Sepa SS ee Snes epee see SS a Soe earety eee seen ear eee Sy aa pS = SSS SS Bisse SS Se = Se oe SSeee see So eee eee = : ee SS = Sa SSS SS Se SS SS SSS == SS ee 5 = ——————— New York State Callege of Agriculture At Gornell University Dthaca, N. B. Library wining ELEMENTARY ZOOLOGY UNIFORM WITH THIS VOLUME. Crown 8vo. ELEMENTARY INORGANIC CHEMISTRY. By G. 8S. Newrn, F.LC., F.C.S., Author of ‘* A Text- Book of Inorganic Chemistry,” ‘‘ Chemical Lecture Experiments,” etc. ELEMENTARY PHYSICS. By W. Watson, B.Sc., Demonstrator in Physics in the Royal College of Science, London, etc. LONGMANS, GREEN, AND CoO. NEW YORK, LONDON, AND BOMBAY. ELEMENTARY ZOOLOGY BY FRANK E. BEDDARD, M.A. (Oxon.), F.R.S. PROSECTOR TO THE ZOOLOGICAL SOCIETY OF LONDON ; LECTURER ON BIOLOGY AT GUY’s HOSPITAL; EXAMINER IN ZOOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON ; LATELY EXAMINER IN THE HONOURS SCHOOL OF MORPHOLOGY IN THE UNIVERSITY OF OXFORD NEW YORK LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON - AND BOMBAY 1898 All rights reserved PREPACE THE present volume contains an account of a few types selected from the chief groups of the animal kingdom, followed and accompanied by a consideration of some of the more general conclusions of biology. It is inevitable in an elementary hand- book to use the type system, but it is not necessary to use it in such a way as to emphasize all the faults of the system. The gravest fault, in' my opinion, is that the student is impressed with the idea that the characters of a given species selected for description are distinctive of a wider assemblage of forms. I have endeavoured to obviate this by emphasizing here and there the differences between allied forms. Many distinguished authorities hold that in treating of animal structure it is desirable to commence with the higher forms and gradually work down to the lowerforms. It is argued that to do this is advantageous, since the student commences with what must be the more familiar part of the subject. Some rough notion of human anatomy is possessed by most persons; whereas the very methods by which the lower animals are studied are new to the beginner. There is, however, no transition between a dissection of a frog with scalpel and scissors and the examination of an amoeba with the microscope. The plunge into an unfamiliar region of the subject must be made some time; and why not at the very commencement? Besides, to begin with the low forms and to gradually work to the higher has the undoubted advantage of presenting the facts in a logical sequence. I therefore begin with the amceba, and deal with the other types in, so far as is possible, an ascending order. vi frejace. The types which are described here, as well as the general facts of animal structure, have been dealt with by so many zoologists in so many text-books, that it is hard to illustrate them by fresh drawings. It is quite useless, for example, to attempt to improve upon the excellent figures of the crayfish appendages to be found in Mr. Huxley’s “The Crayfish.” Such illustrations I have copiously borrowed from various sources, which are duly acknowledged in the case of each cut. Among these will be noticed a few new illustrations, either copied from the original memoirs which they illustrate or modi- fied from existing wood-cuts in text-books; these are Figs. 1-7, 9-13, 22-25, 27-32, 34, 37-41, 43, 45-49, 52, 63, 73) 74, 76. For these I am indebted to the skill of Mr. R. E. Holding. F. E. BEDDARD. CONTENTS CHAPTER I, II. IIT. IV. Vv. VI. VII. VIII. IX. XI. XII. XIII, XIV. XV. THE UNICELLULAR ANIMALS: THE AMG&:BA (AMG@BA, VARIOUS SPECIES) ; THE BELL ANIMALCULE (VoRTI- GELEA) je yw THe Hypra (Hypra VIRIDIS, HYDRA GRISEA, ETc.) . THE EARTHWORM (LUMBRICUS AND ALLOLOBOPHORA, VARIOUS SPECIES) . . . .. . ie eye te THE CRAYFISH (ASTACUS FLUVIATILIS) . . . . . THE COCKROACH (BLATTA ORIENTALIS) . . . . THE METAMORPHOSES OF INSECTS . . ... . THE PonD MussEeL (ANODONTA CYGNHA) . . . . THE SNAIL (HELIX PoMATIA, HELIX HoRTENSIs, ETC.) THE Froc (RANA TEMPORARIA AND RANA ESCULENTA) SKELETAL AND INTEGUMENTARY STRUCTURES IN VERTE- BRATES! C4, Gh fo 8) vids 8) Hs Re Gs we THE EGG, THE SPERM, AND THE DEVELOPMENT OF THE CITICKS ee nee of eae. ie RR! Ge ee Cae ye MORPHOLOGY OF ORGANS. . . . . sw ew ee THE MorpHOLOGY OF TISSUES (HISTOLOGY) . . . . CLASSIFICATION. THE DIFFERENCES BETWEEN PLANTS AND ANIMALS . . . 6. 6 1 ee ee THE CLASSIFICATION OF ANIMALS iy Gat, Se On P< PAGE 13 19 30 50 57 61 67 73 85 118 143 167 173 183 ELEMENTARY ZOOLOGY CHAPTER I. THE UNICELLULAR ORGANISMS. Tue Amasa (Amba, various species). Water from any stagnant pool, particularly if a drop be taken from the surface of the mud at the bottom, will be frequently found to contain examples of.an organism known as Ameba. This term is used rather loosely for a number of creatures— fresh-water, marine, terrestrial (in damp earth)—which agree in being composed of a speck of seemingly jelly-like substance, and in moving by a flowing motion, accompanied by the thrusting out of processes of the irregularly shaped body. Some little experience of these organisms will soon show that there are many kinds of amcebze, which have been classed by naturalists in different species and even genera ;* some, for instance, are larger, much larger, than others. The form of the thrust-out processes of the body—the pseudopodia, as they are termed—differs from species to species; the more or less granular appearance of the body is another character which varies in the different kinds; and there are other points of difference. In all these organisms, however, the body has no fixed form; it is simply an irregular mass of living matter ; hence the name of “Proteus animalcule” was applied to it by some of the earlier observers. When an ameeba, preferably one of the larger species, is examined, it is seen to consist of 1 The terms “‘ genera” and “species” are explained later (see p 173). B 2a Elementary Zoology. a mass of semi-fluid matter, which in the middle of the body is filled with granules, and is clear, or clearer, peripherally. In Fic. 1.—Various species of Amecbzx, highly magnified, characterized by the differing forms of their pseudopodia. (After Mcebius.) the living form sometimes—but it is better shown by staining reagents—a denser central body may be observed of a spherical The Unicellular Organisms. 3 shape, which has no fixed position, but moves about with the movements of the animal; this is termed the wc/eus. In many amcebz a clear vesicle also exists (the contractile vacuole), which, if kept under the eye for a few minutes, will be noticed to suddenly contract—apparently expelling its fluid contents— and to gradually fill out again, repeating the process continually. This is positively the entire anatomy of the amceba, stated, of course, very briefly. No animal simpler in structure than this organism is certainly known. Some amoebz lack the contractile vacuole; but all consist of a mass of the jelly-like living matter (frotop/asm) and a nucleus or nuclei. It has been stated that there are amceboid creatures without a nucleus ; but this does not appear to be by any means certain. And we shall see later that it is improbable. The ameceba not only has the certain definite structure that has just been briefly described ; it also acts; the living matter of which it is built up performs certain functions. We have to consider the physiology (the functions) of the amceba, as well as its structure (its morphology). The movements of the animal have been already referred to. It is continually in motion, and the movements may be retarded by cold, increased by warmth, and again retarded and stopped by too great warmth. Various chemical substances produce similar effects. The living matter, therefore, of which the amoeba is composed is capable of movement, and is irritable—that is, responds to stimuli. If an amoeba be watched for some time it will be seen to feed. It takes in nourishment by simply flowing over and engulphing a minute plant or other organism; it literally gets outside its food. After a time the ingested food particle will be seen to gradually disappear, and the indigestible residue may be seen to be thrust out of the body. Furthermore, the contractions of the contractile vacuole expel from the body other waste substances. If an amceba be kept in filtered water, in which there is no vegetable or animal matter, it will die after a longer or shorter time. Nor will it avail it that the water be impregnated with the various chemical elements, or compounds of them, that make up its body. The amoeba needs organized matter to feed upon. 4 Elementary Zoology. The body of the amceba, therefore, is constantly wasting away and being as constantly renewed by the taking in of food, which must be in the form of living or dead animal or vegetable matter. But the animal performs other functions. When the creature has attained to a certain size, which differs in different species, and even in individuals, it divides into two; the nucleus divides and then the protoplasm, so that where there was one ameceba there are two. This process under favourable circumstances is continually repeated. But it has its limits. After a certain number of generations have been thus produced by simple fission—the number varying with the species, and not being accurately fixed—this method of repro- duction ceases. Another kind of generation comes into play. Two amcebe approach and fuse together, the nuclei joining and the protoplasm being commingled. After a longer or shorter period of conjugation the two may become surrounded in a delicate cyst, and break up into a number of minute spores, which gradually attain to the size of the parents after the rupture of the case; or the two may separate, and, refreshed by the union of the nuclei and protoplasm, go on dividing by the process of fission. This conjugation must not be confused with the ingestion of one amceba by another, though in some cases it is doubtless difficult to distinguish between the two— between hunger and love. The question may be asked, Why should an ameceba divide? why should it not go on growing indefinitely? The question is easier to ask than to answer. In considering the matter it must be borne in mind that possibly the viscid and semifluid protoplasm cannot hold together in droplets above a certain size, and (more important) that the surface does not increase in extent pari passu with the contained mass; hence the power of ingesting food and excreting waste products may be not sufficiently rapid to keep pace with the growing mass. Accordingly the animal divides into more conveniently sized pieces. The amoeba then moves, responds to stimuli, feeds, excretes, grows, and reproduces itself. These are among the The Unicellular Organisms. 5 most important characteristics of living matter; and in all living beings the same phenomena are observable, in addition to respiration, which is the taking in of oxygen.? By these characters living may be distinguished from non- living matter. This living matter, “the physical basis of life,” or protoplasm, as it is usually called, is a viscous semifluid substance with granules interspersed. A certain vagueness of meaning has at times attached itself to the expression proto- plasm. It must not be regarded as a substance of definite chemical composition. It is a mixture of various substances, whose exact relations cannot, from the nature of the case, be accurately ascertained. For in order to manipulate it the protoplasm must be killed; and dead protoplasm is an altogether different thing from living protoplasm. In dead protoplasm the actual elements which compose it can, of course, be accurately enumerated; these have been found to be carbon, hydrogen, oxygen, nitrogen, sulphur, and a few others, such as calcium, phosphorus, potassium, sodium, magnesium, and iron. The first five are combined to form various proteids, ze. albumens, globulins, etc. But any account of the chemical nature of the substances would take too long a space, and requires a detailed treatment at the hands of a chemist. Attention, therefore, will be directed only to the bare outline given above, and to the fact that protoplasm is not a chemical but a morphological expression for a complex substance exhibiting the properties already referred to. Recent microscopical research into the nature of protoplasm has revealed the fact that it has a definite structure, that its particles are disposed in a regular fashion ; but the interpretation of the observed facts has differed greatly. The two principal views of the constitution of protoplasm are known respectively as the “network theory” and the “foam theory.” According to the first view, the protoplasm is disposed in a network of denser protoplasm, the meshes of which are filled by the more 1 This function is separated from the others, since the taking in of free oxygen (= respiration) is not quite absolutely universal. Thus there are the anaerobiic bacteria, which not only do not take in free oxygen, but are killed by it. They must obtain their oxygen from compounds, 6 Elementary Zoology fluid parts; in this case the granules are largely an optical delusion, and represent the nodal points of the meshwork : the granules are also partly “formed” substances, z.e. bodies not protoplasm, but produced by the activity of the protoplasm, just as the droplets of fat in a fat cell are formed substances produced by the activity of the protoplasm. A mass of proto- plasm, then, on this view, may be roughly compared to a sponge. \ 1 Fic. 2.—A portion of the body of a Multinucleate infusorian Ofadina, to show network structure of protoplasm. Very highly magnified. (After Biitschli.) yj Wx According to the foam or honeycomb theory the living substance of a cell is comparable to a mass of froth, of which the air in the bubbles is formed by less viscid, the walls of the bubbles by more viscid, protoplasm. It will be apparent that so far this view of the constitution of protoplasm does not differ widely from the network view. The protoplasm of various cells and certain simple organisms appears to demonstrate that this foam theory is the true explanation of the arrangement of protoplasm. It is especially well seen in an amceboid creature, The Unicellular Organisms. 7 Pelomyxa, not far removed from the more common forms of amcebee, and chiefly differing by the presence of a large number of nuclei. Professor Biitschli, moreover, has succeeded in artificially compounding a substance which has many of the physical properties of protoplasm to all appearance. He made a fine emulsion of oil and:a solution of such a substance as common salt. This mixture not only showed the foamelike Ch.V Fic. 3.—A portion of the body of an Amceboid multinucleate organism (Pelomyxa), illustrative of foam theory of protoplasm. Very highly magnified. (After A. G. Bourne.) N, nuclei; vac, vacuoles; Ch. V., vesicles (tinged green by chlorophyll). structure, but it exhibited movements; and masses of it even divided in a very life-like fashion. In the case of the “artificial protoplasm” the walls of the bubbles were formed by the oil, and the alkaline solution occupied their interior. This mixture, however, bears a relation to true protoplasm like that of a waxwork figure to a man; it is not a kind of monster of Frankenstein. And, ingenious though the experiments 8 Elementary Zoology. undoubtedly are, they really prove nothing, since protoplasm is admittedly not an oily substance, though it may often contain oil drops formed by its own activity. Having considered the protoplasm which builds up the bulk of the amceba’s body, we may turn to the nucleus. The ucleus isa spherical to elliptical structure, encircled by a definite wall, and showing a granular appearance. One or more rounded bodies in its interior are termed zucleoli. At one time the nucleus was regarded simply as a denser bit of protoplasm in the interior of the protoplasm, denser perhaps by reason of its central position and the consequent pressure of the surround- ing substance. It is now known to be a perfectly distinct structure. This opinion is based upon its minute structure, which has been of late years more elaborately investigated, and by the physiological importance which it has been proved to possess. This is evidently incompatible with a mere central thickening of the protoplasm. The nucleus very commonly exhibits a reticular arrangement of its contents, the denser network consisting largely of a substance which has been termed chromatin. But the constitution of the nucleus is more fully entered into below (p. 127) in connection with its multiplication, As to the importance of the nucleus, it seems probable, in the first place, as already stated, that a nucleus is always present in living bodies, There are apparently a few exceptions, such as those minute organisms so often con- nected with disease, and generally known as bacteria. Figures showing a structure like nuclei in those organisms have been published, but not to the satisfaction of everybody. The view has been advanced that the whole body of the minute plant is a free nucleus with but a slender rim of protoplasm. This may be the case, but the matter cannot be fully gone into here; it is sufficient for the present purpose to insist upon the certainly almost (and probably quite) universal presence of a nucleus. The importance of the nucleus in the protoplasm is shown by the striking part that it takes in the division of the amceba; it initiates this division. Experiments have been made with large amcebze, and with allied organisms, which tend to show that when the creature is torn up by fine needles into small The Unicellular Organisms. 9 pieces, it is only those fragments which have a nucleus (in the case of the multinucleate forms such as F¢lomyxa, already referred to), or a fragment of the nucleus where there is but one, that can reproduce the amceba of which they are a fragment. Bits of detached protoplasm, minus all trace of the nucleus, lead for a time what has been described as a “ pseudo-existence,” but ultimately decay; during this brief existence, they may perform movements, but these have an abnormal character without the dominating nucleus to direct them. Again, the nucleus of many tissues in the higher animals has been observed to preside over such functions as the formation of yolk in ova, the budding of plant cells; and, in short, it is clear that the functions of the protoplasm are largely directed by the nucleus. < Before leaving the amceba, there is one important event in its life that must be referred to. At times, when circumstances are unfavourable—if, for example, the medium in which the animal is living becomes too dry—the amceba will surround itself with a delicate skin excreted by the protoplasm. In this encysted condition it can survive a degree of dryness which would be. fatal to it in its naked and unprotected condition. THe Bett ANIMALCULE (Vorticella). The organism known as the Bell Animalcule belongs to the same great division as that which contains the amceba, but to a different group; to this group the name “ Infusoria” has been given, originally from the fact that various members of it were to be found in organic infusions. The Vorticella is a social, but not a colonial, form common in fresh water ; numbers are found living closely together. The creature, as is shown in the accompanying figure (Fig. 4), has somewhat the appearance of a wine-glass supported on a long stalk. When the animal is undisturbed, this stalk is elongated to its full; if the slide upon which a number of the organisms have been placed for examination be jarred, they will contract, and the long stalk is then seen to be thrown into a spiral. At the same time a circle of rapidly moving filaments, which deck the free end of the infusorian, are retracted also. 10 Elementary Zoology. The vorticella is, like the amoeba, an unicellular animal. So far it does not present us with an advance of structure upon the last described organism, which is one of the simplest of organized creatures. But the vorticella is a useful example of a single cell, which is highly specialized in many and different directions ; it is still a unit of structure, but illustrates the very great amount of differentiation which can occur in acell, DISC PERISTOME VESTE CONT. VACUOLE rh FOOD VACUOLE Fic. 4.—Vorticella, highly magnified. (After Biitschli). MTH, mouth ; vest", funnel-shaped vestibule ; mac. Nuc, macronucleus ; MIC. NUC, micronucleus, ' The body of the vorticella is somewhat wineglass shaped ; it has a thick rim above, which is, when the animal is fully extended, slightly everted. This rim is fringed with vibratile cilia. Just inside this rim, on one side of the body, is a funnel- shaped depression leading -some way into the interior of the animal; this is the aperture through which food particles are taken in. The body itself has a delicate, slightly hardened, outer layer, the cuticula; beneath this is a firm layer, the } This anticipates what is dealt with on p. 14. The Onicellular Organisms. II ectosarc, and the rest of the body within this, again, is made up of a more granular and softer protoplasm, the endosarc. The stalk is seen to contain a central core in the shape of a delicately striated filament; at its attachment to the body, the filament spreads out fan-wise, and forms a layer of ex- cessively fine fibres lying in the ectosarc. This is termed the myophan layer,and is of a muscular nature. It is the muscle’ in the stalk which enables the contraction already spoken of to. be effected. The figure (Fig. 4) shows the layer plainly. The vorticella possesses a contractile vacuole; this undergoes Fic. 5.—Conjugating Vorticellz, highly magnified. (After Maupas.) MAC. NU, macronucleus ; Mic. NU, micronuclei. regular expansions and contractions. A nucleus is present, which has a horseshoe form, and near it a smaller body, the micronucleus. The large nucleus is termed the macronucleus, Like the amoeba, then, the vorticella is a mass of proto- plasm with a nucleus; like the amceba also, the protoplasm is distinguishable into an outer and inner layer, and there is a contractile vacuole; but in other respects the vorticella is more advanced in structure: the ectoplasm is again subdivided into a outer cuticular and an inner layer, below which again is 12 Elementary Zoology. the myophan layer, concentrated in the stalk to form the muscle fibre ; while instead of pseudopodia, appearing when and where they are wanted, are definite and persistent outgrowths of the ectosarc, which have the power of independent movement. Yet there is not so wide a difference as might be thought be- tween the heavy, slowly flowing, pseudopodia and the actively vibratile cilia. Some amcebe have very slender pseudo- podia, while organisms belong to the protozoa have long thin fixed processes, from which to cilia is not a long step. Approaching the matter from the other side, cilia have been observed in the living animal to “melt down” into pseudopodia. The vorticella multiplies by division, and also after what may be termed a sexual union. A bell divides down the middle, and two vorticellze are the result. This division results in the production of two kinds of individuals ; one is like the parent form, the other is a locomotive body with a ring of cilia at each end of the body. The locomotive body, after a longer or shorter interval, settles down and becomes a stalked vorticella.| Or one of these locomotive individuals will attach itself to a stalked bell, and become fused with it, the nuclei becoming broken up, fusing with those of the other individual, ultimately reacquiring their original form. Ordinary division occurs after this which may be compared to a sexual process. It has been shown that, after a certain number of generations produced by simple division there is a need for this sexual union to restore the exhausted protoplasm. 1 Sometimes, particularly if the water be too foul, a vorticella will detach itself from its stalk and lead a free existence, forming later a new stalk, and settling down to a sedentary life. CHAPTER II. THE HYDRA (HYDRA VIRIDIS, HYDRA GRISEA, ETC.). Tue Hydra is a small organism, not more than half an inch in length when fully extended—and that would be a large specimen —which is common in ponds and other pieces of still fresh water. The animal exists in, at any rate, two forms in this country ; one set of individuals are green, the others brown or nearly colourless. Hydra has the general shape shown in the accompanying figure (Fig. 6); it consists of a tubular body sur- mounted by a wreath of tentacles which grow out from the base of a conical expansion ; this bears the mouth at its summit. The whole body is retractile, the retracted hydra having an oval to round form. The hydra belongs to a large group of animals containing the jelly-fish of our seas, the “ Portuguese man-o’- war,” and a variety of soft-bodied creatures which, as will be stated later, form one of the primary divisions of the animal kingdom. This creature leads a stationary life, adhering to a leaf of duck-weed or a fragment of stone or stick, and by waving its arms freely in the surrounding water, catches and narcotizes by means of the thread cells—to be described presently— minute worms, crustaceans, etc., which form its food. The green hydra is coloured green by chlorophyli, a pigment which is nearly universal in the vegetable kingdom, being only absent in the fungi and in a few parasitic plants belonging to higher groups. This chlorophyll is also present in a few other animals, even higher in the scale than hydra, such as the two Planarian worms, Vortex viridis and Convuluta schulzei. Tt also exists in a few Infusorians. It is not, however, safe to jump to the conclusion that when an animal is coloured green it is 14 Elementary Zoology. by this particular pigment. Green birds and green lizards, for example, do not in any case owe their colour to chlorophyll. In order to prove conclusively that a given green pigment is or is not chlorophyll, it should be submitted to three tests—chemical, physiological, and morphological. Chlorophyll is, as a rule, associated with certain structures, the chlorophyll corpuscles, which are nucleus, like masses of protoplasm tinged with the chlorophyll. This is not the invariable case; but it is safe to regard a green pigment contained in special corpuscles as being chlorophyll, though the converse cannot be asserted. This may be termed a morphological test. Chlorophyll has certain definite chemical reactions and characters. In the first place it shows a definite absorption spectrum with characteristic dark bands. It is soluble in alcohol, and the solution is fluorescent. By transmitted light it is green, by reflected light reddish. There are, of course, a variety of other chemical methods of deciding whether the pigment is chlorophyll. Finally, there is the physiological method. Protoplasm can, in the presence of chlorophyll, split up the carbonic acid of the air into oxygen and carbon, combining the carbon with the water in the protoplasm to form —usually starch, but sometimes some other substance, e.g. oil, composed of the elements carbon, oxygen, and hydrogen. Tried by the first two of these tests, the green colour of hydra is chlorophyll. Experiments, however, appear to show that it is of no great physiological use to its possessor. Opinions differ as to whether the chlorophyll is really a product of the cells of the hydra (in which case there isa most interesting point of like- ness between animal and vegetable protoplasm), or whether the so-called chlorophyll corpuscles of hydra are not to be looked upon as small unicellular plants. If so, this illustrates what is termed symdzosds ; as the small plants derive advantage (shelter, etc.) from their association with the host, while (though, as already said, this is not so clear) they confer advantages in the shape of starchy or other matters formed by them upon their host. When a transverse or longitudinal section is made through the hydra, the body is seen to consist of two layers of cells surrounding a central cavity, which latter communicates with the exterior through the mouth aperture before referred to. When a hydra is teased up in water with fine needles, these cells are dissociated from each other, and float freely about. Each may then be seen to consist of a piece of protoplasm, with a more or less centrally placed nucleus. The boundaries of the cells may also be recognized in the sections. The whole body of the hydra is thus built up of a number of structural units or cells, each one of which is the equivalent of a single amceba or vorticella. The hydra, and all the animals lying above it in the series, is “ multicellular;” the amoeba and the vorticella are The Hydra. 15 both “unicellular.” The two layers of cells are known respectively as the ectoderm and the endoderm, the names referring to their relative positions. The evéeron, or central cavity, is also ECTODERM CELLS INTERSTITAL bo MUSCLE LAYER MESOGLEA ENDODERM CELL CHROMATPHORES FLAGELLA e@ Fic, 6.—Longitudinal section through Hydra. (After Marshall and Howes.) The asterisks (i) are placed in enteron. continued into the tentacles. The ectoderm is made up of two kinds of cells; there are, in the first place, large cells somewhat 16 Elementary Zoology. narrowed towards the base, where they end in long contractile tails which are in effect muscle fibres (Fig. 7). Between these larger cells are heaps of small zvderstzttal cells, some of which are ganglionic in nature and communicate with fine nerve fibrils. When a living hydra is examined intact, fine bunches of bristle-like processes are seen to protrude on the outside; these are the indications of the thread cells, or, better, cxidoblasts, as they are not cells, which are particularly abundant upon the tentacles. The cnidoblasts (Fig. 8) are formed by the metamor- phosis of the interstitial cells, but when fully formed force their way to the exterior. The fully developed but not used cnidoblast has the characters shown in the diagram. It is enclosed in an interstitial cell which has a delicate process protruding on to the exterior; this is the czidoci? or palpocil. The function of this trigger-like projection is not, as might be inferred from the vernacular name just used, which is sometimes applied to it, the grossly mechanical one ; it is probably of a nervous nature, like the fine end of visual or auditory cell, and communicates an impression to the cell, which then contracts and expels the “thread” from the cnidoblast. The latter is a tough sac formed of a spherical or pear-shaped base, with a fine hollow process ; it has been aptly compared to a glove with one finger, the hand of the glove being, of course, nowhere open. The “finger,” however, is turned inside out within the “hand,” and the whole is filled with fluid. Pressure forces out the thread, which is then seen to be often armed with spines at its base. The “sting” of the jelly-fish is due to similar thread cells ; but whether there is an actual poisonous liquid, which causes the symptoms, or whether it is merely the irritation, like that caused upon sensitive skins by the hairs of certain cater- pillars, does not appear to be certain. From the interstitial cells are also formed the generative tissues, which are rounded swellings nearer the tentacles in the case of the /es¢es, and nearer the base in the case of the ovaries. The testes contain tadpole- like spermatozoa, formed by the divisions of the testicular cells, while the ovary only contains one egg at a time, which is large and full of yolk, and exhibits before it is quite ripe amceboid movements. The ovum nourishes itself upon the small cells of The Hydra. 17 the ovary, which might otherwise become ova. There is thus, it will be observed, a “struggle for existence” among the very cells of a particular organ. ‘Though the egg eats up other cells like an amceba, the process is not really different from that ic. 7.—Isolated ectodermic cells of Hydra. Highly magnified. (After Howes.) Ec, ectoderm cells; cp, muscular process ; CL, supporting lamella; NE, nervous cells ; r~5, cells entirely detached from supporting lamella; v, vacuole. exhibited by other ova, which, as explained elsewhere, are fed by the cells of the follicle. The endoderm is made up of large cells, many of which have one or more vibratile cilia depending from the inner surface ; it is they which bear the chlorophyll corpuscles. Between the two layers is a colourless, thin, structureless membrane, the c 18 Elementary Zoology. supporting lamella, which is simply an excretion of the cells which border it, as is the cuticle of the earthworm an excretion of the epidermis. It will be noticed, therefore, that the hydra’s body is built up of two distinct layers of cells which are different in character and in function; the outer layers provide the sensitive muscular and protective elements, the internal the digestive. The distinctness of these two layers may be impressed upon the mind’ of the student by relating a curious chapter, or rather paragraph, in the history of error, which concerns this animal. An ingenious naturalist of the last century succeeded in inducing hydras to swallow a worm attached to a thread, and then pulling on the thread when the worm was fairly swallowed, turned the creature inside out. He asserted that this reversal made no manner of difference to the animal, who thereupon used its outer Hyda’ Highiomegnifea, COAt as a stomach, and its stomach as The Naeied toon, inthe a covering. But a still more ingenious Japanese naturalist confirmed, it is true, the statement that a hydra could be with ease turned inside out; but found also that when left to itself the hydra quietly reversed matters, and assumed its original condition. The cells, in fact, are specialized to perform their several parts, and could not play any other. CHAPTER III. THE EARTHWORM (LUMBRICUS AND ALLOLOBOPHORA, VARIOUS SPECIES). In dissecting the Earthworm, the beginner will often find slight discrepancies between the descriptions in the book which he uses as a guide and the actual facts of structure before him. This is owing to the existence of a considerable number of different species of earthworms which offer a certain amount of structural differences among themselves. In this country there exist, so far as is known, some twenty species. They are all of small to moderate size, and live in soil, though not impatient of even prolonged immersion in fresh water. They burrow through the earth, swallowing the soil as they go, which is often—after extraction of some, at any rate, of the nutritive substance which it contains—evacuated to form the well-known castings so abundant upon lawns after rain. A certain amount of moisture is necessary for the soft- bodied animals to live, and in very dry weather the worms penetrate deeper into the ground, and often coil themselves into chambers below the surface and surround themselves with a coating of exuded mucus. An earthworm is a soft-bodied animal, obviously segmented, Zé. the body is divided by superficial furrows into a series of similar rings, segments, or somites, as they are variously termed. A closer examination shows that there are other external features which are also arranged in the same ‘ metameric” fashion. If a worm be held in the hand and passed between the finger and thumb of the other hand, it produces a sensation of roughness, which is caused by the implantation of the 20 Elementary Zoology. bristles, or sete. ‘These setze are disposed in a perfectly regular way upon each segment, and have—with a few exceptions to be noted immediately—an iden- tical arrangement in successive segments, In each segment of the body iz Be 4a? ee8 there are eight of these sete 32% disposed in couples, the indivi- 4 nF dual setze of which are more or Zge¢ less closely related ; the greater. ° 8% or less distance which separates = 22 the two sete of a couple is fre- _ 42's quently a specific distinction. 3 3 8.5 On the first segment of the body 2208 the setae are totally absent, as ee 2 also upon a projection of the first gage segment—the prostomium, or % 8 buccal lobe —which overhangs Eee, the mouth, All the other seg- 22% ments have sete. The worm’s g Sy : body is covered externally by a &¢8s delicate transparent cuficl. Ifa Sag worm be allowed to macerate 3 ges for a short time this cuticle be- 2282 comes easily detachable, and gEee when a portion is stripped off : 53. é the setz occasionally come away 5 TBS g with it. The sete, when thus Z Sees isolated, are seen to be of a £ £822 yellow colour, and to be re- 0 $5 € ularly curved like an elongated gg §, or like the mathematical Ze*c sign /, the blunter end being ar that which is implanted in the body wall; the sharply curved hook in which it ends at the other extremity protruding freely on to the exterior. Upon the clitellum the sete are of a different kind; they are longer as well as rather The Earthworm. 21 thinner and less curved. The céi¢el/um, to which reference has been made, is a region of the body which stands out conspicuously by reason of its glandular, smooth, and swollen appearance. It occupies a variable number of segments, and commences at a variable segment, the variability corre- sponding with differént species. It never, however, com- mences further forward than the twenty-third or twenty-fourth segment. The clitellum is sometimes termed the cingulum or the girdle. At the commencement of the clitellum there are _on the ventral surface of the body certain glandular eminences, more conspicuous in immature worms in which the clitellum is not yet formed. These, again, vary in number and position according to the species, and are known as the /edberciula pubertatis. If an earthworm be dried with blotting-paper and then gently squeezed, liquid will be seen to exude or even to spirt out from certain pores placed along the back. These are the dorsal pores, and lie in the middle line of the back between successive segments. They do not begin for some distance behind the anterior extremity, and the exact segment at which they do commence is another of those points which varies with the species. A careful examination, especially of spirit- preserved examples, shows a second series of pores, which, unlike the dorsal pores, are paired, a pair to each segment. These lie laterally, and are placed in front of the more ventral couple of sete. Their position, again, varies somewhat in some species, and they are not apparent on the first two or three segments of the body. These pores are termed the nephridio- pores ; they are the openings of the excretory organs. On the fifteenth segment of the body are a pair of very conspicuous orifices with tumid lips, lying-between the dorsal and the ventral pair of sete. These orifices are those of the sperm ducts, but in some species they are not so obvious as in others. On the segment in front of this—ze. the fourteenth —are two other orifices very minute and not always easy to see, the oviducal pores. Less easy, again, to see are a series of pairs of orifices not fewer than two pairs, lying generally between segments 9 and ro and ro and 11. These are the 22 Elementary Zoology. ; ———_ FO | ae Ve Ap erin DUO ALOR MPAA nD SY CD cr RE FUT QTM atin TOT ort Li Pg oD Ll DDT A AON aes Bint Bea aaa ID Ween aT eH CD ' om Bea) Sp a q ¢ TTA GD a eee. ‘pp queyal iy ammh Sow eS ca , LL Acoma TMM ape q = 9 ‘ mma io ’ gy “ip Sy Tees ap ts oe PERITONEAL MEMBRANE Fic. 10.—Transverse section, body wall of Earthworm. CUTICLE EPIDERMAL CELLS CIRCULAR MUSCLES LONGITUDINAL MUSCLES (After Cerfontaine. ) The Earthworm. 23 spermathecal pores. Finally, there is the mouth in front, already referred to as lying beneath the prostomium, and the anus posteriorly, entirely surrounded by the last segment of the body. Such are the principal external characters of the earthworm. It will be observed that the sete, the dorsal pores, and the nephridiopores show the same plain segmentation as the divisions of the body to which they accurately correspond. When an earthworm is opened by a median incision along the back, and the flaps of skin turned back, the entire anatomy is revealed. The Body-watl—tThis flap that has just been turned back is built up of three layers. Outside there is an epidermis, within that two muscular layers, an outer circular and an inner longitudinal, shown in the accompanying figure (Fig. 10). The alimentary tract is seen passing in a perfectly straight line from end to end of the body, and from it a series of delicate transverse septa reach the body-wall, forming a means of suspension of the digestive tube. These septa divide the body into a set of chambers, through which the digestive tube passes, but which it does not entirely fill. There is left a considerable space, above, below, and at the sides of the tube. This cavity, or series of cavities, is known as the body cavity, or celom, The septa, roughly, not always quite accurately, correspond at their insertion on to the body-wall to the furrows which separate the segments externally. The interior of the worm’s body is therefore segmented like the exterior, and the internal segmentation corresponds to the external. The chambers of the ccelom, which are divided from each other by the successive septa, are not, however, completely separated. On the ventral side the septa are defective, being cut away along a curved line, so that there is an actual communication between the whole series of coelomic compartments, and the fluid which is contained therein can pass from end to end of the body. Lying in each compartment of the ccelom is a pair of delicate coiled tubes, the excretory organs, or xephridia, The term “segmental organ” was originally applied to these glandular tubes in order to express their segmental eae ‘ows TE[NOSNuY TeUIUIIE; OY : q’san ovr uado pus ‘roo payeorfduoo ¥ uLto ‘umnzdas ayy oJe10J1ad 03 UAaS SI aqn3 BY, ‘soYyII0 [eUIa}xa ‘o°x ¢ ouUNy “y (‘ueyueg sayy) ‘“peyluseyy -‘snorquin7] jo umrtprzydeu y— "sr ‘91g Elementary Zoology. The Earthworm. 25 arrangement. The nephridia are tubes which, though very long, do not occupy much space owing to the fact that each tube is coiled several times upon itself in the way illustrated in the figure (Fig. 11). The nephridium commences by an open mouth, the /wnnel; this is immediately continued into a delicate tube which passes backwards and passes through the septum into the segment behind. It then winds about in the way illustrated, and ultimately opens into a wider -sac, which itself opens on to the exterior by the nephridiopore already mentioned. The circulatory system is complicated and highly developed. The main trunk is a dorsal vessel running upon the upper surface of the alimentary tract. This is rhythmically con- tractile, the contractions passing from behind forwards. This communicates by several large, also contractile, “ hearts,” with the ventral vessel which is not contractile. This lies above the nerve-cord; the blood in it flows from before backwards. Beneath the nerve-cord is another longitudinal vessel, and in the cesophageal region a short vessel on either side of the cesophagus which arises from the dorsal vessel. The dorsal vessel in the segments lying behind this lateral cesophageal vessel gives off a series of regularly and segmentally arranged branches to the alimentary canal and to the septa. Branches also arise from the ventral and subneural vessels supplying the nephridia and body-wall. The vascular system of the earthworm is everywhere a perfectly closed system, consisting of tubes of regular diameter gradually decreasing in calibre as the periphery is reached. The con- tained blood is red in colour, the colour being due to hemo- globin dissolved in the plasma. In the plasma float a few corpuscles, which seem to be little more than the nuclei of the cells forming the blood-vessels. The earthworm has zo special respiratory organs. The blood capillaries approach so near the surface of the body— actually into the epidermis in the clitellar region—that the skin itself serves as a respiratory organ. The alimentary canal is a straight tube. The mouth, just below the prostomium, leads into a dzccal cavity, and that Elementary Zoology. 20 (‘satdads 18430 UY Waray! St [9SsaA [esaqE] JO WSO BYT, ‘q'N) “purl snoxraj19yed ySe] 2Y3 Pus JIwdY YsSe] 9q3 pUigqaq Jossea Testop 947 3 woxy SurBurds uaas st [assea [esse] 943 APoeyuW *[assea UelAJoU VyUT*,S°N { [asSaa-poorq UeLAIoU zidns 's-n f wayshs snoAIEU ‘TD £ (11 pue iv-d) sayouerq Jo syas OM} YIM Jassaa [esiop ‘sI fsoursaqur ‘LUNI { plezzZis ‘zg ¢ xuAreyd ‘yg ‘yynow ‘Ww | {sizeoy,, ‘t (‘samoy Joye emsy peurquios y) "WIOMYEY jo sjuauiBes soraque jo youn ArejuouNye puy ‘uraysAs repNIsEA—"ZI “DI yf TANTEI EG sete eta iaraereed, Qo y 3 x h% wa ra Daan anty Ucar eanr tL UL MIAN HELO The Earthworm. 27 again into the pharynx. The pharynx has very muscular walls, and from it arises the esophagus, a narrower tube. Attached to the walls of the cesophagus are one to three pairs of glands, situated in the tenth to the twelfth segments. The first of these, when there are three, as in Fig. 12, opens into the cesophagus, and is sometimes distinguished from the others as cesophageal pouches. The two other glands open into each other and then into the pouch in front, so that on each side of the body there is a chain of glands opening by means of the most anterior one into the cesophagus. These esophageal glands are sometimes termed the calciferous glands, or “glands of Morren ;” their function is to secrete carbonate of lime. The cesophagus dilates in segment 13 into a thin- walled wide cvof. Immediately upon this follows a_thick- walled gizzard with a particularly thick chitinous lining. After this comes the zvzestine, which passes to the anus at the oppo- site extremity of the body. The dorsal wall of the intestine is folded, and the fold projects into the lumen of the gut, thus increasing its secretory surface. This fold is known as the typhlosole. The intestine is covered externally with a yellowish mass (the chZoragogen), which consists of large cells containing excretory products.’ These cells, sometimes, but erroneously, spoken of as the hepatic cells, are simply that portion of the lining of the coelom which lies upon the gut. The earthworm’s nervous system chiefly lies upon the ventral surface of the body within the body cavity. It is a continuous chain, with a swelling to each segment; the swellings are the ganglia, which contain the bulk of the nerve-cells; the inter- mediate thinner parts are the connectives which unite ganglion with ganglion. At the anterior end of the body, lying on the dorsal surface of the gut just in the furrow which divides the buccal cavity from the pharynx, is the double supra-cesophageal ganglion. This is connected with the ventral chain by a com- missure running round the gut. The reproductive organs of the animal are complicated. The essential organs are the ovaries and the estes, collectively spoken of as the gonads. These are simply local proliferations of the lining cellular membrane of the body cavity. The testes are two pairs of little pear-shaped 28 Elementary Zoology. bodies attached on either side of the nerve-cord to the septa, which separate segments 9, 10, rr. The ovaries occupy a precisely similar situation in 13. Besides these essential organs, which produce respectively spermatozoa and ova, there are ducts which carry off the genital products. Opposite to each testis (but wrapped in certain sacs which will be spoken of immediately) is a much-folded funnel- shaped structure—the funnel of the vas deferens. Each vas deferens is continued into a narrow tube ; the two tubes of each side soon unite to form a single tube, which opens on to the exterior by the orifice already referred to upon the fifteenth segment. Similarly, op- IX : . posite to each ovary is a wide and folded funnel x -—the internal aperture of the oviduct. This opens xt into the very next segment —z.e. the fourteenth—on to the exterior. When Xi the earthworm is mature, there are a series of sacs XH developed by growths of septa, termed the sperm _ sacs, or the vesiculze semi- XIV nales. Of these there are 2-4 pairs lying in seg- XV ments 9, 10, 11,12; and,in addition, in ro, 11 a sin- gle median sac, with which Fic. 13.—Reproductive organs of Lumbricus. 4 these paired sacs com- The segments are numbered. 1, testis; sp, sper- - matheca ; V.SEM, sperm sac ; 0, ovary; o’, rudi- municate. Hence there mentary ovary of segment XII.; v.p, sperm . é duct; F, funnel of oviduct ; 0.p, oviduct; us, 18 a considerable space egg sac. taken up within the seg- ments mentioned. In these sacs lie the testes and the funnels of the sperm ducts, Thus the aha which ripens in the sacs cannot fail to find its way to the extérior, when fertilization takes place. A minute body lying on the septum, bounding the thirteenth The Earthworm. 29 segment behind, usually of a reddish colour, is a corresponding sac belonging to the female system ; it is known as the recepia- culum ovorum, or, better and more simply, as the egg sac, or ovisac. It seems, however, that, though this structure corre- sponds to the sperm sacs, it has no important function. Finally, there are two (or three or more) pairs of spermathece, oval to spherical blind pouches opening on to the exterior by pores lying between segments 9, 10 and ro, 11. The earthworm lays its eggs in cocoons, which are chitinous structures fabricated by the clitellum. When formed they are drawn over the head, the worm gradually withdrawing itself. As they pass the generative orifices they receive ova and sperm (the latter being derived from another individual, who deposits it in the form of little cases sticking to the skin, the so-called spermatophores), as well as albumen for the nourishment of the growing embryos; this latter is derived from glands—the capsulogenous or albumen glands—-which are developed during sexual maturity in the neighbourhood of the sexual organs. CHAPTER IV. THE CRAYFISH (ASTACUS FLUVIATILIS). Tue Crayfish is common in many streams of this country, as well as on the Continent. A very slight examination of a specimen will show that it is an animal made up, like the earthworm, of a series of segments. But it will soon be noticed that this segmentation is clearest in the posterior (abdominal) region of the body, and is less obvious anteriorly. Furthermore, the number of segments into which the body is divisible is much less than in any earth- worm, and the number is absolutely constant. Each segment, too, is provided with a pair of jointed limbs, or appendages. It is from this character that the name of the great group to which the crayfish belongs (Arthropoda) is derived. The body is covered with a hard shell externally, which corresponds to the czdézcle of the worm or to the shell of the anodon. Like the latter, it is indurated by the abundant deposition of calcareous salts. Were the whole exoskeleton to be thus permeated with carbonate of lime, locomotion would be clearly impossible ; so we find that the skeleton is ringed, denser tracts alternating with softer tracts; upon the latter the former move. When the abdomen, for instance, is straightened, the softer tracts of the exoskeleton are seen to disappear beneath the calcified plates. In the anterior region of the body, however—the cephalothorax —the entire back and sides of the animal are covered by a continuous hard plate. This represents a number of the separate hard plates of the abdomi- nal region fused together. The term carapace is applied to this anterior plate ; it may be distinguished by an oblique (cervical) The Crayfish. 31 groove into two parts—an anterior cephalic and a posterior thoracic region. If one of the abdominal segments be examined, it will be seen to consist of a ring of calcified cuticle oval in section. The dorsal part of this is the Zergum, the ventral part the sternum; there“is a ventro-lateral flap projecting downwards, and partly concealing the limbs known as the pleuron, Finally, there is at the junction of the pleuron and the sternum a tract of limited dimensions, termed the epimeron. ab tho cg cep r ait br ai @i2 ait a10 Fic. 14.—The Crayfish (Astacus fluviatilis). y, rostrum ; cep, cephalic shield; ¢4o, thorax; cg, cervical groove, between head and thorax ; a4, abdomen; dr, branchiostegite ; az, antenna ; a 9-14, thoracic appendages. This last lies just above the joints for the articulation of the limb on each side. : In the cephalo-thoracic region precisely the same parts can be distinguished, but their several proportions are different. A section through the thorax is shown in Fig. 25, p. 48. The tergum and the sternum are more limited ; while the pleura are great laterally descending plates, forming a covering for the subjacent gills, and are hence termed the dranchioszegites ; finally, the epimera are long plates of thinnish and imperfectly calcified membrane. In addition, this region of. the body possesses what is called the endosternal skeleton ; this is formed of ingrowths of both the sterna and the epimera; the nervous system is sheltered by the framework thus formed. 32 Elementary Zoology. It has been said that the cephalo-thorax Tepresents a number of fused segments of the body. This is proved by three principal facts; in the first place, the general structure is the same, divisible into terga, sterna, epimeta, etc.; secondly, Fic. 15.-—Abdominal appendages of Crayfish. (After Huxley.) a, first appendage of male; n, do. of female; c, second pair of male; p, £, succeeding swimmerets ; F, tail swimierets ; ¢7.f, endopodite ; ¢x.f, exopodite ; 5.4, basipodite; cx.p, coxopodite ; a, 4, divisions of endopodite. the sterna are separate, and finally, to each sternum corresponds: a pair of appendages. The posterior end of the body is formed by a two-jointed plate, the /e/son, which is not usually regarded as having the The Crayfish. 33 value of a segment. In the same way the body ends anteriorly in the projecting rostrum. The crayfish, enclosed as it is in this rigid “skin,” could not grow were it not for the fact that the shell is shed at intervals, a new cuticle having been in the mean time formed beneath the old one. Each segment of the crayfish’s body is provided with a pair of appendages, which are not invariably locomotive, but perform, as will be seen, various functions. It will be con- venient to commence with one of the abdominal swimmerets which shows the limb, it is believed, in its simplest form. The third abdominal appendage is shown in Fig. 15, E. It consists of a basal portion, with two branches arising from this ; the basal part is termed the profopodite, its two distal branches, according to their position with reference to the body, the exopodite and endopodite. The limb is thus a biramous structure, shaped like a Y. The protopodite really consists of two separate calcareous pieces separated by a soft tract ; it is, in fact, two-jointed. To these two joints the names coxopodite and basipodite are applied. The endopodite consists of a large terminal joint and a shorter annulated joint; the exopodite, which is smaller, has the same divisions, but the annulated part is proportionately longer. This description applies, not only to the limb from which it has been drawn, but also to the fourth and fifth of the abdominal limbs. The first two abdominal appendages not only differ from this, but differ from each other according to the sex of the individual. In the female the second pair are the same as the limb which has just been described, but the first pair are very rudimentary structures ; they have merely a short protopodite anda feeble flagellum, the whole limb varying much in length, as is common with rudi- mentary organs. In the male, the first pair of abdominal limbs is a stiff scroll-like structure, shown in the figure (Fig. 15, A). The boundaries between protopodite, exopo- dite, and endopodite cannot be distinguished. In this sex the second pair of limbs is particularly strong ; the basal piece of the endopodite is much larger, and ensheathes (Fig. 15, C a, 6) the small flagellum. We have, finally, to consider the tail swimmerets, or wvofoda, as they are often termed. These D 34. Elementary Zoology. are powerful swimming organs, at first sight very different from the comparatively feeble limbs that we have been studying; but they are plainly reducible to the same elements. There is the protopodite, of two joints; then both exopodite and endopodite, of which the former only is plainly two-jointed. The joints, moreover, are strong and flattened plates, not annulated flagella-like outgrowths (Fig. 15, F). Working forwards from the first pair of the abdominal appendages, the next five pairs (Fig. 16) are ambulatory appendages: they are used for walking; and the fact that there are five has given the special group of arthropods to which the crayfish belongs the name of Decapoda. As their function is different from that of the swimming swimmerets, so their structure is also different. If we take one of the last of these limbs, it is seen to be made up of seven joints of vary- ing lengths, which are thus named, commencing with that which articulates with the body: coxopodite, basipodite, ischio- polite, meropodite, carpopodite, propodite, dactylopodite. These joints are to be found in all the five pairs of ambulatory limbs; but in the three first of these appendages the propodite is lengthened out into a process which lies parallel with the dactylopodite, and forms with it a pair of “pincers.” In the first pair of ambulatory limbs these are especially well developed, and the whole appendage is larger, being called the chela. It is not, at first sight, obvious how these ambu- latory limbs can be brought into line with the abdominal swimmerets. They are uniramous, and not biramous. If it be supposed that they are uniramous through the loss of one branch, which branch has been lost, the exopodite or the endopodite? The crayfish itself supplies no answer to this question ; but its near ally, the lobster, does, a fact which shows the necessity of a comparative study for the unravelling of such problems. When the young lobster is newly hatched, its thoracic appendages are in the “schizopodous” condition, as it is termed, z.e. each is biramous; later on the outer branch dis- appears, and that which is left is thus the endopodite. It may be inferred, then, on account of the detailed likeness The Crayfish. 35 between the thoracic appendages of the lobster and the cray- fish, that the thoracic limbs of the latter consist of protopodite and endopodite. We next come to a set of limbs which sub- serve the function of manducation: they assist in comminuting \ Fic. 16.—Thoracic limb of Crayfish. (After Huxley.) 3 dite ; 44, basipodite ; 7, ischiopodite ; 72/, meropodite ; *o. er, tenes 3 ap, A iaptosediiet ex.s, coxopodite sete ; ¢, epipodite. the food. Here again we should expect to find what we do. find, that a difference of function is accompanied by a differ- ence of structure. These limbs are termed—those next to the 36 Elementary Zoology. ambulatory limbs—the maxil/lipedes, consisting of three pairs; then follow two pairs of maxil/e, and finally a single pair of mandibles, The maxillipedes, as their name denotes, are not exclusively manducatory appendages; they are partly ambu- latory, and are intermediate in structure. They are, however, more “typical” in their formation than the ambulatory appendages. They are all biramous. The last pair (see Fic. 17.—Third maxillipede of Crayfish. (After Huxley.) ex, exopodite ; other letters as in Fig. 16. Fig. 17) has a protopodite and an endopodite much like that of the ambulatory limb ; the details of difference can be gathered from an inspection of the figure. But it has, in addition, an exopodite, shorter than the endopodite, and ending in a flagellum like that of the swimmerets. ‘The pair of appendages in front of this, the second maxillipedes — The Crayfish. 37 (Fig. 18, B), is much the same, but the endopodite has com- menced to deteriorate, and the exopodite is now larger. In the first pair of maxillipedes (Fig. 18, A) the protopodite has changed into two perfectly separated flattened plates, which are applied to the sides of the mouth, and are clearly jaws; the endopodite has become quite small, and the exo- podite large in proportion, In front of these are the two pairs of maxille: The last pair of these is not very different from the first maxillipedes; but the two joints of the proto- Fic. 18.—A, first, B, second, maxillipede of Crayfish. (After Huxley.) e, ef, epipodite ; ex, endopodite ; ex, exopodite; other letters as in Fig. 16. podite are still more flattened and jaw-like, and each joint has become partially separated into two. The endopodite is still small, and the exopodite is fused with a flat plate, present in the appendages lying behind, and there associated with the gills (g.v.), the so-called epipodite. In the first pair of maxille the protopodite is all of functional importance that remains of the appendage, but there is still a small endopodite. Finally, there are the mandibles, in which the protopodite forms a massive chewing organ, to which is appended a small and jointed endopodite. We can trace, therefore, in the series of mouth appendages, a gradual change from ambulatory 38 Elementary Zoology. to manducatory limbs, brought about by the growing im- portance of the protopodite and lessening importance of the endopodite and exopodite. It may be mentioned that the con- joined exopodite and epipodite of the second maxilla is often called the scaphognathite ; it serves to create a current of water through the branchial chamber, and thus to renovate the body through the oxygen absorbed by the branchiz. Fic. 19.—Mouth appendages of Crayfish. (After Huxley.) 4, mandible ; 8, first, c, second, maxilla; 4, palp; 5.4, basipodite ; cxf. coxopodite ; en, endopodite ; se, scaphognathite. Besides the appendages that have been already enumerated, the crayfish possesses two other pairs. These are the antenna and the aztennules. The antenne, which form the second pair of these, are shown in Fig. 20, C. Each consists of a basal piece, two-jointed, which may be compared with the proto- podite of other limbs. With this articulate, firstly a scale-like structure, the scaphocerite, which is perfectly movably articulated, The Crapfish. 39 and a long, many-jointed flagellum.' The scale is believed to represent the exopodite, and the flagellum the endopodite, Finally, the antennules have the form illustrated in Fig. 20, B. Each is formed of a basal piece consisting of three joints, which bears distally two approximately equally sized flagella. It is tempting to see here also a biramous limb of the typical character. But it is not clear that this comparison is justifi- able, for it will be observed that the two flagella spring from the third joint, while in all other cases the endopodite and Fic. 20.—Head appendages of Crayfish. (After Huxley.) a, eye-stalk ; B, antennule ; c, part of antenna ; ¢, surface of eye ; ex.p, exopodite ; y.g, orifice of green gland. exopodite arise from a protopodite consisting of two joints. More probably, therefore, the bifid character of the terminal part of the antennules has nothing to do with the biramous limb, but is a simple fission or a secondarily added appendix. Some naturalists have endeavoured to show that the eye, or rather the eye-stalk, is a rudimentary appendage, consisting in this event of the protopodite only. The stalk is certainly movable; but as the structure arises from a different part of ! The use of the term ‘“‘ flagellum” for a piece composed of numerous minute joints will not be confounded with the long vibratile cilium, which, when single or in groups of very few, in Protozoa and other animals bears the same name. 40 Elementary Zoology. the embryo to that whence the other appendages arise, it is probably not justifiable to make this comparison. We thus see that the crayfish body consists of nineteen segments, each provided with a pair of appendages, a rostrum in front and a telson behind. That the segments are less clear anteriorly, as is the case with the earthworm, where their characters are less marked (2.c. absence of sete on the first, absence of distinct septa internally, etc.) ; there is, in fact, by this means, a specialized anterior region-—a Aead formed. The limbs are reducible to a common plan, which is modified in accordance with the varying function of the,limbs. In the course of the dissection necessary to follow the fore- going description, the organs of respiration—the gil/s or branchie —will have been noted. They are feather-like structures lying beneath the branchiostegite, and limited in consequence to the thoracic region of the body. On each side there are altogether eighteen fully developed gills, besides certain rudiments which will be referred to in due course. These gills, however, are arranged distinctly in three series, and according to their position have been termed podobranchs, arthrobranchs, and pleurobranchs. When the thoracic limbs (7-12 inclusive) are removed a gill will be removed also. If care is taken, only one gill will be torn away with each limb. These gills, which are attached to the coxopodites of the limbs, are the podo- branchs. Each consists of a stem which bears a series of filaments on each side. The stem expands above into a lamina which is bent in the form of an open book. On the first maxillipede there is no podobranch; at least, no fully developed podobranch. There is, however, the structure already described as the epipodite. It is nearly certain that this membranous plate is a rudimentary podobranch; and these are the reasons which lead to that inference. In the first place it occupies precisely the same relations to the limb that bears it as do the podobranchs of the six following gills. Secondly, it is a longitudinally folded plate, like the lamina of the gills, and bears certain hooked spinelets upon its surface exactly comparable to spinelets borne upon the lamina of the fully developed gills. But the strongest argument is that in The Crayfish. 41 certain exotic crayfishes this epipodite has a few rudimentary branchial filaments. When the podobranchs have been removed, other gills come into view. These are termed arthrobranchiz, from the fact that they are attached to the articular membrane, to which is also attached the limbs. These gills correspond to the same appendages that bear the podobranchs; there are, however, a pair to each limb, except No. 8, which has but one—that is to say, there are eleven in all. The structure of these arthrobranchiz differs from that of the podobranchiz in that there is no lamina; the stem which bears the gill filaments is not expanded into the folded plate characteristic of the podobranchs. Finally, the last thoracic limb (No. 13 of the entire series) bears a single gill, which is attached at a level above that of the podobranchs and arthrobranchs—to the epimeron, in fact. This is the pleurobranch. In front of it, and occupying a similar position with regard to the two limbs in front, is a tiny unbranched filament which is believed to represent a rudimentary pleurobranch: from the fact that it occupies a similar position, and—more important—from the fact that in various exotic crayfish there are fully developed pleurobranchs corresponding to the limbs in question. The rudiment, such as it is, may be compared to the stem of the otherwise missing gill. It is convenient for the purposes of a ready comparison with the gills of other crayfishes to express the arrangement of the gills as a formula :— Appendages of segt. Podobr. Arthrobr. — Pleurobr. 65. ase oO (ep.) ° oO ae 1 1+0 ° 8. I I+1 oO 9. I I+1 ° 10. I I+I1 Oo Il. I r+1 r 12. I I+! r 13. ° ° I 6+ep. + 645 + eee 18 + ep. + 27 When the body of the crayfish is opened by dissecting off Elementary Zoology. 42 sede sil ‘vaa {suarajep sea ‘aa { Axaize yerueaa ‘vA { apD1ISsO OVIpseooan ‘0D ‘eanqaade pelrieed ean “a aaa OE, t siaso) "2 teusays ‘5 { Arejae peurels ‘vs { wmysos “a $ yOEUIOIs DTIO[Ad ‘Ad f sDISsO O¥IprEoOIDd ‘OLA t aq~podojoad “aa { apIsso OLIo]Adaid ‘oaa fa[orsso oaofAd ‘oa { uomeyd “ta { savpodojoud ‘a { snSeydoseo ‘zo | oinsstwiu109 TeeSeydoszoumno.ts ‘90 { Arar orarpeysqdo “vo ‘ eupod “opus ‘x $4300) uerpaur ‘Lin {euroysejouI ‘SW { eypodosour W { y3003 yeraIE] ‘LT f zoAry “1 { puLys weead ‘M ‘ sirpodorypsi ‘st $ wnaep rojo01d, ") { Azaqze oryeday ‘vH {j2eey Jo wWNTso [eIeqe] ‘H { VI[ZURS jo UrEYyO TerIUSA 1-29 $ eysued jeeseydoseo vidns ‘15 {ofa ‘a { Axajre [euTWOpqe yessop ‘va | spodojAjoep “a { apodoxoo ‘xd !seqpododied “649 ‘aD ¢ e[dIsso OeIpAva foo § wnd209 ‘WO f yYORWOIS Jo Jaqureyd ovipres ‘9 fyonp ayiq jo ainjzede ‘aa foypodiseq ‘a foes Aroypne ‘nv ¢ Axe ‘Areuusjue ‘vv {uojo[eys eMsesrydopua ‘v “JsINZ pue [peyseyy way) ‘qsyAesD ay3 jo worsessip [esued)—"12 “DIT a Wa “Nod a xo gg 6x dd w vw 89 2969 SI . . vs Gn WaA #19 \ ee OAT T gin x aw E W ST RX MS AS Sr LASERS a # zat S THEN 7 ? ae | IN ee oe <, Ix = = = y Ee Z i \ eA G = ? io 9110 1 iniminicinahace Sew y S11 91S Yu a al | wo PaO NT vd aN L \ 1 3 a H ovS va ny 19 09 2 {oan | od oid 92 IW dd Odd The Crayfish. 43 the shell above, the interior of the body is seen to be almost entirely filled with the various viscera. Chinks of an irregular form will be visible between the white masses of the muscles; but there is no spacious cavity like that of the frog or earthworm. In the crayfish, in fact, the body cavity is greatly reduced, and what there is in the way of irregular spaces is not ccelom, but is made up of blood spaces in communication with the heart. There are, however, perhaps traces of the ccelom in two places: in the interior of the generative gland and at the extremity of the renal organ. But these matters will be entered into when the organs in question are described. The muscular system of the crayfish is enormously developed. The muscles form masses which pass from segment to segment and between the various joints of the appendages. Their detailed description, however, would occupy more space than can be allowed here. When the body is opened from above, . . the first organ that comes into view, commencing from above, is the fear¢. This is a thick-walled sac of somewhat hexagonal form. It lies in a thin-walled sac, which is often—but erroneously—called the pericardium. The strong muscular wall of the heart is perforated by six apertures, the ostza. Of these two are dorsal, two lateral, and two more ventral. They permit a free entry of blood from the enveloping sac into the heart; but the valves with which they are provided internally prevent the egress of blood from the heart into the investing sac. This so-called pericardium cannot be compared with the pericardium of the anodon of the frog, for several reasons. In the latter animals the pericardium is a portion (in anodon practically the whole) of the ccelom, which has no relation to the vascular system. In Astacus the so-called pericardium is to be regarded simply as a number of fused veins whose originally separate orifices into the heart are the existing ostia. It is better to term it the “auricle.” It is an auricle opening into the ventricle which happens incidentally to envelopit. The pericardium of the other animals mentioned is not connected by openings with the heart which lies in it. ‘The accompanying diagrams may serve to explain the way in which the peculiar auricle of the crayfish’s heart arose. 44 Elementary Zoology. From the ventricle spring a number of arteries, which are delicate and transparent, and not readily visible until they are injected with some coloured fluid. In the middle line in front is the ophthalmic artery, which runs in a straight course to the eyes. On either side of this is an antennary artery. Each of these supplies the antennz, antennules, and the green gland of its own side. A little further back there springs from the ventricle on either side an hepatic artery, which, as its name implies, goes to the liver. The stomach is provided with blood from the antennary artery. From the posterior margin zull LL a \ ee \ p y VAL ab Fic. 22.—Diagrams to illustrate the formation of the ‘‘ pericardial sac" of Arthropods from the fusion of veins with originally separate openings. (After Lankester.) SOA eer ha i Vac . ~ VALW) \_\ A aaa Ce Oen a a a Parse c of the ventricle arise two arteries; one of these, called the superior abdominal artery, runs straight along the surface of the intestine, giving branches to it and to the adjacent muscles ; the other descends obliquely downwards, and is known as the sternal artery. It passes between two of the thoracic nerve ganglia, and divides immediately into a forwardly running and a backwardly running artery. These give off a regular series of branches, which between them supply the appendages and the ventral musculature of the body. The veins of the crayfish are largely sinuses, z.¢. more or less wide and irregularly shaped channels. ‘The blood in these enters a particularly large sinus lying on the ventral side of the body; and from this the blood passes by a series of veins to the gills, and thence by another series to the heart. The d/s0d of the crayfish is a colourless The Crayfish. 45 fluid which contains a number of floating corpuscles amoeboid in form, The alimentary tract of the crayfish consists of a straight tube, which is not of the same character throughout, and of a pair of large glands, generally called the liver, which open into it. The first part of the alimentary canal is the esophagus, which opens externally by the mouth, and passes upwards from that point to open into the large stomach. The stomach is formed of two compartments, an anterior cardiac and a posterior pyloric portion. The cardiac portion has its inner surface thickened by a number of strong calcified pieces, which together form a masticating apparatus for the animal’s food. The various ossicles of this stomachal skeleton are so arranged that three specially hard “teeth,” borne upon the extremities of some of the ossicles, can be made to converge in the middle line, and effectually break into pieces any hard particle of food. The general arrangement of these various ossicles and teeth may be appreciated by an inspection of the accompanying diagram (Fig. 21); their action can be seen by pulling upon the two ends of the apparatus of the stomach with two forceps. The pyloric part of the stomach is provided with a number of more delicate plates furnished with stiff hairs, the whole forming rather a sifting than a crushing organ. Immediately upon the stomach there follows a very short tract of gut, which differs from the preceding and from the part which follows it, in having soft walls; it is not lined, as is the rest of the canal, by a thick cuticular lining. This mesenteron is produced dorsally into a short cecum, and it receives the ducts of the two hepatic glands, After this comes the find gut, which has a longitudinally folded cuticular lining; it opens on to the exterior by the anus, The excretory organs of the crayfish consist of a single pair of glands known as the green glands ; they are placed in the head region of the body, and open on to the exterior bya pore placed upon the basal joint of the antenna upon a tubercule which that joint bears. The gland, which owes its name to its bright green colour, has been proved to be excretory in function, 46 Elementary Zoology. to contain guanin, an excretory product. The gland consists of a coiled tube opening into a terminal sac, which itself, as already stated, opens on to the exterior. A small “end sac” has been described in connection with the excretory organ, which has been regarded as a remnant of ccelom; in this case, therefore, the green gland of Astacus will be comparable toa nephridium of Zwmbricus, in so far as it is a glandular tube, opening at the one hand into the ccelom, and at the other on to the exterior. If this structure be not a vestige of ccelom St a a i Tc ma Fig, 23.—Dissection of anterior end of Crayfish, to show relations of green gland. (After Marschall.) v, vesicle of green gland, G, glandular portion ; St, stomach; MAN.MUS, mandibular muscle. there is still no final difficulty in comparing the green gland with a nephridium ; for in an animal in which the ccelum is so largely aborted as it is in Astacus, it might be very well supposed that the nephridial funnel had disappeared also. If there be some difficulty in comparing the excretory system of the crayfish with that of the earthworm, there is no difficulty in comparing the nervous systems of the two animals. In the crayfish we have precisely the same plan of central nervous system. There area pair of supra-cesophageal ganglia just below the rostrum; from these arise circum-cesophageal commissures, uniting behind the cesophagus to form a chain of ganglia extending through the body. This chain consists of six separate pairs of ganglia in the cephalothoracic region, and The Crayfish. 47 six in the abdominal. It will be observed that in the abdominal region there is an exact correspondence between ganglia and segments, but not in the cephalothoracic. As, however, there is some fusion between the originally separate segments of the anterior region of the body, it is hardly surprising to find that this characteristic has extended itself to the ganglia of the nervous system. From the circum-cesophageal commissure and from the cerebral (supra-cesophageal) ganglia arise three Fic. 24.—Green gland of Crayfish. (After Marschall.) The upper figure represents the apparatus dissected out. v, vesicle; /.s, terminalsac ; between them lies the glandular part. nerves which unite to form a trunk, which supplies the anterior part of the alimentary tract, and is known as the visceral nervous system. The crayfish is well off for sense organs. The eyes have been already mentioned. They have a complex structure, equally complicated with that of the eye of a man, but of a different character. It will not be possible to describe them 48 Elementary Zoology. here. The ofactory organs appear to be represented by certain hairs upon the antennules. Other hairs which cover the body and the appendages seem to be of a ¢acti/e nature ; the auditory organ is a sac in the anterior of the basal joint ‘of the anten- nules, with an opening to the exterior, and a lining of delicate auditory hairs, between which float tiny particles—the ofoliths, The crayfish is of two sexes: there are males and females. Externally, the two sexes can be readily distinguished. In the Fic. 25.—Diagrammatic transverse section of Crayfish. (After T. J. Parker.) C&L, cavity of gonad; m, muscles; INT, mesenteron with liver appended. female the abdomen is broader and deeper in its excavation below; this serves to pack away the eggs, which when extruded are attached to the swimmerets. In the two sexes the first two pairs of abdominal appendages are modified in the way that has been already described. In the male the reproductive organs open on to the last pair of thoracic appendages ; in the female on to the last but two. The “gland” itself in both sexes is a Y-shaped mass, white in the male and the immature female, brown in the more mature female. It is a hollow structure, and the cavity communicates with the lumen of the The Crayfish. 49 duct, which carries the generative products to the exterior. These ducts are long and much coiled in the male; short in the female. It is believed that the cavity of the generative gland is ccelomic in nature. This conclusion has been arrived at from a consideration of the formation of the gonad in animals. The gonads are, as already pointed out to be, regarded as local proliferations of the ccelomic epithelium, either restricted to a limited area, as in the earthworm, the vertebrate, etc., or spreading through the whole, or nearly the whole, of the body, as in certain marine worms. If this portion where the generative tissue is formed be separated from the rest of the body, and the ccelom obliterated elsewhere than here, we arrive at a structure like that of the crayfish, where, moreover, it will necessarily follow that the oviducts, or sperm ducts, opening into this intra-gonadial region of the ccelom will get the appearance of being merely prolongations of the gonads themselves (see also p. 159). ‘ CHAPTER V. THE COCKROACH (BLATTA ORIENTALIS), THe common Cockroach of our kitchens is the one that is selected as the type for the present chapter; but an American species, Leriplaneta americana, is really a better animal for dissection, as it is larger than the former species, which origi- nally came to us from the East. The most important difference between the two is, that P. americana is winged in both sexes, while in Béatta orientalis it is only the male which has fully developed wings. The cockroach belongs to the same great division of the animal kingdom as does the crayfish. Itis an arthropod ; but it breathes by means of tracheze instead of gills, and is referred, on that and other accounts, to a separate division of the Arthropoda, the Tracheata, in which division it is a representa- tive of the class Insecta. As in the crayfish, the body is clearly segmented, and the segmentation is more obvious in the abdominal than in the thoracic or cephalic regions. There is a plain division between head and thorax, a narrow “neck” joining and marking the line of division between them. It is believed, from a consideration of the appendages, which will be considered presently, that the head consists of three fused segments, lying behind the mouth, and a prostomium, comparable to that of the earthworm, lying in front of it. There are, however, no indications in the sclerites, which form the exo- skeleton of the head, that this is probably the case. The head, which is carried at night angles to the thorax, its appendages lying therefore behind it instead of below it, is covered bya The Cockroach. 51 wide sclerite above (the icranium), divided by a Y-shaped suture into right and left halves; each fork of the Y termi- nates in a round soft spot of unknown function. In front of and below this plate is the d@ypeus, with which articulates still further forward the upper lip, or 4abrum—not an appendage. At the sides of the head are two other pieces, the gene. Above the genz are the conspicuous compound eyes, black in colour, owing to the contained pigment. The ¢iorax evidently consists of three segments : each hasa dorsal plate, the #ergum , a ventral plate, the sternum, and two smaller elements, the episternum and epimeron. From the mesonotum and metanotum, as the terga of the second and third segments are called (the first being ronotum), arise wings. The first pair of these are stiff, and serve as covers (tegmina) for the second pair, which are delicate, and folded when not in use. These wings are processes of the terga, and may so far be compared to the branchiostegite of the crayfish. The abdomen is made up of certainly ten, and possibly eleven, segments; the eleventh is a pair of plates which lie on either side of the anus. Each segment has a tergal and a sternal plate, connected by soft membrane. The segments differ somewhat in size, the eighth and ninth being very small, and usually hidden by the seventh in the female of 2. orientalis. Thus the entire body of Blatta is made up of not more than seventeen segments. The antenne are not equivalent to the following appendages ; they arise from the prostomium, and are therefore probably the equivalents of cirri, which are found arising from the pros- tomium in many aquatic worms. It is true that in the crayfish both antennze and antennules are preoral in position; but a consideration of the structure of the cerebral ganglia which supplies them with nerves has led to the conclusion that it is a compound structure containing not only the true cerebral ganglia, but a pair of primitive postoral ganglia, which have moved forwards. Thus, the first pair of appendages in Blatta are the mandibles ; it is these that correspond to the antennules of the crayfish. The mandibles are strong and jaw-like, consist- ing of a basal piece only. The next appendage can be homo- logized with the typical Crustacean limb, as already explained. 62 Elementary Zoology. Each consists of a protopodite and of an endopodite and exopodite. In the first of these—the fist maxille—the proto- Fic. 26.—Mouth appendages of Cockroach. (From Miall and Denny.) Mn, mandible; Mx’, first maxilla ; Mx", fused second maxilla; Ca, cardo; St, stipes; La, lacinia; Ga, galea; Pa, palp; Sm, submentum. podite has the usual two joints, the cardo and stipes ; with the The Cockroach. 53 latter articulates, first, the endopodite, divided longitudinally into two pieces, an inner /acimia and an outer galea, The exopodite is a five-jointed palp. The second maxille have their protopodites fused to form a basal piece, divided into a submentum above and a mentum below; the endopodite is divided into a faraglossa (or lacinia) and a galea; while, as in the first maxilla, the exopodite is a palp, but three- jointed. The three pairs of thoracic appendages (whence the term “Hexapoda” for insects) are walking limbs with five joints apiece and a pair of terminal claws. The abdomen has no obvious appendages ; but the cerci borne on the tenth segment and the anal styles of the male on the ninth may be vestiges of such structures, The remaining characters of importance to be noted without dissection are the orifices of the tracheze and’ of the scent glands. ‘The former—the stigmata or spiracles—are present to the number of nine or ten pairs—two upon the thorax, and the rest upon the abdomen. They are widish orifices, guarded by hairs, which lie laterally between the segments, and lead into the tracheal tubes, which ramify in the interior of the body, and will be described presently. The scent glands, which have been only lately discovered, are two pouches lying on the dorsal surface of the fifth abdominal segment, between this segment and the next behind. The feart is a dorsally situated tube which ends blindly behind, and is prolonged into an aorta anteriorly. The heart has a series of paired lateral ostia, and lies in a blood space like that of Astacus. But the circulation generally of B/atra is less “ finished ” than that of Astacus. In addition to the pulsa- ting sinus which envelops the heart, there is a ventral sinus, also , pulsating, which covers the nerve-cord. The respiratory organs are the trachee. These are tubes lined with a spirally thickened chitinous membrane, which ramify through the body, as shown in Figs. 27 and 28. Air is thus carried to every organ ; and this complete aeration perhaps accounts for the imperfection of the circulatory system, which is usually the only, or nearly the only, way in which the organs 54 Elementary Zoology. and tissues can receive their supply of oxygen and get rid of their carbonic acid. A considerable portion of the space within the body is taken up by the fat body. This is an irregular ramifying mass ———= IM il +t | . oan | l I ries me | I'1G. 27.—Tracheal systein of Cockroach. Fic. 28.—Tracheal system of Cockroach (After Miall and Denny.) Fi after removal of' alimentary tract. sp, spiracles; ST, stomach; INT, intestine. (After Miall and Denny.) of white tissue made up of cells in which fat globules are deposited. It has also been found to contain uric acid, and therefore it seems to play some part in excretion The Cockroach. 55 The alimentary canal is rather complicated. As in the “crayfish, there is a large anterior section lined with chitin, and ‘termed the stomodsum ; a large posterior section, also lined with chitin, and termed the proctodeum ; and a short middle a section, the mesenteron. The mouth leads through a short ** ceesophagus,- gradually widening into a large crop; this is followed by a thick-walled gizzard, with six large cuticular teeth. Then follows the short mesenteron. The proctodzeum is divisible into a short ileum, a long colon, and a wide terminal rectum whose inner walls are raised into a number of ridges. The alimentary tract is furnished with a number of glands. Firstly, the sadivary glands, which are present on each side of the crop to the number of two. They are white diffuse glands, and between them lies a bladder, the sadvary receptacle. The ducts of all unite to open into the mouth cavity. Opening into the mesenteron (sometimes called the “ chylific stomach”) are the epatic ceca, blind tubes, seven or eight in number. Finally, there are the malpighian tudes, sixty or more, which open into the commencement of the intestine. These, how- ever, are not glands supplying any fluid used in digestion ; they are excretory in function, and uric acid has been found in them. It is doubtful whether these structures can be com- pared with the excretory organs of other animals. There are, however, a few facts which render a comparison possible (see p. 158). But it is necessary, in the mean time, to dwell rather upon their unlikeness to the excretory organs of other animals. The mervous system is constructed precisely on the plan of that of Astacus. There is a supracesophageal ganglion con- nected by a circumcesophageal commissure with a ventral chain of three thoracic and six abdominal ganglia, besides a subcesophageal ganglion in front, which seems to represent three fused ganglia, as it supplies the three first postoral pairs of appendages. There is also a visceral nervous system, consisting of a small frontal ganglion, which is connected with the circumcesophageal commissure by two nerves, and which gives off backwards a single nerve, running back along the crop to a small ganglion on the dorsal surface of the crop, whence two nerves run still further back. 56 Elementary Zoology. The reproductive organs are elaborate. In the male the testes lie imbedded in the fat body. From them arise two vasa deferentia, which open into the wider and muscular eaculatory duct, At the junction of the two are the wesicule seminales, termed collectively the ‘“ mushroom-shaped gland.” The genital aperture, which is upon the last segment, just below the anus, is provided with a complex series of chitinous out- growths, the gonapophyses. In the female each ovary consists of eight tubes, which unite to form the oviduct; the two oviducts open together upon the eighth sternum. Further back, upon the ninth sternum, opens the sfermatheca, which consists of a pear-shaped pouch and a narrow coiled tube; the latter seems to be the equivalent of a second spermatheca. On the following seg- ment open the much-branched colleterial glands. The female is furnished, as is the male, with gonapophyses. CHAPTER VI. THE METAMORPHOSES OF INSECTS. THE young Cockroach is hatched from the egg in a condition in which it hardly differs from the parent insect. It is simply paler in colour ; its eyes are smaller, with fewer facets ; the wings are not developed, and the reproductive organs are immature. The change from the newly hatched insect to the reproductive adult is simply a question of growth. This is naturally accom- panied by several moults, of which there appear to be seven. On the other hand, there are many insects which leave the egg in a condition only very remotely resembling the parent form. There is a vast difference between the “woolly bear” and the “tiger moth,” into which it is ultimately converted. Moreover, in this case, the transition from larva to adult is not a gradual one. There is an abrupt change: from the cater- pillar to the chrysalis, from the chrysalis to,the moth. To these changes the term “ metamorphosis ” is applied ; and insects which exhibit such metamorphoses are termed “ metabolous ” to distinguish them from the ‘“ametabolous” cockroach. Among metabolous insects are the neuroptera (dragon-flies), hymenoptera (ants, bees, and wasps), the diptera (two-winged flies), and lepidoptera (butterflies and moths). In order to illustrate the nature of this metamorphosis, we shall select an example from each of the last two classes of insects; the silk- worm will serve as a type of the lepidoptera, the blow-fly of the diptera. The life-history of the Silk-worm (Bomdbyx mori) is briefly this: From the egg emerges a caterpillar, which feeds upon leaves, and continues to grow until it is immensely larger, but 58 Elementary Zoology. not different in structure from the minute newly hatched larva. It then becomes quiescent, and secretes a dense chitinous coat, which covers its nearly motionless body. This is the pupa stage, which is further protected by the cocoon, the well- known silk produced by the larva from certain glands, which entirely envelops it. From the pupa, after a certain lapse of time, emerges the moth—the imago, or perfect insect, as it is called. The caterpillar is a segmented creature with so great a like- ness between the successive segments that a vermiform appear- ance results. Hence the general term of “worm” popularly applied to insect larvee, especially to those with no legs. The head of the caterpillar has a pair of small antenne, sessile eyes (differing in structure from the compound faceted eyes of the adult), a pair of strong biting mandibles, and a plate behind the mouth which corresponds to the next two pairs of appendages fused together. After the head follow three segments, which constitute the thorax ; each of these has a pair of jointed walk- ing legs. Then comes an abdomen of ten segments (possibly with a terminal eleventh). Not all of these segments have limbs, but the third to the sixth, and the last, have fleshy “ prolegs”’ armed with terminal hooks. When the larva reaches a certain size it becomes a pupa: The pupal stage is quiescent; the pupa, when touched, will exhibit movements, and in many moths which enter the pupal stage in burrows in wood the pupa moves towards the surface just before the time of emergence of the imago. But, broadly speaking, the pupal stage is quiescent. During this period of rest important changes take place. The tissues of many of the organs break down, and are reconstructed into the definitive organs of the imago; to this remarkable histological process the term “histolysis” has been applied. The pupa much more resembles the perfect insect than it does the larva. The appendages of the imago, its wings, and compound eyes, are visible in the pupa, while the external reproductive orifices are visible by marks, The number of segments in the pupa appears to be the same as in the larva, ie. certainly ten abdominal segments and three thoracic. The Metamorphoses of Insects. 59 The hard chitinous integument which envelops the pupa is not the larval skin; it is formed by the hardening of a sticky fluid thrown off at the moment when the caterpillar skin is also thrown off. Ifa mature caterpillar be dissected just before this period, the various organs of the pupa, wings, limbs, etc., can be seen to be perfectly free. After a longer or shorter period of rest the perfect insect breaks through the cuticle of the pupa, and escapes. On reviewing the life-history of this moth it will be observed that a great difference in the way of life accompanies the more complicated metamorphosis. In the cockroach, the young leads practically the same kind of life as the adult. The difference between them is hardly more than that the earlier stages are devoted to growth, the latest to reproduction, This difference in way of life is also seen between the caterpillar and the moth—in a more marked fashion, indeed, for the feeding of a perfect insect is often practically nothing at all; but, whereas the caterpillar eats leaves, the moth sucks the juices of flowers, and has, as a consequence, a totally different arrangement of the mouth organs. It is this diversity of mode of life which has led—in the opinion of some—to not only the extraordinary difference between larva and imago, but also to the existence of the quiescent pupa. To change the biting parts of the caterpillar’s mouth for the sucking proboscis of the moth would necessitate, if the development were gradual, a series of intermediate stages, which would not be serviceable in either capacity. Hence, during a quiescent period, in which no food is taken, and during which the animal is protected by the silk cocoon, the changes could be, and are, brought about. TheBlow-fly (J/usca vomitoria) hasa series of metamorphoses, which do not differ greatly from those of the silk-worm. From the egg is produced a larva, which differs from that of the silk- worm in being apodous; the limbs are absent. The pupa, into which the maggot turns, is again different, in that its outer brown chitinous case is not a new formation, but the shrunken skin of the larva. Within this skin the process of breaking down of the tissues goes on rapidly. The generative organs, however, appear to be continuous with those of the larva, where 60 Elementary Zoology. they may be recognized as rudiments, as in the caterpillar of the moth. Coincidently with this histolysis, or breaking down of tissues, occurs an histogenesis, or building up of tissue. This largely takes place from the so-called “imaginal discs,” which are buds of the outer layer; but other of the internal organs are reproduced by reformation of the disrupted tissues of the corresponding larval organs. It seems to be mainly the external parts that are formed from the imaginal discs. CHAPTER VII. THE POND MUSSEL (ANODONTA CYGNZEA). THE common Fresh-water Mussels really belong to two genera ; but those referable to Anodonta are most usually made use of for purposes of dissection, and most current descriptions refer to that genus. It contains two species that are found in this country, A. cygn@a and A. anatina, They both occur in lakes, ponds, and canals, where the water is sluggish. The animal lies buried in the mud at the bottom of such a piece of water, with the narrower end imbedded in the mud and the broader end freely emerging. A current of water is kept up by the action of cilia through the posterior end of the shell, so that minute organisms are brought towards the mouth at the opposite extremity. At the same time the current of water serves for the aeration of the gills. The anodon is of two sexes, and the males cannot be distinguished externally from the females, as has been often asserted to be possible. So rare, too, are the males, that the females may possibly also develop male products; but there is no positive evidence upon the point. Out of fifty anodons dredged, Mr. Latter found only two males. The life-history of the anodon is interesting. The eggs are shed into the external, not into the internal, gill; the walls of the gill secrete a mucous matter, in which the developing eggs are held. The young are hatched from the eggs in a stage which has been termed Glochidium, on account of the fact that these young organisms were regarded originally as independent organisms. The glochidium has the valves of its shell armed each with a long curved spine, and the foot secretes a bunch of filaments, the dyssus. This 62 Elementary Zoology. byssus serves to moor the young to the gill-plates; it has the interesting feature of being the equivalent of the byssus of the sea-water mussel, by means of which that mollusc can hang on to piles of wood, etc., a fact which seems to show that the anodon is descended from a byssus-bearing bivalve. The young glochidia are ultimately expelled from the sheltering gill cavity, and then fall to the bottom of the pond or Jake, where the parent form happens to be living. They cannot swim at all, but lie at the bottom on the valves of the shell, with the byssus filaments streaming upwards. With these, and with the nipping valves of the shell, they fasten on to some small fish, such as a stickleback. The glochidia appear to have a very keen appreciation of the presence of small fish. On one being introduced into water swarming with glochidia, the latter were observed violently to close and open the valves of their shell ; had the fish swum near enough they might have succeeded in laying hold of it. The advantage of this power of adhering to a fish is that the young glochidium is carried about, and the currents due to the movement bring particles of food within its reach. The adult anodon, like the glochidium, is enclosed in a two-valved shell. But the sed of the adult arises underneath that of the larva, and is marked by a series of lines running parallel with the long axis of the shell, which indicate its growth. ‘These lines about the middle are slightly curved upwards, an effect which is due to the sharp teeth of the glochidium shell impinging upon the soft and growing shell beneath, and checking its growth at that point. ‘The bivalve shell is composed of two separate halves, which are joined near the summit, the wmdbo, by an elastic hinge, which is stretched during life by the action of the great adductor muscles, but which after death is relaxed, and thus suffers the shell to gape. On the inside of the shell are to be seen the strong impressions of the two adductors, and of three other muscles, two retractors and a protractor of the foot, which muscles serve, as their names indicate, for the protrusion and retraction of the foot. Conchologists often distinguish between the shell of a The Pond Mussel. 63 mollusc and the “animal.” ‘This is, it is hardly necessary to point out, an unfortunate use of language, as it implies in a way that the shell is an adventitious structure, like the case of a caddis-worm. The shell is a secretion of the epidermis of the mussel, just as is the shell of the crayfish. So far the two structures are strictly comparable; and the” fishmonger who broadly classifies marine articles of food into’ fish and shell-fish, is not committing so gross a zoological solecism as might be. The part of the body which underlies and forms the shell is known as the mantle, it is simply a fold of the body drawn out so as to cover over the foot and gills. The Fic. 29.—Semi-diagrammatic representation of a dissection of Anodon. (Partly after T. J. Parker.) N.B.—The word anus is placed in cloacal cavity. The parieto-visceral ganglia lie _ below posterior adductor. same thing has happened as in the crayfish; there there is also a fold of skin drawn out, which secretes the branchiostegite. At the posterior end of the body the two mantle-flaps become joined so as to divide an upper exhalent orifice from a lower inhalent orifice. The edges of the mantle at the latter are fringed with stiff papillz, or tentacles, When the mantle-flap of one side of the body is removed, the structures shown in Fig. 29 are brought into view. The mantle-flaps, like the gills which are now evident, are dependences of the body itself, 64 Elementary Zoology. which is divided into a dorsal softer part, containing the viscera, and a muscular projection below and in front, which is termed the foot. This latter is the organ of progression, and in some lamellibranchs is so active in its movements that the animal can execute considerable leaps; this is the case with the common cockle. The movements of anodon, however, are lethargic. The most prominent organs seen are the gids, which have a fenestrated, lattice-like appearance; they are posteriorly continuous with the septum already spoken of, which divides the inhalent from the exhalent orifice at the posterior end of the body. Each of the two gills of each side of the body is really a bag which, in the case of the outer gill, is attached along both its edges to the body-wall; the inner gill, on the other hand, is open at various points, the inner lamella not being continuously attached to the body-wall. In front of the gills, and at the sides of the mouth-opening, which latter lies above the foot, are a pair of ribbed flat plates, not unlike the gills in colour and appearance; these are the labial palps. The only other structures which are visible without further dissection are the various muscles already. spoken of in describing their impressions upon the shell. If the two lamellz of the inner gill be widely separated by cutting through the inner lamella and turning back the cut edge, the openings of the renal organs and the generative gland will be found close to each other. These apertures lie upon the body- wall between the attachments of the two lamellz of the inner gill. The alimentary canal of the mussel is simple. The mouth, just behind the anterior adductor, leads into a short wsophagus, which in its turn opens into a somewhat globular s¢omach (sr), into which debouch the bile-ducts, carrying thither the secretion of brown and much-branched Ziver. The intestine (INT) coils in the substance of the upper part of the foot, and then, rising towards the dorsal surface of the body, runs a straightish course through the pericardial cavity to the axus, which is placed at the end of a slight papilla in the exhalent chamber. The pericardial chamber is so called on account of the fact that it lodges the heart.. It is practically all that exists of the celom in this animal, which is thus much reduced as compared The Pond Mussel, 65 with the earthworm, or with a vertebrated animal. The /eart consists of a thick-walled ventricle, which is coiled round the intestine, with which communicate two thin-walled auricles, and from which arise two main arteries, one anterior and one posterior. ‘The ccelom thus conforms to the ccelomic cavity of other animals (cf. earthworm, frog), in that (1) it contains various viscera—in the present case the heart and a part of the alimentary canal only; (2) it communicates with the exterior HEART INTESTINE -CCLOM AUR NEPHRIDIUM Fic. 30.—Diagrammatic transverse section of Anodon. (Partly after T. J. Parker.) INT, intestine in foot ; AuR, auricle. by means of secretory tubes, the organs of Bojanus, which are the kidneys; (3) the third character of the ccelom is not apparent in the mussel, for the gonads are removed from the ccelomic wall, and have come to lie in the thickness of the body, whence special ducts convey their products to the exterior. The organs af Bojanus, or kidneys (Fig. 29 NEPH, and Fig. 30), already referred to, are the precise equivalents of the nephridia of the earthworm. Each is essentially a tube, opening, on the one hand, by an orifice into the pericardium (= coelom), and by another orifice on to the exterior of the F 66 Elementary Zoology. body. This tube, as is also the case with the nephridia, is separable into a glandular portion which is thick-walled and brownish in colour, and a vesicle, or ureter, which is com- parable to the terminal vesicle of the nephridium of Lumbricus, The xervous system consists of three pairs of ganglia; of these the cerebral lie just under the skin above the mouth, and just in front of the protractor muscle. The two ganglia are con- nected by a fine commissure of nerve-threads lying above the mouth cavity. Each ganglion is also connected by a con- nective with the pedal ganglia in the foot, situated at the point shown in the diagram (Fig. 29), and by other connectives with the parieto-visceral ganglia lying beneath the posterior adductor. The ganglia are especially conspicuous on account of their orange colour. CHAPTER VIIL. THE SNAIL (HELIX POMATIA, HELIX HORTENSIS, ETC.). THE best snail for dissection is (on account of its large size) the so-called “ Roman snail.” But if this cannot be conveniently obtained, the common Garden Snail, Hedix hortensis, will do. The snail belongs to the same great group as the Swan mussel ; but whereas the latter is bivalve, symmetrical, unisexual, and aquatic, the snail is univalve, symmetrical, terrestrial, and hermaphrodite. It has, moreover, a distinct head, from which the tentacles bearing the eyes protrude, and is on this account placed in a separate division of the Mollusca, the Cephalous Mollusca. To prepare the snail for study, it is best to kill by drowning. A number of snails should be placed in a glass vessel, upon the top of which a glass plate can be placed so as to shut out the air. If the glass vessel be then completely filled with water, the snails immersed and the cover placed upon it, they will be found, after twelve or fifteen hours, dead, and in a fully extended condition. Before dissecting the animal there are a number of external characters that should be noted. When the body is fully extended, it is seen to be divisible into two regions: the muscular foo/, with the Acad at the anterior end, and the wsceral hump, which lies within, and has the shape of, the shell. Ina living snail, just beneath the shell, may be seen a round orifice which alternately shuts and opens. This is the pulmonary orifice, and leads into the mantle cavity, a cavity formed by a fold of the integument, as is the mantle of the Anodon. The eyes are borne upon a pair of long and retractile stalks. Below these are a second and smaller pair of éenéacles. The mouth 68 Elementary Zoology. lies below the latter, while behind the tentacles is, on the right side, a pore, connected with an external groove, the generative aperture. Apart from the unilateral generative pore, that portion of the body which lies outside the shell is bilaterally symmetrical. There is no more evidence of segmentation in the snail than there is in the Anodon. The sei of the snail has a spiral cavity; it is formed by the coiling of a shelly tube, the common wall formed by the apposition of the coils being itself at first hollow. The term columella is applied to this. That the shell is univalve implies no marked distinction from the shell of Anodon. It is simply due to the continuous secretion of calcareous matter instead of its secretion in two plates, as in the Anodon. The mantle cavity is a spacious chamber corresponding, of course, to the spaces which lie beneath the mantle-flaps in Anodonta. It is, therefore, morpho- logically a part of the exterior. The name “lung” sometimes applied to this cavity is, therefore, so far misleading. Never- theless, this chamber performs the function of a lung, as the animal has no gills such as are found in other Cephalous Mollusca. The walls of the mantle cavity are thin, and just beneath the integument is an abundant plexus of blood-vessels, whose blood is thus brought into near relations with the air contained in the “lung.” This (the air) is renewed and ex- pelled at intervals, as already mentioned. Into the mantle cavity open the anus and the renal gland. When the snail is dissected the same absence of ca~om that characterizes Anodonta is to be remarked. As in that animal, the only remaining portion of the ccelom is the sericardium, which surrounds the heart. No other organs, however, lie within the pericardium, but the single renal organ opens into it by a reno-pericardial pore. The alimentary canal is more complex than that of Anodon. The mouth leads into a cavity called the buccal mass, which has thick muscular walls, and whose floor is supported by cartilage. Upon this cartilage lies a structure characteristic of all the Cephalous Mollusca known as the radula. This is a stiff ribbon which is developed into a series of calcified teeth, closely set and overlapping each other. The ribbon bearing 69 these teeth is set in motion by muscles attached to the under- lyin The Snaid. From the g cartilages, and the food is thus torn to shreds. i ; ' ‘i : “pues Axearpes “1D ATVS £ dUIJSAIUT LNT SyoeUoys ‘WLS + daar] ‘AT + 9joe]Ua ondo ‘aA TLNaL'dO Syynow “iN Se[npel ‘ava ‘sseur [eon ‘aq !snSeydosa “Dp Csemoy Jaxyy) "[eus Jo [ues Arequsunpy—1€ ong ave , to open into a rather globular Be. S r ALNIL'dO 19 s1vwS eG AN buccal mass leads a narrow cesophagus, which presently widens into a crop; it again narrows 70 Elementary Zoology. stomach. From the stomach arises an intestine which, after two or three turns, becomes the straight rectum leading to the anal orifice. The alimentary canal of the snail is well supplied with glands. There are, first of all, the saéivary glands, one on either side of the crop. From each a fine duct passes forwards, and opens into the buccal mass just at its junction with the cesophagus. The Zver is a four-lobed brown mass, occupying a great portion of the visceral mass, lying within the visceral hump. Its ducts open into the stomach. The fearvt has one auricle and ventricle. Into the auricle is poured the blood derived from the numerous vessels which ramify over the surface of the mantle. From the ventricle arises an artery, which subsequently divides and supplies the body generally. The 4/ooed is faintly blue when exposed to the atmosphere, the blue colour being due to a respiratory pigment (hzencyanin), analogous in its oxygen-carrying powers to hemoglobin, but of a different chemical composition. Its metallic basis is copper instead of iron. The blood contains colourless amceboid corpuscles. The renal organ is single ; it corresponds to one of two organs of Bojanus of the fresh-water mussel. It opens, as already said, into the pericardium ( =ccelom) on the one hand, and on to the exterior, within the mantle cavity, on the other. The mervous system is constituted quite upon the plan of that of Anodon. The ganglia, however, are more crowded together. The cerebral ganglia lie above the cesophagus, and give off each of them a nerve which ends in a small buccal ganglion, lying upon the buccal mass, and innervating it. Connected with the cerebral ganglia by commissures passing round the cesophagus are the pedal ganglia. On the pedal ganglia lie the small auditory sacs; but although these lie upon the pedal ganglia, their supply-nerves come from the cerebral. Behind the pedal ganglia are the chlamydo-splanchnic, also connected by commissures with the cerebral. As is generally the case with hermaphrodite animals (cf. the earthworm), the organs of reproduction are excessively complicated. The essential part of the generative system is the hermaphrodite gland, or ovo-testis, which produces both The Snail. 71 Vy 4 HER.D E p bs i ini i ‘ h 3 Ry: sen aD ie a Cee SS vy \ \ % y %, Cs U9 X a), Qh Ne S Fic. 32.7 Reproductive organs of Snail. (After Howes.) AL.GL, albumen gland ; HER.D, hermaphrodite duct ; HERM.GL, hermaphrodite gland ; sp, spermatheca; Ov.p, oviduct; PR, prostate; FLA, flagellum appended to male duct; GEN.Vs, genital vestibule ; RET.Mus, retractor muscle. 72 Elementary Zoology. spermatozoa and ova. This is connected by a duct with a large gland, the albumen gland. Some time after this the common duct divides into-a vas deferens and an oviduct. To each separate duct a system of glands and diverticula is appended, which are illustrated in the figure (Fig. 32). The snail deposits its ova in the earth; the embryos have, as is usual with terrestrial animals (to which, however, there are plenty of exceptions, ¢.g. insects), no free larval stage. CHAPTER IX. THE FROG (RANA TEMPORARIA AND RANA ESCULENTA), THIs country is inhabited by two species of Frog. One, Rana temporaria, is abundant everywhere—where, that is to say, suitable conditions are to be met with. The other frog, Rana esculenta, the so-called “ edible frog” (but both, as a matter of fact, are eaten), is found in some of the eastern counties. There is, however, more than one reason for regarding this species as hot truly indigenous to this country; it has been artificially introduced at more than one time. R. esculenta may be distin- guished from &. temporaria by the existence in the male of inflatable vocal sacs, by the wer absence of a characteristic dark Fic. 33.—The upper side of the head of the patch of pigment behind the male Heres saertistte: showing the vocal eye, and by the larger prehal- lux in the foot. It is, moreover, a larger species, and is more purely aquatic in habit than its smaller indigenous ally. The frog varies considerably in colour, being in some cases darker and in others lighter. This variation of colour is de- pendent largely upon the animal’s surroundings, and changes with any changes in the environment. Thus, if a frog be kept for some time in a white dish it will become paler in tint ; if shaded, the hues will darken. This power of changing the colour is due to the contraction or relaxing of certain pigment- cells in the skin, and is a phenomenon which is common among 74 Elementary Zoology. the lower vertebrates. It is seen in other Amphibia, in many fishes, and in reptiles, the chameleon being the most familiar instance of the latter. The body of the frog is moist, thé skin secreting a fluid by means of certain glands imbedded therein. A too dry atmo- sphere is ultimately fatal to the animal. The Jody is divisible into head, trunk, and Limbs, to which regions of the skeleton (p. 85) correspond. There is no recog- nisable neck or tail. The large south is provided with teeth, which are limited to the upper jaw. Posteriorly there is but one aperture, the cloaca’. On the head are the three organs of special sense ; the zos¢ri/s anteriorly, the eyes following, and behind these, on either side, a tight drum-like membrane, the tympanum, which covers the tube leading to the ear. The sense organs, it will be seen, are arranged in a segmented fashion. There are some other characters which can be noted with- out dissection. If the mouth be opened, two small orifices, the internal nares, are seen to communicate with the external nares (the nostrils). Considerably further back are the two Eustachian tubes, which communicate with the auditory meatus lying beneath the tympanum. The teeth have been already referred to. The éougue, which is bifid at the tip, and can be stretched to a fair length, is attached anteriorly in the mouth cavity. Itis by the rapid extension of the tongue, and by the viscid fluid secreted from it and covering it, that the frog is enabled to capture its (generally insect) prey. When the skin is removed preparatory to opening the body, it will be seen to be loosely attached to the underlying muscles, This is caused by the presence of large subcutaneous Zymph spaces. When the thin layer of muscles in the abdominal region is cut through, a large cavity is exposed, in which lie nearly all the viscera. This is the body cavity, or ce/om. It extends through the abdominal and thoracic regions, but does not push its way into the head or the limbs. Thus the body of the frog is hollow, the walls consisting of the skin externally and of the skeleton and muscles next, which form together the body-wall. A more careful examination of the ccelom shows The Frog. 75 that in reality the various organs, which are exposed by opening it up, do not lie within it. The alimentary tract, which is the most conspicuous of these, is tied down to the dorsal side of the cavity by thin sheets of tissue, the mesenteries. These mesenteries are double, and embrace the alimentary canal, thus shutting it off from the coelom. The same applies to the kidneys, heart, and other organs. The only organs that can be said to really lie actually in the body cavity are the male and female gonads, the testes and ovaries. They are formed by a proliferation of the membrane which lines the coelom. The mouths of the oviducts in the female frog, moreover, open actually into the ccelom, the lining membrane of which is continuous with their mouths, A special part of the ccelom is separated off from the rest, and forms the pericardium, enclosing the heart. Mention has been made of the subintegumental lymph spaces; these spaces are in communi- cation with a system of vessels which ramify through the body, often accompanying the blood-vessels, and which open into the celom. This lymph system contains a colourless fluid, in which float colourless nucleated corpuscles, similar to those of the blood. Lying beneath the skin of the back, just in front of the cloacal aperture, and between the third and fourth vertebra, are certain muscular contractile sacs, the Zymph hearts, which pump the lymph into the veins in their neigh- bourhood ; the ccelom thus, through the lymph system, is in communication with the vascular system. The alimentary canal of the frog commences with the mouth cavity and ends posteriorly in the cloacal cavity. The mouth and teeth have been already referred to, and the latter will be more particularly described in the section dealing with the skeleton. The mouth cavity is followed by the wsophagus, which leads into the stomach. The stomach is a bent tube, wider at first, and gradually diminishing in calibre as it passes into the small intestine ; the junction of the two is marked by a fold of the lining membrane. The smad/ intestine is narrow and coiled.1 The J/arge intestine is wide and short, ending in 1 The coiling of the intestine allows a large secretory and absorp- tive surface to “be stowed away in a small space. This surface is 76 Elementary Zoology. the cloaca. Into this terminal chamber opens, firstly a large bifid d/adder, and just below it the ureters and (in the female) the oviducts. Appended to the alimentary canal are two large glands. Of these the Zver is much the largest, and is divided into two principal lobes, a right and a left. Attached to the lower surface of the liver is the usually bright green gal/-dbladder. From this a duct leads through the pancreas, in the substance of which it is joined by the ducts of the pancreas, and finally opens into the duodenum, as is called the first part of the small intestine. The circulatory system of the frog consists of a central organ of impulsion, the Zeaz?7, and of a system of tubes leading to and from the heart. Those conveying blood from the heart are termed ar¢eries, and those returning blood to the heart are termed veézs. Peripherally these are united by the minute capi/- Zaries, which permeate all the organs of the body. The vascular system is, as has already been stated, in communication with the coelom through the lymphatic vessels. It contains, however, a different fluid. Besides the “ white” corpuscles which are common to it and to the lymphatic system and ccelom, it has large, red, oval, nucleated discs, the ved corpuscles. The red colouring matter of these is due to a substance called heemo- globin, the importance of which will be considered in connec- tion with respiration. The body of the frog is thus permeated by a second system of cavities distinct from the ccelom. The Aeartis a stout muscular organ enclosed in a special com- partment of the ccelom, the pericardium. Its cavity is divided into several chambers, which are in communication with each other. Behind the heart is the sus venosius, into which the veins bringing back blood from the body generally open. This leads into the right auricle, which is separated from the left auricle Dy a septum. Into the left auricle open the pulmo- nary veins. Both auricles lead into the single ventricle by a common aperture, which, however, is divided into two openings. further increased, without additional room for storage being required, by villi and folds upon its inner surface. The typhlosole of Zumbrvicus and of Anodon is a similar response to a similar need. The Frog. 77 The walls of the ventricle are thick and spongy ; its cavily is in communication with the ¢runcus arteriosus, from which again arise the arteries. The truncus arteriosus is divisible into two regions. That nearest the heart is known as the pylangium ; it is guarded by three semilunar-shaped valvules at its orifice into the ventricle, the effect of which is to prevent the reflux of blood into the ventricle when the former contracts. Along the rest of the pylangium is a free fold attached along one wall, which is believed to represent a series of smaller valves, such as occur in the heart of the lower fishes, fused together. At the end of this, above, three semilunar valves mark off from the pylangium the distal syzangium. From this immediately spring the two (right and left) arterial trunks. These latter, although apparently single trunks, are really divided internally into three vessels on each side. The arterial system of the frog is illustrated in the accompany- ing diagram (Fig. 34). The anterior of the three vessels arising from the synangium is the carotid ; it divides into two branches, the external and internal carotid. At the origin of the former is a little thickened portion of the arterial trunk which is known as the carotid gland. The term “gland,” however, is quite in- accurate ; it is simply a network upon the vessel, the trunk of which divides into,a skein of vessels to reunite again. The middle of the three trunks is the systemic aorta. It passes round the gullet, and joins its fellow of the opposite side beneath the cesophagus, At the points of junction a stout branch is given off, the cceliac, which supplies the viscera of the abdomen. From each half of the aorta before they join a branch arises which again divides into the drachial (supplying the fore limb) and a vertebral artery. The third of the three arches is the pz/mo-cutancous ; it divides into two trunks, one going to the lungs, the other to the skin. The blood is returned to the heart by a system of veins, which is rather more complicated than the arterial system. It is more complicated because there are two subsidiary circulations introduced along the course of the vessels. The blood from the head and from the fore limbs is returned to the right auricle by a series of trunks which are shown in the accompanying 78 Elementary Zoology. C.GL wo a C.C, AO . LEN.INT.V. IL.V FEM-J Sc \ Fic. 34.—Diagram of vascular system of Rana, (Compound figure, after Howes.) The venous system is black, the arterial white. (N.B. the arterial vessels are really deep of the veins, not superficial.) Au, auricles; v, ventricle; ao, aortic arch; c.c, carotids; C.GL, carotid gland; rR, brachial artery; BR (black), brachial vein; M.cUT, cutaneous vein; ANT.AB, anterior abdominal; L, lung; Liv, liver; cat, coeliac; LEN.INT.V, IL.V, portal vein; T, testis; K, renal organ; R.PoR, renal portal ; D.AO, dorsal aorta; FEM, femoral vein; PEL.v, pelvic vein; sc. sciatic vein. ERRATUM. The figure on the opposite page is, unfortunately, inaccurate so far as concerns the anterior abdominal vein. The error is rectified in the above figure. as. The Frog. 79 diagram. The names of the several vessels and the points at which they unite are there indicated, and an inspection will serve instead of a description. In the same simple fashion the blood is returned to the left auricle from the lungs. It should be noted, however, that while generally the veins correspond to the arteries, this is nat the case with the cutaneous vein. This vein joins the jugular system, while the cutaneous artery is derived from the pulmonary artery. The blood from the renal organs and from the liver is also returned directly to the heart by way of the posterior vena cava. From the rest of the body the blood is not returned directly to the heart. The femoral vein bifurcates in the pelvic region into two vessels, of which one runs to the kidney, and there breaks up into a series of capillaries; the other branch joins its fellow of the opposite side to form the anéerior ab- dominal vein, a vein running just below the muscles of the abdomen, which can be seen through those muscles before they are cut. This anterior abdominal vein divides anteriorly into two branches, one for each of the two lobes of the liver, in the interior of which it breaks up into a capillary network. A minute twig, however, has been described as going directly to the heart. The renal portal vein, as that branch of the femoral which goes to the kidney has been termed, is reinforced by the sciatic vein. Besides this renal portal system, as the vessels which pour their blood into the kidneys are collectively termed, there is an hepatic portal system. The veins from the alimentary canal unite to form a largish trunk, ‘the portal vein ; this enters the liver, and there breaks up into a capillary network. Thus all the blood from the hind limbs passes on its way to the heart either through the kidneys or through the liver, with the exception of a small quantity which reaches the heart directly by way of the small branch of the anterior abdominal already referred to. The respiratory organs of the frog consist of a pair of lungs; these open by a short tube, which is strengthened by certain cartilages, into the pharynx. Each lung is a sac with thin walls, which are abundantly supplied with blood capillaries, the branches of the pulmonary arteries. When the 80 Elementary Zoology. frog breathes it fills the mouth with air; the mouth is then closed and the external nares, while the muscles forming the floor of the mouth force the contained air into the lungs. Expiration is effected by the abdominal muscles, which press upon the viscera, and so upon the lungs, expelling the air. It appears, however, that the frog can also breathe by means of the skin, which is, it will be remembered, supplied with blood by the cutaneous artery, a branch of the pulmonary. Respira- tion is essentially an exchange of gases between the blood and the air in the lungs. The hemoglobin, which tinges the red blood corpuscles, has the power of absorbing and entering into loose combination with the oxygen drawn into the lungs ; this oxygen is then given up with equal ease to the tissues through which the blood passes after it has been through the lungs. The carbonic acid which is there absorbed is given up to the outside when the blood returns to the lungs. The zervous system consists of the central nervous system, the drain and spinal cord, from which arise nerves which con- stitute the peripheral nervous system. These nerves end either in the muscles or in sense organs ; they are either motor in function, or sensory. The central nervous system, unlike that of any of the invertebrate types, is entirely dorsal in position. It runs in a canal formed in front by the skull, and behind by the verte- bral column. Furthermore, the central nervous system is really a hollow tube, though the thickness of its walls far exceeds the diameter of the lumen, save in certain regions of the brain. By these two important facts the central nervous system of all vertebrated animals can be distinguished from the central nervous systems of other animals. The drain is divisible into several regions ; in front there is an unpaired region from which arises the two olfactory nerves, going to the nose; this is the o/factory Jobe. Behind this are the paired cerebral hemispheres ; then follows a region, which is depressed below the level of these, and is known as the thalamencephaton. Yrom the upper surface of the anterior part of the thalamencephalon arises a short stalked body, the pineal body ; this structure is the rudiment of an unpaired eye, ‘The Frog. St fully, and possibly even functionally, developed as an eye in certain other vertebrates. Behind the thalamencephalon come the two optic lobes, the corpora bigemina, as they are sometimes termed. Then a narrow band of brain tissue stretches across to form the cerebellum, rudimentary in the frog, but of great importance in the ltigher vertebrates. Behind this, again, is the medulla oblongata, which pradually narrows into the spinal cord. Ten pairs of nerves arise from the brain, which are as follows: (1) olfactory, supplying nose; (2) offic, supplying eyes; (3) motores oculorum, supplying most of the muscles of the eye ; (4) pathetici, supplying the superior oblique muscles of the eye; (5) ¢rigeminus, with three branches running to the skin of the front part of the head and the lower jaw; (6) abducentes, supplying the external rectus and retractor bulbi muscles of the eye; (7) the facials, supplying the roof and the floor of the mouth ; (8) the auditory nerves, going to the ear; (9) the glossopharyngei, to the root of the tongue ; (10) the vag7, supply- ing the dorsal integument of the head and trunk, and the heart, lungs, and stomach. The sfinal cord gives off ten pairs of nerves ; the first is the hypoglossal, which supplies certain muscles at the back of the head ; the next two nerves unite a short way from their origin, and form a trunk supplying the fore limb. This union between the two nerves is known as the brachial plexus, The seventh to the tenth spinal nerves form another plexus, which is con- cerned with the nerve-supply of the hind limb. The sympathetic system consists of a chain of ganglia on either side of the aorta. The renal organs of the frog consist of a pair of kidneys, which really do not deserve the name of kidneys, as they correspond to the mesonephros of the embryo fowl (see p. 140). They are reddish bodies, and on the surface of each is a yellowish band, the adrenal body. The duct of each mesonephros opens into the cloaca, that of the male being provided with a little glandular caecum known as the vesicula seminalis, From the cloaca in both sexes arises a bilobed d/adder. In the male frog there are a pair of egg- shaped ¢estes ; the ducts from these pass through the meso- nephros, and reach the exterior by the mesonephric ducts (the G 82 Elementary Zoology. ‘“‘ureters”). At the anterior end of the testes are a pair of lobed bodies, the fat bodies. The ovary of the female is much more extensive than the testis, but it has the same fat body attached to its anterior end. The eggs are shed freely into the ccelom, and are caught up by the open mouths of the oviduct ; the oviducts are much-coiled tubes which open into the cloaca. Tue Lire-History OF THE FROG. The male frog at the breeding season develops a thick glandular pad upon the index fingers. This assists the male in clasping the female firmly, which is done during the period of oviposition, the milt being shed upon the ova as they are extruded. The eggs are enveloped in a thick transparent coat derived from the walls of the oviduct, which contains mucin, and swells up when brought into contact with water. The actual eggs themselves are smallish round bodies, black at one pole and white at the other. The Tadpole is hatched out at a very early period of its development. It is the rule of the frog tribe for tadpoles to be produced ; but the rule is not without exceptions. In a few cases there is no tadpole stage at all, the young frogs making their way out of the egg. When the tadpole is hatched it has no mouth, and is therefore still dependent upon the yet un- absorbed yolk for its nutrition. It has a horse-shoe-shaped sucker below the future position of the mouth, by means of which it moors itself to leaves or stones. It has pairs of external gills, which are outgrowths of the skin, and possibly represent the simple cutaneous gills of some invertebrate ancestor. In various marine worms, for example, there are gills of this character. Later, the mouth becomes apparent, and its interior is furnished with a series of horny teeth, the precise arrangement of which has been shown to be charac- teristic for the tadpoles of different kinds of frogs. The alimentary canal grows long, and is coiled in a peculiar fashion, somewhat like the spring of a watch; this form of the alimen- tary tract is associated, in the common frog, with a purely The Frog. 83 vegetable diet; but there is no necessary association of the kind, for the tadpoles of the Cape clawed frog (Xenopus lavis) have an equally watch-spring-like intestine, but feed—chiefly, at any rate—upon small crustaceans, The external gills presently shrivel up, and are replaced functionally by the internal gills. At the sides of the pharynx a series of slits appear, which put the interior of the pharynx into communication with the Fic. 35.—The young Tadpole of the com- Fic. 36.—The under side of the mon Frog (Rana temporaria), enlarged, Tadpole of the Frog, showing showing the external gills. the coiled digestive tube, the suckers (not joined), the in- ternal gills, and the respiratory aperture, enlarged. medium in which the animal lives; these gz//-s/i¢ts, or visceral clefts, become fringed with vascular tags, which are the actual respiratory organs. The intervals between the successive clefts are occupied by cartilaginous bars, the gi/-arches, or visceral arches, the function of which is to keep open the clefts, and so 84 Elementary Zoology. allow of a free flow of oxygen containing water over the gills, When the gill-clefts and the associated gills appear, a fold of skin, the operculum, arises from the side of the head, and grows over the gills, remaining open only at one point. This is on the left side of the body, and its margin projects in a spout-like fashion. It will be observed that in this stage of its existence the young frog is practically a fish. If it grew no further, and were to develop sexual organs, it would have to be classified with the fishes. It has the gills of the fish, its circulatory organs are constituted upon a similar plan, and the body is fringed dorsally and ventrally with a continuous fin-fold as in many fishes. But it has no lateral fins corresponding to the paired fins of the higher fishes. These appear later in the form of the limbs of the adult frog. The lungs also soon appear, the tail gets less, respiration is effected entirely by the lungs, and the tadpole leaves the water a frog. CHAPTER X. SKELETAL AND INTEGUMENTARY STRUCTURES IN VERTEBRATES. TuE frog belongs to the group of Vertebrata whose characters are given below. The Vertebrata contrast with any Inverte- brate type by the possession of an elaborate internal skeleton, which is in the lowest forms entirely, or nearly entirely, cartila- ginous, and ossified in the higher types. The characters derived from the study of the skeleton are exceedingly useful in classi- fying vertebrates. In the following pages, therefore, an account will be given of the skeleton in three types: in the frog, the fowl, and the rabbit. The Vertebrata can be also, to a large extent, differentiated by certain external features. The Mammalia, for example, are the only vertebrates which possess hair ; feathers are unknown outside the class of Birds, while the scales of Serpents and Lizards are totally different from the similarly named structures of Fishes. As the teeth are really, as will be shown later, epidermic structures, they will be included in a survey of the principal external (integumental) characters of the vertebrates. VERTEBRAL COLUMN. The vertebral column of the Frog consists of a number of separate vertebree—very few as compared with the other vertebrate types. The frog has altogether only ten vertebrae, exclusive of the long urostyle, which is never broken up into separate vertebree, but appears to be the equivalent of three. The first vertebra is called the a#/as, and it is followed by 86 Elementary Zoology. the second, or avis. After this come seven vertebre with well-developed transverse processes, and an eighth vertebra with particularly strong transverse processes, which support the pelvic bones. The anterior set may be termed “ dorsals,” and the vertebra which supports the pelvis “sacral.” In order to satisfactorily compare the vertebre of the frog with those of the other vertebrates, it will be necessary to enter into the development of the vertebral column. Unlike the skull, the entire vertebral column is formed out of cartilage. The cartilages, when they first appear, appear round the notochord, the development of which structure, the precursor of the vertebral column, is dealt with on another page. There are typically four pairs of cartilaginous elements arranged in sets, of which, however, some may be suppressed. They have been called by the following names: basidorsalia, basiventralia, interdorsalia, interventralia. In the common frog the inter- ventralia are suppressed, and the entire vertebra is formed by the coalesced basidorsalia, interdorsalia, and basiventralia. In the bird’s vertebral column the separate vertebra are formed of other elements. The centrum of the vertebra is formed of the interventralia, the basidorsalia form part of the neural arch, and, finally, the basiventralia are converted into the intercentra, where these latter exist. The same statements hold for the vertebral column of the mammal. In both these latter groups the atlas is peculiar, in that its centrum remains free from the interventralia, and becomes attached to the centrum or the following vertebra, the axis. It is clear, therefore, that a vertebra of the frog does not exactly corre- spond to a vertebra of either a bird ora mammal. Moreover, apart from their being formed out of partly different elements, and therefore not exactly corresponding, there is not even a rough correspondence between the atlas of the frog, on the one hand, and of the bird and mammal on the other. It has been pointed out that the frog has only ten pairs of cranial nerves; the fowl and the mammal have twelve. This naturally leads to the view, supported by other data, that the skull of the higher vertebrates is more extensive than that of the frog. There are, in fact, rudiments of, apparently, at least Vertebre of Fowl. 87 two vertebrze co-ossified with the occipital bone in the higher vertebrate, which must, therefore, be looked upon as the equivalents of the two first vertebrz of the frog, whose atlas, in consequence, is not to be strictly compared with the atlas of either bird or mammal. The vertebral chain of the Fowl] has been found to consist of 46 or 47 vertebra. The number appears to be rather less than this; but it will be noted that the posterior end of the column is formed of a ploughshare-shaped bone, the pygostyle or urostyle, which is really a compound bone made up of separate elements. These separate vertebrae can be divided into four series. There are, first of all, the cervical vertebre, sixteen in number. The term “cervical” is commonly applied to those in front of the first vertebra, which bears a complete rib articulating with the sternum; but really there is no hard-and-fast line of division, since the last two of the cervical series bear free ribs, which, although they do not reach the sternum, increase progressively in length. The cervical vertebrae lying in front of these latter appear to have no ribs; but they really have short ribs, which are firmly coalesced with the vertebree, so as to surround a canal, through which an artery passes. Each cervical vertebra, like the remaining vertebree, except those at the tail end of the body, consists of a centrum, which articulates in front and behind with the preceding and succeeding vertebra by a surface which is concave in the middle, and more convex peripherally ; it has been compared to the outline of a saddle, and the centra of birds have been described as having saddle-shaped articulating surfaces. This method of articulation ensures considerable mobility, and the neck is long. Rising from the centrum is an arch of bone, the meural arch, which projects at the top in a varying degree to form the meural spine; this surrounds the large neural canal, in which lies the spinal cord. The ribs are attached by two heads: by a capitudim to a smooth surface upon the centrum, and by a ¢udberculum to an outgrowth of the neural arch, the ¢rausverse process. In addition to these various processes and regions of the vertebra, there is commonly a ventral median process, the Ayfapophysis, which 88 Elementary Zoology. is simply a downward growth of the centrum, and not to be confounded with an apparently similar ‘downward process of the caudals. The last cervical vertebra is fused with the three following vertebre ; this gives great solidity to this region of the back, which has to support the sternum. The last dorsal vertebra (we restrict the term dorsal to those vertebre which carry complete ribs) is free, and not ankylosed with the preceding. After this follows a large series of vertebrae which are closely attached to each other and to the pelvis, which they support. Here again the fusion of the vertebrae gives great stability to the pelvis anda firm point d@’appui for the articulation of the legs. These fused vertebrae are sometimes termed sacral; but it is better to reserve the term “sacral”. for the two vertebra which in the embryo chick articulate with the ilia; those lying in front of this point may be called Zumbar, and those lying behind caudal. Behind these last there are a number of free caudal vertebrae, and the column terminates in the ploughshare bone (pygostyle), which is the product of a number (six) of fused vertebrae, which thus form a strong basis for the attachment of the strong quills of the tail, the rectrices. . The free caudal vertebra have slight downward processes, arising from the centra ; these structures are really, and actually, in some birds, separate bonelets, the zvterventra, which are independent of the centra to begin with, and only become fused with them later. The first cervical vertebra is known as the a//as, the second as the axis, or epistropheus. The latter is furnished with a peg-like forward outgrowth of the centrum, which fits into a notch of the apparent centrum of the atlas. We say “apparent,” since the peg—the odontoid process, as it is usually called—is really the detached centrum of the atlas. The vertebral column of the Rabbit can be also divided into regions. First of all there is the cervical, with but seven vertebrae (a number which is curiously constant among the Mammalia, there being only three or four exceptions); then follows the dorsal series, then the lumbar, the sacral, and, finally, the caudal. The rabbit has twelve dorsal, seven lumbar, four sacral, and fifteen caudal vertebra. Skull. 89 There are three salient points of structure which distinguish the vertebral column of the rabbit from that of the bird. In the first place, the centrum is ossified in three pieces—a central - one and two epiphyses (one at each end); secondly, the surfaces of articulation are flat instead of saddle-shaped ; thirdly, the first vertebra (the aé/as) articulates with the skull by two facets, instead of only one, as in the bird. The axis, or epistropheus, is like that of the bird in having a peg-like odontoid process—really the centrum of the atlas. The dorsal vertebrae have very long spinous processes, associated with strong muscles to hold and move the heavy head. The lumbar vertebra have very long lateral processes known as metapophyses. The rabbit has twelve, sometimes thirteen, pairs of ribs; of these the eight anterior are borne by two heads, the capitulum and the tuberculum. The former is articulated with a small semi-lunar facet on the junction of each successive pair of centra; the seventh cervical vertebra bears the half of the first of these facets. The last four vertebrae lack the tubercular head upon the transverse pro- cesses ; they have only the capitular head. Seven ribs reach the sternum. SKULL. The skull was at one time regarded as a single structure, formed of a number of coalesced and modified vertebree. The anatomical knowledge and genius of Goethe and Owen succeeded in impressing this view of its nature upon com- parative anatomists, until the theory was finally overthrown by Huxley in 1858. The skull is now known to be composed of a series of elements which have primarily no connection with each other. It is built up of— 1. The originally cartilaginous and afterwards, in all animals above the elasmobranch fishes, ossified drain-case or cranium proper. 2. Of the cartilaginous (or ossified) capsules of the three organs of special sense—the auditory, olfactory, and optic. 3. Of portions of, at least, the first two of the visceral go Elementary Zoology. arches—structures which were all originally bars of cartilage for the strengthening and support of the branchize. 4. Of the Zabsal cartilages. 5. Of certain membrane bones ossified in the skin generally, or limited to the head region. To understand how these various and quite different tracts of cartilage or bone combine to form the solid whole that we term the skull, it will be convenient to briefly trace the development. 1. The drain-case in its early condition consists of a pair of stiff rods situated anteriorly, and known as the ¢radecule cranii ; these lie in front of the notochord, and become fused with each other anteriorly between the nasal sacs, again diverging in front of the area of fusion to form a plate on each side, which dips into and supports the upper lip. As the embryo grows the trabeculze grow backwards and come into contact with a pair of rods lying on either side of the notochord, the parachordal cartilages. The state of affairs arrived at is illustrated in Fig. 37. All these cartilages constitute the brain-case, or cranium, and the brain lies upon the platform thus formed. Ultimately the walls of the brain-case are formed by the growing up of the sides of these cartilages, which come to more or less enclose the brain, fontanelles being left dorsally. In the frog it so happens that the trabecule cranii are formed before the para- chorials, but this is not the rule among the Vertebrata. The separate bones, which are finally formed by the ossification of the cranium, will be mentioned later. We may next consider— 2. The capsules of the sense organs. Of these, the auditory capsules alone have an important share in the building up of the skull. Quite independent of the parachordal cartilages, but abutting upon them, is a roundish mass of cartilage on either side, which contains the auditory organ. This soon becomes continuous with the side walls of the skull, and helps in the formation of its side, and even its dorsal walls. The capsule of the eye has practically no share in the building up of the skull. The sclerotic coat of the eye is cartilaginous in the lower vertebrates, and in birds becomes ossified into a ring of bone- Skull of Tadpole. or lets; but in no animal does the cartilage, or bone, form an integral part of the skull wall. It remains permanently in a condition which is primitive and transitory in the case of the auditory capsule. Finally, there is the capsule of, the olfactory organ. This is also relatively unimportant, but does become an actual, though a small, portion of the wall of the adult skull. The upper nasal wall of the frog’s skull appears to be partly or entirely formed of the capsule. And in other vertebrates it Fic. 37.—Skull of Tadpole. Lateral view.’ (After Marshall.) The different elements which enter into the formation of the skull are indicated by different shading. Cranium, marked with vertical lines; capsule of sense organ, dotted; wisceral arches, shaded with fine lines; labcal cartilages, marked with horizontal lines. c.H, notochord; E.c, auditory capsule; Q. quadrate; 9.0, palato pterygoid; Q.r, articulation of quadrate; H-R, ceratohyal; H.Q, its articulation with quadrate; H.0, urohyal; R.L, trabecule cranii; L.L, 1.v, labials; m.c, Meckel’s cartilage ; Ju, JL, jaws; Lt, LJ, lips; ps, cerebral hemisphere ; pn, fineal body. enters into the formation of the cartilages which protect the nasal organ. 3. In the tadpole, which breathes by means of gills, the side of the throat is pierced by four gill-clefts, which are slits putting into communication the interior of the pharynx with the exterior. The walls of these gill-clefts (for a further description of which see p. 135) are strengthened by the appearance of cartilaginous bars. Of these, the first and part of the second form an integral part of the adult skull. The first arch is termed the mandibular; it sends forward a process which becomes fused with the skull 92 Elementary Zoology. wall in front in the trabecular region; posteriorly it also acquires an attachment to the auditory region of the skull wall. Below the process the arch becomes segmented off into a lower piece, which bears at its end the lower labial cartilage already spoken of. The lower piece is known as Meckel’s cartilage, the upper piece as the palato-pterygoid bar, the actual area of attachment to the skull wall behind being the guadrate cartilage. The second arch will be dealt with later. 4. The labial cartilages are of some importance in the lower Fic. 38.—Skull of Tadpole. Dorsal view. (After Marshall.) The parachordal cartilages are seen ensheathing the notochord. Lettering as in Fig. 37. vertebrates, but diminish greatly—even to disappearance—in the higher forms. They have been looked upon as the vestiges of a cephalic skeleton which preceded the true skull. In the tadpole’s skull there are a pair of upper and of lower labial cartilages. The former enter into the formation of the cartila- ginous covering of the nasal organs in the adult. 5. Finally, there are a set of membrane bones in all vertebrates above the cartilaginous fishes, which ossify quite independently of the rest of the skull, and come to be closely applied to the upper, lower, and lateral surfaces of its cartilaginous walls. Skull of Frog. 93 These are relatively late appearances, and are greatly different in number in the several types of the Vertebrata, whose skulls we have to consider. Up to this point there is a great similarity in all skulls. There is no wide divergence from this plan of structure and development, though, of course, plenty of slight differences in various details. The constitution of the adult skulls of frog, Fic. 39.—Skull of Frog. Dorsal aspect- (After W. K. Parker.) The membrane bones are dotted ; the cartilage bones transversely shaded; the cartilage is left white. This applies also to Figs. 40 and 41. B.0, basi-occipital region; s.o, exoccipital bones; PRO.OT, pro-otic ; PTy, pterygoid ; P, F, parieto frontal; NA, nasal; P.MXx, premaxilla ; Mx, maxilla; TEM, squamosal; QR, overlies position of quadrate cartilage, and in front is quadrato-jugal (shaded). fowl, and rabbit differ principally in the modifications of the visceral arches, in the number of the ossifications in the primordial cartilage, and in the membrane bones surrounding the skull. As the Frog’s séu// is on the lowest level, we will commence with that. The absolute independence of the membrane bones from the underlying, partly cartilaginous and partly ossified, 94 Elementary Zoology. cranium is well seen by reason of the fact that they can without much difficulty be stripped off. Roofing the skull above are a pair of long bones, closely applied to each other in the middle line; these are the fronto-parietals. In front of these are the two zasals partly concealing the olfactory orifice, and in front of these, again, the small prvemaxille. On each side of the skull, in the auditory region, is a hammer-shaped sgzamosa/. On the under surface of the skull the most conspicuous of the membrane bones is the large and dagger-shaped parasphenoid, of which Fic. 40.—Skull of Frog. Ventral aspect. (After W. K. Parker.) pa, palatine ; ETH, sphenethmoid ; PAR.SP, pares icnord ; EU, Eustachian tube; QR, quadrate; stT.uy, stylohyal. ehind pmMx are vomers (dotted), in front of which are internal nares. Other letters as in Fig. 39. the “blade” underlies the greater part of the base of the skull. In front of it, and bearing much the same relation to the internal nares as the nasal do to the external nares, are the small vomers ; to the side the maxil/e, Laterally there is a splint of bone forming the outer arcade of the skull, the quad ‘atojugal, The skull, thus stripped of its membrane bones, is seen. to be chiefly cartilaginous, but with some ossifications. Skull of Frog. 95 These cartilage bones are not so numerous as the membrane bones, On either side of the foramen magnum, through which the brain becomes continuous with the spinal cord, is a bony mass which also bears the occipital condyles for articulation with the vertebral column. These paired bones are the exoccipitals. Continuous with each of these above, so as to appear to form but one bone, is an ossification in the auditory cartilage, the pro-otic, Beneath the anterior pointed end of the paraphenoid is a complete ring of bone, the ethmoid. Qn each side is a P.MX Fic. 41.—Skull of Frog. Lateral view. (After W. K. Parker.) BH, basihyal ; mk, Mechel's cartilage; ar, articular; DENT, dentary ; M-MK, mentomechelian ; PTR, pterygoid. Other letters as in Figs. 39, 40. Y-shaped bone running forwards, the A/erygoid. This is hidden for a short space anteriorly by the tooth-bearing maxilla, which latter abut in front upon the premaxille. The pterygoids are connected in front with the palatines, which lie transversely to the longitudinal axis of the skull. Finally, there is some ossification of the guadraze, which articulates with the lower jaw. The bones, cartilage, and membrane, which have just been enumerated, belong to the skull proper and to the sense capsules“and the upper half of the first visceral arch, the palatopterygoid arcade. There is left a small bone, the columella ‘auris, lying within the ear cavity and representing the top end of the second visceral arch; the lower jaw, which ! The existence of only a pair of ossifications in the occipital ring is not acharacter of frogs in general, but only of certain frogs. In others there are the four ossificaticns referred to above. 96 Elementary Zoology. consists of a rod of cartilage, ossified near to its articulation into the articulare, and at its junction with the corresponding half, the Mentomeckelian - along the shaft it is covered by membrane bone, dentary ; besides these, there is the Ayoid apparatus. Two long processes, one on either side, connect the plate-like hyoid with the skull wall. Posteriorly a pair of ossified rods, the thyrohyals, represent one of the branchial arches proper. It may assist the remembering of this complicated series of bones if they be arranged in accordance with the elements of the skull to which they severally belong, the membrane bones (distinguished by italics) being, placed in their proper relations to the rest— BRAIN-CASE. CARTILAGE BONES. MEMBRANE BONES, Exoccipitals. Larasphenoid. Sphen-ethmoid. Ffrontoparietals. Maxille. Premaxilla. CAPSULES OF SENSE ORGANS. Pro-otic. Sguamosal. Nasals. Vomers. FIRST VISCERAL ARCH Quadrate.' : Quadrato-jugal.? Pterygoid.* Palatine. Articulare. Mento-meckelian. Dentary. SECOND VISCERAL ARCH. Columella. FOURTH VISCERAL ARCH. Thyrohyal. “! The degree in which the quadrate is ossified among frogs varies much. * This is marked as a cartilage bone in my figures. More probably it is a membrane bone ; but see following footnote. * These are marked in the figures as cartilage bones, So they have been said to be. But perhaps the prevailing opinion is in favour of regard- ing them as membrane bones grafted on a cartilaginous substratum. I am unwilling, however, to attempt to decide between Prof. Parker (who colours these bones yellow, by which he mcans carti/age bones) and others. The fact is, it is not always easy to draw a hard-and-fast line between endosteal and ectosteal ossifications, Skull of Fowl. 97 The sku of a Fowl contrasts greatly with that of the frog, but it is formed out of precisely the same elements, save that the labial cartilages are no longer represented. The most obvious difference in the skull of a full-grown bird is its com- plete ossification; it is only here and there that extremely small portions of the original cartilage are left. From this it follows that it is no longer possible to strip off the adherent membrane bones; they are firmly welded to the other bones, and, indeed, in the old skull the boundaries of the bones which form the brain-case are no longer to be detected. In the bird’s skull, not only is the primordial cartilaginous cranium much more completely converted into bone than in the frog, but the number of elements is greater. The hinder region of the skull (see Fig. 42) is a ring of bone surrounding the foramen magnum, and is made up of four originally separate bones—the basz-occipital (Fig. 42, 6.0), the supra-occipital (s.0),and two ex-occipitals (eo). The condyle, by means of which the skull articulates with the backbone, is single, and not double, as in the frog, and it is formed entirely out of the basi-occipital. The cartilaginous base of the skull in front of the basi-occipital is ossified to form three bones, one in front of the other, which are at first distinct; these are the basi-sphenoia, pre-sphenoid, and the mesethmoid (eth). The walls of the skull above these bones are formed by the ak-sphenoids and the orbito-sphenoids (os). The auditory cartilage is ossified to form three bones, the pro-otic, the efi-otic, and the opisthotic. The membrane bones, however, are hardly more numerous than in the frog. Covering the skull above, and lying in front of the supra-occipital, are the large parietals (f), in front of which, but separate from them, are the frozéals (f). At the side of the frontals are the Zacrymads (7), which are, as a rule, not co-ossified with the skull, but are easily detachable. The wide apertures of the nostrils are bordered behind by the bifid nasal (x) bones, below by the maxil/e (m.x), and in front by the long nasal processes of the premaxille (px). On the under surface of the skull, another sét of membrane bones are to be seen. Forming the base of the skull just in H 98 Elementary Zoology. front of the basi-occipital, are the large bones which are usually called dasi-temporals. They appear, however, to correspond to the “handle” of the dagger-like parasphenoid of the frog. Articulating with the quadrate is a narrow bone on each side, which converges towards its fellow, and is attached to a some- what broader bone which runs forward in a straight line; these two bones are the prer-ygoid (pg) (posteriorly),! and the palatine (fa) (anteriorly). Between the palatines is a single compressed bone which is partly bifid posteriorly. This bone is the vomer (v). Its anterior end passes between two inward growths of Fic. 42.—Skull of Fowl. (From Gadow.) (For lettering, see text.) the maxilla, which have received a separate name, though they are not separate bones; they are termed the maxil/o-palatines. Another arcade is formed by a chain of thin and splint-like bones connecting the quadrate with the maxilla; these are the Jugal (7), nearest to the maxilla, and the guadrato-jugal (q/), articulating with the quadrate. Finally the quadrate articu- lates with the last of the series of membrane bones, the sgwa- mosal (sq). There remains for consideration the first two visceral arches, ' As to pterygoids and palatines, see footnote to p. 96. Skull of Fowl. 99 which, as in the frog, enter into the formation of the skull wall. The first arch, the mandibular, is formed out of the guadrate (7), above, a roughly triradiate bone. Below there is, in the young fowl, Meckel’s cartilage, which is surrounded by a set of membrane bones which form the actual lower jaw of the adult ; but the proximal end of the Meckel’s cartilage is ossified to form the aticudare (a.r)—that portion, in fact, which articulates with the quadrate. There is, however, no Mento-meckelian.! The membrane bones which ensheath the rest of Meckel’s UR HY Fic. 43.—Diagram of Bird’s skull. (Mainly after Gadow.) Cartilage bones (white): B.oc, basi-occipital; 1.0c, ex-occipital ; s.oc, supra-occipital ; " PER, periotic (behind it is columella); QR, quadrate; B.SPH, basi-sphenoid; a.sr, ~ali-sphenoid ; pr.sPH, pre-sphenoid ; or.seH, orbito-sphenoid ; TuRB, turbinals ; ETH, prefrontal; sEpT, ethmoidal septum; ART, articulare; CER.BR, UR.HY, hyoid bones. Membrane bones (dotted): pr, parietal; sq, squamosal; FR, frontal; above turbinal is lacrymal (not lettered); Na, nasal ; pmx, premaxilla; vo, vomer; MAX, maxilla ; PAL, palatine ; pry, pterygoid ; B.T, basi-temporal ; suc, jugal ; gr, quadrato- jugal; DENT, dentary; sPL, splenial; anc, angulare; s.ANG, supra-angulare. cartilage are dentary (d), angular (a), supra-angular (s.a), and Ssplenial, The second arch is formed above by the columella, a rod- like bone, which is—as in the frog—associated with . the auditory organ. The Ayoid bone lies in the tongue, and consists of a basal piece and of two lateral outgrowths; of these the ' A Mento-meckelian ossification has, however, been discovered in a hawk. 100 Elementary Zoology. anterior, and much the shorter one, represents the lower bit of the hyoid arch, while the posterior long and upwardly curved rods are the first branchial arch. The fowl is thus considerably further away from the primitive fish-like condition than is the frog, where there are remains of three purely branchial arches, The general plan of the bird’s skull is shown in Fig. 43, where the bones are represented diagrammatically, and in their approximate positions, but disarticulated. As with the figures FM. BO. B.S. PTA. PL. MN. Fic. 44.—Skull of Rabbit. (For lettering, see text.) of the frog’s skull above, the membrane bones are dotted. The cartilage bones are left white. The skull of the Rabbit offers a number of differences in structure from that of the bird. The most important of these are the following :— 1. The skull articulates with the atlas by two instead of by one occipital condyle. 2. The basitemporals are absent. 3. The representative of the quadrate is a small bone within the ear, the malleus.’ ' It must be borne in mind that this is only one view out of many. Some regard the articular surface of the squamosal as the quadrate, others the tympanics. Skull of Rabbit. 10! 4. The lower jaw consists of a single membrane-bone on each side. In other respects the skull of the rabbit does not differ widely in essentials, though, of course, there are numerous differences in detail. The cartilage bones of the cranium are: the four occipitals (B.0, E.0, 8.0); in front of the basi-occipital is the bast-sphenoid (B.S), above and on either side of which are the ali-sphenoids (a.s); in front of the basi-sphenoid is the pre-sphenoid, above which are the orbito-sphenoids (0.8) ; in front of this again is the mesefhmoid. The membrane bones of the cranium are the pariefals (Pp) behind, which enclose a small inéer-parietal ; in front are the /vonfals (F). Of the sense capsules, the auditory consists in the adult of a single periotic bone (P.0), which is formed in the embryo by the ossification of three tracts of cartilage, the fv0-ofic, opisthotic, and the epiotic. The membrane bones connected with the auditory capsule are the ¢ympanics (T) surrounding the meatus audi- torius, and the sguwamosals (s), with which the lower jaw articulates. The optic capsule has no cartilage bone or bones, but the /acrymal (L) may be considered to be the membrane bone connected with it. The olfactory capsules have three complicated and folded cartilage bones, the etAmo- maxillo and naso-turbinals. Of membrane bones connected with the olfactory sense-capsule are the zasa/s (N) above, and the vomer below. The first, or mandibular, arch has for its upper piece the malleus, one of a chain of three bones, which pass between the tympanic membrane and the foramen ovale in the periotic bone ; the chain serves to convey the waves of sound impinging upon the tympanum to the internal ear. Meckel’s cartilage remains unossified in the rabbit, except, perhaps, a small Mento-meckelian element, as in the frog. The palato-quadrate process of the first arch has no cartilage bones developed in it in the rabbit. But the membrane bones, which overlap, or are in connection with this arch, are, firstly the palatines (P.L) and the péerygoids (P.1), which together form the back part of the hard palate: and in front of, and at the outer side of these, the maxilke (mM) and premaxilla (P.M), which bear Elementary Zoology. *aBeVILS saya ‘soul] poi3oq = ‘[esourenbs ye Zuyeyoriy ‘(Aaeyuep --z) ojqrpuew ‘nw {yemsr0R] SvT fyesat ‘at é Bypeeur San Seypixeueid *xwed faout0a “WoA fyeseu ‘vN { equoy ‘aa ‘eaten age Cponep) sotiog ouviqmoyy ‘pros&saad a sunyed ‘Ivd = syeulqin3 ‘ans prouryjasoul ‘Hass ‘ prousyds-aid ‘as‘aud £ proueyds-o71qi0 “as"x0'! prousyds-ye as‘v ! prousyds-iseq ‘as*swa {oruedursy ‘war ‘(snour pue snajjeu aie 1 pue z ut anouad ‘€ fe ‘z fyemdiso0-eidns ‘90's Syeudioo0-xa So'xa ! eydi090-Iseq ‘90-4 i (@ngM) souog aFepyies -aya Yo Sauoq eBe[131¢9 ‘payjop are sauog suviquiaw ‘aiojaq sy (remopg reye Apseg) [ys s,purg jo wesFerp TEMS YUM Uosievduos Joy “|[NYs urlemUMEy yo weiseiq—-Sr -o1q Fore Limb of Frog. 103 the teeth of the upper jaw. The /ugads, which extend from the maxille to the squamosals, are to be looked upon as belonging to the same section of the skull. As to Meckel’s cartilage, it is invested by the den/ary bone (M.N), which, though a single bone, and thereby differing from the ensheathment of the cartilage in the bird, ossifies from a number of centres, which indicate its primitively compound nature. The number of centres appears to correspond to the number of separate bones in the bird. The second arch is represented proximally by the zwcus, the stapes, and the os orbiculare—three of the four “ear-bones already referred to. The rest of the Ayotd arch consists of a median piece, the body, and two pairs of project- ing cornua, of which one is true hyoid, the other a vestige of the’third visceral arch.* THE SKELETON OF THE FORE Limp. The skeleton of the fore limb consists of the pectoral girdle and of the limb which articulates with it. The pectoral or shoulder girdle itself in the adult Frog is a partly cartilaginous, partly bony structure. Each half of the girdle is C-shaped, the upper end, which does not meet its fellow, lying above the vertebral column; the lower ends do meet below. The part of the girdle lying above the glenoid cavity, into which fits the head cf the humerus, is the scapular region. The uppermost part of this is bent at an angle with the lower part, and is not so much ossified: it is called the supra scapula; the lower, more ossified portion, the scapula, On the ventral side of the glenoid cavity are two bars of car- tilage (in the young frog), which meet at first in the middle “line; these are respectively the coracoid and the procoracoid (the most anterior). The extremities of the coracoid and the procoracoid of each side fuse together, the common portion being termed the ¢picoracoid ;, the two epicoracoids overlap. On the procoracoid, but independently of it, a membrane bone, the clavicle, is formed, so that in the adult shoulder-girdle it 1 It has been shown that the cartilages of the larynx are traceable to branchial arches behind these. 104 Elementary Zoology. appears as if there were a bony procoracoid as well as a bony coracoid; the distinctness of the anterior bony bar must be borne in mind. The so-called sfrnum of the frog consists of an anterior bit, the omosternum, lying in front of the coracoids, and formed by a forward growth of the epicoracoids. The hinder part of the sternum (the siphisternum) is formed in- EAE a eye ) TS 6 ase SNE Se f UA 2 ms - Fic. 46.—Hand of Frog. (After Howes and Ridewood.) R.U, fused radius and ulna; N, naviculare (centrale) ; L, lunare (radiale) ; Pp, ulnare; p.o.1, carpal of pollex ; y.o.2, metacarpal of pollex ; 2-4, carpals of digits 2-4 ; 2-5, metacarpals. dependently of two pieces of cartilage, which fuse into a single piece, and become partly ossified. We shall see later that this part alone can be fairly termed sternum. The limb itself consists of a single bone, articulating with the glenoid cavity, the Awmerus ; of two bones lying side by side and fused together, which articulate with this, the radius Fore Limb of Fowl. 105 and wna; of a number of small bones and cartilages (to be more particularly described immediately), the carpus; of a set of longish bones, one for each finger, the mefacarpals ; and, finally, of a number of smaller elements, still several to each finger, the phalanges. Of separate carpal bones, six can be recognized in.the common frog. They are arranged in two rows, a proximal and a distal. In the proximal row are three bones, of which two articulate with the radio-ulna. The innermost of these is the ware, the outermost the centyale. A little below the latter, but really belonging to the radius, is the radial. In most animals this bone is articulated with the radius; but in the frog it has lost this primitive position. The distal row has also three bones. The first of these bears the rudimentary thumb, but also articulates with the second digit ; the second articulates with the second digit only ; the third is much larger, and is really formed by a fusion of two separate cartilages belonging to the two next digits of the hand? The shoulder-girdle and forelimb of the Fowl differs greatly from that of the frog—a difference which is, of course, related to its very different use. The scafuéa is a thin scimitar-shaped bone, which lies along the ribs, parallel to the long axis of the body. Below the glenoid cavity, for the articulation of the humerus, is the cora- coid, a more solid and a shorter bone than the scapula. From the inner face of each coracoid a process grows forward, which is a rudimentary procoracoid. The two coracoids are implanted in grooves upon the anterior edge of the sternum. The cavicles are represented by a curved and U-shaped bone often termed the “furcula.” It is a membrane bone, as is the clavicle of the frog. The sternum of the fowl is a much more solid structure than that of the frog. It grows out below into a thin but strong keel, or carina, which serves for the attachment of the powerful pectoral muscles, which are the chief agents in the downward stroke of the wing during flight. This sternum is 1 It is apparently very rarely the case among frogs that the fifth carpal has a cartilaginous rudiment. 106 Elementary Zoology. formed by a growing together of the lower ends of the ribs, and cannot, therefore, have any relation to the omosternum of the frog, which, as has been said, is an outgrowth of the epicoracoid. It may, however, correspond to the xiphisternum of the amphibian, which zs produced by the concrescence of two plates of cartilage. As ribs are wanting in the frog, it DG.) MC. il fl c 'D.C.3 DG2- 063 0. Fic. 47-—Hand of Fowl. (After Parker.) D.C.2, D.C.3, Carpals ; MC.1-3, metacarpals ; D.G.1-3, digits may be that these lower bars of cartilage are the remains of formerly more extensive ribs. With the glenoid cavity, formed by both scapula and cora- coid, articulates the Awmerus. This is followed by the radius and wna, which are here distinct bones, the ulna being the Ist. DIG : 2nv.DIGT ~— ; Fic. 48.—Hand of young Chick. (After Parker.) H, humerus; R, radius; vu, ulna; m.c, metacarpals; c, carpals. longer of the two, and bowed on its outer side, where the impress of the strong remiges can often be detected. The carpus of the adult bird has only two elements ; but more are present in the embryo. The hand of the fowl, as of all birds, is provided with only three fingers. Of these the three me¢acarpa/s are firmly welded Fore Limb of Fowl. 107 together, with a view, of course, to allow of a strong stroke in flying; that there may be no “giving” of the constituent bones. It will be clear, from the annexed illustrations, that the hand of the chick is much more like that of the frog than is ome, 7 SES eas, zs Fic. 49.—Shoulder girdle osternum of Rabbit. (From Parker ; slightly altered.) Sc, scapula; MAC, metacromion ; Ac, acromion; CL, clavicle ; cR, coracoidal rudi- ments; OMST, distal coracoidal rudiment; p.st, manubrium; mst, middle part of sternum; EP, epiphysis; x, xiphisternum ; 1ST R, first rib. the hand of the adult. The carpal bones are more numerous, and there is not so great a disproportion between the lengths of the fingers. 108 Elementary Zoology. In the Rabbit the shoulder-girdle appears to consist of but a single cartilage bone, the scapula. This is a triangular bone, with a median ridge along the outer surface, ending in a process, the acromion, At the distal end of this is a lateral outgrowth of the ridge, termed the metacromion. The end of the scapula forms the glenoid cavity for the articulation of the humerus ; but on the inner side of this articular cavity is a little process of bone (Fig. 49, CR), which is really ossified by two centres quite separate from the rest of the scapula. These represent collectively part of the coracoid of the bird; but it will be remembered that the coracoid of the bird reaches the sternum. The junction of the scapula with the sternum is effected in the rabbit by the c/avicle, a membrane bone, and by certain ligaments. Outside the clavicle, however, fragments of cartilage (dotted in the figure) have been discovered, which seem to be bits of the otherwise missing distal part of the coracoid. The sternum is a jointed bone, made up of seven separate pieces, or sternebrz, as they are sometimes called. The first of the series is the longest, and forms the manubrium ; the last is a long slender rod, ending in a cartilaginous plate, the xiphisternum. The fore limb itself has precisely the same divisions as in the other Vertebrata. The hand, however, has five fingers, and the carpus is composed of eight separate bonelets. SKELETON OF THE HIND Lime. As is the case with the fore limb, the hind limb consists of a girdle, the pelvic girdle, and of the limb attached thereto. There is, as will be seen, a very close correspondence between the several elements of the two limbs and their girdles, The pelvic girdle of the Frog appears to consist of two separate bones, somewhat spoon-shaped, narrow in front, and expanding posteriorly into a flattened and rounded area. The narrow ends of the two are attached to the wide transverse processes of the sacral vertebra. Each of these bones is in Flind Limb of Fowl. 109 reality composed of three separate elements. The long process and nearly one half of the rounded area is one element, the ilium. The ventral part of the rounded area is cartilaginous, and represents the pubis, while the remainder is the zschium. It will be observed that the three bones take a share in the formation of the acetabular cavity, in which is articulated the Femur. The Bird’s pelvis is strikingly unlike that of the frog, and yet the same elements can be traced in it. The whole pelvic arch seems to consist of one large bone; but embryology Fic. 50.—Pelvis of Chick. (From Fic. 51.—Pelvis of adult Bird. (From Gadow, Gadow, in Newton’s “ Dic- in Newton’s “‘ Dictionary of Birds.”) tionary of Birds.”) Ac, acetabulum ; Pr.Z, prepubic process ; f7s, ischial foramen. shows that it is composed of a right and a left half, and that each of these, again, is made up of three separate elements. The greater part of each of the two zznominates, as each half is called, is made by the substantial z/zm which abuts upon the sacral vertebre. Running backwards, parallel with the ilium, is a bone not quite so strong, the zschium. This is separated from the ilium in the middle by a large foramen, the ilio-sciatic foramen. Parallel with this, again, is a slender bone, the pudzs, which is nearly quite separate from it. All three bones join to form the acetabular cavity, and in front of this the pubis gives off a small forwardly directed process, the prepubic process. In the Rabbit are the same three bones, which again share in the formation of the acetabular cavity; or, to speak more accurately, the part of the apparent pubis within the acetabulum ossifies separately as a small cotyloid bone. The two pubes IIO Elementary Zoology. unite below to form the pubic symphysis. The totally dispro- portionate 2/2 in the bird, and possibly in the frog, also seems to bear some relation to the bipedal mode of progres- sion. By its extension forwards the ilium grasps more firmly the welded sacral vertebrze, and thus gives a firmer support to the hind limbs. The hind limbs of all these animals consist of a femur, corresponding to the humerus of the fore limb, followed by a Vite, (Dif — sf i eK KLG a Z é y; STILL ew GCELUD - 1. 2. a 4. 5. Vic. 52.—Foot of Rana temporaria. (After Howes and Ridewood.) A, astragalus ; C, calcaneum ; N, naviculare (centrale) ; CU, cuboid (= fused distalia 2, 3); Ph, x, 2, 3, prehallux (calcar) ; 1, distale; MAL, tendon of adductor longus primi digiti muscle ; 1-5, digits. tibia and fibula, equivalent to the radius and ulna, a ¢arsus corresponding to the carpus, a mefatarsus and phalanges which have also their equivalent in the fore limb. The femur of the Frog is curved in a slightly S-shaped curve. The #éa and fibula axe here fused together to form an apparently single bone. The two proximal bones of the tarsus, known respec- tively as the astragalus and calcaneum, are greatly elongated. This state of affairs seems to be correlated with the leaping of the amphibian, for a similar modification is to be found in the jumping Jerboa. The distal rows of tarsal bones are cartila- ginous, and there are five complete toes, with a rudiment of a sixth, in the form of a small bone known as the cadcar. fTind Limb of Fowl. Til The Aind mb of the Bird differs in several important par- ticulars from that of the frog. The femur, to begin with, is much shorter than the “dia; the jibu/a is rudimentary, and does not reach far down the tibia. This bone, the tibia, is followed by a long bone, with which the four toes articulate. Thus the fa7sus appears to be wanting. As a matter of fact, a study of the immature chick shows that what is apparently the tibia, is really the tibia /vs the proximal elements of the ‘tarsus; and what appears to be the metatarsus is really the three-fused metatarsals, A/vs the distal elements of the tarsus. Thus the ankle-joint is not, as it isin the frog and in the rabbit, be- =} |Ilz tween the end of the tibia and the Z tarsus, but in the middle of the tarsus. HWM ‘ Hence it is more correct to apply the Ady" ¢ terms “ tibio-tarsus” and “ tarso-meta- eS a tarsus” to the long bones in question. Mig The metatarsal part of the tarso-meta- MiTit tarsus contains, fused together, only @ U I \ three out of the four metatarsals, those J) \ corresponding to the three long toes. 7 q : The short hallux (or great toe) has a , 3 small metatarsal, loosely attached to : y the end of the tarso-metatarsus. The Te Caton. 4 Newiortd bird is bipedal in its progression, as is Be acai as cm the frog toall intents. But the required "% fibulare; c, centrale; td, elongation of the limb is brought about suis: “The righehand Fie ea in the bird by the elongation of the '** ¥*™ tibia and the metatarsus, not of the astragalus and calcaneum. In the Rabbit the “dca and the /fiu/a are complete, though the tibia is the larger of the two. The tarsus consists of seven separate bones. In the proximal row are the cakaneum and astragalus ; in the middle is the cuboid; the distal row is formed by the navicular inside, and by three cuneiforms fol- lowing it. There are only four digits in the foot of the rabbit. 112 Elementary Zoology. INTEGUMENTAL STRUCTURES. Skin structures are either purely epidermal, or partly epidermal and partly dermal. Purely epidermal structures are feathers, hairs, claws, and the scales of birds (on the feet) and reptiles. They are formed by a modification of the cells of the epidermis only, the lower- lying dermis not actually entering into their formation. The frog is totally devoid of any such structures, its smooth skin producing no hairs, feathers, or claws. The rabbit, like — Fic. 55.—Hair-rudiment from an embryo of i six weeks, magnified 350 diameters. (Kél- Fic. 54-—Section of the skin of the head, with liker. From Quain’s “ Anatomy.”) two hair-follicles. Diagrammatic. (Killiker. a, horny, and 4, mucous or Malpighian layer From Quain.) ? oa ale : . of cuticle; z, limitary membrane; 7, cells a, epidermis; 4, corium; c, muscles of (some of which are assuming an oblong the hair-follicles. figure) which chiefly form the future hair. ND all other mammals, is provided with hairs which completely cover the body, and are even found in the mouth cavity; since, however, the inside of the cheeks is formed in the embryo by an ingrowth from the outer covering of the body, it is not surprising to find that those cells which have thus grown inwards have retained the power of becoming modified into hairs. ‘The first appearance of a hair is a slight thickening of the lowest layer of the epidermis, the stratum malpighii; this growth projects downwards, and becomes larger. Ultimately the central cells change their character and become horny, thus forming the hair itself, while some of the peripheral cells grow out into little sac-like structures, which are the sebaceous glands, always attached to hairs, and secreting an oily fluid. A slight papilla of the underlying dermis projects into the centre Feathers. 113 of the hair, but takes no actual part in its formation, merely. serving to bring closer the necessary blood-vessels and nerves. If the skin of a bird be carefully examined, there will be found among the feathers thin and delicate horny shafts, which have every resemblance to hairs. But between these filo-plumes and the most complicated feathers, every inter- mediate stage will be found; and even the filo-plumes them- selves have commonly a few slight branches at the summit. At its origin, however, a feather—even these simplest feathers—is different from a hair. It appears first as a slight outgrowth, a papilla, of the skin, This is surrounded bya depression, out of the middle of which the papilla arises. The papilla consists both of dermis and epidermis; but the epidermis alone enters ee ee, a oe into the formation of the —“Sn'Newtons Ditton ofprde) feather, the dermis becoming P, pulp; Z, Suc, M, different layers of epiderm. the central pulp, with blood- vessels, nerves, etc. The surrounding fossa deepens, and thus the developing feather comes to lie at its base within a sheath. The feather itself is formed purely by a horny change in the epidermic cells, which are separated into three layers. The outermost layer forms a delicate sheath, which is cast off when the feather is fully formed, but which may be often seen encasing a newly formed feather in a moulting bird; the middle layer forms the feather itself, whose complicated form is due to the irregular modification of the cells, as will be explained directly, while the innermost layer of all forms that series of cup-shaped bits which occur in the inside of the quill, and to which the Germans have given the poetical name of “ Federseele.” A feather itself, when most fully developed, such as one of the strong remiges which fringe the wing, or rectrices which form the tail, or contour feathers, as the strong feathers of the general body surface are called, consists of a quill, or calamus, which is hollow, and of a rachis above this; at the junction of the two is a minute perforation, the wmdilicus, and at this point a small I 114 Elementary Zoology. second feather often arises from the main shaft; this is the after-shaft. The rhachis gives off numerous barbs, which in Fic. 57.—Feather. (From Gadow, in Newton's “ Dictionary of Birds.”) D, downy portion ; P, horny cap (‘‘ Feder selle”), continued through umbilicus as F’; to right of this springs aftershaft. turn give rise to secondary branches, the darbudes, and these again to shorter processes, the Jardicels, which are often Feathers. 115 hooked at their extremities, and so interlock with neighbouring feathers. Thus the firm nature of a feather is arrived at. Down feathers, which are, as their name implies, the softer feathers of the body, frequently do not possess the terminal hooks—the amuéi, as they are sometimes called—and therefore do not interlock, hence their softness. Moreover, in down feathers, the barbs frequently arise in a clump from the calamus, the rhachis being absent. The quill, of course, is formed by being moulded upon the feather papilla; but in the natural posi- ; Z tion the rhachis, with its branches, is ye folded so that the extremities of the barbs meet, thus forming a cylinder above: the umbilicus forms the com- munication between this and the cavity of the quill. The feather itself is, there- fore, simply a continuation of the quill cylinder, with an irregular cornification, incomplete in the middle dorsal line, the end, therefore, becoming free di- rectly the feather is fully formed. When “(vom cag” an after-shaft is present, it is formed on Sw, sheath. the opposite side of the feather papilla from the main rhachis. While the scales of lizards and of birds are purely epidermic structures, those of fishes are either mesodermic or are formed by both epidermic and mesodermic elements. The minute scales of the dogfish, which, together with the intervening skin, form the substance known as shagreen, consist of a base of dentine—formed by the mesoderm, and similar in its characters to the dentine of the teeth of the same animal— and of a cap formed by epidermis, which is, in its turn, like the enamel of the teeth. The identity of structure between these body scales and teeth has led to the inference that they are identical, homologous, structures. At first sight it may appear difficult to compare structures lying on the outside of the body with the teeth lying in the interior of the mouth; but remembering the hairs, purely skin structures, which line the cheeks of the rabbit, it will not be difficult to see that in the 116 Elementary Zoology. case of the dogfish also we have simply to do with an involuted part of the skin, zc. the mouth cavity, whose lining epidermis and dermis has retained the functions of those two layers, elsewhere. Just as the teeth are in most animals attached to bones of membranous, not cartilaginous origin, so the base of these scales are ossified. Each scale, therefore, of a shark is literally a tooth attached to a small bone. The comparison of teeth with scales is supposed to hold good up to the Mammalia. But it must be borne in mind that the mammalian tooth consists, in addition to enamel and dentine, of a layer of bone, the so-called cementum. This may be even preformed in cartilage. It has nothing whatever to do with the bone of the jaw to which the teeth ultimately become attached. This general statement, therefore, ‘‘ The bones around the mouth have been recognised as having their origin in tooth-bearing plates derived from fused placoid plates,” must be, possibly, somewhat cautiously accepted. It looks, in the case of the Mammalia, at any rate, as if the dentary and other membrane bones which bear the teeth were not the precise equivalents of the fused bony bases of placoid scales, since the homologues of the latter exist (?) in the cementum of each tooth, Teeth are found in: the frog and in the mammal; but there is no bird in which these structures occur living at present. There were formerly toothed birds; and some of their descendants of to-day have retained rudiments, shown during the development of the jaws, which are regarded by some, though not by all, anatomists, as rudimentary tooth germs. In the frog the teeth are developed upon the mazxille, premaxilla, and vomers; in the rabbit, upon the two former bones and upon the dentary of the lower jaw. The teeth of the frog are all of approximately the same shape and size; they are, moreover, very numerous, and fresh teeth are formed when the first ones get worn out. In the rabbit, on the contrary, the teeth are fixed in number, and only a few; they are varied in form,’ the chisel-like incisors being easily distinguishable from the flat grinding teeth, while most mammals have, in addition, the sharper canines lying between the incisors and premolars. Furthermore, there are only two * “Homodont” and ‘* Heterodont” are the terms used to express the condition of the teeth in the frog and rabbit respectively. Teeth. 117 sets of teeth, the first set being termed the milk dentition,’ and the last the permanent dentition. The permanent incisors and the first few molars, on this account termed premolars, have predecessors in the milk dentition. The arrangement of the teeth in mammalian jaws is commonly expressed by dental formulz, which serve to show at a glance the number of the teeth and their nature. Only one side of each jaw is taken in representing the teeth, so that the number as given in the formule must be doubled in order to give the full number. The milk dentition of the rabbit is represented by the following formula: d¢. 2, de. 2, dm. 2; the permanent thus: 7. 3, ¢ 2, pm. 3, m. 3. 1, 6, pm., m., respectively stand for incisors, canines, premolars, molars; a @ before each of these letters means deciduous, or milk incisor, etc. ! Diphyodont is the technical term for a mammal with two sets of teeth. CHAPTER XI. THE EGG, THE SPERM, AND THE DEVELOPMENT OF. : THE CHICK. Ecc. TuE fowl’s egg, in spite of its size, is a single cell, just as is the microscopic egg of the earthworm. It is wrapped, however, in an adventitious sheath, derived from the walls of the passages through which it makes its way to the exterior when “laid.” The real egg—the term ovum is preferable, as not implying the non-essential coverings—is limited to that part of the egg which is popularly called the yolk. All outside of the delicate membrane covering the yolk is the adventitious sheath. This sheath consists of the “white,” or albumen, which is fluid in the fresh egg, has two spiral thickenings at each pole, the so-called “chalazz,” the use of which is, apparently, to act as springs to prevent the delicate ovum itself from being jarred and ruptured. Outside of the albumen is a tough membrane, and outside that, again, the shell. The albumen is absorbed by the growing chick; the numerous pores in the shell permit of respiration being carried on before the young chick breaks through the eggshell at hatching. The ovum proper is enclosed by a delicate and elastic wételline membrane. The substance of the ovum is chiefly made up of the yolk; but there is a cap of pure protoplasm at one side, in which lies the nzcleus or germinal vesicle. ‘The great size of the ovum is due to the enormous quantity of this yolk present. Ova are classified according to the quantity of yolk present and its distribution. There are three grades, connected, of course, by intermediate conditions, Egg. 119 The akcithal ovum (e.g. Earthworm, Amphioxus, Rabbit) is a minute egg, with only a little yolk in the form of a few spherules scattered uniformly through the protoplasm. The dlolecithal ovum (¢.g. Fowl, Dogfish) has a quantity of yolk massed at one pole, and occupying the greater part of the ovum. The centrolecithal ovum, finally (eg. Astacus), has a quantity of yolk, which lies centrally, and is surrounded by a peripheral layer of protoplasm. SM Y BA Fic. 59.—Egg of Fowl in longitudinal section. (From Marshall.) BA, germinal disc ; y, yolk ; z, vitelline membrane; wa, albumen; wc, chalaza ; sv, air space ; sM, shell membrane ; SH, shell. The frog’s egg is intermediate between the alecithal and the telolecithal; there is a great deal, but not an enormous quantity, of yolk, which is more dense at one pole than at the other, though the protoplasmic pole of the ovum is not entirely free from yolk, as in the fowl. Alecithal ova either produce embryos which are hatched and lead a free larval life at a very early period of development, 120 Elementary Zoology. as in the case of Amphioxus, or the embryo, if not hatched until it is full grown, as in the earthworm and rabbit, has some special means of nutrition independent of the yolk contained in the ovum. The earthworm embryo lives upon the albumen in the cocoon; the young rabbit is nourished by the mother through the placenta. On the other hand, the animals hatched out from eggs with abundant yolk are nourished during growth by that yolk, and are born in a more or less adult condition, as in the case of the chick. The frog is inter- mediate ; the larva is older when it leaves the egg than that of the Amphioxus. Some frogs are not hatched as tadpoles, but as frogs; this is due to a larger and, consequently, more yolk-laden ovum. If there were any doubt as to the unicellular character of the ovum from an examination of its structure when mature, this doubt would be entirely removed by the mode of develop- ment of the ovum. The ova in the very immature ovary can be detected as cells only a little larger than the other cells which form the tissue of the ovary. The larger cells, destined to become ova, are surrounded by a layer which ultimately becomes several layers thick of the smaller, non-generative cells; this layer is termed the fo//icle, and its cells contribute to the nourishment of the egg cell. In certain animals processes of these cells have been seen to grow out and come into contact with the protoplasm of the ovum ; and it has even been asserted, though the view is not generally accepted, that the yolk is actually elaborated in these cells, and then, as it were, eaten by the ovum. COMPARISON OF THE OVUM WITH AN ORDINARY TISSUE CELL AND WITH THE SIMPLEST ANIMALS. It has at various times been attempted to be shown that eggs in some cases are not single cells. It may, however, be taken for granted that the ovum is invariably a single cell. The main reasons for this conclusion as to the morphological nature of the ovum are as follows: In the first place, the young ova, in most cases, are perfectly indistinguishable from cells Egg. 121 that do not become ova. Secondly, the structure of the mature ovum agrees in every point with that of a single cell: it is made up of a mass of protoplasm in which is imbedded a nucleus ; the reticular structure of the protoplasm and of the nucleus is exactly paralleled in other tissue cells; and, finally, the cells of many tissues (all, probably, according to some histologists) possess a centrosome—that body which plays so important a part in cell division. Thirdly, the way in which an ovum divides is like that of other cells; the complicated Fic. 60.—Amceboid movements of young egg-cells. cr to C5, egg ofa Cat, in five successive stages (after Pfliiger); p, ditto of trout; E, of a Hen; F, of Man. process of Karyokinesis is copied, so to speak, in the ovum (cf. Figs. 64, 65), detail for detail, from the method of other cells. It is true that in many cells a simpler mode of cell division, not found in ova, is often met with, 7.e. direct or amitotic division, in which the nucleus simply constricts and divides, without the complicated figures of Karyokinesis (or Mitosis, as it is sometimes termed) ; but the indirect method is the more characteristic mode of division of cells in general. Another highly important generalisation to be borne in mind in considering the single-celled character of the ovum is its consequent likeness to a single-celled organism, such as an Amoeba. The body of the most advanced animal is 122 Elementary Zoology. derived from a single cell, just as we suppose that the ancestor, of all the higher animals was a single-celled organism. More- over, the eggs of many animals exhibit amceboid movements ; and the encystment previous to division of Protozoa, is paralleled by the secretion of a membrane by the ovum when it is ready to divide. MATURATION OF THE Ovum. The egg, then, is a single nucleated cell, which only differs, and that not always, from other animal cells by its greater size and storage of yolk, the two characteristics being responsible for each other. Except in the comparatively rare cases of parthenogenesis, an ovum must be fertilised by a spermatozoon before it can divide, and by its repeated divisions form an embryo. The hen’s egg, on account of its large size and the dif- ficulty of manipulating the yolk, has not been thoroughly studied in the stages of its development preceding fertilisation. The following account is therefore a general account, which, with slight differences in detail, will no doubt serve as a de- scription of the processes that occur in the fowl’s egg. The nucleus is a vesicular structure surrounded by a definite membrane, and in which there is a network of denser and darkly staining matter—on this account termed chromatin—in the meshes of which is a more fluid substance. The general term “ chromatin” has been given to the meshwork in general, but it appears that the meshwork is formed of a groundwork of Zinin, to which are adherent granules of zuclein. It is im- portant to distinguish between these two substances, because their behaviour is different, as will be seen shortly, in the maturing and dividmg ovum. Just before the ovum is ready for fertilisation the nuclein masses itself into a number of short rods, to which the name of chromosomes has been given. The linin arranges itself in the characteristic form of a spindle, as is shown in the annexed figure ; the membrane of the nucleus has in the mean time disappeared, and at the two poles of the Polar Bodies. 123 spindle are two rounded bodies whose origin and fate is at present a mystery, but which have been called the centrosomes ; from the centrosomes radiate a number of lines of granules of the egg protoplasm into the surrounding mass. The entire nucleus advances towards the periphery, and half of the chromosomes, together with a portion of the spindle, sepa- rates itself from the egg, and remains at the surface as the first polar body. A second polar body is then protruded, and the ovum Fic. 61.—Stages in the formation of polar bodies in the ovum ofa Starfish. (After Hertwig.) gv., germinal vesicle transformed into a spindle-shaped system of fibres; ’, the first poiar body becoming extruded ; A, Z, both polar bodies fully extruded ; Spm, female pronucleus, or residue of the germinal vesicle. after this is ready for fertilisation. The extruded polar bodies play no further part ; they remain for some time, and ultimately disappear. Their nature will be discussed later. HISTORY OF THE SPERMATOZOA. The spermatozoa of animals are very different from their ova. While the ova are always relatively large, sometimes enormous, lethargic, showing at most amoeboid movements, the spermatozoa are minute, and nearly always.actively motile. Even where they are immobile, as is the case with the Crustacea, their form is different from that of the ova, and they are greatly smaller. 124 Elementary Zoology. In spite, however, of these differences, which are as marked as those which occur between the cells of the most diverse tissues, there are abundant reasons for regarding the ova and spermatozoa as equivalent and homologous bodies. In the first place, and apart altogether from the special resemblances that will be pointed out in their development, they are both cells. In each is a nucleus, and the nucleus is accompanied by protoplasm. In the case of the ovum the protoplasm has given rise to more or less yolk, and it is consequently inert. The spermatozoon consists of a “ head,” which is the nucleus; and the protoplasm of the cell has been mainly converted into the actively vibratile “tail.” A spermatozoon, in fact, may be looked upon as a flagellate cell in which the flagellum has acquired undue importance. The general correspondence between ovum and spermatozoon is better shown by those rare cases (the Nematoid worms) in which the spermatozoon is not flagellate, but moves by thrusting out pseudopodia. As to the special resemblances between ova and sperma- tozoa, they are both produced in identical “glands,” the gonads. The gonads, whether male or female, are always local proliferations of the lining membrane of the celom. In the tadpole, for example, as an instance of an animal with separated sexes, the gonads appear as a pair of ridges of the peritoneal epithelium, the genital ridges, which are at first absolutely indistinguishable. In hermaphrodite animals either both ova and sperm are produced from the same gonad (e.g. snail), in which no need arises for a comparison, or if two kinds of gonads are present they are clearly homologous. Thus in the earthworm the testes and the ovaries occupy identical positions in the body. Moreover, they appear to correspond exactly in number, for, although the adult worm has two pairs of testes and one pair only of ovaries, there is in the embryoa fourth pair of gonads in the twelfth segment. There is thus a close correspondence, usually amounting to identity, in the situations where the ova and spermatozoa are produced. ‘There is, furthermore, an exceedingly close likeness in the way in which the ova and the spermatozoa respectively develop in those gonads, Fertilisation of Egg. 125 As to sperm formation, the details vary in different animals, -but an instance selected will serve to illustrate the facts upon which it is necessary to lay stress. The nucleus of the sperm mother cell undergoes what is called a “reducing division ;” that is to say, there are at first twenty-four chromo- somes, which are divided into two lots of twelve each when the original cell has divided into two; in the product of the next division each of the four cells formed has a nucleus containing only six chromosomes. Now, when this state of affairs is com- pared with what occurs in the ovum, it is plain that it is not strictly comparable with the division of the ovum after fertilisa- tion, for when that occurs each chromosome in the nucleus divides by splitting, so that each daughter nucleus has the same number of chromosomes as the parent nucleus, though each separate chromosome is but half that of the parent nucleus. But during the formation of the polar bodies events occur which are exactly comparable to that of the sperm cells. Each of the two polar bodies is formed by a reducing division. What has happened in the ovum, therefore, is a division of the egg mother cell into three or four daughter cells, of which, however, only one becomes an ovum. In the case of the sperm all the daughter cells become spermatozoa. FERTILISATION OF THE OVUM. The mature spermatozoon is a tadpole-like body with a head “resembling a conical bullet,” a neck composed of a small sperule, and a long vibratile tail. The head consists of nuclein. By the active lashing of the tail the spermatozoon approaches the ovum—which in the starfish (see Fig. 61), a classical object for the study of the processes of fertilisation, protrudes a little hillock of protoplasm to meet it—and bores its way into the interior, the tail in some cases remaining behind, Occasionally (Fig. 63) more than one spermatozoon enters the ovum. Directly it enters, the ovum secretes a delicate membrane, which prevents the entrance of another spermatozoon. The head of the spermatozoon (composed, it will be remembered, 126 Elementary Zoology. of nuclein) is termed the male pronucieus. In front of it, as it pushes its way towards the female pronucteus (original nucleus of egg-cell minus the polar bodies), is a small clear body round which the granules of the egg protoplasm are beginning to arrange themselves in a radiating fashion, which is the product of the neck of the spermatozoon, and has been called the made centrosome. The female pronucleus has a correspond- ing centrosome. The two pronuclei now approach more a 3. 4. 5. - mpl aa Xt FPP. Fic. 62.—Fertilization of the ovum of an Echinoderm. From Quain’s ‘ Anatomy,” after Selenka.) S, spermatozoon ; 77z.47., male pronucleus ;_4.47., female pronucleus. rapidly, and fuse together. The centrosomes each divide into two; each half unites with the half of the other. Their move- ments have been fancifully termed “the quadrille of the centrosomes.” The definitive nucleus is now formed, and it has, it will be observed, a centrosome at each end. Thus the process of fertilisation is essentially the union of the nuclei of two dissimilar but homologous cells, of which one, the male, is small and active, the other, the female, is large and passive. DIVISION OF THE Ovum. The fertilised ovum now proceeds to divide into two. This process is initiated by the nucleus; but the modus Fertilisation of Egg. 127 operandi is a little different from the process which accom- - panies the production of the polar bodies. I have said that the process is initiated by the nucleus; as a matter of fact, it appears, at least often, to commence with the division of the centrosome; but the two statements are not at present irreconcilable, for, after all, the centrosome is not at the time thoroughly understood. When the centrosome has divided, Fic. 63.—Ovum of an earthworm, showing entrance of three spermatozoa, marked by a whirlpool-like disturbance of the ovum. (After K. Foote.) the chromosome bodies are formed in the nucleus as before, and lie across a spindle (Fig. 64, 4); the number of chromosomes is, in cases where they have been carefully observed, con- stant and characteristic for a given cell. The chromosomes constantly acquire a V shape, the angle of the V lying towards the centre of the nuclear sphere, the ends being thus péripheral. Now, the process of division of the nucleus does not consist, as in the formation of the polar bodies, by a 128 Elementary Zoology. passing over of half of the chromosomes to the daughter nucleus ; but each chromosome splits longitudinally into two parts, and the result is that each of the two nuclei formed out of the original single nucleus contains exactly the same number of half chromosomes. The remaining divisions of the cell go on precisely the same way. In the fowl’s egg the division is limited to the germinal disc. The first furrow formed runs across the disc, but does Fic. 64.—Successive stages in the division of the ovum, or egg-cell, of a worm. ‘ (After Strasburger.) a to d show the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum at ¢: fand g show a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage at i. not quite reach its margin on either side; the second furrow is at right angles to this, and in the same way does not reach the edge. Finally, the disc is broken up by a series of sub- sequent furrows into a mosaic, which is limited to the central region of the germinal disc, and is slightly excentric. At first this cap of cells, thus formed, is a single layer in thickness, the segmentation being confined to the superficial layer of the germinal disc. Later on the segmentation takes place also in the deeper layers of the germinal disc, so that ultimately a cap Segmentation of Egg. 129 of cells, two or three deep, is formed. Of these layers the outermost is soon separable for the deeper layers, and between it and them a cavity—the blastocel—appears. The outer F Fic, 65.—Karyokinesis of a typical tissue-cell (epithelium of Salamander), (After Flemming and Klein. The series from a to 1 represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the ‘‘nucleus-spindle” of an egg-cell, a, resting nucleus; p, wreath-form ; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star ; 1, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked a, and correspond to the ‘‘ equatorial plate ;” 4, achromatin fibres forming the nucleus-spindle, ¢, granules of the cell-protoplasm forming a ‘‘ polar star.” Such a polar star is scen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed of chromatin. layer thus definitely established is known as the epiblast (Fig. 66); the remaining cells may be termed, for the present and BV H — Fic. 66.—Blastoderm of Hen’s egg at time of laying. (From Marshall.) E, epiblast ; H, hypoblast; Nn’, nucleus in yolk, round which a cell will be formed later; zt, lower layer cells; By, subgerminal space; y, yolk. collectively, the lower layer cells. These are increased by new cells out of the yolk (Nn, Fig. 66). K 130 Elementary Zoology. The next important change to occur is the welding together of the lowest layer of shells to form a continuous sheet, to which the name hypoblast is applied. Between the hypoblast and the epiblast are a few cells which take part in the formation ofthe third layer of the embryo, the mesoblast. Soon a median opacity appears along the embryo, the primitive streak. This, when examined by means of transverse sections, is seen to be due to a thick band of cells, which is marked superficially by a groove. The thickened band of cells is produced bya rapid growth of the epiblast cells, which results in a sheet of tissue, as is shown in the accompanying figure (Fig. 67). This is the main portion of the mesoblast, the rest being produced from the proliferation of hypoblast cells ; and some of H E PG ps M Fic. 67.—Section through primitive streak of Chick at 2oth hour. (After Marshall.) PS, primitive streak ; pc, the groove upon it; E, ep blast; mM, mesoblast ; 4, hypoblast. its cells are persistent lower-layer cells, already referred to as left after the separation of the hypoblast. “ Among the cells proliferated off from the hypoblast to form the mesoblast, a number form themselves into a densish rod of tissue, running along the body of the embryo. This rod of tissue is the notochord, which is formed, it will be observed, from the hypoblast. The notochord is the first part of the skeleton to appear. Directly after the appearance of the notochord, the first beginnings of the central nervous system are laid down. In front of the primitive streak is a thickened layer of epiblast, the neural plate. A groove, the neural groove, is formed along its surface, which becomes, later, closed to form a canal, the neural tube. The closure of the groove into a tube com- mences with the head end and spreads backward. The cavity of the central nervous system persists in the adult as the ventricles of the brain and the canal of the spinal cord. It Formation of Nervous System. 131 must be noted that the brain and the spinal cord arise as a continuous structure, and that at first the brain is only to be distinguished by its wider calibre. While the changes in the epiblast are going on, the meso- blast also becomes differentiated. It forms at first a continuous Fic. 68.—Surface view of embryo Chick at 24th hour, (From Marshall.) HD, head; nF, neural fold; vv, vitelline vein; NG, neural groove ; MS, proto-vertebrze ; AD, margin of area pellucida; ps, primitive streak. sheet with the threefold origin already spoken of. The first change to be noted is its splitting into a dorsal and a ventral layer, with the result that a cavity appears between them. This cavity is the celom. The upper layer of mesoblast is known as the somatopleure, the lower as splanchnopleure. 132 Elementary Zoology. At the same time a split appears on each side of the body, parallel to the notochord, which separates the mesoblast into a vertebral and a lateral portion, while transverse clefts break up the vertebral portion of the mesoblast sheet into a series of squarish blocks, the protovertebr, or, better, ‘“‘ mesoblastic somites.” At this stage, therefore, the embryo of the fowl is distinctly segmented, and, it will be observed, the portion of Fic, 69.—Transverse section of Chick at 48th hour. (From Marshall.) NE, spinal ganglion ; xs, spinal chord; cu, notochord ; mp, muscle space with contained coelom (cm); vc, posterior cardinal vein; kc, Wolffian duct; KS, nephrostome; o£, genital epithelium; c, celom; a, aorta; AN, amnion; ME, somatopleure; MH, splanchnopleure ; 4, hypoblast ; vv, vitelline vein. the ccelom lying within the segmented portions of mesoblast is also segmented. There are, in fact, a series of coelomic spaces as in the earthworm, The fowl agrees with reptiles and all animals higher in the scale, to differ from the frog and other animals lying lower in the scale of vertebrated animals in the possession of an organ of protection for the embryo, which is known as the amnion. The first trace of this to appear is the head-fold (Fig. 69, AN), an upgrowth of epiblast first, but later lined with the somato pleuric mesoblast ; then there is a corresponding tail-fold and two lateral folds—these meet and forma thin but double-layered Allantois. 133 sac, entirely covering the embryo above. The embryo sinks further into the yolk, which is being continually absorbed, and is, as stated, covered above by the amnion. Of this double- layered sac the part nearest to the embryo, which it closely invests, is known as the ¢rue amnion ; the outer layer becomes pressed against the egg-shell, and this portion is termed the false amnion. The space between these two layers is clearly ccelom. AN TA HM Fic. 7o.—Embryo Chick at ninth day. (From Marshall.) aN, true amnion ; TA, allantois; Hm, hyomandibular cleft; ys, yolk-sac; wa, white of the egg ; Sv, air chamber. Another embryonic organ, found in the frog as well as in the higher vertebrates, is the allantois (see Fig. 70, Ta). The allantois is an outgrowth of the gut; it is therefore lined with hypoblast, and covered externally by splanchnopleuric meso- blast. It grows into a large thin-walled sac, which comes into close contact with the shell, and serves as a respiratory organ for the growing embryo, absorbing, as it does, into its numerous blood capillaries the outside air. The allantois totally dis- appears in the adult fowl ; in the frog it is permanently retained as the bladder. 134 Elementary Zoology. Certain Transitory Organs.—In the course of development of the fowl, as of other animals, various organs appear which develop up to a certain point and then disappear. These can be compared with organs found permanently in the lower types, and their transitory existence is regarded as tending towards the proof that the animals in which they are found in the rudimentary condition are descended from animals like those in which they are permanent structures. Some of these structures have been already referred to, as was necessary in giving a general account of the early stages Fic. 71.—Formation of the gastrula of Amphioxus. (After Kowalevsky.) a, wall of the ovum, composed ofa single layer of cells; B, a stage in the process of astrulation; c, completion of the process; s, original or segmentation cavity blastoccel) of ovum ; a/, alimentary cavity of gastrula (archenteron) ; ec¢, outer layer of cells ; ez, inner layer of cells; 4, orifice, constituting the mouth (blastopore). in the development. Thus the primitive streak is by most persons looked upon as the equivalent of the blastopore of other embryos, and as the equivalent of the mouth of ccelenterate creatures, such as Hydra. In many animals—for instance, in Amphioxus—the embryo, after a number of cell divisions; be- comes a one-cell-walled hollow sac, the cavity being the blasto- cel, equivalent to the similarly named cavity of the chick embryo. At one point this wall is thrust in, as is shown in the accompanying figures, thus producing a double-layered sac with a new central cavity and an aperture leading to the exterior, Gastrula 135 which is the aperture of invagination. If a hollow india- rubber ball be pressed in with the finger until the two opposite walls meet, some idea can be obtained of the Gastrula, as the embryo in this stage is termed. Now, in the fowl we have a groove formed, and with the formation of this groove coincides the formation of, at least, the greater part of the mesoblast. The mesoblast of Amphzoxus is also established after the forma- tion of the gastrula; but in Amphioxus the invagination of a layer of cells, at first’external in position, is the way in which the hypoblast is formed, this layer being, in fact, the invaginated layer. In the fowl, as we have seen, the hypoblast is formed before the groove is produced. This is a difficulty in the way of considering the groove to be an abortive blastopore. The typical gastrula, such as that of Amphioxus, resembles Hydra in the number of points, which are— 1. It is two-layered. 2. The two layers are arranged in the form of a hollow sac. 3. The sac communicates with the exterior by an aperture, the mouth of the Hydra and the blastopore of the gastrula. It is thought that, in the embryo at the gastrula stage we have a recapitulation of the Aydra-like form which was its remote ancestor. : f Some other transitory organs will be more conveniently dealt with in the following general account of the development of the several systems. We may mention here, as being purely transitory organs, the gi//-clefts. It will be remembered that the tadpole breathes by means of gills, which are vascular tufts arranged as fringes along the margins of certain clefts, which place the pharynx in communication with the outside world. In the embryo chick there are four of these clefts formed, which grow out from the hypoblast lining the pharynx, and come into contact with the epiblast, a perforation being formed at the points of contact. There are thus a series of apertures established. ‘These, it seems necessary to suppose, are the exact equivalents of the gill-slits, though they never perform the part of respiratory organs; nor, indeed, do they appear to perform any function at all, which is a still further argument in favour of their being comparable to the gill-slits of the frog, 136 Elementary Zoology. since no use can be assigned to them, and so account for. their presence. The notochord is an organ of a similar character. In the Amphioxus the notochord is the only part of the skeleton to appear at all, and it persists throughout life. In the chick the notochord is replaced by (is zof developed into) the backbone, This is an interesting case of a frequent phenomenon of what has been termed the “ Substitution of organs.” Very often it happens that an organ is, as it were, manufactured. out of another organ. An example of this is the formation of the sternum out of the ends of the fused ribs, or of the auditory passage out of the first gill-cleft. But sometimes a new organ, having a similar function to one already existing, is formed afresh in the same place. Thus the backbone is not a product of the notochord, but it appears at the same spot, and performs much the same function, strengthening the body in a longi- tudinal direction. In the one case—to use a homely simile— an old coat is cut down into a waistcoat; in the other, an entirely new coat of a different pattern is acquired. ; DEVELOPMENT OF THE ORGANS OF THE Bony. It will be more convenient to deal with the organs separately, instead of following a strictly chronological order and pursuing the course of development of all the systems together. From the epiblast are formed the skin, ze. the epidermis ; the central nervous system (as already mentioned); the stomo- deum and proctodeum ; portions of the eye and of the ear; and the olfactory organs. The stomodeum and the proctodeum are involutions of the epiblast to form the lining membrane of the first and last parts of the alimentary tract. The greater part of the lining epithelium of this tract is hypoblastic. But the mouth cavity is entirely epiblastic, while the posterior aperture is formed by a very short invagination of epiblastic cells. It seems clear that all sense organs should be primitively Development of Sense Organs. 137 external structures, therefore epiblastic. But in the vertebrate (and in some other animals too) the sense-organs are concealed within the body for protection. Nevertheless, their develop- ment shows that in all cases the sense-organs are epiblastic structures—that is, of course, the actual sensory parts of the organs ; for, in the complicated sense-organs of the vertebrates, the eyes and ears, various accessory structures, which have merely a subsidiary function, also exist. The ear first appears as an invagination of epiblast, which ultimately comes to form a closed sac. From this sac are formed the three semicircular canals and the rudimentary cochlea—in fact, all the essential parts of the ear. The mass of bone in which it is imbedded, and the small bones which serve as a conduit to the sound waves, are mesoblastic structures, The eye is formed from three sources. There is, first of all, a pair of outgrowths from the brain in the shape of hollow vesicles. These become curved flattened plates, hollow within and connected with the brain by a hollow stalk. The plates, into which the ends of the primitive outgrowths of the brain expand, form the retina of the eye, while the stalk ends as the optic nerve. The ens of the eye is produced by a direct invagination from the exterior of the body. ‘The rest of the eye structures are mesoblastic. It will thus’ be seen that the retina, which is the essential part of the eye, is epiblastic ; not so directly, it is true, as in the case of the sensitive part of the ear. But, as the brain itself is formed by an involution of the epiblast, all structures derived from it must be also epiblastic. The olfactory organs are also developed as epiblastic in- vaginations, but they have retained, to a greater degree, their primitive ~ position. They still communicate with the exterior. From the hypoblast is developed: the esithelum of the digestive tract, apart from that of the stomodzeum and procto- deum just referred to; the eithelium of all the glands appended to the alimentary canal; the 4ver (including the gall bladder); the pancreas; the lining of the /ungs, which are developed as outgrowths of the pharynx, and of the 138 Elementary Zoology. air sacs, which are but prolongations of the lungs ;* the notochord, The remaining organs of the body are derived from the mesoblast. It will be necessary to go more into detail into the description of the way in which some of these are developed. The Vascular System.—The heart arises as two closely applied tubes, which soon fuse to become a single tube. The walls of this tube are muscular without, and derived from the mesoblast, and there is a lining of cells which have been stated to arise from the hypoblast. It is a re- markable fact that the heart begins to beat before the walls are differentiated into muscular tissue. Later on the heart becomes twisted into an S shape, and constrictions appear, marking it off into the several chambers of the adult heart. The anterior end of the heart gives off a series of aortic arches, which, embracing the gut, unite upon its dorsal surface to form a dorsal aorta, running back along the median dorsal side of the gut. There are, altogether, five of these arches on each side, just as in the tadpole. But, though the chick thus resembles a tadpole or a fish, these aortic arches never send off ramifications in those types, since no traces of actual gills are developed. They simply lie between the gill clefts, and, indeed, are related to the heart on the one hand (a ¢wde, be it remembered, at first) and to the dorsal aorta on the other, as are the so-called hearts of the earth-worm to the ventral blood-vessel and to the dorsal blood-vessel. The truncus arteriosus, as the anterior end of the heart is termed, is put into communication with the dorsally running aorta by these five pairs of circular vessels. Later in development the fish-like character of the aortic arches is lost. The middle parts of the first two arches disappear; the ventral parts persist as an artery, supplying the tongue of the adult, the lingual artery; the dorsal parts, as the carotids, which run up the neck to the brain, The other arches are partially or entirely lost, and the arterial system of the adult is arrived. The dorsal aorta gives off two great branches. The first 1 The muscular and connective tissue investments of these various organs are derived from the splanchnic mesoblast. Development of Excretory Organs. 139 of these is the vitelline, which goes to the yolk sac; the second supplies the allantois, and is known as the allantoic. The veins which return the blood to the heart are to begin with, and partly, arranged after the fashion of those of a fish. There are, as in those vertebrates, an anterior and a posterior pair of cardinal veins, which unite to form a pair of Cuvierian veins, into which blood is poured from the anterior and pos- terior regions of the body. Before these have appeared two large vitelline veins unite to enter the heart by a single Fic. 72.—Surface view of Chick at end of third day, to illustrate vitelline veins. (From Marshall.) sm, vitelline membrane ; aD, area pellucida; av, area vasentosa, with ramifying vitelline veins ; AK, area space; EM, embryo. trunk, and a little later a pair of allantoic veins join them. Still later a median vena cava posterior, the persistent vein of the posterior part of the body in the adult, opens through the same trunk, known as the meatus venosus. The anterior cardinals persist as the jugular veins, and the Cuvierian sinuses as the venz cavee anteriores. , The Excretory Organs-—In order to properly understand the excretory system, it will be necessary to supplement the account of the development of this system in the chick with some account of what occurs in the tadpole. In the latter the first part of the excretory system to appear is a rod of meso- blast, at first solid, afterwards hollow, which opens to the exterior vi@ the cloaca, and in front communicates with the 140 Elementary Zoology. ccelom by three ciliated funnels, or xephrostomes. The exact way in which these are formed appears to be this: the rod is, as said, at first solid ; it then becomes grooved along the inner surface, and ultimately the groove closes and shuts off a cavity, lying therefore within the rod—except at three points, which are the nephrostomes. The portions of the tube bearing the nephrostomes grow out into short tubes; these tubes become later branched and complicated. This structure is called the head kidney, and in the tadpole it plays the part of an excretory organ ; it is later replaced by the mesonephros, and becomes degenerate, ultimately disappearing. The head kidney, or pronephros, as it is better to term it in correspondence with the mesonephros, is interesting as a relic in the frog, for in the marsipobranchs, or cyclostomata, it persists throughout life as a functional organ of excretion. In the chick the pronephros is not only never developed as a functional organ of excretion, but it actually appears later than the mesonephros. As in the tadpole, the first part of the excretory system to be developed is the longitudinal duct, which corresponds to that of the tadpole. It acquires its lumen, however, by canalization, not by the closing of the lips of a groove—a difference, perhaps, of not great im- portance. It may be termed the archinephric, or segmental duct. Into this duct (there are, of course, two ducts, one on each side of the body) open a number of short tubules, the tubules of the mesonephros or Wolfian body. The anterior set of these tubules develops in two sections; a depression of the ccelom is formed, and within the mesoblast a coiled tube; the depression or nephrostome becomes continuous with the tube, and the latter with the longitudinal duct. This anterior set of tubules presently degenerates and disappears. The posterior set of tubules differs in having no nephrostomes. Later, the rudiments of the pronephros put in an appear- ance. They are three depressions of the lining membrane of the ccelum (the number, it will be noted, corresponds to what is found in the frog), which become connected by a ridge of tissue. The anterior of the three funnels persists, and forms the mouth of the oviduct in the female; the ridge of tissue Development of Excretory Organs. 141 becomes the oviduct in the female, and grows back to open into the cloaca. It is present, but rudimentary, in the cock bird. The mesonephros is the permanent excretory organ of the frog, but not of the fowl. Hence it is inaccurate to speak of both as kidney. In the fowl, on the fourth day, the archi- nephric duct gives off a diverticulum, from which is formed, by its elongation, the ureter of the adult. With this com- municates a third set of tubules, also formed in the mesoblast, but without peritoneal openings. These tubules, with the duct, form the kidneys or metanephros of the adult. The original duct, the archinephric duct, persists in the cock bird as the vas deferens ; it acquires connection with the testis, and carries off the sperm. In the hen the mesonephric duct disappears, the Miillerian duct, as already stated, being the oviduct. A tabular statement of the above facts may aid the memory. TADPOLE. CHICK. Archinephric duct = Archinephric duct. aa provephias of oe = Rudimentary pronephros, 3 funnels. Mesonephric tubes = Mesonephric tubes. Mesonephric duct (persistent pro- nephric duct, itself persistent archinephric duct, m7nzs most} = { anterior section), or Wolffian duct Miillerian duct (independent (?)\ _ Coe duct (a growth backwards of pronephric duct) J ~ \ of rudimentary pronephric duct). Mesonephric (archinephric) or Wolf- fian duct. Froc. Fowl. Excretory organ, mesonephros Ureter of male (also functioning §\ — Vas deferens ( = mesonephric duct). as sperm duct) = mesonephric duct No structure present ° = Ureter. ; Oviduct (Miillerian duct) = Oviduct minus funnel. Funnel of oviduct = No structure present. No structure present = Excretory organ, metanephros. Before leaving the excretory organs, their correspondence in the embryo with the typical excretory organs of inverte- brates must be pointed out. We have already seen that the 142 Elementary Zoology nephridia are essentially glandular tubes which open on the one hand into the ccelom by a ciliated mouth, and on the other on to the exterior. That is precisely what we find with the pro- nephric tubes in the frog, and with the mesonephric tubes in both frog and fowl. But while the tubes of the earthworm open separately, those of the Vertebrata join to form a con- tinuous duct, which, however, reaches the exterior by the cloaca itself, an invagination of the epiblast of the embryo. This fusion of separate nephridia does not, it is true, occur in the common earthworm ; but Annelids are known in which several nephridia do unite to form a continuous longitudinal duct. Another apparent point of difference is that, while in the earthworm each segment of the body has but a single pair of nephridia, the mesonephric tubules, at any rate, are, for the most part, more numerous than the segments which they occupy ; here, again, there is no real difference in the matter from segmented worms ; for among the Annelids more than one form is known in which the nephridia show as little regard for the segmentation of the body as is evinced by the mesonephros. CHAPTER XII. MIORPHOLOGY OF ORGANS. In the foregoing pages the structure of a number of types of animals has been considered, and the development of one form has been dealt with. It may be useful to extract from that survey (adding something to the extract on the way) an account of the main facts in the structure of the higher animals from the point of view of the several organs and systems of organs. Hitherto we have characterized different animals by the structure of their different parts. We shall now examine seriatim the various organs themselves. The facts of Zoology, when treated in this way, are often spoken of as comparative anatomy or morphology. The term “Zoology” may be usefully retained for the consideration of animals in their entirety, their anatomy, mode of life, classification, and history, as indicated by fossil remains. The zoologist deals with the azimal as an unit; the comparative anatomist with the ovgan. It is not intended, however, to insist upon any sharp line of division between these two aspects of the zoological side of biology; they obviously overlap. In investigating the modifications of organs and systems of organs through the animal series, it is clearly requisite to be assured as to the exact equivalence of the organs under consideration. And this necessitates the use of terms with a definite meaning, such as is not always afforded by words in common use. It is not pedantic to speak of the jaws of an insect as mouth appendages. To term them “jaws” is to imply something in common with the jaws of a tiger. There is absolutely nothing in common structurally between the two 144 Elementary Zoology. organs, though they perform similar functions. The organs are, in fact, analogous, just as are (to use the most familiar instance of analogy) the wings of a butterfly and of a bird. When organs correspond in structure and development, they are then said to be homologous. Thus there is no doubt that the heart of a man and of a shark are homologous structures. But both these terms require further expansion and definition before they can become really useful. In extreme cases there can be no doubt as to which category a resemblance between two sets of organs can be referred. The leaves of a tree are in a sense the equivalents of the stomach and lungs ofa man. In both organs are food assimilation and respiration carried on. But no one would assert that there is more than a not very strong analogy. On the other hand, we may take such an example as the correspondence or non-correspondence between the heart of an anodon and that of a frog. Now, here we have two organs which play a like part—they are in each case the central organ of impulsion of the vascular system. Furthermore, there is this structural likeness between them, that both are divided into three chambers, of which two receive the blood coming from the veins, while the other, the ventricle, is concerned with the driving of the blood through the arteries. Finally, both organs lie in a pericardium. Yet no competent anatomist will dispute the assertion that these organs are not homologous, in spite of their almost detailed likeness. They are only analogous ; but to an analogy of this kind it is useful to apply the term “homoplasy.” This term signifies a similar moulding, an adaptation to similar needs. There are even more striking instances of, apparently, similar structures, which are yet different. It is not at all certain, for instance, that the two ventricles of the bird’s heart are severally homologous with the two ventricles of a mammal’s heart. It would seem at first sight absurd to doubt that the four cavities of the heart of the higher vertebrates, mammals, and birds are really and truly homologous. But the doubts that exist are due to the great dissimilarities in other points of Morphology of Organs. 145 structure that exist between these two divisions of the group Vertebrata. So great are these, that no naturalist of to-day would accept for a moment the view that the mammals are highly developed birds, or that birds have been produced by a great change in the structure of any mammal-like form. Some even go so far as to believe that, while mammals are descended from some amphibian ancestor, the progenitors of birds were reptiles; but in any case it would probably be held by all that the two groups, if not derived from different classes of reptiles, diverged early and widely from the same type, which must in that case have been a lowly organized form, simple and not highly differentiated. The fact—the unfortunate fact—is that all questions of true homology depend upon what are, for the most part, speculations. It is unquestionably true to say that “to mix up etiological specu- lations with morphological generalisations” induces confusion into morphology; but it is absolutely necessary to mix up the two, for all that. The test of true affinity must be common descent; and there are, and will be always, divergence of opinion about such matters. It has been, therefore, proposed, and with excellent reason, to change the term “homology” into “homogeny.” “Homogeny” signifies community of origin ; and structures which are clearly derived from each other, or from some common parent form, must be really homogenous. It will be clear to any one that the greatest difficulty of the morphologist is to distinguish between homogeny and homo- plasy; in the case of the two ventricles of the bird’s and mammal’s heart impossible—at present, at least. It must, therefore, be carefully borne in mind by the student who reads the following pages, that, while the facts are, it is hoped, correct, the comparisons may be by no means so correct. THE Bopy-waLL (EPIDERMIS, DERMIS, AND MUSCULAR LAYERS), In the earthworm the body-wall is divisible into the three layers which have been described; there is an epidermis L 146 Elementary Zoology. outside, and this is followed by two muscular layers, an outer layer of circularly arranged fibres and an inner layer of longitudinal fibres. The epidermis is the epiblast of the embryo; the layers below are formed by the outer walls of mesoblastic blocks, the innermost layer being the ccelomic epithelium. The body-wall of the earthworm is sometimes spoken of as the dermo-muscular tube. In the crayfish the ‘simple structure of the body-wall is more complicated. In the first place the empidermis, instead of secreting a simple trans- parent cuticle, produces a much thicker cuticle, which is for the most part calcified. The two muscular layers are no longer recognizable in their simplicity. Instead of two continuous sheets of muscle partly interrupted at the septa, which is found in the earthworm, the muscles of the crayfish are broken up into individual masses running in different directions, in which the regular arrangement is no longer visible. Furthermore, there is between the muscles and the epidermis a layer of connective tissue, the dermis, not represented in the earthworm.! The immense thickness of the muscular layers has re- sulted in the nearly complete obliteration of the ccelom, as it has also in the cockroach, which from the present point of view is essentially like the crayfish. The same, too, is the case with the anodon and with the snail. In the Vertebrata all the structures lying outside of the coelom may be regarded as collectively equivalent to the dermo-muscular tube of the earthworm, In both cases it is the product of the somatic mesoblast. But in the vertebrate the complication of this body-wall is much greater. “The epidermis is thicker, and is among ver- tebrated animals modified in many different ways, some of which have been dealt with on p. 112, ef seg. The same state- ment applies to the dermis. The muscular layers are divided up into separate muscles which have often characteristic arrangements serving to separate the great groups of vertebrates from each other. In the fishes, for example, there is a more regular and apparently primitive arrangement. The muscles are to some extent arranged in continuous sheets, only 1 A dermis has been found in certain earthworms Morphology of Organs. 147 interrupted by fibrous partitions. In the higher vertebrates the muscular layers are more thoroughly split up into separate muscles. In vertebrates, finally, we get formed out of the mesoblast of the body-wall, as well as out of the splanchno- pleuric mesoblast in the head region, the skeleton which has been already described in three vertebrate types, and need not be again described here, NERVOUS SYSTEM. The nervous system is entirely a product of the outer layer of the embryo, the epiblast. As a rule, however, much of it becomes removed deeper within the body, leaving only its ter- minal sensory endings in the outer layer of the body; and even these—as in the case of the retina of the eye, for instance—may also be removed from the surface. The nervous system of the higher animals consists of the central nervous system, which is a chain of ganglia or a cord containing numerous ganglion cells, and of a peripheral nervous system consisting of the nerves which arise from the central nervous system. The latter invariably end in sensory cells situated in the skin or elsewhere, in delicate sensory plexuses, or in muscle fibres. Hence the nerves of the peripheral nervous system may be distinguished into sensory and motor. In the simpler animals there is the rigid distinction referred to between the central and the peripheral nervous system. It is only the central nervous system where ganglionic cells occur. But in higher animals peripherally situated ganglia are to be found, such as the plexuses of cells and fibres connected with the alimentary system of vertebrates. As would be imagined on @ Priori grounds, a general 1 Tt must be remembered that the tissues formed between the epiblast and the hypoblast arise from at least two distinct sources, in the opinion of some embryologists ; there are the mesoblastic somites ( protovertebre), (p. 132), and the mesenchyme, budded off as wandering cells from both hypoblast and epiblast. From the latter is formed z#ter a/iaa considerable portion of the skeletal elements. _ If this distinction can really be drawn, some of the above comparisons will fall to the ground. 148 Elementary Zoology. diffused nervous system has preceded a differentiated central and peripheral nervous system. In //ydra there are nerve- cells scattered over the ectoderm, in association, chiefly at any rate, with the cnidoblasts. A diffused nervous system (asso- ciated, however, with a concentrated central nervous system) has survived in certain simply organized worms (the Nemer- tines) where there is a layer of nervous tissue completely surrounding the body. Even in the embryo frog there are more than traces of the same. The epiblast of the young embryo is divided into an outer layer, from which the future epidermis is to be derived, and a deeper layer from which the central nervous system is developed. This layer, however, is not found only where the central nervous system will be ultimately produced, but it is continuous right round the body, and is generally held to represent an archaic state of affairs, where a continuous nerve-sheath was the nervous system. Even in the same group to which Aydra belongs the com- mencement of a central nervous system is to be seen. Round the margin of the “umbrella” of Medusz are special concen- trations of nerve-cells, the forerunner of a central nervous system. In all the higher animals described in the present book a central nervous system is present ; and in all of them it is removed from the epidermis, though—equally in all—it is developed from the epiblast of the embryo. The central nervous system exists in these animals, it will be observed, in two rather different forms. In all the inverte- brate types it consists of a supra-cesophageal ganglion or ganglia, connected with a subcesophageal pair of ganglia, or chain of ganglia by connectives that pass round the gullet. In the vertebrates, on the other hand, the central nervous system, consisting of the brain and spinal cord, is entirely dorsal in position, the alimentary tract being wholly ventral to it. There are, furthermore, these additional differences between the two types. In the vertebrates the central nervous system is not cut up into masses of nerve-cells connected by tracts of nerve- fibres, into ganglia and connectives; secondly, it encloses a central canal. The nervous stem of vertebrates is a tube of nervous matter. In the invertebrate the ventral and chief part Morphology of Organs. 149 of the nervous system consists of separated ganglia,! and is not a hollow structure. There is, however, in spite of the important differences just enumerated, this in common between the two types of nervous system, that they both possess a mass of nerve-cells in the anterior end of the body above the mouth. Thus the brain of the vertebrate, or at least a portion of it, seems to correspond to the supra-cesophageal ganglia of the invertebrate. That this point in common is important seems to be shown by the following considerations. In the lower worms, the Platyhelminthes, Nemertines, and even in a simple type of Annelid, Holosoma, the central nervous system consists of the cerebral (supra-cesophageal) ganglia alone. Here we may possibly have the common starting-point of the vertebrate and invertebrate central nervous system. On this view it will be assumed that out of the nerves arising from the brain of these simple animals two ventral ones, in the case of the inver- tebrate, and two dorsal, in the case of the vertebrate, became respectively approximated, and formed the rest of the central nervous system. ‘This is, of course, hypothesis; but, as a matter of fact, the first stage in proof of such an hypothesis has been discovered in the Nemertine worms ; in some of these there is a dorsal, and in others a ventral, approximation of two especially stout nerves, arising from the brain. There are, however, other views attempting an explanation of the differences between the invertebrate and the vertebrate nervous systems. The matter does not at present admit of dogmatic teaching. As to the invertebrate types, it is quite easy to see the likeness in their nervous systems. In Zumbricus there is a ganglionic swelling for every segment of the body, except the first one or two. But it seems as if a slight growing together of ganglia originally perfectly distinct had occurred in the first of the subcesophageal ganglia. The crayfish presents us with a further modification. In the first place, the ganglia are more 1 In the earthworm the distinction between the ganglia and the connectives is not so marked as in the other types. There are some, but not so many, nerve-cells in the connectives. 150 Elementary Zoology. clearly marked off from the connectives ; in the second place, there is less correspondence between ganglia and segments than in the earthworm. It will be remembered, however, that the segmentation of the crayfish has not been so distinctly preserved as in the earthworm. The excretory organs have nearly, or perhaps quite, lost their segmented character; the same is the case with the thoracic and cephalic segments, etc. Hence it is not surprising to find that the nervous system has shared in this confusion of an originally simple meta- merism. But it is worthy of note that the nervous system, inherited from much more remote ancestors than those which acquired the special characteristics of the Arthropoda, has retained so completely, on the whole, its regular metamerism. The anodon at first sight differs from the types considered. The equal development of three triangularly situated pairs of ganglia tend to confuse the likeness to the worm and to the crustacean. The anodon, it must be borne in mind, is not a segmented animal; there is no trace of segmentation any- where. Hence it could not be expected that the nervous system should be segmented, and it is not. But we have the supra-cesophageal ganglia connected by a circum-cesophageal commissure with a ventral pair, exactly as in Lwmbricus and Astacus. There is, however, in accordance with the absence of segmentation, only one ventral pair. As for the parietovisceral, they are probably to be compared to the visceral nervous system of the two animals mentioned. ‘The same observations apply to the snail. ALIMENTARY CANAL. The alimentary canal of A’ydra is a blind tube, formed of the endoderm, and in close contact with the ectoderm, separated from it only by the structureless supporting lamella. In all the higher animals (with a few exceptions) there is an alimen- tary canal present, which is a tube running from end to end of the body, and usually opening at the posterior end by an anus as well as anteriorly by the mouth. The simplest expression, Morphology of Organs. 151 therefore, of a coelomate animal is that of two tubes, one sur- rounding the other, enclosing between them a space, the ceelom. The first appearance of the alimentary canal in all coelomates is the archenteron. The archenteron is the equivalent of the gastric cavity of Aydra. But the alimentary canal of the adult ccelomate is not the exact equivalent of the gastric cavity of the hydra. For it has been found that the archenteron gives rise, without doubt, in a few animals, to pouches, from which the coelom and its walls are formed ; while in others, if this does not actually occur, it is in the opinion of many a modification of the same process of development which gives rise to the ccelom and the walls of mesoblast; and theory altogether apart, the hypoblast of the embryonic archenteron does give rise to, at any rate, a part of the mesoblast. Further- more, in the ccelomate animals, two additional portions are added to the archenteron (snus the mesoblastic portion), viz. the stomodzeum anteriorly and the proctodzeum posteriorly, both of which are subsequent ingrowths of epiblast. These various layers of cells, however, only give rise to the actual lining epithelium of the alimentary tract and the lining epi- thelium of the glands, such as the liver, appended to it. The alimentary tract has, besides this epithelial lining, a wall com- posed of connective tissue and muscle. These outer layers are derived from the splanchnopleuric layer of the mesoblast. It is clear, therefore, that the alimentary tract of a ccelomate animal is not the precise homologue of the alimentary tract of flydra. There is no doubt that the alimentary tracts of all the higher animals are equivalent structures. They show, however, considerable variation in structure. The proportions of the stomadzeum and proctodzum to the mesenteron vary greatly. The term “ mesenteron,” it should be remarked, applies to that portion of the gut whose epithelial lining is derived from the archenteron only. Thus in the crayfish all is stomodaum to the end of the stomach, while the proctodzeum commences very shortly afterwards, the mesenteron being limited to a very short tract into which the liver opens. In the earthworm, on the 152 Elementary Zoology. other hand, the stomodzum ends with the pharynx,' and the proctodszeum is an exceedingly short tract at the opposite end of the intestine. Combined with these essential points of similarity in the alimentary tract of coelomates are many points of difference. The canal itself is differently specialized in all the types con- sidered in this volume, and the glands appended to it can only be said to generally correspond. It is impossible, for instance, to definitely compare the “liver” of Astacus with the liver of Rana. CCLom. All the animals above the hydra which have been described in the foregoing pages, possess a cavity, or system of cavities, known as the ccelom. ‘This space lies in the mesoblast, and is bounded by its own walls; it is not, that is to say, simply the interspace between various organs. The term “ body-cavity” is sometimes applied to this system of spaces; but the term is ill-advised as applicable to any cavities in the body, as, for example, the quite spacious cavities in the cockroach, which are really a part of the hemoccel. In representatives of three or four groups of animals it has been conclusively proved that the coelom arises as a pair, or more than a pair, of diverticula of the embryonal gut (archenteron). These pouches grow out, and then meet to become fused, at least in part, and so surround the gut, whence they were derived. In the majority of animals, however, ¢,g. in the earthworm, the ccelom does not arise in this way. It appears as a splitting of the mesoblast, the cells imme- diately abutting upon the cavity thus formed becoming the proper wall of the ccelom,. the peritoneum. To a cclom which arises in the former of these two ways the term “ ezéero- cel” has been applied, and “ schizocwel” to a ccelom arising in the second way. Thus Amphioxus possesses an enteroccel, Lumbricus, a schizoceel. The extent to which the ccelom is developed varies greatly among ccelomate animals. It is ' Possibly with the buccal cavity. 5 Morphology of Organs. 153 spacious in the earthworm and in the frog, much reduced in anodon and the snail, more reduced still in Astacus. The ccelom can be distinguished from other spaces, such as the heemoccel, bya number of characters. In the first place the mode of its development already sketched out; in the second place, when it is at all spacious it envelops more or fewer of EPIBLAST TT COLL \ SS Fic. 73.—Diagrammatic transverse section of Earthworm, to illustrate some of the characters of the ccelom. (After Goodrich.) N, NEPH, nephridia. Beneath the central gut are the generative cells budded off from peritoneum of septum (not shown). The dark line surrounding the gut is the splanchnopleuric peritoneum. Peritoneal funnel represents (diagrammatically) oviduct or sperm duct. the organs of the body. In the earthworm all the organs of the body lie within the ccelom, or—to speak more accurately—they are invested by its walls. A glance at Fig. 153 will make this understood. The proper epithelium of the ccelom closely invests the intestine, nephridia, blood-vessels, etc. In anodon (Fig. 30, p. 65), where the ccelom is reduced to the pericardial cavity, the heart and a section of the intestine are enclosed by this coelom in the same fashion. The ccelom of the crayfish (Fig. 25, p. 48), 154 Elementary Zoology. (the interior of the generative organs and the terminal vesicle of the green gland) is so reduced that all the organs of the body are excluded from it. The hzmoccel differs in these points from the ccelom. It is—at any rate, sometimes—formed at first of solid rods of mesoblast (p. 160), which melt down centrally, while the marginal cells form the walls. Finer vessels, the capillaries, have been shown in some animals to be produced by the direct canalization of rows of cells. Or, again, blood- vessels sometimes arise first as irregular chinks and lacune between the mesoblast cells. Blood-vessels are never formed either as archenteric diverticula or as a series of segmented cavities in the mesoblast. Nor do vascular spaces envelop other organs of the body, as does the ccelom typically. There appear to be exceptions to this statement, such as the so-called pericardium of Astacus, and the blood sinuses, which have been described as enveloping parts of the alimentary tract in certain worms. As to the crayfish, it has been already pointed out that we have probably to do with a number of veins which have fused together, and thus constitute an envelopment to the ventricle. In the worms referred to, it seems likely that the continuous sinus (if it really exists, a fact which has been questioned) is formed in a similar way by the coalescence of a plexus of blood capillaries. The ccelom is further characterized by the fact that it communicates with the exterior, either temporarily (frog) or permanently (earthworm), by means of the excretory organs. It may also have other and more direct communications with the exterior, as in the case of the dorsal pores of the earthworm and the abdominal pores of the dogfish. In the case of the heemoccel, it is doubtful whether communications with the outside world exist. Capillaries have been stated to open on to the exterior in certain earthworms and in various Amphibia ; but the facts seem to require a renewed investigation. Curiously enough, nephridial funnels do sometimes appear to open into the heemoccel. Apart from the Nemertine worms, about which there is not an universal consensus of opinion, there is the undoubted fact that in the frog the funnels of the mesonephros, detached from the rest of the organs, create a communication Morphology of Organs. 155 between the ccelom on the one hand and the renal veins on the other. The funnels are coelomic, and a short tube leads. into. the renal veins. This, however, is not precisely comparable to the communication between ccelom and exterior in the typical fashion. Finally, the generative products are local proliferations of the lining membrane of the ccelom; thus the gonads and the mouths of the nephridia and the gonad ducts are the only organs which can be said to lie actually inside the ccelom. It is this reason which leads to the inference that the cavity of the generative organ in Astacus is coelom. These characters do not apply to the hzemoccel. EXCRETORY ORGANS. It seems probable that, throughout the entire series of coelomate animals, the preponderance of those organs which function as the eliminators of nitrogenous waste products are homologous organs. We shall take these organs first, and then discuss briefly certain excretory organs which are not so plainly derivatives of the same common ground-plan. In the earthworm, it will be remembered, there are a series of paired nephridia; these are, apart from details, tubes of a glandular character, opening on the one hand into the ccelom by a ciliated funnel, and on the other hand by a muscular sac on to the exterior of the body. There is thus, by their means, a communication established between the ccelom and the out- side world. The anodon is not a segmented animal. It possesses a single pair of excretory organs, the organs of Bojanus. These are essentially tubes communicating on the one hand with the ccelom (pericardium), and on the other with the exterior. As is the nephridium of Lumbricus, the organs of Bojanus of the mollusc are glandular tubes with a non-glandular terminal point. It is the prevalent opinion that they entirely correspond to a single pair of nephridia of the earthworm. So, too, with the single “kidney” of Helix. In the frog the same style of excretory organ can still be 156 Elementary Zoology. traced. Not in the adult so clearly as in the tadpole. The details have been, to some extent, dealt with in the preced- ing pages upon embryology. The general facts that can be deduced from those details are that in the frog and other vertebrates there are a series of paired excretory tubules, opening into the ccelom, on the one hand, and on to the exterior, on the other, by means of a continuous longitudinal duct. These differences do not, however, invalidate the comparison, for there are worms in which some, at any rate, of the paired nephridia are connected by a longitudinal duct ; while another fact about the segmental tubes of the Vertebrata— that they are not strictly metamerically arranged—is paralleled in other worms. The essential likeness remains, that in vertebrates no less than in annelids, there are a series of tubes which open into the ccelom, on the one hand, and on to the exterior, on the other, and that these tubes in both cases are the organs for the excretion of waste nitrogenous products. The excretory organs of Astacus and of Periplaneta are not so easily to be referred to the same series. They are both animals with but little coelom. The excretory organ of Astacus is so far like that of Lumbricus that it is a tube which is partly glandular, and which opens on to the exterior by a non-glandular portion. Of this non- glandular portion there is an appended sac—a diverticulum. In describing and figuring that organ attention has been directed to the fact that the glandular part of the organ terminates in a little oval sac—the terminal sac. It is held by some that this is a small pocket of ccelom; and in this case the excretory organ of Astacus—the green gland, as it is usually termed—will agree in essentials with a nephridium of Lumbricus. As for the large vesicle, which is a diverticulum of the duct of the nephridium, it presents no difficulties when compared with the nephridium of worms, for in many species of annelids the duct is provided with an appendix essentially similar. There are, however, other arguments which may be advanced to show the correspondence of the green gland of Astacus with the nephridium of an earthworm. In the Morphology of Organs. 157 ' kidney of various vertebrated animals each separate tube, which development proves to be a nephridium, ends in a dilated sac, into which is thrust a coil of blood-vessels, the so-called glomerulus. This dilated sac, which is lined by an epithelium more flattened than that which lines the excretory tubules themselves, has been shown on very good grounds to be a cut-off bit of the ccelom itself. So that, in this case, we have an undoubted nephridium opening into a small sac, which is coelomic, a state of affairs which we have ex hypothesisi in the crayfish. This, however, is rather an argument from analogy ; it shows that it is, at any rate, reasonable to regard the terminal sac of the crayfish’s excretory organ as a fragment of celom. There are stronger arguments still. Peripatus has been shown to be a very archaic form of arthropod animal. Among other characters which it possesses, which show affinities to the segmented worms, are a regular series of undoubted nephridia. These nephridia do not open into a wide space, like the ccelom of Lumbricus, but into terminal vesicles, which have been proved developmentally to be ccelomic pouches ; these, though larger, are exceedingly like the terminal sac of the green gland of Astacus. Finally, in Feripatus, each nephridium opens on to the base of a limb as does'the green gland of Astacus. The limbs correspond, and it seems likely, therefore, that the corresponding position of the nepbridiopores has significance. In the event of its being shown that the terminal sac of the green gland of Astacus is not a ccelomic sac, there is still no decisive reason for removing the green glands from the category of nephridia ; for in that case there will be no cclom at all in the neighbourhood of the green gland, and if they are homologous with nephridia they cannot have an internal opening into the ccelom, for the wey good reason that there is no ccelom to open into! In the same way, from a comparison with other animals, some arguments can be put forward to show that the Malpighian tubes of the cockroach are referable to the same category of organs. At first sight they differ widely, for they are apparently appendages of the gut ending blindly, and com- parable to the hepatic appendages, which no one has attempted 158 Elementary Zoology. to prove to be excretory organs comparable to nephridia. In the first place, however, the Malpighian tubes are appendages of the end gut, which is proctodeum, and therefore of epiblastic origin; morphologically, in fact, they open on to the exterior of the body, just as do the archinephric ducts of vertebrates, which debouch into the cloaca. As to the absence of any communication with the ccelom, there is no ccelom that has been described for them to open into. And since in certain worms, where there is a copious ccelom, the internal aperture of the nephridia is still sometimes obliterated (Cherogaster), it is not difficult to understand that this portion of the nephridium may have disappeared in Leriflaneta. Moreover, in certain worms (the earthworm Acanthodrilus, certain Gephyreans) there are undoubted nephridia which do open into the cloaca, apart altogether from the vertebrates, which are probably further away from the tracheata. There is thus no insuperable objection to regarding the Malpighian tubes as modified nephridia. This, however, is not the same thing as saying definitely that they are referable to the same category. GONADS AND GENITAL DUCTs. In most Ccelomata genital ducts or gonad ducts exist. In some of the more lowly organized forms the genital products, when ripe, escape by the mere rupture of the body-wall; such is the case, for instance, in certain simply organized worms, such as Lolygordius. In other animals the genital products escape through definite pores, which place the ccelom in which they are formed in communication with the exterior. The cyclostomatous fishes are an instance to the point; so, too, certain lowly organized worms, such as -Zolosoma. It seems, however, to be not settled in these cases whether the pores are not the last remains of definite ducts. They certainly appear to be so in such animals as the worm Auchytreus. In those animals which have separate genital ducts there are anatomically two kinds, which seem to be more distinct than they are. In such cases as the oviducts of the frog and Morphology of Organs. 159 of the earthworm, the ducts are altogether independent of the gonads, whose products they convey to the exterior. The mouths of the ducts are at some distance from the gonads, and not nearly even in contact with them. In the other type, exemplified by the male ducts of the frog, the ducts of both sexes in the Anodon, Periplaneta, and Astacus, the ducts have the appearance of simple prolongations of the gonads. It is probable, however, that this difference is not a fundamental one. A consideration of the male ducts and the testes of the earthworm gives the clue to the difference ; it offers an inter- mediate condition between the gonads independent of the duct and the gonads continuous with their duct. In the earthworm the testes are two pairs of bodies perfectly independent of the ducts in the young; later on certain sacs, the sperm sacs, are formed, which envelop both testes and the funnels of the sperm ducts. Thus both are enclosed in a common ccelomic sac, and appear to be continuous structures, an appearance which a careful dissection shows not to be a reality. Now, in those animals with continuous genital organs and genital ducts, there is, for the most part, a reduced ccelom. This is so with Azodon, Helix, Astacus,and Periplaneta. In those animals it is believed that the interior of the generative gland is, as already stated, the remains of a part of the ccelom. Hence the apparent continuity of the duct with the gonad is merely an exaggera- tion of the state of affairs which is found in the case of the male ducts and the testes of the earthworm; the common wall enveloping gonad and duct is the wall of the ccelom. As to the male ducts of the frog and other Vertebrata, the continuity is arrived at by secondary growths putting into connection the originally distinct gonads and ducts, The gonad ducts themselves are usually held to have some relation to nephridia. This connection is emphasized by the usual way in which both systems of organs are often treated in the vertebrates under the general term of the genito-urinary organs. As to the vertebrates, it will be clear from what has been written above concerning the development of the genito- urinary organs, that there is the most intimate connection between the two. Ducts originally part of the excretory 160 Elementary Zoology. system (e.g. pronephric duct) serve in the adult as gonad ducts. It cannot, however, be argued on d f77077 grounds that this is necessarily the case with other animals. And yet there are certain facts which seem to indicate that, generally, there is a connection between nephridia and gonad ducts. In the earthworm there are certain obvious similarities which cannot be passed over. Genital ducts, as well as nephridia, open by open ciliated mouths into the ccelom, and by a pore on to the exterior. In the case of the oviducts the whole tube occupies two segments, perforating the septum as does a nephridium., The male ducts, it is true, are so far unlike nephridia that they traverse several segments on their way to the exterior; but there are many worms belonging to the same large group as that which contain the earthworm in which the male ducts also occupy but two segments. There are some other facts which point in the same direction, but we shall not enter into a description of them here, as they would be strengthened by further investigation. With regard to the anodon, the genital ducts are not only distinct from the nephridia, but they open on to the exterior by a separate pore; but among the Lamelli- branchiate Mollusca there are forms in which the genital duct opens by the same pore, and others in which the two ducts are a common tube for some distance; this looks much as if the gonad duct is really a part of the nephridium split off. The genital ducts of the crayfish have this fact in common with the green glands (hypothetically, at any rate, nephridia), that they open on to the thoracic appendages at a point precisely corre- sponding with the place of opening upon the antennz of the green glands. There is a suggestion here of a series of metamerically arranged nephridia, of which only one—the green gland—has retained its excretory function, while two others remain, one in each sex, as either oviduct or sperm duct. VASCULAR SYSTEM, The vascular system is not found in the simpler ccelomate animals. It consists essentially of a system of spaces, con- taining a fluid, and excavated in the mesoblast. This system Morphology of Organs. 161 is perfectly independent of the ccelom, and generally this independence is shown by the different character of the enclosed fluids. Thus, in the earthworm the blood is red, with a few corpuscles; in the case of the ccelomic fluid we have numerous and large corpuscles floating in a colourless fluid. In order to emphasize its distinctness the term “hemoccel” is often applied to the vascular system in its entirety. But though the hzemoccel is distinct from the Se pe ei Ree ere es BC oe lm ES AO. GD NEO Bates N HAM. Fic. 74.—Transverse section of an embryo Earthworm, to illustrate independence of ccelom and hemoceel. (After Wilson.) Ent, gut; Cat, celom; Ham, hemoceel; n, nerve-cord. ceelom in its development, there is sometimes a connection between the two. Thus, in the vertebrates the lymphatic system opens, on the one hand, into the ccelom, and, on the other, into the hemoccel. In simpler animals the channels of the vascular system have not always proper walls. In the majority, however, they have, and then tend to become tubes of regular calibre. A larger or smaller part of these tubes may M 162 Elementary Zoology. develop muscles in the walls, which then contract, driving the contained blood from place to place. In the earthworm the dorsal vessel and the lateral hearts are thus contractile, and play the part of the heart of higher types. The trunks of the vascular system are usually arranged ina longitudinal direction, the trunks being connected by transversely arranged vessels. Such an arrangement is well seen in the earthworm. In this animal it clearly shares the general segmentation of the body. In the crayfish, which is also a segmented animal, but one in which the segmentation of the internal organs is commencing to be obscured, the metameric arrangement of the vascular Fic. 75.—Isolated capillary network formed by the junction of several hollowed-out cells. (From Quain’s ‘‘ Anatomy.”) c, a hollow cell the cavity of which does not yet communicate with the network ; A, A, pointed cell-processes, extending in different directions for union with neighbouring capillaries. system is still obvious, but not so clear as in the earthworm. There is no longer a metamerism of the vascular system in the anodon. In the vertebrates the metamerism is plainer in the embryo (aortic arches) of the higher types. The central organ of impulsion of the vascular system—the heart—is to be regarded as a local modification of one of the chief trunks of the system. The simplest form of heart that exists is probably that which occurs in an earthworm (dZicro- cheta); it is simply a local thickening of the dorsal vessel, which has here more highly developed muscular walls, and is thus possibly more powerfully contractile than the rest of the Morphology of Organs. 163 dorsal vessel. From this it is not difficult to derive the heart of Astacus, The heart in this animal is continued in front into the ophthalmic artery, and behind into the abdominal; it has thus quite the appearance of a local thickening of a continuous dorsal trunk, comparable to the dorsal vessel on the earthworm. But the heart of the crayfish is complicated by the presence of an auricle, completely surrounding it, into which open the branchial veins. These may possibly be regarded as the equivalents of the circum-cesophageal “hearts” of the earth- worm, like which, they show distinct traces of metamerism. A further stage is seen in anodon. The ventricle of the heart can be looked upon as a thickening of a dorsal vessel, the arteries behind and before which arise from it representing again a non-modified part of an originally simple dorsal vessel. The two auricles opening into the single ventricle would, in that case, perhaps, be comparable to a single pair of the circum-cesophageal vessels of the earthworm, the segmentation of this part of the vascular system being entirely lost, as is that of all the other organs of the body. The same remarks apply to the snail; but here not only is the segmentation lost, but bilateral symmetry also. These invertebrate types differ from the vertebrate in that the heart is dorsal. It is an important morphological difference that in the vertebrate the heart is ventral. Thus, it is probable that the heart in the two series of types is not strictly comparable. Otherwise the concentrated heart of the vertebrate, with its three to four separated cavities, is derivable again from a specialized part of a longitudinal ventral vessel. The heart at first arises as a simple tube, which afterwards becomes twisted. The auricles, therefore, of the frog’s heart, derived from the division of an, at first single, auricle, cannot be directly compared with the auricles of anodon; for in the frog they are morphologically the posterior part of the heart- tube, which by subsequent twisting come to lie‘in front of the ventricle, Even in the vertebrate it is possible that the prevalent circular vessels of lower types are to be recognized in the aortic arches, which are essentially, in the embryo chick, communications between a dorsal and a ventral vessel. 164 Elementary Zoology. When a definite heart is established, it is customary to speak of arteries and veins. To those trunks which convey blood away from the heart the term “artery” is applied; veins are the trunks which convey blood to the heart. Both vessels communicate peripherally, usually by means of finer tubes (the capillaries), in which region the distinction between artery and vein is extinguished. RESPIRATORY ORGANS. In animals with a delicate body-wall it frequently happens that there are no special organs of respiration at all, The thinness of the body-wall permits an zeration of the blood capillaries which it contains. This is the case with the earth- worm, where the entire surface of the body performs the office of a lung, or branchia. In animals with a thicker body-wall, where such an eration cannot take place, special organs occur which are devoted to respiration. While there are certain grounds for believing that the organs for the secretion of nitrogenous waste are homologous throughout the animal series, this is by no means the case with the organs of respiration. In the animals whose anatomy has been described in the foregoing pages four types of respiratory organs are met. 1. Branchi@.—These are outgrowths of the epidermis, with some of the underlying mesoblastic structures included. In the latter course the blood-vessels, which come to be separated from the oxygen-containing medium by a thin epidermis only. Such branchiz, or gills, are commonly arborescent, in order to increase as much as possible, without an undue increase of the length of the organ, the respiratory surface. The branchize of the crayfish and of anodon, and the external gills of the young tadpole, belong to this category. 2. Trachze.—The trachez of the cockroach are respiratory 1 A variation of this form of respiratory organ is found in the echino- derms, and even ina few annelids (Branchiura), in which the branchia contains a prolongation of the ccelom. Morphology of Organs. 165 organs which fit in with a terrestrial life, just as the branchize —delicate structures which would dry up and become inefficient if exposed to dry air—are suited to the aquatic life. The tracheze are essentially tubes which open on to the exterior by the stigmata, and at the other end branch repeatedly and ramify in the body, carrying the air to the tissues of the most distant organs. The traches, therefore, are not respiratory organs in the sense that branchie are. The latter are the organs where—and where only (in the crayfish)—respiration takes place; the tracheze are merely conduits for the air, each organ absorbing from them its own oxygen, and giving up its carbonic acid.? 3. The Gills of Fishes and Tadpoles—A third form of respi- ratory organ characterizes vertebrates. It is one of the most important definitions of this group that, either temporarily or permanently, there are a series of slits putting the pharynx into communication with the outside world. These slits, temporary in Amphibia and all vertebrates lying above them in the series, are permanent in fishes, and there become fringed with vascular tufts, and perform the office of gills. The water taken in at the mouth is passed through these slits, and as it passes the delicate epidermis covering the vascular tufts gives up its oxygen to the contained blood. ‘This same process of respira- tion goes on in the tadpole. 4. Lungs of Vertebrates —In the adult frog, and in all verte- brates lying above it, respiration is effected by a pair of sacs, outgrowths of the pharynx, the lungs. It has been attempted to be shown that these sacs are really homologous with a pair of gill-slits. The latter originate as outpushings of the pharynx to which inpushings of the epidermis correspond. ‘The lungs, it is suggested, are the out- pushings only, which thus never acquire a communication with the outside world. The diversity of respiratory organs contrasts, as has been pointed out, with the uniformity of the excretory organs, It is ‘ In some aquatic insects there is a curious modification of the tracheal system. The trachez, unprovided with an external orifice, lie in out- growths of the body-wall, which are perfectly comparable to branchiz. 166 Elementary Zoology. interesting to inquire why this is so. Possibly it has something to do with the absence of any necessity for special organs of respiration in thin-walled animals. The secretion of nitrogenous waste matters is (largely, at any rate) an elaborate process of chemical manufacture, which requires a special organ for the purpose. Once acquired, there is no 4 priori reason why a different organ should be formed for the same purpose. On the other hand, skin respiration must be regarded as a primitive state of affairs, and special respiratory organs were only needed as the organisms became bulkier and consequently with thicker integuments. Hence respiratory organs were produced at various periods and separately in many groups of animals. Therefore there is no reason why they should be of the same kind. In this connection it is interesting to note that many animals with special organs of respiration still continue to breathe partly by their skin. This is so with anodon ; and it is well known that the skin of the frog is respiratory. In Aedix the mantle-fold has become so vascular, and so well adapted for respiratory purposes, that the branchiae—present in allied forms—have totally disappeared. CHAPTER XIII, THE MORPHOLOGY OF TISSUES (H{STOLOGY). THE bodies of animals can be not only analyzed into organs, but also into tissues and cells. At present the cell is the ultimate unit of structure. That portion of morphology which deals with the microscopic structure of animals, with the forms and arrangements of the cells and of the tissues, is usually called Histology. It has been already pointed out that all animals are com- posed of one cell or of many. Among multicellular animals there is always some differentiation of the cells. Thus in the hydra the cells of the endoderm differ in their characters from the cells of the ectoderm. The latter, again, are to be dis- tinguished into the larger muscular cells, and the smaller interstitial cells ; the interstitial cells, again, are differentiated into cnidoblasts, nerve-cells, etc. But the entire animal is quite obviously composed throughout of cells, which are funda- mentally very similar. If the section of /Zydra, on p. 15, be compared with the figure of a transverse section through the body-wall of an earthworm on p. 22, the cellular constitution of the latter will not be quite so obvious. That the epidermis is made up of definite cells of two kinds is as clear as possible ; but underneath the epidermis are two layers of muscles, which are composed of fibres running in two directions, and imbedded in a granular substance, through which are scattered nuclei. Nevertheless, these layers are really composed of cells. Fig. 76 represents a section through the body-wall of an embryo earthworm. It is composed of the same three layers as those which are shown in the last figure described. But the _inner layer of muscles is clearly a cellular layer; it is made up ‘ 168 Elementary Zoology. of large cells, in the interior of which fibres are being formed. These fibres are seen cut across, so that they, of course, appear as circles. .So that what we have in the body-wall of the adult worm is a group of cells of which the boundary lines have disappeared, and of which the protoplasm has been largely converted into long fibres of contractile substance. The nuclei remain unaltered to tell the story of the metamorphosis of these cells. It may be remarked that, though the figure does not Fic. 76.—Section through body-wall of embryo Earthworm. (From Vejdovsky.) cuT, cuticle; ErI, epidermis; Gt, gland-cell; cir.mus, circular muscles; LoNG.MuS, longitudinal muscles; PERI, peritoneum. show it, the circular layer of muscles at an earlier stage is developed in precisely the same fashion. It is true of the higher animals generally that their tissues, in many cases, depart widely from the typical cellular form; but it is equally true that those tissues are invariably composed of cells. The most important generalization of histology is that the bodies of all animals and plants are composed of cells. We have already several times used the term “tissue.” It is now requisite to define this term as used in histology. A tissue is a cell aggregate—a group of cells—similar in character, with a function corresponding to their character. The Morphology of Tissues. 169 There are four main classes of tissues among animals; but these are again subdivisible. To use a metaphor borrowed from systematic zoology, there are four genera of tissues, and each contains a number of more or less closely allied species. The four genera of tissues are (1) epithelial, (2) muscular, (3) nervous, and (4) connective. We cannot here enter into anything like a full account of these several tissues, or even their principal modifications ; these matters are more suitably treated of in detail ina physio- logical handbook, and will be so treated in a later volume of this series. But a few general facts and conclusions may be suitably introduced into a handbook of zoology. The epithelial tissues may be exemplified by the epidermis of the earthworm figured upon p. 168. Similar tissues are the epidermis of the frog, the cells which line the gut of the same and other animals, the tissues which form the tubules of the kidney, etc. In all these cases the tissue is composed of groups or layers of cells similar to each other, which have preserved the typical cellular form. In the simplest animals, such as Hydra, all the tissues are of this kind. The muscular tissues are characterized by the fact that a large portion of the cells of which they are composed have been metamorphosed into fibres of a contractile substance. All protoplasm is contractile, as is shown by the movements of an ameba and by the circulation of the protoplasm in a vegetable cell. But in these cases there is no line to be drawn between those portions (of the ectoplasm, at least) which are especially contractile and those which are less so. In the Vort- cella, on the other hand, there is a definite layer of the ectoplasm —the myophan layer—in which contractility principally resides. In the ectoderm of the multicellular Hydra, special cells are set apart to perform the part of contractility; but it will be observed that these cells have not lost their typically cellular character. A process, or two processes, have grown out from their under surface which are not, or are hardly, distinguishable from the protoplasm of the rest of the cell; these are the muscular processes of. those cells. In animals belonging to 170 Elementary Zoology. the same great division as that which contains the hydra, we get a further evolution of the muscle-cell. In such cases (see Fig. 77) the fibre is much more important than the cell of which it is a formation. Nevertheless, the cellular constitution of a muscle-fibre is plainly evident. In the higher animals, as has already been pointed out in the case of the earthworm, the cellular constitution of the muscle-fibres is not always evident. It zs, however, during the course of the development of the fibres. Fig. 78, for example, which represents a muscle-fibre of an embryo sheep, is absolutely like the muscle-fibre of an adult medusa. The Fic. 77-—Muscle-cells of Calenterate. Fic. 78.—Developing muscular fibres (From Claus-Sedgwick’s ‘‘ Zoology.”’) of foctal Sheep. Highly magnified. (After Wilson Fox, from Quain’s “ Anatomy.”’) cell here is partly metamorphosed into a fibre. A bundle of the fibres closely associated together with a more complete disappearance of the original protoplasm of the cell is a muscle-bundle. In the case of the medusa, and in others, each cell gives rise to a single fibre only; but in the earth- worm, as already mentioned, several fibres appear in the interior of each cell. But this is not a difference of importance ; neither is the distinction between the striate fibres of all vertebrates and some invertebrates and the smooth muscular fibres of vertebrates and invertebrates a matter of fundamental dis- tinction. The striated fibre consists of muscular substance, The Morphology of Tissues. 171 which is arranged in the form of alternating light and dark bands, concerning the physical explanation of which there is much divergent opinion. These cross striz are not visible in the plain or smooth fibres. The striated fibres of vertebrates are developed in the way shown in Fig. 78, the plain fibres, on the other hand, are simply elongated cells (see Fig. 79), in which all but a small portion near the centrally placed nucleus has become muscle substance. The smooth muscle-fibre is more clearly cellular in its adult condition than is the striated fibre. But though there is this difference in the cases selected, it is not of universal applicability, for in the earthworm we have smooth fibres which are developed as are the striated fibres of the vertebrate, the nucleus remaining outside of the fibre; while, on the other hand, there are striated fibres where the nucleus remains, as it does in the plain fibre of the vertebrate, within. The nervous tissues are essentially composed of cells which have, as a rule, many and much-branched processes. ‘The processes are the nerve-fibres which seem to be always outgrowths of the nerve-cells, The connective tissues do not really form so clearly definable an assemblage as do those already treated of. Their general characters are that they consist of Fic. 80.—Tendon of Mouse’s tail, stained with logwood ; showing chains of cells between the tendon-bundles. 175 diameters. (After Shafer, from Quain’s ‘‘ Anatomy.”) Highly magnified. (After Schafer, Fic. 79.—Muscular fibre-cells from the muscular coat of the small intéstine. from Quain’s “‘ Anatomy.”) aggregates of cells in which the typical cellular character is largely lost through the conversion of a larger or smaller portion of 172 Elementary Zoology. the cells into a connecting substance, which may be of a trans- parent structureless appearance, or may be definitely fibrillar, The accompanying figure (Fig. 80) will illustrate one form of connective tissue exemplifying the fibrillar change, while Fig. 81 of hyaline cartilage will exemplify the first-mentioned type Se ie) - \S j Bek AS NEN yi a a Fic.81.—Articular cartilage from head of metatarsal bone of Man (Cosmic acid preparation). The cell-bodies entirely fill the spaces in the i ‘om “ 1 th matrix. 340 diameters. (After Shiifer, from Quain’s Anatomy.”) a, group of two cells; 4, group of four cells; 2, protoplasm of cell, with & fatty granules; 7, nucleus. of modification. These tissues are always skeletal or protec- tive. Thus bone and cartilage are skeletal tissues, while the layers of more delicate connective tissue, which evolve and permeate the different organs, are rather protective in function. CHAPTER XIV. CLASSIFICATION. THE DIFFERENCES BETWEEN PLANTS AND ANIMALS. In a footnote to p. 1, it was promised to define what naturalists mean by the terms “ genus” and “ species.” A species is a group of animals (or plants) of which the individuals agree in all but perhaps the very minutest details. This may be illustrated by common examples among the earthworms. Under stones and in damp places there is, frequently to be met with an earthworm of a greenish colour and small size ; it is commonly coiled into a crescent, and is somewhat sluggish in habit. On wet morn- ings, and in dryer weather by digging, a different form, of a greyish blue colour and more active habits, is to be found. The green species is known as AWolobophora chlorotica, the bluish worm as A/olobophora cyanea. On examination the green species will be found to have a clitellum occupying segments 30-37, sometimes including also the twenty-ninth ; it has, more- over, three pairs of spermathece. The bluish form has a clitel- lum occupying only segments 29-34, and there are but two pairs of spermathecee. Wherever we find an earthworm with the characters just given, it will be found to agree in all other par- ticulars with other individuals presenting those same characters. Some examples of A. chlorotica do not show the usually characteristic green colour, and in some the clitellum begins a segment earlier than is usual for the species, ze. segment 29. Some naturalists separate off those individuals which show these minute differences from the typical form into a variety or sub-species ; but it is simplest merely to speak of such ex- amples as variations from the type form. It is not possible for 174 Elementary Zoology. the careful observer to confound these two species of AWolo- bophora with each other or with any other earthworms. The two forms have a real existence as definable animals. We may go a step further and complete the definition of the species by stating that they both inhabit the greater part of Europe. The area inhabited by a species is just as mucha part of its definition as is its colour, or shape, or internal structure. Now, each of these worms has two names—AJUolobophora and chlorotica, Allolobophora and cyanea. The second name is the specific name, the first the generic. The two species, and about sixty others, agree with each other to form a larger assemblage of earthworms than a species, to which the term “genus” is applied. All the species of A//olobophora agree in having the male pores upon the fifteenth segment, in which they contrast with another but smaller assemblage of worms in which the same pores are situated upon the thirteenth segment. This distinction is held by systematists to be of more import- ance than variations in the number of spermathecze, position of clitellum, etc.; and, accordingly, the worms with male pores upon the thirteenth segment are separated off into a genus, Alluvus. But while there can be no difference of opinion about the existence of a given species—save as to the use of such terms of description as “‘sub-species” and “ variety ’—a genus has no such definite existence. The animal world consists of so many ‘species which are grouped into fewer genera. The aim in the formation of genera 1s to indicate real affinities, genuine blood relationship. Such relationships, however, can only be sur- mised, they cannot be proved; hence notions as to what constitutes a genus must and do vary with the individual naturalist, for the “‘ personal equation” comes into play. Genera, again, are grouped into larger divisions, termed “families.” The same remark that was made about genera may be also made about families. One naturalist will decline to regard as more than a genus what another will consider entitled to family rank. But both believed, of course, that the family consists of genera which are more nearly allied to each other than any one of them is to a genus belonging to another Genus and Species. 175 family. All earthworms, for instance, which have a clitellum beginning as far back as at least the twenty-second segment, which possess spermathecz without diverticula, in which the gizzard lies immediately in front of the intestine, and which never have more than eight sete to each segment of the body, are grouped together in the family Lumbricide. On the other hand in the East, and in Australia, are worms met with which agree in having a large number of—sometimes as many as seventy or more—setz in each segment, the clitellum begins as early as the thirteenth or fourteenth segment, the spermathecz always have one or more accessory pouchlets, and the gizzard is separated from the intestine by a stretch of cesophagus; these are ranged into a family, Perichetide. Families, again, are combined to form orders; the earthworm belongs to the order Oligocheta. This order contrasts with the order Polycheeta, which includes the marine worms, such as the lug- worm, by possessing no locomotorial processes of the body, containing closely associated bundles of setee, and in a number of characters. Broader divisions still reduce the animal world to a few classes, until ultimately animals can be grouped into two great divisions, the Protozoa and the Metazoa, which will be defined and contrasted later. The classification of the animal world adopted in this book will be found to differ from many schemes of classification in vogue. This is because of the uncertainty of our knowledge, and the consequent variability of opinions. The boundaries of many genera, families, etc., are indistinct; a student of zoology soon becomes familiar with what are known as inter- mediate types. The Dipnoan fishes, for example, have retained many fishlike characters, while they have adopted in addition to these amphibian characters. There are, in fact, in nature no sharply marked lines of division ; and, if there appear to be, it is on account of defective knowledge. Until a year ago it was possible to distinguish, by a number of important characters, the leeches from the earthworms ; but, in 1896, the characters of a remarkable leech, Acanthobdella, were more fully made known. This animal, a leech, in many of its peculiarities has sete, like those of earthworms, and its body-cavity is divided into a 176 Elementary Zoology. regular series of chambers, a feature which formerly absolutely separated all Oligchzta from all Hirudinea. The first proper description of Peripatus broke down a number of the bounda- ries which kept the annelids apart from the arthropods ; and many similar instances might be cited. Facts like these—and they are multiplying every day—show that classification, if possible practically, is theoretically impossible ; the paradox, in fact, is true, that the less perfect our knowledge, the more complete our schemes of classification ; the existence of clear classifications is an expression of ignorance. ‘They also lead to the inference that there has been, and is still, a gradual evolu- tion of forms of life. Could we have before us all the forms that exist and have existed (it is important to bear in mind the immense numbers of totally extinct creatures, mostly only represented by often unintelligible fragments), it might be im- possible to define a line separating man from the ameeba. ANIMALS AND PLANTS. More than this, it is not possible to draw a clear line between plants and animals. The only line that can at present be absolutely drawn is between living creatures and minerals. To divide nature into Organisata and Inorgani- sata is really the only scheme of classification that can be fully proved. As to animals and plants, it may be useful to point out some of the more salient features by which they are separated. But it will conduce towards clearness if, first of all, their essential similarities are enumerated. Plants, like animals, are built up of cells, or else exist as single cells. The cells in both consist essentially of a mass of protoplasm, of similar composition, enclosing a nucleus (or nuclei). The nucleus, in dividing, may or may not, in both, undergo that elaborate series of changes collectively termed karyoki- nesis (see p. 129). All multicellular plants, like multicellular animals, begin life as a single cell, the ovum or oosphere, Animals and Plants. 177 This single cell may, or may not (parthenogenesis), in both require fertilization. Fertilization consists, in both plants and animals, of the union of a small motile sex-cell (spermatozoon or antherozoid) with a larger quiescent sex-cell, the ovum or oosphere, the union in both being chiefly a union between the nuclei of the respective sex-cells. These are some of the more important likenesses between animals and plants, and it is obvious that they are so numerous and so important that plants and animals must have come originally from the same stock. Next for the essential differences. Plants and animals differ in physiological characters as well as in morphological. 1. Plants, as a rule, derive their carbon from the carbonic acid of the atmosphere, by the help of chlorophyll, which is generally prevalent in the vegetable kingdom ; the other sub- stances which build up their bodies are absorbed as inorganic salts from the soil, or from the water in the case of aquatic plants. . Animals, on the other hand, require as food organized substances, living or dead protoplasm, animal or vegetable. Even a creature so low in the scale as Amada would starve if kept in water that contained all the components of its protoplasm in the form of salts in solution. It eats solid particles of animal or vegetable matter. To this rule there are exceptions, both on the animal and on the plant side. In the first place, there are animals with chlorophyll and plants without it. AMydra viridis, certain infusorians, etc., have chlorophyll, and can therefore obtain carbon from the atmosphere. In the case of Hydra we may, it is true, have to do with a symbiotic organism, but there are infusorians in which the chlorophyll seems to be undoubtedly an integral part of the animal. On the other hand, there are the insectivorous plants and the non-chlorophyllaceous plants. The insectivorous plants form a physiological assemblage of dicotyledonous plants belonging to more than one natural order, which agree in the fact that they possess various mechanisms for the capture, digestion, and absorption of insects and other small creatures. They produce a digestive N 178 Elementary Zoology. fluid, which can convert proteids into the diffusible peptones, and they do not thrive so well without as with animal food. The sundew of this country is an example of an insectivorous plant. ‘The fungi are plants which do not contain chlorophyll. Beside fungi, there are more highly organized plants, such as the parasitic dodder, which are also without chlorophyll. These plants prey upon other plants, or upon decaying animal or vegetable substances; the moulds, which cover dying wood, manure heaps, and other masses of organized matter, are examples, ‘These plants feed so far like animals in that they absorb organized compounds which, as an animal, they break down chemically within their protoplasm, and then reconstruct into the protoplasm of their bodies. These fungi, however, do not always live in this animal fashion. Plants without chloro- phyll can live in fluids containing the elements of which their protoplasm is built up combined into salts, but the salts must be, some of them, organic compounds. Such a solution as Pasteur’s solution is fit for the growth of fungi. Its com- position is as follows :— Water, H,O. Cane sugar, C,.H,.0,,. Ammonium tartrate, (NH,)2C,H,O,. Potassium phosphate, K;PO,. Calcium phosphate, Ca;(PO,),. Magnesium sulphate, MgSQ,. An animal such as an amceba cannot live in this fluid. As to the exact way in which the food stuffs are taken into the body, the plant differs from the animal. The ameeba ingests solid particles; it gets outside its food. This mode of nutrition extends to the highest animals. In the intestine of man, for example, fat particles are eaten up by cells of the intestine, the amceboid independence of the individual cells being thus retained. This has been shown to be the case with many animals. Ina hydra many particles of food are devoured by separate cells, and then, after digestion, passed on and used for the common good. Plants do not absorb their food in this fashion. It is taken in as fluid (we are speaking, of course, of the animal-like plants), not devoured separately as solid particles. This difference between animals and plants might, Animals and Plants. 179 at first sight, be supposed to depend upon a structural difference between the two “sub-kingdoms” to be mentioned presently— the general presence of a rigid cell-wall in the plant and its fairly constant absence in the animal. Given an outer cell- wall, the taking in of nutritious matter must necessarily be limited to the process of osmosis. If two fluids (or gases) of different specific gravity be separated from each other by an animal membrane, it will be found, after a time, that the two fluids have, both of them, passed through the membrane. The amount which has passed from one side to the other depends upon their different specific gravities. This is purely physico- chemical osmosis. In living animals and plants the same phenomenon is met with, but the living protoplasm is sup- posed to alter the purely physical nature of the diffusion. It has been shown that the presence of the cell-wall is not the only reason for the fact that a true plant does not take in solid particles of food, for in many motile unicellular plants (such as Haematococcus), where the protoplasm is at times in the plant’s life perfectedly naked, no protrusion of pseudopodia and absorption of particles has been noticed. On the other hand, it must be remembered that the animal body is also fed by similar process of osmosis. The food taken into our stomachs is chiefly thus absorbed. But many of the foodstuffs used by us are non-diffusible substances, This is the case, for example, with albumen, such as white of egg. What happens is that the stomach and other regions of the gut secrete a digestive juice such as gastric juice, succus entericus, etc, which converts these indiffusible substances into diffusible ones. They can then be absorbed by osmosis. This, however, again, is not distinctive of the animal as opposed to the plant. The insectivorous plants have been already referred to. In thema juice is thrown out which actually produces the same effects upon the proteids of a fly’s body, and converts its insoluble proteids into soluble peptones, which are then absorbed. We may conclude, however, by emphasizing the fact that, while plants generally live upon inorganic and organic salts, animals never do; that plants generally absorb their carbon from the atmosphere by the help of their chlorophyll, while 180 Elementary Zoology. animals rarely do; that the nourishment of plants is never taken in as solid particles, while that of animals nearly always is, to some extent. 2. A point of unlikeness between animals and plants that is often emphasized is the power of locomotion possessed by the one group and deficient in the other. Broadly speaking, it is true that animals move from place to place, and that plants do not. With the higher plants it is’ rigidly true. The cause for this is to be found in the histological structure of plants ; each cell being enveloped in a stiff cell-wall is, of itself, sufficient to prevent locomotion. ‘That this is so is proved by the fact that simple naked plants, such as simple alge, “ Flowers of Tan,”! etc., do move, and rapidly, from place to place. But, though there is but little locomotion, there is plenty of movement among plants. We need not refer to the bursting of seed-vessels, which are due to purely physical causes, such as the swelling and consequent rupture of certain parts. But genuine movements of protoplasm, often affecting a considerable part of the plant, take place. It is enough to remind the student of the sensitive plant of the folding and unfolding of flowers at night and morning, etc. These are the principal physiological differences between animals and plants. We shall now discuss their morphological unlikeness. 1. The shapes of animals and plants markedly differ. Apart from the unicellular forms, the animal has usually a symmetrical and very solid body. The plant is, as a rule, not symmetrical ; the body is much made of flat expansions. ‘This difference, though morphological, really depends upon the physiological considerations already dealt with. The plant being stationary, and feeding upon gases and fluids, has to have as large a surface as possible for their absorption, and to ramify as much as possible for the purpose of collecting the food gases and fluids. Animals, on the other hand, are, as a rule, locomotive, and hence their prevailing bilateral symmetry; when they are ’ This organism is, however, considered an animal by perhaps most naturalists. Animals and Plants. 181 stationary, the symmetry, if marked, is rather radial, such as sea anemones, etc. The process of feeding, too, is different : solid food is taken in, which is rendered diffusible by the processes of digestion ; it is thus the internal structures which tend to become complicated. There is no need of so much surface. The animal either goes in search of its food by moving from place to place, or—if fixed—it possesses mechan- isms, such as the tentacles of hydra, for capturing it. It is interesting to note that certain parasitic animals (some Crustacea), which aré imbedded among plenteous nutriment in the bodies of their hosts, often tend to grow in length and irregularly, like plants. 2. Related to the last-mentioned difference between animals and plants is the fact that all the Metazoa—the animals above the unicellular creatures, which will be dealt with later—are either permanently or temporarily two-layered sac-like creatures ; the Metaphyta are never so. 3. It is commonly, but in some ways with insufficient accuracy, stated that animals and plants differ in the prevailing cellulose cell-wall of the latter and its absence in the former. The statement is, indeed, perfectly true, but the emphasis is wrongly laid. This difference is, in the first place, that in animals the motile phase is the more prevalent, in plants the encysted. It will be remembered that the amceba is for the most of its life a naked mass of protoplasm; but that on occasions it becomes encysted. On the other hand, the prevailing conditions of organisms on the same plane as amceba, such as the unicellular Alga, Hematococcus, is usually enveloped in a cell-wall; on occasions this is thrown off, and the creature moves through the water a naked mass of proto- plasm. Animals are, however, to be distinguished from plants by the fact that their cell-wall, when present as a distinct structure, is not made of cellulose.1_ In plants it is, as a rule. It will be gathered from the foregoing brief account of the main differences between animals and plants that there is no 1 It must be remembered that cellulose is not absent from animals ; it occurs, for example, as a constituent of the test of Ascidians. But, when present, it does not form the walls of individual cells. 182 Elementary Zoology. absolute criterion for determining whether a given unicellular or few-celled organism is a plant or an animal. MHence it follows that there are organisms which have been tossed about from the vegetable to the animal sub-kingdom, and back again. To these doubtful organisms the term “ Protista” was applied by Heckel. An organism known as Z£uglena will serve as an instance. It moves about freely by means of a flagellum, and has an cesophageal tube like a Vorticella, and therefore feeds like an animal; but it has also chlorophyll, and at times becomes encysted, the cell-wall being composed of cellulose. There is not, however, much classificatory advantage to be got by using the term “ Protista ;” for it only doubles the difficulty. We have, without this division, only to distinguish between animals and plants; with it, between animals and Protista, and between plants and Protista. The facts simply emphasize the common descent of all living beings from a common stock. CHAPTER XV. THE CLASSIFICATION OF ANIMALS. BErwEEN Ameba or Vorticella, on the one hand, and Aydra on the other, isa great gap. The two unicellular organisms are representatives of the Protozoa, while Hydra and all the other animals dealt with in this book are Metazoa. These two fundamental divisions of the animal kingdom are some- times—and inaccurately—defined as being respectively uni- cellular and multicellular animals. This definition would, it is true, suffice, if we had only the animals described in the present book to deal with. But there are organisms exceed- ingly like Vorticella, which yet form colonies branching from common stalks ; but the individuals forming these colonies are independent of each other, and each feeds itself and propagates its kind on its own account. So that mere multicellularity is not the essential difference between Protozoa and Metazoa. The hydra is not only multicellular, but the cells are specialised in various directions. There are digestive cells, muscular cells, and so forth. Even at this point we have not reached the real difference of metazoon from protozoon ; for, in Volvox, a colonial protozoon, there are special cells set apart for re- production, while something of the same kind occurs in the colonial form Proterospongia, where there are amoeboid and more highly specialised cells imbedded in the same mass of supporting jelly. The essential difference between the protozoa and the metazoa are two. 1. The Metazoa consist, either temporarily or permanently, of a two-layered sac surrounding a central cavity, which opens on to the exterior at one end, the cells of the inner layer 184 Elementary Zoology. (endoderm) being different from those of the outer layer (ectoderm). That Aydra is a simple form of metazoon, in which these characters are not much more than just complied with, will be obvious from the preceding description. It is a two-layered sac, only complicated by the outgrowth of the tentacles round the orifice at the anterior end. That this definition also applies to the remainder of the animals treated of in this volume will be clearer from their development. In all of them the embryo is at one time clearly composed of a two-layered sac, the gastrula, with a central cavity and an aperture at one end of the body. Disguised though this gastrula stage may be, owing to various reasons, but chiefly the mechanical effect of large masses of yolk in the egg, the gastrula stage has been identified in all animals whose- development has been studied. They all pass through a two- layered stage, in which they more or less closely resemble ffydra,_ A-second essential character of the Metazoa may be stated as follows :— 2. The cells of the metazoon body are grouped into tissues which act as a whole, not each cell for itself, and in subordina- tion to the needs of the individual. This character is placed second in place and in importance to the first character of the Metazoa, and for the following reasons :— In the first place all the tissues of the body are not a group or layer of cells which act as a whole and in subordination to the rest of the body. The reproductive tissues are in some respects an exception. The ovum of the hydra, as already said, actually devours the neighbouring cells precisely as if it were a parasitic Ameba. If it be objected to this that this is really an act for the common good of the tissues composing the body, for the organism as a whole, as it is essential for the propagation of the species, it may be replied that the protozoa show precisely the same phenomenon, i.e. the reproductive cells of Volvox and Proterospongia. Inthe same Protozoa andin other multicellular forms it is difficult to suppose that the cells furthest from the surface of the colony are not fed by particles The Classification of Animals. 185 of food captured by and passed on to them by the more super- ficial cells. To this it may, of course, be answered that, after all, each cell does, as a matter of fact, act independently in taking up food particles; that there is no throwing out of a digestive fluid by the cells at large and a subsequent absorption of the digested food, as in the stomach and intestines of the higher animals. But in Aydra the individual cells lining the enteron do take up separately particles of food ; and this process of assimilation appears to go on even in the highest animals —witness the absorption of fat drops by the intestinal cells. Still, on the whole, the second definition of the Metazoa is correct. To these two definitions is sometimes added a third—that sexual generation is universal. This, however, is only a difference of degree, and the intermediate stages offered by such forms as the colonial Protozoa appear to do away with any marked distinction of this kind. (See the remarks on p. 183.) We can thus primarily divide the animal world into two great divisions—the Protozoa and the Metazoa. The vast assemblage of animals which fall into the second division show immense differences among themselves, which permit of a further subdivision. If we compare the lowest representative of the Metazoa with which we are concerned here, the hydra, with any of the higher forms, we find it to be marked off by two features of great importance, which are related to each other. The body of the hydra is built up of two layers of cells only, which surround a central cavity. In all the remaining Metazoa not only are there these two layers present, but also an interpolated layer, which is more or less excavated into a cavity, or set of cavities, lying between the ectoderm and the endoderm; to this cavity or cavities the term c@lom is applied, and the animals which possess it are called Celomata. The group of Metazoa typified by hydra, and the group typified by any of the other forms, used to be distinguished as Diploblastica, or two-layered animals, and Triploblastica, or 186 Elementary Zoology. three-layered animals. But in many of those creatures, which are better termed Ccelentera, there is a third layer of cells, derived from both ectoderm and endoderm, which is interpolated between them in a jelly-like matrix, the supporting lamella. So that, although the two-layered condition is the characteristic one of the Ccelentera, some of them are truly triploblastic. But in none of them is the interpolated Mesoglea excavated by accelom. The one central cavity is both enteron (gut) and coelom. In the ccelomata these two cavities are separate. The Ccelomata contain all the types described in this volume, with the exception of the hydra. And we have described representatives of many of the important divisions, Leaving aside certain small groups whose relationships are a matter of doubt, and would require, therefore, a more elaborate discussion than space can be found for here, the Ccelomata may be grouped round the following types of animals :— (r) Liver fluke (PLArYHELMINTHES); (2) Thread-worms (NemaropA); (3) Sea-urchin (EcHINODERMA); (4) Earth- worm (ANNULOSA); (5) Lamp-shell (BRACHIOPODA); (6) Sea mosses (BRYOzOA); (7) Snail (Motiusca); (8) Cockroach, Astacus (ARTHROPODA); (9) Frog (CHORDATA). It may be useful to sketch briefly the characters of all of these groups. The Mollusca, Arthropoda, and Chordata will be dealt with more elaborately. J. PLATYHELMINTHES. This large group includes, not only the Trematoda, repre- sented by the liver-fluke, but the Planaria (fresh-water, marine, and terrestrial worms), and the Cestoda (the tape-worms). By many the Nemertine worms are included in the same great division. These “worms” are more or less flattened, sometimes elongated in shape. They are not definitely segmented, like the Annulosa, though traces of segmentation are occasionally apparent, as in the metameric arrangement of certain of the internal organs in Gunda segmentata, The coelom The Classification of Animals. 187 is feebly developed, and at most consists of chinks and minute cavities in the mesoderm. They have a pair of cerebral ganglia, but no ganglionated ventral cord. In the free-living forms the body is often ciliated. It is only among the Nemertea that a vascular system is present. The nephridia are branched and complicated, and terminate in “flame cells” —e. a single cell with a single flagellum attached, not a wide and multicellular funnel, as in the Annelids. II, Nematopa, The typical members of this group are the parasitic thread- worms, of which the thread-worms of the horse and the Trichina are examples. But allied to this group, and possibly to be included within it, are the marine and pelagic Chetognatha. The most striking peculiarity—which, however, they share with the Arthropoda—is the complete absence of cilia in the true Nematoda. The body is unsegmented in the latter, and the alimentary canal lies in a spacious cavity which has not all the characters of a true ceelom. The members of this group have often complicated life histories (as have also various parasitic Platyhelminthes), passing part of their existence as free-living worms and part shut up within the bodies of one or more hosts. III. EcHINODERMA. This group of animals includes the starfishes, sea-urchins, sea-lilies and fossil encrinites, sand-stars, and sea-cucumbers, or holothurians. All the members of the group present a more or less pronounced radial symmetry. In all there is a con- siderable, often massive, calcareous skeleton, mesodermic, and regularly arranged in plates and spines. The ccelom is spacious, and a part of the ccelom, in the shape of regularly arranged canals, is known as the water vascular system. 188 Elementary Zoology. IV. ANNULOSA. These “worms” are usually segmented animals; but a number of forms belonging to the group, known as the Gephyrea, and doubtfully included here, show but little traces of segmentation. The earthworms, marine worms, and leeches are, however, all of them, plainly segmented, the internal organs being largely arranged in correspondence with the external segmentation. The Annulosa have a well-developed cceelom metamerically divided, and a complete and closed vascular system. ‘The excretory organs are typically a series of pairs of tubes, also metamerically arranged. The nervous system consists of a supra-cesophageal pair of ganglia, and of a ventral cord connected with this by a circum-cesophageal commissure, which is ganglionated excepting in the Gephyrea. There are usually setee—bristles—imbedded in the skin, which are used for progression. V. BRACHIOPODA. These animals are marine, with the appearance of bivalves. They are, however, more allied to the Annelids in structure. The valves of the shell are dorsal and ventral, not lateral as in the Pelecypoda. The ccelom is spacious, and one or two pairs of quite typical nephridia exist. Setze are sometimes present. VI. Bryozoa, or Potyzoa. These are small and, with one exception, invariably colonial animals, which secrete a thick horny or calcareous skeleton; the numerous “cells” of the colony form incrusta- tions upon plants, stones, etc., or form erect masses of a solid or branched character. The mouth has a circle of tentacles, as in the Brachiopods; there is a spacious ccelom, from which, in a few cases, nephridia have been observed to lead to the exterior; the gut is U-shaped, and between mouth and anus lies a ganglion. They always multiply by buds, as well as by the sexual process, The Classification of Animals. 189 VII. Mo.iusca. The Mollusca are, as their name denotes, soft-bodied animals, which are generally protected by a shell. As a rule, they are entirely unsegmented; but traces of segmentation occur in the divided shell of C#iton, and in the double excretory system of JVautilus. The ccelom is always greatly reduced, the copious lacuna between the organs of the body being a portion of the vascular system, which is well developed, with a heart of comparatively complicated structure. As a rule, the ventral surface of the body projects more or less as a muscular “ foot,” which is the organ of locomotion. Branchiz are present, and consist of processes of the body-wall. The nervous system consists typically of a pair of cerebral ganglia, lying above the cesophagus, connected by a pair of commissures, with the pedal ganglia lying in the foot, and with a pair of chlamydo-splanchnic ganglia in the visceral region. The Mollusca have been primarily divided into the Lipocephala (Mollusca without a head), the Lamellibranchiata, or Pelecypoda, such as Anodon; and the Glossophora, with a head and the characteristic radula: to this group belongs the snail. We shall divide them into four classes—the Amphineura, the Lamellibranchiata, the Cephalophora, and the Cephalopoda— whose characters, and those of their more important sub- divisions, will now be given. Class 1. AMPHINEURA. To this class belongs the genus Cito and its allies; the Chitons .are characterized by the dorsal shell consisting of a number of pieces following each other. The animals of this class are bilaterally symmetrical. Class 2, LAMELLIBRANCHIATA, or PELECYPODA. To this second class belong all the bivalved Mollusca which are without a distinct head and possess no radula. They are bilaterally symmetrical; the generative organs are 190 Elementary Zoology. simple. Examples: with two adductor muscles, Axodon, Mytilus (the marine edible mussel) ; with one adductor muscle, Ostrea (oyster), Pecten (scallop), etc. Class 3. CEPHALOPHORA. These Molluscs have a head and a distinct radula (only wanting in rare cases such as the Nudibranch Doriopsis). The body is symmetrical, and the shell is single. The symmetry of the body is more perfect in some than in other forms. In those in which it has advanced furthest there is but one gill instead of two, and one nephridium. In others there are two nephridia, but one is smaller than the other. The generative organs are generally complicated, but are simpler in the forms which have most nearly retained the primitive bilateral symmetry. The Cephalophora may be divided into the follow- ing subclasses: Prosobranchs, Heteropods, Opisthobranchs, Pteropods, Pulmonates, and Scaphopods. We shall briefly consider the characters of these subclasses. 1. Prosobranchs.—Of this group, which are characterized by the fact that the gills lie in front of the heart, the simplest forms are those which have been termed the “ Zygobranchia ;” in them there is so much of the bilateral symmetry retained that the gills are two—save in the limpet (Paée//a), where there is a nearly complete circle of gills—and the nephridia are also double, though one may be larger than the other. Two auricles, moreover (as in Lamellibranchs), exist in Aa/iotis, In these forms there are no special generative ducts ; the nephridia serve as such. The more advanced Prosobranchs are termed the “ Azygobranchia.” They have but one gill, the right ; only one nephridium, the left, is retained. The sexes are separate, and there are special generative ducts. To this division belong the fresh-water pond-snail, Paludina (Fig. 82), and the bulk of marine univalved “ shell fish,” such as the whelk (Buccinum), cowrie-shell (Cyf/v@a), Conus, the periwinkle (Zittorina), the purple-producing AZurex, etc. 2. Heteropoda.—By some associated with the last division, The Classification of Animals. 1g! the Heteropods differ from the forms already considered in the subdivision of the foot into three regions, pro-, meso-, and metapodium. They are pelagic creatures, and the foot has in Fic. 82.—Paludina vivipara. Fic. 83.-—Limuaa peregra. consequence become a swimming organ. The shell is unim- portant, and has in some species disappeared. 3. Opisthobranchia.—In these the gill (single) lies behind the heart. The shell is often absent altogether, and when present is frequently small and enveloped in the mantle. These Molluscs are hermaphrodite, and have complicated reproductive organs with various accessory glands. The common 4f/ysia represents one section of this group in which the single gill, the Fic, 84.—Planorbis lineatus. Enlarged. mantle and the shell, though a small one, have been retained. These Opisthobranchs are termed “Palliata.” The non-palliate division includes the Nudibranchs, which are devoid of mantle and apparently of gills morphologically corresponding to those of the Palliata. In Doris, for example, there is a circlet of gills surrounding the anus, which seem to be independent structures. These Mollusca have reacquired a certain amount of bilateral symmetry. Examples of the Nudibranchs are Doris, £olis, with numerous processes on the back, which may be 192 Elementary Zoology. respiratory, and which contain branches of the liver, and are armed externally with thread-cells like those of the Ccelentera. 4. Pteropoda.—The Pteropods, allied to the Opistho- branchs, are pelagic creatures of a transparent appearance, The foot is somewhat reduced, but in compensation two lateral processes of the foot—present also in some of the Mollusca already considered—are largely developed, and form the swimming organs; these are known as the epipoda. The Pteropods have complicated reproductive organs, and are hermaphrodite. Though there is a pseudo-symmetry, the nephridium and the auricle are single. Some have, and others have not, a mantle-fold and a shell. 5. Pulmonata.—This group of Molluscs comes nearest to the Opisthobranchs, and includes, not only the terrestrial snails and slugs, but also the fresh-water Planorbis (Fig. 84), and the pond-snail, Lymdea (Fig. 83). The Pulmonates have no gills, respiration being effected by the very vascular walls of the pulmonary chamber, formed by a fold of the mantle. The shell is usually present, but may become atrophied, as in some slugs. The slug Zestacella, however, has a small shell. The generative organs are complicated, and the animals are hermaphrodite. 6. Scaphopoda.—To this group many give equal rank with the larger divisions of the Glossophora. It is, however, more closely allied to the Cephalophora than to any other of the four main divisions allowed here. The Scaphopoda have a characteristic shell, shaped like a truncated elephant’s tusk ; it is open at both ends, one being narrower than the other. This shell has a cylindrical form on account of the fact that the mantle which secretes it has fused along the ventral surface of the body, thus forming a complete cylinder. Two nephridia are present, and the generative gland is furnished with a duct which opens into the right of these: the duct is, however, not complicated. There is no heart present. 4. Class CEPHALOPODA. The cuttle-fishes are bilaterally symmetrical animals with a well-marked head and a radula. There is usually a shell The Classification of Animals. 193 present, which may be external and internal. The foot has grown round the head ; it is broken up into a large number of lobes, the tentacles. In the mantle-cavity is a muscular pro- jection known as the siphon, and probably the equivalent of the epipodia of other Molluscs. The heart is well developed with two or four auricles; there are one or two pairs of nephridia and of gills. The Cephalopoda are divided into the Tetra- branchiata and the Dibranchiata. Tetrabranchiata: these are the Nautilus and the extinct Ammonites, They have a large external shell, coiled or straight, two pairs of nephridia, of auricles, and of gills. The siphon is incomplete, its edges not meeting. The Dibranchiata includes all the squids, Octopus, Argo- nauta, Spirula, etc. The shell is generally internal, but in the paper Nautilus, Avgonaufa, external. The arms bear suckers which are absent in the Tetrabranchiata. The gills, nephridia, and auricles are but a single pair. The siphon forms a com- plete funnel. An ink-sac is present. VIII. ARTHROPODA. The Arthropoda are animals in which the body is clearly segmented, and provided with segmented appendages. They are bilaterally symmetrical, with a hard exoskeleton. The nervous system is on the Annelid plan, a brain connected by a circum-cesophageal commissure with a ventral ganglionated chain. The ccelom is so much reduced that in many forms it seems to be altogether absent. There are never cilia, except- ing only in Perifatus. The Arthropoda are divided into two primary divisions: the Crustacea, breathing by means of gills, and the Tracheata, which breathes by means of trachez, or invaginated “ lungs.” 1. CRUSTACEA. The Crustacea are typically aquatic Arthropods, breathing by means of gills or by the general surface of the body. There 0 194 Elementary Zoology. are two pairs of antenne. The Crustacea are divided into two groups as follows :— 1. ENromostraca, with usually a varying number of segments, 2. MALACosTRACA, with only nineteen segments. The ENnTomostraca are divisible into the four orders which follow :— (1) Phyllopoda.—This order contains the comparatively large Apus, and the minute water-fleas (Fig. 85). There is often a ee enone a carapace in front. The number of segments may be large (Apus) or small (Daphnia). The appendages bear gills. (2) Ostracoda.—These Crustacea are minute, and the body is not clearly segmented. The body is enclosed in a bivalve shell, which has adductor muscles like a Lamellibranch. There are seven pairs of appendages, no gills. (3) Copepoda.—The free-living members of this order have a segmented body without a carapace; the abdomen has no appendages. Cyclops (Fig. 86), of our fresh waters, is an abundant form. ‘The parasites known as fish-lice belong to this division. (4) Cirrhipedia.—This order contains not only the bar- nacles, but a number of parasitic creatures, which have become so degenerate that the general definition used here only partly The Classification of Animals. 195 applies tothem. The carapace of the Cirrhipedes (Figs. 87, 88) is strengthened by a number of separate calcareous plates. The body is not well segmented, and the abdomen is rudimentary. The Matacostraca are divided into three orders, as follows :— (1) Leptostraca.—This order only includes JVedalia, and a few allied forms, which agree with the Entomostraca in having a bivalved shell, closed by special muscles, as in Ostracoda, and Fic. 87.—A Cirrhipede (Zefas). (From Fic. 88.—A Cirrhipede (Balanus). (From Claus-Sedgwick’s ‘‘ Zoology.”) the same source as Fig. 87.) in having a larger number (eight) of abdominal segments than are found in the remaining Malacostraca. The thoracic limbs are much like those of Afws, being multilobate, and not on the biramous type of the higher Crustacea. The last segment has two long processes (“ furcze”), another Entomostracan feature. (2) Thoracostraca.—In this division all or most of the thoracic segments are united with the head by a cephalo- thoracic fold ; the eyes are nearly always stalked. It is again divided into the Cumacea, small Crustacea, with four or five 196 Elementary Zoology. thoracic segments ; the Sfomatopoda, also with a number of free thoracic segments, and with movable eye and antennule seg- ments; Sc4zzopfoda, with biramous thoracic limbs; and Decapoda, with all the thoracic segments fused with the head. The thoracic appendages are uniramous in the adults, ¢.g. crayfish, crab, lobsters. (3) Arthrostraca.—In this group the eyes are sessile, and there is no cephalothoracic shield. It is divisible into the Amphipoda (Fig. 90), with thoracic gills, of which examples are the shore-hopper and the fresh-water shrimp, and the Fic. 89.—Asellus aguaticus. Fic. 90.—The Fresh-water Shrimp. Magnified, Magnified. Lsopoda (Fig. 89), with abdominal gills, of which the best- known example is the terrestrial woodlouse. 2. TRACHEATA, These Arthropods breathe either by trachez or lung-books, which may be external or invaginated. There is but one pair of antennz. They are prevalently terrestrial, while the The Classification of Animals. 197 Crustacea are mainly aquatic. They may be divided into four classes. : 1. Prototracheata.—This group includes only the Myria- opod-like Peripatius, which, while possessing the characteristic jointed appendages of the Arthropoda, and the trachez of the present division of Arthropods, agrees with the worms in having metameric nephridia. It is a caterpillar-like creature found in tropical regions in many parts of the world. 2. Myriapoda.—This group includes the centipedes and millipedes. They have trachez, like Peripatus, but the body is not distinguishable into thorax and abdomen. In the milli- pedes each segment except the first few is provided with two pairs of appendages ; in the centipedes each segment has only one pair. 3. Insecta.—Insects have a distinct head, thorax, and abdo- men; the latter has no appendages except a few doubtful outgrowths, which may possibly belong to this category (cf. p. 50). The animals breathe by trachez. There is an immense variety of insects which are grouped into the orders Aptera (silver fish), a group of very primitive insects, without metamorphosis and without wings; Orthoptera (cockroach, earwig, grasshopper), with biting mouth-parts and but little metamorphosis ; Neuroptera (dragon-flies, May-flies), with four membranous wings and mouth-parts of the biting type, and usually a pronounced metamorphosis ; Lepidoptera (butterflies, moths), with a complete metamorphosis, four partly or entirely scaly wings, sucking mouth (proboscis); Coleoptera (beetles, with biting mouth-parts, complete metamorphosis, and four wings, of which the first pair, as in the Orthoptera, are horny and not used for flight; Hemiptera (bugs), with incomplete meta- morphosis, piercing mouth-parts; Diptera (two-winged flies, é.g. house-fly), with only one pair of fully developed wings, piercing mouth-parts and complete metamorphosis; Hyme- noptera (bees, wasps, ants), with four membranous wings, biting mouth, complete metamorphosis. 4. Arachnida.—This large group comprises Arthropods, which breathe by trachez, as in insects, or by lung-books, which may be external, as in the king crab, or internal, as in the 198 Elementary Zoology. spiders. The head is fused with the thorax to form a cephalo- thorax. ‘There are four pairs of ambulatory limbs. Class 1, Xiphosura,—This class contains the king crabs Fi. 91.— Odisium trombidio- ides. (From Claus-Sedg- wick’s ‘* Zoology.’’) Magnified. (Limulus), by some placed with the Crustacea, as a group Gigantostraca. There are really five pairs of ambulatory limbs. The abdominal appendages bear the gills, which are composed of a large number of thin plates, like the leaves of a book. Allied to the king crab are the extinct Trilobites, Class 2. Scorpionida.—The scorpions have also five pairs of walking limbs; the abdominal appendages of Limudus are wanting, but instead there are invagi- nated lung-books. Class 3. Pseudoscopionida.—These are minute creatures, with large claws like a scorpion, but breathing by means of Fic. 92.—Phrynus reniformis. (From same source.) Kt, pedipalpi; Gé, flagelliform anterior leg. trachee. The little-book scorpion (Fig. 91), is a common example of this class. The Classification of Animals. 199 Class 4. Pedipalpi.—These are large Arachnids (Fig. 92), in form intermediate between a spider and a scorpion. They have clawed chelicerz, like the last two groups, and breathe by means of lung-sacs, Class 5. So/tfuga.—-The head in these animals is distinct from the thorax, and the latter from the abdomen. They breathe by trachee. The tropical Galeodes (Fig. 93) is an example of this class. Fic. 93.—Galeodes araniotdes. (From same source.) Class 6. Araneina.—This class is that of the true spiders , they have an entirely unsegmented abdomen. The first pair of appendages, the chelicerz, are clawed, and bear a poison- gland ; they breathe by pulmonary sacs. Class 7. Phalangida.—This class, that of the ‘‘ Shepherd Spiders,” differs from the last in breathing by tracheze, and that the abdomen is not separated off from the thorax. Class 8. Acarina.—The mites, all of small size, have no 200 Elementary Zoology. separated abdomen ; they breathe by tracheze when breathing organs are present, Allied probably to the last group of the Arachnids are three little understood groups of Arthropods. The Linguatulida are ento-parasites of a vermiform appearance, but of Arthropod characters, Their appendages are represented by hooks. The Tardigrada are rather more degraded, though free-living organisms; they have been confounded with the Rotifera. They have short clawed limbs and no respiratory organs. The third group is that of the Pyexogonida, sometimes placed among the Crustacea ; they are marine and spider-like with a small body and four pairs of sprawling legs. The abdomen is rudimentary. : IX. CHORDATA. The group Chordata does not only include the Vertebrata, but also a number of varied organisms, formerly of doubtful affinity, which are now held to be more or less distantly related to the Vertebrata. All of these animals agree with each other to differ from any of the Invertebrate group in the three fol- lowing particulars :— 1. There are either temporarily or permanently openings from the pharynx to the exterior, the gill-slits. 2. A dorsal rod of skeletal nature exists either temporarily or permanently, and lies above the nervous system; this notochord extends through a greater or less extent of the body, and is hypoblastic in origin. 3. The central nervous system is dorsal in position, and is usually tubular in character. The Chordata, as thus defined, may be divided as follows :-— Class 1. HEMICHORDATA.—Sub-class (a), Entero- pneusta ; sub-class (4), Cephalodiscida ; sub-class (c), Rhabdo- pluerida. Class 2. VROCHORDATA. Class 3. CEPHALOCHORDATA, Class 4. VERTEBRATA. The Classification of Animals. 201 Some of the main characters of these four classes will now be given. Class 1. HEMICHORDATA. Sub-class (a). Enteropneusta.—This sub-class contains only the genus Balanoglossus, a worm-like organism. There is a large proboscis, and the gill-slits extend a long way down the body. The notochord, which is an at first hollow, but after- wards solid, outgrowth of the intestine, only exists in the region of the collar and proboscis. The dorsal nerve-cord is imbedded in the skin, and a ventral cord also imbedded in the skin. The ccelom is spacious, but there is nothing that can be definitely compared to nephridia. There productive organs are sacs imbedded in the body-wall, and opening on to the exterior. Sub-class (4). Cephalodiscida.—To this sub-class is now assigned an organism (Cephalodiscus), showing many resem- blances to the Polyzoa, with which it was at first confounded. Sub-class (c). Rhabdopleurida. — Rhabdopleura is an organism which has also been referred to the Polyzoa. But since, like Cephalodiscus, it has gill-slits and a small noto- chord, it conforms to two out of three essential character- istics of the Chordata. Class 2. TROCHORDATA. These animals, more generally known as Ascidians or Tunicata, are abundant in genera and species. It is only in the larval stages, and in the persistently larval Appendicularia and its immediate allies, that the nervous system is in the form of a dorsal tract, and that the notochord is present. This latter is developed only in the tail, thus contrasting with the Hemi- chordata, where it is only found in the head region. Class 3. CEPHALOCHORDATA. To this group belongs only the Amphioxus, of which there are several species. It is an elongated fish-like animal. It 202 Elementary Zoology. has a dorsal nervous system, swelling out in front to something resembling a brain. This nerve-tube is hollow. There are numerous vertically elongated gill-slits, but the young Amphi- oxus has much fewer, and is more like the Vertebrata in this respect. Excretory organs are present as a series of paired tubes opening into the ccelom on the one hand, and into the atrium on the other—the atrium being a late ingrowth from the exterior which burrows its way among the organs of the body. Class 4. VERTEBRATA, The word “ Vertebrata” is etymologically incorrect when applied to the present group, for the lowest members of it, the lampreys, have no vertebre. The term “ Craniata” often used, is a much more appropriate term. The Vertebrata, or Craniata, are to be distinguished from all the other Chordata by the fact that they possess a definite skull, which is either cartilaginous or bony, or partly cartilaginous and partly bony. The brain is well marked off from the spinal cord. The liver is always a more or less complicated gland, not a mere sacular diverticulum as in Amphioxus, The Vertebrata are divisible into the six following sub-classes :— Sub-class 1. Cyclostomata.—The lampreys and hagfishes are so different from the true fishes that they form a definite group apart from them. They have a persistent notochord (round which, however, vertebrae seem to have existed in the extinct Paleozoic Palgospondylus) and a smooth scaleless skin (unless the Devonian fishes, Picraspis, etc., are ultimately proved to be Cyclostomes); the skull resembles in many particulars that of the embryos of higher forms; there are no true branchial arches, but a basket-work of cartilages, which strengthen the gills. The Cyclostomes have no limbs, unless certain rudiments in the neighbourhood of the cloaca turn out to be degenerate limbs of the hind pair. The alimentary canal is straight, and the pancreas is absent. There are no teeth, only horny plates, and the mouth is a sucking, not a biting mouth. The Classification of Animals. 203 Sub-class 2, Pisces.—The fishes, as is the case with all the remaining Vertebrates, have two pairs of limbs. These limbs are, however, what is called an Ichthyopterygium; they are not formed on the plan of the five-fingered hand, or foot, of the higher Vetebrates, but are made up of a larger or smaller number of cartilaginous, or bony rays, which cannot be securely reduced to the type of the cheiropterygium. This is really the only distinguishing feature which absolutely separates all fishes from all Amphibians. The bulk of fishes, however, are so characterized by the fact that they breathe by gills only, and, if there are simple lungs, by gills also: by the fish-like form, with its unpaired fins, and the great develop- ment of the lateral line. The Pisces are divisible into the four following groups :— 1. Elasmobranchiu.—The sharks and skates. The skin is smooth, with scattered spines forming, in some cases, a close investment (shagreen). The: skeleton is cartilaginous, the notochord largely persistent. The swim-bladder is entirely wanting ; the intestine, furnished with an extensive spiral valve, opens into a cloaca. 2, Holocephali.—This limited group of fishes contains the chimera of northern seas'and some other forms. The skeleton is cartilaginous, as in sharks; but the mandible articulates directiy with the mass of cartilage forming the skull, there being no free palatoquadrate or hyoid suspensorium. There is a dermal flap (operculum) covering the ‘gill-slits, which are thus not exposed as in sharks. The spiral valve is less conspicuous, and the anus is separate from the urogenital pore. There is no swim-bladder. 3. Teleostomi.—This group includes the Ganoids and Tele- osteans. Examples of the former are the sturgeon, Polypierus of the Nile, etc; of the latter, the vast majority of the familiar fishes, pike, perch, sole, herring, etc. The skeleton is sometimes largely, but never entirely, cartilaginous; in the Teleosts it is nearly always greatly ossified. The body is generally invested with bony plates or scales. ‘The intestine may, or may not, have a spiral valve, and be furnished, or not furnished, with pyloric ceca. There is no cloaca; the gill-slits 204 Elementary Zoology. are covered by an operculum. A swim-bladder is always present, and is unpaired, except in Fo/ypierus, where it is even lung-like in texture, as is the unpaired swim-bladder of Lefi- dosteus and Amia (two American Ganoids). 4. Dipnoi.—The Dipnoi, or lung-fishes, form a limited group (at the present day) with but three species, the ‘Ceratodus of Queensland, the Protopterus of Africa, and Lepidosiren of America. They are more akin to Amphibia than are any other fishes. The body is scaly, and the skeleton is largely cartilaginous. The viscera are somewhat shark-like. The intestine has a spiral valve, and there is a cloaca. The swim-bladder is single in Ceratodus, double in the others ; it functions as a lung, and the gills are somewhat reduced. They have the posterior nares (as in zo fish, but in Amphibians and all higher Vertebrates) ; the heart is three-chambered. Sub-class 3. Amphibia.—By some the Amphibia are united with the fishes to form a group, Ichthyopsida. The only positive differences that distinguish them have been already mentioned, They contrast with the higher Verte- brate by the fact that the larva always has gills,’ which may be persistent throughout life, but in that case in conjunction with lungs. The skull is more or less ossified with two occipital condyles for articulation with the vertebral column. There is no spiral valve, but the intestine opens into a cloaca; the heart is three-chambered. As in Pisces, there are ten pairs of cranial nerves. The Amphibia are divisible into (1) the extinct Stegocephali, including the Labyrinthodonts, which were tailed, and had an extensive dermal armature of plates ; (2) the Ceecilia, snake-like Amphibians, of underground habit, with scales; (3) Urodela, tailed Amphibians, with usually per- sisting gills, e.g. newt, salamander, axolotl; (4) Anura, tailless Amphibians, without gills, partly terrestrial in habit—the frogs and toads. Sub-class 4. Reptilia—The Reptiles agree with the Amphibia and with all the higher Vertebrates in possessing the cheiropterygium, which, however, has partly reverted to ’ With a few exceptions. Thus the tree-frogs leave the egg complete frogs. The Classification of Animals. 205 a fish-like condition in the extinct Zchthyosaurus, etc. They never, however, possess gills. The epidermis possesses a skeleton in the form of scales. The skull articulates with the vertebral column generally by one condyle only ; it is almost completely ossified. The gut terminates in a cloaca; the heart is at least three-chambered, and the ventricle may be incompletely or completely divided into two chambers. There are, as in Aves and Mammalia, twelve pairs of cranial nerves. The embryo, like that of the higher Vertebrata, has an amnion. The Reptiles are divisible into nine orders, of which five, viz. the Ichthyosauria, Plesiosauria, Theromorpha, Dinosauria, and Pterosauria, are totally extinct. The orders with living representatives are the Testudinata, Lepidosauria, Rhynchocephalia, and Crocodilia. We will briefly give the characters of the extinct orders first. 1. Lchthyosauria.—These are the well-known “ fish-lizards ” of the middle period of the earth’s history. Large, often very large, creatures, with fish-like paddles, and a tail formed by an expansion of the integument, like the tail of a whale. 2. Plesiosauria,—The long-necked Plesiosaurus is familiar to everybody. It had paddles, in which the individual bones are less numerous than in the /cAihyosaurus. They are also mesozoic in time range. 3. Theromorpha,—These reptiles are interesting on account of the many points of resemblance which they show to the Mammalia. Their teeth, for example, often show much speciali- zation—canines, molars, etc., being distinguishable ; whereas in reptiles generally the teeth are numerous and all similar. 4. Dinosauria.~The last group of extinct reptiles present certain likenesses to the Mammalia. The present group is, in many respects, nearly akin to the group of birds, The dino- saurs are mesozoic and were of varied size; some were as small as a crow, others reached a length of sixty feet or so. Their likeness to birds is mainly in the pelvis, but it seems ‘probable, from certain osteological details, that the bones were permeated with air-spaces., 5. Prerosauria.—In some ways the pterodactyls also re- semble birds. ‘They were flying reptiles with extensive wings, 206 Elementary Zoology. supported, however, by the elongated digits. Some were tooth- less, and appear to have possessed beaks. 6. Zestudinata.—The tortoises and turtles have the well- known and characteristic “shell,” which is formed by bony plates, partly formed by the flattened neural spines and ribs and other bones. Covering this is a horny set of plates, which are epidermic in structure. This character absolutely distin- guishes the Testudinata. 7. Lepidosauria.—This group includes both the lizards and snakes, which are by some made the representatives of two equivalent orders. They agree in the covering of epidermic scales. The teeth are fused to the jaws, and not implanted in distinct sockets. The cloaca opens transversely, and there is a double penis. In both the limbs may be more or less unre- presented ; and these are the only reptiles in which the teeth are sometimes grooved or perforated for the poison-glands (=modified salivary glands). The Gila monster (Hée/oderma) and an Oriental form are poisonous lizards; while’ there are, of course, numerous poisonous snakes. The snakes are separable from the lizards by the incompletely united rami of the lower jaw, which thus permits the swallowing of larger prey; they have no urinary bladder, while the lizards have one. Snakes are more constantly apodous. 8. Rhynchocephatia—This group is represented at the present day by only a single lizard-like animal, the Hattria, or Sphenodon, of New Zealand islands. The skull differs in several points from that of the lizard’s, and the body is pro- vided in addition to the true ribs with abdominal ribs, also found in the Crocodilia. Some of the ribs bear uncinate processes, as in birds. 9. Crocodilia.—These are the most highly organized of living reptiles. The heart is completely four-chambered. The skin has not only epidermic scales, but bony mesodermic plates beneath them in many parts of the body. The teeth are in distinct sockets. The ribs may possess uncinate pro- cesses, and, as already said, there are abdominal ribs. The cloaca has a longitudinal opening, and the penis is single. Sub-class 5. Aves.—By many the birds are placed in one The Classification of Animals. 207 great division with the reptiles, which is then called Sauropsida. They agree with them in the single condyle of ‘the skull, and in the fact that the ankle-joint is in the middle of the tarsus, and not between the tarsus and the tibia and fibula, as in other Vertebrates. Birds, however, differ from reptiles in the peculiar modification of the fore-limb to form the wing (see above), in the presence of feathers, which is itself sufficient to define birds. The blood is hot; the heart is four-chambered. ‘The bones and viscera are permeated with air from the air-sacs, which are prolongations of the lungs, and are developed to an extent never found in reptiles, with the possible exception of the extinct Dinosaurs (cf. p. 205). Sub-class 6. Mammalia.—The Mammals are separated from all other Vertebrates by a number of important points. They differ from all in the presence of mammary glands, with which the young are suckled, and by the hairy covering which is rarely nearly or quite absent, as in the whales. In all Mammals a complete muscular septum, the diaphragm, sepa- rates a cavity containing the heart and lungs from the cavity in which the intestine, liver, and the rest of the viscera, lie. These three characters—or, indeed, any one of them alone— is amply sufficient to define a Mammal. The Mammalia are commonly divided in the first place into three different divisions: the Prototheria, Metatheria, and Lutheria, To the Prototheria belong only the Platypus and the Echidna, both Australian, and distinguished by the fact that they lay large-yolked eggs, that they possess a well-developed coracoid, and a cloaca. The mammary glands open on to the skin, and are enclosed by a pouch, which really represents the teats, not drawn in into a teat-like form. The M@eratheria are the marsupials, the kangaroos, wallabies, native bear, etc., which have, for the greater part, a well- developed pouch, into which the separate teats open, and in which the young are carried. The young of the marsupials are born in a very imperfect condition. The egg, however, is minute, like that of the Eutheria. There is just a vestige of the cloaca remaining. 208 Elementary Zoology. The Lutheria are the rest of the Mammalia. They have no cloaca, or at most, a trace of one. The egg is minute without yolk. A pouch is absent. The coracoids (as also in the marsupials) are minute processes of the scapulz. Of Eutheria there are a large number of existing orders. The Edentata (sloths, armadillos, aard vark, pangolin). Sirenia (manatee, dugong). Ungulata (horses, oxen, deer, antelopes, rhinoceros, tapir, elephant). Cetacea (whales, dolphins, porpoise, narwhal). Rodentia (rabbit, rat, porcupine, guinea-pig). Carnivora (cat, dog, bear). Insectivora (mole, shrew). Chiroptera (bats). Primates (lemurs, monkeys, gorilla, chimpanzee, man). There are besides a number of extinct orders. ee ee SS 2S a Beis a a ee SS ee = Se