently ciliated, is either forked and then simple or branched (Dendroccela), or rod-shaped (Rhabdo- cc<>:l with single sexual opening. The land and fresh-water Planaria be- long to this group. Fam. Planariadae. The body is of a long, oval, flattened shape, and is often provided with lobed processes, more rarely with ten- tacles, and. as a rule, with two eyes, which are provided with lenses. PLtnuria O. Fr. P r, Miiller. two eyes, no tentacles. PL turrit. M. Sch. (divided by O. Schmidt into li///ntiri*- FIG. 252.Plannriii jiofychroa (c), jmli/cJirtnl. and turn/') (tig. 252). PL (Jiinrti liignbri* (I), toi-va (c), about twice f< nilr<><-ii']//in the natural size (after O. Q Distinguished bv the possession of Schmidt). lobed processes on the head, also by the presence of a copulatory organ placed in a special sheath. U. lin-ti'iiin Oerst., Poli/ci'lix iiii/rd. lirtinni-a 0. Fr. Mull. Fam. Geoplanidae.* Land Planarians. They are characterised by their s Besides M. Schultze. Stimpson. Metsclmikoff. Grube. etc.. compare H. N. 316 PLATTHELMINTHES. elongated and flattened body, which is provided with a foot-like ventral surface. Geoplanalapidicola Stimps., Rhynchodesmus terrestrial Gm. {Fasdola tcrrr.xtris, 0. Fr. Miiller), Europe. Geodeftinuis biliiieatus, Metschn.. with thread cells in the integument, found in potter's earth. 2. Dig'Onopora. Dendroccela with double sexual opening. Almost all are marine. The proboscis is often folded and lies within a special pouch. When protruded, it spreads out like a lobe. Fam. Stylochidae. The body is flat and rather thick, and is provided with two short tentacles on the head. There are usually numerous eyes on the tentacles or on the head. The genital openings are posterior. ftfylorJt-tts macu- latiix Quatr. Fam. Leptoplanidse. Body flat and broad, usually very delicate. Cephalic region not distinct, without tentacles. The eyes are more or less numerous. The mouth is usually placed in front of the middle of the body. The genital openings lie behind it. Leptoplana troni'llarix O, Fr. Mull., Mediterranean. Fam. Euryleptidae. Body broad, and cither smooth or furnished with papillae. There are two tentacle-like lobes on the anterior region of the head. The mouth is placed in front of the middle of the body. Numerous eyes are disposed near the anterior margin. Marine. Thi/x/ni/i-nnn Difxinyii, Gr. Mediterranean. Eurylepta auriculntu 0. Fr. Miiller. North Sea. Order 2. TREMATODA.* Parasitic Platyhelminthes with unsegmented, usually leaf-shaped, rarely cylindrical body. They possess a mouth and ventrally placed organ for attachment : the intestine is forked and without an anus. The Trematodes are with great probability to be derived from the Turbellaria, with which group, both in form and organization, they show a close relationship. In connection with their parasitic mode of life they develop special organs for adhering, such as suckers and hooks. Cilia are present only in larval life. The mouth is invariably placed at the anterior end of the body, usually in the middle of a small sucker (tig. 253). It leads into a muscular pharynx with a more or less elongated oesophagus, which is prolonged into a forked intestine ending blindly. Moseley, ' Notes on the Structure of Several Forms of Land Planarians." etc. Journal of Jfio 4 . Kelt-net', vol. xvii. * A. v. Nordmann, ' Mikrographischc Beitriige zur Kenutniss der wirbellosen Thiere." Berlin. 1832. G. G. Cams. " Beobachtung iiber Leucochloridium paradoxum, etc.," Nov. Aft., vol. xvii., 1835. Wagener. " Ueber Gyrodactylus elegans," Muller'x Arcltlv.. 18(iO. Van Beneden, " Memoire sur les vers intes- tinaux." Paris, 18(11. E. Zeller, Untersuchungen iiber die Entwickelung und den Bau von Polystoma integerrimum. Zfifxrhr. f. n-/xx. ZouL, vol. xxii., 1872. E. Zeller, " Untersuchungen iiber die Entwickelung von Diplozoum paradox- urn.'' Ibid., vol. xxiii., 1878. E. Zeller. ' Ueber Leucochloridium paradoxum und die weitere Entwickelung seiner Distomumbrut." Ibid.. Tom XXIV. E, Zeller, Weitcrcr Beitragzur Kenntniss der Polystomeen," Ibid., xxvii., 1876. Compare also the works of G. Wagener and De Filippi. TKEMATOJJA. 31' The excretory apparatus consists of two large lateral trunks and a network of fine vessels permeating the tissues and beginning with small ciliated lobules. The two large trunks open into a common contractile vesicle, which opens to the exterior at the posterior end of the body (fig. 253, Ep). The excretory system contains a watery fiuid with granular concretions. This fluid is probably an excretory product, corresponding to the urine of higher animals. The nervous system consists of a double ganglion lying above the oesophagus, and from it several small nerves and two posteriorly diivcted lateral trunks are said to lie given off. Eye spots with refractive bodies are sometimes present in the larva? during their migrations. Locomotion is effected by the dermal muscular system and the organs of attachment, viz., the suckers and hooks, which present numerous modifications in number, form, and arrangement. In general, the size arid development of these organs are related to the endo- parasitic or ecto-parasitic mode of life. In the endi (-parasitic Trema- todes they are less developed, and usually consist of the oral sucker and a second larger sucker 011 the ven- tral surface, either near the mouth, as in Distoiimm, or at the opposite pole of the body (Amj)histomum). This large sucker may, however, be absent (Monostomuiii). The ecto- parasitic Poli/stomca, on the other hand, are distinguished by a much more powerful armature, for besides two smaller suckers at the sides of the mouth, they possess one or more large suckers at the posterior end of the body (fig. 258), which, moreover, may be supported by rods of chitin. There are -often in addition chitinous hooks, and very frequently two larger hooks among the posterior suckers in the middle line (If)- The Trentatoda are mostly hermaphrodite. As a rule, the male and female generative openings lie side by side, or one behind the other, not far from the middle line of the ventral surface, near the anterior end of the body (fig. 254). The male opening leads into a sac. the FIG. 253. Youns' Dlstomxm (after La Valette). Ex, trunk of the excretory (water vascular) system ; Ej>, excre- tory pore ; O, mouth with sucker ; S, sucker in the middle of the ventral surface ; P, pharynx ; D, forked in- testine. :3is PLATTHELMINTHES. D cirrus sac, which encloses the pvotrusible terminal part (cirras) of the vas deferens. The vas deferens soon divides into two, which lead back to the two large simple or multilobed testes. The supposed third vas deferens, which, according to v. Siebold, runs from one testis to the female sexual apparatus, so as to permit of direct ferti- lization without copulation, has been recognized as a vagina opening to the exterior on the dorsal surface (canal of Laurer). The female organs consist of a convo- luted uterus and of the glands concerned in the preparation of the egg, viz., an ovary and two yolk glands. There is sometimes in ad- dition a special shell gland. The true ovary which produces the pri- mary ova is a round body, and is usually placed in front of the testes. The yolk glands which secrete the TT yolk are much ramified tubular glands, and fill the sides of the body (fig. 254). The yolk particles come in contact with the primary ova in the first portion of the uterus, and surround them in greater or less quantities. Subseqiiently each ovum, with its investment of yolk, is surrounded by a strong shell. The ova in their passage along the uterus become packed together, often in great numbers, and undergo the stages of embryonic development in the body of the parent. Most FIG. 2o4,.DistoHiitm krjm/iritm (after Trematodes lay their eggs ; only a Sommer). O, Mouth ; D, limb of in- r vivirvivons testine; S, sucker; T, testes; Do, ' VlVlpai .1*. viteiiarium ; Ou (utems), oviduct ; Dr, The just-hatched young either accessory glands. , . 7 possess (in most roLystomea) the form and organization of the parent; or they present the phenomenon of a complicated alternation of generations (heterogamy) connected with a metamorphosis (Distomea). In the first case, the large eggs become attached in the place where the mother lives ; in the last case, the relatively small eggs are deposited in a damp place, usually in the water. After the completion of the segmentation and the em- TREMATODA. 319 bryonic development, the contractile, usually ciliated embryos* (fig. 255, ), which already possess the first rudiments of an excretory system and more rarely a sucker with a mouth and alimentary canal, leave the egg and wander about independently in search of a new host. The latter is, as a rule, a snail, into the interior of which they pene- trate and there become transformed into simple or branched Sporocysts (without mouth and alimentary canal, fig. 255, c), or into JRedice (with mouth and alimentary canal, fig. 255, d). These give rise, by means of the so-called germs [cells lying in the body cavity of the D fh FIG. 255. Developmental history of Distomitm (partly after R. Leuckart). u, free swimming ciliated embryo of the liver fluke. 4, the same in a state of contraction with rudimentary alimentary canal (D) and nell mass (Or) (rudiments of the genital glands). Ex, ciliated apparatus of the rudimentary excretory system, c, sporocyst developed from a Distomum embryo, filled with Cercarire (C) ; B. Boring spine of a Cercaria. d Redia with pharynx, (Ph), and alimentary canal (D) ; O, mouth ; Ex, Excretory organ ; C, Cercaria inside Redia. e, Free Cercaria ; S, sucker ; D, alimentary canal. sporocyst or redia], which probably correspond to the germinal cells (primitive ova) of the rudimentary ovary, to the generation of the * As R. Leuckart has rightly observed, the Dicyeniitlre. which were regarded as Jli'Mziiti by Ed. v. Beneden, as well as the Orthonectidce, which ha\v recently been especially investigated by Giard and E. Metsctmikofl:. and which in the reproductive stage do not rise above a form corresponding to the embryos of Trematodes, recall these Distomum larvae. 320 PLA.TTHELMINTHES. tailed Cercarice, or to another generation of ftporocysts or Redice,* which then produce the Cercaria-. The CercaricK are nothing else than Distoniuni lai-vte, which eventually reach (often only after two migrations, an active and a passive one) the final host, where they become sexually mature. They are furnished with an exceedingly motile caudal appendage, frequently with a buccal spine, and occasionally with eyes, and they present in the rest of their organization great resemblances to the adult Distomum, excepting that the generative organs are not developed. In this form they leave independently the body of the Redia or Sjmrocyst and of the host of the latter, and move about in the water, partly creeping and partly swimming. Here they soon find a new host (Snail, Worm, Insect larva, Crustacean, Fish, Batrachian), into which they penetrate, aided by the powerful vibrations of their tail ; they then lose the latter and encyst. The Cercarice from the interior of the snail thus become distributed amongst a number of hosts, and each of them gives rise to an encyste young Distonmm without generative organs. This young Distomum mi- grates passively with the flesh of its host into the stomach of another animal, and thence, freed from its cyst, into the organ (intestine, bladder etc -)- in which it becomes sexually mature. There are, then, as a rule, three different hosts in the organs of which bold). P, Pigment spots; E, redia, in tho interior of the embryo, the different developmental stages (Redia or Sporocyst, encysted form, sexually mature animal) of the Distomum bury themselves. The transitions from one host to another are effected partly by inde- pendent migration (embryos, Cercarise), partly by passive migration (encysted young Distomum). Modifications of the ordinary course of development may, however, take place ; these may be either complications or simplifications. The embryo at hatching may contain a single Redia (as in Monostomum * In Cerftiria cystapliora from Planorliis mari/inatiix ; according to <). Wagoner, the primary asexual individual is a fipowt/st, the secondary a Jfci/itt. FIG. 256. a, Embryo of giibclavattu (after G. Wagener). D, Alimentary canal ; Ex, excre- tory system. I, Embryo of Mo- iwxtomitm miifulilf (after v. Sie- TREMATODA. 321 ,/faviim and mutabile), which it carries about until it enters the first host (fig. 256, b). In other cases the course of development is sim- plified by the omission of the second intermediate host, viz., that which contains the encysted immature Distomum (Cercaria macro- cerca of Distomum cygnoides, also Leucochloridium in the tentacles of Helix s/iccinea). ( 1 ) Sub-order : Distomea. Trematodes with at most two suckers, without hooks. They develop with a complicated alternation of generations. The asexual individuals and the larvae live principally in Mollusca, the sexually mature animals in the alimentary canal of Vertebrates. The sexes are completely separated in Distomum Ita'matobium (from the veins of man) ; individuals of the two sexes being united in pairs (fig. 257). Dimorphic forms are found in certain species of the genera Monostomum and Distomum in connection with the division of labour of the sexual functions ; one individual develops only male sexual organs, and the other only female, the former producing spermatozoa and the latter ova. The rudiment of the functionless generative gland undergoes in these cases a more or less complete degeneration. Such Distomea are morphologically hermaphrodite, but practically of separate sexes. The complete biology and developmental FIG. 257JD'sfomum hiemato- history is unfortunately only satisfactorily known for a few species which can be fol- lowed through all the stages of development. Hum. Male and female, the latter being in the canalis gyntecophorus of the former. S, sucker. Fam. Monostomidae. Of an oval, elongated, more or less rounded form, with only one sucker, which surrounds the mouth. Monostomum Zeder. Sucker surrounding the mouth ; pharynx powerful. Sexual openings but slightly removed from the anterior end. J/. nn/titl>/l<' Zeder, in the body cavity and in the orbit of various water-birds ; viviparous. M.Jiavum Mehlis. in water-birds, develops from Cercnria ephcmrra of Platmrbis. M. lentls v. Nordm., the young form without generative organs is found in the lens of the human eye. If. liipitrtltiiiii Wedl., living in pairs enclosed in a common cyst, the one indi- vidual surrounded by the lobed posterior end of the other ; branchiae of Tunny- fish. Fam. Distomidte. Body lancet-shaped, frequently spread oiit. more rarely elon- gated and rounded with a large median sucker : in front of which lie the genital openings, usually close together. 21 322 PLATYHELMIXTHES. Dhtonwm. Median sucker approached to the anterior one. D. hepaticum L. Liver fluke. With conical anterior end. and numerous spine-like prominences on the surface of the broad leaf-shaped body, which is about HO mm. long. Lives in the bile-ducts of sheep and other domestic animals, and produces the liver disease of the sheep. It is occasionally found in Man. and bores its way into the portal vein and into .the system of the vena cava. The elongated embryo only develops after the egg has remained a long time in water : it has a continuous ciliated envelope with an X-shaped eye-spot. K. Leuckart's re- searches have rendered it probable that the development is passed through in the young Linuueux pereycr and trtuiratiilux. that here the embryo becomes a Xjiurocyxt. and that this produces Redia-, in which it is supposed that tailless Distmnc'd arise. [The life-history of the liver-fluke has been completely worked out by A. P. Thomas (Quart. Journal of Microscopical Xci. 1883, pp. ( J!) 133). He has shown that the ciliated embryo passes into Limitceux triincatuhis, and there gives rise to a sparocyst which produces recliae. The rrtl'ur produce more n-dia or Cercar'ue. The Cercarier, which are provided with long tails, leave the host (Limnceiis tiiincatvlus), swim about for a short time in the water, and encyst on foreign objects, e.ij. blades of grass. In this condition they are eaten by the sheep.] D. crassum. Busk., in the alimentary canal of the Chinese, one to two inches in length, and half-inch broad, without spinous prominences, with a simple forked intestine. D. lanccoliitum Mehlis. Body elongated into the form of a lancet, 8 9 m.m. long, lives in the same place with 2). lii'patlcinn. The embryo develops at first in water, is pear-shaped, and only ciliated on the anterior half of the body, bears a styliform spine on the projecting apex. D. op7i-thalimi>bium Dies. A doubtful species of which only four specimens have been observed in the lens capsule of a nine-months' child. ]). lieteropliyes Bilh. v Sieb. 1 1*5 mm. long, in the alimentary canal of man in Egypt. 1). i/oHatli van Ben., 80 mm. long, in Pti'rolaltciui. Numerous species live in the alimentary canal, lungs, and bladder of the frog. Digtomun JUicolle End. (JJ. Okt-nl Ku'll) in pairs in the mucous sacs in the branchial cavity of Brama Raji. The one individual is cylindrical and narrow, and produces spermatozoa ; the other is swollen in the middle and posterior region of the body, and is filled with eggs. The dissimilar development of the two individuals is probably due to the fact that copulation only leads to the fertilization of one of them, which alone is able to perform the female sexual functions. J). lirrmatoliiinn. Bilh. v. Sieb. (Gyncecqpkorus Dies) (fig. 257). Body elongated ; sexes separate. The female is slender and cylindrical. The male has powerful suckers, and the lateral margins of the body are bent round so as to form a groove, the canalis gynrecophorus, for the reception of the female. They live in pairs in the portal vein, and in the veins of the intestine and of the bladder of man in Abyssinia. According to Cobbold, the embryos are ciliated, and possess a tolerably well developed excretory system. By the deposition of masses of their eggs in the vessels of the mucous membrane of the ureter, bladder, aad great intestine, inflammation is set up, which may cause hfematuria. (2) Sub-order: Polystomea. Trematodes with two small lateral suckers at the anterior end, and one or more posterior suckers, to which two large chitinous hooks are often added. In exceptional TKEMATODA. cases (Tristomui)i coccineuin) transverse rows of bristles are found. Paired eyes are frequently present. In some species the elongated body presents a kind of external segmentation. They are for the most part ectoparasitic, to a certain extent like the Hirudinea, and they develop directly without alternation of generations from eggs which are usually hatched in the locality inhabited by the mother. Sometimes the development is a metamorphosis (Polystomum), and the young larva? live in another place. The development of Polystomum integerri- mum from the bladder of the frog is the best known, owing to the researches of E. Zeller "- (figs. 258, 2.59). The production of eggs begins W in the spring, when the frog awakes from hibernation and proceeds to pair. It lasts from three " to four weeks. It is easy then to observe the Poli/sto- mea in the process of reciprocal copulation. When the are being laid, Ov eggs FIG. 259. Egg with embryo (a, and hatched larva (6) of PoJystomim/ integerrimum ; ZU-, ,1 operculum (after E. Zeller)! forces the E Me- anterior end of the body with the -enital FlG - '^s. qerrimum (after E. Zeller). opening through the mouth of the bladder 'o, mouth; Go, genital nearly as far as the anus. The development P en ' n s ; D , intestine ; C , -I -i . "", copulatory opening the embryo takes place in water and occu- (lateral pads) ; D a , yolk pies a period of many weeks, so that the sland rtuct ; s> sucker ; Or, ovary ; If, hooks. young larvae are only hatched when the tad- poles have already acquired internal gills. The larvae resemble Gyrodactylus, and possess four eyes, a pharynx and alimentary canal, as well as a posterior disc (for attachment), which is surrounded by sixteen hooks. They possess five transverse rows of cilia ; three are ventral and anterior, two dorsal and posterior. There is also a ciliated cell upon the anterior extremity. The larva? now migrate 324 PLATYHELMINTHES. into the branchial cavity of the tadpole, lose their cilia, and are transformed into young Palystomea by the formation of the two median hooks and of the three pairs of suckers upon the posterior disc. The young Polystomum, eight weeks after the migration into the branchial cavity, at the time when the latter begins to abort, passes through the stomach and intestine into the bladder, and there FIG. 260. Young Diplozoon (after B. Zeller). a, Two young Diporpa beginning to attach themselves together, b, After both individuals have attached themselves. O, mouth ; //, fixing apparatus ; Z, papilla? ; G, sucker. only becomes sexually mature after three and more years. In some exceptional cases, and always when the larva has passed on to the gills of a very young tadpole, it becomes sexually mature in the branchial cavity of the latter. The forms then remain very small, are without the copulatory canals and uterus, and die after the production of a single egg, without ever getting to the bladder. a FIG. 261 .-Egg (a) and larva (b) of E. Zeller). Fam. Polystomidse. With seve- ral posterior suckers, which are usually paired and arranged in two lateral rows, and are rein- forced by an armature of hooks. The genital openings are fre- (after quently surrounded by hooks. Many species have a length of only a few lines. Polystomum Zed., with four eyes ; with no lateral suckers at the anterior end, but with oral sucker; with six suckers, two large median hooks and sixteen small hooks at the posterior end. P. integerrimum Rud., in the bladder of Jtana tniijjtn'ai-ia. P. occllittum in the pharyngcal cavity of Einys. In the formation of the testis and the absence of the uterus it resembles the adult form of P. integerrimum from the branchial cavity of thetadpole. OctdbotTirium laneeolatum Duj. Onohocotyle agypendioulata Kuhn, on the gills of Elasmobranchs. Diplozoon v. Nordm. The animal is double, two individuals being fused to TBEMATODA. 325 form an X-shapcd double animal, the posterior ends of which are provided with two large suckers divided into four pits. In the young state they live solitarily as Dlporptt ; they then possess a ventral sucker and a dorsal papilla (21)0 a, G and Z~). In the double animals the formation of ova is confined to a definite period of the year, usually the spring. The eggs are laid singly after the forma- tion of the thread by which they are attached, and two weeks later the embryo (fig. 2(>1, &). which only differs from Dipnri>a in the possession of two eye- spots and a ciliated apparatus upon the sides and on the posterior extremity of the body, is hatched. When an oppor- tunity of fixing itself on the gills of a fresh-water fish occurs, the young animal loses its cilia and becomes a Diporpa, which possesses, besides the characteristic apparatus for attachment, the alimentary canal, and the two excretory canals with their openings at the anterior part of the body (at the level of the pharynx), and sucks the branchial blood. The junction of the two Diporpa soon follows ; and this does not take place, as was formerly believed, by the fusion of the two ventral suckers, but in such a manner that the ventral sucker of each animal affixes itself to the dorsal papilla of the other, and fuses with it (fig. 260. V). D. parailo.ritm v. Nordm.. on the gills of many fresh- water fish. Fam. Gyrodactylidae. Very small Tre- matodes with large terminal caudal disc and powerful hooks. They are viviparous, producing a single young one (first gene- ration) at a time, within which, while still in the body of the parent, another young one (second generation) may be present, and in this yet another (third generation). V. Siebold believed that he had observed a young animal developing from a germ cell of Gyrodtu-t i/ln.*. and that this became pregnant during its development. He regarded the (T //>/>- dactyliis as an asexual form, since he failed to find organs for the production of sperm. G. Wagener, however, showed that the reproduction is sexual, and conceived the idea that the germs from which the second and third generations are formed are derived from the remains of the fertilized ovum from which the first generation is formed. Metschnikoff, too, is of the opinion that the individuals of the first and second generations are formed at the same time from a common mass of similar embryonic cells. Gyrodnctyltix v. Xordm., ff. clegans v. Xordm., from the gills of Gyprinoids and fresh-water fish. FIG. 26:.'. Tienia Kii/jinntn (mediocanellata), natural size (after R. Leuckart). 326 PLATYHELMINTHES. Order 3. CESTODA.* Elongated and usually segmented Platyhelminthes -without mouth or alimentary canal, with organs for attachment at the anterior extremity. The tape-worms, which may easily be recognised by their band- shaped usually segmented bodies, are parasitic in the alimentary canal of Vertebrata, and were formerly taken for single animals. Steenstrupp was the first to introduce a different view, according to which the tape-worm is a colonial animal, a chain of single animals, each segment or proglottis being an individual. There are, however, Gestoda, like Caryophyttceus, which are destitute both of external segmentation and of segmentation of the gene- rative organs : while in other cases the segments of the body are clearly differentiated, and each is provided with a set of genera- tive organs, but they do not attain individual independence. The proglottides, however, usually become separated oft', and in some cases (Echineibothrium) after their separation from the body of the tape-worm continue to live for a long time independently, and even increase considerably in size ; so that although the individuality of the tape-worm may be justly insisted on, yet the subordinate and morphologically more restricted degree of individuality of the proglottis must also be admitted. This is the only satisfactory mode of regarding the Cestoda ; especially as the entire tape-worm, and not the proglottis alone, corresponds to the Trematode, and is to be derived from the latter by a simplification of organization and loss of the alimentary canal. The anterior part of the tape-worm is narrow, and presents a terminal swelling by which it attaches itself. This anterior swollen part is distinguished as the head of the tape-worm, but it is only its external form which entitles it to this name. In Caryophyllceus * Besides the older works and papers of Pallas, Zeder, Brernser. Rudolphi, Diesing, and others, compare van Beneden, ' Les vers cestoi'des on acotyles," Brussels, 1850. Kuchemueister, " Ueber Cestoden im Allgemeineu uiul die des Menschen insbesondere," Dresden, 1853. V. Siebold. " Ueber die Band- und Blasen-wiirnier," Leipzig. 1854. G. Wagoner, " Die Entwicke- lung, der Cestoden," Nov. Act. Leojt.-t'tir., Tom XXIV., Suppl., 1854. G. Wagener, " Beitrag zur Entwickelungsgeschichte der Eingeweidewiiimer," Haarlem. 1857. R. Leuekart, " Die Blasenbandwiinner und ihre Entwicke- lung," Giessen, 185(1. R. Leuekart, ''Die menschlicbeii Parasiteu," Bd. I.. Leipzig, 1862. F. Sommer and L. Landois, '' Ueber den Bau der geschlechts- rcifeu Glieder von Bothriocephalus latus." Zritxelir. f. ir-isx. Zool., 1872. F. Sonmier, ' Ueber den Bau und die Entwickelungsgeschichte der Geschlechts- organe von Taenia mediocanellata und Taenia solium," Ibid., Tom XXIV., 1874. CESTODA. 327 the head armature is very weak, and consists of a lobed fringed expansion. The apex of the head often ends in a conical projection, the rostelluni, which is armed with a double circle of hooks, while the lateral surfaces of the head are furnished with four suckers (Tn'iiia, fig. 263). In other cases only two suckers are present (Bothriocephalus) ; or we find suckers of more complicated structure and beset with hooks (Acanthobot/trium), or four protrusible probosces beset with recurved hooks (Tetrarhyncus) ; while in other genera the head armature presents various special forms. That portion of the animal which follows the head and is dis- tinguished as the neck shows, as a rule, the first traces of com- mencing segmentation. The rings, which are at first faintly marked and very narrow, become more and more distinct and gradually larger the further they are removed from the head. At the pos- terior extremity the segments or pro- glottides are largest, and have the power of becoming detached. After separation they live independently for a long time, and sometimes even in the same medium. The simplicity of the internal or- ganization corresponds with the simple appearance of the external structure. Beneath the delicate external cuticle is a matrix consisting of small cells, in which are scattered glandular cells. Beneath the matrix there is a delicate superficial layer of longitudinal mus- cular fibres, and next a parenchyma of connective tissue, in which strongly-developed bundles of longitudinal muscular fibres, as well as an inner layer of circular muscles, are embedded ; both these muscular layers are traversed, principally at the sides of the body, by groups of clorso-ventral muscular fibres. The power which the proglottis possesses of altering its form is due to the interaction of all these muscles. By means of them it is able to shorten itself considerably, at the same time becoming much broader and thicker, or to elongate to double its normal length, becoming much thinner. In the connective tissue parenchyma of the body, not only the muscles, but all the other organs are embedded. In its peripheral portion, especially in the neigh- bourhood of the head, we find small densely packed calcareous concre- ments, which are generally regarded as calcified connective tissue cells. FIG. 263. Head of Tania soliitm, viewed from the front (apical surface), with rostellum and double circle of hooks. The four suckers are visible. 328 PLA.TYHELMINTHES. The nervous system consists of two lateral longitudinal cords passing externally to the main trunks of the excretory system. They are somewhat swollen in the head, where they are connected by a trans- verse commissure ; these anterior swellings and the commissure may represent a cephalic ganglion. Distinct sense organs are wanting, but the tactile sense may be ascribed to the skin, especially to that of the head and the suckers. An alimentary canal is also wanting. The nutritive fluid, already prepared for absorption, passes endosmotically through the body wall into the parenchyma. The excretory apparatus, on the contrary, attains a considerable development as a system of much ramified canals which are dis- tributed throughout the whole body.* It consists primarily of two longitudinal canals (a dorsal and a ventral), running along each side of the body and connected in the head and in each segment by transverse trunks. According to the state of contraction of the muscular system, these longitudinal trunks and cross branches appear sometimes straight and sometimes bent in a wavy or zigzag manner : their breadth also presents consider- able variation, so that the power of contraction has been ascribed to their walls. The longitudinal trunks only serve as the efferent ducts of a system of very fine vessels which ramify throughout the whole paren- chyma and receive numerous long- tubes : the latter begin in the parenchyma with closed funnels, which contain a vibratile ciliated lappet (fig. 264). In many cases, as in the Ligulidce and Caryo- pliyttims, these longitudinal trunks are broken up into numerous longitudinal vessels, which are connected by transverse anastomoses. In other cases, on the other hand, the two ventral vessels are enlarged at the cost of the two dorsal, which may entirely atrophy. The external opening of the excretory system is, as a rule, placed at the * Compare Tb. Pintner. ' Untersuchungen liber deu Ban des Bandwurm- korpers," Wien, 1880. PIG. 264. A portion of the excretory system of Caryophylleeus mntabilis (after Pintner). TFJ, Ciliated funnels with the nucleus of the cell belonging' to them. OESTODA. 329 posterior end of the body, i.e., at the hind end of the last segment, in which a small vesicle with an external opening receives the longi- tudinal trunks. According to the observations of Leuckart on Tcenia cucumerina, the posterior transverse canals in the segments immediately preceding the last become, by their gradual shortening and the approach of the longitudinal trunks, transformed into the vesicle, which acquires an external opening when the segment behind it is detached. In rare cases the excretory system, possesses additional openings in the anterior part of the body behind the suckers. The generative apparatus is also divided into segments which correspond to the proglottides. Each proglottis possesses its own FIR. 2G5. Proglottis of Tteniit mediaeanellata, with, male and female organs (after Sommer). OL-, ovary ; DS, yolk gland (vitellarium) ; Sd, sliell gland ; Ut, uterus ; T, testes ; T'o!, vas deferens ; Cl, pouch of the cirrus ; S, generative cloaca ; Va, vagina. male and female generative organs, and can therefore, when separated, be considered as a sexual individual of a lower order. The male apparatus consists of numerous pear-shaped vesicles, the testes (fig. 265, T), which are situated upon the dorsal side, and their vasa efferentia open into a common efferent duct (vas deferens). The coiled end of this duct lies in a muscular pouch (cirrus sheath), whence it can be protruded through the genital opening as the so-called cirrus. This cirrus is frequently beset with spines which are directed back- wards, and serves as a copulatory organ. The female generative organs consist of oiwy, yolk gland, shell gland, iderus, receptaciilum, and vagina. The vagina and vas deferens usually open into a common 330 PLATTIIELMINTHES. FIG. 266. Ripe proglottides ready to separate. n, of Tffnia folium ; b, of Tcenia mediocanellata ; We, watervasciilar (excretory) canal. genital cloaca, which lies either on the ventral surface of the segment (Bothriocepalits), or on the lateral margin (T(enia) (tig. 265). In the last case it is placed alter- b nately on the right and on the left side. Nevertheless it may happen that the two genital openings are widely separate, the male opening being placed at the side, the female on the surface of the segment. As the segments increase in size and become further removed from the head, the contained generative organs gradually reach maturity in such a way that the male generative organs arrive at maturity rather earlier than the female. As soon as the male elements are mature, copulation takes place, and the receptaculum seminis is filled with sperm, and then only do the female generative organs reach maturity. The ova are fertilized and pass into the uterus, which then assumes its characteristic form and size. As the uterus becomes distended, the testes and then the ovaries and vitellaria are more or less completely absorbed (fig. 266). The posterior proglot- tides, viz., those which are ready for separation, have alone under- gone full development, and the eggs in their uterus often contain completely developed embryos. Accordingly we can recognize in a continuous series of the seg- ments the course of development passed through by the sexual organs and products in their origin and gradual progress towards maturity. The number of segments between that with the first trace of the generative organs FiG.F[267. Egg with embryo (a) of Tcenia toliuni; (b) of Microtcen itt ; (c), of Sothrio- eephalim lufiis (after R. Leuckart). CESTODA. 331 and the first proglottis with fully developed organs gives us an expression for the number of stages through which each segment has to pass. The tape-worms are oviparous; either the embryo develops within the egg-shell in the body of the mother, or the development takes place outside the proglottis, for example in water (Bothriocephalus). The eggs of the Cestoda are round or oval in shape and of small size. Their envelope is either simple or composed of numerous thin membranes, or else forms a thick and strong capsule, which in Tccnia is formed of densely packed rods united by a connecting substance, and presents in consequence a granular appearance. In many cases the development of the embryo coincides with that of the egg- r-.he]l, so that the egg at the moment that it is laid contains a a FIG. 268. Stages in the development of Tcenia solium to the Ci/xticercus stage (partly after R. Leuckart). a, Egg with embryo. I, Tree embryo, c, Rudiment of the head as a hollow papilla on the wall of the vesicle, d, Bladder-worm with retracted head, e, The same with protruded head, magnified about four tunes. complete embryo with six, or more rarely, four hooks. In BotJirio- cephalus the development takes place outside the proglottis during the long period that the egg passes in water, and the embryo leaves the egg as a ciliated larva (fig. 267, c). The development of the embryo into the tape-worm probably never takes place directly in the same medium in the intestine of the original host. As a rule there is a complicated metamorphosis, which is sometimes (Echinococcus, Cwnurus) connected with alternation of generations ; the successive stages live in different localities, and usually find the conditions necessary to their development in different species of animals, between which they migrate, partly actively and partly passively. The eggs usually leave the intestine of the host with the proglottis, and are deposited on dunghills, on plants, or in the 332 PLATYHELMINTHES. water, and thence pass in the food into the stomach usually of herbivorous or omnivorous animals. As soon as the egg membranes are digested or burst by the action of the juices of the stomach of the new host, the embryos which have been thus set free bore their way into the gastric or intestinal vessels by means of their six (rarely four) hooks, the points of which can be approached and removed from one another over the periphery of the small globular embryonic body. When they are once within the vascular system, FIG. 269. '(, Brood-capsule of Eckinococcus with developing heads (after R. Leuckart). b, Brood-capsule of Echinococcus (after G. Wageuer). c, Heads of Ecltinococcus still connected with the wall of the brood-capsule one is evaginated ; TV, excretory canals. they are no doubt carried along passively by the current of blood, and transported by a longer or shorter route into the capillaries of the different organs, as the liver, lungs, muscles, brain, etc. After losing their hooks, they usually become enveloped by a cyst of connective tissue, and grow into large vesicles with liquid contents and a contractile wall (fig. 2(38). The vesicle gradually becomes a cystic or Madder worm by the formation of one (Cysticercus*) or * Exceptionally two or more heads are found in some Cysticercus forms. CESTODA. 333 several (Ccenurus) hollow buds, which are developed from the walls and project into the interior of the vesicle (fig. 268, c). The armature of the tape-worm head (suckers and double circle of hooks) is- formed on the inside and at the bottom of this invaginatioii of the wall of the vesicle (fig. 268, d). When these hollow buds ;ire evaginated so as to form external appendages of the vesicle, they present the form and armature of the Cestode head, as well as a more or less developed neck, which presents even at this stage traces of segments (fig. 268, e). In some cases (Echinococcus) the irregularly shaped maternal vesicle produces from its internal walls one or two generations* of secondary vesicles which project into it; and the Cestode heads originate in special small brood-capsules on these secondary vesicles (fig. 269, a). In such cases the number of tape-worms which arise from one embryo is naturally enormous, and the parent vesicle may reach a very considerable size, being some- times as large as a man's head. In consequence of this enormous growth the vesicles frequently obtain an irregular shape ; while on the other hand, the tape- worms which are developed from them remain very small, and carry, as a rule, only one ripe proglottis (fig. 270). So long as the tape-worm head (scolex) remains attached to the body of the bladder-worm and in the host of the latter, it never develops into a sexually mature tape-worm ; although in many cases it grows to a considerable length (Cysticercus fasciolaris of the house-mouse). The bladder-worm must enter the alimentary canal of another animal before the head (scolex) can, after separation from the body of the bladder-worm, develop into the sexually mature tape- worm. This transportation is effected passively, the new host eating the flesh or organs of the animal infected with Cysticerci. The tape- worms, therefore, are principally found in the Garnivora, the Insecti- vora, and the Omnivora, which receive the bladder-worms in the flesh of the animals on which they feed. The vesicles are digested in the stomach, and the cestode head becomes free as a scolex. The latter is protected from the too intense action of the gastric juice by its calcareous concretions, and at once enters the small intestine, fastens FIG. 270. Ttsnin Echin oc occus (after R. Leuc- kart), magni- fied 12 to 15 times. * In Cysticerci (0. 1onr/ir ). The young stages are Cysticerci or Cysticercoids, rarely quite without caudal vesicle ; parasitic in warm and cold-blooded animals. Tainiii L. (Cyxtutd'nia R. Lkt). Development takes place with large vesicles. The heads arise from the embryonic vesicle itself. T. suit it in. L. 2 3 metres long. The double circle of hooks is composed of 26 hooks. The ripe proglottides are 8 10 mm. long and 6 7 mm. broad : the uterus has 7 10 dendritic branches. It lives in the human intestine. The Bladder-worms belonging to it (Ci/stirm-int <-<>11i/lo*fr) live principally in the dermal cellular tissue and in the muscles of pigs, but also in the human body (muscles, eyes, brain), in which self-infection with them is possible if a Tfenia is present in the digestive canal ; more rarely in the muscles of the Roe-deer, the Dog. and the Cat. In the human brain the Cyxtirrrcux acquires an elongated form, and sometimes does not produce a head. T. suginatti Goez&=mediocanellata Kiiohenm.. in the intestine of Man. distin- guished by the older helminthologists as a variety of T. soli inn. Head without circle of hooks or rostellura. but with four more powerful suckers. The Tape- worm reaches a length of four metres, and becomes much stronger and thicker. The mature proglottides are about 18 mm. long and 7 1) mm. broad. The uterus forms 20 35 dichotomous side branches. The Cyxticn-ciix lives in the muscles of the ox (fig. 273). It appears to be principally distributed in the warmer parts of the Old World, but is often found in great numbers in many places in the north. T. sen-tit a Goeze. in the intestinal canal of the d< >g. The Cysticercus is known as Oi/sticercvs pisciformis in the liver of the Hare and Rabbit. T. rw.v.v/V///.v Rud. in the Cat. with Oi/sticercws fasewla/ris of the common mouse. T. inari/iituta Batsch. of the Dog (butcher's dog) and Wolf with Oust icerr.it a tenii-icol- FlG - 273. Cy*/jre of ,. ,. . j T>- i 11 mediocanellata. magnified hs from Ruminants and Pigs, and occasionally in about eight til ' nes . Th ehead Man (Cyst, vigceralis). T, crassiceps Rud. in the is protruded. Fox with Cystirt't-cHx lont/icoHix from the thoracic cavity of the Fieldmouse. T. ccenurus v. Sieb. in the intestine of the sheep-dog, with CCCHIII-KX rereln-tills in the brain of one year old sheep. The presence of Cocnvrux in other places has been stated, as for instance in the body cavity of the Rabbit. T. tenuicollix Rud. in the intestine of the Weasel and the Pole-cat, with a f'l/xtlt-rrcnx which, according to Kiichenmeister, lives in the hepatic duets of the Field-mouse. Echinococcifer Weinl. The heads bud on special brood-capsules, in such a way that their iiivagination is turned towards the lumen of the vesicle (fig. 2fi ( .t). T. ecliinofocciix v. Sieb. (fig. 270) in the intestine of the dog. 34 mm. long, forming but few proglottides. The hooks on the head are numerous but small. Its Bladder-worm is distinguished by the great thickness of the stratified cuticula. It lives as Efltinucoecus principally in the liver and the lungs of Man (. liominix) and of domestic animals (-E. rettrinoniin). The first form is also distinguished as E. altru-lparivna on account of the frequent production of primary and secondary vesicles ; it usually reaches a very considerable size and 336 YERMES. has a very irregular shape ; while that form which inhabits domestic animals, E. soolicipariens, more frequently retains the form of the simple vesicle. Finally these echinococcus cysts frequently remain sterile, in which case they are called Acephaloct/sts. Another and indeed pathological form is the so- called multilocular Ecldnococcus. which was for a long time taken for a colloid cancer. It is also found in Mammalia (in cattle), and here presents a confusing re- semblance to a mass of tubercles. The echinococcus disease (Jiythttid plague) was widely spread in Iceland. This disease likewise seems endemic in many places in Australia. T. (Microt&nia). The Cyst icercold form is small, and has but little fluid in the small portion which corresponds to the vesicle. The head is small, but has a small club- shaped or proboscis-like rostellum, and is furnished with weak hooks. The eggs are provided with several membranes. The embryo is usually furnished with large hooks. The Cysticercoid stages live prin- cipally in Invertebrates (in Slugs, Insects, etc.), and more rarely in cold-blooded Vertebrates (the Tench). T. cuciont'ritiu Bloch, in the intestine of dogs (house dogs). The Cysticercoid is entirely without the caudal vesicle, and lives (according to Melnikoff and R. Leuckart) in the body cavity of the Dog-louse [Tricliodcctcs cams). The infection with the Cysticercoids takes place when the dog swallows the parasites which are annoying him. while the para- sites swallow the eggs contained in fasces adherent to the hair of the dog. Nearly allied is T. cUiptica Batsch. in the intestine of the Cat, occasionally in that of Man. T. iiana Bilh. v. Sieb. in the intestine of the Abyssinians, hardly an inch long. T '. flavopunctata Weinl. in the human intestine (North America). The Cysb'cercoids of the Meal-worm are probably developed into tape-worms in the intestines of Mice and Eats. In other partially unarmed Tffnin>t the FIG. 274 a.-Boariocephalua latus (after g enerative organs and development are as yet not accurately known ; such are T. pcrfollata Goeze, and T.plicata Eud. in the horse ; T. pectinata Goeze, in the hare ; T. dispar Rud. in the frog ; T. e,rpansa Im. in the ox. Fam. Bothriocephalidae. With only two suckers, which are weak and flat. The generative organs, as a rule, open upon the surface of the proglottis. The proglottides do not become detached singly. Hydatid stage represented by an encysted Scolex. OESTODA. 337 BotTvriocephalus Brems. Segmented body. Head with two pits, without hooks. The genital openings are on the middle of the ventral surface. The young stage usually in fishes. Ji. latux Brems.. the largest of the tape-worms parasitic in man. twenty-four to thirty feet in length, principally found in Kussia, Poland, Switzerland, and South France. The sexually mature segments are broader than they are long (about 10 12 mm. broad and 3 5 mm. long). They do not become detached singly, but in groups (fig. 27-t). The segments of the hindermost portion of the body are, how- ever, narrower and longer. The head is club- shaped, and is provided with two slit-like pits. The cortical parts of the lateral regions of the body contain a number of round masses of granules, the yolJi-glundx (fig. 275, Daf), the contents of which are poured into the shell glands (coiled glands) through the so-called yellow ducts. The genital openings lie close together, one behind the other, in the midst of the segment (fig. 275. ). The anterior and larger belongs to the male generative apparatus, and leads into the muscular terminal portion of the vas deferens. which is enclosed in the cirrus sheath and can be eva- ginated as the cirrus (fig. 275. ('!>'). The vas deferens just before its entrance into the cirrus pouch is dilated (fig. 275 b) to form a large muscular swelling (the vesicula seminalis .') It then becomes coiled, and passes in the direction FIG. 274 &. Larva of a Bothrio- cephalus from the Smelt (after R. Leuckart). Fia. 275. Generative organs of a sexually mature pvoglottis of Bothriocephalus 7u's (after Sommer and R. Leuckarl) ; a, from the ventral surface, 6, from the dorsal surface. Ov and v, ovary ; Ut, uterus ; fid, shell gland ; Dst, vitellarium (yolk gland) ; !'ilf. 346 NEMATHELMINTHJSS. In almost every case, with the exception of Gordius, two lateral regions remain free from muscle and form the so-called lateral lines or regions, which may equal in breadth the neighbouring muscular regions. These lateral regions are formed of a finely granular nucleated substance, and enclose a clear vessel containing granules. This vessel is connected with that of the opposite side in the anterior part of the body, and the two open by a common transverse slit, the vascular pore, on the ventral surface in the median line. The lateral lines have the value, both as regards position and structure, of excretory organs. Median lines (dorsal and ventral), accessory median lines (sub-median lines), the latter being placed between the principal median line and the lateral line, are also to be dis- tinguished. The so-called ventral cord of Gordius, which may be compared to the median line and has perhaps the significance of an elastic rod, is very large. Cutaneous glands, in the form of unicel- lular glands, have been observed principally in the region of the oesophagus and in the tail. The nervous system, owing to the difficulty which its investigation offers, has only been satisfactorily recognised in a few forms. It con- sists of a nerve ring surrounding the oesophagus, and sending off posteriorly two and anteriorly six nerve trunks (Ascaris megalo- cephala). The posterior trunks run in the dorsal and ventral lines (N. dorsalis, ventralis), to the extremity of the tail ; while of the six anterior nerves, two run in the lateral lines (JV. latcrales), four in the interspaces between the lateral and median lines (N. sub- mediani), and supply the papillae around the mouth. The ganglion cells lie partly near, in front of and behind the nerve ring, partly on the fibrous cords themselves, and are arranged in groups which can be distinguished as ventral, dorsal, and lateral ganglia. There are in addition groups of ganglion cells in the median lines and in the lateral lines in the caudal region. As sense organs we must mention the eyes found in the free- living Nematoda, and the papillae and tactile hairs found principally in the neighbourhood of the mouth. Each papilla is supplied by one nerve fibre, which is swollen to a knob and forms the axis of the papilla. [The Nernatoda possess a body cavity, but are without any trace of a vas- cular system.] Generative organs. The Nematodes are dioecious (with ex- ception of the hermaphodrite Pelodytes, and of the Eliabdonema NEMATODA. 347 (Ascaris) nigrovenosuin, which produces first spermatozoa and later ova). The males are characterised by their smaller size, and by the posterior end of the body being generally curved. Both kinds of generative organs consist of single or paired and often much coiled tubes, at the upper end of which the generative products are de- veloped, the lower ends representing the efferent ducts and recep- tacula of the generative products. The usually paired ovarian tubes, at the upper ends of which the ova arise, terminate in a short vagina, which opens on the ventral surface, rarely near the posterior end of the body. The male generative apparatus, which contains hat-shaped spermatozoa, is almost invariably represented by an unpaired tube, and usually opens on the ventral surface near the posterior end of the body in a common opening with the intestine. As a rule, the common cloaca! portion contains two pointed chitinous rods, the so-called spicula, in a pouch -like invaginatkm. These spicula can be protruded and retracted by a special muscular ap- paratus, and serve to fasten the male body to the female during copulation. In many cases (Strong //lido-) an umbrella -like bursa is added, or the terminal portion of the cloaca can. be protruded like a penis (Trichina) ; in this case the cloacal aperture lies almost at the extreme end but is still ventral (Acrop/talli). In the male papillfe are almost always present in the region of the posterior end of the body, and their number and arrangement afford important specific characters. Development. The Nematoda for the most part lays eggs ; it is only in rare cases that they bear living young. The eggs usually possess a hard shell and may be laid at different stages of the embryonic development or before it has begun. In the viviparous Nematodes the eggs lose their delicate membranes in the uterus of the mother (Trichina, Filaria). Fertilization takes place by the entry of a spermatozoon into the ovum, which is still without a mem- brane. The segmentation is equal, and leads to the formation of a kind of invaginate gastrula. From the two cell layers are de- veloped the body wall and the alimentary canal. The embryo gradually assumes an elongated cylindrical form, and comes to lie rolled up in several coils within the shell. The excretory pore and the rudiments of generative organs, as well as a nerve ring, are present in the embryo, which is also provided with mouth and anus. The free development is a metamorphosis, usually com- plicated by the circumstance that it is not undergone in the habitat of the mother. The young stages or larva 1 , probably of most Nema- 348 XEMATHELMIXTHES. todes, have a different habitat to that of the sexual animal; the young and the adult Nematode being contained in different organs of the same or even of different animals. The larvae live for the most part in parenchymatous organs, either free or encysted in a connective tissue capsule ; the adults, on the contrary, live principally in the alimentary canal. The embryo is almost invariably characterised by the special form of the oral and caudal extremities, but sometimes also by the posses- sion of a boring tooth, or of a circle of spines (Gordius). Sooner or later the skin is shed, and the animal enters its second stage, which may often still be considered as a larval stage; repeated ecdyses precede the sexually adult stage. The post-embryonic development of the Nematodes presents numerous modifications. In the simplest cases the embryo, while still enveloped in the egg mem- branes, is transported passively in the food (Oxyuris vermicularis and Trichocepkalus). In many Ascaridce to judge by the species parasitic in the Cat the em- bryos, which are provided with a boring tooth, first make their way into an intermediate host, by which they are transported into the intestine of the second host with the food or water. More frequently the young forms encyst within the intermediate host, and, enclosed in the cyst, are transferred into the stomach and intestine of the permanent host (fig. 281). For example, the embryos of Spiroptera oUusa of the Mouse, while still in the egg membranes, are taken with the food by the Meal-worm, in the body cavity of which they encyst. In the viviparous Trichina spiral is there is a modification of this mode of development inasmuch as the migration of the embryos and their development to the encysted form found in the muscles (muscle-trichina) take place in the same animal which contains the sexually mature intestinal Trichinas. The development of the Nematode larvse often makes a considerable advance within the intermediate host into which they have migrated. Thus, for instance, in Cucullanus eleyans, the embryos migrate into the Cyclops, and in the body cavity of these small Crustacea undergo two ecdyses and essential alterations of form, obtaining at this early FIG. 281. Sclerostomum tetracanfhitm, en cysted (after R. Leuckart). NEMATODA. 349 stage the characteristic oral capsule of the sexually adult stage, to which they only develop in the intestine of the Perch. According to Fedschenko,* a similar mode of development occurs in Filar ia medinensis. The embryos pass = into puddles of water, and migrate thence into the body cavity of the Cydopidw ; and after casting their skin assume a form which, except for the absence of the oral capsule, resembles that of the larva of Cucullanus. After the expiration of two weeks there is another ecdysis, with which is connected the loss of the long tail. The later history is unknown. It has not yet a FIG. 282.- a, Rhabdonema (Ascaris) nigrovenosum of a/bout 3'5 mm. in length in the stage of maturity of the male products ; G, genital glands ; 0, mouth; D, intestine ; A, anus ; JV, nerve-ring; Drz, glandular cells; Z, isolated spermatozoa, b, Male and female HhubJifis forms from about 1'5 mm. to 2 mm. long ; Oa, ovary ; T, testis ; V, female genital opening ; Sp, spicula. been discovered whether the migration of the Filarian larva into the permanent host (Man, see p. 356) takes place with the body of the Cyclops, or independently after copulating in the free state. The embryos of some Nematoda develop in damp muddy earth, after casting their skin, to small so-called Rhabditis forms with a double * Compare Fedschenko, " Ueber den Ban und Entwicklung xfici, etc. ; some live in decaying vegetable matter, e.g., the vinegar worm in fermenting vinegar and paste. Nevertheless very similar forms occur in the contents of the intestine and in the faeces of different animals and of man (.1. intestinalis, stercoralis). The power possessed by small Nematoda of resisting the effects of pro- longed desiccation and of coming to life again on being moistened is very remarkable. Fain. AscaridsB. Body tolerably stout. With three lips furnished with papillre. One of these lips is directed towards the dorsal surface, while the two others meet together in the ventral line. The posterior end of the, male is ventrally curved, and usually furnished with two horny spicula. FIG. 283. Axcnris lumlrieoides (after R. Leuckart). a, Posterior end of a male with the two spicula (Sp). b, Anterior end from the dorsal side, with the dorsal lip furnished with two pnpilhv. c, The same from the ventral side with the two lateral ventral lips and the excretory pore (P). d, Egg with the external membrane formed of small clear spherules. Ascurix L. Polymyarian, with three strongly developed lips, the edges of which are in the larger species provided with teeth. The pharynx is not sepa- rated as a distinct bulb. The caudal extremity is usually short and conical, and in the male sex invariably provided with two spicula (fig. 283, a). A. lumbricmdes Cloquet, the human round worm, a smaller variety in the pig (A. xnilla Duj.) The eggs pass into water or damp earth and remain there some months, until the embryonic development is completed ; they are probably carried into the alimentary canal of their later host by means of an inter- mediate host. A. mcgal(>cej>h7(i Cloquet (horse and ox); -A. mijstn-i- Zed. (cat and dog), sometimes parasitic in man. O.ri/urix Rud. Meromyarian ; usually with three lips, which bear small papillae. The posterior end of the oesophagus is enlarged to a spherical bulb provided with a masticatory apparatus. The posterior end of the body of the female is thin and pointed, while that of the male has only two praeanal and few postanal papillae, and a single spiculum (fig. 279). O vcriinculi/rix L., in the large intestine of man, distributed in all countries. The female is about ten mm. long. 0. curvula Rud., in the c;ecum of the Horse. 352 NEMATHELMINTHES. Fam. Strongylidae. The male genital opening is placed at the hinder end of the body, at the bottom of an umbrella- or bell-shaped bursa. the margin of which is furnished with a varying number of papillae. Eustrongylus Dies. With six projecting oral papillae, and a row of papillae on either lateral line. The bursa is bell-shaped and completely closed, with regular muscular walls and numerous marginal papillae. There is only one spiculum. The female genital opening is far forward. The larvae live encysted in fishes. (Filaria cystica from Symbra/nchus). E. gigax Rud., the body of the female is three feet in length, and only twelve mm. thick. It lives singly in the pelvis of the kidney of the Seal and Otter, and very rarely in Man. Strongylus Rud. With six oral papillae and small mouth. Two conical cervical papillae upon the lateral lines. The pos- terior end of the male has an umbrella-like incom- pletely closed bursa. Two equal spicula, usually with unpaired supporting organ. The female sexual opening is sometimes approached to the posterior end of the body. They live for the most part in the lungs and bronchial tubes. St. longevaginatus Dies. Body 26 mm. long, 5 to 7 mm. thick. The female sexual opening lies directly in front of the anus, and leads into a simple ovarian tube. Only once found in the lung of a six-year old boy, in Klausen- burg. St. parado-ms Mehlis, in the bronchial tubes of the pig. St.Jilarifi Rud., in the bronchial tubes of the sheep. St. fonnnutntus Dies., in the trachea and bronchial tubes of the hare and rabbit. Xt. aiiricidarix Rud., in the small intestine of BatracJiia. Dtichmius Duj. With wide mouth and horny oral capsule, the edge of which is strongly toothed. Two ventrally placed teeth project at the bottom of the oral capsule, while on the dorsal wall a conical spine projects obliquely forwards. D. duoclcnalix Dub. (Ancylostomuit duodenale Dub.), 10 to 18 mm. long, in the small intestine of Man, discovered in Italy ; very widely distributed in the countries of the Nile (Bilharz and Griesiuger). By aid of its strongly armed mouth it wounds the intestinal mucous mem- brane, and sucks the blood from the vessels. The frequent haemorrhages occasioned by these Dochmia are the cause of the illness known by the name of Egyptian chlorosis (fig. 284). It has lately been established that this worm occurs in Brazil, and that, like D. trigonoceplialus, it develops in puddles of water (Wucherer). D. trujonocephaliis Rud.. in the Dog. Selerostom/um Rud. With characters of Dochmius, but with a different oral capsule, into which two long glanular sacs open. ,S'c. eqitinvni. Duj. = a-nmttum Dies. In the intestine and the mesenteric arteries of the horse. Bellinger* has shown that the phenomena of colic in the horse may be referred to embolic processes proceeding from aneurism of the intestinal artery. Each aneurism contains about nine worms. * Bellinger. " Die Kolik der Pferde und das Wurmaneurysrua der Einge- weidearterien," Miinchen, 1870. FIG. 28-1. Douchmius dodenulis (after R. Leuckart). a, male; O, mouth ; B, bursa. b, Female ; 0, mouth; A, anus; T', vulna. XEMA.TODA. 353 C Sc. tt'fntnixthiim. Mehlis. also in the intestine of the horse. The embryos, after migrating into the intestine, become encysted in the walls of the rectum and caicum, assume within the cyst their definite form, break out from the cyst, and escape again into the intestine, {.'ui-iillinnix rli-yiuix Zed., in the Perch. Fam. Trichotrachelidae. with long neck-like thin anterior portion of the body. Mouth small, without papilhe. (Esophagus very long, traversing a peculiar cord of cells. Tri<-hort'2)haliix Goeze. Anterior part (fig. 285) of the body elongated and whip-shaped: posterior part cylindrical and sharply distinct, enclosing the generative organs, in the male it is coiled up. Lateral lines absent. Main median lines present. The penis is slender and furnished with a sheath, which is turned inside out when the former is protruded. The hard-shelled, citron-shaped eggs undergo the first part of their development in water. Tr. ilixpn.r Rud. In the human colon : these worms do not live free in the intestine, but bury their filiform anterior extremity in the mucous membrane (fig. 285). The eggs pass out of the host with the fasces, as yet without a sign of beginning development, which only takes place after a prolonged sojourn in the water or in a damp place. According to the ex- periments of Leuckart per- formed with Tr. qfKnis of the ^lieep and Tr. crcnatus of the pig, embryos with the egg membranes, if introduced into the intestine, develop into the adult Trieoceplialus ; and we may therefore conclude that the human Tr. d'ixpar is intro- duced directly, and without an intermediate host either in the drinking water or in uncleaned food. The young Tr. rfisjwr is fit first hair-like, and re- sembles a Trichina, and only a FIG. 2S5.Ti-ichocefikaliu: disjjar (after R. Leuckart). 11, Egg ; I, female ; c, male with the anterior part ut' the body buried in the mucous membrane ; Sp, spiculum. gradually acquires the considerable thickness of the hind end of the body. Triolioxoiniun Rud. Body thin, hair-like, but the posterior end of the body in the female is swollen. Lateral lines and the principal median lines are present. The male caudal extremity has a cutaneous fold and a simple penis (spiculum) and sheath. Tr. murix Creplin.. in the large intestine of the house-mouse. Tr. crusxicatula Bellingh.. in the bladder of the rat. According to Leuckart, the dwarfed male lives in the uterus of the female. There are usually two or three, more rarely four or five males in a single female. There is also a second species of Trichosomum found in the bladder of the rat. Tr. Schmidt ii v. Linst.. the larger male of which was formerly taken for that of Tr. crassicauda. Trichina Owen.* Body thin, hair-like. Principal median lines and lateral * Compare the writings of R. Leuckart. Zenker. R. Virchow, Pagenstecher, etc. 23 354 NEMATHELMINTHES. lines are present. The female generative opening well forward. The posterior end of the body of the male has two terminal cones between which the cloaca is \ FIG. 296.- -Trichina, spiralis. a, Mature female Trichina from the alimentary canal; G, genital opening ; E, embryos ; Oo, ovary, b, Male ; T, testis. c, Embryo, rf, Embryo which has migrated into a muscle fibre, already considerably enlarged, e, The same developed into a coiled Muscle Trichina, and encysted. NEMATODA. projected. Tr. xpirulis Owen, in the alimentary canal of Man and numerous, principally carnivorous, Mammalia : hardly two lines in length. The viviparous females begin to bring forth embryos about eight days after their migration into the alimentary canal. These embryos traverse the intestinal walls and body cavity of the host, and migrate, partly by their own movements in the bundles of connective tissue, partly with the aid of the currents of blood into the striped muscles of the body. They pierce the sarcolemma and penetrate into the primitive bundles, the substance of which degenerates, the degeneration being accompanied by an active multiplication of the nuclei. In a space of fourteen days they develop, within a sac-like swelling of the muscle fibres, into spirally coiled worms, around which and within the sarcolemma and its connective tissue investment a clear citron-shaped capsule is excreted from the degenerated muscle substance.. The young Muscle-Trichina can remain liv- ing for years within this capsule, which at first very delicate, gra- dually becomes thickened and hard by the formation of other layers and by the gradual deposi- tion of calcareous matter. If the encysted animal is transferred into the intestine of some warm- blooded animal in the flesh of its first host, it is freed from its cyst by the action of the gastric juice, and the rudimentary generative organs, which are already toler- ably far developed, quickly attain maturity. In from three to four days after their introduction the asexual Muscle-Trichinas become sexual Trichinas. These copulate and produce a brood of embryos which migrate into the tissues of the host (one female may produce FIG. 287. Film-in meiiineiusis (after Bastian and as many as 1000 embryos) (fig. Leuckart >- " Anterior end seen from the oral aur- 286). face ; O, mouth ; P, papilla, (size reduced more than strongly magnified. 4, Pregnant female half), c, Embryos The house rat is especially to be mentioned as the natural host of the Trh-liiiia. This animal does not hesitate to eat the carcase of its own species, and so the Trichina infection is passed on from generation to generation, ('arrases in- fected with Trichinas are sometimes eaten by the omnivorous pig, in whose flesh the encysted Tricliinas are introduced into the intestine of man. and occasion the well-known disease. Trichinosis, which when the migration takes place in number, often has a fatal result. Fam. Filariidae. Body filiform, elongated, often with six oral papillaj, some- NEMATHELMINTHES. times with a horny oral capsule, with four praeanal pairs of papillae, to which an unpaired papilla may be added, with two unequal spicula or with simple spiculum. Filar la O. Fr. Mull. With small mouth and narrow oesophagus. This species, which is sometimes destitute of papillae, lives outside the viscera, usually in connective tissue, frequently beneath the skin (divided by Diesing into numerous genera). F. (Drat.-uncuJiix') nn-ill/iensis* Gmel. the Guinea worm, in the subcutaneous cellular tissue of Man in the Tropics of the Old World, reaches a length of two feet or more. The head is provided with two small and two larger papillae. The female is viviparous, and without sexual opening. The male form is unknown. The worm lives in the connective tissue between the muscles and beneath the skin, and after reaching sexual maturity, occasions the formation of an abscess, with the contents of which the embryos escape to the exterior (fig. 2S7). It has lately been proved (Fedschenko) that the embryos of Filaria, migrate into a Cyclops and there undergo an ecdysis. Whether they are then (in the body of the Cyclops) introduced into man in his drinking water, or whether they first escape and copulate in a free state, is not known. F. 'mini it ix lives in the right ventricle of the dog, and is very abundant in East Asia. It is viviparous. The embryos pass directly into the blood, where, however, they do not undergo their further development. Similar young Hasmatozoa are also found in the blood of man in the Tropics of the New and Old Worlds (F. sa-nf/t/i/i/x hoiiiiitix, F. BaiK-rofti}. Since these animals are also found in the urine, tiieir appear- ance seems to have an aetiological . connection with haematuria. In the East Indies, young Filaria also live in the blood of the street dog, and would seem to be related to the brood of Filaria sanguinalenta, since, according to Lewis, knotty swellings on the aorta and oesophagus are invariably found with these Filaria. F. papillom Hud. in the peritoneum of the horse. F. loa Guyot., in the conjuntiva of negroes on the Congo. F. labial is Pane. Only once observed at Naples. An immature Filaria, described as Filaria li'titia (oculi humani) has been found in the human capsula lentis. Fam. Mermithidae. Aproctous Nernatodes, with very long filiform body, and six oral papillae. The male caudal region is broad, and is provided with two spicula and three rows of numerous papilht-. They live in the body cavity of insects, and escape into the damp earth, where they attain sexual maturity and copulate. Mi-rmis nigrcscens Duj., was the occasion of the fable of the rain worm. J/. albicans v. Sieb. v. Siebold established by experiment the migration of the embryos into the caterpillars of Tinea rconymclla. Sphcerula/ria l/ombi Leon Duf. Fam. Gordiidse. Body elongated and filiform. Without oral papillae and lateral lines, with a ventral cord. The mouth and anterior region of the alimentary canal is obliterated in the adult state. The testes and ovaries are paired and open to the exterior with the anus near the hind end of the body. Uterus unpaired, with receptaculum seminis. The male caudal region is forked, and is destitute of spicula. In the young stage they live in the body cavity of predatory insects, and are provided with a mouth. At the pairing time they pass into the water, where they become sexually mature. The embryos, which are provided with a circle of spines, bore through the egg-membranes and migrate into Insect larvae (Chironomiis-larvcf, Epln-inerido'}. and there encyst. Water * Compare H. C. Bastian, "On the Structure and Nature of the Dracunculus," Trans. Linn. Xociety, vol. xxiv., 1863. Fedschenko 1. c. CHyETOGNATHA. 357 I in 'ties and other aquatic predatory insects eat with the flesh of the EphemeriA larvrr the encysted young forms, which then develop in the body cavity of their new and larger host to young (im-Jii/In'. Gonlhut uqt/atii-iix J>uj. F.-iin. Anguillulidae. * Free living Nematodes of small size. Caudal glands arc sometimes present. The lateral canals are often replaced by the M>-i-allrd ventral glands. Some species either live on or are parasitic in plants ; others live in fermenting or decaying matter. The greater number, however, live free in earth or water. Tyli'iiclius Bast. Buccal cavity small, and con- taining a small spine. The female genital opening lies far back. T. scHiulrns Schn.*=trifici Needham, in mildewed wheat grains. When the grains of wheat fall the dried embryos grow in the damp earth, bore through the softened membranes, and make their way 011 tu the growing wheat plant. Hei-e they remain some time, perhaps a whole winter without alteration, until the ears begin to be formed. They then pass into the latter, grow, and become sexually mature, while the ear is ripening. They copulate and deposit their eggs, from which the embryos creep out. and at length constitute the -sole coii- tents of the wheat grains. T. dijtxitci'K.u\m. in heads of thistles CCardius) T. Dni-e//t/ in/la fit Schn., in damp earth, 3 mm. long. The larva, which is without a mouth, and has two caudal bands, is found in Arion /'inpiricorviii. Anguillula aeeti = glutinis 0. Fr. Miill.. known as the vinegar worm and pasteworm, 1 to 2 mrn. long. Of the many marine Aiiguillvlid(B (Enoplidoe), we must mention Uori/- luhinix iiHi.i'iiini* Butschli, D. stiiliix tridentatus Duj. The abberant families DrxiiinxeoliTiilu- and Clicetosomidte are allied to the Nematoda. THE CH.ETOGNATHA. The G/uetognatha, f containing only the genus Xagitta, are allied to the Nematodes. They are elongated round worms, with a pecu- liarly armed mouth and laterally placed horizontal tins, the mem- branous edges of which are supported by rays. . The anterior portion of the body is sharply separated ofT as a head, and bears in * Davaine. " Eecherches sur rAnguillule du ble nielle," Paris. 1857. Kiihn, l - Ueber das Vorkommen von Auguilluleu in erkrankten Bllithenkopfen von Dipsacus fullonum.'' Zritsrhr.fiir irixx ZnoL. Tom IX.. 1859. Bastian, '-Mono- Graph of the Anguillulidfe or free Xematoids. marine, land, and fresh water.'' London, 1864. U. Biitschli. " Beitrligc 7.ur Kentniss der freilebenden Xema- tuden." -V<"-. Acfn, Turn XXXVI.. 1873. Lad. Ocrley, " Muiiugraphie der Aii-uillulidcn/' Buda-Pest.. 18SO. j Compare A. Krohn. " Anatomisch-physiologisclie Beobachtungen iiber die Saintta bipnnctata." Hamburg. 1844. It. Wilms. " De Sagitta mare germani- cum circ.'i insulam Helgoland incolente.'' Beroliui. 1846. Kowalevski, " Em- ' bryologische Studien an Wiirmern und Arthropodeii." Man. dc TAi-uii. St. Pt'tcrxhii/ii'ij. Turn XVI. 0. Hertwig, - Die Chsetognatha. eine Mono- " graphie." Jena. isso. 358 NEMATHELMIMTHES. the region of the mouth two lateral groups of hooks which function as jaws. The nervous system consists, according to Krohn, of a cerebral ganglion on which the eyes are situated, and a ven- tral ganglion placed in about the middle of the body length. There are in addition two ganglia near the mouth, which may be considered as the subcesophageal gan- glia, and are connected with each other and with the cephalic ganglion by ceso- phageal commissures. On- OdJi L FIG. '2S8.Sngitfa (Spudella) cephaloptt'i'n, magnified 30 times, viewed from the dorsal side (after O. Hertwig). F, posterior fin ; G, supra- oesophageal ganglion ; Te, ten- tacles ; R, olfactory organs ; Oi\ ovary ; Od, oviduct ; T, testis ; Vd, vas deferens ; Sb t vesicula seminalis. [The common view now is that the large ventral ganglion of the middle of the body, which is connected with the cerebral by com- missures, is homologous with the subcesophageal ganglia of other types.] The straight alimentary canal is at- tached to the body wall by a dorsal and ventral mesentery from the oesophagus backwards, and opens to the exterior at the base of the long tail, which terminates in a horizontal fin (fig. 288). [The body Cavity is well developed, and divided by the dorsal and ventral mesenteries into two parts, and again by two transverse verti- cal septa into a cephalic section, a section in the body, and finally a caudal section. Vas- cular and excretory organs are absent.] Reproduction. The Chcetognatha are hermaphrodite, and possess paired ovaries, which open by two apertures at the base of the tail and are connected with seminal pouches. The testes also are paired, and situated posteriorly to the ovaries in the tail; their products pass to the exterior by openings at the sides of the tail. Segmentation is complete, and leads to the formation of a blastosphere. One side of this becomes invaginated so that the segmentation cavity is obliterated and a gastrula is formed, in the entoderm CH.ETOGjSTATHA. 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter 1 tecomes folded in such a way that the archenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the body cavity, while that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite to that at which the blastopore, which is now closed, was situated. There is but one genus, Sagitta Slab., of which several species, e.g., tiagitta bipunctata Krohn, tf. germanica Lkt. Pag. from the Euro- pean seas have been more accurately described. Order 2. ACANTHOCEPHALA.* Elongated round worms with protrusible proboscis furnished with hooks; without mouth and alimentary canal. The saccular, often transversely wrinkled body begins with a proboscis, which is furnished with recurved hooks and can be retracted into a tube projecting into the body cavity (sheath of the proboscis) (fig. 289, R and Rs]. The posterior end of this sheath is fas- tened to the body wall by a ligament, and by retractor muscles. The nervous system (fig. 289, G) is placed at the base of the proboscis, and consists of a simple ganglion formed of large cells. Nerves are given off from the ganglion anteriorly to the proboscis, and through the lateral retractors (retinacula) to the body wall g_ (fig. 289, R). The latter supply partly the muscular system of the body, and partly the Rj genital apparatus, in which there are, princi- pally in the male animal, special nerve centres FIG. 289 Anterior part, . f ,. . , of an Echinorhtinchus. consisting of ganglionic enlargements. R> Proboscis; X*, Sense organs are entirely wanting, as also are sheath of proboscis ; , . , i G, gang-lion ; Le, lem mouth, alimentary canal, and anus. ^ . _ R> retina ; ula The nutritive juices are taken in through the whole outer surface of the body. In the soft granular subcuticular * Besides Dujardin, Diesing, 1. c., compare : R. Leuckart, " Parasiten des Mensohen," Tom II., 1876. Greeff, " Untersuchungen iiber Echinorhyuchus miliaris." Arch, fiii- JVati/ri/rxr/i. 1SU4. A Schneider, " Uebcr den Ban der Acauthocephalen," Mailer's An-hir.. 1808. Also the Sitzungsberichte d<>r (resell xchaf t filr .\|) (hftpTs from that, of f,hp w i ? i 22, proboscis; Xi, Echuwrliynchus g lffas sheath of the probos- vesselsof the lemnisci, are Com- ( aft er A. Andres). Li, cis; Li, ligament; p l ete ly shut off from the latter. G, ganglion ; Le, lem- * nisei; T, testes; iv, Generative organs. The vasa deferentia; Pr, 11 , ,1 n 1-1 prostatic sacs- De b d y Cavity through which vagina; B, lateral ductus ejacuiatorius; fluids circulate encloses the ^ hes f " ie bel l 1; P, penis ; fi, retracted '"'' dorsal ceils at the greatly developed generative 1)as e of the bell ; GI, shapedflocculi; F',F", appendages of the same ; V, uterus ; T', Jmrsa. lateral cells. organs, which are attached to the end of the sheath of the proboscis by a ligament (figs. 290 ACAXTHOCEPHALA. 361 FIG. 292. Embryo of Echin- orhyncliim gif/n" enclosed in the egg membranes (after Leuckart) . and 291, Li). The sexes are s>i>a rate. The male (tig. LMH)) has two testes (T), and the same number of efferent ducts (IV). The latter unite behind to form a ductus fjaculatorius (J)e), which is often fur nislied with .six or eight glandular sacs (TV), and a conical penis (P), at the bottom of a bell-shaped protrusible Imrsa (), situated at the posterior pole of the body (tig. 2'.H>). The generative organs of the larger females (fig. 291) consist of the ovary developed in the ligament ; of a complicated uterine bell, beginning with a free opening into the body cavity ; of the oviduct and the short vagina, which is divided into several portions and opens at the posterior end of the body (tig. 291). It is only in the young stage that the ovary is a simple body en- closed by the membrane of the above-men- tioned ligament. As the animal increases in size, the ovary grows, and becomes divided into numerous spherical masses of eggs, the pressure of which bursts the membrane of the ligament; the ma :-><.- of ova, as well as the ripe elliptical eggs, which gradually become free from them, fall into the body cavity. The egg membranes are not- formed till after seg- mentation, and ought perhapsto be interpret ed as embryo- nic mem- branes. The eggs, which already con- tain em- bryos, pass out of the 1 >ody cavity into the uterine bell, w h i c h is continually dilating and contracting, thence into the oviduct, and through the genital opening to the exterior. a FIG. 293.- Larvae of EchinorJiynehtix jmiteiis from Giiminnr?S^) there is a thickening of the ectoderm, which is called the apical plate. This represents the rudiment of the cerebral ganglion (apical ganglion), and gives off nerves to either side. The wide mouth (0) has a Ms FIG. 294. Development of Polygordim (after B. Hatschek). a, Young larva ; Sp, apical plate with pigment spot ; Pric, praj-oral circle of cilia ; O, mouth ; Pou-, post-oral circle of cilia; A, anus; 3fs, mesoderm ; A"7\ T , head kidney, b, Older larva with commencing segmentation of the body, a second limb is developed in the head kidney, c, Older stage. The body is elongated to the form of a worm, and divided into a number of metameres ; HWk, posterior circle of cilia; Af, eye spot; F, tentacle. ventral position, and leads into an alimentary canal, which opens at the posterior end of the body (A). In front of the mouth there is a strongly developed circle (prreoral) of cilia (Prw) ; and behind 364 ANNELIDA. the mouth a weaker (postoral) circle (Pow) to the right and left there is an excretory canal (head kidney), which begins with a ciliated funnel. By the differentiation of the cephalic region of the larva into prsestomial lobe and oral segment, and by the gradual growth in length of the posterior part of the body and the segmentation of the latter into a number of successive metameres, the originally un- segmented larva is transformed into an Annelid (tig. 294, a (/). There is, therefore, between the segmented adult and the larva a morphological relation similar to that be- tween the cestoid and the simple scolex, from the posterior end of which the proglottides are developed. The body of the Annelida is sometimes flattened, sometimes completely rounded and cylindrical. It is composed of a number of successive segments, which are usually sepa- rated from each other externally by trans- verse constrictions. The segmentation is generally homonomous, in that the segments following the head resemble each other not only in external appearance, but also in internal structure, i.e., they repeat similar sections of the internal organization. The terminal segment with the anus, however, has a special structure inasmuch as it retains the primitive, more indifferent char- acter of the posterior end of the body of the larva, and during the development of the worm gives origin to new segments anterior to itself. The homonomy of the preceding segments of the body is, how- ever, never complete, since certain organs are confined to definite segments. The internal segments, which are separated by dissepiments, either correspond with the external segmentation as marked by the annular constrictions of the integument (Chcet02Joda), or each internal segment corresponds to a definite number (3, 4, 5, etc.) of the external rings {Hirudinea). FIG. 294, d. The young Polrgordius ; G, cerebral ganglion; Wij, ciliated pit ; D, alimentary canal. ANNELIDA. 365 Organs of locomotion. Special organs of locomotion may either have the form of bristle-bearing unjointed appendages (parapodia) on each ring of the body (' 'In* /<>i>,,,ta, Hirudinea} are hermaphrodite ; the marine Chcctopoda, on the contrary, are for the most part of separate sexes. Many lay their eggs in special sacs and cocoons, in which case development is direct, without metamorphosis. The marine worms, on the contrary, undergo a more or less complicated CHJETOPODA. 367 metamorphosis. The Annelida comprise terrestrial and aquatic animal.'-, and they eat, for the most part, animal food. Many of them (Hirudinea] are occasionally parasitic. In the group of the Annelida, three principal divisions may be distinguished, the Chcetopoda, the unsegmented Gephyrea, and the Hirudinea which are adapted for parasitism. The Hirudinea are not in any degree to be regarded as Annelida of a lower grade of organization, but they rather present, at least in the case of some organs, as alimen- tary canal, circulatory and generative organs, a more complicated structure, and agree most closely with the Oligocha?ta, from which they may be derived. D ( FIG. 296. Grubea fusi- fera (after Quatre- fages). Ph. pharynx D, alimentary can;il ; C, cirri ; F, tentacles. Sub-class 1. CHLETOPODA.* Free living Annelida, with paired tufts of seta' on the segments, frequently with distinct head, also with tentacles, eirri, and branchice. The Cha?topoda are divided externally into segments, which correspond with the rnetameres of the internal organs, and are, with the excep- tion of the anterior region, which is distinguished as the head, usually tolerably alike (fig. 296). Parapodia provided with setfe are very frequently present on the segments; their prin- cipal function is that of locomotion, but their va- rious appendages, the branchice and cirri, also discharge tactile and respi- ratory functions (fig. 297). ' Besides the older works of Savigny. Audouin et Milne Edwards, and Quatrefages, compare E. Grabe, " Die Fami- lien der Anneliden." Art-It ir fiir Naturgescli, 1850 and 18.">1. E. Clapim-dc, lii.'ohoivhes anatomique sur les Annelidcs. etc.," Geneve^ 1861. E. Cla- parede, " Les Annelidas cheto- podes du golfe de Napli^." Geneve et Bale, 1868. also Sup- plement. 1870, and " Eecherches sur la structure des Ann61ide8 sedentaires," Geneve, 1873. Fr. Lej'dig, 1. c,, also ' Tafeln zur vcrgl. Anatomic," 1864. FIG. 397. Dorsal (DP) and ventral (I'P) Para- poclinm with bundles of seta? of Nereis (after Quatrefages). Ac, Aciculum ; Re, dorsal cirrus ; Be, ventral cirrus. 368 ANNELIDA. The form of the movable sette varies extremely, and affords a good character for the classification of families and genera. According to the strength, form, and mode of ending (fig. 298), the following a, ic a e. / 5 FIG 298. Setfe of different Polycliietit (after Malmgren and Claparede). a, Hooked seta of Sabdtii ei-uxxicoraig ; l>, of Te rebel I a Dmiielggeni ; c, seta with piral ridge from tStkenelais ; d, lance-shaped seta of Phylloehcetoptemu ; e, of Sabella erassicornis ; f, at Sabella pavonis ; ff, Composite sickle-shaped seta of Nerds cultrifera. forms can be distinguished : hair-setae, hooked-seta?, fiat-setae (pcdece), lance-seta?, sickle-shaped seta?, etc* When the parapoclia and their appendages are com- pletely wanting, the seta? are embedded in pits in the integument, and are arranged either in one or two rows on either side, that is, in a lateral ventral row on either side, or in a ven- tral row and a dorsal row on either side. In such cases the number of seta? is small (Oligo- chceta}. The seta? may, on the contrary, be pre- sent in great number, so that the integument on either side seems to be covered with long hairs and seta?, and a thick felt of hairs shining with a metallic lustre is distributed over the whole dorsal FIG. 299. Anterior end of Pnlynoii fxteiinufa, the first elytron on the left hand being removed (after Cla- parede). The two setae of the oral segment are visible ; El, Elytra. . CHjETOPODA. 369 Fh surface ( .\/>/n-<>.-nt an equally great variety of form and not unfrequently vary in the different parts of the body. They are either simple or ringed tenta- cle-like processes, the cirri, which arc distinguishable into dorsal and ventral cirri. The cirri are for the most part filiform, and sometimes jointed or conical, and then are often provided with a special basal joint. In some cases the dorsal cirri are flattened out as broad scalo and leaves, the elytra, which constitute a protective covering (Aj)hro- clite) (fig. 299). In addition to the cirri, branchise which may be filiform or branched and antler-like, comb- shaped or in the form of tufts, are frequently present ; sometimes they are confined to the middle region of the body, or are extended over almost the whole dorsal surface ; sometimes they are confined to the head or to the anterior segments immediately following the oral segment (cephalic branchiae). The two anterior segments may be regarded as forming the head ; they are fused together, and are, with regard to their appendages, different from the following segments (fig. 245). The anterior segment projects beyond the mouth as the frontal lobe, and bears the tentacles sand palps \jpalps are ten- tacular structures arising from the veiitro-lateral sides of the head, r'ulf. p. 379] and also the eyes ; the posterior cephalic segment or oral segment bears the tentacular cirri. The List segment (anal segment) bears the anal cirri. The alimentary canal is usually straight, and extends from the mouth to the anus, which is terminal and rarely dorsal ; it is divided into oesophagus, intestine, and rectum (fig. 300). There is in most cases a dilated muscular pharyngeal bulb which is armed with papilla- or with movable teeth and can be protruded as & proboscis. The intestine usually preserves the same structure in its entire length and is divided by regular constrictions into a number of 24 D FIG. 300. Alimentary canal of .Ljiln-uJit.' acn/fiifn (after M. Ed- wnrcl.s). Pli, pharynx ; I\ intes- tine ; L. hepatic appendages. :370 CHMTOPODA. divisions or chambers, winch correspond to the segments and dilate again into lateral diverticula and caeca. The constrictions are due to filamentous or membranous septa (dissepimenta), which divide the body cavity into the same number of chambers lying one behind another. The vascular system appears to be closed, so that the clear nutri- tive fluid found in the body cavity, which, like the blood, contains amoeboid corpuscles, does not communicate with the usually coloured contents of the vessels. The dorsal and ventral vessels are not only connected at each end by lateral loops, but also in each segment ; and from these connecting vessels proceed peripheral networks, which extend into the integument, the wall of the alimentary canal, and the branchiae. Special organs of respiration are wanting in alm'ost all the Oligo- chceta. In the marine Worms, on the contrary, branchiae are very generally present, usually as appendages of the parapodia. These branchiae are either simple cirri which have delicate ciliated walls and contain blood-vessels, or are branched (Amphinome) or in some cases are pectinate structures (Eunice) which co- exist with special cirri on the notopodia (fig. 246). The branchiae are sometimes confined to the middle segments (Arenicola), and are sometimes developed on almost all the segments on the dorsal surface, being simplified towards the posterior end of the body (Dorsibrcmchiatq). In the Tubicoltn the branchiae are confined to the two (Pectinaria Sabellidce) or three (Terebellci) anterior segments. The respiratory function is, however, also shared (Capitibranchiata) by a number of elongated tentacles which are grouped in tufts on the head. These are, in the Sabettidce, supported by a special cartilaginous skeleton, and may have secondary twigs developed upon them. They are either simply arranged in a circle round the mouth, or in two fan-like lateral groups (Serpulidce), the base of which is not ^infrequently drawn out into a spiral plate. Such branchial structures, however, also function as organs of touch, as organs for procuring nutriment, and even for building the tubes and shells. Excretory organs. There are usually in all the segments paired segmental organs, which serve as excretory organs. They begin, as a rule, with a ciliated funnel in the body cavity ; they possess a glandu- lar wall, are several times coiled upon themselves, and open to the exterior in each segment by a, lateral pore. These glandular passages serve in general for the removal of matters from the body cavity, and in the, marine. Chcetopoda are used during the NERVOUS SYSTEM. 371 breeding season as oviducts, or vasa deferentia, and permit of tlw passage outwards of the generative products, which have been set free in the body cavit ;/. Amongst the special glands in the body of the Chcetopoda, those cutaneous glands of the Oligochuita which give rise to the thickening (extending over several segments) known as the clitellus or girdle, are especially worthy of remark. The secretion of these glands perhaps assists the intimate connection of the Worms during copula- tion. In the tierpulidcti there are present two large glands, which open upon the dorsal surface of the anterior portion of the body and furnish a secretion used in the formation of the tubes in which the animals live. FIG. 301. Brain and anterior portion of the ganglionic chain, a, of Serpula 1>, of N>->-< ', (after Quatrefages) ; O, eyes ; (?, cerebral ganglion ; c, cesophageal commissure ; Uff, subffisophageal ganglion ; e e, nerves to the tentacular cirri and the mouth segment. Nervous system. -The longitudinal trunks of the ventral cord are often so closely approached that they seem to form a single cord (Oligochcela). In the Tubieolce (fig. 301), on the contrary, they are very widely separated from one another, especially in the anterior part of the ganglionic chain (Serpula). The visceral nervous system consists of paired and unpaired ganglia, which supply the oral region and especially the protrusible proboscis. Sense organs. Paired eyes upoh the surface of the frontal (i.<\ CHjETOPODA. prseoral or cephalic) lobe are widely distributed. Eye-spots may also be present upon the posterior end of the body (Fabric ia], or may. be regularly repeated upon the sides of each segment (Polyophthalnvus). In species of Sabella, pigment -spots with refractive bodies are found even upon the branchial filaments. The large cephalic eyes of the genus Alciope,* are the most highly developed, being provided with a large lens and a complicated retina. The presence of auditory organs seems less frequent. They appear as paired otolithic vesicle> upon the cesophageal ring of Arenicola, Fabric-id, some Sabellidce and young Terebellidce, etc. Besides the tentacles, cirri and elytra, other portions of the surface of the body may be sensi- tive to tactile sensations. On such parts there are either stiff hairs and tactile setae, or, as in Sphcerodorum, special tactile warts with nerve terminations. Reproduction. In the smaller Clwtopoda asexual generation by fission and gemmation may occur. Either (h'ssiparous reproduction) a large number of segments of the parent be- come separate and give rise to the body of the new worm, as for example in Xyllis prolifera, where a series of the posterior segments, which are filled with ova, become separated by a simple transverse fission, after the formation of a head provided with eyes ; or (gemmiparous repro- duction) a single segment only, usually the last, becomes the starting-point for the formation of a new individual. In this way Aid oly tits pro- lifer, one of the $ijUi<.l; asexually reproduces itself, giving rise to a male and female sexual form, known respectively as Polybostrichus Mulleri'f (male) and tiacconereis helgolandica (female). This is a case of alternation of gene- rations, for the asexual form, Autolytus, gives rise by budding in the lout;- axis to the sexual forms (fig. 302). In this case a whole series of segments are developed * Greet!, '' Uehcrdas Auge der Alciopideii. etc.," Marburg. 187(1 :and " Unter- suchungen iiber die Alciopiclen.'' Sor. Act. ili-r A. Ln>/>. AJuuJ.. it,-.. Tom XXXIX., Nro. I'. f Compare besides the- wr+rks of (). Fr. Miiller, Quatrefagcs, Lcuckart. aud Krohn. especially A. Agassi/. - On alternate Generation of Annelid* and the embryology of Autolytus rnrnutus." Jioxtatt Joiini. Trt/. Hint., vol. iii:. 1SII8. FIG. Wl.Autolytiis cor- rtnfux, with the male animal Polyljimtrifh itx (after A. Agassiz). F, Tentacles ; CT, tenta- cular cirri : f, tenta- cles ; ct, tentacular cirri of the male. GENERATIVE ORGANS. 373 in front of the last segment of the asexual form, and tlit-x- Moments, after the formation of a head, constitute a new individual. As this process is repeated, a chain of connected individuals is formed, and these, as soon as they are separated, represent the sexual individuals. Among the freshwater Xa'nln', in Chcetoga&ter, a regular and continued budding in the long axis leads to the formation of chains, consisting of not less than 12 to 16 zooids, each having only four segments, while the sexual individuals consist of a greater number of segments. A similar process occurs in the mode of reproduction observed by 0. Fr. Miiller in Nais pr'oboscidea, from the last segment of which a new zooid is produced. Both generations of JVais, however, become sexually mature. [For a more. complete account of the asexual reproduction of Chaetopoda, ' Mai four. -'Comparative Embryology.'' vol. i, pp. 283. 2S4.] FIG. 303. A parapodium of Tomopteris with a mass of ova and one free ovum (after C. Gegenbaur). The Clwtopoda are, with the exception of the her- maphrodite Oligochceta and certain Serpulidte (e.g., Xjii- rorltis spirillum, Protula Dysteri) of separate sexes. Male and female individuals seem occasionally so strikingly different in the structure of their oi'gans of sense and lo- comotion that they have even been taken for species of distinct genera. Besides the above- mentioned Sacconereis and Polybostrichus, the asexual generation of which is Autolytus, a similar sexual dimorphism lias been shown by Malmgren for Heteronereis, a genus of the Lycoridte, in which the males and females differ both in external form and in the number of their segments. A remarkable case of lieterogamy is also afforded by this genus, in that a generation of smaller animals swimming upon the surface alternates with a generation of larger forms living upon the bottom. The generative apparatus of the Oliyochceta is very highly deve- loped. The. ovaries and test es lie in definite segments, and empty their contents by dehiscence of their walls into the body cavity. Special generative ducts often co-exist with segmental organs in the same segment (0. terricolce), while in other cases the segmen- tal organs are wanting in the generative segments (0. limicolce). In 374 CH.ETOPODA. the marine Chcetopoda, the ova or spermatozoa originate on the body wall (fig. 303) from cells of the peritoneal membrane, either in the anterior segments alone or along the whole length of the body. The generative products then become free in the body cavity, attain maturity, and pass through the segmeiital organs to the exterior. Only a few Chcetopoda, as Eunice and Syllis vivipara, are viviparous, all the rest are oviparous ; many lay their eggs in connected groups, and carry them about with them, while the Oligochceta lay theirs in cocoons. Development. The segmentation is unequal. A primitive streak is very generally developed, though sometimes not until the embryo has left the egg. It arises on the ventral side in consequence of the development of a middle layer and from neutral plates of the upper layer. Excepting in the Oligochceta, the young forms undergo a metamor- phosis and after leaving the egg appear as ciliated larvpe, which are provided with mouth and alimentary canal, and essentially resemble, with some modifications, Loven's larva. The capability of renewing lost portions of the body, more espe- cially the posterior part of the body and different appendages, seems to be generally distributed. The Lumbricince and. certain marine Worms (Diopatra, Lycaretus] are even able to replace the head and the anterior segments, with the brain, cesophageal ring, and sense apparatus. Fossil remains of Chcetopoda are found from the Silurian onwards in the most different formations. Order 1. POLYCH.ETA.* Marine Chcetopoda, with numerous setce embedded in the parapodia, usually with distinct head, tentacles, cirri, and branchiae. They are for the most part dicecious, and develop with metamorphosis. The marine Chcetopoda must be considered as belonging to a higher grade of life, on account of the sharp distinction of the head which is composed of the pra?stomiuni (prseoral lobe) and oral segment (in the Amphinomidce several succeeding segments are also included), and of the presence of the tentacles, tentacular cirri and * Aiuloum et Milne Edwards, ' Classification dcs Annelides et description rles cclles qui habitent les cotes de la France," Annales dcs So. Nat., Tom XXVII. to XXX., 1832-83. Delle Chiaje, " Descrizioni e notomia de-'li animali scnza vertebre della Sicilia citeriore," Napoli, 1841. Quatrefages, " Histoire naturelle des Armeies," Tom. I. and II., 1865. Also the numerous writings of E. Grube and E. Clapare.de. POLTCILHTA. 375 gills, and also of the setse embedded in prominent parapodia, which serve as aids to swimming. The internal organization, however, is in no way more complicated than that of the 0/ii/uc/ni'ta. Neverthe- less all these distinctive characters may be less and less marked, and, indeed, so 'completely vanish that it is difficult to draw a sharp line between the Oligocha'ta and the Polychceta. The parapodia (Capitellidcf^ and also the setre (Tomo'pterid(K) may be wanting. In rare cases, bundles of setse are present on all the segments behind the head ; they are however arranged in a single row and embedded in a single pair of ventral retractile parapodia in each segment. Fio. 301. Head and anterior body segments of Nereix Dumerilli (after E. Clapar^de). O, Eyes ; P, palps ; Ct, tentacular cirri ; K, pharyugeal jaws. This arrangement, which is found in Saccocirrus and its allies, pro- bably represents the primitive state, especially as in these animals the character of the nervous system, which lies in the ectoderm external to the dermal muscular envelope, and of the sense organs, which are reduced to two simple tentacles upon the cephalic lobe and to ciliated pits, indicates lower an'cl more primitive conditions. In another and very remarkable type, Polygordius Schn. and Protodrilus Hatsch., not only parapodia and set* but also the external segmentation are wanting. The segmentation of this achsetous and externally unsegmented worm is entirely confined 376 CH.ETOPODA. T to the internal organization and is, as compared with that of all other Annelida, to a certain extent completely homonomous, inasmuch as the oesophagus is confined to the cephalic segment, and does not extend into the anterior segments of the body. Further, the nervous system is connected with the ectoderm along its whole length, and the cerebral ganglion maintains its primitive position at the anterior end, corresponding to the apical plate of the larva ; and the ventral cord is without ganglionic swellings. In all the above points these forms seem to have preserved the primitive An- nelidan structure, and they have therefore been united by Hat- schek into a special class, the Archian- nelida. In the Pvly- cJui'tn the vascular system is compli- cated by the ap- pearance of bran- chiae, which are provided with blood-vessels. In the forms with dorsal branchiae the branchial blood is derived from the dorsal trunk and re- turned to the ven- tral by special vessels. When, on the other hand, as in the tubicolous capito-branchiate forms, the respiratory apparatus is concentrated on a few segments, the vascular system of that part undergoes greater modifications. In the Tere- bellidfe (fig. 305), the dorsal trunk dilates above the pharynx to a branchial heart from which lateral branches are given off to the branchiae. In the same region the transverse loops connecting the FIG. 305. Terebella nebuloaa, opened from the dorsal side (after M. Edwards). T, Tentacles ; A" Branchia? ; Dy, dorsal vessel or heart. POLYC1I7DTA. '677 dorsal and ventral trunks may perform the function of hearts, as is also frequently the case in the Olitjoc/ueta. Finally the vascular system is in many cases considerably reduced, and, according to Claparede, is entirely wanting in Glycera and CapiteUa, in which the blood is represented by the perivisceral fluid. The generative organs, unlike those of the hermaphrodite Oliyo- ckcetfi, are usually placed in different individuals ; and the males and females are sometimes of very different forms. A number of herma- phrodite Polyclueta are, however, known ; such principally belong to genera of the Serpulidce, e.g., Spirorbis, Protula. The development, unlike that of the Oligochceta, is invariably con- nected with a metamorphosis. Segmentation is, as in the Hincli- nea, usually un- equal, and even the first two M'g- mentation spheres are of unequal size. The smaller (animal) half, which segments more quickly, gives rise to smaller segments, which grow round and envelope the larger segments proceeding from the segmentation of the larger half. In the subsequent development a primitive streak makes its appearance in all embryos of Polychwta, sometimes, how- ever, not unfil the embryo has begun to lead a free life as larva. The ganglia become differentiated later into the ventral chain. In the free-swimming larva' the cilia are rarely distributed over the whole surface of the body (Atroc/Ji//u>i : gcolopendrina Sav. Ocean and Mediterranean. Fam. Eunicidae. Body very long, composed of numerous segments. Pra?sto- mium with several tentacles. Parapodia usually uniramous, rarely biramous, usually with ventral and dorsal cirri as well as branchiae. One upper jaw composed of several pieces, and a lower consisting of two plates ; both lie in a sac, the jaw-sack, on the dorsal surface of which runs the pharyngeal tube. Sta/uroceplialiis vittatus GT., Sialla (Lysidice) pa/rthenopeia Delle Ch., Naples. Diopatra nenpnTitnna Delle Ch., Naples. Eunice Jlt/nixxii And. Ed\v. Fam. Nereidae = i//rf)/vV7l>n Clap., Normandy, Autolytus prolifer 0. Fr. Mull., asexual form. The male has been described as Poly bosti'ic kits Mulleri Kef., the female as &ieciiercix helgolandica Mull. Spfusrodomm l>rrij>t(ti(x Gr., Mediterranean. Fain. Alciopidae (Alciopca). With two large hemispherical projecting eyes. Ventral and dorsal cirri leaf-like. The proboscis is protrusible, the tube of the proboscis being thin walled and its terminal portion thick walled. At * Compare E. Grube, " Die Familie der Lyeorideen.'' Jnkirxbcr. -gig. clieu (li-xcllscluift, 187i!. 380 CH.ETOPODA. iN ii])crture are two hook-shaped papilla?. The larva: are in part parasitic in the Cydippidce. Alrlopa, Cunt nti nil Delle Ch., Naples. Fam. Tomopteridae (Grymnwnjia'). Head well marked, two eyes, ' bifid prasstomium, and four tentacles, of which two in many species are only present in the young. The mouth segment has two long tentacular cirri which are supported by a strong internal seta. The mouth is without proboscis and jaws. The segments are provided with large bi-lobed parapodia without seta?.. ToiHOjiti-rix xcoliyeHflra Kef., Mediterranean. T. nis Esch., northern seas, Heligoland. The genus Myzostoma F, 8. Lkt., a small group of hermaphrodite worms whose affinities are doubtful and disputed, may be placed here. They are small, disc-shaped animals, parasitic on Conn/tula. They possess a soft and ciliated skin, four pairs of laterally placed suckers on the ventral surface, and a protrusible proboscis fur- nished with papillse at their anterior end. also a branched alimentary canal which opens at the posterior end of the body. On the sides of the body are rive pairs of short parapodia, of which each one bears a hook (with one to three supple- mentary hooks) as well as supporting setre. As a rule, , double as many cirri or short wart-like protube- rances- are found on the margin of the body. M. ///(ihnim, clrriferum F. S. Lkt. D Sub-order 2. Seden- 'Ov taria = Tubicolse. * Fra. 30R.-SpirorM> laris (after Claparede). a, The With indistinctly S6pa- aniiual removed from its tube, strongly magnified; rated head and short b, tube ; T, tentacles ; Et, brood-pouch with oper- culum ; Dr, glands, Or, ova ; Oe, oesophagus ; 3f, usually not protl'USlble stomach ; A intestine. proboscis, without jaws. The branchire may be entirely absent and in many cases are confined to the two or three anterior segments following the head. In exceptional cases they are placed on the dorsal part of the middle of the body (Arenicolidw). As a rule, however, they are represented by numerous filiform tentacles and ten- * E. Claparede, " Kecherches sur la structure des Annelides sedentaires," (?enve, 1873. a TUUICOL/E. 3S1 tacular cirri upon the head (Capitifa'anchiata), of which one or more may bear an operculum at its apex to clo.M- (lit- tube (tig. 308). The para podia, are short, and are never used in swimming; the notopodia. usually carry hair-like seta 1 ; the neuropodia are trans- verse ridges with hooked seta j or plates. Eyes are very frequently absent : in other cases thev are present in pairs upon the head or 011 the terminal segment, sometimes even on the branchial tentacles; in the Litter case they are very numerous. The body is often divided into two (thorax and abdomen) or three regions, the seg- ments of which are distinguished by their unequal size. The Tufiicolt.r live in more or less tirm tubes which they construct for themselves, and feed on vegetable matter which they procure bv means of their tentacular apparatus. In the construction of their tubes the animals are assisted in various ways by the long tentacles or branchial filaments of the head ; thus, for example, the Sabellidce are said to accumulate fine ooze at the funnel-shaped base of the branchial apparatus by means of the cilia of their tentacles, to mix it with a cement secreted by large glands, and then to transfer it to the edge of the tube; while the Terebellidce procure the grains of sand for the construction of their tubes by their long and very extensible tentacles. There are also boring Annelids, which pierce limestone and mussel shells, like the horny Molluscs ; ?.irurl>in xpirUltun Pag., the eggs and larva? of "vhich remain within a dilatation of the opercular stalk until the young animals are able to construct a tube for themselves. The free-swimming larva? of most Tubicola 1 , on assuming the form of the worm, lose the ciliary apparatus, while tentacles and parapodia make their appearance. In this condition and sometimes surrounded by delicate membranes, they swim about for some time longer, and, having lost their eyes and auditory vesicles, gradually assume the structure and mode of life of the sexual animal (Terebella). Kam. Saccocirridae. With two 'tentacles on the pvfestomium. two eyes mi. I the same number of ciliated pits. A single row of retractile parapodia, furnished with simple seta}, on either side of the segments of the body. Snrro- cirrus pajnlloccrcus P>obr.. Black Sea and Mediterranean (Marseilles). Fam. Arenicolidae. I'm^tumiiim small and without tentacles. The pro- boM'i-; is beset with papilla?. There are branched gills, on the median and posterior segments. The animals burrow in sand. Ari-niculn niiirhin Lin. (A. />ixt'iniiii Lam.), North Sea and Mediterranean. Fam. Spionidae (Sjiim/cn"'). The small pnvstomium sometimes with tenfacu- 382 CH^ETOPODA. Uir processes, usually with small eyes. The oral segment mostly with two iong tentacular cirri, which are usually grooved. Cirriform branchiae are present. Pol i/iJurii initfiitnitii Clap., Naples. Sp-lo gcticornix Fabr., north seas. Fam. Chaetopteridse. Body elongated and separated into several dissimilar regions. Usually two or four very long tentacular cirri. Dorsal appendages of the middle segments have the shape of wings and are often lobed. They live in parchment-like tubes. Telcj>x,n-ttx Cost urn HI- Clap., Naples. CJia-t^tcnis pergameiitaceus Cuv.. West Indies. Fam. Terebellidae. Body vermiform and thicker anteriorly. The thinner posterior portion is sometimes distinctly marked off as an appendage destitute of sette. The prsestomium is indistinctly separate from the mouth segment. There is frequently a lip above the mouth. Numerous filiform tentacles, usually arranged in two tufts. There are pectinate or branched, rarely filamentous, gills on a few of the anterior segments. Dorsal prominences (uotopodia) fur- nished with simple setse, and ventral transverse ridges (neuropodia) with hooked sets. TerrltcUu. conchiler/a- Pall., English coast, Mediterranean. A martin ivtr GruljL'l Malmgr., Greenland and Spitzbergen. Pectinnriii- aitricoma, 0. Fr. Mull., North Seas, Mediterranean. ^ulicUaria (IL-riiii'lla') spinulosa R. Lkt., Heligoland. Fam. Serpulidae. Body usually distinctly divided into two regions (thorax, abdomen). Prrestomium fused with the mouth segment, which as a rule is pro- vided with a collar. The mouth is situated between two semicircular or spirally coiled plates, from the anterior margin of which spring the branchial filaments. These have secondary filaments arranged in single or double rows, and may be supported by a cartilaginous skeleton, and have their bases connected by a membrane. SpirogrtVplvis Spallanzaiui, Naples. Sabella penieillus Lin., North Seas. lii Risso, Mediterra- nean. Fllii/rana Implexa Berk., Norwegian and English coasts. Serpula noi 1 - cegicu- Gunn.. North Sea and Mediterranean. Sjjinirhtx spirillum Lin., Ocean. Order 2. OLIGOCH.ETA.* Hermaphrodite Choetopoda without pliaryngeal armature and para- podia. There are no tentacles, cirri, or branchice. The development is direct. The cephalic region is composed of the prsestomium, which projects as an upper lip, and the mouth segment. It does not essentially differ from the following segments so as to form a special region (tig. 309). Tentacles, palps, and tentacular cirri are never found on it, but tactile papilla? are present in great number, as are also peculiar sense organs which resemble taste buds. Eyes either fail or are present as simple pigment spots. Besides the small gland cells of the * Besides tin.- works of W. Hoffmeister. D'lJdekem. and others, compare : E. Claparede, " Recherches anatomiques sur les Annelides, etc., observes dans les Hebrides," Geneve, 1860. E. Claparede, "Recherches anatomiques sur les Oligochastes," Geneve, 1862. A. Kowalevski. " Embryo] ogische Studien an Wiirmern uud Arthropoden (Luml-ii-u*, EuiLirtT)." 1 'Petersburg, 1S(>1. I>. Hatschek. Studien liber Entwickhmgsgeschichte der Anneliden." Wien, 1878. Fr. Vejdovsky, " Beitrage zur vergleichenden Morphologic der Anneliden. I. Monographic der Enchytrasiden," 1879. OLKHH'H.ETA. 383 hypodermis there is present in the ditelrus a deeper glandular layer (Saulenschicht Clap.), which consists of finely granular cells embedded in a framework of pigment ed and vascular connective tissue and situated between the hypodermis and the external muscular layer. There are but few set;v present, and they are never disposed on special parapodia, but always in simple pits in the integument, by the cells of which they are secreted. There are small secondary bristles which serve as a reserve. The blood is usually red, as in the Hirudinea. The alimentary canal is often divided into several regions, the relations of which are most complicated in the Lumbricidoe. In Lumbricus, the buccal cavity leads into a muscular pharynx, which is probably used for sucking. This is followed by a long oesophagus extending to the 13th segment, and furnished with a thick layer of glandular cells and several glandular dilated ap- pendages (calcareous sacs). The oeso- phagus is succeeded by a crop, a muscular gizzard, and finally by the intestine itself, the dorsal wall of which is pushed inwards so as to form a longi- tudinal fold, the typhlosole (comparable to a spiral valve). In the Limicolw the alimentary canal is simpler by the absence of a muscular stomach ; a pharynx and oesophagus are, however, always present. Reproduction. The Oliyochwta are hermaphrodite ; they lay their eggs either singly or united in greater num- ber in a capsule ; and they develop without a metamorphosis. The testes and ovaries are paired and placed in definite segments, usually near the an- terior end of the body ; they dehisce their products into the body cavity. The generative ducts possess funnel-shaped openings into the body cavity through which the generative products pass, and may FIG. 309. Lniiilji-iciix rnlrllti* (after G. Eisen). <' ii hujg. Setae elongated, hook-shaped, arranged in four groups in each segment, each group containing two setae. The earthworm lays its eggs in capsules, into each of which several small ova, with sperm from the recep- tacula seminis. are emptied : as a rule, however, only one or but a few embryos are developed. The developing embryo takes up with its large ciliated mouth not only the common mass of albumen, but also the other eggs. L. arjricola Hoffm. = frr/r.vfra Lin.. L. feet iilux Sav., L. miu'i'icanus E. Perr. CrwdrHms I nc n n in Hoft'm. Sub-order 2. Limieolae. Oligoclueta which live principally in water. Without segmental organs in the genital segments. Fam. Phreoryctidse. Long filiform worms, with thick skin and two rows of slightly curved setfe on each side. Phrcoryctex Mrnkeanu* Hoffm. Found in deep springs and wells ; they seem to feed on the roots of plants. Fam. Tubificidae. Aquatic worms, provided with four rows of simple or divided, hooked set SB. Hair-like set as may also be present. The receptacula are in the 9th, 10th, or llth segment. They live in mud tubes, from which they protrude the posterior end of the body. Tulifi:i' rivulorttm Lam. The heart is in the 7th, the receptacula in the 9th segment. T. Boniwti Clap. (Stfnvrix riti-ii'i/atu Hoffm.) The heart in the 8th, receptacula in the 10th segment ; both species live in fresh water. Linntml i-'dit-x Hoffmristi'ri Clap.. L. D" Udekennanns Clap. Is distinguished from Tn bifi-x by the presence of hair-like setae in the upper row of sette. Lumbrii-iifi/g rn-rieijutiix O. Fr. Miill. Every segment is pro tided with a contractile vascular loop and saccular contractile appendages of the dorsal vessel. Fani. Naideae. Small Lim/mlr/- with delicate thin skin and clear, almost colourless, blood. The prajstomium is often elongated like a proboscis and 25 386 ANNELIDA. sc fused with the mouth segment. J\'ttig (Sti/larnt} proboxcidea 0. Fr. Mull. N. jMtrax/'fa Schm. Both species have filiform pnestomium. Cha-toi/nsti-i- rermlcularisQ. Fr. Miill. Sub-class 2. GEPHYKEA.* Worms with cylindrical body, without external segmentation, with terminal or ventral mouth ; with cerebral ganglion, cesophayeal ring and ventral cord. Settv are sometimes present. The Gephyrea possess an elongated cylindrical body and live, as do the Holothuria, in sand and ooze in the sea. The characters which distinguish them as Annelids are the possession of an oesophageal ring connected with a cerebral ganglion and of a ventral cord par- tially surrounded by ganglion cells. The larva? of the Chce- tifera present traces of seg- mentation (see below, p. 391), while in the Achceta the body cavity remains simple. Of sense organs, eye spots have been observed ; these in certain SipuncuUdce lie directly upon the brain ; there are also dermal papillse, into which nerves enter. The structure of the integu- ment is similar to that of the Annelida; the thick upper cuticular layer rests upon a cellular matrix, and is not un- Fie. 3ii. Young Ediiurus from the iiventrai frequently wrinkled. There is side (after Hatschek). 0. Month at the base i , mi of the proboscis; SO, sopha*eal commis- n external segmentation. The sure ; BS, ventral cord ; A, anus ; if, hooks, connective tissue dermis is of considerable thickness and en- closes numerous glandular tubes, which open to the exterior by pores in the epidermis. Below this is the strongly developed dermal muscular tunic, which is regularly composed of an outer layer of circular fibres * Quatrefages," Memoire sur 1'Echiure," Ann. (h-s Sc. Xnt.. 3 Ser., Tom VII. Lacaze-Duthiers, " Recherches sur le Bonellia," Ann. ili-x ,SV>. -\t.. 1858. W. Keferstein. " Beitriige zur anatomischen und systematischen Keimtniss der Sipunculiden," Zcitscltr fur n'/M. Zoologic, Tom XV., 18(io. B. Hatschek. " Ueber Entwickelung?geschichte des Echiurus," etc. Wien, 1880. J. W. Spengel, "Beitrage zur Kenntniss der Gephyreen. I. Mitthi-H. mix dcr zvolo- gischen xtutiun :tt JVeajwl, 1879 ; II. Zcitschr.fur n-ias. Ztwl., Tom XIV., 1881. UEPHYJREA. 387 Te and an inner layer of longitudinal fibres. The latter are connected with the former and also amongst themselves by net-like anastomoses. These dermal muscles cause the folds of the cuticle. Internally to the longitudinal muscles there is another layer of circular muscles. In the <'li't(fera two hooked seta? ai'e present near the genital opening (tig. 311); these assist locomotion. There may also be present one or two circles of setse at the posterior end of the body (Echiurus). In the Chcetifera (fig. 311), the ante- rior part of the body is elongated to form a kind of proboscis, which projects im- movably and cor- responds to the prseoral lobe (pr?e- stomixim) of the A i' nelida. The mouth is placed ventrally at the base of the probos- cis. In the Ac/ueta ( Sipuncudidce) this proboscis is want- ing ; the mouth is placed at the ex- tremity of the an- terior region of the body, which is sur- rounded with cili- ated tentacles and FlG - 312. .SV//Mc/s niulim, laid open from the side (after W. Kefersteiu). 2V. Tentacles ; G, cerebral ganglion ; VG, vru- Can be retracted by tra l ne rve cord; Z, intestine; A, anus ; BD brown tubes means of retractor (ventral glands), muscles (fig. 312). Alimentary canal. The mouth opens into a pharynx, which is sometimes furnished with teeth ; this is followed by a ciliated intes- tinal canal, which is usually longer than the body and disposed in coils in the body cavity. The terminal portion of the intestine is 388 ANNELIDA. muscular and opens to the exterior by a terminal or dorsally placed anus (fig. 312). The vascular system is probably in communication with the body cavity ; it consists of a dorsal vessel, which, as in the Annelida, accompanies the alimentary canal, and of a ventral vessel running along the body wall. There are also branches on the alimentary canal and in the tentacles. The blood is either colourless or red, and moves in the same direction as in the Annelids, the current being maintained both by the contraction of certain parts of the vessels and by the cilia which line the walls of the vessels. The corpusculated fluid of the body cavity differs from this vascular blood. Excretory organs. There are two sets of organs, both of which may be interpreted as segmental organs. One kind, the anal vesicles (fig. 314c, Ab\ are only present in the Cheetifera ; they have the form of a pair of tufted tubes, which open, on the one hand, into the body cavity by numerous ciliated funnels and, on the other, into the rectum. The other kind, known as the brown tubes (fig. 312, J3d) or ventral glands, are placed (one or more pairs) in the anterior part of the body ; they also open into the body cavity by a ciliated funnel, and to the exterior on the ventral surface. The latter, like the seg- mental organs of Annelids, assume the function of seminal vesicles and of oviducts. Generative organs. The Gephyrea are of separate sexes. There are, however, remarkable variations both in the generative glands and their ducts. In Phascolosoma amongst the Achwta (according to Theel) the generative glands lie at the root of the ventral retractor muscles of the proboscis, and form a ridge from which the generative products are set free. Spermatozoa or ova in various stages of development are found in the body cavity, and thence are carried to the exterior through the two brown tubes (segmental organs) which open on the ventral side. In BoneUia among the Chwlifera the ovary, which has the form of a thin cord (fold of the body wall) in the posterior half of the body, is attached by a short mesentery to the nerve cord. From the ovary the ova- fall into the body cavity, and thence pass into the neigh- bouring single uterus (fig. 314, b, U), which is provided at its base with a, trumpet-shaped opening (7V) and opens to the exterior on the ventral surface behind the mouth. This uterus ought probably to be considered morphologically as a segmental organ, which has only been developed on one side. The generative organs of the small Turbellariau-like males which are met with in the uterus of CfEPHYREA. CH.ETIFERA. 389 the female of Bonellia have the same relations (fig. 31:5). These rudimentary males are furnished (in many species) with two ventral hooks, in front of which in the anterior region is placed the external opening of the vas cleferens. The vas deferens corresponds to the uterus of the female, and is in like manner provided with an internal opening into the body cavity. In Echiurus there are two pairs of brown tubes, which function as generative ducts and reservoirs. In Thalassema there are, according to Kowalevski, three pairs of such tubes. The development shows many points of similarity with that of the Annelida. Be- tween the Achceta and Chcetifera, however, there are considerable differences. In both cases a metamorphosis follows the embryonic development. The larvae resemble Loven's larva (larva of Polygordius) ; but in the AclurM they are characterised by a great de- generation of the apical region (prseoral lobe) and the absence of a prjeoral band of cilia. The remarkable larva known as Actino- trochti,, which is the young stage of the tubicolous genus Phoronis,* is distinguished by the possession of a contractile prreoral lobe, behind which there is a circle of ciliated ten- tacles forming a collar. The Gephyrea are all marine. Some of them live in sand and ooze at considerable depths, also in holes in the rocks and in crevices between stones and corals, and in the shells of snails. Their food is similar to that of Holothurians and many tubicolous Annelids. Order 1. CH.ETIFERA = ECHIUROIDEA. FI&. 313. Plauariau - like male of SoneUUt (after Spengel). D. Intestine ; WT, ciliated funnel of the vas deferens (Vd), which is filled with sperm. Gephyrea characterised by the presence, of two st.rong hooked setce on the ventral side and by a terminal anus. The month is placed lit the base of the prworal lobe, which is developed into a, proboscis. The Ecliiuroidea or chretiferous Gephyrea present no external segmentation of their elongated and contractile body; they have, however, in the young state the rudiments of 15 rnetanieres. This * There should be a third order of Grpkyrea for these animals. 390 ANNELIDA. fact, as well as the formation of the prseoral lobe and the develop- ment of the ventral hooked seta?, points to a close relationship with the Clifvtopoda. In the adult animal, however, the internal segmen- tation is very little marked. The dissepiments, with the exception of the first, which forms a partition between the head and the body, are lost, and the segmentation of the ventral cord is only indicated by the distribution of the nerves. The supra-cesophageal ganglion remains at the apical region of the pneoral lobe (proboscis) ; hence the oesophageal commissures are extraordinarily long. The strongly developed piworal lobe forms a proboscis -like FIG. 314. a, female of BoneUhi viriHis (after Lacaze-Duthiers). I, Integument and generative organs after the intestine has been removed. Hd, Cutaneous glands ; Al, anal vesicle'; Ad, rectum ; On, ovary ; TV, ciliated funnel of the titerus (V). c, Anatomy of BoiifUin viridi* (after Lacaze-Duthiers). D, alimentary canal with anal vesicles (Ab) ; M, mesen- tery; U, uterjs; .R, proboscis. appendage which may develop to a considerable length and become forked (Bonellia) (fig. 314 a). A pair of hooked set* (with reserve seta? in the sheath of each seta) are always present on the first segment of the body. In Ecliiurus there are also one or two circles of set* at the posterior end of the body. There are from one to three pairs of anterior segmental organs (so-called brown tubes or ventral glands), which open on the ventral surface and are used for the passage outwards of the generative products. Besides these there is also a pair of GEPHTRBA. CH.ETIFEEA. 391 posterior segmental organs (anal vesicles, fig. -'314, Ab) in the terminal segment, each of which has a immber of peritoneal funnels and opens into the rectum. In Bonellia the segmental organ which performs the function of uterus is, like the ovary, single (tig. 314 ft). Development. The development of the ovum begins with an unequal segmentation. In Bonellidi the small cells of the animal pole grow round the four large yolk spheres, which give rise to the entoderm, leaving a small aperture, the blastopore (fig. 110). The Ecldnrus larva- (iig. 315) are the most accurately known. They present the type of Loven's larva and possess a strongly developed FIG. 315. n, Larva of Echiui-us from the ventral side (after Hatschek). SP, apical plate; Prw, prreoral circle of cilia ; Pow, postoral circle of cilia; En, head-kidney ; Vg, ventral ganglionic cord connected with the apical plate by the long- cesophageal commissures ; AS, anal vesicle. l>, Ventral region of the Echlurus larva with segmented mesodermal hands ; SC, oesophageal commissure ; Dap, dissepiments of the anterior body segments ; MS, mesodermal bands ; A, anus. prseoral circle of cilia (Prw), in addition to which there is also a delicate post-oral circle of cilia (Pow). Early in larval life a seg- mental organ, the head kidney or pi'onephros (A'-V r ), is developed, one on either side; and behind' it a pair of mesoblastic bands makes its appearance and gives rise in the subsequent development to the rudiments of 15 segments (fig. 315 b). In the terminal segment, which is surrounded by a circle of cilia, there appear segmeutal 392 ANNELIDA. organs, which give rise to the a mil vesicles (fig. 315 a, AS). The rudiments both of the cerebral ganglion and of the ventral cord are derived from growths of the ectoderm, the former from the apical plate, the latter as a paired thickening of the ventral ectoderm. The two are connected by the cesophageal ring, which is also provided with ganglion cells. In older stages, after the disappearance of the segments, the ciliary apparatus begins to degenerate and finally vanishes; after which two strong hooked seta? make their appear- ance at the sides of the nerve cord not far from the mouth, and two circles of shorter seta? are formed at the hind end of the body (fig. 316). The pra?oral lobe of the larva becomes the proboscis of the young Echiurus (fig. 311). Fam. Echiuridae. The anterior end of the body above the mouth is elongated into a pro- boscis, the under surface of which is grooved. The long cesophageal commissures lie in the pro- boscis, and meet in front without any cerebral enlargement. Anteriorly and on the ventral side are two setee for attachment, and on the poste- rior end of the body there are sometimes circles of setre. The anus is terminal. Ei-li'nirux Pal- Ittxii Gnerin (Gtirrtnert Quatref., St. Vaast), coast of Belgium and England. Thalassema (/if/nx M. Miill.. Italian coast. linn-cU'ni cii-idix Rolando. Mediterranean. The males are small and rudimentary, and resemble Plaiiarians. They live in the efferent ducts of the female generative organs. BK Order 2. ACHJETA= SIPUNCULOIDEA. Gephyrea with terminal mouth, dorsally placed anus, and without setce. The ante- rior region of the body is retractile. The Sipunculoidea differ from the chsetiferous Gepliyrea in their entire want of all traces of nietameric segmentation^ in the degeneration of the praeoral lobe and in the position of the mouth and anus. The elongated body is destitute of a projecting prseoral lobe, so that the mouth, which is frequently surrounded by a circle of tentacles, comes to be placed at the anterior end of the body. On the other hand, the anus is moved far forward on the dorsal surface (fig. 317). FIG. 316. Older Echiurus larva seen from the side. The head kidney is atrophied. O, mouth ; M, stomach ; A, anus ; -B-Zr, circles of set* ; SC, ceso- phageal commissure ; AS, anal vesicles ; G- , cerebral ganglion, developed from the apical plate; 7"V/, ventral nerve cord ; H, ventral hooks. GEPHYREA. ACII.ETA. 393 The cerebral ganglion, cesophageal ring and ventral cord run inside the dermal muscular tunic. Only one pair of segmental organs, known as brown tubes or ventral glands, is present. The blood vascular system is well developed. Development. The segmentation is com- plete and is followed by the formation of a gastrula by invagination. The blastopore marks the ventral side. The two posterior marginal cells* of the entoderm move in- wards as primitive mesoderm cells, and give rise to the mesoblastic bands which do not undergo segmentation. Invaginations of the ectoderm of the animal pole and ventral sur- face of the em- bryo give rise to cepha- lic and ventral p late s respec- tively, while the remain- der o f the ecto- d e r m cells grow round thes-e and form an external envelope for the embryo of the nature of a serous mem- brane (serosa). Cilia project from the latter through the pores of the vitelline membrane and are employed by the embryo in FIG. 317. Quite young Si- piincnliiK still without ten- tacles (after B. Hatschek). O, mouth ; A, anus ; US, ventral cord ; JV, nephri- dhim (brown tube) ; , apical plate ; A, anus ; PoW, postoral circle of cilia; N, kidney. The cephalic and ventral plates soon grow together. The mesodermal bands split into somatic and * Compare especially B. Hatschek. 394 ANNELIDA. splanchnic layers, and give rise to the rudiments of the two seg- mental organs ; while the oesophagus arises as an invagination of the ectoderm, and a postoral circle of cilia is formed around its opening (fig. 318). The serous membrane is cast off with the egg membrane, and the larva then contains all the essential organs of the adult Sipunculus except the ventral cord and the blood-vessels. At a later stage, during the growth of the larva, the ventral cord is developed from the ectoderm, the circle of cilia disappears, the first tentacles sprout out at the edge of the mouth, and the metamor- phosis of the free-swimming larva into the creeping young fiipun- culus is completed. Fara. Sipunculidae. Body elongated and cylindrical, the anterior part re- tractile. The mouth is surrounded with tentacles, and the anus is dorsal. The intestine is coiled spirally. SijniH-riiln* nudus'L., Mediterranean. Pliascolosoma Iceve Kef.. Mediterranean. Pli. /'lnr/at>tm Kef. St. Vaast. Fam. Priapulidae. Anterior part of the body without circle of tentacles. Pharynx armed with papillae and rows of teeth. Anus at the posterior end of the body and slightly dorsal, above it there usually projects a caudal appen- dage which bears papilla-like tubes (branchia?). The intestine is straight. Priajnilux ruud/itus 0. Fr. Miiller. Ilallrrypttix xpiniilosnx v. Sielx, Baltic, Spitzbergen. Sub-class 3.- -HIRUDIXEA*=DISCOPHORA, LEECHES. Body either with short rings or not ringed, without parapodia, with terminal ventral sucker, hermaphrodite. . The body of the Hirudinea, so far as its external form is con- cerned, recalls that of the Trematoda, with which group the Hirudinea have often been incorrectly connected. Externally the body is marked by a number of transverse rings, which are short and may be more or less indistinct or even entirely absent. These rings correspond in no way with the internal segments, which are separated by transverse partitions or dissepiments ; but they constitute much shorter portions of the body, four or five of them corresponding to one internal segment. The large sucker at the posterior end of the body serves as an organ of adhesion ; and there may be in addition a second smaller sucker, either in front of or * Brandt and Batzeburg. " Medicinische Zoolo.sie." 1829. Moquin-Tandon, "Monographic de la famille des Hirudinees," 2nd. edit.. Paris, 1846. Fr. Leydig. fi Zur Anatomie von Pisciccla geometrica." Zeitschv.f&r irixx. Zm>L. Tom. L, 1849. H. Rathke. " Beitrage zur Entwickelungsgeschichte des Hirudineen." edited by R. Leuckart, Leipzig. 1862. E. Leuckart. " Parasiten des Menschen.'' Brl. I., Leipzig, 18(58. Van Beiiedenet Hesse. ;> Recherches sur les Bdelloides ou Hirudindes et les Trematodes marins." 1863. Robin. " Memoire sur le developpe- ment embryogenique des Hirudinees." Paris. 187"). HIRUDINEA. 395 surrounding the mouth. There are no parapodia few exceptions, are absent. A sharply distinct head is never developed, since the first rings are not essentially different from those following and are never furnished with tentacles or cirri. Alimentary canal. The mouth is situated near the anterior end of the body, sometimes at the bottom of a small anterior sucker (RhynchobdelUdce), sometimes at the base of a projecting spoon-shaped hood, which resembles a sucker (G nathobdellidcr} (fig. 319). The mouth leads into a muscular pharynx provided with glands. The anterior part of the pharynx, which may be distinguished as the buccal cavity, is armed (Gnathobdetti- dce) with three serrated chiti- nous plates (fig. 319, a, b), or more rarely with a dorsal and ventral plate (Branchi- obdellidce), or it is provided with a protru- sible proboscis, which lies free in its anterior part (Shynchobdellidce). The pharynx leads into a stomach, which forms a straight tube in the axis of the body and sometimes shows con- strictions, which correspond with the segments ; sometimes it is produced into a larger or smaller number of lateral caeca. From the stomach a short rectum, which is sometimes also provided with cfeca, leads to the anus. The anus is placed at the posterior pole of the body, dorsal to the sucker. Excretory organs. Segmental organs are pre- sent, one pair to each segment in the middle region of the body. Their number, however, and setje, with a FIG. 319. c Cephalic region of the Medicinal Leech. The three jaws are visible, b, One of the jaws isolated with the finely serrated free edge. FIG. 320. Lonsritudina section through the Medicinal Leech (after R. Leuokart). D, in- testinal canal; G, cerebral ganglion ; Gk, ganglionic chain ; Ex, excretory canals or segmental organs (water vascular sy.~- tem). 896 ANNELIDA. SI, A varies very considerably, since, for instance, Branch iobddl a astaci, parasitic on the gills of the cray-fish, has but two pairs, while the Gnathobdellidce usually possess seventeen pairs. Unicellular glands are present in the Hirudinea in great numbers in the skin and in the deeper layers of the connective tissue. The former secrete a finely granular mucous fluid, which covers the skin ; while the more deeply situated glands, which lie beneath the dermal muscular tunic, secrete a clear viscid substance, which quickly hardens outside the body and is us-ed to form the cocoons when the eggs are laid. These glands are espe- cially numerous in the region of the genital openings. A blood-vascular system is always present, but in different degrees of development. Portions of the body cavity are transformed into vessel- like trunks, and as a result of this organs which lie in the body cavity seem to be enclosed in blood sinuses. The two lateral vessels and the me- dian blood sinus, which always en- closes the ventral ganglionic chain and sometimes also the alimentary canal (Clepsine, Piscicola), may be interpreted in this manner. In most of the Gnathobdellidce the blood is red, the colour being due to the fluid part of the blood and not to the corpuscles. Special respiratory organs are wanting, excepting in Brcmchellion FIG. 321. Anterior end of SintJo (after * iv i i^ i i i Ley.lig). G, Cerebral ganglion with antl S0me allied leecheS > whldl P S - subcesophageai ganglionic mass ; Sp, sess leaf-like branchial appendages. X^f The nervous system* in all cases is highly developed. The cerebral ganglia are characterized by a peculiar arrangement of the nerve cells which give rise to swellings on the surface of the ganglia (described by Leydig as a follicular arrangement) (fig. 321). x Hermann, ;i Das Centralnervensystem von Hirudo medicinnlis." Mimchen. 1875. HIRUDINKA. 397 This is also the case with the ganglia of the ventral cord, and especially with the sub-oesophageal ganglia, on which there are often four longitudinal series of such ganglionic swellings, two median and ventral, and two lateral projecting dorsally. The two longitudinal trunks of the ventral ganglionic chain are invariably closely approached to one another in the middle line, and their ganglia are connected together in pairs by transverse com- missures. In the Gnathobdellidce two nerve trunks are given oft' to the right and left from each pair of ganglia, while from the brain and the last ganglion, which may be called the caudal ganglion and is formed of several ganglia fused together, a much greater number of nerves pass off. The nerves passing oft 1 from the brain supply the sense organs and the mus- cles and skin of the cephalic disc (anterior sucker) ; the nerves of the ventral chain are distributed in their proper segments, and those of the terminal ganglion supply the ventral sucker. An unpaired median longitudinal cord (Faivre, Leydig), which passes from ganglion to ganglion between the two halves of the ven- tral cord, most probably corresponds to the unpaired nerve which Newport discovered in insects. A system of risceral nerves was dis- covered by Brandt. It consists of an intestinal nerve, which arises from the brain and runs close to and above the ganglionic chain and sends branches to supply the cieca of the in- testine. Three ganglia, which in the common leech lie in front of the brain and send their nerve plexuses to the jaws and pharynx, are considered by Leydig as enlargements of cere- bral nerves and very likely control the move- ments which occur in swallowing. Almost all leeches possess simple eyes on the dorsal surface of the anterior ring. In addition there are cup-shaped organs (in Hirndo tnedicinalis about sixty) on the cephalic rings. These probably give rise to a sense perception comparable to the sensation of taste. Generative organs. The Hii-n<]'nn' are hermaphrodite. As in many marine Pltmmene Organismen;" Leipzig. 1838. Dujarctm, " Histoire naturelle des Infusoires," Paris. 1841. Dalrymple, Phil. Tninx. Hoy, Soc. 1844. Fr. Leydig, ' Ueber den Bau unddie systematise-he Stdlung der Kivderthiere," Zr'itxdn: fur MV.V.V. Zool.. Ed. VI.. 18o4. F. hn, "Ueber Radt-rthiere," Zetxchr. fiir wis*. ZvoL. P.d. VJl., 1856. Bd. IX., 1858, Bd. XII.. 18(>2. Gosse, " On the Structure, Functions and Homologies of the Manducatory Organs of the class Rotifera,"/'///'/. Trait*., 1856. W. Salensky, ' Beitriige xAir Entwickelungsgeschichte des Brachionus urceolaris," Xcitxclir. fiir n-i*x. Ziwl.. Tuin. XXII.. 1872. KOTIFKRA. 401 a defined and verv dissimilar regions, but the internal organs show no trace of any corresponding segmentation. There is therefore no true segmentation, i.e., division of the body into metarneres. It is usually possible to distinguish an anterior region of the body, in which the whole of the viscera are situated, and a posterior movable foot-like region, which terminates in two opposed pincer-like styles and is used both in locomotion and for attachment. The broad anterior portion of the body, as well as the narrow posterior region, is often divided by transverse constrictions into several rings, which can be drawn into one another like the rings of a telescope and can be bent more or less freely upon one another. The anterior cili- ated and usually re- tractile apparatus which projects at the anterior end, and is termed the trochdl disc, or from its like- ness to a rotating wheel, the wheel ar- il fin, is an important characteristic of the Rotifera. Very fre- quently, especially in the parasitic forms, this trochal disc is re- duced, and in certain cases entirely aborted (Apsilus). In Notom- innta tardigrada the trochal disc is reduced to a small ciliated lip round the mouth; in Ilydat'tHd (fig. 324) to the margin of the head, the whole circumference of which is ciliated. In other cases the ciliated edge projects over the head and forms the so- called double wheel, e.g., PJiilodina, Brachionus, or becomes a ciliated cephalic shield, e.. iiiilifni-ix Ehrbg.. Euchlnnix triqurtra Ehrbg. Fam. Hydatinidse. Edge of wheel-organ prolonged into numerous processes (multifid) or only sinuous ; skin delicate, often ringed ; foot short, usually forked, with two setas or pincer -shaped. Hydathia Ehrbg.. //. xenta 0. Fr. Miill. with E>t1-('i-/n/li'n lii/flittinrr Ehrbg., as male, yotommatn tardlijrada, Ldg.. N. Brachioniis Ehrbg., JV.jMrfixUti Ehrbg. Fam. Asplanchnidae. The sac-like unarmoured body is destitute of rectum and anus. Aspla/ncJina S/cl/uliHi Ldg.. -1. mi/niit'li'o Ehrbg., Axromorjrfia germanica Ldg. Two groups of small animals are allied to the Rotifera : (1) the Echinoderidae which Dujardin and Greef regarded as connecting links between Vrnii/'s and Arthropoda (EcJvinoderes I)ii/r. Al?o the works of H. Ludwitr and 0. Biitschli. ARTHKOPODA. 405 CHAPTER X. ARTHROPODA. Lateral h/ i/m metrical a'nininln u-itli keteronomously segmented body and jointed x>'i/itt<:ntj" mlniji's ; ftth l>raiit (iniprtKi-sophd.yeal ganglia) and ventral nerve cord (ganglionic chain). The most important characteristic which distinguishes the ArtJ/ro- poilti from the closely allied segmented worms, and is an essential condition of a higher organization and grade of life, is the possession of jointed segmental appendages which serve as organs of locomotion. In place of the unjointed parapodia of the Chcetopoda, jointed appendages more adapted for locomotion and confined to the ventral surface, are present. Every segment may possess a ventral pair of appendages which, in the simplest case, are short and consist of only a few joints (Peripatus) (fig. 325). While in the Annelida loco- FIG. 325. Penpatui nij^-nnis (after Moseley). motion is effected by the movements of the segments and undulatory movements of the whole body, in the Arthropod n the function of locomotion is removed from the chief axis of the body to the secondary axes, i.e., to the paired appendages, with the result of the possibility of a much more efficient discharge of the function. The appendages enable the Arthrt>j>d. A', A" Antenna?; Kf, Kf" the anterior maxillipeds on the cephalo-thorax ; B', B", S'", the three pairs of biramous feet. addition at least one jaw segment, the appendages of which may, in larval life (Nauplius), still function as legs. Usually, however, several of the succeeding segments whose appendages function as jaws form part of the head. The middle portion of the body, or thorax, is likewise distinguished by a relatively intimate fusion of some or all of its segments, as well as by the hardness of its integument. It is sometimes sharply marked off from the head, sometimes fused with the head to form a INTEGUMENT. NEKVOUS SYSTEM. 4l>7 region of the body called the cephalothorax (tig. 327). The thorax bears the appendages which are of most importance in locomotion. The posterior portion of the body, or abdomen, is composed of distinctly separate rings, and is, as a, rule, without appendages. When the latter are present, they serve partly as aids to locomotion (abdominal feet), partly for respiration, or for carrying the eggs and for copulation. More rarely, as for example in the scorpions, the abdomen is divided into a broad anterior region, the prceabdomen, and a narrow movable posterior region, the postabdomen. The skin, as in the J />///rn nc/tiata or Crustacea are the older, the Tracheata the younger types. CLASS I. -CRUSTACEA.* Aquatic Arthropoda, which breathe by means of yills. They have tiro pairs uf antennas; numerous pi if red legs on the tltorax, ml usually also on the abdomen. * Milne Edwards, ' Histoire natnrelle des Crustaces," 3 vol. and atlas. ls:;<- 1840. C. Clans. ' Untersuchungen zur Erforschung der genealogiscnen Grnud- lage des Crustaceensystems." Wien. 187). Then follow one or more pairs of weaker jaws (maxillce), and one or more pairs of maxillipeds, which more or less resemble the legs and, in parasitic forms, are often used for adhering (fig. 331). In parasitic forms, the upper and under lips not unfrequently give rise to a .suctorial proboscis, in which the styliform mandibles are placed. The appendages of the thorax, of which at least three pairs are present (Ostracoda), present an extremely various structure, in accordance with the mode of life and the use made of them. They are either broad leaf - shaped swimming feet (Phijllopoda], or bi- ramous appendages (Copepodci] ; they may serve to produce currents in the water like the feet of the Cirripedia, or they may be used for crawling, walking, and running (Isopoda, Deca- poda}. In the latter case, some of them end with hooks or chelse. Finally the appendages of the abdomen, which frequently itself moves in toto and assists in locomotion, are either exclusively locornotory as jumping or swim- ming feet (Amphipoda), in which case they usually differ from the appendages of the thorax ; or they serve with their appendages for respiration, as well as for carrying the eggs, and for copulation (Decapoda). The internal organization is not le.ss varied than is the external form. In the lower forms, the nervous system often consists of a ganglionic mass, which surrounds the oesophagus and is not further FIG. 331. Young stage (larva) <>f the Lobster (after G' 0. Sars). a, The lurva seen from tlie side ; -ff, ros- trum ; A', A", antennae; Ef" third maxilliped; /", anterior ambulatory leg. I, mandible with palp; c. anterior maxilla with two blades and palp; ', pos- terior maxilla with vibratile plate (scaphognathite) ; e, first,/, second maxilliped. 414 AEXKROPODA. segmented. This ganglionic mass corresponds to the brain and ventral cord and gives off all the nerves. In the higher Crustacea, a distinct brain and ventral ganglionic chain, which is usually elongated and of very varied form, as well as a rich plexus of visceral nerves and ganglia of the sympathetic system are always present. Of sense organs, eyes are the most widely distributed. They may have the form either of simple eyes (paired or unpaired), or compound eyes with smooth or faceted cornea ; in the latter case they are often placed on movable stalks, which are attached to the lateral regions of the head. Auditory organs are also present usually in the basal joint of the anterior antenna, rarely in the caudal plate at the posterior end of the body (jlfysis). The delicate hairs and filaments of the anterior antenna are probably olfactory organs. The digestive canal is, as a rule, straight, extending from the mouth to the anus at the posterior end of the body. In the higher forms the oesophagus is usually dilated in front of the mesenteron (uiidgut) into a stomach or crop, which is armed with chitinous plates. The mesenteron is provided with simple or ramified hepatic ca?ca. Excretory organs. The so-called shell glands of the lower Crustacea are regarded as urinary organs, as are also the glands opening at the base of the posterior antenna in the Malacostraca. Ln the Entomostraca the latter are only preserved during larval life. Short tubes, which correspond to the Malpighiaii tubes of the Tracheata, may also be present on the rectum (Amp/tipoda). The circulatory organs present every possible degree of perfection, from the greatest simplicity to the highest complication of an almost closed system of arterial and venous vessels. The blood is usually colourless, but is sometimes green or even red, and as a rule contains cellular blood corpuscles. Respiratory organs are either entirely wanting, or are repre- sented by branchial tubes on the thoracic or abdominal appendages. In the first case they are often contained in a special branchial cavity at the sides of the cephalothorax. Generative organs. With the exception of the CirripaUa and certain Isopoda, all ( 'rustacea are of separate sexes. The male and female generative organs usually open on the boundary of the thorax and abdomen, either on the last or the antepenultimate thoracic ring, or on the first abdominal segment. The two sexes are very often distinguished by a number of external characteristics. CRUSTACEA. 415 The males are smaller, sometimes even dwarfed, and then attached to the females like parasites. They almost always possess appa- ratuses for holding the females and for transferring the spermaio- plmres during copulation. The larger females, on the other hand, frequently carry the eggs about with them in sacs, the membranes of which are secreted by the so-called cement glands. Development takes place either directly or by metamorphosis. The metamorphosis is sometimes retrogressive. When the develop- ment is direct, the young animals, on leaving the egg, already have the body form of the adult. The larva known as the JWauplius (fig. 332) is of great importance as a point of departure. This larva possesses an oval body, on the ventral side of which are present three pairs of appendages for the sense of taste, the prehension of food, and for locomotion. These appendages correspond to the two pairs of antenna? and mandibles respectively. Parthenogenesis is said to occur in certain groups (P1>i/Uo- poda). Almost all Crustacea are carnivorous. Some of them suck the juices of living animals on which they are parasitic. For the systematic review of this heterogeneous group, it is convenient to divide the numerous orders into two series. 1. The small simply organized Cms- Pl& . - 332 ._ Naaplius Iarva of tacea, the number and form of wlm-i- Snimms, seen from the side. T . -11 i A First appendage (first an- appendages is very various, will be in- teima);^ second appendage eluded as EntOmOStraca (O. Fr. M tiller). (second antenna) ; Mdf, third ,1 j Til 77 appendaqre (mandible); Ob, To this group belong the orders Phyllo- upper lip ; z>, intestine. , Ostracoda, Copepoda, and Cirripedln . 2. The higher Crustacea, characterised by a definite number of segments and appendages, may be grouped together as Malacostraca (Aristotle). In this group are included the orders of Arthrostraca (Amphipoda and Isopoda], and Thoracostraca ((_'utcff( Sf<>/nf/ri. Schizopudn, and Decapoda\ In addition there is the genus Xi'l>l!. which has been hitherto erroneously placed with the Phyllopoda, but which is to be regarded as the representative of an ancient group connecting the PJij/llopoda with thf Miili-uxf,-t-; branchial appendages of the eleven pairs of legs ; T, testis. 418 CRUSTACEA. its base a vesicular branchial appendage. The anterior, or even all the legs (Leptodora) may have the form of prehensile feet, and be destitute of branchial appendages. The Phyllopods possess a large pair of eyes, which are sometimes fused together in the median line. In addition a small median simple eye (Entomostracan eye) may persist. They have a saccular or chambered heart, which controls the regular circulation. Coiled excretory organs, known as shell glands, are sometimes present ; they open to the exterior by a special aperture on the posterior maxilla. The function of respiration is performed by the entire surface of the body, the area of which is much increased by the reduplicature of the skin forming the carapace ; also by the folia- ceous swimming feet, and especially by the surface of the branchial appendages. Reproduction. The Pliyllopoda are of separate sexes. The males are distinguished from the females by the structure of the first pair of antenna? which are larger and more richly provided with olfactory hairs, and also by their anterior swimming feet which are armed with prehensile hooks. In general the males are less fre- quently met with than are the females, and, as a rule, only at definite seasons of the year. The females of the smaller Pliyllopoda (Clado- cerct) are able to produce eggs without copulation and fertilization ; and these eggs, the so-called summer eggs, develop spontaneously and produce generations containing no males. In certain genera of the Branchiopoda, e.g., Artemia and Apus, parthenogenesis is the rule ; the males, indeed, have only been known a few years. The females usually carry the eggs about with them on special appendages, or in a brood pouch beneath the shell on the dorsal surface. The just hatched young either possess the form of the sexually mature animal (Cladocera), or undergo a complicated metamorphosis, leaving the egg membranes as a nauplius larva with three pairs of appendages (Bran- chiopoda}. A few of the Pliyllopoda live in the sea, the greater number inhabit stagnant freshwater ; some of them are found in brine pools. Sub-order 1. Branchiopoda.* Pliyllopoda^ with clearly seg- mented body, often enclosed in a flat, shield-shaped, or laterally compressed bivalved shell, with from ten to about thirty or more pairs of foliaceous swimming feet. * Schiiffer. ' Der krebsartige Kieferfuss." etc. Regensburg. 1756. A. Kozu- bowski, " Ueber den miiiiiiliclien Apus cancrifnrmis." Arclilc fur yatu-njcxi-Ji. Tom XXIII. , 1857. C. Clans. " Zur Kenntniss des Baues und cler Eiitwickehniir von Branckipus und Apus," etc., GSttingen, 1873. PHYLLOPODA. BRAXCHIOPODA. 419 The alimentary canal is provided with two lateral hepatic appen- dages, which are, as a rule, branched and racemose and only excep- tionally short and simple. The heart appears as an extended dorsal vessel with numerous paired lateral slits, and may extend throughout the whole length of the thorax and abdomen (Brcmchipus). The genital organs, which are always paired, are placed by the side of the alimentary canal, and open at the boundary between the thorax and abdomen. In the females the genital openings are small slits ; in the male there may be protrusible copulatory organs at the openings (Brcmchipus). The males are distinguished from the females principally by the fact that the anterior, or two anterior pairs of legs, are armed with hooks (Estheridce), or by the modification of the posterior antenme to form a prehensile apparatus (Brancliipus). Remarkable is the rare occurrence of the males ; they seem only to appear under certain conditions and in definite generations, which alternate with parthe- nogenetic generations. The eggs during development are generally protected within the body of the mother, and are carried about either in a saccular brood-pouch of the abdomen or between the valves of the shell on filiform (Estheria, Branchipus), or in vesicular (Apus) appendages of different pairs of legs (9th to llth). The eggs, so far as is known, undergo a complete segmentation. When hatched, the young animal has the form of a Nauplius larva with three pairs of appendages, of which the anterior (which become the anterior antenna?) are in the Estheridce only represented by slightly de- veloped setigerous prominences. On the other hand, in Apus the third pair is small and rudimentary. Almost all the Bra/nchiopoda belong to inland waters, and prin- cipally inhabit shallow fresh- water pools. When the latter dry up, the eggs, preserved in dry mud, remain capable of development. Some species, as Artemia salina, are found in brine pools. JBrancMpus jpisciformis Schaff = B. xtiujnuritt L.. without a shell, found iu the lakes of Germany, together with Ajttis cdnrriformis. ]!. (tifijihanus Prev., France. Artrmin x///i///i. L.. in salt pools, near Trieste, Montpellier. They sometimes lay eggs with a hard shell, sometimes they are viviparous. Ajnut cancriforiiiix Schiiff. with shield-shaped shell. Germany. The males, which are rare, can lie recognized by the normal formation of the eleventh pair of appen- dages. They live in puddles and fresh-water lakes, together with Jirti/u'Jtijinx. E.-itlu r'ia ay rind a! dcx .loly L.. with perfect shell. Sub-order 2. Cladocera.* Water-fleas. Small laterally com- ! Besides the works already quoted, compare H. K. Strauss. Memuire sur les Dnphnia de la classe des Crnstaces," Mem. d>i M//x. rJi.. 1873. A Weismann, " Beitrage zur Kenntiss tier Daphimiden." I IV.. Leipzi.tr. 1876 and 1877. C. Clans. " Zur Keimtiss tier Organisation in id des feinereji Banes tier Onphniden. Zelt.f. -iritis, zool., Tom XXVII. 187(5. C. < 'laus. " Zur Kenntniss ties Banes nnd tier Oiyani- saton tier Polyphemiden," Wien. 1877. C. Grobbeji. " Die Embryonalentwick- elung von Moina rectirostris," Arliriten ris and Cythere resemble legs, and end with strong hooked bristles, by help of which the animal can attach itself to surrounding objects. In the exclusively marine Cypridinidce and Halocypridte this pair of appendages has the form of biramous swimming feet, which consist of a broad triangular basal plate, a many-jointed endopodite beset with long swimming seta 1 , and a rudimentary exopodite, which, however, is stronger in the male and furnished with hooks of a considerable size. In the region of the mouth beneath and to the side of a tolerably large upper lip there are two powerful mandibles with a broad and strongly toothed biting edge. The mandibular palps, which are leg-like and elongated, are usually three-jointed and can be used as legs (Mdf). In exceptional cases (Paradoxostoma), the mandibles are styliform and are enclosed in a suctorial proboscis formed from the upper and under lips. The mandibles are followed by the first pair of maxilla?, which are in all cases distinguished by the great development of their basal portion and by the reduction of the palp. In the Ci/pridw and Cytheridce the basal joint of the first maxilla bears a large comb-like setose plate, which by its swinging movements aids the function of respiration, but does not itself function as a gill. A similar branchial plate may also occur on the two following appen- dages (the 5th and 6th pair), which sometimes have th5 form of jaws, sometimes of legs. The anterior of these appendages (maxilla of the second pair or better maxilliped, fig. 336, J/jc") functions, in < 'i/pris, chiefly as a jaw, but bears, besides the rudimentary bran- chial appendage, a short, backwardly directed, usually two- jointed palp, which, however, in certain genera and in Halocypris becomes a short, three-jointed or even four-jointed leg. In Cythere it acts ex- clusively as a leg, and represents the first of the three pairs of legs present in this animal. In the Cypridina, however, it has completely the form of a jaw, and is provided with an enormously developed branchial plate (fig. 336 a, MX"). The appendage of the sixth pair is usually modified to an elongated, many-jointed, creeping and ad- OSTRACODA. 425 hering foot. The appendage of the seventh pair is always elongated to the form of a leg ; in Cythera it is formed like the preceding one, A FIG. 336. Cypridina meil'iffi-raiti-it. u, Female; b, male. M, Stomach; If. heart; S3f, adductor muscle ; O, eye ; O', unpaired eye ; G, brain ; Sk, frontal organ ; T, testis ; P, copulatory organ; MJf, mandibular palp; MX', first maxilla ; A/.v'', second maxilla; FK, caudal fork. but in Cypris it is curved upwards, and is furnished with a short claw and terminal seta?. It has probably the same function (Putzfuss) 426 CRUSTACEA. D as the long cylindrical appendage of Cypridina, which arises in place of the seventh pair of legs, almost on the back of this animal. The nervous system consists of a, bilobed cerebral ganglion and a ventral chain with closely approximated pairs of ganglia, which may unite to form a single ganglionic mass. Sense organs. In addition to the already mentioned olfactory hairs there is a median eye (Cypris, Cy there), composed of two (often separated) halves; or there are, in addition to a small unpaired eye ? two larger compound and movable lateral eyes (Cypridina). In Halocypris and Cypridina there is a frontal appendage, which probably functions as a sense organ. Alimentary canal. The mouth, which is frequently (Cypris) armed with toothed lateral bands, leads through a narrow oesophagus into a dilated crop-like portion of the alimentary canal. This is followed by a broad and long stomach, provided with two long lateral hepatic tubes, which project into the lamella? of the shell. The anus opens at the base of the abdomen (fig. 337). Of special glands a club-shaped, dilated glandular tube (poison-glands f) found in Cythere must be mentioned, the duct of which opens to the exterior through a spinous appendage of the posterior antennae. A heart is present in Cypri- dina and Halocypris on the dorsal surface, where the shell is con- nected to the animal. The function of respiration is performed by the whole surface of the body, over which an uninterrupted current of water is maintained by the swinging movements of the leaf -shaped setose branchial appendages. In many Cypridinidce (Asterope) there is a double row of branchial tubes on the back, near the last pair of appendages. Generative organs. The sexes are always separate and are dis- tinguished by well marked differences in their entire structure. The males, in addition to the greater development of the organs of sense, possess apparatuses on different appendages in Cypridina on the second antenna?, in Cypris on the maxilliped for holding the females ; or a pair of legs may be completely modified for this pur- FIG. 337. Alimentary canal and generative organs of a female Cypris (after W. Zenker). Oe, resophagus ; P V, crop ; T", stomach ; D, intestine ; L, liver ; Oe, ovary ; SM, adductor muscle ; R receptacuhim ; I'M, vulva ; Fu, caudal fork. OSTRACODA. 427 pose. Iii addition a large copulatory organ, which may be derived from a modified pair of appendages and often possesses a very compli- cated structure, is always present. The male genital organs consist on either side of several elongated or globular testes, of a vas deferens and the copulatory organ ; the presence in Cypris of a very peculiar paired mucous gland and the size and form of the spermatozoa seem to be worthy of notice (Zenker). The female of Cypris possesses two ovarian tubes which project into the reduplicature of the carapace, two receptacula seminis, and the same number of genital openings at the base of the abdomen. Development. The greater number of Ostracoda lays eggs, which they either attach to water-plants (Cypris), or, as in Gypridinq, carry about with them between the shell valves until the young are hatched. The free development of C;/prls consists of a complicated metamorphosis. The larva;, when hatched, possess, like the Nauplius form, only three pairs of appendages, but are strongly compressed laterally, and are already enclosed in a thin bivalve shell (fig. 338). In the marine Ostracoda the development is simplified, so that the metamorphosis is entirely absent. The Ostracoda feed altogether on ani- mal matter, as it seems especially on the carcasses of different aquatic animals. FIG. 338. very young larva of Numerous fossil forms are known from "'* * aup " us sta " e ' with three pairs of appendages. almost all formations, but, unfortunately, J/, stomach; z>, intestine;. only the remains of their shells are pre- S2f ' she11 7 sck ; ; A> ' A " "* J and second antenna?; -3ft'/, served. mandible. Oypridina-. With heart and large movable paired eye. With deep excava- tion in the edges of the shell for the passage of the antennae. The anterior antennas are bent, furnished with strung seta?, aud have olfactory hairs at their extremity. The posterior antennas are biramous swimming feet. The biting part of the mandible is weak HI- entirely aborted ; palp is five-jointed, pediform. and of considerable length. The seventh pair of appendages is represented by a cylindrical ringed appendage (Putzfus>). ('//jn-iiJiini ini'difrrri/ nt:i. AstcrojH- nl>hi Hi/a dr.. Trieste. Ilt/Juri/^rix Dana. Cythcre 0. Fr. Mlill. Without heart. The anterior antennie are bent at their base and beset with short seta?. The posterior antennae are strongly developed, with hooks on the terminal joint. Three pairs of legs, of which the last is the most strongly developed. The abdomen lias only the caudal fork, of which the two branches are small and lobe-like. The testes and ovaries do not project between the lamella 1 of the carapace. The male genital apparatus has no mucous gland. They are all marine animals. The females often carrv the 428 CRUSTACEA. eggs and embryos about between the valves of the shell. Ci/tJu-rr luti'it O. Fr. Miiller, North Seas and Mediterranean. C. vtridls 0. Fr. Miill., North Seas. ( 't/pris O. Fr. Miill. With median eye, but no heart. The shell valves are light but strong, the anterior antennae have usually seven joints and are beset with long seta?. The antenna of the second pair is simple and pediform. with usually six joints. There are two pairs of legs, of which the posterior smaller pair is bent upwards towards the dorsal surface. The caudal fork is very narrow and elongated, and is provided with hooked setae at the point. The testes and ovaries project between the lamella? of the shell. The male genital appa- ratus has a peculiar mucous gland. Most of them inhabit fresh water. CyjjrisfuscaStr., C.pubn-a 0. Fr. Miill., C.fHxi'dta JUT., and others. Notodromus ininiai-hiift 0. Fr. Miill. Order 3. COPEPODA. * Entomostraca with elongated, usuallij well segmented body, without shell-forming reduplicature of the skin, with biramous swimming feet ; the abdomen is without appendages. The group of the Gopepoda includes a number of very different forms. The non-parasitic members of the groups are distinguished by a constant number of segments and paired appendages. The numerous parasitic forms differ in various degrees from those which lead an independent life ; in extreme cases some of them are so modified, that without a knowledge of their development and the peculiarities of their structure, they would rather be taken for parasitic Worms than for Arthropods. The characteristic swimming feet are, however, usually retained, though often reduced in number, as rudimentary or modified appendages. When they are absent, the developmental history gives a certain indication of the Copepod nature. Appendages. The head seems as a rule to fuse with the first thoracic segment ; and the cephalothorax so formed bears two pairs of antenna?, a pair of mandibles, the same number of maxilla?, and four maxillipeds, which last are only the external and internal branches of a single pair of appendages (fig. 341) ; and finally the first pair of swimming feet, which are not unfrequently modified in form. Then come four free thoracic segments, each with a pair of swimming feet, of which the last pair is frequently reduced and in the male may be modified to assist in copulation. Finally, the fifth pair of feet and * 0. Fr. Miiller, ' Entomostraca seu Insecta testacea. qua? in aquis Dania? et Norvegia? reperit. descripsit," Lipsia?, 1785. Jurine. ' Histoire des Monocles," Geneve, 1820. W. Lilljeborg. " Crustacea ex ordinibus tribus : Cladocera, Ostracoda et Copepoda, in Scania ocrurrentibus, 1 ' Lund., 1853. C. Glaus,." Zur Morphologic der Copepoden," Wfirzb. unf unfits. Zcitxchr., I860. C. Clans., Die freiiebenden Copepoden," Leipzig, ~ COPEPOUA. 429 the corresponding thoracic segment may be entirely absent. The abdomen as well as the thorax consists of five segments, but is with- out appendages and ends in a caudal fork, the branches of which are furnished at their points with several long caudal seta? (fig. 339). In the femalf, the two first abdominal segments usually unite to form a double genital segment, on which the genital openings are placed. The abdomen, especially in the parasitic forms, very fre- quently undergoes a considerable reduction. FIG. 339. Female of Cyclops corona tin, seen FIG. 310. An antenna of the male of from the dorsal surface. D, intestine ; OcS, Cyclops serrnlatiis. Sp, olfactory hairs . ovisacs ; A', A 1 ', antenna 1 . M, muscles. The anterior antenna?, which are usually, many- jointed, bear olfac- tory hairs, but serve in the free-swimming forms for locomotion, and in the male as prehensile arms for catching and holding the female dining copulation (fig. 340). The posterior antenna* are always shorter, and not unfrequeiitly bifurcated and adapted for clinging to surrounding objects. With regard to the oral appendages 430 CRUSTACEA. (fig. 341), two toothed, usually palped mandibles are placed be- neath the upper lip. These function in the free-living Copepoda as masticatory organs, but in the parasitic forms are usually trans- formed into pointed styliform rods, which are used for piercing. In this case they are frequently placed in a suctorial tube formed by the junction of the upper and under lips. The two jaws which follow the mandibles are weaker biting plates, and in the parasitic Copvpoda are reduced to small palp-like protuberances. The maxil- lipeds, on the contrary, are much longer ; they are used to procure food and, especially in the parasitic forms, to attach the body. The thoracic swimming feet consist of a two-jointed basal portion, and two three- jointed setigerous swimming raini, which are comparable to broad swimming plates. In the ArgulidfK these rami are much elongated, and by their numerous joints approximate to the legs of the Cirripedia. Nervous System. In all cases there is a brain giving off sensory nerves, and also a ventral cord, which either develops some ganglia in its course or is concen- trated to a common subcesophageal gan- glionic mass. Of sense organs the median frontal eye, divided into three parts (Cy- clops eye), is pretty generally present. The tactile sense is specially localized in the sete of the anterior antenna?, but is probably also present in many other parts of the body. Olfactory hairs are pre- sent as delicate appendages of the an- terior antenna?, principally in the male sex. The alimentary canal is divided into a short narrow oesophagus, a wide sto- mach which often has two blind diverticula near its commence- ment, and a narrow rectum which opens on the dorsal surface of the last abdominal segment. The surface of the intestine often seems to perform the function of a urinary organ. We find, however, at the same time a shell gland in the cephalo-thorax at the sides of the maxillipeds. In all cases the whole surface of the body performs FIG. 341. Mouth parts of Cyclops. M, Mandibles ; MX, maxilla; Kf, internal; Kf" , external niaxilliped. COPEPODA. 431 the respimfor;/ function. Circulatory organs are either replaced by the regular oscillations of the intestinal canal (( '//clojts, Achtheres), or there is present in the anterior part of the thorax above the intes- tine (Calanidoe) a short saccular heart, which may even be continued into a cephalic artery (Calanella) (fig. 53). Generative organs. The C'opepoda are of separate sexes. Both kinds of genital . organs lie in the cephalothorax and in the thoracic segments, and open right and left on the basal segment of the abdomen. Sexual differences in the form and structure of the different parts of the body are almost uniformly found. These lead FIG. 342. Metamorphosis of Cyclops, a, Xauplius larva of Cyclops serrulntns after hatching. I, Older stage strongly magnified, c, Vry young Cyclops form. AD, antennal plain U ; Ol, upper lip ; JIf, mandibnlar foot ; 3IJ, mandible ; MX, maxilla, JTj/', masilliped ; F', F", first and second swimming feet ; He, urinary concretions ; D, intestine ; Ad, rectum ; A, anus ; G, rudimentary genital organs. in certain parasitic Copepoda (Chondracanthidir, Lemceopodid'Ce) to an extremely striking dimorphism. The males are smaller and move with greater facility; the anterior antennae and the last pair of feet become accessory copulatory organs, the former serving to hold the female, the latter to affix the spermatophores. The sper- matophores are formed in the vas deferens by a mucous secretion which surrounds the seminal mass and hardens to a tough mem- brane. The females are larger than the males and often move 432 CRUSTACEA. more clumsily ; they carry the eggs about with them in sacs, placed to the right and left on the abdomen. Many of them possess a cement gland at the end of the oviduct : the secretion of this gland passes out with the eggs and gives rise to the stiff covering of the ovisacs. During copulation, which is only an external approximation of the two sexes, the male fastens one or 'more spermatophores on to the genital segment of the female, and, indeed, on to special openings through which the spermatozoa pass into the receptaculum seminis, and fertilize the ova either within the body of the mother, or as they pass out into the developing ovisacs. Development takes place by means of a complicated metamorphosis, which, in many parasitic forms, is a retrograde one. The larva?, when hatched, have the Nauplius form, with an unpaired frontal eye and three pairs of appendages. Hooked seta? on the second and third pairs of appendages serve to conduct the food into the mouth, which is covered by a large upper lip (fig. 342, ). The posterior region of the body is destitute of appendages, and terminates with two seta? at the sides of t lie anus; it corresponds to the thorax and abdomen, which are as yet undifterentiated. The alterations undergone by the young larva? in the course of their further growth are connected with a number of successive moults, and consist principally in an elongation of the body and the appearance of fresh appendages. Even in the next larval stage (fig 342, b), a fourth pair of appendages, the future maxilla?, makes its ap- ' pearance behind the three original pairs, which develop into the antenna? and mandibles. In a later stage three fresh pairs of appendages are formed. Of these the first corresponds to the niaxillipeds, while the two last pairs represent the first rudiments of the anterior swimming feet. In this stage (Metanauplius) (fig. FIG. 343. Metanauplius of Cycloysine. O, eye ; G, rudimentary genital organs ; SD, antennal gland ; A\ A 1 ', anteunse ; MJ, mandible ; A/a-, max- illa ; Mf, rnaxilliped. COPEPODA. 433 343), the larva still resembles a Nauplius, and it is only after another moult that it is transformed into the first ( ' ydops-lik.e form. It then resembles the adult animal in the structure *)f the antennae and mouth parts, although the number of the appendages and the body rings is smaller (tig. 342, c). The two last pairs of appendages already have the form of short biramous swimming feet, and the rudiments of the third and fouth pairs of swimming feet have made their appearance as projections beset with seta?. The body consists in this >tage of the oval cephalothorax ; the second, third and fourth thoracic segments : and an elongated terminal portion, which gives rise to the last thoracic segment, and to all the abdominal segments by a pro- gressive segmentation, and already terminates in the caudal fork. d FIG. 344. Adheres pn-c, F, the males, and that it IS Only two pairs of feet; A, eye; On, egg-tubes; Oe, oesophagus ; D, intestine ; M, mouth parts ; T, testis ; Td, vas deferens ; Sp, spermatophore. after copulation that they (the females) become parasitic and undergo the considerable increase in size and modification of form which characterises the female with egg-tubes. COPEPODA. 435 ^F fi m l< 1 ij ' 1. Sub-order: Eucopepoda. Copepoda with swimming feet, the mini of which are two or three jointed. They have biting or piercing and sucking mouth parts. 1. Gnathostomata. For the most part non-parasitic; oral apparatus adapted for mastication ; fully segmented body. Fain. Cyclopidae. Mostly fresh-water animals, without a heart, and with a simple eye. The second pair of antennae are four-jointed and never biramous. The feet of the fifth pair are rudi- mentary in both sexes. The male employs the anterior antennas for prehension. Cyclop* curonntux " Ols., ('nthi>i-(imj>tiis HiintttHs Cls., Jlit r/Hti-tiriix cliflifcr 0. Fr. Miill.. North Sea. Fam. Calanidse. The anterior antennae are very long, only one of them is modified for prehension. The posterior antennae are bira- mous. Heart always present. The feet of the fifth pair are, in the male, modified to assist in copula- tion. Crfiii-hiliix s/'pTe/t trioHiilix Goods.. Uinptoiniix ritxtur Jur. Irritants Pxterxonii Tempi. Fam. Notodelphyidae. Structure of body like that of the Cyclop'uhv. The posterior antenuse modified for attachment. The two last tho- racic segments are fused in the female and form a brood cavity for the reception of the eggs. They live in the branchial cavity of As- oiclians. NotodelpTvys ay His Thor. 2. Parasita* (Siphonosto- mata). Mouth parts adapted for piercing and sucking, usually with incomplete seg- mentation of the bod}- and reduced abdomen. The posterior antennae and maxillipeds end with hooks for attachment. Some of Go. fir FIG. 310. Lcrncea brunch'uilis. a, Male (about 2 to 3 mm long). Oc, Eye; G, brain; T, testis ; M, stomach ; F' to -F' v , the four pairs of swimming feet ; Sj>, spermatophore sac. li, Female (5 to 6 mm. long at the time of copulation). A', A", the two pairs of an- teiimv ; D, intestine ; R, proboscis ; Mi-f, maxillipeil. c, Female of Li-rtitea ljfihtx 0. Fr. Mull. The anterior pair of maxillipeds modified into large suckers. There is a styliform spine apparatus. A.folitici'tix L. (Pou de poissons, Baklner) parasitic on Carps and Sticklebacks. A. coregvni Thor., A f/itja/ttcns Luc., Gynqiclfix Hell. The maxillipeds end in a claw ; styliform spine absent. G. Kullai-i Hell, parasitic on the branchiae of Hydrocynn, Brazil. G. Doradix Corn. i Order 4. CIRRIPEDIA.* Fixed, and for the most part hermaphrodite Crustacea with indis- tinctly segmented body enclosed l>y a reduplication of the skin, and a calcareous valved shell. As a ride, there are six pairs of biramous thoracic appendages. On account of the resemblance of their shell to that of the mussels, the Cirrtpedia were held to be Molluscs until Thompson and Burmeister, by the discovery of their larva-, satisfactorily proved that they belong to the Entomostraca. They are enclosed in a mussel- * Compare S. V. Thompson. "Zoological researches," Tom. I., 1829. H. Burmeister, " Beitriige zur Naturgeschichte der Rankenfussler." 1832. Ch. Darwin, " A monograph of the Sub-Class Cirripedia," 2 vol., London, 1851-1854. A Krohn, ' Beobachtungeii iiber die Entwickelung der Cirripudien," An-liiv fur Natvrycxch I860. C. Glaus. " Die Cypris-ahnliche Larve der Cirripedien, etc," Marburg, 1869. R. Kossmann, " Suctoria und Lepadiiia," Wurzburg, 1873. CIRRIPEDIA. 439 like shell composed of several (4, 5 or more) pieces. These pieces, which originate by the deposition of calcareous matter in the chi- tinous covering of a large reduplicature of the skin (mantle), are distinguished as scuta, terya, and carina. The animal is invariably fixed by the anterior end of the head, which in the LejxididiK (h'g. 348, a) may be drawn out into a long stalk projecting freely from the shell. In the Balanidw, which are without the stalk (fig. 348, 6), the body is surrounded by an external calcareous tube, usually com- posed of six pieces; the aperture of the tube is closed by a sort of operculmn formed of calcareous plates lying inside (fig. 348, I}. In Te Kii. 34S. , Lcpa* after removallof the right shell. A', Anterior antenna? at the end of the stalk ; C, carina ; Te, tergum ; Sc, scutum ; 3/, oral cone ; F, caudal fork ; P, cirrus or penis ; M, muscle. I, Sahnms tint'tnnali'hf,,, (after Ch. Darwin), one-half of the shell has been removed; Tu, Section of the outer shell ; On, ovary; CW, oviduct; Oe, opening of oviduct; Ad, adductor muscle; Sc, scutum ; 2V, tergum ; A', anterior antenna?. both cases the attachment is effected principally by the hardening of the secretion of the so-called cement gland, which opens on the penultimate joint of the small and delicate anterior antenna?; this joint being dilated to form a sort of sucker. The body, which is surrounded by the mantle and its shell-plates, lies with its hinder region stretched upwards so 'that the appendages, which are used to cause currents in the water, may be protruded from the slit-like space left on the ventral side between the paired scuta and terga. Appendages and external features. A head with antennae and 440 CRUSTACEA. jaws can be distinguished from the region of the body (thorax) bearing the biramous appendages, but there is no distinct boundary between these two regions. The anus is situated at the extremity of the small stump-like abdomen, which succeeds the thorax and is often only indicated by two caudal appendages. Posterior antennae are in- variably absent, while the anterior pair persists, even in the adult, as small organs of attachment. The oral apparatus is situated on a ventral prominence of the cephalic region, and consists of an upper lip with palps, two mandibles and four maxilla?, of which the two last unite to form a sort of under lip. On the thorax there are usually six pairs of many -jointed biramous appendages, the elongated cirriform rami of which are richly beset with hairs and seta? and serve to set up currents in the water in which the particles of food are brought to the animal. The stump-shaped abdomen bears an elongated cirrus, which is bent to- wards the ventral surface- between the thoracic appendages, and con- stitutes the male copulatory organ. There are numerous and very pecu- liar variations in the shape of the whole body. Not only may the de- position of calcareous matter in the mantle be wanting, and the bira- mous thoracic appendages be reduced in number or even absent, but the mouth parts and the appendages may also 1)6 lost (Peltogastridce), and the body may be reduced to the form of an unsegmented tube, sac, or lobed disc. Nervous system and sense organs. The Cirripedia possess a paired cerebral ganglion and a ventral chain of ganglia, of which there are usually five pairs, but which are sometimes fused to a common, ganglion mass (Balanidce). There is a double eye, which, although rudimentary, corresponds to the unpaired Nauplius eye. An alimentary canal is absent only in the Rhizocephala. In the FIG. 349. The organization of Lc^ax, after removal of the integument. Cd , Cement gland and duct ; L, liver ; T, testis ; I'd, vas defereus ; OK, ovary ; Od, oviduct ; Cf, thoracic appendages. Other letters as in fig. 348. CIRRIPEDTA. 441 Lepadid(f and the Bcdunidce, the alimentary canal consists of a nai-row oesophagus, a saccular dilated stomach provided with several caecal (hepatic) diverticula, an elongated chyle-forming intestine, and a short rectum, which is only sometimes clearly marked off from the intestine (fig. 349). The Rlnzoceplmla (fig. 354, a), which are with- out an alimentary canal, possess root-like processes of the paren- chyma, which ramify in the viscera, especially the liver of Decapods, and absorb from them endosmotically the nutritive "juices (as in Anelasma). Special glandular organs, the so-called cement glands (peculiar to the Cirripedia), open on the sucker of the persistent (anterior) antenna; : the animal is fixed by their secretion, and the Rhizocephala alone seem to be en- tirelv without v r them. A heart and vascular sys- tem seem to be wanting in all cases. The tubes which are present on seve- ral thoracic ap- pendages o f m: i ny Lepadidce are regarded as branchiae, as are also two plicated lamel- la? on the inte- rior of the mantle of the Balanidce. Generative organs. The Cirripedia are, with a few exceptions, hermaphrodite. The testes are branched glandular tubes, and lie at the sides of the alimentary canal (fig. 349, T). The vasa deferentia which dilate into vesicular seininales reach to the base of the cirri- forrn penis, in which they unite to form a common ductus ejacula- torius opening at the point of the penis (IV). The ovaries in the Balanidce lie in the basal part of the body cavity (fig. 348, Or) ; in the Lepadidce (fig. 349) they are moved into the prolongation of the head, which is known as the stalk. The oviducts, according to FIG. 350. Alci/i/if lamjias (after Ch. Darwin.) a, Male, very strongly magnified; A', antenna? ; T, testis ; I ', seminal vesicle ; D, redu- plicative of the skin; 0, eye; P, penis. 6, Longitudinal section through female ; F, maxilliped ; Cf, the three pairs of legs ; <3o> ovary. 442 CRUSTACEA. FIG. 351. a Later'Nauplius larva. A, anus ; 01, proboscis with mouth; //, frontal horns; D, intestine ; A', A", 1st and 2nd antenna; J.M/', mnndibular foot (third pair of appendages). b, Metauauplius larva of Bcilanug befoie the moult. Beneath the skin are the rudiments of the lateral eyes (O) and all the appendages F' to F"' of the Cypris stage ; Ff\ frontal filament ; 0', unpaired eye ; Dr, gland cells of the anterior horns ; A', the antenna 1 with suctorial disc ; MX rudiment of maxilla. Krohn, open 'on a prominence on the basal joint of the anterior pair of thoracic appen- dages. The eggs accumulate in the cavity between the mantle and the body in large thin - walled flat- tened sacs, which, in the Lepadidce, are attached to a fold of the mantle and are packed to- gether on the dor- sal surface of the- animal. In spite of the hermaphrodit ism, there are, accord- ing to Darwin, in certain genera (Ibla, Scalpettuni) very simply orga- nised dwarfed males of peculiar form, the so-called complement al males, which are attached like para- sites to the body of the hermaphro- dite. There are also dioecious Cir- ripedes with a strongly marked dimorphism of the sexes. This is the case with Scalpd- CIKEIPEDIA. 443 In in oTiiatuni and I hi a Cuminyii; also with the remarkable genera Cryptophialus and Alcippe (tig. 350). The males of these forms are not only small and dwarfed, but also, according to Darwin, have neither mouth, digestive canal, nor thoracic appendages. As a rule, two or sometimes more attach themselves to the body of the female. Development. The eggs, while still within the brood-pouch, undergo an irregular segmentation. The clear cells arrange them-, selves around the food yolk in the form of a blastoderm, the ventral side of which soon becomes considerably thickened in consequence of the appearance of the mesodermic layer. The larvje leave the egg as Nauplii (tig. 351, a, b), of oval or pear-shaped form, with unpaired frontal eye, lateral frontal horns, and three pairs of appendages, of which the anterior is simple, the two next biramous and closely beset with swimming setfe. After several moults, the larva, Avhich has grown to a considerable size, enters on a new stage of de- velopment, the so-called Cypris stage (pupa) (fig. 352). The reduplica- ture of the skin has the form of a bivalve mussel-like shell, through the gaping ventral edges of which the appendages can .be protruded. While the form of the shell recalls that of the Ostracoda, the structure of the body, so far as the segmenta- tion and form of the appendages are concerned, approximates to that of the Copepoda. The anterior ap- pendage of the Nauplius larva has given rise to a four-jointed antenna, the penultimate joint of which has become large and disc-shaped and contains the opening of the cement gland, while the terminal joint bears in addition to tactile set one or two delicate lancet-shaped olfactory hairs. The frontal horns are transformed into two conical prominences near the an- terior margin. Of the two pairs of biramous appendages, those which correspond to the second pair of antenna- are cast off, while FIG. 352. Median section through a pupa nf L inif. A' Attaching antenna ; (', rariua ; 7V-, tergum ; Sc, scutum; Of, ovary ; G, cerebral ganglion ; Gg, ganglionic chain; D, alimentary canal ; Cd, cement gland ; J//,-, oral cone ; Al, abdomen ; P, rudiment of the penis ; M, muscle. 444 CRUSTACEA. the posterior pair becomes the rudiment of the anterior jaws (mandibles) of the oral cone, which is still closed and on which the first rudiments of the maxillee and under lip are already visible. The oral cone is followed by the thoracic region with six pairs of birarnous Copepod-like swimming feet, and a minute three-jointed abdomen, which terminates in two caudal appendages and caudal seta?. The pupa has a large pair of compound eyes at the sides of the un- paired eye-spot, and swims about by means of its swimming feet. It appears not to take in food. The material necessary for its further changes is stored up principally in the cephalic and dorsal regions in the form of a largely-developed fat body. After swimming about for a longer or shorter time, the pupa fixes itself by the suctorial disc of its bent antennae to some foreign body. The parts of the adult Cirripede are now visible beneath the skin, and the cement gland begins to secrete a cement, which hardens and so brings about the permanent attachment of the young animal. In the Lepadidce the region of the head above and be- tween the antenna? grows so much that it projects from the pupal integument, beneath which the calcareous pieces of the shell of the Cirripede can be seen, and after the moulting of the chitiiious skin of the pupa constitutes the fleshy peduncle by which the animal is attached, and into which the rudiments of the ova- ries project (fig. 353). The paired eyes of the free-swimming Cypris larva disappear, while the unpaired pigment spot remains. The mouth parts become fully differen- tiated, and the biramous swimming feet become short, many-jointed cirriform appendages. The Cirripedia are marine animals. The}' attach themselves to various foreign objects. They are found fixed, usually in groups, to logs of wood, rocks, mussel shells, Crustacea, the skin of whales, etc. Some, as Litholrya, Alcippe, and the Cryptopialidce, are able to bore, into Lamniellibranch shells and Corals, while the Rhizocephala are parasitic on Crustacea. In the Rhizocephala the body is saccular, Fi<;. 353. Young Lepas after disappearance of the two horny valves of the shell and the straightening of the anterior part of the head (stalk) , which in the pupa stage is bent. CIRRIPEDIA. 445 and the animal loses all its appendages and its alimentary canal, and extracts the juices of its host (Decapoda] by means of root -like processes (fig. 354). 1. Pedunculata. There is A peduncle and six pairs of biramous feet ; the mantle has usually carina, scuta, and terga. Fain. Lepadidse. Peduncle well marked, and not provided with calcareous plates. There is a membranous mantle, which, as a rule, is provided with rive shell plates, of which the scuta and terga lie behind one another (fig. 348, a). Lcpas L. (Anatifa Brug.), L.faxeJciilnrix Ellis. (?vYm/. Lam.) Found from the Northern Seas to the South Sea. L. anatifcm L., cosmopolitan. Conch oil rr in a, FIG. 35b.a,Saceulina,ywrpwea (after Fr. Mfiller). Or, Aperture of the mantle sac; II", root-like processes; K, genital aperture. l>, Nauplius larva of Kiccitlina. A',A'',Milf, appendages, e, Pupa of Lerna-adixctix /im-rel/tina- (after Fr. Miiller). F, The six pairs of legs ; Ab, abdomen ; A', attaching anteiin;e ; O, eye. Olf. (Ofin/t, Clni-riix Leach.), C. rin/dtn Spengl., frequently attached to C. at/rita L., Am-laxinu- Darwin. The stalk is provided with root -like processes, which grow into the skin of Squalidce. A. squalicola Lovcn. Fam. Pollicipedidae. Peduncle not sharply distinct, scaly or hairy. The shell plates very strong, numerous. The scuta and terga lie close to one another. There are sometimes complements! males. Polliripcx cornni-u/tifi. Leach.. Ocean and Mediterranean. Scitlju'Uiini rttlf/ftrr Leach., North Sea and Mediterranean. &. ornutiim Gray, South Africa. Hilu quadrivalvii Guv., South Australia. ./. Citnilmjii Darw., Philippines. 446 CRUSTACEA. 2. Operculata. The peduncle is absent or rudimentary. The body is surrounded by an external ring of plates at the extremity of which the scuta and terga form an operculum, which is usually freely movable and provided with depressor muscles (fig. 348, b). Fam. Balanidae. Scuta and terga freely movable and articulating with one another. The gills are formed each of a fold. Hal amis tintinnabulv/m L. Widely distributed and found in a fossil form. B. hujjivi-ixiis Darvv. Found in brackish water. Fam. Coronulidae. Scuta and terga freely movable, but not articulating with one another. The two gills formed each of two folds. Tubicinella tntrlu'itlix Shaw., South Sea. Coroiuiln Ixiheunria L.. Antarctic Ocean. C. Hindi' ma- L., Arctic Oceau. 3. Abdominalia. The irregularly segmented body is enclosed in a flask-shaped mantle, and bears on its terminal portion three pairs of cirriform feet. Mouth parts and alimentary canal completely developed. The sexes are separate. They live as parasites buried in the calcareous shell of Cirripedia and Mollusca. Fam. Alcippidse. With four pairs of feet, of which the first pair is palpiform, and the two last are imiramous and composed of few elongated joints. The sexes are separate. The female bores into Mollusc shells. The male is dwarfed, and is without mouth, stomach, or feet. Alc'ijijic lauijxts Hanc., bores into the colmnella of the shells of FIISHS and Bui-clmim. Found on the coast of England. Fain. Cryptophialidae. They have three pairs of feet at the posterior end of the body. OryptopMahts Darw., sexes separate. Cr. miniitus Darw., in the shell of ConrtiolrjHix Pcriirinnu . found on ihe west coast of South America. u' liamtita Noll, lives in excavations in the shell of Hallotis. 4. Apoda. The body is segmented, and is composed of eleven rings. There is no special reduplicature of the mantle. The shape resembles that of a maggot. The attaching antennae are elongated to the form of a band. The mouth is adapted for slicking, and has mandibles and maxillae. Feet absent. The digestive canal is rudi- mentary . They live parasitically in the mantle of other Cirripedia. They are hermaphrodite. Fam. Proteolepadidae with the single genus Pi-oti-oli'jM* Darw.. P>\ liirlncta Darw., West Indies. 5. Rhizocephala* (Suctoria). Body tubular or saccular, without segmentation or appendages ; with narrow, short peduncle for attachment, from which branched, root-like filaments arise. The * W. Lilljeborg, "Les genres Liriope et Peltogaster, " -Vow actn ri-ij. nor. scii'H: T r [ixoda and the Malacostraca. The structure and segmentation of the head and thorax resembles that of the Malacostraca, but the terminal region of the abdomen does not present the special form of a caudal plate or telson. In Nebalia we probably have to do with an offshoot of the Phyllopod-like ancestors of the Malacostraca, which has persisted to the present time. The head includes in all cases, behind the mandibular segment on which two paragnathi form a kind of underlip, the segments of two pairs of maxilla? . The latter preserve more or less the characters of Phyllopod feet. The head, therefore, consists of five segments, each with its pair of appendages, viz., two pairs of antenna-, one pair of mandibles, and two pairs of maxilla?. It is followed by the thorax, 448 CRUSTACEA. which is composed of eight segments. The eight pairs of thoracic appendages may have an exactly similar shape, and possess two separate and many-jointed rami. This form of thoracic appendage is characteristic of the Schizopoda ; in Nebalia* the thoracic appen- l-'ic. 355.Neba/ia Geoffroyi, strongly magnified, a, Female; I, [male ; S, rostrum; O, stalked eye ; M, crop ; D, intestine ; S, shell (?, vas deferens. * Xclalia is best placed in a special group. Leptoxtntru, between the Enttmiox- trtn-it and Malacostraca. The palaeozoic fossil genera Hymenocaris, Peltucurix. etc., would have to be placed in such a group. ARTHROSTRAG'A. 449 dages closely resemble the typical Phyllopod limb. As a rule, how- ever, some of the anterior thoracic legs take part in preparing the food and have a form intermediate between maxilla? and thoracic legs. Such are called foot- jaws or maxillipeds. In the Arthrostraca the anterior pair of thoracic appendages only are so modified, and the segment bearing them joins the head ; the thorax is, therefore, in this group composed of seven segments, each with its pair of appen- dages. In other groups of Malacostraca the next or two next pairs of thoracic legs have the form of maxillipeds, so that there is no sharp division between the head and thorax. The latter is, at least partially, covered by a shield-like reduplicature of the skin, which morphologically corresponds to the Phyllopod shell and forms a more or less extensive carapace, which fuses with the back of the thorax, and under which the posterior, rarely all the thoracic seg- ments may remain separate as free rings. Order 1 . ARTH ROSTR ACA . * Malacostraca with lateral sessile eyes, usually with seven, more rarely with six or fewer separate thoracic segments, and the same number of jxiirs of legs. Without a reduplicature of the skin. The head bears four antenna?, the two mandibles, four maxillae, and a pair of maxillipeds ; in all six pairs of appendages. A small bilobed plate, distinguished as the under-lip, behind the pair of mandibles, marks the boundary of the primary region of the head. The two pairs of maxillse as well as the maxillipeds are secondary cephalic appendages derived from the thoracic region of the body. Behind the head there are usually seven free thoracic rings with the same number of pairs of appendages, which are adapted for creeping or swimming. The number of distinct thoracic segments is in rare cases reduced to six (Tanais] or live (Anceus], the anterior or the two anterior segments of the thorax becoming intimately con- nected with the head. In the latter case a more or less extensive cephalothoracic carapace is formed. The abdomen which follows the thorax includes, as a rule, six segments bearing limbs, and a *imple or split plate without appendages and representing the terminal segment. The number of the abdominal segments and appendages may, however, be reduced (Isopoda), and the entire abdomen may * Besides the works of Latreille, M. Edwards, Dana, and others, compare Spence Bate and J. O. Westwood. ' A History of the British sessiloi-yrd Crustacea." Tom. I. and II., London, 1863-1868. G. O. Sars. " Histoire naturelle des Crustaces d'eau douce de Xorvetre," f'hri^tiania, 1SC7. 29 450 CRUSTACEA. even be reduced to an unsegmeiited stump-shaped appendage (Lcemodipoda). The nervous system consists of a cerebral ganglion and a ventral gan- glionic chain, which is most distinctly composed of two lateral halves. In the Isopoda there is also an impaired visceral nerve. The two eyes are always sessile, compound eyes, with smooth or facetted cornea; they are never stalked. Delicate olfactory fibres are often present on the anterior antenna?, and are especially numerous in the male sex. The alimentary canal begins with a short oesophagus, which parses upwards to open into a wide crop, supported by firm horny bands and often armed with strong chitinous plates. The crop leads into a long intestine provided with two or three pairs of tubular hepatic glands. The rectum, which may possess one or two tubular appen- dages (probably urinary), opens at the posterior end of the body. The antenna! gland opens 011 the basal segment of the posterior antenna, often upon a conical protuberance. Vascular system. A heart is always present as the central organ of the circulation. It may either have the form of a tube extending along the whole length of the thorax (Amphipoda) ; or it may be saccular and placed in the abdomen (Isopoda). In the first case the gills are placed on the thoracic feet as tubular appendages : in the latter, on the other hand, they are placed on the abdomen. From the heart the blood passes through an anterior and posterior aorta, and usually through lateral arteries. The vessels conduct the blood into the body cavity, whence it returns in regular streams to the lateral paired slits of the heart. Generative organs. The Arthrostraca are of separate sexes. The males are frequently distinguished from the females by the modifica- tion of certain parts of the appendages to form prehensile organs, by a greater development of olfactory hairs on the anterior antenna 1 , and by the position of the sexual and copulatory organs. It is rare to find a strongly marked dimorphism of the sexes (Bopyrus, Praniza). The generative organs open either at the posterior part of the thorax or at the base of the abdomen ; the female always on the ante- penultimate pair, the male on the last pair of the thoracic appen- dages or between the first of the abdomen (Isopoda). The ovaries are two simple or branched tubes with the same number of oviducts. The testes similarly seem to be composed of one (Amphipoda) or more (3) pairs of tubes (Isopoda), the efferent ducts of which (vasa deferentia) either remain separate or unite to form a copulatory AMPHIPODA. 451 organ. Appendages of the leg> may also be present as additional aids to copulation. The mature ova are, as a rule, carried about by the female in brood pouches formed by the lamellar appendages of the thoracic feet (oosteyites}. Development as a rule takes place without metamorphosis, but the form and appendages of the young animal not unfrequently differ from those of the adult animal (Phronima). The segments and the appendages may even, be incom- plete in number after birth (Isopoda). Fossil Arthrostraca are found in the Oolite (Archceoniscus). Pro- soponisci's occurs in the Permian, Amphipeltis in the Devonian. 1. Sub-order. Amphipoda.* Arthrostraca v:ith laterally compressed bod//, with gills on the thoracic feet and an elongated abdomen, of which the three anterior segments bear the swimming feet, while the three posterior bear posteriori// di- rected feet adapted for sjtriitt/iit;/ (fig. 356). The .1 i/tph ipoda are small animals, being only in rare case> several inches long (Lysianassa magellanica). They move in the water principally by spring- ing and by swim- ming. The head, which is sometimes small (Crevettina, fig. 356), sometimes large and then much swollen (Hyperina, fig. 357). is sharply distinct from the thorax and is fused with the first of the seven thoracic segments only in the aberrant group of the Lcemod ipoda. The two pairs of antenna? usually consist of a short strong shaft * Besides the older works of. De Geer. Riisel, M. Edwards, etc.. compare C. Speuce Bate, ' On the Morphology of some Amphipoda of the Division Hyper- ina," Ann. of Xaf. Hist.. Ser. 2. vol. xix.. 1857. C. Spence Bate. On the uidincation of Crustacea," Ann. of Nut. H/xt., Ser. 3. vol. i. C. Spence P.atc, " Catalogue of the specimens of Amphipodous Crustacea in the collect i>n of the British Museum." London. 1802. E. van Beneden et Em Bessels. ' Memoire sur la formation du Blastoderme chez les .Amphipod.es, etc," Bruxelles. 1868. C. Glaus. ' Der Organismus cler Phronimiden, Ai-britr/t tin-, dcm iliT IJniri-rx'ititt Wien, Tom II.. 1*7'.'. FIG. 356. Onmmai-1/n m-iilectus (after G. O. Sars), with eggs between the brood lamella? (oostegites) on the thorax. A', A", the two antenna? ; Kf, maxilliped ; F 1 to F~, the seven pairs of thoracic appendages ; Sf, the first swim- mins; foot of the abdomen. 452 CRUSTACEA. and a long multiarticulate flagellum, which, however, may be more or less rudimentary. The anterior antennae, which are always longer in the male, often bear a short accessory flagellum and present numerous modifications in their special form. In the Hyperina they are very short in the female ; while in the male they are of consider- able length and are closely beset with olfactory hairs. The posterior antennae are frequently longer than the anterior : in the male TyphidiK they are folded in a zigzag fashion, and in the Corophiidee Ov I* H G. 357. Pkronima, gedentaria, a, female; I, male. O, eyes; A, A!', the two pairs of an tennse ; Kf, jaws ; D, intestine ; H, heart and aorta ; E, gills ; OB, ovary ; N, nervous system ; Dr, glands in the chela of the fifth pair of legs ; G, genital opening. are modified to form strong pediform appendages. In the female, on the contrary, they may be degenerated and represented only by the basal joint (Pkronima) (fig. 357, a and b). The mandibles are powerful biting plates with a sharp, usually toothed edge and a lower masticating process. They usually possess a three-jointed palp, which is occasionally reduced. The anterior bi- AMPHIPODA. 453 lobed maxilla? al>o have as a rule a short, two-jointed palp, while the maxilla? of the second pair are reduced to two lamella? of considerable size attached to a common base. The maxillipeds fuse to form a ><>rt of underlip, which is either tri-lobed (Hyperina) or bears upon a com- mon basal portion an internal and external pair of lamella?, of which the latter may be considered as the basal joint of a large multiar- ticulate and frequently pediform palp (Crevettina and Lcemodipoda). Delicate lamellae or tubes, which are attached to the coxal joints of the thoracic legs, function as gills : the active movements of the abdominal swimming feet cause a constant renewal of the water around them. In the female there are in addition to the gills lamellar plates (oostegites], which are applied together under the thorax to form a brood-pouch. The males are distinguished from the females not only by the absence of the oostegites, but chiefly by the stronger development of the prehensile hooks on the anterior thoracic feet and the different formation of the antennae. The eggs pass into the brood-pouch and there develop. The yolk sometimes (G. locusta and other marine species) undergoes a com- plete segmentation. Sometimes (G. pulex), after a superficial seg mentation, a peripheral cell layer is separated, which develops into a delicate blastoderm beneath the egg membrane. A ventral primitive streak is then formed, and on the dorsal side, beneath a differentiation which has been erroneously taken for a micropyle, a peculiar globular organ makes its appeai-ance ; this is the first rudi- ment of the cervical gland (dorsal onjan), which is confined to em- bryonic life. The appendages are developed from before backwards on the ventrally flexed body of the embryo. The young animals usually possess at hatching all their appendages and in all essential points have the structure of the adult animal, but the number of joints of the antenna? and the special form of the legs still present differences. In the Hyperina alone the just hatched young may be without abdominal feet and differ so much in their form from the adult that they may be said to undergo a metamorphosis. The Atnphipoda for the most part live in fresh and salt water and lead an independent life (the presence of Arctic species in the Swedish and Norwegian seas is very interesting). Some, however, live in tubes (Cerapus), others in holes gnawed in wood (Chelxrii.). The large size of the deep-sea forms is of special interest ; amoiig>t these a Ga/tnmarid, allied to the genus Iphwnedia, and Ci/stosoma Xeptuni (Hyperidce) become several inches in length. The Hyperina 454 CRUSTACEA. live principally in transparent marine animals, especially in Mech'.sce, and may, as the female PJironima sedentaria, take up their abode with their entire brood in transparent Pyrosoma, whose internal parts they eat up. The CyamidcK among the Lcemodipoda are parasitic on the skin of whales. Ti-lbe 1. Laemodipoda. Amphipoda with cervically placed anterior leys and rudimentary apodal abdomen. The anterior thoracic segment is more or less closely fused with the head and the anterior pair of legs shifted on to the neck. The maxillipeds are modified to form a quadripartite under-lip with long palps. The branchiae are usually confined to the third and fourth thoracic segments, the legs of which are often rudimentary or are altogether wanting. The feet end with hooks for attachment. The abdomen is small and reduced to a short protuberance destitute of appendages. Cmvlla lini'firi* L. Body elongated and thin. They are parasitic on Hydroids and colonies of Brynzoa. ( 'yum us <-cti L. Body broad and flat ; abdomen quite rudimentary ; parasitic on the skin of Cetacea. Tribe 2. Crevettina. Amphipoda with small head, small eyes, and midtiarticidate pedifor m maxillipeds. Both pairs of antennae are long and multiarticulate ; in the male they are larger than in the female. The upper or anterior antennae are usually, as in Gammarus, the longer ; their shaft is composed of several joints and bears a small accessory flagellum as well as the principal one. The contrary may, however, occur, as in Corophium, where the posterior antenna? are elongated and pediform. The maxillipeds in all cases fuse together at their base and form a large under-lip, usually with four lamellae and two jointed pediform palps. The coxal joints of the thoracic- legs have the form of broad and large epimeral plates. The abdomen has always the full number of segments. The three posterior pairs of abdominal feet (uropoda) are well developed and often much elongated. This group, which includes an astonishing variety of forms, is principally distributed in the colder seas. Fam. Corophiidae. The body is not laterally compressed. The posterior antenna? are more or less pediform. The coxal joints of the legs are frequently very small. They move rather by walking. Coropliium lonij 'iconic Fabr.. dig AMPHIPODA. 455 passages iu mud. Ct'rapHxtitlmldris Say., lives in tubes. Podoet'nix ra Leach., English coast. Chiinni tn-i-lti-nns Phil, is allied here, gnaw-, with Lininnriti lii/nornm. wood-work in the sea. North Sea and Mediterranean. Fam. Orchestiidae. Anterior antenna? usually short, always without accessory ramus. The posterior pair of uropoda are unbranched and are shorter than tli'- preceding pairs. They live on the shore, especially on sandy beaches, and move by springing. TaHtntx salt xiniiii and Diphyidce, Mediterranean. Fam. Platyscelidae. Both pairs of antenna; hidden beneath the head : the anterior are small ; in the male with much swollen bushy shaft, and short, 456 CBUSTACEA. slender flagellum composed of few joints. The posterior antennas are in the male very long and folded three to four times together in a zigzag fashion ; in the female they are short and straight, sometimes quite reduced. The basal joints of the fifth and sixth pairs of legs are usually enlarged into great lamellae, which cover the thorax. The seventh pair is generally rudimentary. Entij2)hix (Tt/j)Jris Risso) ovoidex Risso (Platysci'lus -vet-ratios Sp. Bate), Mediter- ranean. OseycepJutlus pisoator Edw., Indian Ocean. 2. Sub-order: Isopoda/ :: Arthrostraca with usually broad, more or less arched body, ivith seven free tho- racic rings, with lamellar leys function- ing as -brancldcn on the short-ringed, often reduced abdomen. The structure of the body, which is flat in shape and covered by a hard, usually encrusted integument, presents a great agreement with that of the Amphipoda, to which the in many respects peculiar Tanaidce are most nearly allied. The abdomen of the Isopods is, however, usually much short- ened and composed of six short seg- ments, which are often fused with one another ; it terminates with a large caudal lamella. The abdominal legs are only exceptionally (Tctnaidce) swimming- feet ; as a rule they have the form of branchial lamellae. The sixth pair may be fin-like or styliform. The anterior antennae are, with a few exceptions, shorter than the posterior and external antennae ; in rare cases (Oniscidce) they become so much reduced that the}' are FIG. 358. AseUnt aquaticus (after hidden beneath the cephalic carapace. G. O. Sars). Female with brood . , pouch, seen from the ventral side. In exceptional cases only (Apseudes) * H. Rathke, '' Untersuchungen Ubcr die Bildung und Entwickelung dtjr Wasserassel," Leipzig, 1832. Lereboullet. ''Sur les Crustaces de la famille des Cloportides, etc," Mem. du Mvxeuni d'liht. >txt. dc Strtixln>iu-(/, Tom. IV.. 1850. N. Wagner. " Eecherches sur le systeme circulatoire et les organ us du la respiration chez le Porcellion elargi," Ann. di'x xc. nat., Ser. 5, Tom. l\.. ]><>.";. A. Uohrn, " Die Embryonalentwickelung des Asellus aquations," Zeitxehr filr wl*x. Z<>ol.,Tom. XY1T., 1867. N. Bobretzky, " Zur Embryologie des Oniscus murarius," Zritxclii-./iir iris*. Zool., Tom. XXIV., 1874. isoroDA. 457 Lrl they bear two fiagella. A.s in the Amphipoda, pale, plumous setse and olfactory cones are present on the antenna-. The mouth parts are in some parasitic Isopoda modified for piercing and sucking. The mandibles (except in Boj)yridw and Oniscidce) often bear a- three- jointed palp. On the other hand, the two pairs of maxilla?, which are usually bi- or tri-lobed, are in general without the palpiform appendage. The maxillipeds form a sort of underlip, but present great differences in the arrangement of their parts (fig. 358). As a rule the seven pairs of thoracic legs are adapted for walking or attachment, and in the female some of them are provided with delicate membranous plates (oostegites) which form a brood pouch. They never bear gills. The branchial function is dis- charged by the delicate inter- nal rami or endopodites of the abdominal limbs (pleo- pods), the anterior pair of which is frequently modified to form a large operculum overlying the following pair.-. In certain of the terrestrial Isopods (Porcellio and Arma- dillo] the opercular plates of the two anterior pairs of abdominal limbs contain a system of air spaces which ap- pear to assist respiration. The heart, unlike that in Amphi- pods, lies (except in Tandidce) in the posterior thoracic seg- ments or in the abdomen. The sexes are (except in Cymotfioidoe) separate, and the position and arrangement of the generative organs correspond in general with those of the Amphipoda. The sexes are distinguished In- external sexual characters, which in some cases (]joj>t/ri Z!, oostegite. i>, Sexual organs from a Cymuthoa : c:iirs of thoracic legs (after R. Walz)- VI, Under lip ; Ats, first abdominal seg- ment; A', A", two pairs of antenna; J/ift. mandible. 4GO CRUSTACEA. Tribe 2. Euispoda. Body with seven free thoracic segments and as many pairs of app&idacjes. Abdomen relatively short and broad, with abdominal feet modified to form branchial lamellce. Fam. Cymothoidae. With biting and sucking mouth parts, broad abdomen with short segments and shield-like caudal plate. The last maxillipeds in the form of an operculum. They live partly as parasites on fish, and partly as free-living animals. Ci/motkoa cextrnm Leach., C. cextroldex Kisso, Mediter- ranean. Anilocra meditcrmncu. Leach., JEfja Incarinata Leach.. Serolis paratJo.i'ti Fabr. Fam. Sphaeromidae. Free-living Ixopoda with broad head and short, very convex body, which can often be rolled up in a ball towards the ventral side. Splusroma fossarum Mont., in the Pontine marshes; nearly allied is the S, granulatum of the Mediterranean. 8. srrratum Fabr., Ocean and Mediterranean. It also lives in brackish water. Fam. Idoteidae. Free-living Isopodu with elongated body, biting mouth parts, and a long caudal shield formed of several segments fused together. The last pair of abdominal feet is modified to form a wing-shaped operculum for the protection of the preceding branchial feet. Itlotea entomon L., Baltic. Fam. Asellidae. Body flattened ; the last pair of abdominal feet (pleopods) are styliform (not shaped like an operculum). Jcera albifronx Mont.. British seas. Aselhix (irjHiitirHx I,., fresh- water form. A. cavaticus Schiodte, in deep springs. Limnona tcri'brtinx Leach. L. I'lgnorum, gnaws wood-work in the sea. Fam. Bopyridae. Parasitic in the branchial chamber of prawns ; the body of the female is disc-shaped, imsymmetrical. and without eyes. The males are very small and elongated, with distinctly separated segments and eyes. Bop//rus x/inillitrunt, Batr., on Palantion eyuilla. Here are allied the Entonlscidoe, which are parasitic in the body cavity of other Crustacea {f'lrripetl ia , Pagiir'lix segments, as well as a caudal plate (telson) ; lint the body is stouter and adapted to a more perfect locomotion and a higher grade of life. The thorax, instead of being composed of seven distinctly separate segments, is covered by a dorsal carapace which effects a firm and intimate fusion between the head and thorax. The degrees of development of this dorsal carapace are various. When most highly developed, it forms the dorsal integument of the anterior or of almost all the thoracic segments ; and its lateral portions only, which have the form of wings and are bent towards the ventral surface, consist of a free reduplicature. The application of the appendages differs from that in the Arthrostraca, and, indeed, varies in the different groups of the Thoracostraca. The cephalothorax has thirteen pairs, and the abdomen seven. The facetted eyes are born on two movably separated stalks. These were for a long time considered as the anterior pair of appendages, while in fact they are merely lateral portions of the head which have become jointed. Both pairs of antennae belong to the anterior region of the head. The anterior antennae or antennules as a rule bear on a common shaft two or three jiayella as the peripheral multiarticulate filaments are called and are pre-eminently sense organs. In the Decapoda the auditory vesicles are placed in the basal joint, and on one of the flagella there are delicate hairs and fibres, which are in connection with nerves and are to be looked on as olfactory organs. The second antenna? are attached externally to and somewhat beneath the antennules. They bear a long flagellum and in the macrurous Decapoda are often provided with a more or less considerable scale. A gland (the green or antennal gland) usually opens on a conical process of their basal joint. The following three pairs of appendages function as jaws ; the powerful mandibles, which are furnished with palps, lie at the side of the upper lip ; further backwards are the two pairs of lobed maxillae, in front of which and behind the mouth is the small bilobed underlip. The following eight pairs of appendages present a very " Malacostraca podophthalma Britannia;." London, 1S17 1821. V. Thompson, " On the metamorphosis of Decapodous Crustacea." Zuol. Joiu-n., vol. ii., 1831, aim Tx/.s', 1834, 1S3(>, 1838. H. Rathke, " Untersuchungen iiber die Bildung mid die Entwickelung des Flusskrebses," Leipzig, 1829. Th. Bell, "A history of the British stalk-eyed Crustacea." London, 1853. Lereboullet, Recherche* d'embryologie comparee sur le developpement du B rochet, de la Perche et de 1'Ecrevisse," Paris. 1862. V. Hensen, ' Studien iiber (Las Gehororgan der Decapoden." Leipzig, 18fi3. 462 CRUSTACEA. different form and adaptation in the various groups. As a rule, the anterior pairs are modified to assist in taking up food and are moved nearer the mouth ; these are the maxillipeds, which, with regard to their structure, hold an intermediate position between jaws and feet. In the Decapoda (fig. 362) three pairs of appendages have the form IML. 362. Male and female of Aafaeiix .fluviat'ilh seen from the ventral side. In the male the ambulatory and abdominal feet of the left side have been removed ; in the female the am- bulatory feet of the right side and the maxillipeds of both sides. A' antennules; A", nntenna? ; PI, scale of antenna ; Md, mandible vs ith palp ; MX', Jl/j", first and second maxillre Mxf to -l/j/ : , the three pairs of maxillipeds ; Goe, genital opening ; Doe, opening of the green gland ; I", 1"" , first and second abdominal foot ; Or, eggs ; A, anus. of maxillipeds, so that there are only five pairs of legs left on the thorax. In the Stomatopoda the first five pairs of thoracic append- ages are modified to form maxillipeds and there are only three pairs THOBA.COSTRACA. 463 of biramous swimming feet, which avi.se from the three posterior free Moments of the tliorax. The thoracic legs are either, at least in part, biramous (with swimming ramus), or as in the Decapods the exopodite is absent and the legs have the form of ambulatory appendages. They then terminate with simple claws : the anterior frequently with large chehe. The terminal joints may however be broad plates, in which case they can be used as swimming feet. The biramous legs of the sixth abdominal segment are. as a rule, broad and fin-like and form, together with the last abdominal segment which is transformed into a large plate (telson), the caudal fin. The feet of the five anterior abdominal segments, on the other hand, are sometimes swimming feet (Stomatopoda), sometimes serve to carry the eggs, or the anterior may assist in copulation (in the male). They may however be more or less rudimentary and some of them absent. With rare excep- tions (J///x/'/^) all the Thoracostraca po-->ess gilfe, which are either tufted or composed of regular lancet-shaped leaves. The gills are appen- dages of the liiub> ; in the Stomatopoda \'ii'. 363. Cephalothorax of Agtacusfluviafilis, after removal they are attached to O f the branchiostegite (after.Huxley). K, Gills ; B, ros the abdominal feet in trum ; < stalked e - ve : M l'> scaphognathite (of the second maxilla) ; Mxf", third maxilliped. the Sehizopoda and Decapoda to the maxillipeds and ambulatory feet. The Cumacea are without gills, except for a single pair on the second pair of maxil- lipeds. In the Decapods they are contained in a special branchial chamber beneath lateral expansions of the carapace (branchiostegite) (fig. 3G3). The organs of circulation also attain a high, degree of development, tin- highest not only among the Crustacea, but in general amongst all Arthropods. A heart and vessels are always present. In the Stomatopoda the heart has the form of an elongated tube, which extends through the thorax and abdomen, possesses numerous paired slits, and in addition to an anterior and a posterior aorta gives off to the right and left several branching arterial trunks. In the Ci'niacea, Sehizopoda and Decapoda the heart has a saccular form and lies in the posterior region of the cephalo-thorax. More rarely, 4f)4 CEUSTACEA. as in the youngest larvae of the Decajwda, only one pair of slits is present and the arterial system has but few branches. In the fully- developed Decapoda the number of paired slits is increased by the addition of a dorsal and a ventral pair, and the vascular system is considerably perfected. An anterior cephalic aorta supplies the brain, the antennae and eyes. Two lateral pairs of arteries send branches to the stomach, liver and generative organs. The posterior abdominal aorta usually divides into a dorsal and a ventral artery, of which the first supplies the muscles of the tail, the latter (known as sternal artery) sends branches to the appendages of the thorax and abdomen (fig. 364). From the ramifications (often capillary-like) the blood flows into larger or smaller canals with connective tissue walls which may be regarded as veins, and from thence into a wide blood space situated at the base of the gills. It thence passes through D Aa to, - i Q Fi<:. 364. Longitudinal section through Affaeus Jluriatilis (after Huxley). C, Heart ; Ac, cephalic aorta ; Aa, abdominal aorta, the sternal artery (Sta) is given off close to its origin; Em, masticatory stomach; D, intestine ; Z, liver; T, testis; Vd, vas deferens; Go, genital opening ; 6f, brain ; N, ganglionic cord ; Sf, lateral plate of the caudal fin. the gills and, having become arterial, passes into other vascular tracts (branchial veins containing arterial blood), which conduct it to a receptacle surrounding the heart, the pericardia! sinus ; from the latter the blood enters the heart through the slits which are provided with valves. The alimentary canal consists of a short (esophagus, a wide saccular crop and an elongated intestine which opens by the anus beneath the median plate (telson) of the caudal fin. The wide crop or masticatory stomach is supported by a firm chitinous framework, to which are affixed several pairs of masticatory plates (derived from thickenings of the chitinous lining). In the Decapoda two round concretions of carbonate of lime (Cray-fish) may be deposited in the walls of the masticatory stomach beneath the chitinous lining ; these are the so-called " eyes" and are found in the spring and summer. THORACOSTRACA. 465 The ducts of the very numerous, multilobed hepatic c-eca open into the anterior part of the elongated intestine. A simple or looped glandular tube (the green gland} opens on the basal joint of the posterior antenna. A shell gland is not developed. The nervous system is distinguished by the size of the brain, which is placed far forwards and gives off nerves to the eyes and antennae. The ventral cord, which is connected with the supra- cesophageal ganglion (brain) by very long commissures, presents very different degrees of concentration. In the brachyurous Decapods this concentration reaches its highest point, all the ganglia being fu.-ed together to form one great thoracic ganglionic mass. The system of visceral nerves is also very highly developed. Sense organs. The eyes are large and facetted. Except in the FIG. 365. Generative organs of Aifacut. oda (Mysidea) and the fresh- water Decapoda (Astacus) leave the egg membranes with the full number of segments and appen- dages. All the Stomatopoda, on the contrary, as well as most of the Decapoda, are hatched as larva? ; the latter in the so-called Zo(ca form with only seven pairs of appendages in the antei-ior region of the body (there are two pairs of antenna?, mandibles, two pairs of maxilla 1 , and two pairs of maxillipeds), without the last six thoracic segments and with a long abdomen destitute of appendages (fig. 366). The two pairs of antenna? of the Zoa-a are short and destitute of flagella. The mandibles are without a palp ; the maxilla? are already FIG. 366. Crab zosea (Thin), after the first moult. ZS, Zosea spine on the back ; Kf , ff, the two pairs of biramous appendages corresponding to the first and second pairs of maxillipeds. THOEACOSTBACA. 407 lobed and used as jaws : the four anterior maxillipeds are biramous and act as birainous swimming feet : and behind them, in the macru- rous Decapods, the niaxilliped of the third pair also appears as a biranious swimming foot. Gills are as yet wanting, being repre- FIG. 367. Larva of Pi-naeus Caftev Fr. Miiller). a, Nauplius form seen from the dorsal sur- face, b, Metanauplius stage seen from the left side; MX', anterior mnxilkr; .1/0-'', pos- terior maxilla? ; Grl, sixth and seventh pairs of appendages or first and second niaxillipeds. c, Zosea stage ; 0, eyes. sented by the thin surfaces of the sides of the cephalo-thoraeic shield, beneath which a continual current of water flowing from lit -hind forwards is kept up. A short heart with one or two pairs 468 CBTJSTACEA. of slits is present. The facetted eyes are of considerable size, but are not stalked. Between the facetted eyes there is in addition an unpaired simple eye, the Entomostracan eye. The Zowa larvae of the short-tailed Decapoda (Crabs) are, as a rule, armed with spinous processes. They usually have one frontal spine, a long, curved dorsal spine, and two lateral spinous processes of the cephalo-thoracic shield. The Zoa, however, is not by any means always the earliest larval stage. Passing over those cases in which the larva has the Zo?ea form but is without the middle maxillipeds, there are Podophthal- mata (Penceus), which leave the egg as Nauplii (fig. 367). Thus FIG. 368.- Zocea of Inachug in advanced stage with rudiments of the third niaxilliped (ff") and the five pairs of ambulatory feet (5Sp) ; C, heart ; L, liver, b, Megalopa. stage of Port units; Ab, abdomen. F' to ,F V first to fifth ambulatory legs. the developmental history proves that the series of forms of Ento- mostraca and Malacostraca are continuous. During the growth of the Zoa?a, the subsequent metamorphosis of which is quite gradual and always different, the six (five) pairs of thoracic legs, which are as yet absent, sproiit out beneath the cephalo-thoracic shield. The abdominal feet also make their appear- ance on the abdomen, and the larvae finally enter the Schizopod-like stage, from which the adult form proceeds. The Crab Zocea, how- ever, after a later ecdysis, enters upon a new larval stage, that of the Megalopa (fig. 368, 6) ; in this stage it already presents the cha- racters of the Brachyura, but still possesses a large abdomen, which is indeed ventrally flexed, but provided with a caudal fin. CUMACEA. 469 The Thoracostraca are for the most part marine, and feed on dead animal matter or capture living prey. Most of them are good swimmers ; others, e.g. numerous species of crabs, walk and run and sometimes move sideways or backwards with great agility. The chelae of the first pair of ambulatory legs (fourth thoracic appendages) constitute powerful weapons of defence. Besides the frequent ecdyses of the larval stages, the sexually adult animals cast their shell once 01- several times in the year (Decapoda}. They then live with the new ;md still soft skin for some time in protected hiding-places. Some Brachyura are able to live for a long time in holes in the earth a\vay from the sea. These land crabs undertake, usually at the breeding season, common migrations to the sea and return later to the land with their fully developed offspring (Gecarcinus ruricola). The most ancient fossil Podophthalmia hitherto known are the mac- rurous Decapoda and Scltizopoda, from the carboniferous formations (Palteocrangon, Palceocarabus, Pygoceplialus). * (1) Sub-order: Cumacea." TJioracostraca with a small cephalo-thoracic shield, (four to] five free thoracic segments, two pairs of maxittipeds, and six pairs of leys, of which at least the two anterior jmirs have the biramous tichizopod form. The abdomen is elongated and composed of six segments, and bears, in the male, two, three or five pairs of swimming feet in addition to the caudal appendages. The Cumacea, the systematic position of which was formerly very differently estimated, have a superficial resemblance to Decapod larva?, which they also recall in the simplicity of their organization ; while in many of their characters, such as the formation of the brood-pouch and.their embyronic development, they approach the Arthrostraca. A cephalo-thoracic shield is always present and includes, besides the segments of the head, the anterior thoracic segments and their appendages ; the four or five posterior thoracic segments, however, remain free. The anterior antenna; are small and consist of a three-jointed basal portion, to the end of which, especially in the male, tufts of olfactory hairs are attached, and of a short flagellum and secondary Hagellum. ' H. Kriiyer. ' Fire nye Arter afgia?gteii Ouma." .Yaf //>//. TitlxxL'r., Torn II f., 1841. H. Krb'yer, " Om Cumaceernes Familie," Xtittirli. 7Vitlra