ibrar. | 4 | COLLEGE OF LIBERAL ARTS. | Boston University. Me From. ages Wares No. SJO..KbIr &: Received. Qed. V{os. igoec A: ar ok NATURAL HISTORY OF LOWER VERTEBRATES. H. C. BUMPUS. THEODORE GILL, Pu. D. E. D. COPE, Pu. D. D. S. JORDAN, Pu. D. SAMUEL GARMAN. J. S. KINGSLEY, S. D. R. RAMSAY WRIGHT, Pu. D. I Y VI ~_ a ber. “ : ’ - ‘ al A a ' re oS fee RIVERSIDE NeGeRAL HISKORY EDITED BY JOHN SEERLING KINGSLEY %s*~ ARTICLES BY C. C. ABBOTT, J. A. ALLEN, W. B. BARROWS, E. A. BIRGE, H. C. BUMPUS, S. F. CLARKE, J. H. COMSTOCK, E. D. COPE, E. COUES, G. DIMMOCK, H. EDWARDS, D. G. ELLIOT, W. FAXON, C. H. FERNALD, J. W. FEWKES, W. F. GANONG, S. GARMAN, T. GILL, A. F. GRAY, R. HITCHCOCK, L. O. HOWARD, A. HYATT, D. S. JORDAN, D. S. KELLICOTT, J. S. KINGSLEY, W. N. LOCKINGTON, S. LOCK- WOOD, G. MACLOSKIE, O. T. MASON, C. S. MINOT, A. S. PACKARD, C. V. RILEY, W. B. SCOTT, L. STEJNEGER, P. R. UHLER, S. W. WILLISTON, C. W. WOODWORTH, R. R. WRIGHT Fustrated BY MORE THAN TWENTY-TWO HUNDRED WOOD-CUTS IN THE TEXT, ONE HUNDRED AND SIXTY-EIGHT FULL-PAGE ENGRAVINGS, AND TWELVE PLATES IN COLOR VOLUME III. TOWEK VERPEBRATES BOSTON UNIVERSITY COLLEGE OF LIBERAL ARTS LIBRARY BOSTON AND NEW YORK HOUGHTON, MIFFLIN AND COMPANY Lhe Riversive Press, Cambridge %, a ‘ Lig! ah ae mr i va = <= a W pay: o ke, Ve te “ P a ‘ Och. \q0S . Copyright, 1885, By S. E. CASSINO AND COMPANY. Copyright, 1888, By HOUGHTON, MIFFLIN AND COMPANY. All rights reserved. i The Riverside Press, Cambridge, Mass., U. S. A. Printed by H. 0. Houghton & Company. PAGE Branco IX.— VERTEBRATA. . . ual Cuass I. — Tunroata. J. S. Kore ley Gao gS Sus-C1iass ite —_ loan. aes OG OrperR I.—CoreLat#. .. . 56 OrperR Il.—Monasciprz. . . 56 Orver III.—Synasciprz# . . 57 Orper I[V.—Luciz. ... . 59 Susp-Crass I].—Tuatiacte . . . 59 Orver I.—Cyctomyaria. . . 59 Orver IIl.— DrsmMomyarIA . . 60 Cuiass IJ. — Acranta. Samuel Gar- TG Oo 62 Crass III. Gcroenomt: cE Ss. Kane ley . oo a he OrpbeEr I. are nH Seine, (OD Orper IJ.—HyprrorreTiA . . 67 Crass I[V.— Exvasmosrancui. J. S. Kingsley . . . 5 5 0 GS Sus-Criass I.— eign eanan Seay hl Sus-Crass IJ.— Puaciostromr. . . 72 ORDER Ie SQUATE sen se ne te ORDER RATA ee sy es) Oe Crass V.—Pisces ... ot pl cl) Sus-Crass I. — Srenen Wey IS Kingsley . 90 Orper I. Larson 92 Orper IJ.— PuLacoperMI. . . 92 OrperR III.—CHONDROSTEI . 92 Orper IV.—PycnoponTini. . 94 Orper V.—Crossopreryeu. . 95 Orper VI.—Ginetymopr . . 96 Orper VII.—HAtecomorpHt . 97 Sugs-Crass II.— Trerkoster ... 98 Orver I.—Symprancuu. Theo- GorexGul: a6 6 aes ey or = 200 Orver II.— Apopes. Theodore Gillie. 5 ess ec 6, 6 ACD) Orver III. — eee Theo- doreGill . . . . ate ot L09: Orver IV. — Ounrions: Theos dore Gill . .. . so ahi) OrveER V. ae irae sine D. Sivordane. sen aes) = 0 LLO OrveR VI.—Scyrnornort. D.S. Jordan . a Sees OrveEr VII. — naceoen PHALI Sus-Orper I. — GyMNONOTI. D. 8S. Jordan . Sus-Orper II. Le eae tH. D.S. Jordan. Sus-Orper III. — Isospon- pyu. D.S. Jordan Sus-—Orper IV. — Hapiomt. D. S. Jordan . Sake Sus-Orper V. — XENOMI. D.S. Jordan . ‘ Susp-Orpver VI. aera warn. Theodore Gill. Sus-Orper VII. — PrErcss- oces. Theodore Gill . Sus-Orper VIII. — Acan- THOPTERYGU. Theodore Gill Sus-Orprer IX. — Hyposto- mIDES. Theodore Gill. Susp-OrpER X. — RHEGNOP- TERI. Theodore Gill . Sus—OrpDER XI. — DiscocEPH- Aut. Theodore Gill. Sus-OrpEer XII. — Tnr0- somr. Theodore Gill. . Sus-Orper XIII. — Xenop- TeryGu. Theodore Gill . Sus-OrpEerR XIV.— ANACAN- THINI. Theodore Gill . Sus-Orper XV. — HETERO- somata. Theodore Gill . Orver VIII. — Hemrerancan. Theodore Gill . Orver IX. — igHeeanee Theodore Gill . Sus-OrperR I. — encase Sus—Orper II. —SyNGNATHI. OrpeER X. — PLECTOGNATHI. Theodore Gill . Sus-Orper I. eee pret 2 Sus-Orper II. — Osrraco- DERMI PAGE . 121 . 121 . 123 . 168 vi CONTENTS. PAGE PAGE Susp-Orper III. — Gymno- OrpverR I. — OpnripiIa . 348 DONTES . ate sy Pea SusB—OrDER I.—OPOTERODONTA 352 Orver XI.— Pepicuuati. The- Sus—Orper II. — CoLusriror- odore Gill . « Wee MIA Mo 5 ay CS! Crass VI.—Drenor. J. S. Kingsley . 299 Sus-Orper III. — Protrro- OrpvER I. —MONOPNEUMONIA 300 GLYPHA . +) 0o we) EERIE LO. Orver II.— DipneumMonrIA 300 Susp—Orper IV.— SOLENOGLY- Crass VII.—Barracuta. E£. D. Cope 301 PHA SECM AG SO that it is from the conjugation of two quite microscopic elements that the adult body results. In view of the facts of heredity, it is safe to say that the sim- plicity of the structure of the reproductive elements is only apparent, and that, although unicellular, they are still, in virtue of their molecular constitution, epitomes of the structure of the adults. The sperm cells are always of small size, but the eggs may be very large, in virtue of having within the ego-membrane a greater or less quantity of ‘food-yolk’ in addi- tion to the formative yolk, which is directly converted into the tissues of the embryo. Further nutritive material, like the white of the bird’s egg, may be furnished by the 4 NATURAL HISTORY OF VERTEBRATES. . walls of the oviduct as the egg escapes to the outside; the lower part of that tube may also provide a protective shell for the egg, the form, texture, and chemical nature of which present wide differences in the various groups of oviparous vertebrates. Eggs which are free of food-yolk, like those of most mammals, divide completely into two equal parts after fertilization, and this division goes on regularly till a mass of small cells results, which eventually arrange themselves into the three embryonic layers. Such eggs are holoblastic; but the presence of a large amount of food-yolk in the egg brings about various complications; it prevents the eggs dividing completely, and consequently the process of segmentation is limited to one pole of the ege, and is on this account styled meroblastic. Other processes, too, are rendered more complex by the presence of the mass of non-segmenting food material, so, as an introduction to the phenomena of development, we shall select the eggs of the toad, which are com- paratively free from food material, and which divide completely, though unequally. The eggs of most mammals are so free from food-yolk that they divide completely and equally, but as the mammals are, probably, descendants of forms with large eggs, the loss of the food-yolk is to be considered as secondary, and as consequent upon the develop- ment of certain other arrangements (the placenta) for nourishing the embryo, which have rendered the other food material unnecessary. But the mammalian eggs have inherited from these earlier types certain complications which render it easier for the beginner to understand the processes of development in the eggs of frogs and similar forms. As is well known, the spawn of toads and frogs is composed of groups of eggs sur- rounded by a transparent jelly-like mass, which, like the white of the fo Js) bird’s egg, is secreted by the walls of the oviducts, but, unlike that, is not used as food by the developing embryos. The egg itself measures about +); of an inch in diameter, and that pole which contains the nucleus or germinal vesicle is always more pigmented than the other. After Fig. 1. — Bag of fertilization the egg becomes completely divided into two equal parts, second segmen- and yery soon thereafter into four cells, two of which are much smaller than the others (Fig. 1). Even at this stage the cells begin to retract from each other, so as to leave a small space between them, which eventually increases as the division of the cell goes on, and becomes the segmentation cavity. The larger cells, which contain the food-yolk, segment more slowly than do those at the other pole of the egg, so that, before the next important stage in development takes place, a spheri- cal multicellular body results, the cells at one pole of which are very much smaller than those at the other pole. At one spot on the line of junction of the small and large cells (Fig. 2, x), the small cells are turned in, or invaginated, so as to form the lining of a new cavity, the beginning of the intestine, and simultaneously, while the rest of the small cells gradu- FiG. 2.—Diagrammatie longitudinal section ally grow over the large ones so as to occupy also the of embryo toad ; e, epiblast, h, hypoblast, ae : . m, mesoblast, s,’ segmentation cavity, 2, Other pole of the egg, the lining of the intestine be- edge of invagination. comes separated from the outer layer by a new middle layer, which grows round from the lips of the orifice of invagination (Fig. 2, m, m’) In this way the three layers of the embryo are formed, to which all the tissues and VERTEBRATES. 5 organs of the body may be traced, viz: epiblast, mesoblast, and hypoblast. Of these, the former is constituted by the outer small cells, which now encase the whole embryo; the latter lines the intestine and draws upon the larger yolk-laden cells, as additions to the layer are required, while these two layers are separated by the mesoblast, which, in its development, is closely connected with the invagination of the intestine. Although this orifice of invagination, or blastopore, is very much restricted in extent in the frogs, it would seem to have been much longer in earlier forms, for it is still represented in some amphibians as a ‘ primitive groove,’ which extends along the dorsal surface of the egg, — that afterwards occupied by the central nervous system. Again, in some tailed amphibians the intestine retains, permanently, a communication with the outside through the blastopore, which is converted into the anus, and it is possible that this communication may have been double in earlier forms; a mouth per- sisting at the front end of the elongated blastopore, while the intervening part of the slit became closed. Neither slit nor groove are to be detected in the frog, but along that surface which is thus occupied in other forms, the epiblast becomes altered into a series of cells, which, at first, form a flat plate on the level of the other epiblastie cells, but afterwards come to lie in a groove below that level. These altered epiblastic cells are des- jy. 3. ‘Transverse section of embryo tined to form the central nervous system. The groove PONtj fo poise ty oye oe is known as the medullary groove; in the middle of its extent it soon becomes converted into a tube, so that the resultant nervous tube or canal then communicates with the outside merely by an anterior and posterior neuropore. During the closure of the medullary groove, which is illustrated by a transverse section of an embryo newt in Fig. 3, the epiblast, which was at the sides of the groove, comes to lie over it, and a seam is thus formed along the middle line above, all traces of which are, however, very soon obliterated. It is very probable, as has been suggested by Sedgwick and Van Wijhe, that the stage of development we are considering now, where the neural tube is open in front and behind, really answers to an ancestral verte- brate form, where both intestine and neural tube were traversed by streams of water sucked in by the ciliated linings of these canals. The anterior neuropore is only very transi- torily open, however (except in Amphioxus, where, even in the adult, such a stream enters the neural tube), but the posterior neuropore retains for some time after that an intimate yy piagrammatic longitudinal section of em- relation with the blastopore (Fig. 4), or when oe eabLants 4 Hy pouIREEe Pema eabeh, that orifice closes, as it does in the case of the toad here described, with the neighboring part of the intestine, so that a canal of com- munication between the neural and intestinal tubes (neurenteric canal) exists for some time. That rudiments of this canal are met with also in the embryos of birds and mammals is strong evidence of the truth of this hypothesis. So far the intestine of the embryo toad only communicates with the exterior by the blastopore, but, when that closes, a new aperture, the anus, is formed further for- 6 NATURAL HISTORY OF VERTEBRATES. wards, and the part of the intestine behind the anus, as well as the neurenteric canal, soon become obliterated. It is at a late stage that the intestine acquires a new open- ing to the exterior, the mouth (Fig. 5), which is formed by the turning in of the epi- blast towards the front end of the intestine. In this way a complete intestinal tube is established. While these changes in the neural and intestinal tubes are going on, important alterations are likewise taking place in the mesoblast. This layer is not continuous underneath the neural tube, but is divided into a right and left half (Fig. 4), separated by the notochord, itself probably derived Fe een yee Co meanator embryo; from the underlying hypoblast. It is impor- canal ; p, pineal gland; ai, alimentary canal; 2, neu- tant that the student should understand the relationship of the mesoblast to the noto- chord at this stage, as it is round the notochord that the important axial skeleton is eventually formed. At present each sheet of mesoblast is plainly formed of two strata, Fig. 3, the outer of which applies itself to the epiblast, the inner to the hypoblast. The space between the two is the future cavity of the body in which the viscera lie; higher up the space dilates considerably in the parts of the mesoblastic sheets lying alongside the notochord, and eventually a complete separation is effected between these upper and lower parts. The upper parts become divided into a number of cubical segments, arranged in pairs on either side of the notochord; these retain their original cavity for some time, and give rise to certain very important organs. They are known as proto- vertebra, or, better, as mesoblastic somites, and are the first indication that the frog is a segmented animal. The lower parts of the mesoblast, or lateral plates as they are now called, remain undivided into segments. All of the processes above described, as well as others to which no reference has been made, take place while the embryo is still encased in the egg-nembrane. After that membrane has been burst, the embryo escapes and swims about as a tadpole, which still differs from the adult by the presence of a fish-like tail, the absence of paired limbs, and the fact that respiration is conducted by external gills instead of by lungs. The change to the air-breathing adult stage (so great that it is called a metamor- phosis) is effected by the loss of the tail and gills, and the development of the legs and lungs. It is by processes similar to those described in the frog that the unicellular egg of all vertebrates is converted into an embryo in which three distinct layers of cells are present, destined to give rise to all the organs of the body. These layers are arranged relatively to each other in such a way that the epiblast which covers the whole body has already given rise to a dorsal nervous tube, separated by the notochord from the primitive intestine, while the mesoblast, situated between the other layers, has sepa- rated dorsally into the metameric cubical somites, while ventrally it splits into two plates, which adhere to the epiblast and hypoblast respectively, leaving between them the body-cavity. These processes have only been described so far as to make intelligible the mode of origin of the different systems of organs, to the account of the general arrangement of which we now proceed. They are generally discussed in the following order: the VERTEBRATES. if skin, the skeleton, the muscular and nervous systems, the intestinal, respiratory, vas- cular, and urogenital systems. THe SKIN IN VERTEBRATES. As we shall presently see, there is considerable difference in the structure of the skin in the aquatic and the air-breathing vertebrates, but both groups agree in the fact that it is invariably composed of two primary layers, the ‘epidermis’ and the ‘ corium.’ The former is derived from the epiblast of the embryo, while the latter is formed from the most superficial stratum of mesoblastic cells. Fig. 6 represents diagrammatically a section through the skin of a fish, in which the two layers are very distinctly seen. In the epidermis we observe that those cells which are closest to the surface are more or less flattened, while those resting on the corium are much more elongated. The cellular structure is always easily recognizable, the individual cells being separated by chinks which are traversed by connecting bands of protoplasm. Occasionally a branched pigment-cell may be seen thrusting its processes into these inter-cellular chinks. Certain of the surface cells are constantly being transformed into mucus- cells, which, becoming larger than their neighbors, and undergoing a chemical and physical change, eventually burst and discharge their mucous contents on the sur- face of the skin. In those fishes, like the eel and cat-fish, where the skin is particularly slimy, this is no doubt due to the abundance of certain very large ‘ clavate’ cells, which pour out their secretion into the intercellular spaces. Such clavate cells are known in the common mud-puppy and other aquatic Amphibia, but they disappear from the skin with the change to a new element in cases like the salamander, which, after a larval aquatic life, become, when adult, wholly terrestrial in their habits. As for the mucus- cells, they also do not occur in the skin of air-breathing animals, although they are abundant in the cavity of the mouth. The epidermis and corium are not in contact with each other by a perfectly flat surface, but the latter is here and there elevated into ‘papille, in which the looped capillary vessels are found, whence the nourishment of the epidermis is obtained, and through which the nerves which are destined for the epidermis course. Below this papillary layer, which is further generally remarkable as a seat of pigment cells, the corium is formed, in the aquatic vertebrates, of parallel bands of fibres, traversed here and there by a stout bundle conveying outwards vessels and nerves. Separating this comparatively dense layer from the underlying muscles, is a varying amount of looser connective tissue, often rich in fat, which is termed the ‘subcutaneous connective tissue.’ In many respects the epidermis of aquatic vertebrates is a higher and more com- plex organ than that of the air-breathing forms. This is especially true of modified tracts of the epidermis, which are developed at the extremities of nerves, and which are on this account known as ‘neuro-epithelial’ structures. Two varieties of these exist in the skin; one, on account of its shape and its relation to a nerve, is known as an ‘end-bud,’ and such end-buds, carried upon papilla, are extremely common over the whole of the skin, but especially on the head and in the cavity of the mouth. Although such end-buds are not found in the skin of air-breathing vertebrates, yet they persist in the mouth, where, from being organs of a more general tactile function, they are specialized as organs of taste. The second category embraces a very important group of sense organs, the nerve-hillocks or neuromasts, which are found in all fishes and aquatic amphibians ; they differ in shape, as well as in the form of the sense-cells, from 8 NATURAL HISTORY OF VERTEBRATES. the above, and are often segmentally disposed. They are either free on the surface, as in the blind fishes, or retracted more or less within sacs, as in the sturgeon and eat- fish, or finally withdrawn from the epidermis entirely, and lying within canals in the corium which course through the head and along the lateral line of the body. Such neuromastic canals are present in fishes, with few exceptions; they are generally strengthened by bone round the place where the neuro-epithelial patch is situated, and, in fact, important scales and bones are developed with such organs as their centre of formation. Fig. 6 represents the three chief types of neuromasts met with in fishes. No such structures occur in the higher Vertebrata, unless the auditory organ is to be conceived as a very highly specialized group of pro- tected neuromasts. Apart from the similarity in the structures of the neuro- epithelium, other theoretical considera- tions favor this view. It has been in- ep n v . . - . Fic. 6.— Diagrammatic section of the skin of a fish; dicated that the skin of air-breathing c, corium ; cl, clavate cells ; eb, end-bud, or goblet organ ; rey 5 ah wT J, free neuromast ; m, mucous cell; n, nerve; p, pigment ver tebr ates differ s m many respects ec ia raaaal neuromast ; sc, mucous canal; v,vein;ep, from that of aquatic forms. Very early that layer of the epidermis which cor- responds to the most superficial stratum in the aquatic forms is thrust off, and underneath it one or more layers of cells dry up, flatten, and undergo a chemical change, which process results in the differentiation of two strata of epidermal cells, the superficial horny and deep mucous strata. The flattened scale-like cells of the former are constantly being rubbed off (or exuviated in greater masses, as in the snake’s slough) and replaced by the cells of the lower layer, which retain the power of dividing. It is obvious that the formation of this horny layer is in direct response to the change of medium in which the animal lives. Associated also therewith is the disappearance of the delicate neuro-epithelial strue- tures discussed above. Although the nerves may still be traced into the cells of the mucous layer, yet specialized end-organs are now found only in the corium, where they occur in very various grades of evolution. In all, however, the sheath of the entering nerve-fibre is very much thickened, while the individual cells of the altered sheath are disposed either longitudinally or transversely in the club-shaped or globular ‘tactile corpuscles.’ The lubrication of the surface effected by special cells in the lower forms is, in the Amphibia, brought about by the secretion of cutaneous glands. These are formed from the mucous layer of the epidermis, which may project inwards to some extent into the corium, and even receive a contractile sheath from it which regulates the discharge of the glandular secretion. They may occasionally take on special functions, as in the poison-glands of certain toads, ete. Such cutaneous glands are rare in the reptiles and birds, but they reappear in several distinct forms in mammals, where they associate themselves with very various functions. Thus the sweat glands of mammals are important organs of excretion, assisting the kidneys in removing waste material from the body. Their deep position in the corium, although formed from the mucous layer of the epidermis, is characteristic. VERTEBRATES. 9 Again, the sebaceous glands serve to lubricate the skin, and are closely connected with the hair follicles. Glands of this character are often largely developed in various groups of mammals, for the purpose of enabling individuals of a species to trace each other’s whereabouts. Finally, the milk glands themselves of mammals are developed in the same way as the sebaceous glands, and, in the lowest division — the monotremes — their arrangement is such as to suggest that structures that at one time had a very different function have gradually been diverted towards that of supplying nutriment to the young. ErmermMat APPENDAGES OF THE SKIN. With the change from an aquatic to a terrestrial life, which results, as above remarked, in the hardening of the exposed layer of epidermal cells with a horny layer, we find that certain additional protective or defensive structures are developed, composed of epidermal cells, which have undergone the same horny transformation, but which are often very specialized and characteristic of the different groups of the air- breathing vertebrates. These structures are always formed by the activity of the mucous layer of the epidermis, which generally retains the power of reproducing them when lost. Their presence always affects the underlying corium, and in many instances they may have important relations to underlying bones, whether these belong to the skin, as in the case of the horny tortoise-shell, which covers the bony carapax of the turtle; or to the internal skeleton, as is the case of the horns of oxen; or of the nails, claws, and hoofs of air-breathing vertebrates. But again, they may be confined to the skin, without entering into such relations, as is the case with most scales. Feathers and hairs, one of the structures referred to, belong to the category of epidermal appendages, as do also the horny pads or eallosities met with in places liable to pressure, throughout all the groups of air-breathing vertebrates. In many cases the cellular structures of these is readily demonstrable, but in others (scales, hairs, horns) the elements are so altered as to give a fibrillated appearance to the tissue. Throughout the chapters which treat of the air-breathing vertebrates, many references will be found as to the extreme variety of form which these protective and defensive corneous structures may assume, here it is very necessary to call attention to the similarity in their mode of development. Even structures so different as hair and feathers have, in this respect, many points in common; only that phase of the development of the feathers in which the papilla of the corium pushes out the over- lying epidermis beyond the free surface, is not found in the hair, where the first stage of development is an inward growth of the mucous layer of the epidermis, into which the papilla of the corium afterwards grows. SKELETAL STRUCTURES. Under this heading are embraced all the hard parts which serve for the support of the body, and likewise for the protection of internal organs. Frequently the skin is the seat of such skeletal structures; when this is the case the structures are termed collectively the exoskeleton ; the term endoskeleton is being reserved for those struc- tures which are developed in deeper layers of the mesoblast. EXOSKELETON. Structures of this category are certainly more frequently found in the lower than the higher Vertebrata. The scales of fishes are all of this character, however sing 10 NATURAL HISTORY OF VERTEBRATES. ular the modifications which they assume. The most primitive exoskeletal structures are the little teeth which beset the skin in sharks, constituting the ‘shagreen,’ derived from them; similar teeth occur in the skin of the sturgeon between the larger bony plates, so characteristic of that fish. The mode of formation of such teeth may be readily understood trom Figs. 7 and 8, which illustrates the development of such dermal teeth in one of the mailed siluroids. The seat of formation is obviously one of the papillze of the corium, but certain connective tissue cells clothing the papilla (od) have the property of secreting a substance which becomes impregnated with salts of lime, and simultaneously of leaving some of their own processes in the hard substance so formed. This is dentine, and the forming cells are termed odontoblasts. The surface of the Fics. 7 ands.— Embryonic and mature teeth in skin young tooth becomes coated with a cap we eicues tent Bjecement: bane juevelue of very hard enamel (e), which is secreted eo, enamel organ ; od, odontoblast ; p, papilla. from the lower columnar layer of epidermal cells, modified at this place into a so-called enamel organ. Eventually the tooth grows out beyond the level of the epidermis, the papilla remains as the pulp cavity of the tooth, and all trace of the epidermal cells are soon rubbed off from the enamel cap. Deeper down in the corium a sort of socket is formed for the tooth by the formation from the connective tissue of a smali plate of bone, so-called cement bone. These structures are of great interest and importance, because the bony plates of the stur- geon, already referred to, are simply formed by the fusion of such cement plates, and even the rhomboid scales of the gar-pike have a similar origin, although they have lost the asperities which characterize the plate of the sturgeon. The various forms of scales met with in the Teleosts are developed in the corium in a similar way, but in the ordinary ‘cycloid’ and ‘ ctenoid’ scales, there is never any enamel or dentine formed on their surface. We have noted above that sim- ilar bony plates may be also formed for the protection of the sense organs lodged in the corium, and it is necessary me 6 : —— —SF Ss . . . z - we yf der SSS to call attention specially to these two forms of dermal am — bones, as we shall find them later entering into such “wi Cc » important relationships with the endoskeleton, that we ae o~ aS shall have to treat of them in that connection. Few exo- PEE ES skeletal structures occur in the Amphibia. Crocodiles, on : GRY Zj On their first appearance in the em- | Y y 4h y y Uf o . . A iA 4 bryo of higher, as well as in the adults 44 y Va aM o o . “4 of lower, forms, these are simply rep- 7 y , ; y y . fT resented by a continuous unsegment- Y. yj yy y . . YO VA ed investing sheath of connective y y i : a A ae % JSG tissue, which has the potentiality of |4% 77 74- 1W444 developing into skeletal structures (the skeletogenous sheath) ; but very soon the segmental character becomes impressed upon this, and is, no doubt, traceable to the influence of the strains of the muscles on the sides of the sheath in locomotion. ne 3 $ FiG. 10.— Horizontal section of a young gar-pike on the level with the notochord (ne); mf, myotomes, separated by septa or myo- commata (mc) attached to the ends of the hemal arches (ha); v, corresponds to the centre of a vertebra, the anterior and posterior faces of which are indieated by a dotted outline. The notochord still is wider intervertebrally, iv. That the segmentation of the sheath and the muscles have a very intimate relation to each other may be seen from Fig. 10, which represents a horizontal slice through the notochord of a young gar-pike. The cartilaginous sheath, although continuous round the notochord, exhibits a distinct segmentation ; it pushes out on either side certain processes (Aq), the hemal arches, which run out between the muscle-segments, and are in the adult continued by the ribs, although at this stage these are not yet to be found in the membranous partitions (the myocommata) which are attached to the ends of the hemal arches, and separate the muscle- segments. Opposite the hamal arches the noto- chord is distinctly constricted, and this point corresponds to the middle of the vertebra of the adult, while in the place corresponding to the interval between two contiguous vertebre the notochord is considerably thicker. Figs. 11 and 12 serve to illustrate more fully the relation of the central skeleton to the notochord ; both rep- resent frontal sections through a young gar-pike, alittle behind the head ; but 12 is through a verte- bra, while 11 is through an intervertebral space. Above the notochord is the spinal cord, giving origin to a pair of spinal nerves, and separated from the surrounding muscles by an arch which, except a triangular block of usp Fic. 11,— Frontal seetion through intervertebral space of young gar-pike in the region of the pec- toral fins; nsp, neural spine ; sp, spinal cord ; m, muscles ; v, veins ; k, kidney ; ao, aorta ; ab, air-bladder ; 0, esophagus ; ¢, ccelom ; pe, peri- cardial cavity ; ht, heart ; pa, pectoral arch. 12 NATURAL HISTORY OF VERTEBRATES. cartilage forming its key, is merely membranous. Below the notochord is the princi- pal artery, the aorta, with the trunk veins and the kidneys on either side. These lie above and outside the coelom, a space which contains the viscera (in this region chiefly the air bladder and the esophagus), and a part of which is partitioned off as the peri- cardial cavity to accommodate the heart. The lateral walls of the ccelom are formed by muscles which attach themselves to the supporting cartilage of the fore fin, the ‘pectoral arch,’ Fig. 12 shows how the chief parts of the permanent vertebra arise. Two cartilaginous hemal arches (called so on ac- count of their relationship to the chief blood yes- sel) jut outwards and backwards, and two neural arches of the same tissue project upwards so as partly to wall in the spinal cord. But bone is beginning to appear chiefly in the membrane sur- rounding the cartilage (perichondrium) and is ex- tending also in the rest of the membranous wall round the spinal cord. In this way are formed the essential elements of a vertebra, viz. a central part surrounding the notochord, the body or cen- trum; two lateral parts, the neural arches, which serve to protect the spinal cord, and are aided in INGid — BatuoW ene GoneM OE aut this by a dorsal neural spine; and, finally, two gar-pike ; ha, cartilaginous hemal and (xa) heemal arches continuous with the ribs, which, in neural arches; the surrounding perichon- 5 drium is giving rise to bone continuous with their turn, help to strengthen the wall of the the osseous neural areh (na) developed in the 5 A i 3 A Fr aE CAE He ROLE ceelom, and may, 1 higher forms, unite In a breast-bone or sternum on the ventral surface of that cavity. Further elements of the vertebre are present in higher forms, serving for the purpose of securing union between the arches of contiguous vertebrie (articular processes), or as additional points of support for the ribs (transverse processes), or for the attachment of muscles. The edges of the centra likewise occa- sionally develop articular processes, but as a rule the connection between contiguous centra (except where they are fused together) is effected by ligaments. The extent to which the notochord persists varies greatly in different groups. Teleosts present little advance upon the stage of the gar-pike described above, the individual vertebrae being deeply hollowed on either face (amphiccelous), and retaining in the hollows a very considerable amount of notochord, while in the gar-pike itself a ball-and-socket joint forms between contiguous vertebrae, the sockets occupying the hinder faces (opisthoccelous vertebre). Little or none of the notochord is left in the interior of each vertebra, and this is likewise frequently the case in reptiles, where the socket often occupies the anterior face of the vertebra, which is then styled procelous. In most Mammalia a special provision is. made for the slow attainment of the adult length of the vertebral column, in that the faces of the vertebrae are fur- nished with ‘epiphyses,’ separate plates of bone, between which and the centra proper a zone of cartilage allows for additions to the length, until, when the adult size is attained, the epiphyses fuse with the centra and then form their faces. In the mammals also well-formed intervertebral discs of fibro-cartilage unite the faces of contiguous vertebrae, and contain some trace of the notochord, eyen in the adult condition. An important difference between the vertebral column of higher and lower forms VERTEBRATES. 13 is observable in the greater distinctness of the regions into which the column is divisi- ble in the higher forms. In a fish one distinguishes between trunk-vertebre and tail- vertebrae, the latter being in that part of the body behind the ceelom, while in most higher forms (Fig. @ 13) five regions are distinguishable. Thus cervical verte- bree are distinguish- ed from the succeed- j , . ji ‘ fis ing trunk vertebrze ip i Wi by the fact that their K=- LS Mit Lf f : PR ribs (w hen pr esent) FIG. 13.— Skeleton of boar ; c, cervical ; 7, dorsal; 7, lumbar ; s, sacral, and ¢, caudal do not unite ventral- vertebrae. lyin asternum. Again, certain sacral vertebrze are specialized by their ribs affording support to the pelvic girdle, which, in its turn, supports the hind limbs. Behind the sacral region is the caudal region; in front of it, the trunk, or dorso-lumbar region, composed of the dorsal and lumbar vertebrie, the latter differing from the former in haying no movable ribs. Throughout the vertebrates we meet with parts of the vertebral column, modified in connection with requirements of the most various character. The union with the head modifies the first (atlas) or first two vertebrie (atlas and axis). In some Teleosts certain anterior vertebre are altered in form to establish communication between the air-bladder and the ear; and in all, the end of the caudal region is modified in connec- tion with the caudal fin. Again, fusion of certain tracts is normal in the sacrum, and may be exceptionally present in other regions, as the tip of the tail in birds, the dorsal region in turtles, etc. Tue Riss anD STERNUM. The mode in which the ribs of the gar-pike are developed, continuously with the heemal arches, has been referred to above. Whether the ribs of the higher forms, which frequently unite in the ventral middle line to form a sternum, are homologous strue- tures, is not yet completely determined. That the second point of contact obtained with the neural arch of the vertebra is a new acquisition of the higher forms has been hinted at above. *In defining the regions of the vertebral column of the higher forms, it has been indicated that in these the ribs are very different in different regions. Cervical ric. 14.—Seetion of the thoracie region of the skele- ribs may be movable, but they are generally ee if SoH URI RE ee runrar spine? so short and so fused with the vertebra, that De enal abe ee tcangeaun eaten 2 eoticdias they merely form a ‘perforated’ transverse = P°°°* process. Dorsal ribs, on the other hand, as already mentioned, unite to form the sternum; this they do by their ventral ends fusing together into two cartilaginous 14 NATURAL HISTORY OF VERTEBRATES. strips or plates, which eventually unite with each other. Fig. 14, which represents a section of the thoracic region of a crocodile, shows the intimate relation that these structures have to each other, to which reference will af- terwards be made. The sternal and vertebral halves of the ribs (7 and s7) may be somewhat in- dependent of each other, and the latter may be locked together by overlapping uncinate processes. After the fusion of the cartilaginous strips into the sternum, the latter frequently becomes in the mammals secondarily segmented into bones to the intervals between which the ribs are attached, (Fig. 15). : The snakes are characterized by the absence ee an Med She erinnoaae of a sternum, which, like the absence of the SES eee ear aibea Siphotiear, limbs, is presumably secondary. A similar ex- Bisge: planation must be given for the absence of the sternal ends of the ribs in Amphibia, where the sternum itself is present and closely connected with the shoulder-girdle. THE SKULL. Just as the vertebral column is derived from the skeletogenous tissue surrounding the notochord, so the groundwork of the skull is derived from similar tissue, which lies on either side and in front of the cephalic end of the notochord. But the demands on this tissue are of a very different character in the cephalic end of the body. The central nervous system in this region swells out into the brain; for this a special protective capsule is required, which, of course, adapts itself in form to its contents. No segmentation like that in the vertebral column occurs in this chon- drocranium, which is further modified by protecting the auditory organ behind, the olfactory in front, and, to a less extent, the eye in the middle. These organs thus mark out regions in the chondrocranium (the olfactory, orbital, and auditory) while a fourth region, the occipital, is that which establishes the union with the vertebral column. Fig. 16 indicates how part of the skeletogenous tissue (p, the parachordal tracts) lies at the side of the notochord, while two other tracts (the trabecul) are entirely prechordal, and thus not comparable to the FiG. 16.— Head of embryo dog-fish; a, tissue around the notochord. The parachordal tracts auditory capsule ; c, trabecular cor- 3 3 nua; c/, visceral clefts ; b, branchie; soon coalesce with the auditory capsules, and tend to e, eye ; 7, infundibulum ; 0, olfactory 5 organ ; ”, notochord ; p, parachordal grow upwards round the brain, as do the trabecular ee eee tracts further forwards. The latter also contribute to the formation of the olfactory capsules, so that a cartilaginous box for the accom- modation of the brain results, the walls of which are, however, in most forms, deficient in certain places (fontanelles). It is this chondrocranium which furnishes the substratum in or on which the cranial bones are developed. Even in its notochor- VERTEBRATES. 15 dal region no evidences can be detected of a division into vertebral segments, unless, indeed, certain arches which are attached to its ventral face are to be so regarded. To understand the nature of these, it is necessary to anticipate here the description of a feature of the alimentary canal, which is common to all vertebrates. In the aquatic vertebrates the anterior part of the alimentary canal communicates with the outside by a series of gill-clefts on either side, through which the water streams for respiratory purposes. Although these gill-clefts entirely disappear, with the exception of the first, in the adults of air-breathing vertebrates, yet they are present in the embryos of all, even the highest. Between the clefts (also known as visceral clefts) are left certain solid pillars, the visceral folds, and these are strengthened by cartilagi- nous supports, the visceral arches, which are only developed to a small extent in the adults of air-breathing forms, but attain the maximum of their development in aquatic species, as the gill or branchial skeleton (Fig. 17.) Many anatomists consider these arches to be homodynamous with the hzmal arches and ribs further back, and thus to represent the hzemal arches of the notochordal part of the skull, but this must still be con- sidered as not sufficiently made out; for the side walls of the head, in which they make their appearance, do not participate in the segmentation, which we shall afterwards see is observable in certain organs of the upper part of the head, although not in the chon- drocranium. The greater part of this visceral orbranch- — | ial skeleton may be conveniently discussed Seeman ey Auesaeeey ae 1a ‘ arches ; ¢, ceratohyal ; f, superior fontanelle ; g, apart from the skull, but the whole of the first — glossohyal;_#, hypohyal; im, hyomandibulat arch, and the upper part of the second arch, smeckeliag Uaciikce nee notgetinnte tae rene Pen 5 foramen ; p, parachordal cartilage ; pp, palato- become so intimately related to the skull that pterygoid: 9, quadrate; s, supra-orbital ridge ; it is necessary to take them into consideration 5 rts a ite, -tegiienicesne along with it. Indeed, an actual fusion gen- erally takes place between the upper (palato-quadrate) part of the first areh, which forms the upper jaw, and the chondrocranium, while the lower, or Meckelian, part (the lower jaw) is hinged upon the upper part (Fig. 17.) That we have also to take into consideration the upper part of the second arch (hyomandibulo-symplectic tract, hm, sy) is due to the fact that it loses to a great extent its union with the lower part, and takes on in many forms the function of the suspension of the lower jaw, being on this account styled the ‘suspensorium.’? But in higher forms this same tract loses that function and becomes subservient to the organ of hearing in a way to be afterwards described. The stage of development of the salmon’s skull represented in Fig. 17 affords a good idea of the cartilaginous substratum in and on which the future osseous struc- ture is erected. To understand how this is effected, it is necessary to recall some points which have been referred to above, as to the development of bones in the skin. The sturgeon’s skull is completely cartilaginous, but is covered on the outside by a series of bony plates formed in the skin, chiefly by the fusion of the cement plates of originally separate teeth; these are cement bones, and they afterwards play avery important part in filling up gaps or fontanelles in the chondrocranium, and in strengthening it from the outside. Cement bones are not confined to the free surface 16 NATURAL HISTORY OF VERTEBRATES. of the head; they likewise occur in the cavity of the mouth, where the teeth more usually persist than on the free surface of the head. It is to be noted that, just as in the case of the scales of many fishes, the cement plates may remain, although the teeth (their raison d’étre) have disappeared, so that cement bones are found on the skull and in the cavity of the mouth, with- out a trace of teeth. Such cement bones are also spoken of as dermal bones, or investing bones, from their relation to the skin, or to the underlying skull respectively ; but it is also usual to speak of another category of bones as dermal, which have an entirely different origin from the above, and which, indeed, are de- veloped forthe protection of the neuromastic canals referred to before. Such neuromastice bones are not confined to the aquatic vertebrates, but retain their relationship to the skull in air-breathing forms, although their original function has disappeared. The cement bones and neuromastic bones are something superadded to the cartilaginous skull, and we shall find that they are not confined tothe skull proper, but extend to those modified visceral arches which, roughly speaking, give rise to the jaws. But certain bones originate in closer relationship to the cartilage; they may first appear in the centre of cer- tain cartilaginous tracts, and gradually replace the cartilage centrifugally, or they may appear in the membrane which clothes the cartilage (the perichondrium), and, eating into it, as it were, replace it centripetally. These are generally spoken of as‘ cartilage bones,’ as distinguished from the ‘membrane’ bones of the foregoing paragraph ; but it is impossible to draw a very sharp distinction between them, for a bone which originates round a neuromastic canal, for example, may eat likewise centripetally into the underlying cartilage. Again it may be impossible, except in the cavity of the mouth, to distinguish between the limits of neuromastic and cement bones; for two bones originating in these two different manners may fuse. It will therefore be sufficient to indicate the thief places where the chondrocranium and the cartilages of the jaws are replaced by Fic. 18.— Head of sturgeon, showing the membrane bones evvering the (dotted) chondro- cranium. the ‘cartilage’ bones, and where they are invested on the outside by ‘membrane’ bones. It has been remarked above that a demarcation of the skull into regions is effected by the sense organs, but these regions are not ‘segments.’ So also the points of escape of the cranial nerves give valuable landmarks in the study of the skull, but neither do these mark off segments; for although many of them, as we shall see, are comparable to spinal nerves, yet they have lost the primitive segmental arrangement which the spinal nerves have retained. In discussing the bony framework of the skull, it will be convenient to take the brain-box first, and the jaws afterwards. The occipital region generally exhibits four bones surrounding the foramen magnum or aperture through which the brain commu- nicates with the spinal cord. These are the basioccipital below, the supraoccipital above, and the two exoccipitals, one at either side (Fig. 19), but the latter may alone appear in this region, as in the frog, the rest of the region remaining cartilaginous. The bones of the auditory region are more numerous ; their limits are generally detectable VERTEBRATES. 17 in lower forms, but in the adults of higher forms they fuse on each side into one mass, the petrosal bone, as do the four occipital elements into the occipital bone. The three most commonly occurring elements are the prootic in the fore-part of the ear-capsule, the epiotic above, and the opisthotic behind, but certain additional elements are found forming part of the wall of the ear-capsule as well as the socket for the suspensorium in Teleosts, the sphenotie (post frontal) and pterotic. These bones may be partly dermal in their origin; the latter being intimately connected with a dermal bone, the squamosal. In the cartilage of the base of the skull, between and in front of the auditory capsules, one or two bones appear, con- tinuing forward the basioccipital ; these are called the basi- and pre-sphenoids. In the same relation to these that the yg. 19 pase of eranium of exoccipitals have to the basioccipital are the ali- and orbito- 4" embryo ehick ; ¢, condyle; s : is ; $ €0, exoccipital; r, frontal; hsy sphenoids, which form part of the lateral wall of the cranium comiahonlne rece in the orbital region. But there is no ossification in the tal: s. sphenotie process; so, : , supraoccipital; sq,squamosal. root of the chondrocranium, here comparable to the supra- occipital, and, indeed, in many forms the basal elements persist only as cartilage. In many forms, as birds, the cranial cavity is very much compressed from side to side in the orbital region. The result is a mere thin partition between the orbits — the interorbital septum — in which the sphenoidal elements are to be sought. But the olfactory region is either entirely in front of the cranial cavity, or two tubular prolonga- tions of the latter convey forwards the olfactory tracts to the neuro-epitheliam to which they are destined. A nasal septum, derived from the coalesced trabecule, separates the olfactory organs of the two sides, and is continuous with lateral plates protecting the organs. If ossifications appear in these septal and. lateral cartilages, they are termed mesethmoid, and lateral ethmoids. When the surface on which the nasal mucous mem- brane is distributed becomes very complex, as it does in mammals, special ingrowths of the olfactory capsule ossify as turbinal bones. Related to the roof of the chondrocranium are mem- brane bones which, when the cartilaginous box becomes insuflicient in size to accom- modate the brain, fill up the gaps, and become thus di- rectly related to the brain. Fra. 20.—Skull of young sage-cock (Centrocercus); an, angular ; ar, articu- sy . lar ; as, alisphenoid ; ¢, dentary; e, exoccipital ; 7, frontal ; im, premax- These are known as parietals, illary; 7, jugal ; me, mesethmoid ; ma, maxillary; 7, nasal; 7p, parietal ; n pl, palatine; pf, pterygoid; g, quadrate; gj, quadrato-jugal; s, supra- squamosals, frontals, prLe=- occipital; san, surangular ; sg, squamosal, frontals, and nasals (Fig. 20). Similarly related to the ventral face of the chondrocranium are the yomer in front, and parasphenoid behind, retaining in many instances the teeth to which they owe their origin. All of these membrane bones persist in the higher classes of vertebrates, some of them, such as the parasphenoid, losing, while others, like the frontals and parietals, gain in importance. But we may observe in the Teleosts many membrane bones VOL. Ill. —2 18 NATURAL HISTORY OF VERTEBRATES. which disappear with the loss of the system of neuromastic canals. Such are, e. g., the bones which surround the eye in most forms, and serve to protect the infraorbital branch of the neuromastic canal system. It is now time to devote some attention to the cartilages of the jaws and their products. The first arch early shows a division into two parts, the upper palato-quad- rate, the lower mandibular. Between the two, the quadrato-mandibular articulation is formed. The cartilages immediately entering into this joint are usually ossified as quadrate above and articular below, but the fate of the remainder of the cartilages is very different in different groups. As many as five different ossifications in addition to the articular may unite to form what we call the mandible or lower jaw. Some of these may be cartilage bones, some membrane, and the greater part of the cartilage may persist in some lower forms as the ‘Meckelian’ cartilage surrounded by these membrane bones. Of these the chief is certainly the dentary, the origin of which from teeth is at once apparent, while others again, such as the coronoid and angular, are developed in response to muscular strains. It is only in the fishes that the upper part of this arch is so independent of the skull (Fig.21). Generally the elements deyel- oped from it are connected with the proper cranial bones so intimately that they con- tribute largely to the formation of the floor and side walls of the skull. In addition to the quadrate, certain cartilage elements known as pterygoids and palatines appear in this arch, and other membrane bones, chiefly of the category of cement bones, are developed upon it. The latter are known as premaxillary, maxillary, jugal, and quadratojugal. But the distinction between membrane and cartilage bones is here again difficult to maintain, for a palatine may be in part of cartilaginous origin and in part a cement bone. The arrangement of the bones developed from the palato-quadrate arch is highly characteristic of the different groups of vertebrates. Frequently two distinct rows of bones are formed leading from the maxillary to the quadrate (Fig. 20), the outer row including the jugal and quadrato-jugal, while the inner row is formed of the palatine and pterygoid. Again, a transpalatine may connect the two rows in many reptiles. The reason of the greater development of the palatine and pterygoid bones, in many higher forms, such as turtles and crocodiles, is to be sought for in the separation of a nasal cavity from the primitive mouth cavity. Such a compartment of the mouth cavity does not exist in fishes; its formation is brought about by the devel- opment of a partition, the palate, which shuts off a greater or less portion of the mouth cavity as the nasal cavity. This partition may in part be destitute of an osseous framework. It is then called the soft palate ; while the hard palate is formed by shelves developed from the premaxillary and maxillary bones, with the assistance of the palatines and pterygoids. The latter in mammals rarely contribute much to the formation of the hard palate, being generally attached to the sphenoid bone as pterygoid processes. It is obvious from the above sketch, that many of the peculiarities of the skulls in the various groups of vertebrates are traceable to the differing modifications of the palato-quadrate arch. This is true of the relationship of the quadrate to the neigh- boring bones. Sometimes it is very independent of the rest of the skull, as in snakes and birds (Fig. 22); sometimes it is wedged firmly in with the neighboring bones, as in turtles and crocodiles. In all Amphibia, Reptilia, and Aves, the suspension of the lower jaw is not effected, as in fishes, by the aid of the following arch, but the quadrate alone articulates with the mandible, We shall consider presently the nature of the articulation of the lower jaw in mammals. ‘soutds jwanou ‘0 *sXvi-uy peur ‘s ‘soutds [vanadida ‘/ *BI10B 19 IO} Yow UB UNMOF O} pagtun uolsat [epneo jo sossaooid Juma ‘soutds [Rmemiaqut "BAIQALIGA ‘p *sABI-UYy [B1]UGA “AT *RINOIAR[O- *sAvi-uy *ourds ‘yored & JO UOJATaTS ‘oroatduids ‘Te ‘unuotedoaid ‘og “umntnoatado “Sz *[usuul ‘0% *STPUIG.1OQNs ‘6T “£IV|[IXVUL ‘ST “prow *jesomunbs ‘ep ‘orjosoqd ‘ZT *eqoried ‘TT *Teqdro00xe ‘oT “orjorsido ‘5 *yeqidrooonsdns *g ‘orqooad *) *ploueydsrand “9 *[equo1j-4sod “fp *plouiqye ‘ge “requosjoid ‘7 *yeqaory *T VERTEBRATES. 19 \ . It has been mentioned above that the upper part of the second or hyoid arch possesses the function in many fishes of supporting partly or entirely the man- dible. Such skulls have been called hyostylic, in contradistinction to those auto- stylic skulls, where the quadrate alone performs this duty. The hyoid suspensorium, when present as such, may be a single stout bar, as in the sharks, or may be divided into two pieces, an upper hyo- mandibular and lower symplectic, which ossify sep- arately. Connected with the interval these is the lower part of the hyoid arch, by a bony or cartilaginous bar, the interhyal (Fig. 17). In those fishes where a gill-cover is developed (Ganoids and Teleosts) the skeleton of the gill- cover is very intimately related to the hyoman- dibular. Generally four bones are present in it, the preopercular, opercular, subopercular, and the interopercular; the first of these is developed round a neuromastic canal, while the others ap- pear to be similar to the branchiostegal rays which support the membranous fold developed from the lower part of the hyoid arch, and assist- ing in the protection of the gills (Fig. 21). The gill-coyer is absent in the Amphibia and higher classes, and the mandible is suspended by the quadrate, but we have still to look for the homologue of the hyoid suspensorium, which is now to be found as a chain of bones more or less numerous, now subservient to hearing, and effecting communication between the membrane of the drum of the ear and the internal ear. This chain is the columella auris, which exhib- Some doubt is entertained by morphologists as to the homology of the chain of bones which perform the same function in mammals, and this doubt extends to the nature of the articula- tion of the lower jaw in that group. between dentary; eo, exoceipital; /, frontal; p, parietal; p/, palatine ; pm, premaxillary; pr, prootic ; pt, pterygoid; g, quadrate; sa, surangular ; sh, stylohyoid ; so, supraoccip- ital; sq, squamosal. its much difference in its form in the amphibians, reptiles, and birds. Unlike the lower classes, where the mandible is articulated with an independent quad- ‘ rate, the mammals have the socket for the lower jaw on a bone of very complex nature, the ‘temporal’ bone. This bone not only contains all the bones of the auditory capsule, but also a tympanic developed in connection with the drum of the ear ; the squamosal, which is separate in the lower forms, and a ‘zygomatic’ process of that part the root of which bears the glenoid socket for the mandible (Fig. 23). Fic. 23.— Auditory region It has recently been : or b of human feetus; 7, ineus; asserted that the malar bone of mammals is likewise of complex nature, representing the postfrontal, jugal, and quadrato-jugal of reptiles; further that the zygomatic process of the squamosal with which it articulates is nothing else than the quadrate of the lower classes; if so, the articulation of the lower jaw is the same in mammals as in lower forms, and this view derives support, according to Professor Cope, from the condition of morphous reptiles. /, lower jaw; m, processus gracilis of the malleus joining the Meckelian cartilage of the lower jaw; m/, malleus; s, stapes, within the tym- panic ring; 2, zygomatic arch of temporal bone, bearing the glenoid fossa. affairs in the thero- On the other hand it has generally been believed that the quadrate is represented in mammals by the malleus, one of the chain of bones in the 20 NATURAL HISTORY OF VERTEBRATES. drum of the ear, and that the other bones in the chain, the incus, orbiculare, and stapes, represent the columella auris of the reptiles. Although this is still a moot point in morphology, the former account is certainly that which offers fewest difficul- ties to the evolutionist. Such complex bones as are referred to above are no excep- tion in the higher vertebrates. The tendency to the fusion of originally separate elements is very marked in birds, where, indeed, all the walls of the cranial cavity early form a continuous mass in which it is impossible to detect the separate elements. Only the bones of the face retain a certain amount of independence. Again in mam- mals we frequently find all of the sphenoidal elements united into a sphenoid bone, just as the separate occipital elements, together with the interparietal, form the com- plex occipital bone. The greatest contrast is to be observed within the different groups, as to the relative size of that part of the skull which comes into relation to the brain (the cranium), and the part which lies in front of it — the face. This is perhaps as well seen in the Mammalia as elsewhere, owing chiefly to the enormous development of the turbinal surfaces clothed by the nasal mucous membrane, and the consequent large size of the olfactory region of the skull. In the above account of the vertebrate skull, the object has been to indicate the nature and arrangement of its component elements. The peculiarities of each group will be discussed under the special description of each. Tue ViIscERAL SKELETON. This term is applied to the series of arches which intervene between the gill-clefts. The first cleft is styled hyomandibular, because it lies between the hyoid areh behind, and the palato-mandibular arch in front. The second cleft is the first branchial, and it separates the hyoid arch from the first branch- ial arch. We have already considered the whole of the first arch and the upper part of the second; there remain for treatment the lower part of the second and the various branchial arches proper. It will be serve at visceral arc +, 244.—Skull and visceral arches of a dog-fish ; a, auditory capsule ; bb, observed that visceral arc h asibranchial ; ¢, ceratohyal ; eb, ceratobranchial ; e, epibranchial ; eb, is a term applicable to all extrabranchial ; Jan, hyomandibular : m, lower jaw; mp, metapterygoid i . ligament; 7, nasal capsule ; q, pterygo-quadrate areade ; pb, pharyngo- of these structures. while branchial; pn, prenasal cartilage ; s, supraorbital ridge; sp, spiracle ; s . ”, . t, pelato-ieanenuls ligament; v, trigeminal foramen ; 1, 2, 3, 4, 5, labial branchial arch is confined cartilages. to the third and succeeding visceral arches, even although in some fishes both mandibular and hyoid arch, in spite of their specialization, have not entirely lost their gill-bearing function. It is to the aquatic vertebrates that we mast look for the full development of these structures. With the loss of gills in air-breathing forms, all of the branchial arches disappear except the first, and this generally is much reduced in size. The lower portion of the hyoid arch is generally more subdivided in fishes than the corresponding segments of the succeeding arches, but a fundamental similarity is nevertheless observable. Thus certain basal elements or copule unite the ventral VERTEBRATES. 24 ends of the visceral arches; the first of these is the basihyal, while the succeeding are styled basibranchials. In most forms the basibyal attains greater importance from its supporting the tongue, into which it sends forwards an entoglossal or glossohyal pro- cess. It is to this relationship to the tongue, that the hyoid arch owes its name, and its persistence throughout the higher forms. The lower part of the arch retains its connection with the upper part, in fishes, by means of an interhyal piece, between which and the basibyal are gen- erally found epiceratohyal, cera. #1625, Tower jay and hyoid arch of salmon, an: angular ar, tohyal, and hypobyal pieces (Fig. J0b¥Alj ddehtary; c epicerciontal 9, glosobyal; 4, hypéayal 25). In the higher forms, where the upper part of the arch has become subservient to the auditory organ, the lower part is attached to the auditory region of the skull by a tym- panohyal, between which and the basihyal the following ossifi- é cations may occur: stylohyal, epihyal, ceratohyal; but some or all of these may remain unossified. When present they are .known as the anterior cornua of the hyoid bone (Fig. 26). The posterior cornua of the hyoid are the remains of the first branchial arches, and may enter into important relations with the larynx. The basibranchials are rarely independently represented, ex- cept in fishes, where, however, they are frequently coalesced. In the arches themselves, the following separate pieces are gen- erally present (Fig. 24): hypobranchials, ceratobranchial, epi- branchials and pharyngobranchials, the latter being loosely at- Fic. 26.—Hyoid appara- - e tus of common fowl; a, tached to the ventral face of the skull, and generally carrying eceratobranchial; ¢, first z = = yasibranchial; e, cera- superior pharyngeal cement bones on them, which are covered tohyal ; h, second basi- : i x dranchial ; p, epibran- with teeth. In most bony fishes the fifth arch supports no gill, chil. ; but bears on the surface, turned towards the mouth, considerable dentigerous cement plates — the inferior pharyngeal bones. THe SKELETON OF THE Linps. Two systems of locomotor appendages are distinguished in the vertebrates —a median unpaired system confined to the fishes and aquatic amphibians, and the two paired anterior and posterior appendages, typical of the whole branch. In certain primitive forms the median unpaired fin may be an almost continuous fold occupying the dorsal and ventral middle line, but the tendency is towards its restriction to certain tracts which will be treated of in detail in the special description of the fishes. In this class alone is the fold supported by fin-rays; these in their turn rest on bones, which are either wedged in between the spines of the vertebral column (hence inter- spinous bones), or are continuous with these and thus answer to the segmentation of the body. They may be present although the corresponding tract of unpaired fin has disappeared. According to the most prevalent theory the paired fins have become developed by 22, NATURAL HISTORY OF VERTEBRATES. similar restriction of primitively continuous right and left lateral folds to two tracts, an anterior and posterior on either side. hese tracts, which are in their most primi- tive condition in certain sharks, have given rise to the paired fins of other fishes, and to the anterior and posterior extremities of the higher vertebrates, Like the unpaired fins they are supported by fin-rays (actinosts) and supporting bones (actinophores), but the latter do not enter into direct union with the vertebral column, although it is prob- able that they haye a metameric significance here as well as there. The actino- phores of both anterior and posterior fins acquire independent supports, which in the-sharks are merely two unsegmented arches of cartilage whose conyexities lie on the ventral surface of the body, but which give rise to the pectoral and pelvic arches of higher vertebrates. It is comparatively easy to understand how the skeleton of the anterior and pos- terior fins of a shark-like form might be converted into that of a bony fish, but it is somewhat more difficult to understand how the limb skeleton of the higher vertebrate is comparable therewith. Nevertheless the condition in the shark forms the best start- ing point for the discussion of the paired limbs of the vertebrate branch. What this is may be seen from Fig. 27. The cartilaginous pectoral arch is composed of a right and left half inti- mately united in the ventral middle line. Half way up, three basal cartilages are attached to it on either side, supporting the cartilaginous rays, which again run out into the finer thread-like rays of thefin. The basal cartilages are known as pro-, meso-, and meta-pterygium, but it is to be noted that the propterygium is seconda- Fic. 27.—Peetoral girdle of a shark (Scyllium); rily dorsal, not anterior, while the metaptery- er, coracoid ; ms, mesopterygium; mf, me- , 2 gee, Las id ey eek sea P, protopterygium ; 7, radial car- au 1S ventral, not posterior op Coons This implies a rotation of the anterior edge of the fin through an angle of 90°. Most of the rays are attached to the metapterygium, very few being on its ventral (primitively posterior) edge. The pro- and meso-ptery- gium may be regarded as rays which have lost their attachment to the metaptery- gium, and have acquired new importance by attaching themselves directly to the pectoral arch, while the metapterygium forms the principal axis, giving origin to rays which are chiefly attached to its anterior border. The posterior fins of the sharks are essentially constructed on the same type as the anterior, although they are decidedly less altered from the primitive condition. In respect of the structure of the limbs as well as in other anatomical features, the sturgeons are intermediate between the sharks and the bony fishes. The latter are characterized by the reduction of the parts corresponding to the above described car- tilages, which results from superficial bones developed in the skin, having largely usurped the function of these. This remark is especially applicable to the pectoral arch, the investing bones of which (developed in part round that portion of the neuro- mastic tract which ascends from the lateral line to the head) constitute the great bulk of the shoulder-girdle in these forms. It is very much otherwise in the higher vertebrates. In these the primary cartila- ginous structures persist as the pectoral and pelvie girdles, which we may now shortly consider in the turtles, whose limbs retain many primitive features, as a convenient VERTEBRATES. 23 starting point. In all of the higher forms we no longer meet with an undivided arch, but the point of attachment of the limb (glenoid fossa on the pectoral, acetabulum on the pelvic girdle) divides the cartilage into upper and lower regions (Figs. 28, 29). ‘The upper is styled scapula in the pectoral, ilium in the pelvic girdle. While the dorsal end of the scapula is uncon- nected with the vertebral column, the ilium acquires such a connection by the intervention of one or more pairs of sac- ral ribs. Although the number of sacral vertebre so formed is primarily small, the ilium may coalesce with vertebra in front of and behind them, in such a way as to secure a great amount of solidity in this region, as is the case with the birds. The region of cartilage below the attach- ment of the limb generally ossifies in two pieces, an anterior and_ posterior, between which in all forms, except Am- Fre. 28.—Half of pectoral girdle of turtle; c, coracoid; cl, ches clavicle ; e, epicoracoid ; y, glenoid fossa; s seapula, phibia, a fenestra occurs. In the pec- ; toral girdle, the bone in front of the fenestra is termed precoracoid or clavicle, that behind it, the coracoid; and these elements correspond respectively to the pubis and ischium of the pelvic girdle, which are similarly related to the fenestra there. Con- necting the elements of either side in the middle line, we find in many forms strips of cartilage (epicoracoid, sym- physial) which may project in front into cartilaginous or osseous processes, styled respectively episternum and epipubis. The term episternum is suggested by the fact that in some Amphibia as well as in other reptiles and in birds, although not turtles, the coracoids acquire a secondary connec- tion with the sternum, which persists in the monotremes, the mammals which are nearest the reptiles in their organization. The condition of these parts in the other reptiles and Amphibia will be readily understood from the foregoing description, but some reference is nec- F : ' 1 ian essary to the modifications characteris- Siar Pai eaniena ps eel prone me ; Santee 2 ae Mee birda andiomenmale. sacral ribs ; sv, sacral vertebrae. The pectoral arch in birds is characterized by the stoutness of the coracoid element, and the slenderness of the clavicular. The latter is developed partly in membrane, and the clavicles of opposite sides meet in the ventral, middle line, 24 NATURAL HISTORY OF VERTEBRATES. forming the so-called fureula, ‘wish-bone,’ or ‘merry-thought.’ This is generally bound to the crest of the sternum, a part of the bone which is largely episternal in its nature. In mammals, on the other hand, the coracoid is, except in the monotremes, merely a hook-like process of the scapula; the latter usually develops a ridge or ‘spine’ for muscular attach- ment, terminating in an acromion process, to which the dorsal end of the clavicle is attached, while the ventral end of each clavicle is attached to the sternum by means of the intervention of more or less important remnants of the episternum (Fig. =D 15). Near these, remnants of the epicoracoid also Fic, 30.— Pectoral girdle of embryo bird; persist, especially in the monotremes where the j cl, clavicle ; er, coracoid; gl, glenoid fossa; : : A s, 8temnal ribs; sc, scapula ; ae ceermnrae », episternum is of large size, and forms a T-shaped beat ‘interclavicular’ support for the clavicles. We have referred above to one of the chief peculiarities of the pelvis in birds, the forward and backward extension of the ilia and their fusion with the long sacrum. The ventral region also offers many peculiarities, the chief of these being the backward direction of the pubis, and the wide separation of the ischia. There is consequently no symphysis or epipubis in these forms. Certain fossil Dinosauria have a double pubis, the anterior branch of which appears to be comparable to the pubis of other reptiles, while the pos- terior branch is homologous with the pubis of birds, in which the anterior branch is merely represented by a ‘pectineal’ process of the pubis. It is fairly certain that the pubis in mammals is likewise comparable to the posterior and not to the anterior branch of the dinosaur pubis, and it is possible that the so-called marsupial bones, well devel- oped in monotremes and marsupials, and present rudimentarily in some higher mam- mals, may be comparable to the anterior branches, and not to the epipubis of other reptiles. In adult mammals the three elements of the pelvis unite to form an innominate bone, both the ventral elements of which unite in a symphysis in the lower forms, while in the higher forms the pubes alone do so. If the anterior and posterior extremities of a snapping turtle be compared, many points of agreement will be readily detected. Each is divisible into three regions, supported by similar bones. Thus in the proximal region we have the humerus of the anterior extremity represented by the fe- r wlca* z Ly 1 eS (ee mur of the posterior; ser i he middle region aes In th e or ~ ST the radius and ulna of a the one represented me 1" by the tibia and fibula of the other; and in the FIG. 31.— Fore limb of turtle; ec, centralia; ca 1—5, carpals; h, humerus ; #, : > intermedium ; mec 1—5, metacarpals; 7, radius; 7! radiale; w, ulna; u/—"', distal region the skele- ulnare ; J—V/J, digits, the sixth imperfect. 5 5 ton of the ‘ manus,’ or hand, containing similar bones to that of the ‘pes’ or foot. All of the vertebrates except the fishes are characterized by similarly arranged limb skeletons. A close inspection of the manus discloses two rows of small bones succeeding the radius and ulna. These are the proximal and distal carpals, of which nine or ten at VERTEBRATES. 25 most are present throughout the vertebrate series, but generally a less number, by rea- son either of the fusion of separate elements, or of the loss of such. In the proximal row we find first an ‘intermedium’ wedged in between the radius and ulna, then a ‘radiale’” and an ‘ulnare’ corresponding to the ends of these bones, and separated by one or two ‘centralia” In the next row are five distal carpals, numbered 1 to 5, from the radial to the ulnar side, and corresponding to the separate digits. The skeleton of each of these is formed of a metacarpal followed by a row of phalanges. The nomenclature of the corresponding parts in the posterior extremity differs but little from the above. Here also we have an intermedium, then a tibiale, fibulare, one or two centralia, and finally five distal tarsals, followed by the metatarsals and pha- langes of the toes. From the Amphibia upwards to the Mammalia the structure of the limb skeleton forms one of the most interesting chapters in comparative anatomy. It is impossible within the bounds of the present introduction even to outline the remarkable modifications to which it is subject. Suffice it to say that these are always in direct response to the nature of the use to which the limb is put, and consist either in the great development of certain elements in comparison with others, in the fusion of primitively separate elements, in the loss of such, or in all three combined. Instead of the terms described above for the carpals and tarsals, others derived from human anatomy are frequently employed, especially in the group of the Mam- malia. The subjoined table exhibits how these correspond. : ANTERIOR EXTREMITY. POSTERIOR EXTREMITY. Radiale Scaphoid Tibiale Sesamoid Intermedium Semilunar Intermedium Astragalus Ulnare- Cuneiform Fibulare Caleaneum Centrale Central Centrale Navicular Carpale 1 Trapezium Tarsale 1 Cuneiform 1 ie oe Trapezoid ae 2 on re Magnum Som aS “ 3 ad 4 “ec 4 qe Unciform oe os Cuboid v « Those familiar with human anatomy will observe an apparently additional bone in the carpus (the central); one in the tarsus (the sesamoid), and one deficient in the carpus (the pisiform). The nature of the last will be presently referred to; as to the central, although independent in the adults of many mammals, it is only separate in the human feetus, being afterwards, except in rare instances, absorbed in the scaphoid. The sesa- moid is an independent bone in some mammals, but is generally absorbed in the navicular, which accordingly is usually equivalent to the centrale + tibiale. A different interest attaches to the pisiform bone; it is to be regarded as a rudi- ment of a sixth digit on the ulnar side of the manus, and is represented also in most turtles as well as on the corresponding border of the pes in some tailed amphibians. Of rarer occurrence is a rudiment of a sixth digit on the tibial side of the pes, but the monotremes, as well as certain rodents, possess one. If the dotted lines in Fig. 31 be now studied, it will be apparent that it is possible to represent the bones of the manus or pes as a principal axis, like the metapterygium of the selachian fin, giving rise to a series of radially arranged pieces. The axis is here represented as passing through the humerus, ulna, intermedium, carpale,! and the radial digit (pollex or thumb). There is abundant evidence to show that the radial border of the limb of the higher vertebrate is comparable to the metapterygial border of the selachian fin, but there is also evidence in the development of the tailed am- 26 NATURAL HISTORY OF VERTEBRATES. phibians to show that the radius itself is not part of the principal axis, but a secondary axis developed beside it in response, perhaps, to terrestrial locomotion. THE MUSCULAR SYSTEM. Muscular tissue is characterized by the property of contractility, which may be evoked by a stimulus applied directly to the muscular fibres, or conveyed to them through the channel of nerve fibres. In the latter case the stimulus may be trans- mitted consciously by the will, or it may be entirely involuntary. A large proportion of the muscular tissue which is not directly under the control of the will differs from that which is, in being histologically simpler, and in replying less quickly to a stimu- lus. A rough distinction of voluntary and involuntary muscles is thus made, the latter being exemplified by the minute muscles in the skin which erect the hairs, etc., and by the greater masses of tissue which effect the propulsion of the food along the alimentary canal, and of the blood through the vessels. The muscles which we discuss under this heading, however, are those that are con- nected directly or indirectly with skeletal parts, and which bring about change of relative position of these. They constitute the flesh or musculature of the body, and are of course chiefly employed in locomotion and prehension. It is only comparatively recently that much attention has been given to this branch of anatomy — Myology, as it is called — but it is obvious, from what has been said above, that the most intimate connection exists between this study and the study of the bones to which the muscles are attached, as well as of the motor nerves which end in the muscles. Reference has already been made to the fact that the muscular system partakes in the segmentation of the body. This can be seen in its most unaltered condition in a fish (Fig. 10), where the flesh is divided up into muscle-flakes or segments (myotomes) separated by membranous partitions or septa (myocommata). When the latter are dissolved by boiling, the flesh readily separates into the flakes, the surfaces of which are often much curved, so that several different flakes appear in the same frontal plane (Fig. 11). In addition to these flakes which constitute the musculature of the trunk, there are other muscles also segmentally arranged in connection with the unpaired fins, but it is not so easy to detect a segmental arrangement in the muscles which perform the more complicated movements of the jaws, the gill-skeleton, and the paired fins. With the change to terrestrial life, the paired limbs, taking on the function of the support of the body, become more complicated in their skeletal parts, and conse- quently also in the muscles destined for their movements. How multifarious these movements are can only be realized by reflecting on the extremely different uses to which the paired limbs are put, in prehension and locomotion. The various kinds of the latter, whether terrestrial, arboreal, aérial, or aquatic, are accompanied by cor- responding modifications of the musculature. It is possible, nevertheless, to detect homologies by careful study of the nerve supply, as well as of the points of attach- ment (origin and insertion) to the bones. The latter feature is that which ought to determine the nomenclature of muscles, although general terms expressing action, such as abduction, adduction, flexion, and extension are convenient and in frequent use, and are sometimes, indeed, modified so as to designate the aspect of a limb on which particular groups of muscles are situated. Thus the biceps muscle occupies the ‘flexor’ surface of the arm. The only guide as to the origin of the limb-muscles from particular segments of VERTEBRATES. oT the body is to be found in the origin of the motor nerves which supply them. These are, however, interwoven in a very complex manner (forming the brachial and sacral plexuses of spinal nerves) before they are distributed to the muscles of the fore and hind limbs. It is somewhat easier to detect segmental arrange- ment in the muscles of the trunk of higher vertebrates. They are no longer simply disposed in myotomes, but are differentiated into groups and layers capable of effecting more complex movements; and this differentiation is of course greatest in those regions —such as the neck — where the most complex moyements are neces- sary. The muscles are not confined to the outer surface of the skeleton, but cer- tain very important muscles are attached to the ventral aspect of the vertebral column. To this system belongs the diaphragm, a muscular partition, complete only in mammals, dividing off the chamber containing the heart and lungs from that containing the rest of the viscera. Although the branchial skeleton largely disappears from the higher forms, yet it cannot be said that the musculature of the head becomes simpler in these, for the development of a tongue is accompanied by a corresponding development of the mus- cles attached to the hyoid bone—the representative of the branchial skeleton of the lower forms. The muscles of the jaws belong naturally to the same series of those of the rest of the visceral skeleton, but are specialized in the same degree as the bony framework to which they are attached. While a considerable part of the trunk muscles on the dorsal aspect are derived from the conversion of part of the walls of the mesoblastic somites —the muscle- plates, the muscles of the lateral and ventral regions are derived from the lateral plates of the embryo. In the head, on the other hand, some of the mesoblastic somites give rise to no muscular structures at all, while others (the three foremost) give rise only to the small muscles which move the eyeball. The greater part of the mus- culature of the head is thus derived from the parts corresponding to the lateral plates in the trunk. In addition to the fibres of involuntary muscles which are present in the skin, there are present in higher forms muscles which, arising generally from deeper parts, are attached to the skin, and have the function of moying it. Such are the muscles of expression of mammals, as well as larger sheets which may extend over consider- able tracts of the body. The spines of the porcupine are erected by such muscles, which are also those employed by the mammals generally in shaking the coat. THE NERVOUS SYSTEM. Three constituent portions may be distinguished — the central nervous system, the peripheral nervous system and the connecting nerves. The last serve merely to trans- mit outwards or inwards (centrifugally or centripetally) messages between the central and peripheral systems. The latter is composed of the end organs of the centrifugal and centripetal nerves. We have already considered some end organs of the centripe- tal nerves — the lower sense-organs of the skin—and we shall describe the higher sense-organs at the close of this chapter. The ends of the centrifugal nerves are in the muscles, whether of the body, intestine, or blood-vessels, and likewise in the vari- ous glands, which are thus excited to secretion. Only nerve-fibres are met with in the nerves, while nerve-centres or ganglia are characterized by the presence of nerve or ganglion cells in addition. It is customary 28 NATURAL HISTORY OF VERTEBRATES. to distinguish between the cerebro-spinal and the sympathetic nerve-centres, but we shall see that the latter system is very closely connected with the former in develop- ment, so that we have first to consider the spinal cord and brain, which together constitute the cerebro-spinal nerve-centres, and then briefly the nerves in connection with these organs. In the preliminary account of the development of vertebrates, the central nervous system was first seen in the form of a plate of modified epiblast — the medullary plate — which then became grooved and eventually transformed into a tube, closed at both ends. Certain dilatations and thickenings of the anterior end of the tube result in the brain, while the spinal cord is formed by a more uniform differentiation in the rest of its length. A comparison of Figs. 3 and 32 will show what the nature of this differentiation is. The cavity of the tube is represented by the now small central canal of the cord lined by the remains of the epiblast, while the rest of the epithelium has been converted into the white and gray nervous matter sur- rounding the canal. The distinction of color is due to the presence of nerve cells m the gray matter, which is chiefly aggregated about the central canal, while the white matter forms the more superficial substance FIG. 32.—Diagrammatie section of spinal cord of of the cord. Two fissures running along mammal; a, anterior, p, posterior rvot of spinal G ri, ae) Merve! (¢, GAannll;/g, \ganetion; (gr. rsa the dorsal and ventral middle lines give a marked bilateral symmetry to the cord, which exhibits great uniformity in its thickness throughout, except where it tapers at the tail, and where, in the higher forms, two swellings give rise to the nerves for the anterior and posterior limbs respectively. The gray matter forms a cres- centic outline on a transverse section of the cord, and nerve roots may be seen to extend inwards towards the horns of the crescent. A segmented appearance is given to the cord by the origin of the pairs of spinal nerves, and this segmen- tation extends to the gray matter, which is more abundant in the planes of the origin of the nerves than in the intermediate planes. The spinal nerves arise in pairs, the nerve of each side originating by two roots, a dorsal and a ventral. According to what is known as ‘ Bell’s law,’ the latter are purely motor, while the former are purely sensory nerves, and some structural difference can also be recognized, for the dorsal roots con- tain fine fibres and have a ganglion developed on them, while the ventral roots have coarser fibres and no ganglion. The roots join shortly after their origin, and give rise to mixed motor and sensory nerves, generally disposed in three branches, a dor- sal branch to the parts lying dorsad, a ventral branch to the parts lying ventrad, and an intestinal branch to the contents of the body cavity. The last-named branch is ganglionated, and as the various ganglia of the intestinal branches become united with each other, 2 double ganglionated chain results on the ventral aspect of the vertebral column —the sympathetic nervous system. Much more complex are the changes which affect the anterior end of the neural tube, and which result in the formation of the brain or encephalon. At a very early period three dilatations appear, which are known as the three primary cerebral vesicles, but the first and third of these soon become subdivided in such a way as to render five regions distinguishable, which are known as the prosencephalon, thalamencepha- lon, mesencephalon, epencephalon, metencephalon. The cavities of these regions, VERTEBRATES. 29 known as ventricles or cceliz, communicate with each other, and with the central canal of the spinal cord. They are distinguished as prosoceele, thalamoceele, mesoceele, epi- eeele and metaceele, and instead of being uniform in diameter, like the central canal of the cord, differ very much from each other in configuration. The same is true of the walls of these various cavities, the growth in thickness of which is by no means uniform, so that the floor of such a cavity may be very thick, while its roof is extremely thin. So also the five regions do not ultimately retain an approxi- mately equal size, but one or other becomes developed in ex- cess of the others, so as to over- lap them. Another modifying factor in determining the shape of the brain is the marked bend in the head of the embryo, which brings the mesencephalon to oc- cupy its extreme anterior end, and which simultaneously short- Fic. 33.— Horizontal sections through head of young gar-pike, don a : J lower, Bona higher plane; chondrocranium dotted ; brain with ens the floor of this region of the diagonal shading ; 1, prosocele ; 2, thalamoccle ; 3, mesoccele ; 4 5 epiceele ; 5, metaccele. brain. As we regard that part of the skull in front of the notochord as different from the hinder part, so we may distinguish the prechordal part of the brain from the hinder epichordal part, the limit between the two being approximately that between the floor of the thalamocele and mesoceele. FIG. 34.— Mesal sagittal section of gar-pike’s brain. The dotted lines, 1—8, indicate the positions of the frontal sections in Fig. 35; r, rhinencephalon ; s, thin septum of prosoceele ; 0, optie nerve ; 2, thalamoceele ; ¢, epiphysis; 4, hypophysis; 3, mesoccele (the curved dotted line indicates the limits of its floor) ; 4, epiccele ; 5, metaca@le. Instead of a mere diagram to explain the relationship of the various regions of the brain to each other, Figs. 33 to 35, which represent sections in different directions through a comparatively primitive brain — that of the gar-pike — will serve as an ad- 30 NATURAL HISTORY OF VERTEBRATES. mirable introduction to the study of this complicated organ. In a horizontal section like that shown in Fig. 33 b, the various cceliz are seen communicating with each other, and with the central canal of the cord; only the prosocele (1) appears in this plane, shut off and divided into two separate cavities, the walls of which are the ‘cerebral hemispheres.’ It will be observed, however, from Figs. 33 A, and 35, 2, that the prosocele really does communicate with the thalamoccele behind, and that the thin double septum which dips down into it is only partial. It is further instructive to notice that only the lateral walls and the floor of the cavity are thick; both roof and septum are very thin, and between the folds of the latter a thin sheet dips down from the vascular membrane (pia mater) which everywhere closely surrounds the brain, and transforms the septum into a nutritive organ for this part of the brain, known as a choroid plexus. Such thin places in the walls of the ceeliz are of great in- terest and importance, because, although thin in one animal, they may be extremely thick in another. Thus the roof of the prosocele in man forms the greater part of the bulk of his brain. From Fig. 33 A it will be noticed that the prosencephalon gives off anteriorly two lobes, the olfactory lobes or rhinencephala, which are almost solid (Fig. 35), except for two cavities, the rhinoceles, which open into the prosoceele behind. Occasionally the rhinencephala are separated from the prosencephalon by long olfactory tracts, but generally they lie quite close to the brain. From a study of these horizontal sections one would conclude that the walls of the thalamocele are almost entirely thin, but that such is not the case an examination of Fig. 35, 4, and Fig. 34, will demonstrate. The former figure passes through the origin of the optic nerve from the thick walls of the thalamocele (the optic thalamt), and also through two thick cushions in the roof (habenwlar ganglia) which are situated on either side of a tubular process which here projects upwards from the cavity. The latter, the ‘ epiphysis, or pineal body, can be best seen in the mesal sagittal section (Fig. 34), which is further well calculated to show that the planes separating the regions of the brain need by no means be frontal. Those between the thalamenceph- alon here and the regions in front and behind it are directed from above and in front, downwards and backwards, so that sections 3 and 5 (Fig. 35), cut through parts of the thalamencephalon as well as the pros- and mes-encephalon respectively. This over- lapping is perhaps more marked ventrally, where the thalamoceele runs back into a long ‘infundibulum,’ connected with the ventral wall of which is the ‘hypophysis’ or pituitary body. Both epiphysis and hypophysis appear to be functionless organs in adult vertebrates; their morphological significance we shall return to afterwards. It was noted above that the roof of the mesocele (8) becomes considerably devel- oped owing to the cephalic flexure. This development persists even after the straight- ening out of the flexure, so that very little of the mesencephalon shows upon the base of the brain. The roof, however, may overlap the region in front and behind (Fig. 35, 6), giving rise to the prominences known as optic lobes (corpora bi- or quadri-gemina) throughout the vertebrate series. Behind the mesencephalon the floor of the brain is remarkably uniform, and gives origin to most of the cranial nerves, but the roof is thickened in front into the ‘ cere- bellum’ (Fig. 35,7), while it is thin further back (Fig. 35, 8). The difference in the roof renders it desirable to distinguish between an anterior epencephalon and a posterior metencephalon; the cavities of which epi- and meta-ceeles, 4 and 5, freely communicate with each other. In mammals the cerebellum develops important lateral lobes which are connected with each other by a bridge-like commissure of nerve-fibres (the pons Varo- VERTEBRATES. 31 lii), on the corresponding part of the brain-floor; this structure, when present, marks off also the floor of the epencephalon (pons) from that of the metencephalon (medulla oblongata), but the latter name is frequently employed for the floor of both regions in the absence of the pons. The regions of the brain named above are as little ‘segmental’ in their nature as the regions of the skull already described. It is possible that all of the b rain in front of the notochord (that is the thalam- and pros-encephalon) is merely an outgrowth of the epichordal part, and that consequently any trace of segmentation ought to be looked for in the latter. We have seen how the segmentation of the spinal cord is expressed by the origin of the spinal nerves, but the cranial nerves, although they originate in a similar manner from the neural tube, do not retain the primitive seg- mental arrangement, but acquire new connections with each other and with the brain, which renders the study of their segmental arrangement possible only in the embryo. It is not known how far the olfactory and optic (first and second) nerves, which Till, FIG. 35.—1-8, frontal sections of brain of young gar-pike; 9, of spinal cord, through the planes indicated in Fig. 34. S originate from the prechordal brain (Fig. 37) agree with spinal nerves, but all the other cranial nerves are certainly segmental. This has been determined with most accuracy in shark-embryos, which exhibit more primitive conditions than those of higher vertebrates. In the notochordal part of the shark’s head nine mesoblastic somites are present, the first three of which give rise to the muscles of the eye, the last three to certain muscles connecting the skull and shoulder-girdle, while the middle three do not develop any muscles. The cranial nerves have certain definite relations to these somites, which explains their distribution in the adult, but their relation- ship to the brain is somewhat different from that of the spinal nerves to the spinal cord. As with the spinal nerves, the ventral root is motor, but it pro- ceeds merely to the muscles derived from the corresponding somite, not to those from the lateral plate below. The dorsal ganglionated root, on the other hand, is mixed, and not only contains sensory fibres, but motor fibres for the muscles derived from the lateral plate. It alone forms dorsal and ventral branches similar to those met with in the spinal region. The destiny of the ventral and dorsal roots belonging to the different segments will now be intelligible. The ventral roots of the first three, forming respectively the third, fourth, and sixth cranial nerves, are distributed to the muscles of the eye, the ventral roots of the last three coalesce to form a nerve (known as the hypoglossus in sharks), which goes to certain muscles of the shoulder girdle, while there are no 32 NATURAL HISTORY OF VERTEBRATES. ventral roots corresponding to the middle three segments. On the other hand, the dorsal roots of the first two segments give rise to the fifth (trigeminal) nerve, of the next two to the seventh and eighth (facial and auditory), of the fifth to the glosso- pharyngeal (ninth) nerve, and of the remaining four to the complex vagus (tenth) nerve. That these nerves cause no marked segmentation of the adult brain is abun- dantly apparent from Fig. 37, which shows how they originate in the brain of a turtle. A short reference is all that our space permits to the course of the dorsal and ventral branches of the dorsal roots. The former appear to. be chiefly distributed to the neuromastic tracts, with the exception of the fourth, which forms the auditory nerve (vide supra, p. 8) while the latter course down the sides of the head, and, when they arrive at the visceral clefts, fork over these, giving a slender anterior branch in front of the cleft, and a stouter posterior branch behind. Intestinal branches are likewise given off, which correspond to the intestinal branches of the spinal nerves considered before. In higher animals two other nerves primitively connected with the spinal cord, the spinal accessory (eleventh) and hypoglossus proper (twelfth) become associated with the ten pairs of cranial nerves referred to above. Such a brain as that of the gar-pike forms a type from which the brains of other fishes and amphibians may readily be studied. In the higher classes, however, there is an ever-increasing tendency towards the su- premacy of two regions of the brain over the others, viz., —the cerebral hemispheres and the cerebellum, —parts, it will be remembered, of the pros- and ep-encephalon. This tendency is not very much marked in the brain of the turtle (Fig. 37), but the prosencephalon even here is very much more developed than in the lower forms. The particular way in which this has taken place can be realized by comparing Fig. 36, which represents a horizontal section through the anterior end of the turtle’s brain, with the representation of a similar section through the gar-pike’s brain (Fig. 33). It will be observed that paired outgrowths are-present from the pros- encephalon in the latter, but the median walls and the roofs of these remain very thin, while in the turtle they are nearly as thick as the rest of EiG. 36. Horizontal section through anterior +h wall of the prosoccele. The lateral outgrowths part of brain of a turtle; m, mesocele; ¢, thalamocele ; p, aula, or axial part of pro- j = a eiae “ rer 2 coatalD Teaditie Cer cheicacin ok Ton iniaeie aaeene ae turtle preponderate over the central un Nene Ee ee a icieral renga divided part, and they grow back beyond the latter or proceeles; 1, lamina terminalis ; r, rhino- sq as to overlap even the mesencephalon. The ventricles or cavities of these lateral outgrowths remain in communication by wide ‘foramina of Monro,’ with the central part of the prosocele (the aula), the anterior wall of which is known as the lamina terminalis of the brain. It will be observed that this lamina is of thickened nervous matter in the turtle, whereas in the gar-pike it is represented only by the posterior edge of the thin double septum of the prosoceele. The roof of the brain in the turtle is not unlike that of many lower forms, for the VERTEBRATES. 33 thin roof of the thalamoceele is free on the upper surface; behind that are the optic lobes, or corpora bigemina, of the mesencephalon, succeeded by the comparatively small cerebellum and the thin roof of the metaccele. A greatly advanced stage of development is to be met with in the birds (Fig. 38), the chief characteristics of the brain of which consist in the great size and shape of the cerebral hemispheres, which, growing backwards towards the cerebellum, cover the roof of the thalamencephalon, and cause the optic lobes to as- sume a lateral instead of a dorsal position on the mesen- cephalon. The cerebellum the first spinal nerve ; 7, rhinencephalon ; p, pro roof of thulamencephalon ; 4, hypophysis ; m, mesencephalon, or optic itself 2rows forward to meet lobes (corpora bigemina) ; ¢, epencephalon with thick roof, the cerebel- - lum ; mz¢, metencephalon, with thin root (choroid plexus). the cerebral hemispheres, and backwards so as to overlap the thin roof of the metacceles, and develops on either side a small lateral lobe which becomes of far greater importance in the mammalian organ. So greatly are the cerebral hemispheres and the cerebellum developed in the mam- malian brain, that the other regions are entirely dwarfed in proportion. This is true of even the lowest forms, but it reaches its maximum in man, where the cerebral hemispheres even cover the cerebellum, as well as all the intervening parts of the brain. Not only does this growth take place backwards, but also forwards, in front of the lamina terminalis, and downwards towards the base of the brain, so that the lateral ventricles, instead of being merely in front of the aula, are continued by so-called ‘cornua,’ into the lobes which result from the growth of the cerebral hemispheres in the directions in- dicated. In all of the lower forms the right and left halves of the different regions of the brain are connected by bundles of nerve fibres, constitut- yg 3.Brain of a gallinaceous bird; ¢, eere- ing transverse commissures; so also longitudi- PE ATR oon h Binae ea Batoe Ten velecee nal commissures are present, connecting the Settee See ee oe ea — XZ are various regions of the brain with each other. The mammalian brain not only possesses representatives of these commissural systems, but others are developed in response to the greater increase in importance of the cerebrum and cerebellum, which reach their maximum in man. Of these we have already alluded to the pons Varolii, found on the ventral aspect or base of the brain, and serving to unite the lateral lobes of the cerebellum, which have become in the lower forms of as great size as the original middle lobe, while in the higher forms they greatly exceed it. Two other important commissures — the corpus callosum and the fornix — are developed in connection with the cerebral hemispheres; but these, instead of being within the ccelie, like the commissures of the lower forms, are really outside this system of cavities. If a triangular patch of the median wall of each cere- bral hemisphere be conceived to remain thin, and to unite with that of the other side, instead of being uniformly thick and independent from its neighbor, as in the turtle (IF 36), we shall realize the way in which the ‘septum lucidum’ of the mammalian brain is formed. Occasionally the union of the two halves of this thin septum is not complete, VOL. 11. —3 ig. 34 NATURAL HISTORY OF VERTEBRATES. in which case a little space in the septum is left, known in human anatomy as the fifth ventricle of the brain. It must be understood that it has really nothing to do with the true ceeliz, but is only a part of the great fissure which separates the two hemi- spheres. The corpus callosum and fornix have important relations to the septum lucidum, for the former is constituted by fibres crossing transversely between the right and left hemispheres of the cerebrum, in front of and above the level of the septum, while the latter arches upwards from the base of the brain behind the septum, and thus reaches the under surface of the corpus callosum. Concomitantly with the great development of the cerebral hemispheres and their ventricles, the other parts of the celiac system become very much reduced in size. This is especially true of the aula, which is very small in the higher forms. The thalamocele is known as the third ventricle, the epi- and meta-ceeliz together as the fourth, while the mesoccle is reduced to a mere ‘iter’ connecting the two, known sometimes as the aqueduct of Sylvius. The roof of the mesocele undergoes a further subdivision than we find in reptiles and birds, being converted into four prominences instead of two, the corpora quadrigemina. Of the greatest importance for the functional activity of the mammalian brain is the circumstance that in all except the lowest forms the surface, instead of remaining smooth, becomes much complicated by the development of fissures and intervening convolutions. This complication of the surface has for its object the accommodation of an increased amount of gray matter, for in the cerebrum, as well as in the cerebel- lum, the white fibrous matter is contained in the inside, while the gray matter con- taining the ganglion cells is superficial. The shape of these convolutions is very dif- ferent in the cerebellum and cerebrum. In the former they are in regular foliations, which determine the curious method of distribution of the white matter, known as the arbor vite, while in the latter they are far less regular, although nevertheless always conforming to the same fundamental type in the same species. Of special interest in connection with the form of these convolutions is the discovery within recent years that particular functions are localized in parts of certain conyolutions which are thus regarded as the cerebral centres of these functions. In addition to the pia mater already referred to as carrying the vessels for the brain, there is an ‘arachnoid’? membrane bounding a lymph space immediately outside the pia, and a dura mater intimately connected with the skull, and in mammals sending partitions between the lobes of the brain, which contain the great veins necessary to drain away its venous blood. Sometimes the dural partitions referred to ossify, as in the case of that separating the cerebrum and cerebellum of many carni- vores which possess an ossified tentorium cerebelii. Before leaving the brain, a few words as to the epiphysis and hypophysis cerebri (pineal or conarial and pituitary bodies) will indicate the present state of our knowledge of these remarkable organs. They are apparently functionless in all living forms, but are no doubt derived from structures perhaps of a sensory character, which were intimately related to the brain in the ancestors of the vertebrates. The epiphysis is a diverticulum of the roof of the thalamoceele, while the hypophysis is developed from the roof of the mouth, but is intimately related to a diverticulum of the floor of the thalamoceele. The relations of these two diverticula suggested to Owen and others that the intestine once com- municated with the upper surface of the head through this ‘ conario-hypophysial tract,’ and opened on the neural surface, as it does in arthropods. According to this theory the conario-hypophysial tract is the remains of a primitive cesophagus, and the VERTEBRATES. 35 surrounding nervous matter corresponds to the supra- and infra-cesophageal ganglia, with their connecting commissures in the arthropods and annelids. There is little evidence to support this theory ; it is far more probable that both epiphysis and hypo- physis are rudiments of sense-organs which have become lost in the higher forms. It has been suggested that the epiphysis is a rudiment of an unpaired eye, projecting upwards from the thalamoceele, as do the optic vesicles laterally, and it is probable that the hypophysis is comparable to certain sense organs possessed by Amphioxus and the tunicates. The connection of the hypophysis with the mouth is very early lost, the duct disappearing entirely except in the Myzonts, where, in the most remarkable way, the ventral end of the duct shifts its position so as to open on the upper surface of the head instead of in the mouth. A further reference to this condition will be found in the description of the olfactory organs of these forms. THE HIGHER SENSE-ORGANS. Tur AupiITory ORGAN. All the higher sense-organs may be regarded as differentiated parts of the skin, the supplying nerves of which have become greatly specialized, and have thus acquired a more marked individuality than the other sensory nerves. | The ends of the nerves are to be sought for in modified tracts of the epiblast, known as neuro-epithelial tracts, where they terminate in sensory cells, generally isolated from each other by indifferent cells. In the case of the olfactory and auditory organs these neuro-epithelial tracts are developed from hollow ingrowths of the ordinary epiblast of the head, but in the case of the eye the neuro-epithelium (retina) is formed as an outgrowth of the first cerebral vesicle, which, however, it must be borne in mind, is epiblastic in origin. Surrounding the epiblast destined to form the neuro-epi- thelium there is always some mesoblastic tissue which carries the necessary vessels for the nourishment of the neuro-epithe- lium, and which likewise forms a more or less complete cap- sule for the organ. We have already seen what an important ae 39.— Head of embryo E ype sh (Ctenolabrus); b, brain ; part these sense-capsules play in modifying the skull. PR es Heeger oper ib A fish forms a convenient starting-point for the study of Sle lens OF Setar yiaae the ear, because only the internal ear or auditory organ proper layers” ot optic vesicle 5 A peas sae which later form the retina. is present, without the additional drum cavity and external ear which we find in higher forms. The epiblastic ingrowth is at first sac-like (Fig. 40), but soon gives out processes which result in the highly complicated structure known as the auditory labyrinth. In sharks the auditory labyrinth is closely surrounded by the cartilage of the skull on every side, but in teleosts it is to a great extent free within the cranial cavity, owing to deficiencies in the inner wall of the cap- sule. It is consequently very easy to dissect out the labyrinth of an ordinary bony fish. The cat-fish will serve as an accessible type (Fig. 40). Here we recognize two parts, an upper and a lower; the upper composed of a vertical tube, the ‘utriculus,’ with a large anterior recess, with which are connected three semicircular canals, the lower of a less capacious ‘sacculus,’ also provided with a recess, — the ‘cochlea.’ Con- necting the utriculus and sacculus is a narrow tube which in many cases 1s completely 36 NATURAL HISTORY OF VERTEBRATES. closed in the adult. The original opening by which the cavity of the labyrinth com- municates with the outside is closed in the adults of almost all vertebrates, but the sharks form an exception to this rule, the endolymphatic duct, as it is called, remaining permanently open. ‘This duct is so named on account of the fluid endolymph contained in the labyrinth, the vibrations of which disturb the hairs projecting from the sensory cells into this fluid. The sensory cells are only accumu- lated at certain spots in the labyrinth, to which the Vee branches of the auditory nerve are distributed; three of foe these are known as ‘ criste acusticz,’ and project into the ‘ 0 2 é : CaS o ‘ampullie,’ which terminate one end of each of the three . semicircular canals, while three others are larger patches, ‘macule acustice,’ which occupy the recess of the utricu- sa lus, the sacculus, and the cochlea respectively. Resting on Fic. 40.— Auditory labyrinth of a : Aaa = cat-tish, right side inner aspect; these three macule are certain ear stones or otoliths formed u, utriculus; /a, otolith in the » é 5 réecessus utriculi; aa,ae,ap,am. Of dense calcareous matter, and known as the lapillus, lize of the anterior, external . 3 tr . < Mad posterior semicircular cane Sagitta, and asteriscus. Like the macule they vary much als respectively; s, sacculus wit - g . : 5 Blaine setae iNacna coehlee iM form and relative size throughout the class of the fishes. ee atta fut, Meleates Of the semicircular canals, two are vertical, while the third point where a transverse tube (external) is horizontal in position ; the anterior of the ver- the labyrinth of the other side tical canals is approximately in a sagittal plane, the pos- terior in a frontal plane. The wall of the labyrinth is formed of a very delicate cartilage, lined by a low epithelium, except at the spots where the sensory cells are accumulated. A quantity of loose tissue containing fluid (perilymph) surrounds the labyrinth, and serves to transmit vibrations to the latter. The chief channel by which vibrations reach the ear in fishes is unquestionably the loose bones of the head and gill-cover, but there may be other channels, notably the air-bladder in the cat-fish and sucker, and their allies, while in some sharks the first gill-cleft apparently serves for this purpose. The higher Vertebrata not only approach the sharks more closely in this respect than they do the ordinary fishes, but also in respect of the complete nature of the capsule surrounding the labyrinth. This capsule, at first cartilaginous, afterwards ossifies, and forms the osseous labyrinth, which agrees in all respects with the enclosed ‘membranous’ labyrinth in form, there being only a very narrow space, containing peri- lymph, left between the osseous capsule and its contents. The only deficiencies in the wall of the osseous labyrinth, apart from the aperture by which the nerve gains admis- sion, are two holes on the lateral aspect, which look into the tympanic cavity or cavity of the drum of the ear, present, with few exceptions, among the higher vertebrates. One of these holes, the ‘foramen ovale,’ is opposite the sacculus, and is occupied by the inner end of the chain of bones that stretches across the tympanic cavity, while the other corresponds to the cochlea in position, and, being occupied by a tense mem- brane, is known as the ‘fenestra rotunda.’ It is chiefly through the chain of bones that sound-waves set the perilymph, and consequently the labyrinth which floats freely in it, in motion; although vibrations are no doubt also transmitted through the bones of the head. aan Before referring more specifically to the method by which vibrations are trans- mitted to the labyrinth in the higher vertebrates, one or two points in which the labyrinth itself differs from that of the fishes must be referred to. In the first place, the otoliths which are so conspicuous in the fishes are barely represented among the VERTEBRATES. yf higher forms; and in the second place, the cochlea gradually acquires independence from the sacculus, and eventually becomes conyerted (in the mammals) into a spirally coiled tube, whence in fact it derives its name. It has been mentioned above that vibrations may reach the labyrinth in some sharks by means of the first gill-cleft. In the smooth hound and similar forms there is a dis- tinct recess of the cleft that stretches up towards the labyrinth. This is exactly what we find in higher vertebrates; only the gill-cleft itself disappears, and the recess stretches up from the cavity of the mouth in the form of the Eustachian tube, to ter- minate under the skin in a dilatation —the tympanic cavity — which lies to the outside of the osseous labyrinth. The portion of skin immediately over the cavity, being thin and tense, is subject to vibration, and may be either quite exposed on the side of the head, as in the frog, or at the bottom of a longer or shorter passage, the external auditory meatus, as in higher forms. An additional provision for collecting the vibrations towards the tympanic membrane is afforded by the external ear, which, however, reaches a high grade of development only in mammals. The question now arises how the chain of bones which serves to transmit vibra- tions from the tympanic membrane to the labyrinth gets into the tympanic chamber. We saw above that it represents in all probability the suspensorial apparatus of the lower jaw in fishes, and further that the loose bones of the head in the latter group form an important channel by which vibrations reach the labyrinth. We may con. clude from this that the function which the chain of bones performs in the higher vertebrates is inherited from lower forms, where they had, however, additional duty to perform. If it be also agreed that the Eustachian tube and tympanic cavity represent the recess of the first gill-cleft in a shark, then we may conclude that the higher vertebrates have inherited both the waye-transmitting recess and the chain of bones from some fish-like ancestor. The position of the chain of bones in the tym- panic cavity can only be explained by assuming that the dilated end of the recess grows round the chain, enveloping it in such a way as to bring them within the cavity, but yet covering them on every side with mucous membrane as it does so. Tur Eyer. The essential difference in the development of the eye from that of the other sense-organs — its neuro-epithelium being a direct outgrowth of the medullary epi- blast — has been noted above. At avery early stage the first cerebral vesicle buds out on either side the optic vesicle, which, however, is soon converted into an optic cup by an ingrowth from the overlying skin to form the lens (Fig. 39). The lens is purely epidermal in its origin, and the little piece of mesoblast which it pushes before it becomes transformed into the vitreous humor, a clear gelatinous tissue which fills the cavity of the optic cup. The wall of the cup next to the lens becomes converted into the retina, 7 (Fig. 41), the other wall remains thin, and in fact only formed of a single layer of epithelium, but this acquires a distinctive character and an important function, by a deposition of black pigment in its cells, which then constitute the pig- mentary epithelium of the retina. The cavity separating the two layers of the retina, at one time continuous with the thalamocele, disappears, as does that of the optic stalk, which becomes converted into the optic nerve. Closely applied to the retinal epithelium is the choroid coat, mesoblastic in origin, and serving chiefly as a carrier of vessels for the nourishment of the eye, and of pigment. This coat has cer- tain muscular fibres in it anteriorly (the ciliary portion), which partly serve to support 38 NATURAL HISTORY OF VERTEBRATES. in position and alter the form of the lens, but also serve to regulate the admission of light to the eye by modifying the size of the pupil (the iris). Light is admitted to the eye through the cornea, a modified piece of skin, the mesoblastic layer of which is continuous with the capsule of the eye, or sclerotic coat. The latter is opaque, and cartilaginous or occasionally ossified in plates. Unlike the auditory capsule, it forms no integral part of the skull, although in some sharks it is continuous by a car- tilaginous stalk with the cranial capsule. The eyeball, composed of the elements just enu- merated, is surrounded in the orbit by a quantity of loose tissue which admits of free movement in differ- ent directions, under the control of the muscles at- tached to the sclerotic coat. These are generally six in number, four of them straight and parallel to the Diagram of vertebrate eye, Optic nerve, the remaining two oblique, and arising in eae ei asain pence) the anterior angle of the orbit. We have already dis- Sache sclerotic; th, thalam- cussed the origin and innervation of these muscles. Protective folds of skin to cover the eyeball are de- veloped in most forms above fishes, being rendered necessary by the change from aquatic life. These folds are the upper and lower eyelids, with frequently a third, which can be pulled over the eye from the anterior angle, and is called the ‘nicti- tating membrane.’ This is kept moist by the secretion of a special gland (Harde- rian), while the secretion of the lachrymal gland, which is situated at the posterior angle of the orbit, is being kept in constant motion over the surface of the eyeball by the movements of the eyelids proper. Between these and the surface of the eyeball is a space, the conjunctival sac, which is put in communication with the nasal cavity by the lachrymo-nasal duct, through which the tears are drained into the nasal cavity. Before leaving the eye it is necessary to consider shortly certain structures of restricted occurrence within the vertebrate series. Many bony fishes have an import- ant vascular organ, the choroid gland, formed round the optic nerve after it has pierced the sclerotic, the function of which is very obscure. It will be more con- venient to refer later to the mode in which it is supplied with blood. Another strue- ture, of the nature of a vascular plexus, is the pecten of reptiles and birds: it projects into the vitreous humor, and may be considered to replace the absent vessel of the retina in the forms which possess it. The obstruction which the blood meets with in flowing through such a vascular plexus leads to the effusion of nutritive material through the thin walls of the vessels of the plexus. Immediately outside the retina, certain structures are developed within the choroid coat, which appear to have the function of rendering that coat more impervious to light. Such are the ‘ tapeta’ of many fishes and of some mammals. The focussing of the lens is not always secured by the ciliary part of the choroid coat, but there Fic. 41. may be a special muscular falciform process projecting into the vitreous humor, and grasping the lens, as in the case of most bony fishes. Apart from the presence or absence of yessels referred to above, the retina may exhibit other structural differences within the different groups. In the retina there is a considerable quantity of indifferent isolating material, which serves for the support of the really nervous elements, and which corresponds to the neuroglia of the brain. The elements which are directly acted upon by the light are the rod-like or cone-like VERTEBRATES. 39 ends of the neuro-epithelial cells, which at first look into the cavity of the optic vesicle, but, when that disappears, become surrounded by branched outgrowths of the pig- mentary epithelial cells, which furnish the rods and cones with the sensitive visual pur- ple. The relative proportion of rods and cones is very different in different groups, and their structure is likewise different — those of the reptiles and birds being partic- ularly remarkable for the brilliantly colored drops of oil which are present in them. Tue OLFAcToRY ORGAN. Like the auditory organ, the olfactory organ first appears as an involution of the skin. It is represented by a ciliated sac in Amphioxus, the position of which is such as to suggest that the organ was related to the anterior end of the open neural canal in the earliest vertebrates, a theory which would explain the source of its nervous sup- ply by the extreme anterior end of the prosencephalon. If correct, this theory would lead us to believe that the unpaired sac met with in Amphioxus and the Myzonts is the primitive condition, and that the paired condition in higher vertebrates is secondary. A very remarkable difference exists between the nasal organ of the hag-fish and the lamprey, which merits attention. In the latter fish the olfactory mucous membrane is lodged in an unpaired sac, opening on the upper surface of the head, by means of the remnant of the hypophysial duct referred to above, and has no communication at all with the cavity of the mouth, while in the former, the hypophysial duct forms « complete tube leading from the upper surface to the cavity of the mouth, and the ol factory mucous membrane is lodged in a posterior recess of this tube. The connec- tion between the nasal and mouth cavities, which obtains in the higher vertebrates, is brought about in a very different fashion. In fishes generally, the olfactory sacs are paired, and lined with mucous membrane disposed in folds, in the grooves between which the special sensory cells are lodged, while the surfaces of the folds are merely clad by ciliated cells which cause a current to stream over them. The roofs of the sacs are frequently strengthened by bone, and there are generally two apertures, often quite close to each other, which may be pro- longed into tubes, through which the current makes its entrance and exit. In certain forms the folds are so disposed as to have created the impression that the olfactory sacs might represent a pair of modified gill-clefts with contained fila- ments, but the theory of their origin, referred to above, is that which is more gener- ally adhered to. In sharks a groove leads from the olfactory organ to the mouth, and no doubt this is the first representative of the similar groove which connects on either side the olfactory pit, and the cavity of the mouth in higher forms, and which afterwards is closed into a canal leading from the olfactory sac into the mouth cavity. The pos- _ terior apertures of these canals form the posterior nostrils or choanw; they are situated directly behind the upper lip in Amphibia, but, by the development of the shelf of the palate referred to above (p. 18), gradually shift further backward until they oceupy a position far back in the cavity of the mouth. Mention has already been made of the complex turbinal surface afforded for the olfactory mucous membrane by the nasal capsule. Only part of the cavity is oceupied by the olfactory neuro-epithelium, the rest serving as a channel for respiration in the higher forms, where the posterior nostrils are brought into close relation with the top of the windpipe. The whole mucous surface is provided with glands, which furnish the moisture necessary to preserve the functional activity of the epithelium. A re- 40 NATURAL HISTORY OF VERTEBRATES. markable recess of each nasal sac is well developed in the snakes and lizards, and in some mammals. It is known as the organ of Jacobson, and is situated between the roof of the mouth and the nasal sac. It is to be regarded as a very specialized part of the olfactory neuro-epithelium, being abundantly supplied with branches from the olfactory nerves. é THE INTESTINAL SYSTEM. In all Vertebrata, part of the anterior region of the intestinal canal is devoted to the function of respiration, the organs in connection with which will form the subject of a special section. Here only those parts of the intestinal system which are con- nected more or less directly with the elaboration of the food will be discussed. The formation of the cavity of the mouth, by the turning in of a pit of epiblast, has been already referred to; the anal end of the tract is similarly formed in many animals, but the rest of the tract is lined by hypoblast in its whole extent, which gives rise to the cells engaged in secretion. Associated with the hypoblast, which in the greater part of the tract is only one cell thick, there is the layer of mesoblast (p. 5) from which is formed the vascular connective tissue, on which the hypoblast rests, as well as the muscular tissue which constitutes the greater part of the thickness of the in- testinal wall. Covering the muscular tissue on the outside is a fibrous membrane, clothed with flat endothelial cells, which is continuous by a double dorsal fold (the mesentery), with the similar membrane which lines the inner surface of the body-wall. This is the ccelomic or pleuro-peritoneal membrane, the ‘ parietal’ layer of which lines the body-wall, while the ‘ visceral’ layer clothes the viscera, which lie in the ccelom. The following regions are recognizable in the alimentary canal of all vertebrates: the cavity of the mouth and pharynx, the cesophagus and stomach, the small intes- tine, and the large intestine. No well-marked boundary exists in many forms between the pharynx and the cesophagus, but the stomach is generally marked off from the small intestine by a distinct pyloric valve, while a similar valve (ileo-czecal) between the small and large intestines likewise prevents the backward passage of the food. From their position we should expect the aperture and cavity of the mouth to be subject to extremely wide modifications. These being always such as are adapted to the food, the modifications are as infinite in their variety as the food itself. To realize this it is only necessary to think of the remarkable differences in this respect between the animals with which we are most familiar. The aperture of the mouth or gape, with its bony framework, is hable to just as much modification as the organs contained in the mouth cavity. It is only in the mammals that we find fleshy lips forming a ves- tibule in front of the bony framework, but this, in certain cases, where cheek pouches are present, may be an important part of the alimentary tract. Within the cavity of the mouth are developed certain organs which secure the food, or masticate it, or propel it into the @sophagus, or chemically alter it. These are the teeth, the tongue, and the salivary glands. We have already seen how teeth are developed, and how cement bones formed in connection with them contribute in a most important manner to the formation of the bony walls of the mouth. The tendency to be observed as we proceed from the lower to the higher animal is to the restriction of these teeth to definite tracts, eventually to the bony margins of the gape, and gradually to specialization not only in num- ber but also in form. The description of the teeth forms such a readily accessible VERTEBRATES. AY structural feature for the systematic ‘zoologist, especially in the case of the mammals, that a full description of the dental system of the different groups will be found under these. The student will thus be able to test the law of gradual specialization to which reference has just been made, and will find that it applies within the smaller groups, as also in proceeding from the lower to the higher classes. The second of the series of organs enumerated above — the tongue — is to be re- garded as a fold of the mucous membrane of the floor of the mouth, supported by the basihyal bone, or its anterior glossohyal process. In most fishes it is nothing more, but in Amphibia and the higher classes, muscular tissue enters into its formation, and it be- comes functional in securing the food, and moving it about within the mouth cavity. The muscles are partly confined to the fold (intrinsic), and partly attached to the hy- oid skeleton (extrinsic), but both form the great bulk of the tongue, comparatively little glandular tissue entering into its formation. It was observed above that the end buds, which are more uniformly distributed in fishes, are confined to the cavity of the mouth in higher forms; the tongue is their usual seat in mammals, where they con- stitute the organ of taste, and are arranged on papillz of peculiar form. In shape the tongue is subject to much variation ; it is sometimes developed equally in all directions round its surface of attachment, but its free portion generally extends either forwards or backwards from that point. Thus, in the frogs, the tongue is attached anteriorly, and its posterior bifid end is that which is thrust rapidly out in securing insects. Again, in mammals, it is attached behind and free in front. Frequently thick and fleshy in appearance, it is in many forms slender and cylindrical; generally cov- ered with a soft mucous surface, it becomes, in the birds, coated with a horny layer, in adaptation to entirely different uses. Its highest degree of specialization in one direction is reached in man, where it enters into important functional relations to the organs of voice and speech. The glands which open into the cavity of the mouth, secreting the saliva for ad- mixture with the food, are not represented in the fishes, although they are present throughout all of the higher groups. In the Amphibia and some reptiles they form more or less continuous tracts, which, however, become restricted to particular spots in the higher reptiles, birds, and mammals. The chief of these are the submaxillary and sublingual glands in the floor of the mouth, the buccal and parotid in the lateral walls. The secretions furnished by these glands are of two kinds, and are formed by cells of different histological character, either they facilitate the swallowing of the food, or they act upon it chemically; but both forms of gland-cells may be united in the same gland. Special functions are sometimes met with, as in the case of the ant- eaters, where the submaxillary glands furnish the sticky secretion which aids the tongue in securing their insect food, or in case of the venomous snakes, where the parotid gland becomes an organ of offence and defence, being converted into the poison-gland. A similar change of function of the ordinary cutaneous glands we have met with in the group of the Amphibia. Before proceeding to describe the divisions of the alimentary canal, a short ac- count of its structure may be useful. Various coats are distinguished, of which the outermost (which looks into the ecelom) is the pleuro-peritoneal membrane. Imme- diately within this is the muscular coat, composed of two layers of generally unstriped fibres which are disposed longitudinally in the outer, but circularly in the inner layer. The muscular coat is connected with the innermost mucous coat by a submucous layer of connective tissue, which contains the larger blood vessels and nerves. he smaller 42 NATURAL HISTORY OF VERTEBRATES. vessels and nerves are distributed in the mucosa proper, and this is clad on the free intestinal surface with the hypoblastic cells, generally disposed in a single layer. That portion of the alimentary canal concerned more directly with the elaboration of the food is, as we have seen, divided into three regions by two well-marked valves — the pyloric and ileo-cxecal. The former separates the first region comprising the esophagus and stomach from the smail intestine, the latter the small from the large intestine. Even in forms which are nearly allied to each other, considerable differences may be observed as to the proportionate length of the intestine. These are attributable to difference in the nature of the food; as a rule carnivorous forms have a simpler and shorter intestinal tract than those which are dependent entirely on vegetable food. An instructive illustration of this principle may be observed in the proportionate length of the intestine in the tadpole and frog. There are few forms in which the intestinal tube is not considerably longer than the body; from the consequent folding of the tube within the ceelom, a corresponding complication of the mesentery which suspends the tube occurs. Increase in length of the whole tube is only one way in which a greater secreting and absorbing surface may be brought about. Frequently this is effected by the mucous membrane alone, which may be elevated into villi, or depressed into follicles, or may be raised into straight or spiral folds of various forms as in the Selachians and Ganoids. Again, blind tubes or ceca may be given off, such as the pyloric appendages of the Teleosts, or the ceca given off by the large intestine in birds and mammals. In different parts of the tract, certain constituents may be specially developed; thus, in the gizzard of the granivoreus birds, the muscular coat is locally thickened, while in the glandular stomach in front of it, the hypoblastic cells are chiefly responsible for the increased thickness of the wail. In many lower vertebrates, the limit between cesophagus and stomach is hardly in- dicated, but in most higher forms the stomach is not only distinguished by its being dilated, but also by a sudden change in the nature of the lining cells, and by increased complexity of the muscular coat. The gastric glands are tubular in form; their fune- tion is the secretion of the gastric juice. Immediately behind the pyloric valve the small intestine is characterized by the reception of the ducts of two important glands, the liver and pancreas, both of which are developed from this part of the intestinal tract, and are formed, to a large extent, of hypoblastic secreting cells. Of these the liver is the largest, and is further dis- tinguished by its paired development, expressed in the adult by the right and left lobes. Only in some primitive forms do we detect the original tubular disposition of the hepatic cells; as a rule, this is concealed in the course of development. The bile secreted by the cells collects in their interstices, which simultaneously are the roots of the bile ducts, through which the secretion is conducted, either directly or with the interven- tion of a gall-bladder, into the intestine. Frequently in close connection with the bile-duct is the pancreatic duct, the rami- fications of which terminate in a racemose manner, like the ducts of the salivary glands. A considerable resemblance is to be detected between the pancreas and the salivary glands, not only in the cells, but also in their secretion. In certain fishes the pancreas appears to be absent. When such is the case, its function is performed by smaller glands of similar nature in the intestinal wall, but it is sometimes only apparently absent, as it follows the ramifications of the bile-duct throughout the liver, although its cells and ducts are quite independent of those of that organ. VERTEBRATES. 43 The large intestine is shorter, as a rule, in the lower than in the higher forms, and has little significance in the absorption of food. It may terminate independently on the surface by a separate anal aperture, or may open into an inyoluted area of the skin (the cloaca), into which the ducts of the urinary and genital organs also empty. THE RESPIRATORY ORGANS. THe GILLs. It has been indicated above (p. 15) that all vertebrates have the anterior part of the alimentary canal perforated by visceral clefts, which primitively allow streams of water to flow over the gills formed on their walls. The clefts originate as hollow out- growths of the alimentary canal, which extend to the skin, which is subsequently per- forated. In air-breathing forms the clefts are only present in the embryo, and disappear entirely or undergo a change of function in the adult. They are much more numerous in Amphioxus, and even in the Myzonts, than in the more typical fishes, where they are ordinarily six in number. Of these, the first is called hyomandibular, as it is situated between the mandibular and hyoid arches; the others are generally termed branchial clefts, and are separated from each other by more or less complete partitious bearing the gill-filaments, and strengthened by the branchial arches described above. ‘The partitions referred to are complete in the sharks, where the clefts open independently on the outside by a series of slits, but in the Ganoids and Teleosts the partitions are very rudimentary, the branchial arches carrying little else than the gill- filaments and the vessels and nerves distributed to them. In this way the gill-clefts are extremely wide in the Teleosts, but they are not visible on the sides of the head as in the sharks, for they are concealed by the gill-cover, a fold of skin which projects back from the hyoid arch, and is strengthened by the opercular bones. This gill-cover not only serves to protect the gill-filaments, which would otherwise be exposed, but aids, by its movements, in promoting a stream of fresh water over their surfaces. Each gillbearing arch, except the first and last, bears two rows of gill-filaments, one of which looks into the pouch or cleft in front, the other into that behind it, but there is hardly any trace of a partition separating the rows in the Teleosts, so that the two series of filaments count as one gill. The hyomandibular cleft loses its respiratory function very early in the vertebrate series, and either disappears or persists with an altered function, as was described under the auditory apparatus. It is present, more or less completely, in many sharks and Ganoids, and frequently opens to the outside by an aperture behind the eye, — the spiracle. From this opening, the cleft is sometimes known as the spiracular cleft, and a rudimentary gill situated on its anterior wall as the spiracular gill; but this gill is not functional, — it receives only already aérated blood, and is therefore known as a false gill or pseudobranch. The hyoid arch carries only one row of gill filaments (the poste- rior) which help in the aération of the blood in Sharks and some Ganoids, but receive only blood already aérated in the Teleosts, and consequently constitute a hyoid or oper- cular pseudobranch. Two very different classes of structures are known as external gills. The embryos of many Selachians possess very long filaments, which extend out from the pouches to the outside (Fig. 16) and apparently have a nutritive function, their epithelial cover- ing serving to absorb yolk particles; but the mud-puppy (WVectwrus) and similar peren- nibranchiate Ampkibia possess permanent, other forms only temporary, external gills, 44 NATURAL HISTORY OF VERTEBRATES. which project widely outwards from the branchial arches, and do not come into relation to the clefts separating them. Tue Arr-BLApDER. Various accessory respiratory organs occur in fishes which are exposed to peculiar physical conditions, such as the drying up of the rivers or ponds in which they live. These exceptional structures will be described under the genera in question; but almost all fish-like forms, with the exception of the sharks, possess a bladder formed from the anterior part of the alimentary canal, which, as it contains air, and has blood- vessels in its walls, is often of respiratory importance. This air-bladder is generally considered to be homologous with the lungs of higher vertebrates, having come first partially, and then entirely, into use instead of the gills. We shall consider this homology after having discussed the form and function of the bladder as it ordinarily occurs. In the Ganoids and more primitive Teleosts (Physostomi) the air-bladder communi- cates with the intestine, generally in front of the stomach, and either by aslit, as in the Ganoids, or by aduct. On the other hand it is completely closed from the intestine in the less primitive Teleosts (Physoclysti). Whether provided with an air-duct or not, it lies between the intestine and the vertebral column, and when it opens into the intes- tine does so, with but few exceptions, on the dorsal wall. Conformably with its devel- opment it presents the same coats as the intestine, viz., a connective tissue coat, a mus- eular coat, a submucous coat containing vessels, and a mucous coat. The last rarely presents any glands, and may often be formed of a single layer of flat epithelium ; so, also, the submucous coat may contain very few vessels, and thus be of no importance for respiratory purposes. But in certain instances the vessels are more numerous, and may be extremely abundant, as in the bow-fin and gar-pike (Amita and Lepidos- teus). Indeed, in the former genus, almost all the blood sent through the last pair of gill-arches is diverted into the walls of the air-bladder; the same is the case in the African Ganoid Polypterus, where the air-bladder opens on the ventral wall of the esophagus, and we have only to conceive the series of gill-filaments on the gill-arches to be absent, and the blood to pass through the corresponding aortic arches without being aérated, to have the air-bladder supplied with blood in exactly the same way as are the lungs in the air-breathing vertebrates. In many forms destitute of an air-duct, the capillary vessels are not distributed regularly over the whole surface of the bladder, but form a dense meshwork at one or more spots, known as retia mirabilia. These are thought to serve for the absorption or secretion of air, which thus comes from the blood, in the absence of a direct com- munication with the outside. Even when a duct is present, it is frequently so narrow and tortuous that it is hard to conceive air being taken in through it, although, ocea- sionally, bubbles are, no doubt, ejected by this channel, so that we have to conclude that also, in such cases, the blood vessels are the source of the air contained in the bladder. But where a wide opening exists, as in the Ganoids, it is obvious that the respiratory importance of the bladder may be much greater. The muscular coat is often undeveloped, but in certain cases it may contain striped fibres, continuous with those of the esophagus, or it may be functionally replaced by muscles arising from the vertebral column and distributing themselves over the blad- der. In many instances, however, the air contained in the bladder does not appear to be controlled by voluntary muscles. Such is apparently the case where the outer VERTEBRATES. 45 connective tissue coat is very thick, as it frequently is. This coat may become closely attached to parts of the vertebral column, or may itself become ossified in part. Apart from the respiratory function, which is only important in a few forms, the air-bladder occasionally becomes subservient to the auditory organ, with which it may be connected directly, or with the intervention of certain modified parts of the anterior vertebree. But its most important function is a hydrostatic one, in virtue of which a fish possessed of an air-bladder is able to alter the amount of the contained air, and, consequently, its own specific gravity, so as to be in equilibrium under varying pressures. As far as form goes, the air-bladder rarely presents a subdivision into right and left sacs, like the lungs. It is more common to meet with a subdivision into anterior and posterior chambers, an arrangement apparently serving to alter the centre of gravity as occasion requires. Tue Lunes. Tn discussing the homology of the lungs of the higher vertebrates with the air- bladder of fishes, a difficulty occurs at the outset, viz., that the latter organ is deyvel- oped from the dorsal wall of the intestinal canal, while the lungs grow out from the ventral wall. Two ways out of this difficulty have been suggested. Either the lungs are not completely homologous with the air-bladder, and are merely similar outgrowths from the same division of the intestinal tract, or they are completely homologous, and one or other organ has shifted its original place of development. That the latter is the true explanation appears to be indicated by the similarity of the blood-supply of the air-bladders in Amia and Polypterus, taken in conjunction with the diversity in the position of the air-duct. The amphibious Dipnoan fishes lead us from the con- dition in Polypterus to that in the Amphibia proper, while the Teleosts more closely resemble Amia in this respect. It has been suggested that the ventral opening is the more primitive position, and that the dorsal position has been secondarily acquired, along with the hydrostatic function. As far as structure goes, the lungs in many of the lower vertebrates present little advance over the air-bladder as met with in the gar-pike and bow-fin. But as we advance to the higher forms, the tendency is towards greater complexity of the sur- face, so that, instead of being simply bladder-like, the lining membrane becomes first raised into folds, and eventually the whole organ becomes spongy by the increased number and subdivision of these partitions. The minute chambers bounded by these folds are then known as air-cells, and the whole system of air-cells communicates with the back of the mouth-cavity, or pharynx, by the wind-pipe, or trachea, which represents the air-duct of the Physostomous fishes. It is often the case in the reptiles, that all of the wall of the lung does not become complicated in this manner. Parts remain thin and bladder-like, and this is the condition of affairs which is so exaggerated in the birds, where the thin parts (air-sacs) are often extraordinarily developed, pushing all yielding parts before them, displacing the marrow of the bones, filling up inter- spaces between the intestines, or forming sacs beneath the skin. With the complication of the lungs in the higher forms, there go hand in hand certain alterations in the wind-pipe, which are partly of respiratory importance and partly subservient to the production of sound. To the former category belong the cartilaginous rings, which prevent the trachea from collapsing, and which are con- tinued to a greater or less extent into the subdivisions (bronchi) which go to each lung. ‘To the latter series of alterations are traceable the formation of the larynx, the 46 NATURAL HISTORY OF VERTEBRATES. upper moditied part of the wind-pipe, which is the organ of voice in most of the higher vertehrates; and of the syrinx, the vocal organ of the birds. The latter is situated at the bifurcation of the trachea, and consists of vibrating folds of mucous membrane with a more or less developed resonating apparatus, in adaptation to which certain of the tracheal and bronchial cartilages are altered. Such is also essentially the structure of the larynx, where folds of mucous membrane — the vocal cords—are set in motion by the current of air passing between them, and are adjusted by certain muscles attached to altered cartilages, here known as thyroid, cricoid, and arytenoid. The resonating apparatus varies much in different forms; perhaps the most remarkable varieties are the huge vocal sacs of the bull-frog, and those of the howling monkeys. There is always a close connection between the larynx and the hyoid-bone, which is established by means of ligaments. Tuyrom anp Tuoymus GLanps. Locally closely related to the respiratory organs, as well as developmentally con- nected with this part of the alimentary tract, are two so-called glands (thyroid and thymus), which, however, functionally belong to the vascular system. These structures are common to all vertebrates, and are formed by modification of the epithelium of the gill-clefts, the former on the ventral, the latter originally on the dorsal aspect. They retain this position in the fishes, but in the air-breathing verte- brates, with the disappearance of the clefts, they are subject to considerable variation in this respect. In man, the thyroid lies at either side of the wind-pipe, the thymus behind the breast-bone. Originally of secretory function (at any rate in the case of the thyroid) these structures have gradually become subservient to the lymph-vascular system, and have thus undergone a change of function. The thyroid persists through- out life in functional activity, while the thymus undergoes degeneration soon after birth. THE VASCULAR SYSTEM. Under this heading we have to consider the canals and spaces in which are con- ducted two of the most important fluid constituents of the body—the blood and the lymph. Both of these constituents are composed of a fluid ‘ plasma,’ containing floating corpuscles, and it is the nature of these which renders the two fluids differ- ent. In the blood there are two sorts of corpuscles,— the one laden with a coloring matter known as hemoglobin, which can combine with oxygen in the respiratory organs and part with it to the tissues; the other destitute of coloring matter, and much less constant in form. It is to the former, or red corpuscles, that the blood owes its color, and this varies with the amount of oxygen present in the corpuscles, that is to say, with the degree of aération of the blood. The colored corpuscles are nearly constant in size and form for each species; with rare exceptions they are small and of circular outline in the Mammalia, but larger, oval, and nucleated in the other classes. In accordance with the tendency to change their shape, and consequently to move, the colorless blood cells or leucocytes are frequently spoken of as amceboid or wan- dering cells. They are identical with the lymph corpuscles, and play a very different part in the economy from the colored elements. Not only do they retain the power of independent locomotion, but they can engulf food-particles, grow, and reproduce themselves by division. Unlike the colored elements, they are not confined to the cavi- ties of the vessels, but can effect a passage through the delicate walls oi the finest of VERTEBRATES. 47 these. They are omnipresent in the body, and are to be found pushing their processes into the spaces which separate the elements of the other tissues. In this way the leucocytes play an important part in the absorption of food from the intestinal canal, as well as in the swallowing up of the products of the waste of the tissues, and in the remoyal of useless parts. The leucocytes so engaged are contained in the lymphatic vessels, which do not constitute a system of closed tubes like the blood vessels, but communicate with all the chinks and crevices of the tissues, and are further interrupted here and there by the lymphatic glands — masses of a peculiar form of connective tissue (adenoid tissue) which effect important changes in the lymph traversing the irregular spaces contained in them. We shall first consider the blood-vessels, as their constancy affords valuable material for study to the morphologist, but it may be convenient to introduce some general statements as to the development and structure of the vessels of both categories. All the vessels are formed in that layer (splanchnoplure) of the mesoblast which associates itself with the hypoblast to form the intestine. They are to be regarded as spaces formed between the cells of the mesoblast by the collection of fluids, and, consequently, if they are lined by cells which are disposed like epithelium; these are, nevertheless, of mesoblastic origin. The only other elements met with in the walls of the vessels are of the nature of muscular and connective tissues. In certain tracts of the vascular system the muscular fibres predominate, and they even acquire in the heart a histological character, approaching in differentiation that of the voluntary musculature; but in other tracts the walls are almost entirely formed of connective tissue, which may be largely of that kind known as elastic. Unlike the lymph vessels, the blood vessels form a system of closed tubes, so that if a large vessel be cut across, and an easily-flowing fluid be injected into one end, it will flow along the whole system without passing through the walls, and escape by the other. The system is formed of central receiving and propelling chambers, con- stituting the heart, from which arteries are given off to the various parts of the body. These subdivide until extremely fine vessels, the capillaries, result, in which changes in the gaseous and fluid constituents can take place, owing to the extreme delicacy of their walls. From the variously-formed meshes resulting from the union of these capillaries, the blood is drained by comparatively thin-walled veins, all of which con- verge towards the heart. From what has been said as to the function of the colored blood cells, it is obvious that the vessels must have important relations to the respira- tory organs. It is necessary to bear this in mind to understand the singular course of development of the blood vessels in the higher forms, and here, as well as in the case of the other systems, it is desirable to start with a comparatively primitive form. Such a type is furnished by the bow-fin (Am/ia) the heart and great vessels of which are represented in Figs. 42 and 43. As we have already seen, the heart is situated in a special chamber of the body cay- ity, —the pericardium (p. 11). The great veins converging towards it generally open into a venous sinus, outside the pericardium, which in its turn has a large aperture into the receiving chamber proper, situated on the dorsal aspect of the heart, and called the atrium. The propelling chamber, or ventricle, lies on the ventral aspect, and projects further caudad than the atrium; it is somewhat pointed behind, and in front is nar- rowed into a conical projection, round which the ear-like appendages of the atrium (auricles) lap so as to appear on the ventral surface. The walls of the atrium are thin, its cavity is subdivided by a sieve-like partition into a smaller left and larger right cham- 48 NATURAL HISTORY OF VERTEBRATES. ber, both of which communicate through the single large atrio-ventricular opening with the much thicker-walled ventricle. This opening is provided with valves, so that when the sponge-like ventricle is dilated with blood, and contracts upon its contents, the blood is prevented from passing backwards into the atrium, and is forced on into the slenderer part of the ventricle known as the conus arteriosus. This also contains valves disposed in three rows, which allow the blood to flow only in one direction, namely, forwards into the arterial trunk that lies on the ventral aspect of the gill-arches. Ordinary Teleosts have this arterial trunk dilated into a bulb next the heart, which looks like the conus of the Gan- oids and sharks, but is not a part of the ventricle like the latter, and is separated from that chamber by a single row of valves. The blood, driven out of the ventricle, flows forwards along the arterial trunk, whence it is distributed to the gills by a series of afferent branchial arteries (a), Fig. 43, which ascend the gill- Fic. 42.—Heart of Amia arches, and break up into the capillary network of the gill-fila- Fa nein? anterior’ ments. There the blood is aérated, and is then collected into an veins; de, ductus Cuvieri; WO hieaeeri lie mri a =A S anny % I hepatic vem; cconug ferent artery (€), which begins low down on the arch, but gradu- arteriosus ; 7, right, /, left auricle; v, ventricle, ally widens as it ascends, and receives more and more of the aérated blood. Having arrived at the top of the arches, the efferent arteries generally unite in a great arterial trunk, the dorsal aorta, which lies immediately beneath the vertebral column, and gives off arteries at intervals to the various parts of the body. Much difference is to be observed throughout the fishes, both in the mode of the origin of the afferent arteries and in the union of the efferent arteries into the dorsal aorta. A closer inspection of the condition in Amia will enable us to proceed towards that in the higher vertebrates. Four pairs of afferent and efferent arteries are present. From the foremost of the latter are continued for- wards carotid trunks for the supply of the head; the rest of the blood of the first pair, with the whole of that of the second, flowing into the dorsal aorta. Little of the blood from the third pair of efferent arteries can reach the aorta, for they unite into a vessel (cceliaco-mesenteric) which supplies the viscera. Finally the blood in the fourth pair is distributed entirely to the air bladder. Fi. 43.—Diagram of ecireula- The aérated blood distributed in this way is soon converted tion in Amia seen from be- , A = = low; a, afferent, and e, effe- into venous blood by contact with the tissues, and is then aOninaly oan ee carried towards the heart through the veins. That from the oe Sacer nonce tah head is poured into the venous sinus through the anterior car- gill-eapillaries; J to [V, gill 4. : c ee al arches. Theunaérated blood Ginal or jugular veins; that from the trunk, in part through aeeaked Borehie Weekes’ the posterior cardinal veins. Part of the blood from the trunk, however, passes through the kidneys, and is collected in a sin- gle median stem, the inferior vena cava, which joins the hepatic vein from the liver before entering the venous sinus. Just as some of the venous blood from the trunk passes through the kidneys, so the venous blood from the intestines passes through the liver on its way to the heart. It is conveyed into the latter organ by the portal vein, which breaks up into a capillary net-work, allowing the blood to be acted VERTEBRATES. 49 upon and altered by the hepatic cells, before it is drained off by the branches of the hepatic vein. We have now to consider the changes which would take place when a fish began to use its air-bladder, in addition to its gills, for respiratory purposes. -Amia is able to live out of water for some considerable time, undoubtedly because it can change the air in its air-bladder, and thus aérate the large quantity of blood distributed to its walls. Although the blood so aérated is mixed with the unaérated blood returning to the heart from the rest of the body, it is obvious that a sufficient change is taking place as long as the blood flows freely through the third and fourth gill-arches. Such a free circulation in the gills is dependent on their being kept moist. While the fish is in water it is unlikely that the air is changed in the air-bladder so frequently, but it is obvious, nevertheless, that the blood returned from the air-bladder must be richer in oxygen than that which has only been through the gills. Now in the Dipnoans, which possess a very similar disposition of the vessels, an arrangement exists whereby the purer blood from the air-bladder is kept separate from the blood returning from the rest of the body, and is sent through the more anterior gills, so that the head receives better aérated blood than does the rest of the body. It is to be understood that neither in Dipnoans nor in Amia are the two methods of respiration equally active simultaneously, but rather that they alternate in accordance with the requirements of the surroundings. The arrangement referred to above / consists In a subdivision of the atrium into a right and left auricle, and of the conus arteriosus into a right and left passage, the latter carrying the purer \ blood from the left auricle to the ante- * rior gills. This arrangement, which is not entirely absent in Ama, is the first step towards a complete separation of the circulation in the lungs from that in the rest of the body, as in birds and mammals. Every conceivable condi- : tion intermediate between that in the PH {4;-Heart of Tizard from, Peneaths, the aizated blood Dignoans andithaymn tae mammals is —ceecending aorta; ¢, carotid artery; 7 lett auricle; 2, to be found either in the embryos or the adults of the amphibians and rep- tiles. If we compare the heart and great vessels of a lizard with those of Amia, we shall at once be able to trace the homologous parts. With the change from an aquatic to a terrestrial life, the gill-filaments disappear, and also the respiratory net-work in these, except where a functionless net-work, or rete mirabile, indicates their former position. Generally, the afferent and efferent branchial arteries of each gill are represented by a continuous aortic arch, uninter- rupted by any net-work. Five pairs of these aortic arches are present in the embryos of all higher forms, the second of which corresponds to the trunk vessels of the first VOL. I. —4 —e——e ~ 111, TV, remnants of branchial arches. 50 NATURAL HISTORY OF VERTEBRATES. pair of gills in the adult Amita. They partly disappear with development, and are partly converted into the great vessels proceeding from the heart; this conversion taking place in all in a fundamentally similar manner, although with numerous minor modifications. Fig. 44 will serve to indicate the mode of the conversion of these in the lizard. Here the atrium is completely subdivided into right and left auricles ; the former receiving the blood from the body, the latter that from the lungs. Both open into the ventricle, which is partially subdivided in such a way that only venous blood is sent through the fifth pair of aortic arches (pulmonary arteries) into the lungs, while mixed blood is sent to the body through the third and fourth pairs of arches. The first and second pairs present in the embryo are unrepresented in the adult. It is easy to proceed now to the last term in the evolution of the vascular system, that which obtains in the mammals. Here the ventricle is completely subdivided into right and left chambers, the former of which propels the yenous blood into the lungs through the pulmonary artery, which is derived from the fifth aortic arch of the left side only, while the latter propels the aérated blood received from the lungs by the left auricle into the systemic aorta, the representative of the left fourth aortic arch of the embryo. The two foremost pairs of aortic arches disappear without leaving any trace, and the third is merely represented by parts of the carotid vessels destined for the supply of the head. In the mammials it is obvious that the aérated arterial, and the unaérated venous, blood, are thus kept unmixed, a condition which is reached also in the birds, and almost perfectly in the crocodiles. Toe Lymwpa-VascuLaR SYSTEM. We have already touched upon the nature of the lymph, and on the roots of the vessels in which it flows: a few words will suffice to indicate the disposition of the vessels themselves. Like the veins, they are thin walled, and course towards the heart, but the principal stem in which they terminate generally opens into one of the great veins, not into the heart itself. Asa rule, the larger lymph vessels are to be found side by side with the blood vessels, although the smaller branches unite much more freely with each other than is the case with the smaller arteries and veins. Many of the lower vertebrates have lympk hearts, which assist in the propulsion of the contents towards the heart; but these do not occur in the mammals. The special way in which the masses of adenoid tissue, referred to above, are interpolated in the course of the lymph vessels, is very different in different forms, but the lower vertebrates have much less distinct and differentiated lymphatic glands than the higher forms. In the latter, we distinguish as mesenteric glands those which are interpolated in the course of the absorbent lymphatic vessels distributed to the intestinal walls. Although they can hardly be said to be differentiated in the lower forms, yet certain masses of the same sort of tissue have taken on special functions, apparently in connection with the elaboration of the blood, so that they are often called blood-glands. The most constant as well as the largest of these is the spleen, but the thyroid and thymus, to which reference has already been made, evidently belong to the same category. It is interesting to note that this sort of adenoid tissue may invade organs of originally very different function, ‘and convert them into lymphoid organs. The thyroid and thymus furnish instances of this, while, in the Teleosts, the foremost part of the kidney may become converted into a lymphoid organ, similar in appearance to, and as bulky as, the spleen. In connection with the lymph vascular system, it is proper to mention that the ccelom and its various compartments are, in VERTEBRATES. 51 reality, large lymph spaces, for minute lymph vessels open freely into them, so that a fluid injected into the eccelom is readily taken up by the small communicating lymphatics. In many lower vertebrates the ceelom communicates with the outside by abdominal pores. These are not always homologous, although they may perform, in certain in- stances, the same function —the discharge of the sexual products. Although the nature of these structures is by no means clear, it appears that they are, in certain cases, remnants of the ducts of the sexual organs, and thus more properly referable to the urogenital system, which we shall now briefly consider. THE UROGENITAL SYSTEM. In many Invertebrata, as well as in Vertebrata, a close connection is observable between the urinary and the reproductive organs. This is notable in the Annelids, where each chamber of the celom communicates with the outer medium by means of a more or less coiled tube. These segmental ducts are sometimes enlarged in special regions, so as to carry off the reproductive elements shed into the celom, thus serving 2 double purpose. Those anatomists who believe that the ancestral verte- brates were anneliddike base their arguments largely on the very similar conditions which exist in vertebrates, for in these the primitive excretory organs are likewise seg- mentally disposed, and parts derived from them serve for the outward conveyance of the sexual products. The essential structure of the kidney, in all vertebrates, is a system of coiled tubules lined with glandular epithelium, derived from the lining membrane of the ccelom, with which, in the lower forms, the end of the tubule may communicate. Into each tubule there grows a tuft of finely divided blood-vessels, which interrupt the flow of the blood in such a way as to cause part of its watery constituents to soak through into the cavity of the tubule. Furthermore, the walls of the tubules are richly pro- vided with capillaries, so that the glandular cells can select from the passing blood the effete matters which must be removed from circulation. In the fishes and Amphibia the kidney is usually divisible into two parts, —a pro- and a meso-nephros, —in both of which a segmental arrangement is detectable; but in the higher forms, a third part—the permanent kidney or metanephros —is devel- oped, in which the tubes never show a segmental disposition. The duct of the meta- nephros, or ureter, never serves for any other purpose than carrying off the urine, but such is not the case with the segmental parts of the kidney. In connection with them are formed two ducts,—the Miillerian and the Wolffian, —the former of which is fully developed in the females of all Vertebrates as the oviducts, but is only rudimen- tarily present in the male. The latter has a more complicated fate; in fishes and Amphibia, the Wolffian duct serves in the female sex merely as a urinary duct, while in the male both the urine and the male reproductive elements are conveyed outwards by it. In the higher forms, the Wolftian duct is barely represented in the female sex, while it serves only as a sperm duct or vas deferens in the male. A considerable difference exists between the male and female sexual glands as to their relationships to these ducts. The ovaries generally shed the ova into the ccelom, whence they are collected by the open mouths of the Miillerian ducts, but the testes become more intimately united with the Wolftian duct by the intervention of certain altered urinary tubules (the epididymis), so that the sperm-cells never fall into the ecelom. AYY gy oLuy WuZg) =/@) ID 17 07? iNN NOLSOg 52 NATURAL HISTORY OF VERTEBRATES. In many forms of reptiles, and in birds, the oviducts are not symmetrically devel- oped, one only becoming functionally active; but in mammals both. oviducts are developed, and generally fuse in their lower parts into a median chamber—the uterus, in which the young are nourished by a placenta for a longer or shorter time. Much difference prevails as to the mode of opening of the urogenital ducts, and their relationship to the intestine. While they open independently of the intestine, as a rule, in fishes, such is not the case in Amphibia, reptiles, and birds, where all open into a common cloaca, which in its turn has a single aperture to the outside. In most mammals, again, the urogenital ducts open in front of the anus into a urogenital sinus, in connection with which are developed the external reproductive organs. In place of opening directly to the outside, the urinary ducts may first expand into a bladder, serving as a reservoir for the urine, or there may be an ‘allantoic’ bladder, developed in a different way, although discharging the same function, with which the urinary ducts have no direct connection except in the higher mammals. The mention of this allantoic bladder renders it desirable to explain a feature of the development of the higher vertebrates, which was not alluded to at the beginnmg of this sketch. In all reptiles, birds, and mammals, a protecting sac is early formed around the embryo, known as the amniotic sac, and consequently these forms have been designated Amniota, in contradistinction to the Anamnia (that is, the fishes and Amphibia), which are destitute of it. The allantois is another fetal membrane, which is, however, developed from the intestine, and, being richly supplied with vessels, serves for the respiration of the embryo, either by approaching the surface of the egg in oviparous forms, or by coming intimately into contact with the blood vessels of the uterus, as in the placental mammals. While the amnion is entirely discarded at birth, this is only true of a part of the allantois, the remainder of which is chiefly converted into the urinary bladder. In the foregoing sketch it has been thought desirable to devote most space and illustration to the discussion of those organs which are most easily studied by the beginner in comparative anatomy, while enough has been said with regard to the other systems of organs to enable the reader to put in their proper place isolated structural details which he may meet with in the following pages. R. Ramsay Wricur. TUNICATES. 53 Cuass I.— TUNICATA. Nowhere in the whole vertebrate alliance is there as good an illustration of degen- eracy as in the group known as Tunicates or Ascidians. Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around, and, as one might say, pursue a downward course, which results in an adult which displays but few resemblances to the other vertebrates. Indeed, so different do they seem, that the fact that they belong here was not suspected until little over a decade ago. Before that time, ever since the days of Cuvier, they were almost universally regarded as molluscs, and many facts were adduced to show that they belonged near the acephals. In the later years, when the facts of development began to be known, this association was looked upon with suspicion, and by some they were placed for a short time among the worms. Anyone who has watched the phases of their development cannot help believing that they belong here, the lowest of the vertebrate series. Before treating of the development, let us consider the structure of the adult, taking as our type one of the common, simple forms, like Ascidia or Cynthia, and illustrating the facts of anatomy by means of a diagrammic figure. The tunicates derive their name from the fact that the whole body is invested with a tough envelope or ‘tunic.’ This tunic or test may be either gelatinous, cartilaginous, or leathery. In some forms it is perfectly transparent, in others it is translucent, allowing enough light to pass to show the colors of the viscera; while in still others it is opaque and variously colored. The tunic is everywhere free from the body proper, except in the region of the two openings now to be mentioned. aa One of these openings occupies a more or less cen- tral position, while the other is usually at one side, or may even be placed at the opposite end of the body. On placing one of the ascidians in a glass dish, and sprinkling a little carmine or indigo into | the water, we can study some of the functions of | the animal. As soon as the disturbance is over, the animals will open the two apertures referred to, when it will be seen that each is surrounded with blunt lobes, the number of which varies with the species. As soon as they are opened, a stream of water will be seen to rush into the central opening, earrying with it the carmine, and a moment later a reddish cloud will be ejected from the other aper- ture. From this we learn that the water passes through the body. Why it does so is to be our next inquiry. On cutting the animal open we find coniel ores he Meares So inieetner that the water, after passing through the first-men- mouth; , nerve; p, branchial sac; s, sto- _ Mueneeeeca) A eA 4 \ —_—_—] ES EG Fic. 45.— Diagram of an ascidian ; a, atrial tioned opening (which may be called the mouth) en- ters a spacious chamber, the walls of which are made up of fine meshes, the whole ap- pearing like lattice-work. Taking out a bit of this network and examining it under a 54 LOWER VERTEBRATES. microscope, we find that the edges of the meshes are covered with myriads of minute hairs, or cilia, which are in constant motion, forcing the water through the holes. Of course the supply has to be made good, and hence more water flows in through the mouth. This large cavity is known as the branchial or pharyngeal chamber, and would seem to be homologous with that of the fishes. On one side of this sac (opposite to the nervous ganglion), between the mouth and the bottom of the chamber, arise two folds with ciliated surfaces, which pass forward and form a closed ring near the mouth, which beneath the ganglion gives rise to a small process extending into the branchial chamber. The other, ventral portion of this organ has glandular walls, and is known as the endostyle. In the living animal, when viewed from the side, it appears like a tube. Usually special gills are developed as folds of the wall of the branchial chamber. The water, after passing through the branchial network, is received into narrow passages and conducted to a larger portion—the cloacal or atrial chamber. The general relations can be seen from our diagram, illustrating a vertical and horizontal section. From the atrial chamber the water flows out into the external world. Now we can readily see how in the older works naturalists were misled in regard to the affinities of the tunicates. They regarded the tunic as the equivalent of the mantle of the molluscs (see Vol. I.), while the incurrent and excurrent openings corresponded to the siphons. In one genus, Chevreulius, the resemblance was even stronger, for there the tunic is in two parts, united by a hinge line, and closed by an adductor muscle. How and why these views were totally erroneous will be seen when we come to consider the development of these animals. At the bottom of the pharyngeal sac is the narrow cesophagus, surrounded with cilia which force a current down into the digestive tract. The branchial meshes serve as a strainer for the water, and the larger particles which it contains fall down until they are within reach of the current going down the csophagus. After passing through the throat, they come to the stomach, where digestion takes place, and then the rejectamenta are carried out through the intestine and poured into the bottom of the atrial cavity. The heart lies on the ventral side of the stomach, and is surrounded by a well- developed pericardium. The most remarkable fact connected with the circulation is that the heart, after beating a short time, forcing the blood through the vessels, will suddenly stop for a moment and then resume its beats; but, strange to say, after the stoppage, the direction of the circulation is reversed, the blood taking an exactly opposite course from that formerly pursued. This most exceptional condition was first seen in the transparent Sa/pa, but it may be witnessed in the young of most genera. We have already referred to the branchial chamber. The walls of this chamber, besides acting as a strainer, are also respiratory organs. The meshes of which they are composed are in reality tubes through which the blood circulates, and thus is brought in contact with a constantly renewed supply of fresh water. The nervous system in the adults of all except the Copelatz seems to be reduced to a single ganglion placed near the mouth, and thus indicating the dorsal side. In forms like Cynthia it holds the same relative position with regard to the mouth, but by the doubling of the body (to be explained farther on) it is also brought near the atrial aperture, where it is shown in our first diagram. The sexes are combined in the same individual, though usually the products ripen at different times. As a rule, the earlier stages of the embryo are passed inside the TUNICATES. 55 cloacal chamber, though in some the development occurs outside the body. As a type ot the development we will consider that of one of the solitary forms, leaving the many curious modifications to be noticed in connection with the species in which they occur. This will be best, since these forms show the relationship to the other vertebrates in the clearest manner. The ege undergoes a total segmentation and a regular gastrulation. Soon a tail appears, and under the microscope the young embryo, which now begins its free life, appears much like the tadpole of the frog. It has a large oval body and a long tail which lashes about, forcing the animal forward with a wriggling motion. Nor is the resemblance superficial; it pervades every part of the structure, as may be seen from the adjacent diagram. The mouth is nearly terminal and communicates with a gill chamber provided with gill clefts. At the posterior end of the gill chamber begins the alimentary tract, which pur- sues a convoluted course to the vent. Just above the intestine are the rudi- mentary urogenital organs, which empty either into the rectum or just behind the vent. In the tail, but not extending to any distance into the Fic. 46.— Diagram of tadpole-like larva of an ascidian; b, oe = o : brain; g, gill chamber; 7, intestine; m, mouth; 2, noto- body, is an axial cylinder — the noto- chord; s, neural cord; w, urogenital system. : chord, which here, as in all other verte- brates, arises from the hypoblast ; and above it is the spinal cord (epiblastic in origin), which extends forward to the brain, above the gill chamber. Besides, the animal is provided with organs of sight and hearing. So far the correspondence between the two types is very close, and if we knew nothing about the later stages, one would without doubt predict that the adult tunicate would reach a high point in the scale of vertebrates. These high expectations are never fulfilled; the animal, on the con- trary, pursues a retrograde course, resulting in an adult whose relationship to the other vertebrates would never have been suspected had the embryology remained unknown. After the stage described, this retrograde development begins. From various parts of the body, lobes grow out, armed on their extremities with sucking dises. These soon come in contact with some sub-aquatic object and adhere to it. Then the notochord breaks down, the spinal cord is absorbed, and then the tail follows suit, the intestine twists around, and the cloaca is formed, the result being much like the diagram near the head of this section. In forms like Appendicularia, this degenera- tion does not proceed so far; the tail, with its notochord and neural cord, persisting through life. ; The tunicates, without exception, are all marine. Some are attached, and others float or swim freely through the water. They feed upon diatoms, small alge, Infu. soria, and minute crustaceans. Their stomachs usually prove rich collecting grounds for the student of the lower forms of life. I have examined them when they were almost entirely filled with the curious cilio-flagellate protozoan, Ceratium tripos, while at other times they have contained large numbers of Radiolaria. Most of the species are small, only a small number attaining a length of afew inches. Many of them are highly phosphorescent, especially the transparent pelagic forms. Owing to the entire absence of any hard tissues, fossil tunicates are unknown. 56 LOWER VERTEBRATES. Sus—-Cuass I. — TeruHyopeEa. Most of the members of this group are attached in the adult condition, though they pass through a stage in their development when they are free-swimming and tadpole-shaped. One of the most important features common to all is, that in the course of their metamorphosis the body is so folded that the mouth and atrial aper- tures are brought close to each other, and are not placed at opposite ends of the body. Many of the group are solitary; that is, during the whole of their life each indi- vidual remains distinct and separate from all others. Next come the so-called social ascidians, in which the original parent sends off buds which develop into new individ- uals, each with its own organs, but still connected to the parent. In the last type, the compound ascidians, the condition is variously complicated. The original individual gives rise to buds which develop into new zooids; but these, instead of being distinct, as in the social forms, become united into one mass with more or fewer of the organs in common. We shall recur to these again. OrpER I.— COPELAT A. In this, the lowest of the orders of tunicates, features characteristic of the larvae of other groups are retained in the adult. The species are all minute, free-swimming animals, which in the course of their development never pass beyond the tadpole stage. In the tail, the notochord and neural cord persist through life, the latter with g@an- glionic enlargements at regular intervals, which is accompanied by a segmentation of the mesoblast. Only a single family, ArpENDICULARID&, is recognized, and the genera are few. Oikopleura (or Appendicularia) is a genus common on the surface of the seas far from land, both in the tropics and in more temperate regions. Two species have been noticed by Mr. A. Agassiz on the southern shore of New England, but they have not yet been described. Among the other peculiarities of these animals we may mention that a cloaca is not formed, the intestine communicating directly with the water, and only two gill slits are present. Some of the species form a transparent gelatinous protective en- velope, known as the test or house. This is secreted very rapidly, and when complete has two anterior apertures, while from the middle arises a spacious chamber in which the tail has ample room to vibrate. According to Fol this house is occupied for only a few hours, and then the animal deserts it and forms another. Placed in this family is a very rudimentary form, called by Fol Aowalevshia, after the eminent Russian naturalist. In this genus there is no heart and no endostyle, while the intestine is wanting. The only known species comes from the Mediterranean. OrpER II. —MONASCIDIL. The term ascidians is frequently employed as synonymous with tunicates, but it is better restricted to embrace the social and solitary forms, the Ascidize Simplices of many authors, and the ‘sea squirts’ of the language of the shore. The family Ascrpmp. includes the solitary forms, the general structure and development of which was sufti- ciently elucidated in our general account. This family embraces the largest species of TUNICATES. T the group, many of which are found on our shores. Some of the most common forms belong to the genus Molgula, and individuals of the various species are found at- tached to rocks and piles, from half-tide mark down to a depth of several fathoms. They are rather small, a large one reaching a diameter of about half an inch, and are usually a pellucid yellow, allowing the color of the viscera to be indistinctly seen, giving the whole animal a very pretty appearance. Possibly most com- mon of all is Wolgula manhattensis, which I have found in favorable places in countless myriads. In this species the young undergo a por- tion of their development in the cloaca, and when the tadpoles hatch they swim out as yellow atoms. They form adhesive processes like those described above, but they cannot use them in becoming attached EU a rl al) to rocks and stones, since they are entirely enclosed in a peculiar en- velope. This envelope, however, is after a while very adhesive, and if the little tadpole happens to touch any part of himself to a stone or shell he is fastened for life. Thus I have frequently seen them adhere by the tail, while the anterior part was making the most violent struggles to escape. Soon, however, they settle down contentedly, absorb the tail, and in a few weeks assume the adult structure. These young possess consid- erable vitality, as I have kept them for several weeks without changing the water. The genus Hugyra embraces a number of small spherical species, fond of muddy bottoms, where they live unattached. The embryology of a species of this genus has been studied by Lacaze Duthiers (he calls it Molgula), and he finds that it does not have a tadpole larva. The fishermen call some of the species of the genus Cynthia by the rather appropriate name ‘sea peach ;’ for they are large and nearly spherical, srs ae while the colors, red and yellow, are disposed much as in the familiar fruit. The name ‘sea squirt’ is also given, fromthe fact that when drawn to the surface these animals will contract still further if touched, sending streams of water out of both oral and atrial apertures. The only other genus which we shall mention is Boltenia, which embraces the “sea pears’ of the fishermen’s terminology. These species are usually yellow or reddish in color, and are supported on long and slender stalks which sometimes attain a length of a foot or more. The body proper is pear-shaped, with the openings on one side. The tunic is tough, wrinkled, and leathery, and the animals are utterly unable to keep themselves clean, but are always covered with numbers of seaweeds, hydroids, and other forms of animal and vegetable life. The family CLAVELLINID® embraces the social ascidians. In structure each of these is much like the members of the last family, the distinguishing feature being that the first individual formed sends out a bud which grows into a root-like stem, developing another individual at the extremity. This second one repeats the operation, and the result is a rather large colony connected by a common stem. In these buds both entoderm and ectoderm are concerned. The latter is derived, of course, from the outer layer of the body, but the entoderm arises as an outgrowth from the branchial sac. The principal genera are Clavellina and Perophora. OrpErR III. —SYNASCIDILA. This is one of the groups of compound ascidians, and is frequently termed Ascidie Composite. In all the species numerous individuals are enveloped in a common 58 LOWER VERTEBRATES. mantle, forming a massive gelatinous and frequently brightly-colored colony attached to some stone, or frond of seaweed. In the different groups the individuals are arranged in a characteristic manner, and the colony increases in size by different modifications of the process of budding. In some the body is short and simple, in others it becomes greatly elongated, and is frequently constricted so that it appears to be composed of two or three segments, to which the names head, thorax, and (third) abdomen are usually applied. In the BorryLiip®, represented on our coast by Botryllus gouldii, the body of the individual zooids shows no sign of segmentation, and in the typical genus they are arranged in a star-like manner around a common cloaca. The general structure of one of the systems or groups of zooids is diagrammatically represented in the accompanying cut, which should be compared with the general figure of a tunicate at the beginning of this article. The central cavity is the com- mon cloaca, into which open the water-tubes coming from the branchial sacs of the various individuals. The arrows show Fria. 49. — Colony of Botryllus . tela composed of three systems, the course of the currents of water. otryllus gouldii is very common on the New England coasts, forming gelatinous masses on eelgrass, alow, and hydoids, which sometimes attain a width of an inch, and a length of eight or ten. The color is very variable. Our figure represents a small col- ony of three systems. The larva was at first considered as a composite, but more recent ‘ aes fat Fic. 50.— Diagram of the structure of Bofry//us. researches show that this is not correct. The The arrows show the course of the water pass- o 2 2s ing in through the mouths and out through larva, on settling down, forms an ascidian, the central cloaca. . which by budding forms a colony. This budding process is very complicated, some three or four generations of buds being formed before a common cloaca is produced. The larva forms a bud and then dies. This bud in turn develops two others and then disappears; the next generation con- tains four individuals, and so on. These buds at first have no connection with each other, but eventually they arrange themselves into groups, and take the structure shown in the diagram. In the Dinrmnim the viscera extend themselves behind the pharyngeal sac, and the body is divided into two portions, dignified by the terms thorax and abdomen. In the next group, the Potyciryip#, three divisions of the body —thorax, abdomen, and post-abdomen — are present, and each individual is very long. Amareecium, a genus common on our coasts, forms large colonies, some of which reach a foot or more in length. The general color is much like that of boiled salt pork, although more translucent, and the fishermen, who occasionally draw them up on their lines, call them sea-pork. The animals are arranged much as in Botry/lus, each system having a common cloaca. The process of budding has been studied in some of the species. The larva attaches itself, and develops a solitary zooid, with its three body divisions. Soon the post- abdomen separates from the rest, and divides into several sections, each of which develops into an independent zooid, the parent in the meantime forming a new post- abdomen. Each of the second generation repeats the process, and the result is that the colony increases rapidly in all directions. TUNICATES. 59 OrprER IV.— LUCIA. In this group are placed a few species of compound ascidians which form thimble- shaped, hollow colonies, the individuals radiating from the axis and being imbedded in a common cartilaginous matrix. The incurrent openings are irregularly disposed on the outer surface, while those for the outgoing stream empty into the hollow of the thimble. Only a single family, the Pyrosomip &, is recognized, and of this the principal genus is Pyrosoma, which derives its name from its highly developed phosphorescent powers. The members of the genus are found in the warmer seas of the world, where they swim slowly through the water. Their mode of locomotion, if such it may be salled, is rather peculiar. Each individual is constantly drawing the water in through the branchial openings on the outside, and pouring it out through the cloaca into the central cavity, and thence back into the sea. This constant streaming forces the whole colony constantly forward in one direction; but there is no adaptation for changing the course. Pyrosoma reproduces in two ways, — by eggs and by budding. The eges hatch and leave the parent, and form the bases of new colonies, which are built up by budding from the zooid produced from the egg. Pyrosoma, as we have said, is highly phosphorescent, and the larger colonies (P. giganteum reaches a leneth of two or three feet) emit a large amount of light ; in fact, the story is told that one naturalist read a description he had written of one of these animals by the light emitted from its body. Each of the individuals has a number of cells near the mouth, the function of which is to produce the light. Susp—Ciass I.—THALIACEA. The members of this division present marked difference from those of the last. All are free-swimming and perfectly transparent, and have the incurrent and excurrent orifices at opposite ends of the body. The branchial cavity is much altered, and the gills take a cylindrical or lamelliform shape, and project into the cavity, dividing it, in Doliolum, into two chambers. Expiration is assisted by a number of muscles like hoops, which, by their contraction, force the water out of the cavity, and at the same time the reaction forces the body forward. The alimentary tract forms a small body (called the nucleus) at one side of the branchial cavity. OrvrErR I.— CYCLOMYARIA. The name used to designate this order, which contains only the family DoLionin, has reference to the character of the muscles which surround the body.. Each one forms a complete hoop. The body is barrel-shaped, with an opening at either end, and the alimentary tract has not attained the concentrated condition found in the next order. The reproduction is accompanied by an alternation of generations, numerous cases of which are described in the lower invertebrates, but which here and in the next order obtains its only representation in the group of vertebrates. The whole of the history is not known: but essentially it is as follows: We have first a sexual form which produces eggs. From each ege a tadpole-like larva hatches out and develops into an asexual individual. This in turn gives rise to buds which 60 LOWER VERTEBRATES. finally separate from the parent and float freely in the water. Of these buds there are two kinds differing considerably in appearance and origin. One kind develops into the sexual forms with which we began; the fate of the other is unknown. After a longer or shorter time the individuals of Dotiolum die, and most of the soft parts soon decay; but the outer gelatinous walls last for some time and fulfil a further use- ful role; for one of the amphipod crusta- ceans, Hyperia by name, uses this dead FiG. 51.—Doliolum denticulatum; a, circular mus- test as a home, living in it much like a eles of pharynx; e, endostyle; g, ganglion; h, 5, > 5 i= 5 heart; i, intestine; m, mouth; 0, esophagus; hermit crab in its shell. One species of 1, reproductive organ; s, stomach; wv, branchiz. the genus has been seen on our shores, but it has not yet been described and honored with a Latin name. OrprEr IIl.— DESMOMYARIA. The members of this, the last order of the tunicates, diverges the most widely from the type described in the foregoing pages. The body is nearly cylindrical, sometimes flattened above and below. The muscular bands do not form complete hoops. The generation in these forms is still more complicated than in Doliolwm, and authorities are still in dispute concerning it. Leaving aside all mooted questions, which are mostly of a technical character, the pro- cess seems to be much as follows: Two kind of individ- uals occur, the solitary and the ‘chain zooids, the latter being united together into long bands, the individuals of which correspond to the links of the chain. From one of the solitary individuals a long process or cord grows out, and then this divides up to form the chain. Each one of the chain zooids contains a single egg, and also the male repro- ductive organs. The egg ripens before the spermatozoa, becomes impregnated, and undergoes its development inside the parent, to which it is connected by a structure compar- able to the placenta of the higher vertebrates. After this young Sapa is far along in its developmental history, the male organs ripen and the spermatozoa are cast into the sea to fertilize other eggs. Each egg finally forms a solitary yyg. 52.—satpa spinosa, one in- Salpa, like the one with which we started. Thus the process Saenz Preciecre terre is an alternation of generations, and, as expressed by the m, mouth; n, nucleus; p, pro- eesses by which the various German poet-naturalist, Chamisso, who first discovered this pine r mi in Ss are extraordinary mode of reproduction, “¢a Sa/pa-mother is not like its daughter, or its own mother, but resembles its sister, its grand-daughter, and its grandmother.” Usually it is thought that the solitary Salpz are asexual and that the chains are sexual; but Dr. W. K. Brooks, who has studied their development very profoundly, thinks otkerwise. He regards the solitary individual as the female; and says that it places an egg in each of the chain salpz, while each member of the chain is to be re- garded as a male. From this point of view no alternation of generation can occur. TUNICATES. 61 The chain Sa/pa, before the ripening of its spermatozoa, is to be regarded as a nurse. To the writer it seems that these differences, apparently so marked, are in reality reconcilable; and that it is partly a question of definition, and partly due to the fact that the tissues of the individuals of the chain are outgrowths from the solitary form, so that it is difficult to say which should be regarded as the parent of either sexual element. The individuals of the chain Sa/pe are variously arranged, being placed with their axis parallel or transverse to the chain, or in some intermediate position. The members of the chain are held together by means of spurs extending from one to another. On our east coast a single species, Salpa spinosa, (often called 8. caboti), occurs, forming chains a foot in length or even more, each chain containing two rows. The body is perfectly transparent, but is frequently tinged with red or blue. At times they are very abundant, filling the water for miles. : In the tropical seas they are more abundant, and a considerable number of species have been described. At present, as we have seen, the ascidians are placed at the bottom of the verte- brate series, but among the so-called worms exists a form, Balanoglossus, which may belong here, occupying a place below the tunicates. This view receives considerable confirmation from the researches of Mr. Bateson on the species which lives in the North Carolina waters. Unlike the previously known forms, this species does not pro- duce a Tornaria larva, but develops directly into the adult by processes which recall those of Amphioxus, next to be taken up. Balanoglossus and its peculiar larva are figured and described in our first volume. J. S. Kinestry. 62 LOWER VERTEBRATES. Crass IT. — ACRANIA. “The lowest of the vertebrates,” Branchiostoma, occurs nearly everywhere in the temperate and torrid regions along the sea coasts, in shoal waters of a couple of fathoms more or less. In the United States it has been found from New York to the Gulf of Mexico. Its shape and size are such that it readily escapes notice, even where very common. , brass, 227 3 calico, 2. channel »» school, 2 sea, 214, 2 Sita Ne » white, 2 white sez Bat-fish, 253, 296 Bathylagus, 144 Bathymasteride, 247 Bathythirissidw, 137 Batrachia, 303 Batrachian snakes, 305 Batrachida, 255 Batrachoseps, 315 Batrachus, 255 Batrachyperus, 311 Bay-cod, cloudy, 260 Bdellostoma, 67 Bellows-fish, 289 Belodon, 464 Belodontidw, 464 Belone, 176 Belonida, 175 480 Berg-lax, 275 Berycide, 182 Beshow, 254 Betta, 245 Bichir, 95 dielaga, 93 Big-head, 250 sill-fish, 97, 173, 203 Black-bass, ae 249 Black-fish, 159, 173, 223, 238 ae 993, Black horse, 132 Black-snake, 365, 366, 382 mountain, 365 Black- ‘swallower, 247 Blanquillo, 247 Bleak, 131 . Bleekeria, 261 Blenniidz, 257 Blennioidea, 257 Blind-fish, 172 Blind-worm, 426 Blinks, 193 Blood-sucker, 411 Blue-back, 136 Blue-fin, 149 Blue-fish, 182, 214, 239 Blue-perch, 259 Blue-pike, 228, 229 Blue-sharks, 82 Blunt-head, 3873 Boa, 359 », dog-headed, 361 » Tinged, 361 Boa constrictor, 360 Boar-fish, 209 soceacio, 249 Bodieron, Bogodidee, 23 Boidie, 359 Boltenia, 57 Bolti, 256 Bombinator, 329 Bone-dog, 76 Bone-fish, 137 Bonito, 198 - belted, 198 Boom-slange, 368 Borborocewtes, 321 Boregat, 253 Boregata, 25: Borer, 67 Bothrophera, 393 Botryllide, 58 3otryllus, 58 Bottle-fish, 289 Bow-tin, 97 Box-fish, 289 Box-tortoise, 450 Blanding’s, 450 Brachycephalus, 340 Brachymystax, 149 Brachysoma, 554 Bramidee, 207 Br ranchiostoma, 62 Bream, 129, 254 5, copper-nose, 284 Breviceps, 659, 540 3revoortia, 136 Brindle, 97 Brontosaurus, 466 3rook-trout, 165 Brosmophycis, 260 3rotulidee, 260 Brotulophidee, 261 Bucephalus, 368 Buffalo-fish, 132 Bufo, 327 Bufonide, 319, 327 Bull-frog, 541 5 North American, 341 INDEX. Bull-head, 116 Bungarum, 381 Bungarus, 381 Burbot, 116, 273 Burgall, 239 Butter-fish, 191 Cabezon, 250 Cacopus, 340 Ceciliida, 305 Calamaria, 362 Calamaride, 362 Calamoichthys, 95 Calamus, 222 California toads, 424 Callechelys, 107 Callichthyide, 113 Callichthys, 114 Callionymide, 257 Callisaurus, 423 Callophides, 384 Callopsis, 384 Calloryuchus, 72 Calloselasma, 393 Callula, 840 Calophrynus, 340 Calotes, 411 Caly ptocephalus, 3 1, 344 Camarasaurus, 465 Campbellite, 235 Campostoma, 125 Candle-fish, 145, 254 ¥9 black, 254 Capelin, 145 Caproide, 209 Capros, 209 Caracanthide, ‘ 254 Carangide, 186 Caranx, 187 Carapax, 441 Carawala, 393 Carcharias, 80 Carcharinus, 82 Carcharodon, 79 Caret, 445 Caribe, 134 Carp, 126, 129 » gold, 180 », leather, 130 5, mirror, 130 Carphophis, 362 Carpiodes, 152 Carrassius, 130 Catadromous, 103 Cat-fish, 110 _ blind, 117 BS channel, 118 A electric, 119 nA gaff-topsail, 119 A lake, 118 a4 Mississippi, 118 ry sea, 118 small, 117 Catostomidi, 131 Catostomus, 132 Caudisona, 397 Caularchus, 267 Caulolatilus, 247 Cayally, 186 Cave-fishes, 172 Cayman, 464 5 black, 464 Orinoco, 464 Cebedichthyide, 259 Centrarchide, 230 Centriscida, 284 Centriscus, 284 Centropomide, 212 Centropomus, 212 Centropristis, 223 Centroscyllium, 76 Centroscymnus, 76 Cephalacanthide, 252 Cephalacanthus, 253 Cephalaspide, 92 Ceratiide, 293 Ceratodidz, 300 Ceratodus, 300 Ceratohyla, 339 Ceratophrys, 326, 331, 332 Cerdalide, 261 Cestracion, 78 Cestracionide, 78 Cetorhinus, 79 Chacide, 115 Cc henopside, 2 59 Chetodipterus, 209 Cheetodon, 210 Chetodontide, 210 Cheetodontoidea, 209 Cheetostomus, 113 Chamezleontida, 440 Chameesaurus, 426 Chameleo, 440 Chameleon, 421, 440 American, 420 Chamide, 137 Chamistes, 152 Chanos, 137 Characinide, 125, 132 Charina, 355 Charr, 164 Py) Floeberg, 165 < Greenland, 167 a red, 150, 165 Chatoéssus, 135 Chauliodontida, 141 Chauliodus, 140, 141 Cheecha, 408 Cheilorhina, 362 Chelonia, 440, 445 Cheloniide, 443 Chelopus, 449 Chelydidee, 455 Chely dobatrachus, 528 Chelydra, 452 Chelydridz, 452 Chelys, 454, 456 Chersydrus, 875 Cheyreulius, 54 Chiasmodon, 247 Chiasmodontida, 247 Chilobranchidse, 100 Chilodipteridee, 235 Chilomycterus, 290 Chimera, 71 Chioglossa, 375 Chiracanthus, 92 Chiridee, 253, 257 Chirocentride, 134 Chirocentrus, 154 Chiromantis, 341, 343 Chirotes, 445 Chitra, 447 Chlamydosaurus, 412, 428 Chlamydoselachidee, 75 Chlamydoselachus, 75 Chologaster, 172 Chondropterygii, 68 Chondrostei, 92 Chonerhinide, 292 Chordata, 2% Chorophilus, 330, 335, 388 Christmas-fish, 279 Chromidide, 235 Chromis, 236 Chrosomus, 127 Chrysemys, 448 Chrysopelea, 369 Chub, 131 ;, mackerel, 197 » Tiver, 129 Chub, silver, 128 Cichlide, 235 Cinosternide, 452 Cinosternum, 453 Cirrhitide, 248 Cirrhosomus, 290 Cisco, 149 Cistudo, 451 Citharining, 133 Clarias, 114 Clariid, 114 Clepsydropus, 458 Clidastes, 403 Climbing-fish, 242 Clinidze, 257 Clotho, 392 Clupea, 135 Clupeide, 135 See ead 431 Cobia, 21 Gobitndes, 125 Cobitis, 126 Cobra, 376 Cobra-da-capello, 3876 Cobra-monil, 392 Cock-paidle, 251 Cod, 268 » bastard, 253 s» buffalo, 258 » cultus, 253 »> green, 253 », rock, 248, 253 tom, 273 Codfish, 253 Cecilia, 318 Ceeciliide, 317 Ceelopeltis, 372 Coleonyx, 410 Colocephali, 101 Colostethus, 340 Colpochelys, 446 Coluber, 365 Colubridz, 365 Colubriformia, 353 Conger-eel, 105, 259 Congers, 105 Congo, 259 Congridz, 105 Congrogadidz, 261 Conodonts, 65 Copelatee, 56 Copper-head, 394 Coregonus, 147 Coronella, 363 Coronellid, 363 Coryphena, 191 Coryphenide, 190 Coryphzenoides, 275 Cottida, 249 Cottus, 250 Crab-eater, 212 Crania of anura, 320, 321 Craniata, 2 Cranopsis, 328 Crappie, 235 Crevalle, 187, 188 Cricotus, 305 Crocodile, 459 a American, 462 ae black, 462 i double-crested, 462 cf false, 462 a Nile, 459 Orinoco, 462 Crocodilia, 459 Crocodilus, 459 eet dag | ae 95 Crotalus, 397 Crotaphytus, 423 pe eee ena, 259 Ctenodus, 9 INDEX. Ctenoids, 91 Ctenolabrus, 239 Ctenoptychius, 78 Cubby-yew, 212 Cultripes, 321, 331 Cunner, 239 Curimatineg, 133 Cusk, 274 Cutlass-fish, 206 Cybium, 201 Cycleptus, 132 Cyclodus, 437 Cycloids, 91 Cyclomyaria, 59 Cyclophis, 365 Cyclopium, 114 Cyclopteride, 250 Cyclopterus, 251 Cyclostomata, 65 Cyclostomi, 65 Cyclura, 418 a Bee uemeces 418 Cylindrophis, 354 Cymatogaster, 241 Cynodon, 133 Cynoscion, 213 Cynthia, 57 Cyprinidae, 126 Cyprinodon, 171 Cyprinodontide, 171 Cyprinus, 129 Cystignathida, 326, 329 Cystignathus, 330 Cyttide, 208 Dactylopteride, 252 Dactylopterus, 255 Dactyloscopide, 254 Dallia, 173 Dalliide, 173 Damalichthys, 240 Darter, 229 >, least, 230 » sand, 230 » striped, 230 Dasybatus, 88 Dasypeltus, 373 Day-light, 278 Deal-fsh, 265 Death-adder, 384 Delma, 411 Dendraspis, 384 Dendrobates, 339 Dendrobatide, 339 Dendrophide, 368 Dendrophis, 368 Dendrophryniscide, 326, 329 Denisonia, 584 Desmognathide, 314 Desmognathus, 314, 329 Desmomyaria, 60 Devil-fish, 189 Diabolichthys, 89 Diadophis, 364 Dibamidee, 440 Dibamus, 40 Dicentrarchus, 225 Dicerobatis, 89 Dicynodon, 458 Didemnide, 58 Didocus, 321 Diemenia, 379 Diemyctylus, 316 Dimorphodon, 468 Dinematichthys, 260 Dinichthys, 92 Dinosauria, 464 Diodon, 290 Diodontide, 290 Diplacanthus, 92 Diplodactylus, 409 Diplodus, 221 Dipneumonia, 300 Dipnoi, 299 Dipsadida, 373 Dipsas, 373 Dipsosaurus, 422 Dipterus, 95, 302 Discocephali, 263 Discoglossidee, 327, 329 Discoglossus, 329 Discophida, 340 Distichodon, 134 Distichodontide, 134 Ditrema, 241 Doctor, 211 Dog-fish, 97 9 black, 76 af bone, 76 ” picked, 76 = skittle, 76 ae smooth, 82 Doko, 301 Doliolum, 60 Dollardee, 234 Dollar-fish, 188, 191 Dolphin, 190 Doradin, 119 Doras, 119 Dorosoma, 135 Dorosomatida, 134 Dory, 208, 228 Draco, 411 Dragon, California, 423 s flying, 411 “A Gabb’s, 428 ey spotted- ailed, 423 Drepanide, 207 Dromiscus, 367 Drum, 215, 216 = beardless, 214 » branded, 214 Drum-fish, 217 Dryophide, 369 Dryophis, 369 Duck-bill cat, 94 Dussumieriine, 136 Dyscophide, 339, 340 Eagle-rays, 89 Echeneidide, 263 Echeneis, 264 Echidra, 392 Echinorhinus, 75 Echis, 392 Eel, 101 » conger, 105 » electric, 124 », pug-nose, 107 » Sand, 137, 261 sea, 105 Eel- mother, 103 Eel-pout, 259 Egg-fish, 289 Elachistodon, 373 Elapide, 876 Elapides, 379 Elaps, 379 Elasmobranchii, 68 Elasmosaurus, 458 Elecate, 212 Elecatidm, 212 Electrophorida, 124 Electrophorus, 124 Elopida, 137 Elops, 137 Elosia, $21 Embiotocidx, 239 Embolomeri, 304, 305 Emperor of Japan, 216 Emydiedid, 447 Emys, 450 482 Enchelycephali, 101 Engraulidide, 134 Engraulis, 134 Engystoma, 340 Engystomide, 339, 340 Enneacanthus, 234 Eperlan, 145 Ephippiide, 209 Epic:ates, 361 Epidalea, 328 Epigonichthys, 64 Epinephelus, 224 Eques, 218 Equuliide, 207 Erimyzon, 132 Erycide, 355 Eryops, 303 Erythrine, 133 Eryx, 355 Esocide, 168 Esox, 169 Etheostoma, 230 Etheostomine, 229 Eublepharidz, 409 Eublepharis, 410 Eugyra, 57 Eulachon, 145 Eulamia, 73, 82 Euleptorhamphus, 175 Eumeces, 439 Eunectes, 361 Euprotomicrus, 76 Eurypharynx, 109 Eusophus, 321 Eutenia, 371 Eventognathi, 125 Exoceetide, 173 Exoceetine, 175 Exocetus, 175 Exoglossum, 128 Exostoma, 113 Fair-maid, 219 Fall-fish, 128 Famocantrata, 410 Farancia, 372 Fario, 151 Fat-back. 179 Fer-de-lance, 396 Feuerkréte, 328 Feylinia, 440 Fierasferide, 260 Fighting-fish, 245 Finnan Haddies, 271 Firmisternia, 339 Fishes, 90 Fishing-frog, 177 Fish of paradise, 245 Fistularidae, 283 Flat-fishes, 275 Flying-fish, 175, 175, 253 Fodiator, ate Fool- tish, Recmnande| o 3 Frog, 317, 341 » bull, 341 » green, 343 s wood, 342 Frost-fish, 206 Fundulus, 171 Gadide, 267 Gadine, 268 Gadopsidze, 259 iS Galaxiide, 143 Galeichthys, 119 Galeocerdo, 82 aleorhinidae, 81 Galeorhinus, 82 Galeus, 82 INDEX. Gambusia, 171 Ganocephala, 304 Ganoidea, 90 Gar, 175 _y, broad-nosed, 97 Gar-fish, ee Garibaldi, 23' Gar-pikes, 96 Garrupa, 249 Gasterosteide, 280 Gasterosteus, 383 Gastrechmia, 318, 339 Gastrobranchus, 67 Gastrostomus, 109 Gavial, 459 Gavialis, 459 Geburtshelferkrite, 329 Gecko, 406 » flying, 409 PA variegated, 410 A St. Lucas, 409 “1 Xantus, 409 Geckonide, 406 Genypterus, 260 Gecbhagae:. 236 Geophis, 36: Geotria, 57. Geotriton, 314 Gerrhonotus, 427 Gerrhosauride, 436 Gerrhosaurus, 436 Gila monster, 428 Gillichthys, 257 Ginglymodi, 96 Glanencheli, 123 Glanis, 115 Glanostomi, 92, 93 Glass-eye, 228 Glass-snake, 427 Khasya, 454 Globe-fish, 289 Glossolega, 317 Glyphoglossus, 340 Gobiesox, 267 Gobiesocide, 267 Gobio, 129, 131 Gobioidea, 255 Goggle-eyed-jack, 187 Goggler, 187 Gold-fish, 130, 237 Goniopholis, 464 Goniosoma, 368 Gonorhynchide, 137 Gonorhynchus, 137 Gonostoma, 140 Goodie, 215 *, Cape May, 215, 217 Goose- fish, 177, 295 Gopher, 453 Gourami, 243 Grammicolepididz, 207 Grande €caille, 137 Grayling, 150 as American, 151 common, 151 Gray-pike, 151 Green-cod, 253 Green-fish, 183 Grilse, 160 Grindle, 97 Gronias, 117 Ground-snake, 362 5 Australian, 363 Grouper, 224 fn red, 224 Grunt, 218 Grypiscus, 321 Grystes, 230 Guaican, 264 Guana, 415 Guasa, 224 Gudgeon, 129, 181 Gular, Gurnard, 252 Gymunarchide, 121 Gymnarchus, 121 Gymnodontes, 289 Gymnonoti, 123 Gymnophiona, 317 Gymnotide, 124 Gyropleurus, 78 Haddock, 270 Hadrosaurus, 465 Hemulon, 218 Hemulonide, 218 Hag-tish, 67 Hair-tails, 206 Hake, 274 “Old England, 27. ig silver, 275 Hake’s dame, 273 Halecomorphi, 97 Halibut, 276 Hallefisk, 27 HaAlleflundra, 276 Halosauride, 142 Halosaurus, 142 Hammer-head, 80 Haplochiton, 143 Haplochitonide, 143 Haplodinotus, 217 Haplomi, 168 Hard-tail, 188 Harpagiferide, 247 Harvest-fish, 191 Hatteria, 456 Helicops, 372 Heleioporus, 330 Hell-bender, 309 Heloderma, 425 Helodermatide, 428 Helwcetes, 237 Hemibranchii, 280 Hemichromis, 236 Hemidactylus, 408 Hemilepidotus, 249 Hemiphractide 327, 339 Hemiramphine, 174 Hemiramphus, 174 Hemisalamandra, 315 Hemiside, 339 Hemisus, 339 Hemitripteride, 254 Heniochus, 210 Hen-paidle, 251 Heptanchus, 74 Heros, 237 Herpetodryas, 325 Herpeton, 369, 372 Herring, 135 =p branch, 135 ay glut, 157 ae lake, 140 a} Ohio, 135 toothed, 138 Heterandria, 171 Heterodon, 364 Heterodontus, 78 Heterosomata, 275 Heterotis, 143 Heterostichus, 257 Hexagrammus, 253 Hexanchide, 74 | Hexanchus, 74 ee 238 - iplopagrus, 22: ELinnooarn plies 285 Hippocampus, 285 Hippoglossus, 276 Histiobranchus, 108 Histiophoridx, 203 Histiophorus, 203 Histiurus, 413 Hoe, 76 Hog-choker, 280 Hog-fish, 229 Holacanthus, 239 Holbrookia, 424 Holconotidx, 239 Holocentride, 181 Holocentrus, 182 Holocephali, 71 Hologerrhum, 374 Holostei, 97 Holostomi, 100 Homalopside, 372 Homalopterid, 125 Hoplichthyide, 254 Hoplocephalus, 381, 382 Hoplopleuride, 142 Horn-fish, 229 Horned-pout, 116 Horned-toads, 425 Horny-head, 129 Horse-fish, 188 Horse-head, 188 Hound, 82 Huchen, 163 Hucho, 163 Hundfisch, 170 Huro, 230 Huso, 93 Hydraspis, 456 Hydrocyon, 133 Hydrocyonine, 133 Hydrophide, 384 Hyla, 335, 337 Hylambates, 341 Hylella, 337 Hylide, 327, 335, 337 Hylodes, 322, 330 Hynobius, 311 Hyodon, 138 Hyodontide, 137 Hyperoartia, 65 Hyperodapedon, 457 Hyperolius, 341 Hyperotretia, 67 Hypnale, 393 Hypomesus, 146 Hypopachus, 340 Hypophthalmidz, 112, 120 Hypostomides, 262 Hypsiboas, 335, 336, 339 Hypsirhina, 372 Hypsifario, 153 Hypsypops, 237 Hysterocardus, 241 Ichthyborine, 154 Ichthyocephali, 100 Ichthyodolurites, 68 Ichthyomyzon, 67 Ichthyopterygia, 457 Ichthyosauri, 457 Ichthyosaurus, 457 Teosteida, 247 Ictalurine, 116 Ictalurus, 118 Ictobius, 132 Id, 131 Tdus, 131 Iguana, 415, 418 Ar horned, 415 os naked-necked, 415 Iguanide, 414 Iguanodon, 465 Inconnu, 149 Ipnopide, 137 Ipnops, 139 Isistius, 76 Isospondyli, 134 INDEX. Tsurus, 79 Ixalus, 341 Jacare, 464 Jack, 186, 249 Jack-salmon, 228 Jew-ftish, 224 ‘John A. Grindle,’ 97 ‘John Dory,’ 208, 249 Jordanella, 171 Kettle-maw, 295 Killifishes, 171 King-tish, 72, 178, 215 King of the herrings, 207 Kneria, 125 Kneriid, 125 Komtok, 301 Kowaleyskia, 56 Kurtid, 207 Labidesthes, 178 Labracide, 225 Labrax, 225 Labridx, 238 Labrine, 238 Labrus, 238 Labyrinthodon, 305 Lacerta, 454 Lacertidx, 433 Lacertilia, 405 Lady-fish, 137 Leelaps, 466 Lemargide, 75 Lafayette, 215, 217 Lagocephalus, 290 Lagodon, 222 Lamna, 79 Lamnide, 79 Lamper-eel, 259 Lampetra, 65 Lampreys, 66 Lamprididz, 207 Lampris, 207 Lancelet, 62 Langya, 181 Lant, 261 Latilidze, 246 Latilus, 247 Latris, 248 Launce, 261 re sand, 261 Lavaret, 148 Lawyer, 97 Leather jacket, 288 Lederkarpfen, 130 Lepidomeda, 129 Lepidopididz, 206 Lepidopleurini, 94 Lepidopus, 206 Lepidosiren, 300 Lepidosteidz, 96 Lepidosteus, 96 Lepidotide, 96 Lepomis, 233 Leptocardii, 62 Leptocephalus, 106 Leptodactylus, 330 Leptognathus, 373 Leptophis, 368 Leptops, 117 Leptoscopidxe, 254 Letharchus, 107 Leuciscus, 131 Liasis, 359 Ling, 212, 260 Liodon, 404 Liolephis, 414 Liopelma, 329 Lirus, 191 Lithodytes, 330 483 Litholepis, 96 Living-tish, 181 Lizard, 407 5. alderman, 423 ” croaking, 409 oa frilled, 412 < Galapagos, 416 + gray, 454 ” green, 435 ag ground, 431, 438, 439 oF Holbrook’s, 424 a many-keeled, 427 > Oregon, 427 A pine, 422 “ safeguard, 431 ye sail, 413 he sand, 434 ay scaly, 434 » six-lined, 431 » Spine-tailed, 418 - striped, 431 % tuberculated, 415 Ms variegated, 431 + water, 429 Lizard-fish, 139 Loaches, 125 Loche, 273 Log-fish, 191 Loggerhead, 444 Loke-Sild, 149 Longe, 167 Look-down, 188 Lophide, 293 Lophius, 177, 294 Lophobranchii, 285 Lopholatilus, 246 Lophopsetta, 278 Loricaria, 115 Loricariidx, i113 Loricati, 248 Losh, 273 Lota, 116, 273 Lotinz, 273 Lucie, 59 Lucifuga, 260 Lucioperea, 228 Lump-fish, 250 Lump-sucker, 251 Lung-fishes, 299 Lutjanide, 222 Lutjanus, 222 Luvarid, 207 Lycodide, 259 Lycodon, 374 Lycodontide, 374 Lyomeri, 109 Mackerel, 191 A big-eyed, 197 s bull, 197 chub, 199 ey) easter, 197 mf frigate, 197 si horse, 183, 100 i snapping, 183, 199 +f Spanish, 201 FA tinker, 197 i yellow, 188 Macrochelys, 452 Macropodus, 245 Macrorhamphide, 284 Macrorhamphus, 284 Macruridi, 275 Macrurus, 275 Madregal, 190 Meenidee, 235 Maigre, 214 Malacanthida, 247 Malacoclemmys, 448 Malacosteus, 141 Malapteruring, 119 484 Malapterurus, 119 Mallotus, 145 Malthe, 296 Manculus, 313 Manta, 89 Marbled Polychrus, 415 Marsbanker, 187 Marsipobranchiata, 65 Massasauga, 397 Mastacembelide, 110, 259 Matheide, 296 Maurolicus, 140 Meagre, 214 Meda, 129 Megvera, 393 Megelobatrachus, 308, 311 Megalops, 137 Melanogrammus, 270 Melanophidium, 354 Menhaden, 136 Menide, 267 Menidia, 178 Menobranchus, 307 Menticirrus, 215 Merlangus, 272 Merluciide, Merlucius, 275 Merou, 249 Mesogonistius, 284 Metapoceros, 417 Micristodus, 79 Microgadus, 273 Microhyla, 340 Microperca, 229, 230 Micropterus, 230 Microstoma, 144 Miller’s-thumb, 250 Minnilus, 127 Minnow, 127, 131, 170 7 mud, 170 - top, 171 Misgurnus, 126 Moceassin, highland, 394 53 water, 395 Mola, 291, 292 Molacanthus, 292 Molge, 315 Molgophis, 305 Molgula, 57 Molide, 291 Mollienesia, 171 Moloch, 414 Molva, 273 Monascidize, 56 Monitor, 480 » _ water, 431 Monk-fish, 84, 295 Monocentridez, 182 Monopneumonia, 300 Monopteridz, 100 Moon-eye, 137 Moon-fish, 188 Mordacia, 67 Morelia, 359 Moringua, 108 Moringuide, 107 Mormyride, 121 Mormyrus, 121 Morone, 226 Mosasaurus, 404 Mossbunker, 136 Moxostoma, 132 Mud-eel, 308 Mnud-fish, 97 Mugil, 103, 179 Mugilide, 178 Mullet, 178 7 big-eyed, 178 54 jumping, 178 a3 sand, 178 = silver, 178 INDEX. Mullide, 245 Mullus, 245 Mummichog, 179 rt blue-fish, 179 Murzena, 107 Murznidz, 107 Murznopsis, 308 Murry, 107 Muskallunge, 169 Mustela, 81 Mutton-fish, 259 Myctophum, 139, 140 Myliobatide, 89 Myliobatis, 89 Myrichthys, 107 Myxine, 67 Myxinide, 67 Myxocyprinus, 132 Naja, 369, 376, 377 Nako, 459 Nannocharacine, 133 Narcacion, 87 Narcine, 87 Nardoa, 359 Natricide, 369 Nauclerus, 190 Naucrates, 189 Necturus, 305 Nemachilus, 126 Nematistiide, 207 Nematogenyine, 120 Nematognathi, 110 Nemichthyidz, 108 Nemichthys, 108 Nemophidide, 257 Nessia, 458 New-light, 285 Nipper, 239 Norway-haddock, 249 Notacanthide, 110 Notzeus, 98 Notemigonus, 129 Notidanide, 74 Nototheniide, 247 Nototrema, 322, 337 Notropis, 127 Noturus, 117 Oar-fish, 266 Odacide, 239 Odontaspis, 80 Q2dipus, 314 Oestocephalus, 305 Oikopleura, 56 Oligodorus, 362 Oligosoma, 438 Ombre chevalier, 165 Oncorhynchus, 151 Opah, 207 Opheosaurus, 427 Ophibolus, 364 Ophichthyidz, 107 Ophichthys, 107 Ophidia, 348 Ophidiide, 260 Ophidioidea, 261 Ophiobolus, 367 Ophiocephalide, 180 Ophiocephalus, 181 Ophiodes, 427 Ophiodon, 253 Ophiophagus, 378 Ophisnerus, 107 Opisthodelphys, 322, 337 Opisthognathide, 247 Opisthomi, 110, 116 Opisthonema, 136 Opoterodonta, 352 Orcynus, 198 Ornithosauria, 477 | Cries 291 - | Orthodon, 127 Orthopristis, 218 Orvet, 426 Osmerus, 144 Osphromenide, 243 Osphromenus, 243 Osseter, 93 Osteoglosside, 142 Osteoglossum, 142 Osteophygis, 446 Ostraciontide, 288 Ostracodermi, 288 Otaspis, 329 Otsego-bass, 148 Oudenodon, 458 Oxudercide, 257 Oxybelis, 368 Oxyglossus, 344 Oxyrhopus, 374 Paddle-fish, 94 Pagrus, 219 Paleobftrachus, 329 Palzoniscus, 96 Paludicola, 328 Panai feri, 243 Pantodon, 142 Pantodontide, 142 Pantosteus, 152 Paradise-fish, 245 Pareas, 374 Paralepidide, 139 Paralichthys, 278 Parr, 159 Parrot-fish, 239 Passerita, 369 Patecide, 259 Pediculati, 292 Pegaside, 262 Pelias, 391 Pelican-fish, 109 Pelicosauria, 458 Pelobatide, 331 Pelomedusa, 456 Pelomeduside, 456 Peltaphryne, 328 Peltopelor, 393 Pempheride, 207 Pentacerotide, 210 Perea, 228 Percesoces, 176 Perch, 227, 240 a3 blue, 239 +) sea, 239 aa trout, 168 white, 226 = yellow, 227 Percide, 227 Percina, 229 Percoidea, 213 Percomorphi, 121 Percophidide, 247 Percopsid, 168 Percopsis, 168 Periophthalmus, 267 Peristidiinz, 252 Perophora, 57 Pesce rey, 178 Petromyzon, 63 Phaneropleuron, 302 Pharyngognathi, 99, 235 Philodryas, 368 Phlegethontia, 305 Phosphorescent fishes, 139 Phoxinus, 151 Phractosomata, 92 Phryniscide, 340 Phryniscus, 340 Phry nocephalus, 413 Phrynomantis, +40 Phrynontis, 340 Phrynosoma, 424 Phrynotitan, 255 Phycinw, 275 Phycis, 275 Phyllodactylus, 409 Phyllomedusa, 336, 339 Phy llopteryx, 286 Physostomus, 99 Pickerel, 169 Pig-fish, 215 Pike, 168, 169 » blue, 228, 229 y> gray, 229 + Sacramento, 151 >, Sand, 229 » wall-eyed, 228 yellow, 228 Pilchard, 136 Pilot- fish, 248, 289 Pimele steride, 235 Pimelodinz, 118 Pin-fish, 222 Pipa, 322 Pipe-fish, 285 Pipide, 522 Pisces, 90 Pityophis, 367 Placodermi, 92 Placoids, 91 Plagiodus, 138 Plagiostomi, 72 Plaice, 278 Plakat, 245 Platacida, 210 Platurus, 376 Platycephalide, 254 Platydactylus, 407 Platy pteridie, 2 aT Platysaurus, 426 Platysomidie, 95 Platystacus, 112 Plecoglossus, 149 Plectognathi, 286 Plectospondyli, 125 Plecturus, 354 Plesiosauri, 458 Plesiosaurus, 455 Plethodon, 315 Plethodontide, 313 Pleurodeles, 316 Pleurodelide, 316 Pleurodira, 456 Pleurolepida, 95 Pleuronectes, 278 Pleuronectide, 276 Plotosidw, 115 Podocnemys, 455 Pogonius, 216 Poisson rouge, 215 Pollachius, 271 Pollock, 271 Polychrus, 415 Polyclinide, 58 Polynemide, 263 Polynemus, 263 Polyodon, 94 Polyodontida, 94 Polypedates, 344 Polypteridz, 95 Polypterus, 95 Pomacentride, 237 Pomatomide, 183 Pomatomus, 183 Pomoxys, 234 Pompano, 188, 191 Porbeagles, 79 INDEX. Pore des riviéres, 244 Porecupine-fish, 290 Porcus, 116 Porgee, 210, 219, 222, 240 Poronotus, 191 Potamotrygon, 89 Potter, 448 Pout, eel, 114, 273 * horned, 116 Prenadilla, 114 Prendemeys, 448 Pride, 67 Prionotus, 2 Pristida, Pristiophoride, 78 Pristiophorus, 78 Pristipomide, 215 Pristis, 85 Promicrops, 224 Propleuride, 446 Prostherapis, 340 Proteida, 305 Proteidx, 305 Proteroglypha, 376 Proterosaurus, 457 Proteus, 305 Protonopsidz, 308 Protonopsis, 308 Protopterus, 301 Protostega, 445 Protostegide, 445 Prototroctes, 143 Psammophide, 372 Psammophis, 372 Psammophylax, 364 Sse D RUE OSS 94 Psetta, 278 Psettidae, 210 Pseudecheneis, 115 Pseudechis, 385 Pseudemys, 448 Pseudis, 330, 331 Pseudobranechus, 306 Pseudochromidide, 247 Pseudonaja, 379 Pseudophryne, 528 Pseudopus, 435 Pseudotropis, 121 Psilodactylus, 410 Psilonotida, 290 Pteranodon, 468 Pterichthide, 92 Pterodactylus, 468 Ptilichthyid, 259 Ptilichthys, 259 Ptychochilus, 129, 131 Ptychozoon, 409 ‘Ptyonius, 305 Puffer, 289 Pumpkin-seeds, 253 Pycnodontida, 95 Py enodontini, 4 Pygopodide, 411 Pygopus, 411 Pygosteus, 283 Pyrosoma, 59 Pyrosomide, 59 Pythonide, 356 Py thonomorpha, 402 Pythons, 356 Pytonius, 305 Rabbit-fish, 290 Racer, 366 Rachiodon, 3735 Rachiodontide, 373 Raia, 87 Rai, 84 Raiide, 87 Rana, 341 Ranidze, 319, 340 485 | Ranidens, 311 Ranula, 344 Rattler, 400 3 diamond, 400 3 horned, 402 a5 water, 401 Rattlesnake, 397 » banded, 401 black, 397 es ground, 397 AY Oregon, 399 prairie, 399 red, 402 nr western black, 399 Ray, 84 Red-bass, 215 Red-eye, 151 Red-fin, 127 Red-fish, 152, 159, 214, 218, 249 Red-horse, 152, 215 Red-seed, 193 Reef-bass, 215 Regalecida, 266 Regalecus, 266 Reinhardtius, 278 Remora, 265 Reptiles, 345 Reptilia, 545 Rerias, 393 Retropinna, 144 Rhachitomi, 304, 305 Rhacochilus Rhacophorus Rhamphorhynchus, 468 Rhegnopteri, 262 Rhina, $4 Rhineura, 453 Rhinichthys, 127 Rhinobatide, 85 Rhinobatus, $5 Rhinodon, 78 Rhinodontide, 78 Rhinoglaninze, 229 Rhinophis, 354 Rhinophrynus, 327 Rhinoptera, ees Rhinotriae Ria checantaiies 450 Rhynchodus, 72 Rhynchosaurus, 457 Rhypticidw, 224 Rhypticus, 225 Ribbon-fish, 265 River-jack, 392 River-perch, 241 River-trout, 161 Roach, 151 Robalo, 212 Robin, 252 flying, 258 » sea, 262 Roccide, 225 Roccus, Rock-bass, 233 Rock-cod, 248, 253 Rock-fish, 225 a brown, 249 oe red, 240 Rock-snake, Natal, 358 5 royal, 358 Rock-trout, 253 Rose-fish, 249 Rothfisch, 163 Round-fish, 148 Roussettes, 83 Rudder-tish, 190, 191 Sabalo, 137 Sabre-tish, 206 Saccopharyngide, 110, Sail-tish, 203 486 Salamander, American, 329 spotted, 313 Salamandra, 315 Salamandrella, 311 Salamandride, 315 Salangide, 143 Salar, 158 Salarias, 257 Salbling, 165 Salmo, 158 Salmon, 151, 158, 228 + blue-back, 152 ns dog, 153 + fall, 156 2 hump-back, 153 = king, 152 5 land-locked, 161 5p lost, 158 50 Mackenzie River, 149 “5 quinnat, 152 a silver, 153 spring, 156 Salmonide, 146 Salmen-trout, 159, 161 Salpa, 60, 250. Saluth, 115 Salvelinus, 150, 164 Sand-piper, 67 Sarda, 198 Sardinade Espana, 136 Sardine, 136 Sauger, 229 Sauranodon, 457 Sauromalus, 423 Sauropleura, 305 Sauropterygia, 455 Saw-fishes, 85 Scabbard-tish, 206 Sead, 186 », big-eyed, 187 Scaphiopus, 533 Scaphirhynchops, 94 Seardinius, 151 Scaridee, 239 Scaro, 239 Scarus, 239 Scatophagide, 210 Scelidosaurus, 465 Sceloporus, 421 Schaap-sticker, 564 Schnapel, 147 Scizena, 214 Scizenide, 213 Scincide, 436 Scincus, 437 Sclerodermi, 187 Sclerognathus, 132 Scomber, 192 Scomberesocine, 173 Scomberesox, 174 Scomberomorus, 201 Scombride, 191 Scombroidea, 182 Scoodlo, 271 Scopelidz, 139 Scorpzena, 249 Scorpzenichthys, 249 Scorpenidee, 248 Scorpene, 249 Scorpion, 249, 250 brown, 422 Sculpin, 249, 250 Scup, 219 Scuppaug, 219 Scyllide, 83 Scyllium, 83 Scymnide, 75 Scymnus, 76 Seyphophori, 121 Scytale, 374 Scytalida, 374 INDEX. Seytopis, 321, 339 Sea-bass, 214, 215, 223 Sea-cat, 72 Sea-devil, 89 Sea-horse, 285 Sea-owl, 251 Sea-peach, 57 Sea-pear, 57 Sea-perch, 239 Sea-pork, 58 Sea-purse, 85 Sea-rat, 72 Sea-raven, 254 Sea-serpent, 75, 266 Sea-snakes, 384 Sea-squirts, 56 Sea-trout, 224, 253 Sea-turtles, 444 Sebastes, 249 Sebastichthys, 249 Sebastodes, 249 Selache, 79 Selachii, 68, 72 Selachostomi, 92, 94 Selene, 188 Semotilus, 128 Seps, 437 Sergeant-fish, 212 Seriola, 190 Serpents, 348 Serranidm, 223 Serranus, 223 Serrasalmo, 133 Serrasalmonine, 154 Sewin, 161 Shad, 135 », allice, 136 s American, 156 » gizzard, 134 s, hickory, 135 thwaite, 136 Shad-waiter, 148 Shark, angel, 84 ry basking, 79 s, bonnet, 81 ss dog, 82 » dusky, 82 » gray, 80 s ground, 76 » gurry, 76 ;, hammer-head, 80 », hound, 82 ss mackerel, 79 ss man-eater, 79, 83 » oil, 74, 82 » Port Jackson, 78 » sand, 80 ; Shovel-head, 81 », Shovel-nose, 80 s, Sleeper, 76 ss Spinous, 75 s swell, $3 + SwWingle-tail, 80 » thresher, 80 » tiger, 82, 83 » whale, 78 s, White, 82 zebra, 83 Shark -barrows, 85 Sheat-fish, 115 Sheepshead, 217, 220 Shield-tails, 354 Shiner, 127 s, blunt-nosed, 188 golden, 129 Shovel-nose, 80 Side-winder, 402 Siganidew, 212 Sillaginide, 247 Siluride, 115 Silurine, 115 Silurus, 115 Silver-eel, 207 Silver-sides, 178 Silybura, 354 Simenchelyide, 107 Simenchelys, 107 Simotes, 362 Siphonognathide, 239 Siranota, 316 Siren, 306 Sirenide, 300, 306 Siscowet, 168 Sisoride, 113 Skate, barn-door, 85 ;, brier, 88 s,» Smooth, 89 », tobacco-box, 88 Skates, 24 Skink, 436, 487 » Bermuda, 439 » blue-tailed, 489 ss Officinal, 437 », Skilton’s, 440 Skipjack, 135, 183, 207 Skipper, 173 Skittle dog, 76 Skulljoe, 271 Smelt, 144, 178 », European, 145 » surf, 146 Smolt, 159 Snake, Aisculapias, 366 » black, 366, 382 oe black and white ringed, ie ie headed, 382 5, bull, 367 a carpet, 39 » Chain, 364 » chicken, 366 coach-whip, 366 » collared, 364 »» coral, 380 » corn, 365 » desert, 372 . diamond, 359, 382 » dwarf, 362 +) €mperor, 309 s fox, 366 ;, fresh-water, 372 » garter, 371 » glass, 427 » gopher, 367 ys» grass, 370 ss gray, 367, 379 > green, 365 » ground, 362 » harlequin, 380 5, hog-nosed, 364 », hoop, 372 », horn, 372 » house, 364 s; indigo, 367 » king, 367 +» milk, 364 orange-bellied, 382 », pine, 367 », Yat, 367 » Ting, 364 », Tinged, 370 » rock, 358 » sand, 355 » scarlet-spotted, 384 » sea, 384 », Shield-tail, 354 » Sshort-tailed, 354 » Spectacle, 377 +» Spotted-necked, 371 » striped, 371 Snake, thunder, 362 » tree, 368 » Wart, 374 » worm, 162 Snakes, 348 Snapper, 452 gray, 222 ” mangrove, 2: aoe “A Pensacola, 22% a red, 222 Snappers, 183 Snipe-fish, 108, 284 Snook, 212 Soap-tish, 225 Sole, 279 Sola, 279 Soleide, 279 Solenoglypha, 376 Solenostoma, 285 Solenostomi, 285 Solenostomidz, 285 Somniosus, 76 Spade-foots, 333 Spanish-mackerel, 201, 254 INDEX. Stizostedion, 228 Stoasodon, 89 Stolephorus, 134 Stomias, 140 Stomiatidz, 140 Stone roller, 128 Storeria, 371 Stromateidx, 191 Stromateus, 191 Sturgeon, 92 ae green, 93 i lake, 93 Stygicola, 260 Stygogenes, 114 Stylophoridx, 267 Stylophorus, 226 Sucker, 152, 263 3» Carp, 132 » hare-lip, 132 », Missouri, 132 Suck-fish, 263 Sucking-tish, 263 Sudis, 139 Surf-fish, 240 Teuthidoide, 211 Teuthis, 211 Thalassochelys, 444 Thalassophryne, 255 Thaleichthys, 145 Thaliacea, 59 Thecadactylus, 409 Theromorpha, 457 Thoracosaurus, 464 Thorius, 314, 315 Thoropa, 321 Thread-tish, 206 Ub under Wort; 433 Thymallus, 150 Ticpolonga, 392 Tilapia, » Tile-fish, 246 Tinea, 131 Tinkers, 193 Toad, 327 48 53 hermit spade-foot, 333 » Nnatterjack » obstetr spade, Sparide, 218 Sparisoma, 239 Sparus, 218 peenlass, 4 Spea, 352 Spear-tish, 203 Spelerpes, 314 Sphzrodactylus, 408 Sphargide, 443 Sphargis, 448 Sphenodon, 546 Sphenophryne, 340 Sphryna, 81 Sphrynide, 80 Sphyreena, 177 Sphyrenide, 177 Spiegelkarpfen, 150 Spikes, 193 Spilotes, 367 Spinachia, 283 Spinacidee, 76 Spook, 72 Spoon-bill cat, 94 Spot, 215, 217 Sprat, 136, 240 Spurge-schlange, 377 Squali, 72 Squalius, 129, 131 Squaloraia, 78 Squalus, 76 Squatina, 84 Squatinidie, 84 Squeteague, 213 Squirrel-fish, 181 Star-gazer, 254 Starling, 253 Steel-head, 162 Stegocephali, 304, 305 Stegophilin, 120 Stegostoma, 83 Stellio, 414 Stenodus, 149 Stenorhina, 362 Stenostoma, 353 Stenotomus, 219 Stephanoberycide, 182 Stereocyclops, 340 Sterlet, 93 Sternoptychide, 141 Sternopygide, 125 Sternotheridie, 456 Sterolepis, 224 Stichzidaw, 259 Sticklebacks, 280 Sting-bull, 254 Sting-fish, 254 Stiny-rays, 85 9 black, 241 Sun-fish, 233, 291 ” ” blue, 234 long-eared, 2 Surgeon, 211 Swell-fish, 289 Swell-toad, 289 Swift, 422 Swingle-tail, 80 Sword-fish, 203, 207 Symbranchia, 100 Symbranchide, 100 Synanseide, 254 Synaphobranchide, 108 Synaphobranchus, 108 Synascidiz, 57 Synentognathi, 173 Syngnathi, 285 Syngnathide, 285 Synodontis, 119 Synodontidz, 139 Synodus, 139 Tadpoles, 327, 343 Teenioid, 206 Tzeniosomi, 265 Tailor, 188 Tantilla, 362 Tapetum, 38 Tarente, 407 Tarpum, 137 Tautog, 238 Tautoga, 238 Teiide, 431 Teius, 431 Teleocephali, 121 Teleosaurus, 464 Teleostei, 95 Telmatobius, 331 Temnodon, 183 Tench, 151 Tenpounder, 127 Terrapin, 448 ” ” geographical, 448 red-bellied, 448 salt-water, 445 yellow-bellied, 448 ” Testudinide, 453 Testudo, 453 Tetragonopterine, 135 Tetragonopterus, 133 Tetragonuride, 207 Tetragonurus, 207 Tetrapturus, 203 Tetrodon, 290 Tetrodontid, 289 Teuthide, 211 » Spade-foot s Surinam, 323 tree, 335 Toad, development of, 4 Toad-fish, 255 Toads, 318, 327 Tobaceo-box, 88 Tobacco-pipe fish, 285 Togue, 139 Tom-cod, 273 Tomistoma, 459 Tope, 82 Torpedo, 85 Torpedinidx, 85 Tortricida, 354 Tortrix, 354 Tortoise, box, 450 AS horse, 449 +, Indian, 454 a land, 454 oF mud, 453 Bs river, 449, 455 sculptured, 449 wood, 449 yellow, 452 Toxotida, 210 Trabu, 136 Trachinide, 254 Trachinus, 254 Trachurops, 187 Trachurus, 186 Trachycephalus, 321, 335 Trachynotus, 188 Trachypteridie, 265 Trachypterus, 2¢ Trachysaurus, 435 Trachystomata, 306 Tragops, 369 Tragycephal Tree-frog, 3: Tree-toads, 335 Triacanthidw, 287 Trichiuride, 206 Trichiurus, 206 Trichodiodon, 291 Trichodontia, 254 Trichomy cteride, 120 Trigger- fish, 288 Trigla, 252 Trigle, 252 Trigline, 25) Ti esecaohatan 396 Trimeresurus, 393 Triodontidm, 289 Trionychide, 446 Triprion, 335, 339 Triton, 316 488 Tropedechis, 383 Tropidolepis, 421 Tropidonotus, 370, 383 Trout, 158 s» black spotted, 161 3, brook, 165 » Dolly Varden, 167 » great lake, 167 s» lake, 161 3 Mackinaw, 167 » rainbow, 162 » Rangeley lake, 165 » red-throated, 163 ss Rio Grande, 165 Sy eels er, | 161 » rock, 255 ;, Salmon, 159, 161, sy sea, 159, 161, 214 steel- head, 162 Trumpeter, 248 Trunk-fish, 289 Trygon, 85 Try gonide, 88 Tuditanus, 305 Tullibee, 149 Tunicata, 53 Tunicates, 2 Tunny, 198 Turbot, 278, 288 Turtle, 440 > alligator, 452 >» bastard, 446 » bearded, 454, 456 ;, fimbricated, 456 », iresh-water, 447 > Greek, 454 s green, 445 > hawk-bill, 445 > land, 454 :, leather-back, 442, 445 >» leathery, 445 » loggerhead, 444 ;; marine, 440 » mud, 453 » musk, 452 » Oregon, 449 55 painted, 445 » Snapping, 452 A soft-shelled, 446 >, speckled, 449 » trunk-back, 445 Turtle, plates of, 445 Tylosurus, 176 Tylototriton, 317 Typhlichthys, 172 Typhline, 440 Typhlonectes, 318 Typhlopide, 352 Typhlops, 352 Ular Sawad, 357 Uma, 422 Umbra, 170 Umbride, 170 Umbrina, 215 Unke, 3829 INDEX. Uranidea, 250 Uranoscopide, 254 Urochorda, 2 Urodela, 307 Urogymnus, 89 Urolophus, 89 Uropeltide, 354 Uropeltis, 354 Uroplates, 410 Uroplatidz, 410 Uta, 422 s, Stanbury’s, 422 Vaagmer, 261 Vampyrus, 89 Varanide, 429 Varanus, 429 Vendace, 149 Vermicella, 584 Vertebrata, 1 Vertebrates, air-bladder of, 44 auditory organs of, 35 blood of, 46 bones of, 16, 17, 18, 19, 28, 24, 25 brain of, 32 description of, 3 development of eggs of, 4 eggs of, 5, 4 embryo of, 5,6 epidermal appendages skin in, 9 eye of, 57 gills of. 43 glands of, 8, 9, 38, 41, 46 heart of, 48 intestinal system of, 40 lungs of, 45 lymph of, 50 muscular system of, 26 nervous system of, 27 notochord in, 11 olfactory organ of, 59 of the origin of organs in, 4, 5,6 respiratory organs of, 43 ribs of, 13 scales of, 10 sexes of, 5 skeletal structures of, 9 skeleton of limbs of, 21 skin in, 7 skull of, 14 sternum of, 13 teeth of, 10 thymus gland of, 45 thyroid gland of, 46 urogenital system of, 51 vascular system of, 46 visceral skeleton of, 20 Viper, 391 >, Indian, 392 Vipera, 391 Virginia, 362 Vomer, 188 Walking-fish, 181 Warsaw, 224 Water-dog, 309 Water-pig, re Weak-fish, 2 ¥ ane 215 Weever, 254 on greater, 254 3 Tesser, 254 Weissfelchen, 148 Weissfisch, 148 Wels, 115 Welshman, 181 Whitebait, 144 White-fish, 147, 183, 247 ay common, 148 = lake, 147 Be Menomonee, 148 re mongrel, 149 a5 Musquaw River, 149 Rocky Mountain, 148 Whiting, 215, 272, 275 “4 lake, 149 a New England, 275 Wide-gap, 295 Wide-gut, 295 Window-pane, 278 Wing-fish, 252 W olf-eel, 259 Wolf- fish, 258 W oodeock-fish, 284 Wrass, 23 Xantusia, 431 Xantuside, 451 Xenelaphis, 368 Xenobatrachus, 340 Xenocephalid, 261 Xenodermus, 376 Xenomi, 173 Xenopeltis, 354 Xenopodide, 322, 326 Xenopterygii, 267 Xenopus, 326 Xenosauride, 426 Xenosaurus, 426 Xiphidiontide, 259 Xiphiide, 203 Xiphiphorus, 171 Xiphosoma, 361 Yellow-tail, 247 Zamenis, 368 Zanclide, 210 Zenarchopterus, 174 Zenide, 208 Zenopsis, 209 Zeus, 208 Zoarceidae, 259 Zoarces, 103, 251, 259 Zonuride, 426 Zonurus, 426 Zootoca, 435 Zygena, 81 Zygonectes, 171 SS 2) c © ° ml oO ~ DS) =) if | SAO Kee SNS | Kin s\e Biers : 10 notucel history nm