LIBRARY OF 1865- IQ56 BASHFORD DEAN. COLUMBIA COLLEGE. ^ .13 s/ ELEMENTARY TEXT-BOOK OP ZOOLOGY GENERAL PART AND SPECIAL PART: PROTOZOA TO INSECT A. DE. C. GLAUS, Profemor of Zoology and Comparative Anatomy in the ZTnivertify of Viennn Director of the Zoological Station at Trieste. TRANSLATED AND EDITED BY ADAM SEDGWICK, M.A., F.R.S, Fellow and Lecturer of Trinity College, Cambridge, and Examiner in Zoology in the University of London. WITH THE ASSISTANCE OP F. G. HEATHCOTE, M.A., Trinity College, Cambridge. FOURTH EDITION. VOLUME I. WITH 491 WOODCUTS. LONDON: SWAN SONNENSCHEIN & CO. PATERNOSTER SQUARE. 1892. PRINTED BY HAZELL, WATSON, AND VINEY, LD., LONDON AND AYLESBVRV. PEEFACE TO THE ENGLISH TEANSLATION. r UNDERTOOK the translation of Professor Glaus' excellent -*- " Lelirbnch 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 ^iven 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, '^l* 1884. O TABLE OF CONTENTS. GENERAL PART. CHAPTER I. Page ORGANIZED AND UNORGANIZED 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 . c . . . 25 Cells and Cell Tissues 29 Nucleijs and Nucleolus 29 Cell-membrane 29 Reproduction of Cells and division of Nucleus .... 30 1. Cells and Cell-aggregates 32 Isolated cells, e.g.^ blood corpuscles, ova, etc 32 Epithelium 34 Epidermal exoskeleton 34 Glandular tissue ' . . . 3(J 2. Tissvcs of the connective stclstanrc 37 Cellular or vesicular 37 Mucous or gelatinous 37 Reticular, adenoid 38 Fibrillar * . . 88 Elastic 39 Cartilage 39 Osseous tissue 40 3. Mnscular tinsve 43 4. Nervous tissue 45 Increase in Size and Progressive Differentiation, etc. . 47 Unicellular stage . . . . ' 48 Multicellular stage 49 Correlation and Connection of Organs 50 Doctrine of Final Causes . . 51 "Type" 52 Scope of Morphology 52 Structure and Function of the Compound Organs . . 52 Digestive organs 53 Salivary glands, liver, pancreas 58 TABLE OF CONTENTS. D Page Organs of circulation i ... 59 Hc.irt 61 Arteries and veins 62 Heart and vessels of vertebrates , 64 Organs of respiration ......... 67 Branchise 69 Limgs, tracheje - .... 69 Tracheal gills . . . . : ■ . , . . . 71 Renewal of extu ...... 152 Sexual selection 152 Sexual dimorphism of i)arasites 153 Polymorphism of animal communities . * . . . .155 from mimicry 155 from rudimentary orgaiis . . . 156 from embryology 157 Retrogressive metamorphosis 158 from the facts of Gcogra [.Ideal BiistrlhnUon 159 Zoological Provinces 160 from. PalcEontology 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 Extinct Mammalia, transitional between living groups . . .172 Extinct transitional Reptiles and Birds . . " . . . .175 Frogressire perfection . 177 Fauna of the various geological ptrio Is 177 Incompleteness of the explanati->n 118 TABLE OF CONTENTS. SPECIAL PART. CHAPTEK VI. Page CHAPTEE Vlil. PEOTOZOA . 180 ECHINODERMATA . . . Rhizopoda . . 181 Crinoidea . . , . Foraminifcr.T, . . . 184 Tesselata . . . . Lobosa . . 1S5 Articnlata Reticulavia ISG Asteeoidea . , . . Heliozoa . Kadiolaria . 187 . 189 Stelleridea Opbiuridca Infusoria . . 191 ECIIINOIDEA . . . . Flasellata . 193 Cidaridea Ciliata . . 198 Cypeastridca . Holotricha Hetorotricha . . 204 205 Spatangidea . Hypotricha . 205 Holothuroidea . Peritricha Snctoria . . 205 205 Pedata . . . . Schizomycetidaj . 205 Apoda . . . . Gregarinidns . 207 Entekopneusta . CHAPTEK VII. CHAPTER IX. CGELENTEEATA . . 209 VERMES Spongiaria = Porifera Platthelminthes Spongia, . 221 Turbellaria My'xospongia . . 221 Rhabdoccela . Dendrocojla Ceraospongia . . 221 Trematoda Halichondria; . Hyalospongia . . 221 . 221 Distomea Polystomea Calcispongia . . 222 Cestoda . . . . Cnidaria Nemertini Enopla . . . . Anopla . . . . AnTHOZOA = ACTINOZO A . 223 Riigosa . . 230 Nkmathelminthes Alcyonaria . 231 Nematoda Hexactinia = Zoanthar a . 231 Chsetognatha . POLTPOMEDUS^ = HYDEOZ OA 233 Acanthocephala HydromedussD . . 236 Annelida . . . . Eleutheroblastere 240 ClicBtopoda . . . • Polychffita HydroooialUae . Tubulariae . 240 . 241 Campannlarise . 241 Errantia . . , . Trachymedusaj , 242 Sedentai-ia Biphonophora . . . 243 Oligochffita Pliysophorids . . 24S Terriool;i3 . . • PliysalidEB , 2-ft) Limicoke ... Calyoophoridaj 249 Gepliyvea . , . . Disooideas . 250 Scyplioraedusro=Acale pha 251 Chfetifera Calycozoa .■ 257 Achrota . . . . ;Maisupialida . Diseophora (Acraspeda) 258 259 H'lrndtnea . . . . Ctenophoea . . 261 ROTATOEIA . . . . Page TABLE OK CONTEXT;^ CHAPTER X. rage ARTHROPODA. . . . 405 Crustacea . 411 Entoinostraca 416 rhyllopoda 410 Branchioiior^.i . 418 Cladocera 419 Ostracoda 42:s Cupcpoda . 428 Eucopepoda . 43.1 Bianchiuia 430 Cirripedia 438 Pedunculata . 445 Opercvilata 44G AMominalia . 440 AiKKla . 440 Rliizocepliala 416 Malacostraca 447 Arthrostraca . 449 Anipliipoda 451 Isuijoda . 456 Thoracostraca 4G0 Cumacea 4G9 Stomatopoda 470 Schizopoda 472 Decapoda 475 Macrura 477 Brachyura 478 G'lgantostraca . 479 Merostomata . 479 Xiphosura Trilobita . . 480 . 483 AKACHNIDA . . 484 Linguatulida . 487 Acarina . . 489 Pygnogonida . 495 Page Tardigrada . 406 Araneida . . . . 498 Tretrapneutnones . 504 Dipneiunoiies . 504 Pbalangiidie . . 505 Pedipalpi . . 506 Hcorjtionidea . 508 Pseudoscorpionidca . . 510 Solifugre . . 511 Onychophoea 512 Myriapoda , , 514 Chilopoda. . 518 Chilognatha . 520 Hexapoda-Ixsecta . 521 Thysanura , . 553 Oithoptera . 534 Ortlioptera gen\iina . . 65i5 Oithoptera Pseudu-Neur op- tela . 558 Neuroptera . 562 Plauipennia . 603 Trichoptera . . 564 Strepsiptera . . 565 Rbynchota . 566 Aptera . . 507 Phjtophthires . . 508 Homopteia-Cicadai ia 570 liemiptera 571 Diptera . . 572 Pupipara 575 Brachycera 575 Nemocera 577 Aphauipteia . 578 Lcpidoptcra . . . 579 Coleoptera . 585 Hymenoptcra , . 590 Terebrantia . 594 AcuIeaU . . , . 605 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 tohich manifests itself in inetabol ism. 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 vaiious 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 imder 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- fO GEXEBAL PAUT. 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,, pressvu'e, 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 ceqidvoca, spontaneous generation) actually takes place, even in the simplest and lowest form^ of life : although very recently some investigators (Pouchet) have been led by results of remarkable but equivocal experiments to the opposite \-iew. The existence of the generatio cequivoca would offer a very important seiwice to our contention for the physico-chemical explanation; it even appears to he a necessary p)ostulate in order to explain the first ajyj^earance 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 recej^tion of food and excretion of waste p)roducts. 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 ard chemically incapable of further simplification) of organic matter are arain found in inoriranic nature. A vital OEOANISED A.XD r>-OEGA>'^lSEI) 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 j)ut in shar]) 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 oi-igination of organised bodies the same forces wei-e in action which are suflicient 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 is 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 are a number of solid excretion products of organismg (shells) whose form is mathematically determinable does not of course ann-j; this distinction. 12 GENERAL PAKT. 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 {corjyora non agunt nisi sohita), and for the modification of the entire form of the organism ; it is not however homogeneous and uniform, but is composed of solid, semifiuid, 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 ])er/ormance 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 diftei-ence 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 Y . ^ - ^ '1r^- ' Ipwer order), and these ^<^^i.t^ a. *X(/;.' ''■'%vJa b again are composed of the Fig. 1.— a, young ova of a Medusa ; b, mother-cells ultimate unit of cell, the of spermatozoa of a Vertebrate ; one of them pre- ^^^^ The Cell last of all sentecl amoeboid movement. • ' > » IS to be traced bacJi 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 semifuid albuminous substance {j)roto- 2ilasm), containing, as a rule, a dense or vesicular structure, the nucleus, and is frequently surrounded by a peripheral structvireless membrane. If the latter is not developed, the presence of life is indicated by a more or less pronounced amoeboid movement, the fiuid 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 OKOAXisjiD AND unokqa::tised 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. \ 41 - , / 1 \ Fig 2 —Amoeba (Protogenes) poi-recta (after Mai Scliultze)! Nor is this conception of the significance of the cell as the ci"iterion 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 ai-e 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). Conseqitently, the living frotoplasTim vAtli 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 PAET. 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 hve; and further, that in the smallest organisms, which are proved to be such by theii- 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- FiG. 3.-Schizomycetes (after F. Cohn). ^is, independently of organization, a. Micrococcus; 4, Bacterium termo, ^e mUSt allow that hypothesis a Bacteria found in putrefying bodies .,.'„,. , . , both in motile and Zoo^itra form. certam 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), were we not obliged to conclude that there is present even in the simplest and most primitive organisms the gei'ms of sensation and consciousness, attributes which we cannot regaid as simply the results of the movement of matter. AXIMALS A^D PLA>"ia. 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 oi-ganism, wliich 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 oi'ganisms, 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 fortn 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 vai-ious glands, (salivary glands, liver, pancreas, etc). The useless solid remains of the food pass out through the anus as ffeces. The nitrogenous waste material is excreted by a special urinary 16 GEXi:ilAL PAET. 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 moi-e 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 moie irregvilar 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 (Coelenterata). 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 ^ll^ Fig. 4. Branch of a Polyparium of Corallium rubrum (after Lacaze Duthiers). P, Polyp. <] ANIMAL AND VEGETABLE TISSUES. 17 and sense organs are totally absent in many organisms, vvliich are looked upon as animals, e.g., in the whole of the Protozoa. With such reduction of the internal organs it is easy to understand that the simpler lower animals, such as colonies of polyps and the Sipho- nophora, should often in their outer appearance and the manner of their growth resemble plants, with which they were formerly con- founded, especially when they at the same time lacked the power of free locomotion (Polyps, Hy- droids, figs. 4, 5). In these cases it is as difficult to limit the idea of "indi- viduaHty" as it is in the vegetable kingdom. 2. Between 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; while in animal tis- sues the cells give rise to extremely different structures, in which the cells as such do not al n-ays i-emain recognisable. The reason for this unlike condition of the tissues must apparently be sought in the different structure of 2 Fig. 5. — Physophora hydrostatica. P«, Pneuma- tophoro ; S, Swimming-bells ; T, Dactylozooid ; P, polypite or stomach with the tentacles, Sf. ; Kk, terminal swellings on the latter provided with thread-cells ; G, Clusters of gonophores 1,^ 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 \'iscous boundary 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. 6.— a, Vegetable parenchyma (after Sachs). 5, Axial-cells from the tentacles oJ Cam- paniilaria. 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 algre and fungi, but also animal organisms which are composed of one simple or complexly differentiated cell (Protozoa). Finally, it is not po.ssible 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 {sjyermatozoon) with the female element {oinim) ; and the form of the^e elements presents in both kingdoms a great agreement, at any i-ate 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 I'egarded as a distin- guishing mai'k, inasmuch as in both kingdoms the greatest dilierence prevails in this respect. METABOLISM IX AJflMALS AXD PLANTS. 19 4. The chemical constituents and the metabolic processes in animals and plants present, on the whole, important features of difference.! Former-ly 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 th© nitrogen. But ternary compounds ai-e found to be largely present iu 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 Qg^ 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 ti.'s: e., are also found in plants (Leguminosse). Of far greater importance is the difference in the nourishment anS metabolic processes. Plants take up Avith 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 acids), kreatin, tyrosin, leucin, urea, etc.; viric 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 respii-atory organs for the maintenance of their meta- bolism. The processes of metabolism and of respiration, therefore, in the two kingdoms are indeed mutually determinant, but have an exactly opposite result. The life of animals depends on the analysis 20 ANIMALS AXD PLA>-T9. 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 essenlially a process of reduction; under the influence of which the energy of warmth -and light is stored up, kinetic energy being converted into potential. Nevertheless, this difference also is not applicable as a test in all cases. Recently the attention of investigators has been turned, especially by Hooker- and Darwin,* to the remarkable nutri- tive and digestive processes in a group of plants which were first observed a hundi-ed years ago (Ellis). The plants in question catch, after the manner of animals, small organisms, especially in- FiG.r.-Leafof Droserarotundifoiia, ^ects, and absorb from them through ■iTith partially contracted tentacles the glandular surface of their leaves the organic matter aft^r a chemical process resembling animal digestion (leaves of the Sun-dew, Drosera rotundifolia, and the fly-catcher, Dioncea muscijnda. 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 aho 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. 21 of carbonic acid goes on. In plants, thei'efore, 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 [Aga7-icus olearius) is accompanied by an active consumption of oxygen. Finally, there are organisms (yeast cells, Schizomycetes) which indeed manufacture both nitro- genous and albuminous compounds, but do not assimilate the carbon of carbonic acid, but rather derive the necessary carbon from pre- pared carbohydrates (Pasteur, Cohn). 5. Voluntary movement and sensation, according to the common view, is the chief characteristic of animal life. Formei-ly, the pov/er 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 pj^. g.-Zoospores, «. of P^T^a,.,,™; 6, of 3/.»o.,^r»™a; nature of the movement Ci of Ulotkruc; d, of Beilogonium ; e, of Vauchcria , 1 • 1 1 1. 1 . (after Reinke). can be decided tor a dis- tinction between the respective movements of animals and plants. As such for a long time was regarded the contractile nature of tlie movement as opposed to the uniform movements of plants carried out with rigid bodies. In the place of muscles, which as a special tissue are absent in the 22 AKIMALS ASD PLAXTS. Tig. 10.— Zoospores of Aethalium eepilcum after de Barj-. a, in condition of hatching; 6, as mastigopods ; c, in the amoeboid stage; d, a piece of Plasmodium. lower animals, there is present an undifferentiated albuminous substance known as sarcode, the contractile matrix of the body. The viscous contents of vegetable cells, known as protoplasm, possesses likewise the power of contractiHty, and re- sembles sarcode in its most essentLal properties. Both present the same chemical reactions and agi-ee 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, Chcetojihora). 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 Scqyrolegnia, and in the amccba-like forms occurring in the development of Myxomycetes, the contractile power is as intense as in the sarcode of Infusoria and RhizojJoda. 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 j^olyioodia (jrrin- 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 interpi-etation of which vnW depend upon the individual judgment of the observer. The faculty of sensation, which is inconceivable as a function of matter and which must be always pre-supposed wherever we have to do with voluntary movement, can by no means be afiirmed with certainty in all animal organisms. Many of the lower animals entirely lack a nervous system and sense organs, and, on stimulation, exhibit ^\ '\ /■ -n f-^Pv ^' / V. % !PlO. 11. — AtncBha Daciylofphwra po7)/j)odia. N, nucleus. Pc, contractile vacuole (after Fr E. Bchulze). lEKITABIUXT OF PLAXTS. 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 {Miinosece), 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 Centcmrea 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 du'ection of animal development. An animal, therefore, is to be defined as an organism provided with the power of free and voluntary movement, and Avith 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 enei'gy into kinetic under the influence of oxidation processes in metabolism, and excretes carbonic acid and nitrogenous waste products. * The formation of an intermediate kingdom for tlie simplest forms of life is neither scientitically justified, nor from practical considerations desirable. On the contrary, the acceptance of the Frutistav^-ovld. only double the difficulty] <■- determining the limit. 24 OEGAXIZATIOX AND DEVELOPMENT OF ANIMALS IN GENEEAL. 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 ANniALS IN GENERAL. In the foregoing comparison of animals and plants for the estabhshment of a con-ect idea of the meaning of the word "animal," the great vai'iety and the numerous grades of animal structure have been hinted at. Just as the complex organism is built up from the ovum by a process of gradual differentiation, and often during its free life passes through conditions which lead in ascending series to an ever higher development of the parts and to a more complete performance of functions; so, if the animal kingdom be examined as a whole, there is apparent a similar law of gradually progressing development, of an ascent from the simple to the complex, manifest both in the form of the body and in the compositicn of its parts as well as in the completeness of the phenomena of life. It is true that the grades of animal structure do not, like those of the developing individual, follow the one upon the other in a single continuous series ; and the parallel between the developmental gradation of types in the animal kingdom as a whole and the suc- cessive conditions of an individual animal breaks down in so far as we distinguish in the former, as opposed to the latter, a number of types of animal structure often overlapping, but still, in their higher development, essentially different from each other. These we regard as the highest divisions of the system. INDIVIDUAL — ORGAN — STOCK. The animal organism, when viewed from a physiological and mor- phological stand-point, presents itself as an independent and indivisible unit, as a " complete individual." Amputated limbs or excised parts of the body do not develop into new animals; in fact we cannot usually remove a single piece of the body without thereby endanger- ing the life of the organism, for it is only as a complex of all its parts that the body can retain its full vital energy. With reference to the property of the indivisibility of the individual, we understand INDIVIDUAL. -25 by the tei-m organ every pai-t of the body which as a unit subordi- nate to the higher unit of the organism presents a definite foi^m and structure, and performs a corresponding function ; that is to say, an organ is one of those numerous instruments on the combined work- ing of which the life of the individual depends. There are certainly among the simpler animals many instances in which the term individual in its usual sense cannot be rightly applied. In such cases we have to do with structures which from their development must be termed individuals, and represent indi- viduals, accordingly, in a morphological sense. A great many of them are, however, fu^ed 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 incomjihte or mor2)hological indivi- duals, which are usually incapable of leading a separate existence ; and, when they differ fi-om 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 Radiata, it is possible to connect two opposite points of the body by an axis, which may be called the chief axis, and to divide the body by sections passing through this axis into a number of equivalent and symmetrical parts known as antimeres. The organs which are not repeated are situated in the chief axis of the body, while the other organs, which are imif ormly repeated in each antimere, are situated peripherally. Each antimere contains, therefore, a definite group of organs and represents a secondary 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 K. Leuckart, " Ueber den Polymorphismus der Individuen tmd die Erscheinung der Arbeitstheilung in der Natur." Giessen, 1851, 26 ouganiza-Tion a>'d development of aisimals in gexekae. can be dx-awn, passing between the antimeres. A vertical section tluough a radial line divides the corresponding antimere into two Fig. 12a.— Sea-urcliin (diagrammatic). J, inter-radius with the double row of interambulacral plates and the genital organs G ; It, radii is-itli the double row of ambulacral-plates perforated by the ambulacral pores. A, anus. Fig. 12i'.— Shell of a Sca-nrcuin seen from above, li, radius with the per- forated plates ; J, inter-radius with the corresponding generative organs and theii- 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. 12a, h, 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. 14rt). For example, an animal with four radii possesses four antimeres, each of which will be cHvided 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 i-adii, 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 ambulacral feet in the radii. BILATERAL STIIMETIIY. divides the antimeres from each other. The first, in Avhich the greater number of organs are repeated, may be designated the transverse jyldne, while the second, corresponding to the median plane of bilateral animals, is known as the sagittal 2)lane (fig. \ih). Fig. 14a.— Acalepha larva (Epliyra). Rlc, marginal body ; Gf, pastric fila- ment. Re, radial-canal ; O, nioutli. Via. 1-lJ. — Ctenopheran seen from above. S, Bagittal plane ; T, trans verse plane ; R, vibratile ijlates ; Gf, gastric canals. In the bilateral arrangement, which is found also in each individual antimere of the Radiata, only one plane, the median j^lci-ne, 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 2mrame7'es. 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 paii-ed 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- dinal direction. This occurs especially frequently in bilatei'al, less frequently in radiate animals (stroLila). The body thus obtains a segmentation, and is divisible into successive sections, the segments or metamer Fig. 15.— Segmented worm (Polychffite). Ph. pharj-nx ; B, ali- mcnt:ny cniial; C, cirrus : F, tentacle 28 OBOANIZATIOX AXD DETELOPME^?.^ OF AXIMALS IX QENEEAL. which are placed one behind the other, and more or less completely resemble each other in structure {Annelids, fig. 15). The si;ccessive segments may in structure and function appear completely equiva- lent, and represent, like the antimeres of the Radiata, individuals of a lower order, which on the severance of their mutual connec- tion can acquire independence and remain alive for a shorter or longer period {proglottis of Cestodcs). 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 homonomy. In the same degree as the metameres acquire an unlike structure, and corresponding to this a varying importance in the life of the oi'gan- ism, they lose their individual independence, and sink more and more to the value of organs. The metameres in the polymorphous colonies are quite analogous to the segments of the individual. In them there follow, one behind the other, similar groups of different individuals, each of which fulfils singly the conditions necessary for existence, and there- fore can continue to live as a colony of a lower order when separated from the stock {Eudoxia, Diphyes, fig. 16). The distinction into a higher and lower order also holds for organs. There are organs which are reducible to a single cell, or to an aggregation of equivalent cells (simple organs), and others in the formation of 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 cells and cell derivates, which are organs of a still lower order. Finally, in the last analysis, we come to the cell or the area of protoplasm corresponding to it, which is the simplest and ultimate organ. On the other hand, we group together organs of different order, which are intimately connected so far as their chief function is concerned, under the name of system (vascular system, nervous system) or cqyparatus (digestive apparatus), although we cannot clearly distinguish them fi-om compound organs. Fio. IC— Portion of Diphyes after E. Leuckart). D, hydrophyllium ; Oa, gono- phore; P, Polyp with tentacle. The groups of individua'.s separate them selves as EuJoxia. CELL NUCLEUS, 29 CELLS AND CELL TISSUES. The constituent parts of whicli 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 pi-o- toplasmic substance, the mcdear 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 pai-ticles 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 pi-otoplasm 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 chancre in the outer form. The cell in this case manifests Pia 17.— Different forms of nuclei (after R. Hertwig). a, nucleus from a cell of a Malpiffhian tubule of a caterpil- lar, b, nucleus of a Heliozoon with a cortical layer and nucleolus in the nuclear fluid. c, nucleus from the eSo' of a Sea-urchin. Nucleolus im- bedded in a protoplasmic fibrous net- work surrounded by nuclear fluid. • Th. Schwann, " Microscopischc Untersuchungen iiber die Ueberein.stimmung in del" Structur uud dem Wacbsthnm der Thiere und Pflanzen." Berlin, 1839. Fr. Lej^dig, " Lehrbuch der Histologic des menschen und der Thiere." Frank- furt a. M. 1857. 30 OEOAXIZATIO^f AXD DETELOPMLXT OF ANIMALS Ef GKNEBAL. 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 formeily supposed, a necessary constituent of the cell, but is merely an indication that the cell has undergone a cex'tain amount of differentiation from its early indifferent condition. It has been already pointed out that the fundamental properties Avhich 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 Myxomycetes) which certainly is not clearly marked off from a process of new formation within the mother cell, and is to be looked upon as a modification of the so-called endogenous cell formation. This leads us to a consideration of the very widely distributed method of cell increase by division. When the cell has reached a certain size by the absoi'ption 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 strife 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 strvictures the new nuclei are formed at the poles of the now dumb-bell shaped nuclear spindle, the strijB of which vanish during the constriction of the protoplasm, which has already commenced and quickly progresses. The division CELL DITISrON-. 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 gi-adually 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 nuclevis brings about a separa- tion of the cell contents into two masses — the daughter cells (fig. IS). 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. ■W- h^-^J \'^m Fro. 18.— Processes of cell division in an embryonic blood corpuscle of a chick (after Biitschli). i", nuclear spindle. Kp, nuclear plate or equatorial thickening. Finally, the term endogenous cell formation is applied to that method of increase in which the cells originate within the mother- cell. In this case the protoplasm does not divide by a progressive constriction and separation into two or more parts, but differentiates itself round the newly formed nuclei, with which the original nucleus may persist. The ovum Avhich 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 sei'ves for the for- mation of the tissues. Groups of originally indifferent and similar cells break up and assume severally a changed appeai-ance. The constituent elements undergo various difTerentiations, 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 oi-gans, which, like the tissues compos- ing them, can, according to the functions which they perform, be divided into organs of vegetative life and organs of animal life. The former have to do with the nutrition and maintenance of 32 OEGAXIZAXIOX A>"D DEVELOPMKXT OF AXIMAL8 IX GENEEAL. the body; tlie latter, on the contrary, serve for movement and sensation, functions which are exchisively 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-aggregatas (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 froe^. /, of the pigeon, f, 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 elUptical or circular (Mammalia Petromyzon) contour, Avith nuclei in the first case, and without nuclei in the !^e:ond (except in the embryo) (fig 19). They contain OliOANIZA-TIOX AND DEVELOPMENT OF AXIMALS IX GEXEEAL. 3;^ the red colouriBg matter of the blood, hcemoglobin, which plays so important a part in respiration. They arise in all probabihty from the colourless corpuscles which are always far less numerous in normal blood. The colourless corpuscles are genuine cells of variable form, and have the power of amoeboid motion {migration into tissues, regeneration of tissues, etc.) ; they come from the lymphatic glands, in which they arise as lymph corpuscles, and eventually pass with the lymph stream into the blood. The ova and spermospores, after Fig. 20.— Spermatozoa, a, cf '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 spermospores, respectively belong to the category of free cells. The form and size of the spermatozoa present great variations. They always consist of a modified cell, frequently of a very small cell with a long llagellum, nucleus, and remains of protoplasm. In many cases the head is elongated into a fibre-like structure, or is twisted like a corkscrew (Bh-ds, Selachians). Sometimes a distinct head is absent, and the spermatozoon is thread-like (Insects). In the Nematodes the sperm- 3 34 UENHRAL 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 (endothelium). 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, c^), which are continuous with the living protoplasm of the cell. If only one flagellum projects from the cell (sometimes a flat cell fig. 21, b) then the name flagellate cell is apphed (collared cell of sponges, fig. 21, e). Finally, in the case of pavement epithelium (fig. 21, a) the cells are flattened; and if there d Fig. 21.— Various kinds of epulielial cells, a. Flat cells, b, flat cells with fiagella (from a Medusa), c, cylindrical cells, d, ciliated cell, e, flagellate cell with collar (from sponge). /, cyhndrical 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 fi-ee sm-face of which is distinguished by a OEGANIZATION AXD 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 tine canals which give it a striated ap- pearance (intestinal epithe- lium, fig, 21,/, epidermis cells of Petromyzon). If these thickened borders fuse to- gether so 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 G ^ Fig 22 —a, C iticle aud lij pjdtnnis ot the lar\a of Corethra b, cuticle and hypodermis of a Gastro pacha caterpillar, with two poison glands beneath corresponding bristles. may exhibit various patterns of difl:erent kinds. Very frequently the surfaces of the individual cells are indicated on tlie 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, aud con- tain as a matrix their special cell or a process of it. Cuticular membranes may obtain a very considerable thickness, and, by the deposit.'on of calcareous salts, a high degree of firmness (carapace of Crustacea) so that they acquire the value of skeletal tissue.^, which, Fig. 22e. — Cu, cuticle with bristles in the condition of ecdysis. Cu', newly-formed cuticle (Brajichipus). 36 GE>'i:iiAL i'AUT. however, it is generally diificult to distinguisli 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- vaginations of tlie epithelium, b, 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 diflferentiation 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. Or.GANlZATION xyn development of animals IX GENEEAL. 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 ceM 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 medusfe, 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 deHcate, 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 umbiHcal 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 tissue but little different in its origin, as a unilateral cell excretion, from cuticular structures (Hydroid Medusje, s^\^imming bells of Siphono- FiG. 25. — Vertebra of larva of a toad (after Gotte) . Ch, notochord cells ; ChS, notochord sheath; Sk, skele- logenous tissue ; N, spinal cord. 38 GENERAL PART. phora). The so-called secreted tissue of young Ctenophora, and the gelatinous tissue of Meduste 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, flbrr,U5 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 JihTillar 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 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. Yery often the Fig. 27.— Filjiillar connective tissue. OEQAXIZATION A>"D DEVELOPMENT OF ANIMALS IN GENJiiiAL. 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 du"ections (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 nucha;, 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 connectivo tissue. It is characterized by the shaj^e 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 cartilat^^e. Elastic fibres, network. Fig. 29.— u, Hyaline cartilage with cells. I, Fibro-cartil v^e. According to its special constitution, three kinds of cartilage may be distinguished, viz., hyaline (fig. 29, a), fibrous (fig. 29, b), and 40 GENEHAL PA.ET. 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 earUer 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 fiequently see in the spaces in the cartilage several generations of cells suri'ounded 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 cartilage, 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 structm-e. Cartilage owes its special usefulness as a skeletal tissue to its rigidity. It is sometimes found in the Invertebrata (Cephalopoda, tubicolous worms such as Sabella, Ccfilenterata), and very generally in the "Vei'tebrata, whose skeleton always contains a cei'tain 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. Z\ a, h, c). The cells occupy spaces in the com- FiG. 30.— Incrusted caxtilafjc, or cartilaRe bone. OBOANIZATtOy AND DET£!.Or.\I 1:NT OF ANIMALS IN GENERAL. 41 pact intercellular substance, which is also traversed by numerous canals, known as Haversian canals. blood-vessels and correspond exactly in their course and branchings to the latter. The intercellular substance consists of lamellre, 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 spongy bones have an irregular distribution. In a second form of osseous tissue the cells themselves remain in the outer part of the excreted intercellular substance, and only their Fig. 31a.— Longitudinal section through a long bone (after KoUiker). G, Haver- Bian canal. Fig si/ - ii m ^elbe section thruUi^h a long b ne (after KolUker) K, bone corpuscles , (?, Haversian canals, X, lamellae. Fig. 31(7.— X, spaces containing the bone corpuscles and their processes— they open into the Haversian canal, JIc (after Kolliker). numerous processes, which run parallel to one another and are of great length, are embedded in it. The intercellular substance, which is hardened by the deposition of calcareous salts, is therefore traversed by a great number of fine tubes. It is deposited on one side only of the cells, and in its origin recalls the hard carapace of the Crustacea, which is similarly traversed by prolongations of cells. This kind of osseous tissue, traversed by fine parallel tubes, is 42 QSXEKAL PART. found teeth in osseous fishes, and quite universally as the dentine of (fig. 32). With regard to its development, bone is preceded by soft connective tissue or by cartilage. In the first case, it develops by the tran;4"or- ^ . , mation of the connective tissue ^^^^W^K ^ ^^^^^ "^^° bone corpuscle-, and by ■^^' ^ the hardening of the intermediate tissue. More frequently it is pre- ceded by cartilage ; and this holds foi' a great part of the vertebrate skele- ton. Formerly great importance was attached to this difference in the oiigin )i Fig. 32.— Section througli *he roocof a tooth (after KoUiker). C, cement ; J, intei'globular spaces D, dentine with dentinal tubes. of bones ; and a primary was distinguished from a second- ary method of bone develop- ment. In reality the two processes resemble each other closely. For in the latter case, in conjunction with a precedent deposition of lime, and partial destruction or reduction of the cartilage, there is a new formation of a soft connective tissue-substance (osteogenic substance) from the centre outwards, the cells (osteoblasts) of which give rise to bone corpuscle <, and the intermediate tissue becomes the hard basis of bone (fig. 33). More- over, cartilage bones grow in thickness at the expense of the Fig. 33.— a section of ossifying cartilage (after Fray), a, Smaller marrow spaces placed in the cartilage ; b, ditto, with cells of the cartilage marrow; c, remains of the calcified cartilage; d, larger marrow spaces ; e, osteoblasts. ORGANIZATION AND UEVELOPJIENT OF ANIMALS IN GENERAL, 43 -Myoblasts of a Medusa (Aarelia) . periosteum, the connective tissue of wliich is directly transformed into bony substance. 3. Muscular tissue. We ascribe the property of contractility to the protoplasm itself of the active cell ; but we observe that, even in the protoplasmic body substance of the Infusoria, a striated arrange- ment obtains in those parts in which the contractile function especially resides. By a similar differentiation of the protoplasm certain cells and aggregations of cells possess in a much higher degree the power of contractility, and give rise to the so-called muscular tissue which serves exclusively for movement. At the moment of their activity these cells undergo a change of shape ; they become shorter and broader than when at rest. In many Ccelenterata, cells are found in which a part only of the cell is developed into a contractile fibre. It is the deeper parts of such cells which give rise to delicate muscular fibres or net- works of fibres, while the superficially placed body of the cell ''' (myoblast), the part which produces the above, performs other functions, and usually bears a cilium. In consequence of their epithelial- like arrangement, the myo- blasts receive the name of muscle-epithelium (fig. 34 a, b). In their further develop- ment the greatest part of the cell protoplasm appears to give rise to contractile muscle- substance ; and sometimes the whole cell becomes elongated into a muscle fibre. Two kind-^ of muscles, which are morphologically and physiologically different, are to be distinguished, viz., the smooth muscles, or con- tractile fibi'e-cells ; and the cross-strijjed muscle-suhstaoice. * These cells have been called iieuro-muscular cells ; a misleading' term, since it cannot be shown that they have had anything to do with the origin of ganglion cells. Fig. 344.— Mu-i .■ i jnr helium of a Medusa (Amelia). 44 OEKEBAL PAET. In the first case we have to do with flat, spindle-shaped, or band- shaped elongated cells, and with layers of su:-h cells. They react slowly to nervous stimuli ; they enter the condition of contraction gradually, and remain contracted for some time. The contractile substance appears for the most part to be homogeneous;, but it is sometimes longitudinally striated. / The smooth muscles have the widest distribution amongst the Invertebrata ; but they are also found in vertebrates, in the walls of numerous organs (vessels, ducts of glands, intestinal wall) (fig. 35). Cross-striped muscle consists of cells, more frequently of multi- nucleated so-called primitive bun- dles. It is characterised by the partial or complete transforma- tion of its protoplasm into a cross- h laiiiiugj tei^i^ Fig. 3G. — a, I'rimitive fibre. J.cross-stripcJ muscle fibre (primitive muscle bundle) of Lscerta with uerve termination. Fig. 35. — a, smooth muscle fibres isolated, b, piece of an artery (after Frey) ; 1, outer connective tissue layer ; 2, the middle ] jyer 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 fibrillte are produced by the deeper parts of the myoblasts, which form a continuous flat surface epithelivim (muscle epithelium) above the layer of delicate fibres (Medu.sfe and Siphonophora) (fig. 34 b). In the higher animals they OROA.yiZATIOX A>"D DEVELOPMENT OF ANIMALS IN GliNEUAL. 4i3 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 fibres, the primitive bundles ; the nuclei at the same time increase in number, and a membrane, the sarcolemma, becomes developed on the outer surface of each fibre. More frequently, however, the primitive bundles arise by the fusion of several cells placed in a row. Either the nuclei come to lie close to the sarco- lemma in a peripherally-placed layer of finely granular protoplasm, or they are arranged in a row in the axis of the fibre in some finely granular non- contractile protoplasm. The finer and coarser muscular bundles are composed of many primitive bundles (fibres) placed close together and held together by connective tissue. The fibrillation of the muscular bundles corresponds to the direction of the primitive bundles (muscles of Vertebrata). Finally, both the simple cells, and the multi-nucleated muscles which arise from them, may be branched (heart of Vertebrata, intestine of Arthropods, etc). 4. Nervous tissue. As a rule, nervous tissue is found with mus- cular tissue, and is the means by which stimuli are conveyed to the latter; but above all, it is the seat of sensation and the will. With regard to this important function it would appear probable that in phylogeny the elements of nervous tissue have not arisen in con- nection with muscular tissue, but in connection with the sense cells found in the skin, i.e., differentiated ectoderm cells, and that then, still remaining connected with the sense-cells, they have travelled inwards into the subjacent tissue ; while the connection with the muscle-cells, which at first possessed an independent irritability, is only secondary. Nerve-tissue contains two distinct structural elem-ents, 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 QE3f£IlAL PAliT. 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 ceutripo- 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 ner\'e fibre with the sheath of Schwann. Fio. 37. — a bipolar ganglion cell. 6, nerve cell, from the human spinal cord (anterior cornu), (after Gerlach). P, pigment body. quently enclosed in a nucleated theath. 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 retractile 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 OEGANIZATION AND DETELOPMENT OF ANIMALS IN OENEBAL. 17 of the latter. They possess in addition an outer sheath, known as the sheath of Schwann (cerebro-spinal nerves of most vertebi'ates). 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 fibrillse, and be, so to speak, resolved into its elements. Finally, the nerves of In- vertebrates very often appear as finely striated bundles of fibrillfe, 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). G-anglion cells are frequently found inserted in the course of the nerve fibres close to their termination (fig. 39, a, h, c.) 'lO. 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 fibrillsB and the sense cells. INCREASE IN SIZE AND PROGRESSIVE DIFFERENTIATION, DIVISION OP 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 43 GI^XEEAL PART. cell membi-ane. The latter is often without an opening foi* the entrance of solid bodies ; the entrance of food being entirely effected by endosmosis. In such cases, e.g., in the Gregarines and parasitic Opalines, the outer body-wall suthces, 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 parenchynaa, and is the seat of the animal and vegetative vital activities. Accordingly there results a definite connection between the functions of the peripheral layer and of the included mass, in which the processes of animal and vegetative life are carried on. This connection pre-supposes a definite relation between the superficial area of the surface and the size of the mass, and this relation changes as growth proceeds. For while the surface increases by squares, the mass increases by cubes ; while the mass increases in three dimensions, the surface only increases in two, and therefore as growth proceeds the relation changes to the disadvantage of the latter. In other words, with increase of size the superficial area becomes relatively smaller. Finally it becomes relatively so small that the vegetative processes cannot be carried on, and it is necessary for the mainte- nance of life that for a given energy of life it should be increased by the production of new surfaces. This holds not only for the simple unicellular organisms, which resemble cells in their nutritive processes, but also for cells them- selves whose size never exceeds certain fixed limits. Further, as the organism increases in size, not only does it divide into several cells, but these cells arrange themselves in such a way as to give the largest possible extent of surface. The cellular organism accord- ingly acquires not only an outer but also an inner surface on which the cells are arranged in a regular layer. With the appearance of an inner surface, a division of labour is established. The outer layer carries on the animal functions and such vegetative processes as those of respiration and excretion, while the inner {digestive cavity) serves for the reception and digestion of food. We thus 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 — VoIvok — Blastosphere) (fig. 40, a, b.) Then, in consequence of the needs of the growing organism. lilE OASTUOLA. 49 a it became necessary that they should be divided, so as to bound two surfaces, into an external and an internal layer ; the one foiming the outer wall of the body and known as ectoderm, and the other lining the central cavity (digestive cavity) knoviTi 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 cylindiical 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 froin 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 4 Fig. 40.— a, Cell colony of young Volt-ox Glohator (after Stein), b, Blastosphcre stage of an Acalepha larva (Aurelia Aur'da). c, Gastrula stage of b- Ec, Ectoilerra; En, Endoderm; o, Blasto pore (mouth of Gastrula). 50 OEGANIZATIOX AND DEVLLOPMEXT OF AXIMALS IX GLXEEAX. tissue, developed from one or both of the piimary layers, primitively serves as a support for the body and forms the skeleton ; and it also gives rise to muscle.-; which inciease the organism's power of move- ment and apply themselves, on the one hand, to the ectoderm (s^omatic 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.f Fi'om 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 (tentacJes, 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 progi-e;sing 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, Avhich is thereby rendered capable of a higher and more complete life. Therefore we find, as a genei"al 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 (t^i^es), as well as the special conditions of life which are limited by them, must be taken into account as compensating factors. CORRELATIOX AXD CONNECTIOX OF ORGANS. The organs of the animal body stand in a mutually limiting rela- tion to one another, not only in their form, size, and position, but also in their actions ; for since the existence of an organism depends upon the blending of the individual performances of all its organs to a united manifestation, the various parts and organs must all, in ♦ U?:iially known as segmentation cavity. — Ed. t Usually known as " body cavity." or " coelom." — En. DOCXRINE OF FINAL CAUSES. Bl 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. From this is deduced the principle to which Geoffroy 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 " Abnormaiites " (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 featvires 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 "^:)n?icz}je des causes finales" (des co7iditions 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 Bcnse as implying the existence of design, renders important and 52 OEGANIZATIOX ASD DEV£LOPMi:>T OF A>IMALS I>' GSNEKAL. indispensable service to the understanding of the comphcated 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 ti/2)es. The term 'Type" was applied by Cuvier to the chief, i.e., the most compre- hensive and general divisions of his system; and each tj-pe was distinguished by the sum of the characters of its form and sti"ucture. 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 gi'oups 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 then* 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. TKE 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 form.er, however, they gi'adually 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 DIOESTIVJi OEGAXS. 53 nutrient fluid (blood) which permeates the body, and is carried in more or less definite tiucts 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 nil I ' / Fig. 4!.— Rotalia veneta (after M. Schultze) with a diatora caught in the pse-jdopndial network. consist of the apparatus for the reception of food and for its 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 eftected J-l OEGANlZATIOy AND DETELOPMEXT OF ANIMALS IN GENEEAI. in the simplest cases, as in the Amcebse and Ehizopoda, by prolon- gations of the sarcode (pseudopodia) suirounding the foi-eign 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 present, which serve the purpose of procuring food (adoral ciliated zone of the Ciliata) (fig. 42). Fig. 42. — Stylonychia mytilua (after Stein) viewed from the ventral surface ; Wz, adoral zone of cilia; C, contractile vacuole ; N, nucleus ; A^'juucle- olus (paranucleus); A, anus. Fig. 43.— Lougitudinal section through the body of an Anthozooid (Octactinia). M, stomachic tube with the mouth open- ing ia the centre of the feather-like tenta- cles ; Mf, mesenteric folds ; G, genital organs. Among the animals with cellular differentiation {3Ieiazoa), the internal cavity of the body in the Ccelenterata (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 ALISILXTAIIY CAXAL. 55 vascular canals). In the larger Polyps (Anthozoa) a tiil)e derived from an invagination of the oral disc projects into the central pai't 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 oesophageal 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. — Aurelia aurita seen from the oral surface. JUA, the four oral tentacles -with the mouth in the centre ; Gl; genital folds; GH, opening of the genital pouches ; RIc, 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 movith (Polyps, Medusae) (fig. 44). Such appendages serving for the capture of prey may also be placed further from the mouth (tentacles of Medusjw, Siphonophora, Ctenophora). When the 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 OEGA>'IZA.TION A>'D DEVELOPMENT OF AJSIMALS IN GENEUAX. (fig. 45), with sharply marked off pharjmgeal structures (Trematoda, Turbellaria), or as a tube communicating with the exterior by an anus (tig. 46). In the last case it becomes divided so as to lead to the distinction of three parts — (1) of the fore-gut (oesophagus) for the reception of the food, (2) of the mid-gut for the digestion of the food, and (3) of the hind-gut for the expulsion of the undigested remains of the food. Sometimes the alimentary canal aborts ; and, as in the mouthless Protozoa (Opalina), the mouth opening may be absent -5 (Acanthocephala, Cestoda, E,hizoce- phala). In the higher animals, usually, not only is the number of the divisions greater, but their shape and struct ui'e becomes more com- plicated. The organs for the seizure of food also become more complicated, and the appendages placed nearest the movith often become modified to subserve this f unc- 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 (Vei^tebrata, 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 ( Ai'thropoda), 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 Fig. 45. — Alimentary canal of Distoinum hepaticum (after R. Leuckart) ; D, alimen- tary caual ; O, mouth. Fig. 46.— Alimentary canal of a young nematode. O, mouth ; Oe, fore-grut (cesophagus) with pharyngeal dilatation, Ph ; 1>, mid-gut : A, anus. NT) repeated mechanical action (masticatory stomach of Cray-fish) or by the secretion of digestive fluids (pepsin) furthers digestion ; or it may, as in birds, subserve both these functions. From the stomach the food passes into the mid-gut. Dilatations and out-growths of the buccal cavity give rise to cheek and throat pouches, of the oeosphagus 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. 49.— Alimentary oanai of modifications by the action of ti-ypsin lt^XlZt7t^Z'. (^-^ting, however, only in alkaline solutions). Oe, oesophagus; s, suckinrj The intestine often attains a great length, t?breS:...?ect^!''''''" and becomes divided into regions _ possessing a difierent 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) ; AT), rectum (hind gut) ; M(J, Mal- pighian tubes. 58 OEGA>-IZATION AJN'B DETELOrMZXT OF ANIMALS IX GXXFBAL. 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 ffeces. It may also possess caecal 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) cseca, 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 organs, anal glands). It may in addition dis- charge other functions, e.g., a respiratory (larvae of Libellulidje) or a secretory function (larva of Ant Lion). The salivary glands, liver, and pancreas are to be regarded as outgrowths of the alimentary canal which have become diffe- rentiated into glands. The secretion of the salivary glands is poured into the buccal cavity, and there performs two functions — (1) it dilutes the food, (2) it has a chemical action upon it, converting the starch into sugar : they are absent in many aquatic animals and are especially developed in herbivorous animals. Fis. i\). — Alimentary canal of a bird. Oe, (Esophagus ; K, crop ; Dni, proventriculus ; Km, gizzard ; D, small intestine ; P, pan- creas placed in the loop of the duodenum ; ^, liver; C, the two creca; i7, ureter; Ov, oviduct ; Ad, large intestine ; Kl, cloaca. OKGAXS OP CinCULATIOX. 59 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 Yerte- brata the liver, as a bile-pro- ducing organ, possesses no knov.'n 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, MoUusca). 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 matei-ial 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 Coilenterata. In these animals the digestive cavity itself extends to the extreme periphery of the body, and serves to distribute the nutritive fluids Fig. 50.- Alimentary canal of Jinn. Oe, ffisaphagus ; M, stomach ; L, sjileen ; JI, liver ; Gb, gall bladder ; P, pancreas ; Dii, duodenum receiving the bile and pan- creatic ducts ; Jl, ileum ; Co, colon ; Coe, cascum with vermiform process, Pv; B, rectum. 60 OaaANIZ.VTION" AVD DEVELOrMEXl or ANIMALS !>• GEXEEAL. ti-0-vascular system of Polyps, so-called vessels of Medusse and Cbenophora). The so-called stomach of the Anthozoa is simply an invagination of the body wall into the central cavity of the animal, and functions only as oesophagus. Wlien a distinct ahmentary 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 jjeneral Fig. 51.— Daplinia with simple heart. C, the slit-like opening on one side is seen; D, alimentary canal ; L, liver; A, anus; G, brain; O, eye ; Sd, shell gland; .Br, brood pouch placed dorsally beneath the carapace. tissue of the body in the acoelomate parenchymatous 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 lacunae 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 conti'actions of the somatic muscles HEA.UT OF LNTEETEBRATES. 61 ^Ascaris), but also by the movements of othei' organs, e.rj., the alimentary canal (Cyclops). At a higher stage of development a vudiment 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- chambered heart or dorsa? vessel .Rg, the lateral openings in which are repeated in every seg- ment. D, intestine ; M, mandible ; Sd, shell gland; Br, branchial appendage of the 11th pair of legs ; T, testis. Fig. 53.— Heart of a Copepod (Calanella) 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 02 OIIOAKJZATION AKD DEVELOPMENT OF ANIMALS IN GENERAL, ostia, provided with lip-like valves, which act so as to allow the blood only to enter the organ. From the heart, as central organ of the circulation, well defined canals, the blood vessels, are thendeveloped, 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- UKiit, and gives rise to vessels which carry the blood back to the Fig. 54.-Heart and blood vessels and gills of the crayfish. C, heart, in a blood sinus ; with Ps several pairs of ostia; Ac, cephalic aorta; A.ah, abdominal aorta; Ag, 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 cidled 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 nE.VUT OF TUETEBEATES. 63 a blood sinus or as a system of blood-lacun£e ; or the arteries and veins are connected by a network of delicate vessels, tlie 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 ; Cr/, cerebral ganglion with eye ; Pg, peilal ganglion with adjacent otocyst ; Vg, visceral ganglion ; P/>g, pharyngeal ganglion ; A, auricle of heart ; Ve, ventricle ; Aa, abdominal aorta ; Aa, 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 longitudinal vessel which runs in the ventral wall of the branchial sac gives off numerous lateral branches, which ascend in the branchial walls. These lateral vessels are contractile at their point of origin (U OEGAKIZATION AXJD DEVELOPMENT OF AXIMALS O OEXEEAL. 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, ti-averses a capillaiy network in the liver. From the hinder part of the ventral pha- ryngeal vessel there is developed, in the higher Yertebrata, 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 lackward 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 netwoik the arteiialised 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 OligochEete worm (Ssenuris) (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. I'ULMOXART CinCULA.TION. C5 With the appearance of hmgs as respii-atory organs (Dipnoi, Perennibranchiate Amphibia, larvse of Salamanders and Batra- chians) (tig. 58), the heart obtains a more compUcated structure, in that the auricle becomes divided into a right and left division, the latter of which receives the arte- rialised blood, returning from the lungs by the pulmonary veins. The septum between the two divisions of the auricle may, how- ever, remain incomplete (Dipnoi, Proteus). The advehent pulmon- ary vessels, the pulmonary arte- ries, always proceed from the Fig. 57. — Diagram of tlie circulatcrf organs of aa osseous fish. V, ventricle ; Ba, 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 ; B, alimen- tary canal ; Lk, portal circulation. Fig. 58. — Gills {Br) 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. posterioi' vascular arch, which, as a rule, loses its relation to the branchial respiration. On the disappearance of the gills, which is completed during the metamorphosis in the S.ilamandrina and .Batrachia, the pulmonary 5 66 OEGANIZATIOK AXD DKTELOPMEXT OF AXrMALS I>' GLXEKAL. 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 arter}''), which now receives through the ventricle venous blood from the right auricle, from the system of anterior arches which give origin to the cephalic vessels and dor- sal aorta and receive arterial blood from the left auricle (mixed, how- ever, with venous blood in the ventricle) (fig. 59). In Reptiles the sepa- ration of the arterial from the venous blood is more complete, in that there is an incomplete ventricular septum Avhicli foreshadows the later division of the ventricle into a right and a left half. From the left division arises the right aortic arch, which gives origin in its further course, to the ai-teries to the head (carotid arteries). A vessel to the lungs and a left aortic arch may also be distinguished. The left aortic arch and pulmonary artery receive only venous blood, while the right aortic arch, and therefore the carotids which proceed from it, receive principally arterial blood from the left side of the ventricle (fig. 60). The venti-icular septum, and consequently the sepaiation of tho right from the left ventricle, is found complete for the first time Fig. 59.— Circulatory organs of the frog. P, left lung, right lung is removed ; Ap, pulmonary artery ; Vp, pulmonary vein ; Vc, vena cava inferior ; Ao, dorsal aorta ; N, kidney ; D, alimentary canal ; Lk, portal circulation. LTMPnATIC SrSTEM. 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 Panizz£e) leading from one into the othei', and through which a communication may take place. It is only in Birds and Mammals, in which, as in the Crocodilia, the right and left ventricle are completely separated, that a separation between the two kinds of blood is completely effected (tig. 61). In Birds the right aortic arch persists, and the left entu-ely disappears ; while in Mammalia the opposite obtains, tlie 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 vesi-els. This system origi- nates in simple tissue spaces, which are without walls, and its main trunks open into the vascular system. The con- tents are derived fi-om 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 ilespiration. 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 closiely connected the excretion of carbonic Fig. go.— Heart and great vessels of a Chelouian. Ad, right auricle ; As, left auricle ; Ao.d, right Ao.$, left aortic arch ; Ao, aorta; Ap, pulmonary arteries. aortic arch ; C, carotids : G8 OHGA>IZATIOX A>-D DETELOPMEM' OF A>-mALS IN GEXERAI,. acid (and water). The exchange of these two gases between the blood and the external medium is the essential part of the respiratory process, and is effected through organs which are suited for carrying on this process either in air or in water. In the simplest cases the exchange of these two gases takes place through the genera] surface of the body; and in all cases, even when special respira- tory organs are present, the outer skin aloo takes part in respiration. Fig. 61.— Diagram of the circulation in an animal with a completely separated right and left ventricle, and a double circulation (after Huxley). Ad, right auricle receiv- ing the superior and inferior vense cavse, Veg, and Vci ; Dth, thoracic duct, the main trunk of the lymphatic system ; Ad, right auricle ; Vd, right ventricle ; Ap, pulmonary artery ; P, lung ; T j:', pulmon- ary vein ; Af, left auricle ; Vs, left ven- tricle ; Ao, aorta ; D, intestine ; L, liver ; yp', portal vein ; Lv, hepatic vein. favourable conditions for the inti direct respiration in air, because Fig. G2.— Diagram of the great arteries of a mammal with reference to the five embry- onic arterial arches (after Eathke). c, common carotids ; c', external carotid ; c", inter- nal carotid; A, aorta. Ap, pulmonary artery ; Aa, aortic arch. Inner surfaces also may be con- cerned in this exchange, especially those of the digestive cavity and intestine, or, as in the Echi- noderms, in which a separate vascular system is developed, the surface of the whole body cavity. Respiration in water obviously takes place under far more un- oduction of oxygen than does the it is only the small quantity of EESPlRATOaX ORGAXS. 69 oxygen dissolved in water which 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 branched processes (fig. 63 a, h), or of is available. Hence this form of Fig. 63a, — Head ami 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. 64j. Fig. 61. — Transverse section througn the gill of a Teleostean fish, i, branchial leaf- let with capillaries ; c, branchial artery con- taining venous blood ; d, brancliial vein con- taining arterial blood. <2, branchial bar. Fig. 63J. — Transverse section through the body of Eu- nice. Br, gill ; C, cirrus ; P, parapodium with a bundle of setaj ; D, alimentary canal ; iV, nervous cystcm 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 cir-b£Viring tubes. In the first case (Spiders, Vertebrates) they consist of spacious sacs with alveolar or spongy 70 OKGAKIZATIOX X^D DEVELOPMrNT OF ANIMALS IX GE.VERAL. walls, traversed by numerous septa and folds which bear an extremely rich network of capillaries. The ah- 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 surroun d e d by a fine tracheal network. Kevertheless, the air tubes in the case of the modification known as fan- trackece present an approximation in their structures to lungs, in that the main stems, without further branching, give rise to flat hollow leaves. FiQ. 65. — Trachere with fine brauches (after Leydig). Z, cellular outer wall ; Sp, spiral thread. K Pig. 6C6.— Lateral view of head and body of an Acridium. St, stigmata ; T, Tympanum. Openings in the body wall are present, placing the organs of aerial respiration in communica- tion with the exterior. These openings may be numerous, and paused, placed symmetrically on the sides Fig. COa. — Tnuleal eye tem of a Diptei.i-.is larva. Tr, Longitudi- nal stem of the right side with tufts of tra- chefe; St', and St", anterior and posterior stigmata; Mh, oral hooks. TEACH E.K. 71 cf the body (fig. 6G a, h) {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 Yertebrates). In the aquatic larvaj of certain fro. 67a.- -Larva -jf an Ephemeral fly with seven pairs -of tracheal cri'is Lf, Hlightly magnified ; Tk, isolated tracheal gill strongly magnified. Fig. 676.— Tracheal sys- tem at the sides of the alimentary canal of an Agrion larva (after L. Dufour). T»/, main tracheal trunk ; Kt, tracheal gills ; Na, the three simple eyes. Insects (Ephemeridae, Libellulidse) the tracheae may be without any external openings. In such cases processes of the body filled with a close network of tracheae, which take up oxygen from the water, and are known as tracheal gills, are developed (fig. 67 a, h). In rare instances tracheal gills are developed on the wall of the rectum, and 72 OnUAl^flZATION A>*D DETELOPMENT OF AXIMALS I>' GEXEEAL. thus ac^juire a protected position (rectal respiration of A.eschna, Libelluk). In other respects the branchial and pulmonary respiratory pro- cesses are essentially the same. In the pulmonate snails (Lymnoeus), 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 sex've 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 cai-bonic acid and the introduction of another portion con- taining oxygen and free of carbonic acid, is effected. In the simplest cases this renewal can, although not very efficiently, be brought about by the movements of the body, or by a continuous oscillation of the respiratory surfaces themselves ; a method which is especially common when the gills are placed in the region of the mouth and function also as organs of food prehension, e.g., the tentacles of many attached animals (Polyzoa, Brachiopoda, tubi- colous Worms, etc.) Very frequently the gills appear as appendages of the organs of locomotion, e.g., of the swimming or ambulatory feet (Crustacea, Annelids), the movement of which brings about a renewal of the respiratory medium around the gills. The move- ments become more complicated when the gills are enclosed in special chambers (Decapoda, Pisces), or when the respiratoiy organs are placed within the body, as happens in the case of tracheze 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 rhji;h- mical contractions and dilatations of the air-chamber, constituting the so-called respiratory movements. The term respimtioii is now not only applied to these movements so obvious to the eye in air- breathing animalsj 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 loater. 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 I'ich in oxygen from blood I'ich 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 natiu"e of the blood-vessel, — those carrying the blood to the heart being called venous, and those carrying it from the heart arterial, — they are aLo used in a physiological sense as an expression for the two conditions of the blood before and after its passage through the respu-atory 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 Jind 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 acti\'ities 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 surroundiug medium, the temperature of the 74 0BGA>'IZA.T10N A^D DETELOPMEXT OF ANIMALS IN GENEKAl. 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 jjoikilotheryiiic* or, as they have less appropriately been called, cold-blooded. The higher animals, on the contrary, in which, on account of their highly developed respiratoiy 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 lising and falling of the temperature of the surrounding medium. Such animals are designated liomotherraic, or tcarm-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 metaboHsm) 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 'Warmebkonomie der Thiere zu ihrer Grosse," Gottingcr Studicn, 1847; also Bergmann und Leuckart, " Anatomisch-physiologische Uebersicht des Thicrreichs," Stuttgart, 1852. unixART oi;oA>-s. 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 racemose and lobulated glands. Among the various substances which by the aid of the epithelial lining of the walls of glands are removed from the blood and some- times utilised further for the performance of various functions, the nitrogenous excretory substances are especially important. The organs by which the excretion of these ultimate products of meta- bolism are effected are the kidneys. In the Protozoa they are represented by the contractile vacuoles ; in the Worms they appear as the so-called ivater- 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, eji). In the segmented worms the paired kidneys are repeated in every segment, and are known as segmental organs (figs. 69 and 70). The shell-glands of Crustacea are in all probability to be traced back to these segmental organs : as are also the paired kidney (organ of Bojanus) of mussels, and the unpaired renal sac of Snails, both of which communicate by means of an internal opening with the ■pericardial division of the body cavity. In the air-breathing Arthropods and some Crustacea (Orchestia) the urinary organs are tubular appendages (Malpighian vessels) of the hind gut. In the Vertebrata the urinary organs or kidneys oljtain a greater independence, and open to the exterior by special Fig. C3.— Young DIstomum (after La Valette). Ex, main stems of the excretory system ; Ep, ex- cretory pore ; O, month with sucker; S, sucker in the middle of the ventral surface ; P, pha rynx ; D, alimentary canal. r6 OIlGA»IZA.lION A.ZiD D£VELOPMI;^■T OF AMMALS !>' GENERAL. opeuings which are usually common to the generative organs ; they consist essentially of a number of coiled tubes, which in the more primitive types of Vertebrates have a ciliated funnel-shaped opening into the body cavity (Dogfish embryo, fig. 71). The individual tubules of which the verte irtr JZz^ r T" -Tn Frs. 70. — Diagrammatic representation of the segmental organs of a segmented worm (after C. Samper). Di, dissepi- ment ; Wtr, ciliated funnels whicli lead into the coiled tubes. Fig. 69.— Longitudinal section through the medicinal Leech (after R. Leuckart). D, ali- mentary canal ; X OF A>IMALS !>" GEXEEAL. coloured and variously shaped shells of the.'-e animals. Integumen- tary glands and aggregations of glands may also acquire a relation to the acquisition of food (spinning glands of Spiders). Finally, mucous glands are very widely present in the skin of animals which live in damp localities (Amphibia, Snails) and in water (Fishes, Annelids, Medus.T). ORGANS OF ANIMAL LIFE. Chd c J^f Jl^y M ^^ *^^ so-called animal functions, that _^M''-^^ ^^ locomotion is the most conspicuous. Animals perform movements for the purpose of procuring food and escaping from their enemies. The muscles used for locomotion are, as a rule, and especially in the simpler forms, intimately united with the skin, and give rise to a muscular body wall (Worms), the alternate shorten- ing and elongation of which brings about a movement of the body. The muscles may also be especially concentrated in parts of the body wall, e.g., in the subum- brellar surface of Medusae beneath the supporting gelatinous tissue, or in the ventral surface of the body giving rise to a foot-like organ (Molluscs), or they may be broken up into a series of successive and similar segments (Annelids, Arthropods, Vertebrates). The latter arrangement prepares the way for the rapid and more complete form of movement found in animals in which the hard parts also, whether exoskeletal (Arthropods) or endoskeletal (Vertebrata), have become divided into a series of longitudinally arranged segments or rings, which offer a firm attach- ment to and are moved by the segments of the muscular system. By this ariangement more powerful muscular actions ai"e 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 pi-otect them. The skeletal structures may be external, in which case they have the form either of external shells Fig. 73.— Alimentary canal with its accessory glands of a beetle (Carabus) (after Leon Dufour). Oe, oesopliasus; J"h, crop; Pv, proventriculus ; Chd, cbylific ventricle ; J/?, ilalpigliian tu- bules; H, rectum; Ad, anal glands with bladder. usually products of the external skin (chit in), or they may be internal (cartilage, hone) and give rise to vertehrce (fig. 74 a, h). In either case the body becomes divided at right angles to its long axis into a series of segments, which, in the simpler cases of locomotion, are homonomous (Annelids, Myriapods, Snakes). As development progresses some of the muscles required for locomotion gradually lose their relation to the long axis of the body, and acquire a relation to secondary axes; and in this Avay conditions are acquired for the accomplishment of more difficult and complete forms of locomotien. The hard parts in the long axis of the body then looC their primitive Fig. 74 a— Diac^ram ot the vertebral column of aTeleosteaa fish, with verte- bral constriction of the notochord. Ch, notochord ; Wk, bony vertebral bodies ; J, membranous intervertebral section. Fig. 74 b — Vertebra of a fish. K, ver- tebral body. Oh, neural arch (neura- pophysis) ; Ub, haemal arch (hsemapo- physis) ; D. neural spine j D', hamal spine ; -E, 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 OnOAKIZA.TION AND DEVELOPMENT OF ANIMALS IN CENEBAL. property, like that of movement, resides in definite tissues and organs which constitute the nervous si/stem. 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 or-ganism possesses the first beginnings of an irritabiUty serving for peiception. 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 wdth that of the nervous tissues, which are developed in connection with the sense epithe- lium of the surface (Polyps, Medusje, 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 INIedusse has been shown by later researches to be untenable. The arrangements of the nervous system can be traced back to three distinct types — (1) the radial ar- rangement found in the radiate animals; (2) the bilateral arrange- ment found in segmented Worms, Arthropods, and Molluscs; (3) the bilateral arrangement of the Vertebrata. In the first case the central organs are radially repeated ; in the Echinoderms as the so- called ambulacral brains or nerves, which are found in the arms and are connected together by a circumoral nervous commissure contain- ing ganglion cells (fig. 75). In the second type the nervous system, in the simplest cases, consists of an unpaiiled or paired ganglionic mass placed in the anterior part of the body above the pharpix, 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 othei-s, and take a lateral cour;-e (fig. 76). Fig. 75.— Diagram of the nervous sys- tem of a star-fish. N, nerve ring which connects together the five am- bulacral centres. NERVOUS SYSTEM. 81 At a higher stage of development a cii-cum-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 FiQ. 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 ner\'es ; D.alimentary canalwith mouth and pharynx. Fig. 77.— Nervous system of Fig. -Nervous system the larva of Coccinella (after Ed. Brandt). G, su- pra-oesophageal ganglion or brain ; Gfr, frontal ganglion ; Sj, suboeso- phageal ganglion ; 0',-G", the eleven gangUa of the ventral 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 couoentration of the nervous system begun 82 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. in the latter case may, by the fusion of the brain and ventral oord, be cai-ried to a still further extent, so that in many cases (numerous Arthiopods) only a sub-oesophageal ganglion is present. In Molluscs, animals in which segments are not de- veloped, the subcRsophageal 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 traver.-ed by a central canal, is anterioily widened and (except in Amphioxus) differentiated into ^ a complicated ganglionic apparatus, the -^ f=^ The so-called sympathetic or visceral nervous system appears in the higher animals (Vertebrata, Arthropoda, Hitu- 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 wiU 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 Vertebrata (fig. 80), the system of visceral nerves consists of a double chain of ganglia, placed on each side of the vertebiial 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 wath the above- mentioned spinal and cranial nerves, and they send nerves to the Fig. 79.— Brain and spinal cord of a pigeon. //, cerebral hemispheres ; Cb, optic lobes ; r, cerebellum; Mo, medulla oblongata. Sp, spinal nerves. BE>-SE OEQAKS. 83 blood vessels and visCvT. of nervous fibres containing here and there ganglion cells. The nervous sys- tem possesses further peripheral apparatus, the sense organs, the function of which is to bring about the perception of certain conditions of the outer world as im- pressions of a definite mode of sensation (specific energy of nerves* Joh. Miiller). These pei-ipheral organs usually have the form of peculiarly arranged aggrega- tions of hair-shaped or rod-shaped nerve tenninations (hair- cells, rod-cells of sen- sory epithelium) con- nected by fibi'illfe 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 a ff e ct s con - which there form a complicated netwoj k * 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). 01 olfactory nerves ; O, eye ; O/), optic nerve ; T'-lZATIO>- 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 compai-ed according to the natui^e of the sensations with those of our own body. We can estimate the sense energies of the lower animals exceedingly vaguely, and only by the insufficient method of com- paring them with our own sensations ; and it is certain that among the lower ani- mals there are many forms of sensation of which we, in consequence of the spe- cialised nature of our own senses, can have no concep- tion. Probably of all the senses, that of touch is the most widely distributed, and with this we certainly often see a number of special sensations united. It is generally distributed over the whole surface of the body ; frequently, however, it is con- centrated on processes and appendages of it. Probably the tentacular appendages of the Coelenterata and Echinodermata have this signifi- cance. In the Bilateralia with a differentiated head there are contractile or stiff" segmented processes on the head, the antennce or feelers which in the worms are repeated as paired cirri on every se<^ment of the body. It is often possible to trace special nei-ves to the skin and to find touch organs containing their endings. In the Arthi'opoda 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. AUDITORS AND VISUAL OEGAKS. 86 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 oi-gan in its simplest form appears as a closed vesicle filled with fluid {endolymph) and one or more calcareous concretions (otoliths) ; and containing in its walls rod or hair cells in which the nerve fibrillas end (fig. 83). Sometimes the vesicle lies on a ganglion of the central ner- vous system (Worms), sometimes at the end of a shoi'ter or longer nerve, the auditory nerve (Molluscs, Decapoda). In many aqua- tic animals the vesicle may be open and its contents communicate directly with the exter- nal medium, in which case the otoliths may be represented by small particles such as sand- grains which have entered it from the exterior (Decapod Crustaceans). In Molluscs a deli- cate sensory epithelium (macula acustica, fig. 83 Cz, Hz.), marks the percipient portion of the inner wall of the vesicle ; while in Crus- tacea the fibres of the auditory nerve end in cuticular rods or hairs which project from the wall of the vesicle, and, like the olfactory hairs of the 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 Locu^tidse, 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. K. Leuckart, " Organologie des Auges," Graefe and Samisch, Hand* buch der Ophthalmologie, Bd. II. Fig. 82. — Tactile papilla from the volar surface with the touch corpuscle and its nerve N. 86 OEGANrZATION AXD DEVELOPMENT OF ATTIMALS IX GENERAL. of perfection. In the simplest cases they are known as eye-sjmts, and consist of irritable protoplasm, i.e., nervous siibstance, containing pig- ment grannies ; 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 lic'ht, i.e., is chemically changed by the light waves and transmits the excitation produced by these movements to the protoplasm or Fig. 83— Auditory vesicle of a Heteropod (Pterotracliea). K, acoustic nerve; Of, otolith the fluid of the vesicle ; Wz, ciliated cells on the inner wall of the vesicle ; IIz, 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 impoiiance in this relation appears the special nature of the nerve endings, through which certain movements, progressing in i-egular 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- EEFIiACriLE MEDIA AND PIGMKNT. 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 / quality v,-ith 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 lays diverging from the various parts of the source of the light are, by means of the refractile media, collected and brought to corresponding foci on the retina or peripheral expansion of the optic nerve, which consists of the rod-shaped ends of the nerve fibres and some more or less complicated ganglionic structures. Lately, in consequence of the discovery of the visual purple * in the outer segments of the rods, it has been attempted to reduce the excitation of the end apparatus of the optic nerve to a photo-chemical process taking place in the retina. The fact that the diffuse pigment (visual purple) of the outer segments of the rods is bleached by the * la addition to the older works of Krobn, H. Miiller, IM. Schultze, of. Boll Sitzuugsberichte der Akad. Berlin, 1876 and 1877, also Ewald and Kliluie. Fig. 84.— Diagram of the auditory labyrinth. I. of a fish. II. of a bird. III. of a mammal (after Wal- deyer). U, utricle with the three semicircular canals ; S, saccule ; US, alveus communis ; C, cochlea ; L, la- gena ; S, aqueductus vestibuli ; Cr, canalis reuniens. 88 OEGAXIZATION AXD DEVELOPMENT OF AMMALS ES" 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 Hght 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, the jmjiil, capable of contrac- ting and dilating. In the higher grades of development the whole eye is, as a rule, enclosed in a hard, connective tis- sue coat, the sclerotic, and thus marked off as an eye bulb. The arrangements by which the shining points of an object act in regular ar- rangement on corre- sponding points of the optic nerve and so render possible the perception of an image vary, and are closely dependent upon the whole structure of the eye. Leaving out of consideration the simplest eyes, such as we find in Worms and the lower Crustacea, two types of eye are to be distin- guished. ^ 1. The first form occurs in the so-called facetted eyes* (figs. 85 & 86) of Arthropods (Crustacea and Insects). The retina of such eyes has a hemispherical form, the convex surface being directed out- wards, and consists of large compound nerve rods, the retinulae Fig 85,— Diagrammatic representation of the compound eyo of a Libellula. C, cornea ; K, crystalline cone ; P, pigment ; -ff, nerve rods of retina ; Fh, layer of fibres ; Gz, layer of ganglion cells ; Rf, retinal fibres ; Fk, crossing of fibres. * See Job. MLiller. "Zur vergleichenden Physiologie des Gesichtssinnes," Leipzig, 1826. H. Grenacher, •' Untersuchungen liber das Sehorgan der Arthro- poden," Gottingen, 1879. UNICOIINKAL EXE. 89 (figs. 85 & 8Q Rf (£.' R), whicli are separated from one another by pigment sheaths. In front of these rods ai*e placed the strongly refractile crystalline cones (Ji), and in front of these again the lens- i^haped corneal facets {C d: F). The eye is enclosed by a firm chitinoiis layer, whicJi, following the sheath of the entering optic nerve, surrounds its soft parts and reaches as far as the cornea. That part of the eye which is known as optic nerve corresponds in a great measure to the retina itself, and contains a layer of ganglion cells and of nerve fibres. A reversed and reduced picture of the object is thrown behind each convex corneal facet (lying far from the sensitive layer of nervous rods), and only the perpendicular rays can be perceived since all the others are absorbed by the pigment. Ac- cordingly the light impressions caused by these axial rays, whose number corresponds with the separate nerve rods, form a mosaic on the retina which repeats the arrangement of the parts of the external object emitting light. The picture which is here formed lacks, however, bi-illiancy and dis- tinctness. 2. The second form of eye, which is widely distri- buted in the animal kingdom (the simple eye, Annelids, Insects, Arachnida, Molluscs, Verte- brates) corresponds to a globular camera obscura with collecting lenses (cornea, lens) on its exposed anterior wall on which the light falls and usually with additional dioptric media filling the optic chamber (vitreous humour.) The simple eye of Insects seems to have originated from the simple metamorphosis of part of the integument, beneath which are placed the end organs of the optic nerve (fig. 87). The cuticular covering {CL) projects as a lens-shaped thickening into the subjacent layer of transparent, elongated, hypodermis cells {Gh), within which are placed elongated rod-like nerve- cells with refractile cuticular portions, closely aggregated to form a retina (fig. 87 Rz). The hypodermis cells surrounding the edge of the lens are filled with pigment, and form an iris-like dark ring Fig. so.— Three fa- cets with retinulaJ from the eom- pounrl eye of a cockchafer (after Grenacher). The pigment has been ilissoh-ed away from two of them. F, corneal facet. K, crystalUne coue. P, pigment sheath. P', chief pigment cells. P", pigment cells of the second order. i?, retinulse. ;0 0EQA^•1ZA1I0^• A^•I; detelopment of akimals in gexeeal. through the opening in which the rays of light enter tlie eye to fall on the terminal segments of the retinal cells (fig. 87). In the more highly developed forms of this type of eye, especially in the Vertebrate eye, the peripheral portion of the optic nerve spreads out so as to form a cup- shaped nervous membrane, the retina, placed immediately behind the refraotile media and surrounded by a vascular pigmented membrane, the choroid. The choroid, again, is surrounded by a tough supporting membrane composed of fibrous connective tissue, and known as the sclerotic, which is continued over the anterior part of the eye, i.e., that pai't through which the light passes, as a thinner transparent membrane. Of the refractile media which are placed behind the cornea and fill the cavity of the optic bulb, viz., the aque- ous humour, the lens (fig. 88 L), the vitre- ous humour {Gl), the leus is the most powei-ful. Grasped by the thickened muscular anterior part of the choroid (the ciliary body (Cc) and ciliary processes), the peripheral part of its anterior face is covered by a forward continuation of the choroid, the iris (Jr), which, as a ring-likj contractile border, forms a kind of diaphragm perforated by a central contractile opening, the pupil, through which the light enters the eye (fig. 88). The reversed image which is formed in the hinder part of the Vertebrat'.' eye on the cup -shaped retina has a very considerable brilliancy and definition. The eyes of many Cephalopods may be looked upon as a modifica- tion of this type of eye. In the eye of Nautilus the lens is absent, and the light enters through a small opening. In this case a reversed, but not brilliant, image is formed on the retina placed on the hinder wall of the eye. To enable the eye to see clearly objects in different directions and Fig. 87.— Transverse section through the simple eye of a beetlj l.irv.i (, 'artly after Greuacher). CL, corneal lens ; Gk; the subjacfint hypodermis cells, the vitreous humoui- of Authors ; P, pigment in the peripheral cells of the lat- ter ; Ez, rtiaal cells. St, cuticular rods of the latter. OLFACIOET ORGAN. 91 at different distances, special apparatuses for its movement and accommodation are necessary. They are represented by muscles which can in the former case move the optic bulb and modify the direction of sight in obedience to the will of the animal, and in the latter act upon the refractile media, and vary their relation to the retina. In many compound eyes (Decapod Crustacea) that part of the iiead on which the eye is placed is prolonged so as to give lise to a movable stalk-like process, which bears the eye at its extremity. The eyes of Vertebra ta possess in addition special protective ari^xngements, e.g., eyelids, lacrymal glands. The position and number of the eyes present very great variations amongst the lower animals. The paired arrangement on the head appears to be the general rule among the higher animals ; nevertheless visual organs sometimes occur on parts of the body far removed / ; from the brain, as for instance, in Euphausia, Pecten, Spondy- lus, and certain Annelids (Sabellidje). In the Radiata the eyes are repeated at the periphery of the body in each radius. In the star fishes they lie at the extreme end of the ambulacral furrow at the tip of the arms, in the Acalephre as the marginal bodies on the edge of the umbrella. The sense of smell appears to be less widely distributed. Its func- tion is to test the quality of gaseous matters r.nJ to produce in consciousness the special form of sensation known as " Smell." Thi^ sense in aquatic animals which breathe through gills cannot be sharply marked off from that of taste. The small j^its, standing in connec- tion with nerves and provided with an epithelial lining of hair- bearing sense cells, are to be looked upon as the simplest form of olfactory organ (Medusa?, Hetercpoda, Cephalopoda). Nevertheless scattered hair cells (Lamellibranchiata) may also have to do with the same sensation. In the Arthropoda the cuticular appendages of the / Fig. 88. — Transverse section througli the human eye (after Arlt). C, cornea ; L, lens ; JV, iris with pupil ; Cc, ciliarj- body ; Gi, vitreous humour; J?, retina ; Sc sclerotic; CA, choroid. 311, macula lutea ; Po, papilla optica; Ao, optic nerve. 92 OEGA.NIZATIOX AND DEVELOPMENT OF ANIMALS IN GENERAL. antennae in which the gangliated swollen extremities of nerves occur are to be explained as olfactory fibres. In the Vertebiata the olfactory organ usually has the form of a paired pit or cavity placed on the under surface of the head (nasal cavity), on the walls of which the ends of the olfactory nerve are distributed. The higher aii'- breathing Yertebrata are distinguished by the fact that in them this cavity communicates with the pharynx, and by the great surface extension (in a confined ai-ea) of the much-folded olfactory mucous membrane. The fibres of the olfactory nerve terminate in delicate elongated cells, bearing a rods or hairs and placed between the epithelial cells of this mucous membi-ane. The special sense of taste is confined to the mouth and pharynx. Its function, from what we know of the higher organisms, is to test the quality of fluid sub- stances, and to bring about the special sensation of taste. The presence of this sense can be demonstrated with certainty in the Ver- tebrata, and it is connected \vith the distribution of a special nerve of taste, the glossopharyngeal, which in man supplies the tip, edges, and root of the tongue and also parts of the soft palate, making these parts capable of the taste sensation. The so-called taste-buds found in special papillse (papilL-e circum- vallatjfi), with their central fibre-like cells, are explained as the percipient organs of this sense (fig. 89 a, b, c). Taste is, as a rule, connected with the tactile and temperature sensations of the buccal cavity, and also with the olfactory sensations. Finally, special organs of taste appear to be present also in the Molluscs and Arthropods as a specific sensory epithelium at the entrance to the buccal cavity. In the lower animals the taste and olfactory organs are still less Fig. 89.— a Transverse section through a circutn- vallate papilla of a calf (after Th. W.Engelmann). N, nerve ; Gk, taste buds in the side-vraU of the papilla, Pc. h, isolated taste bud from the lateral taste organs of a rabbit, c, isolated supporting cells {Sz) and sense cells (&) from the same. PSYCHICAL LIFE AND IJfSTINCT, 93 clearly distinguishable than in the higher, and there are numerous senses of an intermediate character for the purpose of testing the surrounding medium. The sense-organs of the lateral line of Fishes and Salamanders, and the organs resembling taste-buds of the Hirudinea and Chajtopoda have been described as organs of a sixth sense. They probably bring about certain sensations referring to the quality of the water. PSYCHICAL LIFE''- AND INSTINCT. The higher animals are not only rendered conscious of the unity of their organization by their feelings of comfort and discomfort, pleasm-e and pain, but also possess the power of retaining residua of the impressions of the outer world conveyed through the senses, and of combining them with simultaneously perceived conditions of their bodily state. In what manner the irritability of the lower pro- toplasmic organisms leads by gradual transitions and intermediate steps to the first affection of sensation and consciousness is as completely hidden from us as are the nature and essence of the psychical processes which we know are dependent on the movement of matter. We are, however, justified in supposing that a nervous system is indispensable for the development of these internal conditions which may be compared with that condition of our own organization called consciousness. Again, as animals have sense-organs capable of receiving impressions of definite quality from external causes, together with a capacity for retaining in their memory residua of their perceptions, and the power of connecting them with present and with the recollection of past states of bodily sensation so as to form judgments and conclusions, they possess all the conditions essential for the operation of the intelligence; and, as a matter of fact, they do manifest in an elementary form nearly all the phenomena which distinguish human intelligence. The actions of animals are not only voluntary, the result of experi- ence and intellectual activity, but are also largely determined by internal impulses which work independently of consciousness, and cause numerous, often very complicated, actions useful to the organism. Such impulses tending to the preservation of the individual and the * W. Wimdt, " Vorlesucgen iiber die Menschen und Thierseele." 2 Bde. Leipzig, 1863. W. Wundt, " GrundzUge der phjsiologischen Psychologies' Leipzig, 1874. 9-t ORGANIZATION AND DEVELOPMENT OF ANIMAJ.S IN GENERAL. species ai'e called histiacts;* and tliey are usually regarded as a special property of the lower animals, and contrasted with the conscious reason of Man. But just as the latter must be looked upon as a higher form of the understanding and intellect, and not as something essentially distinct from them, so a closer examination shows that instinct and the conscious understanding do not stand in absolute contrast, but rather in a complex relation, and cannot be sharply marked off from one another. For if, according to the genei-al view, Ave recognise the essence of instinct in the unconscious and the innate, still we find tha^ actions which were at first performed under the direction of conscious intelligence become, by constant practice, completely instinctive and are performed u^nconsciously ; and that, in accordance with the theory of descent, which tlie whole connection of natural phenomena renders so probable, instincts have been developed from small beginnings, and have only been able to reach the high and complicated forms which we admire in many of the more highly organised animals (Hymenoptera), when assisted by a certain amount, however small, of intellectual activity. Instinct accordingly may be rightly defined as a mechanism which works unconsciously, and is inherited with the organization, and which, when set in motion by external or internal stimuli, leads to the performance of appropriate actions, which apparently are directed by a conscious purpose. We must not, however, forget that while the intellectual activities are the direct means whereby higher and more complicated instincts arise from simple ones, they themselves depend upon mechanical processes. We may well suppose that the simplest form of instinct is identical with the definite reaction of living matter following a stimulus, or, in other words, with that special form of molecular change Avhich is caused by an external action (as, for instance, the contraction of an Amceba when brought into contact with a foreign body). By the theory of partly instinctive, partly intellectual processes, we nmy explain the phenomena of association in societies so often found among the higher animals,t i.e., the association of numerous * Compare H. S. Reimarius, "AUgemeine Betrachtungen liber die Triebc der Thiere," Hamburg, 1773. P. Flourcns, " De rhistinct et de riutelligence des animaux," Paris, 1851. f The origin of the so-called animal stocks with incomplete or confined individuality among the lower animals is quite different, and merely determined by processes of growth ; at the same time the advantage for the preservation of the species gained by the fusion is the same. Cf. the animal stocks of the Vorticellidae, Polyps, and Siphonophora, Bryozoa and Tunicata. HEPEODUCTIVE OKGAXS. 95 individuals into communities— the so-called animal-polities — which may be complicated by the division of labour (Bees, Wasps, Ants, Termites). In fact here the combined action a[>pears to be mutually assisting or mutually limiting, as we find in the so-called animal stocks, the individuals of which are bound together by continuity of body. The advantages to be gained by this mutual rendering of service are not merely limited to the greater facilities for nourish- ment and defence, and therefore for the preservation of the in- dividual ; but, above all, tend to the maintenance of the offspring, and hence to the preservation of the species. It is for this reason that the simplest and commonest associations, from which the more complicated communities, subdivided by partition of labour, are derived, ai-e generally communities of both sexes of the same species. REPRODUCTIVE ORGANS. On account of the limit set to the duration of the life of every organ- ism, it appears absolutely necessary for the preservation of the animal and vegetable kingdoms that new life should originate. The forma- tion of new organisms might be due to spontaneous generation {generatio equivoca) ; and formerly this was supposed to take place, not only in the simpler and lower organisms, but also in the more complicated and higher. Aristotle thought that Frogs and Eels arose spontaneously from slime ; and the appearance of maggots in putre- fying meat was, till E^di's time, explained in the same manner. With the progress of science the limits within which this svipposition could be applied became ever narrower, so that they soon came to include only the Entozoa and small animals found in infusions. Finally it has been shown by the researches of late years that these organisms also must, for the most part, be withdrawn from the region of the generatio equivoca; so that at present, when the question of spontaneous generation is discussed, it is only the lowest organisms, those found in putrefying infusions, that are considered. The greater number of investigators,* supported by the results of * Cf. especially Pasteur, " Memoire sur les corpuscules organises qui existent dans Tatmosphere " (Ann. des. So. Nat.), 1861 ; also "Experiences relatives aux generations dites spontanees ' (Compt, rend, de I'Acad. des Sciences, tome 50). yb ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. numeroas experiments, have rejected, even, for the latter animab, the idea of spontaneous generation, which, however, still finds in Pouchet* a prominent and zealous supporter. Biogenesis, as opposed to abiogenesis, or spontaneous generation, must be regarded as the usual and normal form of reproduction. Fundamentally it is nothing else than a growth of the organism beyond the sphere of its own individuality, and can be always reduced to a separation of a part of the body, which develops into an indi- vidual resembling the parent organism. Nevertheless the nature and method of this process differ extraordinai-ily ; and various kinds of reproduction can be distinguished, viz., fission, budding (spore- formation), sexual rejvoduction.j Reproduction by fission, which, with that by budding and spore- formation, is included under the term monogenous asexual reproduc- tion, is found widely scattered in the lowest animals, and is also of special importance for the reproduction of the cell. It consists simply of a division of the organism into two parts by means of a constriction which gradually becomes deeper, and eventually leads to the separation of the whole body of the organism into two individuals of the same kind. If the division remains permanently incomplete, and its products do not completely separate from each other, con- pound colonies of animals arise. The number of individuals in such colonies increases by a continuation of the process of incomplete and often dichotomous division of the newly-formed individuals (Vorti- cella, Polyp stocks). The division may take place in various du-ec- tions — longitudinal, transverse, or diagonal. Budding diflfers from fission by a precedent disproportionate and asymmetrical growth of the body, giving rise to a structure not absolutely necessary to the parent organism which is developed to a new individual, and by a process of constriction and division becomes independent. If the buds remain permanently attached to the parent, we have here also the conditions necessary for the foi^mation of a colony (Polyp colonies'). Sometimes the budding takes place at various parts of the outer surface of the body, irregularly or obeying definite laws (Ascidians, Polyps) ; sometimes it is localised to a definite part of the body, separated off" as a Germ- stock (Salpa, stolo prolifer). The cell-layers distinguished as germinal * Pouchet, " Nouvelles experiences sur la generation spontanee et la resist- ance vitale," Paris, 1864. f Cf. R. Leuckart's article, " Zeugung " in R. Wagner's " Handworterbucb der Ph3'siologie." EEPEODUCTIOJf BY SPOUES. SEXUAL EEPRODUCTIOJJ. 97 layers are repeated in the commencing buds, and from tliem the organs are differentiated. The reproduction by spores is characterised by the production within the organism of cells, which develop into new individuals m situ or after leaving the organism. But this conception of spores, which is taken from the vegetable kingdom, can only be applied to the Protozoa and coincides with endogenous cell-division. The cases of so-called spore-formation amongst the Metazoa (germinal sacs of Trematodes) are probably identical with egg formation, and are to be reduced to a precocious maturation and spontaneous development of ova (Parthenogenesis, Ptedogenosis). The digenous or sexual reproduction depends upon the pi-oduction of two kinds of germinal cells, the combined action of which is necessary for the de- ^iVx mm 1 velopmeut of a new or- ganism. The one form of germ cells contains the material from which the new individual arises, and is known as the egg-cell, or merely egg (ovum). The second form, the sperm-cell {spermato- zoon), contains the ferti- lising material, semen or sperm, which fuses with the contents of the egg- ceU, and in a way which is not understood gives the impetus to the de- velopment of the egg. The cell structures from which the eggs and sperm arise are called sexual organs, for reasons which will be evi- dent in the sequel ; the eggs being produced in ihefeynale organ or ovary, and the semen in the male organ or testis. The egg is the female, and the semen the male product. The structure of the sexual organs presents extraordinary diffe- rences and numerous grades of progressive complication. In the simplest cases, both products arise in the body wall, the cells of which give rise at determined places to ova or spermatozoa (Cceienterata). Sometimes they arise in the ectoderm (Hydroid-Medusae), sometimes in the entoderm (Acalepha, Anthozoa). A similar arrangement 7 Fig-. 90.— Generative or°rans of a Heteropod (Pterotra- chea) after R. Leuckart. a, Male-organs ; T, testis- Vd, vas deferens, b, female organs ; Ov, ovary ; El, albumen gland; Its, receptaculum seminis ; Fa, va- gina. 98 ORGAKIZATION AKD DETELOrME>T OF ANIMALS IN GENEIAL. obtains in the marine Polychseta, in which the ova and spermatozoa are developed from the epithelium of the body-cavity (mesoderm), and dehisced into the body cavity. Usually, however, special glands, the ovaries and testes, are developed, which perform no other function than that of secreting ova and spermatozoa (Echinoderms). As a rule, however, there are found associated with the male and female generative glands accessory structures and a more or less com- plicated arrangement of ducts, which discharge definite functions in connection with the development of the generative products subse- quent to their separation from the glands, and ensure a suitable meeting between the male and female elements (fig 90). The ovaries are provided with ducts, the oviducts, which are not rarely derived Fia. m, a. — The female organs of Pulex (after Stein). Oc, ovarian tubes ; Ri, receptaculum seminis ; V, vagina; 01, accessory gland, b. The male generative organs of a water-bug (Xepa) (after Stein). T, testis ; Vd, vasa deferentia ; Gl, accessory glands ; D, ductus ejacu- latorius. from structvires serving quite another purpose (segmental organs). The oviducts, in their course, may receive glandular appendages of various kinds which furnish yolk for the nourishment of the ovum, or albumen to surround it, or material for the formation of a hard egg-shell (chorion). These functions may be sometimes discharged by the ovarian wall (Insects), so that the egg when it enters the oviduct has taken up its accessory yolk and acquired its firm egg- shell. Very often the ducts also discharge these various functions, and are divided into corresponding regions ; they are often dilated at part of their course to form a re.servoir for the retention of the naiRMAPlIUODITISM. eggs or of the developing embryos (uterus). Their terminal section presents differentiations subserving fertilization (receptaculum seminis, vagina, copulatory pouch, external generative organs). The efferent ducts of the testis, the vasix deferentia, likewise frequently give rise to reservoirs (vesicuL-e seminales) and receive glands (pros- tate), the secretion of which mixes with the sperm fluid or surrounds aggregations of the spermatozoa with a firm sheath (spermatophors). The terminal section of the vas deferens becomes exceedingly muscular, and gives rise to a ductus ejaculatorius, which, as a rule, is accompanied by an external organ of copulation to facilitate the conveyance of the semen into the female generative organs. The generative organs present „. %> WM either a radial (Cceienterata, Echinodermata) or a bilate- rally sj^mmetrical arrangement (fig. 91), a contrast which is \dsible in the typical arrange- ment of all the systems of organs. The simplest and most primitive condition of the generative organs is the her- maphrodite. Ova and sper- matozoa are produced in the body of one and the same Individual, which thus unites in itself all the conditions necessary for the preservation of the species, and alone represents the species. Instances of hermaphroditism are found in every group of the animal kingdom. But they are especially nume- rous in the lower groups, and also in animals in which the movements are slow (Land-snails, Flat- worms, Hirudinea, Oligochoeta), or which live singly (Cestoda, Trematoda), or in attached animals which are without power of changing their position (Cirripedia, Tunicata, Bryozoa, Oysters). The hermaphrodite arrangement of the gene- rative organs presents great variation, which, to a certain extent, forms a gradual series tending towards the separation of the sexes. In the simplest cases, the points of origin of the two kinds of generative products lie close to one another, so that the spermatozoa and ova meet directly in the parent body (Ctenophora, Chrysaora). Fig. 92.— Sexual orgiins of a Pteropod (Cyinbulia) (after Gegenbaur.) a, Zd, liermaplirodite gland with common duct ; Rs, receptaculum seminis ; TJ, uterus. 4, Acinus of the hermaphrodite gland of the same. 0, ova ; 8, spermatozoa. 100 ORQAXIZA.TIOX AXD JDEVELOPME>'T OF ANIMALS IN GENEKAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-vascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common to the ova and spermatozoa (fig. 92), but which, as in HeUx (fig. 93), may partially s^eparate into vas deferens and oviduct. In other cases the ovaiies and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trematoda, rhabdocoele Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appeal's to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertiUzation is sufficient for the production of off- sjiring. But this original condition of self-fertiliza- tion appears to be tlie ex- ception in almost all hermaphrodites. In those animals in which the ovary and testis are not com- pletely separated from one another cross-fertilization is rendered necessary, and self-fertilization prevented by the fact that the male and female elements are matured at different times (Snails„Salps). From this form of complete hermaphroditism the generative organs pass through a stage of incomplete hermaphroditism, in which, though the organs of both sexes are present, one of them is i-udi- mentary, to reach the dioecious condition in which the sexes are completely separated (Disiomumjillicolle and hcematohiimi). Animals in which the sexes are distinct not unfrequently piesent traces of an Fig. 93.— Sexual organs of the Roman Snail (Helix pomatia). Zd, hermaphrodite gland ; Zg, its duct ; Ed, albumen gland; Od, oviduct and seminal groove ; Vd, vas deferens ; P, protrusible penis ; Fl, flagellum ; Sg, receptaculum seminis ; X), finger-shaped gland ; L. Spiculum amoria ; Go, common genital opening. BEPAEATION OF THE SEXES. 101 of the male hermaphrodite arrangement ; such, for instance, as may be seen in the arrangement of the generative ducts of the Vertebrata. In the Amphibia both male and female generative ducts, which are secondarily- derived from the urinary ducts, are developed in each individual. The oviduct (Miillerian duct) in the male atrophies, and is only repre- sented by a small rudiment (fig. 96b, Mg) ; while, on the contrary, in the female, the vas deferens (Wolffian duct) is rudimentary, or, as in Amphibia, functions as the efferent duct for the kidney secre- tion (fig. 96a, h(j). With the separation and female gene- rative organs in different indivi- duals the most complete form of sexual reproduc- tion, so far as con- cerns division of labour, is reached ; but at the same time a progressing dimorphism of the male and female individuals be- comes apparent. This is due to the fact that the or- ganization in bi- sexual animals is more and more influenced by the deviating func- tions of the sexual organs, and with the increasing complication of sexual life becomes modified for the performance of special accessory functions connected with the production of ova and spermatozoa. In the first place, the modification of the generative ducts of the two sexes in accordance with the function they have to perform determines the development of secondary sexual characters and of sexual dimorphism. Other organs as well as the generative appa- FiG. 94. — G-enei ative appara- tus of a rhabdocoele Tur- bellarian (Vortex viridis) (after M. Schultze). T, tes- tis ; Vd, vas deferens ; Vs, seminal v esicle ; P, pro- trusible penis ; Or, ovary ; Va, vagina ; M, uterus ; D, yolk gland ; Ks, recep- taculum seminis. Fio. 95.— Generative appa- ratus of the medicinal leech. T, testis ; Vd, vas deferens ; Nh, vesicula seminalis ; Pr, prostate ; C, penis ; Ob, ovaries with vagina and female generative opening. 102 OnOANIZATION AND DETELOPMKXT OF AXniALS IN GENERAL. ratus present differences in the two sexes, being moditied for the Or IN -Hi. Fig. 96a.— L-ft urinary end generative or- gans of a female Salamander without the cloaca. Ov, ovary ; iV, kidney ; hg, urin- ary duct corresponding to the Wolffian duct ; Mj, Miiilcrian duct as oviduct. Fig. 0Gb, Loft urinary and generative OTgans of a male Salamander, more dia«rrammatic. T, testis ; TV, vasa efferentia ; iV, kidney with its collecting tubules ; Mg, Miille- rian duct as a rudiment; If'g, Wolffian duct or vas deferens ; XI, cloaca with ac- cessory glands Dr, of the left side. performance of ppccial functions in the sexual hfe. The female is FUNCTIONS OF MALE AND FEMALE. 103 the passive agent in copulation, merely receiving the semen of the malsi the female possesses material from which the offspring Pig. 97 j.— Male of Aphis platanoides. »c, ocelli ; Jlr, honey tubas ; P, copulatory organ. develop, and accordingly takes care of the development of the fate of the offspring. Hence fertilised Qg§, and of the later the female usually possesses a less active body and numerous arrangements for the protection and nourishment of her offspring, which develop either from eggs laid by the mother and sometimes carried about with her, or in the maternal body and are born alive. ■Jhe function of the male is to seek, to excite, and to hold the female during copulation ; hence, as a rule, he possesses greater vigour and power of movement, higher development of the senses, various means of exciting sexual feehng, such as brighter colour- ing, louder and richer voice, pre- hensile organs, and external organs for copulation (fig. 97, a, h). In exceptional cases, the functions relating to tlie maintenance of Fig. 07i, Apterous oviparous female of the same. 104 OnGANlZ;VTIOX AXD DETELOPMEXT OF ANIMALS IN GENEKAL. the offspring may be discharged by the male, e.g., Alytes and the Lophobranchia. Male birds also often share with the female the labour of building the nest, of bringing up and protecting the young. But it is a rare exception to find, as in Cottus and the Stickleback (Gasterosteus), that the care and protection of the young fall exclusively upon the male, that he only bears the brood pouch and alone builds the nest, — an exception which bears strong witness to the fact that the sexual differences both in form and function were first acquired by adaptation. In extreme cases, the sexual dimorphism may lead to so great a difference in the sexes that without a knowledge of their development Fig. 98.— Chondracantlius gibbosus, magnified about 6 times, a, female from the side, h, female from the ventral surface with the male (F) attached, c, male isolated, under strong magnification. An', anterior antenna ; An", clasping antenna* ; F' and F", the two pairs of feet; A, eye ; Ov, egg sacs ; Oe, oesophagus ; D, intestine; M, mouth parts ; T, testis ; Vd, Tas deferens ; Sp, spermatophore. and sexual relations, the one sex would be placed in a different family and genus to the other. Such extremes are found in the Rotifera and parasitic Copepoda (Chondracanthus, Lernseopoda, fig. 98, a, b, c), and are to be explained as the result of a parasitic mode of life. The difference in the two kinds of individuals representing and maintaining the species, whose copulation and mutual action was known long before it was possible to give a correct account of the real nature of reproduction, has led to the designation " sexes," from •which the term sexual has been taken to apply to the organs and njanrer of reproduction. rAUTUEXOQENESIS. 105 In reality sexual reproduction is nothing else than a special form of growth. The ova and spermatoblasts represent the two forms of germinal cells which have become free, and which, after a mutual interaction in the process of fertilization, develop into a new organism. Nevertheless under certain conditions the egg can, like the simple germ cell, undergo spontaneous development; numerous instances of this mode of development, which is known as partheno- genesis, are found in Insects. The necessity of fertilization therefore Fig. 99. — Viviparous form of Aphis platanoides. Oc, ocelli ; Hr, honey tubes. no longer enters into our conception of the egg-ceU, and no absolute physiological test is left to enable us to distinguish it from the germ- cell. It is usual to regard the place of origin in the sexual organ and in tine female body as a feature distinguishing the ovum from a germ cell, but even with this morphological test we do not in each individual case arrive at the desired result [Bees, BarJc-lice, Psychida;): We have already given prominence to the fact that ovaries and testes, in the simplest cases, consist of nothing more than groups of cells of the epithelium of the body cavity or of the outer skin. These, however, do not acquire the character of sexual organs until, at a higher stage of differentiation, the contrast between the two 106 OEGANIZATION A>'D DEVl:LOPME^■T OF ANIMALS IN GEXEEAL. sexual elements has made its appearance. When the male elements, and with them the necessity of fertilization, are absent, and when, at the same time, the organ which produces the germ cells possesses, in its full development, a structure similar to that of an ovary, it becomes very dithcult to distinguish whether we have to do with a pseudovary (germ-gland), and with an animal which reproduces asexually ; or with an ovary and a true female, whose eggs possess the capacity of developing spontaneously. It is only a comparison with the sexual form of the animal which makes the distinction possible. To take the case of the Plant-lice or Aphides; in these animals we find a generation of viviparous individuals, easily distinguishable from the true oviparous females, which copulate and lay eggs. They resemble the latter in the fact that they are provided with a similar reproductive gland, constructed upon the ovarian type ; but they difiei' from them in this impoi'tant peculiarity, that they are without organs for copulation and ferti- lization (in correspondence with the absence of the male animal) (fig. 99). The reproductive cells of the organs known as pseudovaries have an origin precisely similar to that of eggs in the ova- ries, and only differ from ova in the very early commencement of the embryonic development. The viviparous individuals will therefore be more correctly regai-ded as agamic females peculiarly modified in the absence of organs for copulation and fertilization ; and the reproductive cells are by no means to be relegated to the category of germ-cells (as formerly was done by Steenstrup). We must therefore speak of the repioductive pro- cesses in the Aphides as being sexual and partheno- genetic and not sexual and asexual. A comparison of the mode of reproduction of the Bark-lice with that of the Aphides, especially of the species Pem- phigus terebinthi, puts the correctness of this supposition be^-ond the sphere of doubt. A similar condition is found in the viviparous larva of Cecidomyia. Here the rudiment of the generative glands very early assumes a structure resembling that of the ovary, and produces a number of XA, -e (^ Fig. 100. — Vivipa- rous Cecidomy ia (Miastor) larva (after Al. Pagen- stecher). Tl, Daughter 1 a r v se developed from the rudimentary ovarj-. DEVELOPMEXT. lo: reproductive cells which resemble ova in their method of origin, and at once develop into larvse. The pseudovary is clearly derived from the rudiment of the sexual gland, but without ever reaching complete development (fig. 100). The ovary acquires to a certain extent the signification of an organ for producing gei-m-cells, and it is not improbable that many products (liedia, SjwrocT/st) regarded as spores or germ-cells correspond to embryonic ovaries which produce ova c ipable of spontaneous develop;i:ent. -r>^ Fig. 101.— 0\ im of N'ophelis ( ifter O Ucrtwir^). a, tlie ovum half-an-hour after deposition. a projecti juot tlie protoplasm iiidic ites the commencing f jrmation of the first polar body ; the nuclca. „p.„Jl \....Lle. I, TL., same an hour later, with polar body extruded, and after entrance of the spermatozoon. Sh, male pronucleus, e. The same another hour later without egg membrane, and with two polar bodies and male pronucleus (Si-) ; d, the same an. hour later with approximated female and male pronuclei; i^i', polar bodies. DEVELOPMENT. It follows from the facts of sexual reproduction that the simple cell must be regarded as the starting-point for the development of the organism. The contents of the ovum sponbineously or under the influence of fertilization enter upon a series of changes, the final result of Avhich is the rudiment of the body of the embryo. These changes consist essentially in a process of cell division which implicates the whole protoplasm of the ovum, and is known as segmentation. 108 OEGANIZATIOX AND DEVELOPMEXT OF A^flMALS !>' GEXEEAL. For a long time the behaviour of the germinal vesicle at the commencement of segmentation and its relation to the nuclei of the first formed segments were obscure, and the knowledge of the changes and fate of the spermatozoa which enter the ovum in the process of fertiHzation was, in like manner, in a very unsatisfactory state. Of late years, numerous investigations, especially those of Biitschli, 0. Hertwig, Fol, etc., have thi-own some light on these hitherto completely obscure processes. It was supposed that in a ripe ovum preparing itself for segmentation the gei-minal vesicle disiippeared, ^-Jm Ek Fig. 102, a, J.— Parts of the ovum of Asterias glacialis -n-ith spermatozoa, embedded in the mucilarjinous coat (after H. Fol.) c, upper part of the ovum of Potromyzon (after Calberla). Am, micropyle ; Sp, spermatozoa ; Jm, path of the spermatozoon ; Ek, female pronucleus; Eh, membrane of ovum ; Ehz, prominences of the snme. and a new nucleus was formed quite independently of it ; and that the persistence and the participation of the germinal vesicle in the for- mation of the nuclei of the first segmentation .spheres were exceptional (Siphonophora, Entoconcha, etc.) Thorough investigations carried out on the eggs of numerous animals have, however, shown that as a matter of fact the germinal vesicle of the ripe ovum only experi- ences changes in which the greater part of it, together with some of rERTILIZATION. 109 the protoplasm of the ovum, is thrown out of the egg as the so-called directive bodies or 2}olar cells (fig. 101). The part of it, however, which remains in the ovum retains its significance as a nucleus, and is known as the female pronucleus. This fu-ies with the single spermatozoon (male pronucleus) which has forced its way into the ovum (fig. 102); and the compound structure so formed constitutes the nucleus of the fertilized ovum, or as it is generally called, the first segmentation nucleus. Fig. 103.— Development of a Star-fish, Asteracanthion berylinus (after Alex. Agassiz). 1, Commencing segmentation of the flattened egg— at one pole are seen the polar bodies ; 2, stage with two segments ; 3, with four ; 4, with eight ; 5, with thirty-two segments ; 6, later stage ; 7, blastosphere with commencing invagination ; 8 and 9, more advanced stages of invagination. The opening of the gastrula cavity becomes the aims. This new nucleus, which di\'ides to give rise to the nuclei of the first segmentation spheres, would appear therefore to be the product of the fusion or conjugation of the part of the germinal vesicle, which remains behind in the ovum, with the male pronucleus, which is a derivative of the spermatozoon which has entered the ovum. Fertilization would appear, therefore, to depend upon the addition 110 OnCAXlZAlIOX AND DEYELOPME>'T OF A>"IMALS IX GENERAL. of a new element hriiKjing about the regeneration of the primary nucleus of the ovum or germinal vesicle, and would have impressed its influence on the constitution of the conjugated nucleus. The regenerated ovum is therefore the starting-point of the sub^^equent generations of cells which build up the embryonic body. Both the origin of the polar bodies which takes place in the ripe ovum independently of fertilization, and the division of the seffmen- tation nucleus are accompanied by the appearance of the nuclear spindle and star- shaped figures at the poles of the spindle which are so characteristic of the division of nuclei. The male pronucleus, before it fuses with the female pronucleus, also becomes surrounded by a layer of clear protoplasm, around which a star-shaped figuie appears (fig. 101). In those cases in which segmentation takes place without a precedent fertilization [parthenogenesis), the female pronucleus appears to posj^^ess within itself the properties of the first segmentation nucleus. The fertilization is followed by the process known as segmentation, in which the ovum gradually divides into a greater and greater number of smaller cells. Segmentation may le total, i.e., the whole ovum segments (fig. 103), or it may he partial, in which case only a portion segments (fig. 105). Total segmentation may be regular and equal, the resulting seg- ments being of equal size (fig. 103) ; or it may sooner or later become irregular, the resulting segments being of two kinds — the one smaller and containing a preponderating amount of protoplasm, the other larger and containing more fatty matter. In these cases the seg- mentation is said to be unequal. The process of division proceeds much more quickly in the smaller segments, while in the larger and more fatty segments it is much slower, and may eventually come to a complete standstill. The development of the frog's egg will serve as an example of unequal segmentation, of which there are various degrees (fig. 104). In this egg a dai-k pigmented and protoplasmic portion can be distingii's'ied from a lighter portion containing much fatty matter or food yolk. The former is always turned uppermost in the water, and is therefore called the upper pole of the egg. The axis which connects the upper pole with the lowtr is known as the chief axis. The planes of the two first segmentation furrows pass through the chief axis and aie at right angles to each other. They divide the egg into four equal parts. The thii'd furrow (fig. 104, 4) is equatorial, taking place in a horizontal plane, and cutting the chief axis at right angles. It lies, however, nearer HOLOBLASTIC A-XD MEKOBLASTIC SLOMEXTATION. Ill the upper pole than the lower, and marks the line of division between the upper and smaller portion of the egg from the lower m m Fig. 104.— Unequal stguieuuitioa of the Frog'a tgg (after Kckei; lu teu succesbiVe sUges. and larger portioii, in which the segmentation pioceeils much more slowly than in the former. In partial segmentation we find a sharply marked contrast between the formative and nutritive parts of the egg, inasmuch as the latter does not seg- ment. The terms holohlastie and ms- rohlastlc therefore have been applied to total and partial seg- mentation respec- tively. Nevertheless, in total segmentation also, either groups of segments of a definite quality, or, at any rate, a fluid yolk material may be used Fig. 105. Segmeutation of the gormiual disc of a Fc.wr.'; f ere-, surface view (after Kolliker). A, germinal disc with the first vertical furrow ; B, the same with two vertical furrows crossing one another at right angles ; C and X>, inure ad- , T c i_ , 1 vanced sta^'cs with small central segments. embryo. In fact, the contents of every egg consists of two parts — (1) of a viscous albu- minous protoplasm; and (2) of a fatty granular matter, the lasm, or food yolk. The first is derived from the protoplasm for the nourishment of the developing 112 OEOAXIZi-TION AXD DETELOPilENT OT AXIMALS IX GEXERAL. of the original germinal cell, while the yolk is only secondarily developed with the gradual growth of the first ; and not unf requently it is derived from the secretion of special glands (yolk glands, Trema- todes) ; it may even be added in the form of cells. In the Ctenophora and other Coelenterata we see already in the first-formed segments the separation of the formative matter or peripheral ectoplasm from the nutritive matter or central endoplasm. In eggs undergoing a partial segmentation the formative matter usually lies on one side of the large unsegmenting food yolk. In accordance with this, the segments of such eggs, known as telolecithal, arrange themselves in the form of a flat disc (germinal disc) ; hence this kind of segmentation has been called discoidal (eggs of Aves, Reptilia, Pisces) (fig. 105). The food yolk may, however, have a central position. In such centrolecithal eggs the segmentation is Fig. 106. — Unequal segmentation of the centi'Olecithal egg of Gammarus locusta (in part after Ed. van Beneden). The central yolk mass does not appear till a late stage and undergoes later an " after-segmentation." confined to the j^erii^hery, and is sometimes equal (Pala?mon) and sometimes unequal (fig. 106). The central yolk mass may at first remain unsegmented, but later it may undergo a kind of after- segmentation and break up into a number of cells (fig. 106). Again, in other cases the food yolk, at the commencement of segmentation, has a peripheral position, so that the cleavage pi-ocess is at first confined to the inner parts of the egg, and only in later stages, when the food yolk has gradually shifted into the centre of the egg, appears as a peripheral layer on the surface. This is found especially in the eggs of Spiders (fig. 107). The fir.st processes of segmentation in these at first ectolecithal ova are witiidrawn from observation, since they take place in the centre of an egg covered by a superficial layer of food yolk, until the nuclei with their protoplasmic invest BLASTOSniEliE, ii;: lueut reach the periphery, and the fatty and often dnrkly-granulai food yolk comes to constitute the centi-al mass of the egg (Insects). As various as the forms of segmentation are the methods by which the segments are applied to the building up of the embryo. Fre- quently in cases of equal segmentation the segments arrange them- selves in the form of a one-layered vesicle, the hlastosjyhere, the central cavity of which not rarely contains fluid elements of the food yolk ; or they are at once divided into two layers around a central cavity containing fluid; or they form a solid mass of cells without Fig. 107.— Six stages in the segmentation of a spider's egg (Philodroinus limbatus) after Hub Ludwig. A, egg with, two deutoplasmic rosette-like masses (segmentation spheres) ; B. the rosette-like masses with their centrally placed nucleated protoplasm without egg membrane ; C, ef;g with a great number of rosette-like masses ; X>, the rosette-hke masses have the form of polyhedral deutoplasmic columns, each of which has a ceil of the blas- toderm lying immediately superficial to it ; E, stage with blastoderm completely formed ; F, optical section through the same. The yolk columns form wiihin the blastoderm a closed investment to the central space. any central cavity. In numerous cases, especially when the food yolk is relatively abundant (unequal and partial segmentation) or the food supply continuous, the embiyonic development is longer and more complicated. The embryonic rudiment in such cases has at first the form of a disc of cells lying on the yolk ; it soon divides into two layers, and then gi-ows round the yolk. 8 114 ORGAXIZATIOX AXD l)EyELOPMJ::>"T OF ANIMALS IN GENERAL. The two-layered gastrula is, as a rule, developed from the blasto- spliere by invagination (embolic invagination). In this process the one half (sometimes distinguished by the larger size and more granular nature of its cells) of the cell wall of the blastosphere is pushed in upon the other half (fig. 108), and on the narrowing of the Fig. 108.—^, Blastosphere of Amphioxus ; B, invagination of the saint ; C, gasliula, invagi- nation completed; O, blastopore (after B. Hatschek). aperture of invagination (hlastoijore, mouth of gastrula) becomes the endodermal layer (hypoblast) lining the gastrula cavity. The outer layer of cells constitutes the ectoderm or ejnblast. This mode of formation of the gastrula, which is very common, is found, e.g., in Ascidians, and amongst the Vertebrata in Amphioxus (fig. 108). More rarely the gastrula arises by delamination. This process consists of a concentric splitting of the cells of the blastosphere into an outer layer (epiblast), and an inner (hypoblast) (fig. 109). Fig. 109.— Transverse sections through three stages in the segmentation of Geryonia (after H. Fol.) A, stage with thirty-two segments, each segment is divided into an external finely granular protoplasm (ectoplasm) and an inner clearer layer (endoplasm) ; B, later stage ; C, embryo after delamination; with ectoderm slightly separated from the endoderm, which is composed of large cells surrounding the segmentation cavity. Tlie central cavity of the gastrula in this case is derived fiom the original segmentation ca^^ty, and the gastrula mouth is only secondarily formed by perforation. This method of development PUIMITIVE STREATC. 115 of the gastnila has hitherto only been observed in some hydroid Medusas (Geryonia). finally, when the ineiniahty of the segmentation is very pi'O- nounced, the gastrula is formed by a process known as epibole. In this process of development the epiblast cells, which are early distin- guishable from the much larger hypoblast cells, spread themselves over the latter as a thin layer (fig. 110); and in this, as in the s^econd method of development of the gastrula, the cavity of the gastrula is, as a rule, a secondary formation in the centre of the closely-packed mass of hypoblast cells. The blastopore is usually found at the point where the complete enclosure of the hypoblast is effected. It sometimes happens that a part of the primai-y blastosphere is developed more rapidly than the remainder, and gives rise to a Fig. 110.—^, Unequal segmentation of the egg of Buuellia; B, epibolic gastrula of the same (after Spengel). bilaterally-symmetrical stripe-like thickening placed on the dorsal or ventral surface of the embryo. Frequently, however, such a germinal or primitive streah is not developed, and the rudiment of the embryo continues to develop uniformly. Formerly great importance was attached to these differences, the one being distinguished as an erolutio ex una imrte, and the other an evolutio ex omnibus partibus It is not, however, possible to draw a sharp line between these two methods of development, nor haA^e they the significance which was formerly ascribed to them, for closely allied forms may present great differences in this respect according to the amount of food yolk and the duration of the embryonic development. The Coelenterata, the Echinoderms, the lower Worms and Mol- luscs, Annelids, and even Arthropods and Vertebrates (Amphioxus) present us with examples of i-egular development of all parts of the llG ORGANIZATION AND DEVELOPMENT OF ANIilALS IN GENEBAL. body of the embryo which, if the yolk membrane fails, has no need of a special protective envelope. In this latter group, however, the formation of the germinal streak, which is in close relation with the formation of the nervous system, is accomplished later, during the post-embryonic development, when the larva is free-swimming and can procure its outi food. In like manner many Polychaetes and Arthropods (Branchipus) only acquire a germinal streak ia the course of their later growth as larvae. In all cases in which the embryonic development begins by the formation of a germinal streak, the embryo only becomes definitely limited after the yolk has been gradually surrounded, as a result of processes which are connected with the complete entry of the yolk into the body cavity (Frogs, Insects), or with the origin of a yolk sac from which the yolk passes gradually into the body of the embryo. (Birds, Mammals). The progi-e.ssive organization of this latter, up to its exit from the egg membranes, presents in each group such extraordinary variations that it is not possible to give a general account of them. Of primary importance is the fact that in the rudiment of the germ two cell layers first make their appearance — one the ectoderm, which gives rise to the outer integument; and the other the endoderm, from which arises the lining membrane of the digestive cavity and of the glands opening into it. Between these two layers there is formed, either from the outer or the inner layer, or from both layers, an intermediate layer, known as the mesoderm. From the mesoderm arise the muscular system and the connective tissues, the corpuscles of the lymph and blood, and the vascular system. The body cavity may either be derived from the persisting segmentation cavity, i.e., the primitive space between the ectoderm and endoderm (primary body cavity), or it may be developed secondarily as a split in the mesoderm (coelom), or as outgrowths from the rudiment of the alimentaiy canal (archenteron), in which case it is known as an enterocoele body cavity. The nervous system and organs of sense are probably in all cases derived from the ectoderm, very frequently as pit- or gi^oove-hke invaginations which are subsequently constricted ofi". On the other hand, the urinary and generative organs arise both from the outer and inner layers as well as from the middle layer, which is itself derived from one of the primary layers or from the walls of the primary single -layered blastosphere. Accordingly, as a rule the rudiments of the skin and glandular IIOMOLOGX^ OF TILE GiillJMINAL LAYERS. 117 lining of the alimentary canal are the first ditlerentiations in the embryo ; and many embryos, the so-called Planuhe and Gastrulaj, on leaving the egg, have only these two layers and an internal cavity, the arehenteron. Then follows the development of the nervous and muscular systems, — the latter taking place sometimes contem- poraneously with or after the first appearance of the skeleton, — especially in cases in which a germinal streak is developed. The urinary organs and various accessory glands, the blood-vessels and respiratory organs do not appear till later. The degree of difference between the offspring on attaining the free condition [i.e., at bii-th or hatching) and the sexually mature adults, both as regards form and size as well as organization, varies considerably throughout the animal kingdom. It is a very striking fact that an embryo provided with a central cavity and a body wall composed of only two layers of cells appears in different groups of animals as a freely moveable larva capable of leading an independent life. Having recognized this fact, it was not a great step, especially as Huxley* some time ago had compared the two membranes of the body wall of the Medusae (called later by Allman ectoderm and endoderm) with the outer and inner layers of the vertebrate blastoderm (epiblast and hypoblast), to arrive at the conclusion that there was a similar phylogenetic origin for the similar larvae of very different animal types, and to trace back the origin of organs functionally resembling each other to the same primitive structure. It was A. Kowalewskit who, by the results of his numerous researches on the development of the lower animals, first gave this conception the groundwork of fact. He not only proved the occur- rence of a two-layered larva in the development of the Ccelenterata, Echinoderms, Worms, Ascidians, and in Amphioxus amongst Verte- brates, but also on the ground of the great agreement in the later developmental stages of the larvae of Ascidians and Amphioxus and in the mode of origin of equivalent organs in the embiyos of Worms, Insects, and Vertebrata, protested against the hitherto universally received view implied in Cuvier's conception of types, that the organs of different types could not be homologous with one another. * Tb. H. Huxley, " On the Anatomy and Affinities of the family of Medusee." Philosophical Transactions. London, 1849. t Cf. A. Kowalewski's various papers in the " M6moires de I'Acad. de Peters- bourg, " on Ctenophora, Phoronis, Holothurians, Ascidians, and Amphioxus, 1866 113 OliGANlZATION AND DETELOPMKNT OF ANI^fALS IN GEXEBAL. Inasmuch as Kowalew;A{i,* fi^om the results of his embryological woi-k, drew the -conclusion that the nervous layer and embryonic skin of Insects and Vertebrates are homologous, and that the germinal layers of Amphioxus and Vertebrates correspond with those of Molluscs (Tunicata) or Morms, he was in agi-eement with the long recognised fact that anatomical transitional forms and intermediate links between the different groups or types of animals exist, and that these latter do not represent absolutely isolated planes of organization, but the highest divisions in the system, and he only gave in reality an embryological expression to the claims of the descent theory. In fact, the conclusion which Kowalewski reached was completely correct — viz., that the homologies of the germinal layers in the different types afford a scientific basis for comparative anatomy and embryology, and must be recognised as the starting-point for the proper understanding of the relationships of the tj'pes. For this position we find amongst the vertebrata proofs at every step. But while his own comprehensive embiyologici^l experiences inspired Kowalewski, the founder of the theory of the germinal layers, with a prudent reserve, other investigators, inclined to bold genei-alization, appeared at once with ready theories, in which the results of embryo- logical investigations were interpreted in accordance with the theory of descent. Among these E. Haeckel's gastrsea theory is especially prominent, which raises no less a claim " than to substitute, in the place of the classification hitherto received, a new system based on phylogeny, of which the main principle is homology of the germinal layers and of the archenteron, and secondarily on the differentiation of the axes (bilateral and radial symmetry) and of the ccelom." E. Haeckel t designated the larval form used as the point of depar- tm:e the Gastrula, and believed to have found in it the repetition in embryonic development of a common primitive form, to which the origin of all Metazoa may be traced back. To this hypothetical prototype, which is supposed to have liVed in very early times during the Laurentian period, he gave the name of Gastrcea, and called the ancient group, supposed to be widely scattered and to consist of many families and genera, by the name Gastrceadce. From this sup- position was deduced the complete homology of the outer and inner * A. Kowalewski, "Embryologische Studicn an Wlirmernund Arthropoden.'' Petersburg, 1871, p. 58-60. t E. Haeckel, " Gastraeatheorie." Jen. nat. Zeitschrift, 1874.'' For criticism see C. Clans, " Die Typenlehre and Haeckel's sogenannte Gastraeatheorie," "''■Jenna. ISTt. DilillCr DKVELOPilEJfX AKD METAMORPHOSIS. 119 germinal layers throughout the whole Metazoa ; the one being traced back to the ectoderm and the other to the endodorm of the hypothe- tical Gastrrea ; while for the middle layer, wliich is only secondaiily developed from one or both of the primary layers, only an incomplete homology was claimed. It cannot, however, be said that this tlieory, which is esi^entially an extension of the Baer-llemak theory of the germinal layers from the Vertebrata to the whole group of Metazoa, with its pretentious and hasty speculation has created a basis for comparative embryology ; such a basis can only be obtained as the result of comprehensive investigations. DIRECT DEVELOPMENT AND METAMORPHOSIS. Tlie more complete the agreement between the just born young and the adult sexual animal, so much the greater, especially in the higher animals, will be the du ration of the embryonic development and the more complicated the developmental processes of the embryo. The post-embryonic develop- ment will, in this case, be confined to simple processes of growth and perfection of the sexual organs. When, how- ever, embryonic life has, relatively to the height of the organization, a quick and simple course ; when, in other words, the embryo is born in an immature condition and at a relatively low stage of organization, the post-embryonic development will be more complicated, and the young animal, in addition to its increase in size, will present various processes of transformation and change of form. In such cases, the just hatched young, as opposed to the adult animal, is called a Larva, and develops gradually to the form of the aduK- Fig. 111.— Larval stages of the Frog (after Eckev). a, embryo some time before hatcliing, with wart- like gill papilUe (m the visceral arches, b. Larva some time after hatching, with external branchiae, c, Cider larva, with horny beak and small branchial clefts lieneath the integumentary operculum, with internal branchiae ; iV, nasal pit; S, sucker; A', branchiai ; A, eye; -Hz, homy teeth. 120 OBGANIZATIOX AND DEVELOPMENT OF AXIMALS !>' GENEEAL. sexual animal. The development of larvae, however, is by no means direct and uniform, but is complicated by the necessity for special contrivances to enable them to procui-e food and to protect them- selves ; sometimes taking place in an entirely different medium, under different conditions of life. This kind of post-embryonic development is known as metamorphosis. "Well-known examples of metamorphosis are afforded by the deve- lopmental histories of the Insecta and Amphibia. From the eggs of Frogs and Toads proceed larvae pro\dded with tails, but without limbs, the so-called Tadpoles (fig 111). These, with their laterally compres'^ed tails and their gills, remind one of fitches, and they possess organs of attachment in the form of two small cervicjil suckers by which they can anchor themselves to plants. The mouth is provided with horny plates ; the spirally coiled intestine is surprisingly long ; the heart is simple; and the vascular arches have the piscine relations. Later, as development proceeds, the external branchife abort, and are replaced by new bianchise covered by folds of the integument, the eaudal fin is enlarged, and the fore and hind limbs sprout out ; the fore limbs Temain for some time covered by the integument, and only subsequently break through it to appear on the suiface. Meanwhile the lungs have developed as appendages of the anterior part of the alimentar}^ canal, and supplant the gills as respiratory organs, a doable circulation is developed, and the horny beak is cast off. Finally the tail gradually shrinks and atrophies ; on the completion of which the metamorphosis of the aquatic tadpole into the frog or toad suited for life on land is accomplished (fig. 112). We have then to consider two kinds of development, \'iz., develop- ment with a metamorphosis and direct development, which in extreme cases are distinctly opposed to each other, but are connected by inter- mediate methods. The size of the egg, or, in other words, the amount of food yolk available for the use of the embiyo in proportion to the size of the adult animal appears to be a factor of primary importance in any explanation of these two distinct processes (II. Leuckart). Animals with a direct development require — generally in pro- portion to the height of their organization and the size of their bodies — that their eggs should be provided A^ith a rich endowment of food yolk, or that the developing embryo should possess a special accessory source of nutriment ; they arise therefore either from relatively large eggs (Birds), or they are developed inside, and in close connection with the maternal body, by which arrangement they have a continual supply of food matei-ial (Mammals). Animals, EELATION OF METAMORPHOSIS TO FEETILITY. 121 on the contrary, which pass through a metamorphosis always arise from eggs of relatively small size, are hatched in an immature con- dition as larvae, and obtain independently, by their own activity, the materials which have been withheld from them while in the egg, but which are necessary for their full development. The number of embryos produced in the case of a direct development is, in proportion to the total weight of the material applied by the mother for reproductive purposes, far smaller than in the case of a develop- ment with metamorphosis. The fertility of animals whose young Fig. 112.— Later stages in the development of Pelobates fuscus. a, larva Tvithout limbs with well developed tail; b, older larva with hind limbs ; c, larva with two pairs of limbs; d, young frog with caudal stump j e, young frog after complete atrophy of tail. undergo a metamorphosis, or, in other words, the numl^er of ofTspring produced from a given mass of formative material, is increased to an extraordinary degree, and has, in the complicated relations of organic life, a great physiological significance, though systematically it is of little importance. Some time ago it was attempted to explain these indirect meta- morphoses, in which both processes of reduction and new development take place, as the result of the necessity which the simply organized 122 0EGA5IZATI0X AND DETELOPMEXT OF AXIMALS IX GEXEEAL. larva, hatched at an early stage of development, laboured under ol acquiring special arrangements for its protection and nourishment (R. Leuckart). The proof that such relations do exist between the special larval organs and the peculiar methods of nutrition and protection is an important factor for the full understanding of the£.e remarkable processes, but still is by no means an explanation of them. It is only by aid of the Darwinian principles and the theory of descent that we can get nearer to an explanation. According to this theory, the form and structure of larvae are to be considered in relation (o the development of the race, i.e. phylogeny, and are to be derived from the various phases of structure through which tlie latter has passed in its evolution, and in such a way that the younger larval stages would correspond to the primitive, and the older, on the other hand, to the more advanced and more highly organized animals, which have appeared later in the history of the race. In this sense the developmental processes of the individual constitute a more or less complete recapitulation of the developmental history cf the species, complicated, however, by secondary variations due to adaptation, which have been acquired in the struggle for existence * (Fritz Mailer's fundamental principle, called by Haeckel the funda- mental law of biogenesis). The greater the number of stages, therefore, through which the larva passes, the more completely is the ancestral history of the species preserved in the developmental history of the individual ; and it is the more truly preserved the fewer the peculiarities of the larva, whether independently acquired, or shifted back from the later to the earlier periods of life (Copepoda.) On the other hand, there are certain larval forms without any phylogenetic meaning which are to be explained as having been secondarily acquired by adaptation (many Insect larvfe). The historical record preserved in the developmental history becomes, however, gradually defaced by simplification and shortening of the free development; for the successive phases of development are gradually more and more shifted back in the life of the embryo, and run their course more rapidly and in an abbreviated form, under the protection of the egg membranes, and at the cost of a rich supply of nutrient material (yolk, albumen, placenta). In animals with a direct development, therefore, the complicated deve- lopment within the egg membranes is a compressed and simplitied * Fritz Miiller, " Fiir Darwin." Leipzig. 1803, p. To— 81. ALTJ-RXATION OF GENEEATIONS. 123 metamorphosis, and hence the direct development, as opposed to tlie metamorpho.si^', is a secondary form of development. ALTIIRXATION OF GENERATIONS, POLYMORPHISM AND HETEROGAMY. Both in direct development and indirect development by means of a metamorphosis, the successive stages take place in the life- history of the same individual. There are, however, instances of free development, in which the individual only passes through a part of the developmental changes, while the offspring produced by it a<5complishes the remaining part. In this case the life-history of the species is represented by two or more generations of indivi- duals, which possess different forms and organization, exist under different conditions of life, and reproduce in different ways. Such a manner of development is known as alternation of genera- tions (metagenesis), and consists of the regular alternation of a sexually differentiated generation with one or more generations reproducing asexually. This phenomenon was first discovered by the poet Chamisso* in the Salpid* ; but the observation remained for more than twenty years unnoticed. It was rediscovered by J. Steenstrup, t and discussed in the reproduction of a series of animals (Medusae, Trematoda) as a law of development. The essence of the process consists in this, that the sexual animals produce offspring, which throvTgh their whole life remain different from their parents, but can give rise by an asexual process of reproduction to a gener- ation of animals which resemble in their organization and habits of life the sexual form, or again produce themselves asexually, their offspring assuming the characters of the original sexual animal. So that in the last case the life of the species is composed of three different generations proceeding from one another, viz., sexual form, first asexual form, and second asexual form. The development of the two, three, or more generations may be direct, or may take place by a more or less complicated metamorphosis ; similarly the asexual and the sexual generations sometimes differ but little from each other {e.g. Salpa), and sometimes present relations analogous to those which exist between a larva and the adult animal {e.g. * Adalbert de Chamis-o, * De animalibus quibusdam e classe verminm Linnfeana in circumnavigatione tcrrse auspicante comite N. Eomanzoff duce Ottone de Kotzebue annis 1815, 1816, 1817, 1818 peracta." Fasc, I. De salpa Berolini 1819. t Job. Jnp. Sm. Steenstrup, " Ueber den Generationswechsel, etc," iibersetzt von C. H. Lorenzen. Kopenhagen, 1842. 124 ORGANIZA-TIOX AND DEVELOPMENT OF AIS'IMALS !>' GrXEEAX. Medusae). Accordingly we have to distinguish different forms of alternations of generations, which have genetically a different origin and explanation. The latter form of alternations of generations resembles metamor- phosis ; and we have in most cases to explain it as ha\dng arisen in the following way : — The asexual form corresponds to a lower stage in the phylogenetic history, from which it has inherited the capacity of asexual reproduction, while the sexual rej^roduction belongs entirely to the higher form. . To take as an example the alternation of generations of the Scyphomedusa^. The animal is hatched as a fi-ee-swimming ciliated planula (gastrula with closed blastopore) (fig. 113 a). After a certain time it fixes itself by the pole of its body, d Fig. 113.— Development of tlie planula of Chrysaora to the Scyptistuma biatje, with eight arms, a. Two layered planula with a narrow gastric cavity; b, the same after its attachment with just-formed moath (O), and commencing tentacles; c, four-armed Scy- phistoma polyp ; Ctk, excreted cuticular skeleton ; d, eight-armed Scyi^histoma polyp with wide mouth; M, longitudinal muscles of the gastric ridges; Cak, excreted cuticular skeleton. which is directed forward in swimming, and acquires at its free ex- tremity a new mouth, round which 1, 2, 4, 8, and finally 16 long tentacles soon make their appearance ; while the broad oral region projects as a contractile cone (fig. 113 h, c, d). Inside the gastric cavity there project four gastric ridges with longitudinal muscular bands extending from the foot or point of attachment to the base of the oral cone. When the polyp, which has now become a Scyphis- toma, has under favourable conditions of nutrition reached a certain size (about 2 to 4 mm.), ring-like constrictions are formed at the SCIPmSIOilA, STEOBIL\, EPHYJIA. 125 anterior part of the body, giving rise to a series of segment-like divisions. The anterior part of the body bearing the tentacles is first marked oS ; and following this a greater or less number of sections, the new segments appealing continuously in the direction from before backwai'd. The hindermost or basal swollen club-shaped €uuid of the polyp's body remains unthvided. The 8cyphistoma lias W I \ ■ ( Fig. 113.- -e. Stage of Scyphistoma with sixteen arms C^lightlymngnified) ; Gw. gasuic ridgfcis. f, Commencing strobilization. now become the Strohila, which itself passes through various developmental phases. The tentacles abort ; the successive segments, separated from each other by constrictions and provided with lobe- like continuations and marginal bodies, become transformed into small flat discs, which become separate, and, as Ejijhyrce, represent the larvse of the Scyphomedusae (fig. 113 h-h). 126 OEUAXIZ.VTION AND DEVELOPMENT OF AXIM.VLS IN GENEKAL. In the other cases in which the sexual and asexual forms mor- phologically resemble each other, as in Salpa, the origin of the alternation of generations may, as in the case of the origin of the dioecious from the hermaphrodite state, be traced back to the ten- dency towards the establishment of a division of labour acting upon an animal which possessed the capacity of sexual and asexual repro- duction. It was advantageous for the formation of the regular chain of buds (stolo prolifer) that the power of sexual reproduction should be suppressed, and that the % gonei-ative organs should be- come rudimentary and finally vanish in the budding indivi- duals; while, on the other hand, in the individuals united in the chain, the gene- rative organs Avere early de- veloped, and the stolo prolifei- was aborted and completely vanished. Special forms of alternation of generations may be dis- tinguished in which colonies are formed as the result of the asexual reproduction by fAk\ iMlo V buddinsr from a single anima' the buds remaining attached and developing into individuals which differ considerably in structure and appearance, and each of which pei-f orms special functions in maintaining the colony (nutritive, protective, sexual, etc.) Such a form of alternation of generations is known as j^ohjmorjyhism* and reaches a great complication in the polymorphous colonies of the Siphonophora. A form of reproduction which closely resembles metagenesis, but which genetically has quite a difterent explanation, is the lately Fig. 113.—^, Fully developed Strobilawith sepa- rating: EphyraB. h. Free Ephyra (of about 1 '5 to 2 mm. diameter. * K. Leuckart, " Ueber den Polymorphismus der Individuen oJer die Erscheinunsr der Arbeitsthcilung in der Natur." Giessen, 1851. it::Ti:iiOGAMi. 127 discovered process known as heterotjamy. It is characterised bv the succession of differently organized sexual generations living under -diflerent nutritive conditions. Heterogamy, which was first discovered in certain small Nematodes (R/iahdonema niyrovenosum and Leptodera cqypendiculata), can scarcely be explained otherwise than as an adaptation to changed conditions. For when the embiyo is developed as a puiitsite in conditions favour- able for the acquisition of nutriment, it gives rise to a sexual form so diffeient in size and structure frjm that which arises if the Fig. Hi.— a, Rhabdonema ninrrovenosum of aLout 3o mm. in length at the stage when the male organs are ripe. G, genital trland; O, mouth ; D, alimentary canal ; ^, anus ; A, nerve ring; Dr.:, gland cells ; Z, isolated spermatozoa. B, Male and female Rbabditis, length from about 1 o to 2 mm. ; Oo, ovary ; T, testis ; V, female genital opening ; Sj^, spicula. embryo leads a free existence in damp earth or dirty water {i.e., in conditions not so favourable for the acquisition of nutriment), that we .should, from the difference in their structure, place the two forms in different genera. Ehabdonema nigrovenosum from the lungs of Batrachians and the free-living Rhabditis follow each other in a strictly alternating manner (tig. 114, A, B), Other cases of hetero- gnrny are afforded by the Bark-lice (Chermes), and the Eoot-lice 128 OEGAXIZATION AXB DEVELOPMENT OF ANIMALS IN UENEEAL. (Phylloxera), in that one or more (winged and apterous) female genei-ations are characterised by parthenogenetic reproduction, and consist only of oviparous females; while the generation of females, which lay fei-tilised eggs, appears with the males only at certain times of the year, and can be distinguished by their small size, and by the reduction of their mouth parts and digestive apparatus. Such forms of heterogamy lead back apparently to alternations of generations, especially when the parthenogenetic generations present, in the structure of their generative apparatus, essential difierences from the females which copulate. The plant-lice and gall-fiies afford instances of this. The repro- ductive processes of these animals, on the authority of Steen- strup and V. Siebold, were regarded for a long time as instances of altei-nations of generations, until the view, which was supported by the reproductive processes of the allied bax-k-lice, that they came under the head of heterogamy, received general assent. According to this view, the viviparous forms of the plant-lice (Aphides) are mei-ely modified females adapted for pai'thenogenetic reproduction, and their reproductive gland is nothing more than the modified ovary. There are also cases of heterogamy in which, in the partheno- genetic generations, the development of the %^g commences in the ovary of the larva, reproduction being shifted back into larval life. This form of heterogamy, which resembles alternations of genera- tions, was shown to occur in the larva of Cecidomyia (Miastor) by Nic. Wagner and by O. Giimm in the larva of a species of Chiro- nomus, and is to be looked upon as a case of precocious development of the egg in the parthenogenetic generation. The morphologically undeveloped larva has acquired the power of reproducing itself by means of its rudimentary ovary, a phenomenon which, following the proposal of C. E. v. Baer, has been designated Pcedogenesis. If the reproductive gland of the Cecidomyia larva be looked upon as a germ-gland, and the cells contained in it as germ cells or spores, the reproduction of the Cecidomyia falls into the category of alterna- tions of generations. But the idea involved in the term " spore " is borrowed from the vegetable kingdom, and there is no reason for looking upon these or any other structures in the Metazoa as spores. The above explanation, therefore, is untenable.* The reproductive cells in the Metazoa, which have been regarded in this light, have much more probably oiiginated from masses of cells which represent the rudiment of the ovary, and which are usually visible in early stages of development. DEVELOPMENT OF DISTOME^. 129 Furtlier it cannot be doubted that the development of the Distome.-e, which has hitherto been regai^ded as a case of alternation of generations really represents a form of heterogamy allied to paedogenesis. After the completion of the segmentation and em- bryonic development, the ciliated embryos (tig 115, a, V) pass from the egg into the water, where they swim about, and eventually make their way into the body of a Snail, in which they give rise to sac-like or branched Sporocysts (fig 115, c) or to Redije provided with an alimentary canal (fig, 115, d). These stages in the development of Distomum which are apparently Fro. 115.— Developmental history of Distomum (in pan after R. Leuckart). a. Free- swimming ciliated embryo of the liver fluke.— i, the same contracted, with rudiment of alimentary canal D ; and a^n^resations of cells ; Oc, rudiment of genital gland ; JEx, ciliated apparatus of the excretory system.— c, Sporoc.yst, which has proceeded from Distomum embryo, filled with Cercariae C ; B, spine of a Cercaria.- (i, Redia with pharynx PA ; alimentary canal, D ; Ex, excretory organs ; C, contained Ceroarise.- e, free Cercaria ; S, sucker ; D, gut. comparable to larvae, produce by means of the so-called germ granules or spores a generation of offspring known as Cercariae (fig. 115, e), which become free, and then make their way into the body of a new host, and, after the loss of the oral spine and caudal appendage, encyst (fig 115/). Hence they are carried into the body of the pei'manent host to develop into the sexual adult form. 9 130 ORGANIZATION^ AND DEVELOPMENT OF ANIMALS, It is, however, extremely probivble that the masses of cells from which the Cercaria? arise represent the rudiments of ovaries, the elements of Avhicli develop parthenogenetically without the addition of spermatozoa. The so-called germ sacs (Sporocysts or Redia?) would in this case be larva?, Avhich possess the power of reproduction; and the development of the Distomejo would come under the head of heterogamy. The Cercarioe, however, represent a second and more advanced larval phase. Provided with a motile tail, frequently with eyes and buccal spine, their organization, save in the absence of developed generative organs, presents great simi- larities to the sexually mature adults into which they develop. This development, however, takes place only in the body of another and usually moi'e highly organized host after the loss of the larval organs. If the conception of a spore as an asexual reproductive product be main- tained, it becomes impossible in practice to draw a sharp line between alterna- tion of generations and heterogamy ; since there is no test which enables us to distinguish between a spore and an ovum which develops parthenogeneti- cally. On the other hand, if we inter- pret, as Ave are justified in doing, the so-called spores as precociously developed ova, alternation of generations and heterogamy can be clearly distinguished from one another, since in the former one generation is asexual, and in- creases entirely by budding and division ; while in the latter both generations are sexual, though in one of them the ova may possess the power of spontaneous development. An essential characteristic both of heterogamy and alternation of generations depends upon the different form of the individuals appearing in the generations which usually occur in a regularly alternating manner in the life-history of the species. But there are cases in Avhich two methods of reprodviction may follow each other in the life-history of one individual. This form of the Fig. 115. /,— Young Distomum (after La Valette). Ex, main trunk of excretory system ; Ep, excre- tory pore ; O, mouth opening with sucker ; 5, sucker on mitldlo of ventral surface ; F, phai-j-nx ; J?, limb of alimentary canal. INCOMPLETE HETEROGAMY. 131 reproductive process is of the greatest interest as throwing light upon the way in which the phenomena of alternation of generations and heterogamy may have arisen in that it appears in a certain degree as the precursor of the altei-nating sequence of two or more genera- tions of individuals. The so-called alternation of generations in the stone-corals (Blastotrochus), which in early life reproduce tliemselves by budding, without thereby losing the power of acquiring sexual organs at a later period of life, forms an example of this method of reproduction. In this category of incomplete heterogamy should be placed the reproductive processes of the Phyllopoda and Rotifeiu, in which the female produces summer eggs capable of parthenogenetic develop- ment, and later winter eggs requiring fertilization [Daphnidce). [In the above account the term alternation of generations, or metagenesis is applied to those cases in which sexual and asexual generations alternate ; while heterogamy is applied to those cases in which two sexual generations or a sexual and parthenogenetic generation alternate.] CHAPTER IV. HISTORICAL REVIEW. The origin of Zoology extends far back into a?itiquity. Aristotle (4th century B.C.), who scientifically and in a philosophic spirit worked up the experiences of his predecessors with his own extended observations, must be looked upon as the founder of this science. The most important of his zoological worksf treat of the " Repro- duction of Animals," of the " Parts of Animals," and of the " History of Animals." The last and most important work is, unfortunately, only incompletely preserved. We must not expect to find in Aristotle a descriptive zoologist, nor in his works a system of animals followed out into the smallest * Victor Carus, " Geschichte der Zoologie." Miinchcn, 1872. t Compare Jiirgen Bona Meyer's "Aiistotcles' Tlaierkunde " (Berlin, 185.5). — Frantzius, " Aristoteles' Theile der Thiere " (Leipzig. 1853).— Aubert und Wimmer, " Aristoteles' Fiinf Biicher von der Zeugung und Entwicklung der Thiere, iibersetzt und erlautert " (Leipzig. 18C0). — Aubert und Wimmer, " Aristoteles' Thierkunde." Band L und I L (Leip;ij, 1868). 132 HISTOEICAL EEVIEW. details; a one-sided treatment of science was not the object of this great thinker. Aristotle contemplated animals as living organisms in all their relations to the external world, according to their development, structure, and vital phenomena, and he created a comparative Zoology, which in several respects constitutes the basis of our science. The distinction of animals into animals with Mood (evat/xa) and animals without blood (avaijxa.), which he in no wise used as a strictly systematic conception, certainly depends, according to the meaning of the word, upon an error, since all animals possess blood ; and the red colour can by no means be taken, as Aristotle believed, to be a test of the presence of blood ; but as the possession of a bony verte- bral column was put forward as a character of the animals provided with blood, the two groups established by this distinction coincided in their limits with the two great divisions of Vertebrates and Invertebrates. The eight animal groups of Aristotle are the following : — Animals loith blood, Vertebrates- — (1) Viviparous animals (four-footed, ^cjoTOKowra kv avrois), with which as a special -j^evos was placed the whale. (2) Birds (opw^es). (3) Oviparous four-footed animals (rcrpaTroSa r} arro^a wotokovvto). (4) Fishes {IxOve';). Animals icithout blood, Invertebrates — (5) Soft animals [jxaXaKia, Cephalopoda). (6) Soft animals with shells (^aXaKoo-rpaKa). (7) Insects (evro/xa). (8) Shelled animals (oo-r/^aKoScp/AaTa, Echini, Snails, and Mussels). After Aristotle, antiquity only possesses one zoological wx-iter of note — Pliny the elder — to point to. He lived in the first century, and, as is well-known, was killed in the great Eruption of Vesuvius (79), while captain of the fleet. The natural history of PHny deals with the whole of nature, from the stars to animals, plants, and minerals ; it is, however, of no scientific value as an original work, but is merely a compilation of facts collected from known sources, and is not by any means implicitly to be trusted. Pliny borrowed to a large extent from Aristotle, often imderstood him falsely, and also accepted here and thei^e as facts fable-, which had been rejected by Aristotle. "Without setting vip a system of his own, he divided animals according to the medium in which they lived — into Land- animals (Terrestria), Aquatic-animals (Aquatica), and Flying- SWAMMERDAil, MALPIOUI, KKAUilUIi, ETC. 133 animals (Volatiliu), — a division which was accepted till Gessner's time. With the decline of the sciences, natural history also fell into oblivion. The mind of man, fettered by the belief in authority, felt in the middle ages no need for an independent contemplation of Nature. But the writings of Aristotle and Pliny found an asylum within the walls of the Christian cloisters, which preserved the germs of science developed in Heathendom from complete extinc- tion. In the course of the middle ages, first the Spanish bishop, Isidor of Seville (in the seventh century), and later Albertus Magnus (in the thirteenth century) wrote works on natural history ; but it was not until the renaissance of the sciences of the sixteenth century that the works of Aristotle again came to the fore, and the desire for independent observation and research was also roused. Works like those of C. Gessner, Aldrovandus, Wotton, testified to the newly awakening life of our science, whose scope was continually being in- creased by the discovery of new worlds. The next century, in which Harvey discovered the circulation of the blood, Keppler the revolution of the planets, and Newton's law of gravitation formed the beginning of a new era in physics, was also a fruitful epoch for Zoology. Aurelio Severino wrote his "Zootomia democritaja" (1645), a work which contained anatomical drawings of various animals, more for the use and advancement of human anatomy and physiology. Swammerdam in Ley den dissected the bodies of Insects and Molluscs, and described the metamorphosis of the Frog. Malpighi in Bologna and Leeuwenhoek in Delft appKed the invention of the microscope to the examination of tissues and the smallest organisms (animals from infusions). The latter discovered the blood corpuscles, and first saw the transverse striations of muscular fibres. The spermatazoa also were discovered by a student, Hamm, and called, on account of their movements, sperm-animals. The Italian Redi opposed the spontaneous genera- tion of animals in putrefying matter, proved the origin of Maggots from Flies' eggs, and supported Harvey's famous expression, " omne vivum ex ovo." In the eighteenth century the knowledge of the life-history of animals was enormously enriched. Investigators such as Reaumur, Rosel von Bosenhof, De Geer, Bonnet, J. Chr. Schaeffer, Ledermiiller, etc., discovered the metamorphosis and life-history of Insects and native aquatic animals, while at the same time, by expeditions into foreign lands, a great number of animals from other 134 HISTOmCAL EETIEW. continents became known. In consequence of these, extended obser- vations and a continually grow-ing eagerness to collect curiosities from foreign countries, the zoological material increased so largely that, in the absence of precise distinctions, nomenclature and arrangement, the danger of error vras great, and a general review of the facts almost impossible. Under such conditions, the appearance of the systematiser Carl Linnaeus (1707 — 1778) must have been of the greatest importance for the further development of Zoology. Ray, who is justly placed in the first rank of Linnfeus' prede- cessors, had earlier endeavoured to acquire a basis for the systematic treatment, and \vith a certain amount of success, but he failed to organise a thoi'oughly methodical arrangement. He was the first to introduce the conception of " species " and to consider anatomical characters as the basis of classification. In his work, entitled " Synopsis of Mammalia and Reptilia " (1693), he adopted Ai-istotle's division of the animal kingdom into animals wnth and animals without blood. With regard to the first he laid the basis of the definitions of Linnfeus' first four classes ; the latter he divided into a greater group, containing Cephalopods, Crustacea, and Testacea, and into a smaller containing the Insecta. The importance of Linnaeus' work to the development of science depended not on any far-reaching investigations or important dis- coveries, but on his acute sifting and exact division of the then exist- ing facts, and on the introduction of a new method of more certain diagnosis, nomenclature, and an\angement. By erecting for gi-oups of different value a series of categories based on the ideas of species, genus, order, class, he obtained a means of creating a much more precise system of classification. On the other hand, by the introduction of the principle of binary nomencla- ture, he obtained a fixed and more certain method. Every animal received two names taken from the Latin language — the generic name, which was placed first, and the specific name, which together denote the fact that the animal in question belongs to a definite genus and species. In this way Linnaeus not only arranged the facts then known, but also created a systematic framework in which later discoveries would easily find their proper place. Linnaeus's great work, the " Systema Naturae," which in its thirteen editions received many changes, embraced the animal, vegetable, and minei-al kingdoms, and in its treatment can only be compared to an exhaustive catalogue, in which the contents of nature, like that of a Hbrary, are i-egistered in a definite order vnth a statement of CUVIEB. 135 their most remaikable characters. Every species of animal and plant obtained a place determined by its properties, and was with the specific name inserted under the genus. After the name fol- lowed a short Latin diagnosis, and a list of the synonyms of authors and statements concerning the habits of life, habitiit, the native coiantry, and any special characteristics. Linnreus created for botany an artificial system of classification founded on the characters of flowers. Similarly his classification of animals was artificial, as it did not depend upon the distinction of natural groups, but took isolated features of internal and external structure as characters. Linnajus completed the improvements in Aristotle's classification which had been already begun by Ray, by establishing the following six classes, founded on the structure of the heart, the condition of the blood, the manner of reproduction and respiration. (1) Jfammalia. — With red warm blood, and a heart composed of two auricles and two ventricles, viviparous. The following orders were distinguished — Primates (with the four genera Homo, Simla, Lemur, Vespertilio), Bruta., Ferpe, Glires, Pecora, Bellure, Cete. (2) Aves. — With warm red blood, and a heart composed of two auricles and two ventricles, oviparous — Accipitres, Pica?, Anseres, Grallie, Gallinre, Passeres. (3) Amjjhibia. — With cold red blood and a heart composed of simple auricle and ventricle, breathing by lungs — Reptilia (Testudo, Draco, Lacerta, Rana), Serpentes. (4) Pisces. — With cold red blood, and a heart composed of simple auricle and ventricle, breathing by gills — Apodes, Jugulares, Tliora- cici, Abdominales, Branchiostegi, Chondropterygii. (5) Insecta.— With white blood, simple heart, and segmented an- tennje — Coleoptera, Hemiptera, Lepidoptera, ISTeuroptera, Hymenop- tera, Diptera, Aptera. (6) Vermes. — With white blood, simple heart, and unsegmented antennse — Mollusca, Intestina, Testacea, Zoophyta, Infusoria. While the followers of Linnteus developed still further this barren and one-sided zoographical treatment and erroneously looked upon the framework of this system as an exact and complete expression of the whole of nature, Cuvior, by combining Comparative Anatomy with Zoology, laid the foundations of a natural system. George Cuvier, born at Mompelgard 1762, and educated at the Karlsakademie at Stuttgart, later Professor of Comparative Anatomy at the Jardin des Plant es in Paris, published his comprehensive in- 136 HISTORICAL EETIEW. vestigations in numerous works, especially in his " Lecons d'Anatomie comparee " (1805). lu his celebrated treatise* published in 1812, on the arrangeiaent of animals according to their organization, he established a new and essentially changed classification, which was the first serious attempt to build up a natural system, Cuvier did not, as most zootomists had done, look upon anatomical discoveries and facts as in themselves the aim of his researches, but he contemplated them from a comparative point of view, which led him to the establishment of general principles. By considering the peculiarities in the ar- rangements of the organs in relation with the life and unity of the organism, he recognised the reciprocal dependence of the individual organs, and appreciating fully the idea of the " correlation " of parts already discussed by Aristotle, he developed his principle of the con- ditions of existence without Avhich an animal caniiot live lyjyrincij^e des conditions d'existence ou causes finales). " The organism con- sists of a single and complete whole, in which single parts cannot be changed without causing changes in all the other parts." By com- paring the organizations of many dilferent animals, he found that the important organs are the most constant, and that the less important vary most in their form and development, and even are not univer- sally present. He was thus led to the principle so important for the systematist of the subordination of characters [princijie de la subordination des caracteres). Without being ruled by the pre-conceived idea of the unity of all animal organization, he became convinced, from a conside- ration of the difterences in the nervous system and in the arrangement of the more important systems of organs, that there were in the animal kingdom four main types {embranchements), " general plans of structure on which the respective animals appear to be modelled, and whose individual subdivisions, as they may be called, are only slight modifications based on the development or the addition of some parts, without the plan of the organization being thereby essentially changed." These four groups . (embranchements Cuvier, Typen Blainville) were the Vertebrata, MoUusca, Articulata, and Eadiata. The views of Cuvier, who in knowledge of anatomical and zoologi- cal detail stood far above all his contemporaries, were, however, in opposition to the theories of men of note (the so-called School of * " Sur un nouveau rapprochement a ^tablir entre les classes qui composent Ic r^me animal." Ann. des Mu.icuni d'l/isf. Xat., Tom XIX., 1812. T, HILAIEE, OKEIf, VOX BAEB. 137 Natural Philosophy). In Fi'ance, Etienne Geotfroy St. Hilaire pre- emineutly defemled the idea, which had been already expressed by Buffbn, of the unity of the plan of animal structure, according to which the animal kingdom consisted of an unbroken gradation of animals. Convinced that nature always worked with the same materials, he put forward his theory of analogies, according to which the same pai-ts, though differing in their form and the degree of their development, should be found in all animals ; and, further, his theory of connections {2)ri7icipe des connexions), according to wliich the same parts always appear in the same mutual position. A third funda- mental piinciple was that of the equivalence of organs, an increase in the size of one organ being accompanied by the diminution of another organ. The application of this principle had important results, and led to the scientific foundation of Teratology, His generalizations were, however, in the main hasty, in that they were founded on facts taken only from the Vertebrates ; and if applied outside that group must lead to many rash conclusions, e.g., that Insects are Vertebrates turned on to their backs. In Germany, Goethe and the natural philosophers Oken and Schelling pronounced in favour of the unity of animal organization, but it must be confessed without taking account in a comprehensive manner of the actvial facts. The result of this controversy which in Prance was carried on with considerable vehemence was, that Cuvier's view was victorious, and his principles met with the more undivided assent since it appeared that they were confirmed by C. E. v. Baer's embryological work. Many gaps and errors were certainly discovered by later investigators in Cuvier's classification, and in detail it was much changed, but the establishment of his animal types as the chief groups of the system was retained, and was supported by the results of the developing Science of Embryology. The most essential of the modifications which it has become neces- sary to make in Cuvier's system relate chiefly to the increase in the number of types. The Infusoria were some time ago removed from the Eadiata, and as Pi-otozoa arranged by the side of the four other groups. Lately the number of groups has been increased by the division of the Eadiata into Coelenterata and Echinodermata, and of the Articulata into Arthropoda and Vermes, and of the Mollusca into three groups. In our times, however, Cuvier's view has experienced an essential modification in favour of the Natural Philosophers, and the idea of 138 HISTORICAL EEYIEW. the absolute independence and isolation of each group must be given up. With a more complete study it has been show-n that inter- mediate forms exist connecting the various types, and that conse- quently no sharp line of demarcation can be drawn between them. But just as the transitional forms between animals and plants cannot abolish the distinction between these two most important conceptions of the organic kingdom, so the existence of such transitional forms does not in any way affect the value of the idea of groups and types as the chief divisions of the animal system, but only renders it probable that the different groups have developed from a similar or common starting-point. And to this corresponds the fact, which has become recognised with the progress of embryological knowledge, that similar larval stages and tissue-layers (germinal layers) are found in the develop- mental history of the different types. This fact points to a genetic connection. Likewise the results of anatomical and embryological comparison have rendered it probable that the types are by no means absolutely independent, but are subordinated to one another in more or less close relation, that especially the higher groups are genetically to be derived from the Worms, a group which certainly includes extremely dissimilar forms, and eventually will, without doubt, be broken up into several types. We consider it, under such circumstances, con- venient, in the present state of science, to distinguish nine types as the chief divisions, and to characterise them in the following manner : — (1) Protozoa. — Of small size, with differentiations within the sar- code, without cellular organs, with predominating asexual repro- duction. (2) Ccehnterata. — Radiate animals segmented in terms of 2, 4, or G ; mesoderm of connective tissue, often gelatinous ; and a central body cavity common to digestion and circulation (gastro-vascular (3) Echinodernmta. — Radiating animals, for the most part of pen- tamerous arrangement ; with calcareous dermal skeleton, often bear- ing spines ; with separate alimentary and vascular systems ; and with nervous system and ambulacral feet. (4) Vermes. — Bilateral animals with unseginented or uniformly (homonomous) segmented body, without jointed appendages (limbs), with paired excretory canals sometimes called water-vascular system. (5) Arthrojwda. — Bilateral animals with heteronomously segmented MEAKIXO OP THE SYSTEM. 139 bodies and joiiited appendages, Avith brain aiul ventral chain of ganglia. (G) Molluscoidea. — Bilateral, unsegmented animals with ciliated circlet of tentacles or spirally rolled buccal arms ; either polyp-like and provided with a hard shell case, or mussel-like with a bivalve shell, the valves being anterior and posterior; with one or more ganglia connected together by a perioesophageal ring. (7) Mollusca. Bilateral animals with soft, uni^cgmentod body, without a skeleton serving for purposes of locomotion ; usually enclosed in a single or bivalve shell, which is excreted by a fold of the skin (mantle) ; with brain, pedal-ganglion and mantle-ganglion. (8) Tmiicata. — Bilateral unsegmented animals with sac-shaped or barrel-shaped bodies, and a large mantle cavity perfoiuted by two openings ; simple nervous ganglion, heart and gills. (9) Vertehrata. — Bilateral animals with an internal cartilaginous or osseous segmented skeleton (vertebral column), which gives off dorj-al processes (the neural arches) to sui-round a cavity for the reception of the spinal cord and brain ; and ventral processes (the ribs) which bound a cavity for the reception of the vegetative orgai. ,; never with more than two pairs of limbs. CHAPTER V. MEANING OF THE SYSTEM. Yery different opinions have been held in different places and at different times as to the value of the system. In the last century the French Zoologist Buffon held the system to be a pure invention of the human mind ; while more recently L. Agassiz thought that a real meaning could be attributed to all the divisions of the system. He explained the natural system founded on relationship of organiza- tion as a translation of the thoughts of the Ci-eator into human language, by the investigation of which we become unconsciou'-ly interpreters of his ideas. But it is clear that we cannot call that arrangement, which is derived from the relations of organization founded in nature, an invention of man. Similarly it is preposterous to deny the sub- jective participation of our intellectual activity, since in every system there is expressed a relation of the facts of nature to our comprehen- 140 MEAXIXO OP THE SYSTEM. sion and to the state of scientific knowledge. In this. sense Goethe appropriately calls a natural system a contradictory expression. In establishing systems, that which comes into contemplation consists of the individual forms which are the objects of observation. Every systematic conception, from that of the species to that of the type, depends upon the bringing together of sijiiiliar properties, and is an abstraction of the human mind. Species. — The great majority of investigators, till very recently, were agreed in looking upon the species as an independently created unit with special properties which were retained in propagation, and were really contented with the fundamental idea in Linnreus' defini- tion of species : Tot numeramus species quot ah initio creavit in- finitum ens." This view also accorded with a dogma prevalent in Geology, according to which the flora and fauna of the successive periods of the earth's history were completely isolated, being created afresh at the beginning and destroyed by a vast catastrophe at the end of each period. It was supposed that no living thing could be preserved through one of these catastrophes from one period into the next ; that every species of animal and plant was specially created with definite characters, which it retained unchanged until it was destroyed. This idea was confirmed by the diflerence between the fossil remains of Vertebrates (Cuvier) and Molluscs (Lamarck), and the living forms of these types. As a matter of fact, however, neither in the animal nor in the vegetable kingdom do offspring resemble exactly the parent foi-ms from which they have originated, but present differences more or less considerable, so that the idea of absolute identity must be removed from our definition of species, and agreement in the most essential particulars introduced in its place. The species would ac- cordingly, in close agreement with Cuvier's definition, include all living forms which have the most essential properties in common, are descended from one another, and prodtLce fruitful descendants. All the facts of natural life, however, can by no means be arranged agreeably to this conception, which has for a fundamental principle that all essential peculiarities must be preserved unaltered by repro- duction through all time. The great difficulties in defining species which occur in practice, and which prevent a sharp line being drawn between species and variety, indicate the insufficiency of the conception. Varieties. — Individuals belonging to the same species do not resemble each other in all particulars, but present differences which, SPECIES AXB VARIEXI. 141 on closer investigation, suffice to distinguish the individual forms. Combinations of modified characters are often present in the same species, and occasion important variations (varieties) which can be inherited by the descendants. The more important of such variations which are maintained by reproduction are called constant varieties or subspecies, or races, and are divided into natural races and artificial or domesticated races. The former are found in free natural life, and are usually confined to definite localities. They have arisen in course of time in conse- quence of conditions of climate, and under the influence of variations in manner of life and nourishment. The domesticated races, on the other hand, owe their origin to the care and cultivation of man. They comprise only domestic animals whose origin is still unknown. Varieties, however, which have arisen from one species may differ very surprisingly from one another ; in fact, they may present more important features of difference than do distinct natural species. An example of this is found in the domesticated race of pigeons, whose common descent from Columba livia (the rock pigeon) was shown by Darwin to be very probable. They are capable of such striking alterations, that their varieties, known as tumbler pigeons, fantail pigeons, etc., were held by ornithologists, who were without knowledge of their origin, to be real species, and were even divided into different genera. In the natural state, too, it often happens that varieties cannot be distinguished from species by the quality of their characteristics. It is customary to consider that the essential of a character is to be found in the constancy of its occurrence, and to recognise varieties by the fact that their characteristics are more variable than those of species. If forms which are widely different can be connected by a continuous series of intermediate forms, they are held to be varieties of the same species. But if such intermediate forms are absent, they are held to be distinct species, even when the differences between them (so long as they are constant) are less. Under such circumstances we can understand that in the absence of a positive test, the individual judgment and the natural tact of the observer decides between species and variety ; * and how it is that the opinions of different observers have differed so widely in * The establishment of the conception of sub-species is completely at variance with the common conception of species, and is the most striking proof that systematists themselves recognize that the distinction between species and sub-species is a relative one. 142 MEANING OF TILE SYSTEM. practice. This relation lias been excellently and thoroughly discussed by Darwin and Hooker. As an example of the difference of opinion on this subject, Ntigeli * divided the Ilieracice found in Germany into three hundred species, Fries into one hundred and six, Koch into fifty-two, while other authors recognise hardly more than twenty. Nageli indeed says, " There is no genus of moi-e than four species on which all botanists are agreed, and many examples may be cited in which, since Linnreus' time, the same species have been repeatedly divided up and re-united." We are therefore driven, in order to determine the essential pro- perty distinguishing species and variety, to consider the most impor- tant characteristic for the conception of species, — a characteristic which has hardly ever been used in practice, i.e., the community of descent and the capacity for fruitful interbreeding. This means of determination, however, is also insufficient. It is a commonly known fact that animals which belong to different species pair with one another and pi'oduce hybrids, e.g., horse and ass, wolf and dog, fox and dog. Widely differing species, which are placed in different genei^a, have even been known to ci-oss with one another, and to produce progeny, svich as the he-goat and sheep, and the she-goat and ibex. The hybrids however are, as a rule, sterile. They are intermediate forms with imperfect generative system, with- out the power of propagation ; and even in those cases where there is a power of i-eproduction (such cases are most frequently met with iimongst female hybrids), there is a tendency to revert to the paternal or maternal species. There are, however, exceptions to the sterility of the hybrid which appear to afford weighty proof against immutability of species. The experiments in breeding between the hare and rabbit, made on a large scale in Angouleme by Roux, have shown that their progeny, the hare-rabbit, is perfectly fertile. Half-bred hybrids of the rabbit and hare have been bred, and have been reproductive through many generations of pure in-breeding. In like manner careful inquiries into the hybi'idism of plants, especially the investigations of W. Herbert, lead to the conclusion that many hybrids are as perfectly productive among themselves as genuine species. In a state of nature, too, hybrids of various kinds are found. Such hybrids have frequently been taken for independent species, and have been described as such (Tetrao medius, hybrid of Tetrao * C. Nageli, " Entstchung uud Begriff dcr naturliistorischen Art." Miinicli, 18C5. FEETILITY OF IIY13EIDS. 143 urogallus and Tetrao tetrix ; Ahramidojisis Leuckarti, Bliccopsis ahramorutilus, and others are, according to von Siebold, hybrids.) Sterility of hybrids is not the rule here, for a great number of wild plants have been recognised as hybrid species (Kulreuter, Gartner, Njigeli — Cirsium, Cytisus, Ruhus). This seems to render it the less doubtful that amongst animals which have been domesticated by man, persistent ti";\nsitional forms can be obtained from originally difterent species, by gradual alteration brought about by cross breeding. Pallas, adopting this view, gave it as his opinion that closely allied species, though at iirst they may refuse to breed together, or may produce sterile offspring, will, after long domestication, produce fertile progeny. And in fact, it has been shown to be probable that some of our domestic animals have originated in prehistoi-ic times as the result of the unintentional crossing of different species. Rlitimeyer especially endeavoured to prove this mode of origin for the domestic ox {Bos taurus), which he regarded as a new race resulting from the crossing of at least two ancestral forms [Bos 2}rimi(/enius, hrachyceros). It may be looked upon as certain that the domestic pig and cat, and the numerous breeds of dogs, have originated from sevei'al wild species. In connection with the exceptional cases which have just been discussed, we may lay great stress upon the perfect reproductive capabilities of mongrels, that is, of the progeny produced by the crossing of different varieties of the same species ; though here also we meet with exceptions. Disregarding those cases in which me- chanical causes render the interbreeding of different varieties im- possible, it seems, according to the observations of breeders whose word may be depended upon, that certain varieties have difficulty in crossing with one another ; and further that certain forms which have been bred by selection from a common stock are altogether in- capable of fertile intercourse. The domestic cat introduced into Paraguay from Europe has, according to Rengger, become essentially altered in process of time, and has acquired a marked aversion to the European ancestral form. The European guinea pig does not breed with the Brazilian form, from which it is probably descended. The Porto-Santo i-abbit, which was exported from Europe to Porto- Santo near Madeira in the fifteenth century, is so much altered that it can no longer breed with the Eui-opean race of rabbits. The evident difficulty of precisely defining the conception of species, in presence of the existence of a gradual, almost uninterrupted series 144 MEANING OF THE SYSTEM. of animal forms, and of the results of artificial selection, had alread}', in the beginning of this century, induced illustrious and highly esteemed naturalists to dispute the dominant views on the immuta- bility of species. In the year 1809, Lamark, in his " Philosojyhie Zoologique" broached the theory of the descent of species from one another. He referred the gradual alterations in some degree to changing conditions of life, but mainly to use and disuse of organs. Geoffrey St. Hilaire, too, the advocate of the idea of unity of organization of all animals and the opponent of Cuvier, expressed his conviction that species had not existed unaltered from the be- ginning. While agreeing essentially with Lamark's theory of the origin and transmutation of species, he ascribed a less influence to the inherent activity of the organism, and believed that he could explain the alterations thi'ough the direct operation of changes in the environment [nionde amhiant). The change in the fundamental views of Geology which took place at a later pei-iod must be ascribed to the opinions of these investigators. Lyell endeavoured (Principles of Geology) to explain geological alterations by means of the forces in operation at the present day, working gradually and without interruption through extended periods of time, and rejected the Cuvierian theory of mighty revolutions and catastrophes which destroyed all life. When the geologists with Lyell had given up the hypothesis of periodic disturbance of the course of natural events, they were obliged to assume the continuity of organic life during the successive periods of the formation of the earth, and to endeavour to account for the immense alterations of the organic world by slight influences operat- ing gradually and without interruption throughout long periods of time. The variability of species, the origin of new species from previous ancestral forms in the course of ages, has become, accord- ingly, since the time of Lyell, a necessary postulate of geology in order to explain naturally the differences of animals and plants in successive periods without the supposition of repeated acts of creation. THE TRAXSMUTATION THEORY, OR THEORY OF DESCENT, BASED ON THE PRINCIPLE OF NATURAL SELECTION (dARWINISm). Nevei-theless a more securely grounded theory based upon a firmer standpoint was needed in order to give more force to the Trans- lAutatioD hypothesis which had remained disregarded ; and this WATUEAL SELECTION. 145 service was effected by the English naturalist, Charles Darwin, who employed a mass of scientific material to found a theory of the origin and mutation of species. This theory, which is closely con- nected with the views of Lamark and Greoffroy and in harmony with Lyell's doctrines, has received an almost universal recognition, not only on account of the simplicity of its principle, but also because of the objective and convincing way in which his genius expounded it. Darwin * starts from the principle of heredity, according to which the characteristics of parents are transmitted to their off- spring. But side by side with this, there is an adaptation determined by the peculiar conditions of nourishment, a limited variability of form, without which individuals of like descent would be identical. While heredity tends to reproduce identical characteristics, individual variations appear in the descendants of the same species, and in this way modifications arise, which in their turn are submitted to the law of heredity. Cultivated plants and domestic animals, the individual forms of which vary far more than do those living in a state of natui-e, are especially disposed to alteration ; and capability of domestication is in reality nothing else than the capability of an organism to subordinate and adapt itself to altered conditions of nourishment and way of life. The so-called artificial hretding, by which man succeeds by judicious choice in cultivating in plants and animals definite properties cor- responding to his requirements, depends on the interaction of heredity and individual variation ; and it is probable that the numerous races of domestic animals were in this Avay bred unconsciously by man, just as in our own days large numbers of new varieties are bred by judi- cious choice of the male and female parents. Similar processes are also at work in natural life, calling into existence new alterations and varieties. Here also we find a selection which is occasioned by the struggle of organisms for existence, and may be called a natural selection. All plants and animals are engaged, as Decandolle and Lyell had asserted ten years previously, in a mutual struggle for existence among themselves and with extenial conditions. A plant has to fight against circumstances of climate, season, and soil ; and has also to compete for existence with other plants which, by their superabundant increase, endanger the possibility of its preservation. Plants serve as food for animals, which themselves are engaged in a mutual struggle with each other ; the carnivorous * Ch. Darwin. ' On the Origin of Species by means of Natural Selection," London, 1859. 10 Ii6 MEAXIXG OF THE SYSTEM. feeding very largely vipon herbivorous animals. Then again, all aro struggling to multiply in great numbers. Each organism produces far more descendants than can in general be preserved. With a definite degree of fertility, a corresponding amount of destruction must take place; for in the absence of the latter the number of individuals would so increase in geometrical progression that no locality would suffice for the sustenance of their progeny. If, on the contrary, the protection afforded by fertility, size, special organiza- tion, colour, etc., were removed, the species would soon vanish from the earth. Amongst the complex conditions and interactions of life, even the most remotely connected organisms struggle with each other for existence {e.g., the clover and the mice); but the most violent strife is waged between individuals of the same species, Avhich seek the same food and are exposed to the same dangers. In this strife it necessarily happens that those individuals which are placed in the most favourable situation, through their special propei-ties, have the greatest chance of maintaining themselves and of multiplying, and, in consequence, of reproducing the modifications useful to the species and of preserving them in their descendants, or even sometimes of increasing them. Just as in the breeding of domesticated animals, an artificial selec- tion is made by man to perpetuate and increase advantageous variations; so in the natvu-al breeding of wild animals, in conse- quence of the struggle for existence, a selection is made by nature which leads to the preservation of modifications useful to the species. Since, however, the struggle for existence in closely related forms must be the more violent the more nearly they resemble one another, the most divergent types will have the greatest chance of enduring and of producing descendants. Hence a divergence of characters and an extinction of intermexiiate forms is the necessary consequence. Thus by the combination of useful properties and by the accumula- tion of hereditary peculiarities, which Avere primitively of little im- portance, varieties gradually arise which ever diverge more and more ; and this is what Darwin sought to prove by happily chosen examples. We can now comprehend why everything in the organism is directed towards one end, which is to ensure existence in the most perfect way. The great series of phenomena xichich could hitherto only receive a teleological explanation are thus brought into causal relation, and can he exiilained as the inevitable result of efficient causes, and their natural connection is thus rendered intelligible. CAL'SE OF TAUIATIOXS. 147 This principle of natural selection, which is the basis of the Dar- winian theory, rests, on the one hand, on the interaction of adaptation and heredity, and on the other, on the struxjgle for existence which can be shown to occur everywhere in natni-e. In its fundamental idea the natural selection theory is essentially an application of the doctrine of Malthus to plants and animals. Developed simultaneously by Darwin and Wallace,''' it received from the former a most comprehensive scientific foundation. We must certainly admit that Darwin's selection theory, although supported by what we know of biological processes and of the opera- tion of the laws of nature, is very far from discovering the final causes and physical connection of the phenomena of adaptation and heredity, since it is unable to explain why such or such a variation should appear as the necessary consequence of a change in the vital conditions, and how it is that the manifold and wonderful phenomena of heredity are a function of organised matter. It is cleai'ly a great exaggeration when enthusiastic supporters of the Darwinian theory + say that it I'anks as equal to the gravitation theory of Newton, because "it is founded upon a single law, a single effective cause, namely, upon the interaction of adaptation and heredity." They overlook the fact that we have here only to do with the proof of a mechanical and causal connection between series of biological phenomena, and not in the remotest degree with a physical explanation. Even if we are justified in connecting the phenomena of adaptation with the pi'ocesses of nourishment, and in conceiving heredity as a physiological function of the organism, we still stand and regard these phenomena as " the savage who sees a ship for the first time." While the complicated phenomena of heredity:]: remain completely unintelligible, we are only in a position to explain in general terms certain modifications of organs, on physical grounds, by the altered conditions of metabolism. It is only rarely — as in the case of the operation of use and disuse — that we are able more directly to relate the development or the atrophy of organs to the increase or decrease in their nutritive activity, i.e., to give a chemico-physical explanation. Darwin has been unjustly reproached with having left chance to * Compare also A. R. Wallace, "' Contributions to the Theory of Xatural Selection." t Compare E. Haeckcl, " Xatiirliehe Schopfungsgcschichte. 4. Auflage. Berlin, 1873. X It is clearly a misuse of the word "Law" to represent the numerous Eartially opposed and limiting phenomena of heredity as so manj " laws of eredity," as E. Haeckel does. 143 MEANING OF THE SYSTEM. play a considerable part in his attempt to account for the origin of varieties, with having accounted for everything by the struggle for existence, and with having given too little prominence to the direct influence of physical action on the mutation of forms. This reproach seems to arise from a misapprehension. Darwin says himself that the expression " chance," which he often uses to explain the presence of any small alteration, is a totally incorrect expression, and is only used to express our complete ignorance of the physical reasons for such particular variation. If Darwin has by a series of considerations arrived at the conclu- sion that the conditions of life, such as climate, nourishment, etc., exercise but a small dii-ect influence upon variability, since, for in- stance, the same varieties have arisen under the most different conditions, and different varieties vinder the same conditions, and that the complex adaptation of organism to organism cannot be produced by such influences, still he recognises in the altei^ation of the vital conditions and the mode of nourishment the primary cause of slight modifications of structure. But it is only natural selection which accuiaidates those alterations, so that they become appreciable to us and constitute a variation Avhich is evident to our senses. It is exactly upon the intimate connection of direct physical action with the consequences of natural selection that the strength of the Dar- winian theory rests. The origin of varieties and races would appear, however, to consti- tute only the first stage in the processes of the continual changes of organisms. However slowly the process of selection may work, yet there is no limit to the extent and magnitude of the changes, or to the endless combinations of reciprocal adaptations of living beings if we allow a very long period of time for its operation. With the aid of this new factor of duration of time, which, according to geo- logical facts, cannot be rejected, but stands to an unlimited extent at our service, the gap betAveen variety and species disappears. Since ^ the former are continually diverging with the lapse of time — and the t more they do so and become differentiated in their organization so much the better will they be fitted to fill different places in the eco- nomy of natu,re and to increase in number — they at length attain the value of species, which in a state of nature do not interbreed, or, at any rate, only exceptionally produce progeny. Thus, according to Darwin, a variety is a species in process of formation. Variety and species are connected by continuous series of transitions, and are not absolutely distinct from one another; but are only relatively separated PEOGEESSING BIVEEGEXCE OP ClIAUACTEliS. 149 by the amount of diffei-ence in tlieir morphological and physiological characteristics. This conclusion of Darwin's, which extends the result of natural selection from the production of variety to that of species, is ob- stinately and often bitterly opposed by those who subordinate the phenomena of nature to traditional ideas. Even if they do not deny the facts of variability, and even admit the influence of natural selection on the formation of natural varieties, they yet continue true to the belief that there is an absolute separa- tion between species and race-variety. As a matter of fact, however, we are not in a position to draw such a line of separation. Neither the quality of the distinctive characteristics nor the results of cross- ing aftbid us a distinctive criteiion between species and variety. The fact, however, that loe are not able to give any satisfactory defini- tion of the concejjtion of species, precisely hecatose ice are unable clearly to distinguish heticeen sjiecies and variety, adds so much the more weight to Darwin's argument, since neither the variability of the organism and the struggle for existence nor the great antiquity of life upon the globe can be contested. The variability of foi-ms is a firmly established fact ; so, too, is the struggle for existence. Now if we add the operations of natural selection to these two factors, we are able to understand the origin of varieties. If we imagine the same process which has led to the formation of varieties continued through a greater number of genera- tions and effective through a longer period of time- — and we are the more justified in making use of these long periods of time, since with their help astronomy and geology have been enabled to explain many phenomena — the diverging characteristics will become more and more marked, and will at last gain the value of distinctive species- characters. In still greater periods of time the species will become so far separated from one another by the simultaneous disappearance of the intermediate foi'ms that they will represent different genera. Accordingly the greater differences of organization which are ex- pressed in the higher divisions of the system, such as orders and sub-orders, etc., require a longer interval of time for their accom- plishment, and an extinction of a greater number of intermediate forms. Finally, the different ancestral forms of the classes of a group may be referred to a common starting-point ; and since the different groups of animals are connected by many intermediate forms, the number of the ancestral forms becomes much reduced. 150 MEAXIXa OF THE SYSTEM!. Tlie imdifFerentiated contractile substance, sarcode or protoplasm, was probably the starting-point of all organic life. If these suppositions are cori-ect, species no longer retain the signifi- cation of independent and imviutahle units, and appear, according to the great law of evolution, only as transient groups of forms, capable of change, and confined to longer or shorter periods of time, to definite conditions of life, and preserving, as long as these conditions do not vai-y, a constancy in tlieir essential characters. The difFei'ent categories of the system show the closer or more remote degree of relationship ; and the system is the expression of genealogical relationship founded upon descent. All systems, however, must be imperfect and full of gaps, since the extinct ancestors of organisms living at the present time can only be very imperfectly supplied by the geological record ; numerous intermediate forms are wanting and finally no traces of organic remains from the most ancient periods are preserved. Only the ultimate twigs of the enormously ramified ancestral tree are accessible to us in suflicient nvimber. Only the extreme points of the twigs are completely preserved ; while of the numerous rami- fications of the branches only the existence of a stump here and there has been demonstrated. Hence it appears quite impossible, in the present state of our knowledge, to attain to a sufiiciently sure representation of this natui-al genealogical tree of organisms ; and while we admire the bold speculations of E. Haeckel's genealogical attempts, it must be admitted that at present there is room for inxiumerable possibilities in detail, and that subjective judgment holds a more conspicvious place than objective certainty of fact. Hence we must be contented for the present with an incomplete and more or less artificial arrangement ; although the concej)tion of the natural system theoretically is established. When the fundamental arguments of the Darwinian theory of selection and the transmutation theory founded upon it are submitted to ciiticism, it is soon apparent that direct proof by investigation is now, and perhaps always will be, impossible; for the theory is founded upon postulates which cannot be submitted to direct inquiry. Periods of time which cannot be brought within the limits of human observation are required for the altei-ation of forms under natural conditions of life ; and the extremely complicated interactions, which in the natural state under the form of natural selection are tending to change plants and animals, can only be grasped in a general sense, while in their details they are practically unknown to us. Further, plants and animals which are under the influence of ETIDEXCE FROM MOIlPIIOLOaY. 151 natural selection are entirely inaccessible to the experiments of man, and the relatively few forms which man has, in a greater or less space of time, brought completely within his power, have been and are being altered and modified by the so-called artificial selection. The action of the natural selection, in Darwin's sense, is therefore in general incapable of dii-ect piwof, and even for the origin of vai-ieties can only be illustrated and rendered probable by hypothe- tical examples. Against this we must, however, set the fact that there is a great probability in favour of the correctness (jf the theories of descent and transmutation of species, which have never received better support than from the natural selection theory of Darwin ; and that this probability is supported, not only by the whole Aveight of morphological evidence, but also by the testimony of Palaeontology and of geographical distribution. EVIDENCE IN FAVOUR OF THE THEORY OF DESCENT. If the transmutation of species is to be regarded as an hypothesis, because it is incapable of being demonstrated by direct observation, then its value depends upon its correspondence with the facts and phenomena of natui'e. Evidence from Morphology. — The whole of Morjyhology tends to show the correctness of the theory of transmutation of species. The degrees of resemblance between species which was for a long time expressed by the metaphorical term " relationshij),'" and which rested upon an agreement in more or less important characteristics, led to the establishment of systematic groups, of which the highest, the kingdom or type, was founded upon a similarity in the most general features of organization and development. The agreement of numerous animals in the general plan of their organization, e.g., the common possession by fishes, reptiles, birds, and mammals of a rigid column forming the axis of the body, and the dorsal position in regard to this of the central nervous system and the ventral positioxi of the organs of nourishment and reproduction, are very well explained, according to the theories of selection and descent, by the derivation of all Vertebrates from a common ancestor possessing the characteristics of the type, while the supposition of a plan of the Creator renounces all explanation. In like manner is explained that similarity of characteristics by which the remaining gi-oups and sub-groups, from class to genus, are distinguished, as well as the possibility of dividing all organized beings into groups subordinated the one to the other. 152 MEA>TNG OF THE SYSTEM. The impossibility of a sharply defined classification is also rendered comprehensible by the theory of descent. The theory requires the existence of forms transitional between intimately and remotely allied groups ; and explains, as a result of the disappearance, in course of time, of numerous types which have been worsted in the struggle for existence, the fact that groups of equal value are of such various extent, and are often only represented by single forms. It is not only systematic characters, but also the innumerable facts brought to light by the science of Comparative Anatomy which point to a nearer or more remote relationship between the different groups. For example, if we examine the structui-e of the extremities or the brain of Vertebrates, we find, in ,spite of considerable differ- ences (sometimes bridged over by intermediate forms) in the various groups, that in all they are built upon a common type of struc- ture. This type is found very variously modified and more or less differentiated in each secondary group, according to the different functions which the organ has to fulfil and according to the exigencies of the mode of life to which each species is subjected. In the fin of the whale, in the wing of the bird, in the anterior limb of the quadruped, and in the human arm it can be shown that there are present the same bones, here short and broad and immoveably con- nected, there elongated and jointed in different ways to allow of corresponding movements, sometimes with every part fully developed, sometimes simplified in one way or another, and partly or entirely rudimentary. Evidence from the facts of Dimorphism and Polymorphism.— The phenomena of dimorphism and polymorphism in the same species, and the sexual differences which have been developed in animals originally heimaphrodite, may be quoted as important evi- dence of the extensive influence of adaptation. Male and female forms differ not only in the fact that the former produce spermatozoa and the latter ova, but they exliibit numerous secondary sexual characteristics connected with the different func- tions which the male and female respectively have to perform. The existence of these secondai-y characteristics can in all cases be satisfactorily explained by means of natural selection. We may therefore, in a certain sense, speak of a sexual selection * by means of which the two sexes have been, in course of time, gradually sepa- rated from one another, not only in peculiarities of form and organiza- * Ch. Darwin. " The Descent of Man, and. Selection in Relation to Sex," Vol. i. and II. London 1871. EVIDENCE FROM DIMORPniSM. ] 53 tion, but also in habits of life, in such a way as to favour the preservation of the race. Since the male sex geneially has to take a more active part in the acts of copulation and fertilization it is comprehensible that the male form should differ more from the young than the female which supplies material for the formation and nourishment of the embiyo and is charged with the care of the progeny. Very frequently the male sex is capable of quicker and more facile movements ; in many Insects the male alone has the power of flight, Avhile the female remains without wings (fig. 97). In the strife which the males of similar species have to wage for the possession of the females, those individuals which are most favoured by their organization (in respect of strength, capability for motion, prehensile organs, beauty, organs for production of sound, etc.) will prove the conquerors ; while those females which possess properties especially favourable to the prosperity of the offspring will best fulfil their task. At the same time variations in the duration of development, in the mode of growth and structure, may in a more passive way be favourable under the special conditions of life of the species. The secondary sexual characters may sometimes acquire such importance as to lead to essential and deeply engrained modification of the organism, and to a true sexual dimorphism (males of Roiifera with no digestive tube, dwarfed males of Bonellia, Trichosomum crassicauda). It is a significant fact that dimorphism of sex reaches its highest extreme in parasites. In many parasitic Crustacea (S'i])honostoma) such extreme cases, in which the large shapeless females have lost the organs of sense and locomotion, and even segmentation, while the males are small and dwarfed, are connected by numerous inter- mediate forms; and the circumstances which have operated as the cause of this sexual dimorphism are not far to seek. The influence of favourable conditions of nourishment which parasites enjoy does away with the necessity of rapid and frequent locomotion, increases in the female the capacity of producing reproductive material, and brings abovit such an alteration of form that the power of locomotion is diminished and the organs of movement atrophy and may com- pletely vanish. The body acquires an unwieldy, shapeless character in consequence of the enormous size of the ovary which is filled with eggs, and throws out outgrowths and processes into which the ovaries project, or else acquires an unsymmetrical saclike form. The seg- mentation is lost and the limbs degenerate ; the slender moveable abdomen which, when the animal was free-swimming, was an essen- 154 MEAXIXG OF THE SYSTEM. tial aid to locomotion, is reduced more and more till it becomes a short, unsegmented stump. The appearance of such a parasite is so strange that one can easily comprehend how it was that formerly- one of these abnormal gioups, the Lerncem, was placed among the endoparasitic Worms, or even among the Mollusca, The more the female remains behind the type of its fully-developed, free-living allies, so much the more do the two i-excs become morpho- logically remote from one another, for the form and organization of the male also are affected by the changed conditions of life, but in a different manner.''' In the male sex the more favourable and abundant nourishment may not affect the necessity of locomotion and the development of the locomotive organs in so direct a manner, since the sexual activity of the male and the necessity for locomotion in order to select a female remain unaltered. Even when locomo- tion is reduced and rendered difficult, parasitism does not, in the case of the male, lead either to a complete loss of segmentation or to such unsymmeti'ical growths as we observe in many female parasitic Crus- tacea. The large quantity of generative material produced, which in the female is of the greatest importance for the preservation of the species, and which therefore favours the development of a large, shapeless, unwieldy body, is the less conspicuous in the male because a very small quantity of sperm serves for the fertilization of an enormous number of ova. Thus, then, the extreme degi-ee of parasitism in the male, even when accompanied by a confined and more creeping mode of loco- motion, does not lead to an excessive increase in size nor produce an unsegmented and strange form of body, but, on the contrary, gives rise to the symmetrically formed, dwarfed pigm.nean males. This extreme state is, however, connected with the normal state by numerous intermediate steps. Thus we find in the Lerna^opods that the size of the male Actkeres is only slightly reduced, while the true dwarfed males of the Lerna'ojjoda and Chondracanthidce are attached, liljie small parasites (fig. 98), to the posterior end of the female body, which is relatively enormously large. The preparation of a large amount of sperm which implies the possession of a large body, would only be a useless expenditure of material and time in the life of the species, and this must have been avoided by the influence of natural selection. In addition to this sexual dimorphism we find in various groups of animals — especially in the insects which live together in great * Compare C. C'laus, "Die frcilebcnden Copcpodcn." 1863. ETIDE>'CE FKOil MIMICET. 155 societies, the so-called animal communities — a third group of indi- viduals (sometimes even divided into several series of forms) which are "without generative organs and are incapable of repi-oduction, but Avhich assume the functions of protecting, of providing nourish- ment for the community, and of caring for the young. Adaptive peculiarities suitable for the discharge of these functions are apparent in their structure and organization. These sterile indivi- duals are in the Hymcnojytera aborted females. Among the ants they are divided into workers and soldiers. Amongst the Termites they are derived from both males and females, in which the genera- tive organs are reduced. Sterile individuals are also found amongst animals (Fishes) which do not form communities, and were formei-ly taken for particular species and described as such. Polymorphism i-s most highly developed in the Hydroids which are united in stocks — the Siphmiojyhoi'a. The numei'ous cases of dimorphism and polymorphism in either sex of the same species, shovild be regarded from the same point of view. Dimorphic females among insects have been observed, e.g., in the Malayan Pcqnlionidce (P. Memnon, Pamnon, Ormenus), in cer- tain species of Hydroporus and Dytiscus, as also in the Neurotemis, a genus of the Neurojitera. In these, cases, as a rule, one of the female forms is more nearly related in form and colour to the male form whose peculiarities it has assumed. In other cases the differences are more connected with climate and season (seasonal dimorphism of butterflies), and also affect the male animal. They may be connected with the difterent forms of reproduction (parthen- ogenesis), and lead to the phenomenon of heterogamy {Chervies Phylloxera, Aphis). Much more rarely we find two kinds of males ■vvith dissimilar secondary sexual characters connected with copula- tion, as in the case of the *' smellers " and " claspers " * described by Fritz Muller in the Isopoda {Taiiais dicbiits). Evidence from Mimicry. — Another series of phenomena which may probably be referred to useful adaptation is the so-called mimicry. Certain animal forms come to resemble other widely- distributed species, which are protected by any peculiarity of form and colour, so closely that they seem to have copied them. The cases of mimicry_ which have been piincipally made known by Bates and "Wallace are directly connected with the protective resemblances mentioned above ; that is, the resemblance of many animals in colour and body shape to the objects amongst which they * Fritz Muller, " Facts for Darwin," p. 22. 156 MEAXIXa OF THE SYSTEM:. live. For example, amongst the butterflies certain Lejytalidce resemble in outward appearance and in mode of flight a species of the family Ileliconius (fig, 116), which appears to be protected from the pursuit of birds and lizards by a yellow disagi-eeable-smelling fluid, and share the same locality with the above-mentioned species. The most perfect instances of mimicry are found in the Tropics of the Old World, where the Danaidce and Acneidce are imitated by the Papilionidaj {Danais niavius, Papilio hippocoon — Danais echeria, Papilio cenea — Acrcea gea, Panopcea hirce). Cases of mimicry fre- quently occur between insects of difierent orders ; butterflies imitate the form of Hymenoptera, which are protected by the possession of stings [Sesia homhtjliformis — Bomhus hortorum, etc.) In the same way certain beetles resemble bees and wasps (Charis melipona, Odontocera odyneroides), and the Orthopteran genus Con- dijlodera tricondi/Ioides from the Philippines is like a genus of Cicindehe [Tricondyla). Numerous Diptera have the form and colour of stinging Spthegidce and Wasps. Also among Vertebrates (Serpents and Bii'ds) some examples of mimicry are known. Evidence from Rudimen- tary Organs. — Rudimentary organs, too, which are so common, are satisfactorily ex- plained by the theory of selec- tion as the result of non- employment of such organs. Organs which were formerly functional have gradually or even suddenly become functionless as a result of adaptation to special conditions of life, and, through want of exercise, have, after the lapse of generations, become weaker and finally aborted or degrade'IN(} OF THE SYSTEM. that of the adult ; and we can thus understand how larvfe of many insects belonging to different orders can present great resemblances to one another and be unlike the larvas of insects of the same order. While as a general rule the development of the individual is an advance from a simpler and lower organization to one more complex which has become more perfect by a continued division of labour among its parts — and we shall later find a parallel to this law of perfection of the individual in the great law of progressive perfection in the development of groups — yet the course of development may, in particular cases, lead to numerous retrogressions, so that We may find the adult animal to be of lower organization than the larva. This phenomenon, which is known as retrogressive metamorplios'is {Cirripedia and parasiiic Crustacea), corresponds to the demands of the selection theory, since under more simple conditions of life, where nourishment is more easily obtained (parasitism), degradation and even the loss of parts may be of advantage to the organism. Again, the facts of embryonic development, when considered in relation to the gradations expressed in the system are in complete accord with the theory of evolution. Numerous examples may be cited to prove that features, not only of the simple and more pi'imitive, but also of the more perfectly organised groups of the same type, are reflected in the successive phases of fcetal life. In the case of a complicated free development by metamorphosis, wdiich is usually correlated with an unusual simplification of the fcetal development within the egg- membi^anes, the relation of the successive larval stages to the alHed smaller groups of the system, to the genera, families and orders, is more direct and striking. For example, in the early stages of the embryonic development of mammals certain structures occur, which in the lower fishes endure throughout life. Later stages show peculiarities which correspond to the characters of amphibia. The metamorphosis of the frog begins with a stage which in form and organization and mode of locomotion agrees with the fish type ; and this stage is succeeded by numerous other larval stages in which the characters of the other orders of Amphibia (Pei-enni- branchiata, Salamandrinidos) and of individual families and genera of the same are repeated. This undeniable likeness between the successive stages of individual development and between allied groups of the system allows us to institute a parallel between the former and the evolution of the species. The evolution of the species finds, it is true, a most imper- fect expression in the relationship of the systematic groups, and can GEOOKAPHICAL DISTRIBUTIO.V. 159 only bo inferred from the history of the past for which paUeon- tology affords us but slight materiah Tliis parallel, which naturally piesents numerous greater or smaller variations in detail, is explained by the theory of evolution, according to which the developmental history of the individual appears to he a short and simjylijled repetition, or in a certain sense a recajntiolation, of the course of development of the species/'' The historical record preserved in the developmental history of the individvial must often be more or less blurred and obscure on account of the many adaptations which have occurred during the embryonic development, or during larval life. Especially in those cases where the peculiar conditions of the struggle for existence demand a simplification, the development will take a more direct course from the ovum to the perfect animal, will be thrown back into an earlier period of life, and finally will be completed before the animal is hatched, until, in absence of a metamorphosis, the historical record is completely suppressed. On the contrary, in the cases of progres- sive transformation where the larval states are gradually modified and live under similar conditions of life, the history of the species will be less impei'fectly reproduced in that of the individual. Evidence from the Facts of Geographical Distribution. — Unlike the facts of moi-phology, those of geograjyhical distrihidion raise great difficulties for the theory principally because the phenomena are very complicated and our experiences are still too limited to permit of our establishing general laws. The present distribution of plants and animals over the surface of the earth is clearly the combined result of the earlier distribution of their ancestors and of the geologi- cal changes Avhich have since taken place, the modifications in the extent and position of land and water, which must have had an influence on the fauna and flora. Accordingly the geographical distribution of plants and animals f appears intimately connected with that part of geology which has for its aim the investigation of the most recent occurrences in the formation of the earth's crust and its contents. It cannot, therefore, be confined to an examination of the areas of distribution of the animals and plants of the present day, but must take cogni- zance of the distribution of the remains, enclosed in the most recent formations, of the nearest relations and ancestors of living forms, in *■ Fr. Miiller. "Fiir Darwin,"' Leipzig, 1804. t A. E. Wallace. " The Geographical Distribution cf Animals," London, 1876, P. L. Sclater, " Address to the Biological Section of the Brit. Association,' 1875, 160 MEAXIXa OF THE SYSTEM. order to find an historical explanation of the known facts of distribu- tion. Although in this sense the science of animal geography is still in its infancy, yet numerous and important phenomena of geographical distribution receive a satisfactory explanation according to the theory of transmutation of species on the supposition of migrations and gradual changes brought about by natural selection. It is a most important fact that neither the resemblance nor the want of resemblance of the animals inhabiting different localities can be completely explained as the result of climatic and physical conditions. Closely allied species of plants and animals often appear under very different natui-al conditions, while a completely different fauna and flora can exist in a similar climate and on a similar soil. On the other hand, the extent of the difference between two fauna is closely connected with the limitations of space and the barriers and hindrances to free migration. The Old and New Worlds, which, leaving out of consideration the polar connection, are completely separated, have in part a very different fauna and flora, although with regard to the climatic and physical conditions of existence there are innumerable parallels which would equally favour the prosperity of the same species. In particular if we compare the districts of South America with regions situated in the same latitude and possessing the same climate in South Africa and Australia, we find three faunas and floras Avhich differ considerably, while the natural productions from different latitudes of South America with entirely different climates are closely allied. Here the northern animals are indeed specifically different from the southern, but belong to similar or nearly allied genera with the peculiar stamp characteristic of South America. Zoological Provinces. — The surface of the earth can be divided into from six to eight regions according to the general features of the terrestrial and fiesh-water fauna. These regions can indeed only be considered as a relative expression for large natural districts of distribution, since they cannot be applied to all groups of animals in the same manner, and it is impossible that they should differ in like degree and in the same direction. There must also be inter- mediate regions combining the characteristics of the neighbouring regions with peculiarities of their own ; and the question must arise whether these should not be taken as independent regions. The merit of having first established a natural division of the earth into zoologi&il regions and svib-regions belongs to Sclater. This naturalist founded his system on the distribution of birds, and dis- ZOOLOGICAL PEOYINCES. 161 tinguished six regions, the limits of which agreed fairly well witii the distribution of Mammalia and Eeptilia. These regions ai'e — (1) The Palcearctic Reyion — -Europe, the temperate part of Asia, and North Africii as far as Mount Atlas. (2) Nearctic Region — Greenland and North America as far as North Mexico. (3) The Ethiopian Region — Africa, south of Atlas, Madagascar, and the Mascarenes with South Ai-abia. (4) The Indian Region — India south of the Himalayas, to South China, Borneo and Java. (5) The Australian Reyion — Celebes and Lombok eastward to Australia, and the South Sea Islands. (6) The Neotropical Reyion — South America, the Antilles, and South Mexico. Other naturalists (Huxley) have since shown that the four first of these regions have a much greater resemblance to one another than any one of them has to the Australian or South American regions; that New Zealand is entitled by the peculiarities of its fauna to be considered as forming a region by itself ; finally, that a circumpolar* province should be formed equal in value to the Palse- arctic and Nearctic. Wallace objects to the establishment either of a New Zealand or of a circumpolar reyion, and advocates the adoption of the six regions of Sclater on practical grounds, but suggests the modification that since the South American and Australian are much more isolated, the regions shou.ld not be of equal value. These regions are bounded by extended seas, lofty mountain ranges, or vast sandy deserts, and obviously such boundaries do not constitute effective barriers to the migration of all animals, but allow certain groups to pass from one region to another. The obstacles to immigration and emigration appear in certain places, at all events in the present time, to be insurmountable; * Andrew Murray, on the contrary, in his work on the geographical dis- tribution of Mammalia in 1866, distinguishes only four divisions — the PalEearctic, Indo-African, the Australian, and the American. EUtimeyer recognises in addi- tion to the six provinces of iSclater a Mediterranean and Circumpolar province. J. A. Allen ("Bulletin of the Museum of Comparative Zoology, Cambridge," vol. ii.) proposes to distinguish eight regions, in connection with " the law of circumpolar distribution of life in zones : " — (1) Arctic realm ; (2) North Temper- ate realm; (3) Tropical American realm ; (4) Indo-African Tropical realm; (6) 'Tropical South American realm ; (6) Temperate African realm ; (7) Ant- arctic realm ; (8) Australian realm. n 162 ]iIEA>'IXG OF THJE SYSTEM. but in past ages, when the di\'isions of land and water were different, they must have been, for many forms of life, easily surmountable. The expression "centre of creation," which has long been used in the sense of a tolerably defined district of dis- tribvition — or better still, Riitimeyer's word, "centre of distribution" — has as a fundamental idea the endemic appearance of definite groups of typical species and their gradual extension * towards the boundaries of the said region, a conception which harmonizes well with the theory of the origin of species through gradual alterations. The same laws apply also to the distribution of the inhabitants of the sea. Great seas studded with islands which serve to confine the land animals may favour the migration of marine species, while extended continents, which allow their inhabitants to wander freely over them, confine the sea animals within limits which cannot be passed. A great number of sea animals live only in the shallow water round the coast, and their distribution thus often coincides with that of the land animals ; whereas the animals found on the opposite coasts of great continents are very different. For example, the sea animals of the east and west coasts of South and Centi^al America differ to such a degree that, with the exception of a series of fishes, which, according to Giinther, are found on both sides of the Isthmus of Panav a, only a few forms are common lo the two coasts. In the same way we find that the marine inhabitants of the east insular district of the Pacific differ completely from those of the west coast of South America. If, however, we advance to the west of this part of the Pacific till we come to the coast of Africa in the other hemisphere, we find that the fauna of this extensive district cannot be so sharply distinguished. Many species of fish are found from the Pacific to the Indian Ocean. Numerous Mollusca of the South Sea Islands live also on the east coast of Africa, almost beneath the opposite meridian. In this case the limits of distribu- tion are not impassable, as numerous islands and coasts afford a rest- ing place to wandering inhabitants of the sea. In respect of the different haunts of the inhabitants of the sea, we must make a dis- tinction between the littoral animals, which are distributed along the coasts, and live under different conditions and at different depths on the bottom of the sea, and the pelagic animals, which swim on the suiiace. ♦ Compare Riitimej'er's Essay, " Ueber die Herkanft unserer Thierwelt." Basel and Genf. 1867. EVIDENCi; FltOM PALEONTOLOGY. 163 But there also exists, jit considerable depths and on the bottom of the sea, a rich and varied animal life. This has only lately been brought to our knowledge principally by the deep-sea explorations from North Ameiica, Scandinavia, and England. In place of that want of animal life which we should on a priori grounds expect to find, we see that numerous lowly org»inised animals of the most different groups are able to exist even at the greatest depths. Besides the lowest sarcode animals of the Foraminifera (Globigerina ooze), we find especially silicious sponges, certain corals, Echinoderms, and Crustacea* The representatives of the latter are in part of low type, but gigantic, and many of them blind. It is also a fact of more than ordinary interest, as showing the continuity of living creatures from successive geological forma- tions up to the present time, that the deep sea animals are allied to ancient types which occur in Mesozoic formations, especially in chalk. Evidence from Palaeontology. — The results of geological and palceontological inquiry give us a third great series of facts in support of the theory of slow alterations of species and the gradual development of genera, families, orders, etc. The firm crust of our earth is formed of numerous and enormous rock strata, which have been deposited in a definite series by water in course of time, and also of the so-called volcanic or plutonic rocks, masses which have been forcibly ejected from the molten interior of the earth. The former or sedimentary deposits, which have under- gone numerous alterations in the originally horizontal arrangement of their strata as well as in the petrographical condition of their rocks, contain a quantity of the fossilized remains of former plants and animals which have become buried in them, and thus afford an historical record of a lich fauna and flora which existed during the earlier periods of the earth's development. Although these so-called fossils have made us acquainted with a very considerable number of ancient organisms presenting great diversity of form, yet they only constitute a very small portion of the enormous quantity of living beings which have at all times existed upon the earth. They suffice, however, to teach us that a different fauna and flora existed at the time when each individual deposit was being formed, and that * Compare Wyville Thomson, " The depths of the sea. An account of the general results of the dredging cruizes of the Porcupine and Lightning, during the summer months of 1868, 1869, 1870." London, 1873. Also the results of the Ckallenger expedition 1874-1876. 16-i AlEANING OF THE SYSTEM. the deeper a stratum comes in the series, that is, tlie earlier it appears in the history of the earth, so much the more its fauna and flora differ from those of the pret^ent time. The more nearly one stratum follows another in the series, the closer the relationship between their respective fossils. Every sedimentary formation possesses characteristic fossils which appear very frequently ; and from these, taking into account the succession of strata and the petrograjjliic characters of the rocks, the place occupied by the stratum in the geological system can be defined with tolerable accuracy. Without doubt the characters of the fossils and the relative posi- tions of the strata are the most important aids to the determination of the geological age of the deposit ; at any rate they furnish a more reliable cx^iterion than does the structure of the rocks. The idea entertained in earlier times that rocks of the same period always possessed a similar, and rocks of a difiei'ent period a dissimilar structure, has lately been given up as erroneous. Stratified or sedimentary deposits have arisen in every period vinder similar condi- tions. In past times, as at the present time, they were caused by the deposition of clay, of fine or coarse sand, of fine and coarse debris, by chemical precipitation of carbonates and sulphates of lime and magnesia, of silica and oxide of iron, and by accumulation of solid animal and vegetable remains. These have become transformed only in course of time into such hard rocks as argillaceous and calcareous schists, limestone, sandstone, dolomite, and conglomerates of many kinds ; as the result of many causes, such as mechanical pressure of superincumbent masses, increase of temperature, internal chemical processes, and so forth. Even though the peculiar structure of rocks may in many cases afford good ground for conjecture as to the relative age, yet it is certain that deposits of similar age may show an entirely different petrographical character ; and, on the other hand, that deposits of very different ages may have given rise to rock forma- tions that can be scarcely or not at all distinguished from one another. The old idea that deposits of the same age must everywhere contain the same fossils, could only be maintained as long as geological inves- tigations were confined to small distiicts. Similarly the idea, closely connected with the former, that the various geological formations, characterised by a series of definite strata, are entirely independent of one another, no longer obtains credit. The various forma- GEOLOGICAL PERIODS. 165 tions,* as the group of strata of one district of distribution and belong- ing to one period are named, cannot be divided petrographically or * The following table may serve for a bird's-eye view of the geological periods and their most important formations : — QUARTIARY PERIOD i^'f^ZJo^'^' (aHuvium, marine and fresh-water (Diluvial and Alluvial < ^^ , -,. ^., Formations). ] ^'^^^P^'^'""^'^" or Dihtvial IWiod (erratic boulders, \ glacial period). Pliocene Period (subappeninc formations, bone sand of Eppelsheim, etc.) Miocene Period (Molasse, Tegel near Vienna, brown coal in North Germany, etc). Eocene Period i ^^^^""^^ Nummulitc formation ( of the Paris basin, plaestricht strata, white chalk, •^ upper green sand. Gault, ^ lower green sand, Weald. IPurbeck strata, Portland stone, Kimmeridge clay. Coral Rag, Oxford clay, Great oolite. Lower oolite, Lias (white, brown, and black jura). I Kcuper or upper new red sand- 1 stone, Muschelkalk (upper "{ Muschelkalk, gypsum and i anhydrite, Wellenkalk, Bun- L ter Sandstein). iZechstein, Rothliegcndes. — lower new red sandstone. TERTIARY PERIOD (Cainozoic Formations). SECONDARY PERIOD {Mesozoic Formation). SECONDARY PERIOD ( Mesozolc Formations). Cretaceous Period Jurassic Pe Triassic Period Permian PALEOZOIC PERIOD {Palceozoie Formationx) 'A Carhoniferoiis Period Coal Measures of England, Germany, and North America, Kulmformation, Carboniferous limestone. Devonia7i Period (Spiriferenschiefer, Cypridinen- schiefer, Stryngocephalenkalk, etc. — old red sand- stone.) Silurian Period (Ludlow, Wenlock, strata, etc.) Camhrian Period (slate, etc.) ( Thonschiefer, Laurentian formations. Mica schist, ( Older Gneiss formations. According to Professor Earn say the groups of formations in England have a thickness of 72,-584 feet, i.e., about 13| Englishmiles ; that is, formations of the — Palaeozoic period have a thickness of 57.154 "I Secondary ., „ 13',1 90 1 72,584 feet Tertiary „ „ 2,2^0 J ARCH^AN PERIOD 166 MEANING OF THE STSTEM. palaeontologically from each other in such a manner as to lend support to the hypothesis of sudden and mighty revolutions and catastrophes destroying the whole living world. We may rather assert with cer- tainty, that the extinction of old species and the appearance of new ones has not taken place at the same time at all points of the surface of the earth, for many species extend from one formation into another, and a number of organisms persist from the tertiary period to the present time, but little altered or even identical. Just as the commencement of the recent epoch is hard to define, and cannot be sharply separated from the diluvial period by the character either of its deposits or of its fossils, so it is with the remoter periods of the earth's history, which are founded, like periods of human history, upon great and important occurrences, and yet are in direct con- tinuity. Lyell has proved in a convincing way on geological grounds that there were no sudden revolutions extending over the whole surface of the earth, but that changes took place slowly, and were confined * to separate localities; in other words, that the past history of the earth consists essentially of a gradual process of development, in which the numerous forces which may be observed in action at the present day have, by their long continued operation, had an enormous total effect in transfoi-ming the earth's surface. The reason for the irregiilar development of strata and for the limitations of formations is principally to be sought in the interrup- tion of depositions, which, though >videly distributed, were only of local importance. Were it possible that a single basin of the sea should have persisted during the whole period of sedimentary forma- tion and under singularly favourable circumstances have formed new deposits in persistent continuity, then we should find a progres- sive series of strata interrupted by no gaps, which we should be unable to classify according to formations. Such an ideal basin would include only a single formation, in Avhich we should find representatives of all the other formations of the surface of the earth. * " Every sedimentary formation was extended at the time of deposition over a confined territory, — confined on the one hand by the extent of the sea or fresh- water basin, and on the other by the different conditions favourable to the depo- sition inside the basin. At the same time, in other places entirely or at any rate somewhat differently stratified formations (i.e., formations of the same age, but of different composition) resulted. Thus marine, fi-esh-water, and swamp formations have been deposited at the same time from different rocks and with different fossils, while the land surface has remained free." Comp. B. Cotta, " Die Geologic der Ge»enwart." GAPS IN TlIK GEOLOGICAL EECOED. 167 In reality this ideal continuous series of strata is interrupted by numerous and often large gaps, which determine the petrographical and paheoutological ditferences, often strongly marked, between successive strata, and correspond to periods of inactivity, or, as may happen, to periods when the results of sedimentary action have been again destroyed. These interruptions of local deposits are explained by the constant alterations of level which the surface of the earth has undergone in every period in consequence of the reaction of the molten contents of the earth against its firm crust. As we see in the present time that wide tracts of country are gradually sinking (west coast of Greenland, coral islands), while others are being slowly elevated (west coast of South America, Sweden) ; that strips of coast line are suddenly submerged beneath the sea by subterranean forces, and that islands as suddenly appear; so it was in earlier periods. Elevation and depression were at work, perhaps uninterruptedly, causing a gradual, more rarely a sudden (and then locally confined) interchange between land and sea. Basins of the sea rising with gradual movement became dry land and i-ose up first as islands, and afterwards as connected continents, the different deposits of which, with their included fossils, bear witness of the sea which onee covered them. On the other hand, great continents sank beneath the sea, leaving perhaps their highest moun- tain peaks appearing as islands, and again became the seat of fresh deposition of strata. In the first case there would be an interruption of deposit, while in the latter there wovild result, after a longer or shorter period of inactivity, the beginning of a new formation. Since, however, elevations and depressions, even though affecting districts of great extent, mu.st always be locally confined, the commencement and interruption of formations of equal age have not taken place every- where at the same time. Deposits continued a long time on one tract after they had ceased on another; hence the upper and lower boun- dary of equivalent formations may show great want of uniformity, according to the different locality. This explains how it is that for - mations lying one above the other are composed of strata of very variable thickness, and why we can only in rare cases supply the gaps in the series of these strata from strata found in other countries. The whole succession of formations known to us up to the present time is not sufiiciently complete to form an entire and uninterrupted series of the sedimentary formations. There are still numerous and important gaps in the geological record which we may expect to 168 MEANING OF THE SYSTEM. see filled in future days, when knowledge has iucrefised, and per- haps only when formations now beneath the sea have become known to us. Imperfection of the Geological Record. — After the foregoing dis- cussion we may consider that the continuity of living organisms in the successive periods of the earth's development and their close relationship has been proved partly by geological and partly by palseontological facts. The theory of descent, however, according to which the natural system must be regarded as a genealogical tree, requires still further proof. It requires proof of the presence of numerous forms, transitional not only between the species now existing and those in the more recent formations, but also between the species in all those formations which have immediately succeeded one another in point of time. The theory also demands proof that forms connecting the different groups of plants and animals of the present day have existed. The estabUshment and limitation of these groups can, according to Darwin, only be explained by the extinction, in the course of the earth's history, of numerous and intimately connected species. Palaeontology is only able imperfectly to comply with these demands ; for the numerous closely graduated series of varieties which, according to the theory of selection, must have existed, are, for the greater number of forms, entirely wanting in the geological record. This want, however, which Darwin himself recognised as an objection to his theory, loses its importance when we consider the circumstances under which organic remains were generally deposited in mud, and pi^eserved for succeeding ages in a fossil form ; when we recognise the facts which indicate the extraordinary incomplete- ness of the geological record, and which show that the intermediate forms must have been in part described as species. First of all we can only expect to find in deposits the remains of those organisms which possessed a firm skeleton supporting the softer parts of the body, since it is only the harder structures of the body, such as the bones and teeth of Vertebrates, the calcareous and siHcious shells of Molluscs and Rhizopods, the shells and spines of Echinoderms, the chitinous skeleton of Arthropods, etc., which are able to resist rapid decay, and to undergo gradual petrifaction. Thus the geological record will fail to provide us with any account of the numerous and principally low organisms which are not pro- vided with firm skeletal structures. But also among those organisms whicli ai'e capable of becoming lAlPEUFKCXION OF THE GEOLOGICAL EECOED. 1G9 fossilized, there are large groups which have only exceptionally left traces of their existence : these are the animals which lived on land. Fossil remains of land animals can only have survived when, during grejit floods or inundations, or for some reason or other their carcasses have been carried away by the water, floated hither and thither, and been sin-rounded finally by hardening mud. This explains not only the relative scarcity of fossil Mammalia, but also the fact that of the most ancient Marsupials (Stonesfield slate), scarcely anything is preserved but the underjaw, which, as the body decayed, was easily detached, and, on account of its weight, offered most resist- ance to the current of the water, and was the first part to sink to the bottom. Although it has been shown by such remains that Mammalia existed in the Jurassic period, yet the Eocene forms are the fii'st which give us an insight into the details of their structure. Circumstances must have been more favourable to the preservation of fresh- water animals, and most of all to that of marine animals, since the marine deposits have a much greater extent than the locally confined fresh-water deposits. Thick formations seem in general to have arisen under one of two conditions : either in a very deep sea, protected from the operation of Avinds and waves, no matter whether the bottom was gradually rising or sinking — in this case, however, the strata would be relatively poor in fossils, since only the inhabitants of the deep sea, which is comparatively wanting in animal and vegetable life, would be preserved — or in a shallow sea, in which the bottom underwent a gradual and continued depression during long periods of time favourable to the development of a rich and varied fauna and flora. In this case the sea would have retained uninterruptedly its rich fauna so long as the gradual sinking of its bottom was countei-acted by the continual supply of sediment deposited upon it. Thick formations, all oi most of the strata of which are rich in fossils, must have been deposited in extended and very shallow regions of the sea, during a long period of gradual depression. Thus the great gaps which occur in the series of palseontological remains are explained by a consideration of the mode of origin of deposits. These remains must necessarily be confined to the more recent formations. The lower, more ancient, and very thick succes- sions of strata in which the remains of the oldest fauna and flora must have been buried, seem to have been so completely altered by the heat of the molten interior of the earth, that the organic 170 aiEANINO OF THE SYSTEM. residua which they contain have been completely destroyed, or so altered that they cannot be recognised. In any case it may be regarded as certain, that only a small part of the extinct animal and vegetable world has been preserved in a fossil state, and that of this we only know a small part. Therefore we cannot conclude that, because the fossil remains of intermediate stages cannot be found, they have never existed. It is true that transitional forms are wanting in the strata where they should have occurred, that a species suddenly appears in the middle of a series of strata and suddenly disappears, and that whole groups of species make their appearance and quickly vanish, but the value of these facts as arguments against the theory of selection is diminished by the circumstance that in cei-tain cases series of transitional forms between more or less remotely related organisms have been found, and that many species have been developed in course of time as links between other species and genera ; and again, that species and groups of species not unfrequently increase veiy gradually till they attain an unusually wide distribution, extend into later formations, and then gradually disappear again. Such positive facts have a higher value when we consider the incomplete- ness of fossil remains. It will suffice here to refer to the Ammonites and Gasteropods, such as Valvata multiformis, as examples supplied to us by Palaeon- tology of transitional forms which can be arranged in a gradual sei'ies. Kelation of Fossil Forms with Living Species. — The close i-ela- tionship of the plants and animals of the present time to the fossil remains of recent formations is a fact of great importance. In particular, we find in the diluvial period and in the different tertiary formations the ancestral forms from which numerous living species are directly descended ; and further the characteristic features of the fauna of any particular geographical province in the present epoch are foreshadowed by the fauna of the epoch immediately preceding in the same I'egion ; a fact which is proved by the fossil remains we find buried in the most recent strata. Many fossil Mammalia from the diluvial period and the most recent (pliocene) tertiary formations of South America belong to types of the order of Edentata which are now distributed in that part of the world. Sloths and Armadillos of immense size (Megatherium, Megalonyx, Glyptodon, Toxodon, etc.) formerly inhabited the same continent, the mammalian fauna of whi^h in the present day is so specially charac- SUCCESSION OF SIMILAR TYPJiS. 171 terised by its Sloths, Armadillos, and Anteaters. In addition to these gigantic forms, small and extinct species have been found in the bone caves of Brazil, and some of these are so nearly related to the living forms that we may assume them to have been their ancestors. This law of the " succession of similar types " m the same localities is also exemplified by the Mammalia of New Holland ; for in the bone caves of that country are found many species nearly allied to its present Marsupials. The same law holds good for the gigantic birds of New Zealand, and, as Owen and others have shown, for the Mam- malia of the Old World, Avhich, indeed, is continuous by the circum- polar region with North America ; and ancient types were able, in the tertiary period, to pass into North America, and vice versd by that way. The presence of Central American types {Didelphys) in the early and middle tertiary formations of Europe is to be explained in the same way. It is even more ditficult to distinguish the regions of distribution of the animals of that time than of those of the later tertiary period. The evolution of the ancient forms into those ,of the present time was effected in the case of the lower, simply organised animals at a much earlier period than in the case of higher organisms, Rhizopods, indistinguishable from species living at the present time {globigerina ooze) were already living in the Cretaceous period. The deep sea explorations * have accordingly yielded the interesting result, that certain Sponges, Corals, Molluscs, and Echinoderms now living in the deep sea existed in the Cretaceous period. We meet with a number of living species of Molluscs in the oldest tertiary period, though the mammalian fauna of this period differs completely from that of the present day. The greater number of species of Molluscs found in the recent tertiary period resemble those of the present day, but the Insects of that time dilibred considerably from living species. On the other hand, the Mammalia, even in the post-pliocene (diluvial) deposits, differ in part both in genera and species from those of the present day, although a number of forms have been preserved through the glacial period. On this account, and on account of the relative completeness of the tertiary remains, it is * [Itlt ! zocr inns' Lofotensis — Ajnocrinites, Pleurotomaria, Sijihonia, Micraster, Pomocaris. etc.) Types of earlier and even of the older geological formations have been found preserved in the depths of the ocean, which, in spite of the great pressure, the want of light and deficiency in gaseous contents of the water, are more suited to the development of animal life than was formerly believed. 172 MEAXIXG OF THE. SYSTEM. especially interesting to trace the recent mammalian fauna back through the pleistocene forms to the forms of the oldest tertiary period. It is possible to trace the ancestry of a number of mam- malian species. Riitimeyer was the first to under-take to trace out the ancesti-al line of the Ungulata, and especially of the Ruminantia, so as to obtain a palteontological developmental history, and succeeded in obtaining results, by means of detailed geological and anatomical (deciduous teeth) comparison, which leave no room to doubt that whole series of species of existing mammalia are collaterally or directly related with each other and with fossil species. RUtimeyer's investigations have received corroboration in their essential points from the recent comprehensive works of W. Kowalevski, and have resulted in the establishment of a natural classification of the ungulate animals founded on phylogeny. Pig. 117.— Bones of the feet of the different genera of the EquUltF (after Marsh), a, Foot of Orohipput (Eocene). 6, Foot of Anchitkerium (Lower Miocene), c. Foot of Uipparion (Pleiocene). d, Foot of the recent genus Eguus. In addition to these works we have the recent researches of Marsh, who has completed to an extraordinary degree our knowledge of the genealogy of the genus Equv.s, by numerous discoveries (fig. 117) in America {Wyoming, Green River, White River). The eocene Oroliippus, in which the small posterior toes were present as well as the three principal toes which rested on the ground, was succeeded in the Lower Miocene formation by Anchitkerium with three hoofs ; and the latter was followed by the Hipparion of the Pleiocene formations ; and this is the ancestral form of the existing genus Equus. The origin of most orders of Mammalia, such as Rodentia, Cheirop- tera, Proboscidea, Cetacea, etc., cannot be clearly traced out, but for ce-tain orders, as the Prosimice, Carnivora, Ungulata, and Ro- EVOLUXIOX OF MAMMALIA. 173 dentia, remarkable transitional forms have been discovered among the remains of extinct types. These also appear most prominently among the tertiary remains of North America. In the Eocene period here (Wyoming) lived the Tillodontia with the genvis Tillo- theriuni* characterized by having a broad skull like a bear, two broad incitor teeth like a rodent, and molar teeth like Palceotherium, and feet having five toes armed with strong claws. It thus united in its skeletal structure peculiarities of Carnivora and Ungulata. The Dinocerata (Dinoceras latlcejys mirabile) were powerful Ungulates with five-toed feet with sis hoi^ns on their heads, without incisors in the priemaxillary bone, with strong sabre-like canine teeth in the upper jaw and with six molars. A third type, that of the Brontotheridce attained elephantine proportions, and was provided with transversely placed horns in front of the eyes. In addition to the foregoing there are a number of other groups of Mammals now completely extinct, the remains of which extend back into far earlier strata. Amongst them are the South American Megatheridce [Mylodon, Megatherium), which belong to the order Edentata, and the Toxodontia, whose skull and dentition show relations to the Ungulates, Rodents, and Edentates. Many other types, however, especially of the Ungulates, which during the tertiary period inhabited both hemispheres, are now extinct in America, but still exist in the East. Elephants, Mastodonta, Rhinoceridse, and Equidse existed in America in the diluvial but not in recent periods. Of the Perissodactyles the group of Tapii-s alone is preserved in America. This group has also been preserved in the Eastern hemisphere in the East Indian species. In the palsearctic region also are found the remains of extinct intermediate groups of Mammals which existed during the tertiary period. In the Phosphorites of Quercyt in the south of France are found the remains of the skulls of Prosimise (^Adaiois), the dentition of which is intermediate between the ancient Ungulates and the Lemuridai (^Pachylemurldce), so that the question may be raised whether the Prosimiaj had not a common ancestry with several * Compare 0. C. Marsh, "Principal Characters of the Tillodontia." Amer. Journal of Science and Art, Vol. xi., 1876. 0. C. Marsh, •' Principal Characters of the Dinocerata." A mcr. Journal oj Science and Art, Vol. xi., 187G. 0. C. Marsh, " Principal Characters of the Brontotheridse. " Amer. Journal of Science and Art, Vol. xi., 1876. f Compare H. Filhol, " Recherches sur les Phosphorites du Qucrcy, iltude des fossils qu'on y rencontre et specialement des Mammiferes." Ann. Science.'^ geologiqws, Vol. vii., 1876. J 74 MEANING OF THE SYSTEM. eocene Ungulates {Pachijderniata). In the same locality are found the well preserved remains of the bones of peculiar Carnivora which are well worthy of remark. These are the Hytenodonta. It was for a long time doubtful whether they were Marsupials or not, until Filhol showed from the reserve teeth of their permanent dentition that they were probably of the nature of placental Carnivora. The great agreement of the molars of these Hyjenodonta with those of the carnivorous Mar- supials, as well as the small size of the skull cavity and the rela- tively slight develop- ment of the brain, support the view, which is also rendered probable by many other circumstances, that placental Mam- malia have developed from the Marsupials of the mesozoic period. In the oldest strata of the Eocene forma- tions in both hemi- spheres, the higher placental Mammalia already appear in a rich variety of forms, which contrast mai'k- edly with one another {Artiodactyla, Peris- sodactyla). There is, however, no ground for regarding the immeasurable period from the oldest Eocene to the Keuper, in which the oldest Mammalian remains (the teeth and bones of insectivorous Marsupials) have been found, as the period in which this higher development of the Mammalian organism has been effected. In other cases also the science of palaeontology has led to the discovery of intermediate forms between groups and even between 119— r/«ro-d number, or as lattice-work cieus. Pv. pulsating vacuole. chambers {Badiolaria), which often bear points and spines, or finally as single and many chambered shells with finely perforated walls {Foraminifera) and one larger opening. Through this last (fig. 123), as well as through the countless pores of the small shells (fig. 122), the slender threads of sarcode pass out to the exterior as pseudopodia, changing without intermission in form, size, and number, and often joining themselves together in delicate networks (figs. 122, 123). The pseudopodia, by their slow, creeping movements, afibrd a means of locomotion, while they also serve for the taking up of nourishment miTzoroDA. 183 by surrounding and transporting into the interior of the body small vegetable organisms ',s Bacillaria. Among the shell-bearing forms, the reception and digestion of food takes place outside the shell in the peripheral threads and networks of sarcode ; for each spot on the Burface can for the time being assume the functions of mouth, and i 'H/// / / ! Mi/'/ // 1 i Mi '/ / / . \\\\<':////y^' Fig. \22.~Sotalia venJa (after M. Schiiltze), with a Dlaton taken in the network of Pseudopodia. also of anus, by rejecting the undigested remnants. The Rhizopoda live for the most part in the sea, and contribute by the accumulation of their shells to the formation of the sea sand, and even to the deposition of thick strata. An innumerable quantity of fossil forms from various and very ancient formations are known. 184 Order 1. — Foraminifeea.* RMzopoda, either naked or with a shell, the shell almost invariably calcareous and usually pierced tvith fine pores for the exit of the pseudopodia. ■ Only in rare cases, for instance Nonionina and Polymorphina, is the shell substance of a silicioiis nature; in all other forras it is Fio. ViZ.—^PHola ienera, with network of pseudopodia (after M. Schultzr). membranous or consitsts of a calcareous deposit in a basis of oi'ganio matter. The shell is either a simple chamber, usually provided with a laige opening, or is many chambered, that is, is composed of numerous chambers arranged upon one another according to definite laws. The spaces of these chambers communicate by means of narrow * Besides D'Orbigny, Max Schultze, 1. c, compare W. C. Williamson, " On the recent Foraminifera of Great Britain." London, 1858. Carpenter, "Introduc- tion to the Study of the Foraminifera," London, 1862. Reuss, "Entwurf einer system. Zusammenstellung der Foraminiferen," Wien, 1861. rOKiJUlNIFEUA. 185 passages and large openings in the partition walls. In like manner those portions of the living sarcode body which are enclosed in the individual chambers are in direct communication with one another by means of processes which pass through the passages and openings in the septa, and connect one portion with another. The quality of the body- substance, the mode of movement and nourishment, agree closely with those which have been depicted as characteristic of the order. Our knowledge of the mode of reproduction is imperfect. Amongst the forms without a shell, fission has been observed as well as fusion, which may perhaps be referred to a species of sexual reproduction (conjugation). The reproduction of shell-bearing Foraminifera such as Miliola and Rotalia has also been observed. The former produces from the protoplasm of its body single chambered, the latter three-chambered, young. Probably this mode of reproduction is preceded by an increase in the number of nuclei, aod the animal divides into as many portions as there are nuclei, eaqh of which becomes a young Foraminifer, and contains but one nucleus. In spite of their small size, the shells of our simple organisms may lay claim to no small consequence, since they not only accumulate in enormous quantity in the sea sand (M. Schultze calculated their number for an ounce of sea sand from Molo di Gaeta at about one and a haK millions), but are also found as fossils in diiferent formations (the cretaceous and tertiary), and have yielded an essential material to the construction of rocks. Silicious nodules of Polythalamia are even found in Silurian deposits. The most remarkable, on account of their considerable size, are the JShtmniulitp.s (fig. 124) in the thick formation of the so-called Nummulite liniestonL' (Pyrenees). A coarse chalk of the Paris basin, which makes a i excellent building stone, contains the Triloculina trigonula [Miliolite chalk). The greater number of Foraminifera are marine, and move by creeping on the bottom of the sea, but Globigerina and Orbulina have been met Avith on the sui-face. The bottom of the sea at very consider- able depths is also covered with a rich abundance of foi-ms, especially with Globigerina, the remains of the shells of which give rise to an enduring deposit. 1. Sub-order: Lobosa [Amoihiformes). — Amoeba-like fresh-water Rhizopoda, usually with pulsating vacuole, sometimes naked, some- times with a single-chambered firm shell. The sarcode body consists as a rule of a tougher exoplasm and a fluid granular endoplasm. The pseudopodia ai-e lobed or finger-shaped processes of considerable •earns 186 PBOTOZOA, size, occasionally tougher slender processes without granule sti (figs. 125 and 126). Amwha princeps Ehrbg., A. terrlcola Greef., Petalo]/us diffufiiens Clap. Lachm. Here should also be placed the famous Bathybius Haecheli Huxl., which is found in the' deep sea mud of the Atlantic Ocean, if it is indeed a living organism (and not simply a deposit of Gypsum). Arcella vulgaris Ehrbg., Bifflugia proteiformis Ehrbg., Etiglyplia glohosa Cart, have shells and tough, pointed, dichotomously branching pseudopodia (fig. 125). Fig. 121-. — Xiunmulitic Limeptcme, with horizontal section of N. d'.dans (ufter Zittell). Fig, 126.—Dlffluri ol?owja (after Steii.). Fig. V15. — Euglypla ghbc (after Hei twig and Lesse: ). Fig. 127. — Acercitlii.a g!,.bo.-a (alter M. Schultze). 2. Sub-order : Reticularia (Thalamophora). Principally marine Rhizopods with extremely slender anastomosing pseudopodia, with granule streams in the latter, rarely naked [Protogejies, Lieher- kiihnia), usually -with membranous or calcareous shell, which is single-chambered (Jfonol/udamia) or many-ch;.^nbered {Poljthalamia) (fig. 127). nZLIOZOA. 187 1. Impevforata. With membranous or calcareous shell, which is without fine pores, but possesses, in one place, an opening, either simple or sieve-liJce, through which the pseudopodia project. To these belong the Gromidce, with a mem- branous chitinous shell : Gromia oviformis Duj,, and MiliolidcP, with a porcellanous shell : Cormi^inra planorlis M. Sch., Miliola cyclostoma M. Sch., from the Miliolite chalk. 2. Perforata. The shell, which is usually calcareous, is invariably pierced with innumerable fine pores as well as by one larger opening, and has complicated passages in the partition walls of its chambers. The Lagenidce have a hard shell, with a large opening surrounded by a toothed lip : Lacjcna vulgaris Williamson. The GlohigerinidcB on the contrary have a hyaline shell pierced by large pores, and a simple slit-like open- ing : Orbulina universa D'Orb., Globigerina bulloides D'Orb., Rotalia D'Orb., Textularia D'Orb. The greatest size is attained by the Mimmulhiidep, which possess a firm shell and an in- ternal skeleton, which last is pierced by a complicated canal system : Polystomella Lam., Nummulina D'Orb. Order 2. — Heliozoa.^' Fresh-water Rhizo2Jocls usually with pulsating vacu- ole, and one or more nuclei. A radial silicious skeleton sometimes pi'esent. The sarcode body sends out in all directions tough radiating pseudopodia (fig. 128), When a skeleton is secreted, it consists either of radially arranged silicious spines [Acanthocystis) or of latticed silicious shells {Clathrulina), and so closely resembles the skeleton of the Radiolaria that the Heliozoa have been actually described ^s fresh-water Radlolaria. They differ from the Radiolaria in the absence of the complicated FiO. 128.— Young Actin single nucleus (af tei F. '. :phiE}-ium, still with a , Schultze). iV, Nucleus. * L. Cienkowski, " Ueber Clathrulina." Areliiv. fur mikrosk. Anatomie, Tom III., 1867. R. Greeff, "Ueber Radiolarien und radiolarieniihnliche Rhizopoden des slissen Wassers." Tom V. & XT. R. Hertwig und Lesser, " Uber Rhizopoden und denselben nahe stehende Organismen." Suppl. Tom X., 187 1. Also Archer and F. E. Schultze, etc. 188 PROTOZOA. differentiations of the sarcode, particularly of the central capsule. One or more nuclei may be present in the central mass. An im- portant distinguishing mark is afforded by the presence of the pulsating vacuoles, which have not been observed in any marine Radiolarian. The reproduction very frequently takes place by fission, occasional!}'- %f^//!j|ilili|w\ / / I Fig. 129. — ThalassicoUa pelagica, with central capsule and single large nucleus, also numerous alveoli in the protoplasm (after E. Haeckel). after previous conjugation of one or more indi\'iduals, also during encystment. Multiplication by spores has also been obterved {Clathrulina). In the Actinophnjidoe there is no skeleton secreted : Aetinosphcerium Eichhornii Ehrbg. The central matter contains numerous nuclei. Actinoplirys sol Ehrbg. of small size, with a single central nucleus. In the Acanthocystidce slender silicious spikes are found : Acanthocystis tpinifevd Greeff. with .silicious spikes and needles. In Clathry Una there is a latticed silicious shell, and 1he body has a stalk Clathrulina eltgans Cienk. EADIOLAEIA. 189 Order 3. — RadioLaria.* Ifarine Ehizojjoda with comjjlicated differentiation of the sarcode body, loith central capsule and radial silicious skeleton. The sarcode body contains a membranous porous capsule (the central capsule), in which is contained a tough slimy protoplasm with vacuoles and granules {intracapsular sarcode), fat and oil globules, and albuminous bodies, and more rarely crystals and con- cretions. The intracapsular mass contains also a single large nucleus or several small nuclei. The sarcode which surrounds the capsule and which emits on all sides simple or anastomosing pseudopodia, contains numerous yellow cells, sometimes pigment masses ; and in some cases delicate trans- parent vesicles, or alveoli, are found in the peripheral layer between the radia- ting pseudopodia {Tlialas- sicolla 2)elagica, fig. 129). Many Radiol aria form colonies, and are composed of numerous individuals. In such colonies the al- veoli are placed in the common protoplasm, which contains in itself, not as in the monozoic Radiolaria a single cen- tral capsule, but a number of capsules. Only a few species remain naked and without firm deposits ; as a rule, the soft body possesses a silicious skeleton, which either lies entirely outside the central capsule (Ectolithia) or is partially within it {Entolithia). In the most simple cases the skeleton consists of small, simple, or toothed silicious needles (spicula) united together, which sometimes give rise to a fine sponge work round the periphery of the proto- plasm, e.g., Physematium. In a higher grade we find stronger hollow silicious spicules, which radiate from the middle point of the body to the periphery in regular number and order, e.g., Acanthometra * Joh. Mliller, " Ueber die Thalassicollen. Polycystinen und Acanthometren," Alh. dcr Bcrl. AMcl. 1858. E. Haeckel, " Die Radiolarien," Eine Monographic Berlin, 1862, Fig. 'iZO.—Acanfhomftra MiiUeri (after E. Haeckel). 190 (fig. 130). A fine peripheral framework of spicules may be added to these. In other cases simple or compound lattice-works, and pierced shells of various external form (like helmets, bird-cages, shells, etc.) are found, and on the periphery of these, spicules and needles, and even external concentric shells of similar shape may be formed, e.g., Pohjcystina (figs. 131 and 132). Up to the present time but little has been made out about the reproduction of these animals. Besides fission {Fohjcyttaria), the formation of spores has been observed. These are formed from the contents of the central capsule, and, after the bursting of the latter, become free-swimming mastigopods. Radiolaria are inhabitants of the sea, and swim at the surface, but are also able to sink to deeper levels. Fossil remains of Ra- diolaria have been made known in great numbers by Ehrenberg, e.g. from the chalky marl and polii^hing slate found at certain parts of the coast of the Mediterranean (Caltanisetta in Sicily, Zante and -^gina in Greece), and in particu- lar from the rocks of Barbados and Nikobar, where the Radiolaria ■have given rise to mdely extended rock formations. Samples of sand also from very con- siderable depths have shown themselves rich in Radiolarian sheila. yiG. 131— JTfnosjLi.^fEra echinohhs (after E. Haeckel). I. liadiolaria monozoa. Kadiolaria which remain solitary. 1. Fam. ThalassicoUidae. Skeleton absent or consisting of single spicules not joined together. Thalassicolla (without skeleton) nucleata Euxl., Physe- matium Miilleri Schn. 2. Fam. Polycystinidae. The skeleton consists of a simple or divided latticed shell, the long axis of which is bounded by two poles of different structure. IMiosplicera. EucyrtUlium galea E. Haeck. 3. Fam. Acanthometridae. The skeleton consists of several radial spicules which pass through the central capsule and unite in its centre, without forming INFUSORIA.. 101 a latticed shell. The extra-capsular cells [yellow bodies] are wanting. Acantho- metra pellucida Joh. Miill. II. Pohjcyttaria. Kadiolaria which form colonies with several central capsules- Amongst the Sphierozoa a skeleton is wanting or consists of single pieces not joined together. Collozoum iiierme E. Haeck. Sphcerozoum punctatum Joh. Miill. In Collo^phcera the skeleton consists of simple latticed spheres, each of which encloses a central capsule, Qollosphara Iluxleyi Joh. Miill, Fig. 132 — Eucyrtidium ides (after E. Haeckel). CLASS II.— INFUSORIA.* Protozoa with a definite form and provided with an external membrane, bearing either flag ella or cilia. Mouth and anus usually, contractile vacuole and one or more nuclei always present. Infusoria were discovered towards the end of the 17th century * Ehrenberg, " Die Infusionsthierchen als vollkommene Organismen," 1S3S. Balbiani, "Etudes sur la Eeproduction des Protozoaires," e/ywrw. de la Phys.. Tom. III. Balbiini, "Kecherches sur les ph^nomenes sexuels des Infusoires,"' 192 PKOTOZOA. in a vessel of stagnant water by A. von Leeuwenhoek, who made use of a magnifying glass for the examination of small oi'ganisms. The name Infusoria, which was at first used to denote all animalculse which appear in infusions and are only visible with the aid of a microscope, was first brought into use by Ledermiiller and Wrisberg in the last century. Later on the Danish naturalist O. Fr. Miiller made valuable additions to our knowledge of Infusoria. He observed their conjugation and their reproduction by fission and gemmation, and wrote the first systematic work on the subject. O. Fr. Miiller included a much larger number of forms than we do now-a-days, for he placed among the Infusoria all invertebrate water animal- culse without jointed organs of locomotion and of microscopical size. The knowledge of Infusoria received a new impulse from the comprehensive researches of Ehrenberg. The principal work of this investigator, "Die Infusionsthierchen als vollkommene Organismen," discovered a kingdom of organisms hardly thought of. These were observed and portrayed under the highest microscopic powers. Many of Ehrenberg's drawings may even yet be taken as patterns, and are hardly surpassed by later representations, but the significance of the facts observed has been essentially corrected by more recent investi- gations. Ehrenberg also conceded too great an extent to the group of Infusoria, including not only the lowest plants such as Diatomacecv, Desmidiacece, under the name of Poli/gastrica anenteral but also the much more highly organised Rotifera. As he chose the organization of the last-named for the basis of his explanations, he was led into numerous errors. Ehrenberg ascribed to the Infusoria mouth and anus, stomach and intestines, testis and ovary, kidneys, sense-organs, and a vascular system, without being able to give reliable proofs of the nature of these organs. There very soon came a reaction in the way of regarding the Infusorian structure ; for the discoverer of the Ehizopoda, Dujardin, as well as von Siebold and Kolliker (the latter taking into consideration the so-called Nucleus and Nucleolus), refeired the Infusorian body to the simple cell. In the subsequent works of Stein, Claparede, Lachmann, and Balbiani numerous differentiations were certainly shown to exist, which, however, can all be referred to differentiation of the body of the cell. This view is supported by Journ. de la Pliys., Tom. IV. Claparede und Lachmann, " Etudes sur les Infusoires et les Khizopodes," 2 vol. Geneve, 1858 — 1861. E. Haeckel, " Zur Morphologie der Infusorien" Jen Zeitschrift, Tom. VII., 1873. 0- Butschli, "Studien iiber die ersten Entwickelungsvorgangedes Eizelle, die ZeUtheilung und die Conjugation dos Infusorien," Frankfurt, 1876. FLAGELLATA. 193 the more recent work of Biitsclili, Avho has shown that the repro- duction of these animals is essentially similar to that of the cell. The outer boundary of the body is usually formed by a cuticle, a delicute, transparent membrane, the surface of which is beset with vil>i-:itile and moving appendages of various kinds ari-anged in regular oi-der. In the smallest Infusoria, the Fhujellata, we hnd only one or two long whip-like cilia ; while the more highly differentiated C'diata are usually richly provided with cilia. According to the varying thickness of the external membrane, which cannot in all cases be isolated, and according to the different condition of the peripheral parenchyma of the body, we get forms which change their shape, forms which have a fixed shape and armoured forms. If the simply organized Flagellata, which present nvimerous affinities and transitional forms to the Algfe and Fungi, are not entirely i-emoved from the region of the Infusoria, the two principal groups to be distinguished are the Giliata and Flagellata. Oj'der 1. — Flagellata.* Infusoria of small size, characterised by jMssessing one or more long vhijJ-like cilia, visually placed at one end of the oval body. A row of cilia sometimes and a nucleus always present. The Flagellata are Infusoria the locomotive oi-gans of which consist of one or more whip-like cilia, rarely with an accessory row of cilia. They pass through an inactive stage, and in their develop- ment as well as in their mode of nourishment are allied to certain Fungi. The reasons for regarding the Flagellata as Protozoa are — the perfect contractility of the body, which is not surpassed by Myxomycetes in the mastigopod stage ; also the contractility of the cilia, the apparently purposed and voluntary movements, the occurrence of contractile vacuoles, and, as has been established in many cases, the reception of solid substances into the body through an opening at the base of the flagellum. Nevertheless these phenomena are by no means a test of animal organization. The Monadince are a large group of Flagellata, found for the most part in putrefying infusions, and are hard to distinguish from the monads usually regarded as fungi. They reproduce themselves by * Besides Ehrenberg, Claparede, and Lachmann, loc. cit., compare Stein, '• Organismus dor Infusionsthiere," Tom. III., 1878. Biitschli. " Beitriige zur Kenntniss der Flagellaten," Zeiti^chr. fur Wixs. Zool.. Tom. XXX. Dallinger and Drysdale. "Researches on the Life-history of the Monads," Monthly Micri'xeoju Journal, Tom. X. — XIII. 13 194 PEOTOZOA. transverse fission, and also by spore formation in an encysted condition; the latter method seems in many forms to be preceded by conju- gation. The best known species are Cercomonas Duj. and Trichomonas Donne, of which the first is characterised by the possession of a caudal filament, while Trichomonas has an undulating low of cilia close to the flagella, which are usually two in number (fig. 133). They live principally in the intestines of Vertebrates, but are also found in Invertebrates. Cercomonas intestinalis Lumbl. and Trichomonas vaginalis Donne, are found in Man. The Monads,* which cannot be sharply separated from the Monadina!, are simple cells free from chloi-ophyll, the swarm spores of which iTsually pjiss into an amoeboid stage, and after I'eceiving nourish- ment enter u^xin a motionless stage characterised by the possession of a firm cell-membrane. A number of them (Monas, Fseudos])ora, doIpodeUa), the so-called Zoospores, are mastigopods resembling the mastigopods (swarm spores) of !Myxo- s^^ Y^L^ih-' mycetes, and, with the exception of ^^ CoIpodeUa, grow up to ci-eeping Amcebje which protrude pointed pseudopodia. In this stage they may also be simply regarded as small plasmodia, especially Avhen, as in Jlonas amyU, several masti- FiG. i33.-a, Cerco«u,nas ini.sth.aih. gopods f usc together to form the amoeba. h, Triehomonat vaginalis (after R. They thcu take — in Colpodella M-ithout Leuckart). . first entering the amoeba stage — a globu- lar form, their surface develops a membrane, and in this cyst they break up by division of protoplasm into a number of segments which pass out as swarm spores and rej^eat the course of development {Coljyodella jnignax to Chlamydom.onas, Pseudospora volvocis). Other Monads, the so-called l'etrap)lasta [Vamjyijrella, Xuclearia), do not pass through the mastigopod (swarm spore) stage. Their pro- toplasm during the inactive encysted stage gives lise by division into two or foui-, to the same number of Actinophrys-like Amabfe, of which some, like Colpodella, suck their nouiishment from alga cells (Spirogyra, Oedogonia Diatomacea, etc.), and some envelope ex- traneous bodies. In mode of nourishment and locomotion the monads are allied to the Rhizopods, but also to lower fungus forms like Chytridium. * L. Cienkowski, " Bcitrago zur Kcntniss der Monadcn," ArcJtiv fir MicrusV. Anatomic, Tom. I., ISfij. L. Cienkowski, '• Uber Palmellacoen und einisre Flasrellatcn,'' Tom. YI., 1S7U. voLvociNiD.T:; — astasiad.t:. 195 In their whole developmental cycle they agree very closely with uni- cellular algie and fungi; still the analogy to the developmental processes of many Infusoria, Amphileptus, is not to be passed over. iSjntmella vuhjaris {termo Ehrbg.) of Cienkowski shows a somewhat ditierent development and cyst formation ; it receives solid food (by aid of the food vacuoles) and is lixed by a libre, as also Chromulina nebulosa Cnk., and Ochracea Ehrbg. A second group nearly allied to the Algse (Frotococcacea) is that of the Volvocin'uke. These organisms consist of colonies of cells united by a common gelatinous substance, and the following characteristics indicate their close relationship to the Algai : — (1) in the inactive stage tliQy possess a cellulose membrane ; (2) they exhale oxygen ; (3) they possess an abundance of chlorophyll and of vegetable red or brown coloured oils. Fig. l^i.—EiigUna i Idh. a ami i.free swimming, in different states of contraction, encysted and in process of division. c,d,e. During the motile stage they possess the power of reproduction, since the individual cells give rise to daughter colonies inside the mother colony. A sexual reproduction (conjugation) has also been shown. Certain of the mother cells increase in size and divide into numerous microgonidia corresponding to spermatozoa ; others grow to large ovicells, which are impregnated by the former, and then surround themselves with a capsule, and sink to the ground as large star-shaped cells. They also reproduce themselves during their period of inacti%'ity by fission within the cellulose capsule, while at the same time a change of colour takes place. Amongst the best known of the Volvocina are Volvox glohator, Gonium pectorale, Ste- phanospli(sr(i pliivialis. The Astasiadce are contractile unicellular Flagellata, which are allied to the Volvocinidce in their life phenomena, but they take up 196 pnoTOZOA. solid nutriment. The best known genus is Eughna, which, according to Stein, has a mouth and gullet. In their inactive stage they i-ecrete a capsule and divide up into parts which pass out as mastigopods. Euylena viridis (fig. 134), E. sanguinolenta. Another genus, also with a mouth, is Astasia Ehrbg. A. trichophora Ehrbg., with rounded posterior end, a very long flagel- lum, and an abruptl}' terminated anterior end. The genera Salpmr/oeca and Codosiga described by Clark were included by Biitschli under the name Cylicomastiyes, on the ground that they possess a well-marked collar surrounding the basis of the flagellum, and corresponding to the collar on the entoderm cells of the Sponges (hence Clark regarded the Sponges as most nearly related to the Flagellata); Codosiga Botrytis Ehi-bg, forming colonies, possessing food vacuoles which contain the solid bodies taken up as nutriment, with nucleus and contractile vacuole. Salpingoeca Clarkii BUtsch. (the individuals of this species possess a shell). Another grouji, the Ciliojlagel- lata* is characterised by the posses- sion of a row of cilia, situated in a furrow of the hard cuticular exo- FiG. \ro.-Ceratium tripos (after skeleton (fig. 135), in addition to Nitzsch). the flagellum. The Peridinice, some of which are of peculiar appearance, with large homed processes of the shell, belong to the group, and are allied, so far as their development is known, most neaily to the EuglentB. The mouth lies in a depression ; there is sometimes a kind of gullet, at the end of which the nourishing materials pass into a vacuole. In addition to the locomotive and armoured forms, there are also some without shell or organs of locomotion ; and again there are encysted stages in the interior of wiiich a number of small young forms are said to take their origin [Ceratium corniUum Perhg., Peridiniicni tahulatum Ehrbg). Finally Noctiluca t is included in this group. It is an inhabitant • R. S. Bergh, " Der Organismus der Cilioflagellaten," Morph, Jalirh. Tom. TIL L. Cienkowski, " Ueber Koctihica miliayii," Arcli'n: fui- micronh. Ana- tonle, 1871 and 1872. NOCTILUCA. 197 of the sea, and possesses a peach shaped body which is surrounded by a cuticular envelope, and bears a tentacle-like appendage, A furrow- like invagination is situate at the base of this appendage, at one end of which is the mouth close to a tooth-like prominence and a slender vibratile flagellum. The soft body consists of a central mass of conti-actile protoplasm, connected by fine and anastomosing threads with a layer of the same substance which lines the cuticular envelope of the body. In the central protoplasm lies a clear body, the nucleus; and the spaces between the radiating processes, which exhibit the phenomena of granule currents, are filled with fluid. The contractile substance extends into the appendage, and there assumes a cross- striped appearance (fig. 136). .—Noctihica miliaria (partly after Cienkowski). N, Nu- cleus, a, Single animal, h, conjugation of two individuals. c and d, swarm spores. The reproduction takes place by means of fission (Brightwell), pre- ceded by division of the nucleus ; or by spore formation (Zoospores). In the latter case, the flagellum is absorbed or thrown off, and the Noctiluca assumes a spheroidal shape. After the disappearance of the nucleus, the sarcode contents accumulate on the inner side of one region of the cuticle, divide into from two to four masses which ai'e not sharply separated from one another, and the cuticular envelope is thrust out into a corresponding munber of protuberances. These buds increase and form numerous wart-like prominences, the future spores. They arise, therefore, at the expense of the protoplasmic contents of the disc, which is gradually exhausted in their for- 198 mation. The buds separate themselves from the membrane and become free as small spores, with nucleus and cylindrical appendage, to assume the Noctiluca form under circumstances which have as yet not been closely observed. According to Cienkowski, conjugation may take place between normal forms as well as between encysted forms. The Noctiluca owe their name to their power of producing light, — a power which they share with numerous sea animals, such as Medusje, Pyi'osoma, etc. The light proceeds from the peripheral layer of protoplasm. Under certain conditions they rise from the; deep to the surface of the sea in such enor- mous numbers as to cause wide tracts of the sea to give out a reddish light. It is after sunset, and especially in the evening, when the sky is overcast, that we get the beautiful phenomenon of the phosphorescent sea. The species distributed in the North Sea and in the Atlantic Ocean is Noctihica ■iiiiliaris. Neaily allied is the Mediterranean Leptodiscus medusoides R. Hertwig. Oi-der 2. — Ciliata.* Ciliated Ivfusoria with mouth and anus, sarcode body of complicated structure (with endoplasm and exojjlusni), toith nucleus and paranucleus (nucleolus). Fig. i37.-%/o„y.;, a n,,t,.u.. ^hc locomotive cuticular appendages that (after Stein), (seen frou: v,Q most frequently meet with are slender ventral side). TT r, Artoral .,. i • i <>, .1 1 i p <• zone of cilia; c, eontractiie ^il^-'^^ ^^'^"'^^ o^^en cover the whole surface of vacuole ; N, nucleus ; i\'', the body in close rows, and give it a striped paranucleus : ^, anus. mi -i- n ^ appeai-ance. Ihe cilia are usually stronger la the region of the mouth, and are here grouped so as to form an adoral zone of large cilia, which, during swimming, causes a whiil- pool, and conducts the matter which serves as nourishment into the mouth (fig. 137). This adoral zone is more highly developed in fixed Infusoria such as the hell aniinalcide, the sm-face of which has no uniform coating of cilia. In these animals there are * Besides Ehrenbergr, Claparedc. Lachmann, Butschli, 1. c. compare especially Fr. Stein, " Der Organismus dcr Infusionsthiere." I. and II., Leipzig, 1S59 and 1867. 199 one or more rings of large cilia round the edge of a raised lid- like flap which is capable of being shut down. There is also an in- ferior row of cilia upon this flap running to the mouth. The free-swamming Infusoria often possess in addition to these delicate cilia and zones of cilia, thicker hairs and stiff" bristles, and more or less bent hooks, which are em- ployed in locomotion and for attachment. Certain fixed Infusoria as Stentor (fig. 138) and Cotlmrnia secrete external coverings or shells, into which they retract themselves. Nourishment is taken in in a few cases by endosmosis through the whole surface of the body, e.g., the parasitic Ojmlina. The Acineta feed themselves by sucking the body of their prey. They are without a mouth, and are incapable of taking in solid food. But they possess a number of long, nai-row, contractile t^entacles, which radiate from the surface of their bodies, and have the form of delicate tubes, presenting a structureless *»xternal wall and a semi-fluid gi-anular axis. The Acineta applies one or more of these organs to the body of an extraneous organism, when the substance of the latter travels down the interior of the granular axis of the tentacle into the body of the Acineta (fig. 139). By far the greatest num- ber of Infusoria possess an oral aperture, usually near the anterior pole of the body, and a second aperture which acts as anus, and which can be seen in a definite part of the body as a slit during the exit of the excreta. The body parenchyma, which is bounded by the external membrane, is divided into a viscid exoplasm and Fig. 13S.— Stentor lirtcUi Ehrbg. (after Stein). O, oral aperture witk Kiillet; PV, pulsating' vacuole ; iV, nucleus. Fig. in9. — Aeiiirla ferruihfqitinnm Ehrhjr., which is sucking the body of a small Infusorian (Euchelys) (after Lachmann). T, sucking tentacle ; V, vacuole ; iV, nucleus. more fluid endoplasm, into- 200 which a slender oesophagus, rarely supported by firm rods {Chilodon, JS^assula), often projects (fig. 140). Through this the food stuff passes into the endoplasni, in which it gives lise to food vacuoles. The latter undergo a slow rotating movement round the body in the endoplasm, which is caused by the contractility of the sarcode. During tliis process the food is digested, and finally the solid, useless remainder is ejected through the anal aperture. A digestive canal, bounded by distinct walls, exists no more than do the numerous stomachs which Ehrenbei-g, who was deceived by the food vacuoles, . asciibed to his Ivfusoria jwlygastrica. In all cases where a digestive canal has been described, we have to do with peculiar strings and trabeculje of the internal parenchyma which enclose spaces filled with a clear fluid. The more viscid exoplasm is pre-eminently to be regarded as the motor and sensory layer of the body. In it we find differentiations i-esembling muscles [Stentor, the stalk of Vorli- cella). Sometimes small rod-shaped bodies are present {e.g., Bursaria leucas, Xassula), which ai-e comparable to the thread cells of I'arhellaria and Cidenterata. The contractile vacuoles appear as further differentiations of the external layer, structures which to the number of one or more are found in quite definite portions of the body. They are clear, mostly spherical spaces filled with a fluid ; they diminish suddenly and then vanish, but gradually reappear and increase to their original size. These pulsating vacuoles are usually connected with one or more vessel- like lacuna?, which swell considerably during the contraction of the vacuole. These structures have been compared to the water vascular system of Rotifera and Turhellaria, and have been explained as excretory — an interpretation which has in its favour the fact that the contractile vacuoles in certain cases open to the exterior through a fine pore at the surface, through which granules pass to the exterior. The nucleus and nucleolus lie in the exoplasm of the infusorian body. The nucleus, which ten years ago was compaied to the nucleus of the simple cell, is a structure of variable shape but with a definite position in the body. One, or more than one, may be present. It is sometimes round or oval, sometimes elongated, being drawn out Fig. \iO.—ChiIodim cucul- his (after Stein), with prullet re.semblinpr a fish-basket. iV^ nucleus with nucleolus;,excrcta are passing out of the anus. REPBODUCTIOX OF CILIATA. 201 to the shape of a horse-shoe or a band, and may be broken up into a number of fragments. It contains a granular viscid substance, is bounded by a delicate membrane, and, a h according to the erroneous views of Stein and Balbiani, gives rise to ova or to germi- nal spores. The nucleolus or paranuclevxs also varies in form, position, and number in different species. It is always much smaller than the nucleus, and is strongly refractile ; it usually lies close to the nucleus, or even sunk in a cavity of the latter. Both play an important part in the reproduction of the Infusoria. The most usual method of reproduction in the Infusoria is by fission. When the forms reproduced remain together and connected with the parent, a colony of Infusoria is formed, e.g., the stocks of Epistijlis and Carchesium. Fission usually takes place by a trans- verse division (at right angles to the long axis), as in the Oxytrichidce, Pig. 111. — a, Anpidisca lyncaster (iifter Stein), b, Anpidisca polysty. la, during fission (after Stein). Fig. 142. — Podophrya gemmipai-a (after R. Hertwig). a, with extended suctiou-tube.s and pre- hensile tentacles, with two contractile vacuoles, h, the same with ripe bud.s, in which processes of the branched nucleus N enter, c, free young form. Stentoridoe, etc., and, obeying definite laws, follows conjugation and division on the one hand of the nuclei, and on the other of the nucleoli (fig. 141). Less frequently (Vorticella) the fission takes place through the long axis (fig. 143, a, b), and far more rarely in a diagonal dii-ection. The asexual reproduction is often preceded by encystment, which appears to be of great importance for the 202 PEOTOZOA, preservation of the Infusoria from desiccation. The animal retracts its cilia, contracts its body to a globular mass, and then secretes a transparent cyst, which hardens and protects the animal, thus en- abling it to survive in damp air. In the water, the contents of the cyst divide into a number of parts, which attain freedom by the bursting of the cyst, each one becoming a young animal. Moreover, many Infusoria (Actnetce) produce with participation of the nucleus a number of buds asexually, which separate them- selves from the walls of the parent body (fig. 142). The broods of Splijerophrya make their way into the inteiior of other Infusoria, as Paramecium and Stylonychia, nourish ihemselves at the cost of the enlarged nu- cleus, and form em- bryos by fission. These embryos swarm out, and were for a long time taken by Stein for the embryo broods of Stylonychia (fig. 144, b). The process of con- jugation observed by Leeuwenhoek and O. Fr. Miiller is very general, and is con- nected with changes of the nucleus and nucleolus. These changes, which gave rise to the erroneous interpretation of the two structures as ovary and testis, are in reality simply pieparatory to a process of regeneration of the nucleus by parts of the paranu- cleus, a process comparable to the phenomena of the fertilization of the ovum in sexual reproduction. The conjugation of two Infusoria occurs in very different ways, and leads to a more or less complete fusion, which, after regeneiution of the nucleus, is followed by an increase in the frequency of fission, Paramcfciian, Stentor, Spirostoma, during conjugation, become con- Fig. 113. — VortieeUa mierottoma (after Stein), a, In process I'.t fission ; N, nucleus ; the mouth apparatus in each por- tion is formed afresh, of, gullet, h, Fission is completed, the smaller product is set free after the formation of a posterior ring of cilia ; «•, adoral zone of cilia, c, Vorti- eeUa in process of bud-like conjugation ; JTi'the bud-like individuals attached. COKJUOATIOX OF CILIATA. 203 nected by their ventral surfaces ; other Infusoria with a flat body like Oxytrichina, Chilodon, by their sides ; while EncheJijs, Ilaltcria, Coleps, join together the anterior extremi- ties of their bodies, giving the appear- ance of transverse fission. A lateral conjugation also takes place not un- frequently in Vorti- cella, Trichodina, etc., between individuals of unequal size, the smaller one having the appearance of a bud (bud-like conju- gation) (fig. 143, c). The alterations which the nucleus and "paranucleus un- dergo during and after conjugation have been especially worked out vcLraramcecium^Xi^ Stylonychia (fig 144 a, 145). When several nuclei are present they Fig. 144. — a, Stylonychia myliUif, in process of conjugation. The nucleus is depicted t uriiip division (Bnlbiani's so- called ova); the nucleoli have divided into four spheres (sup- posed spermcapsules) . b, Stylonychia filled with parasitic Spharophrya (after Balbiani). Fig. lio.— Stylonychia myfiliis in process of conjugation, slightly magnified, (treated with acetic acid), (after Biitschli). a. Stage of conjugation with two nucleoli (paranuclei); JVi, the four pieces into which the nucleus has divided in each individual, b. Stage of conjuga- tion with four nucleoli, of these iV becomes the nucleus, and «' the two nucleoli ; 2VJ, the lour remaining jjieces of the old nucleus, c, Stylonychia on the sixth day after conjuga- tion with nucleus and two nucleoli. fuse together to form a single oval body (Balbiani), the substance of which takes a finely fibrous structure previous to further fission, 204 PROTOZOA, like the substance of a true cell nucleus, when undergoing division. The paranucleus too increases in size and becomes striated, and divides into a number of bodies by a single or re- peated di\asion. Some of these bodies produced by the division of the nucleus and paranucleus disappear or are cast out, and others are employed in the formation of the new nucleus and paranucleus. The processes of regene- ration are for the most part not com- pleted until the conjugating animals have separated. Conjugation is probably followed by a repeated division (fig. 146). The mode of life of the Infusoria, which principally inhabit fresh water, is very various. Most of them lead an independent life, and take up larger or smaller bodies, even Rotifera, as nourishment. Some, as Amjyhihjytus, select fixed Infvisoria, as Ejnstylis and for their prey, and swallow them down as far as the origin of the stalk ; they then, while fixed on the stalk, secrete a capsule, and diAade up into two or more individuals, which pass out. Certain Infu- soria, as the mouthless Opalina, and many Bursa- ridfe, are parasitic in the intestine and bladder of Vertebrates. To these belongs the Balantidium coli from the large intestine of Man (fig. 147). 1. Sub-order : Holotricha. — Body uniformly covered with cilia, which are arranged in longitu- dinal rows, and are shorter than the body. Longer cilia are sometimes found in the region of the mouth, but these do not form an adoral zone. Besides the parasitic Opalinae (^Oj)alina! ranarvm'), with- out mouth or anus, the following families belong to this group :— Fam. Trachelidse. Body of changeable shape prolonged into an anterior neck-like process. Mouth ventral, without longer cilia. Trachdius ovum Ehrbg., Amj)7iilrj>tu.^ fasci- cola Ehrbg. Fam. Colpodidse. Form of body definite. ^louth ventral, in a depression, Fig. 146. — Faramepcium Bursaria about one hour after conjugation (after Biitschli). n, nucleolus; 2f, nucleus; PV, contractile vacuole. Two of the nucleoli have become clear spheres. Carchesium, 1.147. — Balantidium coli with two pulsa- ting vaciioles (after Stein). Under the nucleus lies a starch-granule that has been eaten, a ball of e.\crement is passing out of the anus at the poste- rior end. CILIA.TA. 205 always furnislied with long cilia or undulating membranes. ParamcEchim AurclM Fr. ^Mailer, P. Bursaria Focke, Coljfoda cucidlus Ehrbg., Glaucoma scintillans Ehrbg. 2. Sub-oi'der : Heterotricha. — Body uniformly covered with line cilia, which are arranged in longitudinal i-ows, with a distinct adoral zone of cilia. Fani. Bursaridae. The adoral zone of cilia is oa the edge usually of the left half of the body. liiirsarla truncatclla 0. Fr. Mull., Balantldtuni coli Malmst.. parasitic in the colon of man ; Spirostomum amhiguum Ehrbg. Fam. Stentoridae. At the anterior end of the body is a peristomial space with a funnel-shaped depression, without any distinct gullet. Stentor polijmorjjhux, O. Fr. Miill., St. cwridcus Ehrl)g. 3. Sub-order : Hypotricha.^ — Body with sharply defined dorsal and ventral surface. The convex dorsal surface is usually naked, the ventral covered with cilia and beset Avith styles and processes, mouth on the ventral surface. Fam. Oxytrichidae. Body elongated to an oval. On the left half of the ventral surface is a peristomial region, with an adoral zone of cilia. The ventral surface is beset at either edge by a marginal row of cilia, and also with bristles and hooks. Styloiiychia pusUdata Ehrbg., with eight anterior styles, five ventral, and five aual cilia. Oxytriclia gibha O. Fr. Miiller. Fam. Chilodontidae. Body usually armoured, with gullet in the form of a fish-basket. Chd.odon cucuUus Ehrbg. 4. Sub-order : Peritricha. — Infusoria with cylindrical or bell-shaped partially ciliated body. The cilia are placed on an adoral ciliated disc, and frequently on a ring-like zone. Fam. Vorticellidae. Peritricha with adoral spiral of cilia, without a shell, attached by a stalk, usually forms colonies. Vorticella microstoma Ehrbg., Epistylis ^;Z/ca^!^7^5 Ehrbg., Zoothamnhim arbuscida Ehrbg., CarcJuidum, polypinuvi Ehrbg. Fam. Trichodinidse. Peritricha with adoral spiral of cilia, and circle of cilia as well as an apparatus for attachment at the posterior end. Trichodinapediculiis Ehrbg. Fam. Halteriidae. Near the adoral spiral of cilia is an equatorial zone of longer cilia. Haltcria volvox Clap. Lachm. 5. Sub-order : Suctoria. — Body usually without cilia, with knobbed tentacle-like processes which serve as sucking tubes. Fam. Acinetidae. Acineta mystacina Ehrbg., Podophrya cyclopum Clap, Lachm., Splicurophvya Clap. Lachm. As ail appendix to the Protozoa we will now proceed to consider the Schizo- mycetidce, which approach more nearly to the Fungi, and the Girgarinida, 206 PBOTQZOA. 1. The Schizvmijetiiaa;* (Bacteria) arc small globular or rod-shaped bodies which are found in decaying matter, and are especially numerous on the surface of putrefying fluids, where they give rise to a slimj' film (fig. 148). They are most nearly allied to the fungus of yeast, with which they also agree in their manner of nourishment, in that they make \ise of ammonia and organic com- pounds containing carbon. Like the yeast fungus they excite and maintain the fermentation or, as may happen, putrefaction of organic matter by with- drawing its oxygen or by attracting oxygen from the air (reduction or oxyda- tion ferments). But they are clearly separated from the fungi by their deve- lopment, for thcij iiicrraxe hij dicliViiig into two halrrn, while the yeast fungus {Saccharonijiccs. Uormincium) forms buds which separate off as spores. The transverse division takes place, after the cell has become elongated, by a con- striction of the protoplasm and by the secretion of a cross partition wall. The daughter-cells either divide at once, or remain united and produce chains of Bacteria (filiform Bacteria) by afresh fission. Sometimes the successive genera- tions of cells remain connected by a gelatinous substance, and so produce irre- gular shaped gelatinous masses izoogloea'). Sometimes they become free and are dispersed in swarms. They may also settle on the bottom in the form of a Fig. 118.— Scliizomycetes (after F. Cohn). a, Micrococcus, b. Bacterium termo, bacteria of putrefying fluids, both in the motile and zoogloea form. granular precipitate, as soon as the nourishment in the fluid is exhausted. The greater number have a motile and a motionless stage ; in the first they rotate themselves about their long axis, but are also able to bend and extend, but never to serpentine. Their activity seems to be connected with the presence of oxygen. Owing to the absence of sexual reproduction, the division of Bacteria into genera and species is beset with such difficulty that we must content ourselves with establishing, in an artificial fashion, form species and physiological species and varieties without always being able to demonstrate their independence. F. Cohn distinguishes four groups : — (1) Globular Bacteria, Microeoccva (^Monas and Mijeodcrma) ; (2) Eod Bacteria {Bacteriuin) ; (3) Filiform Bacteria {Bacillus and Vibi-io') ; (4) Spiral Bacteria {Sjnrillum and SjnrochfPta). The Globular Bacteria are the smallest forms, and only exhibit molecular movements. They cause A'arious forms of decomposition, but not putrefaction. * F. Cohn. "Beitriigc zur Biologic der Pflanzen." Heft 2 and 3, 1872 and 1875. '• Untcrsuchungen fiber Baktericn," 1, 2, and 3 (Bacterium termo). Com- pare further the works of Eberth and Klebs, BACTERIA — GEEGABINID.i:. 20: They can only be divided, according to their various methods of development, into cliromogenous (pigment), zymogcnous (fermentation), and pathogenous (contagion) divisions. The tirst appear in coloured gelatinous masses and vegetate in the Zooglocaform, c.ij., M. jinnlii/in.'oi.i Ehbrg. in potatoes, etc. To the Zymogenous belong M. vrra; urine ferment ; to the Pathogenous J/. naccuKP, the Pox Bacteria, M. xc2>tlritK of py;umia, J/, d'qihthcrlcux of diphtheritis. The Rod Bacteria form small chains or threads, and exhibit spontaneous motions, especially in the presence of abundant nourishment and oxygcs. Here belongs Jiacteriiim termo Ehrbg. distributed in all animal and vegetable infusions and the necessary ferment in putrefaction, just as yeast is in alcohol fermentation ; also B. Lincoln Ehrbg. of considerable size, which exists in spring water and in standing water, in which there are no products of putrefaction, and, as well as the former, has a zoogloea jelly. Another Bacterium form acts as ferment of lactic acid, ^ according to Hoffmann. Of the Filiform Bacteria the motile Bacillus (vibrio') Kubtilis Ehrbg. occasions butyric acid fermentation, but is also found in infusions together with B. tcrinn. Very nearly allied and hardly to be distinguished is the motionless BnciUiix anthracis of inflammation of the spleen. Vibrio rinjuln and gc7-j)cng arc charac- terised by constant undula- tions of the chain. Finally these lead to the spiral forms of which Sjnrvclurfa resembles a long and flexi- ble but closely wound, and !<2>irilliiv), a thick, short, and coarse screw. Sj)iril- lum tenax, vndula, volutans, the last with a flagellum at each end. 2. The Grcffarini(7(V*aTe unicellular organisms which lire as parasites in the intestine, and in the internal organs of the lower animals. The body is fre- quently elongated like that of a worm, and consists of a granular viscid central mass surrounded by a delicate external membrane (s( metimes with a subcuticular layer of muscle stripes). The nucleus, a round or oval clear body, is embedded in * N. Lieberkiihn, " Evolution des Gregarines," 3/c'iii com: dc VAcad. dc Bclg. 1S5.5. N. Lieberkiihn, " Beitrag zur Kcnntniss der Gregarinen." Arch, fiir Aunt, vnd Phyniol., 186.5. E van B.cnedcn, "Rechcrches sur revolution des Gregarines." Bulletin, de VAcnd. roij. dc Bclgiqne,2 Scr. xxxi., 1871. Aime Schneider, " Contributions a Thistoire des Gregarines des Invcrtebres dc Paris et dc Roscoff." Arch, dc Zool. Eu'in-ri merit., Tom IV., 1875. Fig. no. — Greonrina (after Stein nml Kolliker). a, Sty- lorhynchns oUgacanih us out of the intestine QiCuUopteryx. h, Greffarina (Clepsidrina) poli/mor/iha from intestine of the meal beetle, during conjugation, c. The same in process of encystment. d, Encysted Gregnrina. e. Stage of formation of PseudonavicelhT?. /, Pscudo- navicellacyst with ripe Pscudonavicella;. 208 PEOTOZOA. the central mass. The structure of the body may be complicated by the pi-e- sence of a partition wall which parts ofiE the anterior end from the main mass of the body. The anterior portion of tlie body gets in this way the appearance of a head, upon which there may be formed here and there prominences in the form of hooks and processes for the purpose of attaclinient, Nourishment is effected by endosmosis. through the external walls. Motion is confined to slow gliding forward of the feebly contractile bod}-. In their fuIl-gro\A-n state the Gregarina are frequently seen fastened to one another, two or more together. This connected state precedes reproduction (fig. 149). The two individuals lying with their long axes in the same straight line contract and surround themselves with a common cyst, and after undergoing a process resembling segmentation, divide into a number of small spore- like balls, which become spindle- sliapcd bodies (pseudonavicellas). The cyst secreted round the conju- gating individuals, or, as is often the case, round a single individual, be- comes a pseudonavicella cyst, and by its bursting the spindle-shaped bodies reach the exterior. The contents of each Pseudonavicella sometimes gives rise to a small amoeboid body, as may be inferred from Lieberkiihn's obser- vations on the Psoro.<s * R. Leuckart, " Ueber die Morphologic und Verwandschaftsverhiiltnisse niederer Thiere," Braunschweig, 1848. 14 210 OffiLENTBEATA. and Medusce, and have included the former in the group of the Coelenterata. The Porifera were for a long time taken for plants, and more recently for Protozoon-stocks. While, however, the Polyps and Medusfc are distinguished as Cnidaria and are characterised by the possession of nematocysts and by the higher differentiation of their tissues, the Porifera or Spongiaria present more simple forms of tissue in the spongy structure of their body, and are without nemato- cysts. The entire structure of the body may, generally speaking, be described as radial, although the radial symmetry does not appear in tuost SDonges, and among the Cnidaria transitions towards lateral symmetry are ap- parent. Similar organs are usually repeated round the body axis four or six times or in multiples of these numbers. Four distinct types of body form are met with in the group Calente- rata, viz., that of the Sponge; of the Poll/]) ; of the Me- dusa; and of the Ctenop/iora. The Sponge type.— The sim- plest form of Sponge is represented by a fixed cylindrical tube, with an exhalent opening, the Oscidicni, at the free end (fig. 152). The contractile wall is supported by skeletal spicules, and is pierced by numerous inhalent pores, through which water and small food particles pass into the ciliated internal space. By the fusion of separate indi- viduals, and by reproduction by gemmation, the latter being the more fi'eqvient mode, widely different Sponge stocks -n-ith compli- cated canal systems are formed. The polyzooid natiire of these is made apparent by the presence of many oscula. The Polyp type.— The Polyp has the form of a cylindrical or club-shaped tube, of which the posterior end is fixed and the opposed Fig. ISS.— Tonng S^cuii (after Fr. E. Schulze). O, Osculum or exhalent pore ; P, pore in the wall. MEDUSA — CTENOPnO R. 211 Fig. 153. —Sagartia nicca (after Gosse). free pole pierced by an oral opening sitvxated on a flat or conical prominence, the oral cone. The mouth is surrounded by one or more circles of tentacles, and leads into a simple cylindrical body cavity (Hydroidpoli/ps), or through an oesophageal tube into a compli- cated gastrovascular cavity {Anthozoa, fig. 153). The disappearance of the tentacles gives rise to the so-called polypoid form, whicli consists of a simple hollow tube fur- nished with a mouth. The Medusa type. — The fx-ee-swim- ming Medusa consists of a flattened disc or arched bell of gelatinous or cartilaginous consistence, from the under surface of which hangs a central stalk, the manubrium, bearing at its free end the mouth. This manubrium is frequently prolonged in the neighbourhood of the mouth, into numerous lobes and tentacles, while from the edge of the disc arise a varying number of thread-like tentacles. The central cavity of the body, into which the hollow manubrium leads, is called the gastric cavity, and from it peripheral pouches or radial canals, the so-called vessels, run to the edge of the disc, where, as a rule, they are con- nected by a circular vessel. The movements of the Me- dusa are eft'ected by the alter- nate contraction and dilatation of the muscular under surface of the bell, i.e. of the subvim- brella. Rudimentary Medusae, in which the manubrium and marginal tentacles are absent, are found. They are called Medusoids, and do not acquire individual independence, but remain attached to the body of the Medusa or Polyp from which they are budded. The Medusse and Polyps, in spite of the important differences between them, are but modifications of the same plan of structure. A Medusa may be compared to a free, flattened Polyp, possessing a large gastric cavity and a muscular and enlarged oral disc. The Ctenophor type has fundamentally the form of a sphere, Fig. 151.— Medusa of the Podocoryne carnea with four tentacles at the edge of the disc, ovaries and manubrium, immediately after separa- tion from the stock. 212 CCELENTERATA. beset with eiglit meridional rows of vibratile plates, which, working like oars, serve for locomotion (fig. 155). The body parenchyma in the Sponges consists principally of arareba-like cells, which frequently bear flagella, but which never produce stinging threads. In the Cnidaria (Polyps and Medusae), in certain cells the peculiar struc- tures known as thread cells (fig. 1 5 G )arc developed. They consist of small capsules filled with fluid, and containing a sharp-pointed, spi- rally coiled thread; they are developed in cells which may be called cnido blasts. Under cer- tain mechanical conditions, e.g. under influence of the pressure pro- duced by contact with a foreign body, these cap- sules burst, and the thread is sud- denly protruded, and either fastens on to the cause of disturbance or pierces it, carrying -Cy3!ppe (RormipUra) -^^.^ j^ ^ ^^^^^ ^j the fluid contents of the capsule. In many parts of the body, and especially on the tentacles, which serve for the cajiture of prey, these small micro.'ccpic weapons are collected in masses, and are often united in a peculiar arrangement to form batteries of thread cells. Pig. 155. plumnsa (after Chun), znoutta. Neinutocyst3 and cnidoblasts of Sipkonophora. a and 6, with the cnidocil of the cell, c to e, Nemato- cysts with evaginated thread. DEVELOPM EN T. 213 The tissues (whicli are composed of cells) are generally arranged in two or three layers, of which the external layer is known as ectoderm and forms the outer skin, while the internal layer, tJitt e)uloderm, lines the gastric cavity. Between the two there is developed a delicate homogeneous sup- porting membrane or a stronger layer of connective tissue, in which the skeletal elements are developed. This intermediate layer is known as the mesoderm. The skeletal formations present great variations in structuie and arrangement. Muscles are formed in the deeper part of the ectoderm as cell- processes (the so-called neuromuscular fibres), but often penetrate within the mesoblast as independent cell structures. Sense epi- thelium, nerve fibrillaa, and ganglion cells also appear as differentia- tions of the ectoderm. The endoderm cells, on the other hand, often bear cilia, and are principally concerned in the processes of digestion and secretion. Arsexual reproduction by fission and gemmation is prevalent in these animals, constituted, as they are, on the whole of homogeneous tissues. If the individual forms so produced remain united, they give rise to the colonies which are so widely distributed amongst the Polyps and Sponges, and which, by the continual multiplication of their individuals, may in course of time attain a very considerable size. But we also meet everywhere with the sexual reproduction, in that ova or spermatozoa are produced in the tissues, usually in the region of the gastrovascular cavity, in a definite portion of the body. As a rule, the ova come in contact with the spermatozoa away from the place where they are prodviced ; either within the body cavity or outside the parent body, in the sea-water. In a few cases only do both the sexual elements originate in the body of the same indivi- dual, as, for example, in many of the Spongicma, some Ant/iozoa, and in the hermaphrodite Ctenophora. As a rule, in the colonies of Anthozoa the monoecious arrangement of sexes obtains, the indivi- duals of the same stock being partly male, partly female. Some are dioecious, e.g. Veretillum, Diphyes, Aj^olemia. The development of the Coelenterata for the most part consists of a metamorphosis. The just hatched young differ from the sexual animal in the form and structure of the body, and pass through larval stages. The greater number of them leave the egg as ciliated larvae, which resemble somewhat an Infusorian in external appearance. They acquire a mouth, body cavity, and organs for obtaining food, either during theii" existence as free larvse, or after 214 CffiLBNTBEATA. attachment to solid siiiTOimding objects in the sea. If the young forms, which diller from the sexual animal, gain the power of re- producing by budding, the development leads to various forms of alternation of generation. Suii-uKuui-s.— I. SPONGIARIA*=PORIFERA. The body has a spongy consistence and is coinjjosed of masses of cells capable of anueboid movements and si02)ported by a solid, calcareous, silicious, or horny skeleton. There are external ]}0)-es, an internal canal system, and one or many exhalent o])euings (oscula). The sponges are at present univei-sally regaixled as Ccelenterata, and in this group they are distinguished from the Cnidaria (Polyps and Medusae). They are composed of a contractile tissue, which is usually supported by a framework composed of spicules and fibres ; the whole being arranged in such a manner that there exists on the external wall of the body larger and smaller openings ; and in the interior a system of canals and spaces in which a continuous stream of water is maintained by the vibratile motion of cilia. Amoeba-like cells, net-like membranes of sai-code, flagellated cells, spindle cells, ova, spermatozoa, and tissues derived as excretions from cells are present as the histological elements of the Sponge body. The chief mass of the contractile parenchyma is composed of the amoeba like cells. These are gi'anular cells, which, P;g. 157. - Amceba-iike cell of j^j^^ Amoebae, have no external membrane, Spongilla. ' ' can protrude and retract processes, and ♦^ake into their interior foreign substances (fig. 157). The framework or skeleton, which we find wanting only in the soft * Literature : Nardo G. D., " System dor Scliwiimmc," Isis, 1833 and 1831. Grant, " Observations and Experiments on the Struct, and Funct. of Sponges," Edin. Phil. Jonrnal, 1825—1827. BoAverbauk, "On the Anatomy and I'hysio- logy of the Sporigiada;," Fliilos. Trans., ISoS and 18G2. Lieberkiihn, " Bcitrage zur Entwickelunffsgeschichte der Sponsillen,'" Milller\', amongst which are reckoned the unpaired ten- tacles of the long axis, alternate with the same number of smaller ones, and we have two cii-clos of six tentacles of the first and the same number of the second order. The asexual reproduction by gemmation and fission is of great significance. Buds can be formed in various positions, even at the oral end, in which case a strobila-like form appears. In Blastotrochus the buds appear at right angles to the axis of the parent animal (fig. I'l). If the individuals so produced remain connected with one another, a polyp-stock is formed, which may attain very vai^ious forms and great size. As a rule the individuals are imbedded in a common body mass, the ccenenchyrn, and their gastiic cavities communicate more or less directly, so that the juices acquired in the in- dividual polyps penetrate into the collective stock. This stock afibrds us an excellent example of an animal community built up out of similar members. The foi'mation of the generative products alone is distributed, as a » rule, to different individuals, which, however, unite in dis- charging all animal and vege- tative functions together (tis;. 172). The skeletal formations of the fjg. 172. polyps are specially worthy of remark {polyj)aria). In almost every case, with the exception of Ac- tinia, there is a deposit of solid calcareous matter in the mesoderm, and * Like the first tentacle of the .young Scyphistoma polyp among the Ihjdre- Mi-duscB. -Branch of a Polyparium of Coralliua (after Lacaze Diithiers). I', I'olyp. 228 COELE>'TEEATA, according to the density of this deposit, there is produced a leathery, chalky, or even stony framework. If isolated needles or toothed rods (fig. 173) of calcareous substance are distributed beneath the epidermis and the coenenchyma, the pol}-p-stock has a fleshy, leathery nature {Alcyonaria) ; but if, on the contrary, the calcareous structures are fused together or are cemented together in a larger mass, a solid, moi-e or le.-s firm, often stony cal- careous skeleton is developed {Madrejjoraria). In the individual animals the formation of this sub-epidermic skeleton begins on the foot surface, and advances thence in such a manner that near the calcareous foot-plate there is foi'med in the under part of the polyp body a more or less cup-shaped theca, from which numerous perpendicular plates, the se^ita, radiate in- wards. In the cup-shaped cal- careous framework of the individual polyp, the structure of the gastrovascular cavity is repeated, Avith the exception that the calcareovis septa cor- respond to the interspaces of the mesenteries (fig 174). The number of the i-epta in- creases as does that of the mesenteries and tentacles with the age of the polyp according to the same laws. At the same time a great number of systematically important modifications of the skeleton are effected by further differentiation. A column-like, calcareous mass sometimes arises in the axis of the cup (eolumdla), and in its neighlx)urhood a circle of calcareous rods (pcdi), which &re separate from the .septa (fig. 175). Thei'e may further be foimed between the lateral sui-faces of the septa processes of calcareous substance as interseptal rods or horizontal shelves (dissepimeiita) ; also on the outer side of the wall of the theca ribs [costai) projecting beyond its external surface, and similar dissepi- ments may be produced between these. Fig. 173.— Calcareous liodies (Sclerodtrm'tff) of Alcyonaria (after Kolliker). a, of PlexaurMa. h, of Gorgonia. e, of Alcijonium. ACnXOZOA. 220 The important diversities of form in the polyp stocks are not only occasioueJ by thj ditierences of structure of the skeleton of the \)l/// Fio. 175.— Vei-tical section through the cup of Cyathi- tia Cyathut (after Milno Edwards). S, Septa ; P, pali ; C, columella. polyp, but are also the resultant of varying methods of growth by gemmation and impev- Fio. 17i. — Vertical section throui^h a polyp of Affro'ulea caJycularh (after Lac.ize Duthiers). The mouth open- ing and cesophaLToal tul)e are seen as well as the me- senteries fastened to the same ; also the calcareous seiita betvreen the mesenteries, and the columella of the skeleton, Sk. feet fissioH. According to the method, nume- rous modifications of branched stocks are dis- tinguished, e.g., Madre- j)ores (fig. 17G), Oculi- iiidce (fig. 177), and the lamellar and massive stocks as Astrcea (fig. 178) and the Mctan- drinidce (fig. 179). The Anthozoa are all inhabitants of the sea, arul live mostly in the warmer zones, but certain types of the fleshy Fia. 17G.— afarfr coaa after Ed. H. Fig. 177. — Brarch of Ock- lina speciosa (after Ed. H). 230 COILENTEKATA. Octactinia and Actinia are distributed in all latitudes. The polyps which build banks and reefs are confined to a zone extending about 28 degrees on either side of the equator, and only here and there extend beyond these bounds. They live for the most part near the coast, and produce there in coiirse of time rocky masses of colossal extent by the uccumulations of their stony calcareous frameworks. These masses may form coral reefs {atolls, harrier reefs, fringing refs), which are perilovis to ship- ping, and may also become the foundations of islands. In both cases a gradual alteration of level, the raising of the bottom of the sea, assists the work of the coral animals. The presence of the coral banks in the deep sea is, on the other hand, due to a continual sinking of the sea-bottom The part which the Anthozoa take in the alteration of the earth's surface is considerable. In the pi^esent time they protect the coast from the consequences of the breaking of the waves and assist in the formation of islands and i-ocks by pi-oducing immense masses of calcareous matter. In earlier geological epochs they have played a still more important part judging from the great thickness of the Palaeozoic period and of the Jurassic Fig. \'!i.—Ai.fi-idvgorgia) flahellum L., or composed of alternating horny and calcarc-ous segments, as, e.g., Isis hijqmris Lam., Melithcea ochracea Lam., or stony and formed of calcareous matter. The red coral, CoralUum ruhrum Lam., falls under the last head, and yields the coral stone which is used in jewellery. This species is found in the Mediterranean, on the rocky coasts of Algiers and Tunis, and there forms an important object of industry. 4. Fam. Tubiporidae, organ coral. The polyparia resembling the pipes of an organ. The animals are placed in parallel calcareous tubes connected by hori- zontal plates. Tuh'qjora Hemprlchtll Ehrbg. Order 3. — Zoantiiaria = IIexactixia. Polyps and polyp stocks, lohose tentacles usually alternale in several circles, and are either six or some multiple of six in nuiuLer. 232 C(ELEXTEKATA. The body is seldom quite soft, or with a leatheiy framework ; as a rule it has a calcareous stony polyparium with radial striations. Separated sexes are the rule, but hermaphrodite polyps (Actinia) are not seldom to be met with. The polyps very generally retain their embryos for a long time, so that they are born eight or twelve rayed, with rudimentary tentacles. Many give rise to coral reefs and islands (figs. 175 — 179). 1. Antipatharia. Mostly -with only six tentacles, and horny skeletal axis. Fam. Antipathidae. Polyp stocks with soft non-calcareous body, but with dmple or branched axial skeleton. Only six tentacles surround the mouth, e.g., Aiitqjathes Tall. 2. ACTiNiAKiA, with no hard structure. Fam. Actinidae, with soft body ; sometimes single animals with several alternating circles of tentacles, Actinia L. ; sometimes connected in stolons and aggregated to form stocks, Zoanthus Cuv. The former are able, by means of their contractile foot, to leave their place of attachment and to move freely. Many reach a relatively considerable size, and possess beautiful colours. Under the name of sea anemones they are the ornaments of salt water aquaria. Actinia mescmhnjantliunnim L. The skin sometimes secretes a glutinous mass filled with nematocysts or a kind of membrane, Ccriantlnis Delle Ch. 3. Madeepoeakia with continuous hard calcareous skeleton. {a) Ajioros-a. Fam. Turbinolidae. Mostly single polyps with compact calcareous frame- work, imperforate thecns, and well developed septa, the spaces between which are open to the bottom. Tiirhinolia Lam., Flahellum Less., Caryo])hyUia Lam., C. cijathus Lam., lilastotrochus Ed. H. Fam. Oculinidae. Polyp stocks with hard usually branched polrparium, with coenenchyma rich in calcareous matter, and but few septa in the cup of the individual. Ocidina virrjinea Less., Indian Ocean. Awj/hiJicUa cculata L. white corals of the Mediterranean. Fam. Astraeidae, Star corals, Mostly massive polyp stocks with fused thecre, and without coenenchyma. The septa have sometimes cutting edges, sometimes toothed edges. The interseptal spaces are filled with horizontal partition walls. EunDiilia Edw. The single animals are produced by fission and remain connected only at their bases. They produce a cespitous polyparium, the septal edges of the cup being cutting. GaUu'ea Oken. The single cups arise by gemmation, are free at the upper edge ; the septa have cutting edges. Astrcpa Lam., single cups fused throughout the entire wall. The septal edges of the cup are jagged. Mceandrina Lam., the neighbouring cups fused to form long valleys. M. Crassa Edw. H. Fam. Fungidae. Mui^hroom corals. Usually with large flat single cups, some- times polyp stocks ; without thecje, with numerous strongly developed septa, toothed and connected liy synapticuloj. JFicngia discus Dana,, JJuloinitra Dana., Loj)Jioscris Edw. H. {h) Perforata. Fam. Madreporidae, Madrepores. Polyps and .polyp stocks with porous coenenchyma and perforated thec?e. Gastric cavity oiten at the bottom and communicating with the central caual in the axis of the branched polyparium. HTDROZOA. Septa but slightly developed. Midrcpora ccrvicornls Lam. ratnoa Edw., Mediterranean, Astroides ealycularis Pall. 233 Dandroj^ihijllla CLASS IL— POLYPOMEDUS.E.* [HYDEOZOA.] PoIi/2)s loithout oesophageal tube, with simple gastrovascular cavity. The generative elements are developed in medusoid forms which may he either free-swimming, or permanently attached to hydroid forms. This class includes the small polyps and polyp stocks, and the Medusce which form the sexual generation. The Folyjwmedusce have always a simpler structure than the Anthozoa to which they are also usually infe- rior in size. They lack cusophagus, septa, and gastrovas- cular pouches. Only the polyps of the a- sexual generation of the Scyphomedu^ie [Acra?peda], known as S'cyj^histoma, pos- sess a remniint of the gastric folds as four gastric ridges from which filaments are developed. The polyp stocks develop in rare cases (Jlille- poridce) a compact calcareous framework comparable to the poly par ium. When skeletal formations are present they con- sist as a rule of more or le.-s horny secre- tions of the ectoderm, which as dehcate tubes svirround the stem and its ramifications, and sometimes form small cup-like structures surrounding the fo'yp, and known as * Escholtz, "System der Acalephen," Berlin, 1820. Th. Huxley, "Memoir on the Anatomy and Affinities of the Medusae," I^Iiil. Tranx., London, 1849. Fig. 180 o.— Branch of an Obelia-stock (0, ge'athwaa). O, Mouth of a nutritive polyp wi h extended tentacles. M, Medusa buds on the body of a proliferous polyp (blasto- style) ; Th, bell-shaped tup (thcca) of a nutritive polyp. 234 C(ELENTEIIATA. hydrothecae (fig. ISO a). A more or less stiff mesoderm lamella is also developed in the interior of the body wall, between the ectoderm and the endoderm. This serves to support the soft parts of the animal, and, in the Mediisce, is in part represented by the gelatinous connective tissue of the disc. The Medusa (fig. 180 b) is without doubt morphologically higher than the Polyp, since it represents the mature sexual individual, while the Polyp performs the nutritive and vegetative functions. The Medusa, in correspondence with its power of free locomotion, possesses an ectodermal nervous system and sense organs. The nervous system consists of nerve fibres and ganglion cells, and is usually specially concentrated round the edge of the disc, where it forms a. double ring of fibres running parallel to the circular vessel. The sense organs are the so-called marginal bodies. The generative pro- ducts of the Medusae cither have their origin in the ectoderm, in which case they may be developed on the under surface of the disc (sub- umbrella) in the ecto- derm immediately un- derlying the radial canals (Eucopicke), or in the ectoderm of the manubrium (Oceanidce) ; or they may arise from the endoderm of the under surface of the umbrella (>Sc)/2}homedus(je). Both Polyps and Medusae frequently remain at a lower grade of morphological differeatiation, the former becoming polypoid appen- dages, the latter medusoid buds enclosing the generative products. In either case they are situated on the stem or on some part of the Polyp. The individuality of such appendages appears limited ; the medusoid or polypoid animal sinks, physiologically speaking, to the value of a portion of the body or of an organ, while the entire stock L. Agassiz, " Coutribntion to the Natural History of the United States, Aca- lephae," vol. iii., 1S60, vol. iv., 1S62. E. Haeckcl, "System der Medusen," Tom. I. and II.. Jena, 1880 and 1881. Fig. ISO S.— Free Medusa of Oloelia gelatinosa, as yet without generative organs ; ff, auditory vesicles. IIYDROZOA. 2i55 nppro.achcs more nearly to a single organism. The more completely polymorphism and division of labour are impres>ed upon the polypoid and medusoid appendages, so much higher becomes the unity of the whole which is morphologically a colony of animals. In these cases it is often difficult to distinguish between budding and simple growth. For a long time it was considered as a remarkable circumstance, hardly admitting of a satisfactory explanation, that organisms which differed so widely as Polyps and Medusfe — they had, indeed, been systematically separated as different classes — should only form dif ferent stage.-; in the life-history of a single cycle of development and thus be united in the closest genetic connection. The theory of " Alternation of Generations " contained only a def^cription of the matter, and offered no explanation. The discovery of the mode of origin of the Medusa as a bud on the body of the Polyp first clearly demonstrated the direct relation of the two forms, for it proved that the Medusa is a flattened, dlsc-shajyed Fohjp with a shallow hut wide gastric cavity, the perij^heral part of which has, hij the fusion of its upper and loiver loalls along four, six, or eight radiating areas, become divided into the vascular pouches {(jastric p)ouches), or, as they are called, radial canals, which correspond to the gastrovascular pouches of the Anthozoa. The differences consist, in connection with the discoidal form, mainly in the position of the gastric tube as an external appendage, the manu- brium, and in th-e great reduction in height of the radially extended septa (mesenteries), which are traversed by a layer of endoderm cells, the vascular or endoderm lamella. This layer is derived from the fusion mentioned above of the aboral with the oral layer of the endoderm of the peripheral part of the gastro-vascular cavity. At the same time the oral disc becomes enlarged and concave to form the cavity of the bell, the ectodermal lining of which gives rise to the muscles of the subumbrella. The supporting substance of the arched (after it is freed from its attachment) aboral surface of the disc becomes very much thickened and gives rite to the gelatinous substance (mesodermic), which sometimes contains cells; while that of the oral surface keeps the character of a thin but firm lamella, and serves as a support for the muscles on the under surface of the disc. The tentacles accordingly arise near the edge of the disc, and become the marginal tentacles of the Medusa. In addition to these, four simple or branched oral appendages appear as outgrowths from the manubrium. In addition to the sexual reproduction, asexual multiplication is 236 COiLKXTEKATA. widely distributed, especially amongst the polypoid forms, in wliich it leads to the formation of polymorphous animal stocks. The two forms of reproduction alternate for the most part in regular order, f-o as to produce different generations. There are, however, 3Iedusce (Aegmopsis, Pelarjia) which proceed without alternation of genera- tions and develop directly from the ovum by continuous development with metamorphosis ; but, as a general rule, the e^^ of the Medusa (phanero-codonic gonophore) or the medusoid generative bud (adelo- codonic gonophore) produces a Polyp, and this Polyp either at once, by transverse fission {HcijiAomedusob), or later, after a longer period of growth, in which a sessile or free-swimming polyp stock is pro- duced, gives rise to a generation of free-swimming Medusas, or of medusoid buds which never become separate from the polyp stock. The Hydromeduste feed entirely on animal substances, and for the most part are inhabitants of the warmer seas. The free-moving Medasoe and Siphonophora are phosphorescent. Order 1. — Hydromedus.e.* Colonial forms, the individual Pohjps ofiohich are tcitJiout (VSO})hageal tube or mesenteric folds. The sexual gaieration has the form either of small free-swimming Medusae, provided ivith a velum (^Ci'aspedote Medusa) or of medusoid generative buds (rudimentari/ Medusce) which remain attached to the hydroid colony. The Polyps and polypoid forms are the asexual individuals. They form small moss- or tree-like stocks which are fi'equently surrounded by chitinous or horny tubes (cuticular skeleton). These exoskeletal structures may become extended into cup-like hydrothecte surrounding the individual Polyps. The stem and ramified branches [ccenosark] contain a central canal which commiinicates with the gastric space of each individual Polyp and polypoid appendage and contains the common nourishing fluid. The Polyps have no oesophageal tube, and the ciliated gastric cavity is undivided by mesenteries. As a rule, the ectoderm and entoderm remain simple, and are only separated by a thin interposed supporting lamella which does not contain cells. The presence of elongated muscle fibres as processes of the ectodermal epithelial cells is very general [Hydra, Podocoryne). These muscles may, however, * L. Agassiz, " Contributions to the Natural History of the United States of America,"' vol. ii. — iv., 1800 — 1802. G. J. Allman, "A Mono^fraph of tlie Gymnobla-itic or Tubularian Hydroids," vol. i. and ii., London, 1871 and 1872. N. Kleinenberg, '-Hydra," Leipzig, 1872. 0. and R. Hertwig, "Das Ncrven- sys'em und die Sinnesorgane der Medusen," Leipzig, 1878. UYDKOZOA, 237 be separated as an independent layer of nucleated fibre cells below the epithelium. The Polyps are not invaiiably alike, proliferous Polyps (or Blastostyles) being frequently found as well as the nutritive ones. The proliferous Polyps develop generative buds on their walls. The sterile Polyps may differ from one another in the number of tentacles and in their entire form, so that different kinds of individuals may be found on a single stock. Thus we find tiie polymorphism of the Siplionophora foreshadowed amongst the Hydroidea {Podocoryne^ Pluinularia). The generative products are only exceptionally developed in the Polyp body itself, in which case they are produced in the ecto- derm {Hydra). This exception is probably to be looked upon as an extreme case of degeneration of a medusoid bud. As a rule the generative products are de- veloped in special medusoid buds [gono- phores] formed from both cell-layers. In the most simple cases the budding in- dividuals of the sexual generation contain a diverticulum of the gastric cavity of the polyp-shaped parent or of the axial cavity of the hydroid stock. The generative products become accumulated around this diverticulum {Hydractinia echinata, Ckiva squamata). In a more advanced stage we find a mantle-like envelope enclosing the bud, and con- stituting the rudiment of the umbrella, with a continuous vascular lamella or with more or less developed radial vessels {Tuhidaria coronata, Eiulendrium raviosum, Van Ben.) Finally, at the highest stage, the buds develop into small Meduste {CamjKinidaria gelatinosa van Ben., Sarsiu tuhulosa), which become free, and sooner or later, 'Pig. \%\.—Poclocorynei'aynea (after C. Groljbcn). P, Polyp; M, Jledusa bud ou the proliferating polyp ; 8, spiral- zooid; Sk, skeleton Polyp (compare the free Medusa, fig. 154). 238 CCELE>'TEBA'l A. often only after a long period of free life, in which they become much larger and undergo a metamorphosis, reach sexual maturity. The Medusae belonging to the order Hydromedusai are, with but few exceptions, distinguished from the Acalephce (Scyphomedusaj) by their smaller size — although certain forms, for example Aequorea, may attain such a size as to have a diameter of more than a foot — and by their simpler organization. The number of their radial vessels is smaller (4, 6, or 8), their sense organs (marginal bodies) are not covered by folds of membrane (hence G ijmnophtltalmata Forbes), and they have a muscular velum (hence Craspedota Gegenbaur) (fig. 182). The generative products are always formed from the ectoderm, and originate on the walls of the radial canals or of the manubrium, but never, as in the Acalepho, in diverticula of the gastric cavity. The hyaline gelatinous substance of our Medusfe is, as a rule, structureless, and contains no cellular elements ; there may, how- ever, be tibres running per- pendicularly through it (Liriope). These fibres are probably derived from cell processes of the ecto- derm and entoderm, and have arisen contemporane- ously with the gelatinous disc, Avhich is itself to be looked upon as an exci-etion product of the adjoining ectoderm and entoderm epithelium. The nerve-ring is placed at the edge of the disc at the point of insertion of the velum. It is covered by a sense epithelium com- posed of small cells bearing senee hairs, and has the foi-m of a double fibrous cord containing ganglion cells. The larger upper nerve-ring runs above the velum, while the weaker nerve-ring, on the other hand, is placed below it. The lower nerve-ring is composed of larger tibres and larger ganglion cells ; bundles of fibrillar pass off from it to supply the muscles of the velum and subumbrella, where they form a sub-epithelial plexus interspersed ynth ganglion cells, between Fig. i&2. — Thialadium fariaii/*' represented from the underside of the umhreVa. T", Velum ; O, mouth ; Oo, ovary ; Oh, auditory vesicle ; Ef, tentacles on the margin of the disc; Ew, marginal swellings. 23& Ihe muscular epithelium and the jabrous layer. The ganglion cells in the upper nerve-ring are smaller, and the fibrillar given off from it pass to the tentacles. The fibrillar of the sense nerves may be derived from both rings. The marginal bodies have long been recognised as sense organs, and are either eye spots (ocelli) or auditory vesicles ; hence the Hydromedusoi may be divided into two groups, the Ocellata or Vesiculata. In the Vesiculata the auditory vesicles are situated at the edge of the under side of the umbrella, and contain one or more concretions (otoliths) ^\hic]i-Ave formed in the interior of cells. Peculiar sense cells surround each vesicle-like cell containing a concretion. The curved hairs of these sense cells (auditory hairs) are in contact with the con- cretion vesicle, A nerve tibrilla enters the basis of the auditory S3). The audi- tory organs of the Tra- clii/mediosce are placed above the velum, and are in con- nection with the upper nerve ling ; they have the form of small projecting tentacles furnislied with otoliths and auditoiy hairs. The tentacle may either project freely on the surface {Trachijnema), or, as in Geryonia, it may be placed in a vesicle (fig, 184) which lies in the gelatinous substance of the disc and close to the edge of the latter. Sepai-ate sexes are almost invariably the rule, but it is rare to find that the colonies are dioecious, i.e., that male and female medusoids are developed in different colonies [Tubularia). Gemma- tion has occasionally been observed among the Medusoi {Sarsia j)roKfera) and division [StomohracJiium mirablle). The larvse of Cunina, which are parasitic on the Genjonidai, may also there give rise to a cluster of Duas, Fig. 183.— Sens I ou the nerve-ring and circular vessel of Octorchi) (after O. and E. Hertwig). Rh, Sense organ ; O, O', two otoliths ; Hh, audi- tory cilia ; H:, auditory cells ; Nv, upper nerve-ring ; Eg, cir- cular vessel. (Type of the audi- tory organ of the Vesicuhi/a.) Fig. 181.— Auditory vesicle of Giyiy- onia (Caniuiriiui), seen from the surface (after O. andR. Hertwis). iV and N', The auditory nerves ; Ot, otolith ; Hz, auditory cells ; Hh, auditory cilia (type of the auditory organ of the Trachy- medusa:). 240 cuclenteeata. The development of the ovum, which is, as a rule, naked {i.e., with- out a vitelline membrane), has hitherto only been completely followed out in a few cases. In every case the segmentation Sicems to be com- plete, and leads to the formation of a segmentation cavity and a single-layered blastoderm [a single-layered blastosphere]. The latter gives rise to a second endodei-mal layer of cells, which lines the segmentation cavity. The segmentation cavity thus becomes converted into the gastric cavity of the future polyp. The spherical or oval larva now either attaches itself and gives rise by budding to a small hydroid stock, or swims freely and develops directly into a small Medusa [Track ymedus(e). The Medusa, after becoming free, usually undergoes a more or less fundamental change of form, which concerns not only the alteration caused by the enlargement of the umbrella and manubrium, but also the increase, according to definite laws, of the marginal tentacles, sense organs (Tima), and the radial canals (Aequorea). "We must remark, however, that the sexually complete ]Medusfe exhibit very considerable variations in size, number of sense organs and tentacles [Phyalidium variahile, Chjthia voluhilis). The difficulty of systematic arrangement is augmented by the fact that closely allied Polyp stocks can produce diftei-ent sexual forms. Thus, for example, Jlonocauhis gives rise to sessile generative buds and Corymorjiha to free Medusce (Steenstritpia). Medusae of identical structm^e also, which one would place in the same genus, may form the sexual genera^tions of hydroid stocks belonging to different families {iso(jo7iism). There are also cases in which we find Medusai of closely allied genera, some developed from hydroid stocks by an alternation of generations, and othei's developed directly. Hence it appears just as little satisfactoiy to found a classification entirely upon the sexual generations as to pay attention to the asexual generation alone. (1) Sub-order: EleutherohlastecB. Simple hydroid Polyps without medusoid buds ; both generative products are developed in the body- wall of the Polyp. Fam. Hydroidae. Hydra, the fresh-water polyp. II. viridh L.. Il.fusca L., remarkable for great powers of reproduction. (2) Sub-order : Hydrocorallice. Coral-like hydroid stocks with cal- careous coenenchyma and tubular hydrothecse opening to the exterior by pores. Some of these contsiin the larger nutritive animals, while others contain animals without a mouth and beset with tentacles. nVDKOZOA HYDEOirEDUS-E. 241 The latter are arranged usually in the form of a circle round each of the nutritive animals. The polyparia are found in the fossil state Fam. Milleporidae. Millcpora L. M. alcicornis L. Fam. Styiasteridae. (3) Sub-order : TuJndarioi (Ocellata). Polyp stocks which are either naked or clothed by a chitinous pei-iderm without cup-shaped hydrothecse surrounding the polyp head. The generative buds arise on the body of the Polyp or on the stock. The Medusae, which are set free belong to the genera Oceania, Sarsia, etc., and have ocelli. Fam. Clavidae. Polyp stocks with a chitinoufs periderm. Polyp club-shaped, with scattered, simple, filiform tentacles. The generative buds arise on the Polyp body and for the most part remain sessile. Covihjloplwra AUm. The stock is branched ; there are stolons which grow over external objects. Oval gonophores covered by the perisarc. The animals are dioecious. In fresh water — C. lacustvis Allm. alhicola Kirchp., Elbe, Schleswig. The following are marine genera — Clara 0. Fr. Mliller. Allied are the Eudcndridce with Eudendrium ramoxiim. L. Fam. Hydractinidae. Polyp stocks with flat extended coenenchyma and firm encrusted skeletal excretions. Tlic Polyps are club-shaped, with a circle of simple tentacles. In addition to the latter there are large tentacle-shaped Polypoids (Spiralzooids). Hijdractlnia van. Ben. The medusoid buds sessile on the proliferous animals, which are without tentacles. H. echlnata Flem. Podocorijiw Sars. (fig. 181). The generative buds are freed as Oceanidce. P. caimea Sars. Fam. Tubularidae. Polyp stocks clothed with a chitinous periderm. The polyps possess a circle of filiform tentacles on the proboscis inside the external circle of tentacles. The generative buds arise between the two circles of tentacles. Tuhularia L. The hydroid stocks form creeping root-like branches at the bottom, from which arise simple or branched twigs with the terminal polyp heads ; the generative buds are sessile. T. (Tlmvinocnidia Ag.) coronata Abilg. dioecious. Corijmorpha Sars. The stalk of the solitary polyp is clothed with a gelatinous periderm, attaches itself by root-like processes, and con- tains radial canals which lead into the wide digestive cavity of the Polyp- head. The freed Medusa is bell-shaped, with one marginal tentacle, and bulbous swellings at the end of the other radial canals. C. nutans Sars., C, nana Alder. (4) Sub-order: Camjxmidarke (Vesicvilsit^). The chitinous skeletal tubes widen out round the Polyp-head to form cup-like hydrothecai. The Polyp-head, the oral cone (proboscis), and tentacles can be in most cases completely retracted into these hydrothecai. The generative buds arise almost regularly on the walls of the proliferous individuals, which have neither mouth nor tentacles. The buds are sometimes sessile, and sometimes become separated off IG 242 GSLENTERATA. as small vesiculate Medusce, with generative organs on the radial canals {Eiicopidce, Geryonojysidce, Aequoridce). Fam. Plumularidae. The hydrothecae of the branched hydroid-strcks are arranged in single rows ; those of the nutritive Polyp have small accessory calyces filled with nematosysts (nematocalyces). Pluviular'ui cristata Lam., Aiitcnnular'ui untcnn'tna Lam. Fam. Sertularidse. Branched Polyp s'.ocks, the Polyps of which project ia flask-shaped hydr(itheca3 on opposite sides of the stem. Bijnamena pumila L., Sertularia ahictinu. cvpresslnfL L. Fam. Camp^nularidae-Eucopidae. The cup-shaped hydrothccce are placed at the end of ringed stalks. The Polyps possess a circle of tentacles below their conical proboscis. Campanularla Lam. The proliferous individuals are situated on the branches and give rise to free McduscB, bell-shaped, with a short manubrium with four lips, four radial canals, the srr.e number of marginal tentacles, and eight inter-radial marginal vesicles. After separation the inter-radial tentacles are formed. C. QCl(/thia) Johnstonl = voluhUis Johnst., probably with Encope xarlahxlU Cls. Ohelia Per. Les., is distinguished from Camjxinularia by its Meduscs. These are flat, disc-shaped Medusce with numerous marginal tentacles, but with eight inter-radial vesicles. 0. dichotoma Jj. = (^Ccwijjaniiliiria f/elatinosa van Ben.), C. gcnictdata L., Laomcdea Lamx. The generative buds remain sessile in the hydrotheca of the prolifci-ous polyps. L. ealicvlafa Hincks. Fam. Aequoridae Medusa' with numerous radial vessels and maiginal tentacles. Aeqnorca Forsk. The Gertjonojts'uhe are allied here. OctorcJiis E, Haeck. Tima, (5) Sub-order : Trachy medusce. Medusce with firm, gelatinous umbrella, supported by cartilaginous ridges with stiff tentacles filled with solid rows of cells ; these may be confined to the young stage (larvfe of Geryonidce). Development by metamorphosis without hydroid asexual individual. Fam. Trachynemidae, with stiflE mnrginal tentacles, which are scarcely capable of motion. The genital organs are devel'opcd on vesicle-like swellings of the eight radial canals. Trachynema ciliatum Ggbr. lihopalonema relatum Ggbr., Messina. Fam. Aeginidae. The hard cartilaginous umbrella has a flat, discoid shape. The extended digestive cavity has pouch-like enlargements in pla?e of the radial vessels. The circular vessel is usually reduced to a row of cells. Cunina alhescens Ggbr., Naples. Aegincta flacescens Ggbr. Fam. Geryonidae. Umbrella with cartilaginous mantle ridges and four or six hollow tube-shaped marginal tentacles. The manubrium is long, cylindrical, or conical, with a proboscis-like oral portion, and four or six canals which lead into the radial canal. The generative organs lie on the radial canals ; eight or twelve marginal vesicles. Liriope Less., with four radial canals, four or eight tentacles and eight vesicles. L. tetraijJiylla Cham., Indian Ocean. Geinjonia P6r. Les., with six radial canals without lingual cone. G. uinhella E. HaecK. , Carmarina E. Haeck., with six radial canals and a lingual cone, E. Haeck. C, hast at a, Nice. IITDEOZOA — SIPIIOXOPIIORA. 243 Order 2. — Sipiionophora.* Free-swimming loohjmorphous hydroid-stocks ivith contractile stem, with polypoid nutritive indi- viduals and medusoid buds, usually also with nectocaly- ces, hyrophyllia and dactylo- zooids. Morphologi- cally the /SiyiViO- nophora are directly allied to the hy- droid-stocks ; but they possess to a much greater extent than the latter the characters of individuals, in consequence of the highly developed poly- morphism of their polypoid and medusoid appendages. The functions of the latter seem so inti- mately con- nected and are so essential for the preserva- tion of the entire colony that we may regard each colony of Sipho- * Besides Kolliker, C. Vogt, Huxley and others, compare C. Gegenba\ir, •' Beobachtungen liber Siphonophorcn," '/ieitschrift fur wIuk. Zool., 1853, C. Fig. 185.— Diagram of a colony of Pht/sophortda. St, Stem; Ek; ectoderm; En, entoderm; f'/i, Pneumatophor; Sk, nectocalj'X beins budded off; S, nectocalys ; D, hydrophyllium ; a, gono- phore ; T, dactylozooid ; Sf, tentacle ; P, polyp ; O, mouth of the latter ; i\rt, battery of nematocysts. 244 C(ELENIERATA. nophoi-a physiolcgiciilly as an organism and its appendages as organs. In this connection we may mention that the sexual medii- soid generation is so little independent that it only exceptionally {VeleUidce) reaches the morphological grade of the free-swimming Mednsa. lu place of the attached and ramified hydroid-stocks we find in the Siphonophora a free-swimming* con- tractile unbranched stem (hydrosoma), which is rarely provided with simple lateral branches. The upper end of the hydro- soma is frequently dilated to the form of a flask (pneumatophore), and contains an air chamber [pneumatocyst] (fig. 185). In every case there is a central space in the axis of the stem in which the nutritive fluids are kept in constant motion by the contractility of the walls and by the move- ments of the cilia. The air sac or pneu- matocyst at the apex of the hydrosoma is connected to the chamber which contains it by radial septa, and in many cases attains a considerable size {Physalia). It func- tions as a hydrostatic apparatus, and in those forms, which have a long spiral hydrosoma {Physoplioridcti), serves to keep the body in an upright position. In some cases the gaseous contents can escape freely by one or more openings. The appendages which are attached to the spirally twisted bilaterally symmetrical stem and whose cavities communicate with that of the stem are of at least two kinds — (1) The polypoid nutritive animals with their tentacles ; (2) the medusoid sexual buds. The nutritive Polyps (hydranths) are simple tubes provided with a mouth, and never Gegenbaur, " Neue Beitrage zur Kenntniss der Siphonophoren," Nova Acta., Tom. XXVIL, 18.59. K. Leuckart, " Zoologiscbe Untersuchungen," I., Giessen, 1853. K. Leuckart, " Zur niiheren Kenntniss der Siphonophoren von Nizza," Arcliiv. fiir Naivrgcsch, 1854. C. Clans, '• Uebcr Halistemma tergestinum n. s. nebst Bemerkungen liber den feineren Ban der Phj'sophoriden," Arhritcn atts dem Zoologisclmn Institvt. der Unir. W'lcn, etc., Tom. I., 1878. E. Met- schnikoff, " Studien Uber die Entwickehmg der Medusen und Siphonophoren," Zeitsch.fiir mss. ZooL, Tom. XXIV., 1874. Fig. 18C.— a portion of the stem and appendages of Halistemma itrgesthuim. St, Stem; X>, hy- drophyllium ; T, dactylozooid; Sf, tentacle of the latter ; Wg, female, Mg, male, gonophores. II YDEOZOA SirHO>'OPUOBA . 245 possess a circle of tentacles. They always, however, have a long tentacle arising from their base. This tentacle can be extended to a considerable length, and be retracted into a spiral coil. It rarely has a sinaple form, but, as a rule, it bears a number of unbranched lateral twigs, which are also very contrac- tile. These tentacles are invariably beset with a great number of nema- tocysts, which in many places are closely packed and have a regular arrangement. These aggregations of thread-cells are especially found on the lateral branches of the tentacles, and give ri-e to large, brightly-coloured swellings, the batteries of nematocysts. The batteries show consideraljle variations Fig. 1 Group of buds of a Fhysophor at the bottom of the ]>neuma!ophore. C, Central cavity ; Sk, nectoealyx bud with the ectoJormal ingrowth. Fig. 1S8.— Development of Aijahnojilf .^arsil (after Metschnikoff). a, Cilinted larva, b, Stnpe with developing hydrophj-llium (D). c. Stage with cap-shaped hydropliyllium (X>) and deve'iO]iing pneumatophore (i/). d, Stage with thi-eo hydrophyllia, (I>, D', D"), polyp (P), and tentacle. in form in the various species, genera, and families, and such varia- tions afford valuable characters for systematic lacssification. 246 C(EliE>T£EATA. separately in differently shaped buds, The second form of appendage, the cjonopliores, usually possess a bell-shaped mantle containing cii-cular and radial vessels, and surround- ing the central stalk or clapper (manubrium), which is filled with ova or spermatozoa. They usually arise in clusters at the base of the tentacles, more rarely from the nutritive Polyj)s themselves {e.g. in Velella). The male and female generative products always arise but are usually found closely approximated on the s; m ) stock (fig. 18G). There are, however, also dioecious Sipho- nojyhora, or if the medusoid buds or gonophores be regarded as generative organs, Sij^hono- ->hora of distinct sexe.~, e.g., Apolemia uvaria and Diphyes acuminata. The ripe sexual Medusoids frequently become separated from the stock, i.?. after the development of the generative products, and only rarely become liberated as f-mall Medusce {Chrysomitra in the Velellidce), which produce generative products during their free life. Besides the constant nutri- tive Polyps and medusoid gonophores, there are incon- stant appendages, which are also modified Polypoids or Medusoids. These are the mouthless worm-like dactijlo- zoids (fig. 186), which, like the Polyps, are provided with a tentacle, which is, however, shorter and simpler, and has no lateral branches or aggregations of nematocysts ; also the leaf -shaped hard cartilaginous hi/drophyllia, which serve to protect the polyps, dactJ^dozoids, a-nd gonophores ; and finally the appendages known as nectoculyces, which ai-e placed beneath the pneumatophore. The nectocalyces have a structure similar to that of the Medusoe, though their bilateral symmetry is apparent ; Fi&. 1S9.— Small larval stock of AQalmopnh niter the type of Afhuri/hia. Lf, Pneumatophore ; D, hydrophyllium ; Nk, groups of nemato- cysts ; P, polyp. nrDROZOA — siPiioxopnoRA. •247 ttiey are, however, without maiiul)riuin, mouth, tentacles, and sense organs. The deeply concave sub-umbrella surface of the nectocalyx is largely developed and has a very powerful muscular covering in rela- tion to its exclusively locomotive function. All the appendages are developed as buds formed of ectoderm and endo- d rm, and containing a cential cavity which communicates with the central sp ice of the stem. In the nectocalyces anl gonophores an ecto- dermal ingrowth gives rise to the coveiing of the sub-umbrella and to the generative products respectively ^(fig. 187). The ova, of which there is often only one in each female gono- phore, are large, and have no vitelline mem- brane, and, after im- pregnation, undergo a complete and regular segmentation. A nectocalyx [Diphi/es) is the first structure formed in the free-swim- ming larva, or the upper part of the body of the larva gives rise to a cap- shaped protective cover or hydrophyllium as well as a pneumato- phore, and the under part becomes the primary nutritive polyp {Agalmopsis, fig. 188). Since new buds give rise to leaf -shaped hydrophyllia, a small stock with Fig. IdO.—Phi/sophora hydrodatica. Fn, Pneumatoph.'-e ; iS, nectocalyces arranged in double rows on the swim- ming column; T, dactylozoid ; F, polyp (nutritive individual) with tentacles, Sf; Nk, gi-oups of nemato- cysts on the latter j G, clusters of generative buds. 24S CCEUIXXEllATA. Fig. l^T.—II,n!ttemma tergcsir, S. Nectocalyx; P, polyp; Pn, pneumatnphore ; aydroplij-llium ; J^7c■, { provisional appendages is formed which allows us to regard the develop- ment of the Siphono' phora asa metamoi-phosis (lig. 188 and 189). The croAvn of hydro- phyllia, which is com- pleted by the addition of fresh hydrophyllia after the appearance of a tentacle with provisional groups of nematocysts, persists only in Atliory- hia, where a swimming column with nectocalyces is never formed. In A(jalmoj)sis and Thysophora the primary hydrophyllia of the larva fall off as the stem be- comes larger, and are replaced by nectocalyces. (1) Sub-order: Fhyso- plioridce. Stem short, extended in the form of a sac (fig. 190), or elongated spirally (fig. 191), with a pneumato- phore, usually nectocaly- ces, which are arranged in two or more rows on a swimming column below the pneumatophore. Hydrophyllia and dacty- lozooids are usually pie.-ent, and alternate with the polyps and gonoph(;res in regular order. The body of the larva usually develops voups of nematocjEts. nTBROZOA. — SIPIIOXOrUOEA. 249 first a polyp with pneumatopbore and tentacle beneath an apical bydi'opbyllium. The female gonopbore has only one Qgg. Fam. Athorybiadae. "With a bunch of hydrophyllia in i)lace of the swim- ming cokimn ; resembling a persistent larval stage. Atlwrijhia rosacea Esch., Mcditenanean. Fam. PhyaoplioridsB. s. str. Stem short and enlarged to a sjiiral sac beneath the swimming column with its double row of nectocalyccs. No hydrophyllia but instead two outer bunches of dactylozooids with gonoblasfeidia, nutritive polyps and tentacles lying beneath them. Phi/aojiliora Forsk., Ph. hydvostattca Forsk., Mediterranean (fig. 190). Fam. Agalmidae. Stem unusually elongated and spirally twisted. Swimming column with two or more rows of nectocalyces. There are both hydrophyllia and tentacles. Fvrsl-nlta eo7itorta M. Edw., Ilalhtcmvta. Dactylozooids and hydrophyllia directly connected with the stem. In the ciliated larva a pneumatophore is first developed at the upper pole. //. ruhrum Vogf, Mediterranean. H. tergedinwni Cls. (fig. 191). Arial- inoj7sis Sarsii KolL, Ajjoleiitla uvarla Le^s., Mediter- ranean, Dicecious. (2) Sub-order: P/ti/.salidce.— 8t(^m. dilated to form a large chamber, the pneumatophore lying almost horizontally, containing a very large pneumatocyst opening to the exterior. Necto- calyces and hydrophyllia absent. On the ventral line of the sac are situated large and small nutritive polyps with strong and long tentacles. There are also clusters of gonophores attached to the tentacle-like polyps. The female buds seem to become free-swimming Medusce, Fam. Physalidae. With the characteristics of the group Physalla Lam., P. cararclla Esch. {Arcthvsa Til.^, wliifftca, utriculus Esch., Atlantic Ocean. (3) Sub-order : Cahjco'plioridoi. Stem long and fig. 192.— i'i>.\i/es acu. without pneumatophore. Swimming column mmata, magnified ■^ ^ /^ ^ about 8 times. Sh^ with double row of nectocalyces (Hippopodidfe) Fluid reservoir in tbe or with two large opposed nectocalyces, more yPP°'\ fectocaiys >^ i-i: .- J (somatocyst). rarely with only one nectocalyx. There are no dactylozooids. The appendages arise in groups arranged regularly, and can be retracted into a cavity of the nectocalyx (fig. 192). Each group of individuals consists of a small nutritive polyp, a tentacle with naked kidney-shaped groups of nematocysts, and gonophores. 250 CCELEXTERATA. To these is usually added a funnel or umbrella-sliaped hydrophyl- Hum (fig. 192). These groups of individuals may in some Diphyids become free, and assume a separate existence as Eudoxia (fig. 193). The gonophores contain numerous ova in the manubrium, which often projects as a cone from tlie aperture of the bell. In the larva the upper nectocalyx is th^e first formed. Fam. Hippopodidae. The swimming cohimn has two rows of nectocalyccs, and is situate on an upper lateral branch of the stem. The male and female gonophores are grouped in clusters and are situate at the base of the nutritive polyp. Gh'ha Hippojms Forsk., Mediterranean. Fam. Diphyidae. With two very lari^e nectocalyces at the. upper end of the stem and opposite to each other. Bipliyes acuminata Lkt., dicccious ; with Eudoxia campanulata. Ahyla jyentagona Esch., with Eudoxia cuboides, Mediterranean. Sjjhceronectes 'H.-a.±\.. = 3lonophyes Cls., Sp. f/racilis Cls. with LipJo- pJtijsa inermis, Mediteiranuan. (4) Sub-order: Discoidece. Stem compressed to a flat disc, with a system of canal-like spaces (.eutral cavity). Above lies the pneumatocyst in the form of a disc-.shaped reservoir of car- tilaginous consistence composed of concentric canals opening to the exterior. The polypoid and medusoid appendages are situate on the under side of the disc. In the centre is a large nutritive Polyp, around which are a number of smaller ones. To tlie base of these small Polyps are attached the gonophores. The dactylozooids are not far from the edge of the disc. The gonophores are set free as small Medusce {Chry- somitra), Avhich do not produce the generative material till long after separation. Velulla S2)irans Esch., Mcditer- i\ Fig. 103. -Part of a Di- phyd (after H. Leuck- art). D, Hydropliyl- lium ; GS, genital nectocalyx ; P, polyp with tentacles. The individual groups se- parate as Eudoxia. Fam. Velellidae. ranean. Porjjita vwditerranca Esch. Order 3. — Scyphomedus.e = Acalepiia.* Medusm of considerable size, with gastric filaments. The edge of the umbrella lobed. The sense organs covered. The embryonic stages are not hydroid stocks but Scyphistoma and Strobila forms. The Medusce of this order are distinguished from those of the hydroid group by their considerable size and the great thickness of * Besides the works of Erandt, L. Aga«siz, Huxley, Eysenhardt, compare V . Siebold, " Beitriigre zur Naturijeschichte der wirbellosen Thiere,'' 1839. M. Ui'DUOZOA — SC1PIIOMEDUS.E. 251 their umbrella, the gelatinous coun?ctive tissue of which is richly developed and contains a quantity of strong fibrillar and a network of elastic fibres, which structures confer upon it a greater firmness and rigidity. Another ch vracteristic of the group is derived from the structure o" the ed^e of the umbrella. This is divideJ by a regular number MA Fig. 191.— Aurelia aur'ifa, from the oral surface. JilA, Tlie four oral tentacles with the mouth in the centre; Glc, generative organs; GH, aperture of suli-irenital pit; iJt, sense organ (marginal body) ; RG, radial vessel ; T, tentacle at edire of the disc. of indentations usually into eight groups of lobes between which the sense organs are contained in special pits (fig. 194). - - The marginal lobes of the Acaleph*, like the continuous velum of the Hydromedusce, appear to be secondary formations at the edge of the disc. In the young stage known as Epliyra, which is common at least to all the Dlscopliora, they arc p-e_e:it as eight pairs of Sars, "Ueber die Entwicklung der Medusa aurita und Cyanea capillata," Archie, fiir Naturqcseh, 18 U. H. J. Clark. - Prodiomus of the History, etc., of the Order Lucern'aria," Journ. of Bost. Foe. of Nat. Hid.. 18G3. C. Claus, " Studien iiber Polypen und Quallcn der Adria," JJcnlmchriftcn dcr k. Akadnnic der WusemcJi. Wirn, 1877. C. Clans, '• Uiitersuchuncren iiber ('harybdea marsupialis," Arhclten aiia dcm Zvul. Iii.s/itiit, Win, 1878. Also E. Haeckel, 1. c. 252 COiLENTEEATA. relatively long tongue-like processes, and grow out from the disc-like segments of the Strob'da as marginal cones. An undivided mar- ginal membrane (the velarium), differing from the velum of the Craspedota [in containing prolongations of the canals of the gastro- vascular system], is present in the Charyhdeidce alone. The Acalejiha differ from i\xQ II jdromedusce in possessing, as a rule, large oral tentacles at the free end of the wide manubrium. These may be regarded as being derived from an unequal growth of the edges of the mouth. They grow as four arm-like processes of the manubrium from the angles of the mouth, and are placed radially, Fig. 195. — Diagrammatic lons-itudinal section through a lihizafoma. U, Umbrella; 3T, gastric cavity; S, sub-umbrella; G, genital band; Sih, sub-genital pit ; F, filament; SUf, muscle system of the sub-umbrella ; Ji,//, radial vessels ; Hk, sense Grfjars ; JRg, olfactory pits; Al, ocular lobe; Sir, shoulder tufts ; Dk, dorsal tufts ; I't, ventral tufts of the eight arms ; Z, terminal parts of the arms. i.e. they alternate with the genital organs and gastric filaments. In some cases the arms become forked at an early period, and four pairs of arms are formed, the lobed tufted edges of which may again divide and sub-divide into many branches. In this case, the margins of the mouth and the opposed surfaces of each pair of arms fuse in early life in such a way that the original central mouth becomes obliterated, and in its place there are developed a number of .small tufted orifices on the peripheral parts of the arms, through which nutriment is taken in (Rhizostomido'. fig. 195). HTDEOZOA — SCYPnOMEDUS.'E. 253 The form of the gastrovascvxlar apparatus exhilnts considerable differences, which in the Dlscophora may be considered as modifica- tions of the Ephyra tj-pe. The flat disc of the Ephjjra, which is split into eight pairs of lobes, contains a central gastric cavity into which the canal of the short, wide, four-cornered manu- brium leads. From this central cavity there diverge eight canal- like peripheral diverticula (radial pouches), between which there are formed sooner or later in the vascular lamella the same number of short intermediate canals (intermediate pouches). The radial and intermediate canals sometimes become enlarged, as in Pelagia and Fig. 196. Section through the olfactory pit, the sense-organ (marginal body) and its nerve centre, of Aurelia aurifa. S, Olfactory pit ; X, lobe of the umbrella covering the sense organ ; P, eye spot ; Ot, otolith of the auditory sac ; Z, cells after solution of the otoliths ; £n, entoderm ; Ec, ectoderm with the underlj'ing layer of nerve fibrillar, F. Chrysaora, so as to form unusually broad gastric pouches separated by thin septa and without any communication with each other at the periphery. Sometimes, however, they become transfoi-med into narrow vessels, between which, in the broad intervening septa, there is secondarily developed during the subsequent growth by a separa- tion of the two layers of the vascular-lamella, a rich network of anastomosing canals, and near the edge of the disc a circular canal {^Aiorelia, Rhizostovia). 254 CCELENTEEATA, The gastrovascular apparatus of the cup- or bell -.• haped Cahjcozoa and Charyldeklce differs from the tj-pes above described, and re- sembles that of the more primitive Scyphistoma stage, in that the gastric cavity presents only four peripheral vascular pouches, which are very wide, and separated by extremely thin septa. The worm-like movable tentacles of the gastric cavity, the gastric filaments, which are not found in any Ilyclromedusce afford an im- portant distinctive mark. They correspond to the so-called mesenteric filaments of the Anthozoa, and afford the same aid to digestion through the secretion of their glandular entodermal covering. In every case they are attached to the sub-umbrella wall of the stomach, and fall in the four radii of the" generative organs (radii of the second order), which alternate with the radii of the angles of the mouth, or radii of the first order. They usually follow the inner edge of the generative organs in a simple or convoluted curved line. The existence of the nervous system of the Acalepha has only recently been demonstrated with certainty. It has been proved that the centres of the nervous syst em are contained in the ectoderm of the stalk and base of the margin 1 bodies, and consist of a considerable layer of nerve fibrillfe deep in the ciliated ectodermal epithelium, the nerve cells of v/liich are elongated in the form of a rod, and bend round at their basal extremities to be continued directly into the nerve fibrillse (fig. 196). There is in addition a widely distributed and important peripheral nerve plexus intlie muscles of the sub-umbrella. Up to the present time no investigations have completely elucidated the manner in which this nerve plexus is related to the nerve centres of the marginal bodies, and how the latter are connected with one another. The existence of a nerve ring on the sub-umbrella surface has been proved only for the Charijhdeidce, in which the edge of the disc is not notched (fig. 169). The antimeres of the Acalepha show in all cases a great degree of individuality, and, when cut oft', are able to live for a considerable time. The marginal bodies, as well as the pit-like depressions on the dorsal side of the excavations in which the mai-ginal bodies are placed (olfactory pits), must be considered as sense-organs. The marginal bodies are morphologically the remnants of reduced tentacles. They may be seen on the under side of the umbrella in the stage of the Ephyra^ and are overgrown by portions of the edge of the umbrella {Htenanopldlmlmata). [They contain a central canal lined by endoderm and continuous with the gastro-vascular system of the cRsc, fig. 196]. They appear in all cases to unite *he functions HTDROZOA — SCYPHOMEDUS-E. 255 of ocular and auditory apparatus. The auditory function is provided for by a large sac containing crystals, which originates from the cells of tlue entoderm ; while the eye consists of a mass of pigment lying on the dorsal or ventral face, and nearer the end of the stalk. In some exceptional cases {Faiisitlioe) it is provided with a refractile cuticular lens. But it is in the Charybdeidfe that the sense body reaches the highest development ; for in them, in addition to the terminal Fac of otoliths, there is also present, in the wall of the dilated vascular space of the papilla, an extremely complicated visual organ, formed of four small paired and two large unpaired eyes, in which lens, vitreous body, and retina can he distinguished. The four genei-ative organs of the Acalepha can be easily dis- tinguished in consequence of their size and their bright colouring. In some cases, at any rate in the Disco2)hora, they protrude as folded bands into special cavities in the umbrella, the so-called sub-genital pits (hence the term Phanerocarjxe Esch.) In all cases these bands lie on the lower (sub-umbrella) wall of the digestive cavity (figs. 194, 195), from which they originate as leaf -like prominences. The upper surface is covered with gastiic epithelium ; the under, which is turned towards the sub-umbrella, with germinal epithelium, the elements of which, in the process of development, pass into the gelatinous substance of the band. The formation of the cavities in the sub-umbrella of the Disco'phora is due to a local growth of the gelatinous substance of the sub-umbrella; in some cases, however, they may be completely absent [Discomedusa, Nausithoe). The mature generative products are dehisced into the gastric cavity, and pass out through the mouth; but in many cases the ova undergo their embryonic development either in the ovary {Chrijsaora) or in the oral tentacles (Aurelia), Separate sexes are the rule. Male and female individuals, however, apart from the colour of their generative organs, have only slight sexual differences, as, for instance, the form and length of the tentacles (Aiirelia). Chrysctora is hermaphrodite. In the Discophora the development is generally accompanied by an alternation of generations ; the asexual generations being repre- sented by the Scypldntoma and Strohila ; but in exceptional cases it is direct (Pelar/ia). In all cases a complete segmentation leads to the formation of a ciliated larva, the so-called j^^O'^ida, which attaches itself by the pole vWiich is directed forwards in swimming. This pole is, however, opposite to the gastrula mouth, which in the meantime becomes closed, while round the mouth, which is 256 CCELEXTEEATA. foi'med as a perforation at the free eiid, the tentacles appear. As in the embrj-o jiclinia, two opposite tentacles first make their appearance ; not, however, simiUtaneously, the one appearing after the other, so that the young larva about to develop into the Scyphis- toma presents a bilaterally symmetrical structure. Subsequently the second pair appear in a plane at right angles to the plane of the first tentacles. These four tentacles mark the radii of the first order. Then alternating with these, but in a less regular suc- cession, the third and fourth pairs appear ; and soon after in the plane of these latter four longitudinal folds of the gastric cavity are developed (i-adii of the second order or of the gastric filaments and genital organs). The eight-armed ScyjMstonia soon proiluces eight fresh tentacles, which succeed one another in irregular succession, and alternate with the tentacles already present. Their position determines the inter- mediate I'adii of the future young Biscophor or Ephyra. After the formation of the ciBcle of tentacles and the secretion of a clear basal periderm {Chrysaora), the Scyjihistoma is capable. of reproduction by fission and gemmation. At first the Scyphisfoma appears to multiply only by budding ; the second mode of reproduction, the process of strobilization, begins later. This consists essentially in the fission and division of the anterior half of the body into a number of segments, thus changing the Scyp>histovia to a Strohila. The separation of the segments progresses continuously from the anterior end to the base of the Strobila, so that after the disappearance of the tentacles, first the terminal segment, then the second, and so forth, attain independent existence. Each segment becomes an Ephyra, developing eight pairs of elongated marginal lobes, with a marginal body in the notch which separates the two lobes of the same pair. It is these marginal lobes which give to the edge of the umbrella of the Ephyra its characteristic appearance. The young Ephyra gradually acquires the special peculiaiities of form and organization of the sexually mature animal (vide figs. 113 a — h). The number of nematocysts accumulated on the upper surface of the disc and on the tentacles of many Medusce enable them to cause a perceptible stinging sensation on contact. Many, e.g. Pelagia, are phosphorescent. According to Panceri, this phenomena originates in the fat-like contents of certain epithelial cells on the surface. In spite of the delicacy of their tissues, certain large Medusce have left impressions in the lithographic slate of Sohlenhofen (Mediisites circularis, etc.) sciniOMEDUs.i: — caltcczoa. 057 ti) Sub-oruer: Calycozoa (Cylicozoa). Cup-shaped Acalepha attached by their aboral p)oh. They have four wide vascular pouches sejxirated by narrow walls, and eiyht arm- Uke processes beset with tentacles on the edge of the umbrella. The Calycozoa are best considered in their relation to the Scyhis- toma. They may be looked upon as Sr-yphistoma deprived of their tentacles, Avhich indeed are only transitory structures, and elongated so as to assume the form of a cup, and changed in several particulars which are characteristic, of the medusa stage. The four septa arise by the fusion of the four gastric folds with the Avide oral disc, which becomes drawn in and concave like a sub- umbrella. These four septa separate the same number of gas- " b Fig. 197. — o, A Calycozoon (Luccrnar'.a) fvoTO. the oral surface maguified about 8 diameters. S, Septa of the four gastric pouches ; L, longitudinal muscle fibres ^T^th the genital band ; Rt, marginal tentacles. I, The Calycozoon seen from the side ; G, Genital organs ; Gn, gastric fold in the stalk ; at the base is the foot gland. trovascular pouches ; while the margin of the cup is drawn out into eight arm-like processes, from which groups of short, knobbed tentacles arise (fig. 197). The genital organs extend on the oral wall of the umbrella into the arms as eight band-shaped, plicated ridges. They run along in pairs at the lower part of each septum in the gastric cavity. The ovum, according to Fol, undergoes a complete segmentation, which i-esults in a single-layered blastosphere. This becomes an oval, two- layered larva, Avliich becomes ciliated, swims freely about, and finally attaches itself. The further development probably takes place T>ple gastric sacs, without oral arms, but with ei'^ht marginal tentacles. The genital organs (in four pairs) do not lie in umbrella cavities. E.prlayica Koll., Mediterranean and Adriatic. Fam. Pelagidae. Pelagiii Per. Les. With wide gastric pouches and eight long marginal tentacles in the iuterradii. No alternation of generaticms. P. noctiluca Per. Les., Mediterranean. Chrijmora Vkv. Les., with twenty-four long marginal tentacles. The radial and intermediate gastric pouches are per- ceptibly different. Chr. hysoscdla Esch. Hermaphrodite, North Sea and Adriatic. Fam. Cyaneidae. Cyanca Per. Les. The tentacles are united in bundles on the under surface of the deeply lobed thick disc. There are sixteen (eight radial and eight intermediate) more or less wide gastric pouches, which break up near the end of the marginal lobes into small ramified vessels. C. cajrillata Esch. Fam. Aurelidse. Diaroiiu-dnxa Cls. With large oral arms, with branched vessels and 24 marginal tentacles. Subgenital pits present. D. lohata Cls., Adriatic. AvrcUa Per. Les., with branched radial vessels and edge of disc fi-inged with small tentacles. A. anrita L. {Mi'dusa aurlta L.), Baltic, North Sea, Adriatic, etc. A.f:ividula Ag., coast of North America, 2. Rhizostomece. No central mouth, funnel-shaped slits in the eight oral arms and eight, rarely tv^elve, mai-ginal bodies on the lobed margin of the disc. There are no marginal tentacles. The central mouth, which is at first present, becomes closed during the larval development by the fusion of the edges of the lips. Funnel-like splits are formed on the folded edges of the four pairs of arms, the so- called suctorial mouths, by means of which microscopic bodies are received into the canal system of the oral arms (fig. 19.5). Rliizostoma Cuv. The arms end in simple tubular prolongations, and bear accessory tufts at their bases. Rh. Cuvieri Per. Les., Cepliea Per. Les. The branched oral arms have groups of nematocysts and long filaments between the terminal tufts. Cepliea Per. Les. (^Cas.-^iopca^ borhonica Delle Ch., Medi- terranean and Adriatic. CLASS III.— CTENOPHOPA.* MeduscB of spherical or ci/lindrical, rarely hand-sliapzd form ; vnth eight meridional rows of vibratile jylO'tes formed of fused cilia. They * C. Gegenbaur, "Studien Uber Organisation und Systematik der Cteno- phoron," Archiv. fur IViiturgeKch., 18.56. L. Agassiz, " Contributions to the Natural History of the Unitel States of America," vol. iii., Boston. 1860. A, Kowalevski, "Entwickelungsgeschichte der Rippenquallen," Petersburg, 1866. H. Fol, " Ein Beitrag zur Anatomic und Eutwioklung-sgeschichte einiger Rip- penquallen," Inaugural dissertation, Jena, ISiiD. A, Agassiz, "Embryology of th'3 Ctennphorae," Cambridse. U.S., 1874. C. Chun. "Die '^tenoohoren dea Golfes von Neapol," Leipzig, ISSO. 262 C(£Li:XlERATA. Fig. 201. — Cyd'ppe, Been from the apical pole. S, Sagittal plan?; T, transverse plane ; R, swimnung plates; Gf, gastro-vascular system. an oesophageal tube and a gastro-vascular canal system. Two lateral tentacles, which can be retracted into pouches, are often present. The Ctenophora possess a shape which can in all cases be reduced to a sphere. They are radially symmetrical free-swim- ming Coilenterata of gelatinous consistence. The body is often bilaterally compressed, so that it is possible to distinguish two planes passing through the long axis at right angles to one an- other ; these arethesa^t^ irt^planeand the transverse plane, and are analogous to the median (longitudinal vertical), and lateral (longitudinal horizontal) planes of bilaterally symmetrical animals (fig. 201). The arrangement of the internal organs bears a relation to these two planes. All parts of the body which occur in pairs, as the two tentacles, the gastric canals, the hepatic bands of the stomach, and the vessels which give oingin to the eight lateral canals, all lie in the transverse plane, while the sagittal plane coincides with the longer axis of the oesophageal (gastric) tube (whence also called the gastric plane), the two so- called pol r-fields, and the terminal vessels of the infundibulum. The infundibulum is so compressed that its longest diameter falls in the lateral plane, which on this account is sometimes called the infundibular plane. Since these two planes divide the body into halves, which correspond with one another, and since there is no division into dorsal and ventral surfaces, the arrangement of the body may be said to be bi-radially symmetrical, but cannot be called Fig. 202.— Cyif '>;)p (llormiphora) phnnosa (after Chun). O, Mouth. CTENOPnOKA, 2fi3 bilaterally symmetrical, although each half possesses this property. The body is divided by these two perpendicular pianos into four similar quadrants. Locomotion is principally effected by the regular vibration of the hyaline swimming plates, which are dis-posed over the surface of the body in eight meridional rows, in such a way that each quadrant possesses two rows of plates, a transverse and a sagittal (fig. 202). Locomotion is also assisted by the contractility of the muscle fibres of the gelatinous tissue; this contractility in the band-shaped Cestidce causes an undulating motion of the whole body. The mouth, which is sometimes surrounded by umbrella-shaped lobed processes of the gelatinous tissue, leads into a wide [Beroe) or narrow oesophageal tube, which in the latter case soon becomes flattened and broad. The oesophageal tube is furnished with two hepatic bands, and com- i Fig. 2'3. -Aboral end of Calllan.ra llalnfa (after R. Hertwifr). x. The two polar spaces ; w, the bchora live in the warmer seas, and, under favourable conditions, often appear in great quantities at the surface. They feed on marine animals of various size, which they capture with their tentacles. Many, as the Beroiclte, which do not possess tenta- cles, are compensated for this deficiency by the possession of an unusually large mouth (fig. 205), by means of which they are able Fig. 2ro — B-ros oiatw. Of, Litliocyst, at its sides are the small tentacles of the polar areas ; Ti; infundibulum. 266 l:ClirN'ODEEMA.TA. to receive relatively large bodies, even fishes, into the wide oesophageal tube, and to digest them. Although the average size is small, some of them, as Cestum, Eucharis, reach the length of a foot. Fam. Cyd'ppidae. Body slightly compressed, spherical or cylindrical, with extremely regular development of the swimming plates. Their structure is therefore apparently octoradial . They possess two tentacles ; the vessels of the stomach and swimming plates end blindly. Cyd\ppc liorimplwra Ggbr. = Ilorm'iplwra jilumom Ag., Mediterranean. Esclischoltzia cordata Koll., Mediterranean. Fam. Cestidae. Body elongated to the form of a band in the direction of the sagittal plane. Two tentacles. Vctilhtin parallelum Fol., Canary Isles. Ct'stum Veneris Less., Venus' Girdle, Mediterranean. Fam. Lobatae. The laterally compressed body possesses two umbrella-like lobes near the mouth, and has relatively small tentacles. Eurhamphaea vcril- ligera Ggbr., Mediterranean and Atlantic Ocean. CMaja papUlosa, M. Edw. {Alcime papillosa Delle Ch. = Xrapolitana Le.j., the madreporic plate, the stone canal, heart, etc., always remains single, and does not fall in the axis of the body, it will be only those planes, in the radius or inter-radius of which the unpaired organs fall, which can fulfil the ic. 2(ij.— The shell of a regular Sear archin seen from above. B, Radius with double row of perforated plates ; J, Inter-radius with the genital organs and their pores. In the rij;ht ante- rior inter-radius is the madrepcric p^ate. 2G3 Ecni>'or«:r.MA'rA. conditions Avhich admit of the body being divided into two exactly symmetrical halves. Even these planes do not exactly fulfil these con- ditions, since the re- maining organs are not strictly symmetrical in regard to such a plane. Very frequently one of the rays differs in size from the others, and then we have an itvegu- larity in the external form of the Echinoderm, ■which renders the bi- lateral symmetry visible even from the exterior. The pentamerous body of the Echinoderm may become bilateral, the plane of the unpaired ray forming a median plane, on each side of which two paii-s of equal rays are repeated. We can distinguish an upper sur- face (apical pole) and an undei (oral pole), a right and left bide (the two paired rays and Iheu inter-radii), an anterior end (un- paired radius) and a posterior (unpaired inter-radius). In the irregular Sea-urchins, the biUte rally symmetrical form is still more strongly marked. Not only is the unpaired radius of abnormal size and form, and not only are the angles at which the principal ray and the accessory rays cut each other equal only in pairs, but also in the Clypeastridea (fig. 207), the anus is removed from the dorsal pole to the ventral half of the body in the unpaired inter-radiu Fib. 207. — ChjpeaKUr rosaccun, from the dorsal side. The madreporic plate is situate in the centre and is sur- rounded by five genital pores and by the five-leaved rosette. The unpaired radius is directed forwards. At the side is the median portion of the ventral sur- face. O, mouth ; A, anus. / Fio. Zn^.—Si-hzagter {Spatangidcc) , frorr'. fhe ventral side. O, mouth ; A, anus ; P, pores of the ambulacral feet. BITIUM— TRITIUM. 2G9 while, in Sjiatangidce, both poles, or only the oral pole, are shifted in the direction of the unpaired radius, and become eccentric (fig. 208). Only a few of the regular Echinodermata have the means of loco- motion on all the five rays, and seldom then along the whole length of their meridians ; far more frequently the area surrounding the oral pole becomes with regard to the position during movement the ventral surface; it is flattened and mainly or entirely possesses the organs of locomotion (cim- hulacral surface). These re- lations always obtain among the irregular EcJdnoder- mata which do not move indifferently in the direction of all five rays, but princi- pally in that of the unpaix-ed one. In these animals the mouth, and therewith the oral pole, being pushed to- wards the anterior edge, the two posterior radii (biviiom) seem principally concerned in the formation of the ventral surface (S'pa- tangidce). It is otherwise in the case of the cylindrical Holothurians. Their mouth and anus preserve the nor- mal position at the poles of the elongated axis, and the body is not unf requently compi-essed in the direction of the axis in such a manner that three radii (irivium) with their organs of locomotion are placed on the foot-like ventral surface. On the body of these Holothurians one unpaired and two paired radii can be distinguished, only in this case the unpaired radius with its inter-radius marks, not the anterior and posterior, but the dorsal and ventral surfaces. In many Echinoderms (Echinoidea) the oblate spheroidal form is the prevalent one. The principal axis appears shortened, the apical pole may be either pointed or flattened, and the ventral half is Fig. 209. — Cuciimaria -with extended dendritically branched tentacles {TJ. Af, ambulacral feet. J 70 ECHIXODEEMAl.*. Fig. 210.— Calcareous bodies from the infp;rwment of Holothu- riaus. o, calcareous wheels of Chiroduta ; i, anchor with supporting plate of Synapta ; c, chair-hke bodies ; d, plates of Hulothuria impatlens ; e, hooks of Chiroduta. flattened out to form a more or less extended surface. The cylin drical form is obtained by an elongation or cue «2ii [Ilolothuroidea) (fig. 209), the round form by a shortening of the same and the penta- gonal disc by the latter process combined with a simultaneous elonga- tion of the radii. If the radii are elongated till they are two or more times the length hiurid(e ten lobed generative glands composed of a number of blind tubes are developed around the stomach ; their products pass through special passages into pouches, and from thence to the exterior through paired slits on the ventral side between the arms. The generative glands of the Crinoidea are concealed in the arms and pinnules. In tlie Ilolothurians, the generative organs are reduced to one branched gland, the duct of which opens to the exterior not far from the anterior pole of the body on the dorsal side (fig. 219). The develojyment of the Echinodermata presents as a rule a complicated meta- morphosis, and is characterised by the possession of bilateral larval stages. Fig. 223.— lart of the inter-radius of Many Hdothurians are developed with- V:^!^^^;^;'^^^:^^^ out passing through the e larval stages, of pores (sieve plates) on the dor- 1 j_ • CI 1 • sal skin (after J. Miiller and Troo- as also are certain bea-urchms, as ^,J^gJ^ Anoclianus, Ilemiaster, and some Aste- roidea, which are either viviparoas {^Amjihiura squamata) or lay only a small number of eggs, and protect them during their development in a brood pouch. In these cases also the first stage is a ciliated embryo, which is either developed directly or passes through a much simplified metamorphosis. In the cases of a complicated metamorpho.sis, the ovum, after under- going a nearly equal segmentation, gives rise to a spherical embryo, the cellular wall of which is ciliated and encloses a central gelati: 0 is substance (fig. 103). A pitlike depression of the cellular wall gives rise to the first rudiment of the alimentary canal, and the opening of this depression (gastrula mouth) to the anus. The ciliated embryo becomes elongated and gradually takes the form of a long, oval, more 280 ECHI>*ODEBM±TA. or lesa pear-shaped larva, in whicli a slightly arched dorsal, t"wo symmetrical lateral, and a saddle-shaped ventral surface can be dis- tinguished. The cilia which are concentrated upon the raised edge of the ventral depression give rise to a continuous ciliated band which serves as a locomotive apparatus. [This band first appears as two f-eparate ciliated ridges placed transver.-^ely, one in front of, and the other behind the mouth (fig. 224, 3). These soon become con- nected laterally.] The alimentary canal, which has now acquired an anterior opening, the mouth, consists of three portions, — the oeso- phagus, the stomach, and the intestine. The wide mouth leading into the oesophagus is situated within the band of cilia on the ventral surface ; the anus is also ventral, but external to the ciliated band in the region of the posterior pole. Before the appearance Pia. 221. — Larval development of Atteracanthion heiylintig (after A. Agassiz) (for earlier stages see fig. 103). 1, stage where the mouth (O) has just appeared, represented in profile; ^.blastopore (anus); 2>, intestine; 1'p, vaso-peritoneal sac. 2, Somewhat older stage in surface view with two separated vaso-peritoneal sacs. 3, Later stage, from the ventral side, with two transverse ciliated ridges (W) ; the sac on the left side has an excretory pore. 4, Young Bipinnaria with double band of cilia (IVJ. of the mouth, another organ is separated horn, the alimentary canal : this is a sac-like ciliated tube, which opens to the exterior by a pore on the dorsal surface, and represents the first commencement of the ambulacral system, A second organ, which also has its origin from the rudimentary digestive canal, consists of the disc-shaped lateral sacs (fig. 224), from the walls of which the peritoneal lining of the body cavity is produced. With their progressive development the larvte of the Sea-urchin, the Starfish, and the Hohthurian diverge more and more widely from one another. The raised edge of the depression just mentioned, with its band of cilia, oecomes bent and prolonged into processes (fig. TTPES OF LAEViE. 281 225) of different form. These processes are arranged with a strict regard to bilatei*al symmetry, and their number, position, and size essentially determine the special shape of the body. An anterior and a posterior ventral region of the band of cilia can be distinguished from the lateral parts which form the dorsal portions ; the latter curve round and pass into the former at the anterior and posterior ends of the body (fig. 225, b). The dorso-lateral parts may, however, unite anteriorly with one another mthout passing into the anterior ventral band; in this case the anterior continuations of the latter pass directly into one another so as to form an independent' pra^oral ring, while the dorso-lateral and posterior ventral portions of the origin- ally continuous band form a longitudinally directed post-oral ring. This ai-rangement is characteristic of the larvae of the uisteridea (Bipinnaria, Brachiolaria). In all other forms a single longitudinal band of cilia only is pre- sent. In the larvae of Holo- thurians, the Auricularia {fig. 225), the processes re- main short and soft ; they are found on the dorso- lateral edges and on the posterior dorso-ventral arch of the band of cilia; they also appear on the posterior ventral (umbrella) and the anterior ventral (oral shield) parts of the band. The processes have a similar disposition in Bipinnaria, where, however, they are often much longer, but are in this case also not provided Avith calcareous rods. The Brachiolaria are distinguished from the Bip)innaria by the possession of three anterior arms, which are placed between the anterior portions of the two rings of cilia, and serve as a fixing apparatus. The bilateral larvse of the Ophiurids and Sea- Urchins, the so-called Pluteus forms, are distinguished by their large rod- shaped processes, which are supported bv a svstem of calcareous rods. Fig. 22$.—Auricidarta larvse (after J. Miiller). a, from the dorsal side ; b, from the ventral side. 0, mouth beneath the oral shield ; Ot-, CESO- phagus ; M, stomach ; T>, intestine with anus [A) ; P, peritoneal sac ; V, Water-vascular rosette with pore ; K, calcareous wheel-like bodies. 282 ECHINODEEMATA. Tlie Fluteus larvte of the Ophiurids possess long lateral arms on the anterior dorso- ventral arch of the' band, on the dorso- lateral edge, and on the edge of the pos- terior ventral hood. The Pluteus larva of the Sea-urchin has no lateral arms, but pro- cesses are developed on the edge of the anterior ventral hood (fig. 226). The larvre of the Sjyatanfjidce are characterised by an unpaired apical rod, and those of Echinus and Echinocidaris by the presence of ciliated epaulettes (fig. 227). The transformation of the laterally symmetrical larva with its bilateral processes and com- plicated organization into the body of the adult Echino- derm is not in all cases effected in the same mnnner. In the Sea-uichins and Star- fishes the young animal is developed by a process of new formation within the body of the larva, the stomach, intestine, and dcreiil sac alone persisting; while the transformation of the Auricularia into Synapta takes place without the lc33 of so many parts of the larv.i, the young passing through a pupadike intermediate stage. Fig 22r-P/«f.«.iarva of rcHn«WuW«. with fo-ir i- r ° ciliated epaulettes (TTe) (after E. Metschmkoft> In the first case a mass of from the ventral side. O, Muuth ; 4, anus. -Tlutcus of a Spaiangiig with •o-?allcd apical rod (StJ (after J. Muller). METAMonrncsis. 2srj interstitial tissue filled with round cells is formed external to the lateral discs, and with participation of the thickening skin. This tissue becomes the seat of calcareous deposits, and forms the dermal skeleton of the adult Echinoderm (fig. 228 a, h). The canal of the dorsal pore has in the meantime changed its simple form and developed into the circular vessel with diverti- cula, which are destined to become the ambulacral trunks. As development progresses, the young animal appears as a more or less spherical or pentagonal body, or as a star with short arms, in propor- f"iG. 22B.~B'pinnar'a frcm Trieste forminpr a stase in the develf pment of the Stir-Ssh (St) (after J. Muller). a. Earlier stage. M, stomach; A, anus; V, ambulacral rosette with ciliated tube opening by the dorsal pore, h, Older stage. tion as it predominates over the larva. Finally, after the sprouting out of the ambulacral feet, the young Echinoderm becomes separated from the larval body, which not unfrequently remains attached to the former, like the remnants of a broken-down framewoi'k. The stomrch, which is taken into the interior of the body of the Echinoderm, is torn from the oesophagus of the larva [Biinnnaria), and acquires a new oesophagus and mouth. The dorsal pore becomes the pore of the madreporic plate. The SynaiAvloi, on the contrary, are formed by the transformation of the entire body of the Aicricularia. Five tentacles appear in front 284 ECIIINODEUMATA. of the stomach and the circular vessel, which is formed from the dorsal tube. They are at first enclosed in a cavity, from which later on they penetrate to the exterior. The larva retracts its lateral lobes and transforms itself into a barrel- shaped body with five transverse rows of cilia, and loses the mouth and dorsal pore (fig. 229). The ambulacral system gradually de- velops further, the intestine be- comes longer, the first five tentacles break through to the exterior, the mouth appears at the anterior pole, and the first suctorial foot with its ambulacral vessel is seen on the ventral surface (fig. 230). The animal gradually loses the bands of cilia, and as a young Holothurian creeps about by means of its ten- tacles and of the first ambulacral foot, which is soon followed by a second new one. In the more direct development the bilateral larva seems to be more or less completely suppressed, and the time of free-swimming life shortened or altogether dispensed with. In these cases, protective arrangements, such as brood pouches, are always present in the mother. The brood pouch of Pteraster militaris is the mo;it carefully pro- tected. It lies above the anus and generative open- ings ; its walls are highly charged with calcareous matter, and they are raised above the spicules on the back. From eight to twenty ova (1 mm. in diameter) pass into the interior of the brood pouch, and are there developed into oval embryos, which acquire several sucking feet and assume later the form of a star with five rays. Fig. 229. — Auricularia pupa of Si/napta seen in profile (after E. Metsclmikofl). The mouth is ah-eady large, so that the tentacles (T) can be protruded. IfV, Ring- of cilia ; Pi?, Ft, somatic and visceral la.yers of the peritoneal sacs ; Ob, auditory vesicle ; Fo, pore of the water-vascular system; R, cal. careous wheel-shaped body. Fig. 230.— Young Holothurid with extended ten- tacles (T), swimming and creeping (after J. Miiller). PROTECTED DEVELOPMENT. 285 The formation of tl:e embryo takes place thus : four shield-like thickenings are formed upon one segment of the o\aTm, and beneath them several ambulacral feet make theu- appeai-ance. The star is developed by the increase in size and number of these discs and ambulacral feet. At this period of development Ave can distinguish the circular ambulacral vessel surrounding a central hemispherical projection of the oral disc, also the five vascular trunks and 2 — 3 pairs of sucking feet in each ray. In other cases, the brood pouch is formed upon the ventral surface of the Star-fish, e.g., Ecldnaster Sarsii, and the embryo, which is completely ciliated, is provided at the anterior end with a knobbed process. The latter is divided into several structures (Haftzapfchen), which serve as organs to attach the body of the embryo to the wall of the brood pouch. Suctorial feet are now formed in each ray, two paired and one unpaired, the latter lying nearest to the angle of the pentagon. The five angles come to project more and more, and acquire eye spots and ambulacral groove.-;. Spicules appear, and the mouth perforation is formed, the fixing organ aborts, and the em- bryos escape from the maternal brood pouch ; and being at this time capable of creeping and of nourishing themselves independently, they gradually develop into small star-fishes. The mode of deve- lopment is the same in Asteracanthion Mdllerii, and some Ophiurids, as Amphiura squamata. Amongst the Holothurids {H. tremula) the simple and more direct development was first observed by Danielssen and Koren, and hiter by Kowalevski, in P/ojllojjhorus ibrna, and by Selenka, in Cucumaria doUolum. In the first case the embryo leaves the egg in the form of a ciliated larva, which soon assumes a pear shape, and develops the cii-cular vessel of the water- vascular system, and five tentacles round the mouth. The ahmentary canal and the dermal skeleton make their appearance before the five tentacles have assumed the function of locomotion in place of the cilia which have disappeared. Later on, w-ith the progressive gi^owth, the tentacles become branched, and two ventral feet appear, which put the bilateral symmetry of the larva beyond all doubt. In all cases, even in the cases of a more direct development, the radial symmetry of the adult Echinoderm appears to he preceded by a bilateral symmetry of the larva. All the Echinoderms are inhabitants of the sea ; they are capable of a slow, creeping movement, and feed on marine animals, especially on Mollusca, but also on Fuci and sea-weeds. Some are found near the coast at the bottom of the sea, others occur at considerable ECn I STODERMATA. depths. IMany possess a great reproductive power, and are able to replace lost parts, such as arms, with all their apparatus of nerves and sense organs. CLASS I.— CEINOIDEA.* Globular or cup-shapsd Echinodermata loith segmented arms fur- nished with pinmdte. They are usually attached by a segmented calcareous stalk. The shin up)on the aboral side is provided vAth 2)!ates, the ambulacral appe7idages have the form of tentacles, and are situated in the amhidacral furroios of the calyx and of the segmented arms. The greater number of Crinoidea are cha- racterised by the pre- sence of a segmented stalk bearing cirri. This stalk arises from the apical (dorsal) pole of the calyx, and is attached at the in- ferior end to surround- ing objects (fig, 231). in some few living genera, as Comatida (fig. 232) and Actino- metra, this stalk is only present in the young form. The body with the contained viscera appears, therefore, as the calyx at the upper end of the stalk, and only in exceptional cases is directly * J. S. Miller, "A Natural History of the Crinoidea or Lily-shaped Animals," Bristol, 1821. J. V. Thompson, " Sur le Pentacrinus Europaeus, I'etat de jeunesse da frenre Comatula," L'iiistitut, 1835. J. M'.iller, " Ueber den Bau von reiitanrinus caput Mcdusffi," Ahluindl. dcr Bcrl. Ahad., 1841. J. Wliller, *' Ueber die Gattung Comatula und ihre Arten," AhJiandJ. dcr JBcrl. Alum., 1847. Leop. v. Buch, " Ueber Cystideen," Ahhandl. dcr JBcrl. AMd., 1814. Ferd. Eomcr, " Monographie der "fossilen Grinoideen familie der Blastoideen," Fig. 2Z\.—Pini(icrimts caput Medusa: (after J. Miiller). O, mouth ; A, anus, of the disc, which is represented from the oral side. CBINOIDEA. 287 attaclied by its dorsal apex. The segments of the stalk, which are mostly pentagonal, are connected by bands of tissue, and are pierced by a central canal, which serves for nutrition, and contains a central a:id five peripheral blood vessels ; at certain distances they bear hollow and segmented cirri, which are arranged in whorls. The dorsal surface of the calyx is covered externally by regularly arranged calcareous plates, while the upper (ventral) surface, on which the mouth and anus are situate, is clothed with a leathery Fig. 232 —Comatida mediterraiiea represented from the ventral side. O, mouth ; A, anus. The pinnulse are filled with the generative products. skin. At the margin of the cup there arise movable, simple or forked, and often branched arms, which are su2:)p3rted by a solid framework con-isting of dorsally placed calcareous plates, which are movable upon one another by special muscles. In almost Arch, fiir Naturgesch, 1851, W.Thompson, "On the Embryology of the Antedon rosaceus," Phil. Trans. Roj. Soc, Tom 15.5, 18G5. W. B. Carpenter, "Researches on the Structure, Physiology and Development of Antedon rosaceus," Hid., Tom 1.56. A. Gotte, " Vergl. Eatwickelungsgeschichte der Gumatula Mediterranea," Archiv. fiir miekrosh. Anatomie, Tom XII. H Ludwig, " Merphol. Studien an Echinodermen," Leipzig, IcTT. 288 ECniKODERMATA. every case the arms bear, either on their main stems or on their branches, lateral appendages, the pinnules, which have an alternate arrangement on each side, one being attached to each segment of the arms. Essentially the pinnules represent the ultimate rami^cations of the arms. The. mouth, as a rule, lies in the centre of the cup. From it certain furrows, the amhulacrcd grooves, traverse the disc (fig. 231) Fi(i. 23a.— Developmental stages of Comatulz (Anfedon), much enlarcted. a, free-3wimaiin» larva vrith tuft and rings of cilia (Wr), also with rudimentary calcareous plates. 6, At- tached Pentacrinoid form of the same animal. O, Oralia ; R, Radialia ; B, Basalia ; Cd, Centrodorsal plate, c, Older stage described as Fentacrinus europaeus with arms and cirri (after Thomson) . and pass on to the arms, and their branches and pinnules; they are lined by soft skin, and carry the tentacle-like ambulacra! appendages. The anus, when it is present, lies excentrically on the ambulacral (ventral) surface of the disc. The development of the living genus Comatula, which begins with a barrel -shaped larva with four bands of cilia and leads to the fixed stage of the Pen- OIIINOIDEA. 289 tacrhms form [P. Europ(tus) (fig. 233), consists of a complicated metamoi'phosi.s. The greater number of Crinoids belong to the oldest periods of the Jiistory of the earth (the Cambrian, Silurian, Devonian, and the Cavbonifer-ous formations). Existing forms live mostly at considerable depths. We distinguish two orders, the Tesselata and the Articulata. The latter is represented by numerous fossil forms, bvit by only a few living genera as Penta- crinus, Ilolojms, and Comctula (fig. 234). The cup is always less completely provided with plates than in the fossil Tesselata. Articulata. Fam. Peatacrinidae. Crinoids with ten arms, several times bifurcated. There is a pentagonal stalk with whorled cirri. Pcutaerlmis caput Mcdvscr, Mill, from the Antilles. P. Miilleri Oerst., West Indian Ocean. The fossil forms are : Encrinits liliiformis Schl. (fig. 234) from the Muschelkalk ; also A2Jiocrimis, allied to the ! i existing Rliizocrimis lofotensis Sars, and to Batliycrinnx '■/'ODEBMATA. CLASS II.— ASTEROIDEA (STARFISHES).* Echinoderms vnth dorso-ventrcdly compressed 2^e'>^tf'i/onal or star- shaped body. TJie amhxdacral feet are confined to the ventral surface. Internal skeletal 2'>i^ces in the amhidacra articulated together like "vertehrcB. The Star-fishes are chiefly characterised hj the predominating pentagonal or star-like discoidal shape of the body, to the ventral Fig. 233. EcTiinasfo- seiifiis, from the oral surface (after A. Agai?si7,). ' 0, mouth; Af, ambulacral feet. surface of which the ambulacral feet are confined (fig. 235), The radii are long in compai-ison with the inter-radii, which are very short in consequence of the divergence of the interambulacral rows of plates; they constitute more or less projecting movable arms, with movable skeletal structures. These latter consist of transversely airanged, paired calcareous plates (ambulacral ossicles), * 'J, Miiller and Troschel, '• System cler- Asteriden," Brunswick, 1841. Com- pare besides the nnmcrons papers of Krohn, Sars, Llitken, L. Agnssiz, etc. ASTEKOIDEA. 291 which reach from tlie mouth to the end of the arms, and are articulated together like vertebra?. The skeleton of the Asteroidea is distinguished from the globular or flattened shell of the Echinoidm by the fact that the ambulacral and interambulacral plates are confined to the venti-al surface, and that on the outer side of the former there is a deep amhidacral groove, which contains, outside the ossicles and beneath the soft skin (which in OjMurids possesses special calcareous plates), the nerve trunks, the peri- lu-emal canals with the blood-vessels and the ambulacral trunks. In the Oplduridea the ambulacral groove is covered by the dermal plates so t'Aat the ambulacral feet project at the sides of the arms. Upon the dorsal surface the dermal skeleton appears leathery; it is, however, as a rule, filled with small calcareous plates, on which are placed spines, protuberances, and papilke, constituting a covering of a most varied kind. At the mar- gin in" the dorsal integu- ment there is usually a row of larger cal- careous plates {superior mar- ginal plates) (fig. 236). I '^ ' Fig. 23G.— 8keletal plates of A^i ropecfen Hnvprichtii (after J. Muller). U pon the ven- dh^ Dorsal marginal plates ; VR, ventral marginal plates, Ap, am- tral surface bulacral ossicles; Jp, intermediate interambulacral plates; Adp, anterior adambulacral plates forming an angle of the mouth. Ave can distin- guish, in addition to the internally placed ambulacral ossicles, inferior marginal ossicles (fig. 236, VR), also the adambulacral plates [Adp), and the intermediate interambulacral p)lates {Jp). The two last corre- spond to the interambulacral plates of the Ecliinoidea, where they occur as two or more rows, which are united along the whole length of the inter-radius : in the Asteroidea, however, they separate from one another at an angle, and are disposed along the opposed sides of ad- jacent arms. The ambulacral ossicles are calcareous bodies articulated together like vertebrae, with spaces between their lateral processes for the passage of the vessel connecting the ampullte with the radial vessel and the tvibe feet. The right and left pieces of each double row are either {Ophiuridea) immovably connected by a suture, or are 202 ECHIXODEEMATA. (Stelleridea) movably articulated by teeth, which fit into one another at the bottom of the ambulacral groove ; the latter only (Stelleridea) possess ti'ansverse muscles on the ambulacral ossicles, and are able to bend their arms together towards the ventral surface. The Ophiuridea are provided with longitudinal muscles only, by means of which they are able to bend their arms to the right and left in a horizontal plane with a serpentine movement. The mouth is always placed in the centre of the ventral surface in a pentagonal or star-shaped depression, the edges of which are usually beset with stiff papillte. The inter-radial angles are marked by the junction of two adambulacral plates, and frequently function as organs of mastication. The anus may be wanting ; when present, it invariably lies at the apical pole. The madreporic plate, of which there may be one or more, is situated inter-radially and dorsally {Stelleridea), or on the inner surface of one of the buccal •plates {Oijhiuridea), on the exterior of which a pore may be present. Development in certain oises takes place without the interposition of a bilateral larval phase with bands of cilia. When such larvse are developed, they have the form of a Pluteus (Ophiurid) or Bijnn- naria and Brachiolaria (Stellerid). The great power of regeneration possessed by Starfishes is not confined to the reproduction of lost arms, but may lead to the new formation of portions of the disc, or even of the entire disc from a single separated arm. This process thus amounts to a species of asexual reproduction by fission, and has been especially observed in forms with six (Op)hiactis) or more than six (Linckia) arms. Fossil star-fishes are found as far back as the lower Silurian strata (Palceaster), where intermediate forms between Stelleridea and Ophiuridea [Protaster) make their appearance. Sub-Class 1. — Stelleridea (Asteridea) Starfishes. Asteroidea whose arms are prolongations of the disc, and contain the hepatic apj^endages of the alimentary canal, and also the generative organs. They jmssess a deep, uncovered ambulacral groove running along the ventral surface, in v)hich groove the ambulacral feet are disposed in rows. The Stelleridea usually possess broad arms, and are chai*acteiised by the fact that the ambulacral ossicles of the two sides are connected by transverse muscles and are movable upon one another. The anus lies at the aboral pole, but may be wanting in certain genera {Astropecte7i). The madreporic plate and the genital pores are STELLEBIDEA. 293 situate inter-radially and upon the dorsal surface. The multilobed branched diverticula of the stomach extend into the cavities of the arms (fig. 218). On the ventral surface of the latter, two or four rows of ambulacral feet project from the deep ambulacral groove, the edge of which is beset with papillse (fig. 235). Pedicellarice are also found, and dermal gills projecting through the tentacular pores of the dorsal surface. They feed principally upon Mollusca, and, by means of their ambulacral feet, ci-awl slowly upon the bottom of the sea. Some few of them are developed by a very simple process of metamorphosis within the brood-pouch of the mother; but the greater number of them pass through the free larval stages of Jjipinnaria and Brachiolaria (figs. 22-i and 228), Fam. Asteriadae. The cylin- drical ambulacral feet end in broad suctorial discs, and are usually arranged in four rows along each ambulacral groove. Astcvia.i L. (^Asteracanthion'), A. rjlacialis 0. F. Mliller., Hi- Uastcr lii-Viantlnis Gray. Fam. Solasteridae. The cylin- drical ambulocral feet are dis- posed in two rows. Eays long, often more than five Solastcr jyaijposnis Retz., Ecli'master seposltns Eetz., Opkkliaster Ag., Llnckia Nardo. Fam. Astropectinidae. Am- bulacral feet conical, and with- out suctorial disc, arranged in two rows. There auranticicus Thil. Lnidia Forb., Ctcnodiscns Mlill. Tr. Fam. Brisingidae. Body shaped like an Ophiurid. Eays distinct from the disc with only a narrow internal cavity. Brisinga coronata Sars. Sub-Class 2. — Ophiuridea {Brittle Stars). Asteroidea characterised hy the absence of an amis, and by the pos- session of long cylindrical arms which are sharjyly distinct from the disc, and do not contain c^ppendages of the alimentary canal. The ambulacral groove is covered by the dermal p)lates so that the ambulacral feet p)i'oject at the sides of the arms. The Ophiuridea can be at once distinguished by the flexible cylindrical arms, which are sharply distinct from the disc, and enclose Fig. 237.- Axferiscus verntcidatiis, with the dorsal skin removed. Ld, Ralial appendages or hepatic tubes of the stomach ; G, generative glands. no anus. Astro2>ecten 294 rCnrN'ODERlIATA, no diverticula of the alimentary canal. The movements of the arms are principally in the horizontal plane, and in many cases permit of a creeping locomotion amongst marine jilants. The ambulacral groove is always covered by special dermal plates, and the ambulacral feet project laterally between the spicules and plates on the upper surface (fig. 238). In a few cases the arms are branched, and can be rolled up in the direction of the mouth. In such cases the ventral groove is closed by a soft skin {Astrojohyton). The anus is always wanting, as are 'pedicellarke. The generative products pass into genital pouches (bursse), and from these directly to the exterior through inter-radial paired slits. The madreporic plate lies upon the ventral sur- face in one of the buccal plates. Some few Ophiu- rids are viviparous, e.g., Avijihiura squamata; these do not undergo metamor- phosis. Most pass through the Pluteus larval stage, e.g., OiMoglyplia Lym., {Opldoleiiis) ciliata with larval stage Pluteus ixtradoxus. Fig. 2Z9.»-Ophiotli>-ijrfragiIig. The ends of the rays have been removed. &S, Slits of the genital pouches ; K, masticatory ossicles. Fam. Ophiuridae. With simple unbranched arms, and with ventral plates to the ambulacral groove. They are divided into special genera according to the pec-aliar character of the dermal covering and of the buccal armature. Ophiothrix Miill. Tr. The back is provided with granules, hairs, or spicules. The lateral plates of the arms bear spicules. Oiih. frafjilis 0. Fr. Miiller. OjjJitura Lam. (Oj)hio(Ierina'). Two pairs of genital slits in each interbrachial space. 0. loiujiccmda Link., OjJhivlejns Liitk., AvqMnra Forb. Fam. Euryalidae. Mostly with branched arms which can be curved towards the mouth and are without plates ; the ventral groove closed with soft skin. Astrojjhyton verrucosnm Lam., Indian Ocean. A. arhorescens Eond., Mediter- ranean. Astcronyja Lovcni Miill. Tr. CLASS IIL— ECHINOIDEA,* SEA-URCHINS. Spherical, heart-shaped, or disc-shaped Echinodermsioith immovable skeleton com20osed of calcareous plates. The skeleton encloses the body * Besides the works of J. Th. Klein, compare E. Desor, " Synopsis des Achinides fossiles," 1854 to 18.58. S. Lov6n, "Etudes sur les Echinoid^es." Stockholm 1874. Al. Agassiz, "Revision of the Echini," Cambridge, 1872- 1874, EClII^'OIDEA. 295 after the manner of a shell, and carries movable sjjuies. There is invariably a mouth and anus, and locomotive and often respiratory ambidacral appendages. The deruaal skeletal plates are connected togetliei' so as to form a firm immovable shell, which has no arm-like prolongations in the direction of the rays, and is sometimes regularly radial, sometimes irregular or symmetrical. With some rare exceptions among the fossil Perischccchinidoi, as Lepidocentrus, the calcareous plates ai'e firmly connected with one another by sutures, and are usually arranged in twenty meridional rows. These rows (fig. 206) are disposed in pairs, and correspond alternately with the radii and the .inter-radii. The five radial pairs are the ambulacral plates, and are piei"ced by rows of fine poi-es for the exit of tube feet (fig. 212, P), and bear, as do the broad interambulacral plates, spherical promi- nences and tubercles to which the differently shaped spines are movably articulated. The body form of the Sea-urchins, as con- trasted with that of the Star-fish, depends upon the meridional arrangement of the rows of plates, and, at the same time, on the continuity of the interambulacral rows. The position of the nerves and ambulacral vascular trunks beneath the skeleton is the special -chai-acteristic of the internal organization. Pedicellarice are found between the spicules, and are especially- numerous in the region of the mouth. Some Cidaridea are provided with branched respiratory tubes. The genital pores are disposed inter-radially on the genital plates near the apical pole. One of these genital plates is, as a rule, also the madreporic plate. The ocular plates, which are radial in position, are also pierced (figs. 206, 212). The regular Sea-urchins are often symmetrical, one radius being longer or shorter than the others, which are equal to each other. So we find long oval forms which are laterally symmetrical, having the mouth and anus centi-al, and an anterior unpaired radivis l^Acrocladia, Echinometra). In the irregular Sea-urchins the anus is thrust away from the apical pole into the unpaired radius {Clypea- stridce). The mouth also often has an eccentric position in front (Spafancjidce, fig. 208), in which case the masticatory apparatus is always wanting. In many regular forms all the ambulacral feet have the same shape, and are provided with a suctorial disc supported by calcareous bodies; in others the dorsal feet are unprovided with a disc, and are pointed and often have an indented edge. In addition to the ambulacral feet, the irregular Sea-urchins almost all possess ambu- 296 ECHINODEH M ATA, lacral branchite upon a rosette formed of large pores on the dorsal sui'face (fig. 239). The locomotive feet are very small in Chjiyeastridce, and are distributed either over the whole surface of the ambulacra, or are confined to branching rows upon the ventral sui^face. In the SpataiifjicUe there are peculiar bands upon the upper surface, the fascioles or semitce (fig. 239), upon which, in place of the spicules, knobbed bristles with active cilia [clavuke) are distributed. Develop- ment takes place with a Pluteus larval stage, in which the larva is pro\'ided Avith ciliated epaulettes or with an apical rod. The Sea-urchins live, as a rule, near the coast, and feed on molluscs, small marine animals, and Fuci. Some species of Echinus have the power of boring holes in the rocks in which they live. We find many fossil shells, especially in the chalk. Fig. iZ^.—Brlssopsie lynfera with the fascioles or Semites surround- ing the rosette. A, anus. Order 1. — Cidaeidea= Regular Sea-uechixs. Echinoidea with central mouth and egual hand-like amhvlacra ; with teeth and masticatory apparatus ; with suh-central anus in the apical space. Farn. Cidaridse. "SVith very narrow ambulacral and broad interambulacral areas, on both of which are large perforated tubercles and club-shaped spines. There are no oral branchije. Cidaris mctularia Lam., Phyllacantlms imxjerialis Lam., East Indies. Fam. Echinidae, Sea-urchins. The pores are grouped in transverse rows ; there is a round, thin shell, broad ambulacral spaces bearing tubercles and spines, which are mostly short and pear-shaped. Oral branchias are present. To-voj)- nciistcK variefjatu-t, Lam., Ecldmis melo Lam., Strongylocentrotus Uvidus Brit. sa.ratilis Lin., Mediterranean. Fam. Echinometridae. AVith long oval shell, imperforate tubercles and oral branchia\ Eclilni'mctva olloiiga Blainv., Podopliora atrata Brdt., Acrochidla friffonaria Ag., Pacific. Order 2. — Clypeastridea. Irregular Echinoidea compressed into the form of a shield. Mouth central and furnished icith masticatory apparatus. Very broad amhur- lacra, five-leaved amhidacral rosette round the ajyical pole, and very UOLOTIIUROIDEA. 297 small tube /ceL Five genital 2^0)-es in the region of the madre^wrio 2)late. Fam. Clypeastridae. The edge of the disc without indentations. Clijpcaster romccus Lam. (fig. 207), Echinocyamvs 2}uslllvs 0. F. Miiller, Mediterranean. Fam. Scutellidae. Flattened EchmoUlva with a shell often lobed or per- forated, with rows of pores for the ambulacral feet. Loho])Uora hifora Ag., Sot Ilia RunqMi Klein, Africa. Order 3. — Si'ATangidea. « Irregidar Echinoidea of a more or less hearl-shajjed form, with eccentric mouth and anus. There are no teeth or masticatory ap2)aratns, and there is usually a four-leaved amhidacrnl rosette and four genital plates. As a rule there are semitse and four genital pores, but the number of the latter may be i^educed to three and two. Fam. Spatangidae. Spatangus 2'^'rjmrcus 0. Fr. Mlill., Mediterranean ; Schizaster canaliferus Ag., Brissvs Klein. CLASS IV.— HOLOTHUROIDEA.* Wormlike elongated Echinoderms loith a leathery hody loall, ivith contractile tentacles surrounding the mouth ; anus terminal. The Holothuria approach the worms in possessing an elongated cylindrical shape and a bilateral symmetry, which is expressed in many ways. In particvilar they possess so striking a resemblance, so far as their extei-ior is concerned, to many Ge2'>hyrea that formerly they were included in the same group. The body-covering never consists of a firm calcareous shell like that which we find in other Echinoderms, but always remains soft and leathery, the calcareous matter being confined to a few isolated particles of definite form. In rare cases [Cuvieria), scales are present in the doi-sal skin. These are arranged like the slates on a roof, and may even take the form of spicule-like aj)pendages {Echinocucumis^. The bilateral symmetry results not only from the existence of un- paired organs, but from the contrast which is often very distinctly expressed between the dorsal and ventral surfaces. The ambulacral feet are not in all cases regularly arranged in the five meridional * G. J. Jaeger, " De Holothuriis,"' Dissert, inaug. Tnrici. 1833. J. F. Brandt, " Prodromus descriptionis animalium ab H. Mertcnsio in orbis terrarum circumnavigatione observatorum," Fasc. I. Petropoli, 1835. J. Miiller. •' Ucber Synapta digitata uud iiljer die Erzeugung von Schnecken in Holothurien," Berlin, 1852. A. Baur, '• Beitriigf zur Naturgeschiehte der Synapta digitata," Dresden, 18G4. C. Semper, " Keisen im Archipel der Pliilippinen." Tom J., Leipzig, 1868. 298 ECIIIKODERMATA. rows from the oral to tlie anal pole, but may be principally or altogether confined to the three rays of the so-called trivium. In this latter case the Holothurid moves upon a more or less foot-like ventral surface. The ambulacral feet may also be distributed uni- formly over the surface of the body, especially on the ventral side. As a rule, the tube-feet have a cylindrical shape, and terminate -nith a suctorial disc : in other cases they are conical, and the suctorial disc is absent. The tentacles, which are in communication with the water-vascular system, and represent specially modified ambulacral appendages, are simple or pennate, or even dendritic (Dendrochirota) or shield-shaped (^Asjndochirota), that is, provided with a disc, which is often divided into many parts. In certain genera (Sijnajita), the ambulacral feet are altogether wanting, and the tentacles re- main as the sole appendages of the ambulacral system (fig. 240). Locomotion is effected by the strongly developed dermal mus- cular system, the longitudinal fibres of Avhich are attacked to the calcareous ring surrounding the oesophagus. It is charac- teristic of the water-vascidar system that the stone canal, which is usually simple, hangs freely in the body cavity, ending in a calcai-eous framework com- parable to the madreporic plate. The I'espiratory trees at the end of the intestine perform the func- tion of resjnration, while certain glandular appendages (organs of Cuvier), which open into the rectum, may be regarded as excretory orrjans: these, as well as the respiratory trees, may be wanting. The generative organs consist of a bundle of branched tubes, the duct of which opens on the dorsal surface in the region of the mouth. The genus Synajita is hermaphrodite. The development is in many Holothurians direct (as e.g. in Holotlmria tremula according to Koren and Danielssen) ; Avhere there is a com- plicated metamorphosis, the larvre have the Auricidaria form, and pass through a barrel-shaped pupa stage. Fig. 240. — S^mijifa hihterens (after Quntre- fages). 0, Mouth ; A, amis : tlie intestine can be seen through the skin. noLOTnuROiDEA. 299 The IToIothurians are partly nocturnal in their habits, and live at the bottom of the sea, for the most part in shallow places near the coast, where they crawl slowly upon the bottom. The Synaptidce, which have no feet, burrow in the sand. They feed on the smaller mai'ine animals, which, in the Dendrochirota, a.ve carried to the mouth by means of the branched, tree- like tentacles. The As2ndo- chirota fill their intestine with. sand, which they eject from the anus by means of the current of water from the respiratory trees. It is worthy of notice that they (especially the Asjndochirota) can eject through the anal openipg the intestine, which breaks off easily behind the vascular ring, and are able to renew it. The Synapta, when irritated, are able to break their body into several pieces by violent muscular contractions. Order 1. — Pedata. Numerous amhulacral feet, which are sometimes arranged regularly in the meridians, and sometimes distributed over the whole surface. Fam. Aspidochirotae. With shield-shaped tentacles. Holotlutria L. With scattered ambulacral feet, which are conical on the dorsal side, and are without suckers. //. tuhvlom Gmel., Adriatic and Mediterranean ; //. cdul'is Less., the edible Trepanj^ of the East Indian seas. Fam. Dendrochirotae. With tree-like branched tentacles. Cvcnmaria Blainv. Ambulacral feet arranged in regular rows. C. frond osa Gr. Psolus Oken. Ambulacral feet confined to the foot-like ventral surface of the trivium. Ps. phantapus. Gr. Order 2. — Apoda. No amhulacral feet ; as a rule without respiratory trees ; the tentacles are usually branched or plmiate. Fam. Synaptidee. Hermaphrotlite and without respiratory trees. In the skin there are wheel-shaped calcareous bodies or projecting masses shaped like anchors, and affixed to calcareous plates. Synapta digitata Mntg. with anchor- shaped calcareous bodies. J. Miiller discovered in their bodies {jarasitic cylin- drical animals with spermatozoa and ova, which latter develop into small shell-bearing Gastropods (^Entoconcha mirabiUs). Chirodota Esch. Skin beset with rows of small tubercles bearing calcareous wheel-shaped bodies. The genus 3Iolpadia Cuv. is furnished with respiratory trees. ENTEROPNEUSTA. The remarkable genus Balanoglossus must be placed here. It is the representative of a class, Enteropneusta Gegenb.,* allied to the Echinodermata, but usually classed with the Vermes, and presenting * A. Kowalevski, " Anatomic des Balanor/lo-fsus Dellc Chiaje," Memnires de V Acad, imper. des Sciences dc St. Pctcrshmrg, Turn X., No. 3, 18GG. L. Agassiz, 300 ENTEnOPNEFSTA. Pig. 241.— Young Balanoglotmt, strongly mngnifled. boscis, the numerous branchial slits arc visible. an affinity to the Tunicnta by the mode of respiration. This in- teresting worm was discovered by Delle Chiaje, and its organization and development have been recently investigated by Al. Agassiz and Kowalevski [more recently by Bateson, Q. J. Mic. Sci. 1884] (fig. 241). The most inte- resting point about this form is the structure of its larva, which renders its relationship to the Echinodermata probable. The larva was described by J. Miiller asTornaria, and was regarded by him as an Echi- noderm larva. It does, in fact, possess a double band of cilia, like Bipinnaria. Of these two rows of cilia, one, the prpeoral, forms a ring round the pra3-oral lobe, while the other is larger and runs in a more longitudinal direction so as almost to join the former near the apical pole. There is also a transverse prae-anal ring of cilia (fig. 242, a, h). Internally a diverti- culum of the ar- chenteron gives rise to an independent sac forming the water-vascular sys- tem, while two pairs of diverticula furnish the first rudiments of the body cavity. A pulsating heart, is developed from a thickening of Fig. 242, a, h.—Tomiaria larva (after E. Metschnikoff). a, Seen from the side ; b. from the dorsal surface. O, mouth ; A, anus ; S, apex, W, rudiment of water vascular system ; C, heart ; P, P', peritoneal sacs. " The History of Balanoglossus and Toraaria," Mvmmrs of the American Academy of Arts and Sciences, Vol. IX., 1873. E. MetschnikofiE, Zeitsehr. f. missensch. ZooL, Tom XX., 1870. BALANOGLOSSUS. 301 the ectoderm, and sinks into a depression of the water vascular vesicle. At the apical pole there is a thickening of the ectoderm resembling the apical plate of the larval Worms and containing two eye-spots. The development of the larva into the adult Balanojlossus was first traced by E. Metschnikoff and then by A. Agassiz. The band of cilia gradually disappears, the pra3-oral part of the larva becomes the proboscis, while the oral portion gives rise to the collar. The trunk is formed from the posterior elongated portion on which the posterior transverse ciliated band still persists. The anterior portion of the alimentary canal becomes pierced by paired branchial slits (figs. 243, 244). The body or the adult animal is worm-like and completely cili- ated ; it can be divided by the external features into a number of different regions. The anterior end of the body is indicated by a proboscis well marked off and projecting like a head. This is fol- lowed by a muscu- lar collar. Poste- rior to the collar there is a longer portion of the body, the branchial region, which may be divided into a median distinctly ringed part (branchiae) and two lobed lateral portions usually filled with yellow glands. At the boundary, between the median portion and the two lateral lobes, there are on either side rows of openings which serve for the exit of the water from the branchial chamber. Then follows a third division of the body, the gastric i-egion, upon the upper side of which there are four rows of yellow glands {generative glands). Fig. 243.— Stage in the con- version of Tornaria into Bala- jjoglossus, with one pair of branchial slits (after B. Met- schnikoff), seen from the side. So, external branchial open- ing ; P, peritoneal sac ; Vc, circular vessel ; O, mouth ; C, he\rt. Fig. 244.— Stage in the conver- sion of Tornaria into Balano- glossus, with four pairs of branchial slits (after Al. Agassiz). Letters as in figs. 242 243. 302 EXTEEOPNEUSTA. Between these, brownish-green promintnees are visible (the hepatic appendages of the intestine), which, towards the posterior extremity where the yellow glands disappear, are larger and more closely aggregated. Finally there follows a distinctly ringed caudal region, at the hind end of Avhich is the anus. The contractile proboscis serves not only as a siphon to maintain respiration, but also as a locomotory organ. It projects above the level of the mud in which the animal is buried, and is said to take in water by a terminal aperture (the existence of this opening has been recently disputed) [and to pass it out into the mouth through a pore at its base]. The mouth lie.s behind the anterior margin of the so-called collar, and leads into a buccal cavity, the walls of which contain a great number of unicellular mucous glands. The portion of the alimen- tary canal which follows the buccal cavity bears the branchial frame- work, and is divided into a dorsal and ventral part by two longitudinal folds, so that it almost presents in transverse section the appearance of a figure of 8. The intestine does not hang freely in the body cavity, bvit, except in the region of the tail, is fastened to the body wall by connective tissue; it is, however, always very closely attached in the two median lines. Beneath the dorsal and ventral median lines, where the two principal vascular trunks are visible through the skin, two grooves, beset with strong cilia, run along the whole length of the intestine. From these grooves secondary grooves are given off, and as it were divide the whole surface of the intestine into islands. Some distance behind the branchial region, on the upper side of the intestine, the peculiar cell masses begin, which gradually assume the form of sac-like diveiticula with ciliated internal walls. These " hepatic appendages " are either disposed in a simple row along each side {B. minutus Kow.), or densely aggregated together (Z>. clavifjerus Delle Ch.) The branchial basket-work which is placed at the commencement of the alimentary canal projects on the anterior flattened part of the body in the form of a transversely ringed longitudinal fold, and con- tains a system of chitinous plates, which constitute its framework and are connected in a peculiar manner by transverse rods. The water taken in through the mouth passes through special openings in the wall of the anterior portion of the alimentary canal into the ciliated branchial spaces, to issue thence through the two rows of lateral pores on the dorsal sui-face of the branchial region. Tlie vascular system consists of two median longitudinal trunks, EALA.NOGLOSSUS. VERMES. 303 which give off numerous transverse branches to the walls of the intestine and body, and of two lateral trunks. The branchia3 receive their rich vascular supply entirely from the lower trunk. The upper trunk, in which the blood flows from behind forwards, divides at tho posterior end of the branchiaj into four branches, of which the two lateral ones pass to the lateral portions of the anterior part of the body. Certain fibrous cords, running directly beneath the epidermis in the doi'sal and ventral median lines and branching into a net- work of fine fibrillar, have lately been interpreted as nervous centres. These coixls are described as being connected at the posterior end of the collar by a ring-like commissure. The (/enerative orrjans are arranged in single rows in the branchial region, but posterior to this in double rows. During the breeding season they are extraordinarily developed, and the male and female can be easily distinguished by the ditTerence in their colour. Each ovum is contained in a capsule, which is provided with nuclei, but is otherwise homogeneous. The eggs are probably laid in strings like those of Nemertines. These animals live in fine sand. They saturate the sand in their immediate vicinity with mucous. They fill their alimentary canal mth sand, and move themselves by means of their proboscis, which, alternately elongating and retracting, draws the body after it. Both the species named were found in the Gulf of Naples, A third northern species of Balanoglossus was discovered by Willemoes-Suhm, and described as B. Kupfferi. CHAPTER IX. Vermes. Bilateral animals loith imsegmented or uniformly (Jiomonomous) segmented body. There are no segmented lateral appendages. A dermal muscular system and 2^«'ired excretory canals [loater-vascular system) are piresent. Since the time of Cuvier, a number of groups of animals all characterised by the possession of an elongated laterally symmetrical body and by the absence of articulated limbs have been classed together as Vermes. This group includes such a variety of forms that attempts have already been made to break it up, and it will perhaps be necessary at some futux'e time to separate the unseg- 304 mented from the segmented forms, under the respective heads of Vermes and Annelida. The form of the body, Avhich is soft and adapted to hve in damp media, is usually elongated, flat, or cylindrical, sometimes without rings, sometimes ringed, and sometimes divided into segments (meta- meres). In every case we can distinguish a ventral and a dorsal surface. It is on the first that the animal moves or attaches itself to foreign objects. The mouth is usually placed ventrally at the end of the body which is directed forward in locomotion. The con- trast between the flat, shorter form of body and the cylindrical and elongated seems, especially in the case of the non-segmented worms {Vermes s. str.), to be of importance, so that on this ground we can establish the classes of Platyhelminthes or flat worms, and of Xemathelminthes or round worms. The segmented worms [Annelida) possess a ventral chain of ganglia in addition to the brain, and a segmentation of the organs which corresponds more or less with the external segmentation. The portions of the body which are primitively alike and are known as segments or metameres do not by any means always re- main homonomovis. In the most highly developed segmented worms, the two anterior seg- ments unite to form a division of the body which foreshadows the head of the Arthropoda, and, like the latter, is pierced by the mouth, contains the brain, and bears the sense organs (fig. 245). In the succeeding metameres there are also frequently vai-iations of form which disturb the homonomy. The skin of worms presents very different degrees of consistence, and covers a sti-ongly developed muscular system. In the skin we can distinguish a layer of cells {Jiypodermis) or, at any rate, a nucleated layer of protoplasm which functions as a matrix, and a superficial homogeneous cuticular layer which is secreted by the first- named layer or matrix and in the lower worms is extremely thin and delicate. In the Nematlielmlnthes it is often laminated, and can FcG. 215.- Head and anterior segments of Eunice seen from the dorsal surface. T, Tentacles or antennae of the prEestomium ; Cf, tentacular cirri; C, cirri of the parapodia ; JBr, branchial appendages of the parapodia. even be separated into several layers. It is of considerable thickness in many Annelida {Ghaitojwda), and may be perforated by pores. Cilia are found principally in the larval stages of Platijhelminthes and Annelida. Where there are no cilia, the superficial cuticular mem- brane, which may project in the form of tubercles or spines, consists of a substance allied to the chitin of the Arthropod skin, like which it may bear cuticular formations of many kinds, such as hairs, bristles, hooks, etc. In many Nemathelminthes, as well as in segmented worms, this fij-m cuticula gives rise to a kind of exo-skeleton, which opposes the contractions of the dermal muscular envelope. In the Chfctopoda among the Annelida, but also in the Rot if era, the tough integument is divided into a number of sections Ijing one behind the other. These, like the segments of Arthropoda, are connected by soft portions of skin and moved by the dermal muscles, which are divided into corresponding groups. The cutaneous segments of the Rotifera are not true metameres, since there is no segmentation of the internal organs. Cutaneous glands are very widely distributed ; they are sometimes unicellular, vsometimes polycellular, and are sometimes situated directly beneath the epidermis, sometimes in the deeper tissues of the body. The tissue which lies beneath the hypodermis, and which we may call the cutis, contains in all cases longitudinal and in some cases also circular muscles, and so constitutes a muscidar cutaneous envelope, the principal locomotory organ of worms. Taking into consideration the importance of this dermal muscular system in the locomotion of worms, we must attribute a ceitain systematic value to the special forms which it assumes, a value which, however, we must be careful not to exaggerate. The stratification and the direction of the fibres of this dex-mal muscular system is most complicated in the Platyhel- minthes and in the Hirudi/iea amongst the Chcetopoda, for here we find the circular and longitudinal muscles, which are embedded in a basis of connective tissue, crossed by muscle fibres, which run in a dorso-ventral direction (sometimes also by fibres running obliquely). To these may be added groups of muscular fibres, which serve to attach the internal organs to the integument. The suckers of the parasitic worms, the pits and the parapodia with their setse of C/ioetopocZrt, must be looked upon as special difterentiations of the dermo- muscular envelope. These aids to locomotion are mostly developed upon the ventral svirface. The suckers and their accessory hooks, etc., are situated either near the two ends or in the middle of the 20 306 VERMES. body; the parapodia are distributed in pairs on the individual seg- ments along the whole length of the body, and belong to the dorsal as well as to the ventral surface, so that each segment bears a dorsal and a ventral pair of parapodia. The internal oiganization of Worms is extraordinarily various. In those flat and round worms which live in the chyme or the other organic juices of the higher animals, as, for instance, the Cestoda and the Acantliocephala, the whole of the digestive apparatus, including the mouth and anus, may be wanting ; the nutrition in such cases taking place by osmosis through the body-wall. When the alimen- tary canal is present, the mouth is usually situated ventrally in the anterior region of the body, while the anus is placed either terminally at the posterior end of the body, or near it on the dorsal surface. The alimentary canal is generally simple, and is only exceptionally divided into numerous portions corresponding to the special functions. A muscular pharynx can most often be distinguished, also a well developed stomach and a short rectum opening at the anus. The nervous si/stem appears in its most simple form as an unpaiied ganglion or, when the two parts of which it is composed are separated, as a pair of ganglia (fig. 76), which are placed near the antei-ior pole of the body above the oesophagus and genetically may be referred to the apical plate of Loven's Chsetopod larva. The nervous system has more rarely the form of a nerve ring surrounding the oesophagus and connected with groups of ganglion cells (^Xematoda). The nerves given ofi" from the supra-oesophageal ganglion are distri- buted symmetrically forwards and laterally ; they supply the sense organs, and form two strong lateral nervous trunks, which run back- wards. In still higher tyjies two larger ganglia appear, which are con- nected by an inferior commissui-e {Neniertinea). In the Annelids with degenerated metameres (Gephjrea) there is in addition to the supi'u- oesophageal ganglion (the brain) a ventral nei-ve cord connected with the supra-oesophageal ganglion by an oesophageal ring. This nerve cord is in all other Annelids divided into a series of paired ganglia, which, in most cases, correspond to the segmentation. The lateral nerve trunks approach each other in the middle line below the ali- mentary canal, and constitute, together Avith their ganglia, which are connected together by transverse commissures, a ventral chain of ganglia, which is connected with the brain by the circum-oesophageal commissures, and is continued to the hind end of the body, giving off in its course paired nerves to the right and left. The sense organs are represented by eyes, auditory apparatus, and SENSE ORGANS. TASCULAR SYSTEM. 307 tactile organs. The latter are joined to nervous expansions and special integumentary appendages (tactile hairs), and are present even in the parasitic Worms as papilla} of the outer skin connected with nerves. In the fi-ee-living worms, these tactile organs fre- quently take the form of filiform, tentacle-like appendages on the head and segments (cirri). Auditory organs are not so generally present, and are represented by auditory vesicles (otocysts) either lying on the brain (some TurheUaria and Ifeniertinea), or on the oesophageal ring (certain branchiate Worms among the Annelida). The organs of sight are simple pigment spots in connection with nerves (eye-spots), and may be provided with refractive bodies. The ciliated pits of the Nemertiaea have been regarded as organs of smell. The cup-shaped organs of the Ilirudinea and Gephyrea are also sense organs. A blood vascular system is wanting in the Nemathehninthes, the Rotifer a, and the Platyhelminthes with the exception of the Xemertinea. In these cases, the nu- tritive fluid passes endosmotically into the body parenchyma or into the body cavi- ty, and penetrates the tissues as a clear chyle, sometimes containing cellular elements. In the Nemertinea a blood vascular system is present, as also in the Gephyrea and Annelida. In the latter it obtains the highest development, and may have the form of a completely closed vascular system provided with pulsating trunks. In most cases a dorsal contractile longitudinal trunk and a ventral vessel can be distinguished ; the two being connected in each segment by arched transverse vessels, which are sometimes pulsatile. Where a vascular system is present, the blood does not always appear clear and coloui-less like the fluids of the body cavity, but sometimes has a yellow, greenish, or more frequently red colour, which is in some cases connected with the presence of blood corpuscles. The function of respiration is usually performed by the general external surface of the body. Among the Annelida, however, we Fig. 216. — Section through a body segrmeiit of Eunice. Bi\ branchial appendages ; C, cirri ; P, parapodia with tuft of bristles ; D, intestine ; iV, nervous system. 308 TEBMi-S. find in the large marine Chcctopoda filiform or branched gill>, which are usually appendages of the parapodia (fig. 246). A respiratory function may also be attiibuted to the tentacles of the Gepliyrea. The excretory organs are represented by the so-called water-vas- cular system, which consists of canals of different sizes, symmetrically arranged and filled with a watery fluid in which granules are sus- pended ; they communicate -with the exterior through one or more openings. The canals begin either as small passages in the tissues of the body, or free funnel-shaped openings in the body cavity. In the last case, they may subserve other functions ; for example, they may conduct the geneiative products out of the body cavity. In the segmented Vermes they are paired, and are repeated in each segment as nephridia or segmented organs (fig. 70). A diiierent arrangement is presented by the two lateral canals of the Xematoda, which lie in the so-called lateral lines or areas, and open by a common excretory pore in the region of the pharynx. In addition to sexual reproduction an asexual multiplication by means of gemmation and fission (rarely by formation of germinal cells) is widely distributed, especially among the lower forms. This asexvial reproduction is, however, frequently confined to the larv?e, which differ fi*om the sexually mature animal in foi-m and habitation, and play the part of an asexual generation in the cycle of development. Almost all the Platijhelminthes and numerous Annelida are hermaphrodite ; the yemathelminthes, the Gepliyrea, and Rotifera, and also the branchiate Annelids are of separate sexes. Many Worms pass through a metamorphosis ; the larvae are charac- terised by the possession of a prseoral ring of cilia (Loven's larva), or of several rings of cilia. In the Cestoda and Trenuitoda, which possess in their embryonic stage the capability of asexual reproduc- tion, the metamorphosis assumes the form of a more or less com- plicated alternation of generations which is often characterised by the difference in the habitat of the two successive stages of development and by the alternation of a pai-asitic and free life. The vital activity of the Worms is in general of a low order, corresponding with their habitat. Many of them (Entozoa) live as parasites in the interior of the organs of other animals, some as ectoparasites on the external surface of the body, and feed on the juices of their hosts. Oth.ers live free in damp earth, or in mud ; others, and these are the most highly organized forms, inhabit fresh and salt water. TUnUELLABIA. 309 CLASS I.— rLATYHELMINTHES Vermes tolth a fiat, more or less elongated hody, loith cerebral gan- glion. They are often jirovided tcith suckers and hooks, and are usually hermaphrodite. The series of forms included under this class are mostly Bntozoa, or else live in the mud and beneath stones in the water. In their organization they occupy the lowest place among the worms. Their body is more or less flattened, and is either unsegmented or is divided by transverse constrictions into a number of successive divisions, which, although forming parts of one animal, yet have a strong tendency towards individualisation, and frequently attain to separa- tion and lead an independent life. These segments are products of growth in the direction of the long axis of the body, and stand in a special relation to reproduction. They are by no means to be con- sidered as necessarily indicating a high grade of organization, as does the segmentation of the Annelida. The alimentary canal may be altogether wanting (Cestoda), or, if present, may be without an anus {Trematoda, TurheUaria). The nervous system is usually composed of a dovible ganglion above the oesophagus, giving off small nerves anteriorly and laterally, and two stems backwards. In many Platyhelminthes simple eye-spots occur, either with or without refractive bodies, and more rarely thei'e is an auditory vesicle. Blood-vessels and organs of respiration are found only in the Nemertinea. The excretory (water vascular) system is everywhere developed. With the excep- tion of the Zlicrostomidce and Nemertinea, hermaphroditism is the rule. The female generative glands consist of distinct yolk-glands and ovaries. The development very frequently takes place by a very complicated process of metamorphosis connected with alternation of generations. Order 1. — Turbellaria.* Free living Platyhelminthes with oval or leaf-shaped hody, with, soft skin covered tcith cilia. They possess a mouth and apiroctous * Dagos, " Picchcrches sur I'oi'ganisation et les moeurs de Planaires," Ann. (h'f! Sc. JS'at,, Ser. I., Tom XV. A. S. Oerstedt, "Entwurf ciner systcmatisclien Eintheilung luid speciellen Bescbreibung der Plattwlirmern," Copenhagen, 1844. De Quatrefages, " Memoire sur quelques Planariecs marines," Ann. des Sc. Nat., 1845. M. iSchultze, " Beitriige zur Katurgeschichte der Turbellarien," Greifswald, 1851. L. Graff, ''Zur Kenutniss der Turbellarien," Zcituchrijt fur in.?.>\ Zud?., Tom XXIV. L. Graff, "Neue Mittheilungen iiber Turbellarien." Zcitsch. f. miss. Zoul., xxv., 1875. P. Hallez, "Contributions a I'histoirc uatnrellc des Tui-bellarics," Lille, 1879. 310 PLATIIIELMINXnES. alimentary canal. Hooks and suckers are absent. A cerehral (jangVion is ])resent. The Turhellaria usually possess an oval flattened body, and reach only a small size. The uniform ciliation of the body is connected with their existence in fresh and salt water, beneath stones, in mud, and even in damp earth. Only in exceptional cases do we meet with apparatuses for adhering, viz., small hooks and suckers. The skin consists of a single layer of cells, or of a finely granular layer containing nuclei, which is sup- ported by a stratified basal membrane, and covered externally by a special homogeneous membrane bearing cilia and comparable to a cuticula. Peculiar integumentary strvxctures, which have the form of rods or spindles, and, like the nematocysts in Ccelenterata, take their origin in cells, are not unfre- quently present. Various pigments are also often found embedded in the epi- dermis, and of these pigments the green- coloured vesicles, in Vortex viridis for example, which are identical Avith chlo- rophyl corpuscles, are specially wor'thy of remark. Pear-shaped mucous glands are also present. Beneath the conspicu- ous basement membrane which supports the epidermis lies the dermis. It con- tains the strongly developed dermal muscular system embedded in a connec- tive tissue layer formed of round, often l)ranclied cells. A body cavity between the body wall and the alimentary canal, is, as a rule, absent ; it may, however", in many cases be recognised as a system of lacunne, or as a continuous cavity surrounding the alimentary canal. The nervous system consists of two ganglia connected by a com- missure, and giving off nerve fibres in various directions ; of these, two especially large lateral trunks run backwards, one on either side (fig. 247). The latter are connected at regular intervals by delicate transverse trunks. In a number of dendroco^lous Turbellarians a Pig. 247.— Alimcntiu-y canal and ner- vous system of Zlnostomum Ehren- herijil (after Graff). G, the two cerebral ganglia with two eye spots; St, the two lateral nerve trunks ; D, alimentary canal -with mouth and pharynx. TUUBELLAEIA. 311 diverticulum of the stomach runs forward above the transverse commissure in a groove between the two cerebral lobes {Leptojylana). In some genera of Planarians, a ring-shaped double commissure has been shown to exist in the brain (^Polycelis), and ganglion-like swellings, from which nerves ai-e given off, have been observed on the lateral nerve-trunks {Sphyrocephcdus, Polycladus). With regard to sense organs, eye sjJOts are tolerably widely distri- buted among the Turhellaria. They either lie in pairs upon the cerebral ganglia or are connected with short nerves given off from the latter. More frequently two larger eyes with refractive struc- tues are developed. Otocysts are but rarely present, e.g., in Monocelis among the lihabdocoela a single one is present lying upon the cerebx-al ganglion. The integument is without' doubt en- dowed with a highly developed tactile sense ; the large hairs and stiff bristles which project between the cilia may possibly be of importance in this relation. Lateral ciliated pits, which may also be explained as sense organs, are in rare cases present at the anterior end of the body (compare the Nemertinea). Mouth and digestive apj)aratus are never wanting but the former is frequently removed from the ventral surface of the anterior end of the body to the middle or, indeed, even to the posterior region. According to Metschnikoff and Ulianin, a stomach may in some cases be absent {Convoluta, Schizo- 2)rorci), and be replaced, as in Infusoria, by a soft internal pai'enchyma. The mouth leads into a muscular pharynx, Avhich can usually be protruded after the manner of a proboscis. The alimentary canal, of which the internal wall is frequently ciliated, is either forked and then simple or branched {Dendrocoda), or rod-shaped {Rhahdo- ccela). An anus is never present. A peculiar tube capable of being evaginated as a proboscis, and without connection with the pharynx is sometimes also present [Prostomum). The excretory (water-vascular) system consists of two transparent lateral trunks and innumerable side branches, which begin with closed ciliated funnels, and are furnished with vibratile cilia, which Fia. 2i3.—3Z:crosto- mum Uneare, after Graff. A chain produced by fis- sion ^OyO', mouth openings. 3i: PLATYHELMIXTHES. project here and there freely into the vessels. As a rule, several openings occur on the main trunk of this excretory appai-atus. Reproduction may take place asexually by transverse fission, e.g., Berostomea (Catenida) and Microstomea (fig. 248). With the exception of the Microstomea, the Turhellaria are hermaphrodite: but steps intermediate between the hermaphrodite and the dioecious condition seem by no means to be wanting, for, according to MetschnikofF, in Prostomum lineare the male genei-ative organs are sometimes developed, while the female remain rudimentary ; and sometimes, on the other hand, the reverse holds. In Acmostomum dioecum also the sexes are separate. In the her- maphrodite forms the male sexual organs consist of testes, which mostly lie as paired tubes at the sides of the bod}^, also of vesi- cular seminales, and of a protru- sible copulatory organ beset with hooks. The female organs con- sist of ovaries, yolk glands (vitellarium), a receptaculum seminis, a vagina, and a uterus (fig. 249). The male copula- tory organ and the vagina open as a rule by a common orifice upon the ventral surface. Some- times, as in the Rhabdoccele genus Macrostomum, the vitella- rium (yolk gland) and ovary are united; the ova being produced at the upper blind end of the ovary, and the yolk at the lower end of the same gland. In the marine Dendrocoela, on the other hand, the vitellarium is generally absent. After fertilization, a hard, usually reddish-brown shell begins to be foi-med round the ovum. In such cases, the hard-shelled eggs are laid; but among the Rhabdocoela, in Schizostomum and certain Mesostomea [M. Ehren- hergii), transparent eggs furnished with thin, colourless capsules, and undergoing development in the body of the parent, are often produced. According to Schneider, the production of these thin- 249. — Generative apparatus of ileeoito- mum Ehrenht-rgii (combined from Graff and Schneider). S, Pharynx ; Go, sexual openings ; Ov, ovary ; r7, uterus, with winter eggs ; Do, yolk gland ; Dg, duct ofyolkglaml; T, testis; T'J, vas deferens ; P, penis ; A'», receptaculum seminis. TUEBELLA.RIA. 313 shelled or summer eggs invai-iably precedes that of the hard-shelled or loiater eggs, and the svimmer eggs are normally self-fertilized. In rare cases the hermaphrodite generative organs pi-esent a segmentation recalling that of frhe Cestoda (Akmrina com.posita). The freshwater Turhellaria, as well as many of the maiine forms, undergo a simple direct development, and in the young state are often difficult to distinguish from Tn/nsoria. Other marine Dendrocoda undergo a metamorphosis, the larvt\3 being characterised by the possession of finger-shaped ciliated lobes (fig. 251). (1) Sub-order : Rhahdocoela. The body is round and more or less flat. The intestine is cylindrical, and there is usually a protrusible pharynx. They are usually hermaphrodite. The Rhabdoccelous Turbellarians are the smallest and most simply organised forms. The intestine is cylindrical and elongated, and is sometimes provided with lateral diverticula. The position of the mouth varies exceedingly, and has been employed as a principal characteristic for distinguishing the various families. Sometimes salivary glands are present, opening into the pharynx. According to TJlianin's discovery, which has been several times confirmed, the alimentary canal may be wanting in many forms, and be replaced by a central cavity, filled with a substance containing numerous vacuoles and rich in oil globules {Convoluta, Schizo2}rora, Nadina). In those Rhahdoccela which possess an alimentary canal, interstices and spaces in the connective tissue parenchyma are often present : these must be related to a body cavity. In some cases (in Prostomum) the body cavity may be recognised as a continuous space filled with, fluid and surrounding the alimentary canal. The Rhahdocoela live on the juices of small worms and of the larvse of Entomostraca and Insecta, which they envelop with a cutaneovis secretion containing small rods, and afterwards suck. They are mostly inhabitants of fresh water, and only a few of them are to be met with in the sea or upon the land {Geocentrophora sphyroctipliakt). Fam. OpisthomidaB. The mouth is placed at the posterior end of the body and leads into a . tubular pharynx, which can be protruded like a proboscis. J/onoceUs agilis M. Sch., Ojmthomvm jmlUduvi O. S. Fam. Derostomidae. Mouth placed slightly behind the anterior margin ; pharynx barrel-shaped. Derostumum Schinldtlamim M. Sch., Vortex tlridls, M. Sch., Catenida JemncB Dug. Fam. Mesostomidae. Sleuth placed nearly in the middle of the body, pharynx ringlike, cylindrical or resembling a sucker. Mesostomuni Ehnnhcrgii Oerst., with two eyes. Fam. Convolutidae, (Acocla). 'Without alimentary canal. The ovaries and 314 PLAXrnELMIJJTUJiS. Concoluta Ocrst. C. imracloxa Oerst., North yolk glands are not separate. Sea, Baltic. Schkojjrora O. S. Fam. Prostomidae. The mouth, which is situate on the ventral surface, leads into a muscular pharynx. At the anterior end there is a protrusible tactile proboscis furnished with papill.x, Prostomniii Oerst. (Gt/7-ator Ehrbg.), P. lineare Oerst. With pointed pcnial spine at the posterior end, imperfectly hermaphrodite, living principally in fresh water. Pr. helgolandicim, Kef., completely hermaphrodite. Fam. Microstomidae. llhahdncala with separate sexes. The small but very extensible mouth lies near the anterior cud of the body. There are laterally placed ciliated pits near the anterior end of the body. Formation of metamercs and transverse fission fre- quently occur. Microsto- III urn linear c Oerst. (fig. 248). (2) Sub-order : Den- drocoela. The body is broad and flat, and the lateral margins are often plicated. There are ten- tacle-like processes at the anterior end. There is a branched alimentai'y canal and a muscular pharynx Avhich is usu- ally protru.sible. They are, as a rule, herma- phrodite. The Dendroccela are mostly marine, but also live in fresh water and on land. In their ex- ternal appearance they resemble the Trema- todes, and the branching of their straight or forked intestine is a character common to the larger species of the latter. Compared with the Rhahdoccela, they are distinguished by the greater develop- ment of their bi-lobed cerebral ganglion, as well as by the greater number of their eyes (fig. 250). The rows of papillae, or the tentacle-like processes at the anteiior end of the body have Fig. 2oO. — Anatomy of FoJijcelis pallida (after Qnatre- fiiRes). G, Cerebral ganglion with the nerves given off from it ; O, mouth ; D, branches of intestine ; Of> ova; Od, oviduct; V, vagina; W.Goe, female gene- rative oiicuing; T, testes; M.Goe, male generative opening. Trm3ELLABIA — DENDROCCELA. 315 Fig. 25' vidata ,— Larva of Jiurjlepta after Hallcz. probably the function of tactile organs. The mouth usually lies in the middle of tlie body, and leads into a wide and protrusible pharynx. The skin is often provided with glands, the secretion of which in certain land Planaria [Biixdium, Rhijnchodesmus) hardens to a fibrous web. They are almost always heimaplu'odite. The fresh-water forms possess a common generative opening, while in the marine forms the generative openings are usu- ally separate (fig. 250). In the latter case a separate vitellarium is absent. In some marine forms development takes place with metamorphosis, as is shown by the larva discovered by J. MUller, which possessed six provisional finger-like ciliated lobes (fig. 251). In the fresh- water Pianarians develop- ment is direct. The cocoon, when laid, contains four to six small eggs. At the close of segmentation there is developed a layer of cells, which is said to split into two layers, an upper or animal layer, from Avhich are derived the body wall and muscular system, and a lower or vegetative, from which the alimentary canal is formed. The marine Dendroccela fre- quently deposit their eggs in the form of broad bands. 1. Monogonopora Stimps. Den- droccela with single sexual opening. The land and fresh-water Planaria be- long to this group. Fam. Planariadse. The body is of a long, oval, flattened shape, und is often provided with lobed processes, more rarely with ten- tacles, and, as a rule, with two eyes, which are provided with lenses, rianaria O. Fr. Miillcr, two eyes, no tentacles. Fl. torra, M. Sch. (divided by O. Schmidt into h/f/7/hris, 2)ohjchroa, and torva) (fig. 2.52). PZ. dioica Clap., with separate sexes. Dcndroceelum Oerst. Distinguished by the possession of lobed processes on the head, also by the presence of a copulatory organ i)laccd in a special sheath. D. ladcnrn Oerst., Folycdis ntijra, hruniiea 0. Fr. Miill. Fam. Geoplanidae.* Land Pianarians. They are characterised by their * Besides M. Schultze, Stimpson, MctschuikofE, Grube, etc., compare II. N. Fig. 252— P/a: luguhrU (J), toi the natural Schmidt). polycliroa (a), I (e), about twice size (after O. 316 PLATTUELMIXTILES. elongated and flattened body, which is provided with a foot-like ventral surface. Gcoplana lajjidicola Stimps.. Illujiichodegmus tcD'e.itr'ui Gm. (^Fascloln terrcntru, O. Fr. Miiller), Europe. Gcodexmus I'duicatns, Metschn., with thread cells in the integument, found in potter's earth. 2. Digonopora. Dendroc(£la with double sexual opening. Almost all are marine. The proboscis is often folded and lies within a special pouch. When protruded, it spreads out like a lobe. Fam. Stylochidae. The body is flat and rather thick, and is provided with two short tentacles on the head. There arc usually numerous eyes on the tentacles or on the head. The genital openings are posterior. Stylochns macn- laUi^s Quatr. Fam. Leptoplanidae. Body flat and broad, usually very delicate. Cephalic region not distinct, without tentacles. The ej^es are more or less numerous. The mouth is usually placed in front of the middle of the body. The genital openings lie behind it. Lcptoplana trcmellarii^ 0, Fr. Miill., Mediterranean. Fam. Euryleptidae. Body broad, and either smooth or furnished with papillae. There arc two tcutaclc-lilce lobes on the anterior region of the head. The mouth is placed in front of the middle of the body. Numerous eyes are disposed near the anterior margin. Marine. Thysanozoon Diesingii Gr. Mediterranean. Eurijlcpta anricuhita 0. Fr. MUllcr, North Sea. Order 2. — Trematoda.* Parasitic Platyhelminthes vnth unseijinented, usualhj leaf-sluq-)ed, rarely cylindrical body. Tliey possess a viouth and ventrally 2^lf-t'C'id organ for attachment : the intestine is forked and ivithout an anus. The Trematodes are with great probability to be dei-ived from the T'urhellaria, with which group, both in form and organization, they show a close relationship. In connection with their parasitic mode of life they develop special organs for adiiering, such as suckers and hooks. Cilia are present only in larval life. The mouth is invariably placed at the anterior end of the body, usually in the middle of a small sucker (fig. 253). It leads into a muscular pharynx with a more or less elongated oesophagus, which is prolonged into a forked intestine ending blindly. Moseley, " Notes on the Structure of Several Forms of Land Planiuians," etc. Jovrnul of JMicr. Science, vol. svii. * A. V. Nordmann, " Mikrographische Beitragc zur Kcniitniss der wirbellosen Thiere," Berlin, 1832. G. G. Carus, " Beobachtung iiber Lcucocliloridium paradoxum, etc.," Nov. Act., vol. xvii., ]S:55. Wagener, " Ueber Gyrodactylus elegans," MiiUer's ArcJtiv., 1860. Van Beneden, " Memoire sur les vers intcs- tinaux," Paris, 1801. E. Zeller, " Untersuchuiigen iiber die Entwickelung und den Bau von Polystoma intcgcrrimum, iTr/YA*?///-. /. JivVv. Znol., vol. xxii., 1872. E. Zeller, " Untcrsuchungcn iiber die Entwickelung von Diplozoum paradox- um," Ibid., vol. xxiii., 1873. E. Zeller, " Ucber Lcucochloridium paradoxum unci die weitere Entwickelung seiner Distomumbrut." Hid.. Tom XXIV. E. Zeller, " Weiterer Beitragzur Kenntniss der Polystomeen," Ihid., xxvii., 1876. Compare also the works of G. Wagener and De Filippi. TEEMATODA. The exci-etory apparatus consists of two large lateral trunks and a network of fine vessels permeating the tissues and beginning with small ciliated lobules. The two large trunks open into a common contractile vesicle, which opens to the exterior at the posterior end of the body (fig. 253, E'p). The excretory system contains a watery fluid with granular concretions. This fluid is probably an excretory product, corresponding to the urine of higher animals. The nervous system consists of a double ganglion lying above tlio oesophagus, and from it several small nerves and two posteriorly directed lateral trunks are said to be given off". Eye sjoots witli refractive bodies are sometimes present in the larva) during their migrations. Locomotion is effected by the dermal muscular system and the organs of attachment, viz., the suckers and hooks, which present numerous modifications in number, form, and arrangement. In general, the size and development of these organs are related to the endo- parasitic or ecto-pai-asitic mode of life. In the endo-parasitic Trema- todes they are less developed, and usually consist of the oral sucker and a second larger sucker on the ven- tral sui-face, either near the mouth, as in Distomitm, or at the opposite pole of the body [Amphistomum). This large sucker may, however, be absent (3lonostomum). The ecto- parasitic Polystoraea, on the other hand, are distinguished by a much more powerful armature, for besides two smaller suckers at the sides of the mouth, they possess one or more lai-ge suckers at the posterior end of the body (fig. 258), Avhich, moreover, may be supported by rods of chitin. There are often in addition chitinous hooks, and very frequently two larger hooks among the posterior suckers in the middle line (//). The Trematoda are mostly hermaphrodite. As a ride, the male and female generative openings lie .'-ide by side, or one beliind the other, not far from the middle line of the ventral surface, near the anterior end of the body (fig. 254). The male opening leads into a sac, the --E^ Fig. 253. — Younpr BUtomum (after La Valette). Ex, trunk of the excretory (water vascular) system ; .Ej), excre- tory pore ; O, mouth with sucker ; 5, sucker in the middle of the ventral surface ; P, pharynx ; D, forked in- testine. 318 PLATTnELMiyTIIES. cirrus sac, which encloses the pi-otrusible tei-minal part (cirrus) of the vas deferens. The vas deferens soon divides into two, which lead back to the two large simple or multilobed testes. The supposed third vas deferens, which, according to v. Siebold, runs from one testis to the female sexual apparatus, so as to permit of direct ferti- lization without copvilation, has been recognized as a vagina opening to the exteiior on the dorsal surface (canal of Laurer). The female organs consist of a convo- luted uterus and of the glands concerned in the preparation of the egg, viz., an ovary and two yolk glands. There is sometimes in ad- dition a special shell gland. The true ovary which produces the pri- mary ova is a round body, and is usually placed in front of the testes. The yolk glands which secrete the yolk are much ramified tubular glands, and fill the sides of the body (fig. 254). The yolk particles come in contact with the primary ova in the first portion of the uterus, and surrovind them in greater or less quantities. Subsequently each ovum, Avith its investment of yolk, is surrounded by a strong shell. The ova in their passage along the uterus become packed together, often in great numbers, and undergo the stages of embryonic development in the body of the parent. Most Trematodes lay their eggs ; only a few are viviparous. The just-hatched young either possess (in most Polystomea) the form and organization of the parent; or they present the phenomenon' of a complicated alternation of generations (heterogamy) connected with a metamorphosis {Distomea). In the first case, the large eggs become attached in the place where the mother lives; in the last case, the relatively small eggs are deposited in a damp place, usually in the water. After the completion of the segmentation and the em- FlG. 2a\.—DUtomuM hepaiicim (aftei- Sommer). O, Mouth ; D, limb of in- testine ; S, sucker; T, testes; Do, vitellarium ,■"<)" (uterus), oviduct ; Di-, accessory glands. lilJSMATODA. 819 bryonic development, the contractile, usually ciliated embryos* (fig, 255, a), which already possess the first rudiments of an excretory system and more rarely a sucker with a mouth and alimentary canal, leave the egg and wander about independently in search of a new host. The latter is, as a rule, a snail, into the interior of which they pene- trate and there become transformed into simple or branched S'])orocysts (without mouth and alimentary canal, fig. 255, c), or into Reclice (with mouth and alimentary canal, fig, 255, d). These give rise, by means of the so-called germs [cells lying in the body cavity of the Fig. 255,— Developmental history of Dhfomum (partly after R. Leuckart). n, free swimming ciliatetl embryo of the liver fluke, b, the same in a state of contraction %\-ith rudimentary alimentary canal {DJ and cell mass (Oo) (rudiments of the genital glands). Ejt, ciliated apparatus of the rudimentary excretory system, c, sporocyst developed from a Distomum embryo, filled with Cercarias (C) ; JS. Boring spine of a Cercaria. il Eedia with pharynx, (Ph), and alimentary canal (D) ; O, mouth ; Ex, Excretoi-y organ ; C, Cercaria inside Redia, e, Free Cercaria ; S, sucker ; D, alimentary canal. sporocyst or redia], which probably correspond to the germinal cells (primitive ova) of the rudimentary ovary, to the generation of the * As R, Leuckart has ri,t^htly observed, the Dlcyemidfc, which were regarded as Mi'xozoa by Ed. v. Beneden, as well as the Ortlioncctidcv, which have recently been especially investigated by Giard and E, Metschnikoff, and which in the reproductive stage do not rise above a form corresponding to the embryos of Trematodes, recall those Distomum larvae. 320 PLAXrUELMlNTIIJCS. tailed Cercarice, or to another generation of Sjwroojsts or Redica* which then produce the Cercarise. The Cercarice are nothing else than Distomnm larvae, which eventually reach (often only after two migrations, an active and a passive one) the final host, where they become sexually mature. They are furnished with an exceedingly motile caudal appendage, frequently with a buccal spine, and occasionally with eyes, and they pi'esent in the rest of their organization great resemblances to the adult Distomum, excepting that the generative organs are not developed. In this form they leave independently the body of the Redia or Sporocyst and of the host of the latter, and move about in the water, partly creeping and partly swimming. Here they soon find a new host (Snail, Worm, Insect larva, Crustacean, Fish, Batrachian), into wliich they penetrate, aided by the powerful vibrations of their tail ; they then lose the latter and encyst. The Cercarice from the interior of the snail thus become distributed amongst a number of hosts, and each of them gives rise to an encysted young Distomum without generative organs. This young Distomum mi- grates passively mth the flesh of its host into the stomach of another animal, and thence, freed from its cyst, into the organ (intestine, bladder etc.), in which it becomes sexually mature. There are, then, as a rule, three different hosts in the organs of which the different developmental stages [Redia or Sjiorocyst, encysted form, sexually mature animal) of the Distomum bury themselves. The transitions from one host to another are effected partly by inde- pendent migration (embryos, Cercarife), partly by passive migration (encysted young Distomum). Modifications of the ordinary course of development may, however, take place ; these may be either complications or simplifications. The embryo at hatching may contain a single Redia (as in Jfonostomum * In Cercaria cystoj)liora from Planorhis marginatus ; accordlDg to G. Wagener, the primaiy asexual individual is a Spowcyst, the secondary a Bcdra. Fig. 256. — a, Embryo of uijiIodiHcug tiibclavafiis (after G. Wagener). -D, Alimentary canal ; JBx, excre- tory sy.stem. h, Embryo of Mo- nostomiim mutahile (after v. Sie- bold). P, Pigment spots ; R, redia in the interior of the embryo. TUEMATODA. 321 flavum uiid mutabile), which it carries about until it enters the first host (fig. 256, h). In other cases the course of development is sim- plified by the omission of the second intermediate host, viz., that which contains the encysted immature Distomum {Cercaria macro- cerca of Distomum cygnoides, also Leucochloridium in the tentacles of Helix succinea). (1) Sub-order : Distomea. Trematodes with at most two suckers, without hooks. They develop with a complicated alternation of generations. The asexual individuals and the larvse live principally in Mollusca, the sexually mature animals in the alimentary canal of Vertebrates. The sexes are completely separated in Distomum hcematobiicm (from the veins of man) ; individuals of the two sexes being united in pairs (fig. 257). Dimorphic forms are found in certain species of the genera Monostomum and Distomum in connection with the division of labour of the sexual functions ; one individual develops only male sexual organs, and the other only female, the former producing spermatozoa and the latter ova. The rudiment of the functionless generative gland undergoes in these cases a more or less complete degeneration. Such Distomea are morphologically hermaphrodite, but practically of separate sexes. The complete biology and developmental r oj J. Yia. 257— Diefomtim hamato- history is unfortunately only satisfactorily uum. Male and female, known for a few species which can be fol- ^^^ 1^*^^^' ^«^\ '"^ *^^ J^ canahs gynsecophorus of lowed through all the stages of development. the former, s, sucker. Fam. Monostomidae. Of an oval, elongated, more or less rounded form, with only one sucker, which surrounds the mouth. Monostovium Zeder. Sucker surrounding the mouth ; pharynx powerful. Sexual openings but slightly removed from the anterior end. M. mutalile Zeder, in the body cavity and in the orbit of various water-birds ; viviparous. M.fiavum Mehlis, in water-birds, develops from Cercaria ephemera of Planorhis. M. lentts v. Nordm., the young form without generative organs is found in the lens of the human eye. M. bijmrtitum Wedl., living in pairs enclosed in a common cyst, the one indi- vidual surrounded by the lobed posterior end of the other ; branchiae of Tunny- fish. Fam. Distomidie. Body lancet-shaped, frequently spread out, more rarely elon- gated and rounded with a large median sucker ; in front of which lie the genital openings, usually close together. 21 322 PLATYHELMrN-THES. Distomnm. Median sucker approached to the anterior one. D. hcj^aticum L, Liver fluke. With conical anterior end, and numerous spine-like prominences on the surface of the broad leaf-shaped body, which is about 30 mm. long. Lives in the bile-cu'::ts of sheep and other domestic animals, and produces the liver disease of the sheep. It is occasionally found in Man, and bores its way into the portal vein and into the system of the vena cava. The elongated embryo only develops after the Q^g has remained a long time in water ; it has a continuous ciliated envelope with an X-shaped eye-spot. K. Leuckarf s re- searches have rendered it probable that the development is passed through in the young Llmnceiis 2>crefier and truncatvlus, that here the embryo becomes a Sjwrocyst. and that this produces Rcdice, in which it is supposed that tailless Distomca arise. [The life-history of the liver-fluke has been completely worked out by A. P. Thomas (Quart. Journal of Miaroscojncal Sci. 1883, pp. 99 — 133). He has shown that the ciliated embryo passes into Limncevs triincatulvs, anil there gives rise to a sjjorocyit which produces redias. The n-diw produce more rcdlcs or Ccrcarlce. The Ccrcaricr, which are provided with long tails, leave the host (^Limncens triincatvhts), swim about for a short time in the water, and encyst on foreign objects, e.g. blades of grass. In this condition they are eaten by the sheep.] D. crass^im Busk., in the alimentaiy canal of the Chinese, one to two inches in length, and half-inch broad, without spinous prominences, with a simple forked intestine. D. lanceulatnm Mehlis. Body elongated into the form of a lancet, 8 — 9 m.m. long, lives in the same place with D. luyaticum. The embryo develops at first in water, is pear-shaped, and only ciliated on the anterior half of the body, bears a styliform spine on the projecting apex. D. opJtthahnobhim Dies. A doubtful species of which'only four specimens have been observed in the lens capsule of a nine-months' child. B. hctcmpluji's Bilh. v Sieb. 1 — 1-5 mm. long, in the alimentary canal of man in Egypt. D. f/oJiath van Ben., 80 mm, long, in Ptcrolalcena. Numerous species live in the alimentary canal, lungs, and bladder of the frog. Distomim Jilicolle Rud. (Z>. Olteni K611) in pairs in the mucous sacs in the branchial cavity of Brama Raji. The one individual is cylindrical and naiTOw, and produces spermatozoa ; the other is swollen in the middle and posterior region of the body, and is filled with eggs. The dissimilar development of the two individuals is probably due to the fact that copulation only leads to the fertilization of one of them, which alone is able to perform the female sexual functions. B. lupmatohinm. Bilh. v. Sieh. (G>/nceco2?Iio7'vs Dies) (fig. 257). Body elongated ; sexes separate. The female is slender and cylindrical. The male has powerful suckers, and the lateral margins of the body are bent round bo as to form a groove, the canalis gynjecophorus, for the reception of the female. They live in pairs in the portal vein, and in the veins of the intestine and of the bladder of man in Abyssinia. According to Cobbold, the embryos are ciliated, and possess a tolerably well developed excretory system. By the deposition of masses of their eggs in the vessels of the mucous membrane of the ureter, bladder, and great intestine, inflammation is set up, which may cause hematuria. (2) Sub-order: Polystomea. — Trematocles with two small lateral suckers at the anterior end, and one or more posterior suckers, to which two large chitinous hooks are often added. In exceptional thematoba. 323 cases {Tristomum coccineum) transverse rows of bristles are found. Paired eyes are frequently pi'esent. In some species the elongated body presents a kind of external segmentation. They are for the most part ectoparasitic, to a certain extent like the IlirucUnea, and they develop directly without alternation of generations from eggs which are usually hatched in the locality inhabited by the mother. Sometimes the development is a metamorphosis {Fobjstomum), and the young larvte live in another place. The development of Pohjstomum integerri- mum from the bladder of the frog is the best known, owing to the researches of E. Zeller (figs. 258, 259). The production of eggs begins in the spring, when the frog awakes from hibernation and proceeds to pair. It lasts from three to four weeks. It ■^• is easy then to observe the Polysto- mea in the process of reciprocal copulation. When the eggs are being laid, the parasite forces the anterior end of the body with the genital Fig. ns.-Poiysfomnm inte- ° fferrimum (after E. Zeller). opening through the mouth or the bladder o, mouth; Go, genital nearly as far as the anus. The development opening; D, intestine; •' _ '■ ly , copulatory opening of the embryo takes place in water and occu- (lateral pads) ; D^, yolk pies a period of many weeks, so that the orovaJ^ • 'zr hooTs!^'' ' young larvfB are only hatched when the tad- poles have already acquired internal gills. The larvae resemble GyrodactTjlus, and possess four eyes, a pharynx and alimentary canal, as well as a posterior disc (for attachment), which is surrounded by sixteen hooks. They possess five transverse rows of cilia ; three are ventral and anterior, two dorsal and posterior. There is also a ciliated cell upon the anterior extremity. The larvaa now migrate Fig. 259.— Egg with embryo{o),and hatched larva (i) of PoJystomum integerrimum ; Dk, operculum (after E. Zeller). 324 PLATYHELMINTnES. into the branchial cavity of the tadpole, lose their cilia, and are transformed into young Pnhjstomea by the formation of the two median hooks and of the three pairs of suckers upon the posterior disc. The young Polystomum, eight weeks after the migration into the branchial cavity, at the time when the latter begins to abort, passes through the stomach and intestine into the bladder, and there Fig. 260.— Young Diplozoon (after E. Zeller). a. Two young JDiporpa beginning to attach themselves together. 6, After both individuals have attached themselves. O, mouth ; H, fixing apparatus ; Z, papillje ; G, sucker. only becomes sexually mature after three and more years. In some exceptional cases, and always when the larva has passed on to the gills of a very young tadpole, it becomes sexually mature in the branchial cavity of the latter. The forms then remain very small, are without the copulatory canals and uterus, and die after the production of a single egg, without ever getting to the bladder. Fam. Polystomidae. With seve- ral posterior suckers, which are usually paired and arranged in two lateral rows, and are rein- forced by an armature of hooks. The genital openings are fre- FiG. 26l.-Egg (a) and larva (6) of Diplozoon (after quently surrounded by hooks. E. Zeller). Many species have a length of only a few lines. Polystomum Zed., with four eyes ; with no lateral suckers at the anterior end, but with oral sucker; with six suckers, two large median hooks and sixteen small hooks at the posterior end. P. inteijerrlmnm Eud., in the bladder of Rana tcmjwrarla. P. ocellatuvi in the pharyngeal cavity of Emys. In the formation of the testis and the absence of the uterus it resembles the adult form of P. integerrimum from the branchial cavity of the tadpole. Octohothrium lanceolatiim Duj. OncJiocotyle appendiculata'Kxxla.n, on the gills of Elasmobranchs. Diplozoon V. Nordm, The animal is double, two individuals being fused to TEEMATODA. 325 form an X-sbaped double animal, tbe posterior ends of which arc provided with two large suckers divided into four pits. In the young state they live solitarily as Blporpa ; they then possess a ventral sucker and a dorsal papilla (2G0 a, G and Z'). In the double animals the formation of ova is confined to a defimte period of the year, usually the spring. The eggs are laid singly after the forma- tion of the thread by which they arc attached, and two weeks later the embryo (fig. 261, *), which only differs from Dipiu'pa in the possession of two eye- spots and a ciliated apparatus upon the sides and on the posterior extremity of the body, is hatched. When an oppor- tunity of fixing itself on the gills of a fresh -water fish occurs, the young animal loses its cilia and becomes a Biporpa, which possesses, besides the characteristic apparatus for attachment, the alimentary canal, and the two excretory canals with their openings at the anterior part of the body (at the level of the pharynx), and sucks the branchial blood. The junction of the two Dijwrpa soon follows ; and this does not take place, as was formerly believed, by the fusion of the two ventral suckers, but in such a manner that the ventral siicker of each animal affixes itself to the dorsal papilla of the other, and fuses with it (fig. 260, V). B. imracloxum v. Kordm,, on the gills of many fresh- water fish. Fam, Gyrodactylidae. Very small Tre- matodes with large terminal caudal disc and powerful hooks. They are viviparous, producing a single young one (first gene- ration) at a time, -within which, while still in the body of the parent, another young one (second generation) may be present, and in this yet another (third generation). V. Siebold believed that he had observed a young animal developing from a germ cell of Gyrodactylus, and that this became pregnant during its development. He regarded the Gyro- dactylus as an asexual form, since he failed to find organs for the production of sperm. G. "Wagener, however, showed that the reproduction is sexual, and conceived the idea that the germs from which the second and third generations are foiTned are derived from the remains of the fertilized ovum from which the first generation is formed. Metschnikoff, too, is of the opinion that the individuals of the first and second generations are formed at the same time from a common mass of similar embryonic cells. Gyrodactylus v. Nordm., G. clegans v. Nordm., from the gills of Cyprinoids and fresh-water fish. FIG.2C2. — Tcsnia taginata (mediocaneUatd), natural size (after R. Leuckart). 326 PLATIILEL\riKTILES. Order 3.— C.sroDA* Elongate! and vsually segmented Platyhelminthes loWiout mouth or aliiiientary caned, with organs for attachment at the anterior extremity. The tape-worms, which may easily be recognised by their band- shaped usually segmented bodies, are parasitic in the alimentary canal of Vertebrata, and were formerly taken for single animals. Steenstrup was the first to introduce a different view, according to which the tape-worm is a colonial animal, a chain of single animals, each segment or 2iroglottis being an individual. There are, however, Cestoda, like Caryophyllceus, which are destitute both of external segmentation and of segmentation of the gene- rative organs ; while in other cases the segments of the body are clearly differentiated, and each is provided with a set of genera- tive organs, but they do not attain individual independence. The proglottides, however, usually become sepai\ated off, and in some cases {Echineihothrium) after their separation from the body of the tape-worm continue to live for a long time independently, and even increase considerably in size ; so that although the individuality of the tape- worm may be justly insisted on, yet the subordinate and morphologically more restricted degree of individuality of the proglottis must also be admitted. This is the only satisfactory mode of regarding the Cestoda ; especially as the entire tape-worm, and not the proglottis alone, corresponds to the Trematode, and is to be derived from the latter by a simplification of organization and loss of the alimentary canal. The anterior part of the tape-worm is narrow, and presents a terminal swelling by which it attaches itself. This anterior swollen part is distinguished as the head of the tape-worm, but it is only its external form which entitles it to this name. In Caryopiliylloius * Besides the older works and papers of Pallas, Zeder, Bremser, Rudolphi, Die-?iiig, and others, compare van Beneden, " Les vers cestoi'des ou acotyles," Brussels, 1850. Kiichenmeister, " Ueber Cestoden im Allgemeincn und die des Menschen insbesondere," Dresden, 1853. V. Siebold, " Ueber die Band- und Blasen-wurmer," Leipzig, 1854. G. Wagener, " JDie Entwicke- lung, der Cestoden," Nov. Act. Leop.-Car., Tom XXIV., Suppl., 1854. G. Wagener, " Beitrag zur Entwickelungsgeschichte der EingeweidewUrmer," Haarlem, 1857. R. Leuckart, " Die Blasenbandwiirmer und ihre Entwicke- lung," Giessen, 1856. E. Leuckart, " Die menschlichen Parasiten," Bd. L, Leipzig, 18G2. F. Sommer and L. Landois, " Ueber den Bau der geschlcchts- reifen Glieder von Bothriocephalus latus," Zeitschr. f. iviss. Zool., 1872. F. Sommer, " Ueber den Bau und die Entwickelungsgeschichte der Geschlechts- organe von Taenia mediocanellata und Taenia solium," Ihid., Tom XXIV., 1874. CESTODA. 327 the head armature is very weak, and consists of a lobed fringed expansion. The apex of the head often ends in a conical projection, the rostellum, which is armed with a double circle of hooks, while the lateral surfaces of the head are furnished with four suckers {Tcenia, fig, 263). In other cases only two suckei's are present (Botkriocephalus) ; or we find suckers of more complicated structure and beset with hooks {Acanthohothrium), or four protrusible probosces beset with recurved hooks {Tetrarhynchus); while in other genera the head armature presents various special forms. That portion of the animal which follows the head and is dis- tinguished as the neck shows, as a rule, the first traces of com- mencing segmentation. The rings, which are at first faintly marked and very narrow, become more and more distinct and gradually larger the further they are removed from the head. At the pos- terior extremity the segments or pro- glottides are largest, and have the power of becoming detached. After separation they live independently for a long time, and sometimes even in the same medium. The simplicity of the internal or- ganization corresponds with the simple appearance of the external structure. Beneath the delicate external cuticle is a matrix consisting of small cells, 1 . 1 ^, 1111 n Fig. 368.— Head of TfTwia »o7;ttm, viewed m which are scattered glandular cells. ^^^^ ^^^ ^^^^^ ^^^.^^^ ^^^^^^^^ ^^.^^ Beneath the matrix there is a delicate rostellum and double circle of hooks. „ . , , „ T . , T 1 The four suckers are visible. superficial layer of longitudinal mus- cular fibres, and next a parenchyma of connective tissue, in which strongly-developed bundles of longitudinal muscular fibres, as well as an inner layer of circular muscles, are embedded ; both these muscular layers are traversed, principally at the sides of the body, by groups of dorso-ventral muscular fibres. The power which the proglottis possesses of altering its form is due to the interaction of all these muscles. By means of them it is able to shorten itself considerably, at the same time becoming much broader and thicker, or to elongate to double its normal length, becoming much thinner. In the connective tissue parenchyma of the body, not only the muscles, but all the other organs are embedded. In its peripheral portion, especially in the neigh- bourhood of the head, we find small densely packed calcareous concre- raents. which are generally regarded as calcified connective tissue cells. 328 PL ATTHELM INTHES. The nervotis system consists of two lateral longitudinal cords passii^ externally to the main trunks of the excretory system. They are somewhat swollen in the head, where they are connected by a trans- verse commissure; these anterior swellings and the commissure may represent a cephalic ganglion. Distinct sense organs are wanting, but the tactile sense may be ascribed to the skin, especially to that of the head and" the suckers. An alimentary canal is also wanting. The nutritive fluid, already prepared for absorption, passes endosmotically through the body wall into the parenchyma. The excretory apparatus, on the contrary, attains a considerable development as a system of much ramified canals which are dis- tributed throughout the whole body.* It consists primarily of two longitudinal canals (a dorsal and a ventral), running along each side of the body and connected in the head and in each segment by transverse trunks. According to the state of contraction of the muscular system, these longitudinal trunks and cross branches appear sometimes straight and sometimes bent in a wavy or zigzag manner: their breadth also presents consider- able variation, so that the power of contraction has been ascribed to their walls. The longitudinal trunks only serve as the efferent ducts of a system of very fine vessels which ramify throughout the whole paren- chyma and receive numerous long tubes : the latter begin in the parenchjTua with closed funnels, which contain a vibratile ciliated lappet (fig. 264). In many cases, as in the Ligulidce and Caryo- phylloPMS, these longitudinal trunks are broken up into numerous longitudinal vessels, which are connected by transverse anastomoses. In other cases, on the other hand, the two ventral vessels are enlarged at the cost of the two dorsal, which may entirely atrophy. The external opening of the excretory system is, as a rule, placed at the * Compare Th. Pintner, " Untersucliimgen iiber den Bau des Bandwurm- korpers," Wien, 1880. Fig. 26i.— a portion of the excretory system of Caryophyllaeug mutabilis (after Pintner). Wb, CiHated funnels with the nucleus of the cell belonging to them. 329 posterior end of the body, i.e., at the hind end of the last segment, in which a small vesicle with an external opening receives the longi- tudinal trunks. According to the observations of Leuckart on Tcenia cucumerina, the posterior transverse canals in the segments immediately preceding the last become, by their gradual shortening and the approach of the longitudinal trunks, transformed into the vesicle, which acquires an external opening when the segment behind it is detached. In rare cases the excretory system possesses additional openings in the anterior part of the body behind the suckers. The generative apparatus is also divided into segments which correspond to the proglottides. Each proglottis possesses its own Fig. 2C5.— rroglottis of Tania mediocaneUata, with male and female organs (after Sommer). Oc, ovary ; DS, yolk gland (vitellarium) ; Sd, shell gland ; Ut, uterus j T, testes ; Vd, vas deferens ; Cb, pouch of the cirrus ; K, generative cloaca; Va, vagina. male and female geneiTitive organs, and can therefore, when separated, be considered as a sexual individual of a lower order. The male apparatus consists of numex'ous pear-shaped vesicles, the testes (fig. 265, T), which are situated upon the dorsal side, and their vasa efFerentia open into a common efferent duct {vas deferens). The coiled end of this duct lies in a muscular pouch {cirrus sheath), whence it can be protruded through the genital opening as the so-called cirrus. This cirrus is frequently beset Avith spines which are directed back- wards, and serves as a copulatory organ. The female generative organs consist of ovary, yolk gland, shell gland, uterus, receptaculum, and vagina. The vagma and vas deferens usually open into a common 330 PLATTlIELMINTItES. Fig. 2GG.— Ripe proglottides ready to separate. a, of Trcnia solium ; h, of Teenia mediocanellata ; Wc, water-vascu'ar (excretory) canal. genitcal cloaca, which lies either on the ventral surface of the segment {Bothriocejxdus), o^ on the lateral margin {Tcenia) (fig. 2G5). In the last case it is placed alter- nately on the right and on the left side. Nevertheless it may happen that the two genital openings are widely separate, the male opening being placed at the side, the female on the surface of the segment. As the segments increase in size and become further removed from the head, the contained generative organs gradually i-each matuiity in such a way that the male generative oi'gans arrive at maturity n\ther earlier than the female. As soon as the male elements are mature, copulation takes place, and the receptaculum seminis is filled with sperm, and then only do the female generative organs reach maturity. The ova are fertilized and pass into the uterus, which then assumes its characteristic form and size. As the uterus becomes distended, the testes and then the ovaries and vitellaria are more or less completely absorbed (fig, 266), The posterior proglot- tides, viz,, those which are ready for separation, have alone under- gone full development, and the eggs in their uterus often contain completely developed embryos. Accordingly we can recognize in a continuous series of the seg- ments the course of development passed through by the sexual organs and products in their origin and gradual progress towards maturity. The number of sesrments between that with the first trace of the eenerative organs ^^'^WB'i!'': ^iiiP Fig* 2G7.— Egg with embryo (n) of Tan solium; (J) of Microttenia ; (c), of Both cephalus latus (after R. Leuckart). CESTODA. 331 and the first proglottis with fully developed organs gives us an expression for the number of stages through which each segment has to pass. The tape- worms are oviparous; either the embryo develops within the egg-shell in the body of the mother, or the development takes place outside the proglottis, for example in water i^Bothriocephalus). The eggs of theCestoda are round or oval in shape and of small size. Their envelope is either simple or composed of numerous thin membranes, or else forms a thick and strong capsule, which in Tcvnia is formed of densely packed rods united by a connecting substance, and presents in consequence a granular appearance. In many cases the development of the embryo coincides with that of the egg- shell, so that the Qg^ at the moment that it is laid contains a Fis. 2C9.— Stages iu the development of Tama solium to the Cysticercus stage (partly after B. Leuckart). a, Egg with embryo, b, Free embryo, c, Rudiment of the head as a hollow papilla on the wall of the vesicle, d. Bladder-worm with retracted head, e. The same with protruded head, magnified about four times. complete embryo with six, or more rarely, four hooks. In Bothrio- cephaliis the development takes place outside the proglottis during the long period that the egg passes in water, and the embryo leaves the egg as a ciliated larva (fig. 267, c). The development of the embryo into the tape-worm probably never takes place directly in the same medium in the intestine of the original host. As a rule there is a complicated metamorphosis, which is sometimes [Echinococcus, Ccemtrics) connected with alternation of genei-ations ; the successive stages live in different localities, and usually find the conditions necessary to their development in different species of animals, between which they migrate, partly actively and partly passively. The eggs usually leave the intestine of the host with the proglottis, and are deposited on dunghills, on plants, or in the 332 rLATTlIELMINTUES, water, and thence pass in the food into the stomach usually of herbivorous oi- omnivorous animals. As soon as the egg membranes are digested or burst by the action of the juices of the stomach of the new host, the embryos which have been thus set free bore their ■way into the gastric or intestinal vessels by means of theii- six (rarely four) hooks, the points of which can be approached and removed from one another over the periphery of the small globular embryonic body. When they are once within the vascular system, Fig. 2CS.— <7, Brood -crppule of FMnococcus with developing heads (after R. Leuckart). h. Brood-capsule of JEchinococcua (after G. Wagener). c, Heads of Echinococcus still connected with the wall of the brood-capsule — one is evaginated ; Vc, excretory canals. they are no doubt carried along passively by the current of blood, and transported by a longer or shorter route into the capillaries of the different organs, as the liver, lungs, muscles, brain, etc. After losing their hooks, they usually become enveloped by a cyst of connective tissue, and grow into large vesicles with liquid contents and a contractile wall (fig. 268). The vesicle gradually becomes a ci/stic or Madder worm by the formation of one {Cysticercus*) or * Exceptionally two or more heads are found in some Cysticercus forms. 333 several (Coenums) hollow buds, which are developed from the walls and project into the interior of the vesicle (fig. 268, c). The armature of the tape-worm head (suckers and double circle of hooks) is formed on the inside and at the bottom of this invagination of the wall of the vesicle (fig. 268, d). When these hollow buds are evaginated so as to form external appendages of the vesicle, they present the form and armature of the Cestode head, as well as a more or less developed neck, which presents even at this stage traces of segments (fig. 268, e). In some cases {Echinococcus) the irregularly shaped maternal vesicle produces from its internal walls one or two generations* of secondary vesicles which project into it; and the Cestode heads originate in special small brood-capsules on these secondary vesicles (fig. 269, a). In such cases the number of tape-worms which arise from one embiyo is naturally enormous, and the parent vesicle may reach a very considerable size, being some- times as large as a man's head. In consequence cf this enormous growth the vesicles frequently obtain an irregular shape ; while on the other hand, the tape- worms which are developed from them remain very small, and carry, as a rule, only one ripe proglottis (fig. 270). So long as ^h.Q tape-wonn head (scolex) remains attached to the body of the bladder-worm and in the host of the latter, it never develops into a sexually mature tape- worm ; although in many cases it grows to a considerable length {Cysticercus fasciolaris of the house-mouse). The bladder- worm must enter the alimentaiy canal of another animal before the head {scolex) can, after separation from the body of the bladder-worm, develop into the sexually mature tape- worm. This transportation is effected passively,, the new host eating the flesh or organs of the animal infected with Cysticerci. The tape- worms, therefore, are principally found in the Carnivora, the Insect i- vora, and the Omnivora, which receive the bladder-worms in the flesh of the animals on which they feed. The vesicles are digested in the stomach, and the cestode head becomes free as a scolex. The latter is protected from the too intense action of the gastric juice by its calcareous concretions, and at once enters the small intestine, fastens FlG.270. — Tcenia Echin oe oceits (after R.Leuc- kart), magni- fied 12 to 13 times. * In Cysticerci QO. longicollis, tenuicoUi.") also sterile daughter vesicles an sometimes budded off. 334 PLATYHELMIN-TILES. Fig. . 271.— Ci/stlcercoid of Tania cueumerina, magni- fied 60 times (after R. Leuckart). itself to the intestinal wall, and grows by gradual segmentation into a tape-worm. From the iScoIex the chain of proglottides proceeds as the result of a growth in length accompanied by segmentation, a process which is to be looked upon as a form of asexual reproduction (bud- ding in the direction of the long axis). Since, however, it is the body of the Scolex which undergoes growth and segmentation, it seems most natural to assume the individuality of the entire chain, and to subordinate to this the individuality of the proglottides. The development of the tape-worm is then to be explained as a metamorphosis, characterised by the individualization of certain stages of the development. It is only in those cases in which the young form produces a number of heads that the development can be ex- plained as a case of alternation of genei-a- tions. The development of some tape-worms pre- sents considerable simplijfications. In the cysticercus stage the vesicle frequently dimin- ishes to an excessively small appendage, and the Cysticercus becomes a cysticercoid form, in which one portion bearing the embryonic hooks is distinct from a larger pai^t which represents the scolex (figs. 271, 272). In other cases the embryo becomes a Scolex directly without passing through a cystic stage, so that the Scolex stage is merely a late stage of the embryo {Bothrio- cephalus). The segments produced from the Scolex also show very different degrees of individuality, and finally are sometimes not deve- loped at all. In the latter case {Garyophyllfeus) the head and body cannot be sharply distinguished from one another, and represent only one single individual comparable to a Trematode and characterised by its single generative apparatus. Its development is to be looked upon as a metamorphosis completing itself in one individual. Fam. Taeniadae. The armature of the head consists of four muscular suckers, to which is frequently added a single or double circle of hooks on the rostdluin. Fig. 272.— ^cAt«ococeus-like Cyttkercoid from the body cavity of the Earth- worm (after E. Metschnikoff). a. Brood-capsules with three Cysticer- coids. h, Cysticercoid with evaginated head. 835 The pro.Gflottidcs have a marginal sexual opening. The vagina is usually long, separated from the uterus, and enlarged at the end to form a recejitaculum seminis (fig. 265). The young stages are Cysticerci or Cysticcrcoids, rarely quite without caudal vesicle ; parasitic in warm and cold-blooded animals. Tccnia L. ( Cystotwnia E. Lkt). Development takes place with large vesicles. The heads arise from the embryonic vesicle itself. T. solium. L. 2 — 3 metres long. The double circle of hooks is composed of 26 hooks. The ripe proglottides are 8 — 10 mm. long and 6 — 7 mm. broad ; the uterus has 7 — 10 dendritic branches. It lives in the human intestine. The Bladder-worms belonging to it {Ci/stirercvs ccUuIoskv) live principally in the dermal cellular tissue and in the muscles of pigs, but also in the human body (muscles, eyes, brain), in which self-infection with them is possible if a Tcsnia is present in the digestive canal ; more rarely in the muscles of the Eoe-decr, the Dog, and the Cat. In the human brain the Cysticcrcvs acquires an elongated form, and sometimes does not produce a head. T. saglnnta Gocze — mcdiocaiu'Uafa Kvichenm., in the intestine of Man, distin- guished by the older helminthologists as a variety of T. solium. Head without circle of hooks or rostellum, but with four more powerful suckers. The Tape- worm reaches a length of four mfitres, and becomes much stronger and thicker. The mature proglottides are about 18 mm. long and 7 — 9 mm. broad. The uterus fonns 20 — 35 dichotomous side branches. The Cysticercvs lives in the muscles of the ox (fig. 273). It appears to be principally distributed in the warmer parts of the Old World, but is often found in great numbers in many places in the north. T. scrrata Gocze, in the intestinal canal of the dog. The Cysticercus is known as Cijsticcrcus j^iscifonnis in the liver of the Hare and Eabbit. T. crassicollis Eud. in the Cat, •with.KCysticcrciisfascwlaris of the common mouse. T. marfjinata Batsch. of the Dog (butcher's dog) and Wolf with Cysticercus tenuicol- 'S'la-'^n.—CyMc lis from Euminants and Pigs, and occasionally in Man {Cyst, fisceralis). T. crassiccj)S Eud. in the Fox with Cysticercus longicollis from the thoracic cavity of the Fieldmouse. T. ccennrvs v. Sieb. in the intestine of the sheep-dog, with Cmnvrtts cevehralis in the brain of one year old sheep. Tlie presence of Coe7ivrus in other places has been stated, as for instance in the body cavity of the Eabbit. T. temdcollis Eud. in the intestine of the Weasel and the Pole-cat, with a Cysticercus which, according to Klichenmeister, lives in the hepatic ducts of the Field-mouse. Echinococcifer Weinl. The heads bud on special brood-capsules, in such a way that their invagination is turned towards the lumen of the vesicle (fig. 269). T. echinococctis v. Sieb. (fig. 270) in the intestine of the dog, 3 — 4 mm. long, forming but few proglottides. The hooks on the head are numerous but small. Its Bladder-worm is distinguished by the great thickness of the stratified cuticula. It lives as Echinococciis^xmci^vMjin the liver and the lungs of Man (£". hominis) and of domestic animals {E. vcterinorum). The first form is also distinguished as E. altricijmrtcyis on account of the frequent production of primary and secondary vesicles ; it usually reaches a very considerable size and ercua of Tccnia mediocaneUaia, magnified about eight times. The head is protruded. 336 \EriiIES, has a very irregular shape ; -while that form which inhabits domestic animal3, E. scoliciparicns, more frequently retains the form of the simple vesicle. Finally these echinococcus cysts frequently remain sterile, in which case they are called Acejuialocyxts. Another and indeed pathological form is the so- called multilocular Echinococcuf, which was for a long time taken for a colloid cancer. It is also found in Mammalia (in cattle), and here presents a confusing re- semblance to a mass of tubercles. The echinococcus disease {hydatid plagne') was widely spread in Iceland. This disease likewise seems endemic in many places in Austi'alia. T. (Microtaenia). The Cysticercoid foi-m is small, and has but little fluid in the small portion which corresponds to the vesicle. The head is small, but has a small club- shaped or proboscis-like rostellum, and is furnished with weak hooks. The eggs are provided with several membranes. The emb]:70 is usually furnished with large hooks. The Cysticercoid stages live prin- cipally in Invertebrates (in Slugs, Insects, etc.), and more rarely in cold-blooded Vertebrates (the Tench). T. cucumerina Bloch, in the intestine of dogs (house dogs). The Cysticercoid is entirely without the caudal vesicle, and lives (according to Melnikoif and R. Leuckart) in the body cavity of the Dog-louse (^Trichodectes canis). The infection with the Cysticercoids takes place when the dog swallows the parasites which are annoying him, while the para- sites swallow the eggs contained in faeces adherent to the hair of the dog. Nearly allied is T. ellljytica Batsch. in the intestine of the Cat, occasionally in that of Man. T. nana Bilh. v. Sieb. in the intestine of the Abyssinians, hardly an inch long. T. ilavopunctata Weinl. in the human intestine' (North America). The Cysticercoids of the Meal-worm are probably developed into tape-worms in the intestines of Mice and Eats. In other partially unarmed Tcenias the generative organs and development are as yet not accurately known ; such are— 7'. perfoliata Goeze, and T. pUcata Eud. in the horse ; T. pectinata Goeze, in the hare ; T. dispav Rud. in the frog ; T. expansa Im. in the ox. Fam. Bothriocephalidae. With only two suckers, which are weak and flat. The generative organs, as a rule, open upon the surface of the proglottis. The proglottides do not become detached singly. Hydatid stage represented by an encysted Scolcx. 274 a.—Bothriocephaliis Jatiis (after R. Leuckart). C'JESTODA. 337 Bothrioccphalufi Brems. Segmented body. Head with two pits, without hooks. The genital openings arc on the middle of the ventral surface. The young stage usually in fishes. B. latus Brems., the largest of the tape-worms parasitic in man, twenty-four to thirty feet in length, principally found in Russia, Poland, Switzerland, and South France. The sexually mature segments are broader than they are long (about 10 — 12 mm. broad and 3 — 5 mm. lonq-). They do not become detached singly, but in groups (fig. 274). The sc'^nicnts of the hindermost portion of the body are, how- ever, narrower and longer. The head is club- shaped, and is provided with two slit-like pits. The cortical parts of the lateral regions of the body contain a number of round masses of granules, the yolk-glands (fig. 275, Dst^, the contents of which are poured into the shell glands (coiled glands) through the so-called yellow ducts. The genital openings lie close together, one behind the other, in the midst of the segment (fig. 275, a). The anterior and larger belongs to the male generative apparatus, and leads into the muscular terminal portion of the vas deferens, which is enclosed in the cirrus sheath and can be eva- ginated as the cirrus (fig. 275, Cb). The vas deferens just before its entrance into the cirrus pouch is dilated (fig. 275 V) to form a large muscular swelling (the vesicula seminalis ?). It then becomes coiled, and passes in the direction Fig. 274 J.— Larva of a Bothrin- cephalus from the Smelt (after R. Leuckart). Via Vo — Geaerattve orf^ins f a sexuilly mature proglottis of Bothriocephalus lutus (after feommor and R Leuckarl) a from tho ventral surface, i, from the dorsal surface. Ov and r o\ary , Ut, nleras . SJ, shell „1 n 1 , Dsf, vitellarium (yolk gland); Va, vagina with opening ; T, testis ; Cb, pouch of the cirrus ; Fif, vas deferens. of the long axis of the segment on the dorsal surface and divides into two side branches. These receive the efferent canals of the delicate testicular sacs, which occupy the lateral parts of the middle layer (7). The female genital opening (fig. 275 a) leads into a vagina (Fa) situated behind the pouch of the cirrus, and frequently filled with semen. This vagina runs as a tolerably 22 333 PLATTHELMINTKES. straight median canal on the ventral surface, and opens by a short, narrow tube into the oviduct. The vagina also functions as. a recqitaculum seminis. There is yet a third opening (fig. 275, a), situated at some distance behind the other two ; this is the opening of the tubular uterus CUt), the convolutions of which give rise to a peculiar rosette-shaped figure in the midst of the segment (^Wa2)j}enUlie Pallas). Close to the hind end of the segment the ducts of the yolk-glands (DsQ and of the ovaries (Ov) unite with each other and open into the uterus ; the cells of the shell-gland (Sd) surround and open into the point of junction of these structures. Behind the uterus, and partly among its posterior lateral horns, lie the so-called coiled glands ; and at its sides are the so-called lateral glands (Eschricht). The latter are, according to Eschricht, the ovaries or germaria (formerly held by Leuckart to be the vitellaria). The coiled glands (Leuckart's ovaries), an aggregation of pear-shaped cells, were considered by Stieda, with whom Landois and Sommer are in accord, to be a shell gland (fig. 275). The ova are for the most part developed in water, and escape from the upper pole of the egg-shell through a lid-like valve. The escaped embryo is covered with cilia, by means of which it swims about for a long time. Hence it is probable that the later stages of development take place in an aquatic animal. It is unknown how and in what host the embryo with six hooks becomes a Scolex ; and the question how this tape-worm gets into the human body — in spite of the researches of Knoch, who maintained that they appeared there directly and without the intervention of an intermediate host — is still un- decided. B, cordatus Lkt. With large, heart-shaped head, without a filiforra neck ; with numerous deposits of calcareous bodies in the parenchyma. It attains a length of about three feet and lives in the intestines of man and of the dog in Greenland. Schistocephalus Crepl. Head split, with a sucker on each side. The body of the cestoid form is segmented. S. solidus Crepl. Lives in the body cavity of the stickleback, escapes into the water, and becomes sexually adult in the intestine of water-birds. Tricenojihorus Eud. Head not distinct, with two weak suckers and with two pairs of tridentate,hooks. The body has no external segmentation. The generative openings are marginal. T. nodulosus Rud. In the intestine of the pike. Asexual encysted form in the liver of Cyprinus. Fam. LigvLliise (^Pscvdo_i)7njUid(B'). Without real suckers. Hooks are either present or absent. The Cestoid has no segmentation, but the generative organs are repeated. They live in the body cavity of Teleosteans and in the intestine of birds. Ligula Bloch. Body band-shaped and unsegmented. L. simpli- cissima Bud., in the body cavity of fishes and in the intestine of aquatic birds, i. tuba V. Sieb., in the intestine of the Tench. The families of the Tetrarhynchidae (Tetrarhynchus llngualis, Cuv., passes its young stages in Soles, and is matured in the intestine of Rays and Dog-fish), and Tetraphyllidse {Ech'meibuthrium minimmn van Ben.) are allied here. Fam. Caryophyllaeidae. Body elongated and unsegmented. The anterior margin is plicated. There are no hooks, and there are eight sinuous longitu- dinal canals of the excretory system. Generative organs single. The develop- ment is a simplified metamorphosis. Caryopliyllmus mutabilis Rud., in the intestine of Cyprinoids. The young form possibly lives in Tubifex rivnlorum^ if the Helminth observed by d'Udekem was the same. In this worm, however, there lives another parasite, which was observed by Ratzel and has recently been more closely investigated by E. Leuckart, who has shown that it is NEMEETIXI. 339 a sexually mature Cestoid still fixed by an appendage bearing the embryonic hooks. Arclugetcs Sieholdii Lkt. With two weak suckers and a caudal appendage. Order 4. — ]N'emertini*= Riiynchoccela. Elongated, frequently hand-shcqyed Platijhehninthes, with straight alimentary canal opening hy an anus, and with a separate protrusible proboscis. Usually loith two ciliated j)its in the cephalic region. The sexes are separate. The Nemertines are distinguished not only by their elongated form, but also by their con- siderable size and high organization. Thick layers of muscles, traversed by connective tissue, are spread beneath the integument, which con- tains pigment as well as flask-shaped mucous glands. The external layer of longitudinal muscles^ strongly developed in the Anojjla, is Avanting in the Enopla (Nemertines, the probos- cis of which is armed with stylets), in which group there is only an outer layer of circular muscles and an inner layer of longitudinal muscles. A long tubular protrusible proboscis, which is sometimes armed with stylet-shaped rods, is always found at the anterior end of the body above the buccal cavity, and projects through a special prjeoral opening (fig. 276), and can be retracted into a special muscular sheath separate from the body cavity. At the bottom of the principal portion of the proboscis, there is in many Nemertines (Enopla) a large spine, which is directed forwards, and at its sides numei-ous small secondary spines in pouches. The posterior glandular portion of the proboscis, to which retractor muscles are attached, is, according to Claparede, to be regarded as a poison apparatus. When the proboscis is pro- * A. de Quatrefages, " Memoire sur la famille des Nemertines," Ann. des Sc. Nat.. Sen 3, Tom. VI., 18i6. Mcintosh, " On the Structure of the British Nemerte- ans," Transact. Edinh. Royal Soc, Tom XXY., 1 & 2. Barrois, " Memoire sur I'Embryologie des Nemortes," Paris, 1877. Hubrecht, '• Untersuchungen iiber Nemertinen, etc.," Niederl. Archiv,. Tom. II. Fig. 276. — Tetrastemma ohscurum (after il. Schultze). Young specimen about 3 lines in length ; O, mouth ; D, intestine ; A, anus ; Bg, blood vessels; if, proboscis armed with stylet ; Ex, lateral trunks of the e.xcre- tory system; P, ex- cretory pore; G, ciliated pit ; Nc, nerve centre ; Ss, lateral nerve trunks ; Oe, eyes. 340 PLATTHELMINTIIES. truded, it is inverted like the finger of a glove, so that the blind end at which the spines are placed becomes the extreme fi-ont end of the protruded proboscis. The hrain attains a considerable development. Its two halves are connected by a double commissui^e which embraces the proboscis, and in them several lobes, usually a dorsal and ventral, may be distin- guished. The two ventral lobes are produced into the two lateral nerve trunks, which in certain cases (Oerstedtia) may approach each other on the ventral surface. The nerve trunks contain not only fibres but also a superficial layer of ganglion cells, which may give rise to ganglion-like enlargements at the points of exit of the nerve branches. In the embryos of Prosorochnus Clcqyaredii the nerve trunks are said to end in an enlargement. In the cephalic region there are two strongly ciliated depressions known as the cephalic slits, beneath which special lateral organs, supplied with nerves from the brain or it may be posterior lobes of the brain itself, are placed. These structures are probably sense organs. The cephalic slits were formerly erroneously taken for the openings of respiratory organs. Eyes are widely distributed, and usually consist of simple pigment spots which rarely contain refractive bodies. Exceptionally, as in Oerstedtia jK^ida, two otolithic vesicles are found on the brain. The Nemertines, unlike all other Platyhehninthes, possess a blood- vascular system. This consists of two sinuous lateral vessels in which the blood flows from before backwards, and a straight dorsal vessel in which the blood flows in the reverse direction. This latter is connected with the ventral vessel at the postei-ior end of the body and in the region of the brain by wide loops, and in the rest of its course by numerous narrower transverse anastomoses. These vessels lie in the body cavity and have contractile walls. The blood is usually colourless, but in some species it is red. In Aviphijwrus splendens, Borlasia splendida, the red colour (haemoglobin) is con- tained in the oval disc-shaped blood corpuscles. The Nemertines are, with some few exceptions {Borlasia herma- phroditica), dioecious. The two kinds of generative organs have the same structure, and are sacs filled with ova or spermatozoa lying in the lateral portions of the body between the pouches of the intestine, and opening to the exterior by paired openings in the body wall. The ova, when laid, fretjuently remain connected by a gelatinous substance, and are deposited in irregular masses or in strings, from the middle of which the animal creeps out, like the leech out of its KEMEETOI. 341 Fig. 277.— Filidium (after E. Metschnikoff). a, free swimming larva vnih invaginated cavity ; b, later stage, helmet-shaped ; E, E' the tvfO pairs of ectodermal invaginations ; D, alimentary canal. cocoon. Some forms, as Prosorochmus Claparedli and Tetrastemma ohscurum, are viviparous. Some of the Anopla develop with a metamorphosis. The larva is ciliated and m ay pass through a free - swimming stage, in which case it is known as the Fili- dium, or it may be without such a stage {Type ofDesor). In both cases the perfect worm is deve- loped within the skin of the ciliated larva. The Pilidium larva is helmet-shaped, and was formerly described as the species of a supposed independent genus, Pilidium, and presents many analogies to the Echinoderm larva. In the case of the Pilidium, the segmentation is regular, and results in the formation of a spherical ciliated em- bryo, which is hatched and becomes a free-swimming larva; the archenteron is then formed by invagina- tion ; and at the side of the embryo, opposite the blasto- pore, a long flagellum is developed (fig. 277, a). On each side of the mouth a broad lobe grows out, the edges of which are fringed with cilia (fig. 277, h). Two pairs of invaginations of the ectoderm now make their appear- Pio. 278.— Later stage of Pxhilaim, with tuft of cilio and enclosed Nemertme (after Butschh), Oe, oesophagus; D, alimentary canal ; wftm, amnion; R, rudimentary proboscis of the Nemertine; So, lateral pit. 342 PLATrilELMIXTUrS. ance, forming the first rudiment of the Nemertine body. The four discs so formed fuse together and give rise to a ventral germinal plate, which gradually grows lound the alimentary canal of the PUidium to form the skin of the future Nemertine. The proboscis arises as an invagination of the anterior end of the germinal plate (fig. 278). The young Nemertine subsequently breaks through the larval skin. The Nemertines live principally in the sea, under stones in the mud, but the smaller species swim about freely. There are also forms which live on the land, as well as pelagic forms. Certain species form tubes and passages, which are lined with a slimy secre- tion. The food of the larger species principally consists of tubicolous worms, which they extract from their habitations by means of the proboscis. There are, however, parasitic Nemertines which infest Crustacea or live on the mantle and gills of Mollusca. In this case they are, like the Hirudinea, furnished with a posterior sucker (^Malacohdella). The Nemertines are distinguished by their repro- ductive capacity and by their tenacity of life. Mutilated parts are quickly regenerated, and the parts into which certain species readily break are said to have the capacity, under favourable conditions, of developing into new animals. 1. Sub-order : Enopla. — The proboscis is armed with stylets. The short, often funnel-shaped cephalic slits are connected with lateral organs, which correspond to the posterior cerebral lobes of the Anopla. In the brain the upper lobes are slightly elongated posteriorly leaving the ventral lobes, from which the lateral nerves arise, quite free. Development takes place without metamorphosis. Fam. AmphiporidaB. The ganglia are more rounded, the lateral nerve trunks are placed inside the dermal muscles. The mouth is on the ventral surface near the anterior end of the body, in front of the commissures between the ganglia. The lateral organs are separated from the brain and connected .with it by fibres ; they contain a narrow water canal. A?)ij>hijni7-us lactifloTeus Johnst. Lives under stones, and is distributed from the North Seas to the Mediterranean, 3 — 4 in. long. A. spectaMlis Quatr. JBorlasia spleiulUla Kef., Mediterranean, and Adriatic. Tetraatcmma oiscurum M. Sch. Viviparous : Baltic. T.agricola Will. Suhm., terrestrial. Kemertcs gracilis Johnst. 2. Sub-order : Anopla. — The proboscis is unarmed. The long cephalic slits occupy the whole side, or the anterior part of the head, and lead into the lateral organs, which are direct processes of the upper lobes of the brain. Development frequently by means of ciliated larvaa. NEMATnELMIXTHES. 343 Fam. lineidse. Ganglion elongated. The head has deep slits on either side. Lincus mar huts Mont., L. Icngissimus Sim. (sea long-worm, Borlasia anglica Oerst., Nemertes Borlasii Cuv,), grows to a length of 15 feet and more. English coast. Cercbratnlus mnrginatv.s = Mcckelia somatotojims F.S. Lkt., Adriatic and Mediterranean. Micrura fasciolata Ehrbg., North Seas to the Adriatic. Fam. Cephalotrichidae. Cephalic slits and lateral organs are wanting. Head not distinct, very long and pointed. Cejihalothrix hwculata Oerst. Sund. Malacobdclla grossa O. Fr. Miill. Body broad and flat, with posterior sucker. Is parasitic in the mantle cavity of various MoUusca, as Mya, Ci/2>ri?ia, etc. CLASS II.— NEMATHELMINTHES. Hound, worms loith tubular or filiform bodies. The cuticle is fre- quently ringed. The anterior pole is either armed with hooks or 2>rovided with pajnllce. The sexes are separate. The unsegmented body is rounded, more or less elongated, tubular or filiform, and both ends are, as a rule, tapei-ed off. Appendages are always wanting, as are, with few exceptions, movable bristles. On the other hand, special organs for attack and attachment, such as teeth and hooks, are not unfrequently present on the anterior end of the body ; and in some cases small suckers, which serve for attachment during copulation, may be developed on the ventral surface. As a rule, the integument possesses a cuticular layer of relatively considerable thickness, and a well developed muscular layer, which permits not only of the body being knotted, curved, and bent, but, in the thin filiform Nematoda, of undulatory movements. The body cavity is enclosed by the muscular body wall, and con- tains the blood fluid and the digestive and generative organs. Blood vessels and resinratory organs are wanting. A nervous system is, however, always present. Of se7ise organs simple eyes are not unfrequently present in the free Kving forms. The sense of touch is probably distributed all over the surface of the body, particulaily on the anterior end, especially when papillte and lip-like prominences or bristles are found on it. While in the Acanthocephala mouth and alimentary canal are completely absent, the Xematoda possess a mouth placed at the anterior pole of the body, an oesophagus, and an elongated straight digestive canal, which usually opens by the anus on the ventral surface near the pos- terior end of the body. The excretory organs have various forms, and always differ considerably from those of the Platodes. In the JS'ematoda they consist of paired canals, which open by a common pore and lie in the so-called lateral lines. In the Acanthoce- 344 NEMAXnELMIXTlIES. phala they are branching subcutaneous canals. With a few excep- tions the Nemathelminthes have separated ?exes, and develop directly without metamorphosis. The larvie and sexual animals are not unfrequently distributed in two different hosts. The majority of the Nemathelminthes are parasites either during the whole pei'iod of their life or at different stages. There are, however, also free living forms which often show the closest relationship to the parasitic members of the group. Order 1. — Nematoda (Thread-worms).* Nemathelminthes, with mouth and ali mentary canal. They are lyrincipalhj parasites. The Nematodes possess an extremely elongated thread-like body, which may be provided with papillae at the anterior pole in the region of the mouth, or with hooks and spines within the oral cavity. The mouth leads into a narrow oesophagus, which usually has thick muscular walls, a chitinous lining, and a triangular lumen, and is frequently dilated behind to a muscular bulb (pharynx). In cei'tain genera [Rhahditis, Oxyuris), the chitinous lining of the pharynx is raised into ridges or tooth-like prominences, to which the * Besides the older writings of Eudolphi, Bremser, Cloquet, Dujardin, compare Diesing, " Systema helminthum," 2 Bde Wien, 1850-51. Diesing, " Eevision der Nematoden," Wiener Sitznngsherichte, 1860. Claparcde, "De la for- mation et de la fecondation des ceufs chez les vers Nematodes," Geneve, 1856. A. Schneider, " Monographic der Nematoden," Berlin, 1866. R. Leuckart, " Untersuchungen iiber Trichina spiralis," Leipzig and Heidelberg, 1866. 2nd edition ; also " Die menschlichen Parasiten," etc., Tom. II., Leipzig and Heidelberg, 1876. C. Claus, " Ueber Leptodera appendiculata," Marburg, 1868. 0. Blitschli, '• Untersuchungen iiber die beiden Nematoden der Periplaneta orieutalis," Zeitzsclir. fur W'tss. Zool., Tom. XXI., 1871. And " Beitrage zur Kenntniss des Nervensystems der Nematoden," Arch i v. fiir MUtr Anatomic, Tom X. Fig. 270.— 0;py«r is vermwularit (after R. Leuckart). a, female ; 0, mouth ; A, anus ; V, genital opening ; 6, male with curved posterior end ; c, the latter enlarged ; Sp , gpiculum ; d, egg with enclosed embryo. 345 radial muscles converge in the form of conical bundles. Accord- ing to its function, the oesophagus is essentially a suctorial tube, which pumps in fluids, and by peristaltic action passes them on to the intestine. The intestine follows the pharynx, and opens by the anus not far from the hind end of the body on the ventral surface (fig. 279). Its walls are formed of cells and are non-muscular, except behind, where they have a special investment of muscular fibres which render the terminal portion contractile. Muscular fibres passing from the body wall to the wall of the rectum are also frequently present. In certain Nematodes the anus may be want- ing (Mermis) ; and in Gordius even the alimentary canal undergoes degeneration. Beneath the stiff cuticle, which is often trans- versely ringed, and is composed of several layers, lies a soft granular nucleated sub-cuticular layer (^hypodermis), which is to be regarded as the matrix of the former. Beneath this lies the highly deve- loped muscular layer, in which band-shaped or fusi- form longitudinal muscles predominate. The surface of the body may present mai'kings, as for instance polyhedric spaces and longitudinal ribs, also pro- cesses in the form of tubercles, spines,* and hairs. Ecdyses, i.e., shedding the cuticular layer, seem only to occur in the young forms. The muscles are each composed of a single cell, in which two parts are distinguishable, — a clear, sometimes a granu.lar protoplasmic portion (medullary sub- stance), which projects into the body cavity and is often prolonged into processes ; and an external fibrillated layer (fig. 280). The Nematodes may be distinguished as Meromyaria or Pohjmyaria, according to the arrangement of their muscular system. In the Meromyaria the number of muscle cells (which are arranged according to definite laws) in the cross section is small (eight), while in the Polymyaria their number is considerable. In the latter the muscle cells are often connected together by transverse processes of the medullary substance, which unite on the so-called median lines to form a longitudinal cord. WW Fig. 280. — Muscle- cell of a Nematode. * There may also be prominences of various kind-", and even in some cases a complete coverinij of spines {Cheiracantlms D'ws = Gnatho.stoma Ow., Ch. hiijridinn Fedscb.) 346 KEJIATHELMrVinES. In almost every case, with the exception of Gordius, two lateral regions remain free from muscle and form the so-called lateral lines or regions, which may equal in breadth the neighbouring muscular regions. These lateral regions are formed of a finely granular nucleated substance, and enclose a clear vessel containing granules. This vessel is connected with that of the opposite side in the anterior part of the body, and the two open by a common transverse slit, the vascular pore, on the ventral surface in the median line. The lateral lines have the vahie, both as regards position and structure, of excretory organs. Median lines {dorsal and ventral), accessory median lines (sub-median lines), the latter being placed between the principal median line and the lateral line, are also to be dis- tinguished. The so-called ventral cord of Gordius, which may be compared to the median line and has perhaps the significance of an elastic rod, is very large. Cutaneous glands, in the form of unicel- lular glands, have been observed principally in the region of the oesophagus and in the tail. The nervous system, owing to the difiiculty which its investigation ofiers, has only been satisfactorily recognised in a few forms. It con- sists of a nerve ring surrounding the oesophagus, and sending off posteriorly two and anteriorly six nerve trunks (Ascaris megalo- cejyhala). The posterior trunks run in the dorsal and ventral lines {N'. dorsalis, ventralis), to the extremity of the tail ; while of the six anterior nerves, two run in the lateral lines {N. laterales), four in the interspaces between the lateral and median lines (iV. suh- mediani), and supply the papillce around the mouth. The ganglion cells lie partly near, in front of and behind the nerve ring, partly on the fibrous cords themselves, and are arranged in groups which can be distinguished as ventral, dorsal, and lateral ganglia. There are in addition groups of ganglion cells in the median lines and in the lateral lines in the caudal region. As sense organs we must mention the eyes found in the free- li\-ing Nematoda, and the papillte and tactile hairs found principally in the neighbourhood of the mouth. Each papilla is supplied by one nerve fibi-e, which is swollen to a knob and forms the axis of the papilla. [The NematoJa possess a bodj- cavity, but are without any trace of a vas- cular system.] Generative organs. The Nematodes are dioecious (with ex- ception of the hermaphodrite Pelodytes, and of the Rhahdonema NEMATODA. 347 {Ascaris) nigrovcnosum, which produces first spermatozoa and later ova). The males are chaiacterised by their smaller size, and by the posterior end of the body being generally curved. Both kinds of generative organs consist of single or paired and often much coiled tubes, at the upper end of which the generative products are de- veloped, the lower ends representing the efferent ducts and recep- tacula of the generative products. The usually paired ovaiian tubes, at the upper ends of which the ova arise, terminate in a short vagina, which opens on the ventral surface, rarely near the posterior end of the body. The male generative apparatus, which contains hat-shaped spermatozoa, is almost invariably represented by an unpaired tube, and usually opens on the ventral surface near the posterior end of the body in a common opening with the intestine. As a rule, the common cloacal portion contains two pointed chitinous rods, the so-called spicula, in a pouch-like invagination. These spicula can be protruded and retracted by a special muscular ap- paratus, and serve to fasten the male body to the female during copulation. In many cases (Strongi/lidce) an umbrella-like bursa is added, or the terminal portion of the cloaca can be protruded like a penis (Trichina) ; in this case the cloacal aperture lies almost at the extreme end but is still ventral (^Acro2)halli). In the male papilloe are almost always present in the region of the posterior end of the body, and their nvimber and arrangement afford important specific characters. Development, The Nematoda for the most part lays eggs ; it is only in rare cases that they bear living young. The eggs usually possess a hard shell and may be laid at different stages of the embryonic development or before it has begun. In the viviparous Nematodes the eggs lose their delicate membranes in the uterus of the mother {Trichina, Filaria). Fertilization takes place by the entry of a spermatozoon into the ovum, which is still without a mem- brane. The segmentation is equal, and leads to the formation of a kind of invaginate gastrula. Fi-om the two cell layers are de- veloped the body wall and the alimentary canal. The embryo gradually assumes an elongated cylindrical form, and comes to lie rolled up in several coils within the shell. The excretory pore and the rudiments of generative organs, as well as a nerve ring, are present in the embryo, which is also provided with mouth and anus. The free development is a metamorphosis, usually com- plicated by the circumstance that it is not undergone in the habitat of the mother. The young stages or larvce, probably of most Nema- 848 NEMATHELMHS'THES. todes, have a different habitat to that of the sexual animal; the young and the adult Nematode being contained in different organs of the same or even of different animals. The larvae live for the most part in parenchymatous organs, either free or encysted in a connective tissue capsule ; the adults, on the contrary, live principally in the alimentary canal. The embiyo is almost invariably characterised by the special form of the oral and caudal extremities, but sometimes also by the posses- sion of a boring tooth, or of a circle of spines {Gordius). Sooner or later the skin is shed, and the animal enters its second stage, which may often still be considered as a larval stage; repeated ecdyses precede the sexually adult stage. The post-embryonic development of the Nematodes presents numerous modifications. In the simplest cases the embiyo, while still enveloped in the egg mem- branes, is transported passively in the food {Oxyuris vermicularis and TricJioce2ihalus). In many AscaridcB — to judge by the species parasitic in the Cat — the em- bryos, which are provided with a boring tooth, first make their way into an intermediate host, by which they are transported Fi*. 2.«i.-Scie,-ostomnm ietvacanthum, en- in^-Q ^\^q intestine of the sccond eysted (after R. Leuckart). host with the food or water. More frequently the young forms encyst within the intermediate host, and, enclosed in the cyst, are transferred into the stomach and intestine of the permanent host (fig. 281). For example, the embryos of Spiroptera ohtusa of the Mouse, while still in the eg^ membranes, are taken with the food by the Meal-worm, in the body cavity of which they encyst. In the viviparous Trichina spiralis there is a modification of this mode of development inasmuch as the migration of the embryos and their development to the encysted form found in the muscles (muscle-trichina) take place in the same animal which contains the sexually mature intestinal Trichinas. The development of the Nematode larvae often makes a considcnrable advance within the intermediate host into which they have migrated. Thus, for instance, in Cucullanus elegans, the embryos migrate into the Cyclops, and in the body cavity of these small Crustacea undergo two ecdyses and essential alterations of form, obtaining at this early NJiMATODA. 349 stage the characteristic oral capsule of the sexually adult stage, to which they only develop in the intestine of the Perch. According to Fedschenko,* a similar mode of development occurs in Filaria medinensis. The embryos pass < into puddles of water, and migrate thence into the body cavity of the Cyclopidce ; and after casting their skin assume a form which, except for the absence of the oral capsule, resembles that of the larva of Gucullanus. After the expiration of two weeks there is another ecdysis, with which is connected the losf of the long tail. The later history is unknown. It has not yet Pig. 232.— a, Shabdcjiema (Ascaris) nigrovenosuin of about 35 mm. in length in the stage of maturity of the male products ; O, genital glands ; 0, mouth ; D, intestine ; A, anus ; N, norve-ring; Brz, glandular cells; Z, isolated spermatozoa. 6, Male and female iJAaMiYw forms from about I'o mm. to 2 mm. long ; Oc, ovary ; T, testis ; V, female genital opening ; Sp, spicula. been discovered whether the migi-ation of the Filarian larva into the permanent host (Man, see p. 356) takes place with the body of the Cyclops, or independently after copulating in the free state. The embryos of some Nematoda develop in damp muddy earth, after casting their skin, to small so-called Rhahditis forms with a double * Compare FedschenTendiculata, which lives in the slug Avion empiricortcm, also presents in its development a like alternation of heteromorphic generations, which, however, are not strictly alternating, inasmuch as numerous generations of the Rhabditis form may succeed one another. The Le]itodera are peculiar in that the form parasitic in the snail is a larva characterised by the absence of a mouth, and by the possession of two long band-shaped caudal appendages; it quickly attains maturity, but only after a migration into damp earth and after losing the caudal appendages and casting the skin. TJie Nematoda feed on organic juices, some of them also on blood, and are enabled by their armed mouth to inflict wounds and to gnaw tissues. They move by bending their body with a rapid undulatory movement towards the ventral and dorsal sui'faces, which thus seem KEMATODA. 351 to be the lateral surfaces of the moving animal. Most JSfematoda are parasitic, but lead an independent life in certain stages of their life history. Numerous small Nematoda, however, are never parasitic, but live freely in fresh and salt water and in the earth. Some Nematodes are parasitic in plants, for example, Anguillula tritici, dipsaci, etc. ; some live in decaying vegetable matter, e.g., the vinegar worm in fermenting vinegar and paste. Nevertheless very similar forms occur in the contents of the intestine and in the faeces of different animals and of man {A. intestinalis, stercoralis). The power possessed by small Nematoda of resisting the effects of pro- longed desiccation and of coming to life again on being moistened is very remarkable. Fam. Ascaridae. Body tolerably stout. With three lips furnished with papillte. One of these lips is, directed towards the dorsal surface, while the two others meet together in the ventral line. The posterior end of the male is ventrally curved, and usually furnished with two horny spicula. Fig. 283.— ^»cari« lumbricoides (after R. Leuckart). a, Posterior end of a male with the two spicula (Sp). b. Anterior end from the dorsal side, with the dorsal Up furnished with two papillae, c. The same from the ventral side with the two lateral ventral lips and the excretory pore (P). d. Egg with the external membrane formed of small clear spherules. Ascaris L. Polymyarian, with three strongly developed lips, the edges of which are in the larger species pi'ovided with teeth. The pharynx is not sepa- rated as a distinct bulb. The caudal extremity is usually short and conical, and in the male sex invariably provided with two spicula (fig. 283, a). A. lumbricoidcs Cloquet, the human round worm, a smaller variety in the pig (j1. suilla Duj.) The eggs pass into water or damp earth and remain there some months, until the embryonic development is completed ; they are probably carried into the alimentary canal of their later host by means of an inter- mediate host. A. nncgalocepliala Cloquet (horse and ox) ; A. mystax Zed. (cat and dog), sometimes parasitic in man. Oxyurii Eud. Meromyarian ; usually with three lips, which bear small papillce. The posterior end of the oesophagus is enlarged to a spherical bulb provided with a masticatory apparatus. The posterior end of the body of the female is thin and pointed. Avhile that of the male has only two prteanal and few postanal papillje, and a single spiculum (fig. 279). 0 vermicularis L., in the large intestine of man, distributed in all countries. The female is about ten mm. long. 0. curvida Kud., in the cscum of the Horse. 352 KEilATHELilLNTlLES. Fam. Strongylidse. The male genital opening is placed at the hinder end of the body, at the bottom of an umbrella- or bell-shaped bursa, the margin of which is furnished with a varying number of papillse. Eustrongylus Dies. With six projecting oral papillae, and a row of papillae on either lateral line. The bursa is bell-shaped and completely closed, with regular muscular walls and numerous marginal papillae. There is only one spiculum. The female genital opening is far forward. The larvae live encysted in fishes. (_Filarla cystica from Symbmnchus'). E. gigas Rud., the body of the female is three feet in length, and only twelve mm. thick. It lives singly in the pelvis of the kidney of the Seal and Otter, and very rarely in Man. Strongylus Rud. With six oral papillae and small mouth. Two conical cervical papillae upon the lateral lines. The pos- terior end of the male has an umbrella-like incom- pletely closed bursa. Two equal spicula, usually with unpaired supporting organ. The female sexual opening is sometimes approached to the posterior end of the body. They live for the most part in the lungs and bronchial tubes. *S^. longevag hiatus Dies. Body 26 mm. long, 5 to 7 mm. thick. The female sexual opening lies directly in front of the anus, and leads into a simple ovarian tube. Only once found in the lung of a six-year old boy, in Klausen- burg. St. paradoxus Mehlis, in the bronchial tubes of the pig. St. Jilaria Rud., in the bronchial tubes of the sheep. St. comnmtatus Dies., in the trachea and bronchial tubes of the hare and rabbit. St. auricularis Rud., in the small intestine of Batracliia. Dochinius Duj. With wide mouth and horny oral capsule, the edge of which is strongly toothed. Two ventrally placed teeth project at the bottom of the oral capsule, while on the dorsal wall a conical spine projects obliquely forwards. D. duodcnalis Dub. {Ancylostomun duodenale Dub.), 10 to 18 mm. long, in the small intestine of Man, discovered in Italy ; very widely distributed in the countries of the Nile (Bilharz and Griesinger). By aid of its strongly armed mouth it wounds the intestinal mucous mem- brane, and sucks the blood from the vessels. The lujdenalis fi'cquent hasmorrhages occasioned by these Dochmia are the cause of the illness known by the name of Egyptian chlorosis (fig. 284). It has lately been established that this worm occurs in Brazil, and that, like B. trigonocephalus, it develops in puddles of water (Wucherer). D. trigonocephalus Rud., in the Dog. Sclcrostamum Rud. With characters of Dochmius, but with a different oral capsule, into which two long glanular sacs open. Sc. ecpdtnim Duj. = annatum Dies. In the intestine and the mesenteric arteries of the horse. Bollinger* has shown that the phenomena of colic in the horse may be referred to embolic processes proceeding from aneurism of the intestinal artery. Each aneurism contains about nine worms * Bollinger, " Die Kolik der Pferde und das Wurmaneurysma der Einge- weidearterien,' Miinchen, 1870. FlO. 284.— DocAmi (after R. Leuckart). a, male; O, mouth ; B, bursa, b. Female ; 0, mouth; A, anus; V, vulva. NE3IA.T0DA. *AZ Sc. tetraranthum Mehlis, also in the intestine of the horse. The embryos, after migrating into the intestine, become encysted in the walls of the rectum and csecum, assume within the cyst their definite form, break out from the cyst, and escape again into the intestine. Oucnlla/ius elegam Zed., in the Perch. Fam. Trichotrachelidae, with iong neck-like thin anterior portion of the body. Mouth small, without papillae. (Esophagus very long, traversing a peculiar cord of cells. Trichocephalus Gocze. Anterior part (fig. 285) of the body elongated and whip shaped: posterior part cylindrical and sharply distinct, enclosing the o-enerative organs, in the male it is coiled up. Lateral lines absent. Main median lines present. The penis is slender and furnished with a sheath, which is turned inside out when the former is protruded. The hard shelled, lemon-shaped eggs undergo the first part of their development in water. Tr. dixpar Rud. In the human colon : these worms do not Kve free in the intestine, but bury their filiform anterior extremity in the mucous membrane (fig. 28.5). The eggs pass out of the host with the fseces, as yet without a sign of beginning development, which only takes place after a prolonged sojourn in the water or in a damp place. According to the ex- periments of Leuckart per- formed with Ti: affiiiis of the sheep and Tr. crenatus of the pig, embryos with the egg membranes, if introduced into the intestine, develop into the adult Tricocephalus ; and we may therefore conclude that the human Tr. disjmr is intro- duced directly, and without an intermediate host either in the drinking water or in uncleaned food. The young Tr. dhpar is at first hair-like, and re- sembles a Trichina, and only gradually acquires the considerable thickness of the hind end of the body. TricJiosomum Rud. Body thin, hair-like, but the posterior end of the body in the female is swollen. Lateral lines and the principal median lines are present. The male caudal extremity has a cutaneous fold and a simple penis (spiculum) and sheath. Tr. muris Creplin., in the large intestine of the house-mouse. Tr, crassicauda Bellingh., in the bladder of the rat. According to Leuckart, the dwarfed male lives in the uterus of the female. There are usually two or three, more rarely four or five males in a single female. There is also a second species of Trichosomum found in the bladder of the rat. Tr. Schmidtil v. Linst., the larger male of which was formerly taken for that of Tr. crassicauda. Trichina Owen.* Body thin, hair-like. Principal median lines and lateral * Compare the writings of R. Leuckart, Zenker. R. Virchow, Pagenstecher, etc. 23 Fig. 2%5.—TncliocepTialui- dispar (after R. Leuckart). a. Egg ; b, female ; c, male with the anterior part of the body buried in the mucous membrane; Sp, spiculum. 354 KXM A'i lIELMrS'THES. lines aro present. The female generative opening well forward. The posterior end of the body of the male has two terminal cones between which the cloaca is Fig. 280.— Trichina tpiralis. a, Mature female Trichina from the alimentary canal ; G, genital opening ; E, embryos ; Oo, ovaiy. b, Male ; T, testis, c, Embryo, d. Embryo which has migrated into a muscle fibre, already considerably enlarged, e, The same developed into a coiled Muscle Trichina, and encysted. >'EMATODA.. 355 projected. Tr. spiralis Owen, in the alimentary canal of Man and numerous, principally carnivorous, Mammalia ; hardly two lines in length. The viviparous females begin to bring forth embryos about eight days after their migration into the alimentary canal. These embryos traverse the intestinal walls and body cavity of the host, and migrate, partly by their own movements in the liundles of connective tissue, partly with the aid of the cun'ents of Ijlood into the striped muscles of the body. They pierce the sarcolemma and penetrate into the primitive bundles, the substance of which degenerates, the degener.ition being accompanied by an active multiplication of the nuclei. In a space of fourteen days they develop, within a sac-like swelling of the muscle fibres, into spirally coiled worms, around which and within the sarcolemma and its connective tissue investment a clear lemon-shaped capsule is excreted from the degenerated muscle substance. The young Muscle-Trichina can remain liv- ing for years ^^'ithin this capsule, which at first very delicate, gra- dually becomes thickened and hard by the formation of other layers and by the gradual deposi- tion of calcareous matter. If the encysted animal is transferred into the intestine of some warm- blooded animal in the flesh of its first host, it is freed from its cyst by the action of the gastric juice, and the rudimentary generative organs, which are already toler- ably far developed, quickly attain maturity. In from three to four days after their introduction the asexual Muscle-Trichinas become sexual Trichinas. These copulate and produce a brood of embryos which migrate into the tissues of the host (one female may produce Fig as many as 1000 embryos) (fig. 2S6). Th 2 house rat is especially to be mentioned as the natural host of the Trichina. This animal does not hesitate to eat the carcase of its own species, and so the Trichina infection is passed on from generation to generation. Carcases in- fected -ndth Trichinas are sometimes eaten by the omnivorous pig, in whose flesh the encysted Trichinas are introduced into the intestine of man, and occasion the well-known disease. Trichinosis, which when the migration takes place in number, often has a fatal result. Fam. Filariidae. Body filiform, elongated, often with six oral papillte^ some- S7.—Filaria medinensU (aiter Bastian and Leuckart). a, Anterior end seen from the oral sur- face ; O, mouth ; P, papilla, b, Pregnant female (siz3 reduced more than half), c. Embryos strongly magnified. 356 NEMATHELMINrHES. times with a horny oral capsule, with four praeanal pairs of papillae, to which an unpaired papilla may be added, with two unequal spicula or with simple spiculum. Fdaria 0. Fr. Miill. With small mouth and narrow oesophagus. This species, which is sometimes destitute of papillje, lives outside the viscera, usually in connective tissue, frequently beneath the skin (divided by Diesing into numerous genera). F. {Dranmculus') mcdinensis* Gmel. the Guinea worm, in the subcutaneous cellular tissue of Man in the Tropics of the Old World, reaches a length of two feet or more. The head is provided with two small and two larger papillae. The female is viviparous, and without sexual opening. The male form is unknown. The worm lives in the connective tissue between the muscles and beneath the skin, and after reaching sexual maturity, occasions the formation of an abscess, with the contents of which the embryos escape to the exterior (fig. 287). It has lately been proved (Fedschenko) that the embryos of Filaria migrate into a Cyclops and there undergo an ecdysis. Whether they are then (in the body of the Cyclops) introduced into man in his di'inking water, or whether they firet escape and copulate in a free state, is not known. F. immitis lives in the right ventricle of the dog, and is very abundant in East Asia. It is viviparous. The embryos pass directly into the blood, where, however, they do not undergo their further development. Similar young Hasmatozoa are also found in the blood of man in the Tropics of the New and Old Worlds {F. sanguinis Juminis, F. Bancvdfti). Since these animals are also found in the urine, their appear- ance seems to have an etiological connection vnih. haematuria. In the East Indies, young Filaria also live in the blood of the street dog, and would seem to be related to the brood of Filaria sanguinolenta, since, according to Lewis, knotty swellings on the aorta and oesophagus are invariably found with these Filaria. F. pajjillo.'ia Rud. in the peritoneum of the horse. F. loa Guyot., in the conjunctive of negroes on the Congo. F. laMalis Pane. Only once observed at Naples. An immature Filaria described as Filaria lentis (^ocidi hiimani) has been found in the human capsula lentis. Fam. Mermithidae. Aproctous Nematodes, with very long filiform body, and six oral papilliv. The male caudal region is broad, and is provided with two spicula and three rows of numerous papilla. They live in the body cavity of insects, and escape into the damp earth, where they attain sexual maturitj' and copulate. Mcrviis iwjrescens Duj., was the occasion of the fable of the worm rain. M. albicans v. Sieb. v. Siebold established by experiment the migration of the embryos into the caterpillars of Tinea cvonymvlla. Sphcerularia bvinhi Leon Duf. Fam. Gordiidae. Body elongated and filiform. Without oral papillae and lateral lines, with a ventral cord. The mouth and anterior region of the alimentary canal is obliterated in the adult state. The testes and ovaries are paired and open to the exterior with the anus near the hind end of the body. Uterus unpaired, with receptaculum seminis. The male caudal region is forked, and is destitute of spicula. In the young stage they live in the body cavity of predatory insects, and are provided with a mouth. At the pairing time they pass into the water, where they become sexually mature. The embryos, which are provided with a circle of spines, bore through the egg-membranes and migrate into Insect larvae {Chironomm-larves, Fphemerida;). and there encyst. Water * Compare H. C. Bastian, "On the Structure and Nature of the Dracanculus," Trans. Linn. Society, vol. xxiv., 1863. Fedschenko I. c. CHJETOGKATnA. 357 beetles and other aquatic predatory insects eat with the flesh of the EpkemerUl Icurvce the encysted young forms, which then develop in the body cavity of their new and lar2:er host to young Gordlidce. Gordius aqvaticus Ihij. Fam. Anguillulidae.* Free living Nematodes of small size. Caudal glands are sometimes present. The lateral canals are often replaced by the so-called ventral glands. Some species either live on or arc parasitic in plants ; others live in fermenting or decaying matter. The greater number, however, live free in earth or water. Tylcnclms Bast. Buccal cavity small, and con- taining a small spine. The female genital opening lies far back. T. scandcns Schn. = trltici Needham, in mildewed wheat grains. When the grains of wheat fall the dried embryos grow in the damp earth, bore through the softened membranes, and make their way on to the growing wheat plant. Here they remain some time, perhaps a whole winter without alteration, until the ears begin to be formed. They then pass into the latter, grow, and become sexually mature, while the ear is ripening. They copulate and deposit their eggs, from which the embryos creep out, and at length constitute the sole con- tents of the wheat grains. T. dipsacl Kuhn, in heads of thistles (Cardius) T. Davaviii Bast, on roots of moss and grass. Heterodcra Schachtii Schmidt., roots of the beet-root, also of the cabbage, of wheat, barley, etc. RhahdUia Duj., divided by Schneider into Lrptodera Duj. and Pelodera Schn. Rh. flexilis Duj,, head very sharply pointed, mouth with two lips, in the salivary glands of Limnx cinrrcus, Rh. arKjiostoma Duj. Rh. aj)j)e?idiculata Schn., in damp earth, 3 mm. long. The larva, which is without a mouth, and has two caudal bands, is found in Arion c7n2nriconnn. Angulllula uccti = ghitinis 0. Fr. Miill.. known as the vinegar woim and pasteworm, 1 to 2 mm. long. Of the many marine Angitillnlidce (^EnopUdoe), we must mention Dortj- laiinns maximus Biitschli, D. stagnalis Duj., found in mud everywhere in Europe. Eachelidhim marinum Ehrbg., Enojjlus trUIentatus Duj. The abberant families Besmoscolecxdce and Chatosomidce are allied to the Nematoda. The Ch.etognatha. The Chcetognatha, f containing only the genus Sagitta, are allied to the Nematodes. They are elongated round worms, with a pecu- liarly armed mouth and latex'ally placed horizontal fins, the mem- branous edges of which are supported by rays. The anterior portion of the body is sharply separated off as a head, and bears in * Davaine, " Eecherches sur I'Anguillule du ble nielle," Paris, 1857. KUhn, " Ueber das Vorkommen von Anguillulen in erkrankten Bliithenkopfen von Dipsacus fuUonum," Zittschr.filr luiss Zuol., Tom IX., 1859. Bastian, '• Mono- graph of the Anguillulidfe or free Nematoids. marine, land, and fresh water," London, 1864:. O. Biitschli, •' Beitrage zur Kentniss der freilebenden Nema- toden," Kov. Acta, Tom XXXVI., 1873. Lad. Oerley, " Monographic der Anguilluliden," Buda-Pest., 1880. f Compare A. Krohn, " Anatomisch-physiologische Beobachtungen iiber die Sagitta bipunctata," Hamburg. 1844. R. Wilms, " De Sagitta mare germani- cum circa insulam Helgoland incolente," Berolini, 1846. Kowalevski, "Em- bryologische Studien an Wiirmern und Arthropoden," 21cm. de VAcad. St. Peterslourg, Tom XVI. 0. Hertwig, " Die Chcetognatha, eine Mono- graphic," Jenr,, ISSO. 358 NEM ATIIELM ES' THES. the region of the mouth two lateral gioups of hooks which function as jaws. The nervous system consists, according to Krohn, of a cerebral ganglion on Avhich the eyes are situated, and a ven- tral ganglion placed in about the middle ■S 1 1^^°=^. / °^ ^^^^ body length. There are in addition two ganglia near the mouth, which may be considered as the suboesophageal gan- glia, and are connected with each other and with the cephalic ganglion by oeso- phageal commissures. Od^ :^ ^A ?i FlO. 288.—Sagitta (Spadella) cephalopUra, magnified 30 times, viewed from the dorsal side (after O. Hertwig). F, posterior fin ; (?, supra- cesophageal ganglion ; Te, ten- tacles; R, olfactory organs; On, ovary ; Od, oviduct ; Tt testis; Vd, vas deferens; Sh, vesicula seminalis. [The common view now is that the large ventral ganglion of the middle of the body, which is connected with the cerebral by com- missures, is homologous with the suboesophageal ganglia of other types.] The straight alimentary canal is at- tached to the body wall by a dorsal and ventral mesentery from the oesophagus backwards, and opens to the exterior at the base of the long tail, which terminates in a horizontal fin (fig. 288). [The body cavity is well developed, and divided by the dorsal and ventral mesenteries into two parts, and again by two transverse verti- cal septa into a cephalic section, a section in the body, and finally a caudal section. Vas- cular and excretory organs are absent.] Reproduction. The Chcetognatha are hermaphrodite, and possess paired ovaries, which open by two apertures at the base of the tail and are connected Avith seminal pouches. The testes also are paired, and situated posteriorly to the ovaries in the tail; their products pass to the exterior by openings at the sides of the tail. Segmentation is complete, and leads to the formation of a blastosjihere. One side of this becomes invaginated so that the segmentation cavity is obliterated and a gastrula is formed, in the entodei'm CH.ETOGKAinA. 359 of which two cells may already be recognised as primitive generative cells. As soon as these make their appearance in the entoderm, the latter becomes folded in such a way that the ai-chenteron is divided into a median and two lateral cavities. The layer of cells lining the lateral cavities becomes the mesoderm, and the contained cavities the two lateral compartments of the bod}^ cavity, while that of the middle cavity gives rise to the wall of the mesenteron or alimentary canal. The permanent mouth is formed at the end opposite to that at which the blastopore, which is now closed, was situated. There is but one genus, Sagitta Slab., of which several species, e.rj., Sagitta hiinmctata Krohn, S. germanica Lkt. Pag, from the Euro- pean seas have been more accurately described. Order 2. — Acanthocepiiala.* Elongated round loorms with jjrotrusible 2'>rohoscis furnished loith hooks ; without mouth and alimentary canal. The saccular, often transversely wiinkled body begins -with a proboscis, which is furnished with recurved hooks and can be retracted into a tube projecting into the body cavity (sheath of the proboscis) (fig. 289, R and Rs). The posterior end of this sheath is fas- tened to the body wall by a ligament, and by retractor muscles. The nervous system (fig. 289, G) is placed at the base of the proboscis, and consists of a simple ganglion formed of large cells. Nerves are given ofiT from the ganglion anteriorly to the proboscis, and through the lateral retractors (retinacula) to the body wall (fig. 289, R). The latter supply partly the mviscular system of the body, and partly the Rj genital apparatus, in which there are, princi- pally in the male animal, special nerve centres consisting of ganglionic enlargements. Sense organs are entirely wanting, as also are mouth, alimentary canal, and anus. The nutritive juices are taken in through the whole outer surface of the body. In the soft granular subcuticular * Besides Dujarclin, Diesing, 1. c, compare : R. Leuckart, " Parasiten des .Menschen," Tom II., 1876. Greeff, " Untersuchungen ilber Echinorhynchus miliaris," Arch, fiir NaUirgesch, 186-t. A Schneider, ** Ucber den Bau der Acanthocephaleii," Midler''^ Arehiv., 1868. Also the Sitzungslcrichte der Oherhessischen Gesellschaft fiir Natur- mid EeUkundc, 1871. Fig. 289— Anterior part, of an Echinorhynchus. Jt, Proboscis ; Es, sheath of proboscis; G, ganplion ; Le, lem- nisci : E, retinacula. 3G0 NEM ATlI£LMi:S THES. layer of the integviment lies a complicated system of canals, filled with a clear fluid containing granules. Beneath the internal layer of the integument, which layer is often very extensive and of a yellow colour, is placed the powerful muscular tunic ; it is composed of external transverse and internal longitudinal fibres, and bounds the body cavity. The complicated ramified system of dermal canals, of which two principal longitu- dinal trunks may be recog- nised, is filled with juices, and probably functions as a nutritive apparatus. The portion of this system which extends into two bodies (the lemnisci, fig. 289, Le) project- ing behind the proboscis through the muscular tunic into the body cavity, probably acts as an excretory organ, ^^ since the contents of the fre- quently anastomising canals of these lemnisci is usually of a brown colour, and consists of a cellular mass rich in concretions. According to Schneider, the vessels of the lemnisci open into a circular vessel in the integument, and only communicate with the network of canals in the cephalic region, while the other dermal vessels (nutritive Fig. 290.— Male of J?eA»- , \ ,i . . c ttt^ wi norh^ncn. angudatu, apparatus), the COntCUtS of ^'«- -^^ which differs from that of the vessels of the lemnisci, are com- pletely shut off" from the latter. (after R. Lcuckart). B, proboscis ; Rs, sheath of the probos- cis ; Li, ligament ; Q, ganglion ; Le, lem- . nisei; T, testes; Yd, Generative organs. The vasa deferentia; Fr, ^ , ■. through which prostatic sacs; De, J J » ductus ejacuiatorius ; fluids circulate encloses the Lra"''^'''''^''"" greatly developed generative oro-ans, which are attached (ienerative ducts of a female i:c'hinorhynchus ffiffaa (after A. Andres). Li, ligament ; J'^, d i s c- shapedfioccuU; F',F", appendnges of the same ; U, uterus ; V, vagina ; B, lateral pouches of the bell ; Gd, dorsal colls at the base of the bell; Gl, lateral cells. to the end of the sheath of the proboscis by a ligament (figs. 290 ACANTHOCEPIIALA. 361 FiQ. 292.— Embryo of Echin- orhynchiig g'- -s 5, The two lateral trunks of the nervous system; G, ganglionic arises from two lateral nerve cords, layer of the same: B, alimentary which probably correspond to the ^ana^^J^r, nephridium; ilf, muscles"; lateral nerve trunks of the Ne- mertines. These two cords are continuous with the oesophageal commissures, and, like the latter, are uniformly covered with ganglionic cells. This form of the nervous system may persist, as may also its ectodermal position (Archiannelida, Protodriius) (fig. 295). In most Annelida, however, this is only a transitory condition; for at a later stage the lateral cords become separated from the ectoderm, come together in the median line, and acquii-e a segmentation corresponding to the metameres of the body. The nerves of the sense organs arise from the cerebral ganglion ; the other nerves pass out from the parts of the vential cord or, as the case may be, from the ganglia of the ventral chain and from the longitudinal commissures between the latter. There is in 366 AKXELIDA. almost all cases a viscei-al nervous system {symjKithetic). The following sense organs are found : paired eye sjjots with refractive structures, or larger more complicated eyes; also auditory vesicles upon the oesophageal ring (branchiate worms), and tactile organs. The latter have, in the Chcetojmda, the form of tentacles and tentacular cirri on the head and of cirri on the parapodia. When tentacles and cirin are absent, the anterior end of the body and the region of the mouth seem to function as tactile organs. Vascular system. A blood vascular system is very commonly present ; in many cases, however, it seems not to be completely closed, but to communicate with the body cavity, which contains blood. Two main vascular trunks, a dorsal and a ventral, connected with one another by numerous transverse anastomoses, are generally present. The blood is usually coloured (green or red), and its cir- culation is effected by the contractility of the walls of certain vessels ; sometimes the dorsal vessel, sometimes the ventral, and sometimes the transverse connecting vessels are contractile. Lateral longi- tudinal vessels are often present in addition to the above. In the Hirudinea these, as well as the median contractile blood sinus, are probably to be regarded as isolated parts of the body cavity. Special respiratory organs are found amongst the Chcetopoda in the branchiate worms. The excretory organs, corresponding to the water-vascular or excretory system of the Platyhelminthes, have the form of coiled canals (segmental organs or nephridia), which are repeated in pairs in each segment. Each nephridium usually begins with a ciliated, funnel-shaped opening into the body cavity, and opens to the exterior by a lateral pore (fig. 70). These may assume in certain segments the function of generative ducts, e.g., the nephridia of the Gepliyrea, which, however, are much reduced 'n number. In the cephalic segment or haad there is also a segmental organ (head kidney), which in the larva functions as a kidney and later disappears. Reproduction. — Considering the independence of the segments, to which we ascribe the value of a subordinate (morphological) indi- viduality, the occurrence of asexual reproduction by fission and gemmation in the long axis {Chcetopoda) is not surprising. Nume- rous Annelida {OligochcpM, Hirudinea) are hermaphrodite ; the marine Chcetopoda, on the contrary, are for the most part of separate sexes. Many lay their eggs in special sacs and cocoons, in which case development is direct, without metamorphosis. The marin* worms, on the contrary, undergo a more or less complicated Cn.^TOPODA. 867 metamorphosis. The Annelida comprise terrestrial and aquatic animals, and they eat, for the most part, animal food. Many of them {nirudinea) are occasionally parasitic. In the group of the Annelida three principal divisions may bo distinguished, — the Chmto'poda, the vinsegmented Gephyrea, and the Hirudinea which are adapted v A . -?'' for parasitism. The Hirudinea are not in any degree to be regarded as Annelida of a lower grade of organization, but they rather present, at least in the case of some organs, as alimen- tary canal, circulatory and generative organs, a more complicated structure, and agree most closely with the Oligochteta, from which they may be derived. Sub-class 1. CH.ETOPODA.* Free living Annelida, tvith 2}«'i'''6d tufts of setce on the segments, frequently tvith distinct head, also with tentacles, cirri, and hranchice. The Chastopoda are divided externally into segments, which correspond with the metameres of the internal organs, and are, with the excep- tion of the anterior region, which is distinguished as the head, usually tolerably alike (fig. 296).- Parapodia provided with setse are very frequently present on the segments ; their prin- cipal function is that of locomotion, but their va- rious appendages, the branchice and cirr/, also discharge tactile and respi- ratory functions (fig. 297). * Besides the older works of Savigny, Audonin et Milne Edwards, and Quatrefages, compare E. Grubo, " Die Fami- lien der Annelidon," Arcltiv fur Naturgesch, 1850 and 1851. E. Claparede, '■ Eecherches anatomique sur les Ann Glides, etc.," Geneve, 1861. E. Cla- parede, " Les Annelides ch(^to- podes du golfe de Naples," Geneve et Bale, 1868, also Sup- plement, 1870, and " Eecherches sur la structure des Annelides scklentalrcs," Geneve, 1873. Fr. Lcydig, I. c, also '• Tafeln zur vcrgl. Anatomie/' 1861. Fig. im.—Grulea fusU /era (after Quatre- fages). Ph. ph.aryns D, alimentary canal ; C, cirri; F, tentacles. Fig. 297.— Dorsal (DP) and ventral (VP) Para- lifdium with bundles of setae of Nereis (after Quatrefages). Ac, Aciculum ; He, dorsal cirrus ; Jic, ventral cirrus. 368 The form of the movable setae varies extremely, and affords a good character for the classification of families and genera. According to the strength, form, and mode of ending (fig. 298), the following Pii 29S.— Setfe of difTerent Polt/chceta (after Malm^reu and ClapareJe). a, Hooked seta of Sabella crassicornis ; h, of Terelella Daniehseni; c, seta with spiral ridge from SthenelaU ; (J, lance-shaped seta of PhyUochretopterug ; e, of Sabella crassicornis ; f, ot Sabella pavonlf; g, Composite sickle-shaped seta of Nereis ctiHrifera. forms can be distinguished : hair-setae, hooked-setse, flat-setse (palece), lance-setae, sickle-shaped setae, etc. When the parapodia and their appendages are com- pletely wanting, the setae are embedded in pits in the integument, and are arranged either in one or two rows on either side, that is, in a lateral ventral row on either side, or in a ven- tral row and a dorsal row on either side. In such cases the number of setae is small {Oligo- chceta). The setae may, on the contrary, be pre- sent in great number, so that the integument on either side seems to oe covered with long hairs and setae, and a thick felt of hairs shining with a metallic lustre is distributed over the whole dorsal ^IG. 209.— Anterior end of Polsnoe exfeiiuafa, the first elytron on the left hand being removed (after Cla- parede). The two seta; of the oral segment are visible ; El, Eljtra. CII.ITOPODA. 369 surface {Ajyhrodite). The appendages of the parapodia present an equally great variety of form and not unfrequently vary in the different parts of the body. They are either simple or ringed tenta- cle-like processes, the cirri, which are distinguishable into dorsal and ventral cirri. The cirri are for the most part filiform, and sometimes jointed or conical, and then are often provided with a special basal joint. In some cases the dorsal cirri are flattened out as broad scales and leaves, the elytra, which constitute a protective covering {^Aphro- dite) (fig. 299). In addition to the cirri, branching which may be filiform or branched and antler-like, comb- shaped or in the foi-m of tufts, are frequently present ; sometimes they are confined to the middle i*egion of the body, or are extended over almost the whole dorsal surface; sometimes they are confined to the head or to the anterior segments immediately following the oral segment (cephalic branehife). The two anterior segments may be regarded as forming the head ; they are fused together, and are, with regard to their appendages, diiferent from the following segments (fig. 245). The anterior segment projects beyond the mouth as the frontal lobe, and bears the tentacles and pa^is [jyalps are ten- tacular structures arising from the ventro-lateral sides of the head, vide p. 379] and also the eyes ; the posterior cephalic segment or oral segment bears the tentacular cirri. The last segment (anal segment) bears the anal cirri. The alimentary canal is usually straight, and extends from the mouth to the anus, which is terminal and rarely dorsal; it is divided into oesophagus, intestine, and rectum (fig. 300). There is in most cases a dilated muscular pharyngeal bulb which is armed with papillae or with movable teeth and can be protruded as a proboscis. The intestine usually preserves the same structure in its entii-e length and is divided by regular constrictions into a number of 24 Fig. 300. — Alimentary canal of Aphrodite aculeata (after M. Ed- wards). Pk, pharynx ; D, intes- tine ; L, hepatic appendages. 370 CHyi:TOPODA. divisions or chambers, which correspond to the segments and dilate again into latei-al diverticula and cfeca. The constrictions ai-e due to filamentous or membranous septa (dissepimenta), which divide the body cavity into the same number of chambers lying one behind another. ' The vascular system appears to be closed, so that the clear nutri- tive fluid found in the body cavity, which, like the blood, contains amceboid corpuscles, does not communicate with the usually coloured contents of the vessels. The dorsal and ventral vessels are not only connected at each end by lateral loops, but also in each segment ; and from these connecting vessels proceed peiipheral networks, "s^diich extend into the integument, the wall of the alimentary canal, and the hranchice. Special organs of respiration are wanting in almost all the Oligo- chceta. In the marine Worms, on the contrary, branchige are very generally present, usually as appendages of the parapodia. These branchiae are either simple cirri which have delicate ciliated walls and contain blood-vessels, or are branched {Amphinome) or in some cases are pectinate structures [Eunice) which co-exist with special cirri on the notopodia (fig. 246). The branchiae are sometimes confined to the middle segments [Arenicola), and are sometimes developed on almost all the segments on the dorsal surface, being simplified towards the posterior end of the body [Dorsibranchiata). In the Tuhicolce the branchiae are confined to the two {Pectinaria,SabelUdce) or three (Terebella) anterior segments. The respiratory function is, however, also shared {Ccqntibranchiata) by a number of elongated tentacles which are grouped in tufts on the head. These are, in the Sabellidce, supported by a special cartilaginoiis skeleton, and may have secondary twigs developed upon them. They are either simply arranged in a circle round the mouth, or in two fan-like lateral groups (Serjmlicke), the base of which is not unfrequently draAvn out into a spiral plate. Such branchial structures, however, also function as oi'gans of touch, as organs for procuring nutriment, and even for building the tubes and shells. Excretory organs. — There are usually in all the segments paii-ed segmental organs, which serve as excretory organs. They begin, as a rule, with a ciliated funnel in the body cavity ; they possess a glandu- lar wall, are several times coiled upon themselves, and open to the exterior in each segment by a lateral pore. These glandular passages serve in general for the removal of matters from the body canty, and in the viarine Chcetopoda are tcsed during tJie NEEVOUS SYSTEM. 571 breeding season as oviducts or vasa deferentia, and jjcrmit of the 2)assa(/e outwards of the (jenerative 2Jroducts, lohlch have been set free in the bodij cavity. Amongst tlie special glands in the body of tlie Chaitopoda, tliose cutaneous glands of the Oligochceta which give rise to the thickening (extending over several segments) known as the clitellus or girdle, are especially worthy of remark. The secretion of these glands perhaps assists the intimate connection of the Worms during copula- tion. In the S'erpididce there are present two large glands, which open upon the dorsal surface of the anterior portion of the body and furnish a secretion used in the formation of the tubes in which the animals live. Fig. 301.— Brain and anterior portion of the ganglionic chain, a, of Serpula ; h, of Nereis, (after Quatrefages) ; O, eyes ; G, cerebral ganglion ; c, oesophageal commissure ; TTg, suboesophageal ganglion; e e, nerves to the tentacular cirri and the mouth segment. Nervous system. — The longitudinal trunks of the ventral cord are often so closely approached that they seem to form a single cord (OUgochoiia). In the Tid)icolce (fig. 301), on the contrary, they are very widely separated from one another, especially in the anterior part of the ganglionic chain (Seiyida). The visceral nervous system consists of paired and unpaired ganglia, which supply the oral region and especially the protrusible proboscis. Sense organs. — Paired eyes upon the surface of the frontal {i.e. 372 Cn.ETOPODA. prscoral or cephalic) lobe are widely distributed. Eye-spots may also be pi-esent upon the posterior end of the body [Fahricia), or may be regularly repeated upon the sides of each segment {Poli/o])hthaImus). In species of Sabella, pigment-spots with refractive bodies are found even upon the branchial filaments. The large cephalic eyes of the genus Alciope* are the most highly developed, being provided with a large lens and a complicated retina. The presence of auditory organs seems less frequent. They appear as paired otolithic vesicles upon the oesophageal ring of Arenicola, Fahricia, some Sahellidce and young Terebellidce; etc. Besides the tentacles, cirri and elytra, other portions of the surface of the body may be sensi- tive to tactile sensations. On such parts there are either stiff hau-s and tactile setse, or, as in Sphcerodorum, special tactile warts with nerve terminations. Reproduction. — In the smaller Chcptopoda asexual genei'ation by fission and gemmation may occur. Either (fissiparous reproduction) a large number of segments of the parent be- come separate and give rise to the body of the new worm, as for example in Syllis prolifera, where a series of the posterior segments, which are filled -with ova,, become separated by a simple transverse fission, after the formation of a head provided with eyes; or (gemmiparous repro- duction) a single segment only, usually the last, becomes the starting-point for the formation of a new indi\ddual. In this way Autolytus pro- lifer, one of the Syllida:, asexually reproduces itself, giving rise to a male and female sexual form, known respectively as Pohjbostrichiis Miillerif (male) and Sacconereis helgolandica (female). This is a case of alternation of gene- rations, for the asexual form, Autolytus, gives rise by budding in the long axis to the sexual forms (fig. 302). In this case a whole series of segments are developed PlO. %C^.—Antolytm cor- nutut, with the male animal Pohjbosirichm (after A. Agassiz). F, Tentacles ; CT, tenta- cular cirri : /, tenta- cles ; ct, tentacular cirri of the male. * GreefE, " Ueberdas Auge derAlciopiden, etc.," Marburg, 1876 ;and " Unter- suchungen iiber die Alciopiden," Kov. Act. der K. Leon. Akad., etc., Tom XXXIX., Nro. 2. t Compare besides the works of 0. Fr. Miiller, Quatrefages, Leuckart, and Krohn. especially A . Agassiz, " On alternate Generation of Annelids and the embryology of Autolytus cornutus," Boston Jonrn. Nat. Hist., vol. iii., 1863, CrXIlRATIVE ORG.VXS. 373 in front of the last segment of the asexual form, and these segments, after the formation of a head, constitute a new individual. As this process is repeated, a chain of connected individuals is formed, and these, as soon as they are separated, represent the sexual individuals. Among the freshwater Naidce, in Chcetogaster, a regular and continued budding in the long axis leads to the formation of chains, consisting of not less than 12 to 16 zooids, each having only four segments, whDe the sexual individuals consist of a greater number of segments. A similar process occurs in the mode of reproduction observed by O. Fr. Miiller in Xais jjrohoscidea, from the last segment of which a new zooid is produced. Both generations of Nais, however^ become sexually mature. [For a more complete account of the asexual reproduction of Ch;ctopoda, ride Balfour, "Comparative Embryology," vol. i., pp. 283, 28-t.] The Glicetopoda are, with the exception of the her- maphrodite OligochcBta and certain Serpididce {e.g., Sjn- rorhis S2nrillum, Protula Dysteri) of separate sexes. Male and female individuals seem occasionally so stiikingly different in the structure of Fig. 303.— a parapodium of TomopterU with a their organs of sense and lo- ™^^^ °* "^'^ ^""^ °°^ *^^e o^""^ (^"e'- C- ° Gegenbaur). comotion that they have even been taken for species of distinct genera. Besides the above- mentioned Sacconereis and Polyhostrichus, the asexual generation of which is Autolyt'ios, a similar sexual dimorphism has been shown by Malmgren for Heteronereis, a genus of the Lycoridce, in which the males and females differ both in external form and in the number of their segments. A remarkable case of heterogamy is also afforded by this genus, in that a generation of smaller animals swimming iipon the surface alternates with a generation of arger forms living upon the bottom. The generative apparatus of the OlujocJueta is very highly deve- loped. The ovaries and testes lie in definite segments, and empty their contents by dehiscence of their walls into the body cavity. Special generative ducts often co-exist with segmental organs in the same segment {0. terricolce), while in other cases the segmen- tal organs are wanting in the generative segments {0. limicolce). In 374 cn-T:Toi'ODA. the marine CluHo'poda, the o\'a or spermatozoa originate on the body wall (fig. 303) from cells of the peritoneal membrane, either in the anterior segments alone or along the whole length of the body. The generative products then become free in the body cavity, attain m'^turity, and pass through the segmental oi-gans to the exterior. Only a few Ch(et02:>ocla, as Eimice and Syllis vivipara, are viviparous, all the rest are oviparous ; many lay their eggs in connected groups, and carry them about with them, while the OUcjochctta lay theii-s in cocoons. Development. — The segmentation is unequal. A primitive streak is veiy generally developed, though sometimes not until the embryo has left the egg. It arises on the ventral side in consequence of the development of a middle layer and from neutral plates of the upper layer. Excepting in the Oligochceta, the young forms undergo a metamor- phosis and after leaving the egg appear as ciliated larva?, which are provided with mouth and alimentaiy canal, and essentially resemble, with some modifications, Loven's larva. The capability of renewing lost portions of the body, more espe- cially the posterior part of the body and different appendages, seems to be generally distributed. The Lumbricince and certain marine Worms [DiojKttra, Lycaretus) are even able to rej^lace the head and the anterior segments, with the brain, oesophageal ring, and sense apparatus. Fossil remains of Cluntoiwda are found from the Silurian onwards in the most different formations. Order 1.- — PoLYcn.ETA.* Marine Chcetopoda, loith numerous setce embedded in the partqjodia, usually xoitli distinct head, tentacles, cirri, and branchice. They are for the most fart dimcious, and develop %oith metaviorphosis. The marine ChcetojJoda must be considered as belonging to a higher grade of life, on account of the sharp distinction of the head which is composed of the pra?stomium (prseoral lobe) and oral segment (in the Amp)hinomidce several succeeding segments are also included), and of the presence of the tentacles, tentacular ciiTi and * Andouin et Milne Edwards, " Classification dcs Annenicillus Lin., North Seas. S. KoUikeri Clap., Mediterranean. Protula Rudolplil Piisso, Mediterra- nean. Filtgrana im2)lcxa Berk., Norwegian and English coasts. Serpula nor- 'cegica Gunn., North Sea and Mediterranean. Sjjtrorlns sjririlhim Lin., Ocean. Order 2. — Oligoch.eta.* Hermaphrodite ChoRtopoda witlwut pharyngeal armature and para- podia. There are no tentacles, cirri, or hranchicB. The develojyment is direct. The cephalic region is composed of the prajstomium, which projects as an upper lip, and the mouth segment. It does not essentially differ from the following segments so as to form a special region (fig. 309). Tentacles, palps, and tentacular cirri are never found on it, but tactile papillse are present in great number, as are also peculiar sense organs which resemble taste buds. Eyes either fail or are present as simple pigment spots. Besides the small gland cells of the * Besides the works of W. Hofifmeister, D'lJdekem, and others, compare : E. Claparede, '• Eecherclies anatoniiques sur les Annelides, etc., observes dans les Hebrides," Geneve, 18G0. E. Claparede, "Eecherches anatomiques sur les OligochfBtes," Geneve, 1862. A. Kowalevski, '■ Embryo! ogische Studien an Wiirmcrn und Arthropoden (^Lvmhrictis, Eiia.rcs),'" Petersburg, 1861. B. Hatschck, " Studien uber Entwicklungsgeschichte der Anneliden," Wien, 1878. Fr. Vejdovsky, " Beitrage zur vergleichenden Morphologic der Anneliden. I. Monographic der Enchytra^iden," 1879. oligocu-i;ta. 383 hypodermis there is present in the clitellus a deeper glandular layer {Sdulenschicht Clap.), which consists of finely grannlar colls embedded in a framework of pigmented and vascular connective tissue and situated between the hypodermis and the external muscular layer. There are but few setae present, and they are never disposed on special parapodia, but always in simple pits in the integument, by the cells of which they are secreted. There are small secondary bristles which serve as a reserve. The blood is usually red, as in the Hirudlnea. The alimentary canal is often divided into several regions, the relations of which are most complicated in the Lumbricidce. In Lumhricus, the buccal cavity leads into a muscular pharynx, which is probably used for sucking. This is followed by a long oesophagus extending to the 13th segment, and furnished with a thick layer of glandular cells and several glandular dilated ap- pendages (calcareous sacs). The oeso- phagus is succeeded by a crop, a muscular gizzard, and finally by the intestine itself, the dorsal wall of Avhich is pushed inwards so as to form a longi- tudinal fold, the tyiMosoU (comparable to a spiral valve). In the Limicolce the alimentary canal is simpler by the absence of a muscular stomach : a pharynx and oesophagus are, however, always present. Reproduction. — The OUgochceta are hermaphrodite ; they lay their eggs either singly or united in greater num- ber in a capsule ; and they develop without a metamorphosis. The testes and ovaries are paired and placed in definite segments, usually near the an- terior end of the body ; they dehisce their products into the body cavity. The generative ducts possess funnel-shaped openings into the body cavity through which the generative products pass, and may Fig. m^.— Lumhricus yuldlus (after G. Eiseu) . a, The whole worm ; CI, Clitellus. h. Anterior end of the body from the ventral side, t, Isolated seta. 3S4 CU^DTOPODA. co-exist in the same segment with segmental organs [Lumhricidui). In the earth-worm, whose generative organs were first accurately described by E, Hering, the female apparatus consists of two ovaries in the 13th segment,* and two oviducts, which begin with trumpet- shaped openings into the body cavity, contain several eggs in a dila- tation and open to the exterior on either side on the ventral surface of the 14th segment. There are in addition in the 9 th and 10th segments two pairs of receptacula seminis, which open at the junction of the 9th and 10th and 10th and 11th segment respectively. They are filled with sperm in copulation (fig. 310). The male genital organs consist of two pairs of testes in the 10th and 11th segments, and two vasa defe- rentia, each of which opens inter- nally by two fvin- nels and to the exterior in the 15th segment. Copulation takes place in June and July on the sur- face of the earth at night. The worms apply their ventral surfaces to one another and lie in opposite directions, in such a manner that the openings of the re- ceptacula seminis of one worm are opposite the clitellus of the other. During copulation sperm flows out from the openings of the sperm duct and passes backwards in a longitudinal groove to the clitelJus, and thence into the receptaculum seminis of the other worm. In Tuhifex and Enchytrceus the ovaries may break up into groups of ova which float free in the body cavity. Special albumen glands and also glands which secrete the substance of the shell of the cocoon are often present. In the breeding sea.=^on the above-mentioned * The head (prsestomium and buccal region) being reckoned as the first segment. Fig. 310. — Generative organs of Lumbriom in segments VIII. to XV. (after E. Hei-ing). T, Testes ; St, the two funnels of the vas deferens on either side ; Yd, vas deferens ; Or, ovary ; Od, oviduct ; Be, receptacula seminis. OLIQOCn.^TA. 385 girdle or clitellus, which is formed of a thick ghxnduhir layer, is almost always present. The embryonic development of the Oligochceta presents many relations to that of the Ilirudinea. The unequal segmentation, which is very much alike in the two groups, and the similarity in the method of origin of the mesoderm, from two large cells near the blastopore at the posterior end of the embryo, point to a close relation- ship between these two groups of Annelids. A few Oligochceta, as for example Chcetogaster, are parasitic on aquatic animals ; the rest of them live, some free in the earth, some in fresh water, and some in the sea. Sub-order 1. Terricolae. Oligochagta which live principally in the earth. They have segmental organs in the genital segments. Fam. Lumbricidse. Large earthworms with compact skin and red blood. Without eyes. Tufts of vessels surround the segmental organs. Their activity in boring into the earth is of the greatest importance, loosening and exposing the soil to the action of the weather. Lumhricug L., Earthworm. Prjestomium distinct from the mouth segment. The clitellus includes a series of segments, and is situated nearly at the end of the anterior quarter of the body/a?- bclimd tJw genital opc/tin/js, Setce elongated, hook-shaped, arranged in four groups in each segment, each group containing two setas. The earthworm lays its eggs in capsules, into each of which several small ova, with sperm from the recep- tacula seminis, are emptied ; as a rule, however, only one or but a few embryos are developed. The developing embryo takes up with its large ciliated mouth not only the common mass of albumen, but also the other eggs. L. agrlcola 'iioS.m.. = terrestris Lin., L. fcetidits Sa,v., L. americaims E. Perr, Criodrilus lacmim HofEm. Sub-order 2. Limicolae. Oligochseta which live principally in water. Without segmental organs in the genital segments. Fam. Phreoryctidae. Long filiform worms, with thick skin and two rows of slightly curved setss on each side. Ph rconjetes Menheamts Hoff m. Found in deep springs and wells ; they seem to feed on the roots of plants. Fam. Tubificidae. Aquatic worms, provided with four rows of simple or divided, hooked set.-e. Hair-like setse may also be present. The receptacula are in the 9th, 10th, or 11th segment. Thej' live in mud tubes, from which they protrude the posterior end of the body. Tulifex rivulorvm Lam. The heart is in the 7th, the receptacula in the 9tfe segment. T. Bonneti Clap. {ScemirU varivgata Hoffm.) The heart in the 8th, receptacula in the 10th segment ; both species live in fresh water. Limnodrilus Hoffvieisterl Clap., L. D'Udekemianns C\a}^ Is distinguished from Tublfex hj iho. absence of haii'-like setae in the upper row of sctfe. Luiiibricvhis variegatus 0. Fr. Miill. Evei-y segment is provided with a contractile vascular loop and saccular contractile appendages of the dorsal vessel. Fam. Naideae. Small Limicolw with delicate thin skin and clear, almost colourless, blood. The prsestomium is often elongated like a proboscis and 25 AlfNELIDA. fused with the mouth segment. Nais (Stylaria) prohoscidea O. Fr. Miill. N. parasita Schm. Both species have a filiform piaestomium. Chatogaster vermicnlarig 0. Fr. Miill. Sub-class 2. — Gephyrea.* Worms with cylindrical body, without external segmentation, with terminal or ventral mouth; ivith cerebral (jamjlion, oesoj)harjeal ring and ventral cord. Setae are sometimes present. The Gejyhyrea possess an elongated cylindrical body and live, as do the Holothuria, in sand and ooze in the sea. The characters which distinguish them as Annelids are the possession of an oesophageal ring connected with a cerebral ganglion and of a ventral cord par- tially surrounded by ganglion cells. The larvae of the Ghce- ti/era present traces of seg- mentation (see below, p. 391), ■while in the Achceta the body cavity remains simple. Of sense L * \ organs, eye spots have been '^ % \ observed ; these in certain Sipuncididce lie directly upon the brain ; there are also dermal papillae, into which nerves enter. The structure of the integu- ment is similar to that of the Annelida; the thick upper cuticular layer rests upon a cellular matrix, and is not un- FiG. 311.— Young jScMurus from the ventral frequently wrinkled. There is side (after Hatschek). 0, Mouth at the base ^^ external segmentation. The of the proboscis ; SC, CEsophageal commas. _ _ ^ Bure; £5, ventral cord; ^, anus; IT, hooks, connective tissue dermis is of considerable thickness and en- closes numerous glandular tubes, which open to the exterior by pores in the epidermis. Below this is the strongly developed dermal muscular tunic, which is regularly composed of an outer layer of circular fibres * Quatrefages," M^moire sur TEchiure," Ann. dcs Sc. Xnt.. 3 Scr.. Tom VII. Lacaze-Duthiers, " Kecherches sur 1» Bonellia," Ann. dcs Sc. Kat., 1858. W. Keferstein, " Beitriigc zur anatomischen unci sj'steniatischen Kenntniss der Sipunculiden," Zeitschr fur wis.t. Zoulogie, Tom XV.. 1805. B. Hatschek, *' Ueber Entwickelungsgeschichte des Echiurus," etc. "VVien, 1880. J. W. Spengel, "Beitriige zur Kenntniss der Gephyrccn. I. Mittheil. av-i der zoolo- fjiHch-n station zv Nrapd, 1879 ; II. Zcitachr. fur n-'tss. Zool., Tom XIV., 1881. 3S7 and an inner layer of longitudinal fibres. Tlie latter are connected with the former and also amongst themselves by net-like anastomoses. These dermal muscles cause the folds of the cuticle. Internally to the longitudinal muscles there is another layer of circular muscles. In the Chcetifera two hooked setce are present near the genital opening (fig, 311); these assist locomotion. There may also be present one or two circles of setfe at the postei'ior end of the body (Echiurits). In the Chcetifera (fig. 311), the ante- rior part of the body is elongated to form a kind of proboscis, which projects im- movably and cor- responds to the prseoral lobe (pra3- stomium) of the Annelida. The mouth is placed ventrally at the base of the probos- cis. In the Achcata (StpuncuUdce) this proboscis is want- ing; the mouth is placed at the ex- tremity of the an- terior region of the body, which is sur- rounded with cili- ated can be retracted by means of retractor muscles (fig. 312). Alimentary canal. — The mouth opens into a pharynx, which is sometimes furnished with teeth ; this is followed by a ciliated intes- tinal canal, which is usually longer than the body and disposed in coils in the body cavity. The terminal portion of the intestine is tentacles, and Fig. 312.— Sipuninil us mulus, laid open from the side (after W. Keferstein). Te, Tentacles ; G, cerebral ganglion ; Fff, ven- tral nerve cord ; D, intestine ; A, anus ; BD brown tubes (ventral glands). 388 ANNELIDA, muscular and opens to the exterior by a terminal or dorsally placed anus (fig. 312). The vascular system is probably in communication with the body cavity; it consists of a dorsal vessel, which, as in the Annelida, accompanies the alimentary canal, and of a ventral vessel running along the body wall. There are also branches on the alimentary canal and in the tentacles. The blood is either coloui-less or red, and moves in the same direction as in the Annelids, the current being maintained both by the contraction of certain parts of the vessels and by the cilia which line the walls of the vessels. The corpusculated fluid of the body cavity differs from this vascular blood. Excretory organs. — There are two sets of organs, both of which may be interpreted as segmental organs. One kind, the anal vesicles (fig. 314c, Ab), are only present in the Chcetifera ; they have the form of a pair of tufted tubes, which open, on the one hand, into the body cavity by nvimerous ciliated funnels and, on the other, into the rectum. The other kind, known as the brown tubes (fig. 312, Bd) or ventral glands, are placed (one or more pairs) in the anterior part of the body ; they also open into the body cavity by a ciliated funnel, and to the extei'ior on the ventral surface. The latter, like the seg- mental organs of Annelids, assume the function of seminal vesicles and of oviducts. Generative organs. — ^The Gephyrea are of separate sexes. There are, however, remarkable variations both in the generative glands and their ducts. In Phascolosoma amongst the Achmta (according to Theel) the generative glands lie at the root of the ventral retractor muscles of the proboscis, and form a ridge from which the generative products are set free. Spermatozoa or ova in various stages of development are found in the body cavity, and thence are carried to the exterior through the two brown tubes (segmental organs) which open on the ventral side. In Bonellia among the Chcetifera the ovary, which has the form of a thin cord (fold of the body wall) in the posterior half of the body, is attaejied by a short mesentery to the nerve cord. From the ovary the ova fall into the body cavity, and thence pass into the neigh- bouring single uterus (fig. 314, b, U), which is provided at its base with a trumpet-shaped opening (TV) and opens to the exterior on the ventral surface behind the mouth. This uterus ought probably to be considered morphologically as a segmental organ, which has only been developed on one side. The generative organs of the small Turbellarian-like males which are met with in the uterus of GJarUYEEA. CH.ETIFEEA. 389 the female of Bondlia have the same relations (fig. 313). These rudimentary males are furnished (in many species) with two ventral hooks, in front of which in the anterior region is placed the external opening of the vas deferens. The vas deferens corresponds to the uterus of the female, and is in like manner provided with an internal opening into the body c;ivity. In Echiurus there are two pairs of brov/n tubes, which function as generative ducts and reservoirs. In Thcdassema there are, according to Kowalevski, thi-ee pairs of such tubes. The development shows many points of similarity with that of the Annelida. Be- tween the Achceta and Chcetifera, however, thei'e are considerable differences. In both cases a metamorphosis follows the embryonic development. The larvae resemble Loven's larva (larva of Polygordius) ; but in the Achceta they are characterised by a great de- generation of the apical region (pi-aeoral lobe) and the absence of a prseoral band of cilia. The remarkable larva known as Actino- trocha, which is the young stage of the tubicolous genus Phoronis* is distinguished by the possession of a contractile prseoral lobe, behind which there is a circle of ciliated ten- tacles forming a collar. The Gephyrea are all marine. Some of them live in sand and ooze at considerable depths, also in holes in the rocks and in crevices between stones and corals, and in the shells of snails. Their food is similar to that of Hoiothurians and many tubicolous Annelids. Order 1. — Cii.etifera = Eciiiuroidea. Fio. 313. — Planarian- like male of Bonellia (after Spengel). i). Intestine ; WT, ciliated funnel of the vas deferens {Vd), which is filled with sperm. Gephyrea characterised hy the loresoice of two strong hooked setae on the ventral side and hy a terminal anus. The mouth is 2)laced at the base of the prceoral lobe, which is developed into a proboscis. The Echiuroidea or chsetiferous Gephyrea present no external segmentation of their elongated and contractile body; they have, however, in the young state the rudiments of 15 metameres. This * There should be a third order of Gephyrea for these animals. 300 AKJTELTDA.. fact, as -svell as the formation of the prfeoral lobe and the develop- ment of the ventral hooked ?et£e, points to a close relationship with the Clicetopoda. In the adult animal, however, the internal segmen- tation is very little marked. The dissepiments, \Ai\ the exception of the first, which forms a partition between the head and the body, are lost, and the segmentation of the ventral cord is only indicated by the distribution of the nerves. The supra-cesophageal ganglion remains at the apical region of the prasoral lobe (proboscis) ; hence the (Esophageal commissures are extraordinarily long. The strongly developed prajoral lobe forms a proboscis -lUce Fig 3U-a female of BonelUa viridu^ (after Lacaze-Duthiers). b. Integument and gcncrativo oro-ans after the intestine has been removed. Hd, Cutaneous glands ; Ab, anal vesicle ; Ad rectum ; Or, ovary ; Tr, ciliated funnel of the uterus (17)- C Anatomy of Boiullia viridU (after Lacaze-Duthiers). D, alimentary canal with anal vesicles (,Ab) ; M, mesen- tery ; U, uterus ; B, prohoscLs. appendage which may develop to a considerable length and become forked {Bonellia) (fig. 314 a). A pair of hooked sette (with reserve setos in the sheath of each seta) are always present on the first segment of the body. In Echiurus there are also one or two circles of sette at the posterior end of the body. There are from one to three pairs of anterior segmental organs (so-called brown tubes or ventral glands), which open on the ventral surface and are used for the passage outwards of the generative products. Besides these there is also a pair of OEPIITEEA. CU.DTIFEBA, 391 posterior segmental organs (anal vesicles, lig. 31 4, Ah) in the terminal segment, each of which has a number of peritoneal funnels and opens into the rectum. In Bonellia the segmental organ which performs the function of uterus is, like the ovary, sin^e (fig. 314 6). . Development.— The development of the ovum begins witli an unequal segmentation. In Bonellia the small ceils of the animal pole grow round the four large yolk spheres, which give rise to the entoderm, leaving a small aperture, the blastopore (fig. 110). The EcMurus larv.-B (fig. 315) are the most accurately knoAvn. They present the type of Loven's larva and possess a strongly developed Fig 315 -a Larva of Eclnurus from flie ventral side (after Hatschek). SP, apical plate j 'prw pv£eoral circle of cilia; Pov^, postoral circle of cilia; i-», head-kidney ; T.?, ventral r-an-lionic cord connected with the apical plate by the long oesophageal commissures ; AS "anal vesicle. 6, Ventral region of the Echiiivns larva with segmented mesodermal bands; SC, oesophageal commissure; Dsp, dissepiments of the anterior body segments; 2IS, mesodermal bands ; A, anus. prteoral circle of cilia {Prio), in addition to which there is also a delicate post-oral circle of cUia {Pow). Early in larval life a seg • mental organ, the head kidney or pronephros {KN), is developed, one on either side ; and behind it a pair of mesoblastic bands makes its appearance and gives rise in the subsequent development to the rudiments of 15 segments (fig. 315 h). In the terminal segment, which is surrounded by a circle of cilia, there appear segmental 391 organs, which give rise to the anal vesicles (fig. 315 a, AS). The rudiments both of the cerebral ganglion and of the ventral cord are derived from growths of the ectoderm, — the former from the apical plate, the latter as a paired thickening of the ventral ectoderm. The two are connected by the oesophageal ring, which is also provided with ganglion cells. In older stages, after the disappeai-ance of the segments, the ciliary apparatus begins to degenerate and finally vanishes; after which two strong hooked setae make their appear- ance at the sides of the nerve cord not far from the mouth, and two circles of shorter setae are formed at the hind end of the body (fig. 316). The prseoral lobe of the larva becomes the proboscis of the young Echiurus (fig. 311). Fam. Echiuridae. The anterior end of the body above the mouth is elongated into a pro- boscis, the under surface of which is grooved. The long oesophageal commissures lie in the pro- boscis, and meet in front without any cerebral enlargement. Anteriorly and on the ventral side are two setse for attachment, and on the poste- rior end of the body there are sometimes circles of setffi. The anus is terminal. Echiurm Pal- lasii Guerin (^Gacrtneri Quatref., St. Vaast), coast of Belgium and England. Thalasscma f/iffas M. Miill., Italian coast. Bonellia riridis Rolando. Mediterranean. The males are small and rudimentary, and resemble Planarians. They live in the efferent ducts of the female generative organs. Order 2. — Ach.«ta= Sipunculoidea. Gephyrea with terminal mouth, dorsally placed anus, and without setce. The ante- rior recjion of the body is retractile. The Sipuncidoidea differ from the chsetiferous Gephyrea in their entire want of all traces of metameric segmentation, in the degeneration of the prjeoral lobe and in the position of the mouth and anus. The elongated body is destitute of a projecting prseoral lobe, so that the mouth, which is frequently surrounded by a cu-cle of tentacles, comes to be placed at the anterior end of the body. On the other hand, the anus is moved far forward on the dorsal surface (fig. 317). Fig. 316.— Older 'Echiurui larva seen from the side. The head kidney is atrophied. O, mouth ; M, stomach ; A, anus ; BK, circles of setse ; SC, oeso- phageal commissure ; AS, anal vesicles; ff, cerebral ganglion, developed from the apical plate; Vg, ventral nerve cord ; if, ventral hooks. OEPUTEEA. ACniETA. 393 The cerebral ganglion, oesojiliageal ring and ventral cord run inside the dermal muscular tunic. Only one pair of segmental organs, known as brown tubes or ventral glands, is present. The blood vascular system is well developed. Development.- — The segmentation is com- plete and is followed by the formation of a gastrula by invagination. The blastopore marks the ventral side. The two posterior marginal cells* of the entoderm move in- wards as primitive mesoderm cells, and give rise to the mesoblastic bands which do not undergo segmentation. Invaginations of the ectoderm of the animal pole and ventral sur- face of the em- bryo give rise to cepha- lic and ventra 1 plates respec- tively, while the remain- der o f the ecto- d e r m cells grow round these and form an external envelope for the embryo of the nature of a serous mem- brane (serosa). Cilia project from the latter through the pores of the vitelline membrane and are employed by the embryo in swimming. The cephalic and ventral plates The mesodermal bands split into somatic and Compare especially B. Hatschek. Fig. 317.— Quite yoimg Si- punculus etill without ten- tacles (after B. HatscheW O, mouth ; A, anus ; liS, ventral cord ; 2f, nephri- dium (brown tube) ; G, cerebral ganglion ; Bg, blood vessel. Fig. 316 —Larva of Sipunculut (after Hats cUek). O, Mouth ; Sp, apical plate; A, anus ; Fo IF', pestoral circle of cilia ; N, kidney. soon grow togethe 394 AN'XELIDA. splanchnic layers, and give rise to the rudiments of the two seg- mental organs ; while the cesophagus arises as an invagination of the ectoderm, and a postoral circle of cilia is formed around its opening (fig. 318). The serous membrane is cast off with the egg membrane, and the larva then contains all the essential organs of the adult Sijnmcichis except the ventral cord and the blood-vessels. At a later stage, during the growth of the larva, the ventral cord is developed from the ectoderm, the circle of cilia disappears, the first tentacles sprout out at the edge of the mouth, and the metamor- phosis of the free-swimming larva into the creeping young Sipun- culus is completed. Fam. Sipunculidae. Body elongated and cylindrical, the anterior part re- tractile. The mouth is surrounded with tentacles, and the anus is dorsal. The intestine is coiled spirally. Sipuyiculus nudm L., Mediterranean. Phascolosoma lave Kef., Mediterranean. Ph. clonr/atum Kef. St. Yaast. Fam. Priapulidae. Anterior part of the iDody without circle of tentacles. Pharynx armed with papillrc and rows of teeth. Anus at the posterior end of the body and slightly dorsal, above it there usually projects a caudal appen- dage which bears papilla-like tubes (branchiae). The intestine is straight. Priapulus caudatus 0. Fr. Miiller. Uallcryptus sjrhiulosus v. Sieb., Baltic, Spitzbergen. Sub-class 3. — Hirudinea*=Discophora, Leeches. BocIt/ either ivith short rings or not ringed, without j^ci^c-l^odia, with terminal ventral sucker, hermaphrodite. The body of the Uirudinea, so far as its external form is con- cerned, recalls that of the Trematoda, with which group the Eirudinea have often been incorrectly connected. Extei'nally the body is marked by a number of transverse rings, •which are short and may be more or less indistinct or even entirely absent. These rings correspond in no way with the internal segments, which are separated by transverse partitions or dissepiments ; but they constitute much shorter portions of the body, four or five of them corresponding to one internal segment. The large sucker at the posterior end of the body serves as an organ of adhesion ; and there may be in addition a second smaller sucker, either in front of or . * Brandt and Ratzeburg, " Medicinische Zoologie," 1829. Moquin-Tandon, "Monographic de la famille des Hirudin^es," 2nd. 6dit., Paris, 1846. Fr. Leydig,"'- Zur Anatomic von Pisciccla gcometrica," Zcitschr.fur wiss. Zool., Ton;. I., 1849. H. Rathke. " Beitrjigc zur Entwickelungsgeschichte des Hirudiueen," edited byR. Leuckart, Leipzig, J 862. R. Leuckart, "Parasitcn des Menschen," Bd. I.. Leipzig, 1863. Van Bcncdenet Hesse, " Recherches sur les Bdelloides ou Hirudindes et les Tr6matodes marins," 1863. Robin, " MemoLre sur le developpe- ment embryog(3nique des Hirudinees," Paris, 1875, HIRFDnfEA. 395 V surrounding the mouth. There are no parapodia few exceptions, are absent. A sharply distinct head is never developed, since the first rings are not essentially different from those following and are never furnished with tentacles or cirri. Alimentary canal. — The mouth is situated near the anterior end of the body, sometimes at the bottom of a small anterior sucker {RhyncJiohdellidce), sometimes at the base of a projecting spoon-shaped hood, which resembles a sucker {Gnathohdellidoi) (fig. 319). The mouth leads into a muscular pharynx provided with glands. The anterior part of the pharynx, which may be distinguished as the buccal cavity, is armed {Gnathohdelli- dce) with three serrated chiti- novis plates (fig. 319, a, b), or more rarely with a dorsal and ventral plate [Branchi- ohdellidce), or it is provided with a protru- sible proboscis, which lies free in its anterior part {Rhynchohdellidce). The pharynx leads into a stomach, which forms a straight tube in the axis of the body and sometimes shows con- strictions, which correspond with the segments ; sometimes it is produced into a larger or smaller number of lateral ca!ca. From the stomach a short rectum^ which is sometimes also provided with ca;ca, leads to the anus. The anus is placed at the posterior pole of the body, dorsal to the sucker. Excretory organs. — Segmental organs are pre- sent, one pair to each segment in the middle region of the body. Tiieir number, however. and seta?, with a /»//*/ a Tj Fig. 319.— rt Cephalic region of the Medicinal Leech. The three jaws are visible, b, One of the jaws isolated with the finely sen-ated free edge. Fig. 320.— Lonoritudinal section throuEfli the Medicinal Leech (after R. Leuckart). D, m- testinal canal ; G, cerebral ganglion ; Gk, ganglionic chain ; Ex, excretory canals or segmental organs (water -vascular sys- tem). 396 varies very considerably, since, for instance, Branchiohdella astaci, parasitic on the gills of the cray-fish, has but two pairs, while the Gnathobdellidce usually possess seventeen pairs. Unicellular glands are present in the Hirudinea in gi'eat numbers in the skin and in the deeper layers of the connective tissue. The former secrete a finely granular mucous fluid, which covers the skin ; while the more deeply situated glands, which lie beneath the dermal muscular tunic, secrete a clear viscid substance, which quickly hardens outside the body and is used to form the cocoons when the eggs are laid. These glands ai'e espe- cially numerous in the region of the genital openings. A blood- vascular system is always present, but in difterent degrees of development. Portions of the body cavity are transformed into vessel- like trunks, and as a result of this organs which lie in the body cavity seem to be enclosed in blood sinuses. The two lateral vessels and the me- dian blood sinus, which always en- closes the ventral ganglionic chain and sometimes also the alimentary canal (Clepsine, Piscicola), may be interpreted in this manner. In most of the Chiathohdellidce the blood is red, the colour being due to the fluid part of the blood and not to the corpuscles. Special respiratory organs are wanting, excepting in Branchellion and some allied leeches, which pos- sess leaf-like branchial appendages. The nervous system* in all cases is highly developed. The cerebral ganglia are characterized by a peculiar arrangement of the nerve cells which give rise to swellings on the surface of the ganglia (described by Leydig as a follicular arrangement) (fig. 321). ' Hermann, " Das Centralnervensystem von Eirudo medicinalis," Munchen, 1875. Fig 321.— Anterior end of Sinulo (after Leydig). G, Cerebral ganglion with suboesophageal ganglionic mass ; Tohoscidca Ehrbg., Steplianoccrox Eiehhornli Ehrbg., TuMcolaria najas Ehrbg., McUccrta ringen.'i L., Conochilus rolcox, Ehrbg. Fam. Philodinidae. Free, often creeping (in a looping manner) Rotifera ; with double-wheeled rotatory organ, and jointed, telescopically retractile foot, without gelatinous investment. CalUdina elegans Ehrbg., Rotifer vulgaris Oken (^R. redivivvs Cuv.), Philodina erythi'pphthalma Ehrbg. Fam. Brachionidse. Rotifera with bifid or multifid wheel-organ ; with broad, shield-shaped armoured body ; and foot ringed, or with short segments. BracMomis Balteri 0. Fr. Miill., B. militaris Ehrbg., Euclilanix trirpietra Ehrbg. Fam. Hydatinidse. Edge of wheel-organ prolonged into numerous processes (multifid) or only sinuous ; skin delicate, often ringed ; foot short, usually forked, with two setse or pincer-shaped. Ilijdatina Ehrbg., II. se/tta 0. Fr. Mull, with Enterojylea hydatince Ehrbg., as male. Kotovunata tardigrada Ldg., N. BracMonus Ehrbg., N. parasita Ehrbg. Fam. AsplanchnidsB. The sac-like unarmoured body is destitute of rectum and anus. Asjjla7ichna Sieholdii Ldg., A. myrmeleo Ehrbg., A.seomorplia germanica Ldg. Two groups of small animals are allied to the Eotifera : — (1) the Echinoderidae which Dujardin and Greet regarded as connecting links between Vermes and Arthropoda (EeJmwderes Dvjarditiii Clap., E. sctigera Greet) ; and (2) the Gastrotricha * or Ichthydina (Chcetonotvs). * Compare E. MctschniTtoff, " Ueber einige wenig bekannte niedere Thier- formen," Zcitschr. fiir iviss. ZooL, Tom. XV., 1865. Also the worka oi H . Ludwig and 0. BUtschli. ' AETHBOPODA. 405 CHAPTER X. ARTHROrODA. .Laterally symmetrical animals with heteronomously segmented body and jointed segmental ajjpendages ; with brain {supravesopJiageal ganglia) and ventral nerve cord [ganglionic chain). The most important characteristic which distinguishes the Artkro- poda from the closely alKed segmented worms, and is an essential condition of a higher organization and grade of life, is the possession of jointed segmental appendages which sei've as organs of locomotion. In place of the un jointed parapodia of the Choitopoda, jointed appendages more adapted for locomotion and confined to the ventral surface, are present. Every segment may possess a ventral pair of appendages which, in the simplest case, are short and consist of only a few joints [Peripatas) (fig, 325), While in the Annelida loco- FlG. 325. — P(ripiiluf rapeimie (after MoseleyX motion is effected by the movements of the segments and undulatory movements of the whole body, in the Arlhropoda the function of locomotion is removed from the chief axis of the body to the secondary axes, i.e., to the paired appendages,- with the result of the possibility of a much more effic'eat discharge of the function. The appendages enable the Arthropoda not only to swim and creep with much greater ease and speed, but also to execute various kinds of more complicated movement, e.g., running, climbing, spi-inging, and flying. The Arthropoda are, therefore, true terrestrial and aerial animals. » The high development of the organs of locomotion as paired appendages leads of necessity to a second essential property, viz., to the het€ronomy of the segmentation, and in connection with this to the hardening of the outer layer of the skin to form a firm exo-skeleton. If the function of the limbs is to be perfectly discharged, there will be need of a considerable mass of muscle, the points of attachment of which can only be furnished by the integument of the body. The insertions of the appendages and their muscles, therefore, require 406 AETHEOPODA. rigid surfaces, which are oLtainccl partly by the development of internal chitinous tendons and plates, and partly by the hardening of the integument and the fusion of several segments to form larger armoured regions. It is only when the movements are simpler and resemble those of Annelids, that all the segments remain independent and bear similar appendages along the whole length of the body (larva?, Myriajoocla). In general, three regions of the body can be distinguished, the head, the thorax, and the abdomen, the appendages of which possess respectively a different structure andfunction(fig.326). The head constitutes the short and compact anterior region of the body, is covered by a hard integument, encloses the 'brain and bears the sense organs and mouth-parts (jaws). The appendages of this region are modified to form the antennce and jaivs. The head of Arthropods, as compared with that of Annelids, contains, besides the frontal (pra?oral) or antennal segment and the oral segment, in Fig. 326.— Head, thorax and abdomen of an Acridium, scon from the side. St, Stigmata ; T, tympanum. 327.— SquiUa mantis. A', A" Antennn? ; Kf, Kf" the anterior maxillipcds ou the cephalo-thorax ; B , B", £'", the thiee pairs of biramous feet. addition at least one jaw segment, the appendages of which may,- in larval life (N'aiqdius), still function as legs. Usually, however, several of the succeeding segments whose appendages function as jaws form part of the head. The middle portion of the body, or thorax, is likewise distinguished by a relatively intimate fusion of some or all of its segments, as well as by the hardness of its integument. It is sometimes sharjDly marked off from the head, sometimes fused with the head to form a IXTEOUMEXT. XERTOUS STSTEil. 407 region of the body called the cephalothorax (fig. 327). The thorax bears the appendages which are of most importance in locomotion. The posterior portion of the body, or abdomen, is composed of distinctly separate rings, and is, as a rule, without appendages. When the latter are present, they serve partly as aids to locomotion (abdominal feet), partly for respiration, or for carrying the eggs and for copulation. More rarely, as for example in the scorpions, the abdomen is divided into a broad anterior region, the prceabdo7nen, and a naiTOW movable posterior region, the postabdomen. The skin, as in the Annelida, consists of two difFerent layers, — an external firm, usually homogeneous chitinous layer, and an internal soft layer, which is composed of polygonal cells [matrix, hypodermis) and secretes in layers the at first soft chitinous cuticle (fig. 22). The latter usually becomes hardened by the deposition of calcareous salts in the chitinous basis, so as to form the firm exoskeletal armour, which, however, is interrupted between each segment by thin connecting membranes. The various cuticular appendages of the skin (fig. 22, a, h, c), which may have the form of simple or pennate hairs, of filaments, setse, spines and hooks, originate as processes and outgrowths of the cellular matrix. The chitinous cuticle together with its appendages is from time to time, principally in the young stage during the period of growth, renewed, the old cuticle being cast off as a continuous membrane (ecdysis, or moult). The muscular system never constitutes a continuous envelope, but the mviscles are usually broken up into segments which corre- spond with the segmentation of the animal. The muscles of the body are arranged in longitudinal and transverse bundles in the difFerent segments, and are frequently interrupted. There are in addition large groups of muscles, which move the appendages. The muscular fibres are always cross-striped. The internal organization is allied to that of the Annelida, but does not present such a well-marked internal segmentation. The nervous system consists of brain, oesophageal commissures and a ventral cord. The latter usually has the form of a ganglionic chain (fig. 328), and is placed beneath the alimentary canal. Some- times, however, it exhibits great concentration, and may have the form of an vinsegmented ganglionic mass beneath the oesoj)hagi;s. The segmentation of the ventral ganglionic chain presents in details the greatest variations ; in general, however, it corresponds to the heteronomous segmentation of the animal, in that in the larger reaions of the bodv, which have arisen by fusion of several segments, 408 AETHEOPODA. au approximation or fusion of the corresponding ganglia has taken place. In one case only, viz., in the Pentastomidce, which in form and grade of life resemble the intestinal worms, the dorsal part of the oesophageal commissure is not swollen out to form a cerebr-al ganglion, and the central parts of the nervous system are com- pressed together into a common gangli- iCtjfi onic mass beneath the oesophagus. In j^■J^,' all other cases the brain is a large gangli- onic mass lying above the oesophagus) and connected by means of the oesophageal ring with the anterior ganglion of the ventral chain, which is usually placed in the head and is known as the suboeso- phageal ganglion (fig. 328). The sense nerves arise from the brain, while the ganglia of the ventral chain send nerves to the muscles, organs of locomotion and the body covering. Visceral nervous system. — In addition to the brain and ventral ganglionic chain, which are comparable to the cerebro- spinal system of Vertebrata, we can distinguish in the larger and more highly oiganised Arthrojioda a visceral nervous system [si/mpathetic), which consists of special ganglia and plexuses connected with the other system and specially distributed to the alimentary canal. In the higher Ar- thi'ojwda, paired and unpaired visceral nerves are very generally present, both of which have their origin in the brain. Sense organs. — Eyes are most generally distributed, and are only absent in a few parasitic forms. In their simplest form they are paired or unpaired structures placed upon the brain, provided with re- fractive bodies, and with or without a simple lens (stemmata, or simple eyes), hich are always paired, are much more distinguished by the presence of nervous Fig. 328.— Nervous system of the larva of Coccinella (after Ed. Brandt;. Gfr, Frontal ganglion ; G, brain ; Sg, sub- oesophageal ganglion; O' to G", ganglia of the ventral chain in the thorax and abdomen. The compound eyes, w complicated. They are rods and crystalline cone.';, and may be divided into faceted eyes ALIMEXTARY CAXAL. EXCRETORY ORGAXS. 409 and eyes with smooth cornea {Claaocera). The formei' possess numerous lenses, and are sometimes placed on movable stalks {Dectqmda). Occasionally accessory eyes are found on other parts of the body, on the jaws and between the legs of the abdomen (^Eiqjhausia). Auditory organs are found most frequently in the Crustacea as auditory vesicles with otoliths in the basal joint of the anterior antenn.ne, or rai-ely in the appendage of the abdomen known as the fan (tail of Mysis). In Insecta, auditory organs of a very different structure have been discovered. Olfactory organs are also widely distributed. They are situated on the surface of the antennce, and consist of delicate tubes or peculiar conical projections, beneath which the sense nerves end in ganglionic swellings. Tactile organs. The antennae and palps of the oral appendages and the ends of the limbs have a tactile function. These parts are provided with peculiar hairs and setse, beneath which nerves end in ganglionic swellings. Alimentary canal. — An independent digestive apparatus is always present, but its structure and degree of development are very various. The alimentary canal is only exceptionally degenerated and absent {Rhizoce]yhala). The mouth is placed on the ventral surface of the head. It is furnished with a projecting upper lip, and usually with paired appendages, which are used either for masticating or for piercing and sucking. A narrow or wide cesophagus leads into the intestine, which either simply traverses the axis of the body or is disposed in several coils. The oesophagus and midgut (chyle stomach) may even be divided into several regions, and may possess salivary glands and hepatic appendages of various size. Excretory organs. — Urinary organs are mdely distributed. In the simplest form they appear as cells on the surface of the intestine (lower Crustacea), in a more highly developed state as tubular filiform diverticula of the hindgut {Alaljnyhian tubes) (fig. 329). In the Crustacea, glands are present in the shell (shell glands) and in the base of the posterior antennae; they are regarded as the morphological equivalents of segmental organs. The circulatory and respiratory organs present the gi-eatest differences in the various groups of the Arthropoda. In the simplest case the clear, more rarely coloured blood fluid, which is often corpusculated, fills the body ca\dty and the interstices of all 410 AETHEOPODA. the organs, and is circulated in an in-egular manner by the move- ments of the different parts of the body. Not unfrequently (Achtheres and Cyclops) the circulation is effected by the regularly repeated movements of certain organs (intestine, vibratile plates, etc.) ; in other cases, a short saccular heart is present dorsally above the intestine ; or a long vascular tube (the dorsal vessel), divided into chambers, serves as a propelling organ. From this, vessels [arteries) may arise, which conduct the blood in definite directions. Vessels for returning the blood {veins) may also be present. These either begin in the body ca\ity, or are connected with the ends of the arteries by capillary vessels. The vascular system seems never to be completely closed, since even when the circulation is most complete, lacunar spaces of the body cavity are found inserted in the course of the vessels. Respiration is very frequently effected, especially in the smaller and . more deli- cate species of Arthrojioda, by means of the entire surface of the body. In the larger aquatic forms, the function of respi- ration is assumed by special tubular, usually branched appendages of the limbs [brcmchice) ; while in the air-breathing Insects, Centi2}edes, Scorjnons, and Spiders, respii'ation is performed by means of in- ternal branched tubes filled with air (trachece) or by pulmonary sacs {fan trachece). The reproduction of the Arthrojioda is usually sexual, but sometimes takes place by the development of unfertilized ova {2yartheno(j€nesis). Ovaries and testes are in their origin paired, as are also the gene- rative ducts, which often have a common terminal portion and open by a median generative apertvire {Insecta, Arachnoidea). "With a few exceptions [Cirripedia, Tardigrada), the sexes are sepai-ate. Males and females frequently differ essentially in their entu-e form and organization. In rare cases, for example in the parasitic Fig. 329.— Alimentary canal of Fontia hramiccB (after iVew- port). B, Proboscis (Maxilla?) ; Sp, salivary glands ; Oe, oeso- phagus ; S, sucking Btomach ; Mr/, llalpighian tubes; Ad, rectum. CKUSTACEA. 411 Crustacea, there is such a marked .sexual dimorphism that the males remain small and dwarfed, and are attached like parasites to the body of the female. During the act of copulation, which i^i often limited to the external union of the two sexes, the spermatophores are fastened to the female genital segment or thrust into the vagina by the organ of copvilation, whence they sometimes pass into a special receptaculum seminis. Most Arthrojyoda are oviparous, but in almost every group there are viviparous forms. The eggs are frequently carried about by the mother, or deposited in protected places where food may easily be obtained. The embryonic development (i.e., development within the egg) is characterised, except in the case of the small stout embryos of the Cyclopidce, Pentastomidce and Acarina, by the presence of a ventrally placed primitive streak, from which especially the ganglionic chain and the ventral parts of the segments proceed. The moi^e or less complex embryonic development is usually followed by a complicated metamorphosis, during which the young form as larva undergoes several ecdyses. Numerous seg- ments and parts present in the adult are not unfrequently wanting in the just-hatched larva ; in other cases, all the segments of the adult are indeed present, but are not as yet fused together to form regions. In such cases, the larvse resemble the Annelida in their homonomous segmentation, and in their locomotion and mode of life. The meta- morphosis may however be retrogressive ; the larvte are hatched with sense organs and appendages, but in the further course of develop- ment they become parasitic, lose their eyes and organs of locomotion, and develop into strange unsegmented (Lernceo^) or entozoon-like (^Pentastomidce) forms. The Arthropoda are no exception to the general rule that the aquatic forms which breathe by gills are lower and, from a genetic point of view, older than the air-breathing members of the same group, inasmuch as the Brancliiata or Crustacea are the older, the Tracheata the younger types. CLASS I.- CRUSTACEA.* Aquatic Arthrojioda, u-hich breathe hy means of gills. They have tii'o pairs of antennce ; momerous 2^(>'ired legs on the thorax, and usually also on the abdomen. * Milne Edwards, " Histoire natiirelle des Crnstaces," 3 vol. and atlas, 1838- 1840. C. Glaus, " Untersuchungen zur Erforschung der gcnealogisclien Grund- laje des Crustacecnsystems," Wien, 187G. 412 AETnEOPODA. The Crustacea, whose name is derived from the body-covering (which is often hardened), are principally aquatic animals. Some forms, however, can live on land, and possess respiratory organs adapted for breathing air. An important character of the group is the great number of paired appendages. The appendages of all the segments, even those of the head, may be used in locomotion (fig. 330). As a rule, the head fuses with the thorax, or at any rate ^\ith one or more of the thoracic segments, to form a cej)h(dot]iorax ; which is followed by the remaining free thoracic segments. Some- times, however, these two regions of the body remain distinct. The head and thorax are seldom so sharply maiked off from one another as, for example, in the Insecta : usually certain appendages, the so-called maxillipeds, occupy an intermediate position between legs and jaws, and being placed at the boundary between the two regions may be rec- koned either as be- longing to the head or the thorax. The fusion of the seg- ments may be veiy extensive; not only may the head and thorax be tmited, but the boundary be- tween thorax and abdomen may vanish, and the segmentation may even disappear. As a general rule, the form of the body presents extraordinary differences in the various groups. A redupli- cature of the skin arching over the thorax and covering the body as a shell is frequently present. This fold of the integument constitutes, in extreme cases, a mantle-like investment, which may develop calcareous plates and occasion a certain resemblance to Lamelli- branchs {Cirri2')edia). In other cases the body has quite lost it3 segmentation, and the animal resembles a worm (Zerncece, Saccidhia). On the head there are usually two pairs of antennae, which function as sense organs and sometimes also as organs of locomotion or of prehension. There is a pair of large jaws (the mandibles), one on each side of the mouth, over which a small plate, kno\\Ti as the upper lip, often projects. The mandibles are simple but very rigid and hard masticating plates, which are usually toothed and correspond Fl8. 330.— Cam marKs n^glectus (after G. O. Sara). A', A", The two antennffi ; Kf, maxilliped ; F F\ first to seventh thoracic feet ; Sf, anterior swimming feet. CRUSTACEA. 413 morphologically to the coxal joint of a limb, the following joints developing into a palp-like appendage {mandibular 'palp). Then follow one or more pairs of weaker jaws {inaxillai), and one or more pairs of maxillipeds, which more or less resemble the legs and, in parasitic forms, are often used for adhering (fig. 331). In parasitic forms, the upper and under lips not unfrequently give rise to a suctorial proboscis, in which the styliform mandibles are placed. The appendages of the thorax, of which at least three pairs are present (Ostracoda), present an extremely various structure, in accordance with the mode of life and the use made of them. They are either broad leaf - shaped swimming feet [PhyUopoda), or bi- ramous appendages {Co2Je2yoda) ; they may serve to produce currents in the water like the feet of the Cirrijjedia, or they may be used for crawling, walking, and running {Isojwda, Deca- poda). In the latter case, some of them end with hooks or chelse. Finally the appendages of the abdomen, which frequently itself moves in toto and assists in locomotion, are either exclusively locomotory as jumping or swim- ming feet (Amphijjoda), in which case they usually differ from the appendages of the thorax ; or they serve with their appendages for respiration, as well as for carrying the eggs, and for copulation [Deccqjoda). The internal organization is not less varied than is the external form. In the lower foi'ms, the nervous system often consists of a ganglionic mass, which surrounds the resophagus and is not further Fig. 331.— Young stage (larva) of the Lobster (after G. O. Sars). a, The l.-trva seen from the side; R, ros- trum ; A', A", antenna; ; Kf" third ma.xilliped ; F, anterior ambulatory leg. b, mandible with palp ; c. anterior maxilla with two blades and palp; d, pos- terior maxilla with vibratile plate (scaphognathite) ; e, first, /, second maxilliped. 414 AirruROPODA. segmented. This ganglionic mass con-esponds to the brain and ventral cord and gives off all the nerves. In the higher Crustacea, a distinct brain and ventral ganglionic chain, which is usually elongated and of very varied form, as well as a rich plexus of visceral nerves and ganglia of the sympathetic system are always present. Of sense organs, eyes are the most widely distributed. They may have the form either of simple eyes (pan-ed or unpaired), or compound eyes with smooth or faceted cornea ; in the latter case they are often placed on movable stalks, which are attached to the lateral regions of the head. Auditory organs are also present usually in the basal joint of the anterior antenna, rarely in the caudal plate at the posterior end of the body {2Iijsis). The delicate hairs and filaments of the anterior antenna are probably olfactory organs. The digestive canal is, as a rule, straight, extending from the mouth to the anus at the posterior end of the body. In the higher forms the cesophagus is usually dilated in front of the mesenteron (midgut) into a stomach or crop, which is armed with chitinous plates. The mesenteron is provided with simple or ramified hepatic ca}ca. Excretory organs. — The so-called shell glands of the lower Crustacea are regarded as urinary organs, as are also the glands opening at the base of the posterior antenna in the Malacostraca. In the Entomostraca the latter are only preserved during larval life. Short tubes, which correspond to the Malpighian tubes of the Tracheata, may also be present on the rectum {A7n])hipoda). The circulatory organs present every possible degree of perfection, from the greatest simplicity to the highest complication of an almost closed system of arterial and venous vessels. The blood is usually colourless, but is sometimes green or even red, and as a rule contains cellular blood corpuscles. Respiratory organs are either entirely wanting, or are repre- sented by branchinl tubes on the thoracic or abdominal appendages. In the first case they are often contained in a special branchial cavity at the sides of the cephalothorax. Generative organs. — With the exception of the hermaphrodite Cirripedia and Isopoda, all Crustacea are of separate sexes. The male and female generative organs usually open on the boundary of the thorax and abdomen, either on the last or the antepenultimate thoracic ring, or on the first abdominal segment. The two sexes are vei-y often distinguished by a number of external characteristics. CKUSTACKA. 4To The males are smaller, sometimes even dwarfed, and then attached to the females like parasites. They almost always possess appa- ratuses for holding the females and for transferring the spcrmato- phores during copulation. The larger females, on the other hand, frequently carry the eggs about with them in sacs, the membranes of which are secreted by the so-called cement glands. Development takes place either directly or by metamorphosis. The metamorphosis is sometimes retrogressive. When the develop- ment is direct, the young animals, on leaving the egg, already have the body form of the adult. The larva known as the Naujylius (fig. 332) is of great importance as a point of departure. This larva possesses an oval body, on the ventral side of which are present three pairs of appendages for the sense of taste, the prehension of food, and for locomotion. These appendages correspond to the two pairs of antennaj and mandibles respectively. Parthenogenesis is said to occur in certain gi-oups {P/ii/llo- poda). Almost all Crustacea are carnivorous. Some of them suck the juices of living animals on which they are parasitic. For the systematic review of this heterogeneous group, it is convenient to divide the numerous orders into two series. 1. The small simply organized Crus- „ „„„ xr ,• , ^ •' tj Fig. 332.— Nauplms larva of tacea, the number and form of whose Baiamn, seen from the side. appendages is very various, will be in- 1«T "ioXS,."; eluded as EntomOStraCa (0. Fr. Mliller). (second antenna) ; JIf.//, third To this group belong the orders Phyllo- 7w^tp^ b^m^^! °*' pc-TOMOSTBACA. of Crustacean orders, for the most part fossil and ])e\onging to tlie oldest formations, which present in their development no certain trace of the Nauplius form so characteristic of the true Crustacea, and are in all probability related to the Arachnoidea. These orders, which may be grouped together as the Gigantostraca, are the Merostomata and Xiphosura, to which the Trilohita are possibly allied. 1 .— ENTOMOSTRACA. Order 1. — Phyllopoda.* Crustacea loith elongated and often distinctly segmented body ; usually vnth a fiat, shield-like carapace, or laterally compressed bivalve shell, formed by a reduplicature of the skin. There are, at least, four pairs of leaf -like lobed svnmming feet. The animals belonging to this order differ very considerably in form and size, in the number of their segments and appendages, as well as in their internal structure. They all, however, agree in the structure of their lobed, leaf -like feet. In their form, internal organization and development they appear to be the most primitive of Crustacea, and may be regarded as the least modified descendants of ancient types. The body is either cylindrical, elongated and clearly segmented, without free reduplicature of the skin, e.g. Branchiinis (fig. 333), or it may be covered by a broad and flattened shield, which only allows the posterior part of the body to project uncovered, e.g. Apus. In other cases the body is laterally compressed and is enclosed by a bivalve shell, from which the anterior part of the head projects {Cladocera) ; or finally the laterally compressed body is completely covered by a bivalve shell (^Esther idee). Sometimes the head is more sharply distinct, while the thorax and abdomen are not so clearly distinguishable from each other. As a rule, the posterior segments only are without appendages. The hind end of the abdomen is very often curved ventralwards and forwards, and bears two rows of posteriorly directed claws, the two last of which arise at the point of the caudal appendage, and are by far the * Besides the works of O. Fr. Miiller, Jurine, M. Edwards, Dana, compare Zaddach, " De Apodis cancriformis anatome et historia evolutionis," Bonnse, l.Sil. E. Grube, " Bemerkungen liber die Phyilopoden," Arcliiv fiiv Natnrgesch, 1853 and 1855. Fr, Leydig, " Monographie der Daphniden," Tubingen, 1860. PUTLLOPODA. 417 strongest. In other cases a pair of fin-like appendages are presenl constituting the caudal fork [Branchijms). Appendages. — On the head there are two pairs of antennae, whicl* however, in the adult animal, may be rudimentary or peculiarly modified. The anterior antennje are small, and bear the delicate olfactory hairs. The posterior antennre frequently have the form of large biramous swimming appendages, but in the male may also have a prehensile function, e.g., Branch'qnts. In other cases {Ajyii^) they are rudi- mentary and may even be enirely absent. Two large mandibles are always present beneath the well developed upper lip; they possess a toothed, biting edge, and in the fully developed condition are invariably destitute of palps. The mandibles are followed by one or two pairs of slightly developed maxillfe. A kind of under- lip is in many cases present, in the form of two promi- nences behind the mandibles The legs, which are placed on the thorax, are usually very numerous, and are smaller towards the poste- rior end of the body. They are lobed, leaf-like, bira- mous structures, and func- tion as swimming feet ; they also assist in procuring food. They consist of the following parts : a short basal portion, which is usually provided with a masticatory process and is followed by a long foliaceous stem with setse on its inner edge ; this is continued into the multilobed internal branch [endopodite] of the biramous limb, while it bears on its outer de the external ramus [exopodite] with marginal sette, and nearer Fig. 333.— Male of Branchipus sfagnalis. Eg, Heart or dorsal vessel with a pair of slit-like openings in each segment ; D, intestine ; M, mandible ; Sd, shell gland ; Br, branchial appendages of the eleven pairs of legs ; T, testis. 418 CRUSTACEV. its base a vesicular branchial appendage. The anterior, or evep all the legs {Le2)iodora) laay have the form of prehensile feet, and be destitute of branchial appendages. The Phyllopods possess a large pair of eyes, which are sometimes fused together in the median line. In addition a small median simple eye (Entomostracan eye) may persist. They have a saccular or chambered heart, which controls the regular circulation. Coiled excretory organs, kno^^^l as shell glmuls, are sometimes present; they open to the exterior by a special aperture on the posterior maxilla. The function of respiration is performed by the entire surface of the body, the area of which is much increased by the reduplicature of the skin forming the carapace ; also by the folia- ceous SA\-imming feet, and especially by the surface of the bi-anchial appendages. Reproduction. — The Thyllo'poda are of separate sexes. The males are distinguished from the females by the structure of the first pair of antennfe which are larger and more i-ichly pro^■ide(i tvith olfactory hairs, and also by their anterior swimming feet whii^ are armed Math prehensile hooks. In general the males are less fre- ^juently met with than are the females, and, as a rule, only at definite seasons of the year. The females of the smaller Phyllopoda {Clado- cera) are able to produce eggs Avithout copulation and fertil'- ■■.ation ; and these eggs, the so-called summer eggs, develop spontaneoudy and produce generations containing no males. In certain genera of the BrancMoi)oda, ejj., Artemia and Ajnis, parthenogenesis is the rule ; the males, indeed, have only been knoA\Ti a few years. The females usually carry the eggs about with them on special appendages, or in a brood pouch beneath the shell on the dorsal surface. The just hatched young either possess the form of the sexually mature animal {Cladocera), or undergo a complicated metamorphosis, leaving the e§^ membranes as a nauplius larva with three pairs of appendages {^Bran- chiopoda). A few of the F/iyUopoda live in the sea, the greater number inhabit stagnant freshwater ; some of them are found in brine pools. Sub-order 1. Branchiopoda.* P/iyllojJoda, with clearly seg- mented body, often enclosed in a flat, shield-shaped, or laterally compressed bivalved shell, with from ten to about thirty or more pairs of foliaceous swimming feet. * Schaffer. " Der krebsartigc Kiefcrfuss," etc. Eesrcnsbiirg. 17.56. A. Kozu bowski. ••Ucber den mannlichen Apus cancriformis," Archir fiir Katur/jciich, Tom XXIII. . 1857. C. Glaus, " Znr Kenntniss dcs Baues und der Entwickelung yon Branchipus und Apus," etc., Gottingen, 1873. rnTLLOPODA. BRANCIIIOPODA. 419 The alimentary canal is provided with two lateral hepatic appen- dages, which are, as a rule, branched and racemose and only excep- tionally short and simple. The heart appears as an extended dorsal vessel with numerous paired lateral slits, and may extend throughout the whole length of the thorax and abdomen [BrancJiijms). The genital organs, which are always paired, are placed by the side of the alimentary canal, and open at the boundary between the thorax and abdomen. In the females the genital openings are small slits ; in the male there may be protrusible copulatory organs at the openings (Brcmchipus). The males are distinguished from the females prmcipally by the fact that the anterior, or two anterior pairs of legs, are armed with hooks {Estherldce), or by the modification of the posterior antenna? to form a prehensile apparatus [Branchipus). Remarkable is the rare occurrence of the males ; they seem only to appear under certain conditions and in definite generations, which alternate with parthe- nogenetic generations. The eggs during development are generally protected within the body of the mother, and are carried about either in a saccular brood-pouch of the abdomen or between the valves of the shell on filiform {Estheria, Branchijms), or in vesicular {A2ni.s) appendages of different pairs of legs (9th to 11th). Tho eggs, so far as is known, undergo a complete segmentation. When hatched, the young animal has the form of a Nauplius larva with three pairs of appendages, of Avhich the anterior (which become the anteiior antennre) are in the Estheridce only represented by slightly de- veloped setigerous prominences. On the other hand, in Aims the third pair is small and rudimentary. Almost all the Branchiojwda belong to inland waters, and prin- cipally inhabit shallow fresh-water pools. When the latter dry up, the eggs, preserved in dry mud, remain capable of development. Some species, as Artemia scdina, are found in brine pools. BrayicUijms pisciformis Schaff = B. staijnaHs L., without a shell, found in the lakes of Germany, together with A2}us cancriformis. B. dlaijlLaims Prev,, France. Artemia salina L., in salt pools, near Trieste, Montpellier. They sometimes lay eggs with a hard shell, sometimes they are viviparous. J^w.* eancrifovmis SchafE, with shield-shaped shell, Germany. The males, which are rare, can be recognized by the normal formation of the eleventh pair of appen- dages. They live in puddles and fresh-water lakes, together with Branchijms, Estheria eycladoides Joly L., with perfect shell. Sub-order 2. Cladocera.* Water-fleas. Small laterally com- * Besides the works already qiioted, compare H. B. Strauss, "Memoirc sur les Daphina do la classc dcs Crustaccs," Mem. du 3Ius, d'hist nat., Tom V. and 420 CRUSTACEA. pressed Phi/Ilopoda, whose body, with the exception of the head, which projects freely, is usually enclosed in a bivalve shell. They have two large antenna?, which are used in swimming, and four to six pairs of swimming feet. The Cladocera are small simply organized Phyllopods, whose resemblance to the larva; of . the shelled Branehiopoda, particularly to the larva of Estheria with its six pairs of legs, gives the best indica- tion of the probable origin of the group. Unlike the anterior antennpe, which are short, the posterior are modified to form bii-amous swimming appendages beset with numerous long setae. The four to six pairs of legs are not always foliaceous swimming feet, but in many cases have the form of cylindrical ambulatory or prehensile appendages. The abdomen, which is ventrally flexed, develops on its dorsal side several prominences, which serve to close the brood pouch. It usually consists of three free segments, as well as the terminal anal portion, which is beset Avith rows of hooks. The anal poi-tion begins with two dorsal tactile setai and ends with two hooks or styles, representing the caudal foi'k (tig. 334). The internal organization is simple in correspondence with the small size of the body. The compound eyes fuse together in the middle line to form a large, continually tiembling, fi-ontal eye, be- neath which the unpaiied simple eye usually remains. A special sense apparatvis, whose function is not quite clear, appears in the region of the neck, in the form of an aggregation of ganglion cells. The heart has the form of an oval sac, with two transverse lateral venous ostia and an anterior arterial opening. Its pulsations are rhythmic, and succeed one another quickly. In spite of the want of arteries and veins, the circulation of the blood, which contains amoeboid cells, is completed in definite tracts marked out by lacunae and spaces in the body. The looped and coiled shell gland is always present. The cervical gland, Avhich functions as an organ of attachment, is less widely distributed. The ' sexual glands lie in the thorax as paired VI, 1819 and 1820, Leydig, " Natnrgeschichte der Daphnidtii," Tubingen, 1860. P. E. Mliller, "Bidrag til Cladocerernes Fortplantings historic," Kjobenhavn, 1808. G. 0. Sars, " Om en dimorph Udvikling samt Generations — vexel hos Leptodora," Videnitk. Sclsk. Fork., 1873. A Weismann, " Beitriige zur Kenntiss der Daphnoiden," I — IV., Leipzig, 1876 and 1877. C, Claus, " Zur Kenntiss der Organisation und des feineren Baues der Daphniden, /!^eit. f. miss, zool, Tom XXVII, 1876. C. Chius, " Zur Kenntniss des Baues und der Organi- saton der Polyphemiden," Wien, 1877. C. Grobben, " Die Embryonalentwick- elung von Moina rectirostris," Arheiten aus dem zool. verol. anatoin. Institut. II Band, Wien, 1879. rUTLLOPODA. CLADOCEIJA. 421 tubes by the side of the alimentary canal. In the ovaries groups of four cells are separated ; one cell of each group becomes an ovum, while the rest are employed as nutritive cells for the nourishment of the ovTim, which increases in size and absorbs fat globules. The ovary is directly continuous Avith the oviduct, which opens dorsally beneath the shell into the brood-pouch. The testes, like the ovaries, lie at the sides of the intestine and are continuous with the vasa deferentia, Fig. 33t. — Daphnia. C, Heart — the slit-like opening of one side is visible ; D, alimentary canal ; Z, hepatic diverticulum ; A, anus ; O, cerebral ganglion ; O, eye ; Sd, shell gland ; Br, brood-pouch beneath the dorsal reduplicature of the shell. which open to the exterior ventrally behind the last pair of appen- dages or at the extreme end of the body, the openings being some- times situated on small slightly protrvisible prominences. The smaller males usually appear in the autumn ; they may, however, also be present at any other time of the year, and, as recent investi- gations have proved in a tolerably satisfactory manner, always when 422 CEUSTACEA. the conditions of life and nourishment are unfavourable. Before the appearance of the males, hermaphrodite forms * sometimes make their appearance Avith an organization which is half male and half female. At the season when males are not present, normally in the spring and summer, the females produce the so-called summer eggs, which contain a large quantity of oil globules and are surrounded by a delicate vitelline membrane. They develop rapidly within the brood- pouch between the shell and the dorsal surface of the mother, and after the space of only a few days give rise to a fresh generation of young Cladocera, which escape from the brood-pouch. The embryonic development takes place accordingly under extremely favourable conditions, which depend upon the rich supply of food yolk in the large eggs, and are sometimes favoured by the secretion of additional food material within the brood-pouch. At the season when the males appear, the females, under the like influence of unfavourable nourishment and independently of copu- lation, begin to produce so-called winter eggs, which are incapable of developing without fertilization. The number of these hard-shelled •winter eggs is always relatively small. They are, therefore, distin- guished from the -summer eggs by their larger size and the greater quantity of food yolk ; and their origin in the ovary is accompanied by much more extensive processes of absorption. The Daphnidce live for the most part in fresh water. Certain species inhabit deep inland lakes, brackish water, and the sea. They swim quickly, and usually with a jumping movement. Some of them attach themselves to solid surrounding objects by means of a dorsally placed organ of attachment, the cervical gland. "When the body is thus fixed, the swimming feet seem to be able by their vibrations to set up currents in which small food particles are swept towards the animal. — • Sida crystallina 0. Fr. Miiller. The six pairs of lamellar legs beset with long swimming setfe. The rami of the swimming antenna two- to three-jointed. Dajilinia 2>ulex De Gear. B. sima Liev. Five pairs of legs, of which the anterior are more or less adapted for prehension. One ramus of the swimming nntennjB is three-jointed, the other four-jointed. PolypJinnvs 2}t'dicidus De Geer. In the lakes of Switzerland, Austria, and Scandinavia. Evadne Kordmannl •Lov^n, North Sea and Mediterranean. Lcptodora hyalina Lillj., in lakes. * Compare especially W. Kurz, " Ueber androgyne Missbildung bei Clado- ceren," Sitzungsher der Ahad. der Wisseiisch. Wien. 1874. Also Schmanke- witsch. OSTEACODA. 423 Order 2. — Ostracoda.* Small, usualbj laterally cooipressed Entomostraca, vnth a bivalve shell and seven j^airs of ajypendages, which function as antennce, jaios, creeping and sioimminy legs. There is a pediform mandibular palp, and a short abdomen. The body of these small Crustacea is unsegmented and is completely enclosed in a bivalve shell, which gives the animal a resemblance to a mussel. The two v^alves of tlie shell join together in the middle line, and are fastened together by an elastic ligament along the middle third of the back. The action of this ligament is opposed by a two- headed adductor muscle, which passes from one valve of the shell to the other and causes impressions discernible from without. The common tendon of the two heads of this muscle lies nearly in the Fio. 335. — Female Cyprif before sexual maturity ; the right valve of the snell has been removed, A', A!', first and second pair of antenna; ; Ob, upper lip ; Md, mandible with pediform palp ; G. cerebral ganglion with unpaired eye ; SM, adductor muscle ; Mx', Mx", first and secoud pair of maxillse ; F', F", first and second pair of feet ; Fu, caudal fork ; M, stomach ; D, intestine ; L, hepatic tulje ; Ge, rudimentary genital organs. middle of the body. The edges of the valves are free at both ends and along the ventral side. In the marine Cypridinidce there is a deep indentation in the edges of the v^alves, to allow the antennas to pass out. When the valves of the shell are open, several pediform appendages can be protruded on the ventral side, which enable the animal to move in the water either by crawling or by swimming. * H. E. Strauss-Diirkheim, " M^moire sur les Cypris de la classe des Cras- tac6s," Mem. du Mv.i (I'liist. nat., Tom VII., 1821. W. Zenker, " Monographie der Ostracodcn," Archiv.fiir JVaturfjcsoh., Tom. XX.. 1854. C. Claus,' "Beitrage zur Kentniss der Ostracoden. Entwickelungsgcschichte von Cypris." Marburg. 1868. C. Claus. " Neuc Beobachtungen Uber Cypridinen," Zeltiirhr. filr orh^s. Zool., Tom XXIII. ('. Claus, "Die Familic der Halocypriden." Schriften zoologixrlien Inhaltx, Wien, 1874. G. S. Brady, "A Monograph of the Kccent British Ostracoda." Transact, of the Lin. Soc.,'Yo\. XXVI. 424 CUL'STACEA. The abdomen can also be protruded ; it either ends in a caudal fork (Cypris and Cylhere), or has the form of a plate armed with spines and hooks on its posterior margin {Cyj^ridina). Appendages. — The two pairs of antennfe are placed on the anterior region of the body (fig. 336, A', A"), and are used as creep- ing and swimming legs. In Gypridina, however, the anterior pair is provided with olfactory hairs. The antennae of the second pair in Cypris and Cythere resemble legs, and end with strong hooked bristles, by help of which the animal can attach itself to surrounding objects. In the exclusively marine C ypridinidce and Halocypridce this pair of appendages has the form of biramous swimming feet, which consist of a broad triangular basal plate, a many -jointed endopodite beset with long swimming seta;, and a rudimentary exopodite, which, however, is stronger in the male and fvirnished with hooks of a considerable size. In the region of the mouth beneath and to the side of a tolerably large upper lip there are two powerful mandibles Avith a broad and strongly toothed biting edge. The mandibular palps, which are leg-like and elongated, are usually three-jointed and can be used as legs {3fdf). In exceptional cases [Paradoxostoma), the mandibles are styliform and are enclosed in a suctorial proboscis formed from the upper and under lips. The mandibles are followed by the first pair of maxilla% which are in all cases distinguished by the great development of their basal portion and by the reduction of the palp. In the Cypridm and Cytheridce the basal joint of the first maxilla bears a large comb-like setose plate, Avhich by its swinging movements aids the function of respiration, but does not itself function as a gill. A similar branchial plate may also occur on the two following appen- dages (the 5th and 6th pair), which sometimes have the form of jaws, sometimes of legs. The anterior of these appendages (maxilla of the second pair or better maxilliped, fig. 336, Mx") functions, in Cypris, chiefly as a jaw, but bears, besides the rudimentary bran- chial appendage, a short, backwardly directed, usually two-jointed palp, which, however, in certain genera and in Halocypris becomes a short, three-jointed or even four-jointed leg. In Cythere it acts ex- clusively as a leg, and represents the first of the three pairs of legs present in this animal. In the Cypridina, however, it has completely the form of a jaw, and is provided Avith an enormously developed branchial plate (fig. 336 a, Mx"). The appendage of the sixth pair is usually modified to an elongated, many-jointed, creeping and ad- OSTRACODA. 425 hering foot. The appendage of the seventh pair is always elongated to the form of a leg ; in C'ljthera it is formed like the preceding one, FiS. ZZ^.—Cypridina mediterranea. a, Female; 6, male. M, Stomach; II. heart; SM, adductor muscle ; O, eye ; C, unpaired eye ; G, brain ; Stz, frontal organ ; T, testis ; P, copulatory organ ; Mdf, mandibular palp ; Mx', first maxilla ; Mx", second maxilla ; Fu, caudal fork. but in Ci/2^ris it is curved upwards, and is furnished with a short claw and terminal setae. It has probably the same function (Putzfuss) 426 CUrSTACEA. as the long cylindrical appendage of Ci/2^ridina, which arises in place of the seventh pair of legs, almost on the back of this animal. The nervous system consists of a bilobed cerebral ganglion and a ventral chain with closely approximated pairs of ganglia, which may unite to form a single ganglionic mass. Sense organs. — In addition to the already mentioned olfactory hairs there is a median eye {Cyj)ris, Cythere), composed of two (often separated) halves ; or there are, in addition to a small unpaired eye, two larger compound and movable lateral eyes {Cyx>ridina). In Halocypris and Cypridina there is a frontal appendage, which probably functions as a sense organ. Alimentary canal. — The mouth, which is frequently {Cypris) armed with toothed lateral bands, leads through a narrow oesophagus into a dilated crop-like portion of the alimentary canal. This is followed by a bi'oad and long stomach, provided with two long lateral hepatic tubes, which project into the lamellse of the shell. The anus opens at the base of the abdomen (fig. 337). Of special glands a club-shaped, dilated glandular tube (poison-glands ?) found in Cythere must be mentioned, the duct of which opens to the exterior through a spinous appendage of the posterior antennfe. A heart is present in Cypri- dina and Halocypris on the dorsal surface, where the shell is con- nected to the animal. The function of respiration is performed by the whole suiface of the body, over which an uninterrupted current* of water is maintained by the swinging movements of the leaf-shaped setose branchial appendages. In many Cypridinidai (Asterojje) thei-e is a double row of branchial tubes on the back, near the last pair of appendages. Generative organs. — The sexes are always separate and are dis- tinguished by well marked differences in their entire structure. The males, in addition to the greater development of the organs of sense, possess apparatuses on different appendages — in Cyp>ridina on the second antennre, in Cypris on the maxilliped — for holding the females ; or a pair of legs may be completely modified for this pur- FiG. 337. — Alimentary canal and generative organs of a female Cypris (after W. Zenker). Oe, oesophagus ; P T, crop ; V, stomach ; 1), intestine; i, liver ; Of, ovary ; ,SiV, adductor muscle ; R roceptaculum ; Vu, vulva ; Fa, caudal fork. OSTEACODA. 427 pose. In addition a large copulatory organ, which may be dex-ived from a modified pair of appendages and often possesses a very compli- cated structure, is always present. The male genital organs consist on either side of several elongated or globular testes, of a vas deferens and the copulatory organ ; the presence in Cypris of a very peculiar paired mucous gland and the size and form of the spermatozoa seem to be worthy of notice (Zenker). The female of Cypris possesses two ovarian tubes which project into the reduplicature of the carapace, two receptacula seminis, and the same number of genital openings at the base of the abdomen. Development. — The greater number of Ostracoda lays eggs, which they either attach to watei"-plants (CyjJris), or, as in Gyj)rid'ina, ciirry about Avith them between the shell valves until the young are hatched. The free development of Cypris consists of a complicated metamorphosis. The lai-voe, when hatched, possess, like the Nauplius form, only three pairs of appendages, but are strongly compressed laterally, and are already enclosed in a thin bivalve shell (fig. 338). In the marine Ostracoda the development is simplified, so that the metamorphosis is entirely absent. The Ostracoda feed altogether on ani- mal matter, as it seems especially on the carcasses of different aquatic animals. fig. 338.— Very youns larva of Numerous fossil forms are known from y^^- Naupiius stage with three \ airs of appendages. almost all formations, but, unfortunately, M, stomach; D, mtcstine; only the remains of their shells are pre- f^' tcLTltL'J'.f 'ij! served. mandible. Cijpridlna. With heart and large movable paired eye. With deep excava- tion in the edges of the shell for the passage of the antennae. The anterior antennfe are bent, furnished with strong sette, and have olfactory hairs at tlicir extremity. The posterior antenna; are biramous swimming feet. The biting part of the mandible is weak or entirely aborted ; palp is five-jointed, pediform, and of considerable lengtli. The seventh pair of appendages is represented by a cylindrical ringed appendage (Putzfuss). Cypridina meditcrranea Costa. Astcvfljfc oUonga Gr., Trieste. Ilalocyyris Dana. Cytlicre 0. Fr. Miill. Without heart. The anterior antennic are bent at. their base and beset with short set.ie. The posterior antenna; are strongly developed, with hooks on the terminal joint. Three pairs of legs, of which the last is the most strongly developed. The abdomen has only the caudal fork, of which the two branches are small and lobe-like. The testes and ovaries do not project between the lamella; of the carapace. The male genital apparatus has no mucous srland. They are all marine animals. The females often carry the 428 carsTACEA. eggs and embryos about between the valves of the shell Cythere Intea 0. Fr. Miiller. North Seas and Mediterranean. C. viridis 0. Fr. Miill., North Seas. Cypris O. Fr. Miill. With median eye, but no heart. The shell valves are light but strong, the anterior antennoe have usually seven joints and are beset with long setae. The antenna of the second pair is simple and pediform, with usually six joints. There are two pairs of legs, of which the posterior smaller pair is bent upwards towards the dorsal surface. The caudal fork is very narrow and elongated, and is provided with hooked setfe at the point. The testes and ovaries project between the lamellie of the shell. The male genital appa- ratus has a peculiar mucous gland. Most of them inhabit fresh water. Oypris fmca^tw, C.puleraO.Yx.WJll., C./wscate Jur., and others. Kotodromvs monachus 0. Fr. Miill. Order 3. — Copepoda.* Entomostraca with elongated, usually loell serjmented body, icithout shell-forming reduj)licature of the skin, with hiramous swimming feet ; the abdomen is without appendages. The group of the Copepoda includes a number of very different forms. The non-parasitic members of the groups are distinguished by a constant number of segments and paired appendages. The numerous parasitic forms differ in various degrees from those which lead an independent life ; in extreme cases some of them are so modified, that without a knowledge of their development and the peculiarities of their structure, they would rather be taken for parasitic Worms than for Arthropods, The characteristic swimming feet are, however, visually retained, though often reduced in number, as rudimentary or modified appendages. When they are absent, the developmental history gives a certain indication of the Copepod nature. Appendages. — The head seems as a rule to fuse with the first thoracic segment ; and the cephalothorax so formed bears two pairs of antennse, a pair of mandibles, the same number of maxilla^, and four maxillipeds, which last are only the external and internal branches of a single pair of appendages (fig. 341) ; and finally the first pair of swimming feet, which are not unfrequently modified in form. Then come four free thoracic segments, each with a pair of swimming feet, of which the last paii" is frequently reduced and in the male may "be modified to assist in copulation. Finally, the fifth pair of feet and ♦ O. Fr. Miiller, " Entomostraca sen Insecta testacea, quse in aquis Daniae et NorvegiEe reperit, descripsit," Lipsiae, 1785. Jurine, " Histoire des Jlonocles," Geneve, 1820. W. Lilljeborg, " Crustacea ex ordinibus tribus : Cladocera, Ostracoda et Copepoda, in Scania occurrentibus," Lund., 1853. C. Claus, " Zur Morphologic der Copepoden," Wurzh. natvrniss. Zeitschr., 1860. C. Claus., " Die freiiebenden Copepoden," Leipzig, 18fi3. COPEPODA. 429 the corresponding thoracic segment may be entirely absent. The abdomen as well as the thorax consists of five segments, but is with- out appendages and ends in a caudal fork, the branches of which are furnished at their points with several long caudal setaj (fig, 339). In the female, the two first abdominal segments usually unite to form a double genital segment, on which the genital openings are placed. The abdomen, especially in the parasitic forms, very fre- quently undergoes a considerable reduction. Fia. 339.— Female of Cydopi coronaim, seen Fig. 310.— An antenna of the male of from the dorsal surface. V, Intestine ; OvS, Cyclops serruiatus. Sf, olfactory hairs . ovisacs ; A', A'', antennae. M, muscles. The anterior antennae, which are usually many- jointed, bear olfac- tory hairs, but serve in the free-swimming forms for locomotion, and in the male as prehensile arms for catching and holding the female during copulation (fig. 340). The posterior antennse are always shorter, and not unfrequently bifurcated and adapted for clinging to surrounding objects. With regard to the oral appendages 430 CEUSTACEA. (fig. 341), two toothed, usually palped mandibles are placed be- neath the upper lip. These function in the free-living Copepoda as masticatory organs, but in the parasitic forms are usually trans- formed into pointed styliform rods, which are used for piercing. In this case they are frequently placed in a suctorial tube formed by the junction of the upper and under lips. The two jaws which follow the mandibles are weaker biting plates, and in the parasitic Copepoda are reduced to small palp-like protuberances. The maxil- lipeds, on the contrary, are much longer ; they are used to procure food and, especially in the parasitic forms, to attach the body. The thoracic swimming feet consist of a two-jointed basal portion, and two three- jointed setigerous swimming rami, which are comparable to broad swimming plates. In the Argulidce these rami are much elongated, and by their numerous joints approximate to the legs of tho Cirrijiedla. Nervous System. — In all cases there is a brain giving ofT sensory nerves, and also a ventral cord, which either develops some ganglia in its course or is concen- trated to a common subcesophageal gan- glionic mass. Of sense orrjans the median frontal eye, divided into three parts (Cy- clo2)s ei/e), is pretty generally present. The tactile sense is specially localized in the setae of the anterior antennse, but is probably also present in many other parts of the body. Olfactory hairs are pre- sent as delicate appendages of the an- terior antennae, principally in the male sex. The alimentary canal is divided into a short narrow oesophagus, a wide sto- mach which often has two blind diverticula near its commence- ment, and a narrow rectum which opens on the dorsal surface of the last abdominal segment. The surface of the intestine often seems to perform the function of a urinary organ. We find, however, at the same time a shell gland in the cephalo-thorax at the sides of the maxillipeds. In all cases the whole surface of the body performs Fig. 341. — Mouth parts o£ Cyclops. M, Mandibles ; Mx, maxilla; Kf, internal; Kf, external maxilliped. COPEPODA. 431 the respiratory function. Circulatory organs are either replaced by the regi^lar oscillations of the intestinal canal [Ci/clopSy Achtlieres), or there is present in the anterior part of the thorax above the intes- tine {Calanidca) a short saccular heart, which may even be continued into a cephalic artery {Calanella) (fig. 53). Generative organs. — The Cojicpoda are of separate sexes. Both kinds of genital organs lie in the cephalothorax and in the thoracic segments, and open right and left on the basal segment of the abdomen. Sexual differences in the form and structure of the different parts of the body are almost uniformly found. These lead Fig. 342.— Metamorphosis of Cyclops, a, Nauplius larva of Ci/clops serrulaius after hatching. b. Older stage strongly magnified, c. Very young Cyclops form. SD, antennal glands ; 01, upper lip ; Mf, mandibular foot ; Md, mandible ; Mx, maxilla, Mxf, masilliped ; J-*, JP", first and second swimming feet ; He, iirinary concretions ; D, intestine ; Ad, rectum ; A, anus ; G, rudimentary genital organs. in certain parasitic Copepoda {Chondracanthidce, Lernceopodidce) to an extremely striking dimorphism. The males are smaller and move with greater facility ; the anterior antennte and the last pair of feet become accessory copulatory organs, the former serving to hold the female, the latter to affix the spermatophores. The sper- matophores are formed in the vas deferens by a mucous secretion which surrounds the seminal mass and hardens to a tough mem- brane. The females are larger than the males and often move 432 CBTJSTACEA. more clumsily ; they carry the eggs about with them in sacs, placed to the right and left on the abdomen. Many of them possess a cement gland at the end of the oviduct ; the secretion of this gland passes out with the eggs and gives rise to the stiff covering of the ovisacs. During copulation, which Ls only an external approximation of the two sexes, the male fastens one or more spermatophores on to the genital segment of the female, and, indeed, on to special openings through which the spermatozoa pass into the receptaculum seminis, and fertilize the ova either within the body of the mother, or as they pass out into the developing ovisacs. Development takes place by means of a complicated metamorphosis, which, in many parasitic forms, is a retrograde one. The larvae, when hatched, have the Nauplius form, with an unpaired frontal eye and three pairs of appendages. Hocked setfe on the second and thu-d pairs of appendages serve to conduct the food into the mouth, which is covered by a large upper lip (fig. 342, a). The posterior region of the body is destitute of appendages, and terminates with two setse at the sides of the anus ; it coi-responds to the thorax and abdomen, which are as yet undifferentiated. The altei-ations undergone by the young larvae in the course of their further growth are connected with a number of successive moults, and consist principally in an elongation of the body and the appearance of fresh appendages. Even in the next larval stage (fig 342, h), a fourth pair- of appendages, the future maxillae, makes its ap- pearance behind the three original pairs, which develop into the antennae and mandibles. In a later stage three fresh pairs of appendages are formed. Of these the first corresponds to the maxillipeds, while the two last pairs represent the first rudiments of the anterior swimming feet. In this stage {Mefcmai(plitis) (fig. Fig. 3-13. — Mctananplius of Cyctoptine. O, eye ; G, rudimentary genital organs ; SD, antennal gland ; -4', A'', antennfc ; Md, mandible ; Mx, max- illa ; Mf, maxilliped. 433 343), the larva still resembles a Nauplius, and it is only after another moult that it is transformed into the first Ci/cIops-\ike form. It then resembles the adult animal in the structure of the antennte and mouth parts, although the number of the appendages and the body rings is smaller (tig. 342, c). The two last pairs of appendages already have the form of short biramous swimming feet, and the rudiments of the third and fouth pairs of swimming feet have made their appearance as projections beset mth set*. The body consists in this stage of the oval cephalothorax ; the second, third and fourth thoracic segments ; and an elongated terminal portion, which gives rise to the last thoracic segment, and to all the abdominal segments by a pro- gressive segmentation, and already terminates in the caudal fork. Fio. 3^.—Actheres percarum.—a, Nauplius form, h, Lar\'a in the youngest Cyclops stage; Kf, Kf", maxillipeds. c. Female seen from the ventral side. Ov, Ovaries ; KD, cement glands, i. The smaller male seen from the side ; Mxf, Mxf", maxillipeds. Many forms of parasitic Copepoda, for example Lernanthropus and Chondracanihus, do not get beyond this stage of body segmenta- tion, and obtain neither the swimming feet of the third and fourth pairs, nor a fifth thoracic segment separate from the stump-like abdomen; others, for example Achtheres, by the loss of the two anterior pairs of swimming feet, sink back to a still lower stage (fig. 344). All the non -parasitic and many of the parasitic Copiepioda pass in the successive moults through a larger or smaller number of de- velopmental stages, in which the still undeveloped segments and appendages make their appearance, and the appendages already 28 434 CKUSTACEA. present undergo further segmentation. Many parasitic Copepoda, however, pass over the series of Nauplius forms, and the larva, as soon as hatched, undergoes a moult, and appears at once in the youngest Cyclops form, with antennae adapted for adhering and mouth parts for piercing (fig. 344). From this stage they undergo a retrogressive metamorphosis, in which they become attached to a host, lose more or less com- pletely the segmentation of the body which grows irregular in shape, cast off their swim- ming feet, and even lose the eye, which was originally pre- sent [Lernccopoda). The males, however, in such cases often remain small and dwarfed, and adhere (fre- quently more than one) firmly to the body of the female in the region of the genital open- ing (fig. 345). In the Lerncea (fig. 346) such pigmy males were for a long time vainly sought for upon the very peculiarly shaped body of the large female (fig. 346, c, d) which caxTies egg tubes. At last it was discovered that the small cyclops-like males (fig. 346, a), lead an independent life, and swim about freely by means of their four pairs of smm- niing feet; and that the fe- males (fig. 436, h), in the copulatory stage I'esemble the males, and that it is only after copulation that they (the females) become parasitic and undergo the considerable increase in size and modification of form which characterises the female with egg-tubes. Tig. 345.— The two sexual animals of Chandra, canthus gihboeux magnified about six diameters. a, Female seen from the side; b, from the ventral surface \vith adhering male ; c, male strongly magnified. An', Anterior antenna^; An'', antenna; for attachment; F', F", the two pairs of feet; A, eye; Ov, egg-tubes ; Oe, oesophagus; D, intestine; M, mouth parts; T, testis; FcJ, vas deferens; Sp, Bpermatopliore. 435 1. Sub-order : Eucopepoda. Copepoda with swimming feet, the rami of which are two or thi'ee jointed. They have biting or piercing and sucking mouth parts. 1. Gnathostomata. For the most part non-parasitic; oral apparatus adapted for mastication ; fully segmented body. Fam, Cyclopidae. Mostly frcsli-water animals, without a heart, and with a simple eye. The second pair of antennae are four-jointed and never biramous. The feet of the fifth pair are rudi- mentary in both sexes. The male emploj-s the anterior antennjc for prehension. Cycloj^s coronattis CIs., Ca)ithocam.j>fiis minutus Cls. , Harpact'icus chalifcr 0. Fr. Miill., North Sea. Fam. Calanidae, The anterior antenna; are very long, only one of them is modified for prehension. The posterior antennae are bira- mous. Pleart always present. The feet of the fifth pair are, in the male, modified to assist in copula- tion. CctocMlus scjifentrionali.f Goods., jyiajytomus castor Jur. Irenccus Patersonii Tempi. Fam. NotodelphyidsB. Structure of body like that of the Cyclopidcc. The posterior antennae modified for attachment. The two last tho- racic segments are fused in the female and form a brood cavity for the reception of the eggs. They live in the branchial cavity of As- cidians. K'otoddpliys agilis Thor. 2. Parasita* (Siphonosto- mata). Mouth parts adapted for piercing and sucking, usually A\dth incomplete seg- mentation of the body and reduced abdomen. The posterior antennse and maxillipeds end with hooks for attachment. Some of Pia. 346. — LerncBa branchuiUt. a, Male (abotif, 2 to 3 mm. long). Oc, Eye; G, brain; T, testis ; M, stomach ; P' to F''', the four pairs of swimming feet; Sp, spermatophore sac. b. Female (5 to 6 mm. long at the time of copulation). A', A", the two pairs of an- tennfB ; D, intestine ; R, proboscis ; Mt/, maxilliped. c, Female of Zenura hranchialis after copulation undergoing metamorphosis j d, the same with egg sacs, natural size. * Besides Steenstrup and Liitken I.e. compare A. v. Nordmann, " Mikro- graphische Beitrage zur Naturgeschichte der wirbellosen Thiere," Berlin, 1832. H, Burmeister, " Beschreibung einiger neuen und wenig bekannten Schmarot* 4.'J6 CTJX'STACEA. them still swim freely, but most of them live on the gills, in the pharynx, and on the outer skin of fishes. Some live A\-ithin the tissues of their host [Penella), and nourish themselves on the blood and juices of the latter, Fam. Corycaeidae. Anterior antenna; short, few jointed, and similar in both sexes. The posterior antennae unbranched, with clasping hooks, usually differ- ent according to the sex. Mouth parts often arranged for piercing. Median eye and lateral eyes often present. They live partly as temporary parasites, Corycavs ehmgatna Cls,, Si/j>j>hi rina fi(I{/ens Thomps. Fam. Chondracanthidse. Body elongated, often without distinct segmenta- tion, and furnished with pointed outgrowths. Abdomen stump-like. The two anterior pair of swimming feet are represented by bifid lobes, the others are wanting. There is no suctorial proboscis, the mandibles are sickle-shaped. The pear-shaped males are small and dwarfed, and attached, often in pairs, to the body of the female. Chondracanthus gihhosns Kr, (on Lophlus). Ch. cornutus 0, Fr, Mlill., on flat fish [PleuronectUla^ (fig. 345). Fam. Caligidas. Body flat, with shield-like cephalothora-v, and very large genital segment which in the female is especially swollen. Aljdomen, on the contrary, is small and more or less reduced. There is a suctorial tube and styliform mandibles. Foi;r paired biramous swimming feet enable the animal to swim rapidly. They live on the gills and the skin of marine fish, and the females have long string-like egg tubes, Callgtis raim.v JCdw., Cccroj^is Latrcillii Leach, Fam, Lemaeidae. The body of the female vermiform or rod-shaped ; unseg- mented, with outgrowths and processes on the liead. Mouth parts piercing with suctorial tube. There are four pairs of small swimming feet. The females become attached to fishes, in which the anterior part of their body is buried. LcrncBocera eyjjrinacea L,, Penclla sagitta L,, Lerncea hrcuicMalis L, (fig. 346). Fam, Lernaeopodidae. Body separated into head and thorax, abdomen rudimentary. Mouth parts piercing with suctorial tube. The external maxilli- peds attain a considerable size, and in the female unite at their points so as to form a single organ of attachment, by means of •which the animal adheres permanently. Swimming feet completely absent. The males, which are more or less dwarfed, have large free clasping feet, and are, like the females, without swimming feet, Achthcres iJercarum l^ovdcm. (fig, 344), Anchorella uncinata 0. Fr. Miill, (on species of Gadus). 2, Sub-order: BrancMura,* Carp-lice, With large compound eyes, and long protrusible spine in front of the suctorial tube of the mouth ; with four pairs of elon- gated biramous swimming feet, zerkrebse," Kova acta Ac. Ccea. Loop., Tom XVII,, 1835, C, Claus, " Ueber den Bau und die Entwickelung von Achtheres percarum," Zeituchr fiiv tvisx. Zool., 1861, C, Claus, " Beobachtungen iiber Lernjeocera, etc, Marburg, 1868, * Jurine, '' M^moire sur I'Argule foliace," Annales du Musevm d'hist. nat., Tom, VII,, 1806, Fr. Leydig, '• TJeber Argulus foliaceus," ZeitKchr fur n-iss. Zool., Tom II., 1850, E. Cornalia, •' Sopra una nuova specie di crostacei sifonos- tomi," Milano, 1860, C, Claus, " Ueber die Entwickelung, Organization und systcmatischc .S tellung der Arguliden," Zeitschrfiir n-^. ovary. 442 CEUSTACEA. Fig. 351.— a Latcr'Nauplius larva. A, anus ; 01, proboscis with mouth ; U, frontal horns ; D, intestine ; A' , A", 1st and 2ud antenna;; Mdf, mandibular foot (third pair of appendages). b, Metanauplius larva of Balanux Ijet'oie the moult. Beneath the skin are the rudiments of the lateral eyes (O) and all the appendages F^ to F'" of the Cypris stage ; Ff, frontal filament ; C, unpaired eye ; JDr, gland cells of the anterior horns ; A', the antennEB with suctorial disc ; iVx rudiment of maxilla. Krohn, open on a prominence on the basal joint of the anterior pair of thoracic appen- dages. The eggs accumulate in the cavity between the mantle and the body in large thin - walled flat- tened sacs, Avhich, in the Lepadidce. are attached to a fold of the mantle and are packed to- gether on the dor- tal surface of the animal. In spite of the hermaphroditism, there are, accord- ing to Darwin, in certain genera {Ihla, Scaljiellum) very simply orga- nised dwarfed males of peculiar form, the so-called complemental males, which are attached like para- sites to the body of the hermaphro- dite. There are also dioecious Cir- ripedes with a strongly marked dimorphism of the sexes. This is the case with Scalpel- CIIlElPiDIA- 443 lum omaturn and Ibla CumirKjii; also Avith the remarkable genera CryptopJualus and Alcippe (fig. 350). The males of these forms are not only small and dwarfed, but also, according to Darwin, have neither mouth, digestive canal, nor thoracic appendages. As a rule, two or sometimes more attach themselves to the body of the female. Development.— The eggs, while still within the brood-pouch, undergo an ii-regular segmentation. The clear cells arrange them- selves around the food yolk in the form of a blastoderm, the ventral side of which soon becomes considerably thickened in consequence of the appearance of the mesodermic layer. The larva) leave the egg as Nauplii (fig. 351, a, b), of oval or pear-shaped form, with unpaired frontal eye, lateral frontal horns, and three pairs of appendages, of which the anterior is simple, the two next biramous and closely beset with SAvimming seta?. After several moults, the larva, which has grown to a considerable size, enters on a new stage of de- velopment, the so-called Cypris stage (pupa) (fig. 352). The reduplica- ture of the skin has the form of a bivalve mussel-like shell, through the gaping ventral edges of which the appendages can .be protruded. While the form of the shell recalls that of the Ostracoda, the structure of the body, so far as the segmenta- tion and form of the appendages are concerned, approximates to that of the Copepoda. The anterior ap- pendage of the Nauplius larva has given rise to a four-jointed antenna, the penultimate joint of wdiich has become large and disc-shaped and contains the opening of the cement gland, while the terminal joint bears in addition to tactile setae one or two delicate lancet-shaped olfactory hairs. The frontal horns are transformed into two conical prominences near the an- terior margin. Of the two pairs of biramous appendages, those which correspond to the second pair of antennai arc east ofl', while Fig. 352. — Median section through a pupa of Lepas. A' Attaching antenna ; C, carina; Te, tergnm ; Sc, scutum; Ov, ovary ; G, cerebral ganglion ; Gg, ganglionic chain ; D, alimentarj- canal ; Cd, cement gland ; Mk, oral cone ; Ab, abdomen ; F, rudiment of the peuJs ; M, muscle. 444 CKUSTACill. the posterior pair becomes the rudiment of the antei'ior jaws (mandibles) of the oral cone, which is still closed and on which the first rudiments of the maxillae and under lip are already visible. The oral cone is followed by the thoracic region with six pairs of biramous Copepod-like swimming feet, and a minute three-jointed abdomen, which terminates in two caudal appendages and caudal set£e. The pupa has a large pair of compound eyes at the sides of the un- paired eye-spot, and swims about by means of its swimming feet. It appears not to take in food. The material necessary for its further changes is stored up principally in the cephalic and dorsal regions in the form of a largely-developed fat body. After swimming abovit for a longer or shorter time, the pupa fixes itself by the suctorial disc of its bent antennae to some foreign body. The parts of the adult Cirripede are now visible beneath the skin, and the cement gland begins to secrete a cement, Avhich hardens and so brings about the permanent attachment of the young animal. In the Lejmdidce the region of the head above and be- tween the antennae giows so much that it projects from the pupal integument, beneath which the calcareous pieces of the shell of the Cirripede can be seen, and after the moulting, of the chitinous skin of the pupa constitutes the fleshy peduncle by which the animal is attached, and into which the rudiments of the ova- ries project (fig. 353). The paired eyes of the free-swimming Cypris larva disappear, while the unpaired pigment spot remains. The mouth parts become fully differen- tiated, and the biramous swimming feet become short, many-jointed cirriform appendages. The Cirripedia are marine animals. They attach themselves to various foreign objects. They are found fixed, usually in groups, to logs of wood, rocks, mussel shells, Crustacea, the skin of whales, etc. Some, as LitJiotrya, Alcijjpe, and the Crijj^tojnalidce, are able to bore into Lammellibranch shells and Corals, while the RMzocephala are parasitic on Crustacea. In the Wiizoce'phala the body is saccular, Fig. 353. — Young l,epa» after disappearance of the two homy valves of the shell 6.n(i the straightening of the anterior part of the head (stalk) , which in the papa stage is bent. Letters as in fig. 3i9. CIHRIPEDIA. 445 and the animal loses all its appendages and its alimentary canal, and extracts the juices of its host (Decapoda) by means of root -like processes (fig. 354). 1. Pedunculata. There is a peduncle and six pairs of biramous feet ; the mantle has iisually carina, scuta, and terga. Fam. Lepadidse. Peduncle well marked, and not provided with calcareous plates. There is a membranous mantle, which, as a rule, is provided with five shell plates, of which the scuta and terga lie bnhintl one another (fig. 348, a). L('j?as L. (Anatifa Brug.), L. fai^cicularh Ellis, (vitrea Lam.) Found from the Northern Seas to the South Sea. L. anatifcra L., cosmopolitan. Cone hod urnui Fto. 354.— hijs. Gcscllsch, Wurzburg, Neuc Folge, Tom. IV. MALACOSTEACA. 447 latter pierce the body of the host, and carry nourishment to the parasite. Mantle saccular, and without calcareous plates, with narrow aperture which can be closed. Mouth and alimentary canal absent. The testes are usually paired, lie between the ovaries, and open into the brood-pouch. The Bhizoceplmla live principally as parasites on the abdomen of the Decapoda, and wind their root-like filaments around the viscera of the latter. Fam. Peltogastridae. Peltogadcr inifjnri Eathke. Saccidiiia earclni Tliomps,, Lcrn, intestine ; S, ehell G, vas deferens. * Nelalia is best placed in a special group, Leptostraca, between the Entomos- traca and Malacostraca. The paljEozoic fossil genera Hymenocaris, Pdtocaris, etc., would have to be placed in such a group. ARTKROSTRACA. 4V\ Ja^es closely resemble the typical Phyllopod limb. As a rule, how- ever, some of the anterior thoracic legs take part in preparing the food and have a form intermediate between maxilla3 and thoi-acic legs. Such are called foot-jaivs or viaxilli2)eds. In the Arthrostraca the anterior pair of thoracic appendages only are so modified, and the segment bearing them joins the head ; the thorax is, therefore, in this group composed of seven segments, each with its pair of appen- dages. In other groups of Malacostraca the next or two next pairs of thoracic legs have the form of maxillipeds, so that there is no sharp division between the head and thorax. The latter is, at least partially, covered by a shield-like reduplicature of the skin, Avhich morphologically corresponds to the Phyllopod shell and forms a more or less extensive carapace, which fuses with the back of the thorax, and under which the posterior, rarely all the thoracic seg- ments may remain separate as free rings. Order 1. — Arthrostraca.* Malacostraca vntK lateral sessile eyes, usually %vith seven, 'more rarely with six or fewer separate thoracic segments, and the same number of jxiirs of legs. Without a redujMcature of the skin. The head bears four antennae, the two mandibles, four maxillae, and a pair of maxillipeds ; in all six pairs of appendages. A small bilobed plate, distinguished as the under-lip, behind the pair of mandibles, marks the boundary of the primary region of the head. The two pairs of maxillae as well as the maxillipeds are secondary cephalic appendages derived from the thoracic region of the body. Behind the head there are usually seven free thoracic rings with the same number of pairs of appendages, which are adapted for creeping or swimming. The number of distinct thoracic segments is in rare cases reduced to six {Tanais) or five (^Anceus), the anterior or the two anterior segments of the thorax becoming intimately con- nected with the head. In the latter case a more or less extensive cephalothoracic carapace is formed. The abdomen which follows the thorax includes, as a rule, six segments bearing limbs, and a simple or split plate without appendages and representing the terminal segment. The number of the abdominal segments and appendages may, however, be reduced (Jsopoda), and the entire abdomen may * Besides the works of Latreille, M. Edwards, Dana, and others, compare Spence Bate and J. 0. Westwood, " A History of the British sessile-eyed Crustacea," Tom. I. and II., London, 186.3-1868. G. O. Sars, " Histoire naturelle des Crustaces d'eau douce de Norvege," Christiania, 18G7. 29 450 CErSTACEA. even be reduced to an unsegmented stump-shaped appendage [LcBmodijyoda). The nervous system consists of a cerebral ganglion and a ventral gan- glionic chain, which is most distinctly composed of two lateral halves. In the Isojyoda there is also an unpaired visceral nerve. The two eyes are always sessile, compound eyes, with smooth or facetted cornea; they are never stalked. Delicate olfactory fibres are often present on the anterior antennre, and are especially numerous in the male sex. The alimentary canal begins with a short oesophagus, which passes iipwards to open into a wide crop, supported by firm horny bands and often armed with strong chitinous plates. The crop leads into a long intestine provided with two or three pairs of tubular hepatic glands. The rectum, which may possess one or two tubular appen- dages (probably urinary), opens at the posterior end of the body. The antennal gland opens on the basal segment of the posterior antenna, often upon a conical protuberance. Vascular system. — A heart is always present as the central organ of the circulation. It may either have the form of a tube extending along the whole length of the thorax (Amphijjoda) ; or it may be saccular and placed in the abdomen [Isojwda). In the first case the gills are placed on the thoracic feet as tubular appendages ; in the latter, on the other hand, they are placed on the abdomen. From the heart the blood passes through an anterior and posterior aorta, and usually through lateral arteries. The vessels conduct the blood into the body cavity, whence it returns in regular streams to the lateral paired slits of the heart. Generative organs. — The Arthrostraca are of separate sexes. The males are frequently distinguished from the females by the modifica- tion of certain parts of the appendages to form prehensile organs, by a greater development of olfactory hairs on the anterior antennae, and by the position of the sexual and copulatory organs. It is rare to find a strongly marked dimorphism of the sexes {Bopyrus, Praniza). The generative organs open either at the posterior part of the thorax or at the base of the abdomen ; the female always on the ante- penultimate pair, the male on the last pair of the thoracic appen- dages or between the first of the abdomen (Iso2}oda). The ovaries are two simple or branched tubes with the same number of oviducts. The testes similarly seem to be composed of one (Amj^hij^oda) or more (3) pairs of tubes (Isojjoda), the efferent ducts of which (vasa deferentia) either remain separate or unite to form a copulatory AMPniPODA. 461 organ. Appendages of the legs may also be present as additional aids to copulation. The mature ova are, as a rule, carried about by the female in brood pouches formed by the lamellar appendages of the thoracic feet (oostegites). Development as a rule takes place •without metamorphosis, but the form and appendages of the youno- animal not unfrequently difter from those of the adult animal (Phronima). The segments and the appendages may even be incom- plete in number after birth [Isojwda). Fossil Arthrostraca are found in the Oolite {Archceoniscus). Pro- soponiscus occurs in the Permian, Am^^Juj^eltis in the Devonian. 1. Sub order. — Amphipoda.* Arthrostraca with laterally comioressed body, toith gills on the thoracic feet, and an elongated abdomen, of lohich the three anterior segments bear the swimming feet, while the three ^)05ie?'io?' hear j)Osteriorhj di- rected feet adapted for springing (fig. 35 G). The Am2)hipoda are small animals, being only in rare cases several inches long [Lysianassa magellanica). They move in the water principally by spring- ing and by swim- ming. The head, which is sometimes small {Crevettina, fig. 356), sometimes large and then much swollen {Ilyperina, fig. 357), is sharply distinct from the thorax and is fused with the first of the seven thoracic segments only in the aberrant group of the Lcemodipoda. The two pairs of antennae usually consist of a short strong shaft * Besides the older works of Dc Geer, Rosel, M. Edwards, etc.. compare C. Spence Bate, " On the Morphology of some Amphipoda of the Division Hyper- iua," Ann. of JVat. Hlxt.. Ser. 2, vol. xix., 18.57. C. Spence Bate, "On the nidification of Crustacea," Ann. of JVat. Hist., Ser. 3, vol. i. C. Spence Bate. '• Catalogue of the specimens of Amphipodous Crustacea in the collection of the British Museum," London, 18G2. E. van Beneden ct Em Besscls, "' Memoire ,-ur la formation du Blastoderme chez les Amphipodes, etc," Bruxelles, 1868. 0. Claus, " Der Organismus dcr Phronimiden, Arheiten avz dcm Zool. Institvt. der L'nivertitdt Wicn, Tom II.. 1879. Fig. 350. — Oammarut iirglecfiis (after G. O. Sars), with ee;;* between the brood l.imella? (oostegites) on the thorax. A', A", the two antenna; ; JCf, maxilliped ; F' to J^", the seven pairs of thoracic appendages ; Sf, the first swim- ming foot of the abdomen. 452 CKUSTACEA, and a long multiarticulate flagellxim, which, however, may bo more or less rudimentary. The anterior antennae, which are always longer in the male, often bear a short accessory flagellum and present numerous modifications in their special form. In the Hyperina they are very short in the female ; Avhile in the male they are of consider- able length and are closely beset with olfactory hairs. The posterior antennae are frequently longer than the anterior : in the male I'yphidcB they are folded in a zigzag fashion, and in the Corophiidm a, 857. — Phrommit sed-cnfaria, a, female; I, male. O, eyes ; A', A", the two pnirs of tin tenn» ; Kf, jaws ; D, intestine ; II, heart and aorta; K, gills ; Ov, ovary ; N, nervons system ; Br, glands in the chela of the fifth pair of legs ; O, genital opening. are modified to form strong pediform appendages. In the female, on the contrary, they may be degenerated and represented only by the basal joint [Phronima) (fig. 357, a and h). The mandibles are powerful biting plates \i\i\\ a sharp, usually toothed edge and a lower masticating process. They usually possess a three-jointed palp, which is occasionally reduced. The anterior bi- AMPniPODA. 453 lobeJ maxillae also have as a rule a short, two-jointed palp, while the maxillae of the second pair are reduced to two lamellae of considerable size attached to a common base. The maxillipeds fuse to form a sort of underlip, which is either tri-lobed (Ili/perhia) or bears upon a com- mon basal portion an internal and external pair of lamellte, of which the latter may be considered as the basal joint of a large multiar- ticulate and frequently pediform palp {Crevettina and Lcemodijwda). Delicate lamellre or tubes, which are attached to the coxal joints of the thoracic legs, function as gills ; the active movements of the abdominal swimming feet cause a constant renewal of the water around them. In the female there are in addition to the gills lamellar plates (oostegites), which are applied together under the thorax to form a brood-pouch. The males are distinguished from the females not only by the absence of the oostegites, but chiefly by the stronger development of the prehensile hooks on the anterior thoracic feet and the different formation of the antennae. The eggs pass into the brood-pouch and there develop. The yolk sometimes [G. locusta and other marine species) undergoes a com- plete segmentation. Sometimes (G. pulex), after a superficial seg- mentation, a peripheral cell-layer is separated, which develops into a delicate blastoderm beneath the egg membrane. A ventral primitive streak is then formed, and on the dorsal side, beneath a differentiation which has been erroneously taken for a micropyle, a peculiar globular organ makes its appearance ; this is the first rudi- ment of the cervical gland {dorsal organ), which is confined to em- bryonic life. The appendages are developed from before backwards on the ventrally flexed body of the embryo. The young animals usually possess at hatching all their appendages and in all essential points have the structure of the adult animal, but the number of joints of the antennae and the special form of the legs still present differences. In the Ilyperina alone the just hatched young may be without abdominal feet and differ so much in their form from the adult that they may be said to undergo a metamorphosis. The Amphijwda for the most part live in fresh and salt water and lead an independent life (the presence of Arctic species in the Swedish and Norwegian seas is very interesting). Some, however, live in tubes (Cerajjus), others in holes gnawed in wood [Chelura). The large size of the deep-sea forms is of special interest ; amongst these a Gammarid, allied to the genus Iphimedia, and Cystosovia Neptuni (ITyperidce) become several inches in length. The Ilyperina 454 CKUSTACEA. live principally in, transparent marine animals, especially in 3fedtt8ce, and may, as the female Phronima sedentaria, take up their abode ■\\ith their entire brood in transparent Pyrosoma, whose internal parts they eat up. The Cyamidce among the Lcemodipoda are parasitic on the skin of whales. Tribe 1. — Laemodipoda. Am.'pMpoda loith cerviccdly placed anterior lecjs and rudimentary apodal abdomen. The anterior thoracic segment is more or less closely fused with the head and the anterior pair of legs shifted on to the neck. The maxillipeds are modified to form a quadi-ipartite under-lip with long palps. The branchiie are usually confined to the thii-d and fourth thoracic segmerits, the legs of which are often rudimentary or are altogether wanting. The feet end with hooks for attachment. The abdomen is small and reduced to a short protuberance destitute of appendages. Caprella linearis L. Body elongated and thin. They are parasitic on Hydroids and colonies of Bryozoa. Cijaitms ceti L. Body broad and flat ; abdomen quite rudimentary ; parasitic on the skin of Cetacea. Tribe 2. — Crevettina. Ampliipoda loith small head, small eyes, and multiarticidate pediform 'maxillip>eds. Both pairs of antennae are long and multiarticulate ; in the male they are larger than in the female. The upper or anterior antennre are usually, as in Gammarus, the longer ; their shaft is composed of several joints and bears a small accessory flagellum as well as the principal one. The contrary may, however, occur, as in Corophium, where the postei-ior antennae are elongated and pediform. The maxillipeds in all cases fuse together at their base and form a large under-lip, usually with four lamellae and two jointed pediform palps. The coxal joints of the thoracic legs have the form of broad and large ejnmeral plates. The abdomen has always the full number of segments. The three posterior pairs of abdominal feet (uropoda) are well developed and often much elongated. This group, which includes an astonishing variety of forms, is principally distributed in the colder seas. Fam. Corophiidae. The body is not laterally compressed. The post>erior antennre are more or less pediform. The coxal joints of the legs are frequently very small They move rather by -walking, Corophium Jongicorne Fabr., dig AMPHIPODA. 455 passages in mud. Cerapiis tnhularis Say., lives in tubes. Pudoccrus variegatus Leach., English coast. Chclura terebrans Phil, is allied here, gnaws, with Luinwria h'f/nonnn, wood-work in the sea. North Sea and Mediterranean. Fam. Orchestiidae. Anterior antennte usually short, always %vithout accessory ramus. The posterior pair of uropoda are unbranched and are shorter than the preceding pairs. They live on the shore, especially on sandy beaches, and move by springing. Talitrus saltator Mont. = 71 locnsta Latr. On the sandy coasts of Europe. Orchestia littorea Mont., North Sea. Fam. Gammaridse. The anterior antennce often have a second ramus, which is always longer than the shaft of the posterior. The coxal plates of the four anterior pairs of legs are very broad. They move more by swimming than by springing. Gammarii.tjmlcx L., G.Jluvlat'dis Kos., G. marinus Leach. In the blind Mphargus Schiodte the crystalline cones and eye pigment are wanting. K. puteanns Koch., in deep springs and lakes (Lake of Geneva). Lysianassa Costce Edw., Mediterranean. L. atlantica Edw. L. magellanica Lillj. Tribe 3. — Hyperina. A7nph{])oda with large swollen head and large eyes, usually divided into frontal and lateral eyes. They have a 2xdr of rudimentary niaxillipeds functioning as underlip. The antennse are sometimes short and rudimentary, sometimes of considerable size, and in the male are elongated into a multiarticulate flagellum {Hyperidce). The posterior antennse may in the female be reduced to the basal joint enclosing the glandular tube [Phromina) ; in the male, on the contrary, they are folded in a zigzag, after the manner of a carpenter's rule {Platyscelince). A paired auditory vesicle may be present above the brain {Oxycephalus, Rhahdosoma\ The maxillipeds form a small bi- or tri-lobed under-lip. The paired legs end in some cases in a powerful chela. The caudal styles are sometimes lamellar and fin-like, sometimes styliform. Development takes place by metamorphosis. They live principally in jelly-fish, and swim very rapidly. Fam. Hyperidae. Head globular, almost entirely occupied by the eyes. The two pairs of antennae have a multiarticulate shaft ; the flagellum longer in the male. The mandible has a three-jointed palp. The fifth pair of feet is gener- ally formed like the sixth and seventh, with claw-like terminal joint. Ilgpcria {Lestrigonus Edw.) medusarum O. Fr. Miill. (Zf. galha Mont. = if. Latreilll Edw.) with Lestrigonus exulans Kr. as male, North Seas. Fam. Phronimidae. Head large, with projecting rostrum and large divided eye. The anterior antennae are short in the female, with only two or three joints, in the male with long multiarticulate flagellum and a shaft closely beset with olfactory hairs. The thoracic limbs have in some cases powerful chelae. Plirosina nicceensis Edw., Phronima sedentaria Forsk. The female lives with its offspring in Pi/rosoma and B'qjhyidce, Mediterranean. Fam. Platyscelidae. Both pairs of antennae hidden beneath the head ; the anterior arc small ; in the male with much swollen bushy shaft, and short, 456 CEUSTACEA. slender flaofellum composed of few joints. The posterior antennae are in the male very long and folded three to four times together in a zigzag fashion ; in the female they are short and straight, sometimes quite reduced. The basal joints of the fifth and sixth pairs of legs are usually enlarged into great lamellae, which cover the thorax. The seventh pair is generally rudimentary. EutyjJhh (TyjjJtis Eisso) ovoidcs Kisso (Platijscdus serratus Sp. Bate), Mediter- raneaa. Ojcycrjjluilus piscator Edw., Indian Ocean. 2. Sub-0-der : — Isopoda.* Arthrostraca with usually broad, more or less arched body, with seven free tho- racic rings, with lamellar legs function- ing as hranchicB on the short-ringed, often reduced abdometi. The structure of the body, which is flat in shape and covered by a hard, usually encrusted integument, presents a great agreement ^^^.th that of the Ani2)hipoda, to which the in many respects peculiar Tanaidce are most nearly allied. The abdomen of the Isopods is, however, usually much short- ened and composed of six short seg- ments, which are often fused with one another ; it terminates with a large c ludal lamella. The abdominal legs are only exceptionally (Tanaidce) swimming feet ; as a rule they have the form of branchial lamellae. The sixth pair may be fin-like or styliform. The anterior antennae are, with a few exceptions, shoi-ter than the posterior and external antennae ; in rare cases (Oniscidce) they bacome so much reduced that they are Fig. %Z9.—A>enu> aquaiicuH (after hidden beneath the cephalic carapace. G. O. Sars). Female with brood , • i i / ^ 7 \ pouch, seen from the ventral side- In exceptional cases Only {Ajiseudes) * H. Eathkc, " Untersuchungen iiber die Bildung und Entwickelung der V.'asserassel," Leipzig, 1832. LerebouUet. "Sur les Crustac^s de la famille ("es Cloportides, etc," 3/em. du Muxeum dliitst.iiat. de Strasl/oiirff, Tom. IV., IS'.O. N. Wagner, " Recherches sur le systume circulatoirc ct les organes de la respiration chez le Porcellion 61argi," Ann. des sc. nat., Ser. 5, Tom. IV., 186.5. A. Dohrn, " Die Embryonalentwickelung des Asellus aquaticus," Zeltschr fiir n-iss. Zool., Tom. XVII., 1867. N. Bobretzky, '• Zur Embryologie des Oniscus murarius," ^extKchr. fiir n-\.%i. ZooL, Tom. XXIV., 1874. 457 they bear two flagella. As in the Amphipoda, pale, plumous setae and olfactory cones are present on the antennae. The mouth parts are in some parasitic Jsopoda modified for piercing and sucking. The mandibles (except in Bopyridce and Oniscidce) often bear a three- jointed palp. On the other hand, the two pairs of maxillse, which are usually bi- or tri-lobed, are in general without the palpiform appendage. Tlie maxillipeds form a sort of underlip, but present great diflerences in the arrangement of their parts (fig. 358). As a rule the seven pairs of thoracic legs are adapted for walking or attachment, and in the female some of them are provided with delicate membranous plates (oostegites) which form a brood pouch. They never bear gills. The branchial function is dis- charged by the delicate inter- nal rami or endopodites of the abdominal limbs (pleo- pods), the anterior pair of which is frequently modified to form a large operculum overlying the following pairs. In certain of the terrestrial Isopods (Forcellio and Arma- dillo) the opercular plates of the two anterior pairs of abdominal limbs contain a system of air spaces which ap- pear to assist respiration. The heart, unlike that in Amphi- pods, lies (except in Tandidce) in the posterior thoracic seg- ments or in the abdomen. The sexes are (except in C ymothoidai) separate, and the position and arrangement of the generative organs correspond in general with those of the Amphipoda. The sexes are distinguished by external sexual characters, which in some cases {Bopyridce) may lead to a strongly-marked dimorphism (fig. 359, a, b). In the male three tubular testes unite on either side to form a dilated seminal vesicle, from which the vasa deferentia are given off. The latter are frequently separate along their whole length and, at the end of the last thoracic segment, each of them enters a cylindrical appendage Fig. 359. — Gi/ge hranch'.alis (after Comalia and Pancori). a, Female seen from the ventral side ; Brl, oostegite ; K, branchifp. h. Abdomen of the same strongly magnified, with adhering male. 453 CEUSTACEA. {Asdlus) or they unite together into a common median penis which lies at the base of tlie abdomen (Oniscidce). A pair of styliform or complicated, hook-bearing apj^endages of the anterior abdominal feet are to be looked upon as accessory copulatory organs ; in addition to these a paii* of outwardly turned chitinous rods on the inner side of the second pair of feet may also be present (Oniscidce). The Cymothoidce are hermaphrodite * (Bullar), but the sexual organs become ripe at different times. In the young stage these animals function as males, and possess three pairs of testes, two rudimentary ovaries internal to the testes, and a paired copulatory organ into which the two vasa deferentia ^ <^ open (fig. 360). After a subse- quent ecdysis and after the fe- laale glands have developed at the expense of the gradually diminishing male glands, the oostegites, which in the meantime have been developed, become free on the thoracic legs and the copu- latory organs are thrown off. Henceforward the animal func- tions only as a female. The embryonic development begins after the entry of the eggs into the brood pouch and is in- troduced by a centro-lecithal seg- mentation, the central part of the egg (food yolk) remaining at first unsegmented. The blasto- derm soon consists of a periphe- ral layer of naked nucleated cells and produces by a rapid growth of its constituent cells the ventrally placed germinal bands, at the anterior end of which the cephalic lobes are first marked off. The rudiments of the trifoliate appendages (dorsal organ) of the Isopod embryos are next formed as two prominences on the cephalic lobes. The physiological and morphological meaning of these structures has not yet been explained. Of the appendages the two pairs of antennae • J. Bullar, " The s:enerative organs of the Parasitic Isopoda," Jovrn. Anat. Physiol., 1876. P. Mayer, " Ueber den Hermaphroditismus einiger Isopoden," Mittheil. aus der Zool. Stat. Nuapel, 1879. Fig. 300.- o, f emale of Cymothoa £uni»»(af ter M. Edwards). Brl, oosiegite. 6, Sexual organs from a Cymothoa tEstridet, 13 mm. in lenjrth (after P. Mayer). T, The three testes ; Ob, ovary ; Od, oviduct ; Vd, vas deferens ; P, penis. 459 are the first formed. After these have made their appearance, a new cuticle, the larval skin corresponding to the Nauplius stage, is formed (as also is the case in Liyia according to Fr. Miiller), While the other appendages are successively developed, the caudal region of the embryo becomes bent towards the dorsal surface. Of the embryonic membranes the chorion is the first to disappear, then the cuticle of the blastoderm, and finally, when the embryo is fully developed, the Kauplius skin. The young animals, when they become free in the brood-chamber (fig. 361), are still Avithout the last pair of thoracic legs; in the TandidcB the abdominal feet are also wanting. They undergo not inconsiderable changes in the form of the appendages until the attainment of sexual maturity. The Isopoda may therefore be said to undergo a metamorphosis which is most complete in Ta- nais, Praniza (Anceus) and the Bopyridce. The Isopoda live some in the sea, some in fresh waters, and some on land {Oniscidce). They nourish themselves on animal matters ; many of them are para- sitic (seldom complete endopara- sites, Entoniscus) principally on the skin and in the buccal and branchial cavities of fishes {Cy- motJioidce) or in the branchial cavity of prawns {Bojyyridce). Tribe 1. — Anisopoda.* Body more or less resemblinrj that of an Amphijwd. The abdomen ivith biramoits swimvmig feet (Tanais), ivhich do not function as gills, or with fin-like feet (Anceus). Fam. Tanaidae. Tanais duhlus Kr., Brazil. Two kinds of males, "smellers and claspers." T. gracilis Kr., Spitzbergen. Fam. Pranizidae, Anceidse. Anceus maxillaris Mont. (^Pr. cwruleata Dcsm.), North and West coasts of Europe. * Compare Spence Bate, " On Praniza and Anceus, etc," Ann. of Kat. Hist., Ser. 3, Vol. II., 1858. Hesse, •' Memoire sur Ics Pranizes et les Ancles." Ann. d. Scien. Nat., Scr. IV., Tom IX., 1864. Fr. Muller, » Ueber den Bau der Sclieerenasseln," ArcTirv. fiir Natiirgcsch, Tom XXX., 186-1. A. Dohrn, *• Entwickelung und Organisation von Praniza maxillaris sovvie zur Kcfnntniss des Baues von Paranthura costana " Zvitschr. fiir vrlss. Zool,, Tom. XX., 1870. FlQ. 361.— Larva of Bopyrm vlrbii, with sis pairs of thoracic less (after E. Walz)- Ul, Under lip ; Ab^, first abdominal seg- ment ; A', A", two pairs of antennffi; Mdb. mandible. 460 CEUSTACEA, Trihe 2.— Euispoda. Body lolth seven free tlioracic segments and as many pairs of appendages. Abdomen relatively short and broad, with abdominal feet modified to form branchial lamellce. Fam. Cymothoidae. With biting and sucking mouth parts, broad abdomen, with short segments and shield-like caudal plate. The last maxillipeds in the form of an operculum. They live partly as parasites on fish, and partly as fi'ee-living animals. Cijmothoa w-ttrum Leach., C. ccxtroides Risso, Mediter- ranean. Anilocra medlterranca, Leach., JSga licarmata Leach., Serolis paradoxa Fabr. Fam. Sphaeromidae. Free-living Isopoda with broad head and short, very convex body, which can often be rolled up in a ball towards the ventral side. Sphferoma fossaritm Mont., in the Pontine marshes ; nearly allied is the S. granulatuni of the Mediterranean. S.scrratum Fabr., Ocean and Mediterranean. It also lives in brackish water. Fam. Idoteidae. Free-living Isopoda Avith elongated body, biting mouth parts, and a long caudal shield formed of several segments fused together. The last pair of abdominal feet is modified to form a wing-shaped operculum for the protection of the preceding branchial feet. Idotca entomnn L., Baltic. Fam. Asellidae. Body flattened ; the last pair of abdominal feet (pleopods) are styliform (not shaped like an operculum). Jara albifrons Mont., British seas. Ascllus aqiiatlcns L., fresh- water form. A. cavaticus Schiodte, in deep springs, Limnoria terebrans Leach. L. lignorum, gnaws wood-work in the sea. Fam. Bopyridae. Parasitic in the branchial chamber of prawns ; the body of the female is disc-shaped, unsymmetrical, and without eyes. The males are very small and elongated, with distinctly separated segments and eyes. Bopyrus squillarum Batr., on Palcemon squllla. Here are allied the Entonhcldoe, which are parasitic in the body cavity of other Crustacea (^Clrrtpcdia, Pagiirida, and Crabs), Cri/pfonis:ciis planarioides Fr. Miill., parasitic on SaccuU>ia j)urpurea of a Pai/unis, Brazil. Cr. pygmceii.'s Rathke, parasitic on Peltogaster. Entoniscus PorceUanai Fr. Miill., lives between the heart and the intestine of a species of Porcellana in Brazil. Fam. Oniscidae. Land Isopods. Only the internal lamellje (endopodites) of the abdominal feet are modified to form delicate branchiaj, the exopalites constituting firm opercula. The two anterior abdominal feet are sometimes provided with air chambers. The mandibles are without palps. They live mostly in damp places on land, Ligla oceanica L., on stones and rocks on the sea coast. Oniscits murarius Cuv., Porcellio scahcr Leach., Armadillo vulgaris Latr,, A. officinarum Brdt, Order 2, — Thoracostraca.* Malacostraca with compound eyes which are usually p>l(f'Ced on movable stalks, with a dorsal shield which connects all or at least the anterior tlioracic segments with the head. * Besides the larger works of Herbst, M. Edwards, Dana, and the essays of Duvernoy, Audouin and M. Edwards, Joly, Couch, etc. compare Leach, XUOEACOSTltACA. 461 The Thoracostraca, like the Arthrostraca, possess a cephalo-thorax composed of thirteen segments and an abdomen composed of six segments, as well as a caudal plate (telson) ; but the body is stouter and adapted to a more perfect locomotion and a higher grade of life. The thorax, instead of being composed of seven distinctly separate segments, is covered by a dorsal carapace which eftects a firm and intimate fusion between the head and thorax. The degrees of development of this dorsal carapace are various. When most highly developed, it forms the dorsal integument of the anterior or of almost all the thoracic segments ; and its lateral portions only, which have the form of wings and are bent towards the ventral surface, consist of a free reduplieature. The application of the appendages differs from that in the Arthrostraca, and, indeed, varies in the different groups of the Thoracostraca. The cephalothorax has thirteen pairs, and the abdomen seven. The facetted eyes are born on two movably separated r>talks. These were for a long time considered as the anterior pair of appendages, while in fact they are merely lateral portions of the head which have become jointed. Both pau-s of antennae belong to the anterior region of the head. The anterior antennaj or antennules as a rule bear on a common shaft two or three fiacjdla — as the peripheral multiarticulate filaments are called — and are pre-eminently sense organs. In the JJecapoda the aiiditory vesicles are placed in the basal joint, and on one of the fiagella there are delicate hairs and fibres, which are in connection with nerves and are to be looked on as olfactory organs. The second antennae are attached externally to and somewhat beneath the antennules. They bear a long flagellum and in the macrurous Decapoda are often provided with a more or less considerable scale. A gland (the green or antennal gland) usually opens on a conical process of their basal joint. The following thi^ee pairs of appendages function as jaws; the powerful mandibles, which are furnished with palps, lie at the side of the upper lip ; further backwards are the two pairs of lobed maxillse, in front of which and behind the mouth is the small bilobed underlip. The following eight pairs of appendages present a very " Malacostraca podophthalma Britaimise," London, 1817 — 1821. V. Thompson, " On the metamorphosis of Decapodous Crustacea," Zool. Journ., vol. ii., 1831, also Zm, 1834, 1836, 1838. H. Rathke, " Untersuchunscn liber die Bildung und die Entwickelung des Flusskrebses,'' Leipzig, 1829. Th. Bell, "A history of the British stalk-eyed Crustacea," London. 1853. Lereboullct, •' Ucchcrchcs d'embryologie comparee sur le developpement du Brochet, de la Perche et de rEcrevisse," Paris. 1862. V. Hensen, •'■ Studien iiber das Gchororgan dcr Decapoden,"' Leipzig, 1863. 462 CEL'STACrA. different form and adaptation in the various groups. As a rule, the anterior pairs are modified to assist in taking up food and are moved nearer the mouth ; these are the maxilHpeds, which, with regard to their structure, hold an intermediate position between jaws and feet. In the Decapoda (fig. 362) three pairs of appendages have the form Fig. 362.— Male and female of Aatacut fluvlatilii seen from the ventral side. In the male the ambulatory and abdominal feet of the left side have been removed ; in the female the am- bulatory feet of the right side and the maxilHpeds of both sides. A' antennules ; A", antenna! ; PI, scale of antenna ; Md, mandible with palp ; Ifx', Mx", first and second maxillse 2IxP to jixj/% the three pairs of maxillipeds ; Goe, Rcnital opening ; Doe, opening of the green gland ; F\ F", first and second abdominal foot ; Ov, eggs ; A, anus. of maxillipeds, so that there are only five pairs of legs left on the thorax. In the Stomatopoda the first five pairs of thoracic append- ages are modified to form maxillipeds and there are only three pairs TnORACOSTEACA. 463 of biramous swimming feet, which arise from the three posterior free segments of the thorax. The thoracic legs are either, at least in part, biramous (with summing ramus), or as in the Decapods the exopodite is absent and the legs have the form of ambulatory appendages. They then terminate with simple claws ; the anterior frequently with lai-ge chelae. The terminal joints may however be broad plates, in Avhich case they can be used as swimming feet. The biramous legs of the sixth abdominal segment are, as a rule, broad and fin-like and form, together \vith the last abdominal segment Avhich is transformed into a large plate (telson), the caudal fin. The feet of the five anterior abdominal segments, on the other hand, are sometimes swimming feet {Stomatopoda), sometimes serve to carry the eggs, or the anterior may assist in copulation (in the male). They may however be more or less rudimentary and some of them absent. "With rare excep- tions [Mysidce) all the Thoracostraca possess gills, which are either tufted or composed of regular lancet-shaped leaves. The gills are appen- dages of the limbs : in the Stomator)oda , ii 1 J J. ^'°- Sl^a.— Cephalothorax of Agfacusjluviafilie, after removal they are attached to of the branchiostegite (after Huxley). K, Gills ; S, ros- the abdominal feet in ^"^^ > ^' stalked eye ; Mp, scaphognathite (of the second ' maxilla) ; Mxf", third maxilliped. the Scliizopoda and Decapoda to the maxillipeds and ambulatory feet. The Cumacea are -without gills, except for a single pair on the second pair of maxil- lipeds. In the Decapods they are contained in a special branchial chamber beneath lateral expansions of the carapace (bi-anchiostegite) (fig. 363). The organs of circulation also attain a high degi^ee of development, the highest not only among the Crustacea, but in general amongst all Arthropods. A heart and vessels are always pi-esent. In the Stomatopoda the heart has the form of an elongated tube, which extends through the thorax and abdomen, possesses numerous paired slits, and in addition to an anterior and a posterior aorta gives off to the light and left several branching arterial trunks. In the Cumacea, Schizopoda and Decapoda the heart has a saccular form and lies in the posterior region of the cephalo-thorax. More rarely, 464 CBUSTACEA. as in the youngest larvae of the Deccqwda, only one pair of slits is present and the arterial system has but few branches. In the fully- developed Deccqwda the number of paired slits is increased by the addition of a dorsal and a ventral pair, and the vascular system is considerably perfected. An anterior cephalic aorta supplies the brain, the antennae and eyes. Two lateral pairs of arteries send branches to the stomach, liver and generative organs. The posterior abdominal aorta usually divides into a dorsal and a ventral artery, of which the first supplies the muscles of the tail, the latter (knowm as sternal artery) sends branches to the appendages of the thorax and abdomen (fig. 364). From the ramifications (often capillary-like) the blood flows into larger or smaller canals with connective tissue walls which may be regarded as veins, and from thence into a wide blood space situated at the base of the gills. It thence passes through F" F' Fio 364 -Longitudinal section througli Asfaeut fluviatilis (after Huxley). C, Heart; Ae, cephalic aorta ; Aa, abdominal aorta, the sternal artery (Sta) is given oS close to its orit'in- Km, masticatory stomach; D, intestine ; X, liver; T, testis; Vd, vas deferens; Go-rgeAital opening; G, brain; N, ganglionic cord ; Sf, lateral plate of the caudal fin. the gills and, having become arterial, passes into other vascular tracts (branchial veins containing arterial blood), which conduct it to a receptacle surrounding the heart, the pericardial sinus : from the latter the blood enters the heart through the slits which are provided with valves. The alimentary canal consists of a short oesophagus, a wide saccular crop and an elongated intestine which opens by the anus beneath the median plate (telson) of the caudal fin. The wide crop or masticatory stomach is supported by a firm chitinous framework, to which are affixed several pairs of masticatory plates (derived from thickenings of the chitinous lining). In the Decapoda two round concretions of carbonate of lime (Cray-fish) may be deposited in the walls of the masticatory stomach beneath the chitinous lining ; these are the so-called " eyes," and are found in the spring and summer. TnORACOSTHACA. 465 The ducts of the very numerous, iuultilol)ed hepatic cieca open into the anterior part of the elongated intestine. A simple or looped glandular tube (the yreen gland) opens on the basal joint of the posterior antenna. A shell gland is not developed. The nervous system is distinguished by the size of the brain, which is placed far forwards and gives off nerves to the eyes and antennae. The ventral cord, which is connected with the supra- CBSophageal ganglion (brain) by very long commissures, presents very different degrees of concentration. In the brachyurous Decapods this concentration reaches its highest point, all the ganglia being fused together to form one great thoracic ganglionic mass. The system of r/'.sceral nerves is I'lso very highly developed. Sense organs. — The eyes are large and facetted. Except in the Pig. 365.— Generative organs of Agfacus. a. Female ; b, male. Ob. ovaries ; Od, oviduct ; Va, vulva on the basal joint of the third pair of ambulatory legs (-F'"); T, testis; Vd, vas deferens; Oe, genital openings on the basal joint of the fifth pair of ambulatory legs(F'). Cumacea, in which the eyes are sessile, they are borne on movable stalks, which morphologically are to be regarded as the lateral parts of the anterior region of the head which have been segmented off. In the larva a median simple eye, equivalent to the unpaired Ento- mostracan eye, may appear between the stalked facetted eyes. In exceptional cases the adult animal may have paired eyes at the sides of the thoracic appendages, and unpaired eyes between the abdominal feet (Ettphausia). Auditory organs are wanting in the Cumacea and Stomalojjoda. In the Decapoda they are present as vesicles containing otoliths in the basal joint of the anterior antenna, and in many Schizopjoda in the lamellaj of the caudal fin. The delicate 466 CIlTJSTACEiL. filaments and hairs on the surface of the anterior antennie have the vakie of olfactory organs; the antennse function as tactile organs, as do also the palps of the jaws, the maxillipeds and the legs. The generative organs are paired and lie in the thorax or in the abdomen (Stomatopoda), and, as a rule, are connected across the middle line by a median portion. The female organs consist of two ovaries and two oviducts, which open on the basal joint of the antepen- ultimate pair of ambulatory legs or on the sternal region between these appendages (fig. 365, a). The testes (fig. 365, b) are composed of numerous sacs and blind tubes, and, like the ovaries, are connected by a median portion; there are two vasa deferentia, often much coiled, which open on the basal joint of the last pair of ambulatory legs, more rai-ely on the sternum, and occasionally on a special copulatoiy organ {Schi- zojyoda). The first, or the first and second, pair of abdominal feet act as intromittent or- gans. The eggs either pass into a brood-pouch formed by lamellar ap- pendages of the thoracic legs (Cumacea, Schizo- 2)oda), or become at- tached by means of the cementing secretion of special glands to the hairy abdominal feet of the female, where they remain until they are hatched (Decajwda). Development. — Most of the Thoracostraca undergo a metamor- phosis which may be more or less complicated. The Cumacea, some Schizojwda (^Mysidea) and the fresh-water Decaj)oda (Astacus) leave the egg membranes with the full number of segments and appen- dages. All the St07nato2)oda, on the contrary, as well as most of the Decapoda, are hatched as lai-vfe ; the latter in the so-called Zo(ea form with only seven pairs of appendages in the anterior region of the body (there are two pairs of antennae, mandibles, two pairs of maxillse, and two pairs of maxillipeds), without the last six thoracic segments and with a long abdomen destitute of appendages (fig. 366). The two pairs of antennse of the Zocea are short and destitute of flagella. The mandibles are without a palp ; the maxilloe are already Tig. 866.— Crab zooea (Thia), after the first moult. ZS, Zowa spino on the back ; Kf, Kf'', the two pair-s* of biramoua appen(laG;es corresponding to the first and second pairs of maxillipeds. inOEACOSTEACA, 467 lobed and iised as jaws ; the four anterior maxilHijeds are biramous and act as biramous swimming feet ; and behind them, in the macru- rous Decapods, the maxilliped of the third pair also appears as a bii-amous stvimming foot. Gills are as jet wanting, being repre- FiG. 367.— Larva of Penaeus (after Fr. Miiller) . a, Nauplius form seen from the dorsal sur- face, h, Metanauplius stanje seen from the left side; Jlfx', anterior maxilla^; Mx'', pos- terior maxilte; 61, sixth and seventh pairs of appendages or first and second maxilUpeds. c, Zotea stage ; 0, eyes. sented by the thin surfaces of the sides of the eephalo-thoracic sliield, beneath which a continual current of water flowing from behind forwards is kept up. A shox't heart with one or two pairs 4G8 CEUSTACEA. of slits is present. The facetted eyes are of considerable size, but are not stalked. Between the facetted eyes there is in addition an unpaired simple eye, the Entomostracan eye. The Zocea larvse of the short-tailed Decapoda (Crabs) are, as a rule, armed with spinous processes. They usually have one frontal spine, a long, curved dorsal spine, and two lateral spinous processes of the cephalo-thoracic shield. The Zo£ea, however, is not by any means always the earliest larval stage. Passing over those cases in which the larva has the Zoaja form but is without the middle maxillipeds, there are Podopldhal- mata {Pencsus), which leave the egg as Nauplii (Qg. 367). Thus I'.G 308.-a,^o.aof J»arf«. in advanced stage ^vith rndiments of the tbird maxilliped (r/") and tbe five pairs of ambulatory feet >J>Bp); C, heart; L, hver. b. Megalopa stage of ^orlunus ; Ah, abdomen. F' to i'" first to fifth ambulatory legs. the developmental history proves that the series of forms of Ento- mostraca and Malacostraca are continuous. During the growth of the Zotea, the subsequent metamorphosis of which is quite gradual and always different, the six (five) paii-s of thoracic legs, which are as yet absent, sprout out beneath the cephalo-thoracic shield. The abdommal feet also make their appear- ance on the abdomen, and the larvae finally enter the Schizopod-like stage, from which the adult form proceeds. The Crab Zoom, how- eve°r,'after a later ecdysis, enters upon a new larval stage, that of the Megalopa (fig. 368, h) ; in this stage it already presents the cha- lacters of the Bracliyura, but still possesses a large abdomen, which is indeed ventrally flexed, but provided with a caudal fin. CUMACEA. 469 The Thoracostraca are for the most p.art marine, and feed on dead animal matter or capture living prey. Most of them are good swimmers ; others, e.g. numerous species of crabs, walk and run and sometimes move sideways or backwards with great agility. The chela) of the first pair of ambulatory legs (fourth thoracic appendages) constitute po\verful weapons of defence. Besides the frequent ecdyses of the larval stages, the sexually adult animals cast their shell once or several times in the year (Decapoda). They then live A\ath the new and still soft skin for some time in protected hiding-places. Some Brachyura are able to live for a long time in holes in the earth away from the sea. These land crabs undertake, usually at the breeding season, common migrations to the sea and return later to the land with their fully developed offspring {Gecarcinus ruricola). The most ancient fossil Podophthalmia hitherto known are the mac- rurous Decapoda and Schizopoda, from the carboniferous formations {Palceocrangon, Pcdieoccwahus, Pygoceplialus). (1) Sub-order : Cumacea.* Thoracostraca with a small cejyhalo-thoracic shield, {four to) five free thoracic segments, two pairs of 7naxillipeds, and six pai7's of legs, of which at least the two anterior pairs have the biramous Schizopod form. The abdomen is elongated and composed of six segments, and bears, in the nude, two, three or five pairs of swimming feet in addition to the caudal a2)j)endages. The Guiiuicea, the systematic position of which was formerly very differently estimated, have a superficial resemblance to Decapod larvae, which they also recall in the simplicity of their organization ; while in many of their characters, such as the formation of the brood-pouch and their embyronic development, they approach the Arthrostraca. A cephalo-thoracic shield is always present and includes, besides the segments of the head, the anterior thoracic segments and their appendages ; the four or five posterior thoracic segments, however, remain free. The anterior antennae are small and consist of a three-jointed basal portion, to the end of which, especially in the male, tufts of olfactory hairs are attached, and of a short flagellum and secondary flagellum * H, Kroyer, " Fire nye Arter af sltegteu Cuma," Naturh. Tidsshr., Tom III. , 1841. H. Kroyer, " Om Cumaceemes Familie," Naturh. Tid.^glir.'k. R., Tom III., 1846. G. 0, Sars, " Beskrivelse af de paa Fregatten Josefihines Exped. fundne Cumaceer," Stockholm, 1871. A. Dohrn, " Ueber den }>au und die Entwickelung dcr Cumaceen," Jen. naturniss. Zeitschr. Tom V., 1S70. 470 causTACEA. In the female the posterior antennje are short and rudimentary, ■while in the adult male they, together Avith their multiarticulate flagellum, may be as long as the body (as in Nehalia). The upper-lip is usually small, while the deeply cleft iinder-lip is of considerable size. The mandibles are A\-ithout palps, and possess a comb of bristles and a powerful masticatory process below their strongly toothed extremity. The anterior maxillae consist of two toothed blades and a cylindrical, flagellate appendage directed backwards. The unpalped maxilla of the second pair is composed of several pairs of masticatory plates lying one above another. The two following pairs of appendages may be distinguished as maxillipeds. The anterior, which corresponds to the palped under-lip of the Isopoda, is five- jointed and may be recognised by the process of the basal joint ; the posterior, which is also usually five-jointed, is of considerable length and the basal joint is cylindrical and elongated. They also bear the large pinnate gill and a peculiar plate. Of the remaining six pairs of thoracic appendages, the two anterior are always formed like the feet of the ScJdzojyoda ; they consist of a six-jointed leg, the basal joint being strongly developed and lamellar, and of a multiarticulate accessory ramus (exopodite) beset with long swimming setfe. The four last pairs of appendages are also six-jointed, but are shorter ; they bear in many cases, \\4th the invariable exception of the last pair, a larger or smaller swimming appendage as exopodite. The very narrow and elongated abdomen is, in the female, entirely ■nithout swimming feet, but bears on the large sixth segment at the sides of the caudal plate long-stalked biramous caudal styles; while in the male two, three or five pairs of swimming feet may in addition be present on the preceding segments. Fam. Diastylidae. DiantyJis liathJdi Kv., North Sea. JD. Edwardsii Kr. Lcucon nasicus Kr., Norway, (2) Sub-order : Stomatopoda. * Elongated Thoracostraca icith short cephcdo-tlwracic sliidd winch does not cover the thoracic segments. There are five pair of maxilli- 2)eds and three 2}air of hiramous thoracic feet. The swimming feet on the strongly developed abdomen bear branchial tufts. * Besides Dana, M. Edwards and others, compare 0. Fr. Mliller, " Bruch- stUck aus der Entwickelungsgeschichtc der Jlaulfiisser," I. and II., ArcJiiv fiir Katurgc^ch., Tom XXVIII., 1862, and Tom XXIX., 1863. C. Claus, "Die Metamorphose der Squilliden," Abluindl. der Gottinger Socict'dt, 1872. C. Grobben, " Die Geschlechtsorgrane von Squilla mantis," Sitzungshcr. der It, Akad. der Wmenssh., Wien, 1876. STOMATOPODA. 471 The sub-order Stomatopoda, with which formerly the Schizopocla, the genus Le^ccifer and the Phyllosomata (which are now known to be the larvte of Scyllarus and Palinurus) were vinited, is confined at the present day to the small and well-defined group of forms included in the Squillidce. They are Thoracostraca of considerable size and of elongated shape, with a broad, well-developed abdomen, which is much more extensive than the anterior part of the botly and terminates in an extraordinarily large caudal fin. The cephalo-thoracic shield, which is formed of comparatively soft integument, is short and leaves at least the three lai-ge posterior thoracic segments to which the biramous swimming feet belong quite uncovered. The short segments of the maxillipeds also are not fused with the carapace. Appendages. — The anterior part of the head with the eyes and antenna) is movable, and the ventral portions of the following segments covered by the cephalo-thoracic shield are capable of limited movements upon one another (fig. 369). The anterior 4 Fio. ZOO.—SquiUa mantU. A', J", antennse ; itf , ^f, the anterior maxillipeds on the cephalothoras ; B', B", B'", the three pairs of biramous leffs. internal antennse consist of a long three- jointed shaft, bearing three multiarticulate flagella. The second pair of antennse has a large scale on the outer side of the multiarticulate flagellum (fig. 369). The mandibles, which are placed far back, are provided with a slender three-jointed palp. The maxillsB are relatively small and weak. The five following pairs of pediform appendages are crowded together close to the mouth, and on this account have been appro- priately described as oral feet. They all bear at their base a discoidal plate, which, in the case of the two anterior pairs, attains a considerable size. The anterior pair alone (first maxilliped) is slender and palpiform ; it ends, however, in a small chela, which serves to seize the prey. The chela in this and all the other masillipeds of the Stomatopoda is formed by the terminal joint turning back and biting gn the penultimate joint. The maxillipeds 472 CRUSTACEA. of the second pair are by far the largest ; they are moved more or less outwards and are pi'ovided with a very large chela. The three following pairs resemble each other in size and structure, each ending in a smaller rounded chela. Accordingly there remain for locomotion only the three pairs of legs of the last three uncovered thoracic segments ; they have the form of biramous swimming feet. The abdominal swimming feet, however, are much more developed and bear the branchial tufts on their external lamella?. The two sexes are only slightly different. The male is, however, easily to be recognised by the possession of the pair of rods at the base of the last pair of thoracic feet, and also by the slightly modified form of the first pair of abdominal feet. Me tarn orphosis. — The post - embryonic development consists of a complicated metamorphosis, which, unfor- tunately, is as yet not com- pletely known to us. The youngest larva} observed (about 2 mm. long) already possess all the segments of the tho- rax ; but the abdomen, except the caudal plate, is still un- developed. They are thus very different from the Zoaea of the Decapoda. Later larval stages are described as Alima and Erichthus (fig. 370). The Stomato2)oda are found exclusively in the warmer seas. They are excellent swimmers and live by preying on other marine animals. Fam. Squillidae. SqnUla mantu Eond., Sq. Desmarestii Eisso, Adriatic and Mediterranean. (3) Sub-order: Schizopoda.* SttuiU Thoracostraca with large, usually soft ceplialo-thoracic shield and eight pairs of biramous thoracic feet, which are similarly formed and frequently hear freely-projecting gills. * G. O. Sars, " Hist. nat. des Crustac^s d'eau douce de Norv^ge," Chrisliania Fig. 370. — Young AUma larva. Af. Abrlominal feet (pleopods) ; Mxf, anterior maxillipeds ; Mxf, the large maxillipeds (second pair). sciiizoPODA. 473 In their outward appearance the tSc/iizopoda resemble the long- tailed Decapods, inasmuch as they possess an elongated and usually compressed body, a large cephalo-thoracic shield covering the thoracic segments more or less completely and a well-developed abdomen. In the structure of their mnxillipeds and thoracic legs, however, they differ essentially from the Decapods and approach the more advanced larvae of the pra\\Tis, which they also resemble in their simpler internal organization. Further, in all the deep sea forms the cephalo- thoracic shield leaves a greater number of the thoracic segments free (Siriella), and in the early larval stages all the thoracic seg- ments are free as in Nehalia. A larger or smaller number of these free segments subseqviently fuse on the dorsal side with the carapace (Gnathophausia). Appendages. — The first three pairs of thoracic appendages (the homologues of the maxillipeds of the Becapodci) are biramous ambulatory legs and resemble in structure the following thoracic legs, which, by the possession of a multiarticulate setigerous exopodite, are adapted both for swimming and for prodvicing currents in the water. The two anterior pairs, however, show a closer relation to the oral appendages by their shorter and stouter form and by the presence of processes on the basal joint [Mysis, Siriella). The principal ramus (endopodite) of the leg is always relatively slender and ends with a simple weak claw or -with a multiarticulate tarsal flagellum. Rarely {Euplmusia) the two last pau-s of thoracic legs are entirely rvidimentary, except as regards the largely developed bran- chial appendages. The abdominal legs are usually small and delicate in the female, but are strongly developed in the male. Sometimes they are of abnormal size and form (to assist in copula- tion), but only exceptionally (male of Siriella) bear gills. The appendages of the sixth segment, which is usually very much elongated, are always lamellar, biramous structures and form with the telson a powerfvil caudal fin (fig. 371). The inner lamella or endopodite of this pair of limbs frequently contains an auditory vesicle. The differences between the males and females are so great that formerly they were placed in distinct genera. The former possess, on the anterior antennae, a comb- shaped prominence bearing a great number of olfactory hairs ; and, o-\ving to the larger size of the 1867. G. 0. Sars, " Carcinologiske Bidrag til Norges Fauna. Mysider," Christiania, 1870 and 1872. R. v. Willemoes-Svihm, " On some Atlant. Crus- tacea," cf. Trans. Lin. Soc, 1875. 474 CUUSTAC£A. abdominal feet, of whicli the anterior may, moreover, be provided with copulatory appendages, they are capable of a more rapid and perfect locomotion than the females, to which fact corresponds again the greater respiratory requirements and the possession of branchial appendages in Siriella. Development. — The females bear on the two posterior {Mijsis) or at the same time also on the median and anterior (Loji/togaster) paii-s of thoracic limbs lamellffi, which form a brood pouch, in which, as in the Arthrostraca, the large eggs undergo their embrj'onic development. In other cases {Eu]jhaiisia), the development proceeds by meta- morphosis. The young Eit- 2)h.ausia is hatched as a Nau- plius larva, on which the three following pairs of appendages (maxilla; and first maxillipeds) soon appear as small promi- nences. The large carapace of the Nauplius, which is curved forwards round the base of the antennse where it has a serrated edge, is the first rudiment of the cephalo-tho- racic shield, and beneath it, at the sides of the unpaired eye, the rudiments of the late- ral eyes are \'isible. The larva then, having moulted, assumes first the form of the Proto- zoasa and then of the Zosea (described by Dana as Calyp- to'pis), which is however pro- vided with only six pairs of appendages and a long, already fully segmented, apodal abdomen. In the numerous succeeding larval stages {Furcilia, Cyrtopia) the remaining appendages are successively developed. Fam. Mysidss. Mysi?: cjdf/aris Thomps., M.flexuosa 0. Fr. Miill., M. incrnm ■Racthk, Northern seas Siriella Edivardsil Cls. Fio. 371. — Mytit oeiilafa. Female with brood lamellae (after G. O. Sars). Ob, Auditory vesicle. DECAPOD A. 475 Fam. Euphausidae. Euphansia splendcns Dana, Atl. Ocean. Thy-sanoixxJa noriccgica Sars. Fam. Lophogastridae. LopliogaHer typicus Sars, Norway. (4) Sub-order: Decapoda.* Podophthalmia loith large dorsal cephalo-thoracic shield, which is risualhj fused ivlth all the segments of the head and thorax. They have three {two) pairs of maxillipeds and ten {twelve) ambulatory limbs, some of which are armed loith chelce. The head and thorax are completely covered by the dorsal carapace, the lateral expansions of which cover the basal joints of the maxil- lipeds and legs, forming a branchial chamber on either side, in which the gills ai'e concealed. Only the last thoracic segment may retain its independence and be more or less movable. The shell is pro- longed into a frontal spine (the rostrum) between the eyes. The firm, calcified integument of the dorsal carapace presents, especially in the larger forms, symmetrical prominences caused by the sub- jacent internal organs : these may be distinguished as regions and named in accordance with the internal organs. The abdomen presents considerable differences both of size and form throughout the sub-order. In the Macrura it is of considerable size, possesses a hard exoskeleton, and, in addition to the five pairs of feet of whidi the anterior are often aborted in the female, is provided with a large swimming fin (the telson and the pair of large swimming feet of the sixth segment). In the Brachyura the abdomen is without a caudal fin and is reduced to a broad (female) or a narrow triangular (male) plate, which is bent up against the concave sternal surface of the thorax. The abdominal feet also are slender and styliform, and in the male are only developed on the two anterior segments. Appendages. — The anterior antennfe in the Brachyxira are often concealed m lateral pits; they usually arise beneath the movably articulated eye-stalks, and consist of a three-jointed basal portion bearing two or three multiarticulate flagella. The posterior antennre * Herbsfc, "Yersuch einer Naturgeschichtc dcr Krabben und Krcbse," 3 Bdc, Berlin, 1782-1804. Leach. " Malacostraca podophthalma Britannia;," London 1817 to 1821. Th. Bell, " A history of the British stalk-eyed Crustacea," London, 1 858. H. Rathke, " Untersuchungen Uber die Bildung und Entwick- elung dcs Flusskrebses," Leipzig, 1829. Spence Bate, " On the development If Decapod Crustacea," Phil. Tram, of the Boy. Soc, London, 1859. C. Claus, " Zur Kcnntniss der Malacostrakenlarven," M'urzh. naturivix.i. Zeitschr., Tom II., 1861. Fr. Miiller, " Die Vcrwandlung der Garneelcn," Archiv fur Naticrgesch., Tom XIX., 1SC3. Fr. Miiller, " FUr Darwin," Leipzig, 1864. 476 CBUSTACEA. are usually inserted externally and somewhat ventrally to the first pair on a flat plate placed in front of the mouth (ejnstom or oral .shield) : they frequently possess a scale-like lamellar appendage. At their base there is always a protuberance Avith a pore at its end, through which the duct of the antennal gland (green gland) opens. The mandibles vary considerably in shape in the different forms, but have, as a rule, a two or three-jointed palp, which, however, Ls absent in many prawns (Carididse). They are either straight and strongly toothed on their thickened anterior edge {Brachyura), or are slender and much bent [Crangon), or else forked at the ends {Palcemonidce and Aljiheidce). The anterior maxillae always consist of two lamellaj and a palp, which is usually simple. The posterior maxillse, on which there are usually four lamellne {two double lamellfe) as well as palps, bear a large respiratory plate with setose edges (scaphognathite). These are followed by three pairs of maxillipeds, which, as a rule, have a flagellate appendage. There remain, therefore, only a five pairs of thoracic appendages for use as legs; of these the two last are sometimes re- duced or may even be entirely absent (Leuci- /er) as the result of Fig. 372.— Young form (larva) of the lobster (after G. retrogressive changes. O. Sars). fi. rostrum; ^', ^", antennoe; r'". third rpj thoncic secments maxilliped; J" anterior ambulatory leg. ^'^^^ tnoracic segments to which the ambulatory legs belong are, as a rule, all or all but the last fused together and form on the venti-al side a continuous plate, which in all the Brachyura is broad. The legs consist of seven joints, which corre- spond to those of the Arthrostraca, and frequently end with a chela or prehensile hand. Development. — The greater number of marine Decajwda leave the e^g membranes in the zoaea form ; in Homarus, amongst the Macrura, the metamorphosis is much reduced and the just-hatched young possesses all the thoracic legs, which are, however, provided with external swimming rami, but it is still without the abdominal feet (fig. 372). Embryonic development. — In addition to the classical researches of Rathke * on the crayfish, more recent works, especially those of * Besides Kathke 1. c. and LerebouUet 1. c, and a Eussian paper of Bobrctzky, DECAPODA — MACRUKA. 477 Bobretzky (prawns and cray-fish) and Reich enbacli (cray-fisli) have yielded important results. The segmentation seems (in all cases?) to be superficial (centrolecithal), that is, to be confined to the peripheral yolk (formative yolk). This divides successively into two, four, eight, and an increasing number of segmentation cells, while the central granular food yolk, which is rich in oil globules, remains unsegmented. The young of Astacus, when hatched, resemble the adult animal, excepting that the caudal fin is still rudimentary. I. — Mackura. The abdomen is strongly developed and is at least as long as the anterior ji irt of the body; there are four or five pairs of abdominal feet and a broad, well-developed caudal fin. The antennules bear two or three flagella, the antennae have one simple flagellum and frequently bear a scale at the base. The maxillipeds of the third pair arc long and pediform and do not completely cover the pre- ceding ones. The Zoma larva, when hatched, is elongated and has usually three pair of bii-amous feet. Fam. Carididae. Prawns. Body laterally compressed, with a thin shell, which is often provided with a median ridge and prolonged into a saw-like frontal process. The posterior (external) antenuna are inserted beneath the anterior (internal) and have a large scale projecting over the stalk. The long and slender anterior pairs of ambulatory legs frequently end in chelje. They live in shoals near the coast. Some genera QPencBus) possess a rudimentary swimming ramus. Palcemon squilla L., Crangon vulgaris Fabr., Pontonta tyrrhena Eisso, lives between the shells of bivalves. Sergcstes atlanticvs Edw. Fam. Astacidae. Tolerably large, usually with a hard shell. The cephalo- thorax is slightly compressed, the abdomen flattened. The antenn?e are attached near the antennules, and bear a small or quite reduced scale at their base. The first pair of amlnilatory feet ends with large chela;, as do in many cases the weaker and smaller second and third pairs. Some soft-skinned forms bury themselves in the mud or sand. Astacus Jlnviatilis Eond., Crayfish. Ilomarvs vulgaris Bel., Lobster. Ncplirops norwegicus L., Gcbia licach., Thalassina Latr., Calliaiiassa stiUerranea Mont., buries itself in sand on the sea-shore. Fam. Loricata. With very hard, rough armour, and large broad abdomen The antennules end with two short flagella ; all five pairs of ambulatory feet with simple claws. The larvre are described as species of Pliyllosuma. Palinurvs quadriccrnis Latr. Scyllarus latus Latr. Fam. GalatheidsD. With broad, rather large abdomen, and well-developed caudal fin. The first pair of legs is chelate, the last is weak and reduced. Galathca sfrigosa L. Fam. Hippidae. Cephalo-tlioracic shield long ; end of the abdomen curved. The first pair of legs usually with a finger-shaped terminal joint ; the last is Kiew, 1873, compare H. Reichenbach, "Die Embryonalanlage und erste Ent- wickelung des I lusskrcbses," Ztitschr.fur wiss. Zool., Tom XXIX., 1877. 478 CBUSTACEA. weak. Ulppa eremita L., lives buried in the sea sand, Brazil. Alhunea, gymnista Fabr., Mediterranean, Fam. PaguridsB. Hermit crabs. AMomen long, usually covered with a soft skin and distorted, with narrow anal fin and rudimentary abdominal feet. The first pair of feet ends with powerful chelie, the two last are reduced. Some of them seek shelter in empty snail shells, to protect their soft-skinned abdo- minal region. Pagurus Btrnhanhcs L., Ccenohita rugosa Edw., Jiirgus latro Hcrbst, said to climb palm-trees. II. — Braciiyura.' With pits for tlie reception of the short internal (anterior) antennas and so-called orbits, i.e., cavities for the reception of the stalked eyes. Abdomen short and reduced, Avithout caudal fin, curved round against the excavated ventral surface of the thorax ; in the male narrow and pointed, with only one, more rarely two pairs of abdominal feet ; in the female broad, with four pairs of abdominal feet. In the female each oviduct dilates to form a bursa copulatrix. The third pair of maxillipeds have broad flat joints and completely cover the anterior mouth parts. The just-hatched Zocea larvse of stout shape, with only two pairs of biramous feet and a dorsal spine ; later they assume the Megalofa form. Many Brachyiira live on land. Fam. Notopoda. Transitional between the Brachyvra and Macrui'a. The two or four posterior thoracic feet are articulated higher up than the four or three posterior pairs, and shifted on to the back. The first pair of feet has large chelaj, the last is often modified to swimming feet. Porccllana 2}itif]/cJtcli;s Penn, Dromia vuhiaris Edw., Litlwdes. Latr. Fam. Oxystomata. With rounded cephalo-thorax. The frontal region does not project. The buccal frame is triangular. The male genital openings are on the basal joint of the last pair of thoracic legs. Calappa granulata L., Ilia mtcleus Hcrbst, Mediterranean. Fam. Oxyrhyncha. Cephalo-thorax usually triangular, with projecting pointed rostrum. There are nine gills on either side. The male genital opening is on the basal joint of the last pair of thoracic legs. The thoracic ganglia are united into one mass. They do not swim but crawl. InacMs xcorjno Fabr., Miija aquinado Piond., Pisa armata Latr., Stenorhynchus Lam. Fam. Cyclometopa. With broad, short cephalo-thorax, rounded anteriorly. Without projecting frontal rostrum. There are nine gills on either side. The male genital opening is on the basal jointof the last pair of thoracic legs. Some of them are good swimmers. Cancer jfcigurns L., Xantho rimilosus Risso, Mediterranean. Carcimis mcrnan L., Purtunvs puicr L. Fam. Catometopa. Quadrilatera. Cephalo-thorax quadrilateral. Frontal region is curved downwards. There are fewer than nine gills. The male genital openings usually lie on the sternum. Some of them live for a long time away from the water. Some live in holes in the earth, as land crabs. Pinnotheres 2)!-')' strongly their pricking the skin disease known as the itch. The young, when hatched, possess only three pairs of legs and undergo several moults. The domestic animals are infected by different species of Sarcpptidcc, which may be temporarily transferred to man. Dcrmatodcctcs communis Fiirst. Sym- hiotcs eqni Gerl. (fig. 38G). Fam. Tyroglyphidae. Cheese-mites. Of more elongated form, with conical proboscis, chelate chelicerje, and three-jointed pedipalpi. The five-jointed legs are tolerably long, and have lol^s for attachment and claws. Large suckers, especially in the male, are often present at the sides of the anus. They live on animal and vegetable matters. Ti/i-o/jlyphvs siro Gerv. Rhizoglyphiis Fio. 383.— Female of Phyiop- pug vitii, from the leaf of the vine (after H. Lan- dois). Or, Ovaries ; A, anus ; Go, genital opening ; B'", S'", third and fourth pair of legs. majrnified ; £l, pedipalpua. ACAEINA. 493 Rohbii Clap., on roots. Glyciphagvs fccnlarum Gu6r., on potatoes. Hyjyvpvft Dug., according to M6gnin and Kobin, contains larval forms, which attach themselves to insects by their suckers. Fam. Ixodidae. Ticks. Larger usually blood-sucking mites, with strong dorsal shield and large, protrusiblc toothed cheliceras. The pedipalpi are three- or four-joiuted and club-shaped ; their Ijases are joined together to form a a d Fio. ZiZ.—Sarcoptct tcaliei (after Gudden). a, Male from the ventral side, b, Female from the ventral 8ide. c, Female from the dorsal surface, d, Larva. J/, Chelicerse; B'", third pair of legs. proboscis, bearing recurved hooks (fig. 387). The slender legs end with two claws. Two simple eyes are often present. Respiration by tracheae. The Ticks live on the underwood in forests. The females crawl on to Mammalia and Man, suck blood, and become much swollen out. The young, when hatched, have three pairs of legs. In tropical countries the Ticks are of considerable size, and are amongst the most troublesome parasites. Ixodes ricinvs L. I. reduviv.i, 494 AEACHXIDA. Vir,. SSC. Si/mhiof'S rqm - Choriopiet ftpnfhi- fern-i, from ventral side (after Megnin). u, Male ; HG, sucker ; J, young female in copulatory stage ; c, female ready to lay. Fio. 387.— Oral apparatus of JaxxJet (after Al. Pagenstecher). B, Pro- boscis ; Kf, chelicera ; Kt, pcdipalpus ; B' first pair of legs. ACA.KINA rrONOOONIDA. 495 Deg., Argas reflextis, Latr., on Pigeons, occasionally on Man. A, jjrrxicns Fisch. Notorious for its bite. Fain. Gamasidae. Beetle-mites. Chclicerie chelate. Pedipalpi five-jointed. The legs end with two claws and a sucker. Tracheas are present. Some of them lead a free life and are predacious, some are parasitic on Beetles and on the skin of I'irds and Mammals, (t/hiuixvs coleoptratorum L., Dervmyiyssux nvii(i)i, Dug., J'feroptii.'t ri'.ijui-filioni.s Herm. Fam. Hydrachnidae, Water-mites. Body globular, often brightly -coloured. Chcliceraj usually with a claw-like terminal joint. They have swimming legs, and two or four simple eyes. There are tracheiTJ. The larvjc, when hatched, adhere with their large suctorial ccme to aquatic Insects, on the blood of which they live. Hiidraclina cructhfa, 0. Fr. Miill. Atax Jionzi Clap., in the mantle cavity of the Unlox. JAmnocluircs holoKcricens Latr. Fam. Trombidiidae (fig. 388). Body brightly coloured and covered with hairs ; the pedipalpus has a claw and a lobe-like appendage. Eyes present. Respiration by trachece. The hexapodous young live as parasites on Insecta Fig. 389. — Pygnogonum littorale, (rerjno animal) ^4i;, pair of legs used for carrying the eggs. Fig. Zm.—TrnmU,UHm ceuin (after Meguin). and Arachnida, sometimes on Mammalia, and on Man, in whom they (^cisLej)tus autumnalis^ produce a transitory affection of the skin. Trombidium. holose- ricenm L. Erythraus jmrictinus Herm. Teti-a7iychiis tclcarlus L. Spinning mite. Fam. OribatidsB. Chclicera; retractile and chelate. Pedipalpus five-jointed, with toothed biting plate on its basal joint. Ocelli absent. Orihatoi alatiis Harm., under moss. Fam. Bdellidae. The cephalic region is elongated to form a proboscis, and is distinct from the rest of the body. The chelicerse are chelate. The pedipalpi are long and thin. The animals creep about on damp ground, Bdella lonyi- en mis L. PYGNOGONIDA.* Milne Edwards and Krciyer placed the Pygnogonida among the Crustacea ; latterly, however, they have been generally placed between * A. Dohrn, " Die Pantopoden des Golfes von Neapel und der angrenzenden Mecrcsabschnitte," Einc Monographic, Leipzig, 188L 496 ARACnxiD.v. the Mites and Spiders amongst the Arachnida, although they possess a greater number of appendages than either, inasmuch as the males have an accessory pair of legs, used in carrying the eggs (fig, 389, A B). They are small animals with a conical suctorial pi-oboscis and rudimentary abdomen (reduced to a tubercle) ; and they live in the sea, and crawl slowly about amongst the sea-weeds. There are four pau's of very long, many- join ted legs, which contain tubular diver- ticula of the stomach and the sexual glands. There are no trache». On the other hand, there is a well-developed heart >vith an aorta Tig. wo. — Amiiiofhra }>i/rj)inr;i»in'(hf (r^frno nniinnl). T>a, proloncnfions of nliment and several lateral ostia. Above the brain lie four small simple eyes. There is a considerable ventral chain, composed of several ganglia. The eggs are carried about on the accessory pair of legs on the thorax of the male (fig, 389) till the larvpe are hatched, Pynnogomim lit f ovale 0. Fr. Miiller, North Sea. Pho.richiUdiuM Edw., Ammothca Leach, A. jyygnotjonoidcs Quatr. (fig. 390). TARDIGRADA,* The Tardigrada constitute a second group, which is often separated as a distinct order. They are small mite-like Arachnida, and may * Doyere, " M^moire sur les Tartligrades," Ann. dcs Sc. Xat., IP S4r., Tom. XIV., XVII.. XVIII. C. A. S. Schultze, ^' Jracrobiotiis Hufelandii. etc," Berolini 1834, C. ,S, Schultze, "Echiniscus Bcllennanni," Berolini, 18-10. Dujardia, TAUDIGKADA. 497 be defined as hermajihrodite Arachnida with suctorial mouth parts, and short stumjjy legs, loithout heart or respiratory organs. The body of these small, slowly-creeping aquatic animals is elon- gated and vermiform, and prolonged at the anterior extremity into a suctorial tube, from which two styliform jaws can be protruded. The four pairs of legs are short tubercles terminated by several claws (fig. 391); the last pair is placed at the extreme end of the body. The nervous system consists of four ganglia connected by long commissures. The first of these ganglia corresponds to the brain and gives off nerves to two simple eyes and to two sensory papillie. Circulatory and respi- ratory organs are entirely ab- sent. The alimentary canal consists of a muscular pharynx and a stomach beset with short Cfecal diverticula. The ducts of two salivary glands of consider- able size open into the suctorial proboscis (fig. 391). The Tardi- grada are hermaphrodite, and possess a pair of testes and an unpaired ovarian sac which open together into the cloacal termi- nation of the intestine. They usually lay large eggs at the time of moulting, which remain enclosed in the old cast-off skin till the young animals are hatched. Development takes place without metamorphosis. The animals live in moss and algse in the gutters of loofs, and also on the sea-beach, and it is specially worthy of i-emark that, like the Rotifera, they can, by the addition of moisture, be called back to life after a long pei^iod of desiccation. Mucrohotns Hv/dandii S. Sch., Milnesinvi tardigradvm Doy., Ecliiimcvi Bellcrmanni S. Sch. " Sur les Taidigradcs et sur nne espece k longs pieds vivant dans I'eau de mer," Ann. des Se. nat. Ser. III., Tom XV. Also the works of Kaufmann, GreefE and Max. S. Schultze. 32 Fig. 30\.— Macrobiotut ScAw/'/if t(afterGreefF), O, Mouth; Vm, pharynx; Md, stomach; Spd, salivary glands ; Ov, ovary ; T, testes ; F», vesicula seminalis. 403 ABACnMUA,. Order 3. — Araneida * ^Spiders). Arachiida with poison glands in the suhchelate chelicerce ; tcith pediforrn pedipalpi and stalked tmsegniented abdomen. They have four or six spinning mammillce, and two or four pulmonary sacs. The peculiar shape of the true spiders is diie to the swollen and unsegmented abdomen, the base of Avhich is constricted to form the stalk by which it is united to the rest of the body (fig. 392). The large subchelate chelicerae, which project beyond the front of the head, consist of a powerful basal poi-tion gi-ooved on the inner side, and a claw-shaped terminal joint at the point of which the duct of a poison gland opens (fig. 393). At the moment of the bite the secre- tion of this gland flows into the wound, and in the case of small animals causes an almost instanta- neous death. The pedipalpus bears on its broad coxal joint, which constitutes a kind of biting- blade (fig. 392 K), a many- jointed palp, the terminal re- gion of which is peculiarly modified in the male and func- tions as a copulatory organ. The mouth is bounded on the under side by an unpaired plate, forming a sort of lower lip. The four pairs of usually long legs, whose form and size vary according to the manner /Ar;«a from the ventral of j^fp^ end with two toothed x/.cheiiceraiX^pedi- claws, to which a Small claw [Tk) and several palpus; ir, basal joint accessory claws may be added (fig. 394). The (jaw) of pedipalpus; •' _ •' _ ^ ° ' p.lungs;S^stifr^laof abdomen in the female is always larger and more S; tdLTtto «^vollen than in the male; at the base (anterior the trachea; ; G, geni- part) of its ventral Surface is placed the unpaired ning^mammiiiffi. '~^'^" sexual opening, at the sides of which are the two slit-like apertures of the lung sacs. There is often a second pair of stigmata behind these openings leading either into the * Besides the works of C. A. Walckenaer, Trevirarms, C. J. Sundevall, T. Thorell, MengCjlJoch, Dugos, Lebert. etc., compare, E. Claparcde, ^" Eecherches sur revolution des Araijrnecs," Geneve, 18G2 ; E. Claparede, "Etudes sur la circulation du sang chez les Aranees du genre Lycose." Geneve, 1863 ; F. Plateau, " Eecberchcs sur la structure de I'appareil digestif et sur les pb6no« m^nes de la digestion chez les Aranees dipneumones," Bruxcllcs, 1877. Fig. 392.— 2)y«(/era ery- Fig. 393.— Poison gland and terminal joint of chelicera of Mygale (regne animal). K, claw ; Gd, poison- gland ; B, poison vesicle. AUAXJilDA. 499 posterior lung sacs {Mygalidce) or into a system of tracheos {Argyro- neta, Dysdcra). The anus is placed ventrally at the end of the abdomen, and is surrounded by four or six wart-hke pro- tuberances (fig. 395, Spv), the spinning or arachnidial mammillae, from which the secretion of the spinning glands passes out. In front Fig. 304.— (7, Leg of the fourth pair of Airuturollw ferox. Ca, CaUimistrum. b, End of foot of Philteus chrysopg with two claws and pencil consisting of spatulate hairs (5). c. End of foot of Epeira dhdema ; E, web-claws ; Th, ambulatory claw ; Gh, toothed bristles (aceessorj- elaws) (after O. Hermann). of these protuberances there often lies a peculiar structure called the cribrellum, ■with a covering of very fine hairs (fig. 395, Cr). The spinning glands (fig. 396) are tubes of various shapes ; they open by fine pores on the surface of the spinning papillte, and secrete a viscid material, which in the air hardens to a fine thread and is woven by the aid of the claws on the feet into the well-known spider's web. Nervous system (fig. 367).— Besides the brain, with the nerves to the eyes and che- licera?, there is a single, usually star-shaped ganglionic mass in the thorax, from which nerves pass to the pedipalpi and legs, and also to the abdomen. Visceral nerves have also been observed on the alimentary canal. As a rule there are eight, or more rarely are disposed in two curved lines or more Fig. 3)5.— Spinning organ of Amaurohhis firox (after O. Hermann). Cr, Cribrellum ; Spw, spinning mammillEe. Fio. 39G.— Lungs (P), sptoning glands (Spd) and generative organs (Vd) of a, male Pholciis phalanrjieta (regno animal). It, Eectum with Malpighian vessels opening into it. six simple eyes, which in a quadrate on the 500 AEACUNIDA, dorsal surface of the cephalic region behind the frontal margin. Their arrangement is very regular, and is characteristic for the different genera (figs. 398 and 399). O OO n O O O o Fig. ^Q7.—Mygale from the ventral side, part of the skin is turned aside (regne animal). X, Chelicera : Bg, thoracic ganglionic mass; P, 1-', Q O OoqO d Fig. 399. — Arrange- ment of the eyes in different spiders (after Lebert). a, Epeira ; b, Tegenaria ; c, Dolomedei ; d, Sal- ticus. Fio. 400.— Alimentarj' can.al of Mygale ^regne animal). G, Cerebral ganglion; Ms, diverticula of stomach ; L, hei)atic ducts ; N, Mal- pighian vessels; Jt, rec turn. lungs ; F, lamellae of Si, Sf spinning papilloe. The alimentary canal (fig. 400) „ „ , . „ *^® ''^^^'^ ' begins beneath the upper lip with an Si, St, stigmata ; Ov, ovary ; Sw, " . fl f ascending pharyngeal portion of the oesophagus, into Avhich a saccular pha- ryngeal gland opens (salivary gland). The naiTOw a^sophagus, befoi-e passing into the midgut or intestine, is dilated to form a suctorial stomach, which is f urni.shed with powerful muscle.s arising from the dorsal part of the cephalo- thorax. The midgut is divided into an anterior part, lying within the cephalo thoracic region and provided \v\i\\ two anterior and foui- latei-al pairs of cseca, and into a narrower abdominal small intestine, into which the ducts of the branched hepatic tubes pour their secretion. The latter Fio. 398. — Anterior part of the cepha- lo-thorax of Mygale with the eyes (O) (regne animal). ARAXEIDA. 501 appears to have a digestive function similax- to that of the pancreatic secretion, inasmuch as it dissolves albumens and ti-ansforms amyloid substances into sugar. The short rectum receives two bi-anched urinary (Malpighian) canals, and dilates in front of the anal opening to the form of a vesicle (fig. 400). Fia. 401.— Heart and vasculnr trunks of Lycota, in lateral and dorsal view (after Claparede). P, Lunj^s ; C, heart ; Ao, aorta ; O, eyes. Fio. 402.— Sexual organs if a Tegenaria (Philoica) domeefwa, with the abdomsn in outline (after Bertkau). T, Testis ; Vd vas deferens ; St. stigma. The vascular system is not less highly developed (fig. 401). The blood flows from the pulsating dorsal vessel placed in the abdomen, through an anterior aorta into the ce- phalo-thorax, and thence into lateral arte- ries, supplying the legs, jaws, brain, and eyes. The blood returns from these organs into the abdomen, bathes the so- called lungs, which are composed of numerous flattened tubes, and then re- turns to the dorsal vessel thi-ough three pairs of lateral slits. The ovaries (fig. 397) are two racemose glands suiTOunded by the liver ; the short oviducts unite to form a single vagina, which is usually connected with two long receptacula seminis and opens on the ventral surface of the anterior part of the abdomen between the anterior stigmata. The testes consist of two long coiled canals with a common terminal duct, which likcAvise opens at the base of the Rhdomen (fig. 402). Fia. 403.— Terminal part of the pedipalpus of Segeslria ( (Cams, Icones). K, Masillse; MJ maxilliped. fVjmft of the terjxa are Fam. Scolopendridae. Antetin^ long and thin with a relatively small number of joints, only a few ocelli. The seg- ments of the body are sometimes equal, sometimes unequal. Scolnpendra, (with nine pairs of stigmata) gigantea L., found in the East Indies. Sc. mord- ta7i.