MARINE BIOLOGICAL LABORATORY. Received v >_ Accession No. Given by /Zcc^t, i^CCc^^L- Place, *^*Ho book OP pamphlet is to be removed from the Liab- oratory uiithout the permission of the Trustees. m nj r^ cr nj a a r-=l , a ; m i a ELEMENTARY TEXT-BOOK OF ZOOLOGY GENERAL PART AND SPECIAL PART: PROTOZOA TO INSECT A. BY DR. C. GLAUS, Professor of Zoology and Comparative Anatomy in the University of Vienna ; Director of the Zoological Station at Trieste. TRANSLATED AND EDITED BY ADAM SEDGWICK, M.A., F.R.S., Felloic and Lecturer of Trinity College, Cambridge, and Examiner in Zoology in tht University of London. WITH THE ASSISTANCE OF R G. HEATHCOTE, M.A., Trinity College, Cambridge. FOURTH EDITION. VOLUME WITH 491 WOODCUTS. LONDON: SWAN SONNENSCHEIN & CO. PATERNOSTER SQUARE. 1892. PRINTED BY HAZELL, WATSON, AND VINEY, LD., LONDON AND AVLESBURY. f PREFACE TO THE ENGLISH TRANSLATION, T UNDERTOOK the translation of Professor Clans' 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 Glaus' 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 UNOEGANIZED SUBSTANCES . . . 9—14 CHAPTER II. ANIMALS AND PLANTS 15—24 CHAPTER III. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL 24-131 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. Cells and Cell-aggregates 32 Isolated cells, u.y., blood corpuscles, ova, etc 32 Epithelium 34 Epidermal exoskeleton 34 Glandular tissue 30 2. Tissues of tlie connective substance 37 Cellular or vesicular 37 Mucous or gelatinous 37 Reticular, adenoid ...... ... 38 Fibrillar 38 Elastic 39 Cartilage 3!) Osseous tissue . 40 3. Muscular tixxm- .......... 43 4. Nervous tixxue .-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" . 52 Scope of Morphology 52 STRUCTURE AND FUNCTION OF THU COMPOUND ORGANS . . 52 Digestive organs . 53 Salivary glands, liver, pancreas 58 TABLE OF CONTENTS. 5 Page Organs of circulation . • .... ^ ... 59 Heart 61 Arteries and veins 62 Heart and vessels of vertebrates 64 Organs of respiration ... ...... 67 Branchiae 69 Lungs, tracheae .... 69 Tracheal gills .... . 71 Renewal of external medium ... .... 72 Venous and arterial blood ... .... 73 Animal heat. . . 73 Orrja >is of secretion , ... 71 Kidneys 75 Water-vascular vessels and segmental organs . ... 75 Vertebrate kidneys . . 76 Cutaneous glands ' . . 77 ORGANS OP ANIMAL LIFE ... 78 Skeletal structures - , .... . . 79 Nen-ous system . . 79 Sense organs. . . 83 Tactile organs . . 84 Auditory organs 85 Visual organs . . c 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 gastrula 114 Primitive streak . . . . 115 Germinal layers 116 Theory of Gastrrca 117 DIRECT DEVELOPMENT AND METAMORPHOSIS 119 Effect of food -yolk on development 120 Explanation of Metamorphosis 121 ALTERNATION OP GENERATIONS, POLYMORPHISM AND HETEROGAMY 123 Metagenesis 123 Explanation of Metagenesis 124 Polymorphism 126 Heterogamy 127 Predogenesis 128 Heterogamy of Trcmatocla 12'J C TABLE OF CONTENTS. CHAPTER IV. Page HISTORICAL REVIEW ........ 131—139 Aristotle Pliny .... Renaissance of Sciences in Sixteenth century . "*j • • 194 Linnaeus ......... • ™. Cuvier St. Hilaire, Goethe, Oken Classification of the present day CHAPTER V. . MEANING OF THE SYSTEM ..... 139—179 Species ..... Varieties ..... Sterility of hybrids . Fertility of hybrids . Sterility and fertility of mongrels Lamark ..... Lyell's influence on Geology . 140 141 142 143 143 144 144 THEORY OF DESCENT BASED ON NATURAL SELECTION (DARWINISM) . 144 Darwin ........ .«««I Natural selection • -1 Origin of vaiieties, races and species Progressing divergence of characters, and disappearance of inter- mediate forms 1 Species according to the theory of evolution 150 Natural system 150 EVIDENCE IN FAVOUR OF THE THEORY OF DESCENT . . . 151 Evidence from Morphology .....•••• 151 frnm Dimorphism and Polymorphism 152 Sexual selection 152 Sexual dimorphism of parasites ....... 153 Polymorphism of animal communities . * . . . .155 from mimicry 155 from rudimentary organs ......... 156 from rmlryvlogy 157 Retrogressive metamorphosis 158 fnoit the facts of Geographical Distribution 159 Zoological Provinces 160 from Piil/>'iiiiti>Jiiiji) 163 Incompleteness of the geological record 1C7 — 168 Transitional forms between allied species ..... 170 Relation of fossil forms to living species . ..... 170 Succession of similar types ........ 171 Kxtinrt, Mammalia, transitional between living groups . . . 172 Extinct transitional Reptiles and Birds 175 Progressive perfection 177 Fauna of the various geological pcrio la 177 Incompleteness of the explanati >n . . . . . . . 118 TABLE OF CONTENTS. SPECIAL PART. CHAPTER VI. CHAPTER VIII. Page ' Pa?e PEOTOZOA 180 " ECHINODERMATA . . 266 RHIZOPODA .... 181 CRINOIDEA . . . . 286 Foraminifera . 184 Tesselata . . . . 289 Lobosa .... 185 Articulata . . . 289 Reticularia ... ISO ASTEROIDEA . . 290 1ST Eadiolaria 189 Stelleridea . , Ophiuridea . . 292 . 293 INFUSORIA .... 191 ECHINOIDEA . . 294 Flagellata 193 Cidaridea . . . 296 Ciliata .... 198 Cypeastridca . . 296 Holotricha ... Heterotricha . . 204 ' 205 Spatangidea . . . 297 Hypotricha . . 205 HOLOTHTJROIDEA . . 297 Peritrioha 205 Pedata 299 Suctoria .... ScMzomycetida; . 205 205 Apoda . . . . 299 Gregarinidre . . . 207 ENTEROPNEUSTA . . 259 . - CHAPTER VII. CHAPTER IX. CCELENTEEATA 209 VEEMES .... 303 Spongiaria = Porifera PLATTHELMINTHES . 309 SPOXGIA 221 Turbellaria . . . 309 My'xospongia . . 221 Rhabdoccela . . Dendroccela 313 314 Ceraospongia . . 221 Trematoda . . . 316 Halichondrire . 221 Hyalosponeia ... 221 Distomea Polystomea . . 321 322 Calcispongia . . . 222 Ccstoda . . . . 326 Cnidaria Nemertini . . 339 Enopla . . . . 342 ANTHOZOA = ACTINOZOA 223 Aiiopla . . . 342 Engosa .... 230 NEMATHELMINTHES : . . 343 Alcyonaria 231 Nematoda . . . 344 Hexactinia = Zoantharia . 231 Chsstognatha . . 357 POLTPOMEDUSJE = HYDROZOA 233 Acanthocephala . . 359 Hydromednsaj . 236 ^ ANNELIDA . 362 Eleutheroblastere Hydrocovallise . . . Tubulariw 240 240 241 Chcetopoda . . . Polychaeta . . . 367 . 374 Campanularire . 241 Errantia . . , 378 TrachymeduBse . . 242 Sedentaria . . sso Siphonophora . . 243 Oligocbseta . . . 382 Physophorids . 248 Terricoli6 . . 385 Pliysalidje 249 IjimicoliW • • 385 Calycophoridre Disco ideas 249 250 Gepliyrca . . . . 386 Scy phomedustc = A calcpha 251 Chsetifera . . . 389^ Calycoxoa 257 Achreta . . . . 392 Jlavsupialida . Discophora (AcraspeCla) 258 259 Hirvdinea . , . 394 CTEXOPHORA . 2fil ROTATORIA 400 TABLE OK CONTENTS CHAPTER X. Page Tardigrada . . . 496 1': •>' Araneida .... 498 ARTHEOPODA. . 4 n,-) Tretrapneumones . . 504 v -3 CRUSTACEA . Jlntomostraca . Phyllopoda . 411 . 416 . 416 Dipneumouea ... Phalangiidas . . . Pedipalpi .... Scorpioiiidea . 504 505 50(5 508 Branchiopix'.i . . Cladocera . . 418 419 Pseudoscorpionide.i . Solifugre .... 510 511 Ostracoda . 423 ONYCHOPHOEA 512 Copcpoda . . 428 MYEIAPODA .... 514 Eucopepoda Branchiura . . 435 436 Chilopoda. . 518 Cin-ipedia Peduncul.ita . . . 438 . 445 Chilognatha . HEXAPODA-INSECTA 520 521 Operculata 446 Thysanura , 553 Abdomiiialia . Apoda . . . 440 410 Oithoptera . , . 534 Rhizoeepliala . 446 Ortlioptera genuina . . 550 Malacostraca . . . 447 Orthoptera Pseudy-Neurop- tera .... 558 Arthrostraca . . 449 Neuroptera . . 562 Ampliipoda . . 451 Planipennia . . . 563 laopoda . 456 Trichoptera ... 564 Thoracostraca . . 4GO Strcpsiptera . . 565 Cumacea . . 469 Ehynchota . . 566 Storaatopoda . . 470 Aptera .... 567 Scliizopoda . 472 Phytopbthirea . . . 508 Decapoda . 475 Homoptera-Cicadavia . 570 JIacrura . . Brachyura Gigantostraca . 477 47S . 479 Heiuiptera . . Diptera .... Pupipara . . . 571 572 575 Merostomata . . 479 Bracbycera ... 575 Xiphosura . . . 480 Nemocera . . . 577 Trilobita . ARACHNIDA . . 483 . 484 Aphauiptera ... Lepidoptera Coleoptera . . . 578 579 585 Linguatulida . . 487 Hymenoptera . . . 590 Acarina . , . . 489 Terebrantia ... 594 Pygnogonida , . . 495 Aculeata .... 695 GENERAL PART. CHAPTER I. ORGANISED AND UNORGANISED SUBSTANCES. IN the world, which is perceptible to our senses, we distinguish between living organised and lifeless unorganised bodies. The former (i.e., animals and plants) are endowed with the power of movement, and they remain the same in spite of manifold changes both of themselves as a whole and of their parts, and in spite of continual change of the matter entering into their composition. Unorganised bodies, on the other hand, are found in a condition of constant rest ; and although this rest is not necessarily fixed and unchangeable, yet they are without that independence, of movement wliicli manifests itself in metabolism. In the former we recognize an organisation, a composition of unlike parts (organs), in which the matter exhibits its activity in a fluid and dissolved form ; in the latter we meet with a mass which is more uniform, though as far as the position and arrangement of the molecules are concerned, not always homogeneous, and in which the various parts continue in a state of resting equilibrium so long as the unity of the body remains undisturbed. The matter of unorganised bodies (for in- stance, of crystals) is in a state of stable equilibrium, while through the organised being a stream of matter takes place. The properties and changes of living bodies are strictly dependent on the physico-chemical laws of matter, and this is recognized more clearly as science advances ; yet it must be admitted that we are entirely ignorant of the molecular arrangement of the material basis of a living organism, and it exists under conditions the nature of which is as yet unexplained. These conditions, which we may designate, as vital without thereby calling in question their depen- dence on material processes, distinguish organisms from all un- /O GEXEBAL PAET. organised bodies. They relate (1) to the mode of origin, (2) to the mode of maintenance, (3) to the form and structure of the organism. Living bodies cannot be manufactured by physico-chemical means from a definite chemical mixture under definite conditions of warmth,, pressure, electricity, etc. ; their existence rather presupposes, accord- ing to our experience, the existence of like or at least very similar beings from which they have originated. It appears that, in the present state of our knowledge, there is no evidence to show that an independent abiogenetic generation (generatio cequivoca, spontaneous generation) actually takes place, even in the simplest and lowest forms of life ; although very recently some investigators (Pouchet) have been led by results of remarkable but equivocal experiments to the opposite view. The existence of the generatio cequivoca would offer a very important service to our contention for the physico-chemical explanation ; it even appears to be a necessary postulate in order to explain the first appearance of organisms. The second and most important characteristic of organisms, and that on which the very maintenance of life depends, is their metabolic power, i.e., the power which they possess of continually using up and renewing the matter composing the body. Every phenomenon of growth presupposes the reception and change of material constituents; every movement, secretion, and manifestation of life depend on the exchange of matter, on the breaking down and building up of chemical compounds. On this alternating destruction and renewal of the combinations of the body substance two properties necessary to living things depend, viz., the reception of food and excretion of waste products. It is the organic substances (so called on account of their occurrence in organisms), i.e., the ternary and quaternary carbon compounds (the former composed of carbon, hydrogen, and oxygen, the latter of these with the addition of nitrogen, and among the latter are included the albumins) which undergo the exchanges characterising metabolism ; they either (in animals) break up under the influence of oxidation into substances of simpler composition ; or (in plants) are built up by substitution from simpler inorganic substances. But just as the general fundamental properties (elasticity, weight, porosity) of organisms agree so closely with those of inorganic bodies, that it was possible to construct a general theory of the constitution of matter, so all the elements (fundamental substances which differ qualitatively, and are chemically incapable of further simplification) of organic matter are again found in inorganic nature. A vital ORGANISED AND UNORGANISED SUBSTANCES. 11 element, i.e., an element peculiar to organisms no more exists than does a vital force working independently of natural and material processes. Also with reference to the method of arrangement of the atoms, organic and inorganic substances have been erroneously put in sharp contrast ; and the whole of the carbon compounds have been contemplated as the products of organisms only. Now, however, it has been shown for some time not only that the atomic arrangement and constitution of both are explained by the same laws, but also that a great many of the former (urea, alcohol, vinegar, sugar) can be artificially built up by synthesis from their elements. These facts point to the probability that many other organic substances will be synthetically produced, and among them, albumin ; and they also permit us to conclude that in the origination of organised bodies the same forces were in action which are sufficient for the formation of unorganised bodies. The functions peculiar to organisms, viz., metabolism, movement, growth, are accordingly to be referred to the properties of the chemical compounds composing them, and particu- larly to the complicated molecular arrangement of living matter. Nevertheless, this important property of living things, viz., meta- bolic action, may under certain conditions be temporarily suppressed, without thereby depriving the organism of the power of existence. By removal of water or of heat it is possible, in the case of many of the lower organisms and their germs, to suspend the vital processes for months and even years ; and then to restore the apparently life- less body to the full exercise of its vital properties by the simple addition of water or warmth (eggs of Apus, Ostracoda, Anguillula tritici, Rotifera — frogs, water insects, plant seeds). Finally, the living body is distinguished by its entire form, and by the manner in which its various parts are connected together ; in other words, by its organization. The form of a crystal, the in- organic individual, is unchangeable, and is1 bounded by straight lines meeting at determined angles, and by plane, rarely spherical surfaces, which are capable of mathematical expression. The shape of organisms,* on the other hand, in consequence of the semifluid con- sistency of the material composing them, is less sharply determinable and is within certain limits variable. Life manifests itself as a con- nected series of ever-changing states ; and the movements of matter are accompanied by growth and change of form. * The fact that there arc a number of solid excretion products of organisms (shells) whose form is mathematically determinable does not of course annul this distinction. 12 GENERAL PART. Tlie organism commencing as a simple cell, the egg or germ, develops by a gradual process of differentiation and change of its parts up to a definite point at which it has the power of reproducing itself ; finally it dies, and breaks up into its elements. The greater part of the substance composing organised bodies is more or less semifluid and liable to osmotic action, — a condition which appears to be necessary both for the carrying on of chemical changes (corpora non agunt nisi soluta), and for the modification of the entire form of the organism ; it is not however homogeneous and uniform, but is composed of solid, semifluid, and fluid parts which exist as com- binations of elements of a peculiar form. Crystals do not possess heterogeneous units subordinated to one another, which, like the organs of living bodies, serve as instruments for the performance of different functions, but are composed of molecules of similar atomic constitution ; the absence of uniformity in their structure in differ- ent directions (planes of cleavage) being due to the arrangement of the molecules, and not to any difference in the molecules themselves. Organs again prove, on examination of their finer structure, to be built up of different parts or tissues (organs of a lower order), and these a ":-°i?,; . %r/ ^ again are composed of the FIG. 1.— a, young ova of a Medusa; J, mother-cells ultimate unit of cell, the of spermatozoa of a Vertebrate; one of them pre- ^ 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, tlie 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. FIG 2.— Amoeba (Protogenes) porrecta (after Mas Schultze)1 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 Hseckel), 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 unknown molecular arrangement, is the only absolute test of the cell and organism in general. While appreciating the essential differences which have been 14 GEXEEAL 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 '% organisms, which are proved to be such by then1 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- sis, independently of organization, we must allow that hypothesis a 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 organised beings, we might look upon the first appearance of life as essentially only the solution of a difficult mechanical problem (with Du Bois Reymond), Avere 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. FIG. 3.— Schizomycetes (after F. Colin). a, Micrococcus ; I, Bacterium termo, Bacteria found in putrefying bodies both in motile and Zooglaea form. ANIMALS AND PLANTS. 15 CHAPTER II. ANIMALS AND PLANTS. THE division of living bodies into animals and plants 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 hand, 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 PAPiT. 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 (Ccelenterata). 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 FIG. 4. Branch of a Polyparium of Corallium 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- tlroids, figs. 4, 5). In these cases it is as difficult to limit the idea of "indi- viduality " as it is in the vegetable kingdom. 2. Hetween 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 sharply - marked bounda- ries : Avhile in animal tis- Fis. 5. — Physophora hydrostatica. Pn, Pneuma- tophore ; S, Swimming-bells ; T, Dactylozooid ; P, polypite or stomach with the tentacles, Sf. ; 2V&, terminal swellings on the latter provided with thread-cells ; G, Clusters of gonophores sues the cells give rise to extremely different structures, in which the cells as such do net always remain recognisable. The reason for this unlike condition of the tissues must apparently be sought in the different structure of AXIMALS 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 bouiidary layer of of its own semi-fluid contents. Nevertheless, there arc 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. G.— 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 unicellular algse 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 (sj)ermatozoon) 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. ilETAEOLISM IX ANIMALS AND PLANTS. 19 4. The chemical constituents and the 'metabolic 2>rocesses in animals and plants present, on the whole, important features of difference. I 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 tL'sre~, 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 compounds, 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 acicls)T kreatin, tyrosin, leucin, urea, etc. ; uric acid, hippuric acid, 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 aa 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 Avere first observed a hundred years ago (Ellis). The plants in question catch, after the manner of animals, small organisms, especially in- Pi0.7.-L'eafof Droserarotundifolia, sects> and absorb from them through with partially contracted tentacles the glandular surface of their leaves (after Darwin). , . 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 up 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 alt-o in the green parts in the absence of sunlight, i.e. at night, a consumption of oxygen and exhalation * Compare especially Ch. Darwin, " Insectivorous Plants." London, 1875. FIG. 8. — Leaf of Dionaea muscipula in expanded condition (after Darwin). MOVEMENT AND SENSATION AS TEST OF ANIMALS. 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 and 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 oleariits) 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 algae, 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 * FIG. 9.— Zoospores, a, of Phyiarum ; b, of Monostroma ; nature Of the movement «. Of Ulothrix; d, of Bedogonium ; e, of Vaucheria can be decided for a dis- (after Reinke)' 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 a- ANIMALS AND PLANTS. lower animals, there is present an undifterentiated albuminous substance known as sarcode, the contractile matrix of the body. The a viscous contents of vegetable cells, FlG. 10.— Zoospores of AetJialium eepticum after de Bary. a, in condition of hatching ; 6, as mastigopods ; c, in the amoeboid stage; d, a piece of plasmodium. 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, Chcetophora). 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 Myxomyceles, 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 vipon the individual judgment of the observer. The faculty of sensation, which is inconceivable as a function of matter and which must be always TIG. ll.—Amaba Dactylotphara polypodia. pre-SUppOSed wherever W6 have w, nucleus. PC. contractile vacuole (after to ^o with voluntary movement, Fr E. Schulze). J 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 ' V ••;•-. 7 •••:.. ..- •'• - * .-.,.: "-.•:•:.•'••-..•.•... -/ IEEITABILITT 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 (Mimosece), 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 Oentaurea contract along their whole length on mechanical and electrical stimulation, and according to the "same laws as do the muscles of the higher animals. Many flowers open and shut under the influence of light at certain 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 products. * The formation of au intermediate kingdom for the simplest forms of life is neither scientifically justified, nor from practical considerations desirable. On the contrary, the acceptance of the Prutist a would only double the difficulty; f-f 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 cornpositicn 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 25 by the term 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 usiial 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* 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 Racliata, 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 antimere, are situated peripherally. Eacli antimere contains, therefore, a definite group of organs and represents a secondaiy unit, which, together with its fellows and 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 * Vide E. Leuckart, " Ueber den Polymorphismus der Individuen and die Erscheinung der Arbeitstheilung in der Natur." Giessen, 1851. 20 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. can be drawn, passing between the antimeres. A vertical section through a radial line divides the corresponding antimere into two A G FIG. 12«,— 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 ambulacral-plates perforated by the ainbulacral pores. A, anus. Fio. 12i.— 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. Eadiate 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. 14o). 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, generative organ in inter-radius; Af, position of the ambulacra! feet in the radii. BILATERAL STMMETEI. 27 divides the antimeres from each other. The first, in which the greater number of organs are repeated, may be designated the transverse plane, while the second, corresponding to the median plane of bilateral animals, is known as the sagittal plane (fig. Gf li FIG. 14a. — Acaleplia larva (Ephyra). Hit, marginal body ; Gf, gastric fila- ment. Re, radial-canal ; O, niouth. Fio. 145. — 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 Tm' tculacl° and is divisible into successive sections, the segments or metameres, c 28 OBGANIZATIOX AXD DEVELOPMENT OF ANIMALS LST GENLEAL. 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 (proyloltis 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 homonorny. In the same degree as the metarneres 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 order also holds for organs. which are reducible to a single cell, or to an aggregation of equivalent cells (simple organs), and others in the formation of which 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 derivator, which are organs of a still lower order. Finally, in the last analysis, we come to the cell or the area of protoplasm coiTesponding 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. distinction into a higher and lower There are organs FIG. 10.— Portion of Diphycs after R. Leuckart). D, hydropuyllium ; O», gono- phore; P, Polyp with tentacle. The groups of individual separate them selves as Eudoxia. CELL NUCLEUS. 29 CELLS AND CELL TISSUES. The constituent parts of which an organ is made up are known as tissues. 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 iiber die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Pflanzen." Berlin, 1839. Fr. Leydig, "Lehrbuch der Histologie des menschen und der Thiere." Frank- furt a.M. 1857. Fis 17. — Different forms of nuclei (after R. Hertwig). a, nucleus from a cell of a Malpighian 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 OEGAXIZATIOX AXD DEVELOPMENT OF ANIMALS TS GEXEBAL. 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 (plasmodium). In such cases we have a process of free cell forma- tion (e.g., spore formation in Myxornycetes) 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 striae, 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 striae 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 (fig. 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 Biitschli). -5T, nuclear spindle. Ep, 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 newly 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 orglins of animal life. The former have to do with the nutrition and maintenance of 32 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. 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 FIG. 19. — Blood-corpuscles (af ..er 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 frop. /, of the pigeon, f1, 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 Swammerdam 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 >e2ond (except in the embryo) (fig 10). They contain ORGANIZATION" AND DEVELOPMENT OF ANIMALS IN GENERAL. 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 coloui-less corpuscles are genuine cells of variable form, and have the power of amreboid 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 sperniospores, after FIG. 20.— Spermatozoa, a, ct Medusa. 6, of a Nematode. c, of a Crab, d, of Torpedo. e, of Salamander (with undulating membrane). /, of Frog, g, of a Monkey (Cerco- pithecus). they have separated from the epithelial layer in the wall of the ovary and testis, as well as the spermatozoa produced from the sperniospores, 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. Ill 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 PART. 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 line its closed spaces (endotheUum). According to tke different shape of the cells composing it, we distinguish cylin- drical, ciliated, and pavement epithelium. In the first case the cell?, 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 vibratile 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 cells, b, flat cells with fiagella (from a Medusa), e, cylindrical cells, d, ciliated cell, e, flagellate cell with collar (from sponge). /, cylindrical cell with porous border (intestinal epithelium). is more than one layer the superficial cells are flat, while those in the deeper layers are more and more rounded. While the cells of the lower layers retain their semi-fluid character, and are occupied in continual cell division and growth ; those of the upper layers possess a firmer consistency, gradually become horny, and are thrown off as scales or continuous flakes, to be replaced by the continuous growth of the lower layers. Thick stratified layers of cornified cells, almost fused with one another, give rise to indurated or horny structures (nails, claws, hoofs), which may form a more or less complete coat for the body and function as a protective exoskeleton (fig. 21, a to/). There are also cells the free surface of which is distinguished by * ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 35 well-marked thickening. The protoplasm of the free surface of such cells becomes hardened so as to give rise to a thick superficial border, perforated at right angles to its surface by a number of fine canals which give it a striated ap- pearance (intestinal epithe- lium, fig. 21, f, epidermis cells of Petromyzon). If these thickened borders fuse to- gether to as to form a con- tinuous layer which obtains a certain independence, we obtain cuticular membranes, which, according to their ori- gin, may be homogeneous or stratified (fig. 22, a, b, c), and a FIG. 22. — a, Cuticle and hypodermis of the larva of Corethra. b, cuticle and hypodermis of a Gastro- pacha caterpillar, with two poison glands beneath corresponding bristles. may exhibit various patterns of different kinds. Very frequently the surfaces of the individual cells are indicated on the cuticle as polygonal figures; and, in addition to the very fine pores, there are also found larger passages pro- duced by out-growth from the cells. These latter lead to the appearance ©f various cuticular appendages, such as hairs, bristles, scales, etc., which are placed on the cuticular pores, and con- tain as a matrix their special cell or a process of it. Cuticular membranes may obtain a very considerable thickness, and, by the deposition of calcareous salts, a high degree of firmness (cai^apace of Crustacea) so that they acquire the value of skeletal tissue.s, which, FIG. 22c. — CM, cuticle with bristles in the condition of ecdysis. Cu', newly-formed cuticle (Branchipus). 36 GENERAL TAUT. however, it is generally difficult to distinguish from certain connective tissues. While cuticular structures are solid secretions which are of use in supporting and giving a definite form to the organism, there are, on the other hand, various fluid secretions proceeding from cells which give rise to no structures, and which are often of considerable importance from a chemical point of view. In this case the epithe- lium becomes glandular tissue. In the simple cases the gland is constituted of a single cell, the secretion of which either passes out through the free surface of the membrane, or a special opening in FIG. 24. — Gastric glands, a, their origin as in- vagiuations of the epithelium. 4, perfect gas- tric glands. it (fig. 23). If several cells enter into the formation of a gland, they are arranged, in the simplest cases, round a central cavity, which receives the secretion. The gland then has the form of a sack or blind tube, derived from an inva- gination of the epithelium, either of the inner or the outer surface of the body, into the subjacent tissue. From this fundamental form the larger and more complicated glands are to be derived, as the result of continued regular and irregular outgrowth. While their form presents great variations, they are univer.-ally characterised by the transformation of their terminal portion into a duct; this differentiation may also appear in the simple glandular tubes, and even in the unicellular glands (figs. 23, 24). FIG. 23.— Unicellular glands, a, goblet cells from the epithelium of the small intestine of a vertebrate, b, unicel- lular cutaneous gland of Argulus with, long duct, c, unicellular cuta- neous gland of insects with cuticular duct. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 37 N 2. The tissues of the connective substance. Under this term there are included a great number of different tissues which morpho- logically resemble each other in the presence of a greater or less amount of intercellular substance, intercalated between the cells (con- nective tissue corpuscles). They connect and surround other tissues, and serve as supporting and skeletal structures. The intercellular substance arises from the cells as a differentiation of the peripheral part of their protoplasm ; it cannot accordingly be genetically clearly distinguished from, the cell membrane and its differentiations, which we have considered in connection with epithelial tissue. The cell walls already produced by the protoplasm may also become fused with the intercellular substance, and so contribute to its increase. The intercellular substance is usually secreted by the whole periphery of the cell, and presents great variations both in its morphological and chemical characters. When the amount of intercellular substance is small, the tissue is called cellular or vesicular connective tissue. This form is found especially in medusae, molluscs, and worms, and to a less extent in verte- brates (notochord, fig. 25), and is not sharply marked off from cartilaginous tissue. Embryonic connective tissue, which consists of closely aggregated embryonic cells, evidently closely re- sembles it. Mucous or gelatinous connective tissue is characterised by possessing a watery hyaline and gelatinous matrix. The condition of the cells in each case is different. Frequently they send out delicate, often branched processes which anastomose with one another and form a network. In addition, however, parts of the intercellular substance may be differentiated into bundles of fibres (Wharton's gelatine in the umbilical cord). Such forms of tissue are found amongst the Invertebrata, e.g., in Heteropods and Medusae, whose gelatinous disc, in consequence of the reduction or complete absence of cells, is reduced to a layer of soft or hardened connective iis.-ue but little different in its origin, as a unilateral cell excretion, from cuticular structures (Hydroid Medusae, swimming bells of Siphono- FIG. 25. — Vertebra of larva of a toad (after Gotte). Ch, notochord cells ; ChS, notochord sheath ; Sk, skele- togenous tissue ; N, spinal cord. GEXEHAL PABT. phora). The so-called secreted tissue of young Ctenophora, and the gelatinoxis tissue of Medusse and Echinoderm larvae, into which cells eventually migrate, being at first absent, has a similar relation (fig. 26). FIG. 26. — Gelatinous tissue of Rhizostoma. F, fibrous network; Z, cells with processes; Z', the same in division. Reticular connective tissue consists of a network of star-shaped and branched cells, the spaces of which contain another kind of tissue element. In the so-called adenoid tissue, which functions as the supporting tissue of the lymph glands, the contents of the inter- cellular spaces are lymph corpuscles. A form of connective tissue very widely scat- tered amongst the Ver- tebrates is the so-called Jibrillar connective tissue (fig. 27). This consists of a large proportion of spindle-shaped, or branched cells, and of a solid intercellular sub- stance, which is totally or partially broken up into bundles of fibres and FIG. 27.— Fibrillar connective tissue. possesses the property of yielding gelatine on boiling. If the protoplasm of the cells is mostly or entirely used up in the formation of fibres, fibrous tissue is produced v.ith nuclei in the position of the original cells. Very often the OEGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 39 fibres have a wavy outline, and are arranged nearly parallel to one another (ligaments, tendons). In other cases they cross one another at an angle in different directions (dermis), or they present a net-like arrangement (mesentery). Fat tissue consists of ordinary connective tissue in which the cells are for the most part round and contain greater or smaller fat globules. If the normal fibrillse and bundles of fibrillae be treated with acids and alkalies, they swell up, and a second form of fibre, which resists these re-agents, comes into view. These are the elastic fibres (fig. 28), so called because they preponderate in tissue which is especially elastic. They present a tendency to branch and to form networks, and often possess great strength (ligamentum nuchae, arterial walls). They may also be spread out and connected together so as to form a perfo- rated membrane (fenestrated membrane). Cartilage is another form of connective tissue. It is characterized by the shape of its cells, which are usually spherical, and its firm intercellular substance. The latter contains chondrin, and determines the rigidity of the tissue. Externally, cartilage is covered by a vascular connective tissue -coat, known as the perichondrium. When the intercellular substance is very slightly developed, we get tissues which are transitional between cellular connective tissue and cartilage. FIG. 28. -Elastic fibres, a; b, network. FIG. 29.— a, Hyaline cartilage with cells. b, Fibro-cartil i.^-e. According to its special constitution, three kinds of cartilage may be distinguished, viz., hyaline (fig. 29, a), fibrous (fig. 29, b}, and 40 GENERAL PART. elastic cartilage ; the latter containing a network of elastic fibres. There are also intermediate forms, approximating to the fibrillar connective tissue, in which cartilage cells may be surrounded by bundles of connective tissue fibres. The cells are placed in spaces, which are usually round, in the intercellular substance, and are sur- rounded by firm layers which are separated off from the latter, and have the appearance of capsules. These so-called cartilage capsules were formerly looked upon as the membranes of the cartilage cells, analogous to the cellulose capsules of plant cells ; a view of them which is not in any way opposed by what is known as to their development as secretions of the protoplasm. Nevertheless, the capsules stand in closer relation to the earlier formed intercellular substance which has been produced in the same way, in that they often fuse with it. The growth of the cartilage is accordingly in the main interstitial. We frequently see in the spaces in the cartilage several generations of cells surrounded by special capsules placed one within the other. In such cases the secreted cap- sules have remained separate from the intercellular sub- stance. Certain kinds of car- tilage, moreover, have spindle- shaped cells, and sometimes the cells are prolonged into numerous radiating processes. Calcareous salts may also be deposited in the intercellular substance in a greater or less quantity. In this way arises the so-called in- crusted cartilnge, or the cartilage bone (fig. 30), which in the sharks is present as a persistent form of skeletal tissue, but in the higher vertebrates only as a transitional structure. Cartilage owes its special usefulness as a skeletal tissue to its rigidity. It is sometimes found in the Invertebrata (Cephalopoda, tubicolous worms such as Sabellfv, Coelenterata), and very generally in the Vertebrata, whose skeleton always contains a certain amount of cartilage, and in fishes may be exclusively constituted of it (cartilaginous fishes). Osseous tissue possesses a still higher degree of rigidity. The intercellular substance is strengthened and hardened by the deposi- tion of carbonate and phosphate of lime, while the cells (the so-called bone corpuscles^ possess numerous fine processes which anastomose with each ether (fig. 31 a, b, c). The cells occupy spaces in the com- 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 Haversian canals. These contain the nutritive blood-vessels and correspond exactly in their course and branchings to the latter. The intercellular substance consists of lamellse, 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. 31a.— Longitudinal section through a spongy bones have an irregular long bone (after KSlliker). e, Haver- OJ sian canal, distribution. In a seoond form of osseous tissue the cells themselves remain in the outer part of the excreted intercellular substance, and only their *i K FIG. 314.— Transverse section through a long bone (after Kolliker). E, bone corpuscles ; FIG. SO. — (?, Primitive fibre. fc.cross-stripeJ muscle fibre (primitive muscle bundle) of Locerta with rierve termination. FIG. 35. — a, smooth muscle fibres isolated, b, piece of an artery (after Prey) ; 1, outer connective tissue layer ; 2, the middle ) iyer formed of smooth muscle fibres; 3, non-nu- cleated inner layer. 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 fibrillse are produced by the deeper parts of the myoblasts, which form a continuous flat surface epithelium (musc-le epithelium) above the layer of delicate fibres (Medusse and Siphonophora) (fig. 34 b). In the higher animals they ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 45 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 Daphnia). Sometimes the cells become elongated into long tibres, 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 PAKT. 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, b, medullated fibres, one of them with commencing coagulation of the axis cylinder, c, medullated nerve fibre with the sheath of Schwann. FIG. 37.— a bipolar ganglion cell, b, nerve cell, from the human spinal cord (anterior cornu), (after Gerlach). P, pigment body. quently 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, on 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-medullated 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 Cyclostomata 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 fibrillre, and be, so to speak, resolved into its elements. Finally, the nerves of In- vertebrates very often appear as finely striated bundles of fibrilla?, 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 end- 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, a, b, c.) Fio. 39.— Rod-shaped sense cells from the olfac- tory organ (after Max Schultze). a, from the frog ; Sz, supporting cell between two ciliated i-od-cells. b, from man. c, from pike. Pro- bable connection between the nerve fibrillaei and the sense cells. INCREASE IN SI2E AND PROGRESSIVE DIFFERENTIATION, DIVISION OF LABOUR AND PERFECTION. The lowest organisms possess neither tissues nor organs formed from cells. The whole organism consists of a single cell. The body of such an animal is composed of protoplasm, and its skin of the 48 GENEEAL TAET. cell membrane. The latter is often without an opening for the entrance of solid bodies ; the entrance of food being entirely effected by endosmosis. 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 inci-eases 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 see that increase in size must not only be accompanied by an increase in the complexity of organisation, but must 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 — Blastosphere) (fig. 40, a, b.} Then, in consequence of the needs of the growing organism, TILE GASTEULA. 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 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 Ccelenterate closely approximates. As the organism increases in size, additional complications ensue. These result partly from a still fur- ther increase of surface brought about by secondary invaginations and partly from the appearance of some intermediate tissue placed be- tween the two primary layers. The secondary invaginations perform special functions and give rise to glands; while the intermediate FIG. 40. — a, Cell colony of young: Tot cox Globator (after Stein). I, Blastosphere stage of an Acalepha larva (Aurelia Aurita). c, Gastrula stage of b; Ect Ectoderm; En, Endoclerm; o, Blasto pore (mouth of Gastrula). 50 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GLNEBAL. 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 (somatic 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 (tentades, 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 groups (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, iu * Usually known as segmentation cavity. — ED. t Usually known as " body cavity," or " ccelom."— En. DOCTRINE OJ? 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. Fromthis isdeducedthe principle to which Geoffrey St. Hiliare gave the name — if he was not the first to recognise it — of the "principe du balancement des organes," and this enabled that investigator to establish the doctrine of " Abnormalites " (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 features 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 OP 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 OEUANS. 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 III!!// , i , I // I i •' / ' ' / / / / [|l/i/y//'/V Fio. 41. — Rotalia veneta (after II. Schultze) with a diatora caught in the pseudopnrtiaj network. consist of the apparatus for the reception of food and for its diges- tion, 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 Rhizopodn, by prolon- gations of the sarcode (pseudopodia) surrounding the foreign body (fig. 41). In the Infusoria, which are covered by a firm cuticle, there is a central semi-fluid mass of sarcode (endoplasm), 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 pre&ent, which serve the purpose of procuring food (adoral ciliated zone of the Ciliata) (fig. 42). Fio. 42.— Stylonychia mytilns (after Stein) viewed from the ventral surface ; Wz, adoral zone of cilia; C, contractile vacuole; N, nucleus; AT/,nucle- olus (paranucleus); A, anus. r— 6 FIG. 43. — Longitudinal section through the body of an Anthozooid (Octactinia). M, stomachic tube with the mouth open- ing in the centre of the feather-like tenta- cles ; Jiff, mesenteric folds ; G, genital organs. Among the animals with cellular differentiation (Metazoa), the internal cavity of the body in the Crelenterata (morphologically identical with the alimentary cavity and not with the body cavity of other animals) functions as a digestive cavity, and its peripheral adially arranged portions as a system of vascular canals (gastro- ALIMENTARY CAXAL. 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 ccsophageal 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 FIG. 44.— Anrelia aurita seen from the oral surface. MA, the four oral tentacles with tho mouth in the centre ; Gk, genital folds; GH, opening of the genital pouches ; lik, mar- ginal bodies ; KG, radial canals ; T, tentacles at the margin of the 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 Medusa;, Siphonophora, Ctenophora). When 1 he 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 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. Ph- (fig. 45), with sharply marked off pharyngeal structures (Trematoda, 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 (oasophagus) 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 of ten 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 hepaticum (after R. Leuckart) ; D, alimen- tary canal ; O, mouth. FIG. 43. — Alimentary canal of a young nematode. O, mouth ; Oe, fore-gut (resophagus) with pharyngeal dilatation, Ph ; D, mid-gut; A, anus. INTESTINE. 57 repeated mechanical action (masticatory stomach of Cray-fish) or by the secretion of digestive fluids (pep.sin) 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 crop, of the stomach to blind sacs which serve as reservoirs for the food • (stomach of Ruminants) (figs. 47 & 48). In the middle section of the alimentary ca- 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. 43.— Alimentary oanai of modifications by the action of trypsin S^5£SIS£; ("*™8. however> onl^ in alkaline solutions). Oe, oesophagus; s, sucking The intestine often attains a great length, stomach ; Mg, Malpighian •, -, ,..,,. tubules; Ad, rectum and becomes divided into regions possessing 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 FIG-. 47. — Alimentary canal and ac- cessory glands of a caterpillar. O, mouth ; Oe, oesophagus ; Sp D, salivary glands ; Se, spinning glands ; MD, intestine (mid-gut) ; AD, rectum (hind gut) ; MG, Mal- pighian tubes. 58 OBGAIxiZATION A>*D DLVELOrMJZXT OF AXI.MALS IX GEXPUAL. K 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 expulsion of the undigested remains of the food, or freces. It may also possess CEecal 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) caeca, and it may be sub-divided into two parts, 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 Ldbellulidse) or a secretory function (larva of PJO. 4-j.— Alimentary canal of a bird. Of, Ant Lion). oesophagus ; Jf, crop ; JDm, proventriculus ; The paliyary glancls Hver and Km, gizzard ; D, small intestine ; P, pan- creas placed in the loop of the duodenum ; pancreas are to be regarded as n, liver; c the two c*ca; IT. ureter } o», growths of the alimentaiy oviduct ; Ad, large intestine ; Kl, cloaca. 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. ORGANS OF CIRCULATION. 59 Oe The liver, distinguished in the higher grades of development by its great size, is an appendage of the first part of the small intestine (duodenum). The first trace of it is met with in the lower animals in the form of a characteristically coloured part of the cellular covering of the gastric cavity or intestinal wall (Coelenterata, worms). In the higher animals it has at first the form of a small blind sac (small Crustacea) ; this, by a process of branching, is con- verted into a complicated struc- ture composed of ducts and folli- cles, which may become connected together in very different ways -so as to give rise to an apparently compact organ. Nevertheless, it must be remembered that, in the different groups of animals, glands, which differ both mor- phologically and physiologically, are included under this term, "liver." While in the Verte- brata the liver, as a bile-pro- ducing organ, possesses no known relation to digestion, in the In- vertebrata the secretions of many glands, which are generally called " liver," but which would be more appropriately termed hepato- pancreas, exercise a digestive action upon starch and albumen, and at the same time contain bye-products and colouring mat- ters similar to those found in the bile of Vertebrates (Crustacea, Mollusca). The Organs of Circulation. The nutrient material or chyle re- sulting from digestion is distributed by a system of spaces to all parts of the body. Excluding the Protozoa, in which the distribution of nutrient material is effected in the same manner as in the cell or tissue unit, the simplest form of vascular system in animals with cellular tissues, i.e., in the Metazoa, is found in the Coelenterata. In these animals the digestive cavity itself extends to the extreme periphery of the body, and serves to distribute the nutritive fluids Cue — f, FIG. 50.- Alimentary canal of Mnn. Oe, oesophagus ; JI/, stomach ; L, spleen ; H, liver; Gb, gall bladder; P, pancreas; Hit, duodenum receiving the bile and pan- creatic ducts ; Jl, ileum ; Co, colon ; Coe, esecum with vermiform process, Pa; R, rectum. 60 ORaA.XIZA.TIOX AXD DEVELOPMENT OF AXIMALS IX G2XEEAL. (gastro-vascular system of Polyps, so-called vessels of Medusae and Ctenophora). The so-called stomach of the Anthozoa is simply an invagiuation of the body wall into the central cavity of the animal, and functions only as cesophagus. When a distinct alimentary canal is present, the chyle is absorbed by the walls of the gut, and passed through them into the coelom or space developed between the gut and body walls (into the general D Br FIG. 51.— Daphnia with simple heart. C, the slit-like opening on one side is seen; D, alimentary canal ; L, liver; A, anus; O, brain; O, eye ; Sd, shell gland; Br, brood pouch placed dorsally beneath the carapace. tissue of the body in the acoelomate parenchyrnatous worms), and there gives rise to a fluid, the blood, in which (with some few exceptions) corpuscles (cellular structures produced in the organism) are found. In this space, or in a system of lacuna? derived from it, the blood circulates. Primitively its movements are quite irregular, taking place with each movement of the body (as in many worms), and are effected chiefly by the contractions of the somatic muscles HEART OF INTEETEBEATE9. 61 'Ascaris), but also by the movements of other organs, e.g., the alimentary canal (Cyclops). At a higher stage of development a rudiment of the central organ of the circulation, appears, in that a special section of the blood path acquires a muscular investment, and as a pulsating heart, comparable to a force and suction-pump. FIG. 52. — Male of Branchipus stagnalis with many- cha.rnbered heart or dorsal vessel Jiff, the lateral openings in \vhich are repeated in every seg- ment. D, intestine ; 3f, mandible ; Sd, shell gland ; Hr, branchial appendage of the llth pair of legs ; T, testis. — A FIG. 53.— Heart of a Copepod (Calanelhi) with an ante- rior artery, A. Os, cstia ; 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 G2 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. o.stia, 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 denned canals, the blood vessels, are then developed, which in the Invertebrata may alternate with lacunse not provided with walls. In the simplest cases it is only the tracts along which the blood travels from the heart which 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 A.ab FIG. 61.— Heart and blood vessels and gills of the crayfish. C, heart, in a blood sinus ; with Pa several pairs of ostia; Ac, cephalic aorta; A.ab, abdominal aorta; As, 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 VtETEBRATES. G3 a blood sinus or as a system of blood -lacunae ; or the arteries and 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 Vertebrata, 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 differentiated in it. At first rhythmically pulsating sections, very frequently the FIG. 55.— Nervous system and circulatory organs of Paludina vivipara (after Leydig). F. tentacle ; Oe, oesophagus ; Cg, cerebral ganglion with eye ; Pg, pedal ganglion with adjacent otocyst ; Vg, visceral ganglion ; Phy, pharyngeal ganglion ; A, auricle of heart; Ye, ventricle; Aa, abdominal aorta; Ac, cephalic aorta ; V, vein; Vc, afferent vessel. Br, gill. dorsal vessel, or the lateral vessels connecting this with the ventral vessel (fig. 56), serve for the propulsion of the blood. Similarly amongst the Vertebrata, the lancelet (Amphioxus) possesses no distinctly differentiated muscular heart, the function of that organ being discharged by various parts of the vascular system which are contractile. The arrangement of the vessels supplying the pharyngeal section of the alimentary tract, which has a respiratory function and is known as the branchial sac, admits of a comparison with the vascular arrangement of the segmented worms, and repre- sents the simplest form of the vertebrate vascular system. The kmo-itudinal vessel which runs in the ventral wall of the branchial O sac gives off numerous lateral branches, which ascend in the branchial walls. These lateral vessels are contractile at their point of origin (54 OEGjLNIZATION AXD DEVELOPMENT OF AXIMALS IN GENERAL, t 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 venous 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 any backward flow of blood from the cardiac aorta into the ventricle in diastole, and from the ventricle into the auricle in systole. In consequence of the insertion of the respi- ratory organs on to the system of the arterial arches, the latter, and at the same time the structure of the heart, assumes various degrees of complication. 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. 50.— Anterior part of the vascular system of an Oligochffite worm (Sanuris) (after Ge- genbaur). In the dor- sal vessel the blood moves from behind forward ; in the ven- tral vessel from before backwards (see ar- rows). H, heart-like dilated transverse lateral vessels. PCLMOXARY CIRCULATION. 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 iG. 57. — Diagram of the circulator/ organs of an osseous fish. r. 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; Lk, portal circulation. FIG. 58. — Gills (B>-} 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 respirat ion. On the disappearance of the gills, which is completed during the metamorphosis in the S.ilamandrina and Batrachia, the pulmonary 5 66 OEGAN1ZATION AND DKTELOPMENT OF ANIMALS IN G^X^ VP which through 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), now receives 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 the rio-ht aortic FIG. 59.— Circulatory organs of the frog. P, left lung, right lung is removed ; Ap, pulmonary artery ; Vp, pulmonary vein ; t'c, vena cava inferior ; Ao, dorsal aorta ; N, kidney ; D, alimentary canal ; Lk, portal circulation. arises 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. CO). The ventricular septum, and consequently the separation of tho right from the left ventricle, is found complete for the first time LYMPHATIC SISTKM. 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, th-it 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 Avails, 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 cloeely connected the excretion of carbonic FIG. CO.— Heart and great vessels of a Chelouian. Ad, right auricle; As, left auricle; Ao.d, right aortic arch; Ao.s, left aortic arch ; Ao, aorta; C, carotids ; Ap, pulmonary arteries. 68 OKGASIZATIOX AND DEVELOPMENT Ol? ANIMALS LN 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 carry in"1 on this process either in air or in water. In the simplest cases the exchange of these two gases takes place through the genera] surface of the body; and in all cases, even when special respira- tory organs are present, the outer skin abo takes part in respiration. PIG. 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 venae cavffi, Ves, and Vci; 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 ; At, left auricle ; T's, left ven- tricle ; Ao, aorta ; D, intestine ; L, liver ; Vp', portal vein ; Lv, hepatic vein. FIG. G2. — Diagram of the great arteries of a mammal with reference to the five embry- onic arterial arches (after Bathke). 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, 01% as in the Echi- nodernis, in which a separate vascular system is developed, the surface of the whole body cavity. Respiration in water obviou^y 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 EESPIRATOHY ORGANS. 09 oxygen dissolved in water which is available. 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 antler- shaped or dendritically bi-anched Hence this form of Ct processes (fig- 63 a, b), or of Fro. 63a .— Head and anterior body segments of a Eunice, viewed from the dorsal sur- f-ace. T, tentacles. Ct, tentacular cirrus. C, parapodial cirrus. Br, parapodial gill. lancet-shaped closely-packed leaves with a large surface extension (fig. 64). FIG. 61. — Transveroe section througn the gill of a Teloostean fish, b, branchial leaf- let with capillaries ; c, 1 ranchial artery con- taining venoua blood ; d, branchial vein con- taining arterial blood. a. branchial bar. FIG. G3i. — Transverse section through the body of Eu- nice. Br, gill ; C, cirrus ; P, parapodium with a bundle of seta3 ; D, alimentary caiial ; A', nervous system The organs of aerial respiration, on the contrary, are internal. They present likewise the condi- tion favourable for an exchange of gases between the air and the blood, viz., a large extent of surface. They have the form either of lungs or sir-bearing tubes. In the first case (Spiders, Vertebrates) they consist of spacious sacs with alveolar or spongy 70 ORGANIZATION 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 trachece (fig. 65) consti- tute a branched system of canals which extend throughout the whole body, and carry the air to all the organs. Thus instead 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- trachece present an approximation in their structures to lungs, in that the main stems, without further branching, give rise to flat hollow leaves. i iia Fio. 65. — Tracheae with fine brauches (after Leydig). Z, cellular outer wall ; Sp, spiral thread. Via. CGi. — Lateral view of head and body of nn 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. GCa. — Tracleal sye tem of a Diptei.-:is larva. 2V, Longitudi- nal stem of the right side with tufts of tra- cheae; St', and St", anterior ami posterior stigmata ; J/A, oral hooks. TRACHE.-K. 71 cf the body (fig. 66 a, b) (stigmata of Insects, Spiders), or they may 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 larva? of certain Fin. 67ff.- -T.arva of an Ephemeral fly with seven pairs of tracher.1 ofi'i Cf, 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). Ttt, main tracheal trunk ; Et, tracheal gills ; Na, the three simple eyes. Insects (Ephemeridse, Libellulidae) 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 72 OEGAiaZATIOX AJS'D DEVELOPMENT OF ANIXIAL3 IN GEXEEAL. thus acquire a protected position (rectal respiration of Aesclma, Libellula). In other respects the branchial and pulmonary respiratory pro- cesses are essentially the same. In the pulinonate 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, Bracliiopoda, tubi- colous Worms, etc.) Very frequently the gills appear as appendages of the organs of locomotion, e.cj., 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 ANIMAL 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 bright 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 al o 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 kinetic 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 erroneously 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 OBGANIZAT1ON AND DEVELOPMENT OF ANIMALS LN GENEEAL. 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 poikilothermic,* 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 homotkennic, or ic arm-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 * Comp. Bergmann, " Ueber die Verhaltnisse der Wiirmcokonomie der Thicre zu ihri-r Grbsse," Gottingcr Studicn, 1847; also Bergmann imd Leuckart, " Anatornisch-physiologische Uebcrsicbt des Tbicrreicbs," Stuttgart, 1852. UEIXAET OEOANS. 75 is mainly discharged by the special secretory organs. These have the form of glands of a simple or complex structure which originate from invaginations of the outer skin or of the intestinal wall, and consist essentially of simple or branched tubes, or of racemo; e 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 i nto 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 seymental organs (figs. 69 and 70). The shett-ylands of Crustacea are in all probability to be traced back to these segments I 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 Fro. CS.— Young Distomum (after La Valette). Ex, main stems of the excretory system ; Ep, ex- cretory pore ; O, month with sucker; its special coveringsoft 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 rnulticell- 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 Maliughian body from, the anterior part of the kidney of Proteus (after Spengel). Nr, kidney tubule; Tr, ciliated funnel; Mk, Malpig- hian body. 78 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 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 gland* 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. Medusae). 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 f or 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 mav also be especially concentrated in parts of the body wall, e.g., in the subuni- brellar surface of Medusze 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 Ad/ FIG. 73.— Alimentary canal with its accessory glands of a beetle (Carabus) (after Leon Duf our). Oe, oesophagus ; Jn, crop ; PC, proventriculus ; Ckd, chylific ventricle ; 3/9, Malpighian tu- bules ; S, rectum ; Ad, anal glands with bladder. ENDO — SKELETON. usually products of the external skin (chit in), or they ruay be internal (cartilage, bone) and give rise to vertebrae (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 honionomous (Annelids, Myriapods, Snakes). As development progresses some of the muscles required for locomotion gradually lose their relation to the lo.jg 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 looe their primitive FIB. 74 a— Diagram ot the vertebra! column of aTeleostean fish with verte- bral constriction of the notochord. Ch, notochord ; If/-, bony vertebral bodies ; J, membranous mtervertebral section. FIG. 71 b— Vertebra of a fish. K, ver- tebral body. Ob, neural arch (neura- pophysis) ; Ub, ha?mal arch (ha?mapo- physis) ; D. neural spine; D', haemal spine ; B, 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, und is attached more or less closely to the axial skeleton. The most essential property of animals is that of sensation. This 80 OEUANIZATION 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 Yertebrata. 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 ganglionic mass placed in the anterior part of the body above the pharynx, and known as the supra-cesophageal 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. 70). 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 FIG. 76. — Alimentary canal and nervous 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>,alimentary canalwith mouth ami pharynx. fi1 C" \ FIG. 77. — Nervous system of FIG. 78. — Nervous system the larva of Coccinella (after Ed. Brandt). G, su- pra-03sophageal ganglion or brain; Gfr, frontal ganglion ; Sg, subreso- phageal ganglion ; &',-&", the eleven ganglia of the yentral chain of thorax and abdomen. of adult Coccinella (after Ed. Brandt). Ag, optic ganglion. 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 f» 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. It innervates the organs of digestion, circulation, respiration, and generation, and it can carry on its functions for a longer or shorter time after destruction of the sensory and motor centres. In the Yertebrata (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.— Bra'.n and spinal oord uf a pigeon. //, cerebral hemispheres ; Cb, optic lobes ; C, cerebellum; Mo, medulla oblongata. Sp, spinal nerves. SENSE ORGANS. 83 blood vessels and visc:r.i, 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 (specifia 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 fibrillffi 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). Olt olfactory nerves ; O, eye ; Op, optic nerve ; Iy. Gn.-serinn ganglion ; Xg, ganglion of vagus ; Spn 1, first spinal nerve ; Br, brachial nerve ; Sgl-10, the ten ganglia of the sym- pathetic system. Js, 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 o\vn 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). FIG. 81. — Nerves with ganglion cells (G) beneath a tactile bristle (TV) from the skin of Corethra larva. AUDITOR r 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 (endolympfi) 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- FlG- 82. -Tactile papi'.u from the volar surface tacea the fibres or the auditory nerve end in with the touch corpuscle cuticular rods or hairs which project from the and its nerve -v- wall of the vesicle, and, like the olfactory hairs of the antennae, bring about the nervous excitations. In the Vertebrata 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 LocusticUe, 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, " Organologie des Augcs," Graefe and Samisch, Hand* buck der Ophtlialmologie, Bd. II. 80 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. 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 Cz FIG. 83 — Auditory vesicle of a Heteropod (Pterotrachea). N, acoustic nerve ; Of, otolith tfce fluid of the vesicle ; Wz, ciliated cells on the inner wall of the vesicle ; Ih, 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 oases in which in the lower animals specific nerve endings cannot be made out, we have probably only to do with a forerui ner of the eye, consisting merely of the pigmented termina- EEFRACIILE 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 must 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 I 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 tlie 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 * In addition to the older works of Krohn, H. Miiller, M. Schultze, cf. Boll Sitzungsberichte der Akad. Berlin, 1876 and 1877, also E \vald and Klilvne. FIG. 84. — Diagram of the auditory labyrinth. I. of a fish. II. of a bird. HI. 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; Or, canalis reuniens. 88 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. 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, thepivpil, 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 oft' as an eye bulb. The arrangements by which the shining points of an object act in regular ar- Fio 85. — Diagrammatic representation of the compound eye of aLibellula. C, cornea; K, crystalline cone ; P, pigment ; S, nerve rods of retina ; Fb, layer of fibres : Gz, layer of ganglion cells ; Rf, retinal fibres ; Fk, crossing of fibres. regular 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 Joh. Miiller, "Zur vergleichenden Physiologic dcs Gesiehtssinnes," Leipzig, 1826. H. Grenacher, " L'ntersuchuugen iiber das Sehorgan dcr Arthro poden," Gottingcn, 1879. TJNICORNiiAL EYE. 89 following -P- '-J?. (figs. 85 & 86 Rf (£• E], which are separated from one another by pigment sheaths. In front of these rods are placed the strongly refractile crystalline cones (k), and in front of these again the lens- shaped corneal facets (C