^^•'^ TEXTBOOK OF GENERAL ZOOLOGY BY THE SAME AUTHORS PUBLISHED BY JOHN WILEY & SONS, Inc. LABORATORY DIRECTIONS IN GENERAL ZOOLOGY 194 pages. 5 1 by 9. 64 figures. Paper. TEXTBOOK OF GENERAL ZOOLOGY BY WINTERTON C. CURTIS Professor of Zoology, University of Missouri AND MARY J. GUTHRIE Associate Professor of Zoology, University of Missouri NEW YORK JOHN WILEY & SONS, Inc. London: CHAPMAN & HALL, Limited 1927 COPTRIGHT, 1927 BY WiNTERTON C. CdRTIS Printed in TJ. S. A. . PRESS OF 0/ 20 ^ BRAUNWORTH &. CO , INC. BOOK MANUFACTURERS BROOKLYN. NEW YORK DEDICATED TO ^tljan ^Uan ^nbretoa WHO, MORE THAN ANYONE ELSE, INFLUENCED THE EARLIER WORK OF THE SENIOR AUTHOR AS A TEACHER OF GENERAL ZOOLOGY, AND WHOSE INFLUENCE DESCENDED TO THE JUNIOR AUTHOR THROUGH GEORGE LEFEVRE AND D. H. TENNENT. PREFACE Although prefaces are commonly forgotten, if indeed they are read by teachers, it seems that one must say something regarding the origin and nature of a new textbook, particularly in a field that is already well occupied. The present volume is the outcome of a work projected some years ago by the senior author as a formal organization of the course in General Zoology that has been devel- oped at the University of Missouri during the past twenty-five years. Historically, it is the descendant of the course in General Biology that was introduced at the Johns Hopkins University by Huxley's student, Newell Martin, and later developed in that institution by E. A. Andrews. From Johns Hopkins and also from its original source in Huxley and later teachers in England, like T. J. and W. N. Parker, this early attempt to teach the prin- ciples of biological science has influenced instruction in many American institutions. At the University of Missouri the course began as General Biology, but was restricted to the field of Zoology with the estab- lishment of a separate department of Botany. The essential fea- ture of this instruction in General Zoology is that a limited number of animals are selected to illustrate certain biological principles and only incidentally as representatives of particular phyla. This is in contrast with what may be called the " phylum " scheme of instruction, which has been widely prevalent during the past twenty years and is represented by well-known textbooks. The later form of instruction seems to have originated in the old- time courses in Natural History, represented by books like Ten- ney's "Manual of Zoology," and to have been transformed into modern garb through the influence of Louis Agassiz and his students. In both the "principles" and the "phylum" courses, the method of instruction by "types" has been utihzed; but in one case the type illustrates principles, while in the other it shows the morphology and physiology typical of a given phylum. vii viii PREFACE Recent discussion of methods of teaching General Zoology has largely centered upon the relative merits of these two systems, and has been influenced by certain extreme departures in the attempt to teach principles. Another influence that is being felt is the " Project Method " which has been developed in many high school textbooks. While much is claimed for the Project Method, it seems to be the opinion of most college teachers that the product of the high schools in which this method flourishes is not such as to inspire full confidence in it, whatever may be the current educational theory of the learning process. It may be that the Project Method is the method of the future; but it has not yet arrived in the colleges, and the writers of the present volume have yet to be convinced that good teachers are not principally " born " and relatively Httle " made," when it comes to instruc- tion of a serious intellectual content. Good teachers arouse the interest of their students, and to be a good teacher one cannot be forever thinking how it is done, else " the letter killeth." When all is said, intellectual work is for the intellectually competent; and, whatever may be the present population of our colleges, one ques- tions whether the Prbject Method does not tickle the incompetent into temporary activity more often than it stimulates the compe- tent to the work necessary in sustained intellectual effort. This leads one to consider how a textbook of college Zoology should be written; whether it should give the student what he thinks he wants to know and can obtain in a way that takes little effort; or give him what he must know in order to understand some- thing of the subject. The authors inchne to the beUef that college and university instruction must have a certain regard for the existing organization of subject-matter, for example, for Zoology as con- ceived by zoologists. As to phraseology, they have attempted to write clearly, but not with undue simplification of vocabulary or expression. It is part of a college training to learn how to read and understand writing that is understandable by educated adults. The only way to learn this is to read such writing. It is better for a student to find places in a textbook a bit difficult than to find it all easy. Whether the authors have succeeded in their attempt to write on a level above primary English, without using a style that is hopelessly beyond those for whom it has been intended, others must say. They profess only the intention. To a certain extent they have been influenced by the idea that PREFACE ix is back of the Project Method. The senior author has always been conscious that such teaching of General Zoology as he has done effectively has been largely influenced by a sense of the " human- istic " aspects of the subject. The broader aspects of this " Humanism ■' of science have been discussed in a popular volume.^ Zoology is full of human interest — not merely bread-and-butter interest, but interest that may be dignified by the term "human- istic." This is better developed individually by the teacher than formally in a textbook, since its effect upon the student depends so much upon the conviction of the teacher. On the other hand, the approach to Zoology through a study of vertebrate structure and physiology, as in the present work, recognizes the desirabihty of introducing the subject by means of the facts most famiUar and interesting to the student. These are to be found in the body of a familiar animal and in the student's own body. To begin with the frog is to begin with man, since all vertebrates are so much akin in structure and function. The purpose of the first half dozen chap- ters is to review the knowledge of human anatomy and physiology that should be part of the training of every high-school graduate, although such is not the case. With this accomplished, and, one hopes, with interest aroused by the human problems involved, the facts and principles of animal Ufe are presented in the formal manner that is current in most textbooks. The " project " in the first part of the work is to teach the student something of the principles of Zoology as illustrated in his own type of animal body; and the " project " in subsequent chapters is to teach him how other animal bodies may be compared with his own and to impart some of the many interesting facts about animals. If many of these facts do not interest him, the authors believe he is hopeless. In the final chapters on Development, Genetics, and Evolution, there is a return to more human problems. Here again, the attempt is made to state the facts and principles as clearly and fully as space permits, in the conviction that the origin of the individual and of the race, and the mode of inheritance are of such compelling interest that the teacher's energy should be directed toward clear presentation of facts and problems, rather than toward overworked schemes for stimulating intellectual laggards. This smacks of a take-it-or-leave-it doctrine in teach- • Curtis, W. C, "Science and Human Affairs." X PREFACE ing; but we take it or leave it all our lives, and perhaps the prin- cipal trouble with college teaching is that we do not make our students feel that college work is a serious enterprise. As it stands, the present volume represents a temporary crys- talUzation of the course in General Zoology as developed in the University of Missouri, although it contains more than the authors are able to offer in a course extending through but one semester. In the Laboratory Directions,^ designed to accompany the present volume, it was possible to include work upon flatworms, molluscs, and echinoderms. These have been omitted from the text, since it is obviously impossible to deal so largely with principles and at the same time present types of all the phyla. A chapter upon the History of Zoology has been omitted in favor of the inclusion of historical references in connection with special topics, since it is the authors' experience that historical chapters are not very effective with students. In general, the aim has been to include the substantial body of well-estabUshed facts concerning the structure and functions of the animals described and to avoid undue inclusion of veiy recent details, however interesting. The authors hope that the book is not out of date in regard to recent biological investiga- tion, but they have not tried to make it so "up to the minute" that it would soon be found to contain premature conclusions from very recent work. Such details are always better left to the teacher as a means of vivifying his instruction. For example, it is well to explain in a text the saUent facts of "endocrine secre- tion," but not to include very recent extensions that have not been verified. It has been assumed throughout that the laboratory work of the course should be definitely related to lectures and text, and not given as though it were a separate course, as is done in some institutions. The authors' view of laboratory study has been discussed at some length in the Remarks to Instructors as printed in the Laboratory Directions. It is the behef of the authors that a textbook should contain a fundamental body of subject-matter that is correlated with the laboratory work and that may be extended by lectures at the discretion of the teacher. If laboratory work means anything, it should mean some measure of first-hand contact with the facts. On this foundation the text 2 Curtis, W. C, and Guthrie.. M. J., "Laboraton^ Directions in General Zoology." PREFACE X? becomes intelligible, and on the basis of laboratory study and text should rest the lectures and other oral discussion. The trouble with " principles " textbooks is that they have no founda- tion in accounts of representative types of animals; and the trouble with "phylum" textbooks is that they have no space for an outhne of principles upon which the teacher can build his own superstructure. The junior author is primarily responsible for the chapters on Metabohsm, Irritabihty, the Cells of Vertebrates, and Genetics, and has collaborated, by critical editing and by advising, through- out the preparation of the remaining chapters as originally pro- jected or written by the senior author. It is hoped that this united effort has resulted in a better textbook than could othei-wise have been produced. It is impossible, in a work of this nature, to acknowledge all the sources from which assistance has been received. There are the more remote influences, such as the authors have acknowledged in the dedication, and those of former colleagues, including G. S. Dodds and George Lefevre. The entire manuscript, as written before the final revisions, was read critically by Professor E. A. Andrews. Others who have read certain chapters are R. H. Wolcott, E. A. Martin, F. L. Hisaw, and J. A. Dawson. Thanks are due to the authors' colleagues in the department at the Uni- versity of Missouri, who have collaborated in other ways, and to George T. Khne and Helen Woelfel, biological artists. Other acknowledgments of figures and of permission to use figures appear in the legends. University of Missouri, W. C. CuRTIS Columbia, Missouri, M, J. GuTHfilE. March 1, 1927. CONTENTS chapter page 1. Introduction 1 2. Natural History of Vertebrate Animals 9 The Frog as a Representative Vertebrate 10 The Vertebrates and Their Environment 27 Classification 36 3. Morphology of the Vertebrate Body 38 External Features and Related Structures 38 General Internal Organization 41 The Structural and Functional Systems of Vertebrate Animals. ... 50 4. Physiology of the Vertebrate Animal: Metabolism 71 The Nature of Protoplasm 71 Assimilation 76 Dissimilation 93 Secretions 9" Blood as the Common Carrier 103 5. Physiology of the Vertebrate Animal: Irritability 106 Refle.x Action 107 Localization of Function in the Nervous System 115 Reception, Transmission, and Discharge 124 Coordination and Irritability 128 Reproduction ^^^ 6. Cells of the Vertebrate Body = ■ 130 Historical Development of the Cell Doctrine 131 Structure of a Typical Cell 1^3 Cell Division 1^^ Histology ^^^ 7. Representative Single-celled Animals 153 The Sarcodina 1^^ The Mastigophora 1^^ The Infusoria J^^ The Sporozoa J^JJ Metabolism, Irritability, and Reproduction in Protozoa 192 XllI 30629 xiv CONTENTS chapter page 8. General Problems Related to Single-celled Animals 195 Colonial Protozoa and the Comparison of Unicellular with Multi- cellular Organisms 195 Biogenesis vs. Abiogenesis 202 Protozoa and Disease 206 9. Reproduction 213 The Reproductive Cycle 213 Modes of Reproduction 219 Processes Related to Sexual Reproduction in Metazoa 221 10. Classification and General Organization of Animals 237 Classification 237 General Organization of the Animal Body 243 11. The Hydra, A Simple Many-celled Animal 247 The Hydra as a Simple Metazoan 247 Other Coelenterata 274 12. The Earthworm and Other Annulata 282 The Earthworm 283 Other Annulata 309 13. The Animal Nervous System 315 The Sensory-neuro-muscular System 316 Forms of Behavior in Animals 318 14. The Crayfish and the Arthropoda 321 The Crayfish 321 Other Crustacea 332 The Phylum Arthropoda 338 15. The Locust 342 The Locust, or Short-horned Grasshopper 343 16. Some Representative Insects 371 Classification and General Organization 371 Some Representative Insects ...._. 377 17. Development op the Frog and Other Vertebrata 397 Development of Amphioxus 398 Development of the Leopard Frog 400 Development of Other Vertebrates 420 CONTENTS XV chapter page 18. Some General Problems of Development 436 Prefonnation and Epigenesis 436 Heredity and Enviroment in Development 439 The Determination of Sex 446 Problems of Mammalian Development 449 19. Genetics 456 The Method of Biometry 457 The Method of Experimental Breeding 462 The Method of Cytolog>- 474 The Method of Experimental Embryology 483 Genetics in Relation to Evolution 485 Genetics in Relation to Human Affairs 487 20. The Theory op Evolution 489 The Origin of Life 489 Organic Evolution 494 The Evidence for Organic Evolution 497 Human Evolution 528 21. The Causes of Evolution 539 The Lamarckian Theory of the Inheritance of Acquired Charac- teristics 539 The Darwinian Theory of Natural Selection or the Survival of the Fittest 544 The Mutation Theory 557 Orthogenetic Theories 561 Isolation as a Cause of Evolution 563 Evolution by Hybridization 564 Index 567 TEXTBOOK OF ZOOLOGY CHAPTER 1 INTRODUCTION An introduction to the science of Zoology may be secured in a variety of ways. One may become interested in the life of field and stream as a hunter and fisherman, or as an amateur naturalist through the collection of specimens. The farmer's son who watches the insects that devour the crops and who protects insect-eating birds may become sometliing of a zoologist unawares. The boy or girl who studies Anatomy and Physiology in school becomes famil- iar with structure and function as found in the bodies of higher animals. A high-school course in Zoology or in Biology offers a more comprehensive introduction. Whatever may have been the student's previous experience with zoological science, it is desirable that he make the most of it throughout the study outlined in the present volume. The Biological Sciences. — The term science may be applied to any body of exact knowledge. The Natural Sciences are those dealing with the facts of nature, in contrast with the Social Sciences, which deal Avith the facts of human nature. According to such a classification, the Social Sciences are contrasted with the Natural Sciences, as though the two were distinct. But the human rela- tionships with which the Social Sciences deal are facts of nature no less than the phenomena of Chemistry or Physics. Hence it is better to say that the Social Sciences are conveniently set off, as deahng with human relationships, over agamst all other sciences, which are called the Natural Sciences. These Natural Sciences may be divided into two groups : the Physical Sciences, which are concerned with non-Hving bodies; and the Biological Sciences, which are concerned with organisms, or hving things. Thus Cheni- 2 INTRODUCTION istry, Physics, and Astronomy are Physical Sciences; while Zoology, Botany, and Physiology are Biological Sciences. In its narrower sense, the term Biology includes Zoology, Botany, and the closely related sciences that deal with particular phases of animal and plant life. In its broader sense, the term may include everything related to Uving organisms and their activities. The division of the sciences into a physical and a biological group is arbitrary. Even the Social Sciences may be grouped as " biological," if we choose to regard them as such, because they are concerned with the activities of living beings. The Hst of Biological Sciences is. there- fore, extensive; and what, if anything, is to be excluded depends upon the point of view of the person making the classification. Zoological Science. — Biology is, therefore, the science of Uving things, both plant and animal. Botany is the science of plants. Zoology is the science of animals. The term Animal Biology, which is sometimes used, means the study of animals as living forms illustrating the principles often common to both plant and animal life, rather than the principles related to animals alone. The term Plant Biology might be similarly appUed, but has not come into general usage. The accompanying table (p. 3) indi- cates the larger groupings of problems and subject matter within the field of zoological science. It should be studied in connection with the definitions that follow. If the reader understands the justification for the groupings thus exhibited, he has mastered the definitions and their application. Forfn and Function. — The science of Morphology, which deals with the structure of animal bodies, may be considered inde- pendently, although it is really inseparable from Physiology, which deals with their functions. Gross morphology, as studied in the dissection of the human body by medical students or in the dissection of the bodies of animals by students of Zoology, is com- monly called Anatomy. Embryology has Hkewise its morpho- logical and physiological aspects. The problem of form and function further includes microscopic organization and activities. Histology deals with the more general features of microscopic structure, the forms of the living units or cells, and their arrange- ment in tissues and in organs. Cytology deals with the internal structure of cells in their finer details. Like Anatomy and Embry- ology, Histology and Cytology began principally with descrip- tions of structure, but they have become increasingly physiological ZOOLOGICAL SCIENCES Problems and Inter-relationships of Zoological Sciences Problems Sciences Related Sciences Form and Function Morphology Physiology Anatomy Embryology Histology Cytology Pathology Psychology Origin of the Individual Embryology Anatomy Histology Cytology Physiology Organism and Environ- ment Ecologj^ Taxonomy Zoogeography Morphology Physiology Pathology Classification Taxonomy Morphology Physiology Embryology Zoogeography Paleontology Variation and Heredity Genetics Morphology Physiology Embryology Taxonomy Origin of the Race Organic Evolution Morphology Physiology Zoogeography Embryology Paleontology Ecology Taxonomy as the structural features of cells and tissues have been ascertained and it has seemed desirable to know more regarding functions. Pathology, or the science of abnormal structures and functions, and Psychology, or the science of the mind, are naturally included here. 4 . INTRODUCTION The science of Physiology was first concerned with the activities of adult bodies. These activities are dependent upon structure to the same extent that the activities of a machine, such as a steam- engine or an automobile, are related to the structural relationships of its parts. Physiology and Morphology must, therefore, be studied together as the problem of function and structure, if one desires an adequate knowledge of living bodies. The animal is "something happening" rather than an actionless piece of machinery. One may infer how its parts "work" by an exam- ination of their structure, just as the operation of an engine might be inferred from an examination of its mechanism; but complete knowledge can be obtained only when an understanding of the engine's " morphology " is correlated with observations and experiments upon its " physiology," or manner of action. Since Anatomy, Embryology, Histology, and Cytology have their mor- phological as well as their physiological aspects, the sciences of Morphology and Physiology are everywhere related as the struc- tural and functional aspects of Uving organisms. To understand fully the problem of form without considering that of function is a manifest impossibility. Origin of the Individual. ■ — Although it might be included under the head of Morphology and Physiology, the study of Embryology, or the process of development by which individual animals come into existence, must be ranked as one of the major problems of biological science. Embryology is a phase of Anatomy, since it deals with the origin of structure. In studying it from this aspect, one examines embryos in their successive stages and likewise their histological and cytological organization. However, the more important aspect of the origin of the individual is not the descrip- tion of successive stages, but rather an account of what " happens " in development. Now that the successive stages have become well known in so many animals. Embryology is becoming more of a physiological than a morphological science. There is the same relationship between structure and activities as in the problem of form and function. Psychology and Pathology might likewise be included here. The origin of the individual is, in fact, only the problem of form and function as it appears in other than adult phases. Organism and Environment. — The relation of the animal to its surroundings, or environment, presents many complex problems. ZOOLOGICAL SCIENCES 5 Broadly speaking, the environment consists of the entire universe outside the animal's body, and a great deal that may be internal, such as chemical substances circulating in the blood, or the body heat of a higher animal. Narrowly speaking, it comprises the more immediate surroundings; but factors remote in their origin, such as the sun's light and heat, and even the light of the moon, may be important. In recent years the study of organisms in relation to their environment has come to be designated Ecology. To study animals as thus related to their surroundings, it is neces- sary to classify them. Hence, Taxonomy, or the science of classi- fication, is related to Ecology, as the table shows. Zoogeography, or the distribution of animals over the surface of the earth, is likewise important; and one might include the distribution of organisms in geologic time as shown by the study of Paleontol- ogy. Since functions are everywhere important in relation to environment, Physiology is also concerned, and with it Morphology and Pathology, for the reasons indicated in a preceding paragraph. Classification. — The problem of arranging the various kinds of animals in some orderly manner must have presented itself ever to primitive peoples. The " beasts of the earth," the " fowls oi the air," the " fish of the sea," and " everything that creepeth," represents an early attempt of this nature. Advancing knowledge rendered a more satisfactory organization of the many kinds of Hving things imperative. Thus the science of Taxonomy, or classification, had its origin. In the past, classifications have been based upon various features of animals and plants. To-day the standard form is that based upon structure, because structural resemblance seems to be the most constant and significant feature in animal organization. Hence, Morphology and Embryology, and with them Physiology, are important to Taxonomy. Zoogeog- raphy, or the distribution of animals over the earth's surface, and Paleontology, or the science of fossils, are not branches of Taxonomy; but the classifier must consider the localities in which existing animals are found and must have some knowledge of the animals of the past. Beginning as a local effort to classify the plants and animals of a neighborhood, classification was progres- sively extended and developed into the science of Taxonomy. Variation and Heredity. — Variation may be defined, for pre- Uminary purposes, as the differences between the individuals of a species, and heredity as the resemblance between parents and 6 ' INTRODUCTION offspring; although neither of these definitions is quite accurate. From the tune when his attention was first directed to such matters, man must have noted these facts of difference and resem- blance and considered their significance as applied to his own offspring or to his domesticated animals. In recent years, how- ever, variation and heredity have come to be studied by them- selves in what is known as the science of Genetics, which may be said to have originated when the Mendehan laws of heredity were rediscovered and became generally known, about the year 1900. The relationship of Genetics to other biological sciences is very extensive, as will be apparent upon reflection. Variations of form are frequently studied, and hence an aspect of Morphology is involved. There are also functional variations, which involve Physiology. Embryology reveals the manner in which hereditary features are passed from one generation to another. Taxonomy is involved, because variations determine the limits of species in classification. Moreover, Genetics is the key to the causes of Organic Evolution, since evolutionary changes must originate in variations and be perpetuated by heredity. As with the other groupings of related sciences shown by the table, the list of those bearing upon the science of Genetics might be greatly extended. Origin of the Race. — In ancient times the origin of the many different kinds of animals was commonly ascribed to some form of creation by which they were produced in their existing states, unless indeed it was believed that each individual arose sponta- neously. With the advance of knowledge concerning animal and plant life, beUef in an evolutionary process became inevitable. Organic Evolution is the term applied to the transformations of living things since they appeared upon our planet. It may be com- pared with Inorganic Evolution, which describes the history of non-living things in theories concerning the transformations of solar systems or of chemical elements, or in the more certain theories of geologic evolution. Organic Evolution is the biologist's answer to the question of the historical development of the many and varied forms of life. Within the past century it has come to be supported by over- whelming evidence as the most reasonable explanation of this historic course of events. The sciences of Morphology, Physiology, Embryology, Zoogeography, Paleontology, Ecology, and Taxonomy are important to the student of Organic Evolution. Comparative ZOOLOGICAL SCIENCES 7 Anatomy and Embryology tell of relationships that are often unsuspected. Paleontology tells the history of these relationships. Ecology and Zoogeography enable one to interpret the past in terms of the present, which is a cardinal principle in Paleontology as in the related science of Geology. Taxonomy is a summary of all the conclusions regarding racial origins, since classification, based on structure, indicates the degree of relationship that exists among various forms of life and thus reveals their evolu- tionary history. Definitions Uke those given in the preceding paragraphs are less interesting than many scientific matters -of-f act, but they are desirable for purposes of introduction. An attempt has been made to give greater significance to these definitions by means of the tabulation (p. 3) , which shows how the more important zoological sciences are related to one another and to the broader problems of Zoology. If, after examining the table, the reader understands why each of the sciences Usted in the right-hand column is cited in a particular place, he has mastered the definitions and has secured an outline of zoological science. In such a tabulation it is evident that each subdivision of the right-hand column might contain the names of additional sciences. One's understanding of the definitions may be further tested by considering what other sciences might be so hsted. The unity of life is such that every one of these right-hand subdivisions might contain the names of all the other biological sciences and of the principal physical sciences as well. This is a less striking way of saying that if one could know any single field of nature in all its ramifications one would understand the world in its entirety. Consider, for example, what it would mean to know everything one might conceivably know regarding a single animal: how light from the sun and moon is related to its activities; what physical and chemical changes occur within its body; the nature and origin of the material from which its organs are formed; the process of growth; what makes it, perhaps, an object of beauty in the eyes of man; these and similar questions touch the great problems of the material universe on the one hand and those of the human mind on the other. The subdivisions of zoological science above enmnerated find parallels in the science of Botany. While it is desirable at the outset to obtain the comprehensive survey of subject matter that 8 INTRODUCTION such definitions imply, it would be unfortunate if too much defin- ing should give the impression that Zoology consists of many, isolated fields of knowledge. An understanding of the table corrects such an impression. Anatomy, Embryology, Physiology, and the other sciences are merely different angles from which the study of animal life may be approached. Reflection will show that each overlaps others and that none is isolated by the nature of its subject matter. The problems and sciences defined in the foregoing paragraphs and illustrated by the tabulation will all be considered in the present course of study. Note.— Italics are used throughout the present voKime, as in Chapter 1, to emphasize the names of important parts or processes. Unfortunateh', the font of type used for these itahcs does not show without close inspection the difference between the diphthongs (v (iv) and cc (a-). The student should, therefore, bear in mind this difficult}', and when in doubt as to the spelling of an italicized word containing one of these diphthongs should note the spelling where the word is printed in the regular type. CHAPTER 2 NATURAL HISTORY OF VERTEBRATE ANIMALS Everyone knows something concerning the human body and the bodies of related animals. The more complete this knowledge, the easier is the approach to many phases of zoological science. Because the great majority of students thus possess some knowledge of their own bodies, we shall examine first the structure and activities of an animal that is like ourselves. The frog has the advantage of being familiar in its natural surroundings, interesting in itself, large enough to handle, and man-like to a degree that can be appreciated only upon careful examination. It is sufficiently complex to illustrate the most important features of higher animals, and its structures and functions are relatively well known to science. By studying the frog it is possible to review and extend one's present knowledge and to discover certain biological principles which will be elaborated in subsequent chapters. The term Natural History has no precise definition at the present time. It was formerly used to designate the study of natural objects, not only animals and plants but also non-U ving things hke minerals. During the eighteenth and early nineteenth cen- turies, many naturalists were primarily concerned with the col- lection and classification of animals, although their studies afield brought knowledge regarding habits and distribution. In recent years the science of Ecology has taken over and refined some of the diffuse subject matter of natural history; but the word Ecology, which may be defined as the relation of organisms to their environ- ment, conveys too restricted a meaning to serve as a title for the present chapter, in which an attempt is made to review the principal types of backboned animals, or Vertehrata, and to indi- cate some of the more general features of their organization, behav- ior, and general relationships. While this may seem an impos- sible task within so brief a compass, we shall, nevertheless, be able to consider certain facts of general biological interest by way of 9 10 NATURAL HISTORY OF VERTEBRATE ANIMALS introduction. As in the ensuing chapters on Morphology and Physiology, the frog and man will be most frequently cited for purposes of illustration. The Frog as a Representative Vertebrate Classification. — It is a familiar fact that animals fall into restricted groups called species (singular, species, not " specie ")• E. Cyclostbme Fig. 1.— Representatives of the Sub-phylum Vertehrata {cf. Figs. 2 and 3). Since the most effective manner of explaining what is meant by the term species is through concrete examples of such animal groups and their place in classification, we may proceed at once to THE FROG AS A REPRESENTATIVE VERTEBRATE 11 illustrations. This may be done either by following a larger group of animals through its lesser subdivisions until the individuals comprising the smallest groups, or species, are reached ; or by pro- ceeding in the reverse order. The existence of the group known as the Vertehrata (Figs. 1, 2, 3) was recognized only after it had been discovered that a great array of animals all possessed a backbone composed of vertebrae and hence could be called " vertebrates." Fig. 2. — A representative of the Class Aves of the Sub- phylum Vertehrata: the ostrich in silhouette show- ing endoskeleton of bones and exoskeleton of feathers (c/. Figs. 1 and 3). (After Pander and d'.\Iton.) Fig. 3. — A representative of the Class Mammalia of the Sub-phylum Vertehrata: the lion in silhouette showing endoskele- ton of bones and exoskeleton of hair {cf. Figs. 1 and 2) (After Pander and d' Alton.) These vertebrates have been found, in turn, to possess features that lead one to include them, along with certain other simpler animals (Fig. 4), in one of the major divisions, or -phyla (singular, phylum), of the Animal Kingdom, the Chordata {cf. Table, p. 36), which are so called l^ecause they all possess, at some stage in their development, a notochord, or primitive skeletal axis. In the verte- brates the notochord appears in the embryo, but is later replaced by the segmented vertebral column, or backbone (cf. Fig. 40, p. 69, and Fig. 213, p. 405.) The frog, as shown by the table of classification (p. 36), is a member of the Class Amphibia, a subdivision of the Vertehrata, which includes also the toads and salamanders. The classification of Amphibia is as follows : 12 NATURAL HISTORY OF VERTEBRATE ANIMALS Class, Amphibia Sub-class, Labyrinthodonta or Stegocephali Fossil forms, long extinct; bodies of very large size in some instances and commonly covered with scales (Fig. 5). Sub-class, Lissamphibia Existing forms; no well-developed scales; mostly a soft moist skin, as in frog; none of very large size. Order, Gymnopkiona or Apoda Worm-like creatures without limbs; burrowing in moist ground; not found in temperate America (Fig. 6). Order, Urodela Tailed amphibians; the salamanders, etc. (Fig. 1 B). Order, Anura Tailless amphibians; the frogs and toads (Fig. 7). Following the orders, as one proceeds in classification, come families, then genera, and finally species. Thus, the Order Anura contains, along with some half dozen others, the Family Ranidce, which in turn includes the Genus Rana and others. For example: Order, Anura Family, Ranidce Genus, Rana Species, pipiens, the " leopard " frog (Rana pipiens) In speaking of a species, it is customary to use both the generic and the specific names, much as we use a given name and a sur- name in referring to human individuals, for the purpose of more accurate designation. To be more definite, we say " Smith, John," instead of saying merely " John " or " Smith." In like manner, we refer to the leopard frog as Rana pipiens or R. pipiens, writing the generic name with a capital and the specific name with a small letter. The following are common species of the genus Rana: R. catesbiana, the ''bullfrog"; R. clamitans, the "green frog"; R. sijlvatica, the *' wood frog" ; R. palustris, the " pickerel frog "; and R. pipiens. THE FROG AS A REPRESENTATIVE VERTEBRATE 13 Fig. 4. — Other representatives of the Phylum Chordata. A, an. attached tunicate, Sub-phylum Urochordala. a, larval free-swimming stage of same. B, Amphioxus, Sub-phjlum Cephalochordata. C, Balanoglossus, Sub-phylum IJemichordala. A species may be divided into varieties, if it shows different types that do not warrant specific distinction. The amount of difference between the individuals compos- ing any species can be appreciated only if one examines specimen after specimen and makes exact comparisons. For such a purpose, collections of shells, containing large numbers of individuals, present con- venient illustrative material. In general, the intra-specific differences that consti- tute varieties are slight, as are also the differences between species. Whether a given group shall be a variety of an exist- ing species, a new species, or even a new genus, depends upon the judgment of the individual taxonomist making the classi- fication. Some tend to make many species, others few species, of the same material. Taxonomists exhibiting these tendencies are termed " spHtters " and " lumpers," respectively, by those who pursue the oppo- site course. The foregoing catalogue of unfamiHar names is less interesting than many phases of Zoology, but it illustrates the principles of classification that are consistently fol- P 1^^ mr^ w p- m ^ Fig. 5. — Skeleton of a Permian stegocephal- ian, Eryops, an extinct type of the Class Am- phibia. (Photo, by courtesy of the American Museum of Natural History.) 14 NATURAL HISTORY OF VERTEBRATE ANIMALS lowed at the present time. Beginning with any small group of animals, such as a species of frog or grasshopper, one can follow it into larger and larger groupings until the Phylum is reached, and finally to the Animal Kingdom, which is coordinate with the other great group of living things, the Plant Kingdom {cf. Fig. 1 17 and p. 241). Conversely, if one should begin with the Animal King- dom, one might conceivably follow it to everj^ subdivision until all the species were reached, and thus pass in review all the varied forms of animal life that are known to exist. Our present know- ledge of classification is the result of an enormous amount of study, by virtue of which hundreds of thousands of species have become Fig. 6. — A footless amphibian, Ccecilia, one of the Order fHyinnophiona or Apoda. a, anus. described and arranged in the manner indicated. This group- ing of species is necessary as a means of cataloguing the multitu- dinous organisms that make up the Animal Kingdom ; but classi- fication is further significajit because it attempts to follow the lines of evolutionary relationship and hence becomes equivalent to a " family tree " of living beings. Distribution. — With few exceptions, the Amphibia are con- fined to water and its immediate vicinity, or to a moist atmos- phere. It is this characteristic that has given the name amphibian, which means " leading two lives," to this class of the Vertebrata. Many of the salamanders pass their entire lives in water. Most species of frogs are amphibious. The common toad, Bufo ameri- canus, passes the greater part of its life away from the water, but is quite sensitive to atmospheric moisture. The reader may have noticed that toads are seldom seen in exposed localities in times of drought but will appear after a heavy rain. During the dry inter- THE FROG AS A REPRESENTATIVE VERTEBRATE 15 vals, they either remain in shaded places, in which there is a max- imum of moisture, or return to their " holes " as they do in the day time. The Amphibia are thus dependent upon water, for two obvious reasons: because their respiration is in part effected through the skin; and because their skins are not adapted to pre- vent evaporation. If a frog escapes in the laboratory, particularly if the room is artificially heated, it will usually be found shriveled and dead some hours later. On the other hand, many reptiles, r ' ^^%^''^ m-W^^^^I^ ^^fe^qHp J ^^^pP^ M Bp*^' "^"3 A W _i w .::i^: oSl.' 't *; mji^'^ ^^^^^B ^^^'- rtif' -■J ; SSB^ r- Js^StL vlL^j^w t*-^-- ■ '■ - * • niHL-' - - f ^ » *, •■ *\ * •^ ■ ^«i -^H r -k, ■A.'^ '^^MiL ^ ^^ % •^ - ** ; . ^ kLm^;^ ■ — g ?. "^l^^r ^^ wm^ ■'•■^y^M ♦»:''=? l ^ ^^ Fig. 7,— The American toad, Biifo americanus, a male with buccal sacs expanded in "singing." (From Dickerson, "Frog Book," copyright, 1920, by Doubleday, Page & Co., reprinted by permiasion.) including most Uzards and snakes, are adapted for a hot, dry atmos- phere and react accordingly under artificial conditions. In this connection it may be mentioned that frogs and toads do not " drink " water through the mouth as do many famihar ani- mals. They absorb through their skin such water as enters the body uncombined with their food. This can be demonstrated by exposing a frog first to a dry and later to a moist atmosphere, and weighing at proper intervals. Among the species of frogs common in the eastern and central portions of the United States, the " bullfrog," Rana catesbiana, and the " green frog," R. clamitans, have similar habits and are 16 NATURAL HISTORY OF VERTEBRATE ANIMALS confined rather closely to the vicinity of water. The " leopard frog," R. pipiens, on the other hand, may wander far from water and thus is sometimes called the " grass frog " when found in meadows. The " pickerel frog," R. palustris, may also wander from the water, although it lives mostly in spring beds and in cool, damp places. A more extreme example is the " wood frog," R. sijlvatica^ which is regularly found in damp woods. In New Eng- land this frog is common in beech woods, often a long distance from water. Like the toad, it must come to the water at the breeding season, and in wet weather it has access to temporary ponds. ^^^^ff^..M^i Fig. 8. — The common tree frog, Hyla versicolor, showing mottled ('oloratiou that matches its backgrounds in nature. (From Dickerson, "Frog Book," copyright, 1920, by Doubleday, Page & Co., reprinted by permission.) Acris gryllus, the " cricket frog," is another pond and stream- dwelling species, occurring often in swampy places and in small bodies of water. Hyla versicolor, the " tree frog " (Fig. 8), comes to the water in numbers only at the breeding season, and at other times frequents damp places, chmbing tree trunks to feed upon insects. Its changes in color to match the background are re- markable in their rapidity and diversity, and probably account for the fact that these animals are difficult to locate ; although their presence is often made known by their croaking, particularly under stimulation by the moist atmosphere preceding rain. The general distribution of Amphibia is, therefore, conditioned by certain peculiarities of the body surface that have been de- scribed. They cannot live in arid regions or in regions where the ground remains frozen throughout the year, because they must hibernate during cold weather. They are, so to speak, tied to the water by the nature of their organization and activities and by THE FROG AS A REPRESENTATIVE VERTEBRATE 17 the fact that, with few exceptions, they must lay their eggs in water at the annual breeding season. Food and Feeding. — The feeding habits of frogs and toads must be observed to be appreciated. A toad in a garden at dusk, or one Fig. 9. — Feeding habits of the American toad, showing the sudden protrusion and retraction of tongue in capture of a flying insect and a toad walking around an earthworm preparatory to seizing head first. (From Dickerson, "Frog Book," copyright, 1920, by Doubleday, Page & Co., reprinted by permission.) found squatting near a street lamp to which it has been attracted by the insects, will often furnish an amusing as well as an instruc- tive demonstration. 1 Insects flying near the head are snapped into ^ The popular prejudice against toads is quite unreasonable. They do not produce warts and may be handled with impunity. Certain of their skin glands secrete a fluid which is irritating if brought in contact with the mouth or the eyes but has no effect upon the human skin. This fluid is discharged only when the animal is violently handled or is in grave danger, as when seized by a snake. It is presumably protective. Toads compare with the best of our insect-eating birds in their destruction of garden pests, and they should receive the protection afforded to all harmless and useful animals. In a garden or a field they should be welcome guests and not objects of persecution and de- struction by adults as well as children. They have a marked sense of locality, and individuals appear to return year after year to the same foraging grounds. 18 NATURAL HISTORY OF VERTEBRATE ANIMALS the mouth by a sudden movement of the tongue (Fig. 9), which is ahnost as sticky as fly paper. An object cravvHng on the ground, such as an earthworm, is approached by jumping, and as it is gulped into the mouth the two front feet may alternately comb the wrig- gling prey away from the head. Frogs in the laboratory may be fed upon crayfish or similar animals. In nature their food is of wide variety. Almost anything that is small enough may be devoured, and sometimes, as in the case of crayfish, the size and number of the objects swallowed is amazing. Insects flying in the air; spiders, insects, snails, and earthworms crawling on the ground; insect larvae, crayfish, and other animals in the water; even other frogs and tadpoles — all are eaten with avidity. The writer once observed that some large bullfrogs had devoured a number of English sparrows, which had been inadvertently left in a caged aquarium and had fallen an easy prey with the approach of night. In nature, the bullfrog may devour young chicks and ducklings if they blunder within reach. Generally, only living food is taken by frogs and toads, the animals responding to the moving object, although a frog will sometimes swallow the bodies of dead animals if they are placed in its mouth. For this reason a frog will snap at a fishhook dangling near its head, although such a response usually means a speedy end. Similar responses to objects in motion are typical of many other carnivorous animals. They have doubtless been observed by the reader in fish, which " rise " to the bait that falls across the water with just the right imitation of the natural food; in the lizard, which darts forward only when the fly begins to crawl; or in the stolid barnyard fowl, which eyes suspiciously the insects lying motionless for a second or two after they have been uncovered by scratching, but seizes them as soon as they begin to make off. Motion of this sort is so invariable a sign of something alive, and hence good to eat, that carnivorous animals have come to respond automatically to such stimulation, yet they can learn to make nice distinctions, as with the wily old trout that feeds all day on his natural prey but never takes the most cleverly thrown fly. Movements and Locomotion. — In contrast with the salamanders (Fig. 1 B), which are more typical as four-footed vertebrates, the frogs and toads exhibit a specialization of the Hmbs compar- able with that seen in kangaroos, rabbits, the entire class of birds, and in hmnan beings. These animals are not closely related as THE FROG AS A REPRESENTATIVE VERTEBRATE 19 vertebrates, but they have developed independently in the direc- tion of more specialized uses for their two pairs of limbs than is commonly found. In the familiar hopping of frogs and toads, the hind legs are responsible for the strength of the spring while the front legs serve mainly as guides in pointing the body in the desired direction. The animals are frequently seen crawling in a clumsy fashion, but only for short distances, or to shift the position of the body. In water, likewise, the frog swims by means of its powerful hind limbs, while the fore limbs perform the function of balancers. In the squatting posture which it assumes on the bank, the frog is in a position to leap at a moment's notice, hke an athlete ready for the pistol shot. In the " floating " attitude, the animal is no less able to retreat suddenly. At first glance it seems so awk- wardly placed that it must be at a great disadvantage, but observation shows that the floating individual actually " dives," with surprising rapidity. The reason for this appears when the series of movements is carefully observed. The animal hangs obliquely in the water with eyes and tip of nose exposed, the fore limbs projecting from the body and the hind limbs moderately extended. WTien diving from this position, it first withdraws from the surface by bringing the hind legs into their folded position, thus carrying the body backward beneath the water. While this is happening, the fore limbs give a stroke upward and backward, which, together with the bending of the body, directs the head downward. The hind legs are then extended with their powerful stroke, and the frog shoots away, the whole process occurring so rapidly that it is impossible to recognize the successive changes without repeated observations. Like other active annuals, the frog has a well-developed " sense " of its position. It " knows " when it is wrong side up. If placed on its back the animal rights itself without delay. It may roll either to the right or to the left, but the response is unmediate unless the individual has been fatigued by repeated demonstrations. With the onset of fatigue, the process occurs more slowly and is readily observed. In this connection a pecuhar mode of reaction may be mentioned. If a frog is seized in the hands, laid upon its back, and held a few moments until it has ceased its struggles, it will usually remain motionless for a time, sometimes for hours (r/. Fig. 10). The exact significance of this behavior, in relation to 20 NATURAL HISTORY OF VERTEBRATE ANIMALS death-feigning and to the hypnotic state in man, is a matter of doubt. The use of the hmbs in other vertebrate animals shows many- interesting features. Relationships between structure and func- tion are everywhere apparent. Typically, the skeleton of each limb consists, as in the human body, of an upper portion supported by a single bone, the femur in the leg and the humerus in the arm ; a portion containing two bones, the tibia and fibula of the leg, and the radius and ulna of the arm; a group of smaller bones at the ankle or wrist ; and the bones of foot or hand. This plan of struc- ture, which appears in the generalized condition in man and some Fig. 10. — Death feigning in young toads that have been seized and held for a moment, sometimes called hypnotism but probably not comparable with the hypnotic state in man. (From Dickerson, "Frog Book," copyright, 1920, by Doubleday Page & Co., reprinted by permission.) of the less specialized of the terrestrial vertebrates (Fig. 18, p. 39), is modified in a variety of ways for different uses, but persists throughout as a type to which the most specialized forms can be referred. Parts suppressed in the adult are often found in the embryo. Hence, it is in general supposed that limbs Hke those of the horse or the bird, with their smaller number of bones, have reached their present state by loss or fusion of parts in the course of evolution {cf. Fig. 277, p. 517). Often these fusions are readily seen, as in the radio-ulna or tibio-fibula of the frog (Fig. 40, p. 69). Miscellaneous Activities. — Sound production by frogs and toads is more diversified than is commonly supposed. One who has trained himself in this regard can distinguish the notes of different species as he can those of birds, although there is much less range THE FROG AS A REPRESENTATIVE VERTEBRATE 21 and diversification. In R. pipiens there is a slight difference in the croak of the two sexes, and, in addition to its croaking, the animal gives forth a grunting sound under conditions that seem unusually agreeable. When seized by an enemy it may utter a cry called the pain scream. The croaking is produced by forcing air back and forth between lungs and mouth cavity across the vocal cords, which are stretched on either side of the larynx, or laryngo- tracheal chamber. The latter name is sometimes given to the larynx of the frog (Fig. 23, p. 43) because it is equivalent to both the larynx and trachea, or windpipe, in higher vertebrates. A frog can croak even though it is completely submerged in water, because there is only a sKght loss of air through the nostrils during the process and the air may be driven back and forth a number of times before being expelled. It has been generally assumed that this sound production by means of the vocal cords functions pri- marily as a sex call, by means of which the males and females find one another at the breeding season. This is undoubtedly one of its uses, if not the only one, although the croaking is more or less in evidence at other times. Among the sounds uttered by frogs are community signals, like the alarm cry, which causes other indi- viduals to seek safety in the water. Vocal cords are, of course, present only in air-breathing verte- brates. The fishes have no such organs. It is interesting to find in the Amphibia, wliich are the simplest of the land vertebrates, structures and activities that suggest the origin of the human vocal organs (Fig. 24, p. 44). It may also be remarked that a well- developed sense of hearing appears only in terrestrial vertebrates, and that eardrums, Hke vocal cords, do not occur in fishes. In general, it appears that sound production and the ability to perceive sounds go hand in hand. The ear of a mammal, with its external portion and its drum sunken into a protected position (Fig. 68, p. 125), in correlation with a more delicate sense of hearing, is paralleled by the greater speciaHzation of the sound-producing apparatus which mammals exhibit. Some of the diverse activities of the frog seem to be adaptive, that is to say, useful, to the extent that they tend to protect the individual in time of danger. When threatened by an enemy, a frog or toad often folds the limbs close against its sides and inflates its body by filling the lungs to their maximum capacity. In this condition the animal is almost egg-shaped, and the moist 22 NATURAL HISTORY OF VERTEBRATE ANIMALS slippery surface makes it a difficult object for an animal like a snake to hold between its jaws and begin swallowing. Some- times, instead of endeavoring to escape, the frog will crouch close to the ground and remain motionless, thus tending to evade capture by hiding. When diving into the water, it sometimes circles about and comes up to one side of the observer, near the bank, among the weeds. Or it may hide on the bottom by crawl- ing under some object. The tree frog, Bf^la versicolor, which pos- sesses such unusual powers of changing its color to blend with sur- rounding objects (Fig. 8), is perhaps aided by this reaction in its struggle for life. While it would be difficult to prove beyond a doubt that these varied activities of the leopard frog and its relatives are " adap- tive " in the sense that they are frequently of life and death importance, such responses are similar to the forms of behavior exhibited by many other animals which, like the frog, possess no special means of defense. Adaptation in animals has no doubt been over-emphasized at times in the history of Biology; but the fact remains that living organisms, both animal and plant, exhibit a degree of adjustment to the necessities of their existence which has impressed the biologist ever since Aristotle said that the essence of a living being was '' fitness." Enemies and Parasites. — Being entirely without weapons or defensive armor, save the tough and slippery skin, frogs are preyed upon by many other animals and can find safety only in hiding or flight. Next to man, snakes are undoubtedly their greatest enemies, for, despite their lack of limbs, the snakes that frequent banks of ponds and water courses are very adept at capturing frogs and swallowing them, usually head foremost. Shore-feeding birds such as cranes and herons, the common crow, turtles, such fish as bass and pickerel, skunks, and many other animals all prey upon frogs. Were it not for their rapid multiplication, these Amphibia could hardly survive in the face of such destruction. Like most animals that have been carefully studied, the frog is the " host " for a great number of parasites. Among those likely to be met with even in a single frog are fluke worms of the genus Pneumonwces, in the lungs; Clinostomum, encysted on the inner surface of the body wall; Gorgoderina, in the urinary bladder; and various other flukes within the intestine. In the lungs may also be found a species of roundworm, Rhabdias, and THE FROG AS A REPRESENTATIVE VERTEBRATE 23 immature worms of this type are often found in the intestine and the body cavity. In the large intestine of almost every specimen examined, there will be found, in addition to many bacteria, sev- eral species of single-celled parasites belonging to the group of animals known as Protozoa. The Hst might be still further extended. It is not uncommon to find a dozen or more different species of parasites in a single frog taken at random. Since most parasites are specialists, to the extent that they infest but a single species of host or a few closely related species, and since most well- FiG. 11. — Development of the frog. A, eggs. B, C, D, and E, cleavage, gastrula, and neural fold stages. F, newly hatched tadpoles. G and H, later tadpole stages. I, J, and K, metamorphosis to juvenile frog. known animals have many parasites, it is not impossible that the total number of parasitic species of animals exceeds those that are free-Uving. Parasitism is, therefore, of widespread occurrence and presents many interesting biological problems. There are few, if any, true cases of parasitic vertebrates, probably for the rea- son that most vertebrates are animals of some size, while parasites are, of necessity, smaller animals. Seasonal Changes and Life Cycle. — Following its breeding season in early spring, the leopard frog leads an active Hfe, feeding voraciously to restore the loss entailed by its "winter sleep" and 24 NATURAL HISTORY OF VERTEBRATE ANIMALS by reproduction. There seems to be a less active period in the late summer, followed by further activity preceding the hiberna- tion. With the lowering of the temperature in the fall, the animal goes to the bottom and works its way into the mud or under the bank, where it remains dormant until spring. Shortly after its emergence, the mating occurs. The eggs are laid by the female during sexual union, and fertiHzed by the sperm of the male as they pass into the water. At first each egg is surrounded only by a thin layer of sticky substance. In a few hours, however, this imbibes water and becomes the capsule of jelly surrounding the individual eggs, which he massed together and hghtly attached to submerged objects near the surface (Fig. 11 A). Development proceeds, in the course of three or four months, through the familiar tadpole stages to the miniature adult. The rather sudden change from the tadpole to the young frog is termed the metamor- phosis (Fig. 11 I-K). In the tadpoles, gills hke those of fishes con- stitute the primary organs of respiration, although well-developed lungs are present and assume an increasing importance in later tadpole stages. With metamorphosis, the gill apparatus in part disappears and is in part converted into other organs. The frog tadpole thus resembles a fish, since it develops in the open water and possesses certain fish-like organs. Adaptation. — As has already been indicated, certain activities of the frog are so well fitted to the needs of the animal in its struggle for life as to attract our attention. A like condition pre- vails in other organisms. The leaves of a plant are adapted to perform certain functions; the stems and roots, others. Animals are adapted for many differing modes of hfe, each to its own set of conditions. While it is sometmies argued that non-living things also exhibit what may be termed adaptation, as when we find hydrogen " adapted " to combine with oxygen in the formation of water, the earth adapted to revolution about the sun, or the stones in the bed of a stream adapted to their particular places, this adaptation, or fitness, of inanimate objects is far less compli- cated than that observed in living bodies. For convenience, the adaptations of organisms may be grouped as: (1) Anatomical, or structural; (2) Physiological or func- tional; and (3) Related to behavior. This, however, represents no hard and fast distinction. It would perhaps be better to say that structure and function are everywhere inter-related, THE FROG AS A REPRESENTATIVE VERTEBRATE 25 and are together adapted to the more obvious needs of the organ- ism. Thus, the Hmbs of a vertebrate animal and those of an insect (Fig. 22, p. 42) are structurally adapted to function in a manner that is advantageous for the life of their possessor. A famihar example of what is generally regarded as an adaptive fea- ture is the coloration and bodily shapes of many animals, notably birds and insects, which so resemble the surroundings that the animal seems likely to escape the observation of many enemies (Fig. 8). The behavior of an animal as a whole, as shown in actions like eluding an enemy or seizing a victim, is, therefore, adaptive insofar as it tends to preserve the life of the individual under the normal conditions of its environment. Such adaptation is not perfect, but only sufficient for the needs of the particular case. The snake that swallows a hen's egg profits by the experi- ence, but should it swallow a china nest-egg it might die of indi- gestion. Frogs and toads snap at objects moving in the air near their heads. In nature, motion of this sort is almost invariably a sign of something good to eat. Behavior in frogs and toads is adapted accordingly. If the moving object happens to be a fish- hook, the animal may lose its life. In general, it may be said that the behavior of animals is adapted to the conditions which they and their ancestors have commonly experienced. They are not adapted to untried situa- tions, unless by accident. When animals come in contact with such situations, .one of two things seems likely to happen: either the species is exterminated through its inabiUty to cope with these conditions; or, after a period of wholesale destruction, the sur- vivors become adapted. This adaptation may involve the devel- opment of new structures as well as new functions and modes of behavior, and hence may effect what is clearly an evolutionary modification. By such modifications in relation to changes of the environment, it is beheved that species have reached their present state of fitness. Charles Darwin (1809-1882) designated as '' Natural Selec- tion " this action of the environment whereby the indi\dduals of a species are selected by nature according to their abiUty to meet the demands of an intense struggle for life. Granting the reality of adaptation as a widespread phenomenon among animals and plants, his theory remains the best scientific explanation of the manner in which such fitness has reached its present degree of excellence. 26 NATURAL HISTORY OF VERTEBRATE ANIMALS Adaptation in the Human Species. — The case of man appears to be somewhat different from that of the rest of the animal world. While it appears that animals must become adapted to their environment, must change with the changing demands of exist- ence, or perish, man has succeeded to a surprising extent in alter- ing his environment instead of becoming himself modij&ed by the forces of nature. The beaver that builds a dam and constructs Fig. 12. — American, beaver and "house." A beaver pond in background with dam to left. (Photo, by courtesy of the American Museum of Natural History.) its "lodge" by felling trees and cutting them in pieces (Fig. 12), or the colony of bees with its nest, alters the environment to suit its needs; but even these extreme cases show relatively little control of the environment when compared with that secured by mankind, unless one accepts a view of the purposeful actions cf animals with which many biologists are not in agreement. Civil- ized man has created for himself conditions that may be charac- terized as " artificial " in contrast with those of nature. He does this by virtue of his intelligence, by understanding nature and by modifying natural conditions to suit his needs. By contrast, other animals must conform to changes in environment or perish. Having persisted in his present manner of life, man cannot return THE VERTEBRATES AND THEIR ENVIRONMENT 27 to the order of np,ture, even if that were desirable. It is only pos- sible to go forward to an increasing control. If men are to live herded together in cities, to traffic up and down the seas, to cul- tivate the soil, and to solve the problems of increasing population, it must be through an ever more effective control of their surround- ings. The material problems of human beings are, biologically speaking, the problems of an organism that is struggling to control an environment from which much has been secured, but from which much remains to be wrested if the species is to be made safe in its present position or to reach higher levels. Because scientific knowledge is the key to this situation, science assumes an increasing importance in the life of mankind. The Vertebrates and their Environment Habits, Habitat, and Distribution. — Having examined the natural history of the frog as a representative vertebrate, we Fig. 13. — The climbing perch, a fish that can leave the water and travel for some distance on land by means of an adaptation for storing water above the gills. The fish on land and head dissected to show structure of gill region; 6 a., first branchial arch; l.o., labyrinthiform organ; op., operculum; sb.c, suprabranchial cavity. (Left, cour- tesy of Nature Magazine; right, from The Cambridge Natural History, copyright, 1920, by Macmillan and Co., Ltd., reprinted by permission.) may next consider the natural history of the vertebrates as a diversified group of animals. In connection with the classifi- tion of vertebrate animals (p. 36), reference will be made to the habits and habitats of the several types. The fishes (Fig. 1 C, D, E) are aquatic, and breathe by means of gills, although certain species are adapted for brief excursions from the water. Among these may be mentioned the climbing perch (Fig. 13), 28 NATURAL HISTORY OF VERTEBRATE ANIMALS the flying fish, and a few species that remain on the moist sea- weed between tides or are capable of short journeys on land. These, however, are quite exceptional. The whole organization of fishes shows their fundamental adaptation to an aquatic life. The Amphibia (Fig. 1 B) are animals whose early stages, the tad- poles, are fish-hke in structure and thoroughly adapted for Hfe in the open water, save in a few species where some form of brooding the young occurs (Fig. 14) ; and even in such instances the embryo is surrounded by a fluid during its development. In its final stages the animal may five permanently in the water; but most forms become " amphibious," breathing air by means of lungs, although the skin is also used in respiration (c/. p. 91). Hence, the Amphibia, as a class, are confined to the vicinity of water, as with the frogs, or to regions of moist atmosphere, as with the toads. The Reptiha, on the other ^ ,. , , , . , hand, possess fully developed Fig. 14. — A tree frog, i/wia ooeWii, which , , , • ,i . . . ,, .'•"...,' , lungs and a skm that protects carries the eggr; m an incipient brood- ° ^ pouch on its back until the hatching their bodies from excessive stage, which is probably as late as the evaporation, and are true land time of metamorphosis. animals. The developing em- (From The Cambridge Natural History, bryO is encloSCd in an egg copyright, 1920, by Macmillan & Co., Ltd., i i, i_ • i • i j.u IM reprinted by permission.) shcU whlch IS leathery hkc that of a turtle, not brittle like that of a bird (Fig. 220, p. 423). Since these egg shells resist evaporation sufficiently to allow development in a moist atmos- phere, the reptiles find it unnecessary to lay their eggs in water as do the Amphibia. They are thus "emancipated" from the water. There are a few cases among the reptiles, for instance, in some of the snakes, where the young are born "alive" (c/. p. 423). Birds are similar to reptiles in their manner of development, save for the difference in the egg shell and for the care given the young after hatching. Of all vertebrates, the mammals are the most highly special- ized for terrestrial life. In most species, the early stages of development are passed within the body of the female parent but the most primitive mammals (Fig. 15) lay eggs that resemble those of birds and reptiles (r/. Fig. 224, p. 428). In some THE VERTEBRATES AND THEIR ENVIRONMENT 29 cases, as with the mare's colt, the young can stand and run almost at birth. In others, as in dogs and cats, there is a period of greater dependence. Adult mammals are adapted for a wide range of terrestrial conditions. The chamois, mountain sheep, and goats inhabit the heights; deer and wolves, the forests and open country; members of the horse and cattle families, the great plains; other mammals range along the shores of streams, lakes, or oceans. Among the most interesting cases are those of such mammals as whales, porpoises, and seals, which show varying degrees of adap- tation to life in the water. The same peculiarity is shown, in a Fig. 15. — The duckbill or platypus, Ornithorhyncus anatinus, an Australian mammal that lays eggs. (From Parker and Haswell, "Textbook of Zoology," copyright, 1921, by Macmillan & Co., Ltd., reprinted by permission.) lesser degree, by beavers, muskrats, and many others. These animals are obviously mammals; they have hair and give birth to well-developed young which are nourished by means of mammary glands. They are able, in extreme cases like that of the whales, to spend their entire Kves in the water, coming up at intervals for air when not swimming at the surface. Others, hke the fur seal, may spend most of their lives in the water save for the breeding seasons. The fur seals arrive at the seal islands of Bering Sea early in the summer; they give birth to their young on land at the so- called "rookeries" (Fig. 16); the young pass their early life in and out of the water; and in the late summer the seals " haul back to the sea and no man knows their track," although they return to the beaches year after year. A seal is a very clumsy animal on 30 NATURAL HISTORY OF VERTEBRATE ANIMALS the land, but very graceful In the water. It is "tied" to the land by its manner of breathing and producing young, just as the toads, among Amphibia, are "tied" to the water by the necessity of a moist atmosphere and by their breeding habits. The foregoing examples enable us to understand something of the diversification in habits and habitats of the vertebrate group as a whole. The oceans in all their depths, the land surface even to the poles, and the air constitute the limits of distribution. No other one of the large animal groups, unless it be the Arthropoda YiG. 16.— American fur seals at rookery, and adult male and his "harem" of females. Other similar groups are seen in the background. These seals come to the rookeries in summer, giving birth to the young on land. During the remainder of the year they live in the open waters of the North Pacific. (Photo, by courtesy of the American Museum of Natural History.) (cf. Fig. 117, p. 240), ranges so widely. This much can be said in general. To go further would necessitate detailed references to individual species, some examples of which have been given in the Amphibia. Of all the types of vertebrates, the human kind has ranged most widely. No other single species can compare with man in this particular, unless it be some of the animals associated with him, such as rats and other vermin. Taken as a whole, there- fore, the vertebrates show a great range of habitat. The fishes, which are the simplest vertebrates, are adapted for aquatic Hfe; THE VERTEBRATES AND THEIR ENVIRONMENT 31 the reptiles, birds, and mammals, which are more complex, are adapted for hfe on the land and in the atmosphere ; the Amphibia represent an intermediate state in both structure and habits. Feeding and Food Supply. — Like other large groups of animals, the vertebrates present examples of varied types of diet. Some vertebrates are plant-feeding, or herbivorous; others, flesh-feeding, or carnivorous; while still others are omnivorous, thriving upon a diversified food supply. The differing foods and the differences in Fig. 17. — Timber wolves. Tj'pical of the adaptations that enable such manimals to range from the high mountains to the plains and even from tb.e Arctic circle to the tropics. (Photo, by courtesy of the AmerioMn Museum of Natural History.) feeding habits which they imply are, of course, related to the habits and habitat of each species. The vertebrates, taken as a whole, show a wide range in feeding habits and in adaptations related to feeding. The sources of food supply may be considered in this connection, with reference not only to vertebrates but to animal life in general. Animals take into their bodies masses of material, the food, which is digested and becomes a part of their protoplasm {cf. p. 81). Green plants, on the other hand, take in simple compounds, such as water, carbon dioxide, and mineral salts; with these alone they are able, in the presence of sunlight, to maintain and increase the bulk of their bodies. Closer analysis shows that, strictly speaking, the differ- ence between plant and animal nutrition is not so great as might be supposed; but for the present we are considering only the fact 32 NATURAL HISTORY OF VERTEBRATE ANIMALS that plants are able to maintain themselves upon non-living materials, whereas animals arc dependent for their existence either upon the bodies of other animals or upon plants. When the food of animals is traced to its source, the green plants are found to support not only themselves but also the vast majority of animals. Herbivorous animals feed directly upon green vegetation ; carnivorous animals are usually but one step removed, because they feed upon other animals that are herbivorous. Examples will occur to the reader. Since the green plants depend upon sunlight for the energy by which they combine simple chem- ical compounds into the complex ones necessary for a Uving body, the maintenance, from day to day, of every living thing upon the land is dependent upon the light of the sun and the green sub- stance, chlorophyll, in plants. If the green plants should be sud- denly wiped out of existence, the animals would soon perish, for at the best they could do no more than " eat each other up," like the gingham dog and the calico cat. It is true that plants derive much cf their available food from the decomposition of the bodies of animals, but this does not alter the general fact that animals depend upon plants for their food, whereas plants are not so obvi- ously dependent upon animals. Even more fundamental than this difference in the source of their energy is the contrast between the chemical synthetic powers of plants and animals. Green plants make their own nutrients; animals digest and recombine theirs. The colorless plants, or Fwigi, get their energy from organic matter, as do animals, but they are closely related to plants rather than to animals. In the foregoing pages, the reader's attention has been directed to the life that is found upon the land. The relationships that exist in the ocean are similar, although at first glance there seems to be nothing there to correspond to the mass of verdure that clothes the fertile portions of the land. If we try to call before the mind a picture of the land surface of the earth we see a vast expanse of verdure stretching from high up in the mountains, over hills, valleys, and plains, and through forests and meadows, down to the sea, with only an occasional lake or broad river to break its uniformity. Our picture of the ocean is an empty waste, stretching on and on with no break in the monotony except now and then a flying- fish or a wandering sea-bird or a floating tuft of vegetable life. It contains plant-like animals in abundance, but these are true THE VERTEBRATES AND THEIR ENVIRONMENT 33 animals and not plants, although they are so like them in form and color. At Nassau, in the Bahama Islands, the visitor is taken in a small boat, with windows of plate glass set in the bottom, to visit the " sea-gardens " at the inner end of a channel through which the pure water from the open sea flows between two coral islands into the lagoon. Here the true reef corals grow in quiet water, where they may be visited and examined. When illuminated by the vertical sun of the tropics and by the light which is reflected back from the white bottom, the pure, transparent water is as clear as air, and the smallest object forty or fifty feet down is distinctly visible through the glass bottom of the boat. As this ghdes over the great mushroom-shaped coral domes which arch up from the depths, the dark grottoes between them and the caves under their overhanging tops are lighted up by the sun, far down among the anthozoa or flower animals and the zoophytes or animal plants, which are seen through the waving thicket of brown and purple sea-fans and sea-feathers as they toss before the swell from the open ocean. ^ It appears, however, that what seem to be plants attached to the bottom are in reality attached animals feeding upon other animals that swim freely in the water. In these sea gardens the bottom is like one vast mouth, or rather it is carpeted with innum- erable mouths. Even further examination seems to show that the animal life of the ocean possesses no visible means of support, unless one examines the open water with the aid of a microscope. In this way one discovers that in the ocean, as well as on land, the green plant is the primary source of food supply. In the ocean, however, the green plants consist of microscopic forms. Although the number of species is relatively small, such an enormous number of individuals are present in all the oceans that these organisms furnish an abundance of food for the teeming animal population. The inter-relationships whereby the material from this primary source becomes available for larger animals are no more comph- cated than those by which a sunilar elaboration of the food supply is effected upon the land, beginning with the green plants and ending with the largest carnivorous animals. A similar condition exists in the life of inland waters (cf. Fig. 305, p. 550), although the amount and importance of the larger aquatic vegetation is relatively much greater than in the ocean. The conditions that have been described for land, ocean, and fresh water, constitute a fundamental 2 Brooks, W. K., "Foundations of Zoology," pp. 220--21. 34 NATURAL HISTORY OF VERTEBRATE ANIMALS nutritional relationship, upon which have developed all the mul- titudinous interactions and special devices by which animals and plants maintain themselves upon our planet. Inter-relations between Species. — In concluding this discus- sion of the vertebrates in relation to their environment, reference may be made to the manner in which both animals and plants are inter-related in their daily existence. It is well known that ani- mals depend upon other animals and upon plants for food. The relation of '' eater " and " eaten " is almost universal. A host of other relationships also obtain, whereby species depend upon other species for their very lives; and the chain of events often ramifies in so many directions that we can only imagine its ultimate possi- bilities. Among many examples of such relationships are those cited by Darwin in discussing the " checks to increase " in his " Origin of Species." The relation between cats and clover is one of his most famous cases. Stated in Darwin's own language, this runs as follows : I have also found that the visits of bees are necessary for the fertilization of some kinds of clover; for instance, 20 heads of Dutch clover {Trifolium repens) yielded 2290 seeds, but 20 other heads protected from bees produced not one. Again, 100 heads of red clover ( T. pratense) produced 2700 seeds, but the same niunber of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. It has been suggested that moths may fertilize the clovers; but I doubt whether they could do so in the case of the red clover, from their weight not being sufficient to depress the wing petals. Hence we may infer as highly probable that, if the whole genus of hmnble- bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great measure upon the number of field-mice, which destroy their combs and nests; and Col. Newman, who has long attended to the habits of humble- bees, believes that " More than two-thirds of them are thus destroyed all over England." Now the number of mice is largely dependent, as every one knows, on the number of cats; and Col. Newman says, " Near villages and small towns I have found nests of humble-bees more numerous than elsewhere, which I attribute to the nmnber of cats that destroy the mice." Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district. (Darwin, Chas., " Origin of Species," pp. 90-91.) THE VERTEBRATES AND THEIR ENVIRONMENT 35 In examining such a chain of events, one should remember that it is no stronger than its weakest link. If the connection breaks at any one place, the conclusion does not follow from the pre- mises. It is quite possible that the foregoing relationship between cats and clover may not obtain, because one or more links of the sequence do not actually occur. Probably in no such case can we ever be sure that the relationships we observe are in reality matters of life and death in a large number of cases. Nevertheless, in view of the many glimpses of such relationships that are everywhere disclosed by organic nature, we can be sure that living beings are frequently inter-related in ways that are of the utmost impor- tance to the organisms concerned ; and that such interdependence of life upon life is one of the major factors in the complex of ani- mate nature. As explained under the head of definitions in the preceding chap- ter, the biological science that deals with the relationship of organ- isms to their environment, both living and non-living, is known as Ecology. The ecological study of vertebrate animals presents many problems that are intimately related to problems of human existence. Thus, man, although no longer prej^ed upon by the larger animals, falls a victun to the attacks of parasites in the form of the germs of disease, or is tortured by insect pests which secure food from his body. Insects prey upon the plants that he cul- tivates for food. Insect-eating birds in turn devour the insects and are themselves destroyed by their enemies. From a state in which he lived by dint of holding his own physically with other forms of life, man has come to live by his wits, and in this way has become the dominant species among those now in existence. Within a very brief space of time, this single species. Homo sapiens, has upset the balance of power which had previously existed in the inter-relationships of Hving things throughout the earth and had continued throughout geologic time. A new and terrible thing has come upon the world of Hving beings. Because of man, forests have given place to fertile fields or to bare and eroded hill- sides, the larger forms of animal Hfe that do not serve for domesti- cation are fast becoming extinct, and introduced animals and plants are supplanting native stocks in many localities. The zoologist can but feel that the world will be a far less satisfac- tory place for man, to say nothing of the animals, if the birds become mostly English sparrows and the mammals rats and mice, 36 NATURAL HISTORY OF VERTEBRATE ANIMALS while coincidently the virgin forests are largely converted into Sunday newspapers. Classification The Vertebrates and their Next-of-kin. — As shown by the accompanying table of classification, the vertebrates belong to a larger group of animals, the Phylum Chordata. All chordates agree in certain fundamental points of structure, notably in the possession of the notochord (cf. p. 11), as their name implies. Other distinguishing features are the gill slits, and a dorsal, tubular, central nervous system. The Hemichordata (Fig. 4 C) are worm-Uke marine animals which are simple and yet have gill slits resembling those of vertebrates. The Urochordata, or " sea squirts " (Fig. 4 A), are usually modified for an attached mode of life in the adult; but some are free-swimming, and in early stages (Fig. 4 a) they possess a notochord and other unmistakable chordate features. The Cephalochordata are represented by the individuals of several genera, including Amphioxus (Fig. 4 B), which do not possess a skull ; in other respects they are much like the Vertebrata, which possess a skull and vertebral column, as seen in familiar backboned animals (Figs. 1, 2, 3).^ Members of the Phylum Chordata, therefore, exhibit a wide range of organi- zation, being represented at one extreme by very lowly animals, like the tunicates, and at the other by the most complex of all animals, the mammals. Phylum, Chordata. Sub-phylum, Hemichordata. Balanoglossus, etc. (Fig, 4 C). Urochordata. Tunicates, or sea squirts, etc. (Fig. 4 A). Cephalochordata ( Acrania) . Amphioxus, etc. (Fig. 4 B). ^ It is sometimes convenient to distinguish between " vertebrate " and " invertebrate " animals, the vertebrates being sufficiently important, because of their complexity as well as from a human standpoint, to be placed over against the rest of the animal world. This distinction was originally made by Aristotle. As a matter of scientific classification, however, such a division of the animal kingdom into vertebrates and invertebrates is unwarranted. CTASSIFICATION 37 Vertehrata or Craniata. Class, Cyclostomata. Primitive, aquatic forms without jaws and paired appendages, and with circular mouths, e.g., Lampreys, etc. (Fig. 1 E). Class, Pisces. Aquatic forms with paired appendages, true jaws, and with scales in the skin, e.g., the Fishes. Sub-class, Elasmohranchii. Fishes with cartilaginous skeletons, e.g., Sharks, Skates, etc. (Fig. 1 D). Sub-class, Teleostomi. Fishes with bony skeletons, e.g., the more famihar fresh-water fishes (Fig. IC). Class, Amphibia. Semi-terrestrial and aquatic forms with soft moist skin and tadpole stage in development, e.g., Salamanders, Frogs, Toads, etc. (Fig. 1 B). Class, Kept ilia. Terrestrial forms with scaly bodies, well developed lungs, and other adaptations for terrestrial life, e.g.. Lizards, Alhgators, Tur- tles, Snakes, etc. (Fig. 1 A). Class, Aves. Terrestrial forms adapted for flight, covered with feathers, e.g., Birds (Fig. 2). Class, Mammalia. Animals with hair and mammary glands, and in most cases giving birth to well-developed young, e.g., Mammals (Fig. 3). CHAPTER 3 MORPHOLOGY OF THE VERTEBRATE BODY An understanding of scientific facts, in Zoology as in other sciences, must rest upon a basis of first-hand knowledge. Little knowledge is required for an understanding of the preceding chap- ter, beyond the acquaintance with common animals that is pos- sessed by most readers of a book of this character. In the present chapter, more depends upon what the student may be presumed to have seen in the laboratory, but he must also utihze such general information as he may have concerning the structure of the human body and that of familiar animals. As has been explained in the introductory chapter, the various fields of biological science are closely inter-related. This is particularly true of Morphology and Physiology. The study of structure may interest us, because of the intricacies or the beauties of form that it reveals; yet structure never can become meaningful unless we possess some knowledge of the way in which the parts work. The morphology of an auto- mobile is interesting, but the parts have meaning only in relation to their manner of action. For this reason, the present chapter, which deals primarily with structure, will in many places explain functions as well. Likewise, in Chapters 4 and 5, which deal with functions, we shall find it necessary to explain structure in some detail. All these chapters are intended to serve as a review and extension of the knowledge concerning the functional organization of the vertebrate body which is given in elementary textbooks of human physiology, but which experience shows has not been prop- erly comprehended by the majority of students before entering upon the study of General Zoology. External Features and Related Structures Head, Body, Tail, and Appendages. — Reference has been made in the preceding chapter to the external features of familiar 38 EXTERNAL FEATURES AND RELATED STRUCTURES 39 vertebrates. Head, body, and tail regions are recognizable, with few exceptions. In all but the lowest subdivision of the Verte- brata, the Cyclostomata, there are two pairs of appendages, the fins of fishes and the limbs of amphibians, reptiles, birds, and mammals (Fig. 1, p. 10). A neck, distinguishable externally as a constriction between head and body, is found in certain reptiles and in birds and mammals. Modifications occur (Figs. 1, 2, 3, p. 10), but the same general organization persists throughout. Compare the head, body, and tail in a fish, salamander, hzard, turtle, snake, ostrich, sparrow, seal, elephant, horse, and man. In the snakes and in a small subdivision of the amphi- bians (Apoda, Fig. 6, p. 14), limbs are absent in the adult, although present as rudiments in the embryo. In a few ^_^ vertebrates the tail is rudimentary, as in ^^ | ] JjA frogs and toads, the higher apes and l/irft(^(Ci2H2oOlo)9+(Cl2H220il) soluble starch erythrodextrine maltose This reaction, which is a splitting of the starch and dextrine molecules by the addition of water, continues until the action of the ptyalin is halted by the acidity of the juices in the stomach. The food mass, softened by the water and mucin of the saliva, mixed by chewing, and with its starch components partially digested, is carried down the esophagus by the muscular movements known as swallowing. No digestive changes occur in the esophagus; it is merely a passageway. The digestive juice of the stomach is known as gastric juice and is thoroughly mixed with the food mass by muscular move- ments of the stomach wall. Because of the contraction of the pyloric sphincter, a circular band of muscle located between the stomach and the small intestine, the materials are retained in the stomach for a time. Gastric juice is secreted by the gastric glands in the lining of the stomach, and is strongly acidic because of the presence of hydrochloric acid. It contains three enzymes, pepsin, rennin, and lipase. Pepsin, the action of which is dependent on the presence of the hydrochloric acid, starts the digestion of some of the protein foods and brings about their distintegration into proteoses and peptones. Rennin, which is present chiefly in the ASSIMILATION 83 mxt Inl. stomachs of young milk-feeding animals, has a coagulating effect upon milk, and also digests milk protein or casein. The lipase of the gastric juice acts princi- pally upon lipins and emulsified fats, splitting them into glycerol and fatty acids. As the acidity of the stomach contents increases, the pyloric sphincter relaxes at intervals, permitting the expulsion of the partially digested food mass from the stomach into the small intestine. The muscular activities of the small intestine are of two types, known as peristalsis and segmen- tation (Fig. 48). In peristalsis, a contraction begins at the upper end of the intestine and passes with wave-like effect toward the lower end. This motion has a tendency to cause the passage of the food mass toward the large intestine. Segmentation, on the other hand, consists of a series of contractions occurring close yig. 47.— The digestive tract and together and simultaneously at appended structures of the frog, different levels of the intestine. b.c, buccal cavity; b.d., bile dwt-. This results in the pinching of " • """^^^ j''^*^^"''^ '•'^- '''''"' ^'!"''' '^ cL, cloaca; du., duodenum; e., position the food mass into segments; and of eye; es., esophagus; E.I., Eustachian since these segmentation con- ^^^e; o.b., sail bladder; gi glottis; " /id., hepatic duct; t.n., internal nostril; tractions disappear and reappear int., small intestine; Ig., lung; l.i., large at alternate levels they cause •"'^f'^^:'"- ''^■^'■■' "^Z' '""''"'"'^ teeth; ovd., opening of oviduct; p., pan- a very thorough mixing of the creas; p.d., pancreatic duct; pyl., pyloric intestinal contents. Food in the sphincter; sp., spleen; st., stomach; t., tongue; ur., opening of ureter; v.t., small intestine comes in contact vomerine teeth. (Redrawn with modi- with three digestive juices: bile, ^^^^t'^^^ ^'""'- ^°Tc^, "u'^Z °^ -^"^ ^ •' . ' otomy, copyright, 1902, by Macmillan which comes from the Uver by and Co., Ltd., reprinted by permission.) way of the bile duct; pancreatic juice, entering from the pancreas through the pancreatic duct ; and intestinal juice, from glands in the Uning of the duodenal region 84 THE VERTEBRATE ANIMAL: METABOLISM / \ / \ / \ \ / \ / \ Fig. 48. — To illustrate the muscular activities of the small intestuie. The three upper drawings represent a particular region of the intestine at successive periods during segmentation. The three lower drawings show a particular region of the intestine at successive periods during peristalsis. Arrows within the tube snow direction of movement of food contents. Arrows between drawings call attention to changes in wall of intestine. The vertical dotted lines in upper drawings indicate places of following contractions. ASSIMILATION 85 of the small intestine. The reactions of this region occur in an alkaline medium, owing to the presence of alkahne salts in the three juices. Bile contains no digestive enzymes and serves chiefly as an emulsifying agent for the fats. That is, fat drops in the presence of bile become divided into very fine droplets, thus offering a much greater surface for the activities of the fat- sphtting enzyme. Pancreatic juice contains three digestive enzymes: steapsin, or intestinal hpase; trypsin; and amylopsin, or intestinal amylase. Steapsin brings about the digestion of the emulsified fats into glycerol and fatty acids. Trypsin is a protein enzyme and acts to spUt the undigested or partially digested pro- tein molecules into simpler compounds known as polypeptids. Amylopsin continues the digestion of starch, begun in the mouth by ptyalin and interrupted in the stomach, and converts starch into maltose, a compound sugar. The intestinal juice contains enterokinase, erepsin, and the sugar-splitting enzjTnes. Without enterokinase, trypsin is inactive and has no effect upK)n proteins. Erepsin is the enzyme that finally completes the digestion of pro- teins into their component amino acids. Of the sugar-sphtting enzymes, maltase acts upon maltose or malt sugar; sucrase, or invertin, upon sucrose or cane sugar; and lactase upon lactose or milk sugar. The effect of these enzymes is to produce simple sugars, principally glucose or grape sugar. The final stages of digestion of all foods occur in the small intestine. Some of the materials taken in with the food cannot be changed, or for some reason are not changed. These are carried by peri- staltic contractions toward the large intestine, which is separated from the small intestine by the ileo-caecal valve. In man, about four hours elapse from the time of eating until the first part of the food mass enters the large intestine, and the discharge continues for about two hours. After being retained in the lower part of the large intestine for from ten hours to two days, this undigested and undigestible material, now known as fseces, is discharged from the digestive tract upon relaxation of the anal sphincter. This process is known as defecation, or egestion. The amount of food- stuff egested is about 10 per cent of the amount ingested. In the large intestine of some animals there are many bacteria which digest cellulose, a carbohydrate present in the walls of plant cells, and produce simple sugars from it. While some of this sugar is absorbed, it must be kept in mind that the bacteria digest this 86 THE VERTEBRATE ANIMAL: METABOLISM material for their own nutrition, and it is only incidentally that it affords nourishment for the animal harboring the bacteria. Bac- teria in the large intestine of man also cause the putrefaction of protein elements, forming products many of which are toxic, or poisonous, and which, when absorbed into the body, cause head- aches, drowsiness, and a general feeling of depression, or auto- intoxication. Some idea of the great number of bacteria present may be had when it is stated that from one-quarter to one-half of the dry weight of the faeces consists of bacteria. In summarizing the essential processes of digestion, it may be recalled that in some animals, which possess salivary glands, the digestion of starchy carbohydrates is started in the mouth by the action of ptyalin. This action on starch is continued in the small intestine by the amylopsin of the pancreatic juice, with the result that the starch is converted into maltose, a compound or disaccharide sugar. The digestion of the maltose, together with other compound sugars, is completed by the sugar-splitting enzjones of the intestinal juice, with the formation of the simple sugars or mono-saccharides. Protein digestion, started by pepsin in an acid medium in the stomach, is continued in the small intes- tine in an alkaline medium, by trypsin of the pancreatic juice, and completed by erepsin of the intestinal juice, with the formation of amino acids. Emulsified fats and lipins are split by gastric lipase; and, in the small intestine, after the emulsification of fats by bile, the steapsin from the pancreas is effective in producing glycerol and fatty acids as the end products of fat and lipin diges- tion. The entire result of digestion is the decomposition of com- plex food materials into their structural units, which are simple sugars, amino acids, and fatty acids and glycerol. These com- ponents are thus made available for absorption and assimilation in the maintenance of the protoplasmic system. Absorption. — By the process of digestion, complex food mate- rials are chemically changed into the simpler compounds that can be assimilated by the cells. Before assimilation can occur, the simple nutrients must be absorbed from the alimentary canal into the circulating fluids and be distributed to all cells. Absorption may be defined as the passing of simple food compounds through the mucous membrane of the digestive tract into the blood or lymph. Examination of the wall of the digestive tract reveals that the cavity is lined with a single layer of compactly placed ASSIMILATION 87 cells, known as the mucous membrane. Between this layer and the muscular coats of the tract is a region of loosely arranged cells with interlacing fibers, which make up the submucous layer Fig. 49. — Schematic representation of blood and Ij^mph vessels of the digestive tract. A, blood and lymph vessels in relation to entire wall of tract. B, blood capillaries in a single villus of the human intestine. C, lymph vessel from center of same, a, artery; 6.C., blood capillaries; I, lymphatic vessel; I.e., lymph capillaries; 'ui.l., muscular layers of intestine; m.m., mucous membrane; mes, mesentery; s.m., submucosa; v, vein. or submucosa. It is in this region, separated from the digestive cavity by the mucous membrane, that the delicate lymphatics and the thin-walled capillaries that connect arteries and veins are 88 THE VERTEBRATE ANIMAL: METABOLISM found (Fig. 49). Substances, in being absorbed, pass through the cellular membrane and into the fluid of these vessels. Although certain foods, such as grape sugar, water, and the inor- ganic salts, require no change before they are ready for absorption, there is practically no absorption in the stomach. In the upper part of the small intestine the exposed surface of the mucous mem- brane is greatly increased by the projection of numerous finger- like villi into the cavity. Each villus is covered by mucous mem- brane and has a core of submucosa containing capillaries and lymphatics. It is in this region that by far the greatest amount of absorption occurs. Simple sugars, amino acids, and mineral salts pass directly into the blood stream and are found there and trans- ported as such. The presence of a great deal of ammonia in the cells of the mucous membrane would seem to indicate that some of the amino acids had undergone a chemical change there, but the significance of this is noi clear. The fats and lipins are digested into fatty acids and glycerol in the presence of bile, and the fatty acids combine with certain alkaline constituents of the bile to form soaps. In this form they enter the cells of the mucous mem- brane along with the glycerol. There the soaps are broken down and the fatty acids recombine with the glycerol to form neutral fat droplets, the presence of which can be demonstrated in the proto- plasm of the cells of the mucous membrane. This fat is dis- charged into the lymph vessels of the submucosa. Water is absorbed throughout the length of the small and large intestine. No satisfactory theory to account for absorption has yet been formulated. Such a theory must explain the phenomena by which the digestive juices are passed into the cavity of the tract while solutions of the digestion products are passed in the opposite direc- tion into the blood or lymph. Osmosis, or the passing of water through a membrane to equalize concentration of solutions on the two sides of the membrane, would account for the passing of water into the digestive cavity to decrease the concentration of the food mass, but not for the reabsorption of water in other regions of the tract. Diffusion, which is the mixing of substances in such a way that regions of higher and lower concentrations tend to become equal in concentration, would explain the passage of digestion products from the tract, where they are very concentrated, into the circulating fluids which are constantly changing so that equi- librium is not estabhshed. However, it has been found that if the ASSIMILATION 89 fluid portion of the blood of an animal be introduced into its digestive cavity, this blood serum will very soon be absorbed into the circulating fluid, which must have the same concentration. It is, therefore, evident that factors other than osmosis and dif- fusion must be considered. In the absorption of fat, it is clear that the cells of the mucous membrane are not passive. The fact that the glycerol and fatty acids are resynthesized is evi- dence of protoplasmic activity. Indications of such activity are not so obvious in the absorption of other substances, but it must not be forgotten that in protoplasm we are dealing with a complex colloidal system about which we are still seeking knowledge. The enormous surfaces present in colloidal systems make pos- sible a great deal of adsorption, a well-known physical principle which is responsible for the use of finely divided charcoal in gas- masks. The gases in passing through the charcoal are adsorbed on the surface of the finely divided particles. A consideration of this phenomenon, along with other properties of colloidal systems, is engaging the attention of some investigators who seek to explain the physico-chemical basis for absorption. It is safe to say that no explanation will be found satisfactory that docs not take into consideration the metabohsm and organization of the cells of the mucous membrane. Fate of the Nutrients Absorbed from the Digestive Tract. — The fat absorbed into the Ijinph vessels in the submucosa passes, in man, by way of the lacteals running in the mesentery to the thoracic duct which empties into the left jugular vein. From there the finely emul- sified fat is carried in the blood, and with the amino acids, simple sugars, water, salts, and vitamins is distributed to all the cells in the body by way of capillary networks. The veins carrying blood from the digestive tract unite to form the hepatic portal vein, which emp- ties into the capillary system of the liver. Here the greater part of the simple sugars absorbed from the intestine leave the blood and are synthesized in the liver cells to glycogen, a complex carbo- hydrate known as animal starch. As glycogen, carbohydrates are stored in the liver, and to a lesser extent in the muscles of the body. According to the needs of other cells of the body, glycogen is con- verted into glucose, passed into the blood stream, and distributed. Interference with this control of the amount of sugar in the blood stream results in abnormal conditions, of which diabetes is the best known. The nature of this control will be discussed in a 90 THE VERTEBRATE ANIMAL: METABOLISM O 0) a a a I O 6 M Em ASSIMILATION 91 Explanation of Fig. 60 By means of blood and lymph vessels nutrients and oxygen are distributed to all cells of the body, while excretions and endocrines are collected and transported from place to place. The arrows indicate the direction of flow of the blood. A6. v., abdominal vein; .4 u, right auricle, Aft', left auricle; fi.Z)., bile duct; 5/, urinary bladder; C.A., carotid artery; C-M., ccehaco-mesenteric artery; Cp.D.T., capillaries of digestive tract; Cp.F-L, capillaries of fore-leg; Cp.Hd., capillaries of head; Cp.H-L, capil- laries of hind-leg; Cp.A'ci., capillaries of kidney; C^^.SA;., capillaries of skin; Cu. A., cutaneous artery; C.F., cutaneous vein; £>. A., dorsal aorta; iJ.r., digestive tract; F.V., femoral vein; G, gonad; Gl, glottis; Glm, glomerulus; H.A., hypogastric artery; H.P.V., hepatic portal vein; Hp.V., hepatic vein; I. A., iliac artery; J.V., jugular vein; L.H., lymph heart; Lg, lung: Lib, liver; L.S., lymph sinus; L.V., lymph vessel; P. A., pulmonary artery; Pa, pancreas; P-c.A., pulmo-cutaneous artery; Pcv, pre-caval vein; P-c.V., post-caval vein; P.O., pancreatic duct; Pel.V., pelvic vein; P. F., pulmonary vein; 7Z. P. F., renal portal vein; S, sinus venosus; S.A.. systemic artery; Sb.A., subclavian artery; Sb.V., sub- clavian vein; Sk, skin; Sk.Gl., skin bland; Sp, spleen; T.A., truncus arteriosus; Ur, ureter; F, ventricle. (Redrawn with modifications from Parker and Parker, " Practical Zoology," copyright, 1916, by Macmillan and Co., Ltd., reprinted by permission.) later section (p. 101). Fat, also, may be stored in certain regions of the body, such as the subcutaneous fascia, a layer between the skin and muscles; in the mesentery; between the muscles; and, in an animal like the frog, in special organs, the fat-bodies. As is the case with stored carbohydrate, this fat can pass back into the blood for distribution in case of cellular need. So far as we know, amino acids are not stored in any part of the body but are taken from the blood, as required, by all cells. Because of the ingestion of food, its digestion, absorption, and distribution, the protoplasm of all cells of the body receives a supply of the materials that are necessary for its metabolism. The substances brought to the cells are synthesized under the influence of cellular enz>Tnes, to form the constituents characteris- tic of the particular protoplasm in which the synthesis occur.'-. Then, finally, we have assimilation of the foods that enter the body by way of the digestive tract. Respiration. — The term respiration has been widely used to cover the so-called gaseous metaboUsm of an organism. It seems unnec- essary, and perhaps unwise, to try to separate the metabolism of oxygen from that of other substances, since all metabolic reactions are so closely inter-related. If food is to be defined as any sub- stance necessary for the normal functioning of the organism, oxygen becomes a food, just as water and mineral salts are foods. Respi- ration, then, is used here as a term covering the dehvery of oxygen to the protoplasmic system. As was pointed out in the discussion of the structure of verte- brates, respiratory systems are of two types, depending upon 92 THE VERTEBRATE ANIMAL: METABOLSIM the environment of the animal. Water-dweUing forms have struc- tures known as gills, which are richly supplied with blood, covered by a thin layer of cells and exposed to a constantly changing cur- rent of water. Air-dwelling forms, by the process of breathing, draw air, by way of the air passages, into their lungs, which are cavities separated from a rich capillary network by a cellular mem- brane (Fig. 51). In both types, oxygen passes into the blood stream; but in one case it is dissolved in water, while in the other it is free in the air. This is known as external respiration and is analogous to the absorption of nutrients from the digestive tract. As is the case with ali- mentary absorption, there is no agree- ment as to the phys- ical explanation of the proc3SS. Here, also, simple diffusion has been proposed as the underlying principle. The con- centration of oxygen in the water or air being greater than that in the blood, from which the oxy- gen is constantly removed by the cells of the body, the oxygen would diffuse in the direction of the blood to estabhsh equi- hbrium. However, the fact that the membrane is composed of protoplasmic units cannot be ignored, and all the factors governing the absorption of oxygen into the blood are not understood. In the blood stream the oxygen enters into a loose combination with hoemogJohin, an iron compound carried by the red blood cells, and is distributed throughout the body in that way. This combination with haemoglobin, which is respon- sible for the red color of oxygenated blood, is unstable, and, in the Fig. 51. — Diagram of a section through a portion of the lung. The capillaries, which have been cut across, are shown in black. (From Hough and Sedgwick, "The Human Mechanism," copyright, 1918, by Ginn and Co., reprinted by permission.) DISSIMILATION 93 regions of the capillary networks, free oxygen leaves the blood and passes into the cells. This assimilation of oxygen is designated internal respiration. The bodily processes preUminary to assimilation have now been discussed. These include ingestion, digestion, and absorption of the foods that enter the blood by way of the digestive tract, as well as external respiration, which suppKes oxygen to the blood and is dependent, in terrestrial animals, upon breathing. In addi- tion to the incorporation and synthetic processes that maintain the protoplasmic system, assimilation, in a wide sense, includes also the utihzation of reserves or stored materials, such as glj^cogen and neutral fat, and storage products which may be more tempo- rary and present in all cells. It is possible, also, that the produc- tion of certain secretions should be classed under assimilative metabolism. Dissimilation. — Oxidation. — Dissimilative metabohc activ- ities include those reactions by which protoplasmic constitu- ents are chemically decomposed for the transformation of energy and production of heat, and probably, in some cases, for the elaboration of certain secretions. The reactions that transform energy and produce heat are in the nature of oxi- dations, that is, reactions in which oxygen unites with com- pounds of the protoplasmic system. Oxidation is commonly known as combustion, or burning. When fuel burns, oxygen from the air is combined with the chemical compounds that make up the fuel, producing heat and hberating energy which can be made to do work. It was Lavoisier, in 1777, who first recog- nized the exact similarity between the combustion of fuels and oxi- dation as it occurs in the bodies of animals. The statement is sometimes made that " food " is oxidized in our bodies to produce heat and energy. It is well to keep in mind that this is true only in the sense that our organic foods, broken down into their simple structural units, are assimilated by the protoplasm of our cells. The protoplasmic constituents, synthesized from nutrients deliv- ered to the cells, are the compounds that unite with the oxygen. Oxidation of carbohydrates and fats goes to completion in the body and results in the liberation of energy, some of which is con- verted into heat. The combustion of these classes of compounds is the chief source of animal energy. Proteins are also oxidized to some extent, but the incompleteness of the reaction makes it 94 THE VERTEBRATE ANIMAL: METABOLISM impossible to regard them as primary sources of energy. These oxidations, in addition to the transformation of energy, result in the formation of chemical compounds which are of such a nature that they are no longer of use in the protoplasmic system. The combination of oxygen with carbohydrates, fats, and lipins gives rise to carbon dioxide and water, while in the case of proteins the end products are carbon dioxide, water, and a variety of nitrogen- containing compounds. These materials are the waste products of metaboUsm and are known as excretions. Excretion. — The waste products of the oxidations of carbo- hydrates, fats, lipins, and proteins are carbon dioxide, water, and various compounds containing nitrogen. These excretions must be constantly removed from the cells in order that the normal oxi- dation reactions may continue. It is well known that if ashes are permitted to accumulate, a fire will be put out by these waste products of its burning; and the continuity of all chemical reactions depends upon the removal of end products. The process of removal of the waste products of metabolism is excretion, and the places of removal are chiefly the lungs, skin, and kidneys. The waste products are carried from cells all over the body by the blood stream. In air-dwelling vertebrates, as blood passes through the lung capillaries, the carbon dioxide passes into the cavity of the lung and is exhaled. In aquatic forms, this waste gas is eliminated into the water surrounding the gills. This excretion of carbon dioxide is often included under the heading of respiration. Carbon dioxide is, however, one of the metabolic wastes and it seems more logical to include its discharge under the heading descriptive of the removal of water and nitrogenous excretions. Exhaled air is moist, because the lungs also excrete water. From the skin of man, carbon dioxide, water, certain salts, and minor quantities of nitrogenous substances are excreted as perspiration (Fig. 52). In the frog, the amount of carbon dioxide excreted by the skin is relatively large and the loss of water is considerable. The kidneys are usually spoken of as the organs of excretion, and through them about 50 per cent of the water, traces of carbon dioxide, and the principal part of the nitrogenous waste, in the form of urea, are discharged as urine. Urea is not produced, as such, in the cells throughout the body or in the kidney. It is com- pounded from ammonia in the liver, from which it is carried to the kidneys. It will be recalled that the kidney is made up of tubules DISSIMILATION 95 surrounded by capillaries and ending in a cup containing a knot of small arteries (Fig. 31, p. 53). Evidence as to the function of the various parts of the tube with reference to the ehmination is con- flicting, and final conclusions have not been reached. Some hold that, from the glomerulus, water and inorganic salts filter through Epidennia Dermis Adipose tissue Fig. 52. — Diagram of section through the skin of man showing characteristic structures. B, biilb or root of h.air, where growth occurs; C.N., capillary net surrounding sweat gland; D, dui't of sweat gland; M, erector muscle of hair; P, papilla of hair, where blood vessels and nerve endings associated with hair are found; S, shaft of hair which extends beyond surface of skin; Sb.G., sebaceous or oil gland from which secretion passes to the surface by way of the hair follicle; Sw.G., sweat gland from which perspiration is eliminated. Bowman's capsule, and that the nitrogenous compounds are passed in through the walls of the convoluted tubules. An older view was that all the constituents were filtered out at the glomerulus and that some water was reabsorbed from the tubule. Here again, the physical factors governing the passage of materials through living cells are not completely understood. 90 THE VERTEBRATE ANIMAL: METABOLISM In conclusion, it may be stated that excretion is the ehmination of compounds that have resulted from metaboHc reactions of cells. The materials appearing as excretions have been a part of the protoplasmic system. Excretion, as a process, should not be con- fused with egestion, which is the elimination of undigested or undigestible substances from the digestive tract by way of the anus. Most of the egested materials have never been within the cells of the animal from which they are discharged, but some water and the substances giving odor and color to the faeces may have been. Secretions. — It has been pointed out that the protoplasmic constituents are produced, within cells, from the end products of digestion by the reactions of assimilation. Oxidations that break down compounds in the protoplasm liberate energy, furnish heat, and give rise to waste products known as excretions. In addi- tion to these types of reactions, certain substances that are necessary for the performance of bodily functions are formed in protoplasm. These substances are called secretions, and the process of elaborating and passing them out of cells is termed secretion. There is no evidence to indicate whether these com- pounds are made from food materials entering the cells or from intermediate products in dissimilation. It is possible that both types of reactions occur. Secretions, in the nature of enzymes catalyzing protoplasmic reactions, are formed within all cells and used within the cell where they are formed. Certain cells, however, produce secretions that go out of the cells to be used elsewhere in the body. The juices that are secreted into the digestive tract have already been mentioned. These contain materials that serve to soften the food, to render it acid or alkaline, to change its physical state, as in the case of emulsification of fat by bile, and to catalyze the chemical reactions of digestion. These juices are produced in groups of cells composing the digestive glands, and the secretions are emptied into the digestive tract by way of ducts or slender tubes (Fig. 53). The oil glands of the skin likewise discharge their secretions by way of ducts. Sweat glands and kidney tubules are often said to secrete perspiration and urine; but since the substances passed out are not built up in these regions, and also since they are in the nature of waste products, the processes are not functionally comparable. It is, therefore, better to say that the kidney excretes than to say it secretes urine. SECRETIONS 97 There are a number of glands which do not have ducts by which to discharge their secretions, but which pass their products into the blood stream. Such glands are known as the ductless glands, glands of internal secretion, or endocrine glands; and their secre- tions are called internal secretions, or endccrines, tjecause they are never passed out on surfaces such as the digestive canal or skin (Fig. 54). A well-known example of the way in which endo- crines function in coordinating bodily processes is the control Alvioli Fig. 53. — Diagram of a gland with a duct which passe.s to some surface where the secretion is di.schargcd. (From Hough and Sedgwick, "Human Mechanism," copyright, 1918, by Gion and Co., reprinted by permission.) of the flow of pancreatic juice. This fluid is not passed into the small intestine continually but only when food is present to be digested. It was for a long time supposed that this was due to nervous control. Experimentation revealed the fact that the nerves leading to the pancreas could be cut without affecting the control of flow of the pancreatic juice. It was then discovered that if the blood vessels were tied so that blood did not flow into the pancreas the digestive juice was not discharged when food entered the intestine. Further study revealed that when the 98 THE VERTEBRATE ANIMAL: METABOLISM contents of the stomach enter the small intestine the hydrochloric acid stimulates certain cells in the intestinal mucosa to discharge into the blood an endocrine substance known as secretin. The secretin, reaching the pancreas by way of the blood, serves to stimulate the pancreatic cells to secrete pancreatic juice, which then passes into the small intestine by way of the pancreatic duct. Endocrinology is a relatively new field of study and informa- tion is incomplete concerning many of its phases. The results of investigations are rather confusing in many cases, because of the Fia. 54. — Diagram of a section through the thyroid gland, which does not have a duct. cp, blood capillary which carries the thyroxin secreted by the cells of the follicles ; e.t., connective tissue; se, secretion stored in follicle; t.f., follicle or secretory portion of thyroid. great difficulty in identifying abnormalities in structure or func- tion as being directly due to particular substances. The glands of the endocrine system seem very closely inter-related, and normal function is secured when all are acting in a state of balance. Certain effects are, however, particularly referable to individual glands. The nature of the function of these endocrine glands is determined by removing them from experimental animals, by injection of endocrine extracts into the circulating fluid, by feeding, and by clinical observations in cases of diseased glands. The most important endocrine glands, as now known, are the thyroid, adrenals, pituitary, pancreas, gonads, pineal, thymus, and para- SECRETIONS 99 Fig. 55. — The thyroid thyroids. In addition, there are more or less isolated cells, such as those that produce secretin. The thyroid gland is located on the sides of the trachea pos- terior to the larynx (Fig. 55). Its removal causes death in all cases after a few weeks. However, death can be prevented, and normal function secured after removal, by grafting in a thyroid from another animal. This grafting can be done successfully in any region of the body that affords an abundant blood supply. Injection of thyroxin, as the endocrine of this gland is called, or feeding of thyroid will also prevent death after thyroidectomy, but must be repeated at regular intervals in order to maintain life. Extreme deficiency of thyroid secretion in children results in the disease known as cretinism (Fig. 56) in which neither physical nor mental development is normal. Cretinism can be remedied bv admin- istration of the endocrine from other gland of man showing iti- animals if this treatment is started before position with reference to the individual is too old. Subnormal ^^^ ^'^^-^'^^ ^^'^ trachea, thyroid secretion in adults causes a con- ■^■larynx; r, trachea; Thy, dition known as myxoedema, strikingly characterized by swollen, distended, dry skin, and resulting in mental impairment. Here again, administration of extracts is helpful in some cases. On the other hand, the thyroid may over-secrete; in extreme cases this results in exophthalmic goiter, characterized by enlarged thyroid, rapid pulse, moist skin, protruding eye-balls, and nervous symptoms of excitability and restlessness. This condition may be remedied by removal of a part of the gland so that the amount of secretion is reduced to normal. The adrenal bodies, which in the frog lie on the ventral surface of the kidneys, and in higher vertebrates come to lie anterior to the kidneys, are composed of two regions, a central medulla and a surrounding cortex (Fig. 57). These two regions differ in func- tion, but both are seats of endocrine secretion. The secretion of the medulla, known as adrenalin, is important in increasing the flow of blood carrying augmented sugar supply in cases of 100 THE VERTEBRATE ANIMAL: METABOLISM Fig. 56. — Individuals suffering from thyroid deficiency. The effect of thyroid treatment on the cretin to the left is shown in the central figure. An untreated cretin is shown at the right: age 28, height 34 J inches. (Figures to left and in center from Srhafpr, "The Endocrine Organs," copyright, 1916, by Longmans, Green and Co., reprinted by permis- sion. Figure to right from Murray, "Diseases of the Thyroid Gland," copyright, 1900, by H. K. Lewis and Co., reprinted by permission.) m.o. Fig. 57. — Portion of the medulla of the adrenal gland. cap, blood capillary which carries the adrenalin secreted by the medullary cells; m.c., medullary cells. SECRETIONS 101 muscular emergency. This is due to its effect upon the smooth muscle in the walls of the blood vessels. The cortex secretion is concerned with the regulation of the secondary sexual characteristics, notably the growth of hair. Like the adrenals, the pituitary gland, located ventral to the dien- cephalon, is composed of two parts, the anterior lobe being formed from the hypophysis, and the posterior lobe from the infundibulum (c/. p. 65). The secretion of the anterior lobe, known as tethelin, is active in regulating growth, particularly that of the long bones. Abundant over-secretion results in gigantism in youth and acromegaly when it occurs in adults (Fig. 58). The secretion of the posterior lobe is called pituitrin, and has a marked effect upon smooth muscle fibers, similar to that of adrenalin. The pancreas is composed of cells differing in their functions. The pan- creatic juice, which has already been mentioned, is passed into the small intestine by way of the pancreatic duct and is important in digestion. In ad- dition to the cells elaborating this secretion, there are, scattered through- out the pancreas, groups of cells, known as the islands of Langerhans, producing the endocrine insulin which is discharged into the blood (Fig. 59). Insulin has the important function of regulating the storage of sugar b}' the liver cells. If this secretion is absent, sugar is pres- ent in excessive amounts in the blood and conditions of glycosuria and diabetes result. Injections of insulin have been effective in correcting diabetic symptoms, and its isolation and the technique of its use constitute a recent Fig. 58. — Individual in which gigantism has re- sulted from over-secretion by the pituitary gland. Note the narrow chest, length of arms and legs, and large size of hands and feet. Height, 8 ft., 3 in. (From Gushing, "The Pituitary Body and its Dis- orders," copyright, 1912, by Lippincott and Co., reprinted by permission.) 102 THE VERTEBRATE ANIMAL: METABOLISM medical achievement. As is the case with all endocrine injections, the dosage must be continued as long as results are desired. In the case of the gonads, or sex organs, there is evidence of the production of endocrine secretions by both ovaries and testes. These secretions are responsible for the control of the secondary sex characteristics, but as yet there is too much confusion for assignment of definite functions. Although the effect of other endocrine secretions in association with those of the gonads is Fig. 59. — Section through pancreas showing an island of Langerhans sur- rounded by acini. A, pancreatic acinus, the cells of which secrete pancreatic juice; C, blood capillary which carries the secretion of the cells of the island; 1, cells of the island of Langerhans which secrete insulin; L, lumen of acinus by way of which pancreatic juice passes to the pancreatic duct. estabHshed, the exact relationships are still to be solved. The confusion in data makes it impossible to draw definite conclusions. From this brief sketch, in which some of the glands have been treated individually, the student should not get the impression that each member of the endocrine system acts alone. In cases of " imbalance " a number of glands are always concerned, possibly the entire chain. It is well to remember that the examples given are the result of serious divergence from the nonnal. A vast series of deviations from the typical human reactions seem BLOOD AS THE COMMON CARRIER 103 explicable as disturbances of function in a delicate^ balanced endocrine system. It is probably true that these effects are in the last analysis due to disturbances in the metabolic reactions of the protoplasm, which are dependent in some way upon the presence in certain amounts of the several endocrine secretions. Blood as the Common Carrier. — In this chapter the main topic of consideration has been metabolism, or the chemical phenomena characteristic of the protoplasm found in the cells making up the bodies of animals. In complex multicellular ani- mals, such as the vertebrates, it is impossible for each cell to obtain its food materials directly from the animal's surroundings, or to discharge its excretions into the external environment. Conse- quently, we find cells organized into organs and systems which function in such a way that the individual cells may be able to carry on the metabolic reactions that are responsible for the animal's life. The digestive system serves to prepare nutrients, other than oxygen, so that they can be assimilated by the proto- plasm; respiratory organs obtain oxygen for the organism; excretory organs are the places of discharge for the waste products of metabolic reactions; and endocrine glands produce secretions essential for metabolic balance. Blood and lymph, the circu- lating fluids of the body, place these systems in intimate relation- ship with the individual cells in which assimilation and dissimila- tion occur. The blood, circulating in a system of closed tubes, is composed of a liquid, the plasma, in which float two kinds of cells : red blood cells, or erythrocytes; and white blood cells, or leucocytes. Lymph consists of blood plasma that filters out of the thin-walled capil- laries into spaces between the cells. White blood cells migrate out through the capillary walls and are found in the lymph. Reference to the diagram (Fig. 60) will show that lymph is collected by the dehcate vessels known as the lymphatics (Fig. 20, p. 41). These finally discharge their contents into the veins. The individual cells of the body are surrounded by lymph which is derived from the blood, and, by means of these two fluids, materials are trans- ported to and from all cells. When the simple sugars, amino acids, vitamins, water, and mineral salts are absorbed from the small intestine they pass into the plasma of the blood. Fats and lipins, absorbed into the Ijonph, are soon discharged by way of the great veins into the blood plasma for distribution. Oxygen, 104 THE VERTEBRATE ANIMAL: METABOLISM from the organs of external respiration, enters into combination with the haemoglobin of the red blood cells. These materials, which are necessary for maintenance of the chemical reactions of the protoplasm, are carried into capillary networks all over the body, and pass from the blood into the lymph and so into the cells where they are used. Carbohydrates and fats may be left in large amounts at storage depots, to be picked up later for distribution. Fig. eO.^Schematic figure of blood capillaries and lymphatic vessels in relation to cells throughout the body. A maximum of intercellular space is shown. a, artery; b.c, blood capillaries; c, cells; i.c.l.s., intercellular lymph spaces; phatic vessel; I.e., lymph capillaries; v, vein. /, lym- Carbon dioxide, water, and nitrogenous wastes, produced by oxida- tions within the cell, pass out into the lymph and so into the blood plasma in the capillaries. Water and carbon dioxide are carried directly to places of excretion; nitrogenous wastes are trans- ported to the liver where the urea is formed, to be taken to the kidneys. The exchange of foods and excretions between blood and cells by way of the lymph can apparently be adequately BLOOD AS THE COMMON CARRIER 105 explained on the basis of diffusion from regions of higher to those of lower concentration. Toxic products of intestinal putrefaction are carried in the blood plasma to the hver, lungs, and skin, from which they are discharged. Secretions of the endocrine glands are distributed throughout the body to produce chemical coordinations between metabolic processes in different regions. Finally, in passing through parts where oxidations are occurring rapidly with production of heat, the blood becomes heated, an-d thus acts, in warm-blooded animals, as the agency for heat trans- mission and equalization. In summary, the blood as a common carrier performs four functions: first, it carries the nutrient substances, including simple sugars, amino acids, fats, lipins, vitamins, water, inorganic salts, and oxygon, to all the cells of the body; second, it removes the waste products of metabolism, carbon dioxide, water, and nitrog- enous compounds, from all cells; third, it distributes the endo- crine secretions, the chemical coordinators, to all body cells; and fourth, it transfers heat from regions of rapid oxidation to those of a lower rate. An understanding of the functions that have been discussed in the present chapter must be based on an understanding of the necessities of cells as units of function. Complicated systems of organs exist for the maintenance of metabolism in cells through- out the body. If the physiology of metabolism in vertebrate animals is considered and mastered from the point of view of the chemical reactions occurring in its indi\'idual cells, the study of function in other types of animals will be merely a reaffirmation of the facts brought out in this chapter. The protoplasmic require- ments and reactions are assumed to be in general the same through- out the animal kingdom. Another important fact to keep in mind is that, while the cell is the unit of function, the reactions of the animal as a whole depend upon the coordinated activities of all cells — upon what we shall call the physiological balance of the organism. CHAPTER 5 PHYSIOLOGY OF THE VERTEBRATE ANIMAL: IRRITA- BILITY Metabolism in the vertebrate animal has been discus.sed in terms of the chemical reactions in the protoplasm of cells. In such a complex organism there are systems of organs, each speciahzed and functioning in some particular way to make metabohsm possible in all the cells of the body. Thus, in the digestive system, food is disintegrated into its simple compounds by means of secretions from various glands, and is absorbed into the vascular system, by way of which it is transported to all parts of the body. The respiratory system and the circulation of the blood provide for oxygen intake and distribution. Again, the blood, together with the organs of excretion, eliminates waste products of metabolism. An organism cannot live if any one of these systems ceases to function, and it becomes abnormal if the activities of its organs are not correlated in the usual way. That is, the manifestations of life by the animal depend upon the coor- dination of its many systems. We have seen that in some cases the unification of functions is brought about by endocrines secreted by certain glands and cai-ried in the blood. This is to be thought of as a chemical coor- dination. Another and a far more important integrating mechan- ism is the nervous system. Both chemical and nervous coor- dinations are dependent on irritabihty, which has been defined as the capacity of protoplasm to respond by internal reaction to stimuh or changes in the environment. All protoplasm is irritable. One problem of the multicellular animal is that of keeping individ- ual cells in touch with the environment of all the cells. This has been solved by means of the sense organs, with their highly differ- entiated capacities for sensitivity to changes in the environment; and by the nervous system, which is speciahzed for conductivity. The nervous system of a vertebrate may be divided into two REFLEX ACTION 107 main parts, the peripheral nervous system and the central nervous system. These regions are, oi course, continuous, since the peripheral nervous system, which consists of the cranial, spinal, and autonomic nerves, serves to connect the central nervous system, or the brain and spinal cord, with other systems and organs of the body. The structure and relationships of the divisions of the nervous system have been described in Chapter 3 and should be reviewed in this connection. As our consideration of this phase of physiology progresses, the student must not lose sight of the fact that we are discussing the activities of the nervous system both with respect to the irritability of individual cells and with respect to the coordination of the animal as a whole. Reflex Action Simple Reflexes. — Every nervous coordination is the result of a reaction by some part of the body to a stimulus received by some other region. The simplest type of response is that known as reflex action. This term is derived from the analogy that can be drawn between the most primitive reflexes and the reflection of light from a mirror. When one touches a hot stove with one's finger the muscles of the arm react to withdraw the hand. Some- thing passes from the point stimulated to the central nervous sys- tem, and is passed back to produce the contraction of the arm muscles. Another case is that of the knee-jerk reflex in which the leg is extended as the result of a sharp tap below the knee cap. In both of these examples the response to the stimulus is apparent in the same general region that received the stimulus. This is somewhat comparable with the reflection of hght by a mirror, in that the central nervous system seems to reflect the effect of the stimulus. Complete analysis of these simple reflexes shows that they are but the expression of a nervous mechanism which some investigators beUeve explains all nervous coordination. The origi- nal meaning of the term reflex has, therefore, been extended. The cellular structure within the nervous system that fur- nishes the mechanism for reflex action is known as the reflex arc. A nerve cell, or neurone, has a nucleus surrounded by cytoplasm as have most cells, but this cytoplasm is extended as two or more processes (Fig. 82, p. 148). The processes from two or more cells meet one another at their ends, but do not fuse. The places of con- 108 THE VERTEBRATE ANIMAL: IRRITABILITY tact are known as synapses and furnish the basis for what is known as functional continuity in the nervous system. Extensions of one cell may have synapses with those of many other cells so that nervous connections become very complicated, as will be shown later. However, in the simplest reflex arc (Fig. 61 A) there may be only two neurones involved. The stimulus is received by some specialized group of cells constituting a receptor, which is a general term for any type of sense-organ. As a result of the recep- tion of the stimulus, what is known as a nervous impulse, or exci- tation, is transmitted from the place of stimulation toward the central nervous system, over a nerve-cell process. In the simplest reflex arc, the impulse will travel to the spinal cord. The neurone over which the impulse enters the spinal cord is a sensory or afferent neurone, and is one of the nerve cells that are located in the dorsal root ganglion of the dorsal or sensory root of a spinal nerve. One of the processes of the afferent neurone enters the gray matter of the spinal cord and comes in contact with processes of other nerve cells located there. In the case under consideration the impulse would pass through the synapse between the process of the sensory and a process of a motor or efferent neurone and leave the spinal cord by way of a nerve fiber in the ventral or yyiotor root of the spinal nerve. The process of the efferent neurone extends to a muscle on which it terminates. The place of contact between a nerve fiber and a muscle is known as a neuro-muscular junction (Fig. 70, p. 127). It is by way of such a contact that the impulse is discharged and pro- duces a reaction in the protoplasm of the muscle cell. This reac- tion is the result of, or the effect produced by, the stimulus received. The muscle is, therefore, known as the effector in the reflex arc. The parts of the simplest type of reflex are the receptor, where the stimulus is received; the afferent neurone, over which the impulse passes to the spinal cord; the efferent neurone, over which the impulse passes from the spinal cord; and the effector, where the reaction to the stimulus occurs. When acid is applied to the skin on a frog's back the first response is a contraction of the muscles of the body wall in that region. This is a simple reflex action. The receptor in this example con- sists of certain cells in the skin ; the afferent neurone is one of the nerve cells lying in the dorsal root ganglion of the spinal nerve that supplies this particular region of the skin; the efferent neu- REFLEX ACTION 109 Fig. 61. — A. Diagram of a cross-section of the spinal cord showing a pair of spinal nerves and the essential parts of a reflex arc. The neurones neces- sary for the simplest tv-pe of reflex action are present on the left, while those of a typical reflex arc are represented on the right. B. Diagram of spinal cord to show some relations of neurones in reflex arcs. The brain would lie to the right as the diagram is constructed. Arrows in both diagrams indicate the direction of transmission of the nervous impulses. a.n., afferent or sensory neurone; ad.n.\, adjuster neurone, transmitting impulses ven- trally; ad.n.:, adjuster neurone, transmitting impulses longitudinally; ad.nx, adjuster neurone, transmitting impulses from one side of the cord to the other; d.r., dorsal or sensory root; d.r.g., dorsal root ganglion; E, effector; e.n., efferent or motor neurone; g.m., gray matter; R, receptor; s.n., spinal nerve; v.r., ventral or motor root; w.m., white matter. 110 THE VERTEBRATE ANIMAL: IRRITABILITY rone is one of the nerve cells of the gray matter of that part of the spinal cord; and the effector, a certain muscle lying under the region of stimulated skin. The knee-jerk reflex in man is perhaps even simpler. When the tendon of the muscle that extends the knee is tapped sharply just below the knee cap, the stimulus is received by receptors in the tendon. The impulse is transmitted to the spinal cord over an afferent neurone, and, passing through a synapse in the gray matter, is carried back by way of an efferent neurone to the muscle of the region. This effector, by its reac- tion to the discharge of the impulse, produces the extension of the knee. In both of these examples the impulse is transmitted to an effector which is in the same region as the receptor. The analogy to light reflection is apparent. In the great majority of cases of reflex action, we find the effect produced at some part of the body other than that at which the stimulus is received. If the skin of a dog's back be rubbed with a pointed implement, the animal will respond by attempting to scratch the place of stimulation with its hind leg. The receptors in this instance are located at the roots of the hairs of the region of the back that is stroked. The afferent neurone carries the impulse to the spinal cord over the dorsal root of the spinal nerve that supplies the skin region involved. Within the gray matter of the cord, the sensory neurone has a synapse with a neu- rone of which both the cell body and the processes lie entirely inside the spinal cord. Over the processes of such a neurone the nervous impulse passes posteriorly along the spinal cord to the level of exit of the nerves of the hind legs. Here a synapse occurs with an efferent neurone and the impulse leaves the spinal cord over the ventral root of a spinal nerve, passing to muscles (effectors) that produce the scratching motion. In this type of reflex there are three neurones concerned (Fig. 61 A). The neurone along which the impulse passes within the spinal cord is known as the adjustor neurone. Adjustor neurones are very numerous in the central nervous system and make possible the varied reactions that a single stimulus can produce. For example, when acid is applied to the skin of a frog's back, the first reaction, as has been pointed out, is a contraction of the body-wall muscles in the region stimulated. Very soon, however, this is followed by other reactions which can be easily observed in a frog from which the brain has been removed (Fig. 61 B). Th^ fore leg on the side REFLEX ACTION 111 stimulated will move toward the location of the acid, and this will be quickly followed by movements of the hind leg on the same side. These movements, tending to remove the source of stimulation, are made possible by the passage of the impulse over adjuster neurones that transmit it posteriorly to efferent neurones leading to muscles of the hind leg. The reactions described occur on the side of the animal to which the acid has been applied. If, under such conditions, the hind leg that is contracting be prevented from moving, the hind leg of the opposite side will respond to the original stimulus by contracting. This effect is the result of adjuster neurones that carrj' impulses from one side of the spinal cord to the other and make possible bilateral coordination. Compound Reflexes. — In the examples given we have been concerned with isolated reflexes. That is, particular reflex arcs have been discussed as if they were separable from the remainder of the nervous system. Such is obviously not the case. In fact, if any reflex reaction is completely analyzed it is found to be dependent upon many reflex arcs. This compounding of reflexes, or the interaction between reflex arcs, is the principal method of nervous coordination (Fig. 62). Any organism at any particular moment is being subjected to many different kinds of stimuli; yet its reactions are orderly and exhibit correlation. One may say, without becoming involved in too theoretical a consideration, that if more than one reflex arc be stimulated at the same time one of two things may happen. The reflexes may combine for the production of a harmonious effect, in which case they are said to be allied reflexes. In contrast to this condition, cases are found where stimuli that occur simultaneously do not produce reflexes that reinforce one another. Instead, one of the reflexes may prevent the others from becoming effective, that is, it may inhibit the others. Such a reflex is said to be antago- nistic with respect to the others. When an antagonistic reflex or group of reflexes occurs, such reflexes dominate the animal's reac- tions until the}^ in turn are inhibited or their stimulus is removed. The succession of reflexes, or their occurrence in sequence, is very well understood in some situations and must be a very important factor in reflex coordination. The procedure by which a frog obtains its food involves a sequence or chain of reflexes which has been analyzed. The visual stimulus produced by a moving insect brings about the protrusion of the tongue. If the insect 112 THE VERTEBRATE ANIMAL: IRRITABILITY is captured, its contact with the Uning of the mouth stimulates the closure of the mouth. This, in turn, sets in operation the swallowing reflexes, which occur in sequence. In the cases considered so far, the response to the stimulus has been studied with respect to the usual external conditions that A.N. A.N. -^ C E.N. E.N. A.N. AD.N./; B E.N. ^IG. 62. — Diagrams showing relations of neurones in different reflex arcs. A. A simple reflex arc. B. A typical reflex arc showing possibihty of longitudinal transmission. C. A chain of reflex arcs. D. Compound reflex arcs. If the two effectors produce similar action the reflexes are alUed. If the action of the effectors should )>e opposed one or the other is inhibited and the reflexes are antagonistic. E. Com])Oimd reflex arcs. Here impulses from t.vo receptors use as a "final common path" the efferent neurone to a single effector. A.N., afferent neurone; AD.N., adjuster neurone; C, central nervous system; E, effec- tor; E.N., efferent neurone; R, receptor. (.Redrawn with modifications froia Herrick, "Introduction to Neurology," copyright, 1915, by W. B. Saunders Co., printed by per- mission.) produce the effect. Pavlov discovered that it was possible to produce what he termed " conditioned " reflexes. For instance, the flow of saliva is a reflex action stimulated normally by the sight of food. Under experimental conditions, a bell may be rung whenever food is given to an animal. After a number of such experiments, the mere ringing of the bell, without the sight of food, REFLEX ACTION 113 will result in the secretion of saliva. In this way a stimulus that originally had no effect upon the salivary glands has been associated with one to which the glands did respond. As a result of this asso- ciation, the previously indifferent stimulus becomes effective in producing the reaction. A conditioned reflex has been established. Experiments and analysis of conditioned reflexes make it clear that a very large number of our adjustments are the result of such correlations. Our responses to warning colors, signals, and nationally used signs and symbols are all conditioned reflexes. The same explanation holds for many more subtle and less widely understood adjustments. This compounding of simple reflexes into alHed, antagonistic, and chain reflexes, any or all of which may apparently be con- ditioned, constitutes what is known as the " behavior " of the animal. The study of certain fields of animal reactions has indi- cated that behavior is dependent upon the " pattern " and " order " of the reflexes. Pattern is used to indicate the number of simple reflex arcs involved in the compounding and their locali- zation in the nervous system. A study of pattern is essentially one of the " morphology " of behavior, or the tracing of possible pathways of transmission of impulses. The study of order uncovers the time relations that exist in the succession of the pattern elements or simple reflexes. It is generally recognized that certain forms of behavior, known as instincts, are inherited. The nest-building and migratory instincts of birds, for instance, can be explained only on the assumption that both the pattern and the order of the reflexes in- volved are inherited. Defense instincts of many young animals furnish other examples of inherited behavior. Reactions called emotions, such as fear, rage, and love, seem also to be illustra- tions of the inheritance of both pattern and order. The consideration of behavior also involves a discussion of habit. In habits, both the pattern and the order of reflexes are acquired during the individual's lifetime. Habits of walking, dressing and undressing, eating, and talking are examples in which the pattern and order of reflexes may be determined very early in hfe. In learning to manipulate a machine one establishes reflexes of a particular kind and order. One has to make only a superficial analysis of the routine procedure of operating an auto- mobile, or writing on a typewriter, to trace the formation and 114 THE VERTEBRATE ANIMAL: IRRITABILITY seriation of the reflexes involved. Learning is the putting together of a series of reflexes which, when they are finally established, become a habit. The retention of habits involves memory of some sort. In any detailed discussion of behavior that takes account of the distinctions between emotions and instincts, the formation and retention of habits, and the inter-relations that exist between these divisions, the student must follow the investigations and argu- ments of the psychologists in order to form his opinions. It can be said, however, with an increasing degree of certainty, that analysis of nervous coordination in all its complications depends on knowledge and understanding of the reflex arc. Principle of the Common Path. — The examples of reflex action that have been given are representative. It can be seen that, begin- ning with the simplest type of reflex arc, in which one specific response occurs as the result of a stimulus, the series becomes increasingly complex as the result of the interpolation of one or manyadjustor neurones between the afferent and efferent neurones. Both simple and complex reflexes may enter into allied and antago- nistic combinations or form chains for purposes of coordination. Patterns and orders of compound reflexes are transmitted from gen- eration to generation and give rise to instinctive correlations. The mechanism of the reflex arc obviously makes possible the highest degree of coordination. Sherrington has generalized the facts of nervous coordination in his '' principle of the common path." Each afferent neurone is a special pathway by which impulses from its particular receptor enter the central nervous system. Within the central nervous system the impulse may travel over varied paths, formed by synapses between adjustor neurones, and, theoretically, may produce a reaction in any of the effectors. The efferent neurones, over which impulses travel from the central nervous system to the effectors, differ from the afferent neurones in that they are not private paths for particular impulses. It is a commonplace that many different kinds of stimuli can produce the same reaction or effect. Consider, for example, the many and varied stimuli to which man responds by walking. The efferent neurone is, therefore, a " common path " over which impulses from receptive regions all over the body can be discharged at a particular effector (Fig. 62 E). By means of the adjustor neu- rones of the central nervous system, connections are made pos- LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 115 sible between all the special paths that lead from receptive areas and these final common pathways to effector regions. The con- duction of impulses according to this principle of the common path furnishes the mechanism for the complicated and varied responses that characterize nervous coordination. By means of this mechanism the animal is enabled to behave as a unit in its reactions to the numerous and changing conditions of its environ- ment. Localization of Function in the Nervous System Up to this point, the analysis of nervous coordination has been made in terms of the reflex arc. No particular emphasis has been placed on the position of the neurones involved in these arcs with reference to the morphology of the nervous sj-stem. It now becomes interesting to understand the functions of the different parts of the vertebrate nervous system. Peripheral Nervous System. — The peripheral nervous system consists of spinal, cranial, and autonomic nerves. The general function of these parts of the peripheral system is the transmission of nervous impulses to the central .system from receptive regions, and from the central system to the effector organs. In the case of the spinal nerves, it was pointed out that processes of afferent neurones entered the spinal cord over the dorsal roots of spinal nerves, while the processes of efferent neurones passed out along the ventral roots. Such nerves are called " mixed nerves " and may be considered to represent the primitive condition of nerve trunks. Certain of the cranial nerves, as the third or oculomotor, also carry fibers of both sensory and motor neurones. Others carry processes of but one type of neurone. The eighth cranial nerve is made up entirely of processes of afferent neurones from the auditory receptor; while the eleventh and twelfth cranial nerves, found in the higher vertebrates, contain only efferent or motor fibers to striated muscles. Finally, the autonomic nerves, which include the sympathetic system, are entirely efferent, and con- stitute the " final common paths " to glands and to the muscles of the blood vessels and viscera. Central Nervous System. — The central nervous system is composed of the spinal cord and brain. As has been repeatedly imphed in the discussion of the reflex arc, the general function of the central nervous system is the adjustment of incoming to out- 116 THE \'ERTEBR.\TE ANI^LU.: IRRITABIUTY going impulses. It is in the central s^'stem that afferent neurones have s\-napse^ with adjuster neurones and these in turn with effer- ent neurones. The multiphcity of connections thus made possible furnishes the most important part of the mechanism of integration. It is desirable to consider the nature of the adjustment in the dif- ferent regions of the central ner\-ous system (Fig. 63). Adjustor neurones in the spinal cord are concerned with the simpler and less comphcated of the reflex arcs. In the " scratch "' reflex, for instance, adjustor neurones carr*- the impulse down the spinal cord or may transmit it from side to side. Impulses enter- ing the cord over spinal nerv^es can also pass upward to the medulla, cerebellmn, and diencephalon. The cell bodies of these adjustor neurones are located lq the gray matter of the spinal cord, while their processes, which transmit impulses up and down the cord, are to be found in the fiber tracts of the white matter. The white mat- ter also contains groups of nene fibers which arise from adjustor neurones located in the cerebral cortex, mesencephalon, and medulla. The gray matter of the cord is, therefore, the seat of adjustor neurones which connect dift'erent levels of the cord with one another and with parts of the brain, and which carr^- mipulses from one side of the cord to the other. In addition, the efferent neurones, which send processes out through the ventral roots of spinal nerves, are found in the gray matter of the cord. The spinal cord adjusts simple reflexes and transmits impulses to and from the brain. The primitive brain in vertebrates is comp>osed of the telen- cephalon, diencephalon, mesencephalon, metencephalon, and myel- encephalon. These parts constitute the so-called " brain stem." The changes in the direction of greater brain complexity occur chiefly in the region of the telencephalon and metencephalon. The cerebral hemispheres arise from the dorsal wall of the telencephalon and, by outgrowth and folding, become the most conspicuous part of the brain in mammals {cf. Figs. 39, p. 67, and 288, p. 532). The cerebellum is the dorsal development from the metencephalon. The cerebral hemispheres and cerebellum are the parts in which new functions are added; while the functions of the brain stem remain, for the purposes of this discussion, constant throughout the verte- brate group. In contrast to the arrangement of the white and gray matter of the spinal cord, ■^'ithin the brain the gray matter, which consists of groups of adjustor neurones, occurs in masses, LOCALIZATION OF FUNCTION IN THE N'ERVOUS SYSTEM 117 Peripheral Sensory TranBinission B- Olfactory Nerve (1) Optic Nerve (2) Auditor>- Nerve (b) Cranial Nerves (5.7,y & 10) Spinal Nerves (Dorsal Roots) Central Transmission and Adjustment Telencephalon (Olfactory Lobes and Cerebrum) Olfactory Center I. i^pliralJSprtejc 1 1 Sensory Centers Association 1 Motor Centers \ Centers J—Ai. r V C I Audi "A Diencephalon Pain Center A ? - Mesencephalon (Optic Lobes) ^ , itory Center , Optic Center , ^ ■^ Metencephalon (Cerebellum) I Equilibratory i Bilateral Muscu-i , C^enter j lar Coordination \y I Center ^ (.1 ^ /. Myelencephalon (Medulla) Visceral Control Centers (Respira- i tory movements. Heart rhythm, i Blood Vessels, etc.) Spinal Cord Peripheral Motor Transmission Cranial ■^E Nerves (3&4) Cranial Gray Matter ^ _ A_ I Fiber Tracts (White Matter) ■^E Nerves (5.6.7.9.10 11&12) Spinal Nerves (Ventral Roots) Fig. 63. — Diagram to illustrate some of the possibilities of nervous coordina- tion. No attempt is made to indicate specific reflex arcs. Impulses travel from receptors to central nervous system and thence to effectors as indicated bv the lines and arrows. A, adjustor neurone; E, effector; li, receptor. 118 THE VERTEBRATE ANIMAL: IRRITABILITY known as centers, which are completely surrounded by white matter, as in the medulla, or form a continuous peripheral layer, as in the cerebrum (Fig. 64). It is impossible to give here a detailed account of the functions of the parts of the brain, but the more important localizations may be given. The medulla, into which the spinal cord merges, serves as a Fig. 64. — Diagrams of cross-sections of different regions of the central nervous system to show the distribution of white and gray matter. The regions of gray matter are stippled. A, spinal cord; B, medulla; C, cerebrum. pathway between the cord and other parts of the brain. It also contains the centers that control the reflexes of the tongue and of breathing. In the case of the tongue reflexes, afferent neurones enter over the fifth and ninth cranial nerves, and motor neurones pass out along the twelfth nerve. The adjuster neurones lie within the medulla. The respiratory reflex depends upon the sensitivity of the respiratory center in the medulla to the amount LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 119 of carbon dioxide in the blood. Efferent neurones pass to the mus- cles between the ribs and those of the diaphragm. The rhythm of breathing is likewise controlled in the medulla by way of afferent neurones from the walls of the lungs. The medulla also controls other reflexes of the viscera, pharynx, and larynx. At the level of the auditory nerve, certain reflex adjustments of the head and neck to the reception of sound stimuli are made in the medulla. The ventral part of the metencephalon consists of fiber tracts that transmit from side to side as well as those connecting lower and higher levels. In the cerebellum, or dorsal part of the meten- cephalon, there are important muscle-coordinating centers. These include adjustments involving the body as a whole, such as the reactions occurring in response to stimuli received by the organs of equilibration, the semicircular canals. The adjustments that result in bilateral muscular coordinations are also made in the cerebellum. Such bilateral coordinations are chiefly those of the voluntary movements of the limbs, although the muscles of the eyes, facial expression, and mastication are also believed by some investigators to be bilaterally correlated by centers in the cere- bellum. Finally, the normal tension of voluntary muscle, or what is known as " muscle tonicity," is governed by cerebellar centers. On the dorsal surface of the mesencephalon are found the optic lobes. In all vertebrates, the optic tracts, which extend from the optic chiasma, end here. Centers are present which control certain important visual reflexes, such as the constriction of the pupil of the eye in response to the stimulus of light on the retina. In the higher vertebrates, certain reflexes following sound stimuli are con- trolled in the roof of the mesencephalon. The lateral and ventral regions of the midbrain contain many groups of neurones that pro- vide for numerous connections, and fiber tracts over which impulses are relayed from one level to another. In the diencephalon are many fiber tracts connecting various centers in other parts of the brain with the cerebral cortex. The optic nerves and tracts, carrying impulses from the retinae to the optic lobes, form the floor and part of the lateral walls of the dience- phalon (Fig. 67). In the lower vertebrates, all the fibers from one retina cross the optic chiasma to enter the opposite optic lobe. The crossing in higher vertebrates involves only the medial half of the fibers of each retina (Fig. 65), while the fibers of the lateral halves do not cross. Correlations resulting from olfactory stimuli are 120 THE VERTEBRATE ANIMAL: IRRITABILITY on. o.c. made in the diencephalon, and pain sensations are received there. Other centers are in the nature of important relay stations in the compounding of reflexes. Among the lower vertebrates, the most important parts of the telencephalon are the centers for correlation of impulses trans- mitted from the olfactory organs. The olfactory centers in mam- mals occupy the same relative position, but are overshadowed by the very great growth of the roof of the telencephalon to form the cerebral hemispheres. In the cerebral hemispheres, as in the cerebellum, the neurones that make up the gray matter are found in a con- tinuous superficial layer known as the cerebral cortex. Although the cor- tex is continuous, certain areas are known to be con- cerned with special func- tions. Impulses producing movements of the voluntary muscles are sent out to opposite sides of the body from the motor centers of the cortex; that is, if these particular areas of the cortex are destroyed the animal is unable to use the voluntary muscles on the opposite side of the body. In man, the regions that control movements of the principal parts of the body, from the toes to the face muscles, are known. Another major division of the cortex is concerned with sensory func- tions, and contains the sensory centers, to which impulses are transmitted from visual, auditory, and olfactory organs, as well as from receptors of pressure, pain, temperature, and taste. These areas have been almost completely mapped out for the human cortex. Surrounding and connecting the motor and sensory centers which occupy a relatively small part of the cortex, are the asso- ciation centers. These regions are filled with adjustor neurones which are responsible for the complicated pathways of learning processes and which are involved in memory. Fig. 65. — Diagram of the optic chiasma in higher vertebrates. Notice that only the medial nerv'e fibers cross. o.c, optic chiasma; o.n., optic nerve; o.t., optic tract which ends in the optic lobe; r, retina. LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 121 " Intelligence," which is a very vague expression for something almost intangible, is dependent upon the degree of development of the cerebral cortex and especially upon the neurones of the asso- ciation areas. The ability of an animal to profit by experience, which involves analj'sis of a situation and memory, and which enables the individual to react in a way that is advantageous in a new situation, is determined by its degree of intelligence. This, in turn, is limited by the number of adjustor neurones and the synapses possible between them. It is well known that all the neurones that an animal possesses are present at a very early stage of its development. New synapses are, however, formed through- out the life of the individual and probably depend upon the variety and intensity of the stimuU received by that individual. For example, it has been found that if light be prevented from stimu- lating the eyes of an animal, such as a dog, the adjustor neurones in the optic lobes do not grow and send out processes in the usual manner (Fig. 66). The sensory impulses that reach the cerebrum the motor impulses that pass out from it, and the associations made in it constitute our so-called " consciousness." Sleep and anaes- thetics in some way lessen or completely block the functioning of the cortex and produce unconsciousness. It is sometimes said that the cortex " initiates " or controls " voluntary " movements. While there is no necessity of entering upon a discussion of the " will," it must be remarked that, so far as the physiology of the nervous system is understood, the motor centers of the cerebrum transmit impulses produced by stinuili, just as do other parts of the central system. The sources of the stimuli may be difficult to locate, but that does not seem to be a sufficient reason for depart- ing from the neurone mechanism as an explanation. In order to make clear the functions of the nervous system, the analogy to a telephone system is sometimes employed. The various lines that place individual telephones in connection with one another would correspond roughly to the peripheral system. " Central " occupies a position similar to the central nervous sys- tem. If we had in our telephone system certain hnes that con- nected one central office with another central office, these would correspond to the adjustor neurones. It is also possible to compare the nervous mechanism of the vertebrate to the administrative organization of any efficiently managed commercial enterprise. There are the different phases 122 THE VERTEBRATE ANIMAL: IRRITABILITY of buying, selling, financing, crediting, personnel, etc. In each of these departments, adjustments that concern it alone are con- stantly being made. If one makes a cash-and-carrj^ purchase in a large department store, the process can be compared to a simple reflex action. The clerk making the sale has the package wrapped, obtains the change, and returns these to the purchaser. No other part of the sales department is concerned. If, however, the buyer wishes to shop in several parts of the store and pay when the list of purchases is complete, the floor managers concerned must #• ^ ^ * 't^ fibO -^2^^ .*o^ O -^.^f ^% ^s Fig. 66. — A. Undeveloped adjuster neurones in the optic lobe of a dog, the eyelids of which had been sewed together immediately after birth. B. Fully developed adjustor neurones in the same region of a normal dog of the same age. (From Verworn, "Irritability," copyright, 1913, by Yale University Press, reprinted by permission.) approve the transactions, and payment is finally made in a special part of the sales department. Such a situation can be compared to a series of reflexes adjusted by the spinal cord. The several departments are not independent of one another. Let us suppose that clerks in a certain sales division report to the floor manager that customers are inquiring for some product that is not in stock. The floor manager may relay this information through certain intermediate officials to the sales manager of the estabhshment. The sales manager may consult with the purchasing manager, who will be aware of the source of supply of the desired material and will LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM 123 start the proper movements to acquire some of it, to satisfy the customers' demands. This series of events corresponds in a way to the aUied and chain reflexes that are controlled by centers of the brain other than those of the cerebral cortex. We may next assume that the sales and purchasing managers cannot adjust the situation that is presented to them until the points involved have been passed on by higher officials. A general policy of the firm may have to be considered. In that case the Fig. 67. — .4. Section of a bird's retina showing all the cell-layers. The rods and cones are the blackened cells in layers 6 and 7. The inner ends of their processes have sjmapses, in layer 5, with processes of the bipolar neurones that I)ass across layer 4. In layer 3 the bipolar neurone processes have synapses with dendrites of the neurones of layer 2. The axons of the nerve cells of layer 2 make up the optic ner^-e and pass out from layer L B. Showing three outer layers of retina. The middle cell is a cone, the three on each side are rods. (Both figures from Schafer, "Essentials of Histology," copyright, 1916, by Longmans, Green and Co., reprinted by permission.) information would finalh' be carried to the directors, who with all the details before them would adjust the situation, with refer- ence to previous circumstances, in the way that was best for the company's affairs. This decision of the directors would be placed in effect by the lesser officials who handle many matters without the directors' immediate consent. The actions of the directors can be thought of as being comparable with conscious cerebral adjustments in the higher vertebrates, where pre\'ious experience based on many types of stimuli results in intelhgent coordination 124 THE VERTEBRATE ANIMAL: IRRITABILITY of the animal in a given situation. It should be clear to the student that in any organism the vast majority of adjustments are not conscious, and that, in general, the simplest of these adjust- ments occur in the spinal cord, the more complex in the brain. In summarizing the details of functional locaHzation, it may be said that the central nervous system has the general function of adjustment of impulses, while the peripheral system furnishes transmission paths between all parts of the body and the central system. By the combined functions of the nervous system as a whole, the activities of the organism are correlated so that it behaves as a unit. This result is also known as coordination or integration. The degree of integration of which an animal is capable determines its ability to react successfully to the environ- ment, which is the most important factor in its survival. The increase in the complexity and specialization of the nervous system has therefore been said to be the chief factor in evolution. As Gaskell puts it, " The law of progress is this: The race is not to the swift, nor to the strong, but to the wise." Reception, Transmission, and Discharge It has been pointed out that nervous coordination depends essentially upon three factors. In the first place, the organism must be able to be aware of changes in its environment, that is, to receive stimuh. The stimulus sets up an impulse which must be transmitted. And, finally, the animal responds to the stimulus because the impulse is discharged at some effector. Theories of reception, transmission, and discharge are rather compHcated, and the processes are not well understood. However, certain state- ments can be made. Animals like the vertebrates possess sense-organs which are specialized for the reception of stimuli. These sensory areas, or receptors, are sensitive to special kinds of stimuli. The retina of the eye is a receptor for changes in the environment brought about by light waves (Fig. 67). In the lower fishes, the ear is primarily a receptor for stimuli produced by changes in the animal's equilib- rium. The semicircular canals of the ear in higher vertebrates re- tain this function, while the cochlea becomes specialized to receive sound waves (Fig. 68). Chemical substances in solution stimulate the olfactory epithelium of the nose and the taste-buds of the RECEPTION, TRANSMISSION, AND DISCHARGE 125 mouth (Fig. 69), Certain areas of the skin are sensitive to changes in temperature, others to pressure. In addition to these recep- tors, b}^ means of which the organism is made aware of external environmental changes, there are receptors in all the internal organs of the body. The nature of the specialization involved in receptor surfaces is not understood, but their specificity is well known. Sound waves will not start an impulse in the retina. On the -a Fig. 68. — Diagram of human ear. A, the auditory canal leading from the external ear to the tympanic membrane at B; C, cavity of the middle ear containing the "ear-bones" that transmit vibrations from the tympanic membrane to the inner ear; D, the pharynx with which the cavity of the middle ear is connected by the Eustachian tube; E, semicircular canals of the inner ear; F, cochlea; G, auditory nerve. (From Hough and Sedgwick, "Human Mechanism," copyright, 1918, by Ginn and Co., reprinted by permission.) other hand, if the optic nerve is stimulated mechanically, the impulse started results in a visual sensation. The person who says he "sees stars" when he gets a " black eye " has scientific founda- tion for his assertion. Receptors normally are sensitive to only one type of stimulus, and if they are stimulated by unusual methods the impulse produces the typical sensation. This latter effect may be due to the specificity of reaction of the brain center to which the impulse is conveyed. The reception of a stimulus by a sensory area is followed by the 126 THE VERTEBRATE ANIMAL: IRRITABILITY transmission of a nervous impulse along the afferent neurones asso- ciated with the area. Nerves were at first thought to be tubes that conducted the " animal spirits," which were supposed to be like gas. Later the material carried by the tube was believed to be more like water and was called " animal juice." The nature of the nervous impulse and its transmission still remains a puzzle in many respects and engages the attention of many investigators. Some workers have believed that the transmission of the nervous impulse is a wave of chemical reactions along the nerve fiber. The analogy Fig. 69. — A, Cells of olfactory epithelium from human nose. B, Cells of a taste bud in epithelium of tongue. In both cases the sensory cells ter- minate externally in hair-like processes that are able to receive the chemical stimuli of olfaction or smell, and gustation or taste. ep, epithelium of tongue; yc, gustatory cell; oc, olfactory cell; p, pore or external opening of epithelial cup in which the taste bud is located; sc, sustentacular or supportinj^ non-sensory cell. to the burning of a path of gunpowder is often made. This idea involves metabolic activities of a destructive kind in the neurone process. Such metabolism uses oxygen and liberates carbon dioxide and heat. It also is limited by the amount of combining constituents of the protoplasm. The production of carbon dioxide and heat in sufficient quantities to justify a chemical interpreta- tion of transmission has not been demonstrated. The fact that it is almost impossible to detect signs of fatigue, or loss of capacity to transmit in a bundle of nerve fibers, is also an argument against the metabolic theory. Opposed to the chemical interpretation is the physical theory, which explains transmission as the passage of an electric current. The velocity of nervous transmission varies from 27 to 125 meters per second in the cases measured. This is, of RECEPTION, TRANSMISSION, AND DISCHARGE 127 ms course, much slower than electrical transmission through metals or air. That an electric current occurs simultaneously with the transmission of an impulse has been clearly established, although it must be granted that that does not prove they are one and the same. It may well be that the process is dependent upon both chemical and physical factors. An interesting feature of transmission is that the direction of conduction in the fiber is not reversible. Afferent fibers always carry impulses toward the central nervous system; efferent fibers always carry im- pulses away from the central s}- stem. No fundamental difference in the morphol- ogy of these fibers has been found. It was stated above that it was impossible to fatigue a nerve trunk, that is, to diminish its capacity to transmit impulses. However, fatigue occurs when the im- pulse is permitted to pass over the entire reflex arc and produce an effect. Con- duction through the central nervous sj-s- tem includes transmission through ,, __ ., , ° I'lG. 70. — Neurorimseular synapses that connect neurones. The junction of a medullated evidence indicates that changes occurring nerve and striated muscle. in the synapses are responsible for nerve Note that the medullary fatigue. When the impulse reaches the effector . ^ . ^ • .• . 11, that come mto mtnnate cell. It passes through the neuromuscular ^^^^^^^ ^^^^,^ ^,^^ ^^^^,^^.1^ junction, or end organ, to the protoplasm cell. (Fig. 70). This is the discharge of the rns, medullary sheath; n. impulse, and little is known of its nature. °^"'"''^™'"='; "^- "'^'"^e fiber. . (From Schafer, "Essentials of It has been mentioned before that the same Histology," copyright, loie, by response can be obtained in an effector Loi^gmans. Green and Co.. re- . • I- Trv i printed by permission.) by impulses coming from many different types of receptors. Nervous control may be said to be of two kinds, excitation and inhibition. The effector reacts as a result of excitation. Its reaction ceases as the result of inhibition. Inhi- bition might be brought about by blocking the transmission of the exciting impulse, by altering the mechanism of reaction in the protoplasm of the effector, or by making the neuromuscular junc- tion non-functional. Some evidence indicates that inhibition sheath does not cover the branches of the nerve fiber come 128 THE VERTEBRATE ANIMAL: IRRITABILITY results from an alteration of cell metabolism which affects the junction between the nerve and the effector. It therefore appears that, like receptors, effectors are highly specialized and react in a particular way or not at all. Coordination and Irritability It must not be forgotten that coordination in any organism depends in the last analysis upon the irritability of protoplasm. The unicellular animals respond directly to changes in their envi- ronment which are received and become effective in the protoplasm of the same cell. That certain of the individual cells of multi- cellular organisms retain the capacity to respond directly to changes in their immediate surroundings is clear from the nature of chemical coordination. It will be recalled that endocrines carried in the blood and lymph produce reactions by direct stimulation. How- ever, it is impossible for each cell in a highly organized multicellular animal to be directly stimulated by changes in the environment of the organism as a whole. Integration of the many-celled animal is accomplished by means of the mechanism of the neurone arcs of the nervous system, connecting receptors and effectors according to the principle of the common path. This coordination of higher forms is associated with great specialization of cells and regions. A particular receptor receives only a certain kind of stimulus; neurones conduct impulses in only one direction; and effectors respond in a distinctive manner to discharged impulses. The question of the fundamental nature of this specialization is one that cannot be answered at present. Nor, for that matter, can the essential character of irritability be clearly explained. Certain facts point to a close association between metabolism and irrita- bility. In other words, if the metabolism of a cell is altered its irritability may be changed. Metabolism, in turn, depends in some way upon the physico-chemical nature of protoplasm. The complete explanation of irritability and coordination, therefore, awaits further knowledge of the physics and chemistry of proto- plasm. It is, fortunately, quite possible to understand to a greater or lesser degree the outward expression of protoplasmic irritability in the behavior of animals. REPRODUCTION 129 REPRODUCTION Protoplasm has three distinguishing capacities: metabolism, irritability, and reproduction. Protoplasm always occurs in units called cells. A single cell may be a complete organism, or a group of cells may be associated to form an individual. Reproduction can therefore be defined as the capacity, under varied conditions, of certain parts of organisms to detach themselves, and, either alone or after union with protoplasm of another organism of the same kind, to give rise to new individuals in all essential respects Hke the parent or parents. The function of reproduction differs from metabolism and irritability in that it is not necessary for the main- tenance of the life of the individual; it is the capacity upon which depends the continuity of the race or species. To understand the physiology of reproduction, one must have a knowledge of the differentiation of the gametes or germ cells, the problems of fertilization or the union of gametes in pairs to form zygotes, the development of zygotes in characteristic ways, and heredity and variation in the new individuals. Much more is known of the morphology than of the physiology of these phases of reproduction. As these topics are considered in the special chap- ter on Reproduction and in the chapters on Development and Genetics, no further discussion of reproduction will be undertaken at this point. CHAPTER 6 CELLS OF THE VERTEBRATE BODY In the preceding chapters the functions of metabolism, irrita- bihty, and reproduction in the vertebrate organism were dis- cussed. Such a consideration of function, together with that of the structure or anatomy of the vertebrate, conveys a rather definite idea concerning the animal as an individual, possessing the complex and highly coordinated organization that is familiar in the higher types. It is evident, however, that in order to explain the outward manifestations of Hfe one must understand the activ- ities that are characteristic of protoplasm as it occurs in the animal cell. The individual organism is an association of cells, which make up its structure and determine its functions by their correlated interdependence. A fundamental understanding of cells is neces- sary for the explanation of the normal morphology and physiology of the individual. It is also true that the appearance of cells furnishes the evidence that enables us to analyze and classify the abnormal conditions which exist in diseased organisms, and which are determined by the science of Pathology. As will be pointed out in the chapters on development, the processes whereby the adult individual arises from the germ cells, through all the stages of cell division and differentiation, are, essen- tially and without exception, cell phenomena. Embryology is best understood as the behavior of cells during the development of organisms. In addition, the subject of Genetics, or the study of heredity and variation, can be explained satisfactorily only when one makes clear the cell phenomena that are correlated with the inheritance of characteristics from generation to generation. Hence, the science of Cytology, which deals with all aspects of cell structure and activity, explains phenomena in all fields of biological investigation. For the student of General Zoology, this funda- mental knowledge of cells is best obtained through the detailed consideration of a particular subject. In the present chapter the 130 HISTORICAL DEVELOPMENT OF THE CELL DOCTRINE 131 general facts about cells will be explained, the expansion of these facts and their relation to particular fields being left for later chap- ters on these topics. Fig. 71. — ,-1 and B. Leeuwenhoek's micro-scope (about 1673). C. Hooke's compound microscope (1665). (From Carpenter, "The Microscope and its Revelations," copyright, 1891, by J. and A. Churchill, reprinted by permission.) Historical Development of the Cell Doctrine The discovery of cells was made possible by the use of the micro- scope, which was invented about 1591. This instrument was first used as a toy, but about 1650 it began to be utilized in scientific 132 CELLS OF THE VERTEBRATE BODY studies (Fig. 71). Robert Hooke, one of the early English micros- copists, discovered that cork was composed of small spaces, sur- rounded by firm walls, and in 1665 he gave the name cell to these compartments (Fig. 72). The development of microscopic instru- ments was very slow, and it was not until 1833 that Robert Brown observed, in certain plant structures, that each cell contained a small body, which he called the nucleus. In 1838, Schleiden, a German botanist, proposed the inter- pretation that cells were the units of structure in plants, and Schwann in 1839 extended this conception to the -A. Diagram of cork to show cell walls of gtructure of ani- plant cells. S. Squamous epithelium to show nucleus , rFio- 73") and protoplasm in the animal cell. There are no . ,' n 1 cell walls. This was the first cw, cell wall; n, nucleus; /(, protoplasm. lOmiUlatlOn OI tue Cell Theory. The founders of this theory, and other scientists of that time, believed that the walls that surround plant cells were the essential part of these units. The contents of cells had been observed, but were regarded as unimportant or as waste products. Purkinje, in 1840, and von Mohl, in 1846, gave the name protoplasm to Uie cell contents. Through a series of researches, it became apparent that protoplasm was the essential part of cells, since walls were found only in plant cells. Likewise, the presence of a nucleus was discovered in almost all types of cells, and a cell came to be described as a mass of protoplasm containing a nucleus. As knowledge became more complete it was ascertained that in certain parts of the animal much material which is not in the form of cells lies between them. This material, examples of which will be given later, was shown to be produced by cells, and the Cell Theory was modified by saying that organisms are composed of structural units, called cells, and of cell products. Further study of animals with reference to their activities has revealed the fact that all physiological processes must be understood in terms of the functions of cells. That is, the cell is the unit of function. The Cell Theory has, therefore, been extended and STRUCTURE OF A TYPICAL CELL 133 confirmed, and now stands as one of the fundamental generaliza- tions of biological science, being known as the Cell Doctrine. A complete statement of this unifying conception of Biology would be that all living organisms are composed of cells, which are the units of structure and function, and of cell products. In complex organ- isms these units are not isolated; but the coordination, both struc- tural and functional, between the different kinds of cells, is ex- FiG. 73.— j\I. Schleiden, 1804-18S1 (on the left); and Theodor Schwann, 1810-1882 (on the right). (From Locy, "Biology and its Makers," copyright, 1908, by Henry Holt and Co., reprinted by permission.) pressed in the well-known fact that the animal, as a whole, con- stitutes a higher type of unit, the individual. This is sometimes called the Organismal Theory. It is based upon the observed fact that cells associated in a complex organism are coordinated, by their own activities, in such a way that the animal is an individual. Structure of a Typical Cell A cell may be correctly defined, in a general way, as a mass of protoplasm containing a nucleus. It is true, however, that certain cells, notably the red blood cells of mammals, do not contain nuclei after they reach the final stage in their development. If 134 CELLS OF THE VERTEBRATE BODY the definition is to hold for all cases, it must be modified to the effect that a cell is a mass of protoplasm which, at some stage in its development, contains a nucleus. This statement, together with the definition of a cell as the unit of structure and function, is of fundamental importance in further consideration of cells. Struc- turally, cells are divided into two main parts, the cytosome, or cell body, and the nucleus. In the following account these parts will cm. cso n.m nu f - » "*< y ^i v- T ^ ah V:r lA^ m r, V - ^^'TS nz\-^)t — n.h. c.r. Fig. 74. — Diagram of a typical animal cell. cm., cell membrane; c.r., cytoplasmic reticulum; cso, centriole or centrosome; csp, centrosphere; /, fat; y.b., Gol;?! elements; I, linin; m, mitochondria; n.k., net knot of chromatin granules; n.m., nuclear membrane; n«, nucleolus; )i, vesicles; ^, yolk. be considered as they occur in what may be called a typical cell (Fig. 74). The Cytosome. — The cytosome is limited externally Ijy the cell membrane or plasma membrane, which is generally regarded as a firmer layer of the cytoplasm. In some cases a reticula- tion is indicated in the cytosome, but this is not a constant feature. Lying near the nuclear membrane, a differentiated, STRUCTURE OF A TYPICAL CELL 135 rounded area of cytoplasm, the centros'phere encloses one or two small granules, the centrioles or centrosomes. The centrosphere and centrioles are conspicuous structures during mitotic cell divi- sion (c/. p. 137). A number of bodies found more or less com- monly in the ground substance of the cj^toplasm are grouped under the name of cytoplasmic inclusions. Mitochondria are the most universal members of this group. These are small gran- ules, isolated or arranged in rows; they are typically scattered throughout the cytosome but may be more numerous in some regions. Golgi elements have been observed in many cells. Fatty products of metabolism are stored as larger or smaller drops in the majority of cell bodies. Yolk, built up in the cytoplasm, is stored in the form of j'olk plates or spheres in egg cells. In many cases, vesicles, filled with solutions of unknown compo- sition, are present. When the contents of these vesicles are lost they may appear as vacuoles. Secretions produced in gland cells are stored as secretion granules until they are passed from the cells. Granules of pigment characterize many cells. These cytoplasmic inclusions constitute a rather diversified group, and full under- standing of their origin and function is dependent on information, much of which is yet to be gained, concerning the metabolism of the cell. The Nucleus. — The nucleus, which is usually round and cen- trally located, is everywhere surrounded by the cytosome. It is separated from the cjiioplasm b}^ a continuous bounding mem- brane, the nuclear memhrane, and typically exhibits a fine frame- work known as the linin net. Scattered on the linin are fine granules which are called chromatin, because with some stains they color very intensely. Those chromatin granules are fre- quently aggregated on the nuclear framework to form net knots. Chromatin is the material that has been shown to be closely correlated with the mechanism of heredity. Further description of its behavior will be given in the accounts of cell division and maturation. Lying in the meshes of the linin net are found one or more rounded bodies, the nucleoli or plasmasomes. Nucleoli have been interpreted as waste products of nuclear metabolism, but a growing mass of evidence seems to indicate their more fundamental importance as perhaps temporary storage products of such metabolism. The nuclear components described above are embedded in the ground substance of yiucleoplasm. 136 CELLS OF THE VERTEBRATE BODY Both nucleus and cytosome are necessary for the normal activ- ities of the cell. It is not entirely possible to define the part each plays in the metabolism of the whole. Cells that are deprived of their nuclei are unable to carry on assimilation, although dissimilation goes on until the cytoplasm is exhausted. This fact and other types of experimental evidence would seem to indicate that the nucleus may be a place where enzymes necessary for assimi- lation are produced, but that the cytosome is the principal region of synthetic activity and energy transformation in the cell. Whether or not such a distinction can be sharply drawn, it cannot be doubted that there is very close interdependence between these two morphological divisions, and that the life of the cell depends upon balanced interactions between nucleus and cytosome. Cell Division When cells were first discovered they were thought to arise spon- taneously by a sort of crystallization. The nucleus was inter- preted by some early investigators as a new cell in process of forma- tion. As the microscope was perfected and more observations were made, it was found that new cells were formed as the result of the division of previously existing cells, and in no other manner. It will be recalled that the amount of protoplasm increases when assimilation exceeds dissimilation. During this period the cell is said to be in the vegetative or nutritive stage. The cell is some- times referred to, at this time, as a " resting " cell, but nothing could less adequately describe it during this period of metabolic activity. When the cell has reached a certain size it divides. Whether or not cell size is the only factor governing cell division, it is certainly a very important one. Cells divide by two different methods: amitosis, or direct cell division; and initosis. or indirect cell division. Mitosis is by far the more common method. Amitosis. — In amitosis or direct cell division, the nucleus of the cell becomes somewhat elongated and is pinched into two parts which are about equal in volume. The nuclei of certain types of cells may divide amitotically without division of their cytosomes and thus give rise to multinucleate cells. In complete amitosis, after the nucleus is constricted the cytosome divides and two new cells are formed. In this direct process of division the distribution of cell components is only approximately equal. Such a type of CELL DIVISION 137 nc, nucleolus. (From Kellioott, "General Embry- ology," copyright, 1913, by Henry Holt and Co., re- printed by permission.) division apparently occurs only in cells that are very specialized, very old, or in some abnormal or degenerating condition (Fig. 75). Mitosis. — Mitosis is the typical method of cell division. It is called the indirect method because it involves changes that are more complicated than the simple constriction of amitosis. The process is divided, for purposes of description, into four stages, which are continuous. These are the prophase, the VI ctaphase, the anaphase, and the telo- phase. The structure of a typical vegetative cell should be recalled (Fig. 74). Among the earliest changes in the l'^*-- 75.— Amitosis in tendon cells of :i new- prophase of mitosis are ^^^ ™«"««- '^^^^^ Nowikoff.) those of the centro- sphere and centrioles. If the cell contains only one centriole, this divides and the two halves begin to sepa- rate, passing toward opposite sides of the nucleus (Fig. 76). The centrosphere elongates and gives rise to fine fibers, the spindle fibers, which extend between the centrioles as they migrate, and also to delicate strands, the astral rays, which radiate freely from the centrioles. Collectively, these structures are called the mitotic spindle because of the arrangement of the fibers, the amphiaster, because of the resemblance to a double star, or the achroinatic figure, because these structures do not stain. Within the nucleus the chromatic granules become associated in delicate threads which form what is known as the spireme. It is the forma- tion of these threads that gives the name mitosis (from 7nitos, thread) to this kind of cell division. As the prophase progresses, the chromatin threads condense to form bodies known as chro- mosomes. These chromosomes are constant in number, size, and shape for the cells of any particular species of animal. As migra- tion of the centrioles, formation of the amphiaster, and condensa- tion of the chromosomes progresses, the nuclear membrane and nucleolus disappear. When the nuclear membrane is broken down, the spindle comes to lie in the region of the nucleus and the chro- ne n.m. Prophase 1 Prophase 2 ne n.m. Prophase 3 Metaphase d.chs. Prophase 4 S d.chs a. r. as Anaphase d.chs. d. c. chs. d.c. Telophase 1 Telophase 2 Fig. 76. — Diagram of mitosis, or indirect cell division, showing four chro- mosomes. a.r., astral ray; as, aster; cf, rentriole or oentrosonie; chr, chroiiiHtin; chs, rhromoBom©; cs, centrosphere; d.c, daughter cell; d.chs., daughter chromosome; e.p., equatorial plate; ne, nucleolus; 7i.m., nuclear membrane; s, mitotic spindle; sp, spireme. 138 CELL DIVISION 139 mosomes take up a position in the spindle, halfway between the centrioles, to form the so-called equatorial plate. During the metaphase, the longitudinal division of each chromo- some into halves becomes conspicuous. This splitting of the chromosomes occurs at different periods in the prophase, but is more easily observed during the metaphase as the half-chromo- somes he side by side on the equatorial plate of the division spindle. The fact that the chromosomes divide in this manner is of theoret- ical significance, because regions of chromosomes are believed to differ from one another along the longitudinal axis. The genes, or determiners of hereditary characteristics, located in the chrome- somes are believed to be arranged like beads on a string. The longitudinal division of chromosomes is, therefore, thought to be equal with respect to quality as well as quantity of chromatin. The importance of the chromosomes and their behavior in correla- tion with the mechanism of heredity will be more fully discussed in the chapter on Genetics. Following the period of inactivity of the chromosomes as they lie on the equatorial plate, the halves of each chromosome begin to separate and move toward opposite centrioles. This migration constitutes the anaphase. Xo satisfactory explanation of the physical principle underlying the movement of the half-chromo- somes toward the ends of the spindle has been proposed. That certain of the spindle fibers are attached at specific points to the chromosomes is an established fact. The telophase, or reconstruction of nuclei, begins when the half-chromosomes approach the centrioles. The chromosomes more or less reverse the process of their formation during the pro- phase. They become irregular in outline, and are redistributed as chromatin granules on the linin network of the new nucleus, which is set off by the appearance of a nuclear membrane. In this way, daughter nuclei, containing chromatin equal in amount and kind, are established at each end of the mitotic spindle. As nuclear reconstruction goes on, the astral rays and spindle fibers disappear, and in some types of cells each centriole divides. At the same time the cytosome becomes constricted so as to form approximately equal halves, each of the daughter cells containing one of the daughter nuclei and a centrosphere with one or two centrioles. This process of indirect cell division is regarded as essentially a mechanism fcr insuring equal qualitative and quantitative dis- 140 CELLS OF THE VERTEBRATE BODY tribution of the chromatin of a cell to its two daughter cells. In particular instances, variations in certain features may occur, but the essential significance of mitosis is the same throughout the animal kingdom. Histology Tissues. — In the preceding section a so-called typical cell has been described. If the body of a vertebrate is examined micro- scopically, it will be found that no cell conforms to the diagram of a typical cell. In other words, cells differ among themselves although they all contain certain features in common. Cells differ in respect to size, shape, position in the body, and also in the functions that they are especially fitted to perform. Cells that are similar in structure and function make up groups known as tissues. Tissues, then, are groups of cells specialized in the same way for the performance of the same function, and are classi- fied on the basis of the structure and function. There are four principal classes of tissues: epithelial, sustentative, contractile, and nervous. Epithelial Tissue. — The cells of epithelial tissues are com- pactly placed with but a small amount of intercellular material, and function for the covering and protection of body surfaces, both internal and external, and in secretion and excretion. According to the predominating form of cells, this class is subdivided into squamous and columnar epithelium, each of which is again divided into simple or stratified, depending upon whether it exists in single or multiple layers. The cells of simple squamous epithelium, when viewed from the surface, resemble tiling-blocks, and on edge they are very thin (Fig. 77 A) . Such epithelium, sometimes called endo- thelium, is found lining the calome, that is, forming the peritoneum ; it also forms the lining of blood vessels (Fig. 77 B). In stratified squamous epithelium only the outermost layers are typically flat- tened cells, while the cellj of the deeper layers are progressively more cuboidal (Fig. 77 C). Since blood vessels do not penetrate epithelial layers, it is only the cells of the deeper layers that receive abundant nourishment and consequently divide and replenish the outer layers, which die and are cast off. Stratified squamous epithelium is found in the outer layer of the skin and in the lining of the nasal and mouth cavities of many vertebrates. In simple columnar epithehum, such as that lining the digestive tract, the HISTOLOGY 141 cells are longer than they are wide and are arranged side by side (Fig. 77 D). Stratified columnar epithelium is not abundant, but a modified type is found lining the trachea (Fig. 77 E). Columnar epithelial cells are sometimes modified by having their free sur- faces, that is, the surfaces exposed to the cavity that they line, covered with cilia, which are fine, hair-like cj^oplasmic processes Fig. 77. — Epithelial tissues. A. Simple squumous epithelium from human mouth. B. Simple squamous epithelium (endothelium) from peritoneum. C. Stratified squamous epithelium from lining of nasal canal. D. Simple columnar epithelium from mucous membrane of digestive tract. E. Pseudo- stratified ciliated columnar epithelium from lining of trachea. One cell is shown secreting a drop of mucus. F. Simple cihated columnar epithelium. G. Glandular epithelium from the pancreas. //. Goblet cell with drop of mucus. (A, B, and C from drawings by D. F. Robertson.) (Fig. 77 F). The cilia are vibratile and, by their motion, act to keep the surfaces clean. The epithelium of the air passages in higher vertebrates and the roof of the frog's mouth offer examples of this variation. In the retina and in other places, epithelial cells are sometimes characterized by pigment granules and known as pigmented epi- 142 CELLS OF THE VERTEBRATE BODY thelium. In organs of secretion, columnar epithelium is modified as glandular epithelium for the production of certain types of secretions (Fig. 77 G). Epithehal cells, temporarily modified as gland cells, occur as goblet cells (Fig. 77 H), so called because dis- tended by a drop of mucus, in the mucous membrane of the diges- tive tract. Sometimes glandular epithehal cells occur in groups, and these cell groups may sink below the general surface (Fig. 78). Such groups of cells may form simple tubes, constituting simple tubular glands, like the gastric glands; or flask-shaped struc- FiG. 78. — Diagrams of glands. A. Unicellular glands. The one to the left is shown extending below the surface epithelium. B. A group of gland cells remaining in the surface epithehum. C. A simple alveolar gland. D. A sim- ple tubular gland. E. A compound tubular gland. F. A compound alveolar gland. d, duct; gc, gland cell; se, cells of surface epithelium. tures, making simple alveolar glands, such as those of the frog's skin. Secretion may occur in all cells of these glands; or it may be confined to the basal cells, while those leading to the surface form the duct or tube by way of which the secretion leaves the gland. A compound tubular gland, like the liver, or a compound alveolar gland, such as the pancreas, is formed by outpocketings from an original simple type (Fig. 78). Sustentaf/ive Tissue. — The sustentative tissues are a very heterogeneous group, classed together because they are all derived during development from the same source — the stellate mesen- PIISTOLOGY 143 chyme cells (Fig. 79 B). In general, they function in supporting the body and connecting or binding together its parts. Sustenta- tive tissues are characterized by a large amount of intercellular material, which is produced by the cells. In the vertebrates, it is, for the most part, this intercellular material that furnishes the sup- porting and connecting qualities. Sustentative tissue may be divided into five sub-classes: connective, cartilage, bone, adipose, and vascular. Connective tissues are of three kinds : mucous, in which the inter- cellular material is gelatinous, and which is found in the umbilical cords of mammals (Fig. 79 A) ; reticular, in which there is a mesh- work of connective tissue cells with the interspaces filled with other types of cells, and which forms the groundwork of organs like the spleen (Fig. 79 C) ; and fibrous, in which the intercellular material is composed of fibers, and which is widely distributed as a binding tissue in many organs. The intercellular fibers of fibrous connective tissue are of two kinds, white and elastic. White fibers are very fine and occur in bundles, while elastic fibers are thicker and occur singly. Fibrous connective tissue in which both white and elastic fibers occur is found in the submucous layer of the digestive tract and in the dermis of the skin (Fig. 79 D). Fibrous connective tissue in which white fibers predominate is found in tendons, and that con- taining chiefly elastic fibers is found in the walls of larger arteries and in certain ligaments (Fig. 79 E). The cells of fibrous comiec- tive tissue are spindle-shapetl or irregular in outline and possess relatively little cytoplasm. The second sub-class of sustentative tissues is cartilage, which is a supporting tissue. The intercellular material in cartilage is usually hardened by impregnation with mineral salts, chiefly those of calcium. Here the cells are more or less rounded and lie in spaces known as lacunce. Where the matrix between the cells is translucent and apparently structureless, the tissue is called hyaline cartilage, or gristle (Fig. 79 G). Such tissue is found at the ends of long bones, at the ends of ribs, and in the cartilages of the nose and trachea. The cartilage of the external ear contains elastic fibers in its matrix and is, therefore, known as elastic cartilage (Fig. 79 H), while that found between the vertebrae has white fibers in its matrix and is fibrous cartilage (Fig. 79 I). Bone, or osseous tissue, is characterized by its very hard matrix, which is impregnated with lime salts. There is twice as much inor- 144 CELLS OF THE VERTEBRATE BODY Fig. 79. — Sustentative tissues. A. Cells of mucous connective tissue which occurs in the umbilical cords of mammals. The gelatinous intercellular material is not represented. B. Mesenchyme cells. C. Reticular connec- tive tissue from the spleen. D. Fibrous connective tissue from the sub- mucosa showing both white and elastic fibers. E. Elastic fillers of fibrous con- nective tissue from the nuchal ligament of the ox. F. Adipose tissue showing various stages of storage of fat drops in the cells. G. Hyaline cartilage from the end of a rib. //. Elastic cartilage from the external ear. /. Fibrous cartilage from an intervertebral disc. /. Bone cell lying in a lacuna. K. Bone lacuna; and canaliculi from dried bone. L. Haversian system in which lacunae are arranged concentrically around a central or Haversian canal. Canaliculi connect lacunae and canal. cc, cartilage cell; cl, lacuna in which cartilage cell lies; ef, elastic fiber; fd, fat drop; he, capsule of hyaline cartilage, surrounding cells of elastic and fibrous cartilage; wf, white fiber. (.4, E, H, K, and L from drawings by D. F. Robertson.) HISTOLOGY 145 ganic material as organic in bone. The long bones of the body, such as the femur, have a central marrow cavity filled with bone marrow, where red blood cells are formed. This marrow is, how- ever, not osseous tissue but is merely contained in the cavities of bones. The bone cells (Fig. 79 J and K) He in lacunae within the matrix. A very typical arrangement is that of the Haversian sys- tem. This consists of a central Haversian canal, containing an artery, a vein, and a nerve, surrounded by concentrically arranged rows of lacunae which are in communication with one another and with the central canal by means of minute spaces, the canaliculi (Fig. 79 L). Lymph circulates in these canaliculi and furnishes a passageway for foods and wastes between blood and cells. In adipose tissue there is no intercellular material, and the stel- late mesenchyme cells become transformed into rounded cells which serve as storage depots for fat (Fig. 79 F) . In fully developed adipose cells there is a very large drop of neutral fat with only a film of cytoplasm surrounding it and containing the nucleus. The large drop of fat is formed during the specialization of fat-storing cells, by the coalescence of numerous finer drops that are deposited in the cytoplasm. Vascular tissue, which is subdivided into hlood and lym-ph, is characteiized by its fluid intercellular material, the plasma. In blood, two kinds of cells are suspended in the pla.sma. Of the.se the red cells, or erythrocytes, contain the iron-bearing haemoglobin, in combination with which o.xygen is carried in the blood (Fig. 80 A and B). In mammals, the red blood cells lose their nuclei at matu- rity, are consequently veiy short-lived, and must be constantly replaced. The chief source of red cells is the bone marrow, but some are apparently formed in the spleen. Of the white blood cells, the leucocytes are granular, irregular in form, and move by changing their shapes, in the same manner as the amoeba, one of the single- celled animals (Fig. 80 C) . Hence, they are said to possess amoeboid movement. In this way they are able to migrate through the walls of the capillaries. Leucocytes are also able to take solid particles, like bacteria and other foreign bodies, into their cytoplasm and so remove them from other tissues. They so function in the case of infections in any part of the body. Lymphocytes are non-granular white blood cells (Fig. 80 D). The source of white blood cells is chiefly the lymph glands and spleen. Blood plasma is believed to be the carrier for all substances transported by the blood, save 146 CELLS OF THE VERTEBRATE BODY oxygen. It possesses the capacity of clotting when drawn from the vessels. During clotting, a mass of fine fibers, composed of a simple protein material, the fibrin, appears, and the cells are held in its meshes. The plasma that is left can be separated from fibrin and cells and forms the serum that is used in immunizing against certain diseases. Lymph differs from blood in that it does not contain erythrocytes but is composed of plasma and colorless cells. The importance of lymph as the pathway between blood and tissues should be recalled. Contractile Tissue. — Contractile tissues are known as muscles, and are of three kinds: non-striated, cardiac, and striated. The %a A if, nerve fiber; 7in, nucleus of neurilemma; Rn, node of Ranvier. receiving stimuli, or changes in the environment; for transmitting nervous impulses from one part of the body to another; and for discharging these impulses to other kinds of cells in nerve-con- trolled organs. The general functions can, therefore, be stated as reception, transmission, and discharge. The result of these activities of nervous tissue is the coordination of the organism as a whole. A nerve cell, or neurone, is composed of a nucleus sur- HISTOLOGY 149 rounded by a relatively small mass of cytoplasm which is pro- longed into two or more processes of varying lengths, the Jiei-ve fibers. Where there are only two cytoplasmic extensions, the cell is called a bipolar neurone (Fig. 82 A and B). The process by which nervous impulses travel toward the cell body is called a dendrite, while the one over which impulses pass away from the cytosome is the axon. There is never more than one axon, but there may be many dendrites. Where there is more than one dendrite, the cell is multipolar (Fig. 82 C). The cell bodies of neurones are found in groups, forming ganglia, outside the central nervous system; they also constitute the gray matter of the central system. Nerve fibers, bound together and surrounded in bundles by fibrous connective tissue, form the visible nerves of the peripheral nervous system (Fig. 82 D) and make up the white matter of the central nervous system. Certain nerve fibers are intimately covered by a layer of mj^elin, which contains much lipin, and are known as medullated fibers (Fig. 82 E). A nerve fiber is always a process of a neurone. As will be recalled, neurones are named according to their position in the reflex arc, but these names do not indicate subdivisions of the main class. Organs and Systems. — The tissues that have been described exemplify the various types of specialization that cells undergo in th3 vertebrate body. Each particular tissue is capable of perform- ing its special function alone, but they usually occur grouped in organs. Thus, organs are groups of tissues associated together for the performance of a special function. For example, if the wall of the small intestine is examined microscopically, it is found to consist of layers known as the peritoneum, longitudinal and circular muscle layers, submucosa, and mucous membrane (Fig. 83). The peritoneum consists of simple squamous epithelium and functions as a covering membrane. Both longitudinal and circular muscle layers are of non-striated muscle tissue, bound together by fibrous connective tissue, and their contractions produce the muscular movements that mix the food contents of the intestine and push them along toward the lower parts of the tract (cf. Fig. 48, p. 84). Fibrous connective tissue, containing both white and elastic fibers, is the distinguishing tissue of the submucosa and serves to support the numerous vessels carrying blood and lymph. This layer also provides the elasticity necessary for the expansion of the canal, in addition to carrying the circulatory fluids necessary for absorption. 150 CELLS OF THE VERTEBRATE BODY The mucous membrane is composed of simple columnar epithelium which forms the lining of the tract and functions in secretion and absorption. These several tissues are associated to form the small intestine, in which digestion and absorption occur, and each tissue contributes its part of the function of the whole. In addition to the grouping of cells to form tissues, and of tis- sues to form organs, organs are associated to form the systems described in the discussions of morphology and physiology. An understanding of the vertebrate body as a whole is to be had in ■ TH. \ cap \ m.m era. ^ s-mu Fig. 83. — Wall of small intestine (semi-diagrammatic). cap, capillary; cm., circular muscle; g.c, goblet cell; i.g., intestinal gland; l.m., longi- tudinal muscle; l.v., lymph vessel; m.m., muscularis mucosae, a more or less distinct layer of non-striated muscle cells lying beneath the mucous membrane; mu, mucous membrane; p, peritoneum; s-mu, submucosa; v, villus terms of the cells, which are the units of both structure and function. Let us consider briefly, for the purpose of illustrating this state- ment, some of the activities of a common vertebrate, such as a frog. The structure of such an animal is familiar, and the general relations of parts and their functions are well understood. If we consider a frog that has not recently fed, sitting on the bank of a stream, we know that, as a result of metabolism and the conse- quent using up of the protoplasmic constituents, it will be necessary for the animal to obtain food. At such a time, if an insect comes within the frog's range of vision, the cells of the frog's retina receive HISTOLOGY 151 the stimulus produced by the appearance of the insect. Within these retinal cells nervous impulses are set up and pass along nerve-cell processes toward the brain (c/. Fig. 67, p. 123). In the visual centers of the brain, the impulses are transmitted to other cells of the nervous sj^stem, and eventuall}^ go out along nerve fibers leading to the muscle cells of the frog's tongue. The effect produced is the contraction of certain muscle cells, resulting in the movement of the tongue for the capture of the prey. The contact of the insect with the lining of the frog's mouth causes the closing of the mouth, and this in turn stimulates the act of swallowing. These activities, of course, result from muscle-cell reactions. In the stomach and intestine, the insect is digested by the juices secreted by gland cells occurring in the wall of the tract and in the pancreas and liver. These juices are secreted at the proper time because of the coordinating mechanism of nerve cells and their processes, or because of the production of endocrines by cer- tain cells and their effect upon other cells {cf. p. 97). After digestion and absorption have occurred, the simple foods are car- ried in the blood plasma to capillary networks where they pass out through the endothelial walls of the capillaries to the many differ- ent kinds of cells that compose the organs of the body. In the cells throughout the animal, assimilation occurs and certain types of food may be stored, as is glycogen in liver cells and fat in adipose tissue. During its stay out of water, the frog is carrying on respiratory movements of the nostrils, floor of mouth, and glottis. These movements are the results of muscular cell activities, and are pro- duced and controlled as a result of nerve-cell reactions which are dependent, in turn, upon the production of carbon dioxide by cells in all regions of the organism. Oxygen is forced into the lungs by these respiratory movements and passes through the walls of the lungs into the blood, where it enters the red corpuscles and com- bines with haemoglobin. In capillaries throughout the body, oxygen leaves the red cells and the blood stream to enter the protoplasm of all types of cells, where it produces the oxidative reactions of metabolism. As a result of metabolism, excretions are produced and, reaching the blood by diffusion, are eHminated from the body through cells in the liver, kidney, lungs, and skin. A continuation of this discussion would only add further examples of the same kind. Thus we see that the general activities of ani- 152 CELLS OF THE VERTEBRATE BODY mals are to be explained in terms of simple cell and tissue reac- tions, built up into complex activities as a result of the coordina- tions that produce the phenomena of individuation. The emphasis placed upon the activities of individual cells as the basis for the interpretation of bodily processes as a whole, and the analysis of all structures in terms of cells and cell products should make clear how fundamental is the generalization embodied in the Cell Doctrine. As animals other than the vertebrates are studied, it will be seen that cells, and their requirements and responses, are essentially the same throughout the realm of living organisms. The Doctrine of Organic Evolution alone takes rank with the Cell Doctrine as a unifying conception in biological science. CHAPTER 7 REPRESENTATIVE SINGLE-CELLED ANIMALS The preceding chapters are essentially an extended introduction to the subject of zoology. An attempt has been made to review and expand the student's knowledge of familiar zoological subject matter, namely, the structure and functions of higher animals, with a view to making this the point of departure for a survey of further aspects of zoological science. The Protozoa, or unicellular animals, are naturally chosen to serve as an introduction to this more comprehensive study; although their organization and activities are not so simple as might be supposed from superficial examination. The Protozoa are often referred to as " animals reduced to the lowest terms." This is a good characterization, because of their unicellular state; but when we examine the more specialized Protozoa we see that they are more complex than any single cells that occur in many celled animals. The Protozoa are of further interest because in classification (p. 240) they may be placed over against the rest of the animal world. If we proceed to the classification of animals by asking what is the most fundamental difference between the animals of difTerent kinds, the greatest difference, in the opinion of many zoologists, Hes between the unicellular and the multicel- lular state. The Animal Kingdom may be divided into two great groups: (1) the Protozoa, or unicellular animals; and (2) the Meta- zoa, or multicellular animals. While this distinction cannot be sharply drawn, because of the existence of certain intermediate types which are " colonies " of cells, it is good for practical pur- poses; although it Taa,y seem to over-emphasize the Cell Doctrine and to disregard the organization of animals as individuals irre- spective of their cellular state (c/. p. 194). Classification of Protozoa The Protozoa have been variously classified into groups com- parable with the classes in other phyla of the Animal Kingdom 153 154 REPRESENTATIVE SINGLE-CELLED ANIMALS {cj. p. 241). The scheme most commonly used separates them into the four groups indicated in the following paragraphs. Fig. 84. — Representatives of the four classes of protozoa. A, Amoeba, Class Sarcodinj,. B, Chilomonas, Class Mastigophora. C, Frontonia, Class Infusoria. D, Pudophrya feeding on small ciliate, Class Infusoria. E, Gregarina attached to cell of host's intestine and stages within such cells, Class Sporo^oa. Class Sarcodina. — These are forms in which the cell exhibits more or less temporary processes termed pseudopodia, or "false feet." In one subdivision, the Rhizopoda (Fig. 84 A), which are typically creeping forms, these processes are k)bed or root-like and sometimes branching. In another, the Actinopoda, which are typically floating forms hke Actinophrys (Fig. 85), the pseudo- podia are stiff and more permanent. The name Sarcodina was originally applied to this class of the protozoa, because of their resemblance to the sarcode or "flesh/' as the protoplasm of animal cells was called in the early days of the cell theory. Class Mastigophora. — In these forms the cell possesses, in the dominant phase of its hfe cycle, one or more flagella, or whip-like processes, which are used for locomotion, and in some instances for feeding. The number of flagella is relatively small. The Mastigophora (Fig. 85) are a very diversified group, repre- THE SARCODIXA 155 sented on the one hand by species that are animal-like, the Zoomas- tigina, and on the other by plant-Hke types, the Phytomastigina. Mastigophora means "whip-bearers." Class Infusoria. — This class consists of forms in which the cell has cilia during the whole or a part of the life cycle (Fig. 84 C and D) . With few exceptions, the nuclear material is represented by a larger "vegetative" macronucleus, and a smaller, "generative" micronucleus (Fig. 95 E). The infusoria include many highly specialized protozoa. They were so named because many species of this class are often abundant in " infusions." Phaous Peranema Sphaerella Ceroomonas Monoaiga Fig. 85. — Pi,epre.sentative Sarcodina (upper row) and Mastigophora (lower row.) Mastigamceba, which has a flagellum and pseudopodia, is classified as a mastigophoran. (Drawn by C. W. Wilson.) Class Sporozoa. — These are parasitic forms in which the cell often shows marked degeneration in the locomotor and other structures necessary for free life (Fig. 84 E). Spores or seed-like encysted stages give the group its name, "seed anmials." The Sarcodina Structure of Amceba. — The class Sarcodina includes the sim- plest forms that are found among Protozoa, although there are 156 REPRESENTATIVE SINGLE-CELLED ANIMALS reasons for believing that some of the Mastigophora are more primitive from an evolutionary standpoint. Notable for their simplicity are the members of the genus Amoeba (Fig. 86). The following descriptions of the structure and activities of Amoeba proteus and related species are intended to supplement observa- tions which are presumed to have been made in the laboratory. The amoeba is composed of a gelatinous, semi-fluid substance, with an outer portion, the ectoplasm or ectosarc, which is almost homogeneous even under the highest powers of the microscope, and an inner portion, the endoplasm or endosarc, which contains vacuoles and particles of various sorts in addition to the nucleus. The ectosarc appears as a firmer substance, in spite of its manner of flowing as the amoeba moves, and the endosarc as a more fluid region in which the endosarcal particles are suspended. Some investigators have claimed that there is an outermost layer, so thin that it cannot be seen, but demonstrable because objects which stick to the surface move at a different rate from that of the underlying ectoplasm. Others question the existence of this outer- most layer. Like other cells, the amoeba, therefore, possesses a nucleus and cytosome or cell body, but the cytosome is differentiated into ectoplasm and endoplasm. The nucleus contains chromatin, arranged in a characteristic manner in different species. The cytosome is composed of cytoplasm, in which may be recognized inclusions of various sorts, as is the case with most cells in other animals. The larger bodies suspended in the endoplasm are of several kinds: the /ood vacuoles, in which the digestion of ingested food occurs; the contractile vacuole, connected with the ectoplasm; ordinary water vacuoles', various granules; and, particularly, crystals of definite shapes, which are perhaps distinctive for par- ticular species of amoeba. The significance of these several parts will be discussed in connection with their activities. Movements, Locomotion, and Behavior. — The apparently simple manner in which an amoeba effects locomotion by the " flowing " of its irregularly shaped body has attracted attention since the animal was first observed by the early microscopists, who called it the " proteus animalcule," meaning " changing little animal." We seem to have before our eyes, in the amoeba, the simplest of all forms of protoplasmic movement, and one, perhaps, reducible to physico-chemical phenomena. However, the simpler THE SARCODINA 157 explanations that have been given fail to account for certain com- plexities in behavior which are disclosed by more thorough study. For example, it was at one time supposed that the movements of an amoeba were caused by the same physical phenomena as the changes visible in a drop of clove oil when placed on a slide in a mixture of 3 parts glycerine and one part 96 per cent alcohol. Such a drop will change its form, send out " pseudopodia," and move about much as does an amoeba. Or, a mass of glycerine, 8 S Fig. 86. — Locomotion in amoeba as recently described by Mast. AccordinR to this account, the amoeba consists of an inner granule-containing fluid substance, the plasmasol, surrounded by a more solid layer of similar composition, the plasmagel, which are together equivalent to what is ordinarily termed the endosarc. Out- side there is a hyaline portion of the plasmagel, which is almost homogeneous optically, and an outermost layer, the plasmalemma. These last together constitute the ectosarc. During locomotion the plasmasol and plasmagel become transformed into one another after the manner shown in the figure. The plasmasol moves forward and at the end of the advancing pseudopod becomes transformed into the layer of plasmagel, which in turn is becoming plasmasol at the posterior end, "much as a chimney might be extended by carrying brick and mortar up through it and depositing them on the wall surrounding the opening." (7, region of gelation of plasmasol resulting in forward extension of the granular layer of the plasmagel; (/', region of gelation resulting in formation of hyaline layer of plasmagel; It, nucleus; pg, plasmagel; pi, plasmalemma; ps, plasmasol; s, region of solation resulting in transformation of plasmagel into plasmasol. (Reproduced from the original drawing by courtesy of S. O. Mast.) placed in a watchglass of rather thick lubricating oil, can be made to roll about in a suggestive manner. If soot particles are added to the glycerine they are held in suspension much as are the bodies in the endoplasm of an amoeba, and the movements have an even greater resemblance. These simple imitations now appear to offer inadequate explanations of amoeboid movement, although it may be that the phenomenon is capable of physico-chemical explana- tion in more complex terms. 158 REPRESENTATIVE SINGLE-CELLED ANIMALS Within the past fifty years a number of different interpretations have been placed upon the processes to be observed in the move- ments of various species of amoeba. Thus, amoebas have been supposed to send out pseudopodia, hke jets from a fountain with a current flowing outward in the center and backward on all sides (Berthold, 1886, and others) ; again, it has been thought that the amoeba flows by a roUing motion (Jennings, 1904); or "walks" on stiff pseudopodia (Bellinger, 1906) (Fig. 87). According to the most Fig. 87. — Outlines of successive stages in the movements of a single amoeba as seen laterally. Such "walking" upon the pseudopodia, which can flow out and be retracted, is apparently a method of locomotion in some species. A marks a fixed point on the surface. (Outlined from the photographs by Dellinger.) recent theory (Mast, 1923), an amoeba moves by contractions of the body, which cause a flow of the central fluid portion out into the pseudopodia. This movement is accompanied by character- istic changes of the protoplasm from a sol to a gel state and from gel to sol. Further explanation appears in the legend of Fig. 86. While Mast's theory supersedes many of the earlier interpretations, one hesitates to accept it as final in the absence of confirmations in various species. It is an interpretation of observations upon Arnceha proteus alone and may not be applicable to all amoebas. It seems, however, to be an important step forward in the under- standing of what actually happens in amoeboid movement. The term behavior is used to designate the activities of an organ- ism as a whole in relation to external and internal conditions. Fundamentally, we are deaUng with irritability, or sensitivity, which is one of the important properties of protoplasm; but the unicellular amoeba is an individual and hence may be studied as any other animal that " behaves " in certain ways under given conditions. The feeding reactions are of particular interest in this connection. The amoeba captures living prey in the form of smaller unicellular animals and plants, which are abundant wherever there are many amoebas. The various species differ in THE SARCODINA 159 their selection of food. A wide variety of microscopic organisms may be utilized, and even manj^-celled animals and plants, if they are small enough for the amoeba to lay hold upon, may serve as its food. Under ordinary laboratory conditions, unicellular and other minute green plants among the fresh-water algse are com- mon food for certain species of amoebas and are often seen within the cell in various stages of digestion. Am(Eha proteus normally feeds upon such green plant cells and also upon motile prey such as ciliate or flagellate protozoa and the minute multicellular animals known as rotifers (Fig. 88). As with amoeboid movement, investigators have made repeated attempts to reduce the process of ingestion to a basis that can be imitated by non-living bodies, as when a drop of chloroform is placed in a watchglass of water and made to '' ingest " bits of shellac or paraffin. The majority of observations, however, indicate that the feeding reactions are com- plex and variable according to the nature of the prey and the state of the amoeba. It is found that food bodies that are motionless, like many of the green plant cells, call forth reactions that differ from those induced by active prey, like cilates and flagellates. Moreover, the two types of reaction are not fixed, but each varies with the particular conditions. The reactions, like those of higher animals, tend to be " qualitative " and " in the interests of " the reacting amoeba in a way that does not seem to occur in non-living bodies. Amoebas have been carefully studied with reference to other forms of behavior, such as their responses to light, to con- tact, and to chemicals. These reactions cannot be discussed here, but it may be said in general that the behavior is not so simple as might be supposed. Metabolic Processes. — In common with other animal bodies, the unicellular amoeba carries on the activities related to waste, repair, and growth in a living organism. Food is ingested, digested, and assimilated. Waste products resulting from dissimilation are excreted. At first glance, these metabolic processes in the protozoa seem much simpler than the complex series of events that have been described under metabolism in a vertebrate animal (p. 71). In reaUty, the metabolism of the amoeba is fundamentally the same as that of the many-celled organism when its essential features are alone considered. The amoeba ingests the smaller organisms which serve as food, it digests this food within its cytoplasm, and then assimilates the fluid products of digestion in the same manner that 160 REPRESENTATIVE SINGLE-CELLED ANIMALS a cell in the body of a many-celled organism assimilates the food it receives from the blood. The exact methods employed in the ingestion of food differ con- siderably in different species of amoeba, and even in the same species, according to the conditions. In general, the outer sur- face of the cell flows around the prey, either as finger-like pseudo- podia or by the formation of a cup-shaped depression in which the . !!ka:^^^?5s<'i .-■v-j >^iS;f^:>^ -VV^ 7 1 '.;••_ v.-. "°. • 'i^: \ y Fig. 88. — Feeding habits of amoebas. A and B, ingesting motile prey like small flagellate by means of pseudopodia with forma- tion of large food vacuole. C, ingesting non-motile prey like filamentous algae without formation of food vacuole. D and E, "cornering" a paramoecium against a bit of debris. F, G, and H, cutting a paramoecium into two pieces either by mechanical pressure or perhaps by stimulating the paramoecium so that it cuts itself into two. 7, 1, 2, and 3, ingesting an encysted protozoan. (A to /?, after Kepner, Taliaferro and Whitlock; F and H, after Mast and Root.) prey is finally enclosed (Fig. 88). Thus the food, surrounded by a drop of the external water, becomes included within the endo- plasm, forming a food vacuole. The mechanics of the process are usually such that a portion of the outer surface of the cell is folded in, loses its connection with the surface, and becomes a part of the endoplasm. The process varies somewhat, but such is the THE SARCODINA 161 more common relationship. Food bodies like the elongated fila- ments of certain green plants (Fig. 88 C) may be included in the endoplasm without the surrounding mass of water. Active prey, like another protozoan, is usually enclosed in a conspicuous vacu- ole, formed either during ingestion by the inclusion of external water or by the secretion of fluid into the vacuole after the capture (Fig. 88 A and B). Digestion occurs within the food vacuole as is indicated by the disintegration of the soft parts of the ingested food. The case of green plants, which undergo color changes similar to those observed in masses of such material when subjected to digestion in a test- tube, is instructive. Since enzymes are necessary for digestion wherever it can be studied on a large enough scale, it may be inferred that enzymes are secreted into the digestive vacuoles from the surrounding cytoplasm. It is claimed that, in some pro- tozoa, bodies, like the secretory granules of the gland cells in a vertebrate animal, surround the vacuole at a certain stage and dis- appear as they are converted into the enzyme which enters the vacuole. After digestion, the food vacuole undergoes a shrinkage in bulk which is supposed to indicate the passage into the cytoplasm of the products of digestion. This corresponds to the assimilation of nutrient material by the cells of a higher animal. In such a man- ner, nutrients, consisting certainly of proteins, and perhaps of fats and sugars, but apparently not including starch, are digested and become incorporated into the protoplasm of the protozoan cell. The non-nutrient portions of the food, such as the siliceous skele- tons of the green plants known as diatoms, and the shells of animals, are egested when the food vacuole, with what remains of its original contents, comes to the surface and breaks through the ectoplasm, so that the amoeba flows away and leaves the contents of the vacu- ole behind. In some cases the indigestible contents of several vacuoles unite and are egested together. As with the vertebrates, the more important story is that of the nutrients that become incorporated into the protoplasm upon their passage outward from the food vacuoles. It is supposed that in the single-celled amoeba, just as in each cell of a many-celled organism like the human being, metabolic changes are constantly taking place. Like other cells, the amoeba receives nutrients, incorj)orates these into its protoplasm by a process of assimilation, 162 REPRESENTATIVE SINGLE-CELLED ANIMALS and gives off the waste materials that are produced by dissimila- tion. The contractile vacuole is commonly described as effecting excretion by receiving water containing in solution the chemical products of metabolism and discharging these to the outside, some- what as does the kidney in a vertebrate animal. However, many protozoa are without contractile vacuoles, and some investigators regard this structure as serving to regulate the water content of the cell and only incidentally for purposes of excretion. In protozoa that do not possess contractile vacuoles, like the majority of both marine and parasitic species, excretion must be supposed to take place in a manner similar to the passage of waste products of metab- olism from the cells of a many-celled animal with a well-devel- oped circulatory system (cf. p. 104). Like a cell in the body of a vertebrate (Fig. 60, p. 104), the protozoan cell is surrounded by fluid. If the outer portions of the cell are permeable to substances in solu- tion, an accumulation of such substances within the cell will result in a diffusion outward, into the intercellular lymph in the one case, and into the surrounding water in the other. The process will continue as long as the outer cell region remains permeable and the strength of solution is greater within the cell than on the outside. Thus the mechanism for the continual removal of the waste prod- ucts of dissimilation is provided alike in protozoa and in metazoa. Even in protozoa having contractile vacuoles, such diffusion of excretory material from the entire surface may be the more impor- tant means of excretion. Respiration in amoeba is similar to internal respiration in the cells of higher animals {cf. p. 93). Oxygen consumption can be demonstrated in such protozoa by means of delicate micro-chem- ical tests. As with the cells of a man or a frog, which are sur- rounded by their intercellular lymph (Fig. 60, p. 104), so with the amoeba surrounded by water (Fig. 86), it may be supposed that oxygen diffuses into the cell as fast as it is consumed by chemical combinations within the cytoplasm. The amoeba will die if deprived of oxygen, as will the cells of higher organisms. When one recalls the various processes of ingestion and egestion, digestion, absorption, circulation, assimilation, dissimilation, and excretion, as described for the vertebrate, it becomes apparent that the amoeba lacks only those steps of the process that are necessi- tated by the size and complexity of the multicellular organism. The essential steps in metabolism occur in amoeba in the same THE SARCODINA 163 manner as in man, namely, by assimilative and dissimilative changes within the individual cell. Reproduction and Life Cycle. — Up to the present time, amoebas have been rather difficult forms to maintain for long periods in B-2. Endogenous budding A. Binary Fission in A.proteus .B-1. Naegleria gruberi, binary fission B-3. Exogenous budding B-4. Encystment and emergence from cyst through pore B-5. Flagellate stages C. Flagellulie, Sp. 1 Fig. 89. — Phases in the Life cycles of amcebas. A, A. proteus, a common fresh-water species. B, 1 to 5, Nmgleria gruberi, one of the soil amcebas, various phases not necessarily in sequence as shown. C, flagellulse of an unknown species apparently in conjugation. (A, after Botsford; B, after C. W. Wilson.) laboratory cultures. It is probable that the cells are very minute during part of the cycle and hence difficult to follow with certainty by the methods of pedigreed cultures now used in the study of protozoa. In the large active phase of Amoeba proteus, it is well U>4 REPRESENTATIVK SIXGLE-CELLKD ANIMALS known that ivprodiiotioii occurs by cell din'sion or binaiy fission, by which the coll divides into equal halves (Fig. 80 A). Encyst- metit. or the endosun^ of the cell in a (\v^^/ formed as a secretion after the ama^^a ha^; contracted into a sphere and become quiescent, also occurs in some sjxvies of amcvba that have been studied (Fig. SO B 4). The formation of flagellated cells and their union in conjugation, a process comparable with the union of egg and sperm cells in many-celled animals, is known in some of the Sarcodina (,Fig. SO CV However, the details of the life cycle in all its particulars have not yet been established in any one instance for Amcvba prokus and similar fresh-water species. While more is known regarding some of the parasitic species in this sub- division of the Sarcodina. it is not safe to conclude that the same stages occiu" in free-living forms, although thei-e is doubtless some parallelism. If we now summarize what has been learned concerning the amcvba in the foregoing paragraphs and in the laboratory study which is presumed to accompany this account: The three gi-eat bodily functions are no less important for the ama^ba than for the vertebrate animal. The imicellular organism cannot exhibit the complexities of the many-celled state, but it does exhibit metabol- ism, irritability, and reproduction, within the limits of a single cell, comparable with these activities in the cells of the metazoan. Waste and repair of protoplasm, growth, and reproduction of the cell thus appear to be fundamentally alike at the two ends of the anim;\l kingdom. Response to stimulation in an amoeba may be as complex as it can be demonstrated to be in a majority, if not all. of the cells of more complex animal bodies. The activities of protoplasm and hence of cells are, therefore, similar in the simplest and in the most complex animals. Other Sarcodina. — In the subdivision of the Sarcodina known as Rhizopoda {cf. p. 154), there are many forms that resemble the amcTba. Others, like the genera Arcella and Difflugia (Fig. So), possess shells into which the animal can withdraw and from the mouth of which issue pseudopodia. In another type, called the Foraminifera, there is a shell composed of carbonate of lime, through which pseudopodia pass by one or by numerous openings. These psuedopodia form networks in which the food particles are ingested. With few exceptions, the Foraminifera are marine, occurring in the open ocean in unbelievable niunbers. TIIE SARCODIXA 165 As the animals die their shells sink to the bottonn, fornning the " forarniniferal ooze " found at great depths in regions like the North Atlantic CFig. 90). In spite of their small size, and the slow rate at which they accumulate upon the bottom, such protozoan skeletons have become a part of sedimentary rocks in the same manner as the shells of larger animals. The chalk formations in Fig. 90. — .Shells of marine Foramiriifera. I>eft, foraminiferal ooze from bottom of ocean at 1900 fathoms. Right, piece of nummulitic linieBtone, shouing {(jtaW ehella of the foraniiniferari Summulitft. 'From .Ship- "ey, " Inverebrate Zix^logy," copyright, 1903, by A. and C. Black, reprinted by per- luiiuiion.) various parts of the world, like tho.se of England and France, as .seen in the chalk chffs of the English Channel, are composed almost exclusively of the .shells of Foraminifera that once lived at the sur- face of the ocean, died, and " rained " down upon the lx)ttom, to be later con.solidated into rock and raised up as part of the dry land, where they are now expo.sed as layers, .sometimes hundreds of feet in thickness. Among the Actinopoda, two representative types deser^-e mention: the " sun animalcules," hke Actinophrys sol (Fig. 8.5 j, which is common in fresh wat€r; and the marine forms known as Radiolaria, which are famous for their Vxjautiful .siUceous skeletons. These, like the Poraminifera, have contributed to the formation of sedimentary' rocks. In conclu.sion it may be remarked that the Sarcodina are the simplest of the protozoa in their internal organization and external differentiation. There are, however, rea.sons for believing that they are less primitive, from an evolutionary .standpoint, than the Mastigophora. Many of the Sarcodina, for example, have a 166 REPRESENTATIVE SINGLE-CELLED ANIMALS flagellated stage in their life cycles. The Mastigophora are more primitive in their metabolic activities, since they may combine the modes of nutrition of both animals and plants even within the same individual. Nevertheless, the amoebas and their relatives are fairly called the simplest of all organisms that exhibit the typical animal functions. The Mastigophora Structure of Euglena. — The Mastigophora include types that are distinctly plant-like in their appearance and manner of nutri- tion, and others that resemble animals. The genus Euglena (Fig. 91) is representative, because in a measure it combines the characteristics of the plant-like and the animal-like mastigo- phorans. What follows applies to Euglena viridis and E. gracilis, two of the commonest forms, and also to the other species of this genus that are abundant in fresh water. The cell in typical euglenoids, as members of the Family Eugenoidina are often called (Fig. 91 A), is covered with an outer layer of ectosarc, suflaciently stiff to preserve the contours of the organism as it swims through the water, but flexible enough to allow the extensions and con- tractions that are termed "euglenoid movements" (Fig. 91 B). It is quite characteristic of the euglenoids to have spiral markings of various sorts upon the outer surface of the ectosarc. In some species these are highly developed. When seen in what might be termed the "lateral" view (Fig. 91 C), the anterior end of the euglenoid shows a mouth-like opening, the cytostome, with one side suggesting a projecting upper lip. However, the cytostome is not so much a slit between the lips as it is an oval opening leading into the gullet. The flagellum, a whip-like structure used in locomotion, extends from the gullet into the water. At one side of the gullet is a vacuole system consisting of a reservoir which drains into the gullet and is surrounded by contractile vacuoles (Fig. 91 A). An excretory function can perhaps be ascribed to these vacuoles, as with the vacuoles of the Sarcodina. Also near the anterior end, is a mass of red pigment (Fig. 91 C and C), called the eye-spot because it is probably the part that is most sensitive to light; although it would be diflficult to prove this beyond question. Euglenoids that regularly ingest food particles and pass them into the cell in food' vacuoles have a more highly developed gullet region than Euglena viridis. THE MASTIGOPHORA 167 The inner protoplasm of the cell, which may be referred to as the endosarc, is of a semi-fluid consistency, but sufficiently stiff to hold the various inclusions in place without the obvious flowing that occurs in many other protozoa. The nucleus lies near the center. Embedded in the ajtoplasm are found structures charac- teristic of green plants, the chromatophores, containing chlorophyll, and masses of the carbohydrate paramylum, a substance allied to starch. Fig. 91.— Structure and activities of euglena and related flagellates. A, typical euglenoid. B, 1-3, euglenoid movements of Astasia. C and C, gullet and stigma or eye-spot of euglena. D, spiral course of swimming euglena. E, inge.stion of bacteria by euglena. F, ingestion of another protozoan by Peranema. G, euglena feeding upon mass of bacteria. H, euglena completing longitudinal fission. 6. mass of bacteria; c.e.. food; ch, chromatophore; /, fiagellum; /.v.. food vacuole; (7. gullet; n. nucleus; p, paramylum body; r. reservoir surrounded by contractile vacuoles; r.o., pharyngeal rod; s. striation; st, stigma, or eye-spot. (A, after Walton; C, after Wager; E to //, after Tannreuther.) Movements, Locomotion, and Behavior. — Expansions and contractions of the cell are frequently observed in euglenas that 168 REPRESENTATIVE SINGLE-CELLED ANIMALS are not in active locomotion by means of the flagellum. In some euglenoids there are also movements that resemble peristaltic waves as they traverse the cell body. All such activities are termed " euglenoid," since they are commonly seen in the genus Euglena (Fig. 91 B). Some of the larger euglenas regularly lose their flagella and crawl about by a peculiar motion which is not fully understood, although it has been ascribed to the existence of a surface film of flowing protoplasm. This film is invisible but its presence may be demonstrated by the movements of fine particles adhering to its surface. The most characteristic movements, how- ever, are those effected by means of the flagellum. The flagellum is a cyhndrical structure composed of an axial filament surrounded by a sheath. This filament arises from the hlepharoplast or basal granule, an enlargement within the cell, and is the contractile portion (Fig. 91 A). The sheath apparent!} functions as an elastic covering that tends to keep the flagellum in an extended position. The action of such a flagellum as that of euglena may be compared with the spiral waves which can be made to pass along a rope that is tied at one end and held in the hand at the other. The flagellum, however, is free at one end and makes its own spiral waVes, which cause the cell to move through the water. Careful observation reveals that the euglena pursues a spiral course (Fig. 91 D) which consists of three factors: (1) It moves forward, progression; (2) it turns on its long axis, rotation; and (3) it swings away from the axis of the spiral, swerving. One can understand what happens by taking an object like a cane or a larger cylinder and marking one side to indicate the shorter lip of the euglena which is kept turned toward the axis of the spiral. If the cylinder is then made to describe a cone and rotated to keep this lip toward the axis of the cone, the factors (2) and (3) above will be represented. By walking in the direction of the axis and continuing to describe the cone, the factor (1) is added. In this manner a euglena pursues a spiral path, always keeping the shorter lip toward the axis of the spiral, and proceeding in a course, which, if not a straight line, nevertheless enables it to steer what amounts to a straight course " across the trackless deep." The spiral movement of the organism is, of course, a result of the spiral beat- ing of the flagellum. This mode of locomotion is the common one in asymmetrical organisms, whether protozoa or other minute forms that swim through the open water. By such means, an THE MASTIGOPHORA 169 asymmetrical body is able to move in what is approximately a straight line. f f I I I I n -«« -«¥«t -«« -«« -«4 -«« --^^a, -«« -*a n -*<» < 0.03 mnir Fig. 92. — Euglcna sp. (?) in crawling slate, showing details in process of orientation. a-c, positions of euglena with light from n intercepted; c-m, positions after light from n is turned on and that from o cut off so as to change the direction of the rays; es, eye-spot; n, o, direction of light; v, contractile vacuole. (From Mast, "Light and the Behavior of Organismb," John Wiley and Sons, copyright, 1910, reprinted by permission.) By its movements of crawling and contracting, and by its spiral swimming, the euglena responds to a variety of stimuli. The behavior in response to hght has been most thoroughly studied since it can be easily observed. Euglenas depend upon sunlight for one type of their nutrition, and hence, like green plants, they respond positively to light of optimum intensity. The reactions of a crawling euglena with respect to light are shown by 170 REPRESENTATIVE SINGLE-CELLED ANIMALS which is sufficiently explained by the legend. It will be noted that the individual rotates slowly on its long axis as it proceeds, and that the response is a rather complicated one; although the cell may seem to swing into line with the new source of illumina- tion without much delay. A similar positive response to light occurs in the free-swimming individual (Fig. 91 D). Just how the euglena brings itself into the new position, whether by what might be termed a " trial and error " method or by a more direct means, as when a boat swings directly in response tc a " stimulation " of the rudder, is a disputed question among investigators. This can best be discussed for the protozoa as a whole in connection with a species like Paramcecium, which is larger and more easily observed (rf. p. 178). It is thus apparent that the euglenoid cell responds to stimulation as does all protoplasm and hence gives evidence of an irritability comparable with that shown by the cells of higher organisms. Metabolic Processes. — The term holozoic nutrition is applied by zoologists to the nutritive processes that occur typically in animals and that have been described in a preceding chapter, as they occur in the vertebrate, and again in the protozoan A?7iceba proteus. Food, in the form of living prey or material derived from the bodies of animals and plants, is ingested, and then digested to simpler compounds, which are incorporated into the protoplasm by assimilation. The green plants, on the other hand, exhibit such a relationship to their surroundings that they are able to take in simple compounds, such as oxygen, carbon dioxide, water, and the mineral salts of the soil, and, by the process known as photosyn- thesis because it is dependent upon sunlight, followed by other synthetic processes, to build up the nutrients upon which their existence depends. The substance known as chlorophyll, which gives the green color to plants, is necessary for photosynthesis, which is the initial step in this synthetic process. Nutrition of this kind is referred to as holophytic in textbooks of zoology, although this term is not in common use among botanists. What is known to occur in higher plants can be assumed to take place in any cell that contains chlorophyll, and its occurrence can in part be demonstrated by appropriate experiments upon unicel- lular organisms. Euglena can be shown to liberate oxygen in the presence of sunlight, as a result of photosynthetic processes carried on by virtue of its chlorophyll. Starch, in the form of paramylum^ THE MASTIGOPHORA 171 is formed by the euglenoid cell, and the othe? constituents of protoplasm must be synthesized when euglenas grow and multiply. In this manner the euglena carries on the holophytic nutrition characteristic of all living bodies that possess chlorophyll. But the euglena is further interesting because this is not its sole mode of nutrition. Small organisms, such as bacteria, may be ingested through the gullet (Fig. 91E-G) surrounded by vacuoles, and digested in the cj^oplasm as in the feeding of an amoeba or of other holozoic protozoa. Respiration, comparable with internal respiration in the cells of higher animals, as well as excretion of carbon dioxide, can be shown to occur. A third type of nutrition, according to zoological terminology, is known as saprophytic. In this case the organism is unable to synthesize its food from such simple compounds as do the chloro- phyll-bearing organisms. It must depend upon the decomposition products of other plants and animals, after the manner of the moulds and many bacteria that grow upon organic material. When certain species of euglenas are kept in a low illumination and in a medium rich in decomposing organic matter, they lose most of their chlorophyll; and, since they grow and multiply without other nutritive activities, it may be presumed that they carry on their nutritive processes saproph^-tically. Such is known to be the case with many other Mastigophora. Reproduction and Life Cycle. — As an example of the condi- tions in one of the common euglenoids, the life cycle of Euglena gracilis may be described. This species is well suited for pur- poses of illustration, because its cycle includes an unusual range of possibihties (Fig. 93). As with the majority of fresh-water pro- tozoa, there is an alternation of " active " and " encysted " phases. In the active phase, the euglenas usually multiply by longitudinal division of the cell, but transverse division may also occur. In the former case (Fig. 93, 2), the pigment spot, the reservoir, and the enlargement at the base of the flagellum divide, to form these parts for the two daughter cells, while the division of the nucleus is in progress. The external part of the old flagellum is always cast off before division begins, and a new one formed by each daughter cell as division proceeds. As these internal changes advance, a division of the cytosome begins at the anterior end and proceeds posteriorly until separation is accomphshed. The nucleus divides by a pecuhar type of mitosis (cf. p. 137). In 172 REPRESENTATIVE SINGLE-CELLED ANIMALS transverse division (Fig. 93, 3 and 4), the pigment spot and reser- voir also divide, and one member of each migrates to form the 1. Free-swi moling Stage 2. Longitudinal Fission in Active Sta^e 3. Transverse Fission in Active Stage 4. Transverse Fission in Cyst 5. Encystraent without Fission 6. Encystment with Longitudinal Fission 7. Conjugation in free-swiraming stage Fig. 93. — Phases in the hfe cycle of Euglena gracilis. (After Tannreuther.) pigment spot and reservoir of the posterior cell. While this is taking place the nucleus divides, and a transverse constriction of THE MASTIGOPHORA 173 the cell body is begun. A new gullet and flagellum are formed on the posterior cell. The old flagellum may persist on the anterior cell, or it may be dropped and a new one produced. Separation of the cells finally occurs. Euglena gracilis thus reproduces itself either by longitudinal or by transverse division of the cell during its active phase. Under various external conditions, and perhaps as a result of internal changes, euglenas may undergo encystment by secretion of material that hardens as a thin covering, or cyst, enclosing the cell. In E. gracilis, temporary encystment may occur at the surface of ponds during hot da3^s as shown in Fig. 93, 5. Later, the individuals escape from these cysts without reproduc- tion. Apparently, cysts may also be formed for the purpose of cell division, which in this instance is transverse (Fig. 93, 4). The euglena becomes quiet, loses its flagellum, remains extended, and secretes the cyst. Transverse division now occurs in the same general manner as in the active stage. When all the structures are completely formed for each cell, the individuals escape from the cyst without further division. In the more common form of encystment, however, there may be numerous divisions that are longitudinal. In this instance the euglena casts off its flagellum, becomes spherical, and secretes a delicate cyst. Division then occurs longitudinally. If the culture is kept in sunlight the cells grow as the result of holoph^-tic nutrition, and repeated divisions may ensue, resulting in great distention of the original cyst. Sometimes this is ruptured, the liberated cells form new cysts about themselves, and each repeats the process (Fig. 93 6). When the active phase is to be resumed, flagella are produced, and the euglenas break from the cyst and swim away. A process of conjugation, by which cells unite in pairs and become permanently fused into a single cell, having its single nucleus and c>"tosome derived from this double parentage, is well established in many flagellated protozoa. In the genus Euglena it has been described by some investigators but is ques- tioned by others. Unpublished observations by Tannreuther on E. gracilis indicate what is evidently conjugation of free-swimming euglenas (Fig. 93, 7). The cells unite in pairs, and as they fuse a union of the two nuclei occurs. Thus, one cell is formed by the union of two as in fertilization, which is the term appKed to the union of ovum and spermatozoon in many-celled animals. Such 174 REPRESENTATIVE SINGLE-CELLED ANIMALS conjugating cells of protozoa are collectively termed gametes, as are also the sex cells, ova and spermatozoa, of metazoa. Likewise, conjugation and fertilization, or union of ovum and spermatozoon, are collectively termed syngamy, meaning union of gametes. The single cell formed by syngamy is termed the zygote, which means something " j^oked." In the case of euglena above mentioned, the gametes are all ahke and hence are called isogametes. In some protozoa (Fig. 108, p. 208) they are dissimilar, Uke the spermato- zoon and ovum of metazoa, and are therefore called anisogametes. Syngamj' may thus be a process of isogamy, by the union of isogametes, or a process oianisogamy,hyihe union of anisogametes. Such smaller and larger conjugating cells in protozoa are called viicrogametes and macrogametes, respectively, and are comparable with spermatozoa and ova (c/. Fig. 212, p. 402). The phe- nomenon of sex, which consists in the production of two kinds of gametes, is recognizable in protozoa wherever there are micro- gametes that are smaller and more active, like spermatozoa, and macrogametes that are larger and tend to be food-laden, like the ova of metazoa. Thus in the euglena and other protozoa, reprodudion, which max be defined as the formation of a new indi\'idual, may be asexual, by means of cell di\'ision, or sexual, b}' means of conjugation. Other Mastigophora. — The flagellated protozoa are difficult to subdi\'ide into lesser groups because of the diversified habits and structures of various species. On the one hand, there are flagel- lates, the Phytomastigina, so plant-like that many of them are classified as plants; on the other, there are forms, the Zoomasti- gina, that are distincth' animal-like in their nature. In addition there are forms, like the genus Mastigamxxha (Fig. 85), in which the cell is amoeboid and resembles the Sarcodina, although the presence of a flagellum leads to the classification of such genera with the ]Mastigophora. There are also many instances of amoe- boid stages in the life cj'cles of species that are typical flagellates throughout the greater portion of the life histon,'-, a condition which may be compared with the occurrence of flagellated stages in the hfe cycles of certain Sarcodina. These facts suggest an evolutionary relationship between Mastigophora and green plants on the one hand and Mastigophora and Sarcodina on the other. Turning to specific examples of Mastigophora that are of inter- est, there are many simple flagellates occurring in fresh water THE INFUSORIA 175 (Fig. 85). Of these the euglenoids are perhaps the most common, but others are frequently observed. In forms like Trachelomonas it seems that the cell wall is very firm, and in others, like Peranema, the cell exhibits surprising mobility. An interesting type is the choanoflagellates like Monosiga, in which there is a delicate collar surrounding the flagellum. Another is the marine genus Noctiluca, the "nocturnal light," a very large species having one finger-like flagellum, another smaller one in the region of the buccal groove, and cytoplasm which is highly vacuolated. In this instance the cells produ(;e brilliant luminescence throughout large areas of the ocean. Many of the Mastigophora are parasitic. The posterior region of the digestive tract in man often harbors such forms, and almost any frog or tadpole will show more than one species of these para- sites in its large intestine. A most interesting type is the Iry- panosome (Fig. 109, p. 210), many species of which occur in the blood of vertebrates and in the digestive tracts of invertebrates. These are elongate cells with an undulating membrane along one side, on the edge of which is attached a flagellum that becomes fiee posteriorly and arises anteriorly within the cell. One famous species is Trypanosoma gambiense, the parasite causing one type of the sleeping sickness that is so fatal to man in equatorial Africa. Among the plant-like Mastigophora there are some striking examples of colonial organization which suggest a transition from the unicellular to the multicellular state. Some of these are considered in Chapter 8. The Infusoria Structure of Paramcecium caudatum. — The ciliated Protozoa, or Infusoria (Fig. 94), are grouped into two main subdivisions: the Ciliata, which possess cilia throughout the life cycle; and the Sudoria, in which cilia are present for only a limited period, and are later replaced by the so-called tentacles, which are used in the capture of food (Fig. 84 D) . Among the CiUata, the species Para- mcpcium caudatum (Fig. 95) is one of the simpler types and is a common object of laboratory study. If any one kind can be said to be the omnipresent protozoan of 'fresh water, it is paramce- cium, since no other type occurs more commonly in cultures and under a wider range of circumstances in nature. It is, so to speak, adapted to " slum " conditions, and hence can be main- 176 REPRESENTATIVE SINGLE-CELLED ANIMALS tained in the laboratory during long periods, so long in fact that it may be regarded as an animal that can be thoroughly " domesticated." The size of Paramoecium caudatum varies greatly, because, like all other species of plants and animals that have been exhaus- Prorodon Euplotes DIdlnium Stentor Stylonichia l: Trichomonas / r\ Lacrymaria Spirostomum Podophrya Lionotus Fig. 94. — Common ciliated and other protozoa from fresh water. (Drawn in part by Wiley Crawford.) tively studied, the species is really composed of many races which breed true among themselves but may differ widely when one race is compared with another (c/. Fig. 308, p. 559). The bod}'' is spindle-shaped with the anterior end bluntly rounded and the posterior end more pointed. The older microscopists called paramoecium the " slipper animalcule " because its shape seemed to them to resemble the outline of a slipper. At one side is a depression, the oral or buccal groove, passing diagonally from the anterior end to about the middle of the body and ending in a tunnel-like cavity, the cytopharynx, or gullet. The outer end of the gullet is the cyfostome, or cell mouth. The cilia that clothe the body are of uniform length, save at the extreme posterior end THE INFUSORIA 177 .\N^\MI///,'////., If' ; ' ; ' ill '^■' '-- ,^^ c %fy' ura Structure and activities of paramoecium. A, spiral path of paramoecium. h, paramoecium at rest, showing cilia motionless against a cotton fiber. C, paramoecium at rest against a mass of bacteria (o) showing cur- rents produced by the cilia. D, P. calkinsi, external contours and buccal groove. E. P. calkinsi, internal structure. F, P. caudatum, section perpendicular to surface. G, sur- face of pelhcle showing cilia and trichocysts. H, section through cytostome and gullet, /, trichocysts before (left) and after (right) discharge. c, cilia with basal granules; cu, contractile vacuole; ec, ectoplasm; en, endoplasm; fv, food vacuole; ma, macronucleus; mi, micronucleus; n, nucleus; t, trichocysts which form in endoplasm and migrate to ectoplasm; um, undulating membrane. {A, B, and C from Jennings, "Behavior of the Lower Organisms," reprinted by permission of Columbia University Press; D and E, after Woodruff; F to H, after Maier; I, after Khainsky.) 178 REPRESENTATIVE SINGLE-CELLED ANIMALS and in the buccal groove, where they are shghtly longer. Within the gullet, the cilia are fused together in a plate to form the undu- lating membrane, which aids in the passage of food to the interior of the cell. Since the egestion of fsecal material occurs at but one point on the surface of the cell, there is a cytopyge, or cell anus, as well as a cell mouth. In this regard the paramoecium is more speciahzed than the amoeba, in which ingestion and egestion occur at any place on the surface. The outermost layer of the cell is a thin cuticle, or peUicle, which is like an elastic membrane. This cuticle is striated to correspond to the distribution of the cilia along definite hnes (Fig. 95 F and G). Beneath the cuticle is a thicker, non-granular layer, the ectoplasm, from which the ciHa arise (Fig. 95 E and F), and which contains the numerous trichocysts. The latter are apparently defensive struc- tures since they are discharged as threads upon appropriate stimu- lation (Fig. 95 I). It is the ectoplasm that gives to the paramoe- cium its permanent shape, for the internal portion, or endoplasm, of the cell is semi-fluid. The paramoecium may be pressed out of shape mechanically, but resumes its former outUne because of the elastic ectoplasm. The endoplasm contains a macronucleus, which is related to the vegetative processes of the cell; and a micronucleus, which is the part of the nuclear apparatus that is most intimately related to heredity and reproduction. The endoplasm also con- tains the two contractile vacuoles, with their radiating canals, and the food vacuoles. Larger masses of various sorts may also be found, in addition to the very fine inclusions of the cytoplasm. Movements, Locomotion, and Behavior. — The cytosome in paramoecium has a constant shape, unless it is temporarily dis- torted by external pressure. Locomotion and other movements are effected by the ciha, which beat in such a fashion that the animal pursues a spiral course (Fig. 95 A). The same factors of progression, rotation, and swerving, as described for euglena (cf. p. 168), are to be found in the locomotion of paramoecium. In paramoecium, however, these factors are more easily recognizable. The progression is caused by the backward beat of the cilia, and the rotation by the fact that they strike diagonally. It has been supposed that the swerving results from a stronger beat of the cilia in the buccal groove, but there are some ciliates that follow a spiral course without having any such differentiated area, and hence the stroke of the buccal cilia may not be the only factor. These three THE INFUSORIA 179 factors operating together bring about the spiral course that is pursued in locomotion. There is, however, much more to the behavior than mere forward progression. The various responses to stimuli that constitute the beha\ior of paramoecium are effected by modifications of the three factors just mentioned. Thus, if a • paramoecium is swimming forward and comes in contact with a chemical solution that stimulates the cell Fig. %. — Reactions of paramoecium. A, avoiding reaction. 1-6, successive positions occupied by the animal, not showing the rotation on the long axis. B, path followed by an individual trapped in a drop of acid. C, paramcecium approaching a region containing India ink (shown by the dots). A cone of ink is drawn toward the anterior end and oral groove of the animal. D, para- muecium swinging its anterior end about in a small circle, in a weak avoiding reaction. 1, 2, 3, 4, successive positions occupied. (From Jennings, "Behavior of the Lower Organ- isms," reprinted by permission of Columbia University Presa.) strongly but does not injure it, the animal responds by reversing its course and may swim backward spirally for a considerable dis- tance before the normal progression is again resumed. The more common reaction, however, is that seen in the responses to less violent stimulation. When, for example, the animal strikes an obstacle (Fig. 96 A), it responds by backing off a short dis- tance, stopping the backward movement, describing a cone by continuing to rotate and swerve dorsally (Fig. 96 D), and 180 REPRESENTATIVE SINGLE-CELLED ANIMALS proceeds forward by a resumption of the spiral course. A com- plete cone-shaped figure is not always described, but there is always a portion of such a figure. The angle of the cone may vary to such an extent that in extreme cases the animal swings about almost in one plane, as would be the case if a cone became flattened out to form a plane surface. Under mild stimulation, however, the response is not so extreme. The paramoecium backs away from the obstacle, stops, describes part of a cone by con- tinuing to swerve and rotate, and starts forward once more on a spiral course but with the axis of the spiral at a new angle. If this Une of progress brings it again in contact with the obstacle, it repeats the process of backing off, describing the cone-shaped figure, and proceeding in a new direction. Thus, by a series of what may be called " trials," some of which may be " errors," the paramoecium may eventually succeed in finding its way around an obstacle. This automatic and reflex-like response has been calbd the " avoiding reaction." The same kind of reaction occurs in response to other situations, as when the animal is confined within the trap-like meshes of the cotton fibers sometimes used in laboratory study, or in response to other forms of stimulation likely to be encountered in nature. With electrical and other stimuli not encountered in nature, the responses may be different. When many paramoecia are crowded together, the individuals seem to be merely darting backward and forward in aimless fashion. When, however, the response of isolated individuals is studied vvith reference to particular stimuli, the behavior is found to be as above described. The avoiding reaction is given in response to mechanical, thermal, and chemical stimuli, water currents, gravity, and centrifugal force. This form of activity so much resembles the " trial and error " behavior of higher organisms that it seems justifiable to extend this term even to the protozoa. The use of such a phrase does not imply that the paramoecium acts with any degree of inteUigence, that it " tries " and " errs," and recognizes its mistakes; but only that it responds mechanically with a stereotyped form of behavior, which, if repeated a sufficient number of times, will usually bring the individual under conditions that are favorable for its normal activities. As the animal swims through the water or swerves in giving the avoiding reaction, the beat of the cilia in this region draws a cone of fluid against the buccal groove, which thus " samples " the water THE INFUSORIA 181 in advance of its contact with the entire body. As a result of this, responses may be made before the cell becomes wholly surrounded by new conditions, as when it approaches an area containing differ- ent chemical compounds in solution. The possibility of such " advance information " may be demonstrated by watching a paramoecium as it approaches a drop of India ink (Fig. 96 C). Fig. 97. — Reactions of paramoecium to salts, acids, and heat. A, method of introducine a chemical into a slide of infusoria. B, slide of paramopcia four minutes after the introduction of a drop of 5 per cent NaCI. The drop remains empty. C, a slide of paramoecia is heated to 40 or 45 degrees C, then a drop of cold water 'repre- sented by the outline a) is placed on the upper surface of the cover-glass. The animals collect beneath this drop, as shown in the figure. D, coUectiou of paramoecia in a drop of 1,30 per cent acetic acid. (From Jennings, "Behavior of the Lower Organisms," reprinted by permission of Columbia University Press.) The power of receiving stimulation by "sampling" in advance is no doubt useful to the animal in nature, and is presumably utilized under conditions of experimentation when the cell responds (Fig. 97 B) l)y keeping out of a drop of | per cent salt solution, or, having entered a drop of acid (Fig. 97 D), remains trapped therein because it responds negatively whenever it again comes in contact with the surrounding water. The avoiding reaction is, of course, the means by which all such responses are effected. Responses to stimuli such as those described above are usually known as tropisms; although, like the term " instinct," this word has been used in so many ways that some investigators would prefer to abandon it entirely. Thus, a response to light is positive or negative phototropism; a response to mechanical contacts, 182 REPRESENTATIVE SINGLE-CELLED ANIMALS stereotropism; a response to chemicals, chemotropism; and so on. The word taxis is sometimes used in place of tropism, and thus we have phototaxis, chemotaxis, etc. If one attempts to define a tropism, not much more can be said, in view of the present disputed use of the term, than that almost any response of an animal to a stimulus may be called a tropism; but in practice the word is not so often used for complex responses as for simple reactions. We speak of the phototropism of a protozoan, of a moth flying into the flame, even of a fish reacting to a fisherman's " flare "; but not so commonly of the phototropism of dogs and other higher animals. The important things to understand, in the present state of our knowledge, are the visible manner in which each form responds to a given stimulus, and, where a nervous system is present, the internal processes. Any general theory of " tropisms " must rest upon particular cases. There is still considerable disagree- ment as to how these responses are brought about, whether, for example, by trial and error or by a simpler means. In the case of paramoecium it therefore appears that response to the kinds of stimulation which the organism meets in nature occurs by means of a form of behavior known as the avoiding reaction. This can be modified only within limits; but by per- forming it a sufficient number of times, no matter how blindly, a suitable adjustment to existing surroundings may be effected if any such is possible. Thus the paramoecium gives evidence of the same underlying phenomenon of irritdbility, or response to stimu- lation, that is observed in amoeba and euglena and in higher organisms. In view of the complexities of behavior in this species, it is of interest to know that certain structures recentl}^ discovered in Paramcpcium caudatum are interpreted as a miniature neuromotor system, comparable in function with the nervous system of a many-celled animal. There is an area near the anterior end of the cytostome, called the motor center or motorium, from which minute fibers extend to the peripheral parts of the cell (Fig. 98 A). The ends of these fibers are described as connected with granules at the bases of the cilia and with the trichocysts (Fig. 98 B). Con- nected with the motorium are two lesser centers in the wall of the cytopharynx (Fig. 98 C). Similar systems of more complicated nature have been found in some other ciliates (Fig. 102, p. 194, and Fig. 153, p. 315). If these systems are correctly interpreted as THE INFUSORIA 183 having the function of transmission of impulses within such pro- tozoan cells, they constitute a mechanism by which coordinated action of parts of the cell may be accomphshed. The function of such a mechanism may be compared with that of the nervous fi Fig. 98.— Neuromotor mechanism of Paramcecium caudatum. A, showing fibers radiating from motorium im); B, showing fibers connected with cilia and trichocysts; C, showing lesser centers of coordination in region of cytopharynx. CAfter Rees, University of California Publications, 1922.) system in a multicellular animal. Since it hes within the limits of a single cell, its structure cannot be so compared; unless one is more impressed by the Organismal Theory (c/. p. 194) than by the Cell Theory in making comparisons between single-celled and many-celled animals. Metabolic Processes. — In feeding, the cilia of paramcecium draw a current of water against the buccal region (Fig. 95 C), and thus masses of fine particles, such as bacteria, smaller protozoa, and organic debris, w^hich constitute the food, are passed through the mouth into the gullet. By means of the undulating membrane and by a gulping movement of the protoplasm, such masses of food are passed into the endoplasm, included in a drop of water, and are ingested. Within the endoplasm these food vacuoles move 184 REPRESENTATIVE SINGLE-CELLED ANIMALS along a definite course (Fig. 95 E). As with amoeba, it is presumed that enzymes are secreted into the vacuoles, and that the products of digestion undergo assimilation by the surrounding cytoplasm. Finally, egestion of the indigestible remnants contained within the vacuoles takes place at the anal spot, or cytopyge. As in the many-celled body, the assimilated material becomes a part of the protoplasm. Dissimilation is, of course, always occurring; and excretion of the end products of metabolism may take place by diffusion over the entire surface of the cell or by means of the contractile vacuoles. While the latter have been commonly regarded as having an excretory function, it is possible that they are primarily for the purpose of regulating the water content of the cell. Even so, we should expect the extruded water to con- tain any soluble wastes that might be in solution in the water within the protoplasm. Under suitable conditions the storage of nutrient materials, such as starches and fats, may take place in the cytoplasm. Respiration, as in other protozoa, is the pas- sage into the cell of oxygen from the surrounding water and is comparable with internal respiration in the cells of metazoa. Reproduction and Life Cycle. — As with amoeba and euglena, the life cycle of paramcecium consists principally of asexual repro- duction by cell division (Fig. 99, E and F). Sexual reproduction occurs by conjugation (Fig. 100). The animal seldom, if ever, encysts under laboratory conditions, but encystment has been observed (Fig. 99 A to D). Perhaps it takes place more frequently in nature, since it is difficult to understand how any protozoan can be so universally distributed in fresh water without undergoing at least occasional encystment as a means of tiding over adverse con- ditions. It is true, however, that encysted stages are rarely observed, and some investigators have doubted whether encyst- ment ever occurs. As seen in the laboratory, the life cycle consists apparently of an endless active stage with frequent cell division and occasional conjugation. There also occurs a process of internal nuclear reorganization, termed endomixis. Aside from the problem of encystment, the life cycle is very well known, because of the ease with which the common species of paramcecium can be reared under laboratory conditions. Wood- ruff, for example, has maintained P. aurelia in the laboratory, without conjugation, from May 1, 1907, until the present time, and this race can no doubt be thus maintained indefinitely with- THE INFUSORIA 185 out conjugation. The technique by which such results are obtained consists of isolating single paramoecia in a suitable culture medium, like hay infusion, leaving each individual until it has divided once or twice, and then isolating as many of the progen}^ as it is desirable to retain. As division of the cell usually occurs Fig. 99. — Encystment and binary fission in paramct'cium. A to D, sketches by Curtis from living material observed by MeClendon, showing problematical encystment and emergence a few days later. The animal folded itself together (A) and rotated as shown by arrows. Gradually a covering appeared (/}). Within this the paramoecium seemed to have a truncate outline (C) that persisted for a short time after emergence (D), but disappeared in the course of one or two cell di\'isions as the par- mnecium resumed its normal activities. E and F, binary fission, with dedifferentiation of old gullet and rcdifferentiation of a new one for each daughter cell, and with mitosis of the niicronudeus and amitosis of macronuclexis. from one to four times in twenty-four hours, the multiplication is very rapid. If all could be preserved, they would soon produce an inconceivably large mass of protoplasm (c/. p. 548). Although such increase cannot actually occur either in the laboratory or in nature, it is always a possibility as shown by the rapidity of multipHcation often observed in a culture jar that has been "infected" with paramoecia and allowed to stand for several days in the laboratory. P. caudatum, as well as P. aurelia, lends itself to "domestica- tion" by the cultural methods above described. In the cell division by which asexual reproduction is accom- plished, the macronucleus divides by a simple elongation and constriction (Fig. 99); the micronucleus, by a special kind of mitosis (f/. Chapter 6, p. 137). As division of the two nuclei nears completion, the cell body becomes constricted near its middle and finally separates into two daughter cells. Meanwhile, one 186 REPRESENTATIVE SINGLE-CELLED ANIMALS new contractile vacuole is formed for each " daughter," and new gullets arise in each from the material of the original buccal region. After separation, the daughter cells normally grow to full size before a new division occurs. Under favorable conditions there A. Two individuals unite by buccal grooves. The micrO' nuclei separate from the macronuclei. B. The macronucle'us begins to degenerate. The micTonucleus divides. C. The micronuclei divide again. Three of each four disappear. D. The remaining micronuclei divide to form migratory and stationary nuclei. Exchange of migratory nuclei. f. The migratory and stationary nuclei unite. r. The fusion nucleus Is thus formed. 0. The individuals separate. H. Division of the fusion nucleus. I. Division, as shown. J. Differentiation into macro- and micronuclei occurs and dis- appearance of three micronuclei K. Cells and nuclei divide as shown to produce the original condition. Fig. 100.— Schematic representation of conjugation in Paramoccium caudatmn. (Redrawn from Jennings, "Life and Death, Heredity and Evolution in Unicellular Organisms," copyright, 1920, Richard G. Badger, printed by permission.) THE INFUSORIA 187 may be as many as four such divisions, with the production of sixteen individuals in twenty-four hours. The rate of this division is determined by external conditions, such as food and tempera- ture, and by certain internal factors. This production of " orphan twin sisters " continues until interrupted by conjugation or endo- mixis. Although P. aurelia and probably other paramcecia may con- tinue to live indefinitely without the sexual reproduction that is effected by conjugation, this process apparently occurs at more or less frequent intervals under natural conditions as well as in the laboratory. In some cultures that were long continued (Calkins, P. caudatum), it was observed that the need for conjugation occurred at intervals of some two hundred generations. In others (Woodruff, P. aurelia), it was found that conjugation need not occur even in many thousands of generations, if at all. The details of conjugation in P. caudatum, after the two cells have come in contact in the region of their buccal grooves, are shown in Fig. 100, the explanation of which should be consulted in this connection. The process differs markedly from the permanent fusion of cells that occurs in Sarcodina and Mastigophora. In such protozoa, conjugation consists in the complete and perma- nent union of two cell bodies and of their nuclei, and is thus com- parable with the union of egg and sperm in the fertilization of higher animals (c/. Fig. 116, p. 233). The type of conjugation that occurs in paramoecium is found only among the Infusoria, However, the net results are the same in both cases. Conjuga- tion in one of the Sarcodina, such as amoeba, or in one of the Mas- tigophora, such as euglena, results in a single cell of biparental origin, which is a zygote, comparable with the zygote formed by union of egg and sperm in many-celled animals. From two cells one is formed. In paramoecium a similar condition is brought about, since there arise, as a result of conjugation, zygotes, or cells of bi-parental origin (Fig. 100). The conjugation of paramoecium and other Infusoria may be said to be more efficient, however, because two zygotes, instead of one, result from each conjugation. Although the term " sexual reproduction " is commonly applied to the conjugation of paramoecium, it wiU be noted that the conjugating cells are isogametes and hence there is no evidence of sexual differentiation. Attempts are sometimes made to com- pare the migratory nucleus of conjugation with a sperm nucleus. 188 REPRESENTATIVE SINGLE-CELLED ANIMALS and the stationary nucleus with the nucleus of an egg. In this way conjugation is compared with reciprocal fertilization by two hermaphroditic animals. This is a most confusing comparison unless one has an extensive knowledge of the process of syngamy in both protozoa and metazoa. In addition to conjugation, there has been discovered, first in Paramoecium aurelia and later in P. caudatum, the process of endomixis, which involves nuclear reorganization within the limits of a single cell, whereas conjugation involves two cells (Fig. 100). As with conjugation, however, there occurs during endomixis a disappearance of t he macronucleus and of a considerable portion of micronuclear material. From the single micronucleus that remains in each cell, a new macronucleus and the two micronuclei characteristic of P. aurelia are then formed. Endomixis has, therefore, some resemblance to conjugation, although only one cell is concerned. Its significance in the life. cycle is probably somewhat the same as that of conjugation. There has been much discussion among investigators regarding the significance of conjugation and endomixis. It was originally supposed (Maupas) that conjugation must occur periodically among the Infusoria and that it exercised a " rejuvenating " effect upon paramoecium, since the rate of cell division was described as more rapid just after conjugation and gradually declining until the animal was rejuvenated by another conjugation. Later work has shown that conjugation is not necessary in the life cycle of paramoecium, since P. aurelia has been carried for many thousands of generations without such union of the cells (Wood- ruff). Endomixis has also been shown to be unnecessary, since P. calkinsi has been carried for four years (Spencer) during which time neither endomixis nor conjugation has occurred. What, then, is the significance of conjugation in these ciliated protozoa? The present state of our knowledge seems to justify the following general answer: The ciliate L'roleptus, as studied by Calkins, does seem to be rejuvenated, in the sense that a declining rate of cell division is restored to normal by conjugation. Endo- mixis seems also to have this effect in some cases. But varied enviromnental conditions, such as changes in food supply, may also maintain a species at a proper level of cell division and hence normal metabolism. Moreover, there is the case of P. calkinsi, just cited, where neither conjugation nor endomixis seems neees- THE INFUSORIA 189 sary. Ciliates without micronuclei, and hence possibly without conjugation or endomixis, are also known. It therefore appears that such rejuvenescence as may be necessary in these protozoa can be accomplished by more than one means. The one factor in conjugation that is unquestionable is the union of two diverse lines of descent; for, just as the fertilized egg of a many-celled animal is a single cell derived by union of two different germ cells (Fig. 116, p. 233), so the ex-conjugants of paramcecium are cells of double parentage. Thus, the basis for bi-parental inheritance exists among Infusoria as in other protozoa. Conjugation in the ciliated protozoa, and presumably in other unicellular forms, is, therefore, significant (1) as a means of uniting two diverse lines of descent with the result that variation may be increased; and (2) apparentl}^, under some conditions, as a means of stimulating cell division and other cell functions and hence rejuvenating lines that might otherwise perish. Other Infusoria. — Since the cihated Infusoria, or CUiata, include most of the species of Protozoa that are likely to be seen by the student, it will be useful to indicate the more important subdivisions and the names of representative genera. Ciliata are classified, according to the nature and distribution of their cilia, into the following groups: I. Aspirigera. — Forms without a spiral zone of oral ciHa or membranelles. 1. Order, Holotricha. — Having the cilia of about equal length and evenly distributed in the simpler forms; and with cilia having more complex arrangements in the more specialized forms. With a cj^ostome, except in some of the parasitic types. Opalina, Prorodon, Coleps, Didinium, Amyhilej)- tus, Lionotus, Loxodes, Dileptus, Colpoda, Fron- tonia, and Paramcecium are among the genera of this order that are common in fresh water {cf. Fig. 94). II. Spirigera. — Forms with a conspicuous spiral zone of larger cilia or vibratile membranelles leading to the mouth. 2. Order, Heterotricha. — Usually swimming, some- times attached. Body cilia small or reduced in > 190 REPRESENTATIVE SINGLE-CELLED ANIMALS contrast to the well-developed cilia of the oral region. Spirostomum, Stentor, and Halteria are common genera (c/. Fig. 94). 3. Order, Hypotricha. — Typically creeping forms with a marked dorso- ventral differentiation. Cilia of ventral surface modified to form large leg-Uke cirri. Oxytricha, Stylonichia, and Euplotes are the most common genera (cf. Fig. 94). 4. Order, Peritricha. — Typically attached forms. Oral cilia are continued into a depression in which open the cytopyge and the contractile vacuole, and at the base of which is the mouth. Locomotor ciUa are present only during certain phases of the life cycle. Vorticella, and the colonial forms, Carchesium (Fig. 103 G), Epistylis, and Zodtham- nium, are the most common genera in fresh water. The Suctoria, or Tentaculifera, which constitute the other subdivision of the Infusoria standing on a par with the Ciliata, may be mentioned here. Few of these occur in fresh water, but they are not uncommon at the seashore where they are found attached to various objects. In the adult phase of their Hfe cycle the Suctoria are attached and capture food by means of tentacles to which the prey becomes attached distally and through which its contents are slowly sucked into the body of the suctorian. The fresh-water Podophrya is an example (Fig. 84 D). Suctoria are classed with the Ciliata as Infusoria, because they have a cili- ated phase in their life cycle. This leads us to beheve that in their evolutionary history they have arisen by modification of ciliated forms. The Sporozoa Monocystis. — The Sporozoa are a large group of Protozoa, all of which are parasitic. Like other parasites, they show a degener- ation of the structures necessary for free life, and a specialization of structure and function and of the hfe history, wherever neces- sary to meet the demands of parasitic existence. The genus Monocystis, which inhabits the seminal vesicles of earthworms, is a sporozoan that is easily obtainable, and one that exhibits within its host all the important stages of its life cycle. In structure, the adult monocystis (Fig. 101) is a simple elongated cell with a single THE SPOROZOA 191 nucleus. Living as it does under rich nutrient conditions, the cytoplasm of the cell at this stage is able to assimilate and store up an abundance of food material to be used in later stages when nutritive processes are in abeyance. Such powers of rapid assim- ilation, and hence of rapid growth, are common in parasites during favorable periods of their life cycles. Fig. 101. — Life cycle of the gregarine, monocystis. A, spore consisting of a spore case enclosing eight sporozoites. B, transverse section of same. C and D, liberated sporozoites. E, sporozoite after entering multicellular sperm sphere of earthworm. /' and G, growth in sperm sphere until the fully formed trophozoite is formed surrounded by the degenerate remains of sperm sphere with flagella of sperma- tozoa. H, two trophozoites that have become free of the degenerate sperm sphere and united as gametocytes. I, encystmeut of gametocytes. J, division of nuclei and cyto- plasm to form gametes. K, union of the gametes in isogamous conjugation to form zygotes, residual cytoplasm of gametocytes in center of cyst. L, cyst containing many sporozoites formed by secretion of a spindle-shaped spore case around each zygote, which then divides to form eight sporozoites. These eventually become arranged as in A and B, in which stage they are transferred to another host. (Drawn by Wiley Crawford.) Excretion of the waste products of dissimilation must occur, in monocystis, by diffusion from the cell into the surrounding fluid of the host. Hence, the parallel between a monocystis, living sur- 192 REPRESENTATIVE SINGLE-CELLED ANIMALS rounded by the fluid of the earthworm's seminal vesicles, and the cell of any higher animal (c/. Fig. 60, p. 104), surrounded by intercel- lular lymph, is a close one. Presumably, the same kind of con- structive and destructive metabolic changes occur in either case. Very little can be said regarding the cell behavior in such a form. By expanding and contracting its body, the monocystis effects a slow locomotion, but, living as it does, it has no need for any com- plexity of behavior or for locomotor structures. The life cycle is shown by Fig. 101 and its legend. Metabolism, Irritability, and Reproduction in Protozoa The types of protozoa that have been described illustrate the manner in which the three great capacities of metabolism, irrita- bility, and reproduction are exhibited by the protoplasm of unicel- lular animals. While comparisons can be made between the proto- zoan cell as an individual animal and the individual composed of many cells as in higher animals, the more exact comparisons are to be made between cell and cell. Thus, in parasitic protozoa, the unicellular organism bathed in the fluids of its host's body, from which it assimilates nutrients and into which it excretes the waste products of its dissimilation, presents a close parallel with the cell of a higher animal as it lies surrounded by its intercellular lymph. If one examines the more representative protozoa with holozoic nutrition like that of the amoeba and paramoecium, it appears that they exhibit all the metabolic processes that it is possible for them to possess in view of their organization. Ingestion, digestion, egestion, assimilation, respiration, dissimilation, and excretion are all present on essentially the same basis as in many-celled forms {cf. p. 103). In the nature of the case, absorption through the mucous membrane of a digestive tract, circulation in the blood, and excretion by excretory organs cannot be present in the protozoan. Metabolism is the same, however, whether in an amoeba cr in a human being. Likewise, the response to stimulation, which constitutes irri- tability, appears to be the same kind of process whether in protozoa or metazoa. If we consider the cells of the frog or man individually, they respond to a variety of stimuli: mechanical, chemical, ther- mal, electrical, photic, etc. To all these a protozoan cell may respond equally well, and sometimes to a more marked degree. METABOLISM, IRRITABILITY, AND REPRODUCTION 193 because metazoan cells are limited by their specialization. Even the cells of sense-organs, which are par excellence the cells of irri- tability, have limited powers in the metazoa, for they respond only to particular forms of stimulation — the auditory cells only to sound waves, the retinal cells only to light, the cells of the taste- buds to certain chemicals. Nerve cells may respond more widely, as when one stimulates a nerve by heat, by chemicals, or by elec- tricity; but when a nerve impulse reaches other nerve cells in the central system, the effect may be similar, because the cells seem able to respond only ii- a hmited manner. While the responses of the metazoan body as a whole may be far more complex than those of any protozoan, it is, nevertheless, difficult to make out for the majority of cells in the metazoan even as wide a range cf response to stimulation as occurs in the more active protozoa, for the reason that the cells of metazoa are speciahzed for particular functions while the protozoan cell is speciahzed for all the functions of an individual. In any event, it is clear that irritabihty is the same kind of a process in both protozoa and metazoa. The exact manner in which the reproductive process may be compared in protozoa and metazoa is described in a subsequent chapter. In this connection, however, it may be repeated that conjugation, as it occurs in a majority of the protozoa, by a per- manent fusion of two cells to form one, is comparable with fer- tihzation, or the union of egg with sperm, in the metazoa. From a zygote arising in this manner, many independent protozoan cells are formed by division; while in metazoa the zygote produces, by cell division and differentiation, a many-celled body. Hence, there is a remarkable parallehsm between the cellular cycles even in the extremes of animal life {cf. Fig. 110, p. 215). Reproduction, hke metabolism and irritabihty, is a similar process whether in the highest or the lowest animals. The essential nucure of the cell and its protoplasm and the universality of the distinguishing capacities cf protoplasm become increasingly apparent as we proceed. The foregoing comparison of ceU with cell in protozoa and meta- zoa perhaps does injustice to the protozoan as an individual animal. Protozoa are " cells," but they are also " individuals." As such they may be compared with individuals composed of many cells. In extreme cases (Fig. 102) the single-cehed organism may present features that parallel structures in higher animals and thus may 194 REPRESENTATIVE SINGLE-CELLED ANIMALS I. — >or.ci — cytost. — ocs. Sk.lima tnac.n. mic.n. show an astonishing complexity. Comparisons can be made even in forms as simple as paramoecium, in which, although there is no digestive canal, there is a region in the cytoplasm along which vacuoles move from "mouth" to "anus." Again, the paramoe- cium " behaves as a whole " in the action of its cilia during the avoiding reaction and in swimming forward or backward. These considerations have led some students of the group to empha- size the individuality of the protozoan in- stead of its cellular state, and to disre- gard the comparisons that may be made between colonial pro- tozoa and metazoa — caec, as colonies of cells. This idea of the in- dividual as more im- portant than the cell, has been called the Organismal Theory in contrast to the Cell Theory of organisms. The more reasonable position seems to be a recognition of the : eruL cut. cyTop, Fig. 102. — A complex ciliate, Diplodinium ecauda- tum, showing highly developed organelles. coBC, csecum or rectal canal; cut., cuticle; c.v., contrac- tile vacuole; cytop., cytopyge or cell anus; cytost., cytos- tome or cell mouth; d.m., dorsal membranelle; ect., ecto- plasm; end., endoplasm; mac. n., macronucleus; mic. n., micronucleus; myon. {str. retr. oes.), myonemes, strands ioT retracting uesophagus; neur. m. ap., neuromotor appa- ratus; cEs., CBSophagus; or. cil., oral cilia; sk. lam., skeletal laniime. X750. (After Sharpe.) protozoa as physi- ologically balanced and independent cells, which in some instances have undergone extreme specialization of structure within the limits of their unicellular nature; and of the metazoa as multi- cellular organisms in which the cells are physiologically unbal- anced because of their mutual dependence. CHAPTER 8 GENERAL PROBLEMS RELATED TO SINGLE-CELLED ANIMALS It is evident from the special accounts in the preceding chapter that many important biological problems are intimately related to single-celled organisms. The existence of colonial species among unicellular animals and plants suggests the transitional steps that probably occurred in the development of various organisms from the primitive unicellular forms of life that are supposed to have existed at a very remote period and to have been the ancestors of many-celled animals. The manner in which individuals come into being at the present time, whether by reproductive processes, biogefiesis, or by processes of spontaneous generation, abiogenesis, may be considered here, since unicellular organisms were the last stronghold of the advocates of spontaneous generation. Again, many diseases are caused by protozoa living as parasites in the bodies of men and animals, thus Hnking the protozoa with medical problems. Colonial Protozoa and the Comparison of Unicellular with Multi- cellular Organisms The Colonial Organization. — Although the typical protozoan is an independent and self-su.staining cell, there are manj^ colonial species. These protozoan colonies are produced when the cells arising from the single cells, which occur at one or more phases of the life cycle, remain together, instead of separating after division as in non-colonial forms. During this colonial phase the group of cells constitutes an individual of a shghtly higher order, which is the colony. Such colonies are called gregaloid, if their cells are arranged irregularly (Fig. 103 A) ; linear, if in a line (Fig. 103 B) ; arhoroid, if branchinj; (Fig. 103 G); or spheroid, if in a spherical or globular mass (Fig. 103 E). The cells of a colony are associated but in- dependent, since each cell can reproduce a new colony by division 19.5 196 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS B Fig. 103. — Types of colonial protozoa. A, Microgromia, a gregaloid colony. B, Ceralium, a linear colony. C, Codonosiga; an arboroid colony. D, Spondiilomorum, a spheroid colony. E, Synura. F, Urogtena, G, Carchesium. Hi and H2, portions of Anthophysa colony. (Figs, redrawn as follows: A, from Calkins, "Protozoa," copyright, 1901, by The Macmillan Co., printed by permission. C, from Minchin, "Introduction to Study of Protozoa"; ^copyright, 1912, by Edward Arnold and Co., printed by permission. D and E, after Blochmann; G, after Kent; Hi and Hi after Blochmann.) COLONIAL PROTOZOA 197 or become a gamete at the time of sexual reproduction, and is self- sustaining in its metabolism and irritability. The colony may have a definite shape and size and move as a whole in locomotion, but otherwise its cells are as independent as those of non-colonial species. Their colonial organization is only the remaining together of similar cells to form a mass of characteristic size and appearance. Some coordination may exist, as when the colony swims in a particular direction or contracts as a whole; but the colony is not a many-celled organism in the true sense, because there is no division of labor, as in the metazoan, where there are different kinds of cells and corresponding specializations in function. Colonial Mastigophora. — The comparison of unicellular with multicellular organisms may ho pursued through certain of the Fui. 104. — Chlamydomonas, a non-colonial protozoan, and two simple colonial types. A, Chlamydomonas. B and B', two views of Gonium, sociale, a colony with only four cells. C and C, two views of Gonium ptclorale, a colony with sixteen cells. plant-like Mastigophora, particularly the family Volvocidrr and closely related forms. The members of the genus Chlamydomonas (Fig. 104 A) are simple non-colonial protozoa consisting of a single spherical or oval cell with two flagella, a red pigment spot like that of Euglena, a prominent chromatophore, two contractile vacuoles, and a cell wall. Reproduction is effected by binary fission with immediate separation of the two individuals thus formed. Con- jugation takes place by the permanent union of isogametes. Chlamydomonas, therefore, resembles Type 3 of the series shown in Fig. 110, p. 215. 198 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS A very simple type of colony is seen in Gonium sociale (Fig. 104 B; and cf. Type 4, Fig. 110), one of the Volvocidce. This con- sists of four cells arranged in a single layer and embedded in a gelatinous plate. Each cell has two flagella, a pigment spot, a chromatophore, and a contractile vacuole with one pyrenoid. Locomotion is by a revolution which shows first the surface and then the edge of the colony. The cells are physiologically self- sustaining, like those of non-colonial protozoa, since each can produce a new colony if they are separated. The colonies are produced asexually by division of the cells to form four daughter colonies, which separate, thus ending the existence of the parent colony as such ; or sexually by separation of the cells of the colony to form isogametes {cf. p. 174). In the related species, Gonium pectorale (Fig. 104 C), there are sixteen cells in the colony, but otherwise the structure and life cycle resemble those of G. sociale. Pandorina morum (Fig. 105; and Type 4, Fig. 110), which is slightly more specialized than Gonium, consists of eight or sixteen Fig. 105. — Pandorina morum, a colonial protozoan. A, fully developed colony composed of sixteen flagellated cells. B, formation of daughter colonies by division of each cell to form sixteen. B', daughter colony free of parent colony. C, formation of microgametes. d to g, union of microgamete (d) with macrogamete (d') to form zygote (g). (After Oltmanns.) cells, rarely thirty-two, packed together in an oval mass and sur- rounded by a common envelope. Each cell has two flagella, a pigment spot, a chlorophyll body and a contractile vacuole. The colony swims as a whole, but otherwise its cells are physiologically independent. When fully grown, each cell divides to form minia- ture colonies of sixteen cells, each of which breaks through the envelope of the parent colony and grows to full size, repeating the COLONIAL PROTOZOA 199 process. Preceding conjugation the cells of the colony separate and each cell becomes a gamete. These gametes are isogamous, or slightly anisogamous, thus showing the simplest form of sexual differentiation (c/. p. 174). In Eudorina elegans, another species of the Volvocidce, the colony consists of eight, sixteen, thirty-two, or even sixty-four flagellated cells resembling those of Pandorina. Daughter colonies are Uke- wise formed by division of the individual cells of the parent colo- nies. Conjugation occurs by union of anisogametes, which are formed from all the cells of the colony as in Pandorina, but in differ- ent colonies, and fuse to form zygotes that produce new colonies by cell division. Eudorina is more complex than Pandorina because the colony is composed of a greater number of cells, and because there are " male " and " female " colonies as well as " male " and " female " gametes. Thus in Gonium, Pandorina, and Eudorina, there are gametes; but there is no distinction between body cells and germ cells, because all the cells of these colonies gixe rise to germ cells at the time of conjugation. The cells of such colonies are, therefore, independent in all essential respects and exhibit the condition of physiological balance that characterizes the cells of non-colonial protozoa. In the examples that follow, the distinction may be drawn between body cells, which die a natural death by the disintegration of the colony, and gerrji cells, which are poten- tially immortal since they may unite in fertilization and so con- tinue to future generations. Pleodorina illinoisensis consists of a colony of thirty-two cells differentiated into twenty-eight larger cells, which give rise to gametes or germ cells like those of Eudorina, and four smaller somatic or body cells, which are located at one end of the ellipsoidal colony. There is another species, P. californica, in which there are either sixty-four or one hundred and twenty-eight cells, of which approximately one-half are body cells. Hence a division of labor exists among the cells of such a colony, as shown by their structural differences, and by the physiological differences that may be inferred to exist between cells ha\dng such different fates as do gametes and somatic cells. In the various species of the genus Volvox, this fundamental di\'ision of labor between somatic and germ cells becomes more conspicuous, since the somatic cells of the Volvox colony greatly 200 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS outnumber the germ cells. For example, V. perglobator, a species common in the United States and very similar to the European form V. globator (Fig. 106), is a colony of several thousand cells arranged at the surface of a sphere the interior of which is occupied by a watery fluid. Each somatic cell consists of a central mass Fig. 106. — Volvox globator. A, one-half of the spherical colony. B, fertilized macrogamete or ovum, v.ith male and female nuclei uniting, and protective shell secreted after entrance of the sperm nucleus into the cytoplasm of the ovum. C, microgamete or spermatozoon. Ov, ovum; S.C., somatic cells; Sp. 1-2-3, developing spermspheres. of protoplasm containing a nucleus and connected with the proto- plasm of neighboring cells by radiating strands that perforate the cell walls. Each somatic cell further possesses two flagella, by means of which the colony is propelled, a red pigment spot, and a contractile vacuole. The germ cells of the volvox, which are much less numerous than the body cells, become differentiated from the general mass of somatic cells as the colony develops. In their earlier stages they appear as cells of larger size, projecting from the inner face of the somatic layer but retaining their connec- tions with the outer surface (Fig. 106 A). The macrogametes, or ova, as they are sometimes called, are formed by an increase in size which is principally due to the formation of nutrient material in the cytoplasm; the rnicrogametes, or spermatozoa, arise by the COLONIAL PROTOZOA 201 division of a single large cell to form many small ones, each with two flagella. In V. perglobator these male and female cells are produced in different colonies. In some of the other species, like F. globator, both male and female cells are produced in the same colony, which is therefore said to be hermaphroditic. At the time of fertilization, the microgametes are discharged from the colonies and swim by their flagella until they perish or come in contact with a colony containing ova. The ovum is fertiUzed as it hes in place within the colony. In addition to the somatic cells, and the gametes that take part in sexual reproduction, the Volvox colony contains cells known as parthenogonidia. A parthenogonidium, by repeated divisions, can give rise to a new colony. This is the asexual method of reproduction in Volvox. These colonial Mastigophora are of interest because they show that the line between single-celled and many-celled forms cannot be sharply drawn in existing organisms. Since this is the case among the plants and animals now li\ang, it is not unreasonable to suppose that many-celled forms may have arisen, during the early history of organisms upon the earth, by steps somewhat hke those indicated by Fig. 110. This statement does not mean that Volvox and its plant-like relatives, Pleodorina, Pandorina, and Gonium, are the ancestors of animals. They are plants rather than animals, although classified in the IMastigophora (cf. p. 155). What the series shows is a transition, from the single-celled to the many-celled state, which is so gradual that one sees why distino tions cannot be sharply drawm, although the extremes of the series represent distinct conditions. In the protozoan the cell is an independent unit and therefore in a state of physiological bal- ance with respect to its fundamental capacities of metaboUsm, irritability, and reproduction. In the metazoan it is the whole mass of cells that possesses a phj-siological balance comparable with that of the protozoan. In protozoa, the "individual " is the cell; in metazoa, it is the group of cells. These facts have a bearing upon the Organismal Theory as opposed to the Cell Theory {cf. p. 194). The series of colonial Mastigophora that form the basis of the com- parisons here set forth between unicellular and multicellular organ- isms are further considered in the general discussion of reproduc- tion in the next chapter. 202 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS Biogenesis vs. Abiogenesis HistoricaL — As a result of investigations extending over a period of more than two hundred years, it was shown during the third quarter of the nineteenth century that spontaneous origin of protoplasm does not take place. Protoplasm seems to come from preexisting protoplasm, by a process of detachment, as in repro- duction, or by an increase of bulk as in growth. It was natural for the ancients to believe that animals like rats and mice, frogs, and insects, which suddenly swarmed in certain places, were produced from the mud of the fields under the influence of the sun's rays, or bred spontaneously within the decomposing carcasses in which they were found. It was even supposed that forms Hke the mammals, which developed within the female parent, arose spontaneously under the influence of the male's seminal fluid. The higher animals were, of course, known to have " parents," but there was no concept of the continuity between generations, save as " eggs " like those of birds and reptiles were observed to produce young, and mammals to give birth to living offspring. However, it was gradually established that smaller animals arose from eggs. The ItaHan naturalist, Redi, performed experiments (1688) that showed how maggots originated in meat from the eggs laid by flies. He placed pieces of meat in jars, covering some with wire gauze and others with parchment, and leaving others uncovered. Flies were attracted as the contents of the jars began to decompose, and laid their eggs directly on the meat or upon the wire gauze. Maggots were seen to hatch from these eggs and to develop as they consumed the meat. Eggs transferred from the gauze to the meat behaved in hke manner. The meat in the parchment-covered jars merely decomposed withr out the appearance of any maggots. Redi made other observa- tions upon the development of insects and reached the conclusion that all cases of " spontaneous " generation of Hving organisms were presumably due to the introduction of living germs from without. Had it not been for the discovery of protozoa, bacteria, and other micro-organisms during the latter half of the seventeenth century, the question would perhaps never have arisen again in the subsequent history of biological science. In 1676, the Hollander, Anthony van Leeuwenhoek, discovered with the microscope, which had recently come into use as a toy and BIOGENESIS vs. ABIOGENESIS 203 source of amusement, what he described as " little animals ob- served in rain, well, sea, and snow water as also in water wherein pepper had laid infused." Among other things, he observed some of the larger bacteria, many protozoa, and the passage of blood from arteries to veins through the capillaries; and he was the first to describe, if not the discoverer of, the human spermato- zoon. In one of his pubHcations, Leeuwenhoek writes as follows: In the year 1675 I discovered living creatures in rain-water which had stood but four days in a new earthen pot, glased blew within. This invited me to view this water with great attention, especially those Uttle animals appearing to me ten thousand times less than those represented by jMons. Swammerdam, and by him called water-fleas or water-lice, which may be perceived in the water with the naked eye. The first sort by me discovered in the said water, I divers times observed to consist of 5, 6, 7, or 8 clear globules, without being able to discover any film that held them together, or contained them. When these animalcula or living atoms did move, they put forth two little horns, continually moving themselves; the place between these two horns was flat, though the rest of the body was roundish, sharpening a little towards the end, where they had a tayle, near four times the length of the whole body, of the thickness (by my microscope) of a spider's web; at the end of which appears a globul, of the bigness of one of those which made up the body; which tayle I could not perceive, even in very clear water, to be mov'd by them. These little creatures, if they chanced to light upon the least filament or string, or other such particle, of which there are many in the water, especially after it hath stood some daj^s, they stood entangled therein, extending their body in a long round, and stri\-ing to dis-entangle their tayle; whereby it came to pass, that their whole body lept back towards the globul of the tayle, which then rolled together serpent-like, and after the manner of copper or iron-wire that, having been wound about a stick, and unwound again, retains those windings and turnings. This motion of extension and contraction continued a while; and I have seen several hundreds of these poor little crea- tures, within the space of a grain of gross sand, he cluster'd together in a few filaments. The observations of Leeuwenhoek were greatly extended by other workers during the eighteenth century, until all the more important types of microscopic animals came to be recognized. Although it was supposed that larger organisms arose from eggs or seeds that were in the nature of living " germs," it was still pos- sible to believe that micro-organisms arose spontaneously where conditions were suitable for their production. This belief was not 204 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS unnatural in view of the sudden appearance of innumerable minute forms of life as often observed in laboratory cultures. Some biologists, from Redi onward, reasoning by analogy with higher organisms, always believed that such micro-organisms arose from preexisting forms, although many clung to the idea of spontaneous origin. The process by which hfe was assumed to arise sponta- neously may be called abiogenesis, in contrast to biogenesis or the genesis of life from hfe. The persistent behef in the possibility of abiogenesis led Spallanzani, in 1775, and Schwann, in 1837, to perform extensive experiments, the results of which were against the theory of spontaneous origin. In spite of these repeated fail- ures to find any positive evidence for abiogenesis, the question was reopened on theoretical grounds by Pouchet in 1859. Final Establishment of Biogenesis. — The work of Pasteur and others was stimulated by this final recrudescence of the idea of spontaneous generation, as induced by Pouchet and his followers. Then came the series of brilhant researches by this great French- man, by the German, Koch, and by others, which finally showed how even the smallest organisms arise by cell division from parent forms. Encysted stages of protozoa and spore stages of bacteria were recognized and followed stage by stage until the hfe cycles of representative types were fully estabhshed alike in their active and in their resting stages. The English physicist, Tyndall, in the course of his investigations upon light, studied the " floating matter of the air " and showed that it teemed with spores and other resistant stages which needed only to settle upon a proper medium in order to germinate (Fig. 107). The English surgeon, Lister, and others who investigated the germ theory of disease as applied to surgery, established the fact that the germs found in wounds and in specific diseases were not generated spontaneously when conditions became right for them within the animal body, but were introduced into it as the spores or active stages of such minute organisms might be introduced into a sterile culture medium. The progressive extension of such demonstrations and the further exten- sion of the Cell Doctrine to the origin of higher organisms com- pleted the overthrow of abiogenesis and established biogenesis, or the origin of hving organisms from preexisting organisms, as the true explanation of the source of new individuals, although there will always remain the theoretical possibihty that proto- plasm may be synthesized under conditions of laboratory experi- BIOGENESIS vs. ABIOGENESIS 205 Fig. 107. — Tyndall's apparatus. The apparatus consisted of a chamber with glass front and windows {w) and with test- tubes fitted tightly in the bottom. Air could enter the chamber by the tubes a and b, but the entrance of particles floating in the air, like dust and bacterial spores, was prevented by bending these tubes. A pipette (c) that entered the chamber through a rubber diaphragm could be moved to place material in the different test-tubes. This pipette was plugged with cotton at (p) when not in use. In the experiments, the chamber was tightly sealed and left undisturbed for a few days until the particles floating in the air had settled to the bottom of the chamber, as indicated by the fact that an intense beam of light, when passed through the windows, failed to show its track within the chamber. Various nutrient fluids, like hay infusion, beef broth, etc., were then introduced into the test-tubes by means of the pipette. A brine or oil bath was placed under the chamber and the test-tubes boiled for five minutes. Although the chamber was thereafter placed in a warm room, there was not a single unexplained case in which such an infusion showed any signs of life. That the observed sterility was not due to any lack of nutritive power in the infusions was proved by opening the door of the chamber and permitting free entrance of the external air with its suspended particles, and by introducing contaminated material into individual test-tubes through the pipette. (From Tyndall, "Floating Matter of the Air," copyright, 1888, by D. Appleton & Co., reprinted by per- mission.) 206 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS mentation. Hence, the dictum of an earlier time, " omne vivum ex ovo," every living thing from an egg, and the later one, " omnis cellula e cellula," every cell from a cell, express the facts as now established. The manner in which unicellular organisms arise by processes of reproduction from parent organisms like themselves has been described. The origin of higher animals by reproduc- tion and development is explained in subsequent chapters. The long controversy over spontaneous generation, described in the foregoing paragraphs, was related throughout to the idea that certain diseases spread and multiplied like living organisms. When it was discovered that organisms did cause disease, this idea was confirmed. It is, therefore, appropriate to consider the Germ Theory of disease in the section that follows. Protozoa and Disease The Germ Theory of Disease. — What is known as the Germ Theory of disease postulates that certain diseases are produced by " germs," or minute organisms that hve as parasites in the bodies of plants or animals and are the specific causes of particular dis- eases. The " symptoms " of the disease are, in general, the bodily reactions in response to such parasitic invasions. Some diseases, like the hookworm disease, are caused by larger parasites; others, hke malaria and typhoid fever, by minute parasites which are pro- tozoa or bacteria. In general, all " infectious " diseases are due to organisms which infect the body and whose normal exist- ence in this phase of their life cycle is a parasitic one. Malaria and the Malaria Parasite. — The disease called malaria has been known since ancient times. Some have even regarded it as one of the causes of the decHne of ancient Rome. Certain it is that both Romans and Greeks, and probably other ancient peoples, suffered greatly from this pestilence. Early explorers of the Americas found it estabhshed in the tropical regions of both con- tinents and brought back to Europe the South American Indian's medicine in the form of " Peruvian bark," from which quinine was later extracted. At the present time the seriousness of the dis- ease in all the warmer parts of the world is a matter for states- manship as well as medicine. In India alone there are over a million deaths per year, to say nothing of the uncounted thousands who are incapacitated. Other regions are not much better off, PROTOZOA AND DISEASE 207 save as restricted localities have become subject to sanitary measures. In 1907 it was estimated that there were some 12,000 deaths per year in the United States, principally in the South; and it has been estimated that there may be as many as three milhon cases a year, involving a financial loss of not less than S100,000,000. This condition exists in a country in which there is full knowledge of the causation of the disease and of the preventive measures that have made such regions as the Panama Canal Zone safe habitations. The word " malaria," which means bad air, was originally applied to a group of fevers known to be associated with the air of swampy regions. The idea that such air acts as the causative agent is still prevalent among the ignorant, but if there are no mosquitos to act as intermediate hosts for the parasite there is no malaria. The disease genn in this instance is one of the Sporozoa, a representative of which is the Plasmodium malarioe, which causes the quartan type of malaria. In man the parasite lives in the blood, invading the red corpuscles (Fig. 108), where it fonns merozoites that are Uberated with the destruction of the corpuscles, and in turn invade new corpuscles in which the process is repeated. In this manner a very large number of the red blood cells may be destroyed and the numbers of the parasites greatly increased. Waste products, in the form of melanin gran- ules set free in the blood stream with the Uberation of the mer- ozoites from the disintegrating corpuscles appear to be the specific substances that cause the chills and fever, since the hberation of merozoites and the ague both occur at intervals of about seventy- two hours. After a considerable period of such multiplication, the parasite begins the formation of male and female gametocytes which must be drawn from the blood of man by the bite of a mosquito if they are to undergo the process of conjugation which is necessary for their further development. In the stomach of the new host the final stages of the gametes and the conjugation occur. The resulting cell passes through the epitheUum of the stomach wall and takes up a position as shown in the figure. Then division of the nucleus occurs repeatedl}^ and as the mass grows each nucleus is surrounded by cytoplasm and eventually becomes a spindle-shaped cell or sporozoite. The cyst bursts and the cells thus hberated migrate through the body spaces to the sahvary glands, from which they are ejected with saHva when the mosquito bites a human being. 208 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS Life cycles in which there are similar primary and intermediate hosts are not uncommon among parasites. The present example 5i® Fig. 108. — Life cycle of the parasitic protozoan, Plasmodium, malariw, that is the cause of quartan malaria. 1, the parasite, known as the sporozoite, as it is introduced into the blood of man by bite of mosquito; 2, 3, 4 and 5, sporozoite entering red blood corpuscle where the parasite grows and reproduces new individuals by sporulation; 6, destruction of corpuscle resulting in liberation of these new individuals, known as merozoites, and of excretory or melanin granules into blood plasma; 7, 7a, 8 and 8o, formation of male and female gametocytes in red blood corpuscle of man; 9, formation of macrogamete in stomach of mosquito; 9a and 9b,f ormationof microgametes in stomach of mosquito; 10 and 11, union of macrogamete with microgamete to form zygote in stomach of mosquito; 12, zygote after penetrating epithelium of mosquito's stomach; 13, sporozoites formed by divisions of zygote within its cyst; 14, female of mosquito that transmits malarial parasite; head of male below; 15, external surface of mosquito's stomach showing swellings produced by encysted stages formed by division of zygotes shown in 13; 16, section of salivary gland of mosquito, showing sporo- zoites which have been freed from cyst and have migrated through the body cavity to the gland cells. They lie ready to be discharged into the blood stream of man when the mos- quito feeds. (After the Leuckart-Chun chart, from Hough and Sedgwick, "Human Mechanism," copyright, 1918, by Ginn & Co., reprinted by permission.) PROTOZOA AND DISEASE 209 also illustrates the relationship of insects to disease-producing organisms. The following are representative cases affecting man: yellow fever, the germ of which, like that of malaria, is carried by a mosquito; typhus fever and trench fever, transmitted through the body louse; Rocky Mountain spotted fever, trans- mitted by a wood tick; the Japanese flood fever, transmitted by a small mite ; elephantiasis, in which the parasite is one of the thread worms, transmitted by mosquitos; relapsing fever in man, and Texas fever in cattle, transmitted by ticks; bubonic plague, by fleas. The list might be further extended. It is for this reason that the insects have assumed great importance to medicine, particularly in tropical countries, since the stages of the malaria parasite, the first life cycle of this nature to be discovered, were ascertained in 1898. African Sleeping Sickness and Trypaiiosomes. — Another exam- ple of an insect-borne disease is the sleeping sickness, occurring in equatorial Africa and caused by one of the flagellated protozoa called trypanosojnes (Fig. 109). By the bite of the blood-sucking tsetse fly, the stages of the parasites found in human blood are transferred to the intestine of the insect, which serves as an intermediate host. Here a scries of stages occur, until some three to four weeks later the parasites appear in the salivary glands of the tsetse fly, from which they may be again transferred to the blood of man or some other mammal. In the final stages of the cycle, they invade the cerebro-spinal fluid, causing the sleep that characterizes this disease and finally ends in death. It is a curious fact that while they cause a fatal disease in human beings, they apparently produce no very serious effects when they invade the blood of some of the larger African mammals. Such a condi- tion can perhaps be explained on the theory that the mammals in question have acquired an immunity through being subjected to the infection for many generations, whereas man has perhaps but recently come into contact with these parasites. Protozoa, as well as bacteria, have thus become recognized as the germs of certain diseases, although the majority are harmless or even beneficial to man. A science of Protozoology, comparable with Bacteriology, has arisen in the past twenty-five years. Med- ical schools in which tropical diseases demand special considera- tion have established professorships in this subject. It seems unlikely that the protozoa will ever assume the importance of the 210 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS bacteria in this regard, since so many more bacterial diseases have now been discovered. Nevertheless, the fact that certain protozoa To cerebrospinal fluid causing sleeping aickness and death. T>ansmisston by bite of tsetse fly. ^ Man .Antelope, etc & Xrypanosomea in human blood causing Trypanosome jester- Transmission by bte of tsetse fly^. Tsetse ?\y Forms in salivary glands dy for re- infection. \ZO"'-50"' day) Crlthidial /orms in salivacy glands {3. or a days latert Forms In midauf-,(Va/ after infective meal;. newly arrived form in .salivary aland. U2^toJo"'dcty3.) Long slender torms In proventriculus. f about I0*4ol5*''day5) Fig. 109. — Life history of Trypanosoma gambiense. X 1500. (From Chandler, "Animal Parasites and Human Disease," John Wiley and Sons, copy- right, 1922, reprinted by permission.) are important as causes of infectious diseases, along with the rela- tion of insects to disease, is among the great discoveries of medi- cine in recent years. PROTOZOA AND DISEASE 211 Reactions of Hosts to Parasites in Disease. — In the present connection we may consider some aspects of the host-parasite relationship that have a bearing upon the problems of infectious diseases. Similar relationships probably obtain in other cases cf parasitism, although the knowledge of such conditions is mainly derived from the study of disease in man and the domesticated animals. One feature of general importance is that most para- sites are specialists. One might think that a given species of parasite would invade many hosts. This is the case in some instances; but on the whole the parasitic species are remarkably limited in their range of habitat, since they are able to survive only in a single species of host or a few closely related species. Many interesting reactions might be considered under the head of adjustments by the host to its parasites. The simplest of these is mechanical -protection by the fonnation of barriers against fur- ther invasion or injury, as when cysts are developed by the con- nective tissue of a host in a way that walls off the parasites. Another important means of defense is that known as phagocytosis. The white blood cells, or phagocytes, and, to a lesser extent, other cells such as those of the mucous membrane and connective tissue, regularly ingest and destroy invading organisms like bacteria, as does the amoeba with its prey or the endodenn of a hydra during intracellular digestion {cf. p. 261). This relationship is so well adjusted in the higher animals that the number of white blood cells increases rapidly in response to infection by certain disease- producing organisms. A more complex reaction is seen in what is termed immunity. It is a familiar fact that a patient who has recovered from typhoid fever, or certain other diseases, is immune to a subsequent attack of the same disease. In some manner the individual is not as he was before, because he has almost no chance of ever contracting disease again. Something, which we call immunity, has been acquired by his body. Apparently what has happened is a chem- ical change in the plasma of the blood and IjTnph, but the exact nature of this change is unknown. That the relationships are comphcated and far-reaching will appear from the most general enumeration of the types of immunity that are now recognized. Thus, immunity acquired by success- fully withstanding the course of a disease is called natural immunity, in contrast to the artificial immunity acquired through artificial 212 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS agencies. Natural immunity may be inherited, as when a race of animals shows high resistance to a disease; or acquired during development by any individual having a disease at any period of life. Artificial immunity may be acquired by the various methods of vaccination and inoculation that are common practices in cur- rent medical treatment. For example, in typhoid vaccination the dead germs of the disease are injected into the individual. In response to the stimulus of the organisms introduced during vac- cination, the individual produces substances by his own metabo- lism that protect him against such disease organisms. This is known as the establishment of active artificial immunity because the individual has built up his own protection. As a check against the progress of diseases such as diphtheria, blood serum, pre- pared from an animal, such as a horse, that has been inoculated with the germs, is injected into the patient. In such cases it appears that there is present, in the injected serum, a substance called the antitoxin, which counteracts the toxins or poisons liberated by the bacteria of the disease. This produces passive artificial immunity, so called because the metabolism of the patient takes no part in building up the antitoxin. Immunity thus implies the opposite of " susceptibility " and covers a wide range of phenomena. However, it is to be regarded as but an aspect of the whole series of metabolic events and of the responses of the animal body under stimulation. CHAPTER 9 REPRODUCTION The functions of metabolism and irritability have been exam- ined in our study of a vertebrate animal and of unicellular organ- isms. There remains the third great bodily function, reproduc- tion. We may now consider this in a similar manner, by examin- ing the reproductive processes of protozoa and by extending the examination to multicellular animals. The protozoa arise from pre- existing organisms like themselves, as do higher types of animal life. Both sexual and asexual reproduction occur in protozoa, asexual reproduction being effected by cell division, and sexual reproduction by the union of gametes which may or may not be sexually differentiated as microgametes and macrogametes. In the metazoa, sexual reproduction also occurs by means of gametes, the ova and spennatozoa, while asexual reproduction, by processes of budding and fission, occurs in some many-celled annuals. In addition, there is in metazoa the process called development, which may be distinguished from reproduction for purposes of definition, although the two are inextricably related. The union of ovum and spermatozoon is obviously comparable with the union of conjugating cells in protozoa to form a new individual; but nothing in the protozoan cycle is exactly comparable with the development of a many-celled body from the zygote by cell division and differentiation, except the division of the protozoan cell and the colony formation that appears in a few species. Hence devel- opment occurs only in metazoa, while reproduction occurs in both protozoa and metazoa. By making this tlistinction, the process of reproduction may be considered in the present chapter a,nd the study of development deferred until we have completed our examination of representative many-celled animals. The Reproductive Cycle The Cell Cycle in Single-celled and in Many-celled Organisms. ^The resemblances between the reproductive processes through- out the Animal Kingdom may be shown by a schematic represen- 213 214 REPRODUCTION tation of cell cycles in a series of unicellular and multicellular organisms (Fig. 110). Type 1 of this tabulation is the cycle of a simple protozoan in which there is no process of conjugation. The cycle in this instance consists merely of asexual reproduction by cell division. Changes in the rate of division may occur, and there may be encystment; but these may be omitted for the purposes of the present discussion. There is nothing to com- pare with the cycle in a many-celled organism (Type 6), except the cells and their division. In Type 2 is shown a cycle that occurs in a few species of proto- zoa, in which there is a periodic reorganization of the nucleus, either by autogamy or by endomixis. Autogamy may be com- pared with the nuclear reorganization involving nuclear fusion that occurs during conjugation but not with the part of conju- gation that involves a union of two cells, since it occurs within the limits of a single cell. Endomixis is a nuclear reorganization within a single cell but without nuclear fusion. The life cycle of such a protozan consists of a series of cell divisions with occasional nuclear reorganization. Type 3, in which conjugation occurs, is the most representative protozoan cycle. The species reproduces asexually by cell division, and at times sexually by conjugating cells, the gametes {cf. p. 174), which unite in syngamy to form a single cfll, the zygote} The reproductive cycle in such a protozoan thus consists of the asexual reproduction of new individual cells by division, and occasionally sexual reproduction of new cells by syngamic union or conjugation. The gametes are isogamous, and hence there are no structural differ- ences between them that indicate sexual differentiation, although it is conceivable that functional differences may exist. There are, however, many species among the protozoa, as might be shown by a Type 3a, in which the gametes are anisogamous and, therefore, called " male " and " female " cells because they resemble the spermatozoa and ova of metazoa. Hence, the essential feature of sex, which is the production of male and female gametes, is recognizable in the protozoa. The life cycle of such species differs from that of Type 3 only in the production of anisogametes. * It will be recalled that conjugation by temporary union and mutual exchange of nuclear material, which occurs in ciliates like Paramoecium, is an exceptional form of syngamy even among protozoa {cf. p. 187). THE REPRODUCTIVE CYCLE 215 In Tjrpe 4 is represented the cycle of a simple colonial organism such as the plant-like Mastigophora,^ Gonium or Pandorina (Figs. 104 and 105, p. 197). It will be recalled that such a colony consists of a number of cells which have been formed by the THE CELLULAR CYCLE IN PROTOZOA AND METAZOA TYPES CELL DIVISION MATURATION AND SVNGAMY CELL DIVISION 1. Simple PfOlozoan Coniugal'on unknown Many Flagellates s/ w-0 2. Simple Piolozoan tndomuis or Aulogamy Tnchomasln 3. Simple Protozoan Isogamous Coniugalion Aclinophrys Absent ? (^-® ^® Endomuis or Autogamy -®-0-0-®- ®: As oelore ^®^|— ®®-5S-®- (^-® W-0 %4 Fig. 110. — Cell cycles in Protozoa and Metazoa. division of a single cell and have remained together (c/. p. 195). Such a colony is an individual of a slightly higher order than the single cell of a simple non-colonial organism (Type 3), although it is not truly many-celled because its cells are structurally similar 2 The authors are sensible of the fact that these organisms, Gonium and Pandorina, and also Eudorina, Pleodorina, and Volvox, are plants rather than animals. The inclusion of such plant-like forms as a subdivision of the Mastigophora, the Phytomastigina, has been explained (c/. p. 174). As the multicellular state has probably evolved from a unicellular one along parallel lines in animals and plants, the use of such plant-like organisms seems justifiable for the purposes of the present illustration. 216 REPRODUCTION and physiologically independent. In Pandorina {cf. p. 198), each of the sixteen cells divides into sixteen, forming daughter colonies, which separate, grow to full-sized colonies, and repeat the process. These processes are obviously comparable with similar ones in the preceding types. The only difference is the temporary association of the cells to form the colonies. At the time of sexual reproduc- tion the cells separate and become gametes, which conjugate to form zygotes comparable with those of the non-colonial types. In Type 4 there is, therefore, a period in which the cells are asso- ciated in colonies and another in which the species is represented by isolated cells from which colonies again develop by cell division. Th3 gametes cf such a species may be isogamous or anisogamous. In Pandorina they are usually isogamous, but they may be slightly anisogamous {cf. p. 199), in which case they show the beginnings of sexual diiferentiation. In the related genus Eudorina, there are more cells in the colony and the gametes are markedly anisogamous. Moreover, the male and female gametes of Eudor- ina are produced in different colonies. We might include in the table a Type 4a which would be more speciahzed after the man- ner of Eudorina. Type 5 represents the cycle in organisms like Pleodorina and Volvox (Fig. 106, p. 200), in which colonies arise by division of single cells as in Pandorina, but in which the cells are differenti- ated as body or somatic cells and gametes or germ cells. The unbalanced physiological state of the cell that thus arises has been mentioned in the special account of these colonial Mastigophora (p. 201). From the standpoint of reproduction, the important difference between these two kinds of cells in Type 5 of the table is that the body cells die a natural death with the disintegration of the colony, while the germ cells are immortal in the sense that they may live if they unite in conjugation. In the preceding types the cells may die, as most of them do, by the accidents of nature ; but there is no death that is " natural " in the sense that it is inevitable for some of the cells at one phase of the Ufe cycle. In Type 5 the body cells must perish while the gametes alone are immortal like all the cells in the preceding types. In this respect a form like Volvox can perhaps be called a many-celled rather than a single-celled organism, since the differentiation between body cells and germ cells is the most distinctive step in the series of cell cycles repre- sented by the table. THE REPRODUCTIVE CYCLE 217 In Type 6 the conditions are shown for a simple many-celled organism like the animal hydra, or one of the lower plants. This differs from Type 5 only in the number of kinds of body cells. In Type 5 there is but one kind of body cell. In Type 6 there are several, as found in simple metazoa (Fig. 121, p. 250). In the higher plants and animals the difference consists only in the greater number of kinds of body cells and their organization as tissues, organs, and systems; until in an animal like a vertebrate or one of the flowering plants there are innumerable body cells of many sorts. Functional differences are, of course, correlated with these structural differences. The cell in the multicellular bod}^ is a unit that is largely dependent upon the life cf the organism as a v/hole.^ We may thus distinguish between body and germ cells in many-celled animals and compare the germ cells with cells of protozoa, as the table shows (Fig. 110), without supposing that these two types of cells are always sharph' dis- tinguishable in the manner implied by extreme theories of the independence of the germ cells or germplasm, as they may be col- lectively termed in contrast with the somatoplasm which is the whole mass of body cells. The foregoing comparisons of cell cycles in many-celled and single-celled organisms are, of course, made from the viewpoint of the Cell Theor3\ According to the Organismal Theory {cf. p. 194), the homologies of cells and cell acti\aties that are here indicated should not receive so much emphasis, since the organism must be considered as a whole and not as a mere colony of cells differentiated for various functions. The existence of protozoa (Fig. 102, p. 194) so highly specialized that they are more complex than any other cells further emphasizes the importance of the individual as compared with the cell. As already stated, a more reasonable position would seem to be the recognition of organisms ^ In plants and in animals that are capable of regenerating the whole organ- ism from a part {cf. p. 273), the body cells are dependent upon a certain mass of the whole, which may be capable of regeneration, rather than upon the entire organism. In tissue cultures, small masses of body cells may be culti- vated for long periods, if not indefiniteh'. The cells of some sponges may be completely dissociated by squeezing through bolting cloth and will even then form new individuals. But in none of these cases have the single body cells been showTi to be capable of reproducing the whole as do single germ cells. There is, however, no sound theoretical reason why such cells might not thus reproduce the whole if they could be placed under proper conditions. 218 REPRODUCTION as individuals composed of cells which may exist in a balanced physiological state, as in protozoa, or in an unbalanced state, as in metazoa. Reproduction and Development. — The distinction between reproduction and development, to which reference was made in the introductory paragraph of the present chapter, is now apparent. Reproduction has been defined as the formation of a new individual ('/. p. 129). In protozoa reproduction may occur asexualhj by cell division of various sorts, and sexually by the conjugation, or syngamy, of gametes. In the latter process a new cell is formed by the permanent fusion of two cells, as in the fertilization of many-celled animals. In metazoa what is called asexual repro- duction occurs by budding and fission, but is confined to the lower many-celled animals (cf. Fig. 121, p. 250); while sexual reproduc- tion is effected by the syngamic union of ova and spermatozoa in fertilization. The zygote, or one-cell stage, which is thus formed, then produces the adult individual by the cell divisions and differentiations that constitute develo-pment. By making this dis- tinction between reproduction and development, the sexual repro- duction of protozoa and metazoa may be exactly compared; and it becomes clear that development in the metazoa has no exact parallel in the protozoa, although the cell division of protozoa and metazoa are obviously comparable as shown by the' table (Fig. 110). To compare development, as thus defined, with the cell development that occurs when protozoan cells increase in size and undergo certain differentiations after fission, as in the growth and differentiation of the two cells resulting from division in a ciliate (Fig. 99, p. 185), is confusing, since one is comparing a single cell with a mass of cells. Yet something may be said for such a comparison along the line of the Organismal Theory. The comparison of cell cycles points to the origin of development as a necessary incident of the evolutionary change from the single-celled to the many-celled state. It is, there- fore, possible to consider the reproductive processes of meta- zoa apart from those of development. This is desirable, because we have now become famihar with specific examples of repro- duction in the protozoa, and because the related processes in metazoa are so similar in all many-celled animals that they may be described in general terms. In our subsequent study of animal types, the reproductive and developmental processes will be MODES OF REPRODUCTION 219 further illustrated, and development will be examined in detail as it occurs in vertebrate animals. Modes of Reproduction ^ Asexual Reproduction. — As the name indicates, this form of reproduction involves nothing that may be called sexual, since it concerns only one individual and is not dependent upon the existence of sexual differentiation. In the asexual reproduction of protozoa, the cell may either divide into two cells of the same size, binary fission (Fig. 99, p. 185); bud off smaller cells, budding (Fig. 89 B-3) ; or form many small cells by simultaneous division of the cytoplasm about nuclei that have been produced by division of a single original nucleus, spondation (Fig. 101 J). The asexual reproduction of metazoa occurs by vegetative processes, Hke the budding of hydra (Fig. 121, p. 250) and the fission that occurs in some Flatyhelminthes and Annulata. Modifications are seen in the budding of hydroids (Fig. 133, p. 275), where the new individuals remain attached to the parent and produce colonies; in the internal budding, known as gemmidation, which occurs in fresh- water Forifera and Molluscoida; and in cases of fission, where the individuals remain attached until completely developed, forming temporary chains which resemble linear colonies. In all these instances among metazoa, new Individuals are produced without any relation to the phenomenon of sex, and hence such modes of reproduction may be called asexual. Sexual Reproduction. — As the table shows (Fig. 110), the con- jugating cells in protozoa are obviously comparable with the germ cells in metazoa, and conjugation is equivalent to fertilization insofar as it involves a union of two Unes of descent in the pro- duction of a new individual. It is true that the gametes of pro- tozoa may be isogamous and thus without the sexual differences that characterize the gametes of metazoa, but in many protozoa * The distinction that is here made between sexual and asexual reproduction is open to certain objections. Other classifications of the modes of reproduc- tion might be used, for example: uniparental as compared with biparental reproduction ; reproduction by syngamy as compared with reproduction by cell division. The authors have chosen the conventional distinction between sexual and asexual reproduction, because it is perhaps as good as any other and because confusion always results when an elementary presentation departs too widely from the usage in current books of reference. 220 REPRODUCTION they are anisogamous and thus show the beginnings of sex. The resemblance between the processes of sexual reproduction in pro- tozoa and metazoa is, therefore, as obvious as the homology between cells that exists throughout the Animal Kingdom. For this reason the term gametes is applied to the conjugating cells of protozoa and to the ova and spermatozoa of metazoa; and the term syngamy is used to include both conjugation and fertiUza- tion. If anything be needed to complete the resemblance, the existence in some protozoa of processes resembling the maturation of gametes in metazoa presents conclusive evidence of the funda- mental similarity of these sexual processes in all forms of animal life. As to the functional significance of conjugation and fer- tilization, it is clear that each furnishes a basis for biparental inheritance; but whether conjugation and fertilization have other features in common is uncertain (c/. p. 187). The syngamic union of a male and a female cell to form a single cell, the zygote, which is a new individual, is, therefore, the essen- tial feature of sexual reproduction. In the protozoan this individ- ual remains an isolated cell. In the metazoan it develops into a many-celled organism by cell division and differentiation. As this development proceeds, the biparental nature of the original cell supposedly persists in all the cells of the individual; and in this manner the biparental inheritance that appears in an adult is based upon the bisexual origin of every cell of its body when traced back to the one-cell stage. Thus, sexual reproduction and devel- opment are inextricably related ; but the comparison with protozoa is clarified if sexual reproduction in the metazoa is thus defined as the origin of a new individual by syngamy and distinguished from the development that follows. Modifications of sexual reproduction appear as parthenogenesis, both natural and artificial (p. 235); and as pcedoge7iesis, in which the organism becomes sexually mature in a larval stage in particular generations and may never reach the fully developed adult condition. Examples of pedogenesis are seen in the axolotl, an amphibian which reproduces sexually in a stage comparable with the late tadpole stages of other salamanders; in the liver fluke, which reproduces parthenogenetically through a series of larval generations; and in some insects. PROCESSES RELATED TO SEXUAL REPRODUCTION 221 Processes Related to Sexual Reproduction in Metazoa Sexual Differentiation. — The males and females of many familiar animals are distinguishable by characteristics that may not be directlj^ connected with the reproductive organs. There are, however, many other animals, like starfish and sea- urchins, the jelly-fish and polyps, the sponges, and many of the worms, in which there are no such characteristics by which the sexes may be recognized. In some instances, even ovaries and testes are wanting, and the sex cells are distributed through- out the bod}^ as in sponges. It is evident, therefore, that sexual differentiation in the metazoa consists primaril}- in the produc- tion of male and female reproductive cells which arc comparable with the microgametes and macrogametes of protozoa. The sexes are separate in the great majority of animals; but cases ot hermaphroditism, or the production of both male and female gametes by the same individual, are not infrequent among certain classes. In the higher multicellular animals, the underlying maleness and femalcness, which is indicated by the sex cells, may be so extended to the structure and functions of the body that sex seems a matter of somatic organization. Nevertheless, the pri- mary factor of sexual differentiation in metazoa, as in protozoa, is the production of male and female gametes. From an evolutionary standpoint, sexual differentiation prob- ably originated in the divison of labor between gametes, whereby there came to be male cells, or microgametes, which were small and active with special powers of locomotion; and female cells, or macrogametes, which were large, inactive, and food-laden. The course of such an evolution is indicated by the gametes of protozoa, which illustrate all the stages from isogamy to a very specialized anisogamy (cf. p. 174). If the original forms of life were uni- cellular organisms, it is, therefore, probable that sex originated even before the appearance of multicellular animals, in the manner suggested by the comparison of cellular cycles (Fig. 110). Among metazoa, the primary sexual differentiation of male and female cells has been variously complicated by the evolution of repro- ductive organs and systems, which are known as -primary sexual characters- and by the appearance of secondary sexual characters, which are not directly related to the reproductive organs, although they are expressions of the underlying maleness or femaleness of 222 REPRODUCTION the individual.' Examples of this latter type of characters are the color and size differences between the sexes of many birds, the antlers of deer, and even physiological characters like the greater pugnacity of the males in many of the mammals. Thus, the phenomenon of sex, as indicated by sexual differentiation, occurs in many of the simplest forms of animal life and in all many- celled animals, as well as in the great majority of many-celled plants. Some students of unicellular organisms be- lieve that it may exist as a physiolog- ical difference in the absence of visible structural differentiation. In any case, it is so nearly universal that it must have appeared at a very early period in the evolution of organisms. It is, therefore, not surprising that there should be important differences be- tween the sexes in many animals. The Germ-cell Cycle. — Historical. — Even in ancient times, the "seeds" of plants and the " germs," or embry- onic stages, of animals vv^ere recognized as the material from which many organ- isms arose, although the general nature of development was a mystery. Eggs of larger size, like those of birds and reptiles, were observed to bring forth young; and the seminal fluid of the male was believed to be necessary for conception in the Mammalia. Ari.stotle (384-322 B.C.) wrote with remarkable acumen upon the problem, considering the facts that were then available. In later times the Italian naturalist, Redi, who showed in 1668 that flies arose from eggs, and not spontaneously, in decaying meat, extended this concept of the egg as the initial stage of development, but such observations did not explain the action of the seminal fluid. After the spermatozoa and the microscopic ova of many animals had been discovered, between 1650 and 1700, the role Fig. 111. — Diagram illus- trating the Theory of Ger- minal Continuity. A, B, and C represent succes- sive generations; g, gamete (sperm- atozoJn) produced by another organism; z, zygote. White cir- cles indicate successive cell divi- sions within the somatic tissues, the existence of which terminates with the individual of the given generation. Sohd black circles indicate the germ cells. Dotted circles indicate gametes which may perish, or may unite with those of another organism. (From KelUcott, "General Embryology," copyright, 1913, by Henry Holt & Co., reprinted by permission.) PROCESSES RELATED TO SEXUAL REPRODUCTION 223 of each was long in dispute, the " spermatists " of the eighteenth century maintaining that the embryo arose from the sperm, the " ovists " that it came from the ovum. The answer to the riddle was found only as the result of two discoveries: the recognition that the ovum and spermatozoon are cells, following the estabhsh- ment of the Cell Theory in 1838-39; and the discovery by Oscar Hertwig, in 1875, that fertihzation consists in the entrance of a single spermatozoon into the egg and the union of egg-nucleus with sperm nucleus. An animal was thus shown to begin its existence as an individual at the time of fertilization, and development was recognized as a problem of cell division and differentiation. The gametes were seen to be the basis of the hereditary transmission of characters from one generation to another and the most remarkable of all cells because of their potentialities. Confusion will be avoided if we recall the terms that must be used in this connection. The word ovum is applied to the female gamete of an animal in the final stages of its development and before fertilization, in contrast to the male gamete, which is called the spermatozoon. The word " egg " is used more loosely, as when one speaks of a hen's egg, which is a complex structure including the shell and " white," which are secreted by the oviduct, and the ovum or " yolk," which is a product of the ovary; or, biologically speaking, the word egg may mean the ovum. The word gamete is a general term for both male and female sex cells. The germ cells include not only the full}' fomied ova and spennatozoa, or gametes, but also the antecedent cells from which these gametes are differ- entiated. Since the germ cells may be recognized at early stages of development as the primordial germ cells that are seen in many animals (Fig. 112), the distinction that can be made on theoretical grounds between germplasm, or germ cells, and somatoplasm, or body cells, is actually recognizable in particular cases. What is known as the germ-cell cycle includes the entire history of the germ cells from the time they can be recognized in the embryo until they have become differentiated as ova and spermatozoa. Origin of the Germ Cells. — The relationship between the germ- plasm and the somatoplasm is frequently illustrated by such a diagram as that shown in Fig. 111. According to the theory that is commonly associated with this representation, the germplasm is of primary importance, since it gives rise to both body cells and germ cells in each generation, while the somatoplasm is destined 224 REPRODUCTION to perish with the death of the body. As developed by W'eismann in the last quarter of the nineteenth century, this theory regarded the germplasm as relatively stable and independent of changing conditions in the somatoplasm. It was, therefore, in opposition to the theory that the germ cells were a product of the body cells nc^ n.o. Fig. 112. — Early differentiation of germplasm. A, transverse section of late blastula stage jf a lamellibranch, Sphcerium atriatinum, showing segmentation cavity (s. c.) and the two primordial germ cells (g. c.) from which all the germ cells of the paired reproductive organs seem to originate; B, section of late ovarian egg of an insect, Miastor americana, showing covering of follicle cells, the nurse cells (n. c), the nucleus of the ovum (n. o.), and a differentiated area of cytoplasm (p) that is traceable to the cytoplasm of the germ cells in the subsequent development {A, from an unpublished drawing by F. H. Woods; B, after R. W. Hegner, Jour, of Morph., Vol. 25.) at the time of sexual maturity and subject to many of the influences that affect the somatoplasm. The individual is a " chip of the old block," but the " block " is the germplasm and not the parent's somatoplasm. Although this idea was originally elaborated by Weismann on theoretical grounds, it found concrete support in the later discovery that the germ cells of many animals may be identified at an early stage of the embryo before the reproductive organs have become differentiated as such. Thus, in many instances, it has been possible to trace the primordial germ cells of the individual from the late cleavage stages (Fig. 112 A), and in some instances to discover within the cytoplasm of the unfertilized ovum (Fig. 112 B) substances that become localized in the germ cells when these can be definitely recognized in the subsequent development. More recently, however, it has been found in a num- ber of cases that these early germ cells degenerate; and that the PROCESSES RELATED TO SEXUAL REPRODUCTION 225 functional germ cells are differentiated at a later period. For example, in some vertebrates the cells that actually produce the germ cells may arise from the epithelial cells of the ovaries and tests even in the adult organism. As the matter stands, it calls for renewed investigation, although many instances of early differen- tiation appear to be well established. In general, it may be said that the germ cells are relativelj^ stable, for all their dependence upon the body; and that in many instances they appear as a type of cell that can be recognized in the early stages of the individual's development. In the reproductive organs of the adult animal the relationships of the germ cells are, of course, readily established. For example, in the ovaries of most animals there are germ cells surrounded by smaller cells which have a supporting and nutritive but not a germinal function. Thus, the ovary of the frog, as seen in the fall (Fig. 210, p. 401), shows cells of various sizes: the ova of the coming spring, which appear as large cells; other cells of inter- mediate size, representing approximately those of the next suc- ceeding year; and man}' smaller cells, constituting the reserve from which the eggs of subsequent j-ears will be differentiated. In addition to these there are other cells that form a matrix of tissue in which the ova are embedded. The ovary of a mammal presents similar relationships, except that fewer eggs are produced at each breeding season and hence the stock is relatively less extensive. Nevertheless, there are within the human ovary tens of thousands of potential genu cells from which mature eggs might be developed. The testis presents a similar condition, although the number of spermatozoa is so much greater than the number of ova that the multiplication of the primordial germ cells is accordingly increased. The foregoing explanations of the early appearance of the germ cells and their relationships within the reproductive organs will render more intelUgible the history of these cells. In the case of the male germ cells, the zygote, or oosperm, from which an individ- ual originates, is represented by the uppermost circle in Fig. 113. Within this are included outlines to represent chromo- somes, which are taken as eight in this instance. The unshaded chromosome, which represents the single sex-chromosome of the male, need not be considered in the present discussion. From this zygote there arise many cells by mitotic di^dsion, all with the same 226 REPRODUCTION Zygote of Parental Generation Early Ma/e Germ • cells Spermatogonia Germ - cells — Potentially Immortal "Germplasm'^ II. Maturation Division Fig. 113.— Diagram of maturation in male germ cells, spermatogenesis, showing eight paired chromosomes and a single "X " or se.x chromosome. The oosperm or zygote, which is the fertilized ovum, with its eight paired chromosomes, the autosomes, and a single "X" chromosome, is represented by the upper circle. Such a zygote produces by mitotic cell division, as shown schematically on the left, all the body cells or somatoplasm. The zygote also gives rise to the germ cells or germplasm, as shown PROCESSES RELATED TO SEXUAL REPRODUCTION 227 number of chromosomes ; but sooner or later some of these descend- ant cells can be recognized as the early germ cells of the male, or the spermatogonia. Hence, the body cells, or somatoplasm, and the germ cells, or germplasm, are represented in the figure as two distinct lines of descent among the cells of the individual, although both originally came from the same zygote. The spermatozoa are differentiated from the spermatogonia by the nuclear changes of maturation and by the changes in size and shape of the cell body as described in the following section. The separation of germplasm from somatoplasm in a female is similarly shown in Fig. 114. Maturation of the Male Germ Cells: Spermatogenesis. — The final changes in the maturation by which the spermatogonia and oogonia become transformed into the spermatozoa and ova are known respectively as spermatogenesis and oogenesis. In sperma- togenesis (Fig, 113), there first occurs a union of the chromosomes in pairs, which is known as synapsis. Following this union, there is a cell divison known as the first maturation division, producing two cells as shown by the figure. These resemble the cell shown in synapsis, because each chromo- some splits lengthwise, as is regularly the case in mitosis, and thus no change in number is produced. In the second maturation divi- sion, however, the mitotic cell division is unique; for the chromo- somes do not split lengthwise as in all other mitotic divisions in both body and germ cells. Instead of this, the members of the pairs of chromosomes separate, producing cells in which there are only one-half the number of chromosomes that occur in all the other cells of the organism. In some instances reduction occurs in the first instead of the second maturation division. The final result in on the right. Like the body cells, the earlj germ cells or sperniatogonia undergo mitotic cell divisions in which the number of chromosomes remains unchanged. At the time of synapsis the paired chromosomes, or autosomes, unite in the manner shown, but the sex chromosome remains unpaired, since it has no synaptic mate. In the first maturation division there is no change in the number and relationships of the chromosomes, because each one splits lengthwise as in typical cases of mitosis. In the second maturation division, the pairs of autosomes, which united in synapsis, separate and pass without further division to the two cells that become the spermatozoa, while the sex chromosome passes undivided to one or the other of these cells. In this manner each spermatozoon receives one member of each pair of autosomes, and one-half of all the spermatozoa receive an "X" chromosome, while the remaining spermatozoa are without an "X" chromoso-tie. Hence, at the time of fertilization {c.f. Fig. 114), one-half the oosperms or zygotes receive but one ' 'X " chromo- some and become males, while the other half receive two "X" chromosomes and become females {c.f. p. 448). 228 REPRODUCTION Pi Zygote of Parental Generation Early Female Germ - cells - Oogonia 'Oermplasm" Body-oells "Somatoplasm" Single Oogonium and its Diolsion to form other oSgonia in each of which Qrawth and Synapsis occurs Representative Cell of Female Body Synapsis I. Maturation Division '■'■' m P.B. I Zygote - Female Spermatozoa Fig. 114. — Diagram of maturation in female germ cells, oogenesis, showing eight paired chromosomes and two " X " or sex chromosomes, and fertiliza- tion of ova by spermatozoa to form male and female individuals. As in Fig. 113, the oosperm or zygote, with its eight paired chromosomes, the autosomes, and two "X" or sex chromosomes, is shown giving rise to cells of the body and to the . PROCESSES RELATED TO SEXUAL REPRODUCTION 229 such cases is the same, since the second maturation is then Hke any other mitotic division, with the chromosomes dividing length- wise and producing the same number as in the parent cell. After this second maturation or " reducing " division, a spermatozoon, with this reduced or haploid number of chromosomes, is pro- duced from each of the four cells by changes in the cytosome and condensation of the nucleus (Fig. 115). The essential significance of spermatogenesis is, therefore, the reduction in the number of chromosomes of the mature spermatozoon to one-half the number characteristic of the spermatogonia and all other cells of the animal. Maturation of the Female Germ Cells: Oogenesis. — The matura- tion of the female germ cells, or oogenesis (Fig. 114), is com- parable with that of the male cells so far as the nuclear changes are concerned, although only one of the four cells that are formed by the two maturation divisions becomes a functional ovum. The other three are small cells with the same number of chromo- somes as the ovum, but with a minimum amount of cytoplasm. As these are formed at the so-called animal pole of the ovum and can be recognized as minute globules (c/. Fig. 148, p. c07), they were called the polar bodies by the earlier embryologists before their true significance was ascertained. This difference in the maturation of the male and female germ cells may be regarded as a device v.hereby the cytoplasmic materials that might have been distributed to four equivalent cells are given instead to one of the four, which thus obtains a proportionately greater amount of the nutrient material that is utilized in the early stages of develop- ment. The polar bodies might, therefore, be described as abortive ova that come to naught, because one of their fellows receives the cytoplasm that might have been theirs. Another difference in the development of the ovum and spermatozoon is the amount of oogonia or early germ cells of the female. In synapsis the autosomes unite in pairs and likewise the " X " chromosomes. The two maturation divisions produce from each oogonium a single ovum and three abortive cells, the polar bodies (c/. Fig. 148, p. 307 and Fig. 211. p. 4012). In the first maturation division there is no change in the number and relation- ships of the chromosomes, because each one splits lengthwise as in typical cases of mitosis. In the second maturation division the pairs of autosomes, which united in synapsis, separate and pass without further division to the resulting cells. In this manner the ovum and each polar body receives one member of each pair of autosomes and one "X" chromosome. The fertilization of an ovum by a spermatozoon that possesses an "X" chromosome gives an oosperm or zygote with two "X" chromosomes and hence a female. Fertilization with a spermatozoon that does not possess an "X" chromosome gives a njale (<■/. p. 448). 230 REPRODUCTION growth that occurs in the female germ cell before the maturation divisions. By this means the greater size of the mature ovum is attained. In many animals only two polar bodies are found upon the surface of the egg (Fig. 211, p. 402). Examination shows ac& Fig. 115. — The spermatozoon. A and B, human spermatozoon, two views showing flattening of the "head" (nucleus) region. C, stage in formation of spermatozoon of guinea-pig, showing what is more obvi- ously a cell. D, diagrammatic figure of fully formed spermatozoon of a guinea-pig. aca, acrosome; a./., axial filament; cy, cytoplasm; fi, flagellum; n, nucleus; to p., middle piece. (A and B after Retzius; C and D after Meves. Reproduced from figure in Curtis, "Science and Human Affairs," by permission of Harcourt, Brace and Co.) that these are cases in which the first polar body fails to divide, as shown by the schematic representation (Fig. 114). The time of polar-body formation also varies. In some animals both maturation divisions occur before the egg leaves the ovary and well in advance of fertilization; in others the first maturation division occurs when the egg is mature and the second only when the egg is stimulated by the entrance of a spermatozoon in fertiliza- tion. In still other cases, both maturation divisions are delayed until the time of fertilization. These differences in the time of maturation in no wise affect the fundamental nuclear behavior involved. As a result of the nuclear changes in oogenesis and spermato- genesis, the mature ovum and spermatozoon each contain one-half the number of chromosomes characteristic of all other cells in the PROCESSES RELATED TO SEXUAL REPRODUCTION 231 animal. When ovum and spermatozoon unite in fertilization, the full or diploid number is restored by union of these two haploid groups. It will be recalled that the chromosomes of many cells are present in pairs (c/. p. 234). The members of such a pair in the cells of an adult animal are presumed to have descended, one from the ovum and the other from the spermatozoon, by this union of the two haploid groups in fertilization. IMaturation might be characterized as a device whereby the number of the chromosomes is prevented from being doubled in each generation at the time of fertilization. It is believed to occur with minor variations in the germ cells of all metazoa, and comparable phenomena preceding conjugation can be recognized in many of the protozoa (cf. Fig. 110). As similar reductions in the number of chromosomes in the gametes and restoration of the diploid number by fertilization occur in plants, the phenomenon is well-nigh universal. The Gametes. — The foregoing account of the history of the female germ cells explains the origin of the ovum. In contrast to the spermatozoa the distinctive characteristics of ova are the additions of nutrient material in the cytoplasm and of cell mem- branes that may be variously developed. Some ova are amoeboid and can migrate for short distances by means of pseudopodia-hke processes; but in most instances the egg of a metazoan is a non- motile, food-laden cell, comparable with the macrogamete of a protozoan and hence markedly different from the spermatozoon. The ovum is, indeed, a rather typical cell and easily recognizable as such (cf. Fig. 210, p. 401). The history of the male germ cells likewise explains their cellular nature, but the mature spermatozoon is a much more speciahzed cell than the ovum (Fig. 115 A). In a schematic representation of a type that occurs in many vertebrate animals (Fig. 115 D),the fol- lowing parts can be recognized. The " head," as it was called by the early microscopists in contrast to the " tail," consists of a nucleus composed principally of very dense chromatin and surrounded by a thin layer of cytoplasm. This cytoplasm is continuous with the " middle-piece," which contains a centriole. The " tail," or flngellum, contains an axial filament which extends into the middle-piece to a point near the centriole and is sur- rounded, except at its posterior end, by a sheath of c\i:oplasm. These peculiarities of structure do not appear in the male cells until the last stages of their development, following maturation (Figs. 232 REPRODUCTION 113 and 115 ). When liberated in the proper medium, the sperma- tozoon swims actively until it dies or meets an ovum in fertiliza- tion. The structural and functional resemblances between sperma- tozoa and the more speciahzed forms of microgametes in protozoa are obvious (cf. Figs. 115 A and 108, 9b). Fertilization. — The union of ovum with spermatozoon, which constitutes the " fertilization " of the egg, may be described in general terms, since it occurs in much the same manner in all animals. The process has been most thoroughly studied in eggs that are fertilized in water after discharge from the parent, as with many marine animals; but, so far as the observations go, there seems to be little difference between such cases and the fertilization of a grasshopper's or a mammal's egg in the fluid of an internal cavity of the female reproductive system. The present account has reference to the process as it occurs in the sea-urchin, Tox- opneustes (Fig. 116), in which the maturation of the ovum is delayed until the time of fertilization. After being discharged into the water, the egg remains suspended or slowly sinks to the bottom while the sperm swim about actively by means of the lashing of their flagella. When such a spermatozoon comes in contact with the surface of an ovum and penetrates the egg membrane, the movements of its flagellum cease and the surface of the ovum forms the so-called entrance cone by which the head and middle-piece of the spermatozoon become enclosed. From this time on, the spermatozoon appears as though it were inactive and were being drawn into the ovum by the action of the latter's cyto- plasm rather than by any movements of its own. In many ani- mals the flagellum remains on the outside, and even where it nor- mally enters, as in the birds, it takes no further part in the devel- opment. As the entrance cone develops, the egg produces on its external surface what is known as the fertilization memhrane. This was formerly beheved to be the mechanism by which the entrance of other spermatozoa was prevented, but is now regarded as the incidental result of changes produced in the ovum by the presence of the spermatozoon. These changes in some way render the surface of the ovum impervious to other spermatozoa within a few minutes, or even a few seconds, after the entrance cone has made its appearance. In this manner polyspermy, or the presence of more than one spermatozoon in the ovum, is prevented unless the sperm are so abundant that several reach the surface of the PROCESSES RELATED TO SEXUAL REPRODUCTION 233 egg simultaneously. The nucleus and middle-piece of the sperma- tozoon, having thus entered the egg, move very slowly toward the egg nucleus, gradually rotating through an angle of almost one hundred and eighty degrees, with the result that the centrioles of the zygote, which develop from the middle-piece of the sper- matozoon, finally occupy a position between the male and female Fig. 116. — Fertilization in the sea-urchin, Toxopneustes. A, the spermatozoon showing "head" containing nucleus, "middle-piece," and "tail" or flagellum. B to F, entrance of head and middle-piece into cytoplasm of the egg, showing entrance cone and the rotation of sperm-head during development of aster about the middle- piece (the minute centriole in center of the middle-piece is not shown). G and H, beginning of enlargement of sperm-nucleus and its movement toward the nucleus of the ovum. / and J, development of amphiaster of first cleavage and union of male and female pronuclei. (From Wilson, "The Cell," copyright, 1897, by The Macmillan Co., reprinted by per- mission.) nuclei, which are often called the pronuclei. Meanwhile, the male pronucleus has increased in size, and its chromosomes, which are not recognizable as such in the nucleus of the spermatozoon again become apparent in the haploid number seen at the close of its maturation (cf. Fig. 113). ^Yhi\e these changes are in progress, the maturation divisions 234 REPRODUCTION of the ovum are occurring in cases like Toxopneustes where the polar-body formation normally takes place at the time of fertiliza- tion. When this maturation has been completed, the centriole of the ovum disappears, and the egg-nucleus, with its haploid number of chromosomes, moves to a position beside the sperm- nucleus, while the centriole from the middle-piece of the sperma- matozoon divides and the mitotic spindle of the zygote is formed. As a result of fertilization, the male and female pronuclei, therefore, come to occupy a position on either side of the first- division spindle and their chromosomes are distributed equally to each cell of the two-cell stage. If this process were continued throughout the development, the cells of the adult body would contain paired chromosomes arising in such a manner that one member of each pair would be descended from the corresponding chromosome of the ovum and the other member from that of the spermatozoon. As a matter of fact, such a paired relationship of the chromosomes can be observed in many animals, not only in the first division of the zygote but also in many subsequent cell divisions. Moreover, in the cells of the adult body the chromo- somes are found to occur in pairs wherever their diversity of size and appearance renders such a distinction possible. These facts, together with the theoretical conclusions regarding the chromosomes that are drawn from their behavior in correlation with the inheritance of adult characters, are the basis for the gen- erally accepted conclusion that the chromosomes, as they are found in their diploid condition in the cells of an adult animal, have descended one-half from the pronucleus of the ovum and the other half from the pronucleus of the spermatozoon. If this is correct, one can hardly escape the conclusion that the chromosomes furnish a mechanism for the inheritance that is observed in sexual reproduction. In addition to this union of two lines of descent in the produc- tion of a new individual, the process of fertilization also involves the stimulus which causes the ovum to develop into the adult animal after its union with the spermatozoon. The term fertili- zation is sometimes restricted to this developmental stimulus, which may be called the activation of the egg, since the word fertilization suggests the act of making an egg fertile and hence a stimulation to development. In this discussion the authors have chosen to use the term fertilization to include both the PROCESSES RELATED TO SEXUAL REPRODUCTION 235 union of germplasms, or amphimixis, which is the basis of bi- parental inheritance, and the activation by which the egg is stimulated to develop instead of dying as it would do eventually if unfertilized. Natural and Artificial Parthenogenesis. — In this connection, the phenomenon of -parthenogenesis, in which an ovum develops without the entrance of a spermatozoon, may be described. Such a process occurs normally in a considerable number of the Arthro- poda (c/. p. 241), hke insects and spiders, in some Platyhelminthes and Trochelminthes, and perhaps in some of the lower Verte- brata. Males are known to exist in most of these cases, and fertilization of the eggs occurs in certain generations; or certain eggs may be fertilized while others are not, as in the honey bee. Where males are unknown, it is presumed that the}' have not yet been discovered, although it would be theoretically possible to have the male sex completely eliminated in the evolution of a species with such a mode of reproduction. Hence, it appears that species exhibiting natural parthenogenesis produce, under certain condi- tions, eggs that develop without fertilization and other eggs that develop only when normally fertihzed as in the great majority of animals. The occurrence cf natural parthenogenesis suggests that eggs that develop in nature only after normal fortiUzation may be caused to develop parthenogenetically if suitable stimuli can be applied. This is found to be the case; and the phenomenon is known as artificial parthenogenesis in contrast to the partheno- genesis that occurs under natural conditions. Since the first suc- cessful experiments in artificial parthenogenesis were conducted, about 1900, it has been found that the eggs of many animals, among which are worms, molluscs, echinoderms, and vertebrates, may be caused to develop in the absence of fertilization. Develop- ment, which in some few cases has progressed to a late stage, ensues when such eggs are subjected to very dilute solutions of salts, acids, narcotics, and other substances, to changes in temper- ature, and in some instances even to simple mechanical stimula- tion. It is conceivable that there is no egg of any animal that might not be thus " artificially " started on its development by the application of a suitable stimulus. The reproductive processes of animals, therefore, have certain fundamental resemblances. This is particularly true of sexual 236 REPRODUCTION reproduction, which is comparable from one end of the Animal Kingdom to the other, and also in plants. Not only do unicellular animals reproduce by means of cell division and conjugation; but every multicellular animal is, at one stage of its life cycle, circum- scribed within the Hmits of a single cell, the zygote, which is formed by fusion of the ovum and spermatozoon, and from which the many-celled adult animal develops. One of the ultimate problems in Embryology is to discover how a thinking man can arise from a single cell that bears no resemblance to the adult organism. Reproduction, like metaboHsm and irritability, is reducible to cell activities. As a matter of convenience, reproduc- tion has been separated from the process of development, which is described in later chapters, although the two are but different aspects of the reproductive cycle in metazoa. « CHAPTER 10 CLASSIFICATION AND GENERAL ORGANIZATION OF ANIMALS The principles of classification have been illustrated in particular groups of animals in the accounts of Vertebrata and other Chor- data, and in the Protozoa. The distinction between Protozoa and Metazoa has also been explained. The classification of IMetazoa is indicated in this chapter in order that the student may understand something of the Animal Kingdom as a whole before undertaking the study of representative animals that is outhned in the chapters immediately following. Classification The Principal Types of Animals. — It is necessary to classif}- animals, if only for the purpose of Usting the various types, and hence there have been many classifications in the history of Zoolog3\ Gradually, however, these have been refined to a measure of agreement, until at present there is no wide diversity of opinion regarding the nature of the major subdivisions, or phyla. The Animal Kingdom may be divided, according to most zoologists, into a relatively small number of these larger groups, the phyla, which are in turn variously subdivided. As Usted in most of the current texts, these phyla are as follows : Phylum Protozoa, the single-celled animals. Phylum Mesozoa, a small group of very simple organization. Phylum Pari f era, the sponges. Phylum Coelenterata, the polyps, jellyfish, corals, etc.^ Phylum Platyhelminthes, the flat worms, and nemerteans.^ Phylum Nemathelminthes, the round wonns. ^ The Ctenophora, or sea-walnuts, which would here be listed as a subdivision of the Coelenterata, are classified by some zoologists as a separate phylum. * The nemerteans may be placed in a Phylum, Nemertinea. 237 238 CLASSIFICATION AND ORGANIZATION OF ANIMALS Phylum Trochelminthes, the rotifers. Phylum Molluscoida, the polyzoa, brachiopoda, etc.^ Phylum Echinodermata, the starfishes, sea-urchins, sea-cucum- bers, sea-hhes, etc. Phylum Annulata, the earthworms and other segmented worms. Phylum Arthropoda, the crayfish and other crustaceans, the insects, spiders, centipedes, etc. Phylum Mollusca, the clams, snails, squids, etc. Phylum Chor'data, the tunicates, etc., and the vertebrates (c/. p. 36). Within the Hmits of these major groups, and a few lesser ones that are commonly appended to particular phyla, are included all the manifold forms of animals. The diversity of animal Kfe, which is so bewildering upon superficial examination, is thus seen to be unified by the existence of only a few principal types of organization. In many textbooks of zoology no broader classification is attempted than this division of the Animal Kingdom into phyla. It is possible, however, to make further unifications, although some zoologists doubt the certainty of such relationships. To illustrate what may be done in this regard, let us proceed as follows: Taking the Animal Engdom as a whole and assuming a knowl- edge of its varied types, suppose one asks what is the greatest difference between the various kinds of animals. If all the animals were to be divided into two groups, on what basis would they be separated? As previously explained in introducing the Protozoa (c/. p. 153), one may answer that animals may be first subdivided into Protozoa and Metazoa, as shown by the accompanying tabula- tion (Fig. 117). Unicellular animals may thus be placed over against all others, and the Phylum Protozoa may be written in the right-hand column. Proceeding further, one may ask a similar question for the Metazoa. What is the greatest difference between the kinds of many-celled forms? Some zoologists would not agree to the answer here given, but a majority would probably say that the Metazoa may be divided into two principal groups, according as they have or have not a digestive cavity. There is one large 2 The Phylum Molluscoida is questioned because it includes forms so diverse. There is much justification for its division into at least two phyla, the Folyzoa and Tcntaculata. CLASSIFICATION 239 phylum, the Porifera, or sponges that seems to lack in its devel- opment and in its adult state anything that can be properly com- pared with the digestive tract as found in all higher many-celled animals. A similar condition exists in the small group of animals that is here designated as the Phylum Mesozoa. Hence the Metazoa may be divided into two groups, the Parazoa, which includes the Porifera and Mesozoa; and the Enterozoa, which includes all other Metazoa. Again, if the same question be asked for the Enterozoa, it appears that the most fundamental difference between these forms is the presence or absence of another ca\aty, the ccelome, which surrounds the digestive tract. In some cases, as in the Trochelminthes and Molluscoida, the coelome seems to have degenerated; while in others, the Platyhelminthes and Nemathelminthcs, it may never have existed. In the more important phyla, the Annulata, Arthropoda, MoUusca, Echinodermata, and Chordata, there are either unmistakable signs of its former existence or it is well devel- oped, as in the famihar vertebrates. For this reason, the Enter- ozoa may be divided, as the table shows, into the EnteroccEla, including the Coelenterata, and perhaps the Platyhelminthes, which have no coelome; and the Ccelomoccela, which possess such a body cavity. The Nemathelminthcs, the Trochelminthes, and the Molluscoida may be left as uncertainties; although they are presumably forms in which the coelome has become modified beyond clear recognition. In this manner the phyla may be grouped in larger subdi\dsions according to the classifications maintained by many zoologists. A tabulation showing how the phyla of anunals are divided into classes appears on p. 241. The Basis and Meaning of Classification. — For practical pur- poses of listing and arrangement, the classification by phyla alone is sufficient; but classification has a significance in modern zoology apart from its convenience as a cataloging system. The basis of the classification here indicated is structure, and structural resemblances are beheved to indicate evolutionary relationships. ^Vhen one says that certain animals are all chordates, or coelen- terates, or annulates, one means that they are to be regarded as more akin to one another than to any other group. Hence, the " natural " classification, which zoologists are now attempting to ascertain upon the basis of structural resemblance, is in reality a familij tree of animal life. If certain phyla are placed 240 CLASSIFICATION AND ORGANIZATION OF ANIMALS Animal Kingdom Phylum Protozoa . Protozoa Single- celled animals Parazoa Without gut cavity or enteron Metazoa . . Many- celled animals Enterocoela Without ccElome, dip- loblastic or triploblastic Mesozoa Porifera Diploblastic Ccelenterata Triploblastic Platyhelminlhes Coelome problematical body non-metameric N emathelminth.es Trochelminthes Molluscoida Enterozoa . With gut cav- ity or enteron Ccelomocaela .... Coelome recognizable, Coelome pres- ^ body non-metameric ent or prob- Mollusca lematical, Echinodermata triploblastic Coelome recognizable body metameric Anmdata Arthropoda Chordata Plant Kingdom Subdivided in a manner comparable with that shown for the Animal Kingdom i 1 Fig. 117. — Classification of the principal groups of animals {r:f Fig. 118 and list of phyla on opposite page.) CLASSIFICATION 241 Sub-division of the Phyla into Classes Kingdom, Animalia Phylum, Protozoa Class, Sarcodina Class, Mantigophora Class, Infusoria Class, Sporozoa Phylum, Mesozoa Phylum, Porifera Class, Calcarea Class, Hexactinellida Class, Demospongia Phylum, Coelenterata Class, Hydrozoa Class, Scyphozoa Class, Actinozoa Class, Ctenophora Phylum, Platyhelminthes Class, Turbellaria Class, Trematoda Class, Cestoda Class, Nemertinea Phylum, N emaihelminthes Class, Nematoda Class, Acxinthocephala Class, Chcetognatha Phylum, Trochelminlhes Class, Rotifera Class, Gastrotricha Phylum, ^follusco^da Class, Polyzoa Class, Phoronida Class, Brachiopoda Phylum, Mollusca Class, Amphineura Class, Pelecypoda Class, Gastropoda Class, Cephalopoda Phylum, Echinodermaia Class, Asteroidea Class, Ophiuroidea Class, Echinaidea Class, Holothuroidea Class, Crinoidea Phylum, Annulala Class, Archi-annelida Class, Chxetopoda Class, Sipunculoidea Class, Hirudinea Phylum, Arthropoda Class, Crustacea Class, Ottychophora Class, Myriapoda Class, Insecta Class, Arachnida Phylum, Chordata Sub-phylum, Cephalochordata Sub-phylum, Urochordala Sub-phylum, Hemichordata Sub-phylum, Verlebrata Class, Cyclostomata Class, Pisces Class, Ainphilna Class, Reptilia Class, Aves Class, Maniinalia 242 CLASSIFICATION AND ORGANIZATION OF ANIMALS together as the Enterozoa, it means that they are regarded as closely related in their ancestry. It is, therefore, possible to regard a table of classification like the one under discussion from the standpoint of evolutionary development. Referring to the accompanying table (Fig. 117) as though it were a family tree and to Fig. 118, one may say that the first great step in evolution was the divergence between the forms of animal life that continued in a unicellular state and gave rise to the Pro- tozoa, and those that acquired the many-celled state and pro- duced the Metazoa. Within the latter hne, the next great diver- gence was between forms that continued in a primitive state and without a gut cavity, the Parazoa, whose principal descendants are the Porifera; and those that acquired such a cavity and became the ancestors of the Enterozoa. Next came the acquisition of a coelome by forms that may be termed Coelomocoela; while the more primitive state survived in the Enterocoela, from which the ccelenterates, and perhaps the platyhelminthes, have descended without fundamental changes in their general organ- ization. Among Coelomocoela, there was then a divergence into the ancestors of the great phyla, and following this a specializa- tion within the limits of the several phyla. Thus ended the most profound changes in evolution, as expressed by the classification indicated. If such changes occurred, they must have taken place at a very remote time, since the earliest fossil-bearing rocks that contain an abundance of life show representatives of all the larger groups except the Chordata {cf. Fig. 259, p. 491). If one desires to speculate upon the problem, some such answer as the one just given will probably be accepted by most biologists, although it is recognized that such conclusions regarding major evolutionary changes are speculative and not to be taken as clearly demon- strable. The lesser phases of evolutionary history, for example, the evolution of lesser groups of vertebrates {cf. Fig. 276, p. 515), may be accepted with a higher degree of certainty. The table of cell cycles discussed in the preceding chapter may be consulted in this connection, since it shows how Metazoa might conceivably have arisen from a unicellular ancestry in common with the Protozoa, and since it explains the existence of a unicellular stage in the life cycle of every many-celled animal that reproduces by germ cells. The fact that so many members of the phyla that are grouped as GENERAL ORGANIZATION OF THE ANIMAL BODY 243 Coelomocoela exhibit the gastrula stage {cf. Fig. 148 G, p. 307 and Fig. 209, p. 399) in their development may be taken as evidence for the course of evolution here indicated. The gastrula is a two-layered sac comparable with the Cuelenterata in its type of structure {cf. Fig. 121, p. 250), and perhaps reminiscent of an inheritance from an enterozoan ancestry. In other words, it may be that a gas- trula stage occurs in these higher phyla because they have never completely lost this evidence of their primitive ancestry; and it may be that coelenterates are two-layered because they have never evolved beyond what is fundamentally the enterozoan stage. Similar propositions could be laid down for the one-cell stage in all Metazoa (Fig. 110, p. 215), for the gill-sHt stage in higher verte- brates (Fig. 287, p. 531), and for many features of development that are suggestive of ancestry in the various phyla. According to such an interpretation, the frog (Fig. 213, p. 405) arises from male and female gametes, which fonn a zj'gote, and later passes through its gastrula and gill-slit stages because it has never lost these vague evidences of its ancestral history as a vertebrate and as a member of the Metazoa. Classification upon a basis of structural resemblance has, therefore, a deeper meaning than convenience, since it is fundamentally the study of evolutionary relationships (cf. p. 506). General Organization of the Animal Body Forms of Symmetry. — In this connection the principal struc- tural differentiations found among animals may be explained. Forms like Volvox (Fig. 106, p. 200) have a universal symmetry, since they are symmetrical around the center of a sphere. Any plane that passes through this center will divide the individual into halves that are symmetrical. A few of the protozoa are thus universal in symmetry, but there are no cases among Metazoa. The radial symmetry that exists in animals like the fresh-water polyp, Hydra (Fig. 128, p. 266), and in the starfish (Fig. 118) is characteristic of Coelenterata and Echinodermata. Such animals are symmetrical, like an umbrella or a cj'linder, around a line that is the principal axis of the body. There are a number of planes through this axis that will divide the individual into equal halves. As with plants, radial symmetry is intimately related to an attached mode of life. Radially symmetrical animals that are free-living, like jellyfish and starfish, have 244 CLASSIFICATION AND ORGANIZATION OF ANIMALS probably come from ancestors that were attached and hence radially symmetrical, if they do not have an attached stage in the life cycle. In many cases, animals that are essentially bilateral in type but are attached, like some of the segmented worms, Annulata, show a modification of bilateral organs in the direction of radial symmetry. The bilateral sijmmetry that is characteristic of all the more highly developed animal types is a symmetry on either side of a plane, in contrast to radial symmetry which is through a line. A radially symmetrical body may become bilateral by modification in one of its radii, as when an umbrella has a curved handle or a glass cyhnder is modified to form a beaker, and can then be divided into symmetrical halves by only one plane. Another way to define bilateral symmetry is to say that each of the halves sep- arated by the median plane is a mirrored image of the other, or that the halves are " rights " and " lefts," hke hands and feet. In general, bilateral symmetry is related to going " head-end first," and to the antero-posterior differentiation described in a subsequent paragraph. Some organisms are asijmmetrical; that is, there is no plane that will divide them into symmetrical halves. The most familiar examples are Protozoa like paramoecium. Other Modes of Differentiation. — In addition to being organ- ized according to the foregoing types of symmetry, animals may show 'proximal and distal differentiations, as in attached forms, where the base or proximal end differs from the distal end. An example is hydra, with its " foot " and tentacles (Fig. 121, p. 250). In other cases there is an oral side, which is differentiated from the ahoral, as in a jellyfish (Fig. 127, p. 265) or a starfish. Such proximo-distal and oral-aboral modifications are most typical of radially symmetrical animals. Bilateral animals, on the other hand, characteristically exhibit dor so-ventral differentiation into a "back" and a "belly" side; and antero-posterior differentiation into " head " and " tail " ends. Metamerism, or the segmental condition that appears so clearly in the earthworm (Fig. 137 A, p. 284) and other Annulata, is also characteristic of the Arthropoda and Chordata, as evidenced by the abdomen of the crayfish (Fig. 155, p. 323) and the vertebral segmentation of a vertebrate (Fig. 40, p. 69). T3rpes of Internal Organization. — The internal structure of many-celled animals may in turn be broadly classified as belonging GENERAL ORGANIZATION OF THE ANIMAL BODY 245 to one or another of the following types : the protozoan or unicel- lular type, which occurs only in the Phylum Protozoa; the para- A Fundamental Types \ Enterocoela Mollusca Fig. 118. — Principal types of animal structure and their probable relationships for comparison with Fig. 117. zoan type, found only in the Phylum Porifera or sponges (Fig. 118), and differing from that of other many-celled animals pri