BIOLOGY LIBRARY TEXT BOOK OF VERTEBRATE ZOOLOGY BY J. S. KINGSLEY PROFESSOR OF ZOOLOGY IN TUFTS COLLEGE HENRY HOLT AND COMPANY 1899 Q L (s> C BIOLOGY LIBRARY G GENERAL COPYRIGHT, 1899, BY HENRY HOLT & CO. C. J. PETERS & SON, TYPOGRAPHERS, BOSTON. PREFACE WITHIN recent years the laboratory method has become the basis of instruction in every science. The student is expected to find out a certain number of fundamental facts directly from nature, but while this has in itself great value as a training in observation, the fullest benefit of the study is not obtained unless there be a comprehension of the bearings of the facts observed. Observation and uncorrellated facts do not make a science. Attention can be directed to the relations and signifi- cance of the facts ascertained in the laboratory by means of lectures, but a somewhat extended experience has shown that the average student needs something more than his lecture notes, at least when beginning any subject. The present volume is intended to supplement both lectures and laboratory work, and to place in concise form the more important facts and gen- eralizations concerning the vertebrates. It is also hoped that it may have some value for students of medicine in explaining many peculiarities of the structure of man which seem mean- ingless unless viewed in the light of comparative morphology. When once their meaning is comprehended it is easy to remem- ber them. The first part of the volume is devoted to an outline of the morphology of vertebrates based upon embryology. This treat- ment has been adopted, since the author believes that in this way the bearings of the facts can be most clearly shown and most easily remembered. The remainder of the volume pre- sents an outline of the classification of vertebrates, a subject which, in recent years, has been too much ignored in college work. Here the fossils are included as well as the recent forms, since the existing fauna must be studied in the light of the past. Numerous generic names have been mentioned with- out characterization ; they have been inserted in order that the student may be able to ascertain the relationships of the forms he may find mentioned in collateral reading. iii IV PREFACE. In this second part the author has ventured to differ in some points from the majority of American students. Thus he has been unable to recognise in the so called orders of ornithologists groups of birds of more than family rank, while their families are equivalent to genera in the other classes of vertebrates. Again in the matter of nomenclature well-known generic names have been retained, in spite of the law of priority. These are the names of morphological literature, and to have used Tri turns, Molge, Myctophium, Zaglossus ; to have mixed up Esox and Belone would have served no useful end. A fair proportion of the illustrations are original ; as many more have been engraved for the volume. These latter as well as those borrowed have been credited as far as possible, to the original source. The author would here return his sincere thanks to Professor Robert Wiedersheim, Professor A. S. Pack- ard, and Dr. Bashford Dean for cliches from their works. He would also acknowledge his indebtedness to Professor C. S. Minot, Dr. G. H. Parker, and Mr. F. A. Lucas for assistance in connection with the manuscript. While many hundreds of special articles have been read in the preparation of the work, acknowledgement must be made to the aid received from Wie- dersheim's Anatomy, the Embryologies of Minot and Hertwig, Zittel's Paleontologie, Jordan and Evermann's Fishes, Wood- ward's Fossil Fishes, and Flower and Lyddeker's Mammals. Woodward's Vertebrate Paleontology appeared in time to be of assistance in the correction of the proofs. A work of this character must be largely a compilation. It is impossible to settle all disputed questions by personal inves- tigation, and one can only take those statements which seem the most reasonable, and which appear to have the most sup- port. That the volume will be found free from error is more than can be hoped. The only apology the author can offer for mistakes of judgment or of fact is based upon the large field, the enormous literature, and the conflicting statements upon many points. TUFTS COLLEGE, May 14, 1899. CONTENTS. PART I. -MORPHOLOGY OF VERTEBRATES. PAGE Introduction . . Introductory Embryology . Histology .*'*. " .. * ':' * ' Morphology of the Organs of Vertebrates . . -s Entodermal Organs . . .. - - Mouth . . ., .-.. Teeth . . . . . . . * -- '9 Tongue . . ..... Oral Glands . . ... 2I Gills ..... - . . ..... 22 Air bladder . . . " . ' 2 20 Thyroid Gland .... Thymus Gland . . . ...... 33 Digestive Tract . . . ...... 34 40 Liver ........ ^ Pancreas . . . ***___" Ectodermal Structures ........ Central Nervous System ........ Spinal Cord .......... Spinal Nerves . ..... Brain .... ..... ; Cranial Nerves ......... Sense Organs .. Lateral Line Organs ... ^ Sense Corpuscles ..* Auditory Organs ........ Olfactory Organs ....... '^ Visual Organs ...* ' Epiphysial Organs ........ Epidermal and Dermal Structures ...... Jj Skin . . . . ...... Exoskeleton ....*' Scales .......... 92 Feathers ......... Hair ...... .... Mesothelial Structures ..... v VI CONTENTS. PAGE Mesenteries 103 Splanchnocoele .......... 106 Muscular System . . . . . . . . . 107 Electrical Organs . . . . . . . .115 Urogenital Organs . . . . . . . . .116 Mesenchymatous Structures ........ 132 Skeleton ........... 133 Vertebral Column ........ 134 Ribs ........... 143 Sternum .......... 147 Skull ........... 150 Appendicular Skeleton ....... 167 Organs of Circulation . . . . . . . .178 Heart 184 Aortic Arches . . . . . . -. . . 185 i Arteries 188 \ Veins . . .192 Lymph System . . . . . . . . . 198 The Segmentation of the Head ....... 201 The Early History of the Ovum ....... 205 The Origin of the Vertebrates 215 PART II. CLASSIFICATION OF VERTEBRATES. Sub-phylum Vertebrata . ... . . . . . .218 Series I. Cyclostomata . . . . . , . . .219 Class I. Marsipobranchii . . . . . . .219 Sub-Class I. Petromyzontes ..... 223 Sub-Class II. Myxinoidei . . . . . 224 Ostracodermi ... 224 Order I. Heterostraci . . . . . .224 Order II. Aspidocephali 225 Order III. Antiarcha . . . . . .225 Series II. Gnathostomata . . 225 Grade I. Ichthyopsida . ... 226 Class I. Pisces ......... 227 Sub-Class I. Elasmobranchii ..... 232 Order I. Cladoselachii ..... 237 Order II. Ichthyotomi ..... 237 Order III. Selachii 238 Order IV. Holocephali 240 Sub-Class II. Teleostomi 242 Legion I. Ganoidea ...... 248 Order I. Crossopterygii . 249 Order II. Chondrostei 250 Order III. Holostei . . o . . .251 Legion II. Teleostei ...... 252 Order I. Ostariophysi 254 CONTENTS. vii FAGK Order II. Physostomi . 255 Order III. Synentognathi ..... 257 Order IV. Hemibranchii . 257 Order V. Lophobranchii . . . . . 258 Order VI. Acanthopterygii .... 258 Order VII. Pediculata 266 Order VIII. Plectognathi 266 Sub-Class III. Dipnoi . . ' < . .267 Order I. Arthrodira .271 Order II. Sirenoidea ...... 272 Class II. Amphibia 274 Sub-Class I. Stegocephali ...... 283 Order I. Lepospondyli . . . . . 283 Order II. Temnospondyli ..... 283 Order III. Stereospondyli 284 Sub-Class II. Urodela . . . . .- ~ . 284 Order I. Perennibranchiata .... 284 Order II. Derotremata 285 Order III. Salamandrina ..... 285 Sub-Class III. Anura 286 Order I. Aglossa ...... 286 Order II. Arcifera ...'... 286 Order III. Firmisternia . . . . - . 287 Sub-Class IV. Gymnophiona 287 Grade II. Amniota . . . ... . . . 288 Class I. Sauropsida 291 Sub-Class I. Reptilia . . . . .292 Order I. Theromorpha .. """""^? 34 Order II. Plesiosauria . . * . > . 306 Order III. Chelonia 307 Order IV. Ichthyosauria 312 Order V. Rhynchocephalia . . . -313 Order VI. Dinosauria . . . . ." 314 Order VII. Squamata . . . . . . 317 Order VIII. Crocodilia ... . .326 Order IX. Pterodactylia 329 Sub-Class II. Aves . . ... . . 330 Order I. Saurura3 . . . . . - 343 Order II. Odontormae 344 Order III. Odontoholca? . . . . -344 Order IV. Eurhipidurse ..... 345 Class II. Mammalia 35 2 Sub-Class I. Prototheria 37 6 Order I. Monotremata 37 6 Order II. Protodonta ... f . . . 377 Order III. Multituberculata . . . . 377 Sub-Class II. Eutheria 37 8 Legion I. Didelphia . . . ... . 37 8 Vlll CONTENTS. PAGE Order I. Marsupialia . . . . . . 378 Legion II. Monodephia ...... 381 Order I. Edentata . . . . . .381 Order II. Insectivora 384 Order III. Chiroptera ..... 386 Order IV. Rodentia ...... 388 Order V. Ungulata ...... 391 Order VI. Sirenia ...... 403 Order VII. Cetacea ...... 405 Order VIII. Carnivora 410 Order IX. Primates . 414 TEXT BOOK OF VERTEBRATE ZOOLOGY. PART I. MORPHOLOGY OF VERTEBRATES. INTRODUCTION. divide all animals into two great groups, the Protozoa, in which all the functions of life are performed by a single cell which constitutes the whole animal, and the Metazoa, in which the body is composed of many cells, and these cells are arranged into layers and organs with a corresponding differ- entiation of functions between the many-celled organs. The metazoa in turn are subdivided into several groups or phyla, the highest of which is called Chordata, while the others are frequently spoken of collectively as Invertebrata. The phylum chordata is characterized by the possession of at least three features which occur in no invertebrate, a skeletal axis or notochord arising from the inner germ-layer or entoderm ; the possession of paired gill slits connecting the anterior part of the alimentary canal with the exterior ; and a central nervous system which is entirely on one side of the alimentary canal. Details concerning each of these features will be given on sub- sequent pages. The chordata embrace at least three subphyla, theJJro- chordia or Tunicata, the Cephalochordia or Leptocardii, and the Vertebrata, the subject of the present book. It is possible that a fourth phylum, the Hemichordia or Enteropneusti, is to be included here, but as yet there is not agreement upon this point. INTRODUCTION. The tunicates include a large number of marine animals which show their chordate features most plainly in the young, FlG. I. Diagram of a larval tunicate, after Seeliger. A, atrial opening ; T, notochord ; E, endostyle; G, gill slits ; H, heart ; M, mouth ; N, nervous system ; S, adhesive disks ; SV, sensory vesicle. the adults being remarkably degenerate. These young have tadpole-shaped bodies, with a central nervous system dorsal in position, a notochord which occurs only in the caudal region, while the gill slits occur on the side of the pharyngeal region. In the course of development in all except the Copelatae (Ap- pendicularia, etc.), the tail becomes absorbed, the noto- chord being lost, while the body becomes so twisted that both gill slits and vent empty into a common atrial chamber. The body is usu- ally fixed, and is covered by an outer coat or tunic. MM0 The Cephalochordia are represented by Amphioxus> and one or two other allied genera which are decidedly fish-like in their general ap- pearance. The body is distinctly segmented ; the gill slits are very numerous, extending back along the alimentary canal to FIG. 2. Diagrammatic section of adult tunicate, a, atrial opening ; />, branchial chamber ; h, heart ; i, intestine; m, mouth; n, nerve centre ; r, reproductive organ and duct ; t, tunic ; v t vent. INTROD UCTION. the liver ducts, the stomach thus being entirely absent. The notochord extends along the whole length of the animal. These forms, however, differ from the vertebrates in the absence of vertebrae, in the peculiarities of the central nervous system FIG. 3. Diagram of Amphioxus, chiefly after Boveri. A, atrium ; AP, atrio- pore ; B, branchial clefts ; G, gonads ; Z, liver ; M, mouth surrounded by cirri ; A/Y, myotomes ; N, nephridia ; NC y notochord ; S, spinal cord ; V, vent. and the nerves which arise from it, in the total absence of a heart, of paired eyes, etc., as well as in the relations of excre- tory organs, etc. The species are few in number, and are all marine, being found in the warmer seas of all parts of the world ; on our coasts as far north as the mouth of the Chesa- peake. The Enteropneusti are decidedly worm-like in appearance, and their pertinence to the chordate phylum is denied by many. The so-called notochord is but a small diverticulum from the alimentary tract without skeletal character, while it is not found that the segmentation of the body is the same as that in the other chordates. The best-known form Balanoglossus was long considered a worm. It lives in the sand of the seashore in many parts of the world. Other allies are so dif- ferent in appearance {Rhabdopleura^ Cephalodiscus) that they were long regarded as Polyzoa. For further details concerning these forms reference must be made to works upon invertebrates, and to the special papers dealing with them. With this brief reference they must be dismissed here ; for the purpose of the present work is to deal with the single subphylum, Vertebrata. In the second or systematic portion of this volume, the dif- ferent subdivisions of the group of vertebrates will be defined ; 4 INTRODUCTION. but as it is necessary to use the names of several of the major divisions in the general account of the vertebrates, a tabular statement of classification with familiar examples is given here. Details can be found by reference to the index. VERTEBRATA. Cyclostomata (without true jaws). Myxinoidei (hag-fishes, borers, Myxine). Petromyzontes (lampreys). - Gnathostomata (with jaws), a* Ichthyopsida (with gills in adult or young). Pisces (with paired fins). Elasmobranchii (sharks and skates). Holocephali (elephant-fish, Chimczra). Ganoidea (sturgeon, garpike, etc.). Dipnoi (lung-fishes). Teleostei (ordinary bony fishes). Amphibia (frogs, toads, salamanders, etc.). - Sauropsida. Reptilia (lizards, snakes, turtles, alligators). Aves (birds). \ Mammalia (rats, cats, elephants, whales, man, etc.). MBRYOLOGY. INTRODUCTORY EMBRYOLOGY. IN order to understand clearly the structure of a vertebrate, it is well to begin with a short account of some of the phe- nomena of development, since a knowledge of the history of the parts will make their relations, one to another, more com- prehensible. The following outline is given in the briefest manner and in the most generalized form, the various modifica- tions which are found in the different vertebrate groups being ignored. All vertebrates reproduce by means of eggs. These eggs are specialized cells, produced by the female, which have the capacity, after impregnation, of developing into an animal like that which produced them. The impregnation consists in the union with the egg of a still more specialized reproductive cell, the spermatozoan, produced by the male ; and it is only after this union (called also fertilization) that development is possible. The fertilized egg divides (seg- ments) again and again, the result being that the egg is converted into a many-celled embryo. At first the cells of this embryo are arranged in a single layer, surrounding a cen- tral segmentation cavity (Fig. 4). Next, those cells upon one side of the embryo become pushed inside of the others (invaginated) much in the same way that one might push in one side of a hollow rubber ball, the result being partially to oblit- erate the segmentation cavity, and to differentiate the previous single layer into two. This two- layered embryo is known as a gastrula (Fig. 5). FIG. 4. Section of an early stage ef the egg of Amblystoma showing the smaller cells at one pole, the larger at the other, and at S the segmentation cavity. 6 INTRODUCTORY EMBRYOLOGY. The outer of the two layers of the gastrula is called the ectoderm, the inner the entoderm. 1 The cavity bounded by the FIG. 5. Diagram of a gastrula and of the later closure of the blastopore, a, archenteron; <, blastopore ; ec, ectoderm ; en, entoderm ; S, segmentation cavity. entoderm is the archenteron (stomach), and the opening where ectoderm and entoderm meet, and where the archenteron com- municates with the external world, is the blastopore. Usually FlG. 6. Sagtital section of early embryo (late gastrula) of Amblystoma. b, blastopore; c, beginning of infolding of brain; ^, entoderm; ;//, thickening of ectoderm for mouth, hypophysis, and nose ; ;/, mesoderm ; y, yolk-mass. the blastopore is an elongate slit, its major diameter coinciding with the longitudinal axis of the future animal. Soon after invagination, the blastopore begins to close, the opposite lips i In many English works these two layers are called respectively epiblast and hypoblast, while the mesoderm. to be mentioned later, is called mesoblast. There is no- longer necessity for using these terms. . INTRO D UCTOR Y EMBR YOLOG Y. uniting in the median line. This process of closure begins at one end and proceeds towards the other, the end where the first union takes place being the anterior. In some forms the blastopore never closes completely, but persists in part as the anus of the adult. In those forms where it closes completely the anus later appears in the line of fusion. Another landmark may be noted here, the blastopore closes along the median line of the back ; and the region of this closure is known as the primitive streak, the line of closure being the primitive groove. From the region of the blastoporal lips (primitive streak) there next grows into the segmentation cavity, on either side, a third layer, the meso- derm. In several forms (Fig. 7) this mesoderm clearly arises as an out- growth from the entoderm in the shape of a double fold, its walls bounding a cavity (coelom) which at first is connected with the archenteron. Later the connection between these coelomic pouches and the archenteron is lost, the lips of the outgrowth fusing, and then the mesoderm completely separates from the entoderm. In other cases the mesoderm ap- pears as a solid outgrowth from the same point, and FIG. 7.. Transverse section of Ambly- stonia embryo showing formation of meso- derm (mesothelium). a, archenteron ; c, coelom ; ec, ectoderm (outer layer) ; en, ectoderm (nervous layer); ;//, medullary plate ; n, notochprdal cells ; /, parietal layer of mesothelium ; s, remains of seg- mentation cavity; z>, visceral (splanchnic) layer of mesothelium. later it splits so as to form a coelom comparable to that first described. 1 The result in either case is that the segmentation cavity is still farther reduced by the extension into it, on either side of the embryo, of a flattened, mesodermic sac. In this sac two walls can be 1 The type of coelom in the first case is called an enteroccele ; in the second a schizocoele. 8 INTRODUCTORY EMBRYOLOGY. distinguished : the one turned towards the ectoderm is. called the somatic or parietaMayer ; the one facing the entoderm is the splanchnic or visceral layer. Besides this mesoderm arising thus as a continuous out- growth, another type of mesoderm also invades the segmenta- tion cavity. This arises by the migration or inwandering into this space of single cells, which may separate themselves from either entoderm or mesoderm ; or in some instances, as recent investigations tend to show, from the ectoderm as well. Since these two types of mesoderm differ in their origin, and, as will be seen later, in their character and fate, they have been given different names. That mesoderm which bounds the coelomic cavities and all parts formed from it is called mesothelium ; that which arises from the scattered immigrant cells is mesenchyme. ^ At this point, where the four germ-layers of the embryo are differentiated from each other, it will be interesting to state what portions of the adult vertebrate are derived from each. The ectoderm gives rise to the outer portion (epidermis) of the skin, the outer layer of scales, hair, feathers, the enamel of the teeth, nails, claws, true horn, and the essential parts of all sensory and nervous structures. The entoderm develops into the lining of the alimentary canal and the various cavities gills, lungs, liver, pancreas connected with it ; also to the notochord, and possibly to the lining of the blood-vessels. From the mesothelium arises the lining of the body cavity, reproductive and excretory organs, and the voluntary muscles (including the muscles of the heart). The mesenchyme produces the deeper layers of the skin, the lower portions of scales, and the dentine of the teeth ; invo"P untary muscles, connective tissue, fat, cartilage, bone, blood^ and lymph corpuscles. From the point where the germ-layers are outlined the de- velopment must be traced in two different directions. One line follows out the differentiation of the cells and their grouping into tissues ; the other traces the development of the various , organs of the adult. EPITHELIAL TISSUE. HISTOLOGY. Histology deals with the minute structure, and especially with the characters of the cells and the tissues arising from them. In the adult occur cells varying in shape and size, and adapted for various functions ; those cells which are alike grouped together into tissues. A tissue, then, may be defined as an aggregate of similar cells, together with a varying amount of intercellular substances, usually produced by the cells them- selves. The cells themselves are the living portions of the tissue ; the intercellular substance, by its amount and character, being directly influential in determining the nature of the tissue. Tissues may be solid or fluid ; may form thin sheets or thick masses. All tissues can be grouped under four heads, epithelial, nervous, muscular, and connective. Epithelial tissues are the primitive tissues. An epithelium is a layer of cells covering any free surface on or in the body. Thus in the gastrula both ectoderm and entoderm are epithelia, since the one covers the outside, while the other lines the archenteron. The mesothelium is also epithelial l since it lines the cavity of the ccelom. Epithelia are classified according to< shape, arrangement, or character of the cells. In cubical or columnar epi- thelium the cells have shapes corresponding to their names ; in pavement epithelium the cells are greatly flat- tened, so that each one, while very thin, covers, comparatively, a large amount of surface. Epithelia are simple when the cells are arranged in a single layer; stratified when they form several layers. In some cases the epithelial cells may bear on their free sur- 1 The term epithelium is sometimes restricted to those layers on the outer surface of the body, or, like the epithelium of the lungs and stomach, connected with the exterior. The similar cells in the closed cavities, like the body cavity or the blood-vessels, are then called endothelia. The distinction is of little importance. FIG. 8. Epithelia; A, columnar; B, pave- ment, in perspective ; C, cubical ; Z>, stratified. 10 HISTOLOGY. faces minute vibratile hair-like processes (cilia), whence this type is called ciliated epithelium. Again, epithelia may be grouped according to function, and then cuticular, sensory, and glandular epithelia may be recognized. Epithelia may give rise to important structures, such as hair, feathers, scales, enamel of the teeth, etc., which will be mentioned in the proper places. Nervous tissue arises from the ectoderm, and hence from epithelium. It has for its purpose the recognition and trans- ferrence of impulses, the perception of sensations, and the pro- duction of other impulses which shall affect nervous or other tissues. It has for its essential constituent nerve cells, to which are usually added other cells of a supportive nature. Nerve cells (or ganglion cells) consist of a central nucleated body from which radiate one or more protoplasmic processes which, FIG. 9. Different forms of nerve cells from cat. A, pyramid cell from cerebrum ; B, cell from spinal cord ; D, unipolar cell from spinal gan- glion ; C, glia cell from spinal cord ; a, axis cylinder. after a longer or shorter course, break up into minute branches or fibrillations. It must, however, be kept in mind that these processes are really parts of the cell, although the term cell is frequently restricted to the central mass, while the processes are called nerve fibres, etc. When a nerve cell has two proto- plasmic processes it is spoken of as bipolar ; when more than two as multipolar. 1 One of these processes is of considerable i In some cases ' unipolar ' nerve cells are found ; but the process in these is soon found to divide, its halves going off at right angles to the previous course, thus showing that these cells are really bipolar. NERVOUS TISSUE. II length, and is known as an axis cylinder or axon ; the others are shorter, and as they soon break up into minute branches, they are called dendrites. In most cases the axis cylinders have a similar method of termination. Recent investigations show that the only connection between nerve cells consists in an interlacing of these fibril- Ice ; two nerve cells never join. The axis cylinder is the essential part of a nerve fibre. Of these fibres two kinds are to be recognized. In the medullated fibres the axis cylinder is surrounded by a medul- lary sheath of a peculiar substance (myelin) rich in fat. This sheath, it is to be noted, usually stops before the end of the axis cylin- der, and in most cases it is not continued to the central mass of the cell. In the non-medul- lated fibres the sheath is lacking, and only the axis cylinder is found. Both medullated and non-medullated fibres may have a second sheath (the neurilemma or sheath of Schwann) de- rived from the connective tissue (see below), and containing scattered nuclei. Nervous tissue is made up of these nerve cells. In a nerve proper we have but a bundle of nerve fibres (axis cylinders, medullated or FIG. 10. Por- non-medullated) bound together by connective tions of medulla ted ... - nerve fibres (from tissue, while the bodies of the cells are absent. Martin). The These nerves are but conducting trunks, bear- medullary sheath, ing impulses to or from the central portion of stamed black b Y . ,, . ... osmic acid, is inter- the cell. From their color, those parts which rupted at R ^ tne are formed entirely of nerve fibres are called nodes of Ranvier, the white matter. The cell bodies, together across which the .., ~, . . . - . axis cylinder ex- with fibres, dendrites, etc., unite to form the tends> Outside the gray matter, which may be aggregated in medullary sheath is smaller centres (ganglia) or in larger continu- the Oesenchyme) , , 1-1 i sheath of Schwann, ous tracts, as in the brain and spinal cord. the nudei of which In these parts occur certain supporting cells are seen at c. 12 HISTOLOGY. (neuroglia) derived from the ectoderm, but lacking entirely in nervous properties. These glia cells are extensively branched, their branches running between fibres and cell bodies (Fig. 9, 7). Muscular tissue is the special contractile element in the body. It is of two kinds, different in origin, structure, and ac- tion. The mesenchyme gives rise to the smooth muscle. This consists of long spindle-shaped cells, each usually containing a single nucleus, and being marked with fine longitudinal lines. These cells may occur singly, or may be arranged in small bundles or thin sheets ; and in all cases they are not under control of the will, a fact that gives rise to the name, involuntary muscles, often applied to them. Smooth muscular tissue is slow in its action. Striped muscular tissue, on the other hand, is derived from the mes- othelium by modification from the muscle plates, to be described later. It occurs usually in larger masses than does the mesenchymatous muscular tissue, and is (except in the case of the heart) under control of the will. This striped tissue consists either of separate cells (heart muscles) or cf usually long cylindrical, so-called primitive fibres, each of which contains several nuclei ; i.e., is syn- cytial. In these primitive fibres the bulk of the protoplasm has been altered into a strongly contractile substance marked with fine trans- verse lines. Around each fibre is a struc- tureless envelope, the sarcolemma ; while the fibres are bound together into muscle bun- dles by means of connective tissue envelopes (perimysium) bearing nerves and blood-ves- sels, and continuous with the tendons and () an( j t h e sarco- fascia by which the muscles are attached to lemma, s, where the other structures. The nuclei are oval, with muscle fibre . is torn - From Hertwig, after their long axes parallel to the direction ot Gegenbaur. FIG. ii. Smooth muscle fibres. FIG. 12. Cross THE CONNECTIVE TISSUES. the fibres. In the mammals they are placed upon the periphery of the fibres (Fig. 12, n), in the lower vertebrates near the cen- tre. The muscles of the heart agree in origin and cross-band- ing with the voluntary muscles, but differ in being cellular rather than syncytial, and in being removed from the control of the will. All cross-banded muscles are capable of rapid contrac- tions. The connective tissues are all of mesenchymatous origin, and are characterized by a great development of the intercellular substance, which is usually a pro- duct of the cells. They are the sup- porting tissues of the body, and vary accordingly as this intercellu- lar substance va- ries, and may correspondingly be grouped under Several subheads, FIG> ^ Fibrous non . e i ast ic connective tissue the principal ones ( from Martin). being enumerated below. In fibrous connective tissue (white or non-elastic tissue) the cells are branched or spindle-shaped, and the intercellular sub- stance is more or less fibrous, the fibres being parallel, in- terlaced, or in a network, so that there result sheets, mem- branes, or bundles, accord- ingly as the part to be played varies. In some cases this tis- sue is loose (areolar tissue), such as is found between the skin and deeper parts ; at FIG. 14. Fat. o, oil globules in the connective tissue cells. other times it is much firmer, as in the case of tendons. This type of tissue also gives rise to HISTOLOGY. FIG. 15. Elastic tissue (from Martin). fat (adipose tissue) by the deposition of oil in the protoplasm of the cells. In elastic tissue (yellow connective tissue) the intercellular fibres are larger, and elastic in character. In cartilage the intercellular sub- stance (here called the matrix) is more solid and firmer. It varies considerably in abundance, and in proportion to its amount the cartilage gains as a support- ing tissue. When the matrix is homo- geneous, the result is hyaline cartilage ; but it may be traversed by fibres of white or yellow connective tissue, thus producing fibrous or elastic cartilage. The cells of cartilage are circular, oval, or fusiform in outline ; but they send out very fine protoplasmic processes which traverse the matrix, thus connecting all parts. Cartilage in- creases in size in three ways, by addition of new cells to the outside, by increase in the amount of the matrix, and by division of the cells in the cartilage itself. In almost every sec- tion of cartilage several gen- erations of cells may be readily traced by observing the capsules surrounding them. Cartilage is very closely related to bone, and is frequently converted into that more solid substance by a change (ossification) in its matrix. Cartilage may also be calcified by the deposition of lime upon its surface. Calcified cartilage and bone are entirely distinct. In bone this matrix consists of an organic basis combined with salts of lime (chiefly carbonate and phosphate) ; while car- tilage is usually solid, bone is traversed by tubes (Haversian Wim FIG. 1 6. Hyaline cartilage, the matrix dotted. THE CONNECTIVE TISSUES. canals) bearing nutrient vessels, etc. Arranged in layers con- centric to these canals or parallel to the surface of the bone are the cells, each occupying a space (lacuna) in the dense matrix. These cells are connected by fine, branching, proto- plasmic processes, which run in minute tubules (canaliculi) through the layers (lamellae) of the matrix. Both cartilage and bone are enveloped in a layer of fibrous con- nective tissue, called re- FIG. 17. Bone. A, piece of a long bone Bone. showing the appearance under low power in longitudinal and cross sections; B, a transverse section of three lamellae surrounding an Haver- sian canal, from a slice of dried bone; c, bone corpuscles ; fit, canaliculi ; h, Haversian canal ; /, lamellae. spectively perichondrium and periosteum. Many bones, as has just been said, pass through a cartilage stage in their history, the gen- eral outlines being built up in that more yielding substance. Later the matrix is dissolved little by little, and is replaced by the lime salts, the cells (osteoblasts) becoming enclosed in the hardened sub- stance. Such bones are called cartilage bones. Other bones, however, have no cartilage stage, but arise from the calci- fication of the inter- cellular substance of membranes, and these are called membrane bones. In either case the process of ossification proceeds from fixed spots (centres of ossification) FIG. 1 8. Development of membrane bone (mandible of pig). Around the (black) bone are numerous osteoblasts, some of which are included in the bony substance. i6 HISTOLOGY. extending gradually in all directions until the final result is much the same. In structure cartilage bones and membrane bones are very similar, but the differences in the history is very important, as will be seen later in dealing with the skeleton. Closely allied to bone is the dentine of teeth and scales, the chief differences lying in the greater density of the intercellular substance, and in the fact that the dentine-producing cells (odontoblasts) do not become included in the solid structure. The same fine protoplasmic processes of the cells exist, lying in dentinal canals which pursue a nearly parallel course. Blood and lymph are connective tissues, with a fluid intercel- lular substance (plasma) in which float the cells. In lymph the cells are all of one type, known as leu- cocytes, white in color, and possessed of marked amoeboid powers. In blood, besides the leucocytes (white corpus- cles), there are also numerous red cor- puscles, the source of color of the blood. These red corpuscles have no amoeboid powers, but are merely the means of transferrence of oxygen and carbon di- oxide l to and from the tissues. In the lower vertebrates the red corpus- cles are oval and nucleated ; in the mammals the nuclei are lost, and the corpuscles are usually biconcave, circular disks. The blood plaques may also be mentioned. FIG. 19. a, b, <:, red blood corpuscle of man; d, white corpuscle of man; e, red cor- puscle of frog. Carbon dioxide is also carried by the plasma. VDERMAL ORGANS. MORPHOLOGY OF THE ORGANS OF VERTEBRATES. FROM the point where the four germ-layers are clearly dif- ferentiated from each other, we have now to trace the various derivatives of each ; but it must be kept in mind not only that various organs are in the process of development at the same time, while the necessities of treatment demand that they be arranged in sequence, but that two or more layers not infre- quently contribute to the same organ. In such cases the organ is described in connection with that layer which is most promi- nent or most important in its structure. In the following account the stages of development are traced only with such detail as is necessary for a clear interpretation of the adult structure. For more extended accounts the student must go to the embryologi- cal manuals and special memoirs. ENTODERMAL ORGANS. The differentiation of the entoderm by invagination has been described (p. 5). By this process of gastrulation a layer of entoderm cells comes to lie inside the other or ectoderm cells, and by the closure of the blastopore (usually complete) it assumes the form of a sac, the cavity of which is the archenteron. As the embryo elongates, the sac forms an elongate tube. In the middle line of its dorsal wall a cord of cells, lying between the outgrowing ccelomic pouches (Fig. 7, ;/), becomes constricted off from the rest, 1 and occupies a position between the other entodermal structures and the nervous system (Figs. 7 and 20). This rod is the notochord, the subsequent history of which is given in connection with the skeleton. The rest of the entoderm, after the formation of the noto- 1 In a few forms (e.g., Amblystoma) this cord is at first tubular ; later its lumen is lost. i8 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. chord, gives rise to the lining (epithelium) of the digestive canal (alimentary tract) and its appendages, and of the respira- tory organs (gills and lungs). The first step in the differentiation of the alimentary struc- tures is the formation of an outpocketing on the ventral side, the beginning of the liver (Fig. 20, /). This oc- curs some distance in- front of the middle of the body, and divides the alimentary canal into pre- and post- hepatic portions. The post-hepatic portion gives rise to the intes- FIG. 20. Longitudinal section of Amblystoma embryo. /z, hypophysis; ht> heart, its endo- thelial walls not shown ; z, infundibulum ; in, intestine; /, liver; m, mesenchyme ; n, noto- chord ; /, pineal outgrowth ; pc, pericardial cavity. tine and its various di- visions, including the pancreas ; from the pre- hepatic region are developed the pharynx, with the respiratory structures, the gullet, and the stomach. The Mouth. Besides these entodermal structures, the ali- mentary tract, as usually considered, embraces as well the cavity of the mouth, the lining of which is ectodermal in origin. The mouth arises as an inpushing or involution of the ectoderm 1 at the anterior end of the ventral surface of the body. The in- pushing usually takes the form of a cup, the blind end of which impinges directly upon the closed anterior end of the alimentary canal proper, thus forming a double partition between the two (Fig. 55). These two membranes, one ectodermal the other entodermal in origin, fuse, and then an opening breaks through, thus placing the whole in communication. From this ectodermal oral invagination or stomodaeum are developed the lips, teeth, tongue, and glands. The lips bound the opening of the mouth. In all the lower vertebrates they are merely folds of epithelium, or, as in turtles 1 In some forms this inpushing is plainly a paired structure, a fact which adds no little weight to the view which regards the vertebrate mouth as having arisen from the coalescence of a pair of gill slits. TEETH. 19 and birds, they may be entirely absent. In the mammals fleshy lips moved by muscles first occur, and even here they are lacking in monotremes and cetaceans. In turtles and birds the edges of the jaws, and to a greater or less extent the roof of the mouth, is covered with a cornified epithelium forming the so-called beak, and the same is true of the adult monotremes. The surface of this may be thrown into folds for the purpose of crushing the food, but these structures are not to be compared with true teeth. Teeth. In the formation of teeth two layers, ectoderm and mesenchyme, are concerned. The epithelium lining the mouth becomes inpushed into the deeper layers, where teeth are to be formed (Fig. 21). In the lower vertebrates there is a separate inpushing for each tooth, but in the mammals there is a con- tinuous ingrowth, the dental ridge. In other respects the features of development are essen- tially the same in all. The ingrowth is to be regarded, morphologically, as vesicular ; and the deeper wall of the vesicle becomes pushed in- side the other, so that there results a two- walled cup, the cavity of which becomes filled with mesenchyme. The cells of the inner layer * FIG. 21. Tooth become columnar and form the enamel organ ; germ O f Ambiysto- the immigrant mesenchyme cells constitute a ma. d, derma ; e, dental papilla, the external cells of which are e P iderm l s ; > en ~ amel organ ; /, den- known as odontoblasts, from their power of tal p ap jj ]a- secreting a bone-like substance, the dentine or ivory of the tooth. The inner surface of the enamel organ likewise secretes a cup of denser substance, enamel, upon the outer face of the dentine. By growth of the deeper portions (dentine) the enamel cov- ered tip or crown of the tooth is forced up through the epithelial layers so that it comes into position for use. The deeper por- tion or root contains a central or pulp cavity, in which are remains of the mesenchyme, together with nerves and blood-vessels, these together forming the pulp. In the mammals the root is cov- ered by a second coat, the cement, formed by the surrounding tissues (Fig. 22). In the lower forms the process of tooth formation may con- 2O MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tinue for a long time, even through life, new teeth thus arising to make good the loss of others. In the mammals, however, there are at most but two of these sets of teeth, a temporary or milk dentition, and a second or permanent dentition, the details of which are given in connection with that group. In the lower vertebrates teeth may appear in any part of the mouth where there are solid parts (bones or cartilages) to support them. Thus in fishes and amphibia we may find them not only along the edges of the jaws, but upon any of the bones which lie in the walls of the oral cavity. In the higher vertebrates they are confined solely to the edges of the jaws. Teeth are very variable in shape, a fact largely correlated with differences in food. In the lower vertebrates all of the teeth of an individual are closely similar to each other. This is the hgrnodont condition ; the hetero- dont dentition appears in the mammals, and occasionally in the lower groups, where the teeth in the different regions of the mouth are of different shapes. Usually in the lower vertebrates each tooth possesses but a single root and a single cusp ; while in the mam- mals, besides these simple teeth, there are others, with two, three, or several roots, the crowns also showing a corresponding duplica- tion of parts. In the elasmobranchs the teeth rest upon but are not firmly united to the skeletal parts. In the other ichthyopsida they are usually firmly united to the bones of the mouth by continuous growth, and the same is true of many reptiles. In others they may be implanted in sockets (alveoli) in the jaws, a condition which is universal in the mammals. Besides these true or calcified teeth, horny teeth occur here and there, as in the cyclostomes, where the oral hood and the tongue are armed with such structures resting upon epithelial FIG. 22. Dia- grammatic section of mammalian incisor tooth, c, cement; d y dentine ; a, Dactylethra) a tongue is not formed. 22 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. occur, a sublingual, a submaxillary, and a parotid. None of these are poisonous ; but the saliva which they secrete is for moistening the food during mastication, and for the conver- sion of starch into sugar. From the pharyngeal region are devel- oped the respiratory organs, gills and lungs, as well as certain other struc- tures, the thyroid gland, thymus gland, etc. Gills. Gills arise as a series of paired or bilateral outpushings of the entodermal lining of the pharynx. These push out through the mesodermal and mesenchyma- tous tissues until they reach the ectoderm on the sides of the neck. The two layers now fuse, and then an opening is formed at the point of fusion, so that there arise a series of openings (gill-, branchial-, or vis- ceral-clefts) on either side, connecting the pharyngeal cavity with the external world. In the septa between the clefts are devel- oped skeletal structures (gill- or branchial- arches, see skeleton), and also blood-ves- sels. From the walls of the clefts develop vascular leaves or filaments, the gills proper. These are arranged on the anterior and pos- terior walls of the clefts, those on a side constituting a demibranch. The number of gill pouches differ in different groups. In Bdellostoma a (cy- clostome) there may be 14 pairs ; in the notidanid sharks 7 or 8 ; in other sharks 6, and from this down to 5 in reptiles, and 4 in mammals. In the th^opsida all, or nearly all, of these pouches break through as descriBeH above, but in the amniotes but one or two open to the exterior ; the statements regarding the mammals being conflicting. In the jimniotes these gill pouches or clefts never develop gill filaments ; ancT in the adult all traces of them, except FIG. 23. Horizon- tal section through head and pharyngeal region of Acanthias embryo, show- ing the gill slits. , blood-vessels; <", coelomic cavities of gill arches ; /, developing gill fila- ments; //, hypophysis; , notochord ; 0, oculo- motor nerve ; /, pharynx ; s, spiracular cleft; /, first (mandibular) head cav- ity; I-V, gill clefts. RESPIRATORY ORGANS. of the first, are lost. Their presence in this group can only be explained as inheritances from branchiate ancestors. The first gill pouch in the anura and the higher groups form the Eusta- chian tube (see ear). In elasmobranchs and some ga- noids the anterior visceral cleft is smaller than the others, and opens on the top of the head. This spir- acle bears well-developed gills in the lowest sharks (notidanidae), but in others it may have but a vascular network in its walls. In ganoids and embryonic teleosfs it has a gill-like structure ; but it is here termed a pseudobranch, since it receives arterial blood from the opercular gill. The opercular gill is a secondary and ectodermal structure developed on the inner or posterior face of the operculum (see below). In the typical elasmobranchs the interbranchial septa extend to the outside of the body, and the gill clefts open directly to the exterior, either on the sides of the neck (selachii, Fig. 26) or on the ventral surface (raiae). In the cyclostomes, Myxine excepted, there is also a separate opening for each gill cleft. In the holocephali a fold of skin on either side grows back over the gill clefts, thus en- closing a space into which these empty, and which in turn connects with the exterior by a slit-like gill opening be- FiG. 24. Section through the head of a pig embryo 6.5 mm. long, showing the gill slits (1,2,3,) closed by a thin wall. At the left a small portion enlarged. , Eustachian cleft ; //, hyphophysis ; /)/, man- dibular cleft ; P, pharynx. FlG. 25. Tadpole of frog, showing the external gill opening. hind. In the ganoids and teleosts the same relations occur ; but in these the fold, known as the operculum, has a cartilaginous or bony internal skeleton. In the amphibia the opercular fold is also found, but 24 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. in urodeles and caecilians it develops but slightly. In the anu- ran tadpole, on the other hand, the opercular folds of the two sides unite beneath the throat, thus connecting the extra bran- chial chambers of the two sides, and then the folds unite to the sides of the body, usually leaving but a single opening on the left side through which water is discharged from both right and left gills 1 (Fig. 25). In the cyclostomes the gill slits are narrow tubes widened in the middle into a saccular shape (whence the name marsipo- branchs, pouched gills, often given the group). In these sacs A B FIG. 26. Relations of gill clefts, etc., in an elasmobranch, A, and a teleost B. are the demibranchs. In the elasmobranchs the septa extend to the external surface, the gills not extending so far. In ganoids and teleosts, on the other hand, the septa are reduced to small rods while the demibranchs are greatly enlarged. In the embryonic amphibia external gills occur. These are ectodermal structures developed from the outer surface of the gill septa 2 even before the gill clefts break through. In 1 No operculum is developed in the amniotes ; but there is some plausibility in the view which regards the external ear of the mammals as a derivation of the ichthyopsidan operculum. 2 Relations, blood supply, and nerves go to show that the fleshy processes (so called balancers) of the urodele larvae are the modified external gills of the hyoid arch (see Fig. 199). RESPIRATORY ORGANS. 2$ the perennibranch urodeles these external gills persist through life. 1 In the other urodeles they are lost without replacement. FIG. 27. Head of young Amphitima showing the external gills', partially covered at the base by the backward extension of opercular fold. In the anura, on the other hand, the external gills are early lost, and are replaced by internal gills upon the sides of the clefts, which, however, are said to be of ectodermal origin. Air-bladder. From the pharyngeal or cesophageal region there arises also in most ganoids and teleosts the air- or swim-bladder. It starts as a diverticulum from the dorsal 2 wall of the phar- ynx, the distal portion of which enlarges into a thin walled sac, the air-bladder or pneumatocyst ; the proximal portion forms the pneumatic duct. This dllCt remains FIG. 28. Relations of the air-bladder open throughout life in the to the alimentar y canal > after Dean - A > ganoids and the lower tele- osts, but in the higher tele- and dipnoans. osts it closes and is reduced to a fibrous cord. 3 The bladder itself usually lies dorsal to the aorta and urogenital system next the vertebral column. In 1 Cope, however, claims that in Siren the embryonic gills are lost, and that the per- sistent gills of the adult are new structures. 2 The pneumatic duct empties laterally in some characinidae, ventrally in Polypterus* into the oesophagus; but until the development is known, we cannot say how far this condition is secondary. 3 The teleostei were formerly subdivided into physostomi, with permanent pneumatic duct, and physoclisti with it closed. in most physostomous teleosts; B, in Ery- thrimis; C, in Polypterus, Calamoichthys 9 26 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. shape it varies greatly ; it may be unpaired, or it may consist of two paired lobes. It may be a simple sac, or it may be subdivided into two or several successive chambers. Its in- ternal walls are usually smooth, but they may be considerably convoluted, thus greatly increasing the surface. Occasionally its walls are calcified. Its chief function is that of a hydro- static apparatus. It is not respiratory, as it receives arterial blood and returns venous blood. In some fishes (ostariophysiae) it is connected with the ear by a Weberian apparatus, consisting of a chain of small bones. According to the latest conclusions this apparatus seems to be for a recognition of variations in hydrostatic pressure. The swim-bladder is absent in some bot- tom-living fishes (pleuronectids, etc.). In the pharyngeal region of the elasmobranchs are caeca, which may be the structures from which the swim-bladder has developed. The swim-bladder, in turn, is usually regarded as having given rise, by substitution of functions, to the lungs. On the other hand, there are some who regard the lungs as new formations in the air-breathing vertebrates, and as having arisen i>y modification of a pair of gill pouches which have grown backwards instead of outwards, and consequently have failed to form connection with the ectoderm. The method of origin of the lungs and the relations of the cartilages of the larynx, shortly to be described, favor the latter view. Lungs. The lungs arise as an outgrowth from the ventral wall of the pharynx, just posterior to the last gill pouch. The outgrowth almost immediately divides into right and left halves, which grow back, laterally to the heart, into the anterior part of the body cavity, and the distal portions enlarge into thin walled sacs, the lungs proper. The proximal portions of the paired outgrowths form the bronchi, while the unpaired por- tion gives rise to the windpipe or trachea, the opening by which the trachea communicates with the pharynx being the glottis. In this backward growth there is added to the entodermal epithelium of these organs mesenchyme tissue, while the lungs, invading the coelom, become covered externally with a thin layer of epithelium (peritoneum, see coelom). Between the RESPIRATORY ORGANS. 2/ two epithelia run numerous blood-vessels, arteries, veins, and capillaries, conveying blood to and from the lungs. In the lower amphibia the lungs develop scarcely beyond the condition of simple sacs with respiratory ducts. 1 In other forms, however, there is increase of surface by a folding of the internal wall, to be described later ; and in those still higher there is a division of the primary bronchi into bronchi of sec- ondary and tertiary orders, each of which connects with a separate division of the pulmonary organ. FIG. 29. Alimentary tract of human embryo, A at four weeks, B at five weeks, after His. a, allantois stalk ; l>, bile duct; c, caecum; l the intestine was connected with the yolk. In cseca; s, stomach. these higher forms increase of intestinal sur- face is brought about in part by the lengthening of the intes- tine, and in part by the development of numerous small folds (valvulae conniventes) and minute finger-like projec- tions (villi) resembling the pile of velvet. The hind gut is hardly distinct in fishes, as viewed externally, but from the amphibia on it acquires greater individuality. It may consist merely of a straight tube, rectum, or it may have a terminal rectum connected with the mid gut by a more or less convoluted tube, the colon. Just behind the ileo- colic valve in the forms from the turtles upwards is developed a FIG. 43. Part of small intestine of man, showing the valvulse conniventes, from Martin. 1 The marks on certain reptilian coprolites indicate that some extinct reptiles may have had a spiral valve (Fig. 41). DIGESTIVE TRACT. 39 blind tube, the intestinal caecum, 1 which is clearly connected with increase of digestive surface. In certain birds there may be two of these caeca, and in the ostrich there is developed on its inside a spiral fold. In the mammals the caecum shows great varia- tions. It is lacking entirely in certain groups (edentates, chei- roptera, some carnivores). In the herbivorous forms, on the other hand, it may equal the body in length. In man and some, apes and rodents all parts of the caecum are not equally devel- oped, the terminal portion, known as the appendix vermiformis, remaining smaller than the rest. In the elasmobranchs, dipnoans, amphibians, sauropsida, and the monotremes among the mammals, the rectum does not open directly to the exterior, but into a terminal enlargement, the cloaca, into which the urinary and reproductive ducts also empty ; and from this chamber all contents pass to the exterior through the vent. In the other vertebrates no cloaca is formed. In connection with the cloaca in birds is developed a sac (bursa Fabricii), which comes to lie in the pelvic cavity be- tween the vertebrae and the terminal portion of the hind gut. Its function is unknown. The bursa is said to be of ectodermal origin. The alimentary tract is here placed among the entodermal structures, but only the lining coat is derived from that germ layer. Other constituent parts are derived from the mesen- chyme. Beneath the entodermal epithelium and following closely its contour is a layer of loose connective tissue, the sub-mucosa, 2 which carries blood and lymph vessels. Outside of this are the muscular layers, two in number, an inner circular and an outer longitudinal, each of smooth or non-voluntary muscle. These by their action produce peristaltic movements of the contents of the tube. Where it passes through the body cavity the alimentary canal receives a third or peritoneal layer of pavement epithelium derived directly from the splanch- nic layer of the ccelom. 1 The rectal gland of the elasmobranchs is possibly home logous with the caecum of the amniotes. 2 Occasionally the sub-mucosa may be divided by a muscular layer, in which case that portion nearest the entodermal epithelium is called the tunica propria, the deeper portion retaining the name sub-mucosa. 40 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Liver. The liver, as was said above, develops as a divertic- ulum from the ventral side of the primitive alimentary canal. This outgrowth branches again and again, the result being a greatly branched tubular gland, the proximal portion of the tubes being specialized as the ducts leading to the intestine. In the amphibia and reptiles this tubular condition is retained throughout life, the minute lumen of the glandular portions being known as the gall capillaries. In birds and mammals the tubular condition soon disappears, the gall capillaries run- ning, without much regularity, between the cells. By the in- A C FIG. 44. Hepatic ducts of A, frog; B, emeu; and C, cat. A, ampulla of Vater; B, gall-bladder; C, cystic duct; Z>C, ductus choledochus; H, hepatic ducts; HE, hepatoenteric duct; W, duct of Wirsung (pancreatic). growth of connective tissue the liver glands are divided into lobules, the so-called liver islands.' In this connective tissue run the larger gall ducts (which connect with the gall capil- laries), and also branches of the hepatic artery and of the por- tal vein (see circulation). From their position these vessels are often spoken of as interlobular. In the centre of each island (intralobular in position) is a branch of the hepatic vein, while capillaries extend through the lobules from the inter- lobular to the intralobular blood-vessels. As a rule, there is but a single duct emptying from the liver into the intestine, and this, as a rule, has connected with it by a lateral branch (cystic duct) a thin-walled gall-bladder. When these condi- tions occur, the duct leading from the liver as far as the mouth DIGESTIVE TRACT. 41 of the cystic duct is called the hepatic duct; from that point to the intestines, the ductus choledochus. Again, besides these ducts there may also be a separate hepatoenteric duct leading directly from the liver to the intestine, as well as other modifications not necessary to mention, aside from the numer- ous ducts in lizards and snakes. The liver thus formed is a large compact organ, largest in the lower vertebrates, and larger in flesh (fat) eating forms than in the herbivorous groups. In many fishes it forms a single, undivided mass, but in the great majority of vertebrates two lobes are present, and these in turn may be lobulated. The blood-vessels leading to the liver (portal vein, hepatic artery) enter in close relations to the gall ducts, while the veins (hepatic) leaving it are widely separate from these, in contrast to the conditions occurring in most organs. The liver is sup- ported by a mesentery (gastro-hepatic omentum) which connects it to the ventral wall of the alimentary tract, and which is fre- quently continued below as the suspensory ligament of the liver. The pancreas develops in much the same way as the liver, as an outgrowth from the entodermal walls just behind the liver outgrowth. There is the same increase in size, while branching gives rise to glandular portions and ducts. The pancreas has- recently been found to occur in several vertebrates where its existence was formerly denied, and farther research may reveal one in the cyclostomes where none has yet been found. Thus in certain teleosts its condition as a delicate tube lying in the mesentery, and its position in the dipnoi just outside the muscu- lar walls of the alimentary canal, caused it to be overlooked for a long time. In the elasmobranchs and other teleosts it is a well-marked gland. In other forms it is more complex in its origin. Thus in the ganoids (sturgeon) it arises by two dorsal and two ventral outgrowths ; in the amphibia and all higher forms, from one dorsal and two ventral outpushings, these later uniting into one glandular mass. The ducts can undergo vari- ous modifications, all persisting, or either dorsal or ventral dis- appearing ; or finally the ducts may come into connection with those leading from the liver (Fig. 44 C, 42 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Two other structures, the spleen and the urinary bladder, are closely connected with the entoderm in origin, but they are better described in connection with those structures circu- atory and excretory with which they become associated in later life. NERVOUS SYSTEM. 43 ECTODERMAL STRUCTURES. THE derivatives of the ectoderm may be subdivided into epidejmal, nervous, and sensojy organs, the differentiation of the nervous from the epidermal structures beginning with or even before the completion of the process of gastrulation. THE CENTRAL NERVOUS SYSTEM. The central nervous system begins its development as a structure distinct from the rest of the ectoderm by the forma- tion of a neural or medullary plate on the dorsal surface of the embryo. On either side of the primitive groove (fused blasto- poral lips, p. 7) the ectodermal cells become elongated (cylin- drical or fusiform), while in those regions destined to give rise to the epidermis they retain their more flattened character, the line between the two regions being sharply drawn. Soon after being outlined the lateral edges of the medullary plate begin to bend upwards and inwards, the whole thus form- ing a medullary groove bounded by the medullary fnlHQ rhp niitpr portion of each fold being formed by unaltered ectoderm (Fig. 45). This inward bending of the medullary folds continues until the edges meet, the medullary plate being converted by this process into the walls of a tube, which later develops into brain and spinal cord. The edges of the fold now fuse, neural parts with neural, epidermal with epidermal, so that the nervous portion becomes internal, and FlG. 45. Section through embryo Acanthias before the closure of the medullary groove, g. c, ccelom; cranial nerves. nervous in nature, 1 its outer surface (cortical substance) being composed of ganglion cells. In all the lower vertebrates the surface of the cerebrum is smooth, but in the higher mammals 1 Even in mammals a portion the septum pellucidum retains an epithelial character. 52 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. (educabilia) fissures appear in its surface, separating convolu- tions or gyri, and the higher the mammal the more numerous the convolutions. It will readily be seen that this produces an increase in surface, and consequently of cortical (ganglionic) substance ; and it is noteworthy that this increase is correlated with increase of intelligence. From its anterior ventral region each hemisphere gives off an olfactory lobe (rhinencephalon) into which a part of the ven- tricle may extend. Connected with each olfactory lobe is an olfactory ganglion which may be placed either in the cerebrum itself, or may be carried out towards the end of the olfactory lobe. From these lobes arise the olfactory nerves (see below). In the diencephalon the lateral walls become thickened into large tracts, the thalami, while the dorsal wall as a whole retains its epithelial character, becoming variously folded to form the anterior choroid plexus, which car- ries blood-vessels into the three anterior ventricles. From this dorsal surface are also developed three structures, pinealis, epi- physis, and paraphysis, to be mentioned again in connection with the sense organs. From FIG. 55. Sagittal section through the pinealis a pair of nerve tracts, head of larval Petromyzon, after the habenulae, run along the inner von Kupffer. c. notochord; h, in- , r ,, ,11 ^r-i n vagination for hypophysis ; <, infun- SldeS <* the thalam1 ' TllC fl r dibulum; ;//, mouth cavity; n, nasal of the thalamencephalon gives involution;/, pineal outgrowth. r j se to a hollow Outgrowth, the '^infun dibulum, which extends backwards and downwards, developing from its extremity tis- sue, which unites with other cells, derived directly from the ectoderm, to form the hypophysis or pituitary body. This ecto- dermal portion arises from the ectoderm between the nose and the mouth, or from the mouth itself, and grows upwards and inwards to join the infundibular portion. For a time it retains its connection with the parent layer by means of a cord of cells, the hypophysial duct, which later disappears. The significance of these ventral structures of the twixt brain is very obscure. NERVOUS SYSTEM. I A plausible suggestion is that the infundibulum represents the invertebrate mouth, the ectodermal portion of the hypophysis a modified pair of sense organs. The optic nerves are outlined as hollow outgrowths from the sides of the twixt brain, while on its ventral surface may be developed ac- cessory structures, the lobi inferiores, sacculi vasculosi, cor- i /.%*.*::.. pus albicans (corp. Jfe:k mam mil are) tuber cinereum, etc. A topographic point is to be kept in view, the cerebral hemispheres and the diencephalon are in front of the anterior end of the notochord are prechordal. The mesencepha- lon has its walls thickened so that its contained ventricle, in the higher groups, is reduced to the nar- row aqueduct already mentioned. Its dorsal surface is divided by a longitudinal groove into right and left lobes (corpora bi- gemina), and these in turn may each be subdivided transversely into two (corpora quadrigemina). 1 Leading ventrally and for- wards from these lobes in all except the cyclostomes are the optic tracts connecting with the optic nerves. The floor of the mid brain is formed by a pair of fibre tracts crura cerebri separated by a longitudinal fissure. The cerebellum or metencephalon is a thickening of ner- vous matter on the dorsal anterior end of the hind brain. It may exist as a small transverse fold, or it may be greatly enlarged, extending forwards over part of the mid brain, and backwards over the anterior end of the medulla. It may be 1 In older works the anterior of these lobes were called nates ; the posterior, testes. FIG. 56. Two stages in the development of the hypophysis in the pig ; A in an embryo 10 mm. long, Bin 15.5 mm. long. E, epithelium of roof of mouth; //, hypophysis connected with the mouth cavity in A by the hypophysial duct, in B by the solid hypo- physial stalk HS', /, infundibulum. 54 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. unpaired in appearance, or it may consist of a pair of lateral lobes or hemispheres separated by a median portion or vermis, terminating in a small lobe, the valve of Vieussens, which roofs in the fourth ventricle in front. The myelencephalon or medulla oblongata is the cranial extension of the spinal cord, presenting behind but slight dif- ferences from that structure. In front it widens, while its roof thins out and becomes epithelial and folded, to form the poste- rior choroid plexus for the underlying fourth ventricle. This region of thinning is known, from its shape, as the fossa rhom- boidalis. It is bounded in front by the valve of Vieussens, and on either side by the diverging dorsal columns o.f the cord (p. 44), which are frequently subdivided into a median fasciculus gracilis (Column of Goll) and a more lateral fasciculus cuneatus (Burdach/s Column). (Fig. 46.) Each dorsal column receives in front fibres from the lateral column, the whole forming an enlargement, corpus restiforme, on either side, from which the posterior peduncles of the cerebellum pass forward and upward into the metencephalon. On the ventral surface are the anterior ends of the ventral columns (p. 44), here known as the pyra- mids. These can be followed forward until they pass into the crura cerebri already mentioned. In the higher vertebrates the anterior ends of the pyramids are crossed by transverse bundles, forming the pons Varolii, which act as commissures connecting the two halves of the cerebellum. The medulla oblongata is further noticeable since it gives rise to the greater part of the cranial nerves. The various parts of the brain are connected by longitu- dinal fibre tracts and by transverse fibres or commissures. Some of these have already been mentioned, and some others may be noticed here. The chief longitudinal tracts are those of the pyramids, which may be followed through the crura cerebri to the corpus striatum. Some of the fibres of the lateral column and a part of those of the dorsal column enter the cerebellum through the posterior peduncles of the cerebellum, while the majority from these columns end in the medulla. From the cerebellum, fibres extend forward into the mid brain through two bands of tissue known as the anterior peduncles of NERVOUS SYSTEM. 55 the cerebellum, which enter the posterior portion of the optic lobes. The habenulae are also to be re- garded as longitudi- nal tracts ; while the for nix, a part of which lies ventral to the corpus callosum (infra), is to be placed in the same category, although its fibres seem in places to run transversely. Among the transverse fibres most constant are the anterior and posterior com- missures in the region of the twixt brain. 1 The an- terior crosses from side to side in the anterior wall of this region, the other is nearer the junction of twixt brain and optic lobes. In the higher vertebrates the cerebral hemi- FIG. 57. Diagram of fibre spheres are con- tracts in mammalian brain> nected by a large after Jelgersma. C, cortex of cerebrum ; CB, cortex of cer- ebellum ; CF, centrifugal tract ; FO, centrifugal tract to olivary nucleus; GR, nucleus ruber ; ND, dentate nucleus of cerebellum; NO, nucleus olivarius inferior; OC, crossed connective between olivary nucleus and the vermis; P, gan- glion of the pons; PC, dorsal tract from the pontal ganglion to the cerebellar cortex of the opposite side ; PD, pyramid tract ; PDC, tract from the pedunculus cerebelli to the cerebrum; RT, fibre course from nucleus ruber to optic thalamus; TC, connection of with cerebral cortex ; THO, optic thalamus; VPC, ventral tract from portal to cerebellar cortex of the opposite side. 1 The so-called median commissure is not a fibre tract. thalamus ganglion 56 MORPHOLOGY OF THE ORGANS OF VERTEBRATES, transverse band, the corpus callosum. Traces of this occur in amphibia and reptiles, but it acquires its highest development in the higher mammals. The pons Varolii, passing beneath the anterior pyramids of the cord, similarly connects the cerebellar hemispheres in the higher vertebrates. Here, too, must be num- bered the decussation, or crossing of the fibres of the anterior pyramids from one side to the other. il FIG. 58. Longitudinal section of the brain of a frog, after Gaupp. The epi- thelium blocked, ah, anterior part of hypophysis ; ca, anterior commissure ; cb, cerebellum; cc, corpus callosum; ch, optic chiasma ; e, epiphysis ; fm, foramen of Monro ; he, habenular commissure ; i, iter ; li, infundibular lobe ; Id, lamina ter- minalis, infraneural portion ; Its, lamina terminalis, supraneural portion ; o, olfac- tory lobe ; pc, posterior commissure ; pci, inferior and median choroid plexus ; pcp> posterior choroid plexus; q, posterior portion of mid brain; rn, recessus neuropori ; ro, recessus opticus; v, velum medullare ant. ; y, 4?', third and fourth ventricles. In its earlier stages the brain lies in the same horizontal plane with the spinal cord. Soon, by unequal growth of its dorsal and ventral surfaces, bends or flexures appear. Most constant of these is the cephalic flexure between fore and mid brains, by which the axis of the fore brain is bent ventrally at nearly right angles to the rest. Two other flexures may also appear ; they are most prominent in mammals. The pontal flexure, in the region of the pons Varolii, is in the opposite direction ; the nuchal flexure, in the medulla, is ventral again. In the ichthyopsida these flexures large!/ disappear with growth ; in the amniotes they persist throughout life. In the lower groups the five divisions of the brain are sub- equal in size, but the higher vertebrates are characterized by a great increase in size of the cerebellar, and especially of the Cerebral, regions, so that these completely cover over the twixt NERVOUS SYSTEM. 57 and mid brains. The backward extension of the cerebrum is es- pecially marked in mammals. Connected with this overgrowth is the formation of the fifth ventricle, or pseudo-ventricle, a cavity in no way connected with the true ventricles, but lying morphologically outside the brain, between the septa pellucida, the fornix, and the corpus callosum. The brain and spinal cord are enclosed in envelopes of mesenchymatous origin, which hold them in position, and serve as the bearers of nutrient vessels, etc. These membranes from FIG. 59. Sagittal section through the head of pig embryo of 15.5 mm. length, showing the cranial flexures. AA, axis of brain; C\ cephalic flexure; //, hypophy- sis ; 7/7', heart ; J\f, mouth ; 7', pontal flexure ; 7', tongue. The nervous tissue dotted. outside to inside are the dura mater and the pia mater. Of these the dura is a more dense connective tissue, consisting of two lamellae in the lower vertebrates ; its blood-vessels being distributed to the walls of the spinal canal and the skull. The pia is more delicate, and bears the blood-vessels of the brain and cord. Between the two layers is a large lymph space, and in the amphibia and higher vertebrates this is divided by a third membrane, the arachnoid. The pia enters all the fissures and depressions in the brain and cord, carrying nourishment into the nervous mass. 58 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Cranial Nerves. Like the cord, the brain gives rise to nerves, but these nerves present many differences from those of the spinal region. The last word concerning these has yet to be written, but the following outline summarizes our present knowledge, as well as indicates some of the directions in which modifications of our ideas may be expected. ncl FIG. 60. Base of human brain (from Martin), showing roots of cranial nerves, I-XII. ncf, first cervical nerve. The nerves arising from the brain (cranial nerves) are in pairs, which have received names and numbers in man ; and these have bees transferred to the corresponding structures in the lower vertebrates as follows : I. Olfactory. II. Optic. III. Oculomotor. NERVOUS SYSTEM. 59 IV. Trochlearis (or Patheticus). V. Trigeminal (or Trifacial). VI. Abducens. VII. Facial. VIII. Auditory, IX. Glossopharyngeal. X. Vagus (or Pneumogastric). XI. Spinal Accessory (or Accessory of Willis). XII. Hypoglossal. As has been described, the spinal nerves contain both sen- sory and motor roots. The cranial nerves present some dif- ferences from this. Thus nerves I., II., and VIII. are purely FIG. 61. Diagram of cranial nerves (shark), a, alveolaris; b, buccalis; c, cerebrum; cl>, cerebellum; r/, chorda tympani; e, ear; er, external rectus muscle; /, inferior rectus muscle ; g, Gasserian ganglion; h, hyoid cartilage; Am, hyoman- dibular cartilage ; hmd^ hyomandibular nerve ; ?', internal rectus muscle ; ?'o, post-trematic branch; /, intestinal (pneu- mogastric) branch of vagus ; fr, pre-trematic branch ; // glosso- pharyngeal ; //, hypoglossal ; ?, ramus intestinalis of vagus ; _/', Jacobson's commissure ; /, ramus lingualis ; m, maxillaris ; vict, mandibularis ; o, ophthalmic ; og; otic ganglion; /, palatine; s, submaxillary ganglion; sp, sphenopalatine ganglion; A tympanum. distal to the olfactory ganglion. The connection between the olfactory ganglion and the brain is made by the olfactory tract. The optic nerves, which arise primitively from the ventral sides of the diencephalon, have their ganglia lying upon the superficial portion of the retina (see eye, below). They retain their connection with the thalamencephalon- throughout life in 1 In connection with nerves I. and II. it is to be noted that the posterior cranial and the spinal nerves of selachians are at first hollow outgrowths from the brain (or neural crest). Farther, that the definitive nerve of the adult grows back from the ganglion to join the brain in both. These facts tend to invalidate the distinction drawn between nerves I. and II. and the others. NERVOUS SYSTEM. 6 1 the cyclostomes, but in the higher group they become con- nected secondarily with the optic lobes by means of the optic tracts. These optic tracts are so formed that the nerves cross beneath the thalami, that from the right eye going to the left A B CD FIG. 63. Diagrams of optic chiasma, after Wiedersheim. A, most teleosts; J3, herring; C, Lacerta; D, higher mammals. optic lobe, and vice versa. There may be a simple crossing or an interlacing of fibres, or a complete union of the trunks (optic chiasma). Nerves III., IV., and VI. are purely motor nerves, supplying the muscles which move the eye. The oculomotor arises from the crura cerebri, and supplies the muscles rectus superior, in- ternus, inferior, and obliquus inferior. The trochlearis arises from the posterior dorsal portion of the mid brain, although its centre inside the brain lies ventrally. It supplies the superior oblique muscle. The abducens arises from the anterior pyramids, and is distributed to the externus rectus muscle and to the retractor bulbi, when this muscle is present. The oculomotor is always distinct, but the others may be fused with the fifth, and in some animals their existence has not yet been demonstrated. The trigeminal nerve arises from the anterior end of the sides of the medulla. It is always large, and in the higher vertebrates at least has two distinct roots, 1 the dorsal root bear- ing a ganglion (Gasserian or Casserian ganglion). As its name implies, it has three branches, a, ophthalmicus prof undus, dis- tributed chiefly to the nose and lachrymal region ; b, maxillaris superior, supplying the region of the upper jaw ; and <:, the mandibularis or maxillaris inferior, going to the lower jaw, and in amniotes to the tongue. Frequently the last two are united for a distance as a maxillaris nerve. 2 Branches a and b are 1 In at least some of the ichthyopsida these two roots can be distinguished by micro- scopic study, although not by ordinary dissection. 2 The terminology of the trigeminal and facial used here is believed best to express the relations of the branches. 62 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. largely sensory, most of the motor fibres, together with sensory, going to branch c. Each of these branches may have a secon- dary ganglion connected with it, the ciliary ganglion on a, the sphenopalatine on b, and the otic on c. In many ichthyopsida the seventh nerve is closely connected with the fifth, and by mere dissection- the roots of the two can- not be distinguished. 1 In the higher vertebrates the two nerves are distinct throughout. The facialis is more complicated than the trigeminal, and may contain four components. In the lower vertebrates it is a mixed nerve, but in the higher it is purely vnb- m FIG. 64. Diagram of the relations of the fifth (shaded), seventh, and eighth nerves in an aquatic amphibian, after Strong, b, buccalis ; ga, auditory ganglion ; gg, Gasserian ganglion ; gp, palatine ganglion ; h, hyoid nerve ; m, mandibular nerve ; mx, superior maxillary nerve ; op, ophthalmicus profundus; os, ophthalmicus super- ficialis ; /, palatine nerve ; VII a, aa, ab, the three roots of the seventh nerve j VIII, root of auditory nerve; IX, communication of seventh with ninth nerve (Jacobson's commissure). motor, and is connected largely with the muscles of expression. In its greatest development (ichthyopsida) it gives rise to four branches, a, ophthalmicus superficialis ; ' 2 b, hyomandibularis ; c, buccalis ; and d, palatinus. The first of these has its own ganglion and is purely sensory, supplying the lateral line organs (see sense organs, infra) on the top of the head. It is found only in aquatic ichthyopsida, the frog, for instance, losing it at 1 Microscopic study shows that they are usually as distinct here as in the higher forms. 2 Fibres from the fifth accompany the ophthalmicus superficialis. NERVOUS SYSTEM. 63 the time of metamorphosis. The hyomandibularis soon divides into an anterior or mandibular branch and a posterior division, which supplies the muscles of the gill cover, and some of those of the jaw. When the first visceral cleft or spiracle is present, this division takes place just above it, so that one branch (man- dibularis) is pre-trematic, i.e., is in front of the opening, the other being post-trematic (Fig. 61). The mandibularis goes to the lower jaw; and one of its branches, which unites with the mandibularis branch of the fifth nerve, is known among the higher vertebrates as the chorda tympani. The palatine branch supplies the palate and the roof of the mouth. In the lower forms it is a mixed nerve ; in the mammals it innervates only the muscles of the soft palate. It may unite with either branch, a or b, of the fifth. The buccal branch runs in the upper jaw, uniting with the ophthalmicus profundus. The auditory nerve is closely connected with the seventh, and is often regarded as its dorsal root. It goes directly to the ear, dividing almost immediately into two branches, which may leave the skull through separate foramina. The vagus complex is composed of the ninth, tenth, and eleventh nerves, which are closely connected, and present many similarities to each other. In many features they resemble more closely the spinal nerves, especially in the presence of distinct dorsal and ventral roots. The ear intervenes between these and the nerves in front. The complex arises from the side of the medulla by from four to eight or more roots, the anterior pair being considered as those of the glossopharyngeal. Usually in the aquatic vertebrates its ganglion is fused with that of the vagus. The glossopharyngeal nerve splits into two branches, 1 the anterior going to the pharyngeal region, the other (lingualis) to the muscles and mucous membrane of the gill in fishes, and to the sense organs of the tongue in the mammals, etc. The pharyngeal branch also gives off a nerve (Jacobson's anasto- mosis) which unites with the hyomandibularis of the facial. The vagus or pneumogastric has a wide distribution. In 1 In the branchiate vertebrates the divisicm.occurs above the first true gill slit, so that here, too, we have pre- and paglMfrernat jc bVandife. 'V . ft IIKMVTR^ITY 1 64 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. aquatic vertebrates it divides into two main trunks, a ramus lateralis (possibly equivalent to the r. dorsalis of a spinal nerve), which is lacking 'in the terrestrial forms, and a ramus intestinalis. The lateralis branch runs the length of the body, either close beneath the skin, or deeper in the muscles near the vertebral column. It is purely sensory, and is distributed to the lateral line organs .of the trunk ; and the absence of these structures in the amniote vertebrates explains the disappearance of the nerve. The ramus intestinalis is the pneumogastric nerve of human anatomy. It is largely motor (or better, in- hibitory) in its functions. It is distributed to pharynx, stomach (air-bladder of fishes), and the respiratory apparatus, gills and lungs. Of the branches to the gills there are as many as there are gill clefts behind the one supplied by the ninth nerve. Each branch divides above the gill cleft into pre- and post-trematic branches. The accessory of Willis is apparently a spinal nerve which in the amniotes enters into close association with the vagus. Its distribution is chiefly to the muscles connected with the neck and shoulder girdle, e.g., sternocleidomastoid and trapezius. The hypoglossal nerve is, in the adult vertebrate, purely motor, its branches being distributed to the muscles of the tongue and to some of those of the hyoid region. It is only in the amniotes that this can be considered as a cranial nerve ; in the ichthyopsida it does not enter the skull. It is interest- ing to find that in the larval stages of some forms this nerve has a dorsal ganglionated root, while in certain species two such roots have been found, a fact which tends to show that the nerve is really compound. Within recent years it has been recognized that the compo- nents of the spinal and cranial nerves were more numerous than is implied by the account given on pages 46 and 59. In the spinal nerve it is clear that a distinction must be made between the nerves of the body (somatic nerves) and those of the viscera (visceral nerves). Each of these is made up of sensory and motor parts, so that four components are to be recognized: (i), somatic sensory (general cutaneous) ; (2), somatic motor ; (3), visceral sensory ; and (4), visceral motor. The ganglion cells NERVOUS SYSTEM. 65 of the first are situated in the spinal ganglia, and the nerves terminate in the dorsal horn. The ganglion cells of the somatic motor nerves lie in the ventral horn, and the nerves leave by the ventral roots. The internal relations of the visceral system are not so evident ; but both are possibly related to the lateral horn region, the visceral sensory nerves, whose centres in the trunk region are in the sympathetic ganglia, entering by the dorsal roots, while the visceral motor nerves leave by both dorsal and ventral roots (not proved for mammals) rr red plv-.- FIG. 65. Diagram of the sensory components in the cranial nerves in Menidia, after C. J. Herrick. General cutaneous component white; communis (visceral) dotted; lateralis black; the outline of the brain shaded, b, gill clefts; bg, branchial ganglia of the vagus, the last containing the ganglion of the ramus intestinalis ; ct, pre-trematic branch (chorda tympani) of facialis ; dl, dorsal lateral line ganglion of the facialis ; fc, fasciculus communis ; gg, Gasserian ganglion ; gn, geniculate ganglion; /, general cutaneous (jugular) ganglion of the vagus; Iv, lobus vagi; w 5 , os 1 , ophthalmicus superficial of fifth and seventh nerves; red, ramus cutaneus dorsalis of vagus ; rlv, ramus lateralis of the vagus ; rot, ramus oticus ; rp, ramus palatinus of the facialis ; rr, ramus recurrens of the facialis ; s, ramus supratempora- lis of the vagus; spv, spinal V tract (ascending root of the trigeminal); ta, tuber acusticum ; thm, hyomandibular trunk; tm, inferior trunk, containing the rami maxillaris and mandibularis of the trigeminal, the buccalis of the facial and com- munis fibres; vl, ventral lateral line ganglion of the facialis; /, olfactory; //, optic; VIII, auditory; IX, glossopharyngeal. 66 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. In the cranial region the matter is still further complicated by the appearance of a lateralis system, the nerves of which are distributed to the ear and to the lateral line system, and to no other organs. In terrestrial vertebrates, where the lateral line system is lost, the lateralis nerves, with the exception of the eighth (auditory), are lacking. The fibres of the lateralis com- ponents terminate in the tuber acusticum. The relations of the sensory components of the cranial nerves are shown in Fig. 65, in which, for clearness, the motor elements have been omitted. The somatic motor nerves of the head include only the eye- muscle nerves (III., IV., VI.). Visceral motor fibres are found in the fifth, seventh, ninth, and tenth nerves, SENSE ORGANS. All sensory organs of vertebrates arise from the ectoderm. Some remain throughout life connected with the surface of the bodyj^^-^epidermis, while others sink into special structures for their protection, the sense cap- sules. With few exceptions sense organs are formed of specialized cells, sense cells, each of which is connected by afferent nerve fibres with the central ner- vous system. Between the sense cells there may be other ecto- dermal cells which have a sup- porting function, or which serve FIG. 66. ' Sense cells, after various & authors. A, taste cell of rabbit; B, to isolate the sensory cells from hair cell from lagena of pigeon; C, each Other. These sense organs which are situated in the epidermis are the more generalized, and among them are distributed the sensations of touch, pressure, and tem- perature. In the aquatic ichthyopsida (Fig. 67) these organs are composed of rod-like, club-formed, or pear-shaped cells, the free extremities of which may reach the surface ; but in all olfactory cell of Proteus; D and , rod and cone cells from the human eye. SENSE ORGANS, 6 7 FIG. 67. Lateral line organ of Atnblystoma, showing, be- neath, the nerve fibres ; on the free surface the sensory hairs. terrestrial vertebrates where the surface of the skin is dry, the* sensory structures sink to a deeper position. Lateral Line Organs. Some of these organs are irregularly distributed, while others are grouped into regular series, and form what are known as the lateral line organs. In their early stages these lateral line organs are upon the surface. Later they sink, in the amphibia, info pits, in pisces into lon- gitudinal grooves which may be closed into tubes, with openings at regular intervals. With increase in size of the animal, the number of openings also increases by division. The open- ings frequently perforate scales, while the canals between them may become enclosed in bone, espe- cially upon the head. By the presence of grooves and canals in the skulls of many fossil forms, we infer that they pos- sessed lateral line organs. There is considerable variation in the distribution of the lines of these organs, but the following are the most constant series: (i), the lateral line of the trunk (may be double) which ex- tends the length of the body between the dorsal and ventral musculature ; this series gives the name to the whole system; (2), occipital series, crossing the back of the head and connecting the systems of the two sides ; (3), supraorbital, and (4), in- FIG. 68. Dorsal view of head of sturgeon, showing the distribution of the lateral line canals, after Collinge. de, dermethmoid ; do, dermo- occipital ; ee, dermoectethmoid ; . Membranous labyrinth of thrush ( Turdus}, after Retzius, from Wiedersheim. aa, anterior ampulla; ac, eighth nerve; #/, posterior ampulla; ca, anterior canal; ce, ex- ternal canal ; <:/, posterior canal ; de, ductus en- dolymphaticus ; /, lagena ; mn, macula neglecta ; ms, macula sacculi; mu, macula utriculi ; />/, papilla lagense ; raa, nerve to anterior ampulla ; rap, to posterior ampulla ; rb, basilar nerve ; reC) recessus utriculi; r/, nerve to lagena; ;-//, nerve to macula neglecta ; s, sacculus ; sc, septum conciatum ; sfl, posterior utricular sinus ; ss, superior utricular sinus; /z/, tegmentum vasculo- sum ; u, utriculus FIG. 77. Section through the cochlea of a cat. Bone, shaded. C, organ of Corti ; G, spiral ganglion; N, nerve; SM, S7\ SV, scalse media, tympani, and vestibuli ; R, Reissner's membrane. cartilage, but in all except the lower vertebrates the cartilage is finally replaced by bone. The inner walls of these capsules follow more or less closely the contour of the membra- nous labyrinth, thus con- stituting the skeletal labyrinth, between which and the membranous por- tions is a space filled with the perilymphatic fluid. The walls of these cap- sules are perforated in- ternally for the passage of nerves, etc., while on their lateral surfaces, in all groups above amphibia, are two openings, the fenestra ovalis and the fenestra rotunda (the lat- ter crossed by mem- branes), through which sound waves pass to the parts described. In the mammals the skeletal labyrinth follows very closely the membra- nous portion, and in one part these structures need a further description. That part called the la- gena in the lower verte- brates is greatly devel- oped here, and is drawn out and coiled in a spiral, which is accompanied, SENSE ORGANS. 73 above and below, by similar outgrowths of the perilymphatic space. From the resemblance which these structures present to a spiral stairway these divisions are called scalae, that part con- nected with the membranous labyrinth being the scala media, the upper of the perilymphatic spaces being the scala vestibuli, the lower the scala tympani. This whole structure, from its resemblance to a snail-shell, is called the cochlea. In the scala media the macula lagenae of the lower vertebrates becomes de- veloped into a highly specialized sensory structure, the organ of Corti. Besides ' hair cells ' (sensory cells) and other cells, the organ consists of series of hard rods (pillar cells) arranged like a A at right angles to the axis of the scala. As the spiral qa POH FIG. 78. Organ of Corti in section, after Stohr. AT, auditory tooth; CC 9 cells of Claudius; DC, Deiter's cells; HC, Hensen's cells; HP, head plate; Iff, inner hair cells ; MB, basal membrane ; N, nerve ; OH, outer hair cells ; IP, OP, in- ner and outer pillar cells ; /*, phalanges ; SS, sulcus spiralis ; T, tunnel. The ' mem- brana tectoria,' being decidedly problematic in character and relations, omitted. diminishes in size, from apex to base these A's also diminish in size, a fact which led to the view formerly held that these were in some way connected with the recognition of pitch. The middle ear or tympanum first appears in the anura. 1 It is formed by the expanded end of the first visceral cleft (spiracle of elasmobranchs), which does not break clear through to the exterior, but is closed externally by a thin tympanic membrane, with an external wall of ectoderm, an inner of ento- derm, and a middle layer of mesenchyme. Internally the tym- panic cavity remains in connection with the pharynx by means of the proximal portion of the cleft, here known as the Eusta- chian tube. Sound waves are conducted across the tympanic 1 In the urodeles and caecilians the tympanic cavity is lacking, and there is but a single auditory ossicle, the stapes, which usually articulates with the quadrate. 74 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. cavity by means of auditory ossicles which extend from the tympanic membrane to the fenestra ovalis. In the anura and sauropsida there are two of these ear bones, the stapes, situ- ated in the fenestra ovalis, and the columella, extending from the stapes to the tympanic membrane. In the mammals the columella is replaced bV two bones, the incus and the malleus, neither of which can be'homologized with the columella. FiG. 79- Diagrammatic section of human ear, from Martin after Czermak. A t auditory nerve; a, ampulla; B, b, semicircular canal; G, external meatus; k, carti- lages; M, concha; o, fenestra ovalis; P, tympanic cavity with chain of bones; /*/, scala tympani, r, fenestra rotunda; fi, Eustachian tube; S, cochlea; Vt, scala vestibuli. The stapes arises as a chondrincation, and, later, ossifica- tion of the membrane closing the fenestra ovalis; the columella is post-spiracular, and may in part correspond to the hyoman- dibular ; the incus is apparently the quadrate of the lower ver- tebrates ; while the malleus is the proximal end of Meckel's cartilage (? os articulare) which becomes cut off from the rest. 1 In all anura and in many reptiles the tympanic membrane is on the outer surface of the body, but in higher groups the mem- 1 There is great uncertainty upon some of these points, different students having different ideas of the homologies. The view given here is based upon personal studies. Further details are given in the section dealing with the skeleton. SENSE ORGANS. ?$ brane is placed at the bottom of a tube, the external auditory meatus, the outer end of which is frequently protected by movable dermal flaps. In most mammals an external ear, sup- ported by cartilages, is developed ; and there is considerable evidence to show that this external ear is a derivative from the operculum of fishes, or from the external branchial structures of the amphibia. Recent experiments tend to show that in the fishes the ears are without auditory functions and are solely organs of equili- bration. In terrestrial vertebrates they are both organs for hearing and for the maintenance of the equilibrium. Olfactory Organ. The organ of smell is a single sac in the cyclostomes, paired in all other vertebrates. Its essential por- tion is the sensory epithelium, in which sensory cells are inter- spersed with supporting or isolat- ing cells. Its nerve supply is the olfactory nerve already de- scribed. The powers of smell are directly proportional to the extent of sensory surface, and in Order that this may be increased the surface is folded, usually in the longitudinal direction. In the more primitive forms the sensory surface is not uniformly distrib- uted, but is gathered in patches separated by large masses of iso- lation cells. In some ganoids and amphibians the nasal epithe- u PP er fi s ure Acanthias > showin g them .. .. divided by a movable flap. Lower, hum has a peculiar radiate ap- young Amia . ^ ^ anterior and pos . pearance, as seen in transverse terior narial openings, section. From the amphibia up- wards outgrowths of cartilage or bone (turbinals), either from the ethmoid or lateral walls, tend to divide the cavity still further. In the petromyzontes and pisces only external nos- trils occur, and in the cyclostomes there is but one of these. In the forms with paired cavities there is primitively but a single FlG. 80. Divided nostrils of fishes ; 76 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. nostril to each olfactory sac, but in the selachians and ganoids a fold of skin practically divides each nostril (external naris) FIG. 81. Relations of nasal organs to the cavity of the mouth; A, in fishes; , in terrestrial vertebrates. l>, brain ; i, internal nares ; w, external nares. into two. In many teleosts this is carried farther, and two dis- tinct nostrils may occur on either side. These modifications clearly are to permit a current of water over the olfactory epithelium (Fig. 80). In all vertebrates above fishes both external and internal nares (choana) are present, the latter open- ing into the oral cavity. This condi- tion is foreshadowed in the selachians, where an oronasal groove leads back from the external nares of either side to the angles of the mouth. In the higher vertebrates this groove be- comes converted, during growth, into a tube by the union of its edges. 1 In this way a respiratory tract is formed on one side of the olfactory the respiratory nasal tract lead- Surface, the posterior end of which ing from the nostril, n, into the opens inside the cavity of the mouth. In a similar way a naso-lachrymal duct is formed leading from each eye into the corresponding nasal passage. In terrestrial vertebrates nasal glands are frequently present in connection with the nose, the secretion of which moistens the olfactory epithelium. 1 The process is modified in certain groups, where a solid cord of cells, instead of a groove, is formed, the respiratory passage appearing later in the cord. FIG. 82. Head region of a human embryo, after His, show- ing the method of formation of oral cavity, and the naso-lach- rymal duct leading from the eye, e, into the nasal cavity. />, rudimentary gill clefts ; //, hypo- physial pocket ; /, lungs ; s, cer- vical sinus. SEA'SE ORGANS. 77 Connected with the nose in all vertebrates above the fishes is a pair of accessory sensory organs, the organs of Jacobson. They are outpushings of the wall of the olfactory surface, sup- plied by branches of the first and fifth nerves. In the lower amphibia these organs are placed on the medial side of the nasal cavities ; a little higher they are ventral in position ; in the high- est amphibia they have rotated to the lateral side of the olfactory organ. In the amniotes they are either medial or ventral in position. In the lower forms these sacs are connected only FIG. 83. Section through the nasal region of the Surinam toad, Pipa. t, car- tilage; en, cavum nasale ; d, Jacobson's gland and duct; e, ethmoid cartilage; /, frontal bone ; /, organ of Jacobson ; /, lateral portion of nasal passage ; , nasal bone ; /, branch of olfactory nerve to organ of Jacobson ; nn, branches of nasal nerve of trigeminal. with the nasal cavities ; but in the mammals a duct (Stenson's duct) sometimes leads from them into the mouth through the foramina incisiva, between the premaxillary and the palatine processes of the maxillary bones. In many mammals, however, these foramina are closed by membrane, and are vestigial in character. In the mammals for the first time appears an external nose supported by cartilage. In some, like the tapirs and elephants, this organ becomes enormously developed, and forms in the latter the well-known trunk. 78 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Visual Organs. The sensory portion of the eyes arises from the brain, and in the embryos of some vertebrates (elasmo- branchs, urodeles) optic areas can be recognized in the medullary FIG. 84. Diagrams showing the inversion of the layers in the formation of the retina. In all the figures the nuclei are placed in the morphologically deeper ends of the cells. In A the brain () has been closed in, in B the optic vesicle (z>) has reached the thickening for the lens (/), and on the right side the vesicle is being con- verted into the double-walled cup with, as shown in C, a medial epithelial ( esench y e ; , nervous _ layer of retina ; r, deeper layer of of the structure. These fibres are re tina; .r, optic stalk. arranged in layers, like the coats of an onion, and where they meet on the inner and outer sur- faces of the lens, they produce peculiar figures like a three-rayed star. After the lens is cut off from the ectoderm the latter be- comes a smooth, transparent sheet over the front of the eye, forming the epithelium known as the conjunctiva, continuous with the superficial layer of the skin. 3 In man there are from 250,000 to 1,000,000 rods and cones to a square millimetre of retinal surface. 82 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. We are now in position to describe the eye of the adult ver- tebrate. The eye proper is approximately spherical, although, as in fishes, it may be flattened, or, as in birds, somewhat coni- cal in front. In the ichthyopsida it is without any well-devel- oped external accessories for protection ; l but in the amniotes n sh FIG. 88. Horizontal section through human eye, from Hertwig after Arlt. a, arteria centralis ; ac, anterior chamber of eye; r, cornea; ch, choroid; cj, conjunc- tiva; cp, ciliary process; z, iris; /, lens; m, macula lutea, point of distinct vision; n, optic nerve ; a, ora serrata ; /, papilla of optic nerve ; pc^ posterior chamber of eye; r, retina; 5, sclerotic; sh t sheath of optic nerve; v, vitreous body ; z, zonula Zinnii. movable lids, which can close over the organ, occur. There are typically three of these folds of the skin, an upper and a lower lid, moving in a vertical plane, and inside of these a third transparent lid, the nictitating membrane, which is attached at the anterior or inner angle, and which closes horizontally. In 1 Some salamanders have feebly developed eyelids. SENSE ORGANS. 83 man the nictitating membrane is reduced to a vestigial fold, the plica semilunaris, visible at the inner angle of the eye. The free surface of the eye is covered by the conjunctiva already mentioned ; and beneath this is a thicker, dense trans- parent layer, the cornea, composed of connective tissue fibres produced from mesenchyme cells, which penetrate between the conjunctiva and the lens. Laterally the cornea is continuous with a hard white capsule, the sclerotic coat, which envelops the whole eyeball, its anterior portion being the well-known ' white of the eye.' This sclerotic is usually cartilaginous, and in the sauropsida and in monotremes, bony structures, sclerotic bones, may be developed in it. This sclerotic forms a sense capsule, comparable in a way to those enclosing the ears and olfactory organs, but never, like them, uniting with the skull. Between the cornea and the lens is the anterior chamber of the eye, filled with a watery fluid, the aqueous humor, less dense than the vitreous humor already mentioned. Inside of the sclerotic is a highly vascular layer, the choroid, which carries numerous blood-vessels to nourish the eye. The choroid extends forward nearly to the edge of the optic cup ; but beyond this point it becomes muscular, a portion of it forming the contractile portion of a circular curtain, which extends from the edge of the optic cup into the anterior chamber. This curtain, known as the iris, is opaque, and is usually colored by pigment derived from the edge of the optic cup. The opening in the centre of the iris, the pupil, can be enlarged or contracted by means of the muscles already referred to, and thus the amount of light admitted to the retina can be regulated. Just inside the iris the inner wall of the optic cup becomes developed into a strong ridge, the ciliary process, which ex- tends inwards towards the lens to which it is attached by a feriestrated, suspensory ligament (zonula Zinnii), thus partially separating the anterior from the posterior chambers. Close to this region the choroid develops a layer of ciliary muscles, which by their action can move the lens nearer to or farther from the retina, and at the same time, by stress conveyed by the suspensory ligament, can slightly alter its shape. This 84 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. forms the apparatus of accommodation necessary for viewing objects at different distances. In the lower vertebrates accessory glands connected with the eyes are but slightly developed, but with the assumption of a terrestrial life (amphibia) lachrymal glands for lubricating the surface appear. These arise as inpushings of the epider- mis near the lids. In the lower amphibia the glands are on the lower side of the eye and form a continuous series ; but higher this becomes divided into two, a Haider's gland near the inner angle, a true lachrymal gland at the outer. In reptiles l and birds these remain on the lower side of the eye, but in mammals the lachrymal gland passes to the upper lid. The Harderian gland, which has for its Junction the lubrication of the nictitating membrane, becomes reduced in the mammalia. The lachrymal duct has already been mentioned (p. 76). The eye is provided with muscles which move it as a whole. Some of these are remarkably constant through the whole verte- brate series. There are four rectus muscles, known from their position as the superior, inferior, external, and in- ternal. These arise from the bottom of the orbit near the foramen for the optic nerve, and are inserted at about equal distances around the ball. The two oblique muscles (superior and in- ferior) arise in front of the rectus mus- cles, and are inserted on the ball above and below the internal rectus. Besides, there may be a well-developed retractor bulbi attached near the optic nerve, and serving to pull the eye back into its socket. In the sauropsida are also muscles connected with the nictitating membrane, but these are reduced or ab- sorbed in the mammals. Epiphysial Structures. Several structures which are con- nected with that part of the primitive fore brain which subse- i An interesting fact is the absence of lachrymal glands in crocodiles. FIG. 89. Eye muscles and related nerves in shark. a, abducens nerve ; oi, infe- rior oblique ; os, superior oblique; om, oculomotor nerve ; re, rectus externus ; ri, r. internus; rif, r. infe- rior ; rs, r. superior muscles. SENSE ORGANS. 85 quently becomes the twixt brain are best considered in connection with the sense organs. These are best developed in the lizards, and hence these animals serve as the basis of the following account. At an early stage there arises from the epithelial roof of this region a hollow outgrowth (the epiphysis) directed upwards and forwards, its distal end at first being in contact with the epidermis of the top of the head. The extrem- yjC^'lT' i^titfH - vr ._. a FIG. 90. Section of pineal eye of Ilatteria^ after Spencer from Wiedersheim. a, capsule ; b, lens ; <:, vesicle ; , parietal eye; pa, paraphysis; //, parapinealis. EPIDERMAL STRUCTURES, 87 from the anterior, the parapinealis from the posterior, out- growth. Besides these, a third outgrowth, the paraphysis, may arise in front of the epiphysis ; it never develops sensory elements. The parietal eye has different fates even in the lizards. In some, as Hatteria (Fig. 90), it retains its parts well developed throughout life, and its nervous connection with the brain per- sists, so that here it is apparently to some extent functional. In other lizards it has lost this power, either through the exten- sive deposit of pigment in all parts, or by the degeneration of its nerve supply. In other groups these structures of the pin- eal apparatus are much more rudimentary, and are usually entirely enclosed within the skull ; although at times, as in the anura, the parietal eye may lie between the bones and the skin, but here all nervous connection is lost. In many extinct vertebrates, if we may judge by the large parietal foramen in the skull, the pinealis was well developed and functional. EPIDERMAL AND DERMAL STRUCTURES (SKIN). The Skin. That portion of the ectoderm which remains on the surface of the embryo after the differentiation and in- volution of the central nervous system forms the epidermis, which, together with the underlying mesenchymatous tissue, the derma, makes up the skin. In its earliest stage the epidermis is usually but a single cell in thickness, but later, by division of these cells, other layers are formed on the outside of this first or basal layer. In ganoids, teleosts, and amphibia the epidermis is two cells thick from the first, and in the amphibia (the only instance in vertebrates) the outer layer is ciliated in the young. The basal layer is the active portion, producing by cell division the more superficial layers. In those forms with two-layered epidermis, the basal layer also gives rise to nerves and sense organs, and is therefore often spoken of as the nervous layer, the outer one being the cuticular layer. The derma (often called cutis or corium) is of mesenchy- matous origin, and consists largely of layers of fibrous connec- 88 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tive tissue intermingled with smooth muscle cells, blood-vessels, nerves, etc., the whole being separated from the deeper tissues by a layer of much looser connective tissue, usually containing considerable fat. The structure of the adult epidermis varies considerably in the different groups. In all it becomes several layers in thick- ness, and is thicker in * 3 $ c f the terrestrial than in the aquatic forms. In the ichthyopsida there is slight differentiation be- tween the layers, the cells showing less strat- ification than in the higher groups. Among them are numerous uni- cellular glands, usually spherical in shape, and loaded with a slimy sub- stance (mucus) ; and as these cells approach the surface they break, and their contents spread over the body, producing the slimy condition so familiar in these forms. In the amniotes, on the other hand, the outer layers of the epidermis undergo a hardening pro- cess, and are converted into a horny layer (stra- tum corneum), Fig. 95, the beginnings of which are seen in the frogs. Apparently the first layer to be budded from the basal layer persists through a large part of the embryonic life as a distinct sheet on the outside of the corneum, known as the epitrichium, so called because in embryonic mammals it extends in an unbroken sheet over the developing hairs. The non-corni- Co FIG. 93. Section of skin of lamprey eel ( Petromyzon planert) from Wiedersheim. B, mucous cells ; Co, derma ; CS, cuticular layer ; Ep, epidermis ; F, fat ; G, blood-vessels ; Ko, club cells ; Kb, granular cells ; S, IV, fibres of con- nective tissue running vertically and horizontally. EPIDERMAL STRUCTURES. 8 9 fied layers of cells are known as the Malpighian layer, the cells of which are polygonal in outline and rich in protoplasm In the mammals, be- tween horny and Malpighian lay- ers, is a thin stra- tum lucidum, consisting of ex- FiG. 94. Skin of mammalian embryo, showing the epitrichium, e, after Minot. b, basal layer ; m, Malpighian layer. tremely flattened cells closely compacted together. As will readily be understood, the cells of the basal layer are continually dividing, thus producing new cells, which come to lie between the basal layer and those layers previously formed, and in this way tending to increase the thickness of the epidermis. As these cells grow older they gradually pass into the conditions found in the different layers, lucidum, corneum, and at last are cast off from the outer surface, either a few cells at a time, or in larger sheets, FIG. 95. Diagrammatic section through mam- as in the amphibians malian skin. C, stratum corneum; Z>, derma; and reptiles. The Strata G, sweat gland; //, hair; L, stratum lucidum; corneum an d lucidum A/, Malpighian layer ; A 7 , nerve ; T, tactile corpuscles. are clearly protective in nature, and only in the turtles is there an absence of this shedding of the external layers of the skin. Dermal Glands. The epidermis gives rise to numerous glands. In the fishes the unicellular mucous glands have already been referred to. In some fishes (teleosts) in addition the skin also contains poison glands, sometimes upon the back, 90 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. sometimes on the operculum, and sometimes in the axilla, but always in connection with a strong spine. Of our native fishes the poison glands in the axilla of certain catfishes {Notunts) are best known. In the toadfish (^Batrachus) a gland in similar position is well known, but apparently its secretion is not poisonous. In the amphibia, glands in the skin which secrete an acrid juice are abundant, and in the toads their presence causes the warty skin so noticeable in these animals. In the sauropsida, glands are few in number. In certain snakes stink glands occur in the skin, the secretions of which give these animals a dis- agreeable odor. In the lizards, glands are found only on the inside of the femoral region of the hind limbs, the openings of which (femoral pores) are of considerable value in the classifi- cation of these forms. In the birds, glands are developed on the reduced tail (uropygial glands), the oily secretion of which is used in oiling the feathers. These glands are best developed in the water birds. In the rasores there are in addition glands in the neighborhood of the eye. In the mammals, glands are well developed, and acquire a great variety of form. These glands may be arranged in two categories, the tubular and the racemose, the characters of which are indicated by their names. To the tubu- lar type belong the sweat glands, which extend deep into the derma, and in their deeper portions be- come coiled and con- voluted (Fig. 95). The racemose (acinose) glands, in their simplest condition, form the sebaceous glands, and are normally placed in close -connection with the roots of the hair. In some mammals the sebaceous glands of certain regions of the body become con- verted into scent glands, the secretions of which may serve for TIG. 96. Different types of glands, a, intesti- nal epkhelium with a gland- (goblet) cell; b, uni- cellular gland with duct ; c, simple tubular gland ; J, simple racemose gland ; e, compound racemose gland. EPIDERMAL STRUCTURES. offence or defence ; or again may be of value at the rutting season, as attractions for the other sex. Of the defensive glands those of the skunk and polecat come immediately to mind ; to the other category belong the peculiar glands of the beaver, civet cat, musk deer, etc. These glands may be placed near the eyes (deer), on the back (musk swine), on the legs (ordinary swine), on the ventral surface (musk deer), or near the vent (skunks, etc.). A more extreme modification of 'the sel^aceous glands is found in the milk glands of all mammals except monotremes, 1 the secretion of which serves to nourish the young. These milk glands are in pairs upon the ventral sur- face, the number being roughly correlated to the number of young brought forth at a time. The ducts of each group of glands open upon a limited extent of surface, and this becomes converted into a teat or nipple, either by the elevation of the skin in which the ducts open (Fig. 97, A}, or by the extension of the surrounding skin into a tubular form (B). In the skin, pigment is of common occurrence. It may be found in the epidermis, but is more common in the derma. It may consist of scattered pigment granules, or there may be special pigment cells (chromatophores) which enlarge or con- tract under control of the nervous system, producing those color changes so noticeable in many lizards, and to a less degree in amphibia and fishes. EXOSKELETON. Either or both layers of the skin may participate in the formation of firmer parts constituting a tegumentary skeleton, which may take the form of scales or bony plates ; and it is to 1 The milk glands of the monotremes are apparently derivatives of sweat glands. FIG. 97. Two types of nipple. 92 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. be noted that most of the membrane bones of the skull (infra) belong in this category. Here, too, may be enumerated feathers, hair, horn, claws, etc., as well as cornifications of the skin of more limited distribution. Scales. The most primitive type of this exoskeleton is found in the scales of the elasmobranchs. Here papillae of the derma (dentinal papillae), arranged in quincunx, push up into the epidermis, carrying the basal layer of the latter before them. The external surface of each papilla and its base secretes a little plate of bone or dentine with a central spine ; while the epi- dermis covering the papilla becomes converted into an enamel FIG. 98. Developing scales of dogfish (Aca.nthi.as~}. &, basal layer of ecto- derm; c. derma (corium) ; ., epidermal cornifications) reappear in the birds, but dermal bones are never found. Birds have besides a peculiar epidermal covering, the feathers, which must have more atten- tion. In the following account it must be borne in mind that in the development of the feathers, as in the scale of a snake, a dermal papilla takes the initiative, and that the ultimate structure is entirely formed of cornified epidermis. Feathers occur only in the group of birds, and here three principal types are found, down-feathers, pin-feathers, and contour-feathers, differing much in ap- pearance, but of essentially the same struc- ture. Contour-feathers are those which cover the body in the adult bird, giving it its outlines, and forming the broad expanse of wings and tail. In a typical contour- feather are to be distinguished an axial portion, composed of a proximal hollow part, the quill, and a distal and more solid shaft, the latter bearing on either side lateral outgrowths, the barbs ; shaft and barbs making up the vane. Inside the quill occur thin structureless partitions, the pith, while the shaft bears on its so- called inferior surface a longitudinal groove, the umbilicus. The barbs bear on their sides smaller projections, the bar- bules, which are usually provided with minute hooks ; these, interlocking with similar hooks on the adjacent barbules, convert the whole vane into a continuous sheet. In many cases a second or aftershaft joins the axis of the feather on the lower surface near the Junction of the main FIG. 100. Contour feather, a, quill ; b, shaft c* barbs. EPIDERMAL STRUCTURES. 95 FiG. 101. Part of a feather, enlarged. a r portion of shaft showing a part of a barb with its barbules ; b, two barbules greatly enlarged. shaft with the quill. The vane supported by this after shaft is usually more downy than the others. Down-feathers differ from contour-feathers in the absence of a shaft, the barbs arising directly from the end of the quill ; these barbs never inter- lock, but remain soft and : -ee from each other. In pin-feathers (filoplumes) there is merely the devel- opment of a hair-like shaft without barbs. Except in the pen- guins and some ratite birds, feathers are not uniformly distrib- uted over the whole surface of the body, but occur in well-marked feather-tracts or pterylae, the rest of the surface (apteria) being sparsely covered with down- or pin-feathers. The tertiary penguins also possessed feather-tracts, so that their absence in; existing penguins must be a secondary character. The arrangement of the feather-tracts is of importance in the classification of birds. In development down-feathers pre- cede contour-feathers. There first appears in each spot where a down feather is to develop a rapid multipli- cation of dermal cells, thus producing a rudimentary papilla, over which the epidermis, elsewhere consisting of Feather tracts in b asa ] ] a yer and epitrichium, becomes several cells in thickness. By contin- uous growth the papilla becomes long and cylindrical, projecting from the body, the axial derma form- FlG. 102. young of common crow ( Corvus americanus}. 96 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. ing the pulp of the future quill, while the epidermis surrounds the outgrowth. A circular depression around the base of the papilla is the beginning of the formation of the future feather follicle. In the distal portions of this outgrowth there next appear longi- FIG. 103. Two stages in the development of a feather, after Davies. b, basal layer of epidermis; d, derma; e, epitrichium ; p, pulp; jc, beginning of depression for feather follicle. tudinal ridges of the pulp which gradually encroach upon the epidermis, dividing this layer into a series of cylindrical rods (Fig. 104), which at last are held in position by only the layer of epitrichium. Now the derma retracts into the feather follicle, carrying with it the basal layer of the epidermis, so that there re- mains a hollow epidermal out- growth, the quill, bearing at its extremity a number of epidermal rods. The cells of these portions rapidly dry and become cornified, and, the epitrichium breaking away, the rods separate as the down of the down-feather. Later the contour-feathers are developed from the retracted pulp which grows out again as before. In general these develop like their predecessors, excepting in certain details. The rods of pulp are not longitudinal, but oblique to the axis of the outgrowth, the result being that the cornified rods (which form the barbs) proceed from an undivided portion (shaft) on the dorsal side of the outgrowth ; and when the epitrichium breaks away, these FIG. 104. Transverse section of developing down-feather of tern {Sterna wilsoni}. t>, basal layer of epidermis; bv, blood-vessels; e, epi- trichium ; ep, epidermis ; /, pulp ; r, ridges of pulp extending into epi- dermis. EPIDERMAL STRUCTURES. 97 expand so as to form the vane. One point needs a little more detail. On that side where the shaft is to be formed are two longitudinal thickenings (Fig. 105) ; with growth these become larger and bend inwards to meet each other. Near the tip the result is a solid rod (Fig. 105, A), but farther down the ingrowth FIG. 105. Diagrammatic sections through a developing contour-feather : A at about the middle of the vane, B near the base of the vane, and C through the quill, after Davies. b t barbs ; fs, feather sheath ; /*, / 2 , different portions of the pulp cavity. includes a space (Fig. 105, B and C), so that the proximal por- tion of the shaft is hollow. The umbilicus is formed by these ingrowing ridges. As will readily be understood, the so-called dorsal and ventral sides of the feather correspond to the outer and inner surfaces of the epidermis of the feather papilla. At regular intervals the bird sheds or molts its feathers, the old ones dropping out, while new ones arise to take their place by a repetition of the process just described. Hair is as characteristic of mammals as feathers are of birds. In its formation the epidermis apparently takes the initi- ative, the result being the formation of a solid ingrowth of epidermis into the un- derlying derma, the deeper end of which . F / G ' I0 ?' 7 stage * J in the early development becomes cupped (Fig. 106) to accommo- of the hair of the mouse, date a small collection of dermal cells, the rudiment of the hair papilla. Next a circular depression appears in their in- growth, separating a central portion, the future hair, from the surrounding epidermis which forms the after Maurer. D, derma ; , epithelial hair-forming cells ; F, follicle ; /, hair papilla. 98 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. hair follicle. In both hair and follicle several layers may be distinguished. In the follicle there is the basal layer and the more superficial layers of the epidermis, without, however, any clear differentiation of strata corneum and lucidum. At the bottom of the follicle (root of the hair) these pass directly into the hair itself, on the out- side of which is the so-called inner root-sheath (the walls of the follicle forming the outer root-sheath) composed of two layers of cells (the outer called Henle's layer ; the inner, Hux- ley's layer). This inner root- sheath does not reach the external surface. The hair proper consists of a central core or medulla, around which are several layers of cells, the cor- tex, and on the outside is a cutic- ular layer. The growing point is at the bottom of the follicle, where the basal layer of the epidermis, by repeated cell di- vision, adds to the base of the hair. As the cells grow older they become cornified, and the whole is gradually pushed out of the follicle by additions below. Like feathers, hairs appear at first in well-defined tracts^; but later, by multiplication, this regularity is lost. Hairs are least abundant in the whales, where they may be reduced to from two to eight pairs in the neighborhood of the mouth, and even these sometimes only occur during fcetal life. Hairs may also be enormously developed into organs of defence, as in the case of the ' quills ' of the hedgehog and porcupines, while in the case of the vibrissae ('whiskers') near the mouth, they may serve as sense organs (p. 69). Oil glands of the racemose type are usually found connected FIG. 107. Diagrammatic section of a hair and its follicle, after Maurer. C, cuticle ; CR, cortex ; , epidermis ; F, follicle; G, oil gland; HE, Henle's layer ; HD, Huxley's layer ; M, me- dulla; N, nerves; P, hair papilla; S, outer root sheath (the inner root sheath is composed of Henle's and Huxley's layers) ; V, vein. EPIDERMAL STRUCTURES. 99 with the hair follicles, while a system of smooth muscle fibres (especially strong in porcupines) serves to erect the hairs. Closely related to hair are the nails, claws, and hoofs of mammals, and the horn of sheep, goats, and cattle ; in fact, these structures may be regarded as com- posed of agglutinated hairs. Somewhat different in character are the scales which cover the body in the pangolins (manids), and are found on the tail of the rodent Anomalurus, although these are both of epidermal origin. True dermal bones in the skin occur only in the armadillos among recent forms, where they form an armor upon the dorsal sur- face of the body. In the fossil glypto- dons the body was enclosed in a similar bony case, while some extinct cetacea possessed dermal bones. FIG. 108. Hair tracts on early cat embryos, after Maurer. IOO MORPHOLOGY OF THE ORGANS OF VERTEBRATES. MESOTHELIAL STRUCTURES. THE mesothelial structures, as we left them on a preceding page (p. 8), consisted of a pair of compressed sacs or pouches, one on either side of the entodermal tube. Each pouch consists of an inner or visceral, and an outer or soma- tic, wall, the cavity be- tween them being the primitive ccelom, which is now entirely cut off from all other cavities. FIG. 109. Transverse section of Amblystoma ^, , . . ,. . . embryo after the separation of the mesothelium. a, archenteron; c, ccelom; m, mesothelial walls of mesothelial tissue are coelom; n, notochord ; r, groove of closure of now to be described neural tube; s, spinal cord: /, canal of spinal cord; , olk but it must be kept in mind that there are many exceptions to the details as given below. The state- ments regarding the somites apply most nearly to the elasmo- branchs, but they are generalized in many respects. First in order is the development of the primitive segments or somites of the body. It is to be noted that while other parts are segmentally or metamerically arranged (nerves, blood-ves- sels, skeleton), this metamerism primarily arises in the mesothe- lium, and becomes secondarily impressed upon other structures. The process of somite formation is best seen in the trunk region. As a result, partly of the change in the shape of the em- bryo caused by the infolding of the medullary plate (see ner- vous system), in part of the growth of the mesothelium itself, the ccelomic pouches extend upwards from their primitive posi- MESOTHELIAL STRUCTURES. IOI tion along either side of the notochord and the central nervous system, while below the pouches grow until they meet in the mid ventral line, below the entoderm. In each coelomic pouch three horizontal zones are to be distinguished, a dorsal muscle- plate or myotome zone (epimere), a ventral lateral plate zone (hypomere), and between these a much narrower middle zone (mesomere). FIG. no. Diagram of the mesothelial pouch and the beginning of segmenta- tion, based upon Atnblystoma. a, anus; c, coelom; e, epimere; h, hypomere ; hp, hypophysis ; ;//, mesomere ; mt, mouth ; , notochord ; o, eye ; s, spinal cord. By a series of constrictions not easily described, but readily made out from the figure, epimere and mesomere become divided transversely to the body axis into a series of cubical bodies, the protovertebrae of older authors, the myotomes of recent embry- ology. These divisions do not extend into the hypomere, and so do not divide the ventral part of the coelom. As a result we have below a single coelomic space extending the length of the trunk, which connects with a number of dorsal coelomic diver- ticula, extending, one into each myotome. Later, horizontal con- strictions cut the epimeral portions off from the rest, so that from this region there arises, on either side of the body, a series of completely closed cavities with epithelial walls, the myotomes. To avoid confusion with that portion of the primitive body cavity (metacoele or splanchnocoele) which remains between the lateral plates, and to which the term coelom as usually applied is given, the coelomic pouches in the myotomes have been called the myocoeles. The myotomes give rise to the voluntary muscles of the body in a manner shortly to be described ; the modifi- 102 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. cations in other parts of the coelomic wall must be outlined now. As will be seen by the diagram above (Fig. 1 10), the middle zone becomes segmented at the same time and in the same way as the myotomes, and when the latter cut loose from the rest of the mesothelial tissue, the contained coelom becomes roofed in above. From the inner or visceral wall of these mesomeric segments there is now a rapid proliferation of cells upon the FIG. in. Diagram of a part of the trunk of an embryonic vertebrate showing the development of the mesothelial tissues, a, aorta; g, neural crest (anlage of spinal ganglion); i, intestine; n, notochord; p, rudiment of pronephric tubule; s, sclerotome; sc, spinal cord; so, somatic layer of mesothelium; sp, splanchnic layer. (For later conditions compare with Fig. 127.) deeper surface, the products of which migrate inward around the notochord, where they eventually give rise to the skeletal structures (vertebrae) surrounding the notochord and central nervous system, from which fact these immigrant cells, divided at first into segments like the coelomic walls which gave them origin, are called sclerotomes. From the method of formation budding of separate cells instead of an involution of epithelial tissue these sclerotomes must be regarded as mesenchymatous in nature, and their future fate must be described in connection with that layer. It is only necessary to say here that these MESOTHELIAL STRUCTURES, 103 g cells are not wholly used up in building the solid skeleton, but that some wander in between splanchnic mesothelium and entoderm, where they give rise to the smooth muscles and connective tissue of the alimentary canal, some pass be- tween the myotomes, where they form the partitions (myocommata * ) be- tween these structures, while others press farther and give rise, in part, to the deeper layers (cutis) of the skin, etc. This same middle zone also gives rise to a part of the excretory system (nephridia), which is also pri- marily divided into segments (ne- phrotomes). The lateral plate region (hypo- mere) shows but slight traces of segmentation. From the dorsal por- tion of its splanchnic wall arises the glomus of the pronephros (see below) and the gonads (reproductive struc- tures), but whether or not these lat- ter are truly segmented, and whether we have metamerically repeated go- notomes, is as yet a disputed question. The account of these reproductive and excretory organs will be given later. / Mesenteries. The greater por- tion of the lateral plates develops into the flattened epithelium (peri- toneum) lining the body cavity (splanchnoccele or metacoele), and plays an important part in the devel- opment of the walls of the alimentary tract and the membranous supports FIG. 112. Transverse sec- tion of embryo dogfish {Acan- thias}. a, aorta; c, coelom; ^, ectoderm ; g, ganglion of spinal nerve; A, hypochorda; /, intes- tine; m y mesomere ; me, mesen- chyme ; ms, mesentery ; my t myotome;- n, notochord ; /, pronephric duct ; s, sclerotome ; sc, spinal cord ; so and J/, somatic and splanchnic layers; sn, spinal nerve ; TV, wall of in- testine. The section passes on the left side through the middle of a myotome, on the right near the edge of one. 1 The term myocomma is sometimes regarded as a synonym of myotome ; the usage adopted here is preferable ; myoseptum is another term for it. 104 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. (mesenteries, in the broader sense of the word) which connect the various viscera to the walls of the body cavity. The condi- tions in the abdominal region will be described first. Here the splanchnic layer of the mesothelium applies itself to either side of the walls of the alimen- tary tract, it being of course kept in mind that mesenchy- matous tissue has migrated in between entoderm and meso- thelium in this re- gion (see p. 103), while above and be- low the digestive tract the dorsal and ventral walls of the hypomere press in- wards towards the median line, insinu- ating themselves dorsally between the alimentary canal and FIG. 113. Diagrammatic section of vertebrate through abdominal region, a, dorsal aorta; c, ccelom; g, gonad; gl, glomerulus; i, alimentary canal; /, trally between the liver; ///, mesentery ; mu, muscular layer of myotome; entoc j erm anc j ecto . my, myocoele ; n, nephrostome ; na, neural arch ; nc, notochord ; o, omentum ; s, spinal cord ; so, somatic layer of peritoneum; sp, splanchnic layer of peri- the notOCnOrd, derm. As a result there is formed a toneum ; /, nephridial tubule; vm t ventral mesentery ; Double partition be- w, Wolffian duct. tween the metacoeles of the two sides both above and below the intestine, with a small amount of mesenchymatous tissue between the two epi- thelial walls. These partitions, which thus come to support the alimentary canal (Fig. 1 1 3), are the dorsal and ventral mesenteries. The ventral mesentery is never perfect throughout the abdominal cavity. In the posterior portion the partition walls MESOTHELIAL STRUCTURES. 105 break down, placing the coeloms of the two sides in free com- munication. In front a part of this ventral mesentery persists, binding the liver to the anterior abdominal wall, and in many ichthyopsida carrying the sub-intestinal vein to that organ. Another portion, known as the small omentum (or gastro-hepatic) and the duodeno-hepatic omentum extends from the dorsal surface of the liver to the stomach and duodenum (Fig. 113). The dorsal mesentery is usually far more complete. 1 In it are recognized various regions, named according to the organs which they support, mesogaster, mesentery proper, mesocolon, FIG. 114. Three stages in the development of the alimentary canal and the mesenteries of man, after Toldt and Hertwig. a, appendix vermiformis ; ao, aorta; b, bile duct ; c, csecum ; co, colon ; d, duodenum ; go, great omentum ; me, meso- colon ; me, mesentery ; mg, mesogaster ; /, pancreas ; r, rectum ; s; stomach ; si, small intestine ; sp, spleen ; tc, transverse colon ; v, vitelline duct. The arrow points to the opening of the omentum. mesorectum, etc. It is attached to the dorsal wall in a straight line, and in those vertebrates with a straight alimentary canal the mesentery is a plane membrane, but with increasing con- volution of the alimentary canal, the membrane becomes corre- spondingly plaited. Besides this complication, the mesenteries can form secondary unions with the body wall, or with the 1 In Petromyzon (cyclostome) it has entirely disappeared, except a few shreds in the rectal region. 106 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. mesenterial regions, the details of which must be sought in special works. Those mesenterial folds which bind the various regions of the alimentary canal to each other have received the special name of omenta. The small omentum has just been men- tioned ; the gastro-splenic omentum connects the spleen with the stomach ; while in the higher vertebrates the great omen- tum is a large double fold formed from the mesogaster and the mesocolon, which connects the stomach to the transverse colon. In the region of the heart and of the lungs (when these organs are present) the splanchnic layer of the ccelomic wall becomes similarly related to these structures, and in a similar way similar supporting folds (mediastinum for the lungs, meso- cardium for the heart) are formed. In the abdominal region similar mesothelial folds (mesorchium in the male, mesovarium or mesoarium in the female) support the reproductive organs (gonads). Divisions of Splanchnocoele. So far, that part of the ccelom enclosed between the lateral plates has been considered as a single space on either side, as it is in the early development. Soon, however, the anterior portion becomes cut off from the rest and forms a sac, the pericardium, enclosing the heart, the relations of which are described in connection with the circu- latory organs. In the lower vertebrates the posterior wall of this pericardium is known as the false diaphragm or septum transversum, and in many is perforated by one or more small pericardio-peritoneal canals, connecting the pericardium with the abdominal cavity, a result of incomplete closure. In the mammals the true diaphragm appears, although rudi- ments, sometimes even muscular, appear in some sauropsida. This diaphragm is a transverse muscle, usually described as crossing the abdominal cavity from side to side, completely dividing it into two cavities, an anterior or pleural, in which the lungs are placed, and a posterior or peritoneal cavity con- taining the remaining viscera. This statement is not exactly correct. In the lower forms the liver abuts directly against the septum transversum. In the mammals these relations are MUSCULAR SYSTEM. ID/ the same. The diaphragm is therefore to be regarded as a paired structure, extending from the lateral walls behind the lungs to middle part of the septum transversum. This explains why it is that the pericardium appears as if enclosed in the pleural cavity, although it is morphologically outside of it. FIG. 115. Diagrams illustrating the relations of the pericardium to the rest of the coelom: A in fishes, B in amphibia and sauropsida, C in mammalia, d, dia- phragm; //, heart; /, liver; /, lungs; s, septum (false diaphragm) between peri- cardium and the rest of the ccelom. In B the lungs project into the general ccelom (pleuro-peritoneal cavity), in C, by the formation of the diaphragm, pleural and peritoneal cavities are distinct, while the pericardial cavity, containing the heart, has been shoved backwards between the two pleural cavities. The abdominal coelom is not completely closed off from the outer world ; for the urogenital ducts, to be described later, form a means of communication. Besides these there occur in cyclos- tomes, many fishes, dipnoi, turtles, and crocodiles, from one to two small openings (known as abdominal pores), beside or be- hind the vent, by means of which the coelom is connected with the outside world. Little is known as to their function. MUSCULAR SYSTEM. The history of the muscle plates or myotomes is next to be taken up. After their separation from the other portions of the primitive mesothelial tissue, these form a series of approximately IO8 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. cubical hollow bodies on either side of the notochord and cen- tral nervous system. From the early idea that these bodies gave rise to the vertebrae, they were formerly called proto- vertebrae. We now know that they contribute little ^or noth- ing to the skeletal structures, but give rise to the voluntary musculature of the body. The processes involved in the con- version of these epithelial walls into muscle the histogenesis of muscle must be traced first. In the majority of the vertebrates the cells of the mesal wall (i.e., that towards the notochord and nervous system) rap- idly increase in number, thus obliterating the myoccele. In this process the cells lose their original shape and arrangement as a cylindrical epithelium, and form elongated cylinders, the axes of which are parallel to the longitudinal axis of the body. Each of these primitive muscle cells at first contains but a single nucleus ; but by division several arise, which may either eventually lie in the centre (amphibia) or on the periphery (mammals) of the cell. At the same time the peripheral protoplasm of the cell be- comes differentiated into numbers of fine longitudinal fibrillae, which increase in num- ber so that at last all except a small amount of protoplasm in the immediate vicinity of the nucleus has been converted into these contractile structures, the epithelial cell becomes a muscle fibre. The lateral or outer wall of the myotome does not par- ticipate in this muscle formation, but is said to give rise to the deeper layer (corium or derma) of the skin. The process of the histo- genesis of muscle in the cyclostomes differs in some particulars from that given above. The myotomes, after their separation from the mesothelial tissues, increase rapidly in their dorso-ventral dimensions, and gradually push in between the lateral plate and the ectoderm in FIG. 116. Myo- tomes of Amblystoma in process of conver- sion intomuscle-plates. c, remains of myo- coele; ck, chorda; ^, epidermis; ;;/, muscle developing from inner plate of myotome ; o, outer plate of myo- tome ; s, skeletogenous tissue. MUSCULAR SYSTEM. IC>9 the ventral half of the body, thus giving rise to the musculature of this region, while dorsally their extension is less marked (Fig. 1 1 3). In this process the myocommata also participate, so that the whole body is enveloped on either side by a series of muscle-plates, the fibres of which have a generally longitudinal direction, and are inter- rupted at regular intervals by the in- termuscular ligaments, the derivatives of the earlier myocommata. This primitive condition can readily be rec- ognized in the trunk region of a fish, but it becomes greatly modified in the FIG. 117. Illustrating the downward growth of the myotomes. e, epimere (myo- tome) ; m, mesomere. birds and mammals ; yet even here traces of the primi- tively segmented condition can be made out in the ventral abdominal region and in the intercostal muscles. In the fishes the result- ing muscles of the trunk and tail become subsequently di- vided into dorsal and ventral or epiaxial and hypaxial sys- tems, the line of division between the two following more or less closely the lat- eral line, and being marked by a partition of connective tissue. In the amphibia these epi- and hypaxial por- tions are clearly visible in the tail, but farther forward the hypaxial system is reduced. This reduction is carried to a greater extent in the aminotes, where almost the sole traces FIG. 118. Transverse section through young Aniblystoma, showing the final re- sult of the downward growth of the myo- tomes (deeply shaded) ; a, aorta ; c, car- dinal vein; /, liver; o, oesophagus; /, peritoneum ; pg, pectoral girdle ; r, rib ; s, subvertebral vein. I 10 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. of the hypaxial system are to be found, greatly modified, in the pelvic and neck regions. The subsequent modifications of the primitive musculature in the higher groups cannot be traced here in detail, even were it better known. Only the origin of the limb muscles can be referred to. This is best known in the fishes, there being only FIG. 119. Section through the tail of Am- blystoma, showing ( is of itself sufficient to explain the ab- head cav i t i es . sence of a continuous metaccele. To the muscle fibres, the development of which was out- lined above, other parts of mesenchymatous origin are added in the development of the definitive muscle. This connective 1 There is clearly one pair of coelomic cavities in front of the first recognized by van Wijhe (Fig. 121, ac). 112 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tissue wanders in among the separate muscle cells, forms an envelope (perimysium), and binds these together into bundles, and bundles into muscles. This perimysium, which extends beyond the contractile or true muscular portions, also forms the means of attachment l of the muscles to the parts which are to be moved ; it gives rise to what are known as ten- dons, sometimes, as in the extremities, of considerable length. Tendons may occur not only at the ends, but in the middle of muscular tracts. When forming broad, flat sheets, tendons my v gl o f g t FIG. 122. Anterior end of embryo dogfish, Acanthias, viewed as a trans- parent object, a, anterior head cavity; b, first true gill cleft; e, eye; f, facial nerve ; f(>, fore brain ; g, Gasserian ganglion ; gl, glossopharyngeal nerve ; h, heart ; m, position of mouth; nib, mid brain; my, myotome ; o, auditory capsule; /, pinealis ; s, spiracular cleft; /, trigeminal nerve; v, vagus nerve; I, 2, first and second head cavities of van Wijhe. are called fascia or aponeuroses. Frequently ossification may occur in tendons, familiar examples being found in the patella or knee-pan of man, the bony tendons in the ' drumstick ' of many birds (turkeys), etc. In shape the muscles vary extremely. In the trunk region, as a rule, they are short and more or less flattened ; in the ex- tremities they are usually prismatic or cylindrical, and greatly elongate. They may have one or several heads ' or points of 1 That attachment of a muscle which usually remains without motion in the contrac- tion of a muscle is spoken of as its origin ; the attachment to a movable portion as its insertion. MUSCULAR SYSTEM. origin (biceps, triceps, etc.) ; one or several points of inser- tion (pinnate, bipinnate, ser- rate, etc.). In the fish-like vertebrates the trunk muscles clearly show their myotomic origin, for myo- tomes and the intervening myo- commata are strikingly in evi- dence. Even here there is a tendency toward specialization, for a horizontal connective tis- sue partition divides the muscles of each half of the body into dorsal and ventral portions (p. 109) ; while in the ventral region occurs a subdivision into a me- dian rectus muscle, and a more lateral oblique muscle (the names being indicative of the direction of the muscle fibres). These same features can be traced more or less clearly in the higher vertebrates, compli- cations being introduced by the greater development of those muscles which, while having their origin on the trunk, serve to move the limbs, and by the subdivision of the others, into distinct regions. . Thus the rec- tus may be divided into a rectus FIG. 123. abdominis, extending from the f Necturus. ceps ; C, cora cobrachialis ; ceratohyoid ; EO, external oblique; F, flexor communis; FC, gracilis; Gff, geniohyoid; 1C, ileocaudalis ; LA, linea alba; mylohyoid ; MO, middle oblique ; P, pectoralis ; PC, procoracoid ; Pff, procoracohumeralis ; PT, pectineus ; PY, pyriformis ; KA jRI, rectus internus ; SC, supracoracoid ; SH, sternohyoid ; VI, Ventral muscles A, anus ; B, bi- coid ; CB, cora- ECH, external f emorocaudalis ; G If, masseter; MH, PF, pubofemoralis ; , rectus abdominis ; vastus internus. 114 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. pelvis to the breast-bone, a sterno-hyoid from the sternum to the hyoid region, and a genio-hyoid from the hyoid to the extremity of the lower jaw. Similarly the oblique muscles may be subdivided into three or more layers (internal and external oblique, transverse, etc.); intercostals, between the ribs; scalenes, from the anterior ribs to the side of the neck, and sterno- and cleido-mastoid from the breast-bone and clavicle to the skull. In the dorsal half of the trunk also a large number of separate muscles may be distinguished, spinales, between the spinal processes of the vertebrae ; inter-transversales, between the transverse processes ; longissimus dorsi, arising from the ribs and transverse processes, and extending along the back (con- tinued in the cranial region as the trachelo-mastoid) ; recti capi- tis, etc. The muscles of the diaphragm are indirectly derived from the ventral portion of the myotome. In the gill region of the branchiate vertebrates special mus- cles are developed from the corresponding myotomes to open (levator and depressor arcuum) and to close (constrictors) the gill slits. With the loss of the gills these muscles change in their functions, and become connected with the hyoid or disappear. The jaws are opened by a digastric muscle arising from the base of the skull, and inserted on the angle of the jaw, while closure of the mouth is effected by adductors, called masseter, temporalis, or pterygoid, according to their origin from different regions of the skull. The muscles which move the ball of the eye are, in all vertebrates, six in number, and are derived from the three anterior head somites of van Wijhe (p. in). The most anterior of these devel- ops into three rectus muscles, supe- rior, internal, and inferior, and into the inferior oblique; the second furnishes the superior oblique, and the third the external rectus. It is interesting to note that the nerve supply of these muscles corre- FIG. 124. Eye muscles and nerves in shark, a, abducens; otn, oculomotor; /, trochlearis nerves ; oi, os, inferior and superior oblique muscles ; re, ri, rif, rs, ex- ternal, internal, inferior, and superior rectus muscles. MUSCULAR SYSTEM. 115 spends to this origin (see cranial nerves). These muscles move the eye, and in many forms are re-enforced by a retractor bulbi, apparently derived from the third head segment. Beneath the skin of mammals there occurs a general mus- cular layer, the panniculus carnosus, concerning which our information is none too extensive. From this layer are devel- oped in the facial region ' muscles of expression,' which serve to move the skin, especially that around the mouth and eyes. The fact that these muscles of expression are innervated by the facial nerve would apparently indicate their point of origin as behind the jaws. The muscles which move the limbs are divided into intrinsic (those which have their origin and insertion on - the bones of the limb or of the supporting girdle) and extrinsic (which arise from the trunk and are inserted on the girdle or on the limb). In the fishes neither series acquires extensive development ; but with the more varied movements necessary in a terrestrial life both sets, and especially the intrinsic, attain a high grade of differentiation. Both series may be grouped as dorsal and ven- tral, and these divisions again may be considered accordingly as they are preaxial or postaxial in position ; i.e., accordingly as they are in front of or behind the axis of the limb. The proxi- mal extrinsic and intrinsic preaxial muscles act as protractors, serving to move the limb forwards, the postaxial as retractors, which move it in the opposite direction. The other intrinsic muscles are divided between flexors, which bend the limb upon itself, and extensors, which straighten it after flexion. For the details of these muscles reference must be made to special works. Eletrical Organs. In certain fishes, Torpedo, electrical eel (Gymnotus), Malapteru$, and to a less degree in some skates {Raia), certain of the muscles become metamorphosed into an electrical organ. This organ lies in the Torpedo on either side of the head ; in the others in the trunk or tail near the back- bone. In all the organ consists of a series of capsules of con- nective tissue filled with a gelatinous substance in which are the 'electrical plates,' in which the nerves terminate, and which are apparently the modified motor end-plates of the muscle. Il6 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. The discharge of the organ is under control of the will, and varies in strength according to the size of the organ and its con- dition of fatigue ; in the torpedo and electrical eel it is sufficient to knock a man down, but in the others it is much less in amount. UROGENITAL ORGANS. The excretory and tne repro- ductive organs of the vertebrates are so closely related to each other that it is impossible to treat them separately. The excretory glands (nephridia), reproductive organs proper (gonads), and the ducts to carry away the nitrogenous waste and the reproductive elements, stand in close relation to each other in position, development, and function. Hence We speak of urogenital organs. Nephridia. Under the head of nephridia are to be included three different structures which appear in the vertebrates, a pronephros ( ' head kidney ' of older writers) ; a mesonephros or Wolffian body ; and a metanephros, which is the functional kid- ney in the amniotes. Pronephros and mesonephros appear only as embryonic structures in the amniotes ; but in the lower groups the pronephros is usually functional for a time, the mesone- phros assuming its work in the adult. 1 These organs have a regular succession in time, and hence in our account we follow the order of development and begin with the pronephros. It will be remembered (p. 101) that the walls of the meso- thelial cavities on either side are divisible into three zones, and that segmentation only affects the dorsal and middle of these, the hypomere being unsegmented. The nephridial structures FIG. 125. Diagrammntic sec- tion of electrical apparatus, from Wieclersheim. The arrow points dorsally or anteriorly. BG, con- nective tissue framework ; EP y electrical plates; G, gelatinous tis- sue ; A 7 , nerves entering through the septa; AW, terminations of the nerves. i In the elasmobranchs the pronephros is never functional, while apparently in Bdel lostoma (a cyclostome) the whole excretory organ is pronephric. UKOGENITAL ORGANS. 117 arise almost entirely from the mesomeric segments (nephro- tomes). The pronephros arises from a few l nephrotomes im- mediately behind the head. From the outer wall of each of these an outgrowth occurs, sometimes solid at first, but usually hollow from the beginning, its apex directed towards the skin. These outgrowths form the pronephric tubules, each of which opens at the inner end, by means of the remains of the cavity of the mesomere, into the body cavity, the opening being funnel- shaped and, in its full development, ciliated. These openings B FIG. 126. Diagrams of the relations of pro- and mesonephros, based on Semon. Mesomeric structures shown with conventionalized cells. A, pronephros ; B, mesonephros. a, aorta ; b, Bowmanjs capsule ; d, pronephric duct ; g, gonad ; gl, glomerulus; gs, glomus ; in, inner nephrostome ; ;;/, myotome ; ms, formation of mesenchyme ; //, mesonephric tubule ; n, cavity of nephrotome ; ns, nephrostome ; on, outer nephrostome ; /, pronephric tubule. are the nephrostomes. Distally the tubules of the successive segments fuse together, thus giving rise to a longitudinal tube, the pronephric duct, which gradually extends backwards be- hind the pronephric segments, until at last it fuses with the cloaca or with the skin' immediately adjoining. An opening now forms between the duct and the cloaca, and thus, through the system of tubes leading from the nephrostomes to the vent, the body cavity is placed in connection with the external world. 1 Two in most urodeles and amniotes ; three in lampreys, some sharks, anura, and some amniotes ; four in some sharks, seven or eight in skates, and a dozen in caecilians. It has been pointed out that in general terms the number of pronephric nephrotomes is roughly correlated to the number of segments in the whole body. Il8 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. This backward growth of the pronephric duct is apparently (at least in most forms) the result of growth of the duct itself, without any cellular additions from other sources ; although a few years ago the duct was described in most vertebrates as being wholly of ectodermal origin, a view arising from the fact that the duct in its progress fuses with that layer. A second pronephric element is the glomus. The dorsal arterial blood-vessel (aorta) gives off an arterial twig on either side opposite each nephrostome. Each artery breaks up into a vascular network just beneath the dorsal splanchnic epithelium of the hypomere (Fig. 126, A), and pushes out so that the struc- ture projects into the dorsal portion of the body cavity. This vascular outgrowth is the glomus. In most vertebrates it is unsegmented, but forms a continuous rete mirabile, and projects freely into the coelom. In certain forms, however (e.g., Ichthy- opJiis), the glomus becomes distinctly segmented, while the dorsal portion of the body cavity becomes cut off from the rest, forming a separate envelope (Bowman's capsule) around each glomar segment, so that here we have a series of vascular cap- sules almost exactly comparable to the Malpighian bodies to be described in connection with the mesonephros. There is considerable evidence to show that the pronephros originally had a much greater extent than in most existing forms ; and indeed the structure may have extended nearly to the vent, as is apparently the case in Bdellostoma, if we may judge by recent studies. Its fate in all vertebrates except this cyclostome will be better understood after a history of the meso- nephros. ' The mesonephros or Wolffian body is usually confined to segments behind the pronephros, and is often spoken of as a later generation of excretory structures. The fact, however, that pro- and mesonephric tubules can occur together in the same segment tends to show that the two structures are dis- tinct. The mesonephric tubules are formed in a manner similar to the pronephric tubules, except that they arise from the more dorsal portion of the nephrotome. They grow outwards, and finally connect with the pronephric duct, although they do not UROGENITAL ORGANS. 119 participate in its formation. From this time on the prone- phric duct is usually called the mesonephric or Wolffian duct. The aorta likewise forms segmental twigs, which grow out to- wards the splanchnic layer of the nephrotome, and give rise to a series of vascular networks, the glomeruli, which differ from the glomus of the pronephros in that they project not into the larger body cavity (splanchnoccele), but into the cavity of the sm a cv FIG. 127. Diagram of the development of the nephridial system in the verte- brate. The pronephric system is according to the views of Semon ; it is more prob- able that they arise from the nephrotomes instead of from the somatic layer, a, aorta; c, notochord ; cv, cardinal vein; cl, pronephric duct; g, glomus tf, gonad; z, intestinal epithelium; m, myotome (muscular layer); mb, Malpighk . body; me, myoccele ; mt, mesonephric tubule ; n, nephrotome ; ns, nephrostome ; pt, prone- phric tubule; sc, spinal cord; sg, spinal ganglion; sm, sympathetic ganglion; so, somatic layer ; sp, splanchnic layer ; z, cavity of nephrotome. Compare with Fig. 1 1 1. nephrotome, and in their segmental arrangement. The walls of the nephrotome close over each glomerulus, and are hence- forth known as Bowman's capsule, while the whole complex of capsule and glomerulus form a Malpighian body or corpuscle. The mesonephric tubule opens into Bowman's capsule by means of an inner nephrostome, while the lower portion of the nephro- 120 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tome cavity retains its connection with the metacoele, the open- ing forming the outer nephrostome (Fig. 126, j5). As was stated above, pro- and mesonephros are the only excretory organs in the ichthyopsida. They appear as larval structures in the amniotes, and only in certain reptiles does either of them function after hatching. In the adult ichthyop- sidan the pro- and mesonephros are usually readily distinguished, the pronephros being in front, rudimentary in character, and \v FIG. 128. A single tubule of the mesonephros of Proteus anguineus modified from Spengel. C, begin- ning of collecting tubule ; B, Bow- man's capsule ; 6", glomerulus ; /, ON, inner and outer nephros - tomes. FIG. 129. Section through the meso- nephric region of Amblystoma, 45 mm. long. A, aorta; B, Bowman's capsule, from which the glomerulus has dropped out ; C, carti- lage, and />, bone of vertebral centrum ; (7, gonad ; GL, glomerulus ; M, mesentery ; MA, mesenteric artery.; A 7 , notochord ; T, mesonephric tubules; W, Wolffian duct. separated by a greater or less distance from the Wolffian body, which usually extends along the greater part of the dorsal wall of the body cavity. The pronephros acquires a varying development in different vertebrates. In the elasmobranchs its tubules never become convoluted, and in very early embryonic life the nephrostomes fuse so that a single large opening connects the coelom with the pronephric duct, and forms the anterior end of the Mullerian UROGENITAL ORGANS. 121 duct to be described below. In the amniotes, also, the pro- nephros never advances beyond a very rudimentary condition, and soon degenerates, and, to a greater or less extent, disap- pears. In ganoids, teleosts, and amphibia the pronephros is functional for a time. The tubules become greatly convoluted, and between them is developed a rich plexus of sinus-like blood- vessels. Later it degenerates in all except a few teleosts (Fier- asfer, Dactylopterus), where it remains functional throughout life, while in others it retains its excretory character until the ap- proach of sexual maturity. In these teleosts with functional pronephros the funnels connect with the pericardial cavity (the same condition has been described in cyclostomes), a relation readily understood from the method of formation of the pericar- dial walls. In its degeneration the pronephros contributes to the formation of the supra-renal bodies to be described below. In its development the mesonephros progresses beyond the stage at which it was left above. The tubules, instead of being short and transverse, become greatly convoluted, and they also increase greatly in number, new tubules, with funnels and Mal- pighian bodies, being developed by budding dorsal to the primary tubules ; and after a convoluted course these secondary and ter- tiary tubules join the distal ends of the first, which thus become converted into collecting tubules, emptying into the pronephric duct. With this formation of new tubules the mesonephros largely loses the segmental character that it earlier possessed. With the convolution and increase in number of the tubules blood-vessels enter between these structures, and form a rich capillary plexus surrounding them. The cells of the tubules become cubical and excretory in character. This increase in number and size of the tubules increases the size of the organs, so that they protrude into the coelom as a ridge on either side of the mesentery. The physiological action of pro- and mesonephros is appar- ently as follows : Blood from the aorta enters the glomus or glomeruli, through the walls of which it loses water, which passes (pronephros) into the ccelom or (mesonephros) into Bowman's capsule, and from thence into the tubules. From the glomeruli the blood next passes into the plexus surrounding the tubules, 122 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. and here, by means of the cubical cells, loses its nitrogenous waste (uric acid, urates, etc.). By means of the cilia surround- ing the nephrostomata, watery matter is also taken from the coe- lom, and all of these waste products are passed via the pronephric duct to the exterior. In teleosts and ganoids all of the mesonephros is excretory ; but in elasmobranchs and amphibians the anterior end loses this function and becomes largely degenerate (females), or enters into the service of the reproductive structures (males), as will be described below. In the amniotes the whole mesonephros degenerates and disappears, except in so far as it enters into connection with the gonads, and is represented by the paradidy- mis and parovarium (infra). To compensate for this disappearance a third excretory organ, the metanephros, or kidney proper, is developed in the am- niotes. Its developmental history is not so well known as that of the pro- and mesonephros, and the following statement is only tentative. A hollow diverticulum arises from the dorsal sur- face of each pronephric duct, near its entrance into the cloaca. This grows rapidly forward near the aorta, and develops into the excretory duct (ureter) of the metanephros. As it grows for- ward the mesoderm behind the Wolffian body rapidly proliferates, and becomes richly vascular. When the ureter reaches the hin- der end of the Wolffian body it expands, giving rise to the pelvis of the kidney, and produces, by budding from its tip, cords of cells which soon become tubular, and form the collecting tubules of the kidney. In the proliferated meso- derm other tubules also appear (the method of their formation is not clear) connected with Malpighian bodies, essentially like those of the mesonephros. These metanephric tubules become greatly convoluted, and at FlG. 130. Kidneys (/) and supra- renals (j) of a human embryo, after Wiedersheim. The figure shows the lobulated appearance of the early kidney. UROGENITAL ORGANS. 123 last open into the collecting tubules ; but it is important to note that at no time are nephrostomata developed in connec- tion with them, and the body cav- ity is without communication with this nephriclial system. While this process is going on the whole met- anephros pushes farther forward, dorsal to the pronephric duct, the ureter increasing correspondingly in length. In the subsequent his- tory the kidney becomes strongly lobulated, the lobes corresponding to the groups of collecting tubules of which it is composed. This lob- ular appearance is retained through- out life in the sauropsida, but is subsequently lost in all mammals except the whales and some car- nivores. The kidney never extends through as many segments as does the mesonephros, but forms a relatively smaller and more com- pact body lying within or a little in front of the pelvic region. In the mammals the anterior end of the ureter becomes widened out, inside of the mass of the kidney, into a considerable chamber, the pelvis of the kidney, into which the collecting tubules empty, the openings of these being placed on one or more papillae, which extend into the pelvis renalis, partially dividing it into smaller chambers or calyces. The ureter does not long retain its primary connection with the distal end of the pronephric duct, but acquires its own open- FIG. 131. Urogenital system of male heron (Ardea}, from Wiedersheim. Ao, aorta; F, bursa Fabricii, opening at BF' into the cloaca, Cc; Ep, epididymis; Ho, testes ; A 7 , kidney ; Sr, opening of ureter ; V, furrows for veins on ventral surface of kidney ; Vd, vasa deferentia; Vd', their opening into cloaca. 124 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. ing into the dorsal portion of the cloaca ; and then this cloacal region becomes constricted off from the rest to form a urinary bladder, which is connected directly or indirectly with the ex- terior by a single duct, the urethra. The bladder persists throughout life in lizards, turtles, and mammals, but disappears in the other amniotes. Reproductive Organs. To those structures which are to produce the reproductive cells, eggs and spermatozoa, the term gonads has been given. These are paired (unless fusion or IG. 132. Section of ovary of new-born child, from Hertwig after Waldeyer. /, single egg surrounded by follicle cells ; g, group of egg cells and follicle cells ; ge, germinal epithelium ; /, egg strings ; /, female amniote. c, cloaca; e, epididymis; /, fimbriated extremity of Fallopian tube; g, gonad; h, stalked hydatid; k, kidney (metanephros) ; m, Mullerian duct; mn, mesonephros (Wolffian body); o, ovary; od, oviduct; ot, ostium tuBae; pd, paradidymis; po, paroophoron ; pv, parovarium ; r, rete ; /, testes; , uterus; urn, uterus masculinus; ur, ureter; va, vas aberrans; vd, vas deferens; ve, vasa effer- entia; ?v, Wolffian duct. epididymis (see below), while the posterior end occasionally retains its lumen, and is known as the uterus masculinus. The history of the Wolffian duct is somewhat different. In the female its anterior end degenerates ; and in the amniotes, where the metanephros usurps the functions of the mesone- phros, this degeneration extends to the whole tube. The only portions which persist are, a rudimentary structure behind UROGENITAL ORGANS. I2 9 known as Gartner's duct, and in front where it comes in con- nection with a small body known as the parovarium or epooph- oron 1 formed from the degenerate tubules of the mesonephros. J> " The Wolffian duct persists ' throughout life in the male, where -Of Htt- m/(&f) A B FIG. 136. Scheme of urodele urogenital system based on Triton, from Wied- ersheim after Spengel. A, male; B, female, a, excretory ducts; GN, sexual part of mesonephros; Ho, testis; lg t Leydig's duct (ureter); mg, Mullerian duct (oviduct in j?) ; mg' ', its vestigial end in the male ; A 7 , functional portion of meso- nephros ; Ov, ovary; Of, ostium tubse; Ve, vasa efferentia ; -f- collecting duct of the vasa efferentia (rudimentary in\5). it acquires new functions. Here the anterior end of the mesonephros loses its excretory powers, and enters into con- 1 In amniotes, where the whole mesonephros degenerates, the posterior portion of the Wolffian body in the female forms a parobphoron behind the ovary, a structure of only vestigial importance. 130 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. nection with the testis. Its tubules branch and anastomose, and also connect with the seminiferous canaliculi, forming a system of ducts conducting the spermatozoa into the anterior end of the Wolffian duct. This plexus of tubules nearest the testis is the rete, nearer the duct it forms the vasa efferentia. The Wolffian duct, by this assumption of reproductive func- tions, is converted into a vas deferens, the anterior part of which becomes greatly coiled, this por- tion being known as the epididymis. More distally the duct may develop marked muscular walls and form an ejaculatory structure. As was said above, the posterior portion of the mesonephros retains its excretory powers in the ichthyopsida, and as it pours its secretions into the Wolffian duct, this tube in the male is at once excretory and reproductive in character. In the male amniotes, when the metanephros is developed, the pos- terior part of the mesonephros degen- erates into a small body close to the epididymis known as the paradidymis (organ of Giraldi), and occasionally forms one or more blind tubes (vasa aberrantia), opening into the vas deferens. In some vertebrates the Wolffian and Miillerian ducts open directly into the cloaca, but in most they unite into a urogenital sinus, which, in turn, empties by a single opening into the cloaca. In the mammals (monotremes excepted) the separation of the uro- genital sinus from the hinder end of the alimentary canal is complete, a muscular partition (perineum) separating the vent from the urogenital opening. Connected with the urogenital structures outlined above are many accessory parts, glands, external genitalia, copulatory organs, etc., some of which will be described in connection FIG. 137. Urogenital or- gans of male frog; the testis, T, turned to one side to ex- pose A, the vasa efferentia passing from the testis to M, the mesonephros. F, poste- rior end of fat body ; P, post- cava; S, suprarenal; U y ureter. UROGENITAL ORGANS. 131 with the groups in which they occur. Others will be found in the larger manuals to which reference must be made. Only one of these structures seems to demand attention here. This is the suprarenal body, which derives its name from the fact that in the mammals it forms a capsule-like structure on the anterior end of the kidney. In the sauropsida it is in closer connection with the gonads. In the amphibia (Fig. 137) it is either on the ventral surface of the mesonephros (anura), or upon its medial margin (urodeles). In the teleosts it is either closely connected with the mesonephros, or is farther forward in the region of the degenerate pronephros. In the elasmo- branchs the suprarenal is replaced by two structures: (i), an interrenal, a long, slender body just medial to the ureter, those of the two sides being connected behind ; (2), a series of adrenals, on either side closely connected with the sympathetic ganglia. Development teaches that the suprarenals consist of two portions different in origin and corresponding to the inter- and adrenals of the elasmobranchs. One portion (the cortical sub- stance, interrenals) arises from the mesothelium, and according to recent observations, largely by a metamorphosis of the glomus of the pronephros, the mesonephros possibly contributing to some extent. The medullary substance (equivalent to the ad- renals) is derived in part from the sympathetic nervous system, and contains ganglion cells, and in part is mesenchymatous in nature, this tissue arising from cells proliferated by the septum transversum. 132 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. MESENCHYMATOUS STRUCTURES. As was stated on a preceding page (p. 8), the mesenchyme may arise from ectoderm, entoderm, or mesothelium, either by the separation of isolated cells, as is usually the case, or by the immigration of large masses of cells into the space (i.e., the re- mains of the segmentation cavity) between the other body layers. This immigration in large masses from the mesothelium is shown in the formation of the sclerotomes in Fig. 1 1 2, and from the ectoderm into the region of the head to form the gill cartilages in Fig. 138. The mesenchyme is characterized by the fact that it never gives rise to epithelial structures, 1 and as a rule, by the great development in it of intercellular substance, as seen in fibrous or areolar connective tissue, cartilage, bone, blood, etc. Smooth muscle tissue, however, is an ex- ception in this respect. Besides the connective tissues proper, which extend through all parts of the body, forming a sup- port and connection for tissues and organs, the mesenchyme also gives rise to most of the skeletal and circulatory structures. FIG. 138. Section through the head of an embryo Amblystoma, showing the points, H and Af t where the ectoderm is producing the mesenchyme to form the hyoid and mandibular arches. A, audi- tory ganglion ; C, ccelom of man- dibular arch ; CL, cuticular layer of ectoderm ; MO, medulla ob- longata ; NL, nervous layer of ectoderm; VII, facial nerve. 1 It is possible that the epithelium (endothelium) lining the cavities of the vascular system is of mesenchymatous origin, but the weight of evidence goes to show that some of it at least is of entodermic origin. SKELETON. 133 THE SKELETON. The skeletal structures of the vertebrates may be either membranous, cartilaginous, or bony (osseous) in character ; and in development certain portions may pass successively through all of these phases in attaining the adult condition ; or the cartilage stage may be skipped, the membrane developing directly into bone ; or again, the cartilaginous condition may be the final stage of the skeleton. The membranous skeleton consists of connective tissue cells, and in its highest development forms sheets or masses of fibrous tissue. From it cartilage is developed by a great increase in the number of cells, the tissue in what has been called the procartilage stage consisting of closely compacted polygonal cells with large nuclei. These cells rapidly secrete an intercellular substance (chondrin), and thus the tissue be- comes converted into cartilage, the extent and solidity of which are dependent upon the amount of this matrix. In the conver- sion of cartilage into bone this matrix is dissolved ; and around the margins of the cavities thus produced bone-forming cells (osteoblasts) arrange themselves, and these, secreting lime salts (carbonate and phosphate) around themselves, gradually build up the bone. In the lower vertebrates this process begins upon the outside of the cartilages and proceeds toward the interior ; but in the higher forms, besides this perichondrial ossification, centres of ossification appear within the cartilage, and from these the ossification extends peripherally. In the conversion of membrane into bone there is the same appearance of osteoblasts in and upon the tissue as described above, and these produce the bony substance in the same way. The result in either case is the same, and it is not possible by histological means to distinguish between cartilage bones and membrane bones ; this depends entirely upon development. As will appear later, the distinction between the two is very important. Increase in the size of membranes and cartilage is accom- plished by additions to the exterior as well as by increase in the interstitial substance. In the case of bone this interstitial 134 MORPHOLGY OF THE ORGANS OF VERTEBRATES. increase is impossible. Increase in size is effected here by additions to the exterior, and in the case of the long bones, bodies of the vertebrae, etc., by the appearance of more than one centre of ossification in the cartilage. From these centres ossification extends in all directions, but for a time there remains a cartilaginous region between the ends (epiphyses) and the main portion in which increase in length is possible. Later these epiphyses usually become so united or anchylosed to the main portion that the line of division cannot be traced. The skeleton may be divided into internal and dermal por- tions, and the internal in turn is composed of an axial portion, including the vertebral column, skull, ribs, and breast-bone ; and an appendicular portion, consisting of the skeleton of the appendages and the girdles supporting them. The vertebral column is developed around the notochord (p. 17). This, as will be remembered, is a rod-like structure of entodermal origin which lies between the alimentary tract and the central nervous system, extending from just behind the infundibulum to the posterior end of the body. Its cells gradu- ally become gelatinous, and migrate toward the periphery, where they finally become arranged in a man- ner recalling epithelium ; while the mass of the notochord is composed of a reticulum, in the meshes of which is the rather solid jelly. The cellular envelope thus formed and its derivatives are frequently called the elastica interna. It is clearly of entodermal origin. The notochord has different fates in the various divisions of the verte- brates, as will be detailed later. In the cyclostomes it con- tinues to increase in size throughout life, and constitutes the major portion of the skeletal axis ; but in other vertebrates the development of vertebrae relegates it to a very subordinate position in the adult, where it may persist as a very inconspicu- ous remnant. Fig. 139. Section through noto- chord of embryonic shark {Acan- thias}. C, centrum of vertebra; E, /, elastica externa and interna ; N y neural process. SKELETON. 135 The vertebrae proper arise from me_senchymatous cells, which bud off as sclerotomes (p. 102 and Fig. 1 1 1) from the developing mesothelial tissues. Some of these cells arrange themselves as a continuous envelope around the notochord (the notochordal sheath or elastica externa), while others wander inwards, be- tween the spinal cord, notochord, and muscle plates. It is to be noted that this skeletogenous tissue loses all segmental char- acter, and that the segmentation later to be seen in the vertebras is secondary, and is the result of the relations of myotomes and nerves. In the cyclostomes the notochordal sheath in- creases in thickness with age, and in these forms reaches its highest development. The earliest appear- ance of the segmental skeletal structures is seen as an increasing density of the mesen- chyme between the in- ner surface of each J* G ' I4 ' Section through a developing ver- tebral centre of the pig, showing the multiplication myotome and the Spinal of the mesenchyme cells where cartilages are to COrd. These more dense arise. C, vertebral centrum; Z>, dorsal; F, ven- portions are soon con- tral roots of a s P inal nerve ; ^ S an g lion of dorsal root; A r , notochord ; A', rib; RD,RV, rami dor- verted into cartilage, the salis and ventralis of nerve> result being a series of pairs of backwardly directed rods (the neural processes or neu- rapophyses), which tend to arch in the spinal cord. A little later similar condensations of mesenchyme take place around the notochord, a ring of this tissue occurring opposite to each pair of myotomes. This forms the rudiment of the body or centrum of the vertebra. Its subsequent history varies greatly in different groups ; and the final account cannot be written until we know more of the development, especially in the ichthyopsida. As usually described these membranous rings 136 'MORPHOLOGY OF THE ORGANS OF VERTEBRATES. become directly converted into cartilage and, in the higher forms, into bone; but the little we know of development, to- gether with the conditions occurring in the ganoids, and espe- cially in certain fossil amphibia (stegocephali), make it probable that a vertebral body or centrum is more complicated than it was once thought to be. The most complicated condition known is found in the fossil Archegosaurus. Here there occurs on the dorsal surface of the FIG. 141. Diagram of rhachi- tomous vertebrae, based on Arche- gosaurus. /ia, haemal process ; hc t hypocentrum arcale ; hp, hypo- centrum pleurale ; np^ neural pro- cess ; ns, neural spine ; /, pleuro- centrum ; z, zygapophysis. FIG. 142. Trunk vertebra of extinct stegocephalous Eurycormus speciostiS) showing rhachitomous condition, after Zittell. h, hypo- centrum ; n, neural arch ; p, pleuro- centrum; r, radialia. notochord on either side between two successive neural processes (Fig. 1 41 ) a skeletal plate, the pleurocentrum. On the ventral surface, opposite the base of the neural process, is an arched band, the hypocentrum * (or hypocentrum arcale), which extends across the notochord from one side to the other. Behind this and opposite the pleurocentra are a pair of skeletal plates, the hypocentra pleuralia. More usually (Fig. 142) the hypocentra pleuralia are absent. These forms belong to the rhachitomous type of vertebrae. In the embolomerous type (Fig. 143) a vertebral body is com- posed of two rings, one of which is directly opposite the base of 1 The terms centrum and intercentrum often used for these parts lead to unnecessary confusion ; the intercentrum is in most cases the hypocentrum arcale. SKELETON. the neural process, the other between two of these rings. In development (Amia) this embolomerous condition is derived from the rhachitomous type by the fusion of hypocentra pleura- lia with the pleurocentra to form one ring (centrum, auct.~), while the other (intercentrum) is developed by a dorsal extension of the hypocentrum arcale. In others it may be that no hypocentra pleuralia occur, the centrum arising by a ventral extension of the pleurocentra. In the birds and mammals the vertebra arises at first by what has been called a ver- tebral bow, passing beneath the notochordal sheath and obliquely upwards and back- wards to the posterior limits of the somite, while a little later the centrum proper forms behind the bow. This of course suggests a comparison with the rhachitomous vertebra. Concerning the fates of these parts in the higher vertebrates there is a difference of opinion. American students, as a rule, regard the pleurocentra as giving rise to the body of the verte- brae in the amniotes, the intercentrum appearing as the chevron bones well known in mammals. In amphibia and teleosts, on the other hand, the vertebral body is said to arise from the intercentrum ; i.e., from the hypocentrum arcale. Thus the ver- tebrae cannot be regarded as exactly homologous throughout the vertebrate phylum. Many European authorities, on the other hand, claim that the centrum of the vertebrates arises from the hypocentrum arcale, and that the pleurocentra either contribute or give rise to the anterior zygapophyses to be mentioned later. A third type of vertebra is the phyllospondylous, the rela- tions of which to the foregoing has yet to be made out. In this, the vertebral body is composed of right and left halves. This type is found in the fossil Branchiosauridae (stegocephalous batrachia). To the parts of the vertebrae so far described others may be added. In the embryo a ligament (interspinous ligament) runs the length of the body just dorsal to the spinal cord. Where FIG. 143. Tail verte- brae of extinct stego- cephalous Eurycormus speciosus showing embo- lomerous condition, after Zittell. h, hypocen- trum; /ia, haemal arch; , neural arch ;/, pleuro- centrum (intercentrum).. 138 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. this passes between the dorsal ends of the neural processes it becomes converted into cartilage, thus giving rise to an addi- tional element (spinous process or neural spine), which, together FIG. 144. Fifth to seventh caudal vertebrae of Perameles gunni. arch ; n, neural processes ; t, transverse processes. haemal with the two neural processes, form a neural arch enclosing and protecting the spinal cord. In the caudal region of the ichthy- opsida and some higher forms, the vertebra is completed below by a similar haemal arch, which encloses the caudal artery and vein. This arch is composed of a pair of hae- mal processes (haemapophyses) and a haemal spine. These various parts of the vertebrae arise separately ; but they exhibit in recent forms a tendency to fuse together in the adult, the fusion being most complete in the higher groups. The vertebrae are laid down at an early stage in development, and their number is not subsequently increased. Increase in length of body is therefore Caudal verte- accomplished by longitudinal growth of the centra of the vertebrae. In the additions ar TIG. 145. bra of alligator, az, prezyga- pophysis; c centrum; d, diapophysis ; h, haemal arch; ns, neural spine; pz, postzy- layers, on the circumference of the cen- .^apophysis. trum first formed, each new layer being slightly longer than its predecessor. As a result the centrum becomes concave on either end, is am- l>hiccelous. The parts of the centrum first formed prevent any farther increase- of the notochord in the intravertebral regions ; but intervertebrally it expands, filling up the cavities between the successive vertebrae, and thus assuming the appear- SKELETON. 139 me V=^W ^ FIG. 146. Diagram of method of growth of amphicoelous vertebrae, r, centrum ; /, in- tercentral enlargements of, , notochord; 1-4, successive layers of centra. ance of a string of beads (Fig. 146). In the amphibia we have at first the conditions just described, and in the perennibran- chiate forms this amphi- coelous condition persists ^py~~7^V 7-V~"5^El = -" 4 throughout life. Higher ^^fc^^fc still, there appears an in- tervertebral growth of car- tilage (Fig. 147, A) which produces a secondary series of constrictions in the no- tochord. A later stage in the process is shown in Fig. 147, B, where an absorption of a part of the intervertebral cartilage is taking place in such a way as to result in the formation of a cup at one end of the vertebra, and at the other of a rounded extremity which fits the cup at the end of the next vertebra. The extreme of the process is shown in Fig. 147, C, where the intervertebral cartilage has been cut completely in two, the result being the formation of a ball and socket joint be- c ^ tween the successive vertebrae ; while ossi- fication has extended so far that almost the entire centrum as well as a part of the inter- vertebral cartilage has been converted into bone. When this pro- cess results in a cen- trum rounded in front and hollow behind, we have an opistho- oelous vertebra ; when rounded behind and hollow in front, it is proccelous. A statement of the occurrence of these three types of vertebrae centra may be given here. Amphicoelous : most fishes, most perennibranch urodeles, some salamanders, some stegocephali, gymnophiona, many di- FlG. I47 Diagrams of developing vertebrae of urodeles, modified from Wiedersheim. r, centrum; ck, chorda; e, elastica externa ; i, intercentral carti- lage ; /, ligament ; s, incisure in cartilage. Bone lined, cartilage dotted. I4O MORPHOLOGY OF THE ORGANS OF VERTEBRATES. nosaurs, 1 plesiosaurs, ichthyosaurs, precretaceous crocodiles, geckos, rhynchocephalia, and the fossil birds Arch&opteryx and Ichthyornis. Opisthocoelous : Lepidosteus, most salamanders, Pipa, Dis- coglossus (anura), most dinosaurs, some vertebrae in penguins and auks, and the neck vertebrae of most ungulates. Procoelous : the majority of anura, reptiles, and birds. In the majority of mammals the vertebrae are flat upon each end of the centrum, amphiplatyan. In forms with amphicoelous vertebrae there was no true articulation of the separate elements of the vertebral column ; but with the assumption of pro- or Opisthocoelous conditions the vertebral centra articulate with one another, and frequently 71. S. FIG. 148. Anterior and posterior faces of a vertebra of Python, from Huxley. ns, neural spine ; ptz, postzygapophysis ;/#, prezygapophysis ; tp t transverse process; za, zygantrum; zs, zygosphene. accessory portions are developed to lock the vertebrae more firmly together. Most common of these are what are known as articular processes (zygapophyses). Of these there are two pairs, arising from the anterior and posterior surfaces of the neurapophyses. The anterior or prezygapophyses have their flattened surface turned dorsally so that they can articulate with the ventral surfaces of the posterior process (postzygapophyses) of the vertebra in front. In the snakes and some lizards (igua- nidae) these are re-enforced by articular surfaces developed from the neural spine. On the anterior surface of the base of the spine a wedge-shaped process (zygosphene) projects forward, its 1 In Camarasaurus the first caudal is convex on either end. SKELETON. 141 articular surfaces directed obliquely outwards and downwards. This fits into a corresponding cavity (zygantrum) on the poste- rior surface of the neural spine of the vertebra in front. In all forms above fishes, what are known as transverse processes (pleurapophyses) occur. The homologies of these are not settled. In general terms there may be said to be two of these on either side, a diapophysis connected with the neural process, and a parapophysis connected with the vertebral cen- trum. One or the other of these may excel in development, and occasionally either may be rudimentary. In addition the names anapophysis and meta- pophysis have been given to certain projections upon the neural processes which seem to be without great morpho- logical significance. The vertebral column or backbone is built up of these vertebrae, and in this struc- ture two or more regions can always be clearly distin- guished. In the fishes there FlG ' l ^' Anterior thoracic vertebra of alligator. C y canal ; 677, capitular head are two of these regions, trunk of rib . CT) cen trum; D, diapophysis; />, and Caudal, the Caudal being parapophysis; PO, postzygapophysis ; PR, distinguished by the presence Payga P o P hysis ; s, spinous process; 7Y/, tubercular head of rib ; VC, vertebrar- of a complete haemal arch in terial canal> connection with each verte- bra, while in the trunk the haemal processes diverge and be- come converted into so-called ribs (see below). In the am- phibia two other vertebral regions cervical and sacral occur. The sacrum intervenes between trunk and caudal vertebrae, and gives support to the pelvic arch by which the hind limbs are supported. The trunk vertebrae bear true ribs, while the cervi- cal vertebra lacks ribs and transverse processes, or these are present in a rudimentary condition. The line between cervical and trunk vertebrae is also loosely drawn by the girdle of the fore limb. In the sauropsida (except in the limbless forms) the same regions can be traced as in the amphibia ; but it is to be 142 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. FIG. 150. Skeleton of Necturus. noticed that both sacral and cervical regions are increased in extent, there being two or three sacral and a much larger number of cervical vertebrae. In the mammals these regions are still fur- ther increased by a division of the trunk into a thoracic (' dorsal ') region, the vertebrae of which bear ribs, and a lum- bar region in which ribs are wanting. In certain regions there is a strong tendency towards the fusion of vertebrae. Most frequently those of the sacrum unite into a single piece, while fusions in the caudal region are numerous, and are correlated with the partial or entire disappearance of the tail. In modern birds there results from this a short bony complex, the pygostyle, while in the anura the caudal vertebrae of the tadpole are coalesced into the rod-like urostyle. In other regions this union is less fre- quent ; but the fusion of the anterior vertebrae to form the anterior vertebral plate of the skates and the anchylosis of the cervical vertebrae in the whales, and the occasional fusion of some dorsals in birds, will be recalled. The anterior two vertebrae in the amniotes call for special notice. The first of these, which joins the skull, is known as the atlas, the second as the axis or epistropheus. The atlas bears on its anterior face articular surfaces for articulation with the skull ; its neu- ral arch is well developed, but the cen- trum is absent, there being below but a thin bony arch, regarded by some as the first intercentrum (/>., hypocentrum SKELETON. 143 arcale). It arises in development from the ventral part of the vertebral bow (p. 1 37). The axis is in most respects a normal vertebra, but it bears, projecting "from the anterior face of its centrum, a more or less cylindrical outgrowth, the odontoid pro- cess ; this is morphologically the centrum of the atlas, which has lost its connection with its proper neural arch, and has become secondarily united with the centrum of the second vertebra, forming a pivot about which the atlas turns. In crocodiles, Hatteria, and possibly some mammals, a pair of plates (reptiles) or a single plate occurs on the dorsal anterior portion of the neural arch of the atlas. This is the so-called pro- atlas ; but whether this is the last remnant of a vertebra which has otherwise disappeared from be- tween the existing atlas and the base of the cranium cannot yet be definitely decided. Nor is it pos- sible as yet to say whether the only cervical vertebra of the am- FlG . I5I . Three anterior verte- phibia is homologous with either brae of alligator. , atlas; e, axis; atlas Or axis of the amniotes. ^ dont id process; />, pro-atlas; r Ribs. The name rib has been applied to two different structures, 1 one appearing in the gan- oids, teleosts, and dipnoi, the other in amphibia and amniotes, and apparently in selachii as far as these latter have ribs. The ribs of the fish are the haemal processes of the trunk vertebrae, which, in the region of the body cavity, extend from the vertebral centres towards the ventral surface between the muscles and the coelomic walls. The transitions from these ribs into the haemal arches can be traced in any fish skeleton. In the caudal region of the urodeles haemal arches comparable to those of fishes are present, and besides these the caudal vertebrae also bear transverse processes which extend directly outwards between the epi- and hypaxial muscles. Following; 1 The view of the ribs adopted here is that which appears to have the better basis. Baur and others hold that ribs are homologous throughout the vertebrates, but their reasons, are not conclusive. 144 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. the vertebrae forward, it is seen that the transverse process of the sacral vertebra, considerably enlarged, supports the pelvic arch, while in the presacral vertebrae these same transverse processes bear short articulated elements, the ribs. It follows from this (i) that the amphibian ribs are not equivalent to the haemal processes in these animals, and (2) that they are struc- tures different from the ribs of fishes. This view is farther substantiated by the conditions which ob- tain in the ganoid Polypterus, where both types of ribs, those of fishes and those of the higher vertebrates, occur in the same segment, the latter lying in the connective tissue between the epi- and hypaxial muscular systems; The ribs of the amniotes are clearly homologous with those of the amphibia. They are intersegmental in position, and arise by a condrification and more or less complete ossification of part of the myo- commatous tissue, a mode of development which readily explains their frequent ex- tension to the ventral surface. In the fishes the ribs (sometimes lacking, as in some plectognaths and lo- phobranchs) are usually slender, and are frequently firmly united to the vertebral centra ; or, again, they may be movably articulated to short 'basal stumps.' In many physostomous fishes some of the anterior ribs are modi- fied to give rise to a chain of bones connecting the air-bladder with the ear. Besides these ribs, there frequently occur in fishes slender bones in the fleshy portions, the homologies of which remain to be ascertained. Possibly some of them may represent the ribs of the higher forms. These epimerals, epi- centrals, and epipleurals, as they are called, are stated to be without a cartilage stage. The ribs of the elasmobranchs are small and cartilaginous, and are more or less intimately united with the vertebral centra. FIG. 152. Section through tail of Amblys- toma, showing the two types of rib. ^, epaxial muscles; h, haemal arches ; hy, hypaxial mus- cles; n, notochord; r, true ribs. SKELETON. 145 In their relationships to the muscles they resemble the ribs of the amphibia, and are in no way differentiations of haemal arches. From the amphibia upwards the ribs are typically articulated with the vertebrae by two heads, a dorsal or tubercular head articulating with the diapophysis, a ventral or capitular head resting upon the parapophysis. There is thus formed a skeletal arch (vertebrarterial canal) between rib and vertebra, through which passes a vertebral artery (Fig. 149 VC). In the am- FIG. 153. Anterior end of the vertebral column of Polypterus, showing both kinds of ribs from below, from Wiedersheim. Ps, parasphenoid ; R, true ribs (1-V); WK, vertebral centra; + fish ribs. phibia the two heads are said to arise separately and to unite later. From these typical conditions various modifications may occur. Thus either head may disappear, while the parapophysis (as in many mammals) may be reduced to an articular surface. Again, as in the anura, the ribs may fuse to the diapophysis, or, as in the neck of mammals, to both di- and parapophysis. In crocodiles both tubercular and capitular heads articulate with the transverse process in most of the thoracic ribs. In the amphibia the ribs are usually short, and are confined 146 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. to the region near the backbone. 1 In some forms (Megaloba- trachus and some stegocephalans) the ribs are not confined to the trunk region, but from three to eight pairs may occur in the tail. It is to be noted that the pelvis does not articulate directly with the transverse process of the sacral vertebra, but that connection is effected by the intervention of a sacral rib, distinct in many forms. In the caecilians ribs occur on every vertebra except the first and last. In the amniotes the ribs in the trunk region acquire a much greater development, and, like the hoops of a barrel, extend FIG. 154. Pelvis and sacrum of alligator. /, ilium; R, sacral ribs ; 5', 5", sacral vertebrae. around the body cavity. They may be ossified throughout their extent, in which case each rib is usually divided into several segments (crocodile), 2 but usually a considerable portion re- mains cartilaginous. Ventrally they may terminate freely, or they may connect with a sternum to be described later. In the great majority of the birds, as well as in some reptiles (croco- dilia, rhynchocephalia), each rib bears a backwardly directed uncinate process, which overlaps the rib behind, thus giving additional strength to the thoracic framework. 1 Ribs occur in the ventral region of some stegocephali (see p. 147), and cartilaginous ventral ribs have been described in Necturus and Menopoma (urodeles). 2 The median segment in crocodiles is not truly ossified, but is cartilage partially calcified. SKELETON. 147 In the cervical region the ribs are much shorter. They may be freely articulated to the vertebrae (crocodiles, etc.), but usually they are coalesced to transverse processes and centra, the foramen for the passage of the vertebral artery remaining to show the mor- phological relations. Usually caudal ribs are poorly devel- oped, but in some reptiles they may appear on almost every caudal vertebra. In some stegocephals, as well as in many reptiles (Hat- teria, crocodiles, ichthyosaurs, pterosaurs, etc.), so-called ab- dominal ribs occur. These are chondrifications or ossifi- cations in the ventral wall of the abdomen, usually behind the true ribs, and external to the rectus muscles. From the fact that these are not homologous with the true ribs, the name gastralia has been given them. They may, as in crocodiles, equal the segments in number ; they are twice as many in Hatteria, while in some stegocephals there are several series of ossicles to the somite. Sternum. A sternum or breast bone is absent in all fishes, but occurs in the ma- jority of the higher formes ; _ Y ^' l ^' Pos ! e ' ior v f ; eb ' al "i of : . . Testudo grceca. CR, caudal ribs ; /, ilium ; but it is as yet an open ques- ^ trunk rib s; SR, sacral ribs. 148 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tion as to how far the sternum of the amphibia is homologous with the similarly named structure in the amniotes. In the amphibia the sternum arises as a pair of longitudinal cartilaginous rods in the connective tissue on the ventral sur- face of the body. These rods soon unite, and form an unpaired plate in the median line between the origin of the fore limbs. In the uro- deles the sternum remains as a small plate just behind the ventral portion of the shoulder girdle, but in the anura it extends farther for- ward. Its median portion is caught between the epicora- FiG. 156. Sternum and ventral portion of the shoulder girdle of Rana> after Wieders- coids, and is reduced to a heim. d, clavicle ; co, coracoid ; ec, epicora- very slender thread; but in coid ; g, glenoid fossa ; os, omosternum ; s, ventral part of scapula ; .?/, sternum ; .r, xiphisternum. front of the girdle it expands again in a plate, the so-called omosternum. In the uro- deles the sternum is cartilaginous ; but in the anura portions of the omosternum, as well as of the posterior portion (termed xiphisternum, a term adopted from human anatomy), become ossified. The sternum is lacking in the footless amphibia. In the amniotes the sternum arises from the ventral ends of the ribs. The distal ends of these become separated from the rest, and unite to form a pair of ventral rods, which then unite to form the unpaired structure, which in many forms shows evidences of its origin from a series of elements, sternebrae. The sternum is lacking in snakes and turtles. In the dinosaur Amphiccelias, it is said to have been paired in the adult, the two halves possibly having been united by cartilage. In the lizards it is usually a broad rhomboidal plate. In the birds but few (at most eight) ribs contribute to the sternum, which is a broad plate, and in the ordinary birds bears a strong keel or carina upon its ventral surface. In the flightless birds the keel is absent, and the presence or absence of keel was formerly em- SKELETON. 149 ployed as a means of dividing birds into Ratitae and Carinatae. It is interesting to find a keel existing in the bats and in the fossil pterodactyls. In the mammals the sternum is more elongate, and more ribs contribute to its formation than in the sauropsida. It may consist of as many separate sternebrae as there are ribs connected with it, or these may so unite that but three separate bones can be recognized, a manubrium in front, a body in the middle, and an ensiform process (xiphisternum) behind, the latter extending behind the ribs. FIG. 157. Ster- num of dog, show- ing sternebrae. Fig. 158. Shoulder girdle of Ornithorhynchus . C, clavicle; CO, coracoid; , episternum; EC, epi- coracoid; S, scapula; ST, sternum; J?, ribs. Connected with the sternum in many groups is a structure to which the name episternum has been given. This first appears in the stegocephali, but reaches its highest development in the reptiles. It forms usually an unpaired plate connected with the median ends of the clavicles, and in those reptiles where it occurs it is placed ventrally to the sternum proper. It is expanded in front, and frequently takes the shape of a T, the arms supporting the clavicles, while the shaft connects with or may even be fused with the sternum proper. No episternum 150 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. has been described in the birds ; but in mammals one frequently exists, but here it is placed anterior to instead of ventral to the sternum proper. Where best developed it is T-shaped ; and it may be movably articulated to the sternum as in the mono- tremes (Fig. I 58), or firmly united to it (marsupials). In cer- tain rodents it becomes divided into three parts, while in the primates it is reduced to the intermediate cartilages by which the clavicles articulate with the sternum. The omosternum of the anura was formerly regarded as an episternum, but it is apparently truly sternal in nature. The Skull. The skeleton of the head, the skull, is a very complicated structure ; and in it two regions may be recognized, a cranium for the protection of the brain and sense organs (eyes, nose, ears), and a visceral skeleton which forms the jaws, and gives support to the visceral walls. In the beginning all of these parts are outlined in cartilage ; and in marsipobranchs and elasmobranchs they never pass beyond the cartilage stage, although, as in some sharks, the outer portions of the cartilage may be calcified 1 in the adult. /'In the higher groups this carti- lage may be partially or almost completely converted into bone ; and in all vertebrates above the elasmobranchs there are added to those portions of the skull which are of cartilage origin numerous other skeletal elements which are not preformed in cartilage, but which arise as ossifications of membranes. It therefore becomes necessary to distinguish in the higher verte- brates between cartilage-bones and membrane-bones, but these distinctions can be made only by tracing the development ; there is nothing in the fully developed bone which will decide the question. In the development of the cartilaginous cranium (chondro- cranium) there occurs first the formation of a membranous cap- sule, the primordial cranium, which encloses the brain and sense organs. In no vertebrate have the details of this membranous cranium been worked out. Later there is a chondrification of this primordial cranium which proceeds from several distinct cen- tres, which may be spoken of as the parachordals, otic capsules, trabeculae, and nasal capsules. 1 The distinction between calcined cartilage and bone is important. . SKELETON. 151 As will be recalled, the notochord extends forward as far as the infundibulum, and its' anterior end is concerned in the formation of the chondrocranium. On either side of this struc- ture there develops a horizontal cartilaginous plate, the para- chordal cartilage, which grows out laterally until it unites with a cartilaginous box, the otic capsule, which forms around the sac-like inner ear (p. 71). From this union of parachordals and otic capsules, there is formed a trough which encloses the me- dulla oblongata below and on either side. Later, in the typical conditions, the cartilage gradually extends upwards and inwards FIG. 159. Early chondrocranium of Amblystoma, before the formation of the otic capsules. ap, ascending process of quadrate ; 'bq t body of quadrate ; dp, descending process of quadrate; m, Meckel's cartilage; n, notochord; oc, of, foramina for oculomotor and optic nerves; /, parachordals; /, trabecula; trc, tra- becular crest. From Winslow. from the dorsal surface of the otic capsules forming a plate the synotic tectum which roofs in this region of the brain above. To this region there is added (amphibia) a vertebra or vertebral complex, developed like those of the vertebral col- umn, which becomes finally united to the parachordals and otic capsules, and closes in the cranium behind. Comparative mor- phology would also lead us to regard the parachordals as formed of coalesced vertebral centra ; but in their history, so far as made out, they of themselves afford not the slightest clew as to the number of elements fused together in this region. The trabeculae cranii are a pair of cartilaginous rods which 152 MORPHOLOGY OF THE ORGANS OF VERTEBRAl^ES. extend forward from the anterior end of the parachordals (or of the notochord) on either side of the pituitary body. In front, at about the anterior end of the brain, these trabeculse turn inwards towards each other and fuse into a median mass which, from its future history, is known as the ethmoid plate. Farther forward the trabeculae separate, and turn outward in front of the developing olfactory organs, the diverging horns thus formed being known as the cornua trabeculae. The far- ther development of the trabecular region differs considerably in different vertebrates. In general the trabeculae rapidly increase in height by the development of a crest upon the dorsal surface, FlG. 160. Chondrocranium of embryo trout {Salmo fontinalis}. k, hyoid ; hy, foramen for hyomandibular nerve ; />, foramen for glossopharyngeal nerve ; jv y foramen for branch of jugular vein; in. Meckel's cartilage; ns, nasal septum; rs, foramen for ophthalmicus superficialis ; sb, supraorbital bar ; J/, synotic tectum ; /, trabecula; tc, legmen cranii. From Winsiow. and in the elagmobranchs and some ganoids (sturgeon, etc.) this process is continued until the brain is completely roofed in above. In the teleosts, amphibia, and amniotes no cartilaginous roof (tegmen cranii) is found in this region ; J and in lizards, birds, and certain teleosts the trabeculae retain their condition of simple rods closely applied to each other. In most other ver- tebrates the trabeculae gradually grow together beneath the twixt and fore brains, thus forming~a complete floor. In the 1 The history in the Dipnoi is not known. In nearly adult animals (Protopterus) there exists a longitudinal rod of cartilage in the roof of the skull which may be the remains of an earlier complete cartilage roof. The same may also be true of an isolated cartilage plate in the skull of Polypterus, SKELETON. 153 urodeles, crocodiles, lizards, and many teleosts no such carti- laginous cranial floor is formed, the ventral wall of the skull being formed by membrane bones to be described later. Cartilage walls are also found in the optic and olfactory or- gans. Since motion is necessary in the eye, the optic capsule FlG. 161. Dorsal and lateral views of the chondrocranium of Amphiurna. anp, antorbital process; ap, ascending process of quadrate; c, cornu trabeculae; e, ethmoid plate ; ef, foramen for ductus endolymphaticus ; /, jugular foramen ; /, lamina cribrosa; w, Meckel's cartilage; n, notochord ; oc, foramen for oculomotor nerve; ocp, occipital process (vertebra); of, foramen for optic nerve; /, para- chordal ; pal, foramen for palatine nerve ; //, foramen for ductus perilymphaticus ; , stapedial process of quadrate; t, trabecula; trc, trabecular crest ; V, VII, VIII, foramina for V, VII, and VIII nerves. (sclerotic, p. 83) never participates in the formation of the cra- nium. The nasal capsules, on the other hand, unite with the anterior ends of the trabeculae and with the cornua. They are frequently extensively fenestrated. In the vertebrate series a general law may be observed. The more completely the adult skull is ossified, the less developed is the chondrocranium. 154 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. The visceral skeleton consists of a series of paired bars, always preformed in cartilage, in the walls of the pharynx and the oral cavity. Formerly these arches, which partially or com- ; BH HM iv b HI 4 FIG. 162. Diagram of skull and visceral arches of an Elasmobranch. a, audi- tory capsule ; b, basibranchial ; Wlth . each ther ' and named ' P r - ceeding from above downwards, pharyngobranchial, epibranchial, keratobranchial, and hypobran- chial. Between the two hypobranchials of each arch is devel- oped an unpaired piece, the copula or basibranchial, and these hvoid ; K* keratobranchial; P. SKELETON. 55 copulae become more or less intimately connected with each other, thus forming a support for the whole visceral skeleton. The two anterior arches have somewhat different fates. The second (counting from in front) is called the hyoid arch, and it lies between the first true branchial cleft and the spiracu- lar cleft (Eustachian tube, p. 73). In the fishes this arch is divided into two primary pieces, a dorsal hyomandibular and a ventral hyoid proper. The former loses more or less completely FIG. 164. Skull of cod, the outer membrane bones removed, after Hertwig. A, angulare; AR, articulare ; BR, branchiostegals ; DE, dentary ; , ectethmoid ; EKT, ectopterygoid ; ENT, entopterygoid ; EPO, epiotic; FR, frontal; Si 1 - 3 , hyoid; ffM, hyomandibular; IH, interhyal ; MA t maxillary; ME, mesethmoid; MT, metapterygoid; JVA, nasal; OCB, basioccipital ; OCL, exoccipital ; OCS, supraoccipital ; P, parietal ; PA , palatine ; PR O, prootic ; PS, parasphenoid ; PTO, prootic; Q, quadrate; SPO, sphenotic; SY, symplectic. its connection with the hyoid, and intervenes between the jaws and the cranium, where it forms the whole (elasmobranchs) or a part (ganoids and teleosts) of a suspensor apparatus which supports the jaws. In all forms higher than the teleosts this hyomandibular element has apparently disappeared. 1 The hyoid proper may divide into three parts, the epihyal, keratohyal, and hypohyal, while a copula (basihyal), larger than the basi- branchials, is usually developed, and not infrequently grows for- ward to form an internal skeleton for the tongue. 1 The stapes of the ear may possibly be derived from the hyomandibular. 156 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. Each branchial arch may develop cartilaginous outgrowths the branchial rays which serve as supports for the gills. These may also occur upon the hyomandibular arch in those forms where a spiracular gill is developed ; but in the teleosts the rays of. the hyoid portion of the arch are modified into slender bony rods the branchiostegal rays which support a membrane closing in the gills beneath ; while the rays of the hyomandibular are represented by the opercular bones to be described below. The most anterior of the visceral arches, the mandibular arch, has lost all connection with the respiratory region, and has divided into two portions, which are bent on each other so that they meet at a sharp angle behind. The upper of these is the pterygoquadrate, 1 the lower is MeckePs cartilage. These two cartilages of the two sides form the jaws in the elasmobranchs. Both pterygoquadrate and Meckel's cartilage frequently have accessory labial cartilages developed in connection with them. These have been interpreted as degenerate arches in front of the mandibular arch. The foregoing outline of the cartilaginous skull applies to the gnathostome forms ; but before going more into detail, a FlG. 165. Cranium and branchial basket of Petrotnyzon, after W. K. Parker. , branchial basket ; E, otic capsule ; G, gill slits ; A 7 ", nasal capsule ; N7\ noto- chord. word must be said concerning the cyclostomes. The chondro- cranium is formed of parachordals, otic capsules, and trabeculae, the cranial cavity being partially roofed in by a narrow tegmen, the so-called occipital arch. In front the cranium is closed by a cartilaginous nasal capsule. The branchial skeleton consists 1 The term palatoquadrate sometimes applied to this is a misnomer, since the palatine bone is a membrane bone. SKELETON. 157 of a complicated cartilaginous framework, the vertical bars being united by horizontal rods. There occur in connection with the cranium several cartilaginous bars, while in front a series of plates extend to the end of the head. There is no structure comparable beyond a doubt to a hyoid ; while instead of mova- bly articulated jaws, the mouth is supported by a cartilaginous ring, and a well-developed cartilaginous framework exists in the tongue, while the filaments around the mouth {Myxine) have cartilaginous supports. In the elasmobranchs the skull is never converted into bone, although calcareous deposits may be formed in its wall. The cranium is a closed cap- sule, sometimes carti- laginous throughout, sometimes with places in its roof (fontanelles), which are not chondri- fied, but are closed with membrane. Through the walls are openings for the passage of nerves and blood-ves- sels, but there is no trace of division into separate elements. The pterygoquadrate in the normal sharks is united to the chondrocranium by liga- ments and muscles, and by the hyomandibular suspensor. In tne holocephali, on the other hand, pterygoquadrate and cra- nium are firmly anchylosed in the adult (Fig. 166), although free in the young. Above the elasmobranchs bones appear in the skull, both as ossifications of cartilage and as membrane bones. The more constant and more important of these are as follows : The chondrocranium gives rise to four bones around the large opening (foramen magnum) through which the brain is connected with the spinal cord. These are, below, the basi- occipital ; on either side an exoccipital ; and above, part of a FIG. 166. Skull of Chinuzra monstrosa (drawn from a dry specimen). 158 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. supraoccipital. In the floor of the cranium in front of the basi- occipital is a basisphenoid, and in front of this a presphenoid. Still farther in front, in the region of the ethmoid plate and the nasal capsules, a mesethmoid. In the trabeculae are developed two bones on either side, an alisphenoid in front of the otic cap- sules, and an orbitosphenoid in the neighborhood of the eye. The otic capsules each ossify into three bones, a prootic in front, an epiotic above, and an opisthotic behind. These bones form the floor and a part of the lateral walls of the skull. Not FIG. 167. Base of skull of alligator (Alligator Indus}, bo, basioccipital ; bs 9 basisphenoid; eo, exoccipital ; et> opening of Eustachian tube; fm, foramen mag- num; pao, paroccipital ; pt, pterygoid; q, quadrate; qj, quadratojugal ; so, supra- occipital; sy, squamosal; /r, transversum. all of them are always developed, and again two or more may fuse together or with membrane bones. The pterygoquadrates of either side develop into a pair of pterygoid and a pair of quadrate bones, while Meckel's carti- lage never ossifies, or at most gives rise to an articulare on either side, where the lower jaw articulates with the quadrate. The other visceral arches may ossify to a greater or less extent, but the names of the resulting bones are the same as those given the cartilages. In all terrestrial vertebrates certain cartilages or bones are developed in connection with the ear, and the most diverse views have been advanced regarding the homologies of these ossicula auditus. The following account is based upon personal SKELETON. 159 studies of the development of these ossicles in amphibia, sau- ropsida, and mammalia. In the urodeles, where these elements first appear, the lateral wall of the otic capsule is interrupted by an opening, the fen- estra ovalis, in which a plate, the stapes, is supported by mem- brane. In several urodeles and in all caecilians this stapes is connected with the quadrate by means of a stapedial process (see Fig. 161, s, sp). This may be called the urodele type ; and it is to be noted that here no tympanum (p. 73) occurs, the first postoral visceral cleft undergoing reduction in develop- ment. In the anura and sauropsida the tympanum is well developed ; and this is crossed by a rod, the columella, often differentiated into three parts, which reaches from tympanic membrane to stapes, which is situated as in the urodeles. This columella serves to conduct sound waves across the tympanic cavity to the internal ear. In development it arises FIG. 1 68. Diagram of auditory ossicles and'. related parts in the sauropsida, based on em- mn, mandibular branch of facialis ; q, quadrate ; s, stapes; /, tympanum. behind the tympanum, and When fully developed bryos of Scekporus undulatus. c, columella; it is bound to the pOS- ct, chorda tympani;/, facialis; h, hyoid: hn, terior wall by membrane. h y id branch of facialis ; ' Meckel's Cartilage; It is therefore clearly postspiracular in charac- ter, and its connection with the ventral portion of the hyoid (Fig. 1 68) indicates that possibly it is to be homologized with the hyomandibular of the pisces. In these groups the quadrate acts as a suspensor of the lower jaw, and has only a ligamental connection no articulation with the stapes or columella. In the mammals two 1 ossicula intervene between the tympanic membrane and the stapes. The more internal of these is the incus, the outer the malleus. As will be remembered, the lower 1 Frequently a third element is mentioned, the os orbiculare or os lenticulare, which arises in the ligament between incus and stapes. l6o MORPHOLOGY OF THE ORGANS OF VERTEBRATES. jaw of the adult is without a quadrate suspensorium. In the embryo mammal, however (Fig. 169), Meckel's cartilage is seen to be connected to the otic capsule by means of a quadrate, from which a stapedial process extends backwards to articulate with the outer end of the stapes, in a manner which strikingly recalls the relations in the urodeles. The proximal end of Meckel's cartilage is expanded, and, besides articulating with the quadrate, sends a long process, the future manubrium, backr wards and inwards, be- tween the tympanum and the external auditory me- atus, i. e., into the tym- panic membrane. Later, with the formation of membrane bone (dentary) around the more distal portion of Meckel's car- tilage, the lower jaw ac- quires a new articulation with the skull, on the under surface of the zy- gomatic process, while at the same time the prox- imal end of Meckel's car- tilage becomes segmented off from the rest, and gives rise to the malleus. The quadrate, having no longer to serve as a suspensorium, loses its connection with the otic capsule, and becomes the incus. Incus and malleus extend into the tympanic cavity from in front, i.e., are prespiraeular, and cannot be homologous with the anuran and sauropsidan columella. Further, it will be noticed that the ossicula of the mammal are on the oppo- site side of the chorda tympani from what is found in the rep- tilia (Fig. 168). It is an interesting fact, the bearings of which will be alluded to later, that the quadrate, in both urodeles and mammals, retains its articulation with the stapes throughout life. In the reptiles nothing of the sort occurs. PIG. 169. Diagram of auditory ossicles and xelated parts in the mammalia, based on the embryo rat. a, external auditory meatus; ct, chorda tympani; f, facial nerve; /, hyoid; hm, hyomandibular nerve ; hn, hyoid branch of faci- alis; /, mallear portion of Meckel's cartilage, its process extending down between tympanum and meatus; mn, mandibular branch of facialis; q (V), quadrate, later incus; s, stapes; sm, sta- pedial muscle ; /, tympanum. SKELETON. l6l In the teleostomous fishes an operculum or fold covering the gill slits occurs ; and this is supported by opercular bones, which in their full development may number four on either side, oper- culum, preoperculum, inter operculum, and suboperculum. These are cartilaginous in origin, and are usually regarded as extremely modified branchiostegals of the hyomandibular. The membrane bones which complete the lateral walls and roof in the cranium are : the dorsal part of the supraoccipital (when distinct called paroccipital), and, proceeding forwards, a pair each of parietals, frontals, and nasals, meeting in the ^ -=- , = ;. 170. Hyoid and opercular apparatus of cod. , branchiostegals; E, os sum; ff, hyoid ; //J/, hyomandibular; /, interoperculum; O, operculum; 'oerculum : 5. subooerculum. FIG entoglossum P, preoperculum ; S, suboperculum middle line above, the skull being terminated by a pair of pre- maxillaries, which also appear on the ventral surface. Lodged in the angle between nasal and frontal is a prefrontal on either side, while a pair of postfrontals are placed in a similar position between the frontals and parietals. Pre- and postfrontals may make up the superior or inner margin of the orbit, or a supra- orbital may intervene between them. Below the postfrontal and behind the orbit there may be a postorbital which may extend beneath the orbit, or the posterior margin of the orbit may be formed by a squamosal (temporal), which extends up- wards in front of the otic region to reach the supraoccipital and parietal. A lachrymal bone is more constant than some that 1 62 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. have been named ; it usually enters into the composition of the anterior wall of the orbit, but it may be forced forward by the prefrontal. FlG. 171. Ventral and side views of the skull of Hatteria (Sphenodon*), after Giinther. bo, basioccipital ; bs, basisphenoid ; eo, exoccipital ; fr, frontal ; j, jugal ; /, lachrymal ; mx, maxillare ; n, nasal ; oo, opisthotic ; pa, palatine ; pf, prefrontal ; pni, premaxillary ; po, postorbital ; pof, postfrontal ; pt, pterygoid ; q, quadrate; qj, quadratojugal ; sq, squamosal ; z>, vomer. In the elasmobranchs the upper jaw is formed by the ptery- goquadrate cartilage, but in all higher forms other elements usurp these functions. In front there are a pair of premaxilla- ries already mentioned, and behind these a pair of maxillaries SKELETON. 163 usually occur. These may extend back to the angle of the jaw, or a jugal (malar) and a quadratojugal may intervene, the lat- ter connecting with the quadrate, and in some cases arising in part from an ossification of a process of the quadrate cartilage. In the roof of the mouth in front are usually a pair of vomers, and behind these, and extending back usually to meet the ptery- goids, are a pair of palatines ; while in some groups an os trans- J'a- TZT 0/10 FiG. 172. Skull of Cyclodus from the side and split through the middle, from Huxley. Ar, articulare; BO, basioccipital ; BS, basisphenoid ; Co, columella; D, dentary; O, exoccipital ; EpO, epiotic ; Fr, frontal; Ju, jugal ; MX, maxillary ; Na, nasal; OpO, opisthotic; Pa, parietal; Pf, postfrontal ; PI, palatine; Pmx, pre- maxillary ; Prf, prefrontal ; PrO, prootic ; Pf, pterygoid ; Qu, quadrate ; SO, supra- occipital ; Sq, squamosal ; Vo, vomer ; V, VII, exits of fifth and seventh nerves. versum occurs, connecting the hinder portion of the mandible with the pterygoid. In the ichthyopsida the floor of the chon- drocranium does not ossify ; and here the remainder of the roof of the mouth is formed by an unpaired membrane bone, the parasphenoid. In the ganoids and all higher forms membrane bones form around Meckel's cartilage, and these form the functional lower jaw. In their greatest development there may be several of 164 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. these bones on either side, a dentary in front, a splenial far- ther back on the inner side, and an angulare extending forward from the angle of the jaw to meet the other two. In addition, a supraangulare is sometimes present behind the articulation of the lower jaw with the quadrate. Several of these membrane bones may bear teeth. When teeth are present they almost universally occur on the premaxil- laries, maxillaries, and dentary ; but they may also occur on the vomers, palatines, parasphenoid, and splenials, and occasionally on the pterygoids. This leads to the question of the phylogenetic origin of these membrane bones of the skull. All the evidence goes to show that not only these teeth- bearing bones, but most of the covering bones of the skull, have arisen from the fusion of dermal plates, much like the placoid scales of the elasmobranchs. In the jaws the enamel-capped spines have given rise to FIG. 173. Development of dermal (maxil- lary) bone in Amblystoma by fusion of the the teeth, while the basal bases of teeth. b t bone; c, cartilage; d, plates, fusing together, form dentine of tooth ; e, epidermis ; /, tooth. the bones themselves. In many forms this origin of the bones by the fusion of the bases of the teeth can readily be seen (Fig. 173). In the covering bones of the cranium the dental portion has disappeared. The remaining membrane bones have arisen around the canals of the lateral line system, the suborbital chain of bones being the most constant of these. Through the walls of the skull formed by these cartilage and membrane bones are foramina for the passage of nerves ; and these openings afford important landmarks for the identi- fication of certain bones, especially in those numerous cases where different elements fuse together. The optic nerve passes through the orbitosphenoid. Between the orbitosphenoid and alisphenoid is an opening (sphenoidal fissure or foramen lacerus anterior) through which pass the third, fourth, and sixth, and SKELETON. I6 5 the ophthalmic branch of the fifth nerves. The maxillaris su- perior and mandibularis branches of the fifth nerve leave the FIG. 174. Developing bones in the head of Arnphiuma. Cartilage dotted, bone lined. ao, antorbital process ; an, angulare ; d, dentary ; f, frontal ; fa, fora- men ovale ; mx, maxillary ; o, occipital vertebra ; oc, otic capsule ; pa, parietal ; pm, premaxilla; q, quadrate; sq, squamosal ; st, stapes; /, trabecula; //, etc., exits of nerves. skull through the alisphenoid bone, either through a common opening or through two separate foramina (f. ovale for the mandibular, f. rotundum for the other). The seventh nerve FIG. 175. Diagram of the relations of the bones in the mammalian skull, after Flower. AS, alisphenoid; BH, basihyal ; BO, basioccipital ; BS, basi- sphenoid; CH, ceratohyal ; EH, epihyal ; EO, exoccipital ; F, frontal; ^, jugal ; L, lachrymal ; MD, mandible; MC, Meckel's cartilage; ME, mesethmoid; MX, maxilla; N, nasal; OS, orbitosphenoid ; PA, parietal; PL, palatine; PM, pre- maxilla ; PS, presphenoid ; FT, pterygoid ; S, squamosal ; SH, stylohyoid ; SO, supraoccipital; T, turbinal ; TH, tympanohyal ; THH, thyrohyal; V, vomer ; I- 1 2, exits of the cranial nerves. 1 66 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. passes through the otic bones (petrosal), the ninth and tenth through the jugular foramen formed by the junction of basi- and exoccipital and otic bones. Occasional variations from these conditions occur ; for instance, the optic nerve may pass through a notch in the orbitosphenoid, or, again, the ophthal- mic branch of the fifth may be enclosed in the alisphenoid. While the question of the segments of the head will be taken up in a later section of this volume, it may be well to point out here that the bones of the skull form a series of rings surrounding the brain ; but it is to be noticed that these rings are formed in part of mem- brane bones, in part of cartilage bones. The posterior of these rings is formed of basi-, ex-, and supraoccipitals ; next in front comes a ring formed of the basisphenoid, alisphenoids, and parie- tals-; third, one of presphenoid, orbito- sphenoids, and frontals ; and lastly, one of ethmoid and nasals. In the fishes, stegocephalans, and gymnophiona the membrane bones form a continuous layer on the outside of the skull ; but in the higher verte- brates gaps may occur here and there behind the orbit, the fossae thus formed being bounded by arches of bone. There may be two of these fossae, a more dorsal supratemporal, and a more ventral and lateral infratemporal. These fossae are separated from the orbit by a bridge of bone, usually consisting of postorbital and a process of the jugal. The infratemporal is bounded externally by a zygomatic arch into which the quadrat ojugal, jugal, and maxillary may enter ; while between the two fossae is an arch FIG. 176. Skull of the Dinosaur Hadrosaurus, show- ing supra- (S T~) and infra- temporal fossae (/7 1 ), after Cope. FR, frontal ; y, jugal, N, nasal ; O, orbit ; P, post- orbital ; -PA, parietal; PM, premaxilla; POF, post- frontal ; PJRF, prefrontal ; Q, quadrate ; 5, squamosal ; SO, supraoccipital. SKELETON. I6 7 usually composed of squamosal and postorbital. By alteration in the position or extent of the bones these two fossae may unite into a single temporal fossa, and again, the boundaries between this and the orbit may become broken through, the postorbital arch being imperfect or totally disappearing. (For details see Reptilia.) Appendicular Skeleton. The appendages of the vertebrates (fins or limbs) arise as paired outgrowths from the sides of the body, one pair, the anterior or pectoral, arising a short distance behind the pharyngeal region, the other, or pelvic (ventral) pair, FIG. 177. Developing fin of trout, after Corning, f, fin; ;;/, myotomes; n, notochord ; the myotomes are seen to be proliferating strands of cells into the-fin. a little in front of the vent. In the higher vertebrates each limb grows out as a simple bud, but in some elasmobranchs the appendages arise as differentiations of a continuous lateral fold on either side of the body. Into these outgrowths migrate cells derived from the muscle plates (Fig. 177), which are to 1 68 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. give rise to the muscles of the appendage, and also mesenchy- matous tissue, which becomes transformed in part into the skeleton. This skeleton is, with the exception of the clavicles, preformed in cartilage, the cartilage formation beginning at about the middle of the limb and proceed- ing thence in both directions. The skeletons of both pectoral and pelvic appendages are closely similar in structure. Each consists of a skeletal arch or girdle within the trunk, each girdle supporting the skeleton of the appendage. These girdles are known respectively as the pectoral (shoul- der) and pelvic girdles. The pectoral girdle occurs in its simplest form in the lower fishes, where it is a U-shaped arch of cartilage, the bottom of the U crossing the ven- tral surface of the body beneath the skin, the arms projecting up- wards on either side, and the ends being connected by muscles with the vertebral column. 1 The skele- ton of the fin is articulated to either half of the girdle, the point of articulation being usually exca- vate, and known as the glenoid fossa. This fossa serves to divide FIG. 178. Shoulder girdle and each half f the g irdle int a dor - proximai part of pectoral fin of sal or scapular and a ventral or skate (Kaia). g, right half of coracoid portion. With the appearance of bone (ganoids, teleosts) each half of the girdle develops two cartilage bones, a scapula, and a second, usually regarded as a coracoid; while the two halves of the girdle proper become separated from each other. In the dipnoi, ganoids, and teleosts, these are re- 1 In the skates the pectoral girdle becomes attached to the backbone by means of a so-called suprascapula. In many other fishes it is connected with the skull by a chain of bones. Elsewhere, except in some fossil reptiles, it is free from the axial skeleton. girdle ; ms, mesopterygium ; ;///, metapterygium;/, protopterygium; r, radii of fin. SKELETON. 169 enforced by membrane bones. The chief and largest of these is the cleithrum (usually called the clavicle), developed on the outer anterior surface of the girdle, the cleithra of the two sides frequently uniting below. To this is added above a supraclavicle, which may connect directly, or by the intervention of a posttemporal bone, with the base of the skull. Other membrane bones postclavicle, infraclavicle, etc. sometimes occur. In the amphibia and higher groups other portions may be differentiated in the pectoral girdle, and as yet these cannot all be homolo- gized with the conditions found in fishes. In fact, it is probable that no detailed homology exists. The scapu- lar portion of the arch may ossify throughout, or the os- sification may be restricted to that portion the scapula nearest the glenoid fossa, while the dorsal portion may be a distinct element, partly or entirely cartilaginous, the suprascapula. The ventral portion of the girdle gives rise typically to two elements, a posterior coracoid FIG. 179. Shoulder girdle of carp (Cyprinus carpio}, after Gegenbaur. CL> cleithrum ; E, scapulare ; PC, cora- coid (procoracoid) ; F, foramen between coracoid and cleithrum ; A, attachment of fin. FIG. 180. Pectoral girdles of, A, Archegosaurus and, B, Paltfohatteria. c, coracoid ; d, clavicle ; e, episternum ; s, scapula, after Credncr. FIG. 181. Shoulder girdle, etc., of Bombinator igneus, after Wied- ersheim. c, clavicle; to, coracoid; ec, epicoracoid ; g; glenoid fossa ; pc, procoracoid; .?, scapula; w, suprascapula ; s/, sternum. MORPHOLOGY OF TJfE ORGANS OP VERTEBRATES. and an anterior procoracoid, both extending inwards ; and fre- quently the inner ends of these are united by a longitudinal cartilaginous band, the epicoracoid. To these may be added a clavicle, developed from membrane, in front of the proco- racoid, extending in- wards from the scap- ula, and usually con- necting with the sternum by means of the episternum. These parts undergo various modifications, and some or all of them, with the excep- tion of the scapula, may, here and there, more or less com- pl e t ely disappear. Possibly the most common is the re- placement of the FlG. 182. Shoulder girdle of Ornithorhynchus. C, clavicle; CO, coracoid; E, episternum; EC, procoracoid by the epicoracoid; S, scapula; ST t sternum; ^, ribs. clavicle. The details of these modifications will be given in connection with the groups in which they occur ; but in the majority the two halves of the pectoral girdle are more or less firmly united by means of the sternum. The pelvic girdle presents many similarities to the anterior .arch. In the elasmobranchs there is the same transverse arch as in the shoulder girdle ; and this supports the ventral fins, there being in some cases a dorsal portion extending beyond the fossa (acetabulum) in which the fin articulates. There thus arise a dorsal iliac portion and a ventral ischio-pubic portion in ach half of the arch, the ventral part being perforated by an opening (obturator foramen) for the obturator nerve. In the other fishes the pelvic girdle is much less developed, and in the teleosts the fins are supported by these enlarged basal elements (vide infra). In the amphibia and higher groups the iliac por- SKELETON. 171 tion is well developed ; and when bones are developed in the car- tilage, three elements can be recognized in each half, a dorsal ilium, and, below, an anterior pubis and a posterior ischium, the obturator foramen either forming a part of the opening between these two bones, or passing through the pubis itself. Ventrally these bones can unite with their fellows of the opposite side in a symphysis, while all three of a side meet in the acetabulum. 1 These parts can be well compared with those of the pectoral girdle ; pubis with procoracoid, ischium with coracoid, and ilium with scapula ; but one difference is to be noted, the ilium be- comes connected with the sacral vertebra or vertebrae by the inter- vention of short ribs (p. 1 46). To these parts in the amphibia there is frequently added in front of the pubis a cartilaginous epipubis. This reappears again in certain reptiles ; and in mammals it may be homologous with the so-called marsupial bones, which project forward from the anterior margin of the pubis, there being two views upon this point. These parts may undergo many modi- FIG. 183. Side view of pelvis of opossum, after Minot. Ac, acetabu- lum ; y, obturator foramen ; /Z, ilium ; /S, ischium ; M, marsupial bone. FIG. 184. Modifications of branchial arch and rays according to Gegenbaur's archipterygium theory. fications, but the pelvis is not re-enforced by membrane bones such as play such a part in the shoulder girdle. 1 In many mammals a distinct acetabular bone occurs at the junction of the three. 1/2 MORPHOLOGY OF THE OX CANS OF VERTEBRATES. FIG. 185. Skeleton of pectoral fin of Ceratodus, after Gunther. Before beginning the account of the skeleton of the limbs it may be well to summarize the two prominent theories of the origin of these parts. According to the view of Gegenbaur (which he has lately repeated), limbs have arisen from gill structures which have mi- grated backwards. The gill arches have given rise to the girdles, while the skeletal parts of the appendages have had their origin from the gill rays. With the outgrowth of the limb one of the gill rays near the middle of the arch has cor- respondingly elongated, and in its outgrowth has carried the neighboring gill rays along with it, the result being a skeletal axis to the limb, on either side of which was a series of smaller skeletal pieces (Fig. 184). A fin closely corresponding to the requisites of this view is found in the dipnoan Ceratodus. By sup- pression of almost all of these accessory skeletal parts on one side of the axis, and a modification or suppression of some on the other side of this archipterygium, Gegenbaur derives all types of vertebrate limbs. Fig. 186 shows the relations of the typical pentadactyle leg. The other view, first advanced by Thacher, assumes that the ancestral verte- brate was provided with two longitudinal folds on either side of the body. The more dorsal of these migrated upwards, those of the two sides uniting to form the dorsal part of the FIG. 186. Diagram of amphibian fore limb, after Gegenbaur. c, centrale ; //, humerus ; R, radius ; t, interme- dium ; r, radiale ; U, ulna; , ulnare ; 1-5, carpales. The heavy line is the axis of Gegen- baur's archipterygium, the dotted lines, of the radials of his scheme. SKELETON-. 173 median fin to be described later. From the more ventral folds arose the ventral portion of the median fin, behind the vent ; while the pectoral and ventral fins arose as differentiations from the preanal region of the folds. In fact, several existing elasmobranchs exhibit exactly this condition in their develop- ment. In those parts of the fold where the fins are to form, rod- like cartilage supports arose, possibly agreeing in number with the myotomes concerned in the formation of the appendage. At first these were separate and nearly equal in size, but later FlG. 187. Diagram of the origin of median and paired fins, from Wiedersheim. A, with continuous fin folds; B, with differentiated fins. AF, anal fin; An, anus; BF, ventral fin ; BrF, pectoral fin ; D, dorsal folds ; FF, adipose dorsal ; RF t dorsal fin ; S, lateral folds ; SF, caudal fin. the basal portions became larger and separated from a more distal (radial) part. Such a condition is seen in the extinct elas- mobranch Cladoselache (Fig. 188) ; but usually the basalia fuse into a few larger elements, connecting the radialia together, and giving stiffness to the whole fin. One of these enlarged basalia acquired prominence over its fellows, and growing in toward the median line fused with a similar ingrowth from the opposite side, thus giving rise to the ventral portion of the girdle. But such a bar would prove too rigid, and would prevent the fin from moving freely, so there appeared a joint on either side, the distal portion now articulating with the median or girdle region at a place known as the glenoid fossa or acetabulum. The skeleton 1/4 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. of the fin proper would now consist of the basals and radials plus a number of delicate rods of dermal origin, the so-called actinotrichia. From such an archetypal structure the paired fins of all fish-like forms can readily be deduced ; but the transi- tion between this and the limb of the higher vertebrates offers Vi FIG. 188. A, pectoral, and B, ventral fins of Cladoselache, after Dean, b, basalia ; r, radialia. many difficulties, and the interme- diate forms have not yet been found. The most plausible view of the homologies is that which derives the limb of the higher ver- tebrates from one like that of the ganoid or selachian by the loss of most of the basals and radials, a single basal giving rise to the humerus or femur, the proximal portions of a couple of radials forming the ulna and radius (tibia and fibula), while the distal portions of the same radials, with possibly parts of others, producing the distal parts of the limb. The accom- panying diagram illustrates the general outlines of the process, FIG. 189. Diagram illustra- ting possible evolution of penta- dactyle limb from the ventral fin of fishes ; the shaded portion represents the persistent parts. SKELETON. those parts which are shaded being those which persist in the higher vertebrates. In the fin skeleton of the elasmobranchs two basals, an anterior protopterygium and a posterior metapterygium, occur in the ventral fin, while in the pectoral fin a third basal, the mesop- terygium, is intercalated between the other two. These basals. support a richly developed radial system, the radialia being with few exceptions developed on one side of the axis formed by the basals. In the lower ganoids, on the other hand, the basals are more numerous, and show the primitive conditions more clearly than do the elasmobranchs. In both of these groups the rays of dermal origin are well developed, and reach their extreme in FIG. 190. Typical pentadactyle limbs ; above fore limb, below hind Iim&_ c, centrale ; cp, carpus; /, femur; fe, fibulare ; Ji, fibula; h, humerus; t, interme- dium; me, metacarpals ; ;///, metatarsals ; p' , p" > phalanges; r, radius; re, radiale; /, tarsus ; te, tibiale ; //, tibia ; , ulna; ue, ulnare ; 7-F, digits; 1-5, carpales or tarsales. teleosts in which the cartilage bones of the fin are greatly re- duced. In the pectoral fin they are represented by (usually) four bones (actinosts) which support the dermal rays, either directly or by the intervention of cartilaginous radials. In the dipnoi there is a well-developed and segmented axial skeleton to the fin which may be without other skeletal parts {Protopterus, Fig. 269), or which may bear biserial lateral branches which connect with the dermal rays (Ceratodus, Fig. 185). Throughout the higher vertebrates, from the amphibia to- man, the same type of limb structure is everywhere found, that 1/6 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. of the anterior and posterior limbs being essentially identical. In the fore limb there is in the region corresponding to the upper arm (brachium) of man a single bone, the humerus ; in the fore arm (antibrachium) two bones, the radius on the anterior, the ulna on the posterior side. In the wrist (carpus) there are typically nine small bones, arranged in three series. The first consists of a radiale on the radial side, an ulnare on the ulnar side, and between these an intermedium. The second series consists of a single centrale ; while the distal series is a row of five carpales, numbered from one to five, beginning on the radial (thumb) side. In the hand (manus) are recognized metacarpus (palm) and digits (fingers). There are five metacarpal bones in the palm, while the digits are composed of a number of bones arranged in series (phalanges). The fingers are numbered from one to five, beginning at the radial or thumb (pollex) side. In the hind limb the femur corresponds to humerus ; tibia and fibula to radius and ulna respectively. The ankle (tarsus) consists of tibiale, fibulare, intermedium, centrale, and five tar- sales, and these are succeeded by metatarsals and phalanges, which are numbered from one to five, beginning at the hallux (large toe). 1 These parts can be greatly modified, the chief changes con- sisting of fusion or disappearance of some of these elements. These alterations are usually more marked in the distal por- tions, while those bones nearer the body are less subject to modification. Occasionally bones may be added to these typi- cal ones ; thus, there may be two centrales, and again, there may be membrane (sesamoid) bones added to the wrist or ankle. In cases where the details of reduction can be clearly traced, it is found that the outer digits are the first to disap- pear, the order of disappearance usually being i, 5, 2, 4. In human anatomy different names have been given to the carpal and tarsal bones from those employed here ; and as in the older works this nomenclature has been transferred to other groups, the following table, which shows the usually accepted homologies, may prove of value. 1 The student is referred to special works for a discussion of those cases, like the frog and the mammal Pedetes, which seem to indicate the existence of more than five digits. CARPUS. radiale = scaphoid, intermedium = lunare. tibiale intermedium ulnare cuneiforme. fibulare centrale = centrale. centrale carpale l trapezium. carpale 2 = trapezoid. carpale 3 = magnum. carpale* ) ;= uncifo rm. carpale 5 ) tarsale l tarsale 2 tarsale 3 tarsale 4 tarsale 8 TARSUS. = astragulus. = calcaneum. = naviculare. = internal cuneiform. = middle cuneiform. = external cuneiform. cuboid. The pisiform of the carpus is a sesamoid bone (i.e., a_inem- brane bone developed in a tendon as a result of strain or pres- sure), while the centrale often fuses with the carpale 3 to form the os magnum. The median or unpaired fins which develop from the dorsal pair of lateral folds and the postanal part of the ventral folds occur only in the ichthyopsida. The result of such a union of folds (p. 172) would be to produce a fin in the median line which, beginning on the back, should continue around the tail and forward upon the ventral surface as far as the vent. Such a continuous fin occurs in the cyclo- stomes, larval amphibia, and many other forms ; but usually it is inter- rupted, and thus divided into dorsal, caudal, and anal (on the ventral sur- face) fins. In the amphibia these fins are without skeletal support ; but in the fishes a regular skeleton is formed, consisting of segmentally arranged basalia and radialia like those of the primitive paired fins, and besides, a system of dermal fibrous supports. Occasionally, however, there is to be found an intercalation of radialia, these sometimes being at least twice as numerous as the somites. FIG. 191. Dorsal vertebrae of Pleuracanthus, after Fritsch. <:, notochord; /&, haemal arch; , neural arch ; r, radialia of dorsal fin, showing intercalation of ele- ments. MORPHOLOGY OF THE ORGANS OF VERTEBRATES. ORGANS OF CIRCULATION. The circulatory structures of the vertebrates consist of fluids (blood and lymph) and the vessels in which they flow, certain parts of which are specialized for the propulsion of the contained fluid. The general characters of the blood and lymph have already been described ; details will be given below when necessary. On a priori grounds the lymph system is apparently the older, but it will be more convenient to begin our account with the blood-vascular system. In this we recognize in all vertebrates a central muscular organ, the heart, which propels the blood, vessels (arteries) which convey the blood to the pe- ripheral portions, and other canals (veins) which bring it back to the heart ; the extremities* of the arteries and veins being connected by minute tubes, the capillaries. There is considerable evidence to support the view of Biitschli, that the main trunks of the circulatory system are FIG. 192. Diagram of primitive condition of blood-vessels, a, transverse vessels; d and v, dorsal and ventral vessels. V^Gt ^ / I) (st F\ ~* remnants of the segmentation cavity (p. 5) of the embryo, which has otherwise become entirely obliterated by the ingrow- ing mesoderm. The extension of the coelomic pouches towards the middle line of the body above and below the alimentary tract narrows the segmentation cavity in these regions into two longitudinal tubes, the main circulatory trunks ; while from those portions of the cavity between the myotomes arise semicircular tubes uniting the dorsal and ventral tubes, the result being rep- resented diagrammatically in Fig. 192. A part of the ventral tube near the anterior end becomes specialized as the heart ; it forces the blood forward through the anterior end of the ventral tube which is known as the ventral aorta, then dorsally through ORGANS OF CIRCULATION. 179 the connecting semicircular vessels (aortic arches), and thence back through the dorsal tube (dorsal aorta) to again enter the posterior (venous) portion of the ventral tube, and thence back to the heart. In development much of this probable ancestral history has been masked. Many of the vessels which theoretically should appear as spaces between the myotomes are formed as solid cords of cells (often as a single row of cells), which later become canalized and converted into tubes. We may first describe this system of circulatory vessels as they become developed in the lower vertebrates, taking them up in the order heart, arteries, and veins, and then trace the modifications which occur in the higher groups. This method has the advantage, as it traces the ontogenetic steps by which the amniote circulation arises. In the development of the heart three parts are to be considered, the epi- thelium lining it, its mus- cular walls, and the cavity (pericardium) in which it is suspended. Just behind the place where the first (hyoman- dibular) gill slit is to ap- pear, the descending edges FIG. 193. Section through the throat region of an embryonic Amblystoma, illustrating the Of the lateral plates, COr- early formation of the heart, e, endothelium of responding in length tO heart; , ectoderm;/, fusion of ectoderm and several somites, meet just to f rm thr ugh ^ the jf 11 cleft wil1 / develop later from the gill pouch, g; w, myo- above the ventral epider- tome; ms, remains of ventral mesocardium, mis, while more dor Sally the dorsal mesocardium has not yet formed; they enclose a groove-like n > notochord ; A pericardial wall; p c , peri- cardial cavity ; s, spinal cord. space open to the yolk above. In this groove appear cells which ultimately develop into the epithelium (endothelium) of the heart ; but the origin of these cells is not certainly known. The evidence tends to 180 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. show that they are derived from the yolk (entoderm) ; but the investigation is difficult, and they may arise from the mesothe- lium, or, less probably, they may be mesenchymatous in origin. These cells arrange themselves into a tube which is to form the lining (epithelium) of the heart and ventral aorta, while behind the heart region they extend backwards as a pair of tubes, the omphalomesaraic veins to be described later, on either side of the yolk. . In the heart region the edges of the lateral plates now fuse in the median line above and below the endothelial tube, thus giv- ing rise to two longitu- dinal folds, a dorsal and a ventral mesocardium ; while that part of the lat- eral plates surrounding the endothelium later de- velops the muscular wall (myocardium) of the heart and the ventral aorta. The downward growth of the lateral plates brings the ccelom just outside the myocardium, and this part of the ccelom becomes cut off from the rest, and gives rise to a space, the FlG. 194. Early heart of Amblystoma, after a reconstruction by Dr. F. D. Lambert, a, auricle ; b *, b ^, branchial arches, 1-4 ; dm, dorsal mesocardium j A, hyoid arch; m, man- dibular head cavity; o, omphalomesaraic veins; /, pericardial chamber ; j, sinus venosus ; /, truncus arteriosus ; v, ventricle; I, first aortic arch. pericardial cavity, sur- rounding the heart. At first the dorsal and ventral mesocardia are entire, and while dividing the pericardial space into right and left halves, suspend the tube in this chamber in the same way that the in testine is supported by the mesenteries farther back. Soon the ventral mesocardium breaks down, while a little later the dorsal membrane becomes reduced to a small support for the posterior portion of the heart. At first the tube is straight, and equal in ORGANS OF CIRCULATION. 181 length to the pericardium ; but it rapidly increases in length, and as a result becomes twisted into an S-shaped tube, and with this twisting the heart becomes differentiated. In the S are developed two chambers, an anterior ventricle and a posterior atrium or auricle, the tube between these remaining smaller, the atrio- or auriculo-ventricular canal. In this twisting only the posterior portion of the tube takes part, and the atrium comes to have the more dorsal position (Fig. 194). The anterior straight portion of the tube gives rise to the truncus arteriosus and the ventral aorta. In the truncus region, which immediately adjoins the heart, two parts may be differen- tiated, a posterior conus arteriosus, containing on its inside membranous valves preventing any backward flow of the blood, and an anterior and muscular bulbus arteriosus. Behind the heart the two omphalomesaraic veins unite to form a cavity, the sinus venosus, into which, later, other veins entering the heart come to empty. Valves soon arise in the auriculo-ventricular canal, and a little later other valves are formed at the opening of the sinus into the atrium. These valves are fleshy folds which prevent any backward flow of the blood. In its earlier stages the heart lies in the region of the gill slits (Fig. 122); but as the animal increases in age there is a relative shifting of parts, so that the heart comes to lie behind the gills, and in many forms is removed some distance from them. The conditions so far described are permanent in fishes, and also occur in the younger stages of -all higher forrrts, with the FIG. 195. Early stage in the development of the heart in the tern {Sterna). , anterior end of the ali- mentary canal ; c, coelom, later cut off as pericardium ; e, epidermis; /, kidney; /*', left ORGANS OF CIRCULATION. 1 97 The blood distributed by the hypogastric arteries is returned to the heart by the derivatives of a pair of hypogastric veins which run on the ventral body wall forward to the omphalomes- araic vein. When the hind limbs appear, external and internal iliac veins grow out from the hypogastrics into those appendages, their ultimate distribution coinciding more or less closely with the similarly named arteries. When the posterior cardinals grow back into this region they tap these vessels, and so the blood from the hinder appendages is returned to the heart through them, at first directly, later through the renal portal system (Fig. 206), and in the higher vertebrates by way of the post- cava. The ventral portions of the hypogastric veins retain their connection with the iliacs throughout life in the ichthyopsida (Fig. 206), and either as two vessels or as a single anterior abdominal vein, run forward in the ventral body wall, and enter the portal system (Fig. 203). In the amniotes, with the formation of the allantois, the hy- pogastric veins grow out into this, and are here known as the umbilical veins. In the reptiles they retain their distinctness ; but in birds and mammals one aborts, leaving the other as a single trunk which empties into the omphalomesaraic. Dur- ing embryonic life this system is very large and important, but after hatching or birth it becomes reduced to an inconspicuous condition, Fig. 203. In the fishes the relations of jugulars and the ducts of Cuvier are much as outlined above, with the exception that the jugular veins develop two branches, internal and external. With the formation of lungs (dipnoi and amphibia) this system be- comes unsymmetrical, in that the left Cuverian duct is now compelled to reach the right side of the sinus venosus ; and here, as in the higher groups, the trunks, formed of united jugulars, subclavians, and posterior cardinals (i.e., the Cuverian ducts), are known as the precavae, right and left. Here, too, is to be noticed a shifting of the veins (subclavians) coming from the fore limbs. At first they empty into the posterior cardinals, but later they empty into the jugulars, the common trunks formed by the subclavians and jugulars being known as the innominate veins. In the birds a transverse anastomosis forms between the 198 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. jugulars of the two sides. In the mammals (Fig. 208) a trans- verse connection forms between the precavae of the two sides ; and then the direct connection of the left precava with the heart is lost, so that all the blood from the right side of the head and FIG. 208. Diagram (altered from Gegenbaur) showing the modifications of the venous trunks in mammals, tf , azygos vein ; c t coronary ; d, ductus Cuvierii ; ei t external iliac; ej, external jugular; //, hepatic; ha, hemiazygos ; ic, intercostals; zY, internal iliacs; //, internal jugulars; //, left innominate; Ic, left posterior cardi- nal; /, precava; po, postcava; r, renal; ;r, right posterior cardinal; ri, right innominate; s, sinus; sc, subclavian ; si, superior intercostal. In B the postcava has extended backwards and tapped the right posterior cardinal ; and a transverse trunk has formed between the jugulars of the two sides. In Ca transverse vessel, /, has united the two postcardinals ; and these have lost their other connections, and form the azygos system. the right fore limb passes through the left precava in its way to the heart. The lymph system forms another series of circulatory vessels which are distinct from the blood-vessels, excepting at one or more points where they connect, the lymph flowing from the lymph vessel^ into the venous system. The walls of the lymph vessels are always thin ; in most places they consist merely of ORGANS OF CIRCULATION. 199 epithelium without muscular or adventitial envelopes, and at times, as in the frogs, they expand into large subcutaneous lacunar lymph spaces, or similar spaces around the gonads and in the mesenteries, as in many ichthyopsida. The system is frequently in connection with the coelom by means of openings (stomata) in the peritoneal membrane. The distribution of these vessels varies greatly in the differ- ent groups ; and a detailed comparative study of the system is still a desideratum, while its development is largely unknown. In the fishes there is a rich plexus of lymph capillaries beneath the skin which extends into the connective tissue be- tween the muscles, while around the heart and the ventral aorta the system is richly developed. In the lower ver- tebrates (amphibia, reptiles, and" em- bryo birds) pulsating sacs occur in the course of these vessels, the so-called lymph hearts. These are usually placed near some connection between the lymph and venous systems, as near the pelvis and the caudal vertebrae, or in the tho- racic cavity dorsal to the heart ; but oc- casionally lymph hearts occur at more distant points. For instance, in the urodeles a series of these occur beneath the lateral line ; none are known in mammals. In sauropsida and mammals a special trunk, the thoracic duct, is developed in connection with the digestive tract which takes the lymph from the hinder extrem- ities, the reproductive and excretory or- gans as well as the alimentary canal, and carries it forward, pour- ing it, in the sauropsida, into the right brachiocephalic vein, in the mammals into the left. 1 In birds and mammals valves 1 According to the unpublished studies of Dr. F. D. Lambert, a paired thoracic duct is developed in the young of Amblystoma, but a little later the right of these vessels becomes obliterated. FIG. 209. Urogenital system of tadpole of frog, after Marshall and Bles. A, radix aortae; , gills; F, fore foot ; GF, fat body ; GL, glomus of head kid- ney ; GR, genital ridge ; //, heart ; M, mesone- phros ; P, pronephric duct ; PR, pronephros ; U, ureter. 200 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. are developed in the larger lymph trunks, preventing any back flow. In connection with the lymph system lymphoid tissue is- de- veloped, especially around the genital organs of the ichthyopsida, where, as in the amphibia and reptiles, this forms the prominent 'fat bodies.' Aggrega- tions of such lymphoid tis- sue give rise to lymph glands, which are variously distributed through the ver- tebrate body. Of these the most prominent is the spleen, the cells of which are said to arise, in the tadpole, from the ento- derm. It usually is some- what close to the stomach, and in Protopterus (Fig. 40, sp) it is still enclosed in the gas- tric walls. Among other lymph glands may be mentioned the tonsils, which occur at the beginning of the pharynx in reptiles, birds, and mammals. FIG. 210. Urogenital organs and fat bodies of adult frog. C, cloaca ; F, fat body ; M, mesonephros ; P\ postcava ; T, testis ; U, ureters. THE SEGMENTATION OF THE HEAD. 2OL THE SEGMENTATION OF THE HEAD. SINCE in the vertebrates the region of the body behind the head is made up of segments repeated one after the other (metamerism), there has naturally arisen the question, Is the head itself similarly composed of somites ? If so, how many of these somites are to be recognized ? Various attempts have been made to solve these problems, but with varying results. Only the merest outline of the attempted solutions can be given here. In the trunk and tail regions the parts which are meta- merically arranged are as follows : myotomes, spinal nerves, ver- tebrae, nephridial tubes, and the intersegmental blood-vessels. Each and all of these structures have been employed in the. attempt to carry the segmentation forward into the head. The existence of the problem was first recognized by Oken (1807), who attempted its solution upon a vertebral basis. In the mammalian skull he recognized three vertebras, the centres of which were represented respectively by the basioccipital, sphe- noid and ethmoid bones, while the neural arches were formed by ex- and supraoccipitals, parietals, and frontals. Later stu-- dents recognized four vertebrae in the skull, the increase being effected by recognizing basi- and presphenoid centres. 1 In 1869 Huxley showed that this theory was untenable, and that the * vertebrae ' of the skull could not be homologous with those of the trunk, since they were, in part, composed of membrane bone. He also pointed out that in those vertebrates (elasmo- branchs) where one would naturally expect to find the vertebrae- best developed, there was a continuous un segmented brain case. His attempt at the solution of the problem was based upon the nerves and gill clefts, thus transferring the question from the 1 The interested student will find the extreme development of this ' vertebral theory ot the skull ' in the first volume of Owen's ' Anatomy of the Vertebrates.' 2O2 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. T FIG. 211. Diagram of the head segments in a selachian, after Neal. a, anterior so- mite ; aa, aortic arch ; al>, abducens nerve ; rfn, dorsal nerves ; f, facialis nerve ; g, glossopharyngeal nerve ; gc, gill clefts; A, hypoglossal nerve; if, intestinal branch of vagus ; Iv, lateralis branch of vagus; ;//, mediolateral line; n, neuromeres ; o, otic vesicle ; oc, oculomotor nerve ; apt ophthalmicus profundus nerve ;/>0, post-trematic nerve; fr, pre-trematic nerve; s, spi- racular cleft ; so, mesodermic somites; /, trigeminal nerve; v, vagus nerve ; 7-AY, neuro- meres; l-ii, somites of van Wijhe; 1-7, functional gill clefts. vertebrae to the neural and branchial segments. With these as a basis he recognized nine cranial segments. Two years later Gegenbaur, using the same criteria, also concluded that there were nine segments in the head, al- though his somites and those of Hux- ley do not agree in detail. Both of these authors recognized that the nerves behind the ear (IX- XII.) were like the spinal nerves in the possession of dorsal and ventral roots, and that the ninth divides above the first gill slit into pre- and post- trematic branches (p. 63). The tenth nerve, however, bears similar relations in the ordinary sharks to four gill clefts, and hence is a compound nerve. In front of the ear the facial nerve divides above the spiracular cleft, while the trigeminal nerve splits in a similar way on either side of the angle of the mouth. This last circumstance led Huxley to the view that the mouth has arisen from the coalescence of a pair of gill slits, a view which has re- ceived a certain amount of corrobora- tion from embryology. This left a third division (ophthalmic) of the fifth nerve out of consideration ; this was supposed to represent another seg- ment further indicated according to Huxley's view by the orbito-nasal groove, while Gegenbaur saw traces of it in a pair of labial cartilages. Both recognized an additional segment in front of the ophthalmic, the details of which are not necessary here. THE SEGMENTATION OF THE HEAD. 203 Balfour introduced another element, the mesodermal somites, into the discussion ; and his method, developed by Marshall, 1 and still farther by van Wijhe, is that which has given results most often quoted in connection with this subject. Van Wijhe con- sidered mesodermal somites, gill clefts, and nerves, and tried to utilize the purely motor nerves (III, IV, VI,) as ventral roots of the preauditory nerves. He recognized nine mesodermal segments, and the relations of these to the segmental nerves is given here in tabular form. SOMITE. DORSAL NERVE ROOT. VENTRAL NERVE ROOT. 1. Ophthalmicus profundus, Oculomotor. 2. Trigeminal less op. prof., Trochlearis. 3. ) ( Abducens. 3 \ Acustico-faciahs, 4. ) I None. 5. Glossopharyngeal, None. Not recognizable. a 8 us JHypoglossal. Since van Wijhe wrote, others have tried to add to his structure, and some have claimed to recognize eighteen or nine- teen of these head somites. It is pretty certain that there is at least one somite in front of the first recognized by him (Figs. 121 and 122 a). Others have taken the sense organs as their b.isis, including in this not only ear and nose, but sensory struc- tures developed in connection with the gills, and considering the ficialis as compound, have figured out eleven head segments (Heard). Again, the early condition of the neural tube has shown the existence of nervous segments (neuromeres, p. 49), eleven in number in the head region (Orr, Maclure, Hoffmann) back to and including the vagus. Locy has claimed that the same number of segments can be recognized in the medullary plate of elasmobranchs, amphibia, and birds before it is infolded to form the neural tube, but his conclusions are in dispute. The questions asked at the beginning of this section cannot as yet be fully and finally answered. That the head is truly 1 Balfour recognized eight or nine somites ; Marshall nine (eleven in notidanid sharks). 204 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. segmented can hardly be doubted ; but this region of the verte- brates has been so wonderfully altered and specialized that the original segments have been greatly changed, and in some instances may have disappeared. The postauditory region pre- sents the simplest condition ; the tract in front of the ear is much more complex. We can say with great confidence that there are many more than the three somites recognized by Oken ; while with some probability we can say that the number is not far from ten or eleven. In the discussion of the problem the greatest weight should be given to the positive evidence of the myotomes, since it is probable that segmentation originated in the mesoderm ; next in importance are the cranial nerves, while less weight can be given to gill clefts and their modifi- cations, and even less to the so-called branchial sense organs. THE EARLY HISTORY OF THE OVUM. 205 THE EARLY HISTORY OF THE OVUM. THE formation of the essential parts of the sexual or repro- ductive cells from the germinal epithelium was mentioned upon a preceding page (p. 125). A brief account of their subsequent history follows. Space does not admit of any extended account of the details of the phenomena of reduction division, matura- tion, impregnation, etc., and the theories based upon them ; for these, reference must be made to the special text-books upon cytology and embryology. Both eggs and spermatozoa, as they leave the gonads, are cells specialized for the perpetuation of the species ; and ulti- mate analysis shows that as they leave the parent tissue these cells contain all the absolute essentials for the reproduction of the kind. In the vertebrates, however, as in most other animals, these essentials are variously modified in shape and in composi- tion by the addition of certain secondary features which demand attention. As has already been said, the ovum is a specialized cell, which passes into an ovarian follicle, and receives nourishment from the follicular cells, and grows larger than the other cells of the body. At last it escapes from the ovary, passes into the coelom, and thence to the exterior either through the Mullerian ducts (most vertebrates), through the pori abdominales (some teleosts), or by means of special structures (many teleosts). In its simplest condition an ovum is directly comparable to any other cell of the body, consisting merely of a mass of proto- plasm with a specialized portion, the nucleus, near the centre. In most cases, however, it receives other parts of a secondary character, either from the ovarian tissues, or from the walls of the ducts through which it passes. From the ovarian tissue the egg receives a varying amount of food yolk or deutoplasm. This is a peculiar substance to be 206 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. used later as food by the growing embryo. It appears in the shape of small disks, plates, or globules embedded in the proto- plasm. In color it is usually white, but in birds (Fig. 212) two kinds of yolk, white and yellow, are arranged in a complicated manner. The amount and distribution of this deutoplasm exer- cise an important influence upon the early phases of develop- ment, while the size of the egg is directly dependent upon the amount of this substance. In the higher mammals the deutoplasm is scanty in amount, and is regularly distributed throughout the protoplasm (ale- .tf. & FIG. 212. Diagram of hen's egg, from Hertwig after Balfour. ach, air chamber; bl, blastoderm; ch/, chalaza; ism, inner shell membrane; s, shell; sjn, outer shell membrane; vt, vitelline membrane; w, white; u>y, white yolk; jc, inner layer of white; }'}', yellow yolk. cithal). In the amphibia and in Petromyzon the yolk is much more abundant, and the eggs are consequently larger in size. It is still distributed throughout the whole of the egg ; but a marked polarity is visible, one pole of the egg containing the bulk of the protoplasm, while the other is as strongly deutoplas- mic (telolecithal). In selachians, reptiles, birds, and mono- tremes this polarity is still more marked ; while in many teleosts the extreme is reached, for here protoplasmic and deutoplasmic THE EARLY HISTORY OF THE OVUM. 2O/ portions are sharply distinct, the protoplasm resting as a small cap upon the large sphere of pure food yolk. The egg is surrounded by primary and secondary envelopes, the former arising before the ovum has escaped from the ovary, the latter from the ovarian ducts. In the vertebrates the pri- mary envelopes are at most three in number. These are, from without inwards, (i), a vitelline membrane, structureless in character ; (2), a zona radiata (or zona pellucida) traversed by minute pores ; and (3), a thin and delicate inner membrane. These are not constant, and any one or two may be lacking in a given egg. In some cases (teleosts and Petromyzon) an opening (micropyle) exists, through which the spermatozoon obtains entrance to the egg. Of the secondary envelopes one of the simplest conditions is found in Petromyzon, where the outer surface of the egg is covered with a thin layer of adhesive mucus, which serves to fasten the egg to stones, etc. In the myxinoids the egg envel- ope is more horny, and is provided at either end with anchoring hooks. The descriptions would also imply that at the time of laying there was an outer sheathing capsule. 1 In the amphibia the eggs receive a coating in their passage down the oviduct which swells into a jelly when in contact with water. In the elasmobranchs the eggs are enclosed in a horny capsule, usually quadrangular in outline, while in the reptiles and monotremes the oviducts secrete around each egg a calcareous shell. The birds present the most complicated condition. Here the eggs, after they have entered the oviduct, receive first a layer of albumen (the ' white '), a portion of which, firmer than the rest, is twisted into a spiral chalaza at either end. Outside of this there is next deposited a double shell membrane, and then, by the next division of the duct, the calcareous shell (Fig. 212). The spermatozoa arise in the canaliculi seminiferi of the testes (p. 126), but they present many differences from the eggs. These in merest outline are as follows : In every cell of the 1 No one has yet described the origin of these envelopes of the cyclostome egg ; it may be that they are ovarian in origin, a view which seems the more probable from the absence of oviducts. 208 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. body in any given species the nucleus contains a fixed and definite number of bodies, the chromosomes, so called because they are readily colored by the various microscopical stains. -Attach division of a cell these chromosomes are divided so that each daughter-cell contains exactly as many as did the mother-cell. Each egg before it leaves the ovary contains these chromosomes, and the number in each egg corresponds exactly with the number in any other part of the body of the mother. In the formation of the sper- matozoa, however, there is a peculiar cell division, a so-called reduction di- vision, the results of which are that each resulting spermatozoon contains just half the number of chromosomes normal to the species. The spermatozoa also differ from the eggs in their appearance. The egg is passive, and it contains the nourishment and material from which the young is to be developed. The spermatozoon, on the other hand, must be active; for it must seek out and unite with the egg in order that the latter may develop. To this end it is made as small as possible. Deuto- plasm is entirely absent, and the extra- nuclear protoplasm is reduced to the smallest amount. The chromosomes are compacted into a small body, the so- called head, while the protoplasm is largely developed into a 'tail,' consisting of an axial filament and a lateral mem- brane, by means of which the spermatozoon is able to swim. Impregnation consists of the union of the egg and the spermatozoon, and there is abundant evidence to show that a single spermatozoon is sufficient to impregnate a single egg. This impregnation may take place either outside or inside of FIG. 213. A, human spermatozoon front and side view, after Retzius ; B, diagram of vertebrate spermatozoon, modified from Bohm and Davidoff. &/; axial filament ; e, end piece of Retzius; k, head ; j/i, middle piece ; mb, un- dulating membrane; mf t marginal filament ; sf, sec- ondary filament; /, tail. THE EARLY HISTORY OF THE OVUM. 209 the body of the mother, the latter being the prevailing method, external impregnation occurring only in the cyclostomes and in most teleosts and amphibia. Occasionally, as in some urodeles, the spermatozoa are deposited in bunches (spermatophores) , which are taken into the cloacal opening, effecting internal impregnation ; or, as in most elasmobranchs, the ventral fins of the males are modified into copulatory and intromittent organs (claspers). In the birds the transmission of the sperm to the female is effected by an apposition of cloacal openings, although in a few birds a copulatory organ, the penis, is developed. In the reptiles this structure acquires a greater development, and reaches its extreme in the mammals. After the spermatozoon has penetrated into the egg, there occurs, in all eggs accurately studied, certain phenomena which constitute the process known as maturation. These chiefly concern the nucleus, and are as follows : The nucleus ap- proaches the surface of the egg and undergoes a normal divis- ion, one of the resulting halves, together with a small amount of protoplasm, being cast out of the egg as the first polar globule. The nucleus now divides again ; but this division is a reduction division, half of the chromosomes being cast out in a second polar globule, while half sink back into the egg, which now contains just as many chromosomes as does the sperma- tozoon. These chromosomes now unite with those from the head of the spermatozoon, forming the new nucleus of the egg (the segmentation nucleus), which thus again contains the num- ber of chromosomes characteristic of the species. Not until this process is complete is the egg really impregnated and ready for segmentation. The character of the segmentation varies according, among other things, to the amount and distribution of the food yolk in the egg. This substance is inert, and its presence interferes with the living and active protoplasm. Were no deutoplasm present, the egg would divide in a regular and equal manner, and the resulting cells would be equal in size ; and the same would be true, other things being equal, were the deutoplasm small in amount and evenly distributed through the protoplasm. Such eggs do occur in the non-vertebrate groups, but none are 210 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. known among the vertebrates. The simplest conditions pre- sented by these forms may be illustrated by the amphibia, the outlines which follow being to a measure true of Petro- myzon and some gan- oids. In the amphibia the first two planes of division may be compared to two me- FIG. 214 Two stages in the segmentation of the egg of Amblystoma; i to 5 the successive planes of segmentation. Q right angles to each other. These begin to cut through at the protoplasmic (darker) pole of the egg, and gradually extend to the other. The third plane is at right angles to these, but nearer the protoplasmic than to the deu- toplasmic pole. The result is that the eight resulting cells are unequal in size, four being small and four much larger, This disparity in size is continued in the following divisions, and it also affects the position of the internal segmentation cavity (p. 5), which, instead of being central, is pushed toward the protoplasmic pole (Fig. 215). In the amphibia, then, the whole egg divides into cells. Such eggs are called holoblastic. In elasmobranchs, reptiles, and birds, where the deutoplasm is much more abundant, and the polar differ- entiation of the egg is more marked, the planes of segmentation do not FIG. 215. Early stage of the CUt through the entire egg, but are segmentation of the egg of Am- blystoma, in section, showing the excentric position of the segmen- tation cavity, s. confined to what is called the ger- minal area at the protoplasmic pole. Here occur meridional and circular planes of division, so that the germinal area is converted into cells, while the bulk of the egg remains unsegmented. In these meroblastic eggs the segmentation cavity is still farther THE EARLY HISTORY OF THE OVUM. 211 displaced from the centre, so that it comes to lie immediately beneath the protoplasmic pole. The layer of cells formed by this segmentation is SET known as the blasto- derm. In the teleosts where protoplasm and deutoplasm are sharply distinct, the protoplas- mic portions alone are segmented, the food yolk remaining undi- vided, and the segmen- tation cavity here lies between the blastoderm and the deutoplasmic mass. The little that is known of the development of the eggs of the monotremes shows that in their features of segmentation they are closely like the reptiles and birds, they are mero- blastic ; but the other mammals present several peculiarities in their segmentation, which can best be considered after a descrip- tion of the process of gastrulation in other vertebrates. In the amphibia and similar forms the segmentation cavity is so small that it would be impossible for the larger yolk-loaded FlG. 216. Early segmentation of hen's egg, after Duval. e, ectoderm ; /, lower layer cells ; s, segmentation cavity ; w, white yolk ; y, yel- low yolk. Only a small part of the egg shown; compare Fig. 212. FlG. 217. Diagram of the process of gastrulation in Amblystoma ; the cells to be invaginated as entoderm shaded, a and b, front and side views of the beginning of the invagination, the primitive groove beginning in b', c, a later stage with longer primitive groove ; in d, the process is nearly complete, a small patch of entode/tw being seen behind (yolk plug in the ' anus of Rusconi '). cells to become invaginated in the typical manner (p. 5), and so a modification of the process, not easily described, takes place, the result however being the production of the two-layered gas- trula. In a few words the process may be outlined as follows : Since the large cells cannot be pushed inside the smaller ones, 212 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. the latter grow down over the others at that side of the egg which is later to form the hinder end of the embryo. This downward growth results in the infolding of a portion of the cells as a sort of pocket, the cells on the upper surface of the pocket being small, those on the lower side the larger yolk-laden cells. This pocket is the archenteron ; its walls are entoderm, and its opening is the blastopore. As the process continues, the blastoporal lips of either side come to meet in the median line, producing the primitive streak and groove already described. In the elasmobranchs the process of gastrulation is still far- ther modified by the large amount of unsegmented yolk. Here A B FIG. 218. Diagram of two stages in the formation of the embryo in an elasmo- branch egg ; the inflected rim of the blastoderm divided up into segments so as to illustrate the formation of the embryo by concrescence. the blastoderm forms a small circular patch of cells resting upon the yolk. At the morphologically hinder portion of this blasto- derm the cells begin to turn in between the rest and the yolk, thus differentiating ectoderm and entoderm. As the blastoderm increases in size by continual cell division plus additions from the yolk, this infolded rim grows together, its halves swinging in toward the middle line so that the grooves of either side unite to form a tube, the archenteron, the floor of which is formed by the yolk. The rim of the inflected tissue must be regarded as the lips of the blastopore, and as these lips unite in the median line they give rise to the primitive streak. THE EARLY HISTORY OF THE OVUM. In birds the first phenomenon of gastrulation is the forma- tion of a crescentic or sickle-shaped groove at the margin of the blastoderm, the anterior margin of which is directly comparable to the rim of inflection in the elasmobranch. The edges of the right and left halves of this groove coalesce as in the sharks, and then the blastoderm grows backward beyond the primitive streak thus formed, so that the streak comes to lie like an island in the centre of the blastoderm. In the reptiles much the same conditions occur as in the birds, except that the blastopore is placed within rather than at the edge of the blastoderm. A feature to be noticed in all the foregoing types is that in each case the embryo arisesi rom right and left portions, which at first may be widely separate, and which meet and fuse in the middle line. This A ..^^p^;?^, phenomenon of concrescence consists "^r : '- : ;;;Vx V f8&. in the formation of the dorsal portions J: of the embryo, and all of the struc- p ?Mw > :- tures there developed nervous sys- g^;%-- tern, myotomes, sclerotomes, vascular c J::ffi: system, and ccelomic structures from the union of the blastoporal lips. 1 This process is illustrated in our fig- ures (Fig. 218) where the successive FIG. 219. Early blasto- portions of the germinal ring (i.e., derm of hen ' s e gg> after K5J - edges of the blastopore) are shown liker ' '' crescent ; *' primitive . ' groove ; o, area opaca ; /, area uniting to form the axial portion of pe iiucida. the embryo. In the placental mammals the eggs are very small and the amount of deutoplasm is small, consequently the eggs are holo- blastic. In their segmentation and in the method of formation of the germ layers these eggs present many peculiarities, which are usually explained upon the hypothesis that the mammals have descended from animals with large-yolked eggs, and that the features in which they differ from the other vertebrates as well as from other animals with holoblastic eggs are to be attributed 1 Several authors (Sedgwick, Kastschenko, Morgan, and others) have criticised this theory in one way or another, but the actual facts of development seem to negative their arguments. 214 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. to loss of yolk and consequent modification of processes. From almost the first the segmentation is irregular ; and there results, exactly how is not known, a solid sphere consisting of an outer layer of hyaline cells surrounding a mass of more granular cells, one of which reaches to the exterior through a gap, closed later, between the cells of the outer layer. Now the solid mass expands into a hollow sphere, the blastodermic vesicle, the outer layer of cells becoming greatly flattened, the inner adhering to one side of them in a small len- ticular mass, the cavity of the vesicle being filled with fluid. The lenticular mass becomes three layered, increases in extent, and gradually extends around under the outer layer so that the whole vesicle is eventually two layered throughout. The cen- tral portion of the lenticular mass remains thicker than the rest, and in this place the em- bryo arises, a primitive streak being formed, but nothing that can with certainty be called a blastopore appears. So far there is little dispute as to the facts, but as to their interpretation the views are various, some regarding the outer layer as ectoderm, others as entoderm ; while the inner cell mass is regarded by some as purely entodermal, by others as giving rise to both ectoderm and entoderm. For details reference must be made to special works on vertebrate embryology. FlG. 220. Diagram of mammalian blastodermic vesicle. *', inner cell mass. THE ORIGIN OF THE VERTEBRATES. 21$ THE ORIGIN OF THE VERTEBRATES. THE question as to the ancestors of the vertebrates is one of the most vexed problems of zoology. It has seemed at times as if the solution were near at hand. The recognition of chor- date affinities in the tunicates, and, later, in Balanoglossus, at the time when these were regarded as invertebrates, raised hopes that were disappointed when it was found that these forms were chordates, and that only superficial resemblances had caused their association with the non-vertebrate groups. It would seem that to-day we are not much nearer the answer to the question than we were when the theory of evolution was new. Apparently the problem must be solved, if solved it ever be, upon the basis of comparative anatomy and embryology. Paleontology has never thrown the slightest light upon the matter, and it seems as if it never could, because it is more than probable that the ancestral chordate was a soft-bodied animal of small size, incapable of leaving any definite impress in the rocks. The three most important characteristics of the vertebrates, and of all chordates, are the presence of gill slits, the existence of a notochord, and the occurrence of a central nervous system placed entirely upon one side of the alimentary canal. These features are found in no invertebrate, and we can only speculate upon the way in which they have arisen ; for it is one of the canons of evolution that no organ arises de novo, but only by modification of some pre-existing structure. At present the greater weight of evidence, such as it is, points toward an annelid ancestry. Annelids and vertebrates agree in the possession of metamerism, and the homologies of the metameric structures can be traced with some detail. Mus- cular system, ccelomic pouches, and nephridia agree in their general features, while the fact that the nephridial ducts in both 2l6 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. serve to carry away the sexual cells is also suggestive. These ducts, however, afford some difficulties, as it is not easy to see how the continuous pronephric duct of the vertebrates could have arisen from the separate ducts of the annelid. Again, the ventral nervous chain of the annelid can be closely compared with the spinal cord of the vertebrates, the comparison includ- ing the dorsal roots of the nerves with the spinal ganglia. The same is true of the similarities existing between the transverse blood-vessels of the annelids and the aortic arches and inter- costal vessels of the vertebrates. The most plausible, hypothesis by which to homologize the anterior portions of the nervous system is that which regards the infundibulum and the nervous portion of the hypophysis as representing the invertebrate mouth, while the vertebrate mouth may have arisen by the coalescence of a pair of gill slits. In this case the ' brain ' of the annelid would be represented by the vertebrate fore brain. In certain annelids there exists a subintestinal tube of entodermal origin which has been doubt- fully compared with the notochord, but as yet no structures are known in the annelids which can be homologized with the gill slits. Other but less widely accepted views of the ancestry of the vertebrates are those which would derive the group from some arthropod not far from the limuloids, or from the nemertean worms. It must however be kept in mind that the great- est resemblances between vertebrates and annelids are directly or indirectly the result of metamerism ; and that it is possible that this vegetative repetition of parts may have arisen inde- pendently in the chordate phylum, and that the similarities note:! above may be expressions of convergent evolution, and that the chordates may have descended from non-segmented ancestors. This view receives some support from the fact that metamerism also occurs in the echinoderms, where it could not have been inherited from either annelids or vertebrates. In the following pages are numerous references to the lines of descent of the various groups of vertebrates. The adjacent diagram illustrates some of these. Concerning some points there are differences of opinion. Thus the dipnoans are fre- THE ORIGIN OF THE VERTEBRATES. 217 quently regarded as the ancestors of the amphibia, a view which receives its chief support from the existence of lungs in both forms. In their skeleton and in other features the two groups seem widely remote. Again, in recent years the MAMMALS BIRDS REPTILES TELEOSTS GANOIDS 3OCEPHALS CROSSOPTERYGI1 HOLOCEPHAH ELASMOBRANCHS ? CYCLOSTOMES PRIM. ELAS ? OSTRACODERMS dOBRANCHS FlG. 221. Lines of descent of the different groups of vertebrates. tendency has been to regard the mammals as descendants from theromorphous reptiles, a view which receives its chief evidence from paleontology. More lately still the tendency is to revert to the older view of an amphibian ancestry. PART II. SYSTEMATIC ZOOLOGY. SUB-PHYLUM VERTEBRATA. METAMERIC metazoan animals with a complete alimentary canal, the anterior portion of which, at least in embryonic stages, is provided with gill slits. The central nervous system, consisting of brain and spinal cord, is hollow, and is situated entirely on one side of the alimentary tract. Between alimen- tary canal and central nervous system is a skeletal axis the YiG. 222. Anatomy of a vertebrate, based on Amblystoma. a, anus; , brain; /%, heart ; /, lung ; //', liver ; o , ovary ; od, oviduct ; , (amphibia) connected with in teleosts. a, auricle; />, bulbus ar- the gills. A renal portal Sys- teriosus; r, conus, reduced in B to a tern (p. 194) occurs ; and the circle of valves; * ventncle ' red blood corpuscles are 1-arge, oval, and nucleated. The func- tional kidney is the mesonephros, although rarely the proneph- ros persists. The alimentary canal is comparatively short, and either terminates in a cloaca (p. 39), or the vent is anterior to the urogenital openings. No metanephros is developed, and the ova are frequently large. The Ichthyopsida are divisible into Pisces and Amphibia. CLASS I. PISCES. Ichthyopsida with persistent gills ; paired appendages al- most always present in the shape of fins ; median fins sup- ported by dermal rays ; body usually covered with dermal scales ; postcava, a pseudobranch (p. 23). The hemispheres of the brain are united, and the olfactory lobes are separated from the cerebrum by an elongate olfactory tract. The twixt brain is short, and an optic chiasma occurs. The lateral line system is well developed, and in the skates be- comes greatly branched. On the head are numerous sensory ampullae filled with jelly. The skeleton is cartilaginous ; but in many ci|ses it is ren- dered more dense by the deposition of lime, which, however, never takes the shape of bone corpuscles, there being a sharp 234 CLASSIFICATION OF VERTEBRATES. FIG. 236. Diagrams of cyclospondylous {A} and asterospondylous (Z?) vertebrae. Calcifications of cartilage black. line between calcified cartilage and bone. Membrane bones are absent in all recent forms, but in some fossils the dermal scales united to form an ex- tensive armor. In some the vertebral centra are entirely of cartilage. When lime is deposited in them it may take two shapes, either laid down in concentric areas (cyclospondy- lous type), or in a ra- dial manner (astero- spondylous). In the diplospondyli and in the tail of some skates an embolomerous condition occurs. In all recent forms the neu- ral arch is converted into a closed canal by the insertion of intercalary pieces betweeen the neural processes and spine. The cranium is a solid box without sutures. In its roof there may be one or two gaps (f ontanelles) closed by membrane. The pterygoquadrate is never firmly united to the cranium, but either articulates directly with it (amphistylic, Fig. 237), or is supported by ligaments and by the interven- tion of the hyomandib- ular between the hinder end of the pterygoquad- rate and the otic region of the cranium (hyo- stylic, Fig. 162), thus forming a suspensor for the jaws. The pterygoquadrate forms the upper jaw, but is re- enforced in many spe- cies by labial cartilages. In some extinct elasmobranchs girdles are apparently absent, but in all recent forms they are well developed. The pectoral girdle consists of a simple U-shaped FIG. 237. Skull of Heptanchus, after Giinther ; amphistylic. ELASMOBRA NCHS. 235 arch, usually free, but occasionally (raiae) connected with the vertebral column, or rarely with the skull. The pelvic girdle consists of a transverse ventral bar without dorsal or iliac processes. A con us with two or more rows of valves occurs in connec- tion with the heart ; the aortic arches and the chief arteries and veins are of the primitive type, and a cardinal sinus (p. 195) usually occurs. The eggs are few and large, and in the recent forms undergo internal inpregnation, more or less complicated structures (the ' claspers '), which serve as intromittent organs, being developed in the pectoral fins of the male. The spermatozoa is carried to the exterior by means of the Wolffian duct, while the Miillerian duct serves as oviduct, the fused nephrostomes of the rudimen- tary pronephros serving as the ostium tubae. In many sharks and in some of the rays a' portion of the oviduct becomes enlarged into a uterus, and in some species of CarcJiarias and Mustelns, a placenta is formed, at first sight strikingly similar to that of the mammals, but developed from the yolk sac rather than from an allantois (see mammals). The outer surface of the yolk sac in these forms becomes richly vascular ; and this becomes con- nected with the uterine walls, so that the growing embryo receives nourishment from the blood of the mother. The segmentation is restricted to a portion of the upper surface, i.e., is meroblastic. The result of this is the formation of a circular layer of cells, the blastoderm, resting upon the yolk. Inside of these cells is a space corresponding to the segmen- tation cavity of the typical egg (p. 211). Owing to the great amount of yolk, the process of gastrulation' becomes greatly modified. At one end of the blastoderm the cells turn in be- tween the blastoderm and the yolk, and these ingrowing cells become the entoderm. At the place where this ingrowth occurs an arcuate elevation appears, terminating in a pair of swell- ings, the tail swellings, on the margin of the blastoderm. With growth the distance between the arched elevation (which marks the tip of the head) and the tail swellings increases, forming the rest of the head and the trunk region of the body. At first this embryonic area forms a broad, shallow medullary plate ; but 236 CLASSIFICATION OF VERTEBRATES. FIG. 238. Section through the broadly expanded medullary plate of a shark {Acantkias}. a, archenteron ; c, ccelom ; m, meso- thelium ; w/, medullary plate ; n, notochord. the edges of this rise up and gradually unite, so that the plate becomes converted into the medullary tube. Gradually the embryo thus out- lined is raised above the yolk, and soon becomes so separated from it that only a slender yolk stalk re- mains connecting the two. This stalk carries blood-vessels (ompha- lomesaraics), while the yolk itself is connected with the alimentary canal. The mesothelium arises as in- growths on either side at the point of differentiation of ectoderm and entoderm, these growing in be- tween the two layers. The gill slits break through the sides of the neck in regular succession from in front backwards, the mouth breaking through after all five gill slits are open. For a time the gill filaments protrude from the gill slits. The spi- racle is at first the largest of the clefts, but it soon begins to close at the ventral end so that only the dorsal portion persists. The paired fins start as lateral folds (in some cases continuous, e.g., Acanthias), into which grow cells from the myotomes (p. no). Elasmobranchs are, on the whole, the most primitive of the jawed vertebrates, although in some respects they seem to stand above the other fish-like forms. The sharks are free-swimming forms, seek- ing their prey in all parts of the sea, a few species ascending rivers, and one / being found in Lake Nicaragua. The skates, on the other hand, are bottom feeders, living on molluscs, crabs, etc., and their teeth are modified into crush- ing-plates adapted to such food. None of the species are very small ; but some of them are of enormous size, among the largest FIG. 239. Head of embryo Acanthias, show- ing the gill filaments pro- truding from the gill slits. ELASMOBRANCHS. 237 of living fishes ; sharks reaching, in the basking shark and blue shark, a length of thirty-five or forty feet ; the rays, in Mania, a breadth of fifteen feet. ORDER I. CLADOSELACHII (PLEUROPTERYGII). Notochord persistent ; neural and haemal arches slender. Paired fins with basalia and radialia arranged much as in the median fins of recent fish (Fig. 188). No claspers yet found. FIG. 240. Lateral and ventral veins of Cladoselache, restored by Dr. Dean. Apparently a flap of skin much like an operculum covered the first of the gill slits which were seven (possibly nine) in num- ber. Jaws apparently hyostylic. Lateral line an open groove. The only known member of the group is CladoselacJie, from the Waverley group (lower carboniferous) of Ohio. It is prob- able that some of the fossils with similar teeth (Cladodus) belong here. ORDER II. ICHTHYOTOMI. Notochord persistent ; neural and haemal arches and inter- calary cartilages present. Pectoral fin archipterygial (p. 172). Pelvic fins with claspers, caudal fin diphycercal. No placoid scales, but the head was covered with dermal bones. The best- known genus is Plenracanthus (=Xenacanthiis, Didymodns) from the carboniferous and Permian of Europe and America. 238 CLASSIFICATION OF VERTEBRATES. ORDER III. SELACHII. Elasmobranchs with the notochord more or less completely replaced by vertebral centra. The neural canal completely arched in by neurapophyses and intercalaria. No dermal bones. Paired fins never archipterygial ; claspers always de- veloped in the male. To this order belong all recent as well as many fossil elasmobranchs. SUB-ORDER i. DIPLOSPONDYLI. Selachians with embolomerous vertebrae with two neural arches to each myotomic centre ; a single dorsal fin ; anal fin present ; amphistylic skull, and gill slits 6 or 7. Two families are recognized. The CHLAMYDOSELACHID^E FlG. 241. Chlamydoselachus anguineus, after Garman. has an eel-like body, terminal mouth, nostrils on the dorsal surface, Cla- dodus-\\\Lt teeth, and 6 gill slits. To it belongs the single genus Chlamydo- selachus of the deeper portions of the northern parts of the Atlantic and Pacific. In the NOTIDANID^: the body is shark-like, the spiracle is lateral, the teeth differ in the two jaws. The species are viviparous. Hexanchus has six gills, Heptanchus (Fig. 237) seven. SUB-ORDER 2. EUSELACHII. Vertebrae normal, branchial slits five in number, skull hyostylic. SECTION a ASTEROSPONDYLI. The vertebrae in this group, which include the typical sharks, are asterospondylous ; two dorsals and a single anal fin present. Here belong a large series of forms. In the CESTRA- CIONTID,E, which is represented by a few Pacific species to-day, but which was abundant in past times, the dorsal fins bear spines, while the jaws be- hind bear transverse rows of pavement-like teeth. In the GALEID^: the head is normal, the spiracles are small or lacking, the last gill cleft is above I tf-- : ' V EL'ASMOBRANCHS. 239 the pectoral, and a nictitating membrane is present. Here belong the dog- sharks, grouped under Galeus and Mustelus ; the largest of all sharks, commonly called Carcharias (Charcharinus}, some of which have man-eat- ing reputations; and the tiger sharks, Galeocerdo. In the hammer-head sharks (ZYG^:NID/E or SPHRYNID. post-temporal; Q, quadrate; fi, radialia of fin; S O, sub- ocular chain of bones, also suboperculum ; S, scapula. basalia (see, however, p. 173). The dorsal and anal fins are sup- ported upon small interspinous bones embedded in the flesh and usually alternating with the neural and haemal spines, or they may be more numerous than these. The caudal fin is either heterocercal, or the lower lobe may be so well developed that the homocercal condition occurs (p. 229). The ribs, except in Polypterus, where both types occur (p. 145), are modified haemal arches ; and the flesh is further supported by intermuscular 246 CLASSIFICATION OR VERTEBRATES. bones, called epineurals, epicentrals, or epipleurals, accordingly as they are articulated to neural arch, centrum, or ribs. The mouth is usually armed with teeth ; and these may occur not only on the bones which form the edge of the jaws (pre- maxillary, maxillary, dentary), but on those which form the roof of the mouth (palatine, vomer, pterygoid), and also on the pha- ryngeal bones. The alimentary canal usually has the regions well differentiated, and in the ganoids a spiral valve occurs in the intestine. Pyloric caeca, from a sin- gle one to two hundred in number, are common. There is no cloaca, as the urogenital ducts always open behind the vent. A spiracle occurs only in some ganoids. The gills differ from those of the lower fishes in the reduction of the inter- branchial septum, so that the gills themselves pro- ject beyond the arch into the opercular cavity like the teeth of a comb. In the lophobranchs the gill filaments are replaced by curious tufts. An air- bladder or swim-bladder is usually present. It arises as an outgrowth from the dorsal side of the oesophagus (except in Polypterus), which soon becomes differentiated into bladder and duct. In the lower forms the duct remains open through- out life (ganoids and physostomi), but in the physoclisti it closes later, and the bladder loses all connection with the exterior. In many ganoids and some teleosts the inner sur- face becomes plicated. In most forms it receives its blood- supply from the aorta direct, or by way of the coeliac axis ; but FIG. 247. Relations of gill clefts in a teleost. O, operculum, enclosing a branchial chamber. TELEOSTOMOUS FISHES. 247 in Polypterus it comes from the radix aortse, and therefore it receives only arterial blood. The bladder serves as a hydro- static apparatus, but there is also evidence to show that at least in some fishes it is to some extent respiratory as well. For the relations of the bladder to the ear, see p. 255. The brain is noticeable for the large size of the optic lobes and the cerebellum. The cerebrum is rudimentary, and consists of merely corpora striata and a non-nervous pallium in the tele- osts, but in the ganoids larger hemispheres occur. In the ganoids there is a true optic chiasma, but in the teleosts the optic nerves cross (p. 61). The twixt brain is short. The olfac- tory lobes in most teleosts and in the ganoids are joined to the cerebrum ; but in a few forms a long olfactory tract inter- venes. The urogenital organs of the teleostomes will repay further study, for there are many points as yet in doubt. The perma- nent excretory organ is the mesonephros ; only in Fierasfer and Dactylopterus does the pronephros retain its excretory functions. In all others, while it may be of large size, it degenerates into a lymphatic or adenoid structure. The pronephric ducts never divide into Mtillerian and Wolffian ducts, but serve solely as ureters. Usually the two unite behind and form a urinary bladder of some size, the common opening being, except in a few teleosts, behind the vent. 1 The usually paired gonads vary in the way in which their products reach the exterior. In the female salmonids and eels, the eggs are discharged directly into the coelom, from which they escape into a urogenital sinus by means of a pair of slit-like openings, often called pori abdom- inales, but which are apparently not homologous with the sim- ilarly named openings in the elasmobranchs. In most ganoids and in a few teleosts, two longitudinal folds arise in the peri- toneum, the edges of which unite so that a tube, the oviduct, results, which opens freely into the ccelom. In most teleosts, however, these folds are continued to the ovary, so that the eggs do not pass into the general body cavity, but fall at once into these tubes, the lumen of which is, as is readily seen, a part 1 In Pediculati and some symbranchs and plectognaths the urinary opening is in the hinder end of the intestine. 248 CLASSIFICATION OF VERTEBRATES. of the coelom. In the males there is either a simple tube which connects each testis with the urogenital sinus, or there may intervene between tube and testis a system of smaller canals, the vasa efferentia. Legion I. Ganoidea. Teleostomes in which the body is either naked or covered with ganoid or cycloid scales, or bears bony plates ; the skeleton either largely cartilaginous or well ossified ; the tail diphy-, hetero- or homocercal ; ventral fins always abdominal in position ; fulcra present in most recent and in fossil forms ; swim-bladder with duct ; intestine with a spiral valve ; an optic chiasma present ; heart with a conus arteriosus ; eggs with a total segmentation. The group of ganoids contains but a few recent forms, the remnants of a much larger fauna in past times. Its members are widely distributed over the globe, North America, however, having the greater proportion of the species, most of which are inhabitants of fresh water. In the definition above fulcra are mentioned. These are spine-like scales upon the anterior sur- faces of the fins. So far as they have been studied, the eggs of the ganoids undergo a total segmentation ; but, owing to the presence of a FIG. 248. Segmenting egg of Amia, showing the unequal cleavage, after Dean. FIG. 249. Larva of Amia, about the time of hatching, showing the sucking disk at the tip of the snout, after Dean. large amount of yolk, the resulting cells are very unequal in size, the large cells being at one pole of the egg, the smaller at the other. In the sturgeon the central nervous system develops as a tube ; but in the other forms it is at first a solid keel, and only later does a lumen appear by splitting. The larvae of GANOIDS. 249, Amia and Lepidosteus are noticeable for the sucking disk de- veloped on the front of the head ; the larva of the sturgeon has balancer-like structures between the mouth and nose. ORDER I. CROSSOPTERYGII. Ganoids with diphy- or heterocercal tails ; pectoral fins with a large basal portion covered with scales, the ventral fins usually much like the pectorals, the former abdominal in position ; body covered with rhombic or circular ganoid scales ; a pair of gular (or 'jugal') plates in place of branchiostegals ; no fulcra; dor- sal fins two, or a single one divided into many finlets. The crossopterygians are largely extinct, but two genera persisting to-day. The group first appears in the Devonian. FIG. 250. Diplurus longicaudatus, from Dean. A, position of the calcified air-bladder. The CCELACANTHID^E (AcTixiSTiA) have unossified centra and cycloicF scales, two dorsal fins, diphycercal caudal, and ossified swim-bladder.. Ccelacanthus, carboniferous of Europe and Ohio ; Diplurus, trias of New Jersey. CYCLODIPTERINI (RHIPIDISTIA) have the vertebrae partially ossi- FIG. 251. Head of larval Polypterus, after Steindachner, from Dean. EG, external gill. fied ; tail heterocercal ; scales enamelled and rounded behind ; a third' gular plate between the other two. Holoptychius, Devonian of Europe and America; Onychodus and Etisthenopteron, Devonian of America. RHOM- 250 CLASSIFICATION OF VERTEBRATES. BODIPTERINI, with two dorsals, partially ossified vertebral centra ; diphy-or heterocercal tail; two large and several smaller gular plates. Osteolepis, Devonian, Europe. The POLYPTERID^: (CLADISTIA) with two living genera (Polypterus from the Nile, Calamoichthys, greatly elongate and lacking ventral fins, from Old Calabar) are most closely allied to the last. The vertebrae are ossified, the caudal diphycercal ; the dorsal fin elongate and divided into finlets ; pectorals with a short, scaled basal axis ; the body covered with rhomboid ganoid scales. No fulcra exist. In the skull epi- and opisthotics are not distinct ; there are two sphenoidals and ecteth- moids ; the parietals and frontals are paired, and the gular plate is double. ORDER II. CHONDROSTEI. In the sturgeons and paddle-fish there is but slight ossifica- tion of the cartilage, the vertebral centra being unossified, while in the chondrocranium only otic and ectethmoid ossifications appear. The skull is covered with membrane bones, the parie- tals and frontals being paired, while the large parasphenoid extends back beneath the anterior vertebrae. A premaxilla is absent, and only a dentary is present in the lower jaw. The jaw itself is suspended by sympletic and hyomandibular carti- lages, both partially ossified, the mouth itself being ventral as in elasmobranchs. The operculum is large, but its elements are poorly developed ; the branchiostegals are weak or wanting. The body is either naked or covered with rows of bony plates, 'which are continued upon the upper lobe of the heterocercal tail, upon which fulcra are also strongly developed. The ventral fins have a row of cartilaginous basalia. Two recent families are recognized. In the ACIPENSERID^E or sturgeons the body is covered with five rows of keeled bony plates, the skin between the rows bearing small granules. The mouth is toothless in the adult, and FlG. 252. Common sturgeon, Acipenser stuno^ after Goode. In front of it are four barbels. The gill slits are four in number, and the operculum, which does not completely cover the slits, bears an accessory gill. The air-bladder is large and simple, and the stomach has pyloric GANOIDS. 251 appendages. The dorsal and anal fins are posterior in position, the anal being anterior to the dorsal. In Acipenser there is a spiracle, and the naked skin between the plates extends to the tail. About twenty living species are known, half a dozen from North America. From the ovaries caviare is made, while the air-bladders furnish isinglass. The genus appear in the London clay (eocene). The shovel-nose sturgeons (Scaphi- rhynchus}, one species of which is American, lack the spiracle, have the plates forming a complete armor on the depressed tail, while the caudal fin ends in a filament. In the POLYODONTID^E (SELACHOSTOMI), represented to-day by Polyodon spathula in the U.S., and Psephurus in China, the skin is smooth, the snout is prolonged into a long blade (whence the name paddle- fish), the maxillary is obsolete, a spiracle (lacking pseudobranchs) occurs. The PAL/EONISCID^, which range from the Devonian to the lias, have small conical or styliform teeth, simple dorsal and heterocercal tail, and rhombic scales. Palceoniscus, Europe, U. S. ; Eurylepis, U. S. Allied is Platysomus from the carboniferous of Europe and Illinois. 1 ORDER III. HOLOSTEI. Ganoids with well-ossified skeletons ; tail heterocercal ; body with ganoid or cycloid scales ; fulcra frequently present ; branch- iostegals and operculum well developed, and frequently a median gular plate ; mouth terminal, with teeth ; fins without a scaled basal region ; the ventrals with the proximal skeletal elements reduced, much as in teleosts. The garpikes, LEPIDOSTEID^E (GiNGLYMODi) are closely related, struc- turally, to the palaeoniscid forms of the chondrostei. They have opistho- coelous vertebrae, the body covered with rhombic scales, greatly elongate FIG. 253. Garpike, Lepidosteus osseus, from Tenney. jaws, these, like the vomers and palatines, with sharp teeth ; and numerous pyloric caeca. The living species of the only existing genus Lepidosteus are all American. The common garpike, L. osseus, is widely distributed ; 1 The group ACANTHODID/E, which combines ganoid and elasmobranch characters, may be mentioned here. The cartilaginous skeleton, spine in front of the dorsal, absence of opercular elements, are elasmobranch characters, while the presence of spines in the pectorals, and especially of bones in the orbital region and in the roof of the cranium, and the absence of claspers, recall the teleostomes. These forms occur in paleozoic rocks. Acanthodes occurs in U. S. 252 CLASSIFICATION OF VERTEBRATES. the alligator gar, L. tristaechus of the southern states, reaches a length of ten feet. Allied fossil forms are numerous, Catopterus being represented in the triassic rocks of the Connecticut valley. Lepidotus ranges from the trias to the Jura of Europe. Aspidorhynchus had a snout something like that of the sword-fish. In the AMIID^E (HALECOMORPHI) the .vertebrae are amphicoelous, the scales cycloid, teeth on pterygoids as well as on vomers and palatines, no pyloric caeca. Amia calva, the bow fin of the eastern U. S., is the only living species. The genus dates from the eocene. Allied fossil forms are Eurycormus, Callopterus, Caturus, and Pachycor- mus, ranging from the lias to the Jurassic. Legion II. Teleostei. Fishes with the bony skeleton well developed, the cranium and the vertebral centra ossified, the latter amphicoelous ; tail diphy- or homocercal ; spiral valve and conus arteriosus not developed ; no optic chiasma ; scales, when present, cycloid or ctenoid. The group of teleosts or bony fishes so closely follows the ganoids that some students do not distinguish between them. There are, however, some dis- tinctions between the two groups, while the matter of convenience warrants their recognition. In a few teleosts the skin is naked or covered with bony plates, but usually the body is covered with scales of the cycloid or ctenoid type. In a few the tail is diphycercal, but usually it is hom- ocercal. The fulcra, so characteristic of most ganoids, never occur. The skele- ton is well ossified, this being especially true of the skull, where the cartilages are almost entirely replaced by bone. The operculum and its skeleton are well developed, branchiostegals are pres- ent, and gular plates rarely occur. The paired fins never have a basal lobe ; and the ventrals, when pres- ent, may either be near the vent or far forward, beneath the FlG. 254. Breathing valves of teleosts, after Dahlgren. av^ anterior or oral valves open; g; gills; o, oesophagus ; pv, posterior valves. At expiration the anterior valves close, the posterior open ; the enlarge- ment and contraction of the oral cavity being brought about by motion of the oral walls (black). TELE OS TS. 253 throat. A spiral valve is absent except in the single genus Cheirocentnis, while the conus arteriosus is represented only in the genus Butrinus (see, however, p. 227). The bulbus aortae is large. The pallium of the cerebrum is non-nervous in char- acter (Fig. 53), and the optic nerves cross (Fig. 63) and never unite in a chiasma. An interesting discovery has recently been made, that in many if not in all teleosts breathing-valves exist, one pair at FIG. 255* Five stages in the development of the cunner (^Ctenolabrus). A, two cells, resting upon the yolk ; Z?, surface view of the eight-celled stage ; C, the blastoderm covers about one-third of the yolk, the segmentation cavity (Y) showing through; the embryo (oung brought forth alive. The surf perches (EMBIOTOCID/E) of the Pacific coast form the only members of this group, which finds its nearest relatives in the percoid fishes and in the pharyngognaths (infra}. Cymatogaster, Embio- toca, Holconotus, U.S.; Ditrema, Japan. SUB-ORDER 7. PHARYNGOGNATHI. Nostrils double, lower pharyngeals united, scales cycloid ; oviparous. A single family, LABRID.E, of percoid affinities, most of the species being tropical or sub-tropical shore feeders. Labrus, European wrasses ; Cteno- labrus, cunners ; Tautoga ; Scarus, parrot-fish. Allied are the tropical FlG. 260. Gunner, Ctenolabrns cicrnlcus, after Goode. TELEOSTS. 26l families, CHROMID.E, from fresh water, POMACENTRID^E, marine, in which the nostrils are single. The three sub-orders following form a blennoid series which have sprung from a percoid stem. SUB-ORDER 8. TRACHINOIDEA. Ventrals thoracic or jugular, nostrils single, dorsal spines few, soft dorsal and anal long, body scaled or naked. This sub-order is best de- veloped in the south temperate zone. In the deep-sea CHIASMODONTID/E the body is naked, the mouth very large, and two dorsals are present. The species are noted for their sharp teeth and enormous stomach, swallowing fishes several times their own size. The MALACANTHID^: are represented off our shores by the tile-fish, Lopholatilus, of interesting history. TRA- CHINID^E ; mouth oblique, small conical teeth, lateral line distinct. Trach- inus, weevers ; Uranoscopus, Dactyloscopies, star-gazers ; Trachinus appears in the eocene. SUB-ORDER 9. BLENNIOIDEA. Body naked or with ctenoid or cycloid scales ; ventrals thoracic or jugular, sometimes wanting, the soft rays few in number ; dorsal fin long, the spiny rays numerous; anal long; tail homocercal. BLENNID^, gill openings normal, teeth not molariform ; Blennius* Gunnellus, Cryptacan- thodes. ANARRHICHID.E, posterior teeth are molariform; Anarrhichas, wolf-fishes. SUB-ORDER 10. OPHIDIOIDEA. Closely related to the last, but without spines, except sometimes in the posterior part of the dorsal ; tail diphycercal. The eel-pouts are all marine, and occur in all seas. ZOARCID^E, ventral fins never filamentous, some- times wanting. Zoarces. OPHIDIID^:, ventrals slender filaments, a little behind the eye. Ophidium. FIERASFERID.E, ventrals lacking; vent at the throat. Several species of Fierasfer live as commensals with pearl oysters or in the cloaca of holothurians. SUB-ORDER u. BERYCOIDEA. Ventrals thoracic, soft rays of pectorals more than 5 ; tail diphycercal ; duct of air-bladder, in some cases at least, persistent; body naked or scaly. The berycoids are an archaic group, the genera Beryx, Platycormus, Holo- centrum, etc., appearing in the eocene. The nearest relatives are to be sought in the percoids. The living species are marine, and some belong to the deep seas. BERYCID^E, no barbels on the chin ; dorsal is single. HoLOCENTRiDyE, two dorsals. MuLLiD^E, two dorsals and two chin barbels. Mullus, the surmullets ; Upeneus, the goat-fishes. Less certain in its position is the family ZEID^E which has some berycoid affinities, while it also shows relationships to the squamipinnes. 262 CLASSIFICATION OF VERTEBRATES. SUB-ORDER 12. SQUAMIPINNES. Ventrals thoracic, tail diphycercal ; scales small, ctenoid ; dorsal fin long, scales upon the soft portion ; postorbital usually ossified to the skull. The squaniipinnes have left the main stem somewhere near the point of differentiation of percoid and scombroid groups. In turn they have given rise to the plectognaths. The order is introduced by Pomacanthus, and Asineops in the eocene. In the CH^TODONTID/E the teeth are bristle-like and thick set. Chcetodon, butterfly-fishes ; Holacanthus, emperor-fishes ; Toxodon, the archer fish, has the palatines with teeth. All the forms are tropical or subtropical. TEUTHID^:, doctor-fishes, teeth incisor-like ; caudal peduncle is armed with spines or plates, and frequently becomes an important weapon of defence. Teuthis, tropical seas. SUB-ORDER 13. PERCESOCES. Ventrals abdominal, spined ; dorsal spines few, usually forming a sepa- rate fin, tail diphycercal, third superior pharyngeal enlarged, scales cycloid. The percesoces form another stem, arising probably from the ancestors of the hemibranchs and lophobranchs, and close to the synentognaths. In turn the scombroids have descended from some percesocid form. ATH- ERINID^E, or silver sides, lateral line lacking, teeth small or wanting; head and body elongate; species carnivorous, mostly marine; Atherina (appears in eocene) Menidia. MUGILID^E, mullets, herbivorous ; differ from last in the short and broad head. Mugil dates from oligocene. SPHYR^ENID^E, lateral line distinct; teeth strong. Sphyrcena, the barra- cudas of warmer seas. Scyllcemus, cretaceous. The OPHIOCEPHALIDVE of the rivers of India belong near the percesoces, but differ in absence of spines from all fins. They are capable of aerial respiration, and lead to the labyrinthici. The tropical POLYNEMID^E show some relationships to the mugilidae. Polydactylus. SUB-ORDER 14. LABYRINTHICI. Dorsal and anal spines present, ven- trals thoracic, lateral line interrupted or absent ; a complicated apparatus of bony laminae supporting a respiratory mem- brane in the accessory branchial cham- ber, by means of which the animal can breathe air. All are tropical. Anabas FlG. 261. Head Anabas, showing the labyrinthicine apparatus, is sai d to climb trees. Osphromenus, the after Zograff. gouramy. SUB-ORDER 15. AMMODYTOIDEA. Ventrals absent, no spines in any fins, in other respects much as in the percesoces. A group of uncertain relations, placed here for want of a better place. A single family, AMMODYTID^:, with cycloid scales, no teeth, TELEOSTS. 263 lateral line along the side of the back. Ammodytes, sand-launces, common on sandy shores. SUB-ORDER 16. SCOMBROIDEA. Tail diphycercal, caudal usually strongly forked ; ventrals thoracic ; scales usually small, cycloid, sometimes absent ; dorsal fin usually long. A heterogeneous group, not easily defined ; developing in three main lines. SCOMBRID^E, head normal; spinous dorsal well developed; the dorsal divided up into finlets. Scomber, mackerels, first appear in miocene; Thynnus, horse-mackerel, tunnies (eocene) ; Auxis (miocene), frigate-mack- FlG. 262. Mackerel, Scomber scombrus. erel. TRICHIURID;E, body very long, tapering to a point; no caudal; ventrals rudimentary or absent ; tropical. Trichinrus, cutlas-fishes. The allied Lepidopus appears in the eocene. PAL/EORHYNCHID^:, extinct. XIPHIID^:, bones of upper jaw prolonged into a sword. Histiophorus, sail-fish, possesses scales and teeth ; Xiphias, sword-fish, lacks both. Xi- phiidids date from the upper cretaceous. CARANGID/E, pompanos of warmer seas ; caudal forked ; dorsal not divided into finlets ; jaws normal. Naucrates, pilot-fish ; Seriola, amber- fish (date from the eocene); Caranx, crevalle's (miocene) ; Vomer and Selene, moon-fishes, with greatly compressed bodies. Trachinotns, Platax, cre- taceous. POMATOMID^E, blue-fish. CoRYPH^ENiD^E. dolphins ; date from eocene. STROMATEID/E, with teeth-like processes in the oesophagus ; Rhouibus, butter-fish ; Palinurichthys, rudder-fish. BRAMID^E. SUB-ORDER 17. GOBIOIDEA. Dorsal spines few and weak; ventrals thoracic, usually close together; soft dorsal and anal long; tail diphycercal. Over 600 species, mostly marine, and of small size. Callionymus first appears in the miocene ; Gobius (from eocene onwards) ; Clevelandia. Typhlogobius of Californian shores is blind. SUB-ORDER 18. DISCOCEPHALI. With the dorsal fin modified into a flat, transversly laminated oval sucker on the top of the head; ventrals thoracic. Introduced by Opistho- myzon in the eocene of Switzerland with a smaller sucker than in recent 264 CLASSIFICATION OF VERTEBRATES. forms. Living genera, Echineis and Remora, suck-fishes, which attach themselves by their suckers to fish, ships, etc. FIG. 263. Suck-fish or remora, Remora brachyptera, after Goode. SUB-ORDER 19. ANACANTHINI. No spines in any fins, ventrals jugular in position. GADID.E, one of the most important families of fishes ; caudal fin present, scales cycloid, chin with barbels except in Merlucius. Gadus, cod and haddock ; Pol- lachius, pollock ; Microgadus, tomcod ; Lota, burbot (fresh water) ; Phycis ; Merlucius, hake. MACRURID^E, tail tapering to a point, without caudal. Macrurus. The Anacanthini are represented by Nemopteryx in the oligo- cene, while Gadus and Phycis appear in the miocene. FIG. 264. Cod, Gadus morrhua, after Storer. SUB-ORDER 20. T^ENIOSOMI. Body elongate and ribbon shaped ; ventrals thoracic ; dorsal high, and running the length of the back ; mouth small, teeth weak ; caudal, when present, directed upwards. The ribbon-fishes are deep-sea forms, reaching a length of 15 or 20 feet. REGALECID^;, ventrals reduced to a single filament. Regalecus. TRACHYPTERID^E, ventrals normal or wanting. Trachypterus ; Stylephorus. SUB-ORDER 21. HETEROSOMATA. Fins without spines ; dorsal and anals very long ; ventrals thoracic ; tail diphycercal ; head twisted so that both eyes appear on the same side. The flat fishes are among the most remarkable of fishes from the torsion of TELEOSTS. the head. In early life they are sym- metrical ; but very soon, in some spe- cies before reaching the length of an inch, they turn over upon one side, and the eye of the lower surface grad- ually works around to the upper side, twisting the bones of the skull in its passage. The group is nearest the gadoids, and probably these have both descended from some common ances- tor. Many of the species are valuable as food. All are bottom feeders, and some come from the deeper seas. PLEURONECTID^:, preopercular mar- gin distinct ; mouth large or moderate. Hippoglossus, halibut of northern seas ; Paralichthys ; Pseitdopleuro- nectes, winter flounders ; Pleuronectes, plaice ; Lophopsetta, window pane. Psetia (Rhombus}, turbot (dates from the eocene). SOLEID^E, preopercular covered by skin and scales in front; mouth small and twisted. Achirus, American soles ; Salea (dating from the oligocene), European sole. FIG. 265. Cranium of a plaice (Platessa\ from Huxley, showing the distortion of the bones; the dotted line, ab, being the middle line. EpO, epiotics; Er, frontals; Etk, ethmoid; Pa, parietal; Prf, pref rental; SO, supraoccipital^ Cr, position of eyes. FIG. 266. Winter flounder, Pseudopleu- ronectes americanits, after Goode. SUB-ORDER 22. HAPLODOCI. Gill arches reduced to three : head large ; post-temporal undivided f dorsal fins two, the dorsals, pectorals, and ventrals spined in front, the 266 CLASSIFICATION OF VERTEBRATES. ventrals jugular ; scales cycloid or wanting. A single family, BATRACHID^E, mostly from warmer seas. Batrachus, toad-fish ; Thalassophryne, poison toad-fish. Porichthys, midshipman, of Pacific coast, with numerous dermal organs, in structure resembling phosphorescent organs, but not luminous. ORDER VII. PEDICULATI. Pectorals broad, suspended by an < arm ' formed by the elongation of the basilar bones ; head and anterior part of body very large, without scales ; spinous dorsal far forward, the spines often like tentacles ; gill opening a small foramen in or near the axilla ; ventrals jugular. The most specialized of fishes, with possibly an haplodocan ancestry. LOPHIID^E, large mouth ; strong teeth ; ventrals present. Lophius, angler or goose-fish ; the genus dates from the eocene. ANTENNARIID.E, pectorals bent at an elbow-like angle ; ventrals jugular ; Pterophryne, in gulf-weed. Antennarius. MALTHID^E, mouth small, usually inferior. Malthe, sea-bats. ORDER VIII. PLECTOGNATHI. Bones of upper and lower jaws each co-ossified ; post- temporal simple ; ventrals reduced or wanting ; gills pectinate ; gill opening narrow, just in front of pectorals ; spinous dorsal small or wanting. The plectognaths have arisen from near the squamipinnes (above), the teuthids being very near the trigger fishes. FIG. 267. Swell-fish, Chilomycterus geometricus, after Goode. SCLERODERMI, jaws with distinct teeth ; spinous dorsal present ; body with scales or movable plates. Batistes, file-fish or trigger-fish ; Mona- canthus, file-fishes; Acanthoderma, eocene; Altitera, unicorn-fish. Os- TRACODERMI, jaws with distinct teeth ; body enclosed in a three, four, or DIPNOI. 26 7 five angled box composed of polygonal body plates, firmly united. Ostracion Lactophrys, trunk-fishes. Ostracion first appear in the eocene. GYMNO- DONTI, spinous dorsal lacking ; scales spiniform or absent ; jaws with enamelled plates, but without distinct teeth. Tetrodon, Diodon, Chilomyc- terus, etc., swell-fishes or globe-fishes, etc., the common names arising from the powers of inflation possessed by them. Mola (Orthagoriscus), sun-fish, the most bizarre of fishes, seemingly but a large head with fins attached. Fossils doubtfully referred to Mola occur in the British creta- ceous ; Diodon appears in the eocene. FlG. 268. Sun-fish, Mola rotunda, after Putnam. SUB-CLASS III. DIPNOI (DIPNEUSTES). Fishes with partially ossified cartilage, numerous membrane bones, and persistent notochord ; skull autostylic ; a membra- nous operculum ; tail diphycercal ; paired fins archiptergial or reduced ; heart with multivalvular conus, a spiral valve in in- 268 CLASSIFICATION OF VERTEBRATES. testine ; a cloaca present, air-bladder single or paired, function- ing as a lung. The dipnoi, or lung-fishes, are frequently regarded as belong- ing to the ganoids, and have attracted great attention from the fact that they are often considered as intermediate in position between the lower fishes and the amphibia. In past time the group was richly represented ; but in the existing fauna of the earth but four species are known, these having that wide and discontinuous distribution so frequently characteristic of the survivors of an ancient group. They all have an elongate fish-like or eel-like body, covered in the recent species with overlapping cycloid scales ; while in the fossils ganoid scales frequently occurred. The fins are sup- ported by horny dermal rays. The axial skeleton consists of the persistent notochord, around which (except in the tail region of some forms) vertebral centra are not developed, segmenta- tion being shown only in the neural arches and ribs. FIG. 269. Skull of ProtopteruS) after Wiedersheim. #, angulare ; , palatopterygoid ; q, quadrate ; s, supracranial bone; sq, squamosal ; so, supraoccipital ; /- F7, branchial arches. The skull consists of the largely persistent chondrocranium plus a number of membrane bones not easily homologized with those of other vertebrates. In the chondrocranium exoccipitals alone are developed. In the lower groups the membrane bones are very numerous ; but in existing forms, as in the extinct DIPNOI. 269 arthrodira, they are few. In the existing species parasphenoid, vomers, palatoquadrate and squamosal, as well as dentary, angu- lar and opercular in the lower jaw, are more or less certainly to be recognized ; but beside these there are several bones in the cranial roof which are not to be homologized with those in other groups. The operculum is supported by bones (operculum, inter- operculum), and the hyoid and (four or five) branchial arches are cartilaginous. The pectoral and pelvic arches are cartilaginous, the former with membrane bones of the fish-type (p. 174) weakly developed. The pelvis consists of a median plate with (in recent forms) three anterior horns. The fins themselves are very prim- itive, and consist of an axial portion, from which, in Ceratodus, biserial cartilaginous fin rays arise. In many fossils the ante- FlG. 270. Restoration of Dinichthys, from Dean. rior part of the body is enclosed in a strong armor of bony der- mal plates, there being a hinge between the dorsal plates and the base of the skull. The brain differs from that of the teleostomes in the ner- vous character of the cerebral mantle. The two hemispheres are united in Ceratodus, but in Protopterus they are distinct back to the anterior commissure. The mid brain is paired in Ccmtodus, simple in Protopterus ; the cerebellum is but a small transverse fold. The pineal structures have a long stalk, while the envel- opes of the brain are richly developed, and in Protopterus these enter above the fossa rhomboidalis into close connection with the endolymphatic system of the ear. An optic chiasma is present. The teeth are few in number, and are apparently formed by the fusion of several primitive teeth. Of these there is a pair of larger grinding plates borne on the palatopterygoids, a much 2/0 CLASSIFICATION OF VERTEBRATES, FIG. 271. Tooth of ceratodan, Sageno- dus, after Woodward. smaller pair on the vomerine region, while the lower jaw has a pair on the splenial region. The alimentary canal is nearly straight, and is char- acterized by the pres- ence of a well-devel- oped spiral valve (Fig. 40) in the in- testine. Behind, the intestine empties into a cloaca, which also receives, besides the urogenital ducts, median or paired pori abdominales. There are three (JProtop- terus) or four (Ceratodus) l pairs of in- ternal gills, and besides, in the former, external gills (Fig. 269). Besides these, there are present in each swim-bladders which also have respiratory functions. In Cera- todus this lung is single, in Pro- toptcrus it is paired ; but in both its duct or ducts arise from the ventral surface of the pharynx. Internally these or- gans are sacculated, while the blood comes to it by true pul- monary arteries, which arise either (Ceratodus) from the pOS- FIG. 272. Heart and anterior part terior branchial, or (Protopterus) of the lun s s of Ceratodus, after Rose. ,. a, aortic arches and auricle; <:, post- from the radices aortae. cardinal vein . ^ conus; ^ h tic The heart has both the sinUS veins: ;, lung; /, jugular vein; ji, in- and the atrium partially divided ferior jugular vein; oe, oesophagus; /, .' . . i i r^ i i i pulmonary arteries; s, subclavian vein into right and left halves by an and sinu / venosus< incomplete septum, thus fore- shadowing the conditions found in the amphibia, while a true atrio-ventricular valve is lacking. The con us is spirally twisted, and contains several rows of valves, and in Ceratodus is partially, in Protopterus completely, divided into venous and arterial halves. 1 Ceratodus also has a hyoid pseudobranch (p. 23) . DIPNOI. 2/1 In the venous system the most marked advance is the presence of a postcava, while but a single (left) postcardinal comes to complete development. A renal portal system is present. The mesonephros is elongate in Protopterus, short in Cerato- dus ; and nephrostomes are lacking in the adult. Its duct is thick walled, and is apparently a Wolffian duct, although em- bryological evidence is as yet lacking. The gonads are elon- gate, and attached to the lateral parts of the mesonephros. The oviducts are elongate and contorted, and open into the coelom far forward by narrow ostia. Posteriorly they unite just in front of the cloaca. In Ceratodus no vasa deferentia occur, the spermatozoa apparently passing out through the pori abdomi- nales. In Protopterus a well-developed duct occurs in connec- tion with either testis, each passing behind into the rudimentary Miillerian duct, and thence by a common trunk into the cloaca. The cloaca also bears an azygos diverticulum (Fig. 40), usually regarded as an urinary bladder (cf. however, the rectal gland of elasmobranchs). Of the development of Protopterus nothing is known. The segmentation and external development of Ceratodus have been studied, and show striking similarities to that of Petromyzon, and especially to the amphibia. The egg undergoes a total but unequal segmentation, while gastrulation is effected by over- growth as in the amphibia, the result being, as in that group the formation of an elongate primitive groove, on either side of which the medullary folds arise. These close in, and gradually the embryo arises as a ridge above the yolk. So far as is known no metamorphosis occurs. ORDER I. ARTHRODIRA. Body in front covered with large bony plates, a dorsal pair articulating by a hinge joint with the cranium ; paired fins rudi- mentary or absent ; pelvis represented by a pair of club-shaped plates. The relationship of the arthrodira to the other dipnoi is not beyond question. The group is restricted to the palaeozoic rocks, and remains are abundant in Europe and in America. 272 CLASSIFICATION OF VERTEBRATES. Coccostens appears in the Silurian of Europe, and occurs in the Devonian of Ohio. In the Devonian and carboniferous of Ohio FIG. 273. Coccosteus, restored, after Woodward, from Dean. A, articulation of head with trunk ; DB, basalia ; DR, radialia of dorsal fin ; //, haemal arch and spine; MC, lateral line canals; JV, neural arch and notochord; (7, unpaired plate; VB, VR, basalia and radialia of ventral fin. also occur some gigantic forms belonging to the genera Dinich- thys, Titanichthys, and Macropetalichthys, the latter genus occur- ring in Germany as well. FIG. 274. Dorsal wall of skull of Dinichthys, after Claypole. , ethmoid ; 0, exoccipital; F> frontal; M, marginal; N, nasal; P, parietal; PO, postorbital ; PR, preorbital ; S, supraoccipital ; sensory canals dotted. (The homologies of some of the bones with the similarly named elements in other groups is doubtful.) ORDER II. SIRENOIDEA. Body never with bony plates, usually covered with cycloid scales ; paired fins archipterygial ; pelvis a single median plate. The recent forms are subdivided into MONOPNEUMONIA and DIPNOL 273 DIPNEUMONIA. In the first, typified by the Australian genus Ccmtodus, there is but a single air-bladder (lung), and the fins have the secondary rays of the archipterygium (Fig. 185) well developed. Ceratodus forsteri of Australia attains a length of five feet ; it lives in fresh water in places where it is apt to be- come stagnant, and at such times calls its lung into function. FIG. 275. Lung-fish, Protopterus annectans, from Boas. The genus Ceratodus also occurs fossil in the triassic and Juras- sic of Europe, India, and Colorado, the peculiar dental plates being very characteristic (Fig. 271). The Dipneumonia have two air-bladders, and the paired fins retain only the axial part of the archipterygial skeleton (Fig. 269). The living genera are Protopterus from African rivers, and Lepidosiren from South America. At the time of drought the African form burrows into the mud at the bottom of the pools where it lives, and by the aid of the mucus from its body forms the earth into a ' co- coon,' in which it lives in a state of sus- pended animation until the return of the rainy season. Allied to these living animals are a num- ber of fossil forms characterized by the pres- ence of numerous plates in the cranial wall. These occur in the palaeozoic rocks. In Dipterus and Phaneropleuron from the De- FlG - 2 7". Dorsal r T-. i A i view of skull of Dip' voman of Europe and America, jugular ,, rw> after Pander . plates are present ; in Ctenodus and Sageno- dus, from the carboniferous of both hemispheres, jugulars are lacking. 274 CLASSIFICATION OF VERTEBRATES. CLASS II. AMPHIBIA (BATRACHIA). o Ichthyopsida in which lungs are present and the gills are usually lost in the adult. Median fins never supported by dermal rays ; paired appendages in the shape of legs ; body without scales except in caecilians and stegocephals ; a stapes always present, and an Eustachian tube in the higher forms ; nostrils communicating posteriorly with the mouth ; a post- cava always present. The amphibia are readily distinguished from the fishes by the absence of paired fins, their place being taken in most forms by legs built upon the pentadactyl type, like those of amniotes. Occasionally, as in Siren, one pair of limbs may be absent, or again, as in the gymnophiona and, among the stegocephals, the aistopoda, both pairs are lacking. Median fins, confined to the caudal region, occur in the young of all, and in the adults of many aquatic species, but these are never sup- ported by dermal rays, while the tail is diphycercal in character. The skin is largely without cuticular structures, but its outer layers become cornified and are periodically shed. The deeper layers contain numerous glands ; the secretions of some of these are acrid, and in some cases poisonous ; upon these depends the safety of these otherwise unprotected animals. In some cases the skin is smooth, in others it is roughened and covered with warts, in part due to local thickenings, in part to the presence of these defensive glands. Epidermic nails occur on the toes of a few forms. In a few living species of anura, calcareous deposits occur in the dermis, and occasionally (Ceratophrys, etc.) bony dorsal plates may be developed in the same layer. In most gymno- phiona semicircular dermal scales envelop the body, giving it a ringed appearance externally. This dermal skeleton was better developed in the extinct stegocephals, where we usually find from one to three large ventral bony plates and a number of smaller ventral scales, but occasionally this armor extended over the back and limbs. The mouth is always terminal ; and teeth, when present, occur on the margins of the jaws (premaxillaries and maxilla- AMPHIBIA. 27$ ries), and usually upon the vomers. In the urodeles teeth may also occur upon the palatines, and occasionally upon the para- sphenoid. In all cases they are firmly anchylosed to the sup- porting bones. In the anuran tadpoles the jaws are covered with horny plates. In the urodeles the tongue is rudimentary. It is lacking in one division (aglossa) of the anura ; but in the rest it is fixed in front, the bifid free end being turned back in the mouth. It is capable of extension beyond the jaws, and, covered with adhesive mucus, is used in the capture of food. The central nervous system has all of its parts lying in one plane. The cerebrum is larger, and differs from that of all fishes, even of the dipnoi, in the greater development of the pallium. The olfactory lobes are in close connection with the cerebral hemispheres. The cerebellum, on the other hand, is reduced to a small transverse fold. The Gasserian and acus- tico-facialis ganglia are distinct in urodeles and gymnophiona ; but in the anura they are closely united, and the roots of the corresponding nerves are not distinguishable by ordinary dissection. There is a similar union of the glossopharyngeal and vagus ganglia, and the common trunk of the ninth and tenth nerves passes from the skull by a single foramen. In all aquatic forms and larvae the lateralis branches of the seventh and tenth nerves is well developed; but with the assumption of a terrestrial life these are lost, together with the lateral line system which they supply (p. 67). The nasal passages form complete tubes, opening into the oral or pharyngeal cavities by internal nares or choana in its roof. Connected with the olfactory organs are well-developed organs of Jacobson (p. 77). The epiphysial structures do not extend beyond the skull in urodeles or gymnophiona ; but in the anura the parietal eye lies between the skull and the skin, all connection between it and the brain being lost. In the stego- cephals there is a large parietal foramen in the skull, which is interpreted as having contained a functional parietal eye. The ears show an advance upon those of the fishes in the development of a distinct lagena, 1 while the spiracular cleft, 1 The lagena is the seat of audition, and recent experiments show that hearing first appears in the amphibia. 2/6 CLASSIFICATION OF VERTEBRATES. in the anura, enters into the accessory auditory structures, forming an Eustachian tube leading from the tympanic cavity to the pharynx. A stapes is also developed, and in the anura this is joined to a columella (possibly derived from the hyoman- dibular) which stretches across the middle ear to the tym- panic membrane. In the urodeles and caecilians the columella and Eustachian tube are absent, and frequently the stapes articulates directly with the quadrate. The oral and pharyngeal cavities are ciliated ; and into them open, in front, the internal nares, and behind, the slit-like glottis, communicating with the more or less elongate trachea. In the young, and in phanerobranchs and derotremes, gill slits occur in the pharyngeal region. Of these, three open to the exterior, while one (or in some cases two) pouches behind these never break through. The alimentary tract may be nearly straight in the elongate forms, or be greatly convoluted in those with shorter bodies, the convolutions reaching their extreme in the herbivorous tad- poles of the anura. The rectum is short, and opens into the cloaca. The liver is two-lobed, and in the anura the left lobe is more or less completely sub-divided. The pancreas is flattened and lobulated. In the young of all external gills occur, and these may per- sist throughout life (perennibranchs). These gills are ecto- dermal in origin, and arise as outgrowths from the side of the neck before the gill slits break through. Usually they are more or less branched and feathered, but in Concilia compressicauda they are large sacs. The ento- dermal gills are a later appear- ance, and arise from the walls of the gill clefts. These clefts at first open freely to the exterior ; but in the adults of most they become closed, remaining permanently open only in the peren- nibranchs and derotremes. In the anuran tadpole an oper- FIG. 277. Head of young Die- myctylus viridescens, showing lateral line openings and remains of gill clefts. AMPHIBIA. 277 cular fold, traces of which are found in some urodeles, grows back over the gill slits in such a way as to enclose them in an extrabranchial or atrial chamber on either side, the two com- municating by a passage beneath the throat, and opening to the exterior usually by a single opening upon the left side. 1 The lungs, which are absent from several salamandars which respire by means of the skin, are thin-walled sacs, which may be either smooth internally or folded into alveoli and infundibula (Fig. 33). The shape is correlated with that of the body, elongate in the longer species, shorter in the more compact forms. Occasionally (gymnophiona, Amphiuma) the left lung is small or rudimentary. The trachea may be long or the bronchi may unite just behind the glottis. The glottis is supported by a pair of arytenoid cartilages, and in the anura a ring-like cricoid is added. In many stegocephals the vertebral column is poorly de- veloped, the centra being sometimes represented by pleuracen- tra and hypocentra arcale and pleuralia (rhachitomous type, p. i 36) ; or again by an embolomerous condition where centralia and intercentralia alternate. These two conditions, sometimes used as a basis of sub-division, may occur in the same species. In the living species the centra are well developed, and are either amphiccelous (perennibranchs, gymnophiona, and some salamanders), opisthoccelous (most salamanders and aglossate anura), or precocious (most anura). The number varies from 9, plus the urostyle, in living anura to 250 or more in the gym- nophiona. At most but four regions can be recognized, cer- vical, trunk, sacral, and caudal ; the single cervical is without ribs, but bears in front an odontoid process derived from an an- terior vertebra which early fuses with the skull. There is also a single sacral vertebra in all except one group of fossil anura (Palseobatrachidae), where there are two. In the urodeles the vertebrae bear dia- and parapophyses (p. 141), but in the anura only the diapophysis persists. The ribs are small, bicipital in urodeles and gymnophiona, anchylosed to the vertebras in the anura. In the stegocephals they are larger, but in no case do they reach the ventral surface. 1 Paired openings occur in the agl^g^^^^ij^^hal opening in a few forms. jf^' OF THf ^ { UNIVERSITY 1 CLASSIFICATION OF VERTEBRATES. FIG. 278. Skeleton of Necturus. The sternum is lacking in the gymno- phiona ; in the urodeles and arciferous anura it is a median plate grooved to receive the epicoracoids in front. In the firmisternous anura the sternum ex- pands in front of the procoracoids and clavicles into an omosternum, behind the coracoids to a xiphisternum which may be partially ossified. The skull is noticeable for the great extent to which .the chondrocranium persists, and for the wide interval be- tween the trabeculae. This persistence of cartilage accounts for the small num- ber of cartilage bones found in all groups except the gymnophiona. Thus in the anura only a prootic occurs in the auditory region ; in the urodeles an opis- thotic is added. In the occipital region there usually occur but the two exoccip- itals, each bearing an occipital condyle. The quadrate forms the sole suspensor of the jaw, and is more or less closely connected with the otic capsule. If a hyomandibular be present, it is modified into the stapes. In the urodeles no eth- moid ossification occurs, while an orbito- sphenoid is the only bony element in the trabecular region. In the anura a ring-like sphenethmoid occurs (os en ceinture). In the gymnophiona the eth- moid is very large and has large lateral wings. The Mpmbrane bones are more nu- merous ii&ie gymnophiona, stegoceph- als, and aWra than in the urodeles, the skull being very complete in the first two groups, while in the anura a large AMPHIBIA. 2/9 gap appears between the cranium and the quadrat oj ugal-malar arch. This latter arch is entirely absent in the urodeles. The roof of the mouth is formed by vomers, palatines, and a para- sphenoid, the latter element not reappearing in the higher groups. In the caecilians the parasphenoid fuses indistinguish- ably with occipital elements. All of these bones may bear teeth, as may also premaxillaries and maxillaries, the latter element FIG. 279. Skull of Ichthyophis glutinosus, after the Sarasins. b, basal, com- posed of the coalesced parasphenoid and the occipitals ; , ethmoid ; /, frontal ; /, jugal ; mp y maxillopalatine ; n, nasal ; /, parietal; pf, prefrontal ; pm, premaxil- lary ; po, postf rental ; pt, pterygoid ; s, suspensorium ; s/, stapes; / (in front), turbinal ; (behind) tentacular groove. occasionally being absent. The quadrate is overlaid by a squamosal. In the shoulder girdle coracoid, procoracoid, and scapular elements are formed ; in the urodeles the procoracoid usually extends directly forward, but in the anura the ventral ends are connected by an epicoracoid, and the procoracoid is more or less completely replaced by a membrane bone, the clavicle. The amount of ossification varies indifferent forms. The pelvis is characterized by the developmem of the ilium, which is very strong in the anura. Ventral^ there is frequently a continu- ous ischiopubic plate in which a distinct pubis rarely ossifies. Epipubic processes are common in the urodeles. The limbs 280 CLASSIFICATION OF VERTEBRATES. are typically pentadactyl, with primitively a simple carpus and tarsus. In the anura there is a fusion of ulna and radius, while in the hind foot the proximal elements of the tarsus (astragalus and calcaneum) become greatly elongate. In the heart, which, except in the gymnophiona, is far ante- rior, there is always a single ventricle. In the perennibranchs and lungless salamanders the auricles are incompletely sepa- rated, but in the other amphibia two distinct auricles occur. The right auricle receives venous blood, while, when the lungs are functional, the left receives arterial blood. In the lungless forms the pulmonary vein is absent. In the gymnophiona two rows of valves occur in the conus, but elsewhere this region is reduced to a single circle of semilunar valves. The bulbus is well developed, and in the anura contains a longitudinal valve which, by changes in position, directs the first blood to leave the ventricle (arterial blood) into the carotids and the general cir- culation, while the venous blood which follows it is sent into the pulmonary artery, and thence to the lungs. Four pairs of aortic arches appear in the later larvae, the blood at first passing through them to the gills, and thence to the dorsal aorta. With the metamorphosis the branchial circu- FIG. 280. Diagram of venous circulation in an amphibian, av, anterior abdominal vein; al t caudal vein; cv, posterior cardinal veins; //, hepatic veins; ht, heart; z, interrenal vein; iv, iliac vein; /, jugular vein; /', mesonephros; />, portal vein ; pc, postcava ; s, subclavian vein. lation is lost ; but in the urodeles all four arches persist, the first supplying the carotids, the second and third forming the radices aortae, while the fourth go to the lungs. In the gymno- phiona and anura the third of these disappears. In the venous system the most marked feature is the appearance of a hepatic- portal system (p. 192) lacking in the other ichthyopsida. The pronephros is a transitory organ. It is confined to two AMPHIBIA. 28l (most urodeles) or three somites (anura) or several segments (gymnophiona, Amphiuma.) It is replaced later by the perma- nent mesonephric kidney, the anterior end of which in the male becomes subsidiary to reproductive purposes (p. 129). In the gymnophiona it is markedly segmental. The ovaries are long bands in the gymnophiona, elongate sacs in the urodeles, and shorter sacs divided by transverse partitions in the anura. The eggs, in their passage through the Miillerian ducts, become en- veloped in a gelatine which swells in contact with the water. The Miillerian duct always persists in the male. The sperm passes through the anterior part of the kidney, and thence to the exterior by the way of the urinary duct. In many urodeles it becomes enclosed in packets (spermatophores). Connected with the reproductive organs are branched 'fat bodies' which probably are connected with the nutrition of the reproductive structures (p. 200). Fertilization by means of the spermatophores is internal in urodeles, external in the anura. The eggs are laid in the water, and left without further care by most forms. A few, however, have interesting breeding habits. Thus Amphiuma and IchtJiy- ophis wrap the cords of eggs around the body ; in Alytcs the male wraps the cords around his legs. In Rhinoderma there is a large gular fold into which the eggs are received, while irr Nototrema and Notodclphys a brood pouch, open behind, is formed by a duplication of the skin of the back. In the Surinam toad, Pipa, the eggs are spread upon the back, the skin of which thickens around each egg so that it assumes the character of Honeycomb, each cell being occupied by an egg which devel- As in this position until the adult characters are assumed. A P^^P species {Salamandra atra, S. macnlosa, Ccecilia compressi- cauda) bring forth living young, while Amblystoma tigrinum fre- quently breeds in the larval or ' Siredon ' stage. The eggs contain a large amount of yolk, and undergo a total but unequal segmentation (Fig. 214), the result being the formation of a blast ula with small cells on one side and larger (entodermic) cells on the other, and an eccentric segmentation cavity (Fig. 215). The gastrula arises in part as an inpushing, in part as the result of an overgrowth of the ectoderm, and be- .-282 CLASSIFICATION OF VERTEBRATES. fore this process is completed, the differentiation of the central nervous system begins. The medullary plate infolds into a tube, and at the same the egg begins to elongate into the em- bryo. The head now becomes differentiated, and the outlines of the eyes are seen, while the tail begins to extend behind, the ventral surface of the embryo being swollen by the large amount of yolk. On the sides of the neck appear small swellings, the rudiments of the external gills, two pairs in the anura, three in the urodeles and some caecilians. Besides these, the anuran develops a pair of suckers beneath the head, while the uro- dele is characterized by the formation of c. J a pair of slender rod-like ' balancers' in front of the external gills, these balancers being apparently the gills of the hyoid arch. After escape from the egg into the water the gill clefts break through. The limbs make their appearance later than the external gills. In most amphibia there is a metamor- phosis, most marked in the anura where there is a tailed larva, the tadpole, with small toothless mouth. The external gills disappear ; the tail is absorbed, its vertebrae being reduced to the urostyle ; the internal gills appear, and the gill slits first become enclosed in a gill chamber formed by the backward growth of the opercular fold, and then close up completely. The mouth en- larges, and the tadpole assumes the adult form. All the facts of structure and development go to show that the amphibia have arisen from the crossopterygian ganoids, and that existing groups have descended from the stegocephali, each by its own line of ancestry. The view that the anura have descended from urodeles has little morphological evidence in its favor, while there is much against it. TIG. 281. Larva of -Amblystoma punctata, enlarged, showing the .balancers. AMPHIBIA. 283 SUB-CLASS I. STEGOCEPHALI (LAB YRINTHODONTIA) . Extinct amphibia with well-developed tail ; skull solid, with numerous dermal bones, including paired supraoccipitals, supra- temporals, and postorbitals ; the lower surface of the body usually with three large ventral bony shields, and frequently with smaller scales which may extend over the dorsal surface and limbs ; a separate pubic ossification. The stegocephali ap- pear in the carboniferous l and became extinct in the triassic. Some were of gigantic size, and in some the dentine of the teeth was so folded as to give these animals the name of laby- rinthodonts. ORDER I. LEPOSPONDYLI. With vertebral centra consisting of bony envelopes surround- ing the persistent notochord ; teeth simple, with large pulp cavities. Branchiosaurus (one species about four feet long) had persistent gills, and the ventral surface of body, limbs, and tail with oval scales. European carboniferous. Melanerpeton. The MICROSAURIA, with pointed heads and weak fore limbs, are well represented in the carboniferous of Nova Scotia {Hylerpe- ton, Hylonomus) and Ohio (Tutidanus, Colosteus)> as well as of Europe (Keraterpetori). In the AISTOPODA the body was snake- like and limbless. DolicJiosoma, Ophiderpeton, European car- boniferous ; Phlegethontia (coal of Ohio) lacked ribs. ORDER II. TEMNOSPONDYLI. Vertebrae embolomerous or rhachitornous, dentine of teeth radially folded. RHACHITOMI, with rhachitornous vertebrae. Archcgosaurus, the best known stegocephalan (European car- boniferous), five feet long. Trimerorhachis (Texas Permian) had a skull five feet long. Eryops from the same beds was half as large. EMBOLOMERI, embolomerous vertebrae. Crtcotus, Permian of Texas and Illinois. 1 Foot-prints, possibly of a stegocephalan, have recently been found in the Devonian of Pennsylvania. 284 CLASSIFICATION OF VERTEBRATES. ORDER III. STEREOSPONDYLI. Vertebrae amphicoelous, occipital region ossified, teeth labyrinthine. GASTROLEPIDOTI. With ventral elongate scales. Baphetes (coal, Nova Scotia), Platyops (Permian of Europe). LABYRINTHODONTID^E, no ventral scales. Trematosaurus, Laby- rint/wdon, Mastodonsaurus, etc., Europe. Jfa. FIG. 282. Skull of stegocephal, Trematosaurus,irom. Huxley. EpO, epotic; Fr> frontal ; Ju, jugal ; La, lachrymal; Mn, mandible (of several bones); Mx^ maxilla; Na, nasal ; Or, orbit; Pa, parietal; Pmx, premaxilla; Prf, prefrontal; Ptf, postf rental; PtO, postorbital; Qt\ quadrate jugal ; SO, supraoccipital ; Sq, squamosal ; St, supratemporal. The grooves shown were for lateral line organs. SUB-CLASS II. URODELA. (GRADIENTIA.) Amphibia with persistent tails ; usually two pairs of limbs ; skull without ethmoid, supraoccipital, postorbital, or supra- temporal ; no parietal foramen. Vertebrae amphicoelous, never embolomerous or rhachitomous. Skin naked. ORDER I. PERENNTBRANCHIATA (PHANEROBRANCHIA). With persistent, bushy, external gills and gill slits ; maxilla usually lacking; teeth on vomers and palatines. SIRENID^E, hind AMPHIBIA. 285 limbs lacking ; Siren, the mud eel of southern United States, has jaws armed with horny sheaths. PROTEID.E, hind limbs present ; jaws with teeth. Proteus of Austrian caves nearly blind ; Necturus (Menobranchus), the mud puppy of the central United States. ORDER II. DEROTREMATA (CRYPTOBRANCHIA). External gills lost, a spiracle on the side of the neck, lead- ing to persistent gill slits. AMPHIUMID.E, limbs rudimentary ; Amphiuma, one species, the congo eel from the southern states. CRYPTOBRANCHID^:, legs strong; body salamander-like. Men- opoma (Cryptobranchus), hell-bender, from U. S. Megalobatra- chus, giant salamander from Japan, three feet long. Andrias scheuchzeri, European miocene, described over one hundred and fifty years ago as a relic of the legendary Noachian deluge. ORDER III. SALAMANDRINA (MYCTODERA). Gill slits and external gills lost in the adult ; vertebrae fully ossified. LECHRIODONTA, palatine teeth in a transverse row or posteriorly converging series. Amblystoma, toothless parasphen- oid, toes four in front, five behind ; many species in U. S. PletJwdon, teeth on parasphenoid ; premaxillaries separate. Spelerpes, premaxillaries fused ; Dcsmognathus t with parasphen- FIG. 283. Plethodon erythronotus. oid teeth and opisthoccele vertebrae. The species of Amblys- toma are remarkable for the length of time that their larvae (Siredori) retain their gills, some species (A. tigrinum) and the axolotl of Mexico breeding in the siredon stage. Most of the lungless salamanders (p. 27) belong in this family. MECOD- ONTA, parasphenoid toothless, palatine teeth in two rows diver- ging behind. Diemyctylus, our common newt. In Europe Triton, 286 CLASSIFICATION OF VERTEBRATES. Salamandra, Pleurodeles, etc., the first two genera dating from the European miocene. Megalotriton, eocene. FIG. 284. Siredon lava of Atnblystoma^ from Hertwig, after Dumeril and Bibron. SUB-CLASS III. ANURA (SALIENTIA). Tailless in the adult condition, the caudal vertebrae being reduced and fused to a urostyle ; vertebrae usually procoelous ; frontoparietals fused ; sphenethmoid present ; hind legs elongate and fitted for leaping, the proximal row of tarsals greatly elon- gate ; a marked metamorphosis, the tadpoles being vegetarians, the adults carnivorous. The anura contains the frogs, toads, tree-toads, etc., the group being best developed in North Amer- ica and in the tropics. Its origin is uncertain, but probably was from some stegocephalian ancestor. ORDER I. AGLOSSA. Tongue lacking ; the Eustachian tubes open together into the pharynx ; epicoracoids free, but not overlapping. Xenopus {Dactylethrd), from Africa; Pipa, the Surinam toad (p. 281), from South America. ORDER II. ARCIFERA. Tongue well developed ; shoulder girdle arciferous (p. 278), the coracoids of the two sides overlapping ; Eustachian tubes widely separate. The BUFONID^: includes the toads, in which the jaws are toothless, the toes webbed, but without suckers at the tips ; parotid glands prominent. Bufo, the common toad. The genus appears in the eocene. The PELOBATID.E differ in having teeth. Pelobates first appears in the miocene. Scapin- AMPHIBIA. 28 7 opus includes the burrowing spade-foot toad which is rarely seen except at the breeding-season. Allied European genera are Alytes and Bombinator. The HYLID.E have teeth, while the tips of the toes are expanded into sucking-disks. Our tree-toads be- long to Hyla, Acris, and Choro- philus ; Notodelphys and Nototrema, tropital America. The extinct PAL^OBATRACHID^: (oligocene) are noticeable for two sacral ver- tebrae. ORDER III. FIRMISTERNIA. FIG. 285. Shoulder girdle of Bombinator igneus, showing the ar- ciferous type, after Wiedersheim. f, clavicle ; c0, coracoid ; ec, epi- coracoid ; g, glenoid fossa ; /<:, pro- Tongue well developed; epi- corac< ? id; '' scapula; JJ ' supra - scapula ; sf, sternum. coracoids firmly united in the median line. The ENGYSTOMID.E, or toothless frogs, occur in our southern states. Engy- stoma. The RANID.E, or true frogs, have smooth skin, and teeth in the up- per jaw. Rana contains our species including the; bullfrog (A*, catesbianay the largest known frog. Sternum and R ana fi rs t appears in the the Boulder j Numerous other after Wieders- the firmister- families in the tropics, in- FIG. 286. ventral portion of girdle of Rana, heim, illustrating nous type of sternum, cl, clavicle ; co, cora- eluding the DENDROBA- coid; ec; epicoracoid ; g, glenoid fossa ; as, TIDJE w hich have tOCS like omosternum; s, ventral part of scapula; st, sternum; x, xiphisternum. tne tree-toads, Hylldae. SUB-CLASS IV. GYMNOPHIONA (CffiCILLffi). Limbless amphibia of worm-like shape ; tail lacking; vertebrae amphicoelous ; skull well ossified, with well-developed ethmoid ;. body externally ringed, and bearing semi-circular dermal scales. Frequently a protrusible tentacle in a tentacular sheath between 288 CLASSIFICATION OF VERTEBRATES. the orbit and nostril. The caecilians are tropical, occuring in South America, Africa, and Ceylon, where they burrow in the earth, preying upon small invertebrates. The eyes, in consequence of this life, are hidden under the skin. Little is known of the development, except of the Cey- lonese species, Ichthyophis glutinosus, in which the larva has three pairs of pectin- ate external gills. In the larval TypJiIo- nectcs the gills are saccular. Other genera are C&cilia, Rhinotrema, and Hypogeophis. No fossil species are known, but the dis- tribution as well as the characters of the skeleton point to a great ancestry for the group. Within recent years it has been supposed to be related to Amphinma, but this is clearly not the case. The aisto- poda (p. 283) suggest themselves in this connection. FIG. 287. Tentacle of Cecilia oxyura, after \Viedersheim. do, duct of orbital gland; a large sac (which re- ceives the excretion of the primitive kidneys), the outer surface of the sac fusing with the chorion. The result of this, in ovipa- rous forms, is that the allantoic structures come to lie close FIG. 289. Diagram of embryonic circulation in an amniote, the amnion omitted for clearness. A, allantois ; A A, allantoic artery; C, carotids ; CA, caudal artery; CV, caudal vein; DA, dorsal aorta ; DC, ductus Curvierii; H, heart; HA, hypogastric artery; L, liver; OA, OR, omphalomesaraic artery and vein; UV, umbilical vein ; V, vent; VV, vitelline vein. The outline of the alimentary canal blocked. beneath the shell, and hence, with their rich blood-supply, they form efficient organs of foetal respiration. In the higher mam- mals this allantois enters into close connection with the uterine walls, thus giving rise to a structure both nutrient and indirectly respiratory in character, the placenta, the features of which will be described in connection with that group. Basi- and presphenoid bones are present, and a parasphenoid occurs only in some reptiles (ophidia) as a small plate. The SAUROPSIDA, 2 9 I ribs are developed in connection with the transverse processes, and the skeleton of the limbs (when present) is reducible to the pentadactyl type. A sternum is present except in some apodal forms, and is developed in connection with the ribs ; and the branchial arches are much reduced and modified. Gill pouches occur, and some of these may break through to the ex- terior; but in no case are gills devel- oped in connection with them, and they never serve in connection with respiration. The alimentary canal either terminates in a cloaca, or the vent is behind the urogenital open- ings. The heart always has two au- ricles, the sinus venosus becoming FlG 2QO Human embryo included in the right of these, while with the floor of the mouth the ventricle, either partially or com- removed, after Hertwig. b, , , ,. . , , . , . ,. , branchial clefts: s, cervical pletely divided by a longitudinal sep- sinus; ^ eye; ^ hypophysia i turn, is at least physiologically divided pocket; /, lungs; n, nostril, into arterial and venous halves. In the adult true kidneys (metanephros) are developed, the renal portal system is reduced or lacking in the higher forms, and the posterior cardinals become greatly reduced (p. 196). The amniotes are divided into the Sauropsida and the Mammalia. CLASS I. SAUROPSIDA (MONOCONDYLIA). Amniote vertebrates with one occipital condyle ; lower jaw suspended by the free or fixed quadrate; ankle joint between the first and second rows of carpals or tarsals ; coracoid well developed ; external surface covered, at least in part, with ecto- dermic scales ; corpus callosum rudimentary ; heart three or four chambered, red blood corpuscles small, oval, nucleated ; a cloaca present ; the eggs are large, and undergo a partial (meroblastic) segmentation ; all except a few forms are oviparous, and the eggs are enclosed in a more or less calcareous shell. Besides the features of the diagnosis, several other points are CLASSIFICATION OF VERTEBRATES. characteristic of the sauropsida. The skin is remarkably defi- cient in glands, these, when present, usually occurring upon the legs or upon the tail. The characteristic scales are cornifica- tions of the epidermis, and are occasionally re-enforced by bony plates developed in the dermis. The single occipital condyle is situated on the basioccipital, the exoccipitals contributing to its pao. FIG. 291. Base of skull of alligator, showing the single occipital condyle. bo, basioccipital ; l>s, basisphenoid ; eo, exoccipital ; et, opening of Eustachian tube ; fm t foramen magnum ; pao, paroccipital ; //, pterygoid ; q, quadrate ; qj,. quadratojugal ; sq, squamosal ; tr, transversum. formation to a varying extent. The mandible consists of a single cartilage bone, the articulare, and at most five membrane bones, dentary, splenial, coronoid, angulare, and surangulare. The cervical ribs are usually well developed, the neck passing insen- sibly into the thorax. The ovarian ducts have their inner ends entire as in the ichthyopsida. The sauropsida contains the Reptilia and the Aves. SUB-CLASS I. REPTILIA. Cold-blooded amniotes ; the external surface of the body (except in a few fossil forms) covered with horny epidermal, scales or bony dermal plates ; anterior appendages, when present, ambulatory (except in pterodactyls), the carpals and meta- carpals numerous ; sacral vertebrae usually two ; pubic and is- chiadic bones united by symphysis, except in some dinosaurs ; persisting right and left aortic arches. REPTILES. 293 The living reptiles in their external form present three types: (i), the quadrupedal long-tailed form represented by the lizards and alligators ; (2), the cuirassed forms of the turtles ; and (3), the apodal forms of the snakes and footless lizards. If the fossil groups also be taken into consideration the range of shape is still greater ; for it includes not only the swimming- groups, the plesiosaurs and ichthyosaurus, but the flying reptiles, the pterodactyls. A few of the fossil forms apparently had naked skins ; but in the rest the body is more or less completely covered by scales, which differ from those of the ichthyopsida, in that they are cornifications of the superficial layers of the epidermis. These are re-enforced in many by dermal ossifications, which may be minute as in certain lizards, or larger scutes, as in the croco- diles and in many extinct groups ; whereas in some fossil croc- odiles (Teleosaurus) and dinosaurs, they form a complete armor for the body. In the turtles this formation of armor reaches its extreme, for here the dermal plates are usually united with the ribs to form a firm carapace and plastron. Usually there is no pigment in the epidermis ; but the derma contains pigment cells, which in certain lizards (Anolis, Chameleo) are capable of proclu- F,c. 292. Section and medial view of jaw of Anotts, showing pleurodont cmg marked color changes under dentition. control of the nervous system. Epidermal glands are rare. In some turtles scent glands occur beneath the mandibles or on the side of the plastron ; in the snakes and crocodiles similar glands are connected with the cloaca ; while in most lizards there is a row of glands on the ventral surface of the femur. Teeth (lacking in turtles and some .pterodactyls and anomo- dontia) are usually restricted to the premaxillary, maxillary, and dentary bones ; but in snakes and some lizards they may also occur upon the palatines and pterygoids. These teeth are usually simple, without folding of enamel, and only in the therio- dontia are they differentiated into incisors, canines, and molars. 294 CLASSIFICATION OF VERTEBRATES. In the majority of reptiles the teeth are either anchylosed to the edge of the jaws (acrodont), or by their sides to the wall of a groove (pleurodont) , while in crocodiles and many dinosaurs they are implanted in sockets or alveoli (thecadont) ; usually the teeth are in a single row. In the snakes the teeth are grooved, and in the poisonous species the grooves in one pair may be very deep or completely converted into a canal, which is to convey the poison into the wound made by these fangs. As a rule the teeth are used for the prehension of the prey, and only in the herbivorous orthopoda are they of value in the comminution of food. In the turtles, and apparently in the extinct edentulous forms, the jaws are covered with an epider- mal horny beak. Salivary glands are lacking in the marine chelonians and in the alligator, while in the crocodiles they occur only on the tongue. In other reptiles lingual, sub-lingual, palatine, and labial glands may occur, the poison glands of ophidians being modified labials. The tongue is either spatulate and immobile, as in crocodiles, turtles, and a few lizards, or bifid and exten- sile in other forms ; its variations of shape being of value in the classification of the lacertilia. In the alimentary canal the most noticeable features are the wide oesophagus, correlated with the swallowing of the food entire, and the large intestine, frequently provided with a caecum near the ileo-colic valve. In the turtles the oesophagus is armed with numerous papillae pointing backward. The liver is usually bilobed, but in the snakes and snake-like amphisbae- nians it is unilobular and elongate. At no time is there a branchial respiration, the lungs being the sole organs of exchange of gases. 1 The glottis is supported by well-developed cricoids and arytenoids ; the trachea is long, and in crocodiles and turtles may be bent into a loop. The tracheal and bronchial rings are better developed than in the amphibia. The lungs show variations in shape and size ; and in the elongate reptiles the left lung is the smaller, and may even be reduced to a rudiment (snakes). In these forms the 1 Experiments go to show that the pharyngeal epithelium of certain North American and Australian turtles has a respiratory function. REPTILES. 295 posterior dorsal portion or the right lung is supplied with blood from the dorsal aorta. In the chameleons and geckoes the lungs give off large saccular projections, recalling the air sacs so characteristic of birds. In some dinosaurs the bones exhibit a marked pneumaticity, and it is- supposed that in these the air sacs penetrated the bones. In the snakes the chambering of the lung is restricted to the peripheral portion, the centre being occupied by a large air space, and about the same conditions occur in most lizards. In the chameleons, however, each bron- chus, on entering the lungs, divides into three parts, and the proximal portion of the lung is sacculated, while distally all three bronchi connect with a common space, without alveoli. In crocodiles and chelonians the sub-division of the lungs is carried farther. The brain presents advances in several points upon the con- ditions in the amphibia. Thus there is here developed a cere- bral cortex of gray matter containing pyramidal cells. The cerebrum exhibits a tendency to ex- tend backwards, covering in the thalamencepha- lon. The olfactory lobes may be seated directly on the cerebrum, or an elongate olfactory tract may intervene. The olfactory fibres do not ex- tend back to the corpus striatum, but a distinct / olfactory centre is developed in the pallium. / . Hippocampal lobes occur in a few forms {Hat- M teria, crocodiles, chelonians). The twixt brain is at a lower level than the rest, the infundib- ular region being well developed. The mid brain is large, and its two halves rarely exhibit a tendency towards division into four. In the cerebellum there is a great range of structure, from forms in which it is merely a transverse i fold, up to the crocodiles, where it consists of Brain of garter- two lateral lobes and a median portion, recall- snake, Eutainia ing the vermis of the mammals. In the medulla occurs the characteristic nuchal flexure. In the cranial nerves the marked feature is the distinct ori- gin of nerves, the roots of which are closely approximate in the 296 CLASSIFICATION OF VERTEBRATES. amphibia ; thus the facialis is distinct from the trigeminal ; the eye-muscle nerves have distinct roots ; the glossopharyngeal is distinct from the vagus ; the accessorius is a distinct nerve, except in ophidia, and the hypoglossal becomes a cranial nerve, passing through a foramen in the cranial wall. The nostrils are usually terminal, but are just in front of the orbits in ichthyosaurs and plesiosaurs. In the lizards the nasal passage is divided into an anterior vestibule and a posterior olfactory region, and in these, as in the ophidia, the amount of olfactory surface is increased by the presence of a turbinal bone. In the turtles, and still more in the crocodiles, the nasal pas- sage is divided horizontally into an upper olfactory and a lower respiratory tract. Glands occur in connection with the nose in most reptiles, while in all except crocodiles and turtles an organ of Jacobson occurs. The eyeball is nearly spherical ; the sclerotic which sur- rounds it is cartilaginous, and in it are frequently developed (as in many birds) a ring of bony sclerotic plates. A tapetum is developed in the lacertilia, but the argentea, so characteristic of lower vertebrates, is lacking. In many there is an internal structure, the pecten, homologous with the process falciformis of the fishes. Eyelids are usually present, the third being fre- quently developed. In snakes and some lizards the lids are transparent, and their edges are united together so that a lachry- mal space is enclosed between them and the conjunctiva. In many lizards and in Hatteria the parietal eye (Figs. 90 and 92) is extremely well developed, and is situated in a foramen in the roof of the skull. Many fossil reptiles belonging to different orders have a similar parietal foramen, thus suggesting the for- mer presence of a visual organ in these forms. In the inner ear the lagena is large, and in the crocodiles shows the beginnings of a spiral coiling, recalling the cochlea of the mammals. With its increase in length the macula lagenae is correspondingly elongated. The middle ear and Eustachian tube are lacking in adult snakes and amphisbaenans, the colu- mella auris in these forms being embedded in the flesh. The stapes is continuous with the columella, and in rhynchocephalia the columella is connected with the hyoid. In many lizards and REPTILES. 297 chelonians the tympanic membrane is exposed ; in some lizards it is partially covered by a flap developed from in front, while in the crocodiles the flap is movable and the tympanum is some- what sunken, the beginnings of the auditory meatus of the higher vertebrates. In the skeleton the ossifications are far more extensive than *H the amphibia. The notochord does not persist, except inter- vertebrally in a few forms (geckoes and rhynchocephalia). The vertebrae are usually precocious ; but amphicoelous vertebrae occur in some or all theromorpha, ichthyosauria, plesiosaurs, rhyncho- cephalia, geckoes, theropoda, orthopoda, and ornithopoda, while in a few dinosaurs they are flat (amphiplatyan). In many groups the neural arches are anchylosed to the centra, or again, as in ichthyosaurs, turtles, and crocodiles, they are united by suture. Haemal arches occur in snakes, lizards, and crocodiles. Trans- verse processes, when present, are borne on the neural arch (i.e., are diapophyses). At most five regions can be distinguished in the column ;. but in the snakes, where no limbs are formed, only trunk and caudal vertebrae can be distinguished. In the plesiosaurs axis- and atlas are fused ; the proatlas of the crocodilia has beert referred to (p. 143). Usually there are two sacral vertebrae. Ribs are usually present, and may be either with a single head or bicipital. In the snakes they may extend the whole length of the trunk with the exception of the atlas. In the crocodilia and Hatteria thoracic and abdominal ribs are dis- tinguished, the latter developing in the myocommata of the ventral surface, and not extending to the vertebrae (see p. 147). Cervical ribs are entirely lacking in the turtles, while in the same group the thoracic ribs are united to the dermal plates forming the carapace. A sternum is lacking in plesiosaurs, ichthyosaurs, turtles,, snakes, and some snake-like lizards, while there is little evidence as to its structure in the theromorphs and dinosaurs, where it was apparently largely cartilaginous. When present it is tri- angular or rhomboidal in outline, and contains no membrane bone. In the flying reptiles (pterodactyls) it had a strong ven- tral keel for attachment of the wing muscles. The episternum .298 CLASSIFICATION OF VERTEBRATES. .(lacking in chameleons) is usually well developed, and affords a support for the ventral ends of the clavicles. In many turtles it, together with the clavicles, enters into the formation of the plastron. The skull of recent reptiles differs in many respects from that of existing am- phibia ; but when the fossil groups are con- sidered, the distinctions largely disappear, the skulls of stegocephalans and thero- morphs being strikingly similar. In these lower reptiles the top of the skull forms a continuous roof above the attachment of the jaw muscles ; but in other groups gaps or vacuities may occur, so that these mus- cles are exposed from above. These va- cuities or fossae exhibit the following vari- ations : (i), between the parietals and postorbitals (supraternporal fossa) ; (2), between postorbitals and squamoso-jugal (infratemporal fossa) ; (3), between the post-temporal and the exoccipital and op- isthotic (post-temporal fossa) ; (4), the line of bones (arcade) between i and 2 may be interrupted, producing one large temporal fossa; (5), the squamosojugal arcade may be discon- tinuous. As a rule the cartilage of the primordial cranium is largely replaced by bone, the ethmoid and parts of the sphenoid alone being incompletely ossified. Except in a few theromorphs there is but a single occipital condyle, which is either formed by the basioccipital alone or with the participation of the exoccipitals. Either basi- or supraoccipitals may be excluded from the for- amen magnum. In the ear region a fenestra rotunda appears ; of the otic bones the prootic is always distinct, the epiotic is fused with the supraoccipital, while the opisthotics (free in turtles) are usually united to the exoccipitals. While in some the brain extends forwards between the orbits, it frequently does not reach so far forward, and the orbits them- FIG. 294. Pectoral girdle and sternum of lizard, L&manctus lon- gipes, after Parker, c, coracoid ; cl, clavicle ; .*?, epis'ternum ; g, glenoid iossa ; /, procoracoid ; ^', rib ; s, scapula ; st, sternum ; x, xiphister- num. Cartilage dotted. REPTILES. 299 selves are separated by a more or less complete interorbital sep- tum. Correlated with this is the frequent absence of all- and orbitosphenoid ossifications, their places being taken by vertical processes of parietals (turtles) or frontoparietals (snakes) ; while the frontals frequently take no part in roofing in the cra- nial cavity, but are placed above the interorbital septum. The membrane bones of the cranium are numerous, the frontals and parietals of the two sides being frequently fused in the median line. Between the parietals in Hatteria and most FIG. 295. Lateral view of the skull of Hatteria (Sphenodon}, after Giinther. _//', frontal ; y, jugal ; /, lachrymal; true, maxillare ; n, nasal; oo, opisthotic ; pa, palatine ; pf, prefrontal ; pm, premaxillary ; po, postorbital ; pof, postfrontal ; //, pterygoid; q, quadrate; mesocoracoid ; msc, mesoscapula; sc, scapula; St, sternum; xst, xiphisternum. The hyoid and branchial arches are variously developed, but at no time have they gill-supporting functions. Frequently the first, or first and second, branchial arches are well developed, giving rise to long cornua attached to the well-developed copula. The pectoral girdle is developed in all reptiles even the limbless lizards with the exception of the ophidia. Scapula, coracoid, and precoracoid are almost always present, the latter lacking in ichthyosaurs, plesiosaurs, and dinosaurs, while in 1 The fixed or free condition of the quadrate has been employed in dividing the reptilia into monimostylica and streptostylica. REPTILES, 301 FlG. 297. Pelvis of Hatteria, after Wiedersheim. /o, obturator foramen ; i7, ilium ; ts, ischium ; /, pubis ; //', prepubic process. others, except theromorphs, they are represented by processes upon the coracoids. The scapula, except in chelonia, is ex- panded dorsally, while the coracoids are flattened, and either meet in the middle line as in the ichthyosaurs and plesiosaurs, or they may connect with the sternum. A clavicle is^usually present ; in the turtles it may enter into the composition of the plastron (Fig. 305). An epi- coracoid occurs in some lizards and turtles. A pelvis is more constant in appearance than is the shoulder girdle, vestiges of it (ischia) appearing in certain snakes. It is characterized by the great development of the ilium and by marked variations in the pubic bone, which in all except croco- diles and pterodactyls participates in the formation of the ace- tabulum, In many dinosaurs the pubis is differentiated into pre- and postpubic portions (Fig. 298) ; and traces of the pre- pubis may be recognized in many other groups, and also in birds, as anteriorly directed processes arising from the pubis. The fore and hind limbs are much alike in their general struc- ture, and distinctively reptilian features are most marked in the distal portions. In the lower rep- tiles, as in chelonians, the carpal bones are much as in amphibia ; but elsewhere there is a tendency to fusion, intermedium and cen- trales uniting with the radiale, while the carpales are similarly reduced in number by fusion. In the hind limbs much the same features can be seen, except that the tarsal bones can fuse to an even greater extent. In both carpus and tarsus there is a tendency for the proximal row to become closely united to the FIG. 298. Pelvis of Jguanodon, after Dollo. a, acetabulum ; ?7, ilium; is, ischium; po, postpubis; pr, prepubis. 302 CLASSIFICATION OF VERTEBRATES. radius and ulna or tibia and fibula, while the carpales and tar- sales in the same way become associated with the metacarpals or metatarsals, thus producing an intracarpal or intratarsal joint. The modifications of metacarpals, metatarsals, and phalanges are more varied ; and we may have walking-feet, as in most reptiles, swimming-feet or paddles, as in ichthyosaurs, plesiosaurs, pythonomorphs, and some turtles, or, as in the pterodactyls, the anterior pair may be modified into supports for the organs of flight. In the swimming-feet there is fre- quently a reduction in length of the proximal bones, while the number of phalanges may be indefinitely increased. The heart is farther removed from the head than in the ichthyopsida, and the sinus venosus becomes connected with the right au- ricle. Into the sinus empty the post- and the two precavae, except in the ophidia, where the left precava opens directly into the au- ricle. The greatest advance is seen in the development of a partial or complete (croco- diles) septum, dividing the ventricle into right and left halves. Even when the septum is incomplete the ventricle is actually divided in contraction, the right side containing only venous blood, while the left receives that re- turning from the lungs. Associated with the division of the ventricle is a corresponding division of the ventral aorta of the ichthy- opsida into three trunks, two connected with the right and one with the left ventricle. One of those arising from the right ventricle forms the pulmonary artery, blood passing through it, by means of the last aortic arches, to the lungs. The other right ventricular trunk con- nects by means of the fourth arch of the left side with the left aortic root. Thus, as will be seen, venous blood is forced from the right ventricle of the heart into the lungs and into the dorsal aorta. The aortic trunk arising from the left ventricle FIG. 299. Arte- rial trunks of turtle (Emys^y after Wied- ersheim. a, left aor- tic arch; b> bronch- us ; /, to fore limbs ; h, to hind limbs ; r, renals; s, to stomach. REPTILES. 303 connects by means of the fourth aortic arch of the right side with the right radix aortae, and also, by both of the third arches, with the carotids. This insures the supply of arterial blood to the brain, while a part of the same is carried to the dorsal aorta, which consequently contains both venous and arterial blood. In Lacerta and a few other forms the third arch of either side remains in connection with the radices aortae, but in all other groups this connection is lost. Among other peculiarities of the circulation are the per- sistence of a ductus Botalli (p. 187) in some chelonians and crocodilia, and the varying position of the origin of the sub- clavians, which may arise either from the third (carotid arch) of either side, or from the right radix aortae. Subclavians are lacking in the ophidia. A renal portal system occurs in all except the chelonia, and in chelonia there are two hypogastric veins ; in lacertilia and ophidia but one. In the latter group the hypogastric breaks up into a plexus connected with the 'fat body ; ' passing thence to the portal vein. The permanent kidneys of the adult reptile are the meta- nephridia ; they are usually small, compact, or tabulated, but in snakes the lobulation may be carried so far that the lobes are connected only by the ureter. In lizards the metanephridia of the two sides are sometimes united behind. The mesonephros; and the Wolffian duct are more or less degenerate, never func- tioning in the adult. Their remains are more evident in the female than in the male, the mesonephros forming the so-called 'golden yellow body.' A urinary (allantoic) bladder is con- nected with the cloaca in turtles and lizards, but in other rep- tiles it is lacking. The gonads vary in shape with the shape of the body, being broad in the chelonia, long in others. In many forms, and this is especially true of the ophidia, the right gonad is larger and in advance of the left. The ovaries are penetrated with a vas- cular network of connective tissue. The oviducts are long, folded or contorted, and have smooth margined ostia. The ducts themselves are muscular and glandular, the glandular portion secreting the shell. Accessory reproductive organs of two types occur. In liz- 304 CLASSIFICATION OF VERTEBRATES. .ards and snakes there are a pair* of eversible sacs (hemipenes) opening into the cloaca, and when in repose retracted under the skin of the tail. In chelonians and crocodiles there is but a single penis, formed by a thickened portion of the ventral wall of the cloaca, which is composed of erectile tissue, and can be protruded from the vent. Both types are grooved for transmis- sion of the seminal fluid. The hemipenes of embryo snakes have often been described as rudimentary hind limbs. Hatteria lacks a penis. The eggs of reptiles are large and undergo a partial (mero- blastic) segmentation ; the subsequent phases of development being much like that of birds. Most reptiles are oviparous, the eggs being deposited in sand or soil, and left to hatch by the heat of the sun. Some lizards and many snakes, however, are viviparous. The following classification of the Reptilia follows most closely that of Lyddeker. The late Professor Cope recognized several more orders, which seem to be but sub-divisions of the theromorpha. ORDER I. THEROMORPHA. Extinct reptiles, with amphicoelous vertebrae, the notochord frequently persisting intervertebrally ; with a sacrum composed of from two to six vertebrae ; ribs bicipital, their articulation with the vertebrae as in mammals ; quadrate immovable ; teeth in alveoli, and showing much differentiation (occasionally teeth are lacking) ; no sternum ; girdles solid, the pubic and ischiatic bones fused into a continuous os innominatum ; humerus with a foramen (entepicondylar) above the inner condyle. The theriomorphs were mostly terrestrial vertebrates, and are especially interesting, since they show features which make many regard them as having been the ancestors of the mammalia. The order appears in the Permian, and dies out in the triassic. SUB-ORDER i. PAREIASAURIA (COTYLOSAURIA). Teeth homodont, numerous, without diastema; no temporal fossa; one occipital condyle ; vertebra with remains of notochord; two sacral vertebrae. Pareiasaiirus, South African Permian ; Empedias, Permian of REPTILES. 305 Texas ; Elginia, triassic of Scotland. Isodectes, from the coal of Ohio, is the oldest known reptile. This sub-order is regarded by Cope as ancestral to all other reptiles. FlG. 300. Pareiasaurus baini, after Seeley. SUB-ORDER 2. ANOMODONTIA. Large lizard-like, five-toed reptiles ; toothless, or with a single pair of canine-like teeth ; no intercentra ; five or six sacral vertebras ; a single occipital condyle ; supratemporal fossa present. Dicynodon and Ouden- odon, South African Permo-trias. FIG. 301. Skull of Dicynodon, after Seeley. bo, basioccipital ; c, columella; f, frontal ; in, infranasal ; j, jugal ; /, lachrymaj ; mx, maxilla ; n, nasal ; o, orbit ; /, parietal; pf, postfrontal ; //, palatine; pm, premaxilla; pr> prefrontal ; pt t pterygoid ; q, quadrate ; s, squamosal ; /, temporal fossa ; v, vomer. 306 CLASSIFICATION OF VERTEBRATES. SUB-ORDER 3. PLACODONTIA. Palatine teeth large, pavement-like ; premaxilla with incisors, maxilla with rounded molars ; lower jaw with incisors and pavement teeth ; one oc- cipital condyle. The rest of the skeleton is unknown. Placodus, European trias. SUB-ORDER 4. THERIODONTIA (PELYCOSAURIA). Teeth differentiated into incisors, canines, and molars ; intercentra fre- quently present ; supra- and infratemporal fossae developed ; two or three sacral vertebrae ; carnivorous. Clepsydrops, Permian of Texas and Illinois; Dimetrodon and Naosaurus from the Permian of Texas, both with enormous spinous processes ; in the latter these bear several transverse bars ; Gale- saurus, trias of Africa. ORDER II. PLESIOSAURIA (SAUROPTERYGII). Extinct aquatic reptiles, apparently with naked skin ; the tail short, the neck long ; a single occipital condyle ; temporal fossa present ; teeth in alveoli, quadrate immovable ; anterior nares separate, near orbit ; a parasphenoid sometimes pres- ent ; no sclerotic ring in orbit ; vertebrae amphiccelous or flat ; ribs with a single head ; abdominal ribs present ; sternum and FIG. 302. Restoration of Plesiosaurus, after Dames. precoracoid absent, the coracoids meeting in the middle line ; feet pentadactyl and usually modified into swimming-organs. The plesiosaurs were large carnivorous reptiles, sometimes reaching a length of forty feet. In Nothosaurus and Lario- saurus the feet were fitted for creeping, and the animal was lizard-like ; triassic of Europe. In Plesiosaurus the limbs were flipper-like, the phalanges being greatly increased in number, while the neck was extremely long. Allied genera are Cimolio- REPTILES. 307 saurus and Pliosaurus from the Jurassic and cretaceous of Europe, America, and New Zealand. ORDER III. CHELONIA (TESTUDINATA). Recent and fossil reptilia in which the trunk is enclosed in a bony framework, composed of a dorsal carapace and a ven- tral plastron, these parts of dermal and partly of endoskeletal origin ; the quadrate is fixed ; teeth are lacking, the jaws being covered with a horny sheath. The anterior bony nares are FIG. 303. Pectoral girdle of Plesiosaurus, after Zittel. c, coracoid; cl, clavicle ; *?, episternum ; s, scapula. FIG. 304. Dorsal view of carapace of green turtle, Chelone midas, showing the ribs, RI extending beyond the costal plates, C. M, marginal plates; Ntt y nuchal; Py, pygal plates. From Huxley. united, and open at the tip of the snout. A temporal fossa is frequently present. The scapular and pelvic arches are internal to the ribs. The feet have five digits and, while usually fitted for walking and provided with claws, are occasionally modified into flippers. The turtles and tortoises are strongly marked off from all other reptiles, the armor surrounding the body being especially characteristic. In most cases head, legs, and tail can be re- tracted into this, and in the box tortoises the plastron is hinged 308 CLASSIFICATION OF VERTEBRATES. so that it can still further protect these parts. In the atheca the body is covered by a thick leathery skin ; but in the others it bears bony epidermal scales or plates, the arrangement of which is of systematic importance. These plates in one species (Eretmoclielys imbricata) furnish the well-known ' tortoise shell.' There is a median and a pair of lateral rows of plates on the dorsal surface, while around the edge is a series of margi- nal plates. Beneath these scales comes the dermal skeleton of bony plates ,which, however, do not correspond in position to the epidermal coating. In the atheca this dermal skeleton is free from the ribs and vertebrae, and consists of longitudinal rows of polygonal dermal ossicles. In all others the dorsal portion of the armor, the carapace, consists of a median row of (usually eight) neural plates, each being the expanded end of a neural spine of a vertebra. In front of the first neural is a nuchal plate, while be- hind the last are two or three pygal FIG. 305. " Plastron of plates, these being unconnected with Chelone midas, after Zittel. the vertebrae. On either side and cor- < clavicle; ,, episternum; ndi to the neurals are the CQstal ///, hypoplastron ; hy, hyo- plastron; .r, xiphopiastron. plates, each fused to a rib; around the margin of the carapax is a series of marginal plates, the nuchal and posterior pygal forming parts of the series. The ventral portion of the armor, the plastron, usually consists of nine plates, in front a median episternum (entoplastron) flanked on either side by a clavicle (epiplastron), while behind, on either side, follow hyoplastron, hypoplastron, and xiphiplastron. Occasionally the episterum is lacking. All of these plates, except neurals and costals, are membrane bones. Besides the characters quoted in the diagnosis, the absence of ali-, pre-, or orbitospheneid ossifications ; the distinct pro- and opisthotic bones ; and the absence of an os transversum, are distinctive. The epiotic is fused to the supraoccipital ; a tem- poral fossa is usually present, but as in chelydosauria, it may be absent, or again, as in Chelone, it may be arched over by an expansion of the parietal reaching to the squamosal. REPTILES. 309 The vertebrae are mostly procoelous, but some of them may have plain faces. There are two sacral vertebrae. The position of the girdles inside the ribs is secondary, and is produced during growth by the forward and backward ex- tension of the carapace. The ribs have but a single head, and extend into the caudal region (Fig. 155^). The procoracoid is fused to the scapula, the carpus is primitive, but the tarsus is modified by fusion of its ossicles. Five digits always occur, but the number of phalanges is not constant. The brain has large hemispheres which cover the twixt and partly the mid brain ; the cerebellum is a slightly arcuate trans- verse fold. The facialis and acusticus nerves are united at their origin. Both Harderian and lach- rymal glands occur, the latter being at the poste- rior angle of the orbit. FIG. 306. Skull of turtle, Chrysemys picta. The tympanum is well de- ^' basiocci P ital ; / frontal; ". exoccipital; /, jugal; /, maxillary; /, naso-prefrontal ; veloped, the membrane is /, parietal; //, postfrontal; //, pterygoid; q, visible externally, and the quadrate; s, squamosal; so, supraoccipital. Eustachian tube is large. The ventricular septum is poorly developed ; the third aortic arches are not connected with the radices aortae, and the left radix gives off the cceliac artery before joining with its fellow. A renal portal system is lacking, the caudal vein connecting with the epigastrics. The sexual and urinary ducts empty into the neck of the urinary bladder. The penis is an unpaired structure arising from the dorsal wall of the cloaca, and in it are two canalicular extensions of the coelom, which open on two papillae to the exterior. The eggs are covered with a leathery calcareous shell, and are buried in the sand, being hatched by the heat of the sun. Some of the chelonia are herbivorous, some feed on insects, molluscs, etc., and some are strictly carnivorous. All are rather slow in their motions ; and the group is best represented in the tropics, the colder temperate regions having but few species. In cold climates the species undergo a hibernation, and in the 310 CLASSIFICATION OF VERTEBRATES. tropics the terrestrial species sleep through the dry season The group appears in the Permian of North America, and has continued until the present. % SUB-ORDER i. CHELYDOSAURIA. No temporal fossa ; carapace of transverse osseous arches in close con- tact, extending across the back from side to side. Vertebrae amphicoalous ; limbs ambulatory. This sub-order, represented by Otoccelns from the Per- mian of North America, is regarded by Cope as ancestral to the other chelonia and the pseudosuchian crocodilia. SUB-ORDER 2. ATHEC^E. Turtles without scales but with a leathery skin, carapace of polygonal dermal bony plates arranged in rows, unconnected with ribs and vertebrae ; plastron poorly developed, with large central fontanelle ; episternum lacking. Skull without descending process df parietals. Feet flipper-like, claws lack- ing. Dermochelys (Sphargis) coriacea, the leather-back tortoise, occurs in all warmer seas, extending north to Cape Cod. It weighs occasionally 1,500 Ibs. The sub-order appears (Psephoderma) in the trias. Protostega, cretaceous of Kansas. SUB-ORDER 3. TRIONYCHIA. Turtles with the carapace poorly ossified, ribs and vertebrae being connected with it. Scales lacking, the body covered with a leathery skin ; marginal bones few or absent. Plastron with episternum and a large me- dian fontanelle ; sacral and caudal ribs articulating with neural arches. A descending process of the parietals present. Feet webbed, three claws on each foot. The sub-order appears in the upper cretaceous of New Jersey, and is represented by over thirty species to-day, all inhabitants of fresh water, and best developed in the Oriental regions. All are carnivor- ous. Four species of leather turtle (A my da) and soft-shell turtles (As- pidonectes} in the U. S. SUB-ORDER 4. CRYPTODIRA. Turtles with well-ossified carapace, connected with internal skeleton ; epidermal scales and marginal ossicles present ; an episternum ; pelvis free from plastron ; caudal ribs articulated to vertebral centra. A descending process to the parietals. The species are numerous, the more important families being the following: CHELONID^E, with heart-shaped carapace, and paddle-like feet, bearing at most two claws. The costal plates do not reach the marginals. Thalassochelys caretta, the loggerhead turtle, weighs 450 Ibs. Eretmochelys imbricata, the tortoise-shell turtle, is smaller. The green turtle, Chelone my das, may weigh 850 Ibs. It is highly esteemed as REPTILES. 3 I I food. All of these occur in the warmer Atlantic, the tortoise-shell ranging to the Indian Ocean, and all occasionally occur on our shores. TESTUDI- NID;, carapace strongly arched; plastron very broad; five toes in front, four behind. Terrestrial, represented in southern U. S. by the gopher turtle, Xerobates polyphemus. Here also belong the giant tortoises (Tes- tudo elephantopus, etc.) of the Galapagos Islands and Mozambique, and the colossal fossil, Colossochelys atlas, of the upper miocene of India, which was 18-20 feet long, the carapace being 8 feet high. KINOSTER- FiG. 307. Snapping-turtle, Chelydra serpentina, from Huxley. with free toes, short tail; 9 or 1 1 plates on plastron, with our mud- turtle (Kinosternon pennsylvanicuiri) and our musk-turtles (Aromochelys}. EMYD.E with 12 plates on plastron, including about 80 species, among them the wood and spotted tortoises (Chelopus], the painted turtle (Chrys- ernys}, the box-turtles (Cistudo) with hinge in plastron, and the various terrapins, including the famous 'diamond back' (Malacleinmys pahistris}. CHELYDRID^E with long tail, and 9 plates on the plastron. Embraces three species, two, the snapping-turtle (Chelydra serpentind} and the alligator snapper (Macrochelys lacertina] being the fiercest of reptiles inhabiting the U. S. The Cryptodira are found in all ages from the Jurassic down. SUB-ORDER 5. PLEURODIRA. Turtles with epidermal scales ; carapace united to skeleton ; marginals present ; caudal ribs articulated to centrum ; descending process of parie- tals present; neck bending horizontally; pelvis anchylosed to carapace and plastron; plastron always with 13 epidermal plates. 1 Contains over 50 species confined to the southern hemisphere, mostly South American, among them Podocnemis, Chelys, Pelomedusa. Sternothcerus is African. While the living forms are very distinct, the fossils show intergradations between the Cryptodira and Pleurodira. Proganochelys, triassic of Germany ; Both- remys, upper cretaceous of New Jersey. 1 Except in Carettochelydae, in which epidermal plates are lacking. 312 CLASSIFICATION OF VERTEBRATES. ORDER IV. ICHTHYOSAURIA (ICHTHYOPTERYGIA). Extinct aquatic reptiles with naked skin, large head, short neck, long bilobed tail and flipper-like appendages. Amphi- coelous vertebrae ; no sacrum ; vertebral column extending into lower lobe of tail ; no sternum ; ribs bicipital, abdominal ribs present; quadrate immovable ; jaws long and pointed, the upper jaw composed chiefly of premaxillae ; teeth usually numerous (absent in Raptaiwdori) and seated in a common groove. FIG. 308. Skull of Ichthyosaurus, after Zittel. a, angulare; d, dentary; j, jugal ; /, lachrymal ; 7/uc, maxillary ; , nostril ; na, nasal ; pa, parietal ; pf t post- frontal ; pnt, premaxillary ; po, postorbital ; p, prefrontal ; qj, quadratojugal ; s, squamosal ; sa, supraangulare ; st, stapes. The neural arches of the vertebrae are united by suture to the centra ; the caudal vertebrae have chevron bones ; supra- temporal fossa and parietal foramen are present. The orbits are very large, and contain a ring of sclerotic bones ; the exter- nal nostrils are just in front of the orbits. The prefrontals are as large as, or larger than, the frontals ; the pterygoids extend forward between the palatines to the vomers, and a large para- sphenoid is present. The coracoids meet in the middle line ; procoracoids are lacking. The pelvis is entirely free from the vertebral column, and its elements are reduced. The limbs are very short and paddle-shaped, the radius, ulna, tibia, and fibula being reduced to polygonal bones, distinguishable only by position from the metapodial elements. The digits are usually five, but this num- ber is sometimes apparently increased either by fission or by formation of marginal rows ; the phalanges are very numerous. REPTILES. 3 1 3 Ichthyosaurian coprolites (Fig. 41) show that these animals pos- sessed a spiral valve, while the finding of embryos within the fossil skeleton shows that at least some species were vivipa- rous. Ichthyosaurians were widely distribut- ed, fossils having been FlG> 309> Restoration of Ichthyosaurus, found in all parts of after Fraas. the globe except South America. In time they ranged from the upper triassic to the upper cretaceous. Some reached a length of 30 or 40 feet. Over 50 species have been described. Ichthyosaurus, Mixosaurus, and Baptanodon (Jurassic of Wyoming) are the best known. ORDER V. RHYNCHOCEPHALIA. Lizard-like, scaly reptiles with long tail ; amphicoelous verte- brae with frequent intercentra ; ribs one-headend, with uncinate processes ; abdominal ribs ; sternum and episternum present ; two sacral vertebrae ; quadrate immovable ; supra- and infratem- poral fossae present ; no procoracoid ; limbs pentadactyl, vent transverse ; heart, lungs, and brain as in lacertilia. This order, is represented to-day by but a single living species, Sphcnodon (^Hatteria) punctata, from the New Zealand region. While in general appearance it is lizard-like, it differs much from them in structure, and finds its nearest relatives in fossil forms which range from the trias to the present time. From the fact that all the remaining groups of reptiles have probably sprung from a rhynchocephalian ancestry, the order becomes very important, despite its small size. The vertebrae are usually amphicoelous, and remains of the notochord occasionally persist intervertebrally. Sometimes they are flat, and in Proterosaurus the cervicals are opisthocoelous. Intercentra occur in the caudal and cervical regions, and occa- sionally in the region of the trunk. A proatlas (p. 1 43) occurs. The premaxillae are never anchylosed ; the jaws bear acrodont 3 14 CLASSIFICATION OF VERTEBRATES. \ teeth or are toothless, and occasionally teeth occur on the ossi- fied palatines (Fig. 171). The feet are either fitted for walking or for swimming. SUB-ORDER i. SPHENODONTINA. Small terrestrial forms with amphicoelous vertebrae. Here belongs the living Sphenodon. The fossil forms, Hom after c P e < see F[ %' or pleurodont ; cerebellum very small, optic lobes approximate ; ventricles of heart incompletely sep- arated ; vent a transverse slit ; two hemipenes. Lizards and snakes are frequently regarded as constituting two distinct orders ; but in spite of the absence of feet and some other characters, the two groups (together with the ex- tinct pythonomorphs) have so many points in common that the order here recognized is justified. The body is covered with horny epidermal scales, and frequently these are re-enforced by dermal ossifications. In only rare instances are the vertebrae amphiccelous. The nasal apertures in the skull are separate ; the lungs are simple sacs ; limbs, when present, are ambulatory or natatory. FIG. 311. Skull of Hadro- CLASSIFICA TION OF VER TEBRA TES. fr FIG. 312. Side and sectional veins of skull of Cyclodus, from Huxley. Ar, articulare; BS, basisphenoid ; BO, basioccipital ; Co, columella; D, dentary; EO, exoccipital; Fr, frontal; EpO, epiotic ; Ju, jugal ; MX, maxillary ; Na, nasal; Pa, parietal; Pf, postfrontal ; Pmx, premaxillary ; PI, palatine; Pt, pterygoid ; PrO, prootic; OpO, opisthotic ; Prf, pref rental ; Qu, quadrate; SO, supraoccipital ; Sq, squamosal ; Tr, transversum ; Vo, vomer ; V, VII, passages for fifth and seventh nerves. SUB-ORDER i. LACERTILIA (SAURII). Scaled or plated reptiles usually with two pairs of feet; vertebrae rarely amphiccelous ; premaxilfa single or paired. Postorbital arcade some- times entire, jugal arch never complete. Ali- and orbitosphenoids not ossified ; shoulder girdle always present. Sternum and episternum usually present. Feet sometimes rudimentary or absent; when present, usually five-toed and ambulatory ; the -friaxilla, palatines, and pterygoids cannot move on the bones of the skull, and the mouth can be opened to but a moderate extent. Movable eyelids, tympanic cavity and membrane usu- ally occur. The arteries supplying the alimentary canal are extremely- variable. The lizards in their outward appearance resemble closely the crocodiles and Sphenodon, but in structure they have many and important points of difference. The apodal forms are strikingly snake-like ; but these may be distinguished in most cases at a glance by the presence of eyelids and small scales instead of broad abdominal scutes on the ventral surface of the body. The lizards are largely insectivorous, and only one has the repu- REPTILES. 319 tation of being poison. Most of them lay eggs enclosed in a leathery shell. Most of the 1,200 living species are confined to the warmer re- gions of the earth. The sub-order appears in cretaceous, but the fossil forms are few. A natural classification of the sub-order is still a desid- eratum. That adopted here, based primarily upon the tongue, associates together widely diverse forms. SECTION I. VERMILINGUIA. Old-world lizards with vermiform, highly extensile tongue ; tongue papillose, its enlarged tip sheathed ; body covered with small chagreen scales ; tail coiling vertically, and used as organ of prehension. No anal or femoral pores. Orbits closed behind by process of jugal ; teeth acrodont ; no teeth on palatines. Feet with the FIG. 313. Head of Chamelon with the tongue extended. toes in two groups. Only genus, Chameleon, with about 30 species. Trie chameleons are noted for their color-changes, a feature which is shared to a marked extent by the American genus Anolis (infra) . There are two pigment layers in the skin, an upper bright yellow and a deeper dark brown or black layer. The pigment cells in these layers are under control of the sympathetic system, and according as one or^he other becomes prominent the color of the animal changes. SECTION II. CRASSILINGUIA. Lizards with thick, short, fleshy tongue, usually rounded at the tip (never strongly emarginate), not protru- sible, papillose or smooth ; tympanic membrane usually free. ASCALA- BOTjE or geckoes have the feet with adhesive disks on the underside, and usually granular or spinose scales. Teeth pleurodont ; no teeth on pal- atines or pterygoids; a circular fold in place of eyelids. Vertebrae amphi- coelous. The geckoes receive their common name from their cry. They occur in all the warmer regions of the world except the northeastern part of the U. S. Possibly the group should be more strongly marked off from other forms. Phyllodactylus occurs in California. Other genera are Platy- dactylus, Ptychozoon, and Ascalabotes ; 200 species known. All are in- sectivorous and have great powers of climbing. IGUANID/E, lizards of considerable size, without adhesive feet ; body compressed ; limbs long and slender ; often a comb of spines on the back ; pleurodont teeth ; 3 20 CLASSIFICATION OF VERTEBRATES. teeth usually on pterygoids. Usually a large brightly colored sac beneath the throat connected with the hyoid. Over 300 species known, all but a few from the new world. Anolis includes the 'chameleon' (A caroli- nensis) of our southern states. Sce- leporus contains the common lizard or swift (S. undulatus) of the eastern states north to Connecticut and Mich- igan. The various species of ' horned toad ' belong to Fhrynosoma. The AGAMID^E replace the Iguanidae in the eastern hemisphere. One hundred and fifty species are known. These all have acrodont teeth. Chlaniydo- saurus includes the frilled lizard, C. kingii, of Australia, with a broad dermal fold or collar about the neck ; Draco volans of Java has the ribs greatly elongate, supporting a fold of skin which acts as a parachute. SECTION III. BREVILINGUIA. Tongue short, thick at base, no sheath ; tip smaller and more or less emarginate ; only slightly protrusible ; pleurodont dentition; feet often re- duced, two or none, the toes also fre- quently reduced in number; but in all cases pectoral and pelvic girdles are present. Over 400 species are known, but few of them inhabitants of the U. S. The SCINCID.E have a more or less snake-like body, covered with smooth bony scales ; tongue two-pointed. Eumeces, with teeth on the palate, contains our blue-tailed lizard (E. fasciatus} ; and our weak- legged ground lizard belongs to the genus Oligosoma. Scincus, with five toes, contains the true skinks. In Seps the toes are three in number. Scelotes has only hinder extremities, and in Anguis and Typhline legs are lacking. Cyclodus. The ZONURID^: may be recognized by a finely scaled groove along the side of the body. All except our ' glass snake,' Ophi- saurus ventralis, belong to the old world. This species, which is limbless, derives its common name from the brittleness of its tail. SECTION IV. FISSILINGUIA. Tongue long, slender, protrusible, its tip deeply split ; eyelids well developed ; tympanic membrane visible ; legs well developed. VARANID/E, pleurodont, tongue retractile into sheath ; Varanus (Monitor] contains about 30 old-world species. LACERTID^E, pleurodont, no tongue sheath; usually femoral pores; Lacerta, Tropido- FiG. 314. Green lizard, Anolis > from Lutken. REPTILES. 321 saurus. All the species belong to the old world. HELODERMID^E, pleu- rodont ; tongue with papillae at base ; no femoral pores. Heloderma, with two species, horridum and suspectum, from the Mexican region, contains the only poisonous lizards. TEID^E, acrodont, tongue two-pointed, covered with imbricate scales ; tympanic mem- brane visible ; usually two transverse folds on throat. Limbs present, rarely rudimentary. About 70 American spe- cies. Cnemidopkorus, with rounded tail, eyelids developed, small scales and large ventral plates, includes the six-striped lizard (C. sexli^eatus} of the eastern U.S. Tejus teguixin of Central Ameri- ca reaches a length of 6 or 7 feet. SECTION V. ANNULATA. Body covered by quadrangular scales, ar- ranged in rings around the body. Body vermiform, limbless, or with small fore limbs. Teeth acrodont or pleurodont, no palatine teeth ; tongue short, thick, non-protrusible ; eyelids and tympanic membrane lacking. About 50 species, half of them belonging to Amphisbcena. All the species tropical or subtropical ; they live burrowing in the earth, and feed especially on insects and worms. The .Lacertilia are poorly represented as fossils, the group appearing in the cretaceous. Most of the fossils are referred to existing families, but the Dolichosauria from the cretaceous of Europe, differ from all recent lizards in having more than nine cervical vertebrae. SUB-ORDER 2. PYTHONOMORPHA. Large, extinct, extremely elongate reptiles with four flipper-like extremi- ties ; vertebrae proccelous, with or without zygantra and zygosphenes ; usu- ally no sacrum ; supratemporal fossa present, jugal arch incomplete ; teeth large, conic, acrodont, fused to maxillae and pterygoids ; a parietal foramen ; both girdles present ; feet pentadactyl, without claws. The Pythonomorpha occur in the upper cretaceous of America, Europe, and New Zealand. The vertebrae number more than a hundred ; the cer- vicals bear strong hypapophyses, the caudals with chevron bones. The skull was lizard-like, the cranial cavity being open in front. The parietals are fused in the middle line, and were connected with the alisphenoids and prootic by lateral processes. The quadrates are large, and are articulated to a supratemporal. The premaxilla unpaired, the rami of the lower jaw FlG. 315. Skull of ffeloderma, after Giinther. /, frontal ;/, jugal ; ;//, maxillary ; n, nasal ; p, parietal ; //, postfrontal ; //', prefrontal ; //, pterygoid ; /.r, premaxillary ; s, squamosal ; so, supraoccipital. 322 CLASSIFICATION OF VERTEBRATES. united by ligament at the symphysis. Sternum and episternum but rarely occur, the coracoids (which bear a procoracoid process) meeting in the middle line. The pelvis was weak, and in most forms the ilium was without connection with the ver- tebral column. The bones of the limbs were short. In most forms the skin was naked, or at least lacked dermal ossicles. Two groups are recognized, the PLIOPLATE- CARPID.-E, with a sacrum of two fused vertebrae and an episternum ; and the MOSASAURID^E, in which a sacrum was lacking. Plioplatecarpus occurs in the rocks of Maestricht. Mosasaurus (species of which occur in Holland, England, and the eastern U. S.) was first found over a hundred years ago. Clidastes from Alabama, and Platecarpus from Kansas, are pretty well known. Liodon, from the cretaceous of both continents. Some of the phythonomorphs were over 40 feet in length. FIG. 316. Skull of Liodon, after Owen. b, basioccipital ; /, frontal ; j, jugal ; /, lachrymal ; mx, max- illa; w, nasal; /, parietal, with large parietal foramen ; pm, premaxilla ; pf, prefrontal ; pof, post- frontal ; , anterior wall of supracoracoid sac ; Va, anonymous vein ; *, entrance of bronchi into lung ; t, anterior, tt, posterior diaphragramatic sac. 340 CLASSIFICATION OF VERTEBRATES. nerves are distinct in origin. A characteristic is the double condition of the sympathetic in the region of the neck, one portion following the vertebrarterial canal, the other the carotids. The nostrils, except in Apteryx, are near the orbits. Con- nected with the olfactory organ is a nasal gland, usually situated in the frontal bone, its duct emptying into the respiratory chamber. The eyes are large and highly developed, and are spherical behind, obtusely conical in front. Except in Apteryx there is developed a peculiar fold, the pecten or marsupium, which is vascular, and projects into the posterior chamber in the line of the choroid fissure. The nictitating mem- brane is large, transparent, and is moved by two special muscles, the quadratus and the pyramidalis. The associated Harderian gland is large, the lachrymal, at the external angle of the orbit, being small. The eye muscles proper are small ; and the eyeball is some- what limited in its motions, this being com- pensated by the flexibility of the neck. The ear has a large semicircular canal ; and the lagena is long and slightly coiled, its distal end being somewhat expanded. It recalls the cochlea of the mammals, but a Corti's organ is lacking. The tympanic cavity sends prolongations into the surrounding bones, one, the siphoneum, penetrating the lower jaw. The tympanum is crossed by the slender columella, which bears at its membranal end a discoid stapedial plate. The external ear is surrounded by a circle of feathers ; and in some birds (e.g., owls) these may be moved, like a valve, by appro- priate muscles, The heart is completely divided into right and left halves; and the dorsal aorta is supplied only by the right aortic arch, the left of the normal pair being converted into the innominate artery. There is no mixture of arterial and venous blood in the heart. The blood returns to the heart usually by two precavae and a single postcava, these emptying separately into the right auricle in which the sinus has become merged. There is no FIG. 335. Brain of bird. 34i renal portal system. Characteristic of birds is an arterial plexus beneath the skin of the ventral side, which becomes greatly enlarged at the time of incubation. The red blood corpuscles are oval and nucleated. The permanent kidneys are metanephridia. Usually they consist of three lobes, each lobe lying in a cavity bounded by the vertebrae and the transverse processes. Frequently the kidneys meet and even fuse posteriorly. The ureters open separately in the cloaca. No urinary bladder occurs. The urine is white and semi-solid. The left ovary is never functional, and it and its duct are usually aborted. The right ovary is strongly lobulated on ac- count of the large size of the eggs. The corresponding ovi- duct has a large funnel-shaped opening, and' is divided into three regions, the middle of which is glandular and furnishes the white, while the posterior is both muscular and glandular and secretes the egg-shell. From the fact that the egg remains some time in the latter division, this is sometimes spoken of as the uterus. The testes are usually equally developed ; they lie in front of the kidneys, and the vasa deferentia have a convo- luted course, opening separately into the cloaca. A copulatory organ is usually absent. In the ostrich there is a solid retrac- tile penis like that of alligators and turtles, while a few other ratites and aquatic birds have the ventral wall of the cloaca thickened, with a median groove which serves as a sperm duct. Secondary sexual characters are common. The male may be either larger or (more rarely) smaller than the female. Fre- quently he is distinguished by brighter colors, by the develop- ment of certain feathers, etc. The eggs are very large, and are incubated by the parents ; the period of incubation varying from eleven days to seven weeks (ostrich). The nest-building habits vary greatly. The development of different birds shows few and unim- portant variations, and the history of the common chick is well known. The early phases of segmentation are passed through before the egg is laid. This segmentation affects at first only a small portion of the upper surface of the yolk (i.e., is mero- blastic). The resulting blastoderm is several cells in thickness, 342 CLASSIFICATION OF VERTEBRATES. a layer of superficial ectoderm cells, and beneath this the lower layer cells of undifferentiated mesoderm and entoderrn. The blastoderm next exhibits two areas, a translucent central area pellucida and a marginal area opaca. At the edge of the blas- toderm there now appears an elongate depression, the primitive streak (an extremely modified blastopore), the axis of which, corresponding to the future axis of the embryo, lies at right angles to the major axis of the egg. In front of this arise a pair of medullary folds enclosing the medullary groove, the hinder ends of the folds embracing the anterior end of the primitive streak. While the embryo is thus being outlined the blastoderm in- creases in size, and soon becomes differentiated into embryonic and extraembryonic portions, the former giving rise to the whole of the embryo, the other to a cellular yolk sac which eventually embraces the whole yolk. At the edge of the embryonic area arises the amniotic fold which closes in over the embryo from all sides, thus enclosing it in a double-walled sac, the inner layer being the amnion, the outer the serosa. While the am- nion is being closed in, the embryo begins to be cut off from the yolk, at last only a narrow yolk stalk connecting the two. The allantois grows out from the alimentary tract behind the yolk stalk. At its base it is small, but it expands distally into a large vesicle. Both yolk sac and allantois have blood-vessels developed in them, and form important organs of nutrition in the broader sense. The blood-vessels of the yolk sac appear first as outgrowths from the omphalomesaraic arteries and veins in the area pellucida ; but they gradually extend over the area opaca, branching and forming a plexus, the function of which is to take up the yolk and carry it into the circulation. The allantoic circulation is respiratory in character. Its vessels are outgrowths from the anterior abdominal vessels of the non- allantoidan vertebrates. The allantois also serves as a reservoir of urinary waste. In the general growth of the embryo the most striking feature is the close similarity, until a late stage, with the rep- tiles. The gill slits three or four in number never bear gills, and the appendages in the early stages are distinctly paw- BIRDS. 343 or flipper-like. As the yolk is absorbed, the yolk sac is drawn into the body cavity, and the abdominal walls close. Then the shell is broken, in most birds by means of a calcareous or horny growth at the tip of the upper jaw (egg-tooth), and the young begins its free life. As far as is at present known, birds appeared (A rchceopteryx, Laopteryx) in the Jurassic. In the cretaceous the genera IcJithy- ornis and Hesperornis are found, while in the tertiary the forms are more numerous, although at all times fossils belonging to the group are rare. The order of birds is so uniform in its structural features that it is difficult to find important characters to differentiate the twelve thousand known species into convenient groups. As a result, ornithologists have raised a number of minor groups into so-called orders, which are scarcely of family rank, if we are to accept the rules in vogue in other groups of vertebrates. The group by most authors is sub-divided into Ratitae and Carinatae, divisions based upon the presence or absence of a keel to the sternum ; but these divisions are artificial, and do not indicate the phylogeny of the forms concerned. ORDER I. SAURUR^E (ARCH^EORNITHES). Extinct birds with elongate tail consisting of many vertebrae ; up- per jaw with teeth (low- er unknown) ; vertebrae amphicoelous ; feathers of the normal type, those of the tail in pairs, a pair to each vertebra. Only two specimens are known, both coming from the lower Jurassic slates of Bavaria. These belong to the genus Archceopteryx, but may represent two distinct species. One is about the size FIG. 336. Restoration of Archccopteryx, after Pycraft. 344 CLASSIFICATION OF VERTEBRATES. of a crow, the other considerably larger. Laopteryx, known from a few fragments from the Jurassic of Wyoming, may be- long here. ORDER II. ODONTORM^:. Extinct carinate birds with normal avian tail (pygostyle) ; teeth thecodont ; presacral vertebrae amphicoelous ; quadrate with a single articular facet ; rami of lower jaw united by cartilage. To this order belong a few birds arranged in the genera Ichthyornis and Apatornis, pigeon-like in size, found in the middle cretaceous of Kansas and Colorado. They had very large skulls, strong wings, and small legs, while the succession of the teeth was vertical as in the dinosaurs. This and the fol- lowing order are frequently united as odontornithes or toothed birds. ORDER III. ODONTOHOLC^E. Extinct ratite birds with teeth in alveolar grooves ; vertebral centres saddle-shaped ; quadrate with one articular facet ; skull dromaeognathous ; rami of lower jaw united by cartilage; wing reduced, only the humerus retained. The birds belonging to this order occur in the same beds as do the odontormae. In general appearance they were some- what like grebes. The cranial bones were firmly united, the pre- maxillary bone was without teeth, while the teeth of the maxillae and lower jaw had a lateral succession as in the pythonomorphs. There was no true pygostyle, but the caudal vertebrae were broadly expanded, forming a paddle-like tail, only a few of the distal bones being fused. The clavicles were not united into a wish-bone ; the acetabulum resembled that of the crocodiles ; ilia, ischia, and pubes were not united posteriorly ; and the feet were apparently fitted for swimming only. Here belong Hes- perornis and Lestornis. H. regalis was about six feet long ; L. crassipes considerably larger. BIRDS. 345 FIG. 337- Restoration of Hesperornis, after Marsh. ORDER IV. EURHIPIDUR^:. Toothless birds with pygostyle and saddle-shaped centra to> the presacral vertebrae. The rami of the lower jaw are firmly united* To this order belong all living and many extinct birds. \ SUB-ORDER i. DROM^EOGNATHI. Eurhipidurine birds with dromasognathous palate ; ischium and ilium-, not united behind ; sternum either ratite or carinate ; wings rudimentary or of no use in flight. SECTION I. STRUTHIONES. Large ratite birds with elongate hind legs and neck ; bill broad at base ; mouth deeply split ; toes three or two. Restricted to the southern hemisphere. The STRUTHIONID^:, containing the single species Struthio camelus, the ostrich of Africa, has but two toes. The South American RHEID^:, which have three toes, contain the nandu,. 346 CLASSIFICATION- OF VERTEBRATES. Rhea americana, the feathers of which are familiar in feather dusters. The CASUARID^E of the Oriental region have three toes and a helmet-like development on the head. The family contains the emeus (Dromaius) and the cassowaries (Casuarius). A fourth family, the DINORNITHID^E, the extinct moas of Australia and New Zealand, were birds of gigantic size. SECTION. II. yEPIORNITHES. Extinct ratite birds of large size, formerly inhabiting Madagascar. ^Epiornis. FIG. 338. South American Ostrich, Rhea americana, from Liitken. SECTION III. APTERYGES. Dromaeognathous ratite birds with rudimentary wings ; no clavicle ; four toes ; vomer united to palatines and pterygoids ; bill long, nostrils near the tip. Four existing species of kiwi, belonging to the genus Apteryx, inhabit New Zealand. SECTION IV. CRYPTURI. Dromaeognathous carinate birds with clavicle and functional wings. About 50 species from Central and South America. Crypturus, Rhynchotus, Tinamus. SECTION V. GASTORNITHES. Extinct carinate birds from the eocene of France and Belgium. Gastornis. ? Diatryma from New Mexico. BIRDS. 347 SUB-ORDER 2. IMPENNES. Aquatic birds with short, paddle-like wings used only for swimming. Pterylae and apteria not differentiated ; no differentiated remiges ; dorsal vertebrae opisthocoelous and movable; synsacrum poorly developed ; skull schizognathous ; uncinate processes not anchylosed to ribs ; pollex absent ; pubis not united to ischium behind ; four toes ; feet plantigrade. The pen- guins, of which there are several gen- era (Aptenodytes, Spheniscus, etc.) are confined to the colder portions of the southern hemisphere. They are flight- less, but use their wings, which are covered with scale-like feathers, as paddles in swimming. They feed upon fish and shell-fish, and make their nests upon uninhabited islands. Palceeudyptes occurs in the eocene of New Zealand. . 339. Penguin, Aptenodytes longirostris, after Liitken. SUB-ORDER 3. EUORNITHES. Non-dromaeognathous birds with (usually) saddle-shaped centra to the dorsal vertebrae; distal caudal verte- brae united to a pygostyle ; quadrate with two articular facets ; ilium and ischium united behind, enclosing an iliosciatic foramen; pollex free; pterylae and apteria differentiated. To this sub-order belong the great majority of living birds, over twelve thousand in number. They are usually sub-divided into a number of groups commonly regarded as ' orders,' but which are only of family rank, the so- called families being equivalent to genera in other groups of vertebrates. Reference must be made to special works on ornithology for details, as space will only allow mention of families here, with such features as will allow of correlation of other works with the system here adopted. SECTION I. DESMOGNATH/E. Birds with desmognathous pala- tine structure (p. 334). The STEGANOPODES are strong flying, web-footed birds, in which all four toes are directed forwards, while basipterygoid processes are lacking. The tropic birds (Phaethoti) have all the toes con- nected by a web, while in the frigate birds (Fregata} the web is scarcely developed. The pelicans (Pelecanus}, with twenty-four tail-feathers, are characterized by the enormous pouch connected with the lower jaw. The gannets (Sula), the cormorants (Phalacrocorax}, and the darters (Anhinga} also belong here. The CHENOMORPH^E have three toes directed forwards, 348 CLASSIFICATION OF VERTEBRATES. and in most cases, as in the ducks (Anas), geese (Anser), swans (Cygnus), and flamingoes {Phenicopterus}, webbed and fitted for swimming, while in the screamers (Anhima) the web is lacking. The young when hatched are feathered, and able to feed themselves. The HERODII includes altrical forms (p. 330), in which the legs are very long, the toes, of which three are directed forwards, are usually without webs, and these birds, like the grallae of .the schizognathous section, are wading forms. The various species of Ibis, the spoonbills (Platalea), storks (Ciconid), herons (Ardea, Herodias*}, and bitterns (Botaurus}, are familiar examples. The ACCIPITRES (Raptores) are recognized by their hooked bill and claws, the toes, three of which are di- rected forwards, being with- out webs. There is no ba- sipterygoid process, and the young are altrical in char- acter. The hooked beak is shared by the parrots, but the toes at once dis- tinguish the two groups. The birds of prey include the vultures and buzzards (Cathartes, Gyps, Sarcor- hainphns}, the eagles {A guild], hawks (Buteo, Ac- cipiter), and falcons {Falco], forms which are closely alike in structure and differ con- FIG. 340. Wood-duck, Aix sponsa, from siderably from the nocturnal Tenney, after Audubon. owls * {Strix, Bubo, Scops, etc.), which compose the rest of the family. The COCCYGOMORPH^E, as a rule, have three toes directed forwards, but in the cuckoos and toucans the first and fourth toes are turned backwards, while in the colies (Colius] all four toes are directed forwards. In all the rostrum is movable. Typical genera are the plan- tain-eaters {Musophaga), the cuckoos (Cuculus, Geococcyx}, the night-hawks {Caprimulga, Chordediles), the rollers (Coracias], bee-eaters (Merops], motmots {Momotus), todies (Todus], kingfishers {Halcyon and Alcedo], the hornbills (Bitceros}, hoopoes {Upupd}, puffbirds (Monasa, Bucco}, tou- cans (Rkampkastos), and honey guides {Indicator}. The TROGONID^E are characterized by having toes one and two directed backwards. The STEATORNITHID^E resemble the rollers in many of their characters, but they differ from them, as from all desmognaths except the parrots, in the opisthocoele character of the vertebrae. Three toes are directed forward. The oilbird (Steatornis caripensis] of South America is the 1 The owls may be more nearly related to Coracias than to the Accipitres. BIRDS. 349 only species. The parrots or PSITTACI agree with the last-mentioned family in the vertebral centra and movable rostrum, but have the first and fourth toes turned backwards, while the beak is hooked. Conurus, Psittacus, Cacatna, Trichoglossus are typical genera. FIG. 341. Carolina paroquet, Conurtts carolinensis, from Tenney, after Wilson. SECTION II. SCHIZOGNATH/E. Birds in which there is a schizog- nathous palatal structure (p. 355), and the vomer is narrowed and acute in front. The families to some extent parallel those of the desmognathae in appearance and habits. The CECOMORPHiE are swimming-birds, in which the feet are webbed, three toes pointing forwards, and the external nostrils are prolonged backwards as a fissure. The family includes the grebes (Colymbus and Podiceps), loons (Urinator), sun-grebes (Heliornis), auks {A lea), guillemots (Uria), gulls (Larus). terns (Sterna), and skuas (Ster- corarius). The TUBINARES, including the albatrosses (Diomeded), petrels (Procellarid) , and fulmars (Fulniarus), closely resemble the cecomorphae, except in the tubular nostrils. They are oceanic in their habitat. The GRALL^E are long-legged wading birds in which the toes (three directed forwards) are not usually webbed. The nostrils are either as in the ceco- morphae, or they are closed behind by a rounded edge. The pratincoles, plovers (Charadrius), Jacana, snipes (Scolopax), cranes (Grus), and rails (Rallus), and their allies, are littoral forms, while the bustards (Otis) have lost their wading habits and are truly terrestrial. The OPISTHOCOMI of South America, like all the remaining schizognathous families have three toes directed forwards. In general appearance the single species recalls the Gallinae, but differs in the absence of the basipterygoids, the union of 350 CLASSIFICATION OF VERTEBRATES. lachrymals to the rostrum, etc. The GALLING (Rasores, Alectoromorphas) includes the quail (Coturnix), partridge (Perdix), grouse (Tetrao, Bonasa), jungle-fowl, including our domestic fowl (Callus), pheasants (Phasianus, Thau- malea), turkeys (Meleagris), peafowl (Pavo) . These have the hallux rudimen- tary and elevated above the other toes and two carotid arteries. The COLUMB^E (Pullastrae) have usually two carotids and the hallux well developed and near the ground. The group is hardly to be distinguished as a family from the Gall- inae. It contains the doves and pigeons (Coliunba, Gonra, Didunculus), as typi- cal members, while the mound-birds (Megapodius), the curassows (Crax), and the sand-grouse (Pterocles')^ are more aberrant. The dodo (Did?ts), extermi- nated about two centuries ago, was an aberrant pigeon. The humming-birds form the family TROCHILID^E, which has relations with the picarian birds. The toes, however, are three, directed forwards as in the preceding groups. FIG. 342. Wilson's snipe, Gallinago wi/som, from Tenney, after Wilson. FIG. 343. Bird of Paradise {Paradisea apoda), female, from Hertwig, after Levaillant. Other characters are the presence of basipterygoid processes and the existence of a single carotid. In the PICARI^E the first and fourth toes are directed backwards, while the palate is of the saurognathous type BIRDS. 351 (p. 335) . Here belong the woodpeckers (Picus, Colaptes) and the wrynecks SECTION III. yEGITOGNATELE. Birds with the maxillopalatines not united, the vomer single, broad, and notched in front. In the family PASSERES, which embraces over half the known species of birds, three toes are directed forwards. These birds group themselves in five divisions,, FIG. 344. Bird of Paradise {Paradisea apoda}, male, from Hertwig, after Levaillant. typified by the following forms : The lyre-bird {Menurd), the broad-bills (Eurylamia}) the tyrant-birds and king-birds (Tyrannus), the ant-shrikes {Formic aria), and the sparrows {Passer}, those allied to the last being fre- quently known as Oscines or singing-birds. Of these the number is legion \ no attempt can be made here to even enumerate their names. 352 CLASSIFICATION OF VERTEBRATES. The Euornithes date back no farther than the eocene, in which period representatives of the cormorants, pelicans, flamingoes, falcons, kingfishers, and pheasants appear. The colics, oilbirds, and opisthocomi are not known .as fossils, while the other groups appear with the miocene. CLASS II. MAMMALIA. Hair-bearing amniotes with two occipital condyles ; lower jaw suspended directly from the cranium without the interven- tion of the quadrate ; ankle joint between the tibia and fibula and the first row of tarsal bones ; brain with well-developed corpus callosum ; a complete diaphragm ; heart four-chambered ; only one (left) aortic arch persisting ; red blood corpuscles non-nucleate, usually circular in outline ; eggs (except in monotremes) minute, and undergoing a total segmentation, the embryonic development taking place inside the mother ; the young, when born, nourished by milk secreted by the mammary .glands of the mother. The skin in the mammals has a well-developed stratum -corneum (p. 88), which is never molted as a whole, as in the reptiles and lower vertebrates, but comes away piecemeal. The skin is as a rule pigmented, and the pigment may occur in either the deeper (dermal), or more superficial (epidermal) portions. The epidermis gives rise to various structures, the most noticeable and most characteristic of which is hair, the structure and development of which is described elsewhere (P- 97)- The hair is usually abundant, and covers most of the body. The other extreme is reached in the cetacea, where it may be reduced to from two to eight pairs of bristles in the mouth region, these occurring in some cases only in foetal life. Frequently one can distinguish two kinds of hair, one straight and stiff, covering a deeper woolly hair. Hair can undergo con- siderable modifications. It may be straight or curly ; it may develop into bristles, or even into strong protective spines, as in .the porcupines, etc. Frequently certain hairs about the mouth (vibrissae) have tactile functions, their roots being enveloped in a rich plexus of nerve fibres. In some cases the hair seems to persist throughout life (tail and mane of horses), but usually it falls out and is replaced by new hair, this molting occurring MAMMALS. 353 gradually, or, as in the case of many inhabitants of colder climates, before and "after the winter season. In the case of some arctic species this molting is accompanied by color changes, the winter pelage being white. Frequently the epidermal layers of the skin becomes greatly thickened and cornified, producing callosities, or thickened, hair- less surfaces like those found on the soles of the feet. Cornifi- cation of the epidermis also results in the formation of horn, such as that found in the cavicornia (cattle, etc.), and in the rhinoceros, as well as nails, claws, and hoofs. Some of the relations between the epidermis of the ap- pendages and the claws are interesting. In the human finger FIG. 345. Diagrams of the relations of nails, claws, and hoofs, after Boas. A, in man; B, in unguiculates; C, in ungulates, ^unmodified epidermis; SU, subungual epidermis (Sohlenhorn); U, nail. (Fig. 345 A), there exists beneath the nail a peculiarly modified epidermis, su, the subungual portion, while the usual epidermis, s, covers the ball of the finger. In the unguiculates (B) the subungual portion is more developed, and forms the lower sur- face, su, of the claw. In the ungulates (Q, the unguis becomes much wider, and is rolled into a hoof, on the lower surface of which is still to be recognized the subungual epidermis, forming, for instance, in the case of the horse, the sole, into which there projects behind the frog of the foot, which, in reality, corre- sponds to the ball of the finger in man. Scales are not frequent in the mammals. They occur upon the tails of certain rodents, and upon the feet of these and some 354 CLASSIFICATION OF VERTEBRATES. other forms (insectivores, marsupials, etc.). More rarely the whole body may be encased in them as in the pangolins. Again, as in the armadillos, dermal bones are developed in connection with the scales, while in some embryonic cetaceans similar features are seen. Glands are far more abundant than in the sauropsida, and include, besides the common sebaceous and sweat glands, numer- ous modifications, usually in the line of scent glands. These are mostly modifications of sebaceous glands, and in many car- nivores, rodents, and edentates, are most abundant in the anal or inguinal regions. In other groups they may have widely diverse positions ; in the occipital region (camel), in the lachry- mal bone (many ruminants), upon the face (bats), on the legs (swine), in the temporal region (elephants), etc. Here, too, belong the problematical glands connected with the spur on the hind legs of the monotremes. The mammary or milk glands of the mammals are also modified dermal glands, those of the monotremes most closely resembling sweat glands, those of other mammals sebaceous glands. In their development a marked ridge, the 'milk line/ appears along the side of the body, certain portions of which become developed into the glands, the intervening portions aborting. Connected with the glands are the teats or nipples, which are of two kinds ; the one produced by a protrusion of that part of the surface upon which the lacteal glands open ; or (ungulates) by a similar elevation of the surrounding surface, the openings of the ducts remaining at the bottom of the tube thus formed (Fig. 97). The number of teats varies between one and eleven pairs (Centetes). These may be distributed along the length of the trunk, or may be restricted to either thoracic or abdominal region. Except in a very few forms (hares) there is a layer of fat (panniculus adiposus) between the skin and the muscles. Be- sides, there is usually a layer of skin muscles (panniculus carno- sus, p. 115). This is distinct from the isolated smooth muscles connected with the hair follicles. In the skeleton there never occurs that pneumaticity found in birds and some extinct reptiles, the cavities of the bones being MAMMALS. 355 filled with marrow. All of the bones except those of the skull, the elements of the sternum, and some of those of the carpus and tarsus, are provided with epiphyses, separate portions which unite later in life with the rest of the bone. As a rule five regions cervical, thoracic, lumbar, sacral, and caudal are differentiated in the vertebral column, but in the cetacea, where the sacrum is lacking, the line cannot be drawn between lumbars and caudals. The cervicals, which are almost constantly seven l in number, are free, except in most cetacea and some edentates, where they are greatly flattened and fused. In some rare cases they bear movable cervical ribs, but usually these are firmly fused to dia- and parapophyses, leaving the ver- tebrarterial canal to betray their true nature. Usually the faces of the centra are flat, but opisthoccele vertebrae are common in the necks of ungulates. The trunk or dorso-lumbar vertebrae usually number nineteen or twenty, and as a rule, increase in the number of thoracics is correlated with a reduction of the number of lumbar vertebrae. The extremes in the region are found in the armadillo, which has fourteen, and Hyrax, which has thirty dorso-lumbar vertebrae. The number of thoracic vertebrae is usually thirteen, but it is lower in bats and armadillos, and reaches eighteen in the horse, nineteen or twenty in the rhinoceros and elephant, and twenty- three or four in the three-toed sloth. The lumbars vary from two in the monotremes, manatee, and two-toed ant-eaters, to nine in Stenops, the usual number being six or seven. The sacral vertebrae are primitively two in number, but others taken from the lumbar and caudal regions may unite by synostosis with the ilium, giving a total number of sacral verte- brae of eight or nine in the sloths and armadillos. The caudals are extremely variable in number and are usually numerous, the number being greatly reduced only in the anthropoid apes and man. The ribs (corresponding in number to the thoracic vertebrae) are bicipital, being furnished with tubercular and capitular heads, the former articulating with the diapophysis (the transverse 1 Manatus australis and Choloepus hoffmanni have six, Bradypus torquatus has eight, and B. tridactylus nine cervicals. 356 CLASSIFICATION OF VERTEBRATES. process of human anatomy) the latter with articular facets (re- duced parapophyses) on the centra. The ribs usually present bony and cartilaginous portions, the latter reaching the sternum. In floating ribs the union with the sternum is lacking, while false ribs are without vertebral connections. The sternum frequently retains throughout life the separate elements or sternebrae of which it is composed ; but these may fuse into an elongate plate, the corpus sterni or mesosternum, with an anterior portion, the manubrium or presternum, and a posterior xiphisternum or ensiform process with which no ribs articulate. The episternum (p. 149), which is laid down in carti- lage, is placed in front of the sternum, but retains its distinctness only in the monotremes and some edentates and rodents. In other forms it fuses with the sternum. In the mammalian skull there is a more intimate relationship between the cranial and facial elements than is the case in the lower vertebrates. There is also a marked tendency to the fusion of bones which are distinct in the lower vertebrates, but usually the process of co-ossification is not complete except in the hyoid and lower jaw, many of the bones being suturally united throughout life. 1 The floor of the skull is preformed in car- tilage, its roof of membrane bones. No interorbital septum occurs, its remnants being found in the crista galli process of the ethmoid, while the lateral walls continue forward to the ethmoid region. Basi- and presphenoids frequently fuse, and from the sphenoid thus formed a greater and a lesser wing arises on either side, these being the ali and orbitosphenoids respectively of the lower forms. The pterygoids also unite with the sphenoids, forming the pterygoid processes. Basi-, ex-, and supraoccipitals may remain distinct, or they may fuse, sometimes late in life, into a single occipital bone which bears a pair of occipital condyles arising from the exoccip- itals, the basioccipital but rarely contributing to their formation. The sides of the skull are formed in part by the sphenoidal alae, in part by the temporal bones, each of which is a complex of several elements, the petrosal (fused pro-, epi-, and opi- 1 The obliteration of .sutures has progressed the farthest in monotremes, the weasel and some apes. MAMMALS. 357 sthotics) squamosal, mastoid, and tympanic. These are frequently fused. The tympanic, surrounding the external auditory mea- tus may develop into a saccular auditory bulla. In front the temporal gives off a zygomatic process which extends forward to join the jugal, or, as it is called, the malar bone. An interparietal may be distinct, as in rodents, or it may fuse with the supraoccipital, or, more rarely, with the parietals. The frontals are usually paired, and in the ungulates they may e.n FIG. 346. Skull of young Tatusia, from Wiedersheim ; cartilage dotted. aty, tympanic annulus; bhy, basihyal; chy, ceratohyal; cr, cricoid; d, dentary; ehy, epihyal; en, external nares; eo, exoccipital; f, frontal; hhy, hypohyal; /', jugal; in, incus; Ic, lachrymal; mk, Meckel's cartilage; ml, malleus; mx, maxillary; ;/, nasal; occ, occipital condyle; /, parietal; pa, palatine;/.*-, premaxilla; sq, squa- mous part of temporal; st, stapes; stm, stapedial muscle; so, supraoccipital; th, thyroid cartilage; tr, trachea; II, V, passages of nerves. develop bony horns. Frequently each gives off a postorbital process which, approaching or meeting the jugal, partially or completely separates the orbit from the temporal fossa. Post- frontals are lacking. The cranium is closed in front by the ethmoid bone in which may be recognized a median (mesethmoid) plate which divides the nasal cavity into right and left halves, and on either side a lamina cribrosa perforated for the passage of the olfactory nerve. 358 CLASSIFICATION OF VERTEBRATES. The cribrosa, together with the superior turbinal bones in the nasal passage, apparently represents the prefrontal of lower vertebrates. The nasal cavity is bounded externally by the nasal bones, which are small in cetacea and fused in the old-world apes. Inside the cavity, besides the superior turbinals already referred to, are the inferior turbinals which, beginning as separate ossi- fications, fuse with the maxillaries. The septum of the nose, established by the mesethmoid, is continued by the vomer, in which the paired bones of the lower forms are fused, and are entirely cut off from the roof of the mouth. Among the most characteristic features of the facial skele- ton are the close union of the maxillopalatine region with the rest of the skull, and the suspension of the lower jaw direct from the temporal bone without the intervention of a quadrate. The premaxillae may fuse with the maxillae, while maxillae and pala- tines send off horizontal palatine processes, which, meeting in the middle line, form the hard palate, bound the nasal cavities below, and carry the choana far back. The pterygoids may also contribute to the hard palate (some edentates). The lower jaw consists of but a single bone, the dentary, on either side, or the two halves may anchylose at the symphysis. In the middle ear are three small bones which form a sound- conducting apparatus leading from the tympanic membrane to the fenestra ovalis. In order from outside in these are the mal- leus, incus, and stapes. Concerning their homologies the most diverse views are held, both incus and malleus having been regarded as the missing quadrate. The views of the homologies given on p. I 59 seem to be in full accord with the results of the studies of most students who have approached the subject from the embryological standpoint. In the mammals the brain exercises great influence upon the shape of the skull. As it nearly fills the cranial cavity, increase in its size can only be accommodated by an outgrowth of the cranial walls. To measure the extent of this outgrowth and thus approximately to obtain an index of cerebral develop- ment, the facial angle is employed. According to the system of Camper this is the angle formed by two lines, one passing MAMMALS. 359 from the auditory meatus to the base of the nose, the other from the base of the" nose to the most prominent part of the frontal bone. Less used is the system in which the lines inter- sect at the insertion of the teeth in the upper jaw. In man Camper's angle varies from 70 to nearly 90. In monkeys from 60 (Chrysothrix) to 35 or 30 ; in other mammals it is 25 or lower. v The hyoid arch is connected dorsally with the otic capsule, ventrally with the first branchial (Fig. 31). In it ossifications take place which proceed to varying extents. The whole arch may ossify, giving the separate elements, basi-, hypo-, cerato-, epi-, and stylohyal, or the median portion on either side may be converted into a stylohyal ligament, the stylohyal element fus- ing with the skull and forming the styloid process, while the basi- and epihyals fuse to form the body and lesser horn of a single hyoid bone. The first branchial arch also fuses with this, contributing to the body, and forming a greater horn on either side. There is also a ligamentary connection with the second and third branchial arches (thyroid cartilage, p. 28). The pectoral girdle shows many variations from the typical condition, for while the scapula is always present, the coracoid, A B FIG. 347. Pelvis (A^) of young Ornithorhynchus ; (ff~) of calf, after Boas. A, acetabulum; IL, ilium ; IS, ischium; M, marsupial bone; P, pubis. except in the monotremes, is reduced to a small element the coracoid process fused to the scapula. The presence or ab- sence of a clavicle is correlated with habits ; flying, digging, and climbing mammals having it, while it is absent in whales, ungu- lates, and many others. In rodents and carnivores it is reduced, and has only ligamentary connections. The pelvis, on the other 360 CLASSIFICATION OF VERTEBRATES. hand, is more normal, except in the cetacea, where it is apparently absent, the one or two rib-like bones which occur imbedded in the muscles free from the vertebral column being usually inter- preted as femur and tibia. In all others the ilia are united to the sacral vertebrae, while with rare exceptions the pubes of the two sides, and usually the ischia as well, unite in a symphysis below. In monotremes and marsupials, marsupial bones, prob- ably epipubic in character (p. 171), are developed from the cephalic side of the pelvic girdle. In the skeleton of the free appendages great variations occur, especially in the direction of reduction of digits, etc.; and for details of these reference must be made to the accounts of the separate orders. In general it may be said that the tendency is towards a reduction in the number of digits, and towards an alternation and interlocking of the carpal and tarsal bones. In the swimming-forms there is also a shortening of the limbs, the reduction going so far that in the case of the sirenia only the elbow joint is functional, while in the whales even this joint is lost. On the other hand, in these forms, as in the ichthyosaurs and plesiosaurs, an increase in the number of phalanges is more or less marked. In the bats, on the other hand, the elongation of most of the digits of the hand, and their utilization as sup- ports of the wing, is noticeable. In detail : the humerus may be either long or short, the con- ditions here being usually in reversed correlation to those found in the metacarpus. The ulna and radius are usually longer than the humerus, the ulna being produced beyond the hinge of the elbow as an olecranon process. The radius is more closely related to the carpus, and is capable of turning more or less freely around the ulna in the process of pronation and supination of the manus. Occasionally radius and ulna coalesce. The femur usually bears two or three enlargements (tro- chanters) for the attachment of muscles. At the knee joint a patella or knee-pan usually occurs. The tibia and fibula are usually longer than the femur, and in the marsupials are capable of marked rotation. On the other hand, the fibula is frequently reduced, and united more or less closely to the tibia. The greatest variations occur in the carpus and tarsus and MAMMALS. 36 L more distal portions. The carpal and tarsal bones are in twcr or three rows, those "of the distal row being opposite or alter- nate with the others. In the tarsus the os calcis and astragalus are the most prominent, the former being the fibulare, the latter fused tibiale and intermedium (p. 176). The digits are typically five in number, but Pedetes presents structures usually inter- preted as a sixth toe. The tendency is constantly towards re- duction in the number of digits, disappearance being preceded by a reduction in length, in which case the metacarpals are shortened and are occasionally reduced to splint bones. In certain groups there fre- quently occurs a fusion of the two middle metacarpals. The phalanges in the digits never exceed three, ex- cept in the whales. Mammals are spoken of as plantigrade, digitigrade, or unguligrade, accordingly as they walk upon the whole metacarpal or metatarsal region, as in the bear and ' .... . , , FIG. 348. Fore (right) and man ; or upon the distal phalanges, hind (le f t ) f ee t of tapir. ,. as in the Cats and dogs ; Or, again, astragalus; c, cuneiforme in . calcaneum in hind. fore foot ; c", c'" , cuneiforme ; cb Y cuboid ; 7% femur ; /, lunare ; upon the nails (hoofs), as in the horse and cow. The most Striking feature of the m, magnum; , naviculare;: nervous system of existing mammals *> pisiforme; A\ radius; * ...... scaphoid ; /, trapezoid ; 7'., is the great size of the brain, and es- tibia . tt> unciforme; u, ulna, pecially of the cerebrum and cere- bellum, the former overarching twixt and mid brains and reach- ing the latter. In the lower mammals the cerebral surface is smooth, but in the higher it is marked by gyri and convolu- tions, the effect of which is to increase the amount of sur- face and consequently of gray matter. 1 This great increase of the cerebrum is largely an increase of the pallium, only a small portion of which remains epithelial in character. In 1 By some authors the mammals are sub-divided into two groups, Ineducabilia, With, smooth cerebra (Fig. 349), and Educabilia. in which the surface of the cerebrum is convo- luted (Fig. 54). 362 CLASSIFICATION OF VERTEBRATES, the aplacental mammals the anterior commissure is especially well developed, and forms the chief connection between the two sides of the brain, while the corpus callosum remains more rudimentary, as in sauropsida. In the placentalia, on the other hand, the corpus callosum or commissure between the two hemispheres becomes the most important connection between the right and left sides, the anterior commissure remaining behind. The longitudinal commissures, the fornix, and the cornua ammonii, are also well developed. The lateral ventricles are large, and in them several sub-regions are distinguished, anterior and descending, and in the higher mammals posterior cornua. The olfactory lobes are comparatively small, and in the whales are absent. In development a diverticu- ff lum of the lateral ventricle extends into each olfactory lobe, but except in a few forms, like the horse, this disappears in the adult. The tvvixt brain and op- tic lobes (corpora bi- or quadrigemini) are poorly developed, and are covered in by the hinder lobe of the cerebrum. The epiphysis is small and lacks any sensory structures. In the cerebellum the vermis is large in the apla- centals, but in the placentalia the lateral lobes of the cere- bellum are in the ascendancy. A corresponding increase from lower to higher is seen in the pons varolii. The spinal cord extends back only to the sacral region, the posterior part of the spinal canal being occupied by a cauda equina formed of the more posterior nerves before their exit. JUI Wu NF1 FIG. 349. Brain of rabbit from above, 'from Wiedersheim. Bol, olfactory bulb; Fip, pallial fissure; Gp, pinealis; ////, cere- bellar hemispheres ; A"//, medulla ; VH, ^cerebral hemispheres; Wu> vermis. MAMMALS. 363 The olfactory organ is noticeable for the great increase of the olfactory epithelium and the corresponding complexity of the bony labyrinth which supports it. In the formation of this labyrinth small bony processes play the greater part, the upper (superior turbinal) being an outgrowth from the ethmoid, the lower (inferior turbinal) usually uniting with the maxillary. The two cavities are separated behind by the ethmoid and vomer, the partition being continued to the tip of the nose by cartilage. Connected with the nasal cavities are numerous sinuses in the frontal, maxillary, and sphenoid bones. The ol- factory nerve breaks up into numerous fibres before leaving the cranial cavity, and these pass through the perforations in the cribiform plate of the ethmoid. The eyes vary according to the habits, being small in bur- rowing forms, or even occasionally without muscles and beneath the skin {Spalax, Chrysochloris). As a rule the eyeball is ap- proximately spherical, except in the whales, where it is flattened. It is placed in orbits, usually incomplete, and these are more or less lateral in position. Sclerotic bones are never developed. Besides the upper and lower lids, a nictitating membrane is usually well developed, but sometimes, as in man, this is reduced to a small muscleless fold, the plica semilunaris, at the inner angle of the eye. In the sirenia the eyelids act as an iris-like diaphragm. Frequently a seventh muscle of the eye, a re- tractor bulbi, is present, and in the carnivores this is four-divided. In carnivores, dolphins, ungulates, and some marsupials, a metal- lic lustre is developed in a part of the choroid (tapetum). The ears are marked externally, except in monotremes, ce- tacea, and some seals, by the development of a conch, supported by cartilage, and moved by appropriate muscles. From this the external meat us leads inward to the tympanum, which is crossed by the three ossicula auditus malleus, incus, and stapes al- ready mentioned (p. 358). From the tympanum the Eustachian tube (p. 73) leads to the pharynx, except in the whales, where it enters the nasal passages. The inner ear is characterized by the great development of the lagena which coils with two or three turns to form the spiral cochlea. The monotremes have the inner ear more on the sauropsidan plan. 364 CLASSIFICATION OF VERTEBRATES. Except in the monotremes, which are provided with a horny beak, the mouth of the mammals is bounded by movable lips near the margins of the jaws ; between the lips and the jaws on either side are the cheeks, and in many apes and rodents these are developed into large cheek-pouches, sometimes hairy on the inner surface. A tongue is always present. In the cetacea it is immovable, in all others it is mobile and sensory, and in the giraffe it is even prehensile. At its base it bears the papillae circumvallatas, special aggregations of taste buds. Beneath the tongue is a single or double under-tongue or sublingua, especially developed in the insectivores, which may be the ho- mologue of the tongue of the lower vertebrates, the functional tongue being a new formation. Three pairs of salivary glands are present (parotid, submaxillary, and sublingual) except in the carnivorous cetacea. The secretion is most abundant in the her- bivorous mammals. A soft palate is alway present, behind which are the choanae. The teeth show a greater range of variation than is found in any other group of vertebrates. They are lacking in but few forms, as Echidna, Mams, and the baleen whales, while in the adult Ornithorhynchus they are replaced by cornified teeth, al- though true teeth are present in the young. They never have such an extensive distribution as is found in reptilia and ichthy- opsida, but are confined to the margins of maxillary, premaxil- lary, and mandibular (dentary) bones. In all cases they are thecodont, i.e., are situated in sockets or alveoli, although, as in the dolphins, the sockets may run together into a continuous groove, while in some shrews the molars in the adult are firmly anchylosed to the jaws. In a few cases, as in the denticete whales, the teeth are all similar in form (homodont), but usually they are differentiated (heterodont) into incisors, canines, and molars. The incisors are placed in the premaxilla, 1 and in a corresponding position in the lower jaw. The canines are placed behind the suture sepa- rating maxillary and premaxillary bones, and never exceed one on a side in either jaw. Behind the canines are the molars. In- cisors and canines have single roots, and the crown is usually a 1 In some armadillos the premaxillary teeth cannot be incisors. MAMMALS. 365 simple cone or chisel. The molars vary greatly in shape and structure, and may have several roots, a feature not found in other living vertebrates. The shapes and modifications of the molars are of great value in classification, and a few definitions may prove of use in reading the descriptions of systematic works. In the more primitive teeth, each tooth, no matter where placed, h'as the shape of a simple cone, as in the denticete whales (haplodont), but usu- ally the crowns of the molars present crests, prominences, tuber- cles, etc. There are two views as to the origin of the more complex condition. According to the one, the typical mam- malian molars have arisen by the fusion of several simple teeth, like those of many reptiles. The other view is that accessory prominences have been developed upon the primary tooth, a view which has much in its favor. First to appear of these more complex teeth was the tricono- dont, in which secondary prominences cones in the upper jaw, conids in the lower were developed in a straight line, a paracone (para- FIG. 350. conid) in front of the primary or protocone, and Bunodont tooth a metacone behind. In the tritubercular tooth, baboon) which came next in turn, the three cones are arranged in a triangle, the protocone on the inner side, the pro- toconid on the outer. This part of the tooth forms the trigon, and from this modifications may be developed in different direc- tions. Thus, while retaining its tubercular character, a posterior lower heel or talon may be formed, and when this develops a single tubercle it is known as the hypocone or hypoconid, the former occurring at the postero-internal angle of the upper molars, the latter at the postero-external angle of the lower. Protoconule and metaconule are smaller intermediate cusps, while the crests which connect the cones and conids are known as lophs. Again, peripheral cusps or styles may arise outside these from the girdle or cingulum of the tooth. The tubercles and lophs of the teeth also vary in character. When the surface is calculated for cutting, the tooth is secodont, when for crushing, it isbunodont; with the development of prom- inent transverse crests the tooth becomes lophodont. When the 366 CLASSIFICATION OF VERTEBRATES. crests, crescentic in character, have a longitudinal direction, the tooth is selenodont. These characteristics may be combined, giving the types called bunolophodont, bunoselenodont, etc. The teeth first formed may be the only ones to appear during life, when we speak of a monophyodont condition as in the monotremes, cetacea, and edentates ; or again, we may have a first or milk dentition to be replaced later by a permanent set (diphyodont 1 ). Milk and permanent dentitions are not exact repetitions of each other, more molars appearing in the perma- nent than in the milk dentition. This leads to a differentiation of the molars into premolars (bicuspids of the dentist) which occur in both sets, and molars proper, which appear only with the permanent dentition. In homodont dentition the number of teeth is very large, and may vary between one and two hundred. In heterodont types this number is greatly reduced. It is greatest in the marsupials, where there may be five incisors and six molars on either side in either jaw. In placenta! mammals the incisors never exceed three, and the full dentition may be stated as including 44 teeth. JMot infrequently, as in rodents and rumi- nants, the canines may be "lacking, producing a gap or diastema between incisors and premolars, while not infrequently the incisors may not be developed in the adult. To express in concise form the number of teeth present in any mammal a matter of great importance in classification a dental formula has been adopted, in which the kinds of teeth are represented by the letters z, c, p, and m, while the number of teeth in upper and lower jaws are represented by figures above and below a line. Since the two halves of either jaw are mirror-like repetitions, only the teeth in one side are repre- sented. The dental formula of the adult man is expressed thus:- i f, c },/ |, m I = 32; that of the horse, i , c \, p }, m % = 40 ; that of the cow, i , c %, p |, m I = 30. 1 Stirling has described teeth in the marsupial Myrmecobius, formed before the milk set, which, taken in connection with the studies of Kuchenthal and Rose, show that the marsupials, like most other mammals, are diphyodont, and may lead to the conclusion that the milk dentition must be a second set, and the permanent teeth a third. MAMMALS. 367 A few other facts concerning the teeth may be added. Occasionally, as in rodents and elephants, the incisors may have persistent pulps, and hence may continue to grow through- out life. The enamel is lacking in a few forms, like the eden- tates and the dugongs. The milk dentition is lacking in some rodents ; in the guinea-pig the milk dentition is shed in utero, and in the seal it never cuts the gums. Finally, there is such correlation between the teeth and other structural features, that the dentition affords an index to the classification, and hence becomes of great assistance to the paleontologist. FIG. 351. Diagrams of stomachs of, A, horse; B, pig; C, Lagenorhynchus ; D, ziphioid whale; E, seal; F, rat. d, duodenum; o, oesophagus; /, pylorus; oesophageal region horizontally lined ; cardiac gland region obliquely lined ; fundus gland region dotted; pyloric gland region with crosses; after Oppel. The oesophagus is greatly elongated, extending from the pharynx through the diaphragm to the stomach. Usually the stomach is regarded as the saccular enlargement of the alimen- tary canal, lying between the oesophagus and the intestine ; but when histological and physiological features are taken into account, it is seen that frequently the lower end of the oesoph- agus expands, and takes part in the formation of the gastric enlargement, and that the stomach proper begins only where the gastric glands appear. Of these glands three kinds are recognized, cardiac, fundus, and pyloric, for the characters of 368 CLASSIFICATION OF VERTEBRATES. which reference must be made to histological text -books. On the basis of these glands the stomach may be divided into regions, when it is seen that in the monotremes the morphological stom- ach is entirely lacking, the enlargement which occurs being resophageal. More frequently the cardiac glands are lacking. Speaking of the stomach in the more usual sense, it may be said that usually its axis lies at right angles to the axis of the body, and that only exceptionally, as in the seals, is it longitu- dinal. 1 The stomach may be a simple sac, as in man, but in the cetacea and ruminants it becomes divided into several cham- bers, the extremes of differentiation being reached in the rumi- nants, where (the tylopoda and tragulina excepted) four distinct - regions, the rumen (paunch), reticulum (honeycomb), psalte- rium, or omasum (manyplies), and abomasum may be recog- nized ; but of these only the last is a true glandular stomach, the others being oesophageal enlargements. The intestine is differentiated into small and large divisions, the line between them being marked by the ileocolic valve. The first part of the intestine, the duodenum, is characterized by receiving the ducts of liver and pancreas, and also by the presence of Brunner's glands in its walls. The small intestine is greatly convoluted. The large intestine is of larger diameter than the small, and its walls show outsackings or lobulations. It presents two divisions, a rectum, situated in the pelvis, and comparable to the large intestine of the lower vertebrates, and a much longer colon, which appears for the first time in mam- mals. Just beyond the ileocolic valve is a blind diverticulum, the caecum, which undergoes great variations in size. It is largest in the herbivora, where it may equal the body in length, while in the edentates, carnivores, toothed whales, bats, etc., it may be small or even absent. In many rodents, apes, and man, the distal part of the caecum becomes reduced in size, and forms the appendix vermiformis. The rectum terminates, except in the monotremes, in an anus dorsal to the urogenital opening. In that group it and the urogenital system empty into a cloaca as in the sauropsida. 1 In the seals (Fig. 351 ), it is only the oesophageal part of the stomach that is longi- tudinal, the true stomach being transverse. MAMMALS. 369 The liver, often lobed in a complicated manner, is divided into right and left halves by the ligamentum teres, itself a ves- tige of the earlier umbilical vein. The left half is frequently sub-divided into left and central lobes, while the right also is usually sub-divided, and may have a caudate lobe laterally placed, while a spigelian lobe projects dorsal to the entrance of the portal vein. A gall bladder, which arises as a diverticulum of the hepatic duct, is rarely absent (horse, whales, and some rodents). The pancreas is usually compact, but in some rodents it is diffuse. Its duct, as a rule, unites with the hepatic duct ; but occasionally these may empty into the duodenum at points widely remote from each other. As in the birds, the heart is four-chambered ; the divisions occasionally are visible from the outside as in the dugong. Its ma- jor axis is horizontal except in the anthropoids and man. The arch of the aorta bends to the left, a condition ref- erable back to the fact that it is the persistent (fourth) primitive arch of the left side. From the proximal portion of the aorta there are given off, first, the coronary arteries (usually paired), which go to the walls of the heart, and then the subclavians and carotids, the arrangement of which shows many variations ; the most usual condition being first a right arteria anonyma, dividing later into right subclavian and the two carotids, and then the left subclavian. Other arrange- ments can be seen from the diagram. In all cases the right subclavian is in part the persistent right fourth arch of the embryo. The internal carotids enter the cavity of the brain either through the periotic (petrosal) bone, or between it and the base of the skull. FIG. 352. Heart of dugong, after Macal- lister, showing the double character of the ventricles, l>, e ; a, d, auricles ; c, pulmonary aorta ; /, systemic aorta. 3/0 CLASSIFICATION OF VERTEBRATES. In the venous system the most noticeable points are the pres- ence of valves, at least in the veins of the extremities. Both pre- and postcavae empty directly into the right auricle without the presence of a sinus venosus. In rodents, monotremes, and the elephants, two precavae occur. Rete mirabilia are frequent in various situations. The red blood corpuscles are anucleate and, except in tylopoda where they are oval, are circular disks. The lymph vessels, which contain numerous valves, empty by means of a principal or thoracic duct into the precava near the subclavian vein. In their course are numerous lymph glands. Closely related to the lymph system are a couple of masses of adenoid tissue, the tonsils, peculiar to the mammalia, placed at the entrance of the pharynx. The entrance to the trachea (glottis) is covered by a fleshy fold, the epiglottis. The larynx is well developed, with aryte- FiG. 353- Modifications of the aortic arch and its vessels in mammals, from Wiedersheim. Ao, aorta ; c, carotid ; s, subclavian ; tb, truncus brachiocephalicus (anonyma); tbc, truncus brachiocephalicus communis. noid, cricoid, and thyroid cartilages (p. 28), and these, moved by appropriate muscles, put various tensions, etc., upon the vocal cords. The cartilaginous tracheal rings are usually in- complete behind ; the trachea itself is never convoluted, and it divides behind into two bronchi, with occasionally a secondary bronchus on the right side. Inside the lung the rule is a single eparterial bronchus arising above the entrance of the pulmonary artery, and nine hyparterial bronchi on either side. Air sacs never occur. The inspiration and expiration of air is effected in part by the intercostal muscles, which by their action alter the size of the thoracic cavity, and in part by a transverse muscular par- tition, the diaphragm, which divides the abdominal cavity of the lower vertebrates into an anterior or pleural cavity containing MAMMALS. 371 the lungs, and a peritonial cavity containing the other viscera (see p. 1 06). This diaphragm, while foreshadowed in some sauropsida, is only developed in the mammals. The functional kidneys (metanephridia, p. 122) are small compact organs, only occasionally, as in some seals and whales, showing lobulations. The ureters leading from them empty into the dorsal part of the urinary bladder. The urethra lead- ing from the bladder either empties into the cloaca (mono- tremes) or into the urogenital sinus. No renal portal system occurs. In the monotremes and whales the testes remain in their primitive position near the kidneys, but in all other mammals they sink into the pelvis. In the elephants they do not proceed farther ; in the rodents, bats, and some insectivora, they emerge during the breeding-season into a temporary sac or scrotum, and after this time is passed are retracted again by a cremaster muscle. In the other mammals the testes remain permanently in the scrotum after their descent, and the opening through which they descended closes. The cremaster muscle persists, but with more limited functions. Closely connected with the male genitalia are the prostate and Cowper's glands, the ducts of which empty into the genital duct (vas deferens), the secre- tion being added to the spermatozoa, rendering the whole more fluid. The ovaries are relatively small, and are always abdominal in position. The oviducts have their inner ends wide, the inter- nal apertures being usually fimbriate. In each duct three re- gions occur, (i) a somewhat narrow Fallopian tube leading to (2) a uterus with muscular walls, and (3) an external canal or vagina. In the lower mammals the ducts of the two sides may remain distinct, but in the higher fusion begins at the lower end, resulting in a single vagina and a uterus, which shows more or less clearly traces of its double origin. In the monotremes the large eggs, covered by a flexible calcareous shell, pass to the exterior, but in all other mammals the embryo passes through a considerable portion of its devel- opment in the uterus, and is brought into the world in a more or less perfect condition. 3/2 CLASSIFICATION OF VERTEBRATES. The development of the mammals pursues two distinct types. In the monotremes there are eggs which are laid, and which un- dergo their development outside the body of the mother, as do those of birds and most reptiles. These eggs are large (about two centimetres in diameter). They have a large yolk, and the segmentation is restricted to a small portion of it, just as is the case in the sauropsida ; i.e., they are meroblastic. In all other mammals the egg is much smaller, even micro- scopic in size, and the early stages of development are passed inside the mother, the young being born alive. These smaller eggs undergo a total segmentation, all parts dividing ; i.e., they are holoblastic. During this process the egg increases greatly in size by the absorption of fluid which fills the central cavity. As a result the egg is converted into a large sphere (blastula) covered by a single layer of cells except at one pole, where there are a number of ' inner cell-mass cells ' beneath the others. Concerning these layers there is much difference of opinion, due to the great difficulty surrounding the subject. According to one view the outer cells are ectoderm, the inner entoderm ; according to another the inner cell-mass is ectoderm, the outer entoderm, while a third view sees both ectoderm and entoderm in the inner cell-mass. Certain it is that the region of the inner cell-mass eventually becomes two-layered, and later the embryo is outlined here, only a portion of the blastula being utilized in its formation, the rest forming a yolk sac, in the walls of which omphalomeseraic vessels are developed later, although no yolk occurs. This development of a complete yolk apparatus, as well as several other peculiarities, is to be explained upon the hypothesis that the mammals have descended from forms like reptiles or amphibia which were oviparous, and the embryo had to de- pend upon the food stored up in the yolk. Subsequently, as a result of an internal development and a supply of nourishment from the mother, the yolk was lost ; but heredity has caused certain features not incompatible with uterine development to be retained. The mechanism by which this nourishment from maternal sources is transferred to the embryo has now to be outlined. MAMMALS. 373 Like the other amniotes the mammalian embryo forms the foetal structures amnion, serosa, and allantois (p. 288). Of these the serosa is the outermost, and necessarily comes in con- tact with the uterine walls. In most marsupials the develop- ment goes little farther. From the uterus is secreted nutrient fluid which passes through the serosa by osmosis, and is thence taken up by the embryo, furnishing it with the material for growth, which in oviparous forms is supplied by the yolk. In Perameles, one of the marsupials, and in all the higher mammals, a more intimate union occurs between the embryo and the uterine walls in the following manner. From the sur- face of the serosa (which from this time on is known as the chorion) numerous outgrowths or villi are formed. These villi are variously arranged in different mammals. They may be distributed evenly over the whole chorionic surface (diffuse), or they may be collected in tufts scattered over the surface, the intermediate regions of the chorion being smooth (cotyledonary)- ; again, they may form a girdle around the chorion, the ends being free from villi (zonary) ; or, lastly, they may be restricted to a more or less circular patch on one side of the chorion (discoidal). These villi enter into more or less intimate connection with the uterine walls in ways to be described below. The allantois (p. 289) grows out from the body, and finally reaches the inner surface of the chorion, carrying with it the allantoic blood-vessels. The union of chorion and allantois is coextensive with the development of the villi upon the outer surface, and the resulting structure forms the embryonic por- tion of the placenta. The blood-vessels of the allantois may be confined to that structure, or they may extend out into the cho- rion, but in either case they carry away from the embryo waste which passes, by osmosis, to the maternal tissues, and at the same time bring back to the growing young nourishment and oxygen, which pass into the foetal blood by osmotic action. In no case is there a direct connection between maternal and foetal blood-vessels ; but the exchange is always of the character indicated here. It must, however, be noted that the relations of the allantois to the chorion follow two types. In the unguiculate mammals 374 CLASSIFICATION OF VERTEBRATES. the allantois early grows out to join the chorion, and brings with it its blood-vessels, which then ramify through the chorion, which therefore has its own circulation, although this is depen- dent upon the allantois. In the ungulates the allantois, although well developed, re- mains for a consid- erable time distinct from the chorion, and only later, when its expansion brings it in contact with the lat- ter, does the chorion receive its vascular supply. These two types are known re- spectively as the al- lantoic and chorionic placenta. In many mammals the union between the villi of the chorion and the uterine walls is slight, and at the time of birth the two separate, only the em- bryonic placenta be- ing cast off. These forms including the ungulates, cetacea, si- renia are called non- deciduata, or indecid- uata. In others the union is far more intimate, the branched villi entering into such close connection with the uterus 1 that, at the time of birth, a portion or all the uterine walls (the decidua) is cast off with the embryonic, or foetal, placenta. In some mammals, as in man, the decidua exhibits certain peculiarities. At the time of at- i This forms the uterine placenta. FIG. 354. Diagram of human uterus and placenta, based on Wiedersheim. Foetal parts lined, uterine dotted, the decidual portions darker. A, cavity of the amnion; /', foetal placenta; Z, chorion laeve; 7\, decidua reflexa; 6", maternal placenta (decidua sero- tina); 7', entrance of Fallopian tube; F, decidua vera. MAMMALS. .375 tachment of the ovum to the walls of the uterus, these walls rise up over and enclose the egg, thus coming in contact with it on all sides. From but one side, however, is the (discoidal) pla- centa developed, and in this region the decidua is spoken of as the decidua serotina, while that which covers the smooth or non- villous portion of the chorion (chorion laeve) forms the decidua reflexa. The rest of the uterine walls, which do not connect with the ovum, are also cast off at birth, and these form the decidua vera. In older books the eutheria of the following pages are fre- quently divided -into the Implacentalia, including the marsupials, and the Placentalia, including the remaining orders ; but the recent discovery that at least one genus of marsupials (Pera- meles) has a true allantoic placenta tends to break down this line. Still the distinction is one of convenience, and has been used in these pages, the term placentalia including all the orders from edentates to primates, the implacentalia, the marsupials, and frequently the monotremes, when these have not been specially mentioned. There are two views as to the origin of the mammals ; the one that they have descended from the theromorphous reptiles, the other that they have sprung from the amphibia. The first of these receives its chief support from paleontology. The theromorphs have a heterodont dentition, a triple occipital condyle from which the paired condyles of the mammals can be derived by a suppression of the basioccipital portion, as well as several features in the skeleton of the limbs. The advocates of this view suppose that the quadrate has disappeared in the region of the glenoid fossa. The amphibian view receives its support in the double occip- ital condyle, the impossibility of deriving the mammalian ovum from that of any known reptiles, and its easy homology with those of amphibia, and in the relations of the ear bones. This view recognizes the quadrate in the incus, and this articulates with the stapes, a condition repeated in the urodeles, but not derivable from anything known in the reptiles (see p. 1 59). Another difficulty with the reptilian hypothesis is the impos- sibility of deriving the mammalian hair from any exoskeletal 376 CLASSIFICATION OF VERTEBRATES. structures occurring in reptiles ; while there are several features which point to the possibility of their origin from the dermal sense organs of the amphibia. SUB-CLASS I. PROTOTHERIA. Mammals with a single opening for urogenital system and alimentary canal ; sutures of skull obliterated in the adult ; a well-developed coracoid and episternum ; oviparous. ORDER I. MONOTREMATA (ORNITHODELPHIA). Prototheria with small corpus callosum and large anterior commissure ; no teeth in the adult ; epipubic bones present ; ribs with capitular head only ; mammary gland without distinct nipple. The few existing species of monotremes are restricted to the Australasian region, and the only fossils certainly belonging to the order occur in the pleistocene of Australia. These mammals are remark- able for the large number of sauropsidan features which they present. Besides the characters given in the diagnosis the follow- ing may be added. The ossicula auditus are of a low grade, the malleus being large and the stapes columelliform. In the embryo of the duckbill multituberculate teeth occur, but these are lost before maturity, and the adults of all species are toothless. Lips are lacking, and the jaws form horny beaks. The brain is smooth in Ornithorhynchus, convoluted in Echidna, The testes are abdominal in position ; the left ovary is reduced as in birds, the right lobular. There is a horny perforated spur developed on the hind legs in con- nection with a gland. This spur disappears in the adult female duckbill. Family ORNITHORHYNCHIDJE. With duck-like bill, two horny teeth in each jaw; feet pentadactyl, webbed; tail flat; soft, close fur. Ornitho- rhynchus paradoxus, the duckbill of Australia and Tasmania, is the only FIG. 355. Em- bryonic teeth of Ornithorhynchus, after Stewart. MAMMALS. 377 known species. It leads an aquatic, burrowing life, and feeds upon worms and small aquatic animals r using its bill as does a duck. Family ECHIDNID^:. Beak elongate, toothless; tongue elongate, ver- miform ; body with strong spines among the hair. Echidna, with three species from Australia, New Guinea, and Tasmania, has all the toes clawed. In AcantJioglossus, from New Guinea, there are claws on but three toes, and the beak is longer. All of these spiny ant-eaters are burrowing animals, feeding chiefly upon ants. Echidna occurs as a fossil in the Australian pleistocene. FIG. 356. Duckbill, Ornithorhynchus paradoxus, from Liitken. The earliest fossil mammals yet found occur in the triassic of North Carolina, South Africa, and Germany. Little is known of them except of their jaws and teeth. Allied forms are more abundant in later rocks, and some of them persist until the eocene. From peculiarities of the teeth, which pre- sent certain resemblances to the embryonic teeth of Ornitho- rhyncJnis, these fossils are sometimes placed as members of the Prototheria, an example followed here ; although they also present resemblances to the marsupials. ORDER II. PROTODONTA. Incisors reduced, molars with compressed cutting crowns and undivided roots. Represented only by lower jaws of Droma- therium and Microconodon from the American Jurassic. ORDER III. MULTITUBERCULATA (ALLOTHERIA). Incisors enlarged, molars tuberculate with distinct roots. In these forms, which are represented by several genera, the teeth CLASSIFICATION OF VERTEBRATES. are very numerous, ranging from 48 to 68. Plagiaulax from the Purbeck beds (upper Jurassic) of England ; Ctenacodon y American Jurassic ; Chirox and Poly mastodon from the Puerco (lower eocene) of America. The Australian quaternary Thy- lacoleo, usually regarded as a marsupial, may belong here. SUB-CLASS II. EUTHERIA. Mammals with anus distinct from the urogenital opening ; sutures of the skull well marked ; episternum reduced ; coracoid not articulating with the sternum, but reduced and fused with scapula ; viviparous ; mammae with teats. Legion I. Didelphia. Eutherian mammals, with small corpus callosum, usually with marsupial bones (except in Thylacinus). Vaginae partially or completely double. As a rule no placenta developed. ORDER I. MARSUPIALIA. Teeth always present, only one (/ 3 ) replaced by a second dentition, the number usually different in upper and lower jaws ; two precavae present ; mammae abdominal in position and usually enclosed in a pouch in which the very immature young are placed after birth. The order marsupialia and the legion didelphia are coexten- sive. The living species are almost exclusively confined to Australia and the adjacent islands ; the only exceptions being the family didelphidae, which is American. Fossils, however, are found in Europe as well. Forms certainly belonging to the order first occur in the eocene ; but others, possibly related, date from the cretaceous. The order receives its name from the pouch (marsupium) in which, in most species, the young are carried after birth ; but this pouch is not invariably present, the young in these cases being held in the fur covering the abdom- inal region. When first born the young are very immature. They are transferred by the mother to the nipples, to which they adhere closely. Milk is forced into their mouths by mam- mary muscles, and strangulation of the young is prevented by a .MAMMALS. 379 prolongation of the larynx into the choana, much as in the whales. In development the ova pass into the uterus, from which they absorb nourishment without the intervention of a placenta (except in Perameles), no villi being developed on the serosa, and the allantois failing to reach this envelope. An osteological pe- culiarity, present in all except in Tarsipes, is the inflection of the posterior angle of the jaw. In size the marsupials vary from animals the size of a rat, up to the giant kangaroo ; while in the past Diprotodon was as large as FIG. 357. Skeleton of kangaroo, from Macallister. a rhinoceros. In form and habits they show many modifica- tions, usually attributed to the fact that in Australia they have been removed from competition with other mammals, and have developed in every direction, terrestrial, crawling, leap- ing, climbing, and soaring forms. The majority are nocturnal. SUB-ORDER i. POLYPROTODONTIA. Incisors -? -, small, subequal ; canines larger ; molars acutely tuber- 4 or 3 culate. The DIDELPHID.E, opossums, American ; teeth i \,c\tp\ y m%\ feet all five-toed ; tail partially naked and usually prehensile. Didelphys virginiana* north to New England ; other species in the tropics. Chironectes has webbed feet. Didelphys occurs in the eocene of France and America. DASYURHXE -,c-,p~ - -,m-;\ hind feet four-toed. 3 i ^ 2 or 3 4-6 ' Thy I acinus is dog-like, carnivorous, and occurs in Tasmania. Myrmecobius with m f , feeds on ants. Dasyurus, Phascogale. Allied forms fossil in lower tertiary of South America and later tertiary of Australia. The PERAMELID^E, z f , c \, p f * 380 CLASSIFICATION OF VERTEBRATES. .m , include the genera Perameles, in which the feet are much alike, and ^Chczropus, in which the hind legs are very long and the fourth toe alone functional. The bandicoots (Peraweles) are no- ticeable from the existence of a placenta. Fossils, which in some respects closely resemble the polypro- todonts and in some the insectivores, are the TRI- CONODONTA and TRITUBERCULATA, with the genera Amphilestes, (Jurassic, England and the U. S.), Dicrocynodon (Jurassic, Wyoming), Amphitherim (English oolite), Dryolestes (Jurassic, Wyoming), etc. FIG. 358. Opossum, JDidelphys virginiana, after Audubon and JBachman. SUB-ORDER 2. DIPROTODONTA. Incisors _, the central ones large, the others reduced ; canines small or absent ; molars with blunt tubercles or transverse ridges. In the kangaroos and wallabies (MACROPODID/E) the hind legs are very large; the feet as in Perameles : the teeth / _, c or_, ft , m _; tail i o o y 2 or i 4' very large. The larger kangaroos belong to Macropus ; the arboreal tree- kangaroos to Dendrolagus. Macropus, Palorchestes, etc., occur in Austra- lian pleistocene. The PHALANGISTID^E includes climbing and flying '(soaring) forms, with legs of equal size, teeth i . , c -,p -, in -, tail long. I O 21 4 Tarsipes is an aberrant form about as large as a mouse. Pet aunts, Be- lidius, etc., resemble the flying-squirrels in the lateral fold of skin and ilying habits. Cuscus and Phalangista resemble the opossums in their prehensile tail. Phascolarctos, the koala, contains but a single climbing .species two feet long. The THYLACOLEONID^E includes large fossil forms from the Australian pleistocene, with teeth * f , c ^, p f , ;// \. The kan- garoo-rats, or HYPSIPRYMNID/E, with teeth i |, c \, p \ m \, resemble the kangaroos in the disproportionate hind legs. Hypsiprymnus, Bettongia, the last also in the Australian pleistocene. The DIPROTODONTID^: in- cludes only fossil forms of large size from the Australian pleistocene, with the teeth / f , c , p \, m |. Diprotodon australis was larger than a rhi- noceros ; the species of Notothenum somewhat smaller. The PHASCOLO- MYID^E, with a dental formula i \, c ^, p \, m |, differ from all other mar- supials in the presence of persistent dental pulps. The living wombats all belong to Phascolomys, which also occurs in the pleistocene. The extinct Phascnlonus was as large as a tapir. South America has yielded several fossil diprotodonts of eocene or miocene age, and one recent species, ^Ccenolestes obscurus, has been described from Colombia. MAMMALS. 381 Legion II. Monodelphia (Placentalia). Eutherian mammals with well-developed corpus callosumr and small anterior commissure ; no marsupial bones ; vagina single ; foetus nourished by an allantoic placenta. ORDER I. EDENTATA (BRUTA). Placental mammals with the incisors, and occasionally all the teeth, lacking. Teeth when present, usually prismatic ; molars without enamel. Carpals and tarsals usually in linear series (taxeopodous, p. 392) ; digits armed with long, com- pressed, and pointed claws. The Edentata includes a rather heterogeneous assortment of forms, the range of variation being even greater when the fossils are considered. Most of the species are not strictly edentulous, since molars are usually present. These are homo- dont, and except in Tatusia they are monophyodont and have persistent pulps. The skin is covered with hair, horny scales, or bony shields, these sometimes uniting into a more or less complete armor for the body. The mammae are thoracic or abdominal in position. The cerebral hemispheres are small. The placenta shows great variations ; it may be deciduate or not ; in shape it may be diffuse, discoidal, or of discoidal lobes,, or zonary. The edentates are given a position here at the base of the- placental mammals because of their low grade of structure- In some respects, as in the simple condition of the brain, this low grade is primitive ; but in other respects, as in skeleton and teeth, the group is clearly degenerate, although as yet it is uncertain from what group they have sprung. According to* Cope they have probably descended from the group of tillo- dontia of the later cretaceous and eocene. The earliest fossil edentates known occur in the Santa Cruz beds of Patagonia, regarded by Ameghino as eocene, but by some as oligocene ; and it is interesting to note that these early forms retained traces of enamel upon the teeth. The group, as a whole, belongs to the tropics and the south- 382 CLASSIFICATION- OF VERTEBRATES. ern hemisphere, but few species straying north of the tropic of cancer. In times past they had a greater range ; for while the centre even then was in the south, a few species occurred as far north as southern Europe, and to latitude 46 in the new world. The American forms differ from those of the old world In the existence of articular processes, besides the normal zyga- pophyses on the presacral vertebrae. These have therefore been called Xenarthra in contradistinction to the Nomarthra of the eastern hemisphere. To the Nomarthra belong the sub-orders Fodientia and Squamata ; the other sub-orders are xenarthrous. SUB-ORDER i. FODIENTIA. Body covered with sparse, bristle-like hairs, five prismatic molars in each jaw ; femur with a third trochanter, toes four in front, five behind ; placenta zonular. Only the single family, ORYCTEROPODID^E, containing the aardvark, Orycteropus capensis, of South Africa and a fossil species from the miocene of the island of Samos. The aardvark lives a burrowing life, feeding upon ants and other insects. It is about as large as a pig. iro. FlG. 359. Pangolin, Manis longicaudata, from Montei SUB-ODER 2. SQUAMATA. Body covered with overlapping horny scales and scattered hairs ; jaws toothless ; tongue long, vermiform ; feet five-toed ; placenta non-deciduate, diffuse. Contains the single family MANID^E, the scaly ant-eaters or pango- lins of Asia and Africa. Only genus Mam's, which also occur fossil in the pleistocene of Asia. All the species are arboreal and insectivorous ; and have a somewhat reptilian appearance on account of the scaly body and long tail. MAMMALS. 383 SUB-ORDER 3. VERMILINGUIA. Body hairy ; skull very long ; no teeth ; tongue very long and mobile ; tail elongate ; hind feet five-toed ; placenta deciduate, dome-like or dis- coidal. The ant-eaters form the family MYRMECOPHAGID^E, all of which live in tropical America, where they feed upon ants and other insects ; a few are arboreal. Myrmecophaga jubata, the great ant-eater, five feet long, lacks a claw on the fifth fore toe. In Cyclotura only digits 2 and 3 are clawed. Fossils (Scotaops), supposed to belong to this sub-order but with two molar teeth, occur in the Patagonian eocene. SUB-ORDER 4. TARDIGRADA. Body haired ; head short and rounded ; molars _. The BRADYPO- 4 or 3 , or sloths, have cylindrical teeth ; short, weak tail ; long, slender limbs ; digits armed with long, strong claws ; and deciduate, dome-like pla- centa with numerous discoidal lobes. Bradypus, the threfe-toed sloths ; Chol&pus, the two-toed sloths. Both genera have the hind feet three-toed, and are noticeable for the number of cervical vertebrae (p. 355). They are arboreal, and live almost entirely in the trees. Entelops from the eocene of Patagonia. The extinct MEGATHERIID/E includes giant edentates from the pleistocene of both Americas. They had prismatic teeth of peculiar struc- ture ; large, long tails and stout limbs ; feet 3-5 toetl. Megatherium from South America, and one doubtful species from the U. S. The largest species equalled an elephant in size. Megalonyx, first described by Thomas Jeffer- son, and My lotion ranged north to Pennsylvania. Zamicrus, Patagonian eocene. There is some evidence that a species of Mylodon (Neomylodon) still persists in Patagonia. SUB-ORDER 5. LORICATA. Body with armor of bony plates ; teeth prismatic, usually - -- GLYPTODONTID^E ; trunk plates united into a solid carapace, with other plates on the tail ; dorsal vertebrae fused to a continuous tube. Tertiary and pleistocene of South America and north to U. S. Glyptodon, Hoplophoms, Parocthus. These resembled turtles in appearance. One species 12 feet long. DASYPODID^E, dermal armor in three or more movable transverse rows, vertebrae free. These armadillos first appear in the Patagonian eocene, and continue until the present. The living species are small, nocturnal, car- o nivorous forms. Chlamydophorus ; teeth _ ; armor of about 20 trans- 8 to 9 verse bands : body truncate behind. Dasyfius ; teeth -5 - . ; armor of 10 to 9 two shields upon scapular and pelvic regions, with six or seven bands be- tween. Xolypeutes with three bands ; Xennrus with twelve or thirteen. Tatusia has to 7 teeth, all except the last preceded by milk dentition ; o to 7 384 CLASSIFICATION OF VERTEBRATES. seven to nine movable armor bands. Tatusia novemcincta is the only arma- dillo entering the U. S. Chlamydotherium from the pleistocene of Florida and Patagonia stands nearest the glyptodonts. Peltephilus, eocene of Patagonia. FIG. 360. Nine-banded armadillo, Tatusia novemcincta, from Liitken. ORDER II. INSECTIVORA. Small plantigrade mammals, usually with five toes armed with claws ; carpals and tarsals usually in linear series ; denti- tion complete, the incisors never less than two ; canines little differentiated and weak ; molars bunodont or lophodont, the cusps acute ; clavicles almost invariably present ; brain small, cerebrum without convolutions ; placenta deciduate, discoidal. The insectivores owe their name to the fact that the major- ity feed upon insects or other small invertebrates. They are all small, and the structure points to a low stage of organization. The body is covered with fur, and spines are not infrequently developed. The milk dentition is lost at an early date, and rarely is functional. The canine teeth are not sharply differen- tiated from the incisors or premolars, and the latter are sharp sectorial. The upper molars have three or four cusps. The testes are internal, and are never enclosed in a scrotum ; the uterus is bicornuate. In a few genera vertebral intercentra occur in the dorsal region, a condition not paralleled in other mammalia. Among the more superficial but still very charac- teristic features is the prolongation of the muzzle far beyond the lower jaw. Most of the order are nocturnal burrowing animals, only a few being aquatic or arboreal in habits. In external appearance they resemble the smaller rodents ; but in structure they are more like the bats, with other resemblances to the polyproto- MAMMALS. 385 dont marsupials, the creodont carnivores, and the lemurs. They inhabit to-day only the old world and North America ; while the fossils occur only in the northern hemisphere, where they date back to the eocene. The order is one of the most primitive of the placental mammals ; but as yet the fossils are too few and too imperfectly preserved to allow the complete working out of the lines of descent. As here limited the order includes only the Insectivora Vera. By some writers the galago, Gateopithecus, of the East Indies (seep. 415) is included in a second sub-order, Dermaptera. The ICTOPSID^E, from the American eocene and miocene, have skulls much like the hedgehogs, but a simpler dental pattern. The ADAPISORI- CID,E take their place in the eocene of France. The TALPID^E, or moles, with / | to f, c \, PHI % to f , ;// f , snout elongate, tympanic bulla present ; fore limbs modified for digging, with a sesamoid bone (os falciforme) on the radial side ; tibia and fibula united ; are small burrowing animals, of which Talpa is the typical genus, with / f , c \, p |, m f . The species of Talpa belong to the temperate part of the old world. In America occur the genera Scalops, with 36 teeth, and Condylura, the star-nosed mole, with 44 teeth. Talpa dates from the miocene, Talpavus from the miocene. Allied are the MYOGALID.E, in which the falciform bone is absent. Urotrichus, the mole shrew, is the only North American genus. In the TUPAIID^E, in which the lower incisors are never less than two, the tibia and fibula are distinct, and the orbit is encircled by bone. The species are oriental in their distribution, and have arboreal habits. Titpaia. Galerix, from the European eocene. The shrews (SORICID^E), which appear in the eocene, are distributed through the northern hemisphere. They lack the postorbital process, have tibia and fibula fused, and no zygomatic arch ; teeth i \ to f , c ^, p ^-, m \. Sorex\s represented by many species in both hemispheres. Blarina is American. Crocidura, Nectogale. The DIMYLID.E includes miocene species. In the ERINACEID^E, or hedgehogs, the dorsal surface is covered with spines or bristles. All of the species belong to the old world, and are terrestrial and nocturnal. The hedgehogs belong to Erina- cens, a genus which appears in the miocene. The species of MACROSCEL- ID/E from Africa are known as the jumping shrews, from their kangaroo-like gait. The SOLENODONTID^E from the West Indies are remarkable for having the mammae on the buttocks. The tenrecs (CENTETID.E) are from Mada- gascar. The golden moles (CHRYSOCHLORID/E) of Africa have the hair of a brilliant metallic lustre, bronze, green, or violet in color ; the eyes are cov- ered by the integument, and the external ears are concealed by the fur. The last four families have no fossil representatives, but are nearest in structure to the Ictopsidas. 386 CLASSIFICATION OF VERTEBRATES. ORDER III. CHIROPTERA. Flying mammals, in which the anterior limbs are modified into supports for the membranous wings ; dentition complete, the canines- strong, the molars buno-lophodont ; the total teeth never exceeding i |, c 1, / |, m | ; mammae pectoral ; testes abdominal or inguinal ; placenta discoidal, deciduate. The bats must be regarded as highly specialized offshoots from the insectivores, with which they closely agree in all essential points except the development of wings. These last FIG. 361. Skeleton of bat, after Brehm. are membranous folds, supported upon a bony framework com- posed of the modified fore limbs and extending back to the hind legs, while an interfemoral membrane may or may not include the tail when this is developed. Muscles to move the wing are attached to the sternum, which develops a keel similar to that of the birds. The modifications in the fore limbs con- sist in an enormous lengthening of the digits with the exception of the pollex, which remains more normal, and may terminate with a claw. The bones are very light, being slender, with large marrow cavities ; the skull varies considerably, and usually possesses a MAMMALS. 387 complete zygomatic arch, while as generally there are no frontal- postorbital processes. The clavicle is present, and the ulna is rudimentary. The brain is small and smooth. The sense of touch is highly developed, the wings being important in this respect, while in many species a peculiar dermal sensory appa- ratus, the ' nose-leaf,' is developed upon the snout. The shape of this, as well as that of the ears, is very variable, and is util- ized in classification. The teeth are closely similar to those of the insectivores ; the milk dentition is poorly developed, and in some instances is lost before birth. The intestine is short shortest in the insectivorous species ; a caecum rarely occurs. The left lateral lobe of the liver is very large, and a gall bladder is present. About four hundred species of bats are known, all nocturnal, and usually gregarious in their habits. Frequently the colonies are found to be composed of individuals of one sex, the sexes only coming together at the breeding-season. There is some evidence to .show that the males, at least in certain species, take part in nursing the young. Fossils are rare ; they first appear in the eocene. No fossil frugivora are known. SUB-ORDER i. ANIMALIVORA (MICROCHIROPTERA). Small bats with acutely cuspidate molars, index finger reduced, usually with a single phalanx, no claw ; stomach simple, intestine short ; outer and inner edges of ear not meeting below ; tail, when present, connected with the interfemoral membrane. The old-world RHINOLOPHID.E, with a nose-leaf, i \, p f or f , and a long tail, includes about fifty species. Rhinolophus occurs in Europe, Asia, and Africa ; Hipposiderus, Asiatic. Rhinolophus occurs in the eocene of France. Closely allied are Nycteris and Megadenna of Asia, in which a tragus is developed in the ear. The VESPERTILIONID.E have a long tail, lack the nose-leaf, and have a tragus to the ear and a variable number of teeth. Plecotus, with an American representative, has i f, c |, p, | m f ; Antrozous, from California, i \, c \, p \, m f. Vesperugo, the largest genus of bats, is cosmopolitan, one species ( V. serotinns} inhabiting both continents. The teeth are / f or i, c \, p f to i, ;// f . Atalapha (i |, c 1' P I or \-> m f)' exclusively American. Vespertilio (i f , c -J-, p -|, m f), cosmopolitan. Vesperugo, eocene, Wyoming. Thyroptera, Brazil. KM- BALLONURID^:, tropical or subtropical, the middle upper incisors large and close together, no nose-leaf; a distinct tragus, and obliquely truncate muz- 3 88 CLASSIFICATION OF VERTEBRATES. zle. Emballonura, old-world tropics ; Noctilio and Molossns, tropical America. PHYLLOSTOMID^E, tropical America; have three phalanges to the middle finger, nose-leaf present, tragus well developed. Chilonycteris, Vampyrus, Glossophaga. Desmodus includes the blood-sucking or true vampire bats. SUB-ORDER 2. FRUGIVORA (MEGACHIROPTERA). Large bats with smooth-crowned quadrituberculate molars, index finger with three phalanges, clawed ; sides of the ear connected below ; tail, when present, below the interfemoral membrane ; fruit-eating. The only family is the PTEROPODID^E of the East Indies; the species of which are generally known as flying-foxes. About 70 species, 40 being included in Pteropus. ORDER IV. RODENTIA (GLIRES). Placental mammals, with the extremities bearing claws, or more rarely hoof-like nails ; feet plantigrade or subplantigrade, usually pentadactyl ; condyle of lower jaw moving in an elon- gate glenoid fossa; teeth diphyodont ; canines absent ; incisors long, \ or f , with persistent pulps ; molars (including premolars) varying from f to f ; placenta discoidal, deciduate. FIG. 362. Skull of muskrat, Fiber zibithecus. The rodents are as sharply marked off from the other mammals as are the sirenians or whales ; no forms, living or fossil, serving to connect them with the other orders, unless, possibly, with the tillodontia. Especially characteristic of the group are the gnawing incisors, in which the enamel is on the anterior face, the resulting wear keeping these constantly with MAMMALS. 389 a chisel-like edge ; 1 the persisting pulp renews all loss by wear. Between the incisors and the molars is a wide gap or diastema. The molars may be lophodont, bunodont, or prismatic. As a whole, the dentition varies between i \ , c , / f, m f (hares) and i \ y c $, p #, m f ; the most usual being i \, c %, p , m . The milk dentition does not include the incisors as a rule, and in some cases, as the guinea-pigs, is lost before birth. The skin may be covered with the softest fur (chinchillas), or certain hairs may be developed into enormous spines, as in the porcupines ; or again, the spines may be flattened ; not infrequently are there scales on the tail. Sternebrae occur in the sternum. The skull usually presents an interparietal bone; the nasal bones are large and long ; the orbits and the temporal fossae are confluent, and especially characteristic is an infra- orbital canal through the zygomatic process of the maxilla. The clavicle may be present or absent ; the manus is usually pentadactyl, but the thumb may be reduced, while in the hind foot both hallux and minimus may be lost. Usually (except myoxidae) there is a large intestinal caecum; the brain is small, and the cerebral hemispheres, which never cover the cerebellum, are smooth. The testes are inguinal or abdominal in position, while there is either a uterus bicornis or two distinct uteri. The mammae vary extremely between the two found in guinea-pigs and the ten in some rats. About nine hundred living species of rodents are known, and they occur in all regions of the world except the Australian. They are mostly small and are mostly arboreal ; although terres- trial, burrowing, and aquatic species occur. All are herbivorous. The order appears in the eocene, but has its greatest develop- ment in the present time. The genera are not equivalent to those in the preceding orders. SUB-ORDER i. SCIUROMORPHA. One incisor in the upper jaw, molars f ; clavicle present ; tibia and fibula distinct. Mostly belong to the northern hemisphere, where they appeared in the eocene. 1 Similar conditions exist among the diprotodont marsupials, in Typotherium and in some multituberculates. 390 CLASSIFICATION OF VERTEBRATES. SCIURID^E ; molars f or |, fore feet four-toed, hind pentadactyl ; tail covered with fur. To the family belong the woodchucks (Arctomys}, prairie dogs (Cynomys}, gophers (Spermophilus} , chipmunks (Tamms}, squirrels (Sa'urus), and flying-squirrels (Pteromys and Sciuropterus) , the latter sailing, rather than flying, through the air by means of an inter- membral membrane on either side of the body. Sciurus appears in the eocene. The CASTORID^E, or beavers, the habits of which are so well known, have the molars , the feet webbed, and the tail flattened and scaly. Castor fiber, the beaver, formerly ranged over the northern parts of both continents, but has been greatly restricted. The genus dates from the pliocene ; the allied Stenofiber is miocene. The small families HAPLO DONTIDJE and ANOMALURID.E are represented by Haplodon, the sewellel of Oregon, and Anomalurus, a flying squirrel-like form from Africa. The fossil family, ISCHIROMYID^E, occurs in the eocene and miocene of North America. SUB-ORDER 2. MYOMORPHA. Incisors ^, molars f or f ; clavicle usually present; tibia and fibula fused. The DIPODID^E, or jumping mice, including Zapus of the United States, Dipus, the jerboas of Europe, and Pedetes of South Africa, have the hind legs long, the toes being 5, 3, and 4 respectively, in the three genera. A much larger family is the MURID^E, in which there are no pre- molars, the molars are f to f , and the tail generally naked and scaly. Over three hundred living species are known. Cricetus, including the hamsters of the old world, and Hesperomys, the white-footed mice of the new, have the molars f . In Arvicola and its allies the tail is round, and the molars rootless. These are commonly known as field-mice or voles. The migra- tory lemmings of northern Europe belong to Myodes. Fiber includes our muskrat. In Mns, which contains our mouse (M. Hiusculus*), and our rats (M. decumanus, the brown rat, and M. rattns, the black rat, the latter driven out by the former), the incisors are narrower and the molars rooted. The family dates back to the later pliocene. The MYOXID^E of Europe, represented to-day by the seven-sleeper, Myoxus gtis, dates from the eocene. The GEOMYID^:, or pocket gophers, receive their name from the enormous cheek pouches. The legs are fitted for burrowing, and the molars Geo- mys and Thomomys occur in our central region. Farther west is Sacco- mys with much more delicate skull. BATHYERGID/E : Spalax, the blind mole-rat of southeastern Europe, and Bathyergus, the strand-rat of South Africa. Lophiomys, a peculiar arboreal rat with hairy tail, from north- eastern Africa, is nearest to Cricetus. SUB-ORDER 3. HYSTRTCOMORPHA. Skull with very large infraorbital canal ; teeth / \, c , / \, m f ; zygo- matic process large, clavicles perfect or imperfect ; tibia and fibula separate. The hystricomorphs appear in the eocene of Europe and South Amer- MAMMALS. 391 ica, the latter region containing the majority of the sub-order to-day. Some of the species, both living and fossil, are giants among the rodents. In the tropical OCTODONTID^ the clavicles are complete, and the molars have internal and external enamel folds; most of the species are terrestrial. Cteuodactylus, from Africa. Octodon, from South America. Myopotamus coypu, of South America, two feet long. HYSTRICID^E ; porcupines ; stout rodents with spines, molars . The species are grouped in two divisions : those of the old world dwell on the ground ; those of the new, climb. Hystrix, with smooth soles and incomplete clavicles, includes the old- world porcupines ; Erethyzon and Synetheres, with tuberculate soles and complete clavicles, those of the new. The latter genus has a prehensile tail. The ChiNCHiLLiD^E, with very soft fur, complete clavicles ; toes five or four in front, four or three behind ; the molars with simple compressed transverse lamellee, are confined to South America. Chinchilla. Lago- stomns includes the burrowing vizcacha. Megamys, of the miocene of Argentina, was as large as, an ox, the largest known rodent. Somewhat closely allied to them was the fossil Castoroides of N. Y. and Ohio pleisto- cene, and the Amblyrhiza from the pleistocene of the West Indies. The CAVIID^E, also South American, have hoof-like claws on the four toes of the fore feet and the three of the hind ; incomplete clavicles, and the molars |- and rootless. Cai'ia contains the guinea-pigs; Hydrochcerus, the capybara, the largest existing rodent. Both these and other genera date from the miocene. The agutis (Dasyprocta) and the paca {C&logcnys) are South American forms with hoof-like claws and semi-rooted molars which form the family DASYPROCTID^E. SUB-ORDER 4. LAGOMORPHA (DUPLICIDENTATA). Infraorbital canal small ; dentition i | , c g, p \ to f , m ; the enamel of the upper incisors extending on to the sides ; molars high, prismatic, without roots ; tibia and fibula distinct. The lagomorphs are readily distinguished by the two pairs of incisors in the upper jaw. The seat of the sub-order is in the northern hem- isphere, but they extend into South America as well. The LEPORID,E, with the premolars f , long ears and incomplete clavicle, includes the hares, and rabbits. Lepus, the principal genus, appears in the miocene of Oregon. About 20 living species. In the LAGOMYID^: the premolars are \ or f , the ears short, and the clavicle complete. The picas (Lagomys} inhabit high altitudes, one species occurring in the Rocky Mountains. The genus appears in the miocene of Bavaria. ORDER V. UNGULATA. Placental mammals with heterodont, diphyodont dentition ; molars with broad tuberculate or ridged crowns ; clavicles almost 39 2 CLASSIFICATION OF VERTEBRATES. always lacking, digits with broad, blunt nails or more usually with hoofs ; digits ranging from five to one, radius and ulna free or united ; scaphoid and lunar bones (p. 177) of carpus always free. If only the living forms were considered the characters of the ungulates and the sub-divisions of the order could be easily given, but the fossil forms, which are especially well developed in our western states, have introduced so many annectent groups that boundary lines tend to disappear, while to almost every character exceptions occur. Were the recent forms alone considered the order would contain only the artiodactyls and perisso- dactyls of the following pages, but the extinct species connect these so closely with the ele- phants and Hyrax that B GDCDQ 000S / CDQQ FIG. 363. Types of carpal bones ; A, in series (taxeopodous) ; B, interlocking (diplarthrous). R, radius ; /, ulna ; c, carpales ; *', intermedium ; r, radiale ; u, ulnare ; 1-5, metacarpals. all must be included un- der a common heading. The existing forms are all terrestrial and with few exceptions herbivor- ous, none being dis- tinctly predaceous. For convenience all of these forms may be divided into the true ungulates (Ungulata Vera or Diplarthra) and the Subungulata, the former including the artiodactyls and the perissodactyls, the latter the other sub-orders : Condylarthra, Amblypoda, Probos- cidia, Toxodontia, and Hyracoida. In the Ungulata Vera the feet are never plantigrade ; the digits never exceed four, the first being suppressed ; the molars are quadritubercular. The mammae are abdominal or inguinal in position, and are usually few in number. The placenta is nondeciduate, and is either diffuse or cotyledonary. In the Subungulata the feet are frequently five-toed, and they may be plantigrade, and the bones of the first row of the carpus and tarsus are in a direct row with those of the second, MAMMALS. 393 while in the ungulata vera they alternate. The subungulata also present considerable differences in the placental arrange- ments, allusion to which will be made below. Professor Cope, utilizing the characters presented by the carpal and tarsal bones, has proposed to divide the ungulates into five divisions, Taxeopoda, Toxodontia, Proboscidia, Am- blypoda and Diplarthra, his Taxeopoda, including not only forms usually recognized as ungulates, but the primates as well. SUB-ORDER i. CONDYLARTHRA (MESODACTYLA). Extinct ungulates with five-toed, plantigrade feet ; carpalia in straight rows, not alternating; femur with third trochanter, molars bunodont. The condylarthra are the most primitive of ungulates. From them have doubtless descended the ungulata vera, and, if the views of Cope be correct, the carnivores and primates as well. The group appears in the lowest eocene, and is especially well developed in the lower tertiaries of the western U. S. Four families, PERIPTYCHID.E, PHENACODIDjE, MENISCOTHERIID^, and PLEUR- ASPIDOTHERIID^E recognized, the latter from the eocene of France. SUB-ORDER 2. PERISSODACTYLA (SOLIDUNGULA). Unguligrade ungulates with the middle toe well developed, forming the axis of the foot, carpals alternating ; astragalus with pulley-like surface for tibial articulation ; placenta diffuse. The perissodactyls, which walk upon the very tips of the toes, have the feet, as a rule, with the toes three or four in front and three behind ; but frequently only the third toe on either foot comes to complete development, the others be- ing very rudimentary. The dentition is usually complete, the molars being lophodont or rarely bunodont, while the premolars tend to resemble the molars. The femur (except in Chalicothe- FIG. 364. Fore foot of riuiri) has a third trochanter, and the fibula does two-horned rhinoceros, Ate- not usually reach the calcaneum. The stomach lodus bicornis. is simple ; the intestine has large caecum, and a gall bladder never occurs. The mammae, few in number, are inguinal in po- sition. The living perissodactyls present three very distinct types, horses, tapirs, and rhinoceroses, but in the tertiary period many other forms occurred 394 CLASSIFICATION OF VERTEBRA7ES. which intergrade between these, largely obliterating the distinctions. In no group of mammals have the lines of descent been worked out more com- pletely than here. FIG. 365. A, right fore foot of horse, from Huxley, i, radius ; 3, scaphoid ; 4, lunare ; 5, cuneiforme ; 6, pisiforme ; 7, magnum; 8, unciforme; 9, meta- carpal III ; 10, splint bone (metacarpal IV) ; 1 1, 14, sesamoid bones ; 12, proximal phalanx (fetter bone); 13, middle phalanx (coronary bone) ; 15, distal phalanx (coffin bone). B, left hind foot, i, tibia ; 2, calcaneum ; 3, astragalus ; 4, cuboid ; 5, navicular or scaphoid ; 6, ectocuneiforme ; 7, metatarsal Til (cannon bone) ; 8, metatarsal IV (splint bone) ; 9, u, 12, phalanges (see fig. A) ; 10, 14, sesamoids. MAMMALS. 395 In the horses (EguiD^:) the dentition is z f , c \, p f to |, m\-, there are from four to one toes on the fore feet, three to one on the hind feet. In Hyracotherinm (Eo/iippiis) the toes are four and. three on the fore and hind feet respectively ; eocene. P alee other turn, eocene and miocene of both hemi- spheres, with three toes. Mesohippus, miocene. In Hipparion and Protohip- pus toes 2 and 4 reduced so as not to reach the ground, but furnished with hoofs ; pliocene. Equus, the existing horses and asses, has toes 2 and 4 re- duced to metacarpal splint bones without phalanges. The genus appears in the Indian miocene and a little later in North America. The existing species, including the asses and zebras, all belong to the old world. The PROTO- THERIDJE, with tridactyl feet and incisors ^, range through South American tertiary, as do the MACRAUCHENIID^E with the incisors \ , and with no dias- tema in the jaws. TAPIRID/E, with four toes on the fore feet, three on the hind (Fig. 348), range from the eocene to the present time. Lophodon, Isectolophus, eocene. FiG. 366. Sumatran rhinoceros, Ceratorhinus snmatrensts, from Liitken. Tapirus arises in the pliocene of Europe, from which have differentiated the tapirs of India and tropical America, the genus dying out in Europe in the pliocene. The existing species are of middle size, and live usually in woods or swampy places. The RHINOCERID/E, or rhinoceroses, have three or four toes on the fore feet, the hind feet tridactyl, the teeth i f to , c \ or g, p |- to , m f . Some of the extinct forms were without horns, some had one horn and some had two, either one behind the other, as in the existing two- horned species, or as in Dicer atherium^ from the miocene of Oregon, the two horns were placed side by side. HynicJnus and Hyracodon, from the Ameri- can eocene and miocene, were hornless, as was Aceratherium of the oligocene and miocene, the least differentiated of the true rhinoceroses. The living species are distributed in Ceratorhinns, two horns, from Asia; Atelodus* two horns, from Africa ; and Rhinoceros, a single horn. 396 CLASSIFICATION OF VERTEBRATES. The TiTANOTHERiiDyE of the eocene and miocene of Europe and America were mostly large animals with toes four and three on the fore and hind feet respectively, and the teeth varying between i to f , c }, p f to , m f . Palceosyops (Lymnohyus). Titanotheriuin {Br onto t her ium) has a pair of large bony processes on the snout, probably covered with horns. One spe- cies nearly as large as an elephant. The position of the CHALICOTHERIID^E is uncertain. In the teeth it is distinctly perissodactyl, but its three-toed feet were plantigrade, and termi- nated with long, curved claws. The family ranges from the eocene to the pliocene, and is best developed in Europe. Moropns, Macrotherium. SUB-ORDER 3. ARTIODACTYLA. Unguligrade or digitigrade ungulates in which the toes are symmetrically developed about an axis passing between the third and fourth digits. Fre- quently a reduction from the full dentition of 44 teeth ; premolars unlike the molars, the former with one lobe, the latter with two, except the last, which has three lobes ; femur without third trochanter ; fibula articulating with the calcaneum. Stomach complex ; caecum often present, large and convoluted ; mammae 2 or 4, inguinal ; placenta diffuse or cotyledonary. The artiodactyls are mostly large animals, distributed all over the earth, with the exception of the Australian region. The relations of the axis of the foot produce the well-known cloven hoof so characteristic of the group, while in many there is a tendency towards the loss of the incisors and canines in the upper jaw. Another common feature is the development of bony horn cores upon the frontal bones. The recent forms are frequently sub-divided into four series, Suina, Tragulina, Tylopoda, and Ruminantia (Pecora, or Coty- lophora), but when the extinct species are taken into account, the divisions break down. For convenience the characters of these groups may be given here. Suina : with the families Hippopotamidae and Suidae, with bunoclont molars and distinct or but partially fused third and fourth metatarsals and metacarpals ; i.e., without a cannon bone. Tragulina: with the family Tragulidae, in which a cannon bone usually occurs ; and the stomach is three- chambered, the manyplies being absent ; fibula complete. Tylopoda : includ- ing the Camelidae : with only digits 3 and 4 developed, their metapodials fused above ; manyplies absent, red blood corpuscles oval. Ruminantia : with the families Cervicornia and Cavicornia. In these there are no upper inci- sors ; a cannon bone is present, the stomach four-chambered, and the pla- centa cotyledonary. The processes concerned in rumination may be described here, although a chewing of the cud occurs also in the camels. When feed- ing, the food as swallowed passes into the paunch, and thence to the honey- comb. In both of these it is softened, and then is regurgitated into the mouth, and masticated by the teeth. After this comminution it is swallowed again, but at this time it passes directly to the third stomach, or manyplies, and thence to the abomasum, which is the true digestive stomach. The central stem of the artiodactyls seems to be the PANTOLESTID^E of the American eocene, with bunodont molars, and probably four-toed feet. MAMMALS. 397 The ANTHRACOTHERIID^:, best developed in the European upper eocene, have the teeth / \, /// f , the metapodials distinct, and four toes on each foot, the outer ones in process of reduction. Anthracotherinin, eocene. Hyopotamus, miocene of the U. S. and Europe. The SUID^J, or swine, apparently derivatives of the last family, appear in the eocene of both conti- nents, and continue to the present time. They have the teeth i f or f , c \, P \ to f , m f , the molars bunodont. The feet are four-toed, rarely three- toed, toes 2 and 5 smaller than the others, and the metapodials distinct. The stomach has a pouch developed near the cardiac opening ; the colon is spirally coiled, and a caecum is present. The earlier history of the family is less certain than that of some others, and some of the earlier genera seem to have a carnivorous facies. The family to-day belongs to the old world, only the peccaries (frequently set aside as a distinct family, DICOTYLTD^E) occurring in the western hemisphere. In Achanodon, from the eocene, there are already developed the tusk-like canines so characteristic of mod- ern swine ; in Elotherium they are less conspicuous, while in Chocro- potamus (eocene, Europe) and Leptochcerns (miocene, U. S.) these teeth are smaller. The pec- caries (Dicotyles} appear in the American pleistocene, and two or three species persist in warmer America to-day. They have the teeth i f , c j, p \ , m f ; the fifth toe of the hind feet lacking, and the stomach more complex than in the typical swines. The spe- cies are gregarious and omnivor- ous. The allied Platygonus is pliocene. In the pigs proper Sns, Babirusa. Phacochoeriis the canines are greatly developed and triangular in section, and a large diastema exists between these and the premolars. All are old-world forms, and are dis- tinguished by the dentition : Sus, /-|, c \, p |-, m ^ ; Babirusa, i \,c\,p\* M f ; Phacockcerus* i\, c\,P f, m \- The true swine, Sus, appear in the pliocene and continue as our domestic hogs, descended from the wild boar and other Asiatic species. The single species of Babirusa (Porcits} of the Malay Islands is remarkable in that the upper canines of the male grow upward through the skin of the snout. The wart-hogs of Africa (Phacockcerus] receive their common name from the projections on the face. In the adults many of the teeth are lost, but the canines form enormous tusks, both pairs curving upwards and outwards. The HIPPOPOTAMID^E are large, amphibious, bunodont forms, with teeth / I to f, c \, p |, m f , th^lower incisors very long and rootless. The metapo- dials are distinct, the feet four-toed, the lateral toes being nearly equal to the FIG. 367. Stomach of sheep, after Carus^ and Otto (Oppel). a, abomasum ; o, oma- sum; re, reticulum; ru, rumen. 398 CLASSIFICATION OF VERTEBRATES. others ; the digits bear nail-like hoofs. Restricted to the eastern hemisphere, where they occur fossil in Europe and Asia since the pliocene, the living species are all African. Three genera are distinguished by the number of lower incisors : Hippopotamus with six, Tetraprotodon with four, and Chce- ropus with two. Hippopotamus dates back to the pliocene, Chceropns is living; Tetraprotodon is known only from the African pleistocene. The common species, H. amphibius, has an enormous three-chambered stomach, eleven feet in axial length. These animals are gregarious and herbivorous. The OREODONTID.E lived from the eocene to the pliocene of North Amer- ica. They had the teeth i f , c \, p \, m f , and the feet four-toed ; and in Pro- toreodon, from the eocene, the fore toes were five in number. Agriochcerus and Oreodon miocene, Merychius (Ticholeptus) miocene and pliocene. The CA- MELID^E (Tylopoda), which appear in the eocene, may have descended from either the pantolestidas or the oreodontidae. They have the teeth / f to \, c ^-, p 4 to f , m f , the molars selenodont, with a diastema between the premolars and canines ; the feet four- or two-toed, the lateral toes completely lost in the more recent species ; and in all except the older forms the metapodials fused to a cannon bone. The stomach lacks the manyplies, while rumen and honeycomb are sub-divided into numerous small cavities on the walls. The placenta is diffuse. The living genera, Camelus, which inhabits Asia and Africa, and Aitchenia of South America, appear in the pliocene. They differ in the premolars, these being f and \ respectively. The camels are two in number, the single-humped dromedary, C. dromedarius, and the two- humped bactrian, C. bactrianus. Auchenia contains four species, the llama, alpaca, vifuna, and guanaco. Among the extinct genera are Leptotragnlus, eocene ; PoebrotJierium and Protolabis, miocene ; Procamelus, EscJiatius, and Pliauchenia, pliocene. The ANOPLOTHERIID^E of the European eocene and pliocene are noticeable from the fact that it was in this group that Cuvier made many restorations. Anoplotherium, Dichobune, Ccenotlierium, Xiphodon. The TRAGULID^E, or chevrotains, have the teeth i , c \,p f or |, ;// 1 ; fibula complete, usually a cannon bone ; feet four-toed ; stomach three-cham- bered ; placenta diffuse. These forms have been closely associated with the musk deer. Tragulus of Asia contains the smallest existing ungulates. Dorcatherium (Hyomoschns} from Africa. Leptomeryx, American miocene. The CERVICORNIA, in which the teeth are z'f to f , c \, p f to f m, f , the upper canine being sometimes very large, sometimes small or absent, the molars selenodont, are as a rule characterized by the development of horns or antlers upon the frontal bones of the male, although they are occasionally absent, or. as in the reindeer, they may appear in both sexes. These horns consist of a bony outgrowth from the frontals ; and at first this is covered with skin, which may persist through life, as in the giraffe, or, more usually, is soon worn off, leaving the bone alone. Each year this horn is shed, and a new antler takes its place, the later one displaying a greater number of branches or ' tines, 1 so that these become an index of age. Metapodials 3 and 4 usually form a cannon bone ; the lateral metapodials are reduced, and the toes do not reach the ground. The stomach and placenta are of the ruminant type (p. 396). The species are very numerous, but none occur in MAMMALS. 399 Australian regions, and only the giraffes in Africa. The family is not known previous to the lower miocene of Europe, the earliest forms showing relation- ships to the tragulines and antilopes. The musk deer (Moshus, Hydropotes} are without horns, the upper canine is long and projecting, and the male has a ' musk gland ' situated beneath the skin of the abdomen. The species be- long to Central Asia. The living muntjacs are also Asiatic, but their ances- tors appeared in the miocene of both hemispheres. Cope and Schlosser regard the group as the ancestors of the true deer and of the antilopes as well. Cervulus, muntjacs of Asia. Blastomeryx, American miocene. Coso- ryx, American pliocene. The true deer (Cervus) are characterized by the presence of horns. They are usually sub-divided into many subgenera, Axis, Cariacus, Elaphns, etc., upon characters of minor importance; more dis- tinct are the moose (Alces) and the reindeer or caribou (Rangifer). The deer are largely inhabitants of the northern hemisphere. Con- siderably different is Protoceras from the American miocene, in which there were rudimentary horn cores on the frontals and parietals, and vertical bony plates on the maxillae, while the canines recall those of Tragulus. The giraffes FlG< s68> Successive antlers of the red (Giraffa or Camelopardalis, often deer (Cervus elaphus}, after Gaudry. grouped as a family, Devexa) have long legs, and short non-deciduous horns. Allied to these in structure, but lacking the characteristic long neck, occur in the European and Asiatic miocene Helladotherinm, Saniotherium. Sivatherium, with a single large species from the Indian miocene, combines giraffe and antilope characters. In the family CAVICORNIA the horns are almost always borne by both sexes, and, unlike those of the cervicornia, have the bony horn cores covered with true or epidermal horn. With rare exceptions the horns are never shed ; the teeth are / f , c f ,/ f , m f ; the median metapodials form a cannon bone ; the laterals are greatly reduced or entirely absent. The family, which is rich- est in species of any of the ungulates, appears to have descended from the muntjacs through the antilopes. The species are usually arranged in antilo- pine, ovine, and bovine series, the three being distinct in the pliocene. The antilopes, which appear in the miocene, have the round or triangular horns close behind the eyes, the middle incisors largest. Antilope, India; Saiga, with large inflated nose, Asiatic ; Gazella, Asiatic, the springbok (G. eu- chore), African ; Oryx, thegemsboks ; Catoblephas, gnus ; Rupricapra tragns, the chamois ; closely allied is the Rocky Mountain goat, Haploceras montanus. Antilocapra americana, the prong-horn of western U. S., is remarkable for its deciduous horns. In all over a hundred living antilopes are known. Among the fossil genera are Cosoryx, Antidorcas, Tragelaphus, etc. The ovine series, which includes the sheep and goats, has the laterally com- pressed, transversely ribbed horns with hollow cores borne close behind the eyes, and the incisors similar. None are known before the pliocene. Capra, 400 CLASSIFICATION OF VERTEBRATES. ibex, and goats, the domestic goats supposed to descend from C. agagrus of the eastern Mediterranean region. Ovis, the sheep, with several European species and one American, the big-horn, O. inontana. Ovibos inoschatus, the ' musk ox, 1 a goat rather than an ox, is confined at present to Arctic America. In pleistocene times it ranged over Siberia and Europe, south to France and England. In the bovine series the horns are strong, some dis- tance behind the eyes, often on the pos- terior angle of the head; the frontals large, theparietals small. Fossil species first appear in the miocene of India, later, in the pliocene of Europe and America. Bnbalus, the buffalo of India and Africa ; Bibos, the domesticated Indian cattle, and the yak and ban- teng ; Bison, the aurochs of Europe and the ' buffalo ' of America, both near ex- tinction. To Bos belong the domestic cattle, and the now extinct ' ur ' of FIG. 369. Prong-horn antilope. Antilocapra americana. Europe, which possibly lived as late as the composition of the Nibilungen tales. 1 There is evidence to show that our domestic cattle are descended from several distinct races. SUB-ORDER 4. AMBLYPODA. Large, extinct, semiplantigrade ungulates, the pentadactyl feet having the distal phalanges surrounded by hoofs ; carpals alternating, molars lopho- dont, brain very small. These forms, which begin in the lowest eocene, are regarded by Cope as the ancestors of both artiodactyls and perissodactyls. They also show pro- boscidian affinities. Pantolambda. CorypJwdon, with complete dentition, feet digitigrade in front, plantigrade behind, ranged through the lower and middle eocene. The species of Uintatheriuni (Dinoceras) were elephantine in size, and bore on the head three pairs of large bony processes which may or may not have borne horns. SUB-ORDER 5. PROBOSCIDEA. Pentadactyle ungulates with long proboscis (nose), skull increased in size by vacuities in the bone ; incisors 2 never exceeding a pair in either jaw, frequently the upper or the lower lacking; no canines, molars lophodont ; no clavicles ; radius and ulna, tibia and fibula distinct ; stomach simple, caecum 1 It is often suggested that the cattle of Chillingham Park, England, are descendants. of these wild cattle. 2 Frequently spoken of as canines ; however, they arise in the premaxilla, and only- later do the roots extend back into the maxillae. MAMMALS. 401 large, uterus bicornuate ; placenta zonary, non-deciduate ; mammae pectoral, two in number. The elephants and their extinct relatives form a most interesting group, highly differentiated in some respects, more generalized in others. Most strange is the proboscis with the nostrils at the tip. while the incisors which furnish most of the ivory of commerce are almost solely dentine, the enamel covering the tip being worn off at an early date (elephants) , or forming a band on the outer side (Mastodon). The DINOTHERID.E, with the single genus Dinotherium, occur in the miocene and pliocene of Europe and India. In this only the lower incisors are present, and these grow downwards, the symphysis of the jaw being bent so that the teeth form downward-extending tusks. The succession of molar teeth was normal. The animals were about as large as an elephant. In the elephants and their near relatives (ELEPHANTID/E) the upper incisors form tusks of varying size, the lower are smaller or wanting. The molars bear more than two transverse ridges, and are subject to horizontal FIG. 370. Evolution of the teeth of elephants after Flower. A, Mastodon ; B, Stegodon ; C, Elephas (Loxodon} africanus. Dotted, cement ; obliquely lined, dentine ; heavy black line, enamel. replacement. Owing to the shortness of the jaw, and the large size of the molars, not more than two can be in use at the same time. As the more anterior of these becomes worn, it drops out in front, while another takes, its place from behind. In some Mastodons transitional types of succession occur. Mastodon (with several sub-generic divisions) extended from the upper mi- ocene through the pliocene, and may have been contemporaneous with man. Its teeth, characterized by from three to six transverse ridges, or ridges broken into tubercles, are common in the northern hemisphere, Africa, and South America. The skeletons are less perfectly preserved. Stegodon, from the later tertiaries of India, etc. Elephas (including Loxodori) embraces the existing elephants as well as the extinct mammoths. In these the valleys between the transverse enamel folds are filled with cement. The living species, E. indicus and E. africanus occur in India and Africa, respectively. The genus appears in the miocene, the mammoth, E. primigenius of the pleistocene, which became extinct after the appearance of man, being the best known. The discovery in 1799 of frozen mammoth bodies near the Lena delta should be mentioned. The body was covered with close, woolly hair, while a mane on the neck and the hairs on the head were three feet in length. 402 CLASSIFICATION OF VERTEBRATES. SUB-ORDER 6. TOXODONTIA. Extinct ungulates with tri- or pentadactyl semiplantigrade feet ; alternat- ing carpalia ; femur without third trochanter ; fibula articulating with the calcaneum ; third toe the larger. Canines reduced, sometimes to a great extent. FlG. 371. Skull of Typotherium cristatum. The Toxodons and their allies occur in the tertiary of southern South America, and are as yet imperfectly known. They exhibit a strange associa- tion of resemblances to perissodactyls, Hyrax, elephants, and rodents. Tox- odon, which persisted from the older miocene to the pleistocene, was about the size of a rhinoceros. Nesodon, from the eocene. Sometimes Typotherium and its allies from the same beds are separated as a distinct sub-order Typo- theria. SUB-ORDER 7. HYRACOIDEA. Small plantigrade ungulates with tridactyl hind feet, fore feet four-toed, the carpalia in series, the digits with nails; femur with third trochanter; teeth i ^, c , p , m f ; placenta zonary. Only a single genus Hyrax (with several sub-divisions) known, and this only in the existing fauna. The few species described come from Syria, Arabia, and Africa. They live in holes in the rocks, or in hollow trees, and some of the African species are arboreal. One species, H. syriacus, is supposed to be the ' coney ' of the Bible. SUB-ORDER 8. TILLODONTA (INCLUDING T^ENIODONTA). Extinct plantigrade animals with pentadactyl feet ; teeth zf to f, c {, p | to , /// f , the upper molars with three cones, the lower lophodont. These animals of large or moderate size recall in some respects the car- MAMMALS. 403 nivores, and in some the rodents. They belong largely to the eocene, and the United States has furnished the greater number of specimens, Europe having but few. Cope has united these forms with the insectivores and cre- odonts into an order Bunotheria. Esthonyx* eocene, New Mexico. Tilfo* therium, eocene, Wyoming. Calamodon {Styliiiodoii} . Stagodon, cretaceous. ORDER VI. SIRENIA. Thick skinned, naked or sparsely haired, aquatic, placental mammals, with monophyodont teeth ; with fin-like fore limbs ; hind limbs lacking ; a horizontal caudal fin ; a movable elbow joint ; small brain with few and shallow convolutions ; two pectoral mammae. The sirenia contains a few aquatic mammals, which externally resemble the whales in their fusiform bodies, flipper-like fore limbs, absence of hind limbs, and flattened caudal fin. In more important features they are greatly different, and nothing that is known of development or geological history points to their having descended from a common stock. They have the nostrils separate, and opening forward, small eyes with well-developed nictitating membrane, no conch to the ear, no dorsal fin. The paddle-shaped fore limbs have the digits enveloped in the com- mon integument, and only occasionally are nails present. The bones are very dense, and the long bones are without central cavities. Only occasionally are any of the cervical vertebrae anchylosed, and in the recent forms no vertebrae unite to form a sacrum. The anterior caudal vertebrae bear chevron bones. In the skull the chief features are the great development of the zygomatic arch, the reduced nasals, the downward flexure of the jaws in front, and the lower jaw with an ascending ram us. In the fore arm the radius and ulna are usually anchylosed at their extremities ; the digits are always five, and there is no such in- crease in the number of phalanges as occurs in the cetacea. No clavicles are developed. The pelvis is represented by a pair of bones lying at some distance from the vertebral column. Incisors and molars alone are present in the recent forms, and in one genus (Rhytina) no teeth occur, at least in the adult. Many fossil species had a more heterodont dentition, and in Halitherium there was a milk dentition not known in 404 CLASSIFICATION OF VERTEBRATES. FIG. 372. Restoration of Halitherium schinzii, after Miss Woodward. FIG. 373. Manatee, Manatus americanus, after Elliott. MAMMALS. 405 recent forms. In place of teeth, horny plates are developed in the palatal region and at the front of the lower jaw, and these are masticatory in function. The stomach is divided into two principal chambers, and these in turn may be sacculated. The diaphragm is oblique, the lungs very long, and the heart is bifid at the tip, the two ventricles being partially separated (Fig. 352). The testes are abdominal in position ; the uterus is two-horned. The placentation of the dugong alone is known. This form has a non-deciduate zonary placenta. The living species of sea-cow are few. All are littoral in their habits, but never leave the water. They feed upon sea- weed, or upon the grasses growing in the rivers. They are per- fectly harmless, although they attain considerable size. These animals may afford the grain of truth in the mermaid myth. The PRORASTOMID/E (only genus Prorastonis from the eocene of Jamaica), is known only from the skull. It is remarkable in having a com- plete dentition : z f , c \, p f , m f . The MANATID.E have molars 8 to 10, the first single-rooted; incisors and canines never functional. Manatus (Tri- chechus*) includes the manatees of tropical America and Africa. Fossil in pleistocene of South Carolina. They have but six cervical vertebrae. Here possibly belong the tertiary Manatherium and Ribodon. HALICORID^E with heterodont molars, and either with tusk-like incisors in the upper jaw or these lacking. Halicore includes the dugongs of the Indian Ocean. Halitherium, from the miocene and pliocene of western Europe, gives evidences of a milk dentition. Rhytoidus, Felsinotherium, from the tertiary. RHYTINDI/E, with- out teeth. The only known species, RJiytina stelleri, the northern sea-cow of the northern Pacific, was exterminated in the last century. ORDER VII. CETACEA. Aquatic mammals without distinct neck ; fore limbs paddle- like ; hind limbs absent ; usually a dorsal fin ; caudal fin in two lobes or < flukes,' nostrils 011 the top of the head ; teeth, when present, homodont and monophyodont ; no elbow joint ; brain large, cerebrum complicately convoluted ; placenta non-deciduate, diffuse. The whales, like the sea-cows, form a distinctly circum- scribed group, sharply marked off from all others, so that no clear conclusions can be reached as to their line of descent from other groups, although one is justified in believing that they 406 CLASSIFICATION OF VERTEBRATES. have come from some more normal quadripedal mammal. More recently the view has been gaining ground that the two living divisions, the toothed and the whalebone whales, may have had diverse origins, and their present resemblance may be due to convergence rather than to community of descent. The best guesses as to their ancestors would trace them either to the carnivores or to some long-tailed ungulate, while the presence of dermal ossicles in one species of Neomeris, and possibly in Zeuglodon, needs to be taken into account. The skin is smooth and naked, without hairs ; even the bristles around the mouth may disappear in the adult. There is no neck ; the head is large, and may form one-third of the total length. The eye is small, and without nictitating mem- brane ; the nostrils, separate or with a common crescentic opening, are on the top of the head ; there is no external ear, the small meatus opening close behind the eye. Beneath the skin is the thick layer of fat or * blubber.' The bones are light and spongy. The cervical vertebrae are more or less completely fused, and have the zygopophyses poorly developed. There is no sacrum, but the caudal vertebrae are distinguished from the lumbar by the presence of chevron bones. The sternum is very variable, and is reached by only one (mys- tacocetes), or a few ribs. The skull consists of a nearly spherical cranium, from which the facial part projects like a beak. In the cranium the roof is formed by the supraoccipital and inter- parietal, which extend forward to meet the frontals, excluding the parietals from the middle line. The frontals are expanded laterally to form roofs to the orbits. The maxillae are very large, the nasals very small. The lower jaw lacks an ascending ramus. Clavicles are lacking, and there is no elbow joint. The bones of the wrist and hand remain almost entirely carti- laginous in the whalebone whales. The digits are four or five, the phalanges of the second and third being increased in num- ber up to fourteen, a condition recalling the ichthyosaurs (p. 3 1 2). The pelvis is represented by two bones, free from the vertebral column, which, on account of their muscular relations, are regarded as ischia. In some species rudiments of the skeleton of hind limbs occur, imbedded in the flesh. MAMMALS. 407 The teeth in existing species are homodont and monophyo dont, and never pierce the gums in the mystacocetes. In the fossils there is evidence of descent from heterodont ancestors. Pmx Pmx FIG. 374. Skull of foetal cachelot whale {Physeter~}, from Huxley. AS, alisphenoid ; A u, auditory region; BO, basioccipital ; BS, basisphenoid ; O,exoc- cipital; Eth, ethmoid; Fr, frontal; Ju, jugal (displaced in the side view from its connection with the squamosal); MX, maxillary; N, external nares; Pa, parietal; PI, palatine; Pmx, premaxillary; SO, supraoccipital; Sy, squamosal; Vo, vomer. The number varies greatly. The stomach (p. 367) is several chambered, the intestine is comparatively simple, a caecum being present in some species. The liver has four lobes, and no gall 408 CLASSIFICATION OF VERTEBRATES. bladder is present. The larynx is prolonged so that it enters the choana, a condition recalling the marsupials. The testes are abdominal, the uterus two-horned, and the two mammae are in grooves near the vulva. These are provided with constrictor muscles, by which the milk is forced into the mouth of the young. About two hundred existing species are known. The cetacea are introduced by the zeuglodons in the eocene, the other groups appearing in the miocene. SUB-ORDER i. ARCH^EOCETI. External nostrils at the middle of the muzzle ; nasals long ; temporal fossa elongate ; ribs bicipital ; anterior teeth with single roots, posterior with two roots, the free edges dentulate. Breast bone of several sternebrae. Cervicle vertebrae free. Zeuglodon {Basilosaurus} the only genus (with several subgenera) comes from the eocene of our southern states, Europe, and New Zealand. One species was 60 feet long. SUB-ORDER 2. ODONTOCETI (DELPHINOIDEA, DENTICET^E). Skull asymmetrical ; external nares united at base of snout; nasals very small ; temporal fossa short ; teeth present in both jaws or only in lower, occasionally reduced to a single pair. Olfactory organs absent or rudimen- tary ; anterior ribs bicipital, others with only a tubercular head ; sternum of several sternebrae. The toothed whales are all carnivorous, but much as they have been pursued by man, many questions concerning the features presented by any species at different ages are still unsettled. The SQUALODON T TID^E, with teeth in both jaws, and these differentiated into incisors, canines, and two kinds of molars, occur in the miocene and pliocene. In some respects they seem intermediate between the zeuglodons and the toothed whales, but their skeleton is imperfectly known. The PLATANISTID^: include the fresh- water dolphins, Platanista and Inia, from the Ganges and Amazons ; allied forms occur in the miocene and pliocene of both continents. The true dol- phins (DELPHINID^:) have the snout elongate, no premaxillary teeth, the other teeth variable, usually conical, and with a single root. Teeth are numerous in both jaws of the true dolphins and porpoises {Delphinus, Turswp*, Pho- c after De i ag e. Bones black ; w, After birth a series of flat- whalebone? hanging down into the cavity of the tened plates of horny mate- mouth> m . f> et hmoid ; mx, maxillary ; /, parietals ; rial make their appearance ^ vomer. on either side of the upper jaw. These plates, of which there are several hundred pairs, are triangular in shape, with the inner edges fringed out into hair-like fibres, the plates being at righ't angles to the axis of the body, and the whole forms a very efficient straining-apparatus, by means of which the whales separate the small animals on which they feed from the surrounding water. Morphologically the baleen. 4IO CLASSIFICATION OF VERTEBRATES. consists of large numbers of cornified papillae arising from the ectoderm lining the cavity of the mouth. In the BAL^ENOPTERID^: the head is less than a fourth of the total body length, the ventral side of the body is usually marked by longitudinal grooves, a. dorsal fin (a tegumentary fold without skeleton) is present, the hands have four digits, and the cervical vertebrae have their large centra free. Baltcnop- iera, with the head small and flat, and the grooves extending back as far as the throat region, contains the rorquals, fin-backs, or razor-backs. B. sib- Jjaldi is the largest whale, reaching a length of 85 feet. Megaptera includes the hump-back whales. The family occurs in all rocks since the miocene. The BAL^NID^? or right whales date only from the pliocene ; they have a large head, the ventral surface smooth, the hand pentadactyl, the cervical vertebrae fused, and no dorsal fin. Balocna, Neobalcena, and Rhachianectes belong here. ORDER VIII. CARNIVORA (FER^E). Terrestrial or aquatic flesh-eating mammals with unguicu- late four- or five-toed feet ; incisors usually f, canines |, strong, pointed and recurved ; molars more or less sectorial ; mammae abdominal ; placenta deciduate, almost always zonary. The carnivora receive their name from their flesh-eating habits, but it must be understood that not every species con- forms to this rule, since some live largely upon a vegetable diet. All have diphyodont and heterodont dentition ; the first incisor is smallest, the third largest. The canines are especially charac- teristic ; the premolars are compressed and are usually sectorial, while the molars are occasionally broad, but still have cuspidate crowns. The milk dentition is usually functional for a year after t>irth. The feet have either four or five toes, and may be either plantigrade, semiplantigrade, digitigrade, or in the seals modi- fied into flippers. The claws are usually compressed, but occa- sionally may be rudimentary or absent. In the living species the brain is large and richly convoluted, but in the creodonts it \vas much smaller and nearly smooth. The stomach is a simple pear-shaped sac ; the caecum is small or absent ; the uterus two- horned. The radius and ulna are always distinct, the fibula .always slender. Through the extinct group of creodonts the carnivores are closely related to the insectivores, and possibly to the marsupials. Jn fact Cope has taken the creodonts and united them with in- MAMMALS. 41 I sectivores, and certain forms here placed with the ungulates, into an order, Bunotheria, from which he derives the carnivores and rodents. The carnivores appear in the lowest eocene ; the creo- donts disappear in the miocene, while the pinnipedia are first found in the miocene. SUB-ORDER i. CREODONTA. Extinct digitigrade or semiplantigrade carnivores with small and scarcely convoluted cerebrum ; incisors f or f ; molars never more than 8 ; tail long ; feet usually five-toed ; scaphoid and lunare distinct. The creodonts present marked resemblances to both marsupials and in- sectivores in many structural features, but they differ from both in the strong development of the canines, while the presence of a full milk dentition and absence of an inflected angle of the lower jaw serve strongly to mark them off from the former order. Through the miacidas they seem connected with the canidae {infra}, and thus have given rise to the various lines of living fissi- pedes. From all living carnivores they are marked off by the absence of a carnassial tooth, and by a notch or groove at the tip of the distal phalanges. The OxYCLvNiD/E from the lowest eocene of New Mexico are known prin- cipally by the molar teeth. The ARCTOCYONID/E occur in the lower eocene of both continents, and have quadritubercular upper molars. Arctocyon, France ; Clanodon, New Mexico. The MESONYCHID^E of the American eocene have tritubercular molars ; Mesonyx. Allied is the family LEPTIC- TiDjE of Europe and America ; Proviverra. The PALJEONICTID^E {Palcecnictis, Patriofelis} occurs in the lower eocene of Europe and America. The HY^E- NODONTiDvE were larger animals, much nearer the recent carnivores (fissi- pedia), but were distinguished by the absence of differentiated carnassial teeth, which however occur in the MIACID^E. The hyaenodontidae range through the eocene to the lower miocene of both continents ; the miacidae have only been found in the eocene of America. SUB-ORDER 2. FISSIPEDIA (CARNIVORA VERA). Digitigrade or plantigrade carnivores, with incisors f (rarely f), premolar 4 in the upper and molar i in the lower jaw sectorial, the other molars tuber- culate ; feet four- or five-toed ; scaphoid and lunare fused. The carnivores proper are usually terrestrial in habits, only a few being partially aquatic. Most of them are carnivorous in diet, but some are om- nivorous. Correlated with this flesh-eating habit is the large size of the canines and the shear-like carnassial or sectorial teeth alluded to in the diag- nosis. The brain is convoluted, and the terminal phalanges are never notched at the tip, but they are occasionally retractile along with the claws they bear. The dogs (canidae) seem to be the central stock from which descent has been in one line through the viverriclae to the cats and hyaenas, in another through the ursidae to the mustelidas. It must, however, be mentioned that the viverridae and mustelidae show intergrading forms. The sub-order appears 412 CLASSIFICATION OF VERTEBRATES. in the upper eocene, apparently as descendants of the miacidae or palaeonic- tidae, and all known families persist at the present time. The CANID.E have typically / f , c \, p \ , in f or f , the upper sectorial with two lobes, the lower with an inner weaker lobe in addition ; auditory bulla large, inflated, and undivided ; feet digitigrade, the fore feet four- or five- toed, the hind feet usually four-toed, claws not retractile. The majority of the species belong to the genus Cams (including Lupus, Viilpes, etc.) of cos- mopolitan range, including the dogs, wolves, foxes, jackals, etc. Other living genera are Octocyon and Lycaon of South Africa. The fossil genera are numer- ous in both continents,among them Temnocyon, Amphicyon, and Oligobunus. The URSID^E have plantigrade feet, short and stout body, sectorials scarcely differentiated, some of the premolars lost at an early date, and the auditory bulla flat. Ursus, containing the bears, with molars f , is largely confined to the northern hemisphere. Mclursus is Asiatic. The fossil gen- era Dinocyon, Hyceiiarctus, and Arctoiherinin form a line uniting the dogs with the bears. The PROCYONID.E with plantigrade feet, molars f , tuberculate, and with the tail usually ringed, are largely American ; but two species, the raccoon (Procyon lotor} and the raccoon-fox (Bassaris astutd) enter the U. S. The coati (Nasiui) with long, flexible snout, and Cercoleptes occur in Central and South America. The species of MUSTELID/E are more numerous ; they have the molars \ (| in Mellivora). The otters (Lntra) and the sea-otter (Enhydris} have webbed feet. Mephitis, including the skunks, is American. The badgers (Meles} belong to the old world, while the same common name is given to the species of the American genus Taxidea. The minks, martens, sables, ermines, weasels, and ferrets, belong to Mustela, many of the species, being valuable for their furs. The genus begins in the miocene of Europe. Gnlo, the wolverine, occurs in the northern parts of both hemispheres. A peculiarity of many of the mustelidce is the great development of anal glands which secrete a strong-smelling fluid used as a weapon of defence. The VIVERRID;E, like the remaining fissipecles, have a swollen auditory bulla and digitigrade or sub-plantigrade feet. They have p f or |, ;// | or f , and usually five digits on all the feet. The species are all old-world. Cryp- toprocta, Viverra, the civets; Herpestes, the mongoose. The family ap- pears in the lower miocene. The HY^ENID^E, also an old-world group, is closely related to the viverridae by its fossil relatives. Hycena, Proteles. The FELID.E have retractile claws, stronly developed canines, molars \ in recent species (never exceeding \ in fossils) ; the upper sectorial with a three-lobed blade. Felts includes the majority of the living species, lions, tigers, leopards, panthers, lynxes, pumas, jaguars, and the smaller cats. CyiKelurns, the only other existing genus, contains the cheetah, or hunting leopard, which ranges from India to Southern Africa. The family appears in the upper eocene of America, while species are found in the miocene of both continents. Among the extinct genera are Dinictis, Hoplophoneits, and MacJuerodiis, the latter characterized by the enormous canines, these being,, in one species, seven inches in length. MAMMALS. 413 SUB-ORDER 3. PINNIPEDIA. Aquatic pentadactyl carnivores, with webbed feet fitted for swimming, incisors always less than f , p typically f , ;// }, no differentiated carnassial ; tail very short. The seals and their allies are mostly marine, although some ascend rivers, while one species occurs in Lake Baikal. The body is fitted for an aquatic life ; the basal portion of the fore limbs is imbedded in the general integument, while the web of the toes usually extends beyond the extremity of the clawed digits. The seals are true carnivores, feeding upon fish, of which they destroy large numbers. The origin of the group is uncertain. The eared seals show considerable resemblances to the ursidae, while the true FIG. 377. Harbor seals, Phoca vitulina, after Elliott. seals suggest an origin from some form like Lutra. So it may be that the group is polyphyletic, or again, the pinnipeds may have descended directly from the creodonts. The group first appears with forms allied to Phoca in the miocene, while walrus-like forms occur in the pliocene. The OTARIID^E or eared seals have a small external ear, and the soles of the feet naked ; teeth / f , c \, p f , m \ or \. Otaria includes sea-lions of the Pacific and South Atlantic ; and the fur seals, most familiarly known from the northern Pacific. The TRICHECHID/E, or walruses, have the ears without external pinnae, and the upper incisors developed into immense tusks. The species of Trichechus, one or two in number, are confined to the northern parts of both oceans. The PHOCID.E lack external ears, have the soles of the feet hairy, the testes abdominal, the teeth p , m ^, and 414. CLASSIFICATION OF VERTEBRATES. stiff hair without a woolly fur beneath. Phoca, with incisors ^, embraces the common seals of the northern Atlantic ; MonacJius (i f ) , the monk seals of warmer latitudes. Cystophora (i f-) includes the hooded seals of polar seas with inflatable sac connected with the nostrils. ORDER IX. PRIMATES. Diphyodont, heterodont mammals, with typically i f, m , the molars usually quadritubercular ; the orbits separated from the temporal fossa by a postorbital bar ; clavicles well developed ; ulna and radius always distinct ; feet plantigrade, usually penta- dactyl ; the pollex and (except in man) hallux opposable to the other digits. The placenta deciduate or not ; diffuse or discoidal. The primates, as a group, are not easily denned, especially if the extinct forms be taken into consideration, for these to a great extent bridge over the gap which exists, among recent forms, between the primates and the insectivores and creodonts, while in certain points there are suggestions of marsupial char- acters. According to one view, the order is polyphyletic, the lemurs having had one line of descent, and the monkeys, apes, and man having had another ancestry. This view is based primarily upon placental structures, but it is largely negatived by the fossil history so far as this is known. SUB-ORDER i. PROSIMI/E (LEMUROIDEA). Arboreal primates with opposable great toe ; orbits not completely sepa- rated from temporal fossa ; mammae thoracic, or thoracic and abdominal : uterus bicornuate ; placenta non-deciduate, the whole surface of the chorion, except one end, being covered with villi. The lemurs and their allies have their centre to-day in Madagascar, from which outlying species extend to the African continent and to the Indian archipelago, a distribution which has suggested a former continent, ' Le- muria,' in the Indian Ocean. In former times their range was more extensive, since abundant remains have been found in the older tertiaries of Europe and North America. The living species are mostly nocturnal, and many of them have the eyes peculiarly modified in accordance with their habits. In addition to the characters quoted in the diagnosis it may be mentioned that in some all the digits are clawed, while in others only the second and third of the hind toes are provided with claws, the others bearing nails. The upper molars have four or three tubercules, those of the lower jaw having four or five. The brain is but slightly convoluted, and but slightly overlaps the cerebellum. MAMMALS. 415 PACHYLEMURID.E, z | to f , c \, p -*-, m |. From the eocene and lowest miocene of Europe and America. Adapts of Europe is the best known. Pely- codus, Tomitherium. This family is regarded by Cope as the ancestor of the true apes. The modern lemurs may have sprung from the ANAPTOMOR- PHID.E, in which the lachrymal foramen lies outside the orbit, while the den- tition is / 'I to , c y, p \ to f , m \. Anaptomorphus, from the lower eocene, of Wyoming, resembles Tarsius (infra}. Necrolemur. The TARSIIDVE of the Indian archipelago, with only a single species,. Tarsius spectrum, has a dentition / f , c |, p |, /// f ; digits 2 and 3 of the hind feet with claws. In its placentation Tarsius differs from all other prosimiae and approaches man. The lemurs proper belong to the LEMURID^I. with / f , c i, / | to f , m f , the lower incisors and canines directed forward,, and the first premolar serving as a canine. Indris, Lemur, Galago, Loris^. etc. The CHIROMYID^ with / \, c , p ^, in \ , contains but a single species, the ave-aye, Chiromys madagascarensis, which recalls the rodents in its- incisors and diastema, and is unique in the greatly elongate middle digit of the hand. Two aberrant families, exhibiting some relationships to the lemurs, may be mentioned here. The first is NESOPITHECID^E, based on the fossil Malagassy genius Nesopithecus, which has the premolar dentition of a true lemur, with the orbit of a simian. The second is the GALEOPITHECID/E, with a single genus, Galeopithecus, from the East Indies. It is frequently referred to the insectivores with which it agrees in its deciduate discoidal placenta. It has the fore and hind limbs connected by membrane forming a parachute like, that of the flying-squirrel. SUB-ORDER 2. SIMILE (ANTHROPOIDEA). Arboreal or terrestrial primates, with the digits (except hapalidae) all witfr nails, molars with 4 or 3 tubercles ; orbits completely separated from temporal fossa by a plate of bone beneath the postorbital bar ; cerebrum greatly con- volute, covering or nearly covering the cerebellum ; mammae two, always thoracic ; uterus simple ; placenta discoidal, deciduate. The simiae include the monkeys, apes, and man, and the deeper struc- tural features are rc-inforced by characters of less- importance. Thus the eyes are directed forwards, the ears are much as in man, and in the young the whole appearance of the face of the lower forms is more like that of the human being than is that of the adult, the change being largely effected by a later forward growth of the jaws. Man excepted, the sub-order is confined to the warmer parts of both hemispheres, but fossils are found in Europe as far north as England. The sub-order has developed in two lines in the two- hemispheres, the platyrhine forms belonging to the new world, the catarrhine to the old. In the PLATYRHINI the nostrils are separated by a wide septum, and are directed outwards. The HAPALID/E have /' f , c \,p f , ;;/ f ; all digits, except the great toe, furnished with claws, and the tail non-prehensile. The species belonging to the genera Hapale and Midas have the common name of marmosets. Apparently this family has descended from the CEBID^, ia. 41 6 CLASSIFICATION OF VERTEBRATES. which the tail is frequently prehensile, and the premolars are |. Mycetes in- cludes the howling monkeys ; Pithecia, the sakis ; Ateles, the spider monkeys ; Cebus, the sapajous, species of which frequently accompany the organ- grinder. Homujiculus and Anthropops occur in the tertiary of Patagonia. The CATARRHINI have the nasal septum narrow, the nostrils directed forwards, and a dentition / f , c \, p f , m f . In the CERCOPITHECID^: (Cy- nopithecidae) the tail is usually long, the molars quadrituberculate, and the ischial region presents callosities. All of the tailed apes of the old world belong to this family. Cynocephalus contains the baboons of Africa, in which the tail is of moderate size, and the maxillary bones swollen. Here also belong the drill and mandrill. The macaques (Macacus), are almost entirely FIG. 378. Chimpanzee, Troglodytes niger, after Brehm. Asiatic, one species entering Europe at Gibraltar. Cercopitliecus. Semno- pithecus. The SIMIID,E, or ANTHROPOMORPHA, contains apes in which the tail is lacking, the anterior limbs longer than the posterior, and ischial cal- losities lacking, except in Hylobates, which includes the gibbons of Asia, in which the arms are so long that they reach the ground when the animal is in an upright position. Simia includes the orang-utan of Sumatra and Borneo, in which the great toe is small, the arms long, and the ribs, 12 pairs. Gorilla of Africa has 13 pairs of ribs, and prominent superciliary ridges. Troglodytes includes the chimpanzees, of which there are one or two species, in which the ribs are as in gorilla. They come from western Africa. The HOMINIDJE, or BIMANA, includes man, who is far less remote from the simiidae than these are from the new-world monkeys. The chief charac- ters are the upright position, the lack of opposable toe, the 'enormous size of the brain, correlated with his mental development, and the distribution of MAMMALS. 417 hair upon the body, it being best developed in those places where it is most sparse in the allied forms. Man presents certain features in which he resembles more closely each of the anthropomorphous apes, while in others he differs from them all, so that it is difficult to say which is his nearer relative. Of the genus Homo there is, according to accepted tests, but a single species ; but the question of arrangement of the races affords far more difficulties. For all such discus- sions reference must be made to the works on anthropology. The age of man on the earth is another question which can only be alluded to here. That man has been here far longer than the seven or eight thousand years of history is now beyond a doubt. His remains and his handiwork date back to a time far before any records or any traditions ; to a time when the mam- moth was alive, and when England and continental Europe had a fauna recalling those of the tropics to-day ; when the mastodon, Glyptodon, and Megatherium ranged in South America. But when we attempt to pass back of the pleistocene the evidence is scanty, and not beyond question. The skull of Calaveras and the ' Pithecanthropus erectus ' of Java, like the miocene flint chips of Thenay, need more evidence in their support before they can be accepted as proving the existence of man in tertiary time, no matter how probable such existence may be upon a priori grounds. NDEX Aard vark, 382. Abdominal pores, 107. Abdominal vein, 197. Abducens nerve, 61. Abomasum, 35. Acanthias, 239. Acanthodes, 251. Acanthoderma, 266. Acanthodidse, 251. Acanthoglossus, 377. Acanthopteri, 258. Acanthopterygii, 258. Accessory of Willis, 64. Accipiter, 348. Accipitres, 348. Aceratherium, 395. Acetabular bone, 171. Acetabulum, 170. Achaenodon, 397. Achirus, 265. Acinose glands, 90. Acipenser, 251. Acipenseridae, 250. Acris, 287. Acrodont teeth, 294. Actinistia, 249. Actinosts, 175. Actinotrichia, 174. Adapis, 415. Adapisoricidae, 385. Adder, 325. Adipose tissue, 14. Adrenal, 131. yEgitognathas, 351. ^Egithognathous, 335. ^piornis, 346. ^piornithes, 346. .'Etosaurus, 328. Afferent nerves, 47. Agamidae, 320. Agathaumas, 317. Agkistrodon, 326. Aglossa, 286. Aglyphodonta, 325. Agnatha, 219. Agonidae, 260. Agriochoerus, 398. Aguti, 391. Air bladder, 25. Air sacs, 31. Aistopoda, 283. Ala spuria, 330. Albatross, 349. Albulia, 256. Albumen, 207. Alca, 349. Alcedo, 348. Alces, 399. Alecithal eggs, 206. Alectoromorphae, 350. Alepocephalidae, 256. Alewife, 256. Alisphenoid, 158. Allantoic artery, 190. Allantoic placenta, 374. Allantoidea, 288. Allantois, 373. Alligator, 328. Alligator gar, 252. Alligatoridae, 328. Alligator snapper, 311. Allotheria, 377. Alopiidas, 239. Alpaca, 398. Altrices, 330. Alula, 330. Alutera, 266. Alveoli, 20. Alveoli of lung, 30. Alytes, 287. Amber fish, 263. Amblypoda, 400. Amblyopsidae, 257. Amblyopsis, 257. Amblyrhiza, 391. Amblystoma, 285. Amia, 252. Amiidae, 252. 419 Amiurus, 255. Ammocoetes, 222. Ammodytes, 263. Ammodytidae, 262. Ammodytoidea, 262. Amnion, 288. Amniota, 288. Amphibia, 274. Amphiccelias, 315. Amphicoelous, 138. Amphicyon, 412. Amphilestes, 380. Amphioxus, 2. Amphiplaga, 259. Amphiplatyan, 140. Amphisbaena, 321. Amphistylic skull, 234. Amphitherium, 380. Amphiuma, 285. Amphiumidas, 285. Ampullae, 68, 70. Amyda, 310. Amyzon, 255. Anabas, 262. Anableps, 257. Anacanthini, 264. Anaconda, 325. Anal fin, 177, 228. Anallantoidea, 226. Anamnia, 226. Anapophysis, 141. Anaptomorphidas, 415. Anaptomorphus, 415. Anarrhichas, 261. Anarrhichidae, 261. Anas, 348. Andrias, 285. Angler, 266. Anguilla, 257. Anguillidae, 256. Anguis, 320. Angulare, 164. Anhima, 348. Anhinga, 347. 420 INDEX. Animalivora, 387. Archaeopteryx, 343. Autostylic skull, 241. Annulata, 321. Archaeornithes, 343. Auxis, 263. Anolis, 320. Archegosaurus, 283. Aves, 330. Anomaluridae, 330. Archenteron, 6. Axillary artery, 189. Anomalurus, 390. Archer fish, 262. Axis, 142. Anomodontia, 305. Archipterygium, 172. Axis cylinder, n. Anoplotheridae, 398. Arcifera, 286. Axis deer, 399. Anoplotherium, 398. Arctocyon, 411. Axon, ii. Anser, 348. Arctocyonidae, 411. Aye-Aye, 415. Ant-eater, scaly, 382. Arctomys, 390. Azygos vein, 196. Ant-eater, spiny, 377. Arctotherium, 412. Ant-eaters, true, 383. Ardea, 348. Babirusa, 397. Antebrachium, 176. Area opaca, 342. Baboon, 416. Antennariidae, 266. Area pellucida, 342. Bactrian, 398. Antennarius, 266. Areolar tissue, 13. Badger, 412. Anterior abdominal vein, Armadillo, 383. Balaena, 410. 197. Arterial blood, 184. Balsenidae, 410. Anthracotheriidae, 397. Arteries, 178, 188. Balaenoidea, 409. Anthracotherium, 397. Arthrodira, 271. Balaenoptera, 410. Anthropoidea, 415. Articular process, 140. Balaenopteridae, 410. Anthropomorpha, 416. Articulary, 158. Balancers, 24, 282. Anthropops, 416. Artiodactyla, 396. Balanoglossus,3. Antiarcha, 225. Arvicola, 390. Baleen, 409. Antidorcas, 399. Arytenoid cartilage, 28. Balistes, 266. Antilocapra, 399. Ascalabotae, 319. Bandicoot, 380. Antilope, 399. Ascalabotes, 319. Banteng, 400. Antrozous, 387. Asineops, 262. Baphetes, 284. Ant-shrike, 351. Asp, 325. Baptanodon, 313. Anura, 286. Aspidocephali, 225. Barbus, 255. Aorta, 181. Aspidonectes, 310. Barracuda, 262. Aortic arches, 182, 185. Aspidorhynchus, 252. Basalia, 173. Apatornis, 344. Ass, 395. Bascanion, 325. Apeltes, 258. Asterolepis, 225. Basibranchial, 154. Apes, 416. Asterospondyli, 238. Basihyal, 155. Aphredoderus, 259. Asterospondylous vertebras, Basilosaurus, 408. Apodes, 256. 234- Basioccipital, 157. Aponeurosis, 112. Astragalus, 177. Basisphenoid, 158. Appendage, pyloric, 38. Atalapha, 387. Basitemporal, 334. Appendicularia, 2. Atelodus, 395. Bassaris, 412. Appendicular skeleton, 167. Athecse, 310. Bass, black, 259. Appendix, digitiform, 36. Atherina, 262. Bat fish, 266. Appendix vermiformis, 39. Atherinidae, 262. Bathyergus, 390. Aptenodytes, 347. Atlantosaurus, 315. Bathyergidae, 390. Apteryges, 346. Atlas, 142. Batoidea, 239. Apteryx, 346. Atoposauridas, 328. Batrachia, 274. Apteria, 95. Atrium, 181. Batrachidae, 266. Aqueduct of brain, 50. Auchenaspis, 225. Batrachus, 266. Aqueductus vestibuli, 70. Auchenia, 398. Bats, 386. Aqueous humor, 83. Auditory bulla, 357. Bdellostoma, 224. Aquila, 348. Auditory nerve, 63. Bdellostomidns, 224. Arachnoid membrane, 57. Auditory ossicles, 158. Bead snake, 325. Arapaima, 256. Auk, 349. Bears, 412. Arcades, 298. Auricle, 181, 184. Beaver, 390. Archaeoceti, 408. Aurochs, 400. Bee-eater, 348. INDEX. 421 Belideus, 380. 1 Brachium, 176. Callionymus, 263. Belodon,328. Bradipodidae, 383. Callopterus, 252. Belone, 257. Bradypus, 383. Callorhynchus, 242. Belonida;, 257. Brain, 48. Camarasaurus, 315. Belonorhvnchus, 258. Bramidae, 263. Camelidae, 398. Berycidae, 261. Branchiae, 22. Camelopardalis, 399. Berycoidea, 261. Branchial arches, 154. Camels, 398. Beryx, 261. Branchial clefts, 22. Camelus, 398. Bettongia, 380. Branchial rays, 156. Campanula Halleri, 231. Bibos, 400. Branch iosaurus, 283. Camper's angle, 358. Bicuspids, 366. Branchiostegal rays, 156. Canaliculi, 15. Bill fish, 257. Breast bone, 147. Canidae, 412. Bimana, 416. Breathing valves, 253. Canines, 364. Bipolar nerve cells, 10. Brevilinguia, 320. Canis, 412. Birds, 330. Brevirostres, 328. Cannon bone, 394. Birds of Paradise, 350. Brevoortia, 256. Capra, 399. Bison, 400. Broadbill, 351. Caprimulga, 348. Bittern, 348. Bronchi, 27. Capybara, 391. Black bass, 259. Bronchiole, 30. Carangidae, 263. Black fish, 408. Brontosaurus, 315. Caranx, 263. Black snake, 325. Brontotherium, 396. Carapace, 93, 308. Blarina, 385. Bruta, 381. Carassius, 255. Blastoderm, 211. Bubalus, 400. Carcharias, 239. Blastodermic vesicle, 214. Bubo, 348. Carcharinus, 239. Blastomeryx, 399. Buccalis nerve, 62. Carcharodon, 239. Blastopore, 6. Bucco, 348. Cardiac glands, 367. Blennidae, 261. Buceros, 348. Cardiac region, 34. Blennioidea, 261. Buffalo, 400. Cardinal sinus, 195. Blennius, 261. Bufo, 286. Cardinal vein, 183. Blind fish, 257. Bufonidce, 286. Carettochelydae, 311. Blood, 16. Bulbus arteriosus, 181. Cariacus, 399. Bluefish, 263. Bull frog, 287. Caribou, 399. Boa, 325. Bull head, 255. Carina, 148. Boar, 397. Bunodont, 365. Carinatae, 149. Body of vertebra, 135. Bunotheria, 403, 411. Carnassial teeth, 411. Bombinator, 287. Burbot, 264. Carnivora, 410. Bonasa, 350. Burdach's column, 54. Carotid, 183. Bone, 14. Bursa Fabricii, 39. Carp, 255. Bone, membrane, 15. Bustard, 349. Carpale, 176. Bone, cartilage, 15. Butter fish, 263. Carpus, 176. Bony fishes, 252. Butterfly fish, 262. Cartilage, 14. Bony gar, 257. Buteo, 348. Cartilage bone, 15. Booted tarsus, 331. Buzzard, 348. Casserian ganglion, 61. Borers, 224. Cassowary, 346. Bos, 400. Cacatua, 349. Castor, 390. Botaurus, 348. Caeciliae, 287. Castoridas, 390. Bothremys, 311. Caecilians, 288. Castoroides, 391. Bothriolepis, 225. Caecomorphas, 349. Casuaridas, 346. Bothrops, 326. Caecum, intestinal, 39. Casuarius, 346. Bow fin, 252. Caenolestes, 380. Cataphracta, 326. Bowman's capsule, 119. Caenotherium, 398. Cataphracti, 259. Box turtle, 311. Calamodon, 403. Catarrhini, 416. Brachial artery, 189. Calamoichthys, 250. Cat fish, 255. brachial plexus, 48. Calcaneum, 177. Cathartes, 348. 422 INDEX. Catoblephas, 399. Catopterus, 252. Catostomidae, 255. Catostomus, 255. Cats, 412. Cattle, 400. Caturus, 252. Cauda equina, 48. Caudal artery, 183. Caudal fin, 177, 228. Caudal region, 142. Caudal vein, 192. Cavia, 391. Caviare, 251. Caviidae, 391. Cavicornia, 399. Cebidae, 415. Cebus,4i6. Cement, 19. Centetidae, 385. Centrale, 176. Centres of ossification, 15, 133- Centriscidae, 258. Centrum, 135. Cephalaspis, 225. Cephalochordia, 2. Cephalodiscus, 3. Ceratobranchial, 154. Ceratodus, 273. Ceratohyal, 155. Ceratophrys, 274. Ceratops, 317. Ceratopsia, 316. Ceratorhinus, 395. Ceratosaurus, 316. Cercoleptes, 412. Cercopithecidae, 416. Cercopithecus, 416. Cerebellum, 50. Cerebral hemispheres, 49. Cerebrum, 49. Cervicornia, 398. Cervulus, 399. Cervus, 399. Cestracionidas, 238. Cetacea, 405. Cervical plexus, 48. Cervical region, 142. Chsenomorphae, 347. Chaeropus, 398. Chaetodon, 262. Chaetodontidae, 262. Chalaza, 207. Chalicotheriidas, 396. Chamaeleon, 319, 320. Chamois, 399. Champosaurus, 314. Characinidae, 255. Characinus, 255. Charadrius, 349. Chauliodus, 256. Cheetah, 412. Chelone, 310. Chelonia, 307. Chelonidae, 310. Chelopus, 311. Chelydosauria, 310. Chelydra, 311. Chelydridae, 311. Chelys, 311. Cheropus, 380. Chevrotains, 398. Chiasma, optic, 61. Chiasmodontidae, 261. Chilomycterus, 267. Chilonycteris, 388 Chimaera, 242. Chimpanzee, 416. Chinchilla, 391. Chinchillidae, 391. Chipmunk, 390. Chiromyidae, 415.- Chiromys, 415. Chironectes, 579. Chiroptera, 386. Chirox, 378. Chlamydophorus, 383. Chlamydosaurus, 320. Chlamydoselachidae, 238, Chlamydoselachus, 238. Chlamydotherium, 384. Choana, 76. Chceropotamus, 397. Choloepus, 383. Chologaster, 257. Chondrin, 133. Chondrocranium, 150. Chondropterygii, 232. Chondrostei, 250. Chordata, i. Chorda tympani, 63. Chordediles, 348. Chorion, 288, 373. Chorionic placenta, 374. Chorion laeve, 375. Choristodera, 314. Choroid fissure, 79. Choroid plexus, 52, 54. Chorophilus, 287. Chromatophores, 91. Chromidae, 261. Chromosomes, 208. Chrysemys, 311. Chrysochloridas, 385. Ciconia, 348. Cilia, 10. Ciliated epithelium, 10. Ciliary ganglion, 62. Ciliary muscles, 83. Ciliary process, 83. Cimolichthys, 257. Cimoliosaurus, 306. Cingulum, 365. Circulatory organs, 178. Cistudo, 311. Civet, 412. Cladistia, 250, Cladodus, 237. Cladoselache, 237. Cladoselachii, 237. Clasnodpn, 411. Clarias, 255. Claspers, 209, 235. Clavicle, 169, 170. Claws, 99. Cleithrum, 169. Clepsydrops, 306. Clevelandia, 263. Clidastes, 322. Climbing perch, 262. Cloaca, 39. Clupeidae, 256. Clupea, 256. Cnemidophorus, 321. Coati, 412. Cobitis, 255. Cobra, 325. Coccosteus, 272. Coccygomorphae, 348. Cochlea, 73. Cod , 264. Coelacanthidas, 249. Coelacanthus, 249. Coeliac axis, 190. Coelogenys, 391. Coelom, 7, 101. Coffin bone, 394. Colaptes, 351. Colius, 348. Colon, 38, 368. Colossochelys, 311. Colosteus, 283. J. J. V J-SJ^SL, Coturnix, 350. ^J Cyclodipterini, 249. Colubridae, 325. Cotyledonary placenta, 373. Cyclodus, 320. Colubriformia, 324. Cotylophora, 396. Cycloid scales, 228. Columba, 350. Cotylosauria, 304. Cyclopterus, 260. Columella, 74, 159. Coverts, 330. Cyclospondyli, 239. Columnar epithelium, 9. Cowper's glands, 371. Cyclospondylous vertebrae,*' Columns of cord, 44. Coypu, 391. 234- Coly, 348. Cramp fish, 239. Cyclostomata, 219. / Colymbus, 349. Crane, 349. jCyclotura, 383. Commissures of brain, 55. Cranial nerves, 58. Cygnus, 348. Compsognathus, 316. Cranial vertebrae, 166. Cymatogaster, 260. Concrescence, 213. Cranium, 150. Cynaelurus, 412. Condylarthra, 393. Crax, 350. Cynocephalus, 416. Condylura, 385. Crassilingua, 319. Cynomys, 390. Cone cells, 79. Cremaster muscle, 371. Cynopithethidae, 411. Cone, in teeth, 365. Creodonta, 411. Cynoscion, 259. Conid, 365. Crevalle, 263. Cyprinidae, 255. Coney, 402. Cricetus, 390. Cyprinodon, 257. Conger, 257. Cricoid cartilage, 28. Cyprinodontidae, 257. Congo eel, 285. Cricotus, 283. Cyprinus, 255. Conjunctiva, 81. Crista acustica, 71. Cystic duct, 40. Connective tissues, 12. Crista galli, 356. Cystophora, 414. Conodonts, 223. Crocidura, 385. Conurus, 349. Crocodilia, 326. Dactylethra, 286. Conus arteriosus, 181. Crocodilidse, 328. Dactylopteridae, 260. Convolutions of brain, 52. Crocodilus, 328. Dactylopterus, 260. Copelatae, 2. Crop, 34. Dactyloscopus, 261. Copperhead, 326. Crossopterygii, 249. Darter (bird), 347. Copula, 154. Crotalidae, 325. Darters (fish), 259. - Coracias, 348. Crotalus, 325. Dasyatis, 240. Coracoid bone, 168, 169. Crura cerebri, 53. Dasypaedes, 330. Coracoid process, 359. Cryptacanthodes, 261. Dasypodidae, 383. Coral snake, 325. Cryptobranchia, 285. Dasyprocta, 391. Coregonus, 256. Cryptobranchidae, 285. Dasyproctidae, 391. Corium, 87. Cryptobranchus, 285. Dasypus, 383. Cormorant, 347. Cryptodira, 310. Dasyuridae, 379. Cornua of cord, 44. Cryptoprocta, 412. Dasyurus, 379. Cornu trabeculae, 152. Crypturi, 346. Decidua, 374. Coronary bone, 394. Ctenacodon, 378. Decidua reflexa, 375. Corpora bigemina, 50, 53. Ctenodactylus, 391. Decidua serotina, 375. Corpora quadrigemina, 50, Ctenodus, 273. Decidua vera, 375. 53- Ctenoid scales, 228. Decussation, 56. Corpus albicans, 53. Ctenolabrus, 260. Deer, 399. Corpus callosum, 56. Cubical epithelium, 9. Delphinapterus, 408. Corpus mammilare, 53. Cuboid bone, 177. Delphinidoe, 408. Corpus restiforme, 54. Cuckoo, 348. Delphinoidea, 408. Corpus striatum, 51. Cuculus, 348. Delphinus, 408. Cortex of brain, 51. Cuneiform bone, 177. Demibranch, 22. Cortis' organ, 73. Gunner, 260. Dendrites, n. Coryphaenidae, 263. Currasow, 350. Dendrobatidae, 287. Coryphodon, 400. Cuscus, 380. Dendrolagus, 380. Cosoryx, 399. Cuticular layer, 87. Dental formula, 366. Cottidae, 259. Cutis, 87. Dental papilla, 19. Cottus, 259. Cutlas fish, 263. Dental ridge, 19. 424 INDEX. Dentary, 164. Diphycercal fin, 229. Denticetae, 408. Diphyodont, 366. Dentinal canals, 16. Diplarthra, 392. Dentinal papilla, 92. Diplodocus, 316. Dentine, 16, 19. Diplosaurus, 328. Dercetidae, 258. Diplospondyli, 238. Derma, 87. Diplurus, 249. Dermal glands, 89. Dipneumonia, 273. Dermal organs, 86. Dipneustes, 267. Dermaptera, 385. Dipnoi, 267. Dermochelys, 310. Dipodidae, 390. Derotremata, 285. Diprotodon, 380. Desmodus, 388. Diprotodonta, 380. Desmognathae, 347. Diprotodontidae, 380. Desmognathus, 285. Dipsas, 325. Desmognathous skull, 334. Dipterus, 273. Deutoplasm, 205. Dipus, 390. Devil fish, 240. Discobolus, 260. Devexa, 399. Discocephali, 263. Diaphragm, 106. Discoidal placenta, 373. Diapophysis, 141. Discus proligerus, 125. Diatryma, 346. Ditrema, 260. Diceratherium, 395. Dinosauria, 314. Dichobune, 398. Doctor fish, 262. Diclonius, 317. Dodo, 350. Dicotyles, 397. Dog, 412. Dicotylidae, 397. Dog fish, 239. Dicrocynodon, 380. Dog sharks, 239. Dicynodon, 305. Dolichosauria, 321. Didelphia, 378. Dolichosoma, 283. Didelphidae, 379. Dolphins (mammals), 408. Didelphys, 379. Dolphin (fish), 263. Didymodus, 237. Dorcatherium, 398. Didunculus, 350. Dorosoma, 256. Didus, 350. Dorsal aorta, 182. Diemyctylus, 285. Dorsal fin, 177, 228. Diencephalon , 49. Dorsal nerve roots, 46. Diffuse placenta, 373. Dorsal region, 142. Digestive tract, 34. Dove, 350. Digitiform appendix, 36. Draco, 320. Digitigrade, 361. Drill, 416. Digits, 176. Dromaeognathi, 345. Dimetrodon, 306. Dromaeognathous skull, 334. Dimorphodon, 330. Dromaius, 346. Dimylidae, 385. Dromatherium, 377. Dinichthys, 272. Dromedary, 398. Dinictis, 412. Dryolestes, 380. Dinoceras, 400. Duckbill, 376. Dinocyon, 412. Ducks, 348. Dinornithidae, 346. Duct, cystic, 40. Dinotheridae,40i. " Gartner's, 129. Dinotherium,40l. " hepatic, 41. Diodon, 267. " mesonephric, 119. Diomedia, 349. " Miillerian, 126. Duct, Stenson's, 77. " urogenital, 126. pronephric, 117. " Wolffian, 119, 126. " of Wirsung, 40. Ductus Botallii, 187. Ductus choledochus, 41. Ductus Cuvierii, 183. Ductus endolymphaticus, 70. Dugong, 405. Duodenal artery, 190. Duodeno-hepatic omentum, 105. Duodenum, 35. Duplicidentata, 391. Dura mater, 57. Eagle, 348. Eared seal, 413. Ears, 69. Ear stones, 71. Echidna, 377. Echidnidae, 377. Echineis, 264. Ectethmoid, 244. Ectoderm, 6. Ectodermal structures, 43.. Edaphodon, 242. Edentata, 381. Educabilia, 361. Eel, Congo, 285. Eel, mud, 285. Eel pouts, 261. Eels, 256. Efferent nerves, 47. Egg, 5- Egg, development of, 205. Egg tooth, 343. Elapidae, 325. Elaphus, 399. Elaps, 325. Elasmobranchii, 232. Elassoma, 259. Elastic tissue, 14. Elastica exerna, 135. Elastica interna, 134. Electrical organs, 115. Electric eel, 255. Electric skate, 239. Elephant, 401. Elephantidae, 401. Elephas, 401. Elginia, 305. Elotherium, 397. INDEX. 425 Emballonura, 388. Emballonuridae, 387. Embiotoca, 260. Embiotocidae, 260. Embolomeri, 283. Embolomerous, 136. Emeu, 346. Empedias, 304. Emperor fish, 262. Emydas, 311. Enamel, 19. Enamel organ, 19. End buds, 68. Endolymph, 71. Endolymphatic duct, 70. Endothelium, 9. Engraulis, 256. Engystoma, 287. Engystomidae, 287. Enhydris, 412. Ensiform process, 149, 356. Entelops, 383. Enteroccele, 7. Enteropneusti, 3. Entepicondylar foramen, 304- Entoderm, 6. Entodermal organs, 17. Entoglossum, 335. Entoplastron, 308. Eohippus, 395. Epiaxial muscles, 109. Epiblast, 6. Epibranchial, 154. Epicentrals, 144, 246. Epicrele, 50. Epicoracoid, 170. Epidermal structures, 87. Epidermis, 87. Epididymis, 130. Epiglottis, 370. Epihyal, 155. Epimere, 101. Epimerals, 144, 246. Epiotic, 158. Epiphysis, 85, 134, 355. Epiplastron, 308. Epipleurals, 144, 246. Epipubis, 171. Episternum, 149, 308. Epistropheus, 142. Epithelium, 9. Epitrichium, 88. Epoophoron, 129. Equidae, 395. Equus, 395. Erethyzon, 391. Eretmochelys, 310. Erinaceidae, 385. Erinaceus, 385. Erismatopterus, 256. Ermine, 412. Eryops, 283. Erythrinus, 255. Eschatius, 398. Esocidae, 257. Esox, 257. Esthonyx, 403. Etheostoma, 259. Ethmoid, 357. Ethmoid plate, 152. Eumeces, 320. Eumylodus, 242. Eunectes, 325. Euornithes, 347. Eurhipidurae, 345. Eurycormus, 252. Eurylaima, 351. Eurylepis, 251. Eurypharynx, 257. Euselachii, 238. Eustachian tube, 73. Eusthenopteron, 249. Eusuchia, 328. Eutainia, 325. Eutheria, 378. Exoccipital, 157. Exocoetus, 257. Exocoetidae, 257. Exoskeleton, 91. External carotid, 183. External ear, 24. External gills, 24. Eye, pineal or parietal, 85. Eyes, 78. Facial angle, 358. Falciform process, 231. Falco, 348. Fallopian tube, 127. Fascia, 112. Fasciculus cuneatus, 54. Fasciculus gracilis, 54. Fat, 13. Feather tracts, 95. Feathers, 94, 330. Felidae, 412. Felis, 412. Felsinotherium, 405. Femoral artery, 191. Femur, 176. Fenestra ovalis, 72, 159. Fenestra rotunda, 72. Fer-de-lance, 326. Ferret, 412. Fertilization, 5. Ferae, 410. Fetterbone, 394. Fiber, 390. Fibrillations, 10. Fibula, 176. Fibulare, 176. Fierasfer, 261. Fierasferidae, 261. Fifth ventricle, 57. File fish, 266. Filoplumes, 95. Filum terminale, 48. Fin-back whale, 410. Fins, 167, 177, 228, 229. Firmis;ernia, 287. Fishes, 228. Fissilinguia, 320. Fissipedia, 411. Fistularia, 258. Fistularidae, 258. Flamingo, 348. Flat fishes, 264. Flexures of brain, 56. Floccular lobes, 338. Flounder, 265. Flying fish, 257, 260. Flying fox, 388. Flying squirrel, 390. Fodientia, 382. Fontanelle, 157, 234. Food yolk, 205. Foramen jugulare, 166. Foramen lacerus, 164. Foramen magnum, 157. Foramen of Monro, 50. Foramen, obturator, 170. Foramen ovale, 165. Foramen rotundum, 165. Fore brain, 48. Formicaria, 351. Fornix, 55. Fossa rhomboidalis, 54. Fowl, 350. Fox, 412. Fregata, 347. Frigate bird, 347. 426 INDEX. Frigate mackerel, 263. Gastrosplenic omentum, 106. Frilled lizard, 320. Gastrula, 5. Frogs, 287. Gastrulation, 211. Frontal bone, 161. Gavialis, 328. Frugivora, 388. Gavialidae, 328. Fulcra, 248. Gazella, 399. Fulmarus, 349. Gecko, 319. Fundulus, 257. Geese, 348. Fundus glands, 367. Gemsbok, 399. Fundus region, 35. General cutaneous nerves,64. Furcula, 336. Genital artery, 191. Fur seal, 413. Geococcyx, 348. Geomyidae, 390. Gadidae, 264. Geomys, 390. Gadus, 264. Geotria, 223. Galago, 415. Germ layers, 8. Galeidae, 238. Gibbon, 416. Galeocerdo, 239. Gill clefts, 22. Galeopethecidae, 415. Gill rakers, 230. Galeopithecus, 385, 415. Gills, 22. Galerix, 385. Ginglymodi, 251. Galesaurus, 306. Giraffa, 399. Galeus, 239. Giraldi's organ, 130. Gall bladder, 40. Gizzard, 34. Gall capillaries, 40. Gland, acinose, 90. Gallinae, 350. " dermal, 90. Gallinago, 350. " Harder's, 84. Gallus, 350! " internasal, 21. Ganglion, n. " lachrymal, 84. " Casserian, 61. " milk, 91. " cells, 10. " oil, 98. " ciliary, 62. " oral, 21, " of dorsal roots, 46. " parotid, 22. " ' Gasserian, 61. " racemose, 90. " otic, 62. " rectal, 36. " ' sphenopalatine, 62. " salivary, 22. Ganoidea, 248. " sublingual, 22. Ganoid scales, 228. " submaxillary, 22. Ganoin, 92. " sweat, 90. Ganhet, 347. " lihymus, 33. Gar, bony, 257. " thyroid, 32. Gar pikes, 251. " tubular, 90. Garter snake, 325. Glass-snake, 320. Gartner's duct, 129. Glenoid fossa, 168. Gasserian ganglion, 61. Glia cells, 12. Gasterosteidae, 258. Glires, 388. Gasterosteus, 258. Globe fish, 267. Gastornis, 346. Globiocephalus, 408. Gastornithes, 346. Glomerulus, 119, Gastralia, 147. Glomus, 118. Gastric artery, 190. Glossophaga, 388. Gastrostomus, 257. Glossopharyngeal nerve, 63. Gastrohepa'ic omentum, 105. Glottis, 27. Gastrolepidoti, 284. Glyptodon, 383. Glyptodontidae, 383. GnathostDmata, 225. Gnu, 399. Goat fish, 261. Goats, 399. Gobiesocidae, 260. Gobiesox, 260. Gobioidia, 263. Gobius, 263. Gold fish, 255. Golden moles, 385. Golden yellow body, 303. Coil's column, 54. Gonads, 124. Goniopholidae, 328. Goniopholis, 328. Gonotome, 103. Goose, 348. Goose fish, 266. Gopher, 390. Gopher turtle, 311. Gorilla, 416. Goura, 350. Gouramy, 262. Graafian follicle, 124. Gradientia, 284. Grallae, 349. Grandry's corpuscles, 68. Granular layer, 79. Gray matter, n. Great omentum, 106. Grebe, 349. Green turtle, 310. Gronias, 255. Grouse, 350. Grus, 349. Guanaco, 398. Guillemots, 349. Guinea-pig, 391. Gull, 349. Gullet, 34. Gulo, 412. Gunnellus, 261. Gurnard, 260. Gymnodonti, 267. Gymnopoedes, 330. Gymnophiona, 287. Gymnbtus, 255. Gyps, 348. Gyri, 52. Habenulae, 52. Haddock, 264. Haddock, Norway, 259. INDEX. 427 Hadrosaurus, 317. Haemal arch, 138. Haemal process, 138. Haemal spine, 138. Haemapophysis, 138. Haemulon, 259. Hag-fish, 224. Hair, 97, 352. Hair cells, 73. Hake, 264. Halcyon, 348. Halecomorphi, 252. Halibut, 265. Halicore, 405. Halicoridae, 405. Halitherium, 405. Hallux, 176. Hammer-head shark, 239. Hamster, 390. Hapa]e, 415. Hapalidae, 415. Haploceras, 399. Haplodoci, 265. Haplodon, 390. Haplodontidae, 390. Haplodont, 365. Haplomi, 257. Harderian glands, 84. Hares, 391. Harriotta, 242. Hatteria, 313. Haversian canals, 14. Hawk, 348. Head cavities, in. Head kidney, 116, 118. Head, segmentation of, 201. Heart, 178, 184. Heart muscle, 12. Hedgehog, 385. Heliornis, 349. Helladotherium, 399. Hell-bender, 285. Heloderma, 321. Helodermidae, 321. Hemiazygos vein, 196. Hemibranchii, 257. Hemichordia, i. Hemipenes, 304. Hemispheres, cerebral, 49. Hemispheres of cerebellum, 54- Hemitripterus, 260. Hepatic artery, 190. Hepatic duct, 41. Hepatic vein, 192. Hepato-enteric duct, 41. Heptanchus, 238. Heptatrema, 224. Herodias, 348. Herodii, 348. Heron, 348. Herpestes, 412. Herring, 256. Hesperomys, 390. Hesperornis, 344. Heterocercal fin, 229. Heterodon, 325. Heterodont, 20. Heteropygii, 257. Heterosomata, 264. Heterostraci, 224. Hexanchus, 238. Hindbrain, 49. Hindgut, 36. Hipparion, 395. Hippocampus, 258. Hippoglossus, 265. Hippopotamidae, 397. Hippopotamus, 398. Hipposiderus, 387. Histiophorus, 263. Histology, 9. Hogs, 397. Holacanthus, 262. Holconoti, 260. Holconotus, 260. Holoblastic eggs, 210. Holocentridae, 261. Holocentrum, 261. Holocephali, 240. Holoptychius, 249. Holostei, 251. Homaeosaurus, 314. Hominidae, 416. Homo, 417. Homocercal fin, 229. Homodont, ao, 364. Homunculus, 416. Honeycomb, 35. Honey guide, 348. Hooded seal, 414. Hoofs, 99. Hoopoe, 348. Hoplophoneus, 412. Hoplophorus, 383. Horn, 99. Horn bill, 348. Horned pout, 255. Horned toad, 320. Horses, 395. Horse mackerel, 263. Howling monkey, 416. Humerus, 176. Humming birds, 350. Hump-back whale, 410. Hyaena, 412. Hyaenarctus, 412. Hyaenidae, 412. Hyaenodontidae, 411. Hyaline cartilage, 14. Hydatid, 128. Hydrophidae, 325. Hydropotes, 399. Hydrochoerus, 391. Hyla, 287. Hylerpeton, 283. Hylidae, 287. Hylobates, 416, Hylonomus, 283. Hyoid, 155. Hyoid arch, 155. Hyoruandibula, 155. Hyomandibularis nerve, 62. Hyomoschus, 398. Hyoplastron, 308. Hyopotamus, 397. Hypaxial muscles, 109. Hyperoartia, 223. Hyperodapedon, 314. Hyperoodon, 409. Hyperotretia, 224. Hypoblast,6. Hypobranchial, 154. Hypocentrum, 136. Hypocone, 365. Hypogastric artery, 183, 190. Hypogastric vein, 194, 197. Hypogeophis, 288. Hypohyal, 155. Hypomere, 101. Hypoplastron, 308. Hypophysial duct, 52. Hypophysis, 52. Hypsiprymnidae, 380. Hypsiprymnus, 380. Hypsirhopus, 316. Hyrachius, 395. Hyracodon, 395. Hyracoidea, 402. Hyracotherium, 395. Hyrax, 402. Hystricidae, 391. 428 INDEX. Hystricomorpha, 390. Hystrix, 391. Ibex, 400. Ibis, 348. Ichthyophis, 288. Ichthyopsida, 226. Ichthyopterygia, 312. Ichthyornis, 344. Ichthyosauria, 312. Ichthyosaurus, 313. Ichthyotomi, 237. Ictobius, 255. Ictopsidae, 385. Iguanidae, 319. Iguanodon, 317. Ileocolic valve, 36. Iliac artery, 190. Iliac vein, 197. Ilium, 171. Impennes, 347. Implacentalia, 375. Impregnation, 5. Incisors, 364. Incus, 74, 159. Indeciduata, 374. Indicator, 348. Indris, 415. Ineducabilia, 361. Infraclavicle, 169. Infratemporal fossa, 166. Infundibula, 30. Infundibulum, 52. Ingluvies, 34. Inia, 408. Iniomi, 256. Innominate vein, 197. Insectivora, 384. Insertion of muscle, 112. Intercalary cartilages, 234. Intercentrum, 137. Intercostal arteries, 191. Intercostal vein, 196. Interhyal, 244. Intermedium, 176. Internal carotid, 183. Internasal gland, 21. Interoperculum, 161. Interrenal, 131. Interspinalia, 230. Interspinous ligament, 137. Intestinal caecum, 39. Intestinalis nerve, 64. Invagination, 5. Invertebrates, i. Involuntary muscle, 12. Ipnops, 256. Iris, 83. Ischiatic artery, 191. Ischiromyidae, 390. Ischium, 171. Ischyodus, 242. Ischyrhiza, 257. Isectolophus, 395. Isinglass, 251. Isodectes, 305. Isolating cells, 67. Isospondyli, 255. Isteus, 257. Iter, 50. Ivory, 19. lynx, 351. Ja9ana, 349. Jackal, 412. Jacobson, organ of, 77. Jacobson's commissure, 63. Jaguar, 412. Jerboa, 390. Jugal, 163. Jugular fins, 231. Jugular foramen, 166. Jugular vein, 183. Jumping mice, 390. Jumping shrews, 385. Jungle fowl, 350. Kangaroo, 380. Kangaroo rat, 380. Keraterpeton, 283. Keratobranchial, 154. Keratohyal, 155. Kidney, 122. Killer whale, 408. Killifish, 257. Kingbird, 351. Kingfisher, 348. King of the herrings, 242. Kinosternidae, 311. Kinosternon, 311. Kiwi, 346. Knee pan, 360. Koala, 380. Kogia, 409. Kupffer's vesicle, 254. Labial cartilage, 156. Labridae, 260. Labrus, 260. Labyrinth, 7 1. Labyrinthici, 262. Labyrinthodon, 284. Labyrinthodonta, 283. Labyrinthodontidee, 284. Lacerta, 320. Lacertidae, 320. Lacertilia, 318. Lachrymal bone, 161. Lachrymal duct, 76. Lachrymal gland, 84. Lactophrys, 267. Lacuna, 15. Laelaps, 316. Lagena, 71. Lagomorpha, 391. Lagomyidee, 391. Lagomys, 391. Lagostomus, 391. Lamella, 15. Lamina cribrosa, 357. Lamina terminalis, 50. Lamna, 239. Lamnidse, 239. Lampetra, 223. Lamprey eels, 223. Laopteryx, 344. Laosaurus, 317. Lariosaurus, 306. Larus, 349. Larynx, 28. Laternlis nerve, 64. Lateral line, 68. Lateral plate zone, 101. Leather back tortoise, 310. Leather turtle, 310. Lebias, 257. Lemming, 390. Lechriodonta, 285. Lemur, 415. Lemuridas, 415. Lemuroidea, 414. Lens, 8.1. Leopard, 412. Lepidocottus, 260. Lepidopus, 263. Lepidotus, 252. Lepidosauria, 317. Lepidosiren, 273. Lepidosteidae, 251, Lepidosteus, 251. Lepomis, 259. Leporidae, 391. INDEX. 429 Lepospondyli, 283. Lymnohyus, 396. Leptictidas, 411. Lymph, 16. Leptocardii, i. Lymph glands, 200. Leptocephalus, 257. Lymph hearts, 199. Leptochcerus, 397. Lymph system, 198. Leptolepis, 256. Lynx, 412. Leptomeryx, 398. Lyomeri, 257. Leptotragulus, 398. Lyre bird, 351. Lepus, 391. Lestornis, 344. Macacus, 416. Leuciscus, 255. Macaque, 416. Leucocytes, 16. Machaerodus, 412. Leydig's duct, 126 Mackerel, 263. Limbs, 167. Macrauchenidre, 395. Limbs, origin of, 172. Macrochelys, 311. Lingualis nerve, 63. Macromeres, 222. Liodon, 322. Macropetalichthys, 272. Lion, 412. Macropodidae, 380. Lion, sea, 413. Macropus, 380. Liparis, 260. Macroscelidae, 385. Liver, 18, 40. Macrotherium, 396. Liver islands, 40. Macrurus, 264. Lizards, 318. Macruridae, 264. Llama, 398. Macula acustica, 71. Lobus inferior, 53. Malacanthidae, 261. Loggerhead turtle, 310. Malaclemmys, 311. Longirostres, 328. Malapterurus, 255. Loon, 349. Malar bone, 163, 357. Loph, 365. Malleus, 74, 159. Lophiidae, 266. Malpighian body, 119. Lophiomys, 390. Malpighian layer, 89. Lophius, 266. Malthe, 266. Lophobranchii, 258. Malthidae, 266. Lophodon, 395. Mammalia, 352. Lophodont, 365. Mammoth, 401. Lopholatilus, 261. Man, 416. Lophopsetta, 265. Manatee, 405. Loricaria, 255. Manatherium, 405. Loricata, 259, 326, 383. Manatidae, 405. Loris, 415. Manatus, 405. Lota, 264. Mandibularis nerve, 61. Loxodon, 401. Mandril], 416. Lumbar arteries, 191., Manidae, 382. Lumbar plexus, 48. Manis, 382. Lumbar region, 142. Manta, 240. Lump fish, 260. Mantle, 51. Lunare, 177. Manubrium, 149. Lung fish, 267. Manus, 176. Lung pipes, 31. Manyplies, 35. Lungs, 27. Marmoset, 415. Lupus, 412. Marsipobranchii, 219. Lutjanus, 259. Marsupial bones, 171. Lutra, 412. Marsupialia, 378. Lycaon, 412. Marsupium, 378. Marsupium (of eye) , 340. Marten, 412. Mastodon, 401. Mastodonsaurus, 284. Matrix, 14. Maturation of egg, 209. Maxillaris nerve, 61. Maxillary, 162. Meatus, auditory, 75. Meckel's cartilage, 156. Mecodonta, 285. Mediastinum, 106. Medulla oblongata, 50. Medullary folds, 43. Medullary plate, 43. Medullary groove, 43. Medullary sheath, II. Medullated fibres, n. Megachiroptera, 388. Megaderma, 387, Megalobatrachus, 285. Megalonyx, 383. Megalosaurus, 316. Megalotriton, 286. Megamys, 391. Megapodius, 350. Megaptera, 410. Megatheriidae, 383. Megatherium, 383. Meissner's corpuscles, 69. Melanerpeton, 283. Meleagris, 350. Meles, 412. Mellivora, 412. Merlucius, 264. Melursus, 412. Membrane bone, 15, 164. Menhaden, 256. Menidia, 262. Meniscotheriidae, 393. Menobranchus, 285. Menopoma, 285. Menura, 351. Mephitis, 412. Merkel's corpuscles, 68. Meroblastic eggs, 210. Merops, 348. Merychius, 398. Mesencephalon, 50. Mesenchymatous structures, 132. Mesenchyme, 8. Mesenteric artery, 190. Mesentery, 103. 430 Mesethmoid, 158. Mesoarium, 106. Mesoblast, 6. Mesocardium, 106, 180. Mesocolon, 105. Mesodactyla, 393. Mesoderm, 7. Mesogaster, 105. Mesohippus, 395. Mesomere, 101. Mesonephric duct, 119. Mesonephros, 116, 118. Mesonychidse, 411. Mesonyx, 411. Mesoplodon, 409. Mesoptervgium, 175. Mesorchium, 106. Mesorectum, 105. Mesothelium, 8. Mesothelial structures, 100. Mesovarium, 106, 126. Metacarpus, 176. Metacoele, 101, 103. Metacone, 365. Metaconule, 365. Metanephros, 116. Metapophysis, 141. Metapterygium, 175. Metatarsus, 176. Metazoa, i. Metencephalon, 50. Miacidae, 411. Mice, 390. Microchiroptera, 387. Microconodon, 377. Microgadus, 264. Micromeres,222. Micropterus, 259. Microsauria, 283. Micropyle, 207. Midas, 415. Mid brain, 48. Middle zone, 101. Mid gut, 36. Midshipman, 266. Milk dentition, 20. Milk glands, 91. Milk line, 354. Mink, 412. Mixosaurus, 313. Moa, 346. Moccasin, 326. Mola, 267. Molars, 364. INDEX. Mole rat, 396. Musk deer, 399. Mole-shrew, 385. Musk ox, 400. Moles, 385. Musk rat, 390. Molossus, 388. Musk turtle, 311. Momotus, 348 Musophaga, 348. Monacanthus. 266. Mustela, 412. Monachus, 414. Mustelidae, 412. Monasa, 348. Mustelus, 239. Mongoose, 412. Mycetes, 416. Monimostylica, 300. Myelencephalon, 50. Monitor, 320. Myctodera, 285. Monkeys, 415. Myelin, n. Monocondylia, 291. Myliobatidae, 240. Monodelphia, 381. Mylodon, 383. Monodon, 408. Myocardium, 180. Monophyodont, 366. Myocomma, 103. Monopneumonia, 272. Myocoele, 101. Monotremata, 376. Myodes, 390. Moonfish, 263 Myogalidae, 385. Moose, 399. Myomorpha, 390. Mordacia, 223. Myopotamus, 391. Morgagni, sinus of, 29. Myoseptum, 103. Mormyridae, 255. Myotomes, 101, 108. Moropus, 396. Myotome zone, 101. Mosasauridaa, 322. Myoxidae, 390. Mosasaurus, 322. Myoxus, 390. Moschus, 399. Myrmecobius, 379. Motmot, 348. Myrmecophaga, 383. Motor nerves, 46. Myrmecophagidae, 383. Mouse, 390. Mystacoceti, 409. Mound-bird, 350. Myxine, 224. Mouth, 18. Myxinidas, 224. Mud eel, 285. Myxinoidei, 224. Mud-minnow, 257. Myzontes, 219. Mud puppy, 285. Mud turtle, 311. Nails, 99. Mugilidae, 262. Naja, 325. Mullets, 262. Nandu, 345. Mullidae, 261. Naosaurus, 306. Mullus, 261. Nares, 76. Multipolar nerve cells, 10. Narwal, 408. Multituberculata, 377. Nasal bone, 161. Mummichog, 257. Nasal capsules, 153. Muntjacs, 399. Nasal glands, 76. Muraana, 257. Nasua, 412. Muraenidaa, 257. Nates, 53. Muridae, 390. Naucrates, 263. Murry, 257. Naviculare, 177. Mus, 390. Necrolemur, 415. Muscle, development of, 108. Nectogale, 385. Muscle plates, 109. Necturus, 285. Muscular system, 107. Needle fish, 257. Muscular tissues, 12. Nemopteryx, 264. Muskalonge, 257. Neobalaena, 410. INDEX. 431 Neomeris, 408. Neurilemma, n. Neomylodon, 383. Neuroglia, 12. Nephridia, 116. Neuromeres of brain, 49. Nephrostomes, 117. Neuropore, 44. Nephrotome, 103. Newt, 285. Nerve, abducens, 61. Nictitating membrane, 82. " accessory of Willis, Night hawk, 348. 64. Noctilio, 388. " afferent, 47. Nomarthra, 382. " auditory, 63. Nondeciduata, 374. " buccalis, 62. Non-elastic tissue, 13. " cranial, 58. Norway haddock, 259. 41 efferent, 47. Nothosaurus, 306. " glossopharyngeal. 63. Notidanidae, 238. " general cutaneous, 64. Notochord, 17, 134. " hyomandibularis, 62. Notochordal sheath, 135. " intestinalis, 64. Notodelphys, 287. lateralis, 64. Nototherium, 380. " lingualis, 63. Nototrema, 287. " mandibularis, 61. Notropis, 255. " maxillaris, 61. Nycteris, 387. " mixed, 59. " motor, 46. Oblique muscle of eye, 84, " oculomotor, 61. 114. " olfactory, 60. Oblique muscles of trunk, " ophthalmicus, 61, 62. "3- " optic, 60. Obturator foramen, 170. " palatine, 62. Occipital bone, 356. " patheticus, 59. Octodon, 391. " pneumogastric, 64. Octodontidae, 391. " post-trematic, 63. Oculomotor nerve, 61. " pretrematic, 63. Odontoblasts, 16, 19. " roots of, 46. Odontoceti, 408. " sensory, 46. Odontoholcae, 344. " somatic, 64. Odontoid process, 143. " spinal, 46. Odontormae, 344. " spinal accessory, 59. Odontornithes, 344. " sympathetic, 47. CEsophagus, 34. " trifacial, 59. Oil bird, 348. " trigeminal, 61. Oil gland, 98. " trochlearis, 61. Olecranon process, 360. " vagus, 63. Olfactory lobe, 52. " visceral, 64. " nerve, 60. " cells, 10. " organ, 75. Nervous tissue, 10. " tract, 60. Nesodon, 402. Oligobunus, 412. Nesopithecidse, 415. Oligosoma, 320. Nesopithecus, 415. Omasum, 35. Neural arch, 138. Omentum, 105, 106. " crest, 47. gastro-hepatic, 41. " plate, 43. Omosternum, 148. process, 135. Omphalomesaraic vein, 180, " spine, 138. 192. Neurapophysis, 135. Oncorhynchus, 256. Onychodus, 249. Operculum, 23, 161. Ophiderpeton, 283. Ophidia, 322. Ophidiidae, 261. Ophidium, 261. Ophidioida, 261. Ophiocephalidae, 262. Ophisaurus, 320. Ophthalmicus nerve, 61, 62. Opisthocoelous, 139. Opisthocomi, 349. Opisthoglypha, 325. Opisthomyzon, 263. Opisthotic, 158. Opoderodonta, 326. Opossums, 379. Optic chiasma, 61. Optic lobes, 50. Optic nerves, 60. Optic stalk, 78. Optic thalami, 49. Optic tract, 61. Optic vesicle, 78. Orang-utan, 416. Orbitosphenoid, 158. Orca, 408. Oreodon, 398. Oreodontidce, 398.. Organ of Corti, 73. Organ of Giraldi, 130.. Organ of Jacobson, 77.. Origin of muscle, 112. Ornithopoda, 317. Ornithodelphia, 376. Ornithorhynchidae, 376. Ornithorhynchus, 376. Ornithosauria, 329. Oronasal groove, 76. Orthagoriscus, 267. Orthopoda, 316. Orycteropodidae, 382. Orycteropus, 382. Oryx, 399. Os en ceinture, 278. Os entoglossum, 161. Os lenticulare, 159^ Os magnum, 177* Os orbiculare, 159. Os trans versum, 163-.. Oscines, 351. Osmerus, 256. Osphromenus, 262. Ossicula auditus, 158^ 432 INDEX. Ossification, 14. Ossification, perichondral, 133. Ostariophysi, 254. Osteoblasts, 15, 133. Osteolepis, 250., Osteostraci, 225. Ostium tubae. 127. Ostracion, 267. Ostracodermi, 224, 266. Ostrich, 345. Otaria, 413. Otariidae, 413. Otic capsule, 151. Otic ganglion, 62. Otic vesicle, 70. Otis, 349. Otoccelus, 310. Otocyon, 412. Otoliths, 71. Otter, 412. Oudenodon, 305. Ova, 125. Ovarian artery, 191. Ovaries, 125. Ovibos, 400. Oviduct, 127, 247. Ovis, 400. Ovum, history of, 205. Owls, 348. Oxyclaenidae, 411. Paca, 391. , Pachycormus, 252. Pachylemuridae, 415. Pacini's corpuscles, 69. Paddlefish, 251. Palaeobatrachidae, 287. Palaeeudyptes, 347. Palasonictidas, 411. Palaeonictis, 411. Palaeohatteria, 314. Palseoniscidae, 251. Palaeoniscus, 251. Palseorhynchidae, 263. Palaeotherium, 395. Paloeosyops, 396. Palate, 358. Palatine bone, 163. Palatine nerve, 62. Palatoquadrate, 156. Palinurichthys, 263. Pallium, 51. Palorchestes, 380. Pancreas, 41. Pangolin, 382. Panniculus adiposus, 354. Panniculus carnosus, 115. Panther, 412. Pantolambda, 400. Pantolestidae, 396. Parabronchi, 31. Parachordals, 151. Paracone, 365. Paraconid, 365. Paradidymis, 130. Paradisea, 350. Paralichthys, 265. Paraphysis, 87. Parapinealis, 86. Parapophysis, 141. Parasuchia, 328. Parasphenoid, 163. Pareiasauria, 304. Pareiasaurus, 304. Parietal bone, 161. Parietal eye, 85. Paroccipital, 161. Parocthus, 383. Parovarium, 129. Parrot fish, 260. Parrots, 349. Partridge, 350. Passer, 351. Passeres, 351. Patella, 360. Patheticus nerve, 59. Patriofelis, 411. Paunch, 35. Pavement epithelium, 9. Pavo, 350. Peafowl, 350. Peccaries, 397. Pecora, 396. Pecten, 340. Pecten of eye, 296. Pectineal process, 336. Pectoral fins, 231. Pectoral girdle, 168. Pectoral limb, 167. Pedetes, 390. Pediculati, 266. Peduncles of brain, 54. Pegasus, 260. Pelecanus, 347. Pelican, 347. Pelobates, 286. Pelobatidse, 286. Pelomedusa, 311. Peltephilus, 384. Pelvic girdle, 168. Pelvic limb, 167 . Pelvis renalis, 123. Pelycodus, 415. Pelycosauria, 306. Penguin, 347. Perameles, 380. Peramelidae, 379. Perca, 259. Percesoces, 262. Perch, 259. Perch, surf, 260. Percidae, 259. Percoidea, 259. Percopsis, 259. Perdix, 350. Perennibranchiata, 284. Pericardio-peritoneal canals, 106. Pericardium, 106, 179. Perichondral ossification, 133- Perichondrium, 15. Perilymph, 72. Perimysium, 12, 112. Perineum, 130. Periosteum, 15. Peritoneal cavity, 106. Peritoneal layer, 39. Peritoneum, 103. Periptychidae, 393. Permanent dentition, 20. Peropoda, 325. Peroneal artery, 191. Petalopteryx, 259. Petaurus, 380. Petrel, 349. Petromyzon, 223. Petromyzontes, 223. Petrosal, 166, 356. Phacochcerus, 397. Phaethon, 347. Phalacrocorax, 347. Phalanges, 176. Phalangista, 380. Phalangistidoe, 380. Phanerobranchia, 284. Phaneropleuron, 273. Pharyngeal bones, 244. Pharyngobranchial, 154. Pharyngognathi, 260. Phascalonus, 380. INDEX. 433 Phascogale, 379. Phascolarctos, 380. Phascolomyidae, 380. Phascolomys, 380. Phasianus, 350. Pheasant, 350. Phenacodidae, 393. Phenicopterus, 348. Phlegethontia, 283. Phoca, 414. Phocaena, 408. Phocidae, 413. Phrynosoma, 320. Phycis, 264. Phyllodactylus, 319. Phyllospondylous, 137. Phyllostomidae, 388. Physeter, 409. Physeteridae, 409. Physoclisti, 25, 254. Physostomi, 255. Pia mater, 57. Pica, 391. Picariae, 350. Pickerel, 257. Picus, 351. Pig, guinea, 391. Pigeon, 350. Pigs, 397- Pike, 257. Pillar cells, 73. Pilot fish, 263. Pineal eye, 85. Pinnipedia, 413. Pipa, 286. Pipe fish, 258. Pirate perch, 259. Pisces, 227. Pisiforme, 177. Pituitary body, 52.. Placenta, 290, 373. Placentalia, 375, 381. Placodontia, 306. Placodus, 306. Placoid scales, 92. Plagiaulax, 378. Plagiostomi, 232. Plagiotremata, 317. Plaice, 265. Plantain eater, 348. Plantigrade, 361. Plastron, 93, 308. Platanista, 408. Platalea, 348. Platanistidae, 408. Platax, 263. Platecarpus, 322. Platessa, 265. Platycephalus, 260. Platycormus, 261. Platydactylus, 319. Platygonus, 397. Platyops, 284. Platyrhini, 415. Platysomus, 251. Plecotus, 387. Plectognathi, 266. Plesiosauria, 306. Plesiosaurus, 306. Plethodon, 285. Pleuracanthus, 237. Pleural cavity, 106. Pleural layer, 8. Pleurapophysis, 141. Pleuraspidotheriidae, 393. Pleurocentrum, 136. Pleurodeles, 286. Pleurodira,3ii. Pleurodont teeth, 294. Pleuronectes, 265. Pleuronectidae, 265. Pleuropterygii, 237. Plexus, choroid, 52. Plexus, nerve, 48. Plica semilunaris, 83. Pliauchenia, 398. Plioplatecarpidae, 322. Plioplatecarpus, 322. Pliosaurus, 307. Ploughshare bone, 331. Plover, 349. Pneumatic duct, 25. Pneumatocyst, 25. Pneumogastric nerve, 63, 64. Pocket gopher, 390. Podiceps, 349. Podocnemis, 311. Poebrotherium, 398. Poison teeth, 324. Polar globule, 209. Polistotrema, 224. Pollachius, 264. Pollack, 264. Pollex, 176. Polydactylus, 262. Polymastodon, 378. Polynemidse, 262. Polyodon, 251. Polyodontidae, 251. Polyonax, 317. Polyprotodontia, 379. Polypteridae, 250. Polypterus, 250. Pomacanthus, 262. Pomacentridae, 261. Pomatomidae, 263. Pompano, 263. Popliteal artery, 191. Porcupine, 391. Porcus, 397. Pori abdominales, 107. Porichthys, 266. Porpoise, 408. Portal system, 192. Portal vein, 193. Portheus, 256. Postcardinal vein, 183, 193. Postcava, 195. Postclavicle, 169. Post frontal, 161. Postorbital, 161. Post temporal, 169, 245. Post trematic nerve, 63. Postzygopophysis, 140. Prairie dog, 390. Pratincole, 349. Precava, 197. Precoces, 330. Prefrontal, 161, 358. Premaxillary, 161. Premolars, 366. Preoperculum, 161. Presphenoid, 158. Presternum, 356. Pretrematic nerves, 63. Prezygapophysis, 140. Priacanthus, 259. Primaries, 330. Primates, 414. Primitive streak, 7, 342. Primitive groove, 7. Primordial cranium, 150. Prionotus, 260. Pristidae, 239. Pristiophoridae, 239. Pristis, 239. Proatlas, 143. Proboscidea, 400. Procamelus, 398. Procartilage, 133. Procellaria, 349. Processus falciformis, 231. 434 INDEX. Procoelous, 139. Pterosauria, 329. Procoracoid, 170. Pterotic, 244. Procyon, 412. Pterygoid, 158. Procyonidae, 412. Pterygoid process, 356. Proganochelys, 311. Pterygoquadrate, 156. Pronephric duct, 117. Pterylae, 95. Pronephros, 116. Ptychodrilus, 255. Prong-horn, 399. Ptychozoon, 319. Prootic, 158. Ptyctodontidae, 242. Prorastomidae, 405. Ptyctodus, 242. Prorastomus, 405. Pubis, 171. Prosencephalon, 49. Puff-bird, 348. Prosimise, 414. Pullastrae, 350. Prostate glands, 371. Pulmonary artery, 185. Proteidae, 285. Pulp cavity, 19. Proteles, 412. Puma, 412. Proteroglypha, 325. Pupil, 83. Proterosaurus, 314. Pygostyle, 142, 331. Proteus, 285. Pyloric appendages, 38. Protoceras, 399. Pyloric gland, 367. Protocone, 365. Pyloric stomach, 337. Protoconule, 365. Pylorus, 34. Protodonta, 377. Pyramidalis muscle, 340. Protohippus, 395. Python, 325. Protolabis, 398. Pythonomorpha, 321. Protopterus, 273. Protopterygium, 175. Quadrate, 158. Protoreodon, 398. Quadratus muscle, 340. Protostega, 310. Quail, 350. Prototheria, 376. Prototheridae, 395. Rabbits, 391. Protovertebrae, 101. Raccoon, 412. Proventriculus, 34. Raccoon fox, 412. Provivera, 411. Racemose glands, 90. Psalterium, 35. Rachiodon, 325. Psephoderma, 310. Radial artery, 189. Psephurus, 251. Radiale, 176. Psetta, 265. Radialia, 173. Psittaci, 349. Radius, 176. Pseudob ranch, 23. Radix aortae, 182. Pseudopleuronectes, 265. Raiae, 239. Pseudosuchia, 327. Rail, 349. Pseudo ventricle, 57. Raja, 239. Pteranodon, 330. Rajidae, 239. Pteraspis, 224. Rallus, 349. Pterichthys, 225. Ram us dorsalis, 47. Pterocletes , 350. Ramus intestinalis, 47. Pterodactylia, 329. Ramus ventralis, 47. Pterodactylidae, 330. Rana, 287. Pteromys, 390. Ranidae, 287. Pteropaedes, 330. Rangifer, 399. Pterophryne, 266. Raptores, 348. Pteropodidae, 388. Rasores, 350. Pteropus, 388. Ratitae, 149, Rattlesnake, 325. Rats, 390. Rectal gland, 36. Rectrices, 330. Rectum, 38. Rectus muscle of eye, 84, 114. Rectus muscles of trunk, "3- Reduction division, 208. Regalecidae, 264. Regalecus, 264. Reindeer, 399. Remiges,330. Remora, 264. Renal artery, 191. Renal portal system, 196. Renal vein, 196. Reproductive organs, 124. Rennet, 35. Respiratory tract, 76. Rete, 130. Reticulate tarsus, 331. Reticulum, 35. Retractor bulbi, 84. Rhachianectes, 410. Rhachitomi, 283. Rhachitomous, 136. Rhabdopleura, 3. Rhamphastos, 348. Rhamphostoma, 328. Rhea, 346, Rheidse, 345. Rhinencephalon, 52. Rhinoceridae, 395. Rhinoceros, 395. Rhinolophidas, 387. Rhinolophus, 387. Rhinophis, 326. Rhinotrema, 288. Rhipidistia, 249. Rhombodipterini, 249. Rhombus, 263, 265. Rhynchocephalia, 313. Rhynchodus, 242. Rhynchosuchus, 328. Rhynchotus, 346. Rhytina, 405. Rhytinidae, 405. Rhytiodus, 405. Rib, 143. Ribbon fish, 264. Ribodon, 405. Right whale, 410. INDEX. 435 Rockfish, 259. Scaphiopus, 286. Rod cells, 79. Scaphirhynchus, 251. Rodentia, 388. Scaphoid, 177. Roller, 348. Scapula, 168. Rorqual, 410. Scarus, 260. Rostral bone, 299, 316. Sceleporus, 320. Rostrum (birds), 334. Scelidosaurus, 316. Rudder fish, 263. Scelotes,32o. Rumen, 35. Schizocoele, 7. Ruminantia, 396. Schizognathi, 349. Rumination, 396. Schizognathous, 335. Rupicapra, 399. Schwann, sheath of, 11. Sciaenidae, 259. Sable, 412. Sciatic artery, 191. Saccomys, 390. Scincidae, 320. Sacculus, 70. Scincus, 320. Sacculus endolymphaticus, Sciuridae, 390. 70. Sciuromorpha, 389. Sacculus vasculosus, 53. Sciuropterus, 390. Sacral plexus, 48. Sciurus, 390. Sacrum, 141. Sclerodermi, 266. Sagenodus, 273. Sclerotic, 83, 153. Saiga, 399. Sclerotomes, 102. Sail fish, 263. Scolopax, 349. Salamanders, 285. Scomber, 263. Salamandra, 286. Scomberesox, 257. Salamandrina, 285. Scombridae, 263. Salientia, 286. Scombroidae, 263. Salivary glands, 21. Scopelus, 256. Salmo, 256. Scops, 348. Salmon, 256. Scorpaena, 259. Salmonidae, 256. Scorpaenidae, 259. Salmopercae, 259. Scotasops, 383. Samotherium, 399. Screamer, 348. Sand-grouse, 350. Scrotum, 371. Sand launce, 263. Sculpin, 259. Sapajou, 416. Scutellate tarsus, 331. Sarcolemma, 12. Scutes, 322. Sarcorhamphus, 348. Sea bats, 266. Sargus, 259. Sea cow, 405. Saurii, 378. Sea horse, 258. Saurocephalus, 256. Sea lion, 413. Saurognathous, 335. Sea otter, 412. Sauropoda, 315. Sea robin, 260. Sauropsida, 291. Sea snake, 325. Sauropterygii, 306. Seals, 413. Saurorhamphus, 258. Sebastes, 259. Saururae, 343. Secodont, 365. Saw fish, 239. Secondaries, 330. Savi's vesicles, 68. Sectorial teeth, 411. Scalse, 73. Segmentation cavity, 5, 210. Scales, 92, 99, 228. Segmentation of egg, 5, 209. Scalops, 385. Segmentation of head, 201. Scaly ant-eater, 382. Segmentation nucleus, 209. Selachii, 238. Selachostomi, 251. Selene, 263. Selenodont, 366. Semicircular canals, 70. Semnopithecus, 416. Sense capsules, 66. Sense cells, 66. Sense corpuscles, 68. Sense organs, 66. Sensory nerves, 46. Seps, 320. Septum pellucidum, 51. Septum transversum, 106. Seriola, 263. Serpentes, 322. Serranidae, 259. Sesamoid bones, 176. Seven-sleeper, 390. Sewellel, 390. Shad, 256. Sharks, 232, 238. Sheath of Schwann, n. Sheep, 399. Shoulder girdle, 168. Shovel nose, 251. Shrews, 385. Siluridae, 255. Silversides, 262. Simaedosaurus, 314. Simia, 416. Simiae, 415. Simiidae, 416. Sinus of Morgagni, 29. . Sinus urogenitalis, 130. Sinus venosus, 181. Siphoneum, 340. Siredon, 285. Siren, 285. Sirenia, 403. Sirenidae, 284. Sirenoidea, 272. Sivatherium, 399. Skates, 232. Skink, 320. Skua, 349. Skull, 150. Skunk, 412. Sloths, 383. Small omentum, 105. Smell, organ of, 75- Smelt, 256. Smooth muscle, 12. Snakes, 322. 436 INDEX. Snapping turtle, 311. Snipe, 349. Sole, 265. Solea, 265. Soleidae, 265. Solenodontidae, 385. Solenoglypha, 325. Solenorhynchus, 258. Solenostoma, 258. Solenostomidae, 258. Solidungula, 393. Somatic layer, 8. Somatic nerves, 64. Somites, 100. Sorex, 385. Soricidae, 385 Spade foot toad, 287. Spalax, 390. Sparidae, 259. Sparrow, 351. Spelerpes, 285. Spermatic artery, 191. Spermatophores, 209. Spermatozoon, 5, 207. Spermophilus, 390. Sperm whale, 409. Sphargis, 310. Sphenethmoid, 278. Spheniscus, 347. Sphenodon, 314. Sphenodontina, 314. Sphenoid, 356. Sphenoidal fissure, 164. Sphenopalatine ganglion, 62. Sphenotic, 244. Sphrynidae, 239. Sphyraena, 262. Sphyraenidae, 262. Spinal accessory nerve, 59. Spinal cord, 44. Spinal nerves, 46. Spinous process, 138. Spiny ant-eater, 377. Spiracle, 23, 230. Spiral valve, 36. Splanchnic layer, 8. Splanchnocoele, 101, 103. Spleen, 200. Splenial bone, 164. Splenic artery, 190. Splint bone, 394. Spoonbill, 348. Spreading viper, 325. Springbok, 399. Squalidae, 239. Sub-mucosa, 39. Squalodontidae, 408. Sub-operculum, 161. Squaloraia, 242. Subungulata, 392. Squalus, 239. Suckfish, 264. Squamata, 317, 382. Suckers, 255. Squamipinnes, 262. Suidae, 397. Squamosal, 161. Suina, 396. Squatinidae, 239. Sula, 347. Squirrel, 390. Sunfish, 259, 267. Stagodon, 403. Sun grebe, 349. Stapes, 74, 159. Supra-angulare, 164. Star gazer, 261. Supraclavicle, 169. Steatornis, 348. Suprascapula, 168. Steatornithidae, 348. Supraoccipital, 158. Steganopodes, 347. Supraorbital, 161. Stegocephali, 283. Suprarenal body, 131. Stegodon, 401. Supratemporal fossa, 166. Stegosauria, 316. Surf perch, 260. Stegosaurus, 316. Surinam toad, 286. Stenofiber, 390. Surmullet, 261. Stenostoma, 326. Suspensorium, 155. Stenson's duct, 77. Sus, 397. Stercorarius, 349. Swan, 348. Stereospondyli, 284. Sweat glands, 90. Sterna, 349. Swell fish, 267. Sternebrae, 148. Swift, 320. Sternothasrus, 311. Swimbladder, 25. Sternum, 147. Swine, 397. Stickleback, 258. Swordfish, 263. Sting ray, 239. Sympathetic system, 47. Stomach, 34. Syncitium, 12. Stomodeum, 18. Synentognathi, 257. Stork, 348. Synetheres, 391 . Strand rat, 390. Syngnathus, 258. Stratified epithelium, 9. Syngnathidae, 258. Stratiodontidae, 257. Synotic tectum, 151. Stratum corneura, 88. Synsacrum, 331. Stratum lucidum, 89. Syphostoma, 258. Streptostylica, 300. Syrinx, 29. Striped muscle, 12. Strix, 348. Tactile cells, 68. Stromateidae, 263. Taeniodonta, 402. Struthio, 345. Taeniosomi, 264. Struthiones, 345. Tail coverts, 330. Struthionidae,345. Talon, 365. Sturgeon, 250. Talpa, 385. Style, 365. Talpidae, 385. Stylephorus, 264. Tamias, 390. Stylinodon, 403. Tapetum, 863. Stylohyoid, 165. Tapiridae, 395. Subclavian artery, 188. Tapir us, 395. Subclavian vein, 194, 197. Tardigrada, 383. Subintestinal vein, 193. Tarsiidae, 415. Sublingua, 21, 364. Tarsipes, 380. INDEX. 437 Tarsius, 415. Tarso-metatarsus, 336. Tarsus, 176. Taste, 68. Tatusia, 383. Tautoga, 260. Taxeopoda, 393. Taxidea, 412. Teeth, 19. Tegmen cranii, 152. Tegumentary skeleton, 91. TeidcE, 321. Tejus, 321. Telencephalon, 49. Teleostei, 252. Teleostomi, 242. Telerpeton, 314. Telolecithal eggs, 206. Telosaurus, 328. Temnocyon, 412. Temnospondyli, 283. Temporal bone, 161. Temporal fossa, 167. Tendons, 112. Tenrec, 385. Tern, 349. Terrapin, 311. Tertiaries, 330. Testes, 126. Testes of brain, 53. Testudinata, 307. Testudinidae, 311. Testudo, 311. Tetrao, 350. Tetraprotodon, 398. Tetrodon, 267. Teuthidae, 262. Teuthis, 262. Thalamencephalon, 49. Thalami, 49, 52. Thalassochelys, 310. Thalassophryne, 266. Thaumalea, 350. Thecadont teeth, 294, 364. Theriodontia, 306. Theromorpha, 304. Theropoda, 316. Thomomys, 390. Thoracic duct, 199. Thoracic fins, 231. Thoracic region, 142. Thoracosaurus, 328. Thread cells, 220. Thresher sharks, 239. Thylacinus, 379. Thylacoleo, 378. Thylacoleonidas, 380. Thymus glands, 33. Thynnus, 263. Thyrohyoid, 165. Thyroid cartilage, 28. Thyroid gland, 32. Thyroptera, 387.' Tibia, 176. Tibial artery, 191. Tibiale, 176. Tibio-tarsus, 336. Ticholeptus, 398. Tiger, 412. Tiger sharks, 239. Tile fish, 261. Tillodontia, 402. Tillotherium, 403. Tinamus, 346. Tissues, 9. Titanichthys, 272. Titanotheriidae, 396. Titanotherium, 396. Toad fish, 266. Toad, horned, 320. Toads, 286. Todus, 348. Tody, 348. Tolypeutes, 383. Tomcod, 264. Tomitherium, 415. Tongue, 21. Tonsils, 200. Toothed birds, 344. Torpedinidas, 239. Torpedo, 239. Tortoise, 310. Tortoise shell, 308. Tortoise shell turtle, 310. Tortricina, 326. Tortrix, 326. Toucan, 348. Toxodon, 262, 402. Toxodontia, 402. Trabeculae cranii, 151. Trachea, 27. Trachinidae, 261. Trachinoidae, 261. Trachinus, 261. Trachynotus, 263. Trachypteridas, 264. Trachypterus, 264. Tragelaphus, 399. Tragulidae, 398. Tragulina, 396. Tragulus, 398. Transverse process, 141. Trapezium, 177. Trapezoid, 177. Tree kangaroo , 380. Tree toads, 287. Trematosaurus, 284. Triceratops, 317. Trichechidae, 413. Trichechus, 413. Trichecus, 405. Trichiuridae, 263. Trichiurus, 263. Trichoglossus, 349. Triconodont, 365. Triconodonta, 380. Trifacial nerve, 59. Trigeminal nerve, 61. Trigger fish, 266. Trigla, 260. Triglidae, 260. Trigon, 365. Trimerorhachis, 283. Triton, 285. Tritors, 241. Tritubercular, 365. Trituberculata, 380. Trochanter, 360. Trochilidae, 350. Trochlearis nerve, 61. Troglodytes, 416. Trogonidae, 348. Tropic birds, 347. Tropidonotus, 325. Tropidosaurus, 320. Trout, 256. Truncus arteriosus^ 181. Trunk fish, 267. Trygonidas, 239. Tryonychia, 310. Tuber cinereum, 53. Tubinares, 349. Tubular glands, 90. Tunicata, I. Tunny, 263. Tupaia, 385. Tupaiidae, 385. Turbinal bones, 75, 338. Turbot, 265. Turkey, 350. Tursiops, 408. Turtles, 310. 438 . INDEX. Tutidanus, 283. Twixt brain, 49. Tylopoda, 396, 398. Tylosurus, 257. Tympanic bone, 357. Tympanum, 73. Typhline, 320. Typhlogobius, 263. Typhlonectes, 288. Typhlops, 326. Typotherium, 402. Typothrax, 328. Tyrannus, 351. Tyrant bird, 351. Uintatherium, 400. Ulna, 176. Ulnar artery, 189. Ulnare, 176. Umbilicus, 288. Umbra, 257. Umbridae, 257. Unciforme, 177. Uncinate process, 146. Ungulata, 391. Ungulata Vera, 392. Unguligrade, 361. Unicorn fish, 266. Unipolar nerve cells, '10. Upeneus, 261. Upupa, 348. Ur, 400. Uranidea, 259. Uranoscopus, 261. Ureter, 122. Urethra, 124. Uria, 349. Urinator, 349. Urochordia, i. Urodela, 284. Urogenital ducts, 126. Urogenital organs, 116. Urogenital sinus, 130. Urohyal, 335. Uropeltes, 326. Urostyle, 142. Urotrichus, 385. Ursidae, 412. Ursus, 412. Uterine placenta, 374. Uterus, 127. Uterus masculinus, 128. Utriculus, 70. Vagina, 127. Viveridae, 412. Vagus nerve, 63. Vizcacha, 391. Valve, ileocolic, 36. Vocal cords, 29. Valve, spiral, 36. Vole, 390. Valve of Vieussens, 54. Voluntary muscle, 12. Valvulae conniventes, 38. Vomer, 163. Vampyre bat, 388. Vomer (fish), 263. Vampyrus, 388. Vulpes, 412. Varanidae, 320. Vultures, 348. Varanus, 320. Vas aberrans, 130. Vas deferens, 130. Wagner's corpuscles, 69. Vas efferens, 130. Wallaby, 380. Vater's corpuscles, 69. Walrus, 413. Veins, 178, 192. Wart-hog, 397. Velum, 220. Weak fish, 259. Vena cava, 195. Weasel, 412. Venous blood, 184. Weberian apparatus, 26, Ventral aorta, 181. 255- Ventral fins, 231. Weevers, 261. Ventral limb, 167. Whales, 405. Ventral nerve roots, 46. Whale-bone whales, 409. Ventricles of brain, 49. White of egg, 207. Ventricle, fifth, 57. White matter, n. Ventricle of heart, 181. White fish, 256. Ventricle of larynx, 29. White tissue, 13. Vermiform appendix, 39. White whale, 408. Vermis, 54. Window pane, 265. Vermilingua, 383. Wing coverts, 330. Vertebrae, 135. Wirsung's duct, 40. Vertebral artery, 188. Wish-bone, 336. Vertebral bow, 137. Wolf, 412. Vertebral column, 134. Wolffian body, 116. Vertebrarterial canal, 145. Wolffian duct, 119, 126. Vertebrata, 218. Wolf fish, 261. Vertebrates, origin of, 215. Wolverine, 412. Vesicles of Savi, 68. Woodchuck, 390. Vesperugo, 387. Woodpecker, 351. Vespertilio, 387. Wrasse, 260. Vespertilionidae, 387. Wryneck, 351. Vibrissae, 69, 98. Vicuna, 398. Villi, 38. Xenarchi, 259. Viper, 325. Xenarthra, 382. Vipera, 325. Xenopterygii, 260. Viperidae, 325. Xenurus, 383. Visceral clefts, 22. Xenopus, 286. Visceral nerves, 64. Xerobates, 311. Visceral skeleton, 150, 154. Xiphactinus, 256. Visual organs, 78. Xiphias, 263. Vitelline membrane, 207. Xiphiidae, 263. Vitelline veins, 192. Xiphisternum, 148. Vitreous humor, 79. Xiphodon, 398. Vivera, 412. Xiphiplastron, 308. INDEX. 439 Yak, 400. Yellow tissue, 14. Yolk, 206. Yolk stalk, 236. Zamicrus, 383. Zapus, 390. Zebra, 395. Zeidae, 261. Zeuglodon, 408. Ziphius, 409. Zoarces, 261. Zoarcidae, 261. Zona pellucida, 207. Zona radiata, 207. Zonary placenta, 373. Zonula Zinnii, 83. Zonuridae, 320. Zygaenidae, 239. Zygantrum, 141. Zygapophysis, 140. Zygomatic process, 357. Zygosphene, 140. 14 DAY USE RETURN TO DESK FROM WHICH BORROWED BIOLOGY LIBRARY TEL. NO. 642-2532 This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. 1 VA-7 SEP 1 8 13/U 1* JAN 27 1971 /f ^ ? f <>/' L ^^i?^ 68 UniS^Sfornia - h ' Hl ^A 1940 c^ AUG 28 1948 L D2i-.W y^ / 24 JO 3 THE UNIVERSITY OF CALIFORNIA LIBRARY