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 th