Vertebrate zoölogy

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ALBERT R. MANN LIBRARY

Preface

This volume is intended for use as a text-book in college courses in vertebrate zodlogy such as are required of premedical students and others who have had a course in general zodlogy. The aim of the book is to present those aspects of the subject which are not adequately brought out by laboratory work in comparative anatomy. It is taken for granted that the student who uses this as a text in connection with the lecture and recitational part of a course, shall also pursue a laboratory course in comparative anatomy, using a laboratory manual. It is also believed advisable to use as laboratory references the various text-books on comparative anatomy.

The book is avowedly dynamic in tone, emphasizing the physiological, developmental, phylogenetic, and ecological aspects of vertebrates. Structural features must of course be dealt with extensively, but purely anatomical details are as a rule subordinated to physiologic and evolutionary considerations.

The vertebrates are, moreover, viewed not merely as a group of animals belonging to the present, but, historically, as a very ancient assemblage of related forms, that arose from simple beginnings many millions of years ago and have passed through many vicissitudes involved in the mighty world changes of ancient times. Hence more than the usual attention is given to earlier chapters in the ancestral history of the vertebrate classes, chapters that are often of more dramatic interest than those of the present and that give to the student a new conception of the significance of modern end-products of evolution which, in themselves, are often relatively unattractive and devoid of interest.

The writer has for some years been much impressed with the far-reaching applicability to problems of animal morphology, of the aztal gradient conception of his colleague, Professor Child, and one of the features of the present book is the attempt to interpret vertebrate structures in terms of this conception. In some cases it is probable that the theory has been stretched beyond the limits its author would consider justified; hence the present writer takes en-

vii

Vili PREFACE tire responsibility for the applications of Child’s theory brought out in this volume.

The axial gradient conceptions have appeared to the writer to be strictly in accord with the principles of racial senescence as presented by H. F. Osborn and R. S. Lull in their recent volumes on evolution. The present volume has much to say about adaptive radiation and the various degenerative and senescent conditions so common among vertebrates. The writer has freely made use of the material found in Osborn’s and in Lull’s books. Many figures have been borrowed from both authors, for which the writer is deeply indebted.

Much of the data used has been taken from the several volumes on vertebrates of the Cambridge Natural History, and from the various comparative anatomies such as those of Wiedersheim, of Kingsley, and of Wilder. Kellicott’s ‘‘ Chordate Embryology ”’ has been found excellent for much of the embryological data. A list of about a hundred references to which the writer has had access is presented in an appendix.

The illustrations have been borrowed to a considerable extent from Macmillan Company publications, but the great majority of the figures have been redrawn by Mr. Kenji Toda to whom I herewith express my hearty thanks. Many of the figures are made up of several related illustrations arranged in such fashion that they readily may be put into chart form. In our own laboratory we are already using a considerable proportion of these compound figures as charts. The sources of these redrawn figures are various and the author wishes to acknowledge with thanks the courtesy of those who have permitted their originals to be thus modified and used. The greatest care has been taken properly to acknowledge the source of each borrowed illustration. If in any case the figure has been incorrectly attributed to an author, information regarding the error will be gratefully received.

The author is not unaware of the shortcomings of the present book. Some critics will doubtless feel that much valuable, and to their minds necessary, data has been omitted. Others will probably believe that much that has been included, especially in connection with the notes on the natural history of certain types, might better have been omitted. But the selection of what to present and what to omit has been carefully canvassed and what appears to be a workable compromise has been decided upon. The teacher may readily omit what he feels to be

PREFACE ix superfluous, and will be glad to supply the data that he feels should be included.

Doubtless, errors of various kinds have crept into the text in spite of our scrupulous efforts to avoid them. Although the writer has been indebted to several competent readers of his manuscript, he holds himself entirely responsible for the book with all of its defects and whatever virtues it may have. He will be grateful for criticism and correction on the part of his readers.

H. H. Newman.

Chicago, Il.

May 1, 1919.

Table Of Contents

Cuapter I. PrIincipues oF VERTEBRATE MorPHOLOGY

PAGES The fundamental architectural plan of vertebrates............0...0005 1-8 A physiological interpretation of the fundamental structural plan of vertebratesicvis teaser sa Beebe See edd eae ead yeaa 8-12 Generalized, specialized, senescent, and retarded (pedogenetic) typés of vertebrates: 2. ..cauu siege nae ecde ae Laglemeg. emotes 12-22 Vertebrate: phylogeny: .a ec. aev jaye idiaw sd ties INA Aes WaT LN 22-33 Cuapter II. Tar Poytum Cuorpata Géplalochorda tas. 2 8 idaieu Sakae Bed aura ewe head alvin Sau made 33-48 Urochordatay:osccs- dacvgeees aes eters dees bneees wel Ge eas Sees 48-60 Hemichord tars: conch ice dae te aes pean Sy be ba habe See 60-68 Craniata (Vertebrates). 20 ).ic ee 4 Lads Ree ued mgs aoa eres 68-70 Cuapter III. Tue OrIciIn anD EVOLUTION OF THE VERTEBRATES The: Am phioxus Theory .i02daceecocoaie chee ae ea eeu eee ke 72-75 The, Anriélid Theory: .c.s.cncet aia eee ten Peace es 77-79 The Arthropod. Theory «iis ciaedas eeiosaeeww es eevee eaes Wes ee es 79-82 BUY BT e(0) id Wel st 6) 4: ge eee 82-86 Cuapter IV. Cyciosromata Myxindideas ya ee rises Son ee ee eee ere ee beet enieinenters 89-91 PetromyzOnbiaes «<a ove-Fda dawns Cede aceieaie bola ewitie gala antlesd ain asanine.des’9 91-96 Cuapter V. Pisces (FISHES) Introduction:to Pisces s2.0casceani eee eee ea eta tee Sees 97-112 Hl asinObraviGhutt,. «Aa g-ciedaecvacargtiete o aiee aust lle lsecd ech « mastcnieeene os Ree araeds cs 112-127 "PElEOStOMis- <4 hannah oes en wk RA EO i ie ee we Ee eeNS 127-153 Dipneusth.., ocsekere ie ecacer diet eeeee yee ea eee 153-159 * Generalized and specialized types of fishes and the axial gradient theory of structural relations... ........ 06.6 cece eee eens 159-163 Eggs, reproduction, and breeding habits of fishes ................ 163-168 Appendix to fishes (Ostrachodermi, etc.)....6. 60... beer eens 168-172 x1 xii TABLE OF CONTENTS

CuapteR VI. AMPHIBIA PAGES Tntroduetory: ssc. sciesien seis vA RR Rea hike ONO BER ER ORS 173-179 Extinct-Am phiblovis<sa0-cakewes ga esaeseeees bodies seers wees 180-183 Present-day Amphibia. ....... 0... cece see e cece eee reece renee 183-203 The development of the frog... 0... cece cece ee eee ee eee 203-209

Chapter I Principles Of Vertebrate Morphology

Definition.—The vertebrates may be defined as animals having :— pronounced antero-posterior, dorso-ventral, and bilateral axes; internal metameric segmentation, especially of the mesoblast; a central nervous system dorsal in position and tubular in structure, with a well-defined central canal or neurocoel; a well-defined head, characterized by highly specialized sense organs and by a concentration of nervous tissue into a complex brain; the alimentary tract opening by anterior mouth and posterior anus, and provided with paired pharyngeal clefts; a notochord derived from the primitive endoderm and situated between the central nervous system and the alimentary tract; an open ceelom, at first segmented, but later the segmental ccelomic cavities unite to form the large pericardial, peritoneal, and, in the mammals, thoracic cavities; a closed circulatory system quite distinct from the celom; a post-anal prolongation of the body into a metamerically segmented tail, without coelomic cavity; usually paired appendages, pectoral and pelvic. These characters and a few others serve to mark off the vertebrates quite sharply from all other groups. Several of the most fundamental of these characteristics must now be discussed. The diagrams on the following page (Fig. 1) illustrate most of these characters.

THE FUNDAMENTAL ARCHITECTURAL PLAN OF VERTEBRATES

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The Axis of Polarity (Primary Axis).—A typical vertebrate has an elongated form, with head and tail ends clearly defined. An imaginary line drawn from the extreme anterior to the extreme posterior end indicates the primary structural and functional axis of the body, which is designated the antero-posterior or apico-basal axis. The or-

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20h. ods* £04q2 yes it 73 gans of highest dynamic activity and most pronounced sensitivity are at the apical or anterior end and the organs of lowest dynamic activity and least sensitivity are at the basal or posterior end of this axis. Between these extremes, or opposite poles, of the axis the remaining organs or functions are arranged, at least primitively, in a graded series of diminishing dynamic activity and sensitivity. These geometrical relations serve as an index of an inherent spatial orderliness in the arrangement of the functions with reference to one another, and demonstrate that the organism is based on a single plan—is a coherent entity.

From a purely physiological point of view this gradient represents a linear series of functions, ranging from dominant or controlling functions to subordinate or controlled functions, a series which, broadly speaking, runs somewhat as follows:—olfactory and visual, the most anterior and dominant functions, entirely sensory in character; motor functions associated with movement of the eyes, and motor centers for most voluntary functions; sensory and motor activities associated with feeding, including the sense of taste, and the motor activities of jaws and tongue; sensory and motor activities associated with hearing and equilibrium; the active functions of respiration and circulation, which are closely correlated; the most anterior locomotor functions, associated with the pectoral appendages; the most active phases of the alimentary or digestive functions, associated with the stomach and the larger glands; the excretory and lower alimentary functions, associated with the kidneys, the lower intestine and rectum; the reproductive functions, associated with ovaries and testes, their accessory ducts and copulatory organs; the functions of the tail or post-anal body, which may be considered as a developmental afterthought and as more or less beyond the limits of the original primary axis. The tail has a gradient of its own and does not belong to the primary gra~- dient. This is only a rough outline of the real physiological gradient, but is clear enough for our purposes. The true gradient no longer exists in modern vertebrates because there has been a great deal of secondary concentration at various levels, especially at the anterior end, where the original metameric arrangement of the functional series has been profoundly disturbed and distorted. The primary gradient is further obscured by the fact that various systems of organs, such as the heart and blood vessels, the brain, the alimentary tract, etc., have developed secondary axes of functional activity and structural differentiation that are largely independent of the primary axis and have little reference to the polarity of the body as a whole. In the embryo, however, the axis is less distorted than in the adult.

The Dorso-Ventral Axis (Secondary Axis).—If we take a cross section at any level of the primary axis, we at once perceive that a

Fic. 2.—Diagrammatic transverse section of a vertebrate to illustrate the dorso-ventral (secondary) and bilateral (tertiary) axes of organization. al, alimentary tract; ao, aorta; d.m, dorsal mesentery; my, myotome; nc, notochord; n, nephrotome; 0, omentum; sc, spinal chord; vm, ventral mesentery. (Modified after Kingsley.)

line drawn from the mid-dorsal to the mid-ventral line represents another main architectural axis. In the diagram (Fig. 2) it will be noted that the central nervous system occupies the high point or apical end of the axis and that, at this level, one loop of the intestine occupies the ventral or basal point of the axis. The gradient of functions in between the two poles has been so decidedly disturbed by lateral foldings and secondary displacements, that any attempt to construct a list of graded functions would be futile. We know in general, however, that the dorsal side is the dominant part of the axis and that, during embryonic development, the various functions differentiate almost exactly in the order of their relative dynamic activity. The Bilateral Axis (Tertiary Axis).—The dorso-ventral axis divides any level of the antero-posterior axis into two mirror-image halves (Fig. 2), so that each side is the reversed counterpart of the other. It would appear then that the bilateral axis is a necessary consequence of the dorso-ventral axis. The axis has a single median or apical point and two lateral or basal points. Any vertical level in the dorso-ventral axis may constitute the apical point of a double bilateral axis. A good example of this type of axial organization is seen in connection with the differentiation of the segmented mesoblast of the chick (Fig. 3). The median dorsal part of the somite forms the myotome (segmental voluntary muscles), the most highly dynamic of all mesodermal structures; next comes the dermatome which forms the deeper skin and many complex sensory and glandular structures, etc.; next comes the nephrotome or primordium of the excretory system; next, es spl Pes Gaal: Coe/.

Fic. 3.—Transverse section across the primary axis of a 10 somite chick embryo, to illustrate the bilateral (tertiary) axis of vertebrate organization. Ao, aorta; Coel, ccelom; ect, ectoderm; ent, entoderm; nch, notochord; N. F., neural furrow; N. ph, nephrotome; s, somite; som. Mes, somato-pleure; spl. Mes, splanchopleure. (From Lillie’s ‘‘ Development of the Chick ” [Henry Holt and Co].)

but only at posterior levels, the gonatome or primordium of the reproductive system; next, the embryonic ccelom, from which are derived mainly such passive structures as the peritoneal lining of the body cavities and the mesenteries; and finally, the extra-embryonic body cavity, which has to do primarily with the formation of embryonic membranes that serve as protective and respiratory organs during the life of the embryo, but play no réle in the organization of the adult.

It should be understood that the appendages are, as the name indicates, truly added structures and are not to be considered as part of the original bilateral gradient. Each appendage has its own three gradients, with the free end representing the apical end and the fixed end, the basal. It need hardly be said that the main specializations of the appendages take place at the apical or free end, while the basal parts remain comparatively conservative and undifferentiated.

METAMERISM

The segmental organization, metameric structure, is secondary in importance only to the primary axiate organization and is undoubtedly one of the developmental consequences of the latter. The vertebrates constitute but one of three great metameric groups, the other two being the annelids and the arthropods. Annelids, the lowest of metameric types, show metamerism in its most generalized condition. In them segmentation involves equally both internal and external structures, and there is little or no obliteration of primary metamerism by fusion of groups of contiguous metameres into larger regions. In the arthropods the external metamerism to a large extent retains its primitive condition, especially in regions back of the head or cephalothorax, but the primitive internal metamerism is obscured through the almost complete obliteration of the coelom by the system of venous sinuses.

Metamerism in the vertebrates, especially in the higher groups, is confined largely to certain internal structures and is scarcely at all expressed in external characters. Even the internal metamerism is to a large extent obscured by the compacting of the anterior metameres into a head, and by the fusion of the original segmental body cavities into a few large compound ccelomic chambers. The segmentation is best expressed in the structures derived from the embryonic mesoblast: the myotomes, the nephric tubules, the vertebral units, and their skeletal accessories. The central nervous system also retains its primitive metameric segmentation in post-cephalic regions, but in the cephalic region the metameric elements are very difficult to make out. There are believed to be primitively five (possibly six) metameres, three or four in front of the otic vesicle and one behind it.

Back of the head, however, the vertebrate is composed of a linear series of body segments or metameres, each of which has, theoretically, its own complete set of body organs. There is, however, never an ideal or complete development of all the characteristic segmental organs in any one metamere. In the anterior metameres the organs of lowest dynamic activity, such as those of excretion and reproduction, do not appear; and in the posterior metameres the organs of high-

-est dynamic activity, such as the organs of special sense and the higher codrdinative faculties, do not appear. And there is a graded series between these extremes.

As a rule, a series of two to ten or more metameres unites into a functional region and a concentration of these metameres occurs that makes it difficult to discover the original segmental conditions involved. Even the most primitive vertebrates exhibit wide departures from the primitive metameric plan seen in some of the lower annelids, and these modifications of the generalized segmental arrangement become progressively more pronounced from the lowest to the highest vertebrate classes.

CEPHALIZATION

One of the most significant advances over ancestral conditions that the vertebrates have to their credit has to do with the evolution of intelligence, and this is intimately bound up with the pronounced specialization of the anterior metameres into a head, which is principally a brain and a group of sense organs. This foreshortening, condensation, and specialization of the anterior metameres has been called cephalization. The course of evolution from the lowest to the highest vertebrates has been one of more and more pronounced cephalization involving a progressively larger number of metameres, an increased dominance of the head over the rest of the body, and closer integration of all the functions. The climax of the process of cephalization is Man, who is essentially a dominating intelligence and a set of accessory organs. Various attempts have been made to decipher the highly modified metamerism of the vertebrate head. One of these attempts gave rise to the classic ‘Vertebral Theory ” of the skull by Goethe and by Oken. It was thought that the skull was made up of a modified series of vertebral units and that an analysis of these elements would give the number of metameres. This theory was proven by Huxley to be untenable. The cranial nerves have been taken as evidences of metamerism, but the old arrangement of ten cranial nerves is no longer taken to indicate ten metameres, since some of these nerves are the motor and others the sensory components of single metameric divisions of the neuron. Perhaps the most reliable index of the number of metameres in the head is seen in the mesoblastic somites of some of the fishes and cyclostomes. There are but three somites in front of the auditory capsule, which forms a convenient landmark in that it seems to mark off the original head from the post-cephalic region. Possibly then the primitive craniate had a head of three metameres, a number characteristic of the larva of Balanoglossus, and possibly that of the echinoderms. The most characteristic stage in certain annelid larve has also three metameres. The higher vertebrates have in the process of their evolution gradually appropriated to the head, one after the other, the adjacent body metameres.

The Backward Retreat of the Lower Functions.—Accompanying the concentration of the chief nervous elements in the head there is a progressive series of changes in the opposite direction, consisting of a steady retreat toward the posterior end of the respiratory, digestive, excretory, and reproductive functions. These functions are, in the more primitive vertebrates, present in the metameres just back of the head, but in the higher forms they are steadily pushed back into the posterior metameres by becoming atrophied in the anterior metameres and by the development of new sets of more or less equivalent organs further back. An excellent example of this type of process is seen in the evolution of the kidneys.” In the most primitive vertebrates the embryonic kidney is located well toward the anterior part of the ccelom, and in the hag-fishes this anterior kidney, or pronephros, functions in the adult. In vertebrates of the fish and amphibian grades the kidney of the adult is a mesonephros, located farther back in the body cavity; while in the land vertebrates the functional kidney is a metanephros, situated still farther toward the posterior end.

The farther these organs of lower dynamic activity retreat from the dominant anterior end the more readily they appear to differentiate and the more specialized and condensed they become, just as though they were steadily growing out from under an influence that tends to suppress their full developmental possibilities. The evolution of the urogenital organs of the vertebrates affords an interesting example of this. In the lowest vertebrates the gonads and their ducts are simple, are located far forward in the body cavity, and are to a large extent separate from the urinary elements. In the highest vertebrates, however, these organs have retreated far to the posterior end of the primary axis and have beer intimately involved with the urinary system.

A PHYSIOLOGICAL INTERPRETATION OF THE FUNDAMENTAL STRUCTURAL PLAN OF VERTEBRATES

Professor C. M. Child has developed what appears to the writer to be the most scientific and far-reaching dynamic interpretation of animal structure that has yet appeared. His “axial gradient theory” is a statement in energistic terms of the axiate organization of animals and of the relations of dominance and subordination that are so obvious in such animals as the vertebrates. He has shown experimentally “that the apical or anterior region is primarily the region of greatest dynamic or metabolic activity in the individual. The apical end becomes the most highly specialized and differentiated region of the body, and in those forms which possess a central nervous system, the cephalic ganglion or brain and the chief sense organs usually arise in this region; . . . in other words it becomes the head, and in motile forms usually precedes in locomotion.” ‘The basal or posterior region, on the other hand, is primarily the least active region and in motile forms its activity is more or less under the control of the apical or anterior region.”

Tue AxiaAL GRADIENT IN DEVELOPMENT

There is a remarkable parallelism between the orderly spatial arrangement of structures and functions down the polar axis and the orderly sequence in time of these structures during the development of the individual from the egg. The first structures to differentiate are those of the head; in fact the early vertebrate embryo is nearly all head and the body grows out from it as though it were a mere axial outgrowth of the head. The last part of the body to complete its differentiation is the tail or basal part of the axis of polarity. There is thus an important relation between the degree of dominance and the developmental age of the various levels along the axis. The more anterior part always differentiates before the more posterior part and dominates it at least for a time. Secondary alterations in the relationships of dominance and subordination are of frequent occurrence and result from functional adjustments and through the action of external factors. The same parallelism between spatial arrangement and sequence in time exists for the dorso-ventral axis. In vertebrates, the first structures to differentiate are those in the medullary plate region; in fact, in the higher vertebrates, the early embryo consists almost entirely of the anterior median dorsal region, which is destined to form the central nervous system of the head. The last structures fully to differentiate are the reproductive structures which, functionally at least, belong to the basal part of the gradient. Structures also develop mesio-laterally, differentiating first in the middle and proceeding laterally, so that, in the case of such structures as the paired appendages, the last parts to develop are the terminal elements.

The Nervous System’as a Mechanism for Maintaining the Relations of Dominance and Subordination in the Organism.— The method of maintaining the dominance of the apical parts over basal parts is the method of conduction over nerve paths. There seems to be little doubt now that communications between an apical part and a basal part are electroid in character, so that changes in the chemical state of the apical part are transmitted speedily to the basal part and excite it to functional activity or inhibit its action, according to the character of the apical change. Thus a stimulated condition of a particular segment of the central nervous system involves a change of electric potential between it and the basal organ with which it is connected. The reaction of the basal part is in the nature of an adjustment to meet the change in the apical part so as to bring about a return to equilibrium between the two terminal points of the system. Thus, throughout the whole body there is a very intricate system of controlling and controlled parts, depending on the relative intensity or relative rate of metabolism of the various parts. It has been shown by the most exact types of apparatus, e. g., the Tashiro biometer, which measures exceedingly minute differences in carbon dioxide production in small objects such as nerves, that the apical part of a nerve has a higher rate of metabolism than the basal part.

Relative Susceptibility to Inhibiting Agents of the Apical and Basal Parts of the Axes. Child has demonstrated differences in rate of metabolism between apical and basal parts of an organism by the so-called “susceptibility method.” He finds that apical parts are more susceptible than basal parts to agents that retard metabolic activity (such as anesthetics, potassium cyanide, etc.). If lethal concentrations are used, the apical part dies first and there is a progression of death changes down the axes until the last parts to retain life are the basal parts. These experiments demonstrate that the axial gradient is essentially a gradient in the rate or intensity of metabolic activity. From this we may conclude that the relationship of functional dominance and subordination is one depending on the relative rates of metabolism, the part with the more rapid or intense metabolism stimulating through conduction regions of less rapid metabolism, and exciting them to perform their peculiar functions.

The Inhibiting Influence of Dominant Regions Upon the Development and Degree of Differentiation of Subordinate Regions. Among the invertebrates that have the same three fundamental axial gradients possessed by vertebrates, experiments have shown that during development a dominant region exercises an inhibiting influence over a subordinate region. This may be demonstrated by experiments in regeneration. If a planarian worm is cut transversely across the axis of polarity at almost any level the posterior piece will grow a new head. Evidently then, while the posterior part was organically connected with the anterior piece, head formation was inhibited. As soon as the organic connection with the original head or dominant part has been severed, the inhibition is removed and a head develops. In another flatworm, Microstomum, the dominance of the head becomes less with age so that at a point about halfway down the axis new brain and eyes form. This means that a new head is established, but there is not an immediate isolation of the new individual from the old. As long as the new individual remains a part of the old, it remains a subordinate individual, not only functionally in following the lead of the original head, but structurally in that the eyes do not fully differentiate and the brain remains small. Each of the two individuals divides again into two and in each case the posterior individual is subordinate to the anterior.

In general therefore it would appear that the presence of a dominant part inhibits the full differentiation of subordinate parts in proportion to their relative metabolic rates or intensities and the proximity of the two regions.

Let us now apply these principles to vertebrate development. In the first place it will be recalled that vertebrates are metameric animals in which each metamere, at least back of the primitive head, is serially homologous with all the rest. Each metamere has therefore potentially the developmental capacity of any other. Whatever systems or structures that develop in any one metamere should, ex hypothese, be latent in all of the others. Any failure, therefore, on the part of a given metamere to realize the full extent of its differentiational possibilities must be attributed to some sort of inhibition. Let us examine the main regions of the body of a higher vertebrate with this idea in mind.

The head metameres are characterized by a great specialization of brain tissue and an almost complete suppression of the lower functions.

In the neck region there is very little differentiation of subordinate elements belonging to the ventral ranges of the dorso-ventral axis; muscles, glands, and skeletal elements are present, but no excretory or reproductive organs differentiate in that region. In the thoracic region, where the dominance of the anterior end of the dorsal structures is less intense, the organs of respiration, circulation, and locomotion differentiate fully; but there are no digestive, excretory, or reproductive specializations. Still further back in the upper abdominal region the digestive functions reach their maximum importance and the uro-genital functions begin to appear. And, finally, in the lower abdominal region, the excretory and genital functions develop fully; but the genital tissues do not fully differentiate till comparatively late in the life cycle.

The tail of the vertebrate appears to be a developmental afterthought. It is the last part to develop and in many specialized vertebrates scarcely develops at all, as though the developmental momentum slowed down to such an extent that there was not enough force left to push out the tail. When the tail does form, however, it is usually, though not always, the most primitive part of the body. A tailless condition is very common in highly specialized races with prolonged developmental period.