mi :i m mw; .^,«..^'««'^'^'«*"'*^, .,««*«*'*««»* -^? #■ © >Y\ Jh ^^{>UJiH!th»»i>rt,«uwy>si*art 103 INSTRUCTOR'S COPY Compliments of THE C. V. MDSBY COMPANY ST. LOUIS. MO. and SAN FRANCISCO, CAL. SENT AT THE REQUEST OF MR. FRANK A. VOLK Your opinion of this book will be appreciated when your review of tt has been completed. .B TEXTBOOK ^ OF ZOOLOGY BY GEORGE EDWIN POTTER, Ph.D. Professor of Zoology, Agricultural and Mechanical College of Texas, Formerly Professor of Zoology, Baylor University SECOND EDITION With 445 Text Illustrations -T) \ .A. W l- xin^'L-v !v-*ti Iff- ai:i'Tfii., . ■^ ^F^^' ST. LOUIS THE C. V. MOSBY COMPANY 1947 Copyright, 1938, 1947, By The C. V. Mosby Company (All rights reserved) Printed in U. S. A. Press of The C. V. Mosby Company St. Loviis DEDICATED TO PROFESSOR FRANK A. STROMSTEN A friend and an inspiration to the student / PREFACE TO SECOND EDITION The present edition represents a revision of certain parts of sev- eral chapters, such as those dealing with Annelida, Genetics, Eugenics, Internal Regulation and Endocrines, Physiology, and Phylogenetic Relations of Animals. A brief section on Mammalian Development has been added. Numerous minor corrections or improvements have been made throughout. Several illustrations have been added and others improved. In addition to the acknowledgements included in the preface to the first edition, the author wishes to acknowledge the help of Dr. Kelshaw Bonahm of the Fish and Game Department, Agricultural and Mechanical College of Texas, on the chapter dealing with Pisces. At this point recognition is made of assistance given by Mr. Gordon Gunter in revising the list of animals of the Texas Gulf Coast in the chapter on ]\Iarine Zoology. The author is also indebted to Dr. Fred L. Kohlruss, Biology Department, University of Portland (Oregon), for numerous useful suggestions. Many valued suggestions have been received from individuals in a number of other institutions where the first edition has been in use. The author's indebtedness and appreciation is also expressed to Mr. Phil T. Williams who has furnished a number of the new illustrations. Finally, appreciation is expressed to Agricultural and Mechanical College of Texas for co- operation in numerous ways to assist in making this revision. George E. Potter. College Station, Texas f-i PKEFACE TO FIRST EDITION The important problems of life are common to all animals (includ- ing man) as well as to plants. It should be the purpose of a textbook in general zoology to present the animal kingdom in a logical and natural way and at the same time carry the interpretation of the facts in terms of the principles involved. It is exceedingly difficult to strike the ideal balance between the necessity of presenting sufficient, factual, "type" material in order that the student will have the requisite knowledge of classification, structure, function, development, and organography to appreciate the discussion of principles, and the opposite temptation to go into endless discussions of theories and rules, the comprehension of which is unquestionably beyond the capacity of the student who has not become grounded in fundamentals of animal make-up. Of course it is usually hoped that the laboratory division of the course wiU supply this needed foundation. It seems reasonable that the ultimate aim of the teacher of introductory zoology should be to bring the student to a fundamental and well-grounded understanding of the principles involved in all of the living processes. It is extremely difficult to skim this information from the top of the entire body of zoological knoAvledge, as one can skim cream from a crock of milk, and hand-feed it to the waiting student mind. Ap- parently there must be a certain amount of personal acquisition of the principles of the subject through attaining a clear-cut knowledge of tlie complete biology of a series of representative animals. Each of these representatives, since it is a living organism, demonstrates cer- tain of these principles. In order to bring this out there must be a rather close coordination between the studies of the laboratory and the presentation of principles by the textbook. Based on a recognition of the above-mentioned situation and also on the realization that the majority of students taking elementary zoology plan to go no further in the field, the author has attempted to strike a workable combination of the two schools of teaching and still cover the fundamental knowledge of the subject. There has been a definite effort to lead the student to think of biology as related to humankind and to himself. It is hoped that the book will overlap the laboratory studies just far enough to lift the student out of the laboratory into his o^vn correct interpretation of the facts discovered PREFACE there. It is of course assumed that the teacher will naturally elabo- rate upon particular phases of the topics taken up in the course. The anticipation of this and limitations of space have reduced the volume of detailed information included. Many animals from west of the Mississippi River are featured in this book. There has been no attempt to limit the scope of the work to this region, but since many southwestern and western forms are available and serve as very good illustrative material, they have been utilized. It is hoped this wiU make the book more useful and mean- ingful to students in these regions, as well as more teachable. The introduction of chapters on Animal Anomalies, Animal Re- generation, Biological Effects of Radiation, Marine Zoologij, and Wildlife Conservation is a slight departure from the usual textbook outline, but each of these seems to the author to have enough of special value and current interest to warrant presentation. The chap- ters on Regulatory Glands, Animal Distribution, The Animal and Its Environment, Animal Parasitism, Comparative Emhrijology, Animal Behavior, and Paleontology are also presented with the feeling that they are of exceptional general interest to all students, as well as being thoroughly zoological. The arrangement of the chapters on animal groups has been some- what in the order of complexity and systematic relationships. The chapters are written in such a way, however, that this order may be modified in any manner to suit the teacher. The chapters dealing with typical Protozoa, Hydra, Planaria, Annelida, Arthropoda, and Amphibia are somewhat amplified and include more detail because they are so often chosen as typical groups for study. Throughout the book the genus and species names have been italicized, and many names of structures and functions have also been italicized the first time they occur. The author is indebted and extremely grateful for the cooperation of several teachers and specialists who have contributed manuscript for chapters in their fields. For this service acknowledgment is made to: J. Teague Self, University of Oklahoma, Annelida; Elmer P. Cheatum, Southern Methodist University, Mollusca, and assisted with Marine Zoology; Vasco M. Tanner, Brigham Young University, Arthropoda; Mary Fielding, Public Schools, Waco, Texas, collabora- tion on Elasmohranchii; Rose Newman, Baylor University, collabora- tion on Pisces; Ottys Sanders, Southwestern Biological Supply Co., 8 PREFACE Amphibia, and assisted with Marine Zoology; Leo T. Murray, Baylor University, assisted by James E. Blaylock, Ranger Junior College, Reptilia; Helen Joe Talley, University of Oklahoma, collaboration on Regulatory Glands; T. C. Byerly, United States Bureau of Animal Industries, Ayiimal Regeneration; Titus C. Evans, University of Iowa, Medical College, Biological Effects of Radiation; Willis Hewatt, Texas Christian University, A^iimal Distrilution, and assisted with Marine Zoology; A. 0. Weese, University of Oklahoma, The Animal and Its Environment ; Sewell H. Hopkins, Texas Agricultural and Mechanical College, Animal Parasitism ; J. G. Burr, Texas Game, Fish, and Oyster Commission, marine data; Walter P. Taylor, Texas Cooperative Wildlife Service and United States Bureau of Biological Survey, Wildlife Conservation; A. Richards, University of Okla- homa, Comparative Embryology ; Frank G. Brooks, Cornell College, Genetics and Eugenics; Iva Cox Gardner, Baylor University, Animal Behavior; W. M. Winton, Texas Christian University, Paleontology . To Mr. Ivan Summers goes immeasurable credit for the excellent art work he has put into this edition. Dr. Titus Evans, of the University of Iowa, Medical College, has also been of great service with his excellent talent in creating illustrations. Mrs. Ruth M. Sanders, Miss Joanne Moore, and Mr. Edward O'Malley have each assisted by contributing certain illustrations. The drawings used in Chapter XLII on Genetics and Eugenics were made by Miss Betty R. Smith of Cornell College. The author is grateful to all of these individuals for their valuable services. The author wishes to acknowledge also the friendly and helpful advice which has been offered by Professors D. B. Casteel, T. S. Painter, and E. J. Lund of the University of Texas, and Professor Asa Chandler of Rice Institute. Finally, appreciation is expressed to Baylor University for the cooperation which has made the writing of this book possible. George E. Potter. Waco, Texas. CONTENTS CHAPTEE I PAGE Introduction ___________________ 17 The Biological Point of View, 17; Science and the Scientific Method, 18; Zoologj-, a Biological Science, 19; The Subdivisions of Zoologj-, 19; Classification of the Animal Kingdom, 25; Vital Eelations of An- imals and Plants, 27; Attributes of Life, 30; Balance in Nature, 31; Zoology as Belated to Man, 33; Agriculture and Zoology, 34; Fish- eries and the Application of Zoologj-, 34. CHAPTER II History op Zoology _________________ 36 CHAPTER III Protoplasm and the Cell _______________ 49 Living Matter, or Protoplasm, 49; The Cell Principle, 49; General Characteristics of Protoplasm and the Material of the Cell, 53; Fundamental Properties or Activities of Protoplasm, 54; Physical Nature of Protoplasm, 55; Chemical Nature of Protoplasm, 56; Structure of a Typical Animal Cell, 58; Cell Division, 61. CHAPTER IV Phylum Protozoa in General _____________ 65 Characteristics, 65; Classification, 65; Colonial Protozoa, 75; Tropisms and Animal Reaction, 77; Economic Relations of Protozoa, 77. CHAPTER V Euglena op Class Mastigophora ____________ 81 Habitat and Characteristics, 81; Structure, 81* Food and Assimilation, 81; Respiration and Excretion, 83; Reproduction and Life Cycle, 83; Behavior, 84; Locomotion and Flagellar Movement, 84. CHAPTER VI Amoeba op Ck, Si m o ttl 8 fe o >> o C > o C d M bo •i-t INTRODUCTION 33 whose extermination is easily possible, they will advise against it until all phases of the animal's existence are thoroughly investigated. To wipe out this form might remove the cheek on others that are still more obnoxious. Because of the danger of interfering with the nor- mal balance or equilibrium in nature, our government and many others have placed a restriction on importation of plants or animals. One must have permission to bring either into this country. Zoology as Related to Man The values of the study of zoology may be placed in two classes: cultural and practical. There is hardly a field of endeavor in the realm of human activities which is not greatly influenced by zoology and biology generally. The study of philosophy, the formulation of our conception of religion, the comprehension of social welfare problems, and many other similar intellectual and social accomplish- ments are greatly facilitated by a knowledge and recognition of biological principles. From the purely practical or economic side, of course, agriculture, medicine, and their related sciences have profited enormously. In fact, these fields are in themselves applied biology. Most of the great discoveries as to the nature and control of disease, the manner of inheritance of human characteristics, and the knowledge of fundamental physiological processes occurring in our own bodies have been attained by studies on other animals. What is found to be true in a dog, frog, rabbit, rat, monkey, or guinea pig, usually has its application to man. The lives of these laboratory animals have made untold and inestimable contribution to the welfare and comfort of man. The loss of their lives is con- stantly saving millions of human lives. One of the most obvious uses of other animals is as a source of food supply. All of the phyla and classes of larger animals furnish at least a few species that find places on our menu cards, particularly mammals, birds, turtles, frogs, fish, crabs, lobsters, clams, oysters, and even snails. Many animals are important because of their destructive tend- encies in regard to articles valued by man, or to the health and life of man. Most of the predaceous animals today are not a menace to man directly, but they do destroy many domesticated as well as useful wild animals. It is likely that the parasites which live on and in the bodies of men, and on domesticated plants and animals have been much more costly than the depredations of the more conspicuous predators. I 34 TEXTBOOK OF ZOOLOGY Agriculture and Zoology It may frequently bring a smile to the lips of an onlooker to see a full-grown and perhaps intelligent zoologist enthusiastically at- tempting to learn what, when, and how much a little boll weevil eats or when, where, and how it lays its eggs ; and yet, the discovery of such information may influence the activities of our entire cotton industry. A recent instance of the economic importance of zoo- logical knowledge is found in the saving of the entire citrus industry in Florida from the Mediterranean fruit fly. Injurious insects alone cause an annual loss in the United States of more than one and one-half billion dollars ' worth of products if they could be sold at the price the remaining portion brings. With proper knowledge of animal life and application of this knowledge it is likely that at least half of this loss could be prevented. Losses almost as impor- tant are caused each year by the parasitism of our domestic animals by bacteria, protozoans, worms, and insects. The knowledge and application of parasitology, which is a field of zoology, would avoid this loss. Agriculture has benefited greatly from the application of the principles of heredity to plant and animal breeding. Much funda- mental knowledge has come from the extensive studies on the genetics and breeding of the common fruit fly, Drosophila. It is easily kept in the laboratory and mated. It produces a new gen- eration about once every nine days. More improvement of strains of animals and plants too, can be made in one man's lifetime than was previously possible through ages. The United States Department of Agriculture and the United States Department of Interior have taken the lead in much of this type of zoology. Fisheries and the Application of Zoology A very practical and profitable application of zoology has been made in the fishing industry. The annual salmon catch alone on the Pacific coast has been known to be worth $25,000,000. The fishing industry cultures, collects, and markets not only fish of many kinds but also oysters, clams, lobsters, crabs, shrimp, and even sponges. The United States Fish and Life Service does an extensive and remarkable work in the study, propagation, and care of this natural zoological re- source. Even with this work and that of all the State Fisheries Departments, the natural fish life does not flourish as it might, had INTRODUCTION 35 our public more appreciation of conditions necessary for a fish to live. A fish needs suitable water conditions including proper gas content, salt balance, nesting places, vegetation, and freedom from chemical or oil pollution. The strictly intellectual and cultural endowments which zoology has given man are no less valuable than the tangible gifts. To understand something of the orderly conduct of Nature and to see that her operations are in accord with definite principles, gives one insight to the solution of many of the problems of life. Many of the superstitious dreads of unseen monsters have been eliminated by the knowledge of the fundamental principles of life processes. In recent times, it is probably true that nothing has influenced the thinking of the world more than the ideas, principles, and knowl- edge growing out of biological study. CHAPTEE II HISTORY OF ZOOLOGY This brief chapter is organized to afford a slight preview of the works and lives of a selected few of the historic pioneers of zoology. This is not an attempt to give a complete history of the subject. The works of numerous pioneers in special fields are being con- sidered throughout the text rather than in a given chapter. There were individual persons interested in and studying natural history long before there was any orgajiized field of study recog- nized under the name of natural history or the more limited divi- sions of it, including zoology. Some of the translations from the early Egyptians and later from the Greeks indicate that there had been some concern for the problems of life as well as medicine a number of centuries before Christ. Some of the early Greek scholars believed that the ocean supported all of the original life. Hippocrates, a Greek living from 460 to 370 B.C., was the first to think of medicine on a scientific basis. Aristotle (384-322 B.C.) was an outstanding Greek philosopher and scholar. To him goes the credit for establishing the scientific method of study which is based on gathering facts from direct observations and drawing conclusions from a study of these facts. His observations on the structure and development of embryo sharks, chicks, and many other animals, as well as his introduction of animal classification, are contributions which caused him to be called a biologist. He had the assist- ance of the armies of Alexander the Great in collecting materials. Alexander had been one of Aristotle's pupils and had become inter- ested in the development of scientific endeavor. He made a grant of 800 talents ($200,000 or more) for use by Aristotle in his investi- gations. Thus even in those times endowments were being set up for the support of research. The other Greeks who followed Aris- totle added very little of importance. Early Roman Scholars. — From shortly before the time of Christ and extending for about sixteen centuries was a period of "dark ages" in scholarly endeavor. However, a few contributions of note were made. Pliny (a.d. 23-79), a Roman general, compiled a 37- volume work in which much of the scientific knowledge of the time 36 HISTORY OF ZOOLOGY 37 and traditional superstitions are woven together. His work was limited to compilations, and because of the indiscriminate mixing of fact and fancy it is not scientifically valuable. It does reflect the tendency of the time in that scientific observation had given way to speculation. Galen (a.d. 131-201), coming in the midst of the "dark ages" as he does, should be particularly credited for the contributions he made. He was of Greek ancestry but moved to Rome early and became a successful physician. His anatomical studies were made principally from direct observations on elephants, Barbary apes, and Fig-. 7. — Aristotle (384-322 B.C.), father of naturalists. From a bas-relief found In tlie collection of Fulvius Ursinus. (Visconti, Iconographic grecque.) (From Locy, Growth of Biology, published by Henry Holt and Company, Inc.) swine. During his time it was strictly against the law to make dissections of the human body so he was not allowed this privilege. Unfortunately, Galeai did not take advantage of the work of certain of his predecessors who had been privileged to study human bodies. His conviction in the matter of direct observation as a basis of study handicapped him in this respect. His textbook on anatomy became the authority for the next eleven or twelve centuries. Andreas Vesalius (1514-1564). — The return of interest in zoology came about through the medical schools. Vesalius was aji active 38 TEXTBOOK OF ZOOLOGY Fig. 8. — Galen (A.D. 131-200), anatomist. Acta Medicorum Berolinensium, Vol. 5. 1719. (From Locy, Groivth of Biology, published by Henry Holt and Company, Inc.) HISTORY OF ZOOLOGY 39 young student and was not satisfied to accept the authority of Galen's textbook. Therefore, after beginning his medical education at Brussels, he transferred to Padua where human dissection was Fig. 9. — Vesahus (1514-1564), anatomist. Frontispiere. facsimile edition of 1728. Nortliwestern University Library. (From Locy, Grotvth of Biology, pub- Iislied by Henry Holt and Company, Inc.) then allowed. He later became professor of surgery there. He was the first, since the time of Aristotle and Galen, to prove that direct 40 TEXTBOOK OF ZOOLOGY observation is the only true criterion of knowledge. Vesalius is thought of as the "father of modern anatomy," and his teaching is really responsible for the rapid development of biology and medi- cine following his time. William Harvey (1578-1657). — Following closely upon the epoch- making work of Vesalius and inspired by several of his pertinent observatiojis on the anatomy of the circulatory system, "William Harvey, an Englishman, began experiments on the movement of blood in the vessels. Galen, Vesalius, and three or four others had Fig. 10. — William Harvey (1578-1657), father of physiology. (From Garrison, History of Medicine, published by W. B. Saunders Company.) suspected a circuit of the blood from the heart to the lungs and return, but Harvey was the first to demonstrate circulation, and the first to arrive at an idea of a complete circulation of all of the blood through a closed system of vessels. This new idea was presented in 1628. He also did notable work in embryology. Marcello Malpighi (1628-1694) was a famous Spanish anatomist, histologist, and embryologist. His observation of blood corpuscles in capillaries, studies on glands, and his work on the structure and metamorphosis of the silkworm take rank with outstanding con- HISTORY OF ZOOLOGY 41 tributions to zoological knowledge. Numerous organs of the human body are named for this renowned scientist of his time. Like other early microscopists, he had to build his own microscope. Antonj van Leeuwenhoek (1632-1723) lived almost contemporane- ously with Malpighi and like him made many contributions to the Fig. 11. — Leeuwenhoek (1632-1723), pioneer micromotist. (From a painting by Veekolje, 1685. Reprinted by permission from Locy, Growth of Biology, published by Henry Holt and Company, Inc.) development of the microscope. He is said to have possessed a total of 419 lenses, most of which he had ground. Further study on capillary blood circulation, first descriptions of spermatozoa, ex- tended observations on bacteria and microscopic animals, and his 42 TEXTBOOK OF ZOOLOGY valuable contributions to the development of the microscope are the enviable accomplishments of this man. Carolus Linnaeus (1707-1778) was a very eminent Swedish biolo- gist, who, like many early students of this subject, was educated as a physician. He followed somewhat in the footsteps of Ray (1628-1705), who had paved the way by fixing a definite conception Fig. 12. — Linnaeus (1707-1778), an outstanding Swedish biologist of his time. (Reprinted by permission from Locy, Growth of Biology, published by Henry Holt and Company, Inc.) of a species and introduced the use of anatomical features in dis- tinguishing the larger groups. Linnaeus believed in a rigidly fixed species and had divided the animals into six classes, 32 sub-classes, and numerous genera and species. In spite of his idea of the in- variability of species his classification system was so simple, clear, and flexible that it has persisted to the present time. His was the HISTORY OF ZOOLOGY 43 first natural system of classification, and it is known as the Binomial System of Nomenclature. Each individual not only fits into larger general groups by this sj'stem, but it is specifically known by the genus and species names used together, hence the two names. Lin- naeus is said to have classified and listed 4,378 species of plants and animals. Almost immediately following Linnaeus came the Frenchman, Lamarck (1744-1829), who among other important things is credited with being first to realize that there are different lines of descent and that no living species is absolutely fixed. Much later, in 1866, Ernst Haeckel organized the modification of this system as used in modern times. Georges Cuvier (1769-1832) is credited with establishing the field of comparative anatomj^ He was of French ancestry and largely self-educated by his studies at tlie seashore. A number of anatomi- cal structures bear his name. Karl Ernst von Baer (1792-1876), a Russian biologist, is one who really established embryology as a field of study. His notable paper on the development of the chick was published in 1832. He estab- lished the "germ layer theory,*' thus explaining the unfolding and differentiation of the various organs of the developing animal. The recapitulation theory, Avhich is explained elsewhere, came as a result of his work and thought. Johannes Miiller (1801-1858), a German scientist, is referred to as the founder of comparative physiology and the first to apply the facts of physics and chemistry to living protoplasm. His work was a great impetus to modern physiology. Matthias Schleiden (1804-1881) and Theodor Schwann (1810-1882) are the two Germans who in 1838-1839 arrived at one of the most important generalizations of biology, the cell theory (principle). This is to be discussed further in the following chapter. Louis Agassiz (1807-1873) is commonly regarded as the father of American zoology and a renowned student of comparative anatomy. His great inspiration has permeated through his students to nearly every institution in the land. He was a recognized paleontologist as well as zoologist. He is responsible for one of our first and oldest Marine Biological Laboratories. 44 TEXTBOOK OF ZOOLOGY Charles Darwin (1809-1882), an Englishman, made extensive studies on the problem of the manner and means by which new species of organisms arise. He very effectively developed the thesis that they originate by a process of natural selection. This was based on the idea that no two individuals are exactly alike, that new varia- tions are constantly appearing, and finally that those individuals or groups best suited to their environment would be the ones to persist and produce progeny. His conception of the factors and P^ig. 13. — Agassiz (1807-1873), great American pioneer zoologist. (From Locy, Biology and Its Makers, published by Henry Holt and Company, Inc.) limitations determining the development of new species, pictures a constant struggle for existence among organisms, with those whose natural variations happen to fit them best to the changing features of the environment persisting as dominant species and others being crowded out. Those least fitted to the environment would naturally become extinct. Darwin did not claim originality in his idea. Lamarck, Buffon, and Erasmus Darwin, grandfather of Charles, had presented similar HISTORY OF ZOOLOGY 45 ideas before him. It was the vast accumulation of facts covering a period of twenty years which commanded the attention of sci- entists as well as the public generally. In 1858 he read a joint paper with Alfred Russel AVallace, a contemporary who had reached the same conclusion, on the theory of natural selection. That same year Darwin published his book Origin of Species which is a classic in its field and familiar to all scholars. ' >T.:" Fig. 14. — Charles Darwin (1809-1882), the author of Origin of Species. (From Garrison, History of Medicine, published by W. B. Saunders Company.) Gregor Mendel (1822-1884) was an Austrian monk who carried on experiments with the breeding of garden peas in the cloister garden. From his work there, he derived the original laws of heredity. His results were first published in an obscure Swiss paper in 1866 and were not really discovered and appreciated until 1900. He was the founder of genetics. He crossed different kinds of peas and found that the offspring in the first generation all resembled one parent. When these oft'spring were interbred he found that three-fourths of their progeny resembled one grandparent, and the remainder resembled the other. From these facts he referred to characteristics of the former group as dominant and those of the latter as recessive. The facts which he established are now known as Mendel's Laws of Heredity. 46 TEXTBOOK OF ZOOLOGY ■,(S, C^^f^e^a^-^ Fig. 15. — Greg-or Mendel (1822-1S84), the Austrian monk who pioneered In studies of heredity. Plaque by Theodor Charlemont based on "Fuschia picture" made between 1861 and 1864. The signature is his, taken from an autobiography. (Courtesy of the Journal of Heredity.) HISTORY OF ZOOLOGY 47 Louis Pasteur (1822-1895) was a French scientist who had been trained in chemistry but became one of the outstanding pioneers in applied biology and medicine. In 1861 he put an end to the controversy regarding spontaneous generation of living organisms and established the idea that all life in present times comes from preexisting life. He showed that living organisms cause fermenta- tion and demonstrated that these organisms and others could be killed by heating them to a certain temperature. He showed that materials thus heated and then sealed would not ferment until after they were exposed to the organisms in the air. The pasteurization Fig. 16.— Louis Pasteur (1822-1895), one of the benefactors of mankind. (From Garrison, History of Medicine, published by W. B. Saunders Company.) process grew out of these experiments. He rescued the silk in- dustry of southern Europe by discovering the organism which killed the insects, and he also discovered an immunization process and treatment for hydrophobia. Thomas Henry Huxley (1825-1895) Avas one of the most popular English scientists of his day. He was one of the principal champions of Darwin's ideas and theories. Comparative anatomy and paleon- tology were greatly advanced through his influence. August Weismann (1834-1914) was a German zoologist who started out as a physician after having been trained in that field. He was 48 TEXTBOOK OP ZOOLOGY an outstanding scholar in the fields of heredity and embryology. He is best known for his theory, that there is continuity of germ plasm from generation to generation. Hugo DeVries (1848-1935), a Dutch botanist, brought out the mutor tion theory, which is important to all modern biological conceptions. His idea was that species have not arisen through gradual selection requiring thousands of years for each but by jumps through sud- den, though small, transformations. He is widely known for his experimental studies in plant breeding and genetics, particularly with evening primrose. E. D. Cope (1840-1897) was one of the greatest comparative anat- omists of America. He dealt not only with living forms but with fossil materials as well. The work of all those mentioned and hundreds of others has given us the background for our present knowledge and grasp of zoology and medicine. History is being made so rapidly in these fields dur- ing the current years that it is difficult even to catalogue the im- portant contributions. It is an extremely active field, particularly in the realm of the experimental endeavors. The printed program for the annual meeting of the American Zoological Society, which is made up largely of titles and abstracts of new papers to be pre- sented, is a small book in itself. The works and lives of such prominent pioneer zoologists of the Southwest as Jacob Boll, Gustaf W. Belfrage, Lincecum, Vliet, Walker, "Webb, and others have been described in the recent book by Dr. S. W. Geiser of Southern Methodist University, entitled Naturalists of the Frontier. This book is extremely interesting to read. CHAPTER III PROTOPLASM AND THE CELL Living- Matter, or Protoplasm Little is known concerning the origin of living matter, or proto- plasm, as it is called, but more and more is being learned about its nature, characteristics, structure, and activities. Living matter is ahvays active in some degree, and this activity attracted the atten- tion of scholars at a rather early date, but serious study of the material was not begun until approximately one hundred years ago. A Frenchman by the name of Dujardin, in 1835, realized that some of the simple microscopic animals he was studying were composed of a soft, gummy substance and called it sarcode, which means "flesh." He was able to test its solubility and its behavior with alcohol and acids sufficiently to satisfy himself that it differed from ordinary gelatin or albumin, with Avhich it might be confused. In 1840, Purkinje, a Bohemian biologist, gave living matter the name protoplasm, which comes from the Greek protos, first, and plasma, anything formed or molded. In 1846 von Mohl, a German botanist, saw in plants a granular, viscous substance similar to that already seen in animals', and called it protoplasm. He was instrumental in bringing this name into common use. During these years it had gradually dawned on biologists that this matter is found in all liv- ing things. The Cell Principle Cells had been seen and even superficially described during the latter part of the seventeenth century and numerous times during the eighteenth century, but their significance was not realized. Hooke, an Englishman, in 1665 in observing cork with the micro- scope he had made, saw the spaces in it and called them cells be- cause they reminded him of prison cells. This name later came to be applied to the real cells. It was an unfortunate term, for cells do not have a hollow structure but are typically semisolid bodies. Leeuwejihoek saw spermatozoa and bacteria and included them with single-celled animals as "little beasties"; Malpighi had described 49 50 TEXTBOOK OF ZOOLOGY the nature and appearance microscopically of several organs of the body; Grew had made rather extensive microscopic studies of plants, and in 1831 Kobert Brown had discovered the nucleus of the cell, but not until the work of Schleiden in 1838 and Schwann in 1839 was the cell theory formally enunciated. The former a botanist and Fig. 17. — Matthias J. Schleiden (1804-1881), noted German botanist who helped establish the cell theory. (From Locy, Growth of Biology, published by Henry Holt and Company, Inc.) the latter a zoologist, each working independently, came to the same conclusion and in 1839 collaborated their ideas. This theory, as they gave it, was in substance, All living things (plants and animals) are composed of cells. PROTOPLASM AND THE CELL 51 It is no discredit to this theory or these men that they and many other biologists of the time had erroneous ideas concerning the essential features of the cell. Although Brown had recently dis- covered the nucleus, the cell wall was thought to be the essential part, though now we know it is not a universal structure of all cells since practically no animal cells have a cell wall. The notions of the origin of cells and the functional significance were almost wholly fantastic, yet the cell theory proved to be such a unifying generalization and inspiring stimulus to investigation that it became the turning point in the development of biological study. Fig. 18. — Theodor Schwann (1810-1881^), the German zoologist who, in 1838 and 1839, collaborated with Schleiden in formulating the cell theory. (From Garrison, History of Medicine, published by \Y. B. Saunders Company.) The bare statement that living beings are composed of cells soon became inadequate as studies of cells progressed. It was soon found that some tissues are made up not only of cellular structures but included also certain noncellular materials produced by the cells. The matrix, so abundant between the cells of cartilage, was soon found to be noncellular and to be produced by the cartilage cells which became embedded in it. This matrix is not strictly living matter since it is inactive and passive as far as life processes are concerned. Connective tissue fibers fall in the same category. Since living bodies are composed of such an abundance of this noncellular 52 TEXTBOOK OF ZOOLOGY m aterial produced by the cells, the cell prmciple soon came to be stated thus : all living things are composed of cells and cell products. With the years, the conceptions of the nature of the nucleus, the cell membranes, and the composition of protoplasm itself have all added their contributions to the present understanding of the mean- ing and application of the cell principle. The cell is now regarded as a physiological unit as well as a structural one, and as almost a corollary to the original statement of the principle, namely, that the activities of the organism equal the sum of the activities of its cells. With the embracing of the functional activity of the cell as a part of the principle underlying living processes comes also the inclusion of heredity and development. Cell division, growth, tissue forma- tion, migration of cells, formation of cell products, chromosome rela- tionships and modifications have come to be recognized as being brought about in or by the cells. Through the rather rigid and constant set of developmental changes for which the cells are respon- sible, there is developed a new individual which usually resembles its parents quite closely. The influence of the cell theory on biological thinking and progress as well as its effect on fundamental thinking generally, can hardly be over-estimated. The conception of this idea was one of the great landmarks in development of biological ajid scientific thinking. It was the first great generalization in biology. It is comparable in the field of biology to Newton's law of gravitation in the field of physics. Up until this time there had been no single fundamental idea applied to living material that was recognized as being univer- sally true. This conception focused the thinking of all biologists in the same direction and therefore it had a great unifying influ- ence. Deliberation and meditation on this fundamental idea seemed to prepare biologists for other great generalizations which followed quite rapidly. Many new problems arose with this new knowledge of plants and animals. Comparative morphology was extensively investigated, and physiology now has become physiology of cells as a result of this impetus. An understanding of the permeability of cellular membranes, the transformation of energy by chemical reaction within cells, the roles of electrolytes in living substance, and the principles of heredity are some of the results of this new conception of life embraced in the cell theorj^ PROTOPLASM AND THE CELL 53 General Characteristics of Protoplasm and the Material of the Cell To begin with, it may be said that this substance has a variable degree of fluidity under different conditions. The range of this variation may be from semisolid to semiliquid. It is viscid and gelatinous in consistency. It is more or less granular, nearly color- less, and more or less translucent; however, it is never perfectly transparent. The trauslucency causes a mass of it to have a lustrous gray appearance. As a constituent of protoplasm there is always a considerable percentage of water, which conditions the degree of viscosity. WT. Km / •" •• '■"■ ..' *'■ ■ J'y- '''■■. : i->. -v ♦•"•<.*. '-^J'.' S ■■■•■■ .'.•■..■■ ' ... i- --•■.-.,■.■ -- f . >v^^._.^.J^"^-?'^•%^,^ * ' * - . Fig. 19. — structure of living protoplasm as seen in tlie starfish. (From Wilson, The Cell, published by The Macmillan Company.) Protoplasm is in a colloidal state of the emulsoid type. A colloid is a substance of gelatinous nature, permeable by crystalloid solutions, and diffusing not at all or very slowly through animal or vegetable membranes. In the emulsoid, or colloidal emulsion, the substances are distributed through the more watery or dispersion medium. A colloid is identified by the presence of particles which are groups of molecules dispersed through a more fluid or watery phase. These particles, of course, are larger than molecules, but they are too small to be seen with the ordinary microscope. It is possible for water and substances in solution to enter protoplasm from without, and this is reversible. With loss of water from the dispersion medium the dis- persed particles of the colloid become congested by loss of general fluidity. This condition is known as the gel state. When there is increased water in the dispersion medium and the particles move with greater ease in the more fluid medium, the colloidal state tends to be- 54 TEXTBOOK OF ZOOLOGY come sol. This transfer of water may be due to chemical changes in the dispersed particles or in the dispersion medium of the colloid. The ability of protoplasm, because of its colloidal nature, to change from sol to gel state and back to sol repeatedly is the basis of many of the vital activities, such as utilization of food, disposal of waste, and movement. Fundamental Properties or Activities of Protoplasm In addition to the general characteristics, there may be mentioned and described briefly a number of important activities common to all protoplasm. These properties are: 1. Irritability, which refers to the capacity present in all proto- plasm for responding to changes in environmental conditions, or external stimuli. 2. Conductivity refers to the fact that the impulses produced by stimuli or irritations at one point in protoplasm are conducted to other parts of not only a single cell but also to adjoining cells. 3. Contractility, which is the power of contraction and relaxation that is common to the substance of every cell. 4. Metaholism, the process of continual exchange of food and fuel materials being built into the protoplasm, while, at the same time, materials there are being oxidized to liberate kinetic energy, such as heat and movement, and produce waste by-products. 5. Growth is recognized as any increase in volume. When the rate of the building side of metabolism exceeds the oxidation rate in the protoplasm, there is storage of materials in the mass of the protoplasm and hence growth. All protoplasm has this capacity. 6. Reproduction is the capacity for producing new individuals of the same kind. All living organisms are capable of this by some means. Simple cell division is the most primitive process of repro- duction among animals. Consciousness, which refers to the awareness of one's own exist- ence, is frequently given as a property of protoplasm. It is certain that some protoplasm possesses consciousness, but evidence of this quality is rather intangible. Spontaneity is also considered a prop- erty of protoplasm by some. To be certain that the activity and source of all reaction comes from within is likewise rather difficult of definite proof, so this is simply mentioned here as another prop- erty which is often listed. PROTOPLASM AND THE CELL 55 Physical Nature of Protoplasm Protoplasm is a semifluid material which is heavier than water and somewhat more refractive to light. Its physical constitution is similar to glue or gelatin, rather than to crystalloids, such as sugar or ordinary table salt (sodium chloride). Instead of being in the form of a true solution like salt in water, it consists of sus- pensions of relatively large molecular aggregations varying roughly between 0.0001 and 0.000001 of a millimeter in diameter. These particles keep up an expression of energy in that they move against each other as though they were dancing in a limited space. This activity can be seen only with a special optical arrangement known as the ultramicroscope and the phenomenon is known as Brownian movement (characteristic of colloidal substances). Protoplasm dif- fuses slowly or not at all through animal membranes. It changes from a fluid or sol state to a more solid or gel state and may return in the other direction. Ordinarily the viscosity of the continuous phase or supporting liquid is only three or four times that of water, while with the dispersed particles included it is only eight or ten times that of water. The viscosity of the nuclear fluid is only twice that of water. Since glycerin has a viscosity about a thousand times as great as water, it will be realized that most protoplasm is quite fluid in its active state. Changes in viscosity accompany and are essential to the activity and functioning of it. Protoplasm is not a single compound; it is a colloidal system of a number of chemical compounds existing together. Colloidal systems are known as disperse systems of the emulsoid type. The more watery or continuous part of the system is loiown as the dispersion medium, while the particles or molecular aggregations constitute the dispersed phase. An important consequence of the colloidal systems in protoplasm is the enormous surface of particles exposed to the continuous phase. If a sphere of material has a radius of one centimeter its total surface will be 12.6 square centimeters. Now, if the same volume of material is in colloidal particles of the average size given above, the total surface of these will be approxi- mately 7,000 square meters. This increase in surface is one of the significant effects of colloidal organization of substances, because many important reactions occur at these surfaces. By the presence of salt ions in the continuous phase and these becoming adsorbed 56 TEXTBOOK OF ZOOLOGY upon the surfaces of the colloidal particles, they acquire an electric charge. Protoplasm exhibits these several phenomena because of its colloidal nature. Chemical Nature of Protoplasm Up to the present time, protoplasm has eluded complete and exact chemical analysis. Nevertheless the compounds of living matter are composed of several elements, many of them the most ordinary and abundant in the world. The list of elements necessary to make human protoplasm could be gathered in almost any locality on the face of the earth. As a rule the elements found in protoplasm are oxygen, carbon, hydrogen, nitrogen, sulphur, phosphorus, cal- cium, sodium, chlorine, magnesium, iron, potassium, iodine, and fre- quently others like silicon, aluminum, copper, manganese, bromine, and fluorine. The most abundant of these are found named in the first part of the list. A few of them are usually given as constitut- ing approximately the following percentages of protoplasm : oxy- gen 65 per cent, carbon 18 per cent, hydrogen 10 per cent, nitrogen 3 per cent, calcium 2 per cent, phosphorus 1 per cent, and all others makijig up the remaining 1 per cent. These elements are found combined to form compounds. The organic compounds include car- hohydratcs, fats, proteins, and also enzymes. The inorganic com- povmds consist of several inorganic salts and water. The carbohydrates, which include starches and sugars, are com- pounds of carbon, oxygen, and hydrogen. The proportion of the hydrogen to oxygen in the molecule is the same as found in water, two to one. The principal carbohydrate found in protoplasm is the monosaccharid, or simple sugar, glucose, whose formula is CeHioOe- This is actually built into some parts of the cell, but its chief func- tion is to furnish the most available source of energy by its ready oxidation. When a molecule of glucose is burned, the potential energy is released as kinetic or mechanical energy, and there are formed six molecules of v.'ater (H2O) and six molecules of carbon dioxide (CO2). Glucose is converted to a starchlike substance, glycogen, for storage in the various animal tissues. This substance must be reconverted to glucose before it is available for production of energy. Fats, like carbohydrates, are composed of carbon, hydrogen, and oxygen but in more complex molecular arrangement. There is much more carbon and hydrogen with less oxygen, which allows the fats PROTOPLASM AND THE CELL 57 to combine with more oxygen in oxidation and therefore release more energy. Fat is extremely well adapted as a form of material for storage, since Aveight by weight it contains more potential energy than any of the organic group. Such common substances as lard, butter, tallow, whale blubber, and cottonseed oil are good examples. Fats serve a double function in protoplasm : constitution of a part of the structure of the cell and, secondly, the storage of food. Proteins constitute the bulk of the foundation or framework of the cellular structure and are the most abundant organic constituents. They are composed of carbon, hydrogen, oxygen, and nitrogen, with the frequent addition of traces of sulphur, phosphorus, magnesium, and iron. All of the proteins have large molecules, each being com- posed of thousands of atoms ; as an illustration, take hemoglobin of the red blood corpuscles with its formula C7i2Hii3oN2i40245FeS2. Pro- teins have a slow rate of diffusion, high resistance to electric cur- rent, and usually coagulate upon heating or upon addition of acids, alcohol, or salts to form a clot. Egg albumen, gelatin, and lean meat are common examples of proteins. They are split into numer- ous amino acids which serve as the building stones of the stable portions of protoplasm. Enzymes are substances whose exact chemical nature is not yet known, but whose importance to protoplasm is probably unequaled. Chemically and physically they seem to be more like proteins than anything else. These substances are found not only in the cells, but they are also secreted by cells into the digestive tract and into the blood stream, where they act as organic catalysts. The general function of the catalyzer or catalytic agent is that of facilitating and speeding up certain chemical exchanges without the agent itself entering into the reaction. The well-known example of catalysis is the effect of a small amount of platinum in increasing the rate at which hydrogen and oxygen combine to form water. A particu- lar enzyme is usually specific for one kind of reaction, but not for the species of animal in which it will function. Enzymes taken from one species will usually facilitate the same kind of specific reaction in other species. The digestive enzymes may be thought of as an example. Of these, pepsin will bring about the same gen- eral reaction, whether it is in the stomach of a frog or of a man, under favorable conditions. Since many enzymes influence only one specific type of chemical reaction and since there are numerous 58 TEXTBOOK OF ZOOLOGY types of reactions going forward in active protoplasm, it is seen that there must be numerous enzymes present in the cells of every organism. Water constitutes 60 to 90 per cent of protoplasm and maintains many substances in solution. Water is not only a very efficient solvent; but it is important to protoplasm because of its compara- tively high surface tension, because its presence gives the proto- plasm a consistency compatible with the range of variation neces- sary for metabolism, and because of its high specific heat. This latter point is important in maintaining protection against sudden and extreme temperature changes in the living organism. Young cells contain more water than old ones, young organisms likewise contain more than old ones. The relative amounts of water in rela- tion to other materials of the protoplasm vary in different cells and in different species. The inorganic salts are present in considerable numbers but in relatively small amount. They are electrolytes, and therefore split up in aqueous solution into ions, which are able to combine with all the other substances in protoplasm. The chlorides, phosphates, iodides, carbonates, and sulphates of sodium, potassium, calcium, magnesium, and iron are important salts of living cells. The relative proportion of these salts is kept at a fairly constant level, and slight changes in this balance have regulatory effects on metabolism. From the chemical standpoint, living protoplasm is considered the most complex of all systems of compounds. Even the proteins, as a part of protoplasm, are more complex than any other sub- stances. In a sense, protoplasm is quite unstable in that it changes its composition in response to every change in the environment, and when active it is not the same for any two consecutive moments. The exceeding variability of protoplasm chemically, makes possible all of the necessary adjustments of living matter to its environment. On account of the extreme complexity of protoplasm it is not sur- prising that the chemistry of all of its activities is not yet com- pletely understood. Structure of a Typical Animal Cell The quantity of protoplasm comprising a single cell varies within wide limits ; therefore cells vary greatly in size. The majority of cells, but not all of them, require considerable magnification to be seen. Cer- PROTOPLASM AND THE CELL 59 tain of the single-celled blood parasites are about as small as any cells known. They are barely seen with our highest magnifications. At the other extreme of size we may refer to another parasitic single-celled animal, Porospora gigantea, which lives in the intestine of the lobster, and may reach from one-half to two-thirds of an inch in length. Egg cells, including the yolk, may exceed this size. Some of the nerve cells, though of less mass, may be several feet in length. Muscle cells are relatively long also. Plasma /Atmirane Eciop/asm Chondriosoma Enasis Monosiga Cht lononas Codonosiga. Phaous Trachelmonas Peranema Maatigamoeba Fig. 23. — Group of representative Mastigophora. (Reprinted by permission from Curtis and Guthrie, Textbook of General Zoology, published by John Wiley and Sons, Inc.) (Figure of Chilomonas modified.) These forms are frequently classified as plants b}^ botanists. The class Mastigophora is divided into two groups: (a) the animal-like forms which may be holozoic, saprophytic, or entozoic, and (b) those more plantlike forms which may be holophytic, saprophytic, or entozoic. Holozoic refers to forms which ingest and digest food material. Saprophytic refers to the habit of absorbing nonliving organic matter in solution directly through the surface of the body. Entozoic is a name applied to forms which live within the bodies of other animals, as in the intestine or the blood stream. PHYLUM PROTOZOA IN GENERAL 67 A larg-e number of Mastigophora live in quiet streams, ponds, lakes, and in the ocean, Euglena is a very common!}^ studied fresh-water form. Noctiluca is an interesting marine form which is pelagic (lives at the surface) in its habits and appears as a thick, creamy scum. This soupy mass of organisms may cover an area of hundreds of square rods. When stimulated, these animals are luminescent and at night frequently give up an attractive greenish or bluish white light ; Uroglena is a fresh-water form which is often found in water supply basins and causes a pungent, oily odor and unpleasant taste in the water. Giardia (Fig. 386), Trichomonas, Chilomastix, Retortamonas and Enteronomas are all genera with representatives occurring in the digestive tract of man. Arcella Actinophrya Fig. 24. — Group of typical Sarcodina. (From Curtis and Guthrie, Textbook of General Zoology, published by John Wiley and Sons, Inc.) 2. Class Sarcodina (sarkodi'na, fleshy) or Rhizopoda (rizop'Oda, root foot). — ^A distinctive feature of nearly all species of this class is the capacity to form protoplasmic processes called pseudopodia (false feet) which are temporary structures and can be withdrawn. The ani- mal is able to accomplish locomotion by extending the protoplasm into these pseudopodia. The representatives of this class include many free- living forms as well as numerous parasitic ones. A number of the rep- resentatives of class Sarcodina secrete an external shell of lime, silicon, chitin, cellulose, or some bind in sand or other solid substances with one of the secretions. The class is commonly divided into five orders, (a) Amoebina are irregularly-shaped forms with lobelike pseudopodia. Some of the species are naked, and others are covered by a shell. Amoeba proteus is the free-living naked form which is commonly studied. Endamoeba histolytica is the most common parasitic form, Arcella, which secretes its shell, and Difflugia, which constructs its 68 TEXTBOOK OF ZOOLOGY Shell of sand cemented together by a secretion, are two of the most commonly observed shell-bearing forms, (b) Foramimera is an order of shelled forms whose pseudopodia are very slender a^d reticnlar. The pseudopodia are extended through small pores m the sheU Only a very few of this group live m fresh water. The vast ^^iijA^l^ "s :.M iU- e Fig. 25.-Life cycle of one of the Foraminifera PoZysfomeZZa crw^^A^ spheric individual; B amoeboid 9^11^ .f.^^P^^^ from it ^ youn^^^ dividual developing from amoeboid cell ;D,nmcrosphericin^^^^ Parker and gametes escaping from it ; F, union of gametes. Kearawn Haswell, Zoology, published by The Macmillan Co.) PHYLUM PROTOZOA IN GENERAIv 69 majority are marine, and Glohigerina is a typical example. The dis- integrating calcareous shells of this organism constitute a great mass of material on the bottom of the ocean which is known as globigerina ooze and from which chalk is formed, (c) Mycetozoa, are character- ized as being able to produce enormous plasmodia containing hun- dreds of nuclei and contractile vacuoles, as well as having ability to Fig. 26. — Shells of different Foraminifera. A, Rhahdamina abyssorum (X4.5) ; B, Nodosaria hispida (X18); C, Globigerina buUoides (X55). (From Borradaile and Potts, The Invertebrata, published by The Macmillan Company, after various authors. ) reproduce by spore formation. They live quite commonly in masses of decaying vegetable material upon which they feed, (d) Heliozoa is a group with thin, radially arranged, threadlike, unbranched pseudopodia. Actinophrys sol is a common one found in fresh-water streams and ponds, (e) Badiolaria is a marine group with fine, ray- 70 TEXTBOOK OF ZOOLOGY like pseudopodia and a shell composed largely of silica. The pseudo- podia extend through the relatively large apertures in the shell. 3. Class Infusoria (infuso'ria, crowded).— This group includes those single-celled animals covered with small hairlike, cytoplasmic processes known as cilia. They occur in both fresh and marine waters as free-swimming organisms. There are a few parasitic forms, notably Balantidium coli. Paramecium, Stentor, and Vorticella are -ZT^ffflara 0i^» — ■ ^: Didinium Coleps STEHTOl^ VOKTlceiLA. Fig. 27. — Group of typical Infusoria. (Courtesy of General Biological Supply House.) the commonly studied infusorians. The class is now divided into two subclasses, Ciliata and Sudoria. The first, Ciliata, is composed of four orders, (a) Holotrichida, possess cilia of equal length over the body, or they are restricted to particular regions in specialized forms ; a cell-mouth is present in most forms. Paramecium is our most com- mon genus living in fresh water. Didinium, Frontonia, Chilodon, and Coleps are other common forms. Opalina is a well-known para- PHYLUM PROTOZOA IN GENERAL 71 sitic genus which inhabits the large intestine of the frog, (b) Hetero- trichida possess a well-developed undulating membrane in the cyto- pharynx. The body cilia are small or partially absent, but the cilia of the oral region are well developed. In some forms this oral region possesses membranelles. Stentor, Halteria, and Bursaria are common fresh- water genera while Balantidnim (Fig. 389) is a parasite in the intestine of man and some other mammals, (c) Tlypotrichida possess cirri or structures formed by fusion of cilia ; these are found prin- cipally on the ventral side. The cell is flattened dorsoventrally and Prorodon From ton I a iupiom StyhnyoMa Laorymaria i^m lionotw Fig. 28. — Representatives from class Infusoria. (Reprinted by permission from Curtis and Gutlirie, Textbook of General Zoology, John Wiley and Sons, Inc.) (Figure of Frontonia modified.) most of the genera use creeping as their means of locomotion. Stylony cilia, Oxytricha and Euplotes are common fresh-water genera. Kerona is a parasitic form and is often found creeping over the ex- ternal surface of fresh-water Hydra, (d) Peritrichida is an order composed of sedentary ciliates with a whorl of oral cilia continued into a depression in which are located the oral spot and aperture of the contractile vacuole. At the base of this depression is located the mouth. There are no body cilia in certain phases of the life history. These forms are typically attached by stalks. Vor'ticeJla is probably 72 TEXTBOOK OF ZOOLOGY the most common living genus. Epistylis and Carchesium are well- known colonial genera. Vorticella and Carchesium have contractile stalks while Epistylis is attached by noncontractile branching stalks. The second subclass, Suctoria or TentacuUfera, as it is sometimes called, includes animals that are not ciliated, except during a free- swimming stage which may occur following division or encystment. These are attached forms with protoplasmic projections which are used in the capture of food. Most of them are marine, but Podophrya is an example of a fresh-water genus. 4. Class Sporozoa (sporozo'a, seed animal). — These protozoans in their early stages are often amoeboid, but in the completed life history locomotor structures are wanting. During the life cycle there is a spore stage. The animals of this class are entirely parasitic ^^(^^''f^^T^ Ep i th e Hum ; i ^ NUCl_EUS > f '.'©**« V-''^^S? -^ PSEUDOPODIUM -* U", ■^- ^i^ ECTOPL.ASM Fig. 33. — Drawing to show the appearance and structure of a living Amoeba proteus. inclusions as well as the nucleus. The larger bodies in the cytosome are food vacuoles, single, shiny, contractile vacuoles containing watery fluid and varying in size; water vacuoles; various granules; mitochondria; fat globules; and crystals. Some authors distinguish two types of protoplasm in the endosarc; the inner more fluid, plas- masol in which the streaming movements take place and, surrounding this a more viscous, passive portion, the plasmagel. The nucleus usually appears somewhat dense and granular, and is located in the portion away from the end which is advancing in a moving specimen. Metabolism This refers to the constant building up (anabolism) of living protoplasm and its concurrent oxidation (eatabolism). It includes all activities necessary for maintenance of itself and its race. These AMOEBA OF CLASS SARCODINA 87 phenomena are the same as those found in the highest forms of life but reduced to very simple terms. Here we may study the entire metabolic cycle in progress within the confines of a single cell. Its phases are as follows: Food. — Its prey consists chiefly of smaller Protozoa, small single- celled plants, such as diatoms and desmids, and portions of filamen- tous algae. Bacteria may be used to some extent and rotifers (small Metazoa) are sometimes devoured. Ing-estion. — Amoeba has no definite mouth but the food is taken into the body by engulfing it at any point that comes in contact with it. A pseudopodium is formed at this point, and the end of it flows around the food particle until the particle is entirely enclosed. A droplet of water is included with the food to form what is called a food vacuole. These vacuoles move about in the endoplasm. E^ejfion xcretion Fig. 34. — Diagram to show the phases of the metabolic process as it occurs in amoeba. (Redrawn by permission from Wolcott, Anional Biology, published by McGraw-Hill Book Company.) Digestion. — The food gradually disintegrates and much of it goes into solution in the fluid of the vacuole. The function of digestion is to convert complex materials into a soluble, absorbable form. It is assumed that the surrounding cytoplasm secretes enzymes into the food vacuoles of Amoeba to perform this function, since enzymes serve this purpose in larger animals where exact study can be made on the process. A circulatory system is not necessary since the 88 TEXTBOOK OF ZOOLOGY vacuoles with the food in the process of digestion circulate so widely in the endoplasm that all parts of the cell may receive nourishment by direct absorption. Egestion. — Indigestible material or debris that has been ingested with the food is carried to the surface of the cell and cast out or egested by simply being left behind as the animal moves away. Assimilation. — This is the process of transforming the digested food material into protoplasm. In Amoeba the digested food mate- rial is absorbed directly from the food vacuoles by the surrounding cytoplasm. Since the vacuoles move rather generally through the endosarc, most of the protoplasm of the cell is in rather close con- tact with the dissolved food. Respiration. — This is a process whereby the gas, carbon dioxide (CO2), leaving the protoplasm, is exchanged for oxygen (O2) en- tering it. Such a process is essential to all living protoplasm. In Amoeba this exchange is carried on primarily through the general body surface. The water in which the animal lives must contain dissolved oxygen in order that this diffusion may go on. Amoebae, however, are able to and do live in rather foul water where the oxygen content is rather low and the carbon dioxide high because of the decaying vegetation present. Amoebae may live several hours in water from which the oxygen is removed before asphyxia- tion occurs. The contractile vacuole likely assists in discharging CO2. Catabolism or Dissimilation. — The chemical union of the oxygen with the organic substance of the protoplasm liberates kinetic energy and heat. This is known as oxidation and is a burning process which goes on within the protoplasm. Water, some mineral matter, urea, and carbon dioxide are residual products of this process. Excretion. — These by-products of metabolism in the form of waste liquids must be disposed of. They cannot be allowed to accumulate beyond certain limits in the living organism if life is to continue. Urea and uric acid, which are protein by-products, excess water, and salts, are discharged from the body of Amoeba by way of the con- tractile vacuole along with some carbon dioxide. The contractile vacuole is formed by the union of small droplets of liquid under the plasma membrane. It fills out with liquid which is forced out through the membrane as the vacuole disappears. Its location ap- parently is not fixed in the cell but is often near the nucleus. The contractile vacuole is absent in some forms, and in such cases, ex- AMOEBA OF CLASS SARCODINA 89 cretion occurs only by diffusion through the cell surface. There is likely some excretion by this means in all Amoebae. Growth. — If there is increase in the volume of a body, this is spoken of as growth. In all living organisms growth is accom- plished by addition to the protoplasm. If food is plentiful, more material is added to the protoplasm than is used up in the oxidation which produces active energy. In other words, growth occurs when the rate of anabolism exceeds the rate of catabolism in the organism. Reproduction and Life Cycle The life history of the niajiy-celled animals to be studied later includes a series of changes from egg, through embryo state, to adult. In Amoeba the cycle is likely only partly known, because it Nucleus Contractile iracaole Fig. 35. -Diagram to show fission in amoeba. A, Beginning of the process; B, fission nearing completion. (Drawn by Joanne Moore.) is difficult to maintain cultures in perfectly normal conditions for sufficiently long periods to get this complete story. Ordinarily, the animal grows when conditions are favorable until it attains a cer- tain size; when this limit of size has been reached growth ceases. Why does the cell cease to grow? Why should it not attain the size of a man? Or why should a tree not continue to grow until it reaches the sky, or a man take on the proportions of an elephant? We have not been able to put our fingers on any one factor that completely controls growth. We do know of certain relationships that influence it. It will be recalled that all materials used by a cell must pass through the cell membrane, and likewise all waste substances must be discharged in a similar manner. Mathematics states that the volume of a cell increases according to the cube of 90 TEXTBOOK OF ZOOLOGY its diameter; while its surface increases only according to the square of its diameter. In other words, the amount of material in a growing cell increases approximately twice as fast as the plane surface needed to surround it. It is logical, then, to assume that a point may be reached when the surface area will not be sufficient for the passage of necessary materials into and out of a cell. There is, however, considerable variation in the size of cells; hence it seems there must be other factors besides volume and surface rela- tion in operation. Modified surface and difference in the rate of metabolism certainly would be factors affecting the size of the organism. When Amoeba reaches the limit of size, a division oc- curs. Binary fission, by which two new individuals are produced, has been definitely established, and some other methods of reproduc- tion have been presented. Calkins, an authority on Protozoa, states New cells /Nuclear fragment Pig. 36. — Diagram to show amoeba encysted and undergoing the process of sporu- lation. (Drawn by Joanne Moore.) that Amoeba starts out as a tiny pseudopodiospore which has only one pseudopodium. It then passes through a growth period and increases in complexity until it reaches the full-grown condition. It then divides by binary fission into two daughters. When each daugh- ter has grown to nearly twice its original size, fission is repeated. Environmental conditions and the variety of Amoebae determine the number of times this phase is repeated. Occasionally the fission seems to be an amitotic one. At the close of the fission phase, there is a period of encystment and subsequent sporulation. During the en- cystment the protoplasm undergoes several divisions to produce the several pseudopodiospores which later break from the cyst as infant Amoebae. It is felt that the complete details of the life cycle of many common Sarcodina are not yet available. AMOEBA OF CLASS SARCODINA 91 Behavior All of the activities of an animal which come in response to in- ternal or external stimuli make up the "behavior." The activities of the animal under discussion include the formation of pseudo- podia, ingestion of food, locomotion, and others. Amoeba proteus exhibits either positive or negative reactions to various stimuli. An environmental change to which an animal reacts is known as a stimulus, while the reaction of the animal is called the response. The movements made by an animal in response to stimuli are called tropisms. Amoeba exhibits all of the tropisms discussed in Chapter IV. To physical contact, it responds positively if the impact is gentle ; otherwise the response is negative. It responds negatively to strong light and finds its optimum in a moderately reduced light. When some part of the body surface of this animal comes quietly into contact with food, there is a characteristic response. This part of the protoplasm stops flowing while other parts continue, thus form- ing a pocket around the particle of food. The edges of the pocket fold in, meet, and join so as to enclose the object. This attraction to food is likely a positive chemotropism. Amoeba reacts negatively to concentrated salt, cane sugar, acetic acid, and many other chemi- cals which have been tried. Amoebae have an optimum tempera- ture range between 15° and 25° C. Temperatures approaching the freezing point inactivate the animal, while temperatures above 30° C. (86° F.) also retard their activities and may soon become fatal. A weak electric current has aai effect on the physical condition of the protoplasm on the side nearest the cathode. The tendency is toward the sol state here, hence the animal turns toward the cathode. According to Jennings, who has done extensive research on be- havior of Protozoa, these activities are "comparable to the habits, reflexes, and automatic activities of higher animals." He also feels that Amoeba probably experiences pain, pleasure, hunger, desire, and the other simple sensations. Amoeboid Movement and Locomotion The flowing or streaming of the protoplasm and extending the cell in some direction by the formation of pseudopodia is usually called amoehoid movement. It is so named from the perfect exemplification of such activity by Amoeba. Locomotion is accomplished by the pseudopodia, and the process of their formation in most Amoebae. 92 TEXTBOOK OF ZOOLOGY Successive pseudopodia are formed in the moving Amoeba proteus as it goes in a given direction. The pseudopodia are temporary loco- motor structures. Most zoologists explain this movement as being due to the contraction of the more viscous ectoplasm, particularly in the "posterior" region. This brings about a forward movement in the more fluid eudoplasm (plasmasol) which causes an outflow at points where the ectoplasm is thinnest, or where surface tension is lessened. As this plasmasol approaches the advancing tip of the pseudopodium, it turns to the sides and changes to more solid endo- plasm (plasmagel). This process continues, pushing the advancing tip farther and farther forward. At the opposite side, the plasma- gel continues to become plasmasol to provide for fluent material. At the side of the animal away from the advancing pseudopodium, the cell membrane (plasmalemma) moves upward and over the up- per side of the body ; it continues to move forward to the tip of the - - Particle -"" i Pseudopodium Fig. 37. — Successive positions in the movements of an amoeba viewed from the side. Notice the formation of new pseudopodia and tlae engulfing of the particle on the surface. (Modified from photographs by Bellinger, 1906, Journal of Experimental Zoology.) pseudopodium where it dips down and is laid on the substratum over which the animal is moving and becomes a part of the station- ary portion. If the specimen has several pseudopodia, one or more may be developing while others are receding. In the latter, the flow of plasmasol is back through the centers of the pseudopodia toward the main mass. Temperature and other environmental fac- tors affect the rate of locomotion. Dillinger mounted some of the animals on the edge of a slide in a groove formed by the projecting edges of two cover glasses and observed their movement from side view by tilting the microscope to a horizontal position. He describes their movement as a sort of walking on the progressively forming pseudopodia. The new pseudopodia are formed at the advancing margin of the cell. CHAPTER Yll PARAMECIUM OF CLASS INFUSORIA This animal has been the subject of much study and the victim of considerable experimentation. Paramecium caudatum is probably the species most commonly studied. It is easily available and is large in size, ranging between 0.2 and 0.3 mm. in length. Characteristics and Habitat Paramecium is an active, cigar-shaped animal, just about large enough to appear as small white specks in the water. It has a definite axis and permanent anterior and posterior ends, but it is asymmetrical in shape. Paramecia are easily cultured by collect- ing some submerged pond weeds and allowing them to stand in a jar of the pond water for several days. Or some natural creek or pond water may be placed in a jar with some old dry grass and allowed to stand about ten days. These animals occur abundantly in any water which contains considerable decaying organic matter. They thrive in all streams, creeks, or ponds polluted by sewage. They tend to congregate at the surface and particularly in contact with floating objects, where they frequently form a white scum. This animal is a great favorite in zoology laboratories. Structure Paramecium is sometimes described as being slipper-shaped. The anterior portion, which is blunt but generally narrower, represents the heel part ; while the posterior portion, which is generally broader but pointed, represents the sole portion. At one side is a depression, the oral groove, which passes diagonally from the anterior end to about the middle of the body. It is broad and shallow anteriorly but it becomes narrow and deeper as it ends in a mouth, which leads to the gullet. The groove usually extends obliquely from right to left in P. caudatum as the animal is viewed from the oral side. Occasionally cultures are found in which the majority of the individuals show the groove extending from left to 93 94 TEXTBOOK OF ZOOLOGY right from this view. The body is covered with fine hairlike cilia which are of even length except in the oral groove and at the pos- terior extremity, where they are noticeably longer. The cilia within the gullet are fused together into a sheet, forming the undulating membrane. CON-r RACTIL.E VACUO l_E ORAl_ GROOVE MACRO- NUCI_E.US CONTRACTILE VACUOLE TRICHOCYSTS PEI_l_ICl_E Fig. 38. — Diagram showing the structure of Paramecium, much enlarged. by T. C. Evans.) (Drawn The cell is divided into the outer, tough, nongranular ectosarc which is composed of ectoplasm. The outer surface of it is a thin, elastic cuticle or pellicle which is marked in hexagonal areas by the distribution of the cilia. The cilia are direct outgrowths of the ecto- sarc. There are a great many spindle-shaped cavities located in the ectosarc with their long axes perpendicular to the surface. These structures, trichocysts, are filled with a semifluid substance and each opens to the outside through the pellicle. The endosarc, composed of PARAMECIUM OF CLASS INFUSORIA 95 endoplasm, is within. It contains food vacuoles, two contractile vacuoles, macronucleus, and other granular masses. The numerous food vacuoles are formed, one at a time, at the inner end of the gullet by a mass of food material coming in with a droplet of water, a process similar to that described in Amoeba. The vacuoles circulate through the endoplasm in a rather definite course. This activity is called cydosis. The contractile vacuoles are located near each end of the animal. Each vacuole has several radiating canals entering it. These vacuoles expand and contract alternately. The macronucleus is located slightly posterior to the center and somewhat beside the mouth. It is relatively large and rather bean-shaped. The micro- nucleus is located in the curved surface of the macronucleus and is much smaller. P. aurelia, another species, ordinarily has two micro- nuclei instead of one. Metabolism The same general activities as described in Amoeba and others occur, differing only in certain details. These same vital functions must take place in all living things (organisms). Oral qroove J^l^ Nucleus ^ Mouth % — Gullet if M Anus -Food vacuole Fig. 39. — Cydosis in Paramecium, showing the course of the food vacuoles through the endoplasm while digestion is in progress. Food. — Smaller protozoans, bacteria, and particles of debris con- stitute the principal items on the menu for Paramecia. Ingestion. — This animal hunts its food, and when it locates a re- gion where food is abundant, it settles down and becomes relatively 96 TEXTBOOK OF ZOOLOGY quiet. The food is swept through the oral groove by the beating action of the cilia, and carried back through the mouth into the gullet. Finally it passes by means of the action of the undulating membrane into the endoplasm in the form of one food vacuole after another. These food vacuoles move in a definite course through the endoplasm. Since this course is in the form of a cycle, the circula- tion is known as cyclosis. Dig-estion, Assimilation, Respiration, and Catabolism or Dissimi- lation all occur in a manner very similar to that described for Amoeba. Egestion occurs at a definite anus. Excretion of the waste products of metabolism in solution is by means of the alternate filling and expelling of fluid by the two con- tractile vacuoles, or it may occur to some extent by diffusion through the entire cell membrane. Growth occurs as it does in Amoeba and in all other organisms. Under favorable conditions the storage of nutrient materials, like starch and fats, occurs in the cytosome. Nutrition in this animal is holozoic, and its living process is essentially like that of all higher forms of animal life. Reproduction and Life History The actual reproduction is by transverse binary fission which in itself is asexual. The cell divides transversely into individuals, and this is repeated for long series of generations, one after another. During this division process in P. caudatum, both the macronucleus and the micronucleus divide, the old gullet divides into two, and two new contractile vacuoles are formed by division of the old ones. The micronucleus divides by mitosis, but the division of the macronucleus is not distinctly so. The time required for the completion of a divi- sion ranges between thirty minutes and two hours, depending on en- vironmental conditions. Division is repeated at least once each twenty-four hours and under especially favorable conditions, twice a day. It has been estimated that if all survived and reproduced at a normal rate, the descendants of one individual over a month's time would number 265,000,000 individual paramecia. P. caudatum is a conjugating form of Paramecium, while P. aurelia and others seem not to conjugate. Conjugation is a temporary union of two individuals with exchange of nuclear material. Calkins car- ried some cultures of P. caudatum through a long series of genera- tions and observed that conjugation occurs at intervals of approxi- PARAMECIUM OF CLASS INFUSORIA 97 mately every two hundred generations. When two paramecia are ready to conjugate, they come in contact, with their oral surfaces together, and adhere in this position. A protoplasmic bridge is formed between the two individuals. This union resembles a sexual act and has recently been described as such. The conjugants are usually small, rather unhealthy appearing individuals. Shortly after the adherence of the conjugants the nuclei of each undergo changes. The micronucleus enlarges and divides, forming two micronuclei, while the macronucleus undergoes disintegration and final disap- pearance. Each of these two new micronuclei again divides to form four, three of which disintegrate, but the fourth divides again, forming one large and one small micronucleus. Sometimes the smaller of these nuclei is spoken of as the "male" nucleus and the larger, as the "female.'' In each animal the smaller nucleus moves across the protoplasmic connection to the other animal and fuses with the larger nucleus there. Each individual now has a fusion nucleus. The two conjugants now separate, and very shortly the fusion nucleus of each divides by mitotic division ; each of these divides, forming four nuclei in each animal, and these four divide to form eight. The descriptions of the subsequent events vary somewhat. At least it is known that four of the eight nuclei en- large and become macronuclei; three of the others degenerate, and one remains as a micronucleus. This micronucleus divides, and al- most immediately the entire animal divides by binary fission with two macronuclei and one micronucleus going to each cell. These daughter cells then divide to produce a total of four Paramecia which have the typical number of one micronucleus and one macro- nucleus of the active phase. Following this comes the long series of generations formed, one after the other, by transverse binary fission. The whole series of changes involved in conjugation has been compared to maturation of germ cells and fertilization in sexually reproducing metazoans. The degeneration of the three micronuclei is compared with reduction division in maturation, and the fusion of the small "male" micronucleus with the larger "female" micro- nucleus of the other conjugant is compared to fertilization. A phenomenon, known as endomixis, has been found occurring in P. aurelia by Woodruff. It occurs in a single individual. This species has two micronuclei and one macronucleus. At regular in- tervals of about every forty or fifty generations, the macronucleus 98 TEXTBOOK OF ZOOLOGY Fig. 40. — Conjugation and subsequent divisions in Paramecium, showing activi- ties of the micronucleus. Circles are micronuclei and crescents are macronuclei. The shaded ones have been resorbed. The divisions for micronuclei actually occur within the cells instead of outside, as figured for convenience. (From "White, treneral Biology.) PARAMECIUM OF CLASS INFUSORIA 99 disintegrates, and the micronuclei undergo two divisions which pro- duce a total of eight. Six of these disappear, and then the cell divides; one of the remaining micronuclei goes to each. This nucleus then undergoes two divisions. Two of these four become macronuclei, and two remain as micronuclei. The micronuclei then divide again as the entire cell divides to form daughters, each with two micronuclei and one macronucleus, the typical condition for this species. Endomixis may occur in P. caudatum also. Endo- mixis seems to have about the same effect as conjugation. 11 ■■'. '.'■'■ ',\ ->:.'. • " • '- '."' ','■.•' • • . - ' ■'. ■ '• ".'.■'/•'''.",■• • •• "'- * ' ■ ■ - I » . . • B. * ^^^■^^- — Reaction of paramecia to a drop of 0.5 per cent NaCl. A, Introduction of the drop beneath the cover glass; B, four minutes later. (Prom Jennings. Be- havior of the Lower Ojffanisms, published by The Columbia University Press.) There is still difference of opinion as to the exact function of con- jugation and endomixis, but the chief result of the processes seems to be the reorganization of the nuclear substance. This may allow for variations in the fundamental constitution of the race. Accord- ing to some authors these processes rejuvenate or rencAv the vitality of the individuals. In recent years, not only sexual reproduction but also district sexes have been described for Paramecium.* •Sonneborn, Science News Letter, Aug. 21, 1937. 100 TEXTBOOK OP ZOOLOGY Behavior This animal is an active swimmer and necessarily shows ready response to environmental factors. Its behavior consists of its spiral course in locomotion, avoiding reactions, responses to food material, contact and other minor reactions. Its reactions to stimuli are somewhat similar to those described for Amoeba; however, it seems not to be affected by ordinary light. It reacts either posi- tively or negatively to contact, change of chemical constitution, change in temperature, to gravity, and to electric current. The re- sponse to contact is positive, negative to ultraviolet light, negative a ——7 = 9 ^ ^ :; ^ - s 1 ^ / /^ ' ^ 9 " JS' 19- rj,-?-.V."."-:-t;^-_f ,-_.' . . pSn^-";Y=iV;-".-' Be- ss- Fig. 42. — Reactions of paramecia to temperature, a, Paramecia are in a trough with temperature at 19° C. uniformly through the water. The animals are generally scattered. In & the temperature is held at 26" C. at the left end and 38° C. at the other. The animals are collected in the end of lower temperature. In c, the temperature is 25° C. at one end and 10° C. at the other, and the Para- mecia are congregated in the region of higher temperature. (From Jennings, Hehavior of the Lower Organisms, published by The Columbia University Press.) to sodium chloride, positive to weak acetic acid, and positive to the negative pole of a weak, galvanic electric current. The optimum temperature for Paramecium ranges between 24° and 28° C. (71° P.). Gravity causes the anterior end to point upward, and when placed in moving water, the animals will swim upstream. If Para- mecium comes in contact with a solid object when it is moving, it will back away, swing on its posterior end to a slightly different PARAMECIUM OF CLASS INFUSORIA 101 direction and try again. This may be repeated, and is known as the "avoiding reaction/' Such a reaction really involves simply one or more negative responses. These animals are constantly sam- pling the water and avoiding the conditions which are least favor- able. This may be repeated in all directions. The same type of persistence is practiced in attempting to surmount a solid barrier. i Fig. 43. — Diagram of the course and movement of Paramecium through the water. Notice the spiral path. (From Jennings, Behavior of the Lower Organisins, published by The Columbia University Press.) 102 TEXTBOOK OP ZOOLOGY Such successive attempts to gain the result desired constitute what is known as the "trial and error'' mode of behavior. In an effort to defend itself when severely irritated, Paramecium will discharge the contents of the trichocysts, which harden on con- tact with the water and form a mass of fine threads. These threads will entangle many of the aquatic enemies of these animals. Locomotion The beating action of the cilia against the water serves as the principal means of locomotion. The stroke of the cilia is rather oblique and this coupled with the increased length of the cilia along the oral groove causes the body to turn on its long axis while swim- ming. The total effect of these activities causes the course followed through the water to be that of a spiral. Paramecium may reverse the direction of the stroke of the cilia and thus move backward just as a car can be thrown in reverse. The cilia are contractile outgrowths of the ectosarc. Each has an elastic sheath and a fibrillar core. Contraction of the protoplasmic substance on one side, bends the cilium in that direction. The re- verse stroke is much more passive. The movement of one tier of cilia seems to stimulate the adjacent ones to bring about coordi- nated, rhythmic ciliary activity and movement. CHAPTER VIII METAZOAN ORGANIZATION All animals whose bodies consist of few or many cells functioning as a unit are called metazoans. In most respects the vital activities of Metazoa are similar to those of Protozoa. Since Metazoa are more or less like compound Protozoa with some degree of inter- cellular differentiation, it is thought by many authorities that they arose through organization of single-celled organisms. In some forms of compound or colonial Protozoa, only two cells adhere together after cell division, but in others the cells may remain attached after many di\^sions. The size of different colonies may range from two to two thousand similar cells. In the most complicated protozoan colonies there may be several different types of cells. The representatives of class Mastigophora are the most likely ancestral forerunners of Metazoa. The colonial forms, such as Gonium, Pandorina, Eudorina, Pleodorma, and Volvox, are rather plantlike in character- istics, but a series of this type shows the possibility of the relative complexity of different colonial forms. There are several genera of animals which are intermediate between Protozoa and Metazoa, but for the most part the two groups are fairly distinct. General Characteristics This group includes all of the strictly many-celled animals. The cells are definitely organized and classified morphologically as well as physiologically. There is a well-regulated division of labor. Among the single-celled animals each cell, like primitive man, is largely independent of its fellows, doing for itself all that is neces- sary to carry on living processes. In the many-celled animal, as in a highly developed society of men, certain individual cells become more proficient in doing certain kinds of work, and as a result, a special group is able to care for a particular function necessary to the life of the entire organism. In return, other special groups care for other functions. In this way each exchanges the products of its labor for the products of the labors of the other groups. In human society this becomes more and more complicated as civiliza- 10.1 104 TEXTBOOK OP ZOOLOGY tion advances; so it is Avith development of complexity in meta- zoans. Another characteristic of Metazoa is the presence of a defi- nite center of control localized in a particular group of cells which becomes the nervous system in higher forms. Cellular Differentiation In Protozoa there is seen fair development of intracellular dif- ferentiation, making it possible for one part of a cell to perform a particular function, and for other parts to perform other functions. The complexity of Metazoa is not the result of great complexity of the individual cells, but it is due to the special differences between them. The presence of a variety of cells within one body is spoken of as intercellular differentiation. The modification of metabolic B Fig. 44. — Typical germ cells. A, ovum of the female; B, spermatozoa of the male. activity is the basic factor in the development of all differentiation. Certain groups of cells become specialists in a particular phase of the metabolic activity. Some become protective surface cells, others se- crete special enzymes, still others specialize in excretion, and so on. The entire metazoan body is usually divided into germ cells, which are specialized for reproduction, and somatic cells or body cells, which compose the remainder of the body and are grouped in layers. The germ cells are set aside early in the life of the individual for reproductive purposes. They develop in the reproductive glands or gonads of the two sexes. The protoplasm of these cells is known as germ plasm. The female germ cells are eggs or ova, and those of the male are spermatozoa. When the germ cells reach maturity, they be- come separated from the body and may give rise to a new generation. METAZOAN ORGANIZATION 105 About forty years ago Weismann presented the idea of the continuity of heredity from generation to generation by way of the germ plasm. The germ plasm, according to this idea, gives rise not only to the protoplasm of the germ cells of the new individual but to the somatic cells as well. In Protozoa the entire material of the individual is passed on to the two offspring and, for this reason, this protoplasm is spoken of as being immortal. Potentially, germ plasm is likewise immortal. The protoplasm of the somatic cells is known as somatoplasm. This is rebuilt with each generation, and when the individual dies, all of the somatoplasm perishes. In final analysis, the somatoplasm serves as a means of conveyance for the germ plasm through the current generation. Cellular Organization The simpler Metazoa are composed of only two kinds of somatic cells. These cells are grouped according to kind in two layers. With advanced differentiation, a rather wide variety of cells has been produced. A tissue is an organization of similar cells into a group or layer for the performance of a specific function. A certain amount of intercellular substance is characteristic of most tissues and enhances their usefulness. The entire living mass of the metazoan animal body may be classified under five fundamental (four by some authors) kinds of tissues, and when it is so distributed, there is nothing left. These classes of tissues are: epithelial, protective or covering; sus- tentative, connective or supporting; muscular, contractile; nervous, irritable or conductive; vascular, circulatory. Epithelial Tissue. — A sheet of cells that covers external or in- ternal surfaces of the body is known as an epithelium. The epi- dermis or outer layer of the skin and the layer of column-shaped cells lining the inside of the intestine are good examples. Accord- ing to function, this type of tissue can be classified as protective epithelium, glandular epithelium, and sensory epithelium. The epi- thelium which covers external surface of an organism usually de- velops various protective structures in the different groups of ani- mals : the hard, homy chitin of insects ; scales of fish ; homy plates and scales of reptiles ; feathers of birds ; hair ajid nails of mammals. The glands of the body are developed from epithelium. Secretions 106 TEXTBOOK OF ZOOLOGY from these various glands lubricate the surfaces, contain enyzmes for digestion of food, supply regulatory substances directly to the blood, serve as poison to other animals, and some are repellent to enemies. Sustentative Tissue. — This type comprises all tissues whose func- tion is to bind together or support the various parts of the body. Connective tissue is, in most cases, composed of slender cells with an abundance of intercellular material. This tissue is almost uni- versally present in the various organs throughout the body. Ten- dons, the tough cords that connect muscles to bones, of which the "hamstring" is a good example, and much of the dermis of the skin are composed of connective tissue. Bone and cartilage, which make up the framework of the body and support the other tissues, are called supporting tissues. In crayfishes and grasshoppers the sup- porting tissue is chitin instead of bone or cartilage. Cartilage is com- posed of scattered cells interspersed with abundant, homogeneous, granular, semisolid matrix or intercellular substances. Bone is some- what similar, except that the matrix has been replaced by a heavy deposit of calcium phosphate and calcium carbonate, two solid salts. The scattered cells are present as bone cells. Muscular or Contractile Tissue. — This is distinctive because of its ability to contract and in that way produce movements. Cells adapted to this function are more or less elongated and fiberlike. There are three types of muscular tissue : smooth, involuntary, and nonstriated, as found in the wall of the intestine ; striated, volun- tary, skeletal, as found in the muscle of the arm ; and striated, in- voluntary, cardiac, as found in the wall of the heart. Skeletal, voluntary muscle is made up of large multinucleate (many nuclei) fibers, each composed of many fibrils (myofibrils) along which are evenly distributed dense and light areas, which give the general appearance of stripes across the cell, because the dense areas on the adjacent fibrils come at the same level. The smooth involuntary muscle is composed of individual, spindle-shaped (fusiform) cells, the cytoplasm of which is largely myofibrils but without striations and therefore smooth. There is a single oval nucleus, centrally lo- cated. The outer membrane of a muscle cell is the sarcolemma. The cardiac involuntary muscle is said to be made up of individual cells, highly modified in arrangement. The definition of cells in METAZOAN ORGANIZATION 107 this tissue is rather difficult, but the fibers are faintly segmented by thin intercalary disks which define areas each with a single nucleus. The cells branch laterally to join each other quite frequently, pro- ducing a condition of netlike branching known as anastomosis. Nervous Tissue. — This is specialized to receive stimuli and trans- mit impulses which have been set up by some stimulating agent in some part of the body. The structural features consist of nerve cell bodies and their processes. Two kinds of processes are recog- nizable : (a) the axo7ie, usually a single unbranched fiber except for infrequent collateral branches; and (b) dendrites, frequently much Q O Fig. 45. — Typical cells and tissues from vertebrate animals. 1, Squamous epithelial cells ; Z, section through a portion of bone showing Haversian canal (in center), bone cells, lacunae, canaliculi, and matrix; 3, section of hyaline cartilage showing cartilage cells in lacunae, and matrix between lacunae ; i, sec- tion of tendon composed of white fibrous connective tissue ; 5, longitudinal view of smooth (involuntary) muscle cells; 6, striated (voluntary) muscle; 7^ motor nerve cell, showing process; 8, human red blood (nonnucleated) corpuscles and human white (nucleated) corpuscles. (Drawn by Titus Evans.) branched and arborlike. An axone may be several feet long, e.g., one extending from the spinal cord to the hand or foot. Dendrites may be lacking. The impulses are conducted toward the cell body over the dendrites and away over the axone. A nerve cell body together with its processes is called a neuron. The neurons approach each other and pass impulses from one to the other at the synapses, where 108 TEXTBOOK OF ZOOLOGY the brushlike ending of the axone of one comes into close proximity with a dendrite of another. In this way an impulse can be trans- mitted from one part of the body to other parts. The chief function of the nervous tissue is to relate the organism to its environment. Vascular Tissue. — This is fluid tissue consisting of cells known as corpuscles in a fluid medium called plasma. The cells are the red corpuscles (erythrocytes) and white corpuscles (leucocytes), while the plasma or fluid is the intercellular substance. Blood and lymph are the two common vascular tissues. Lymph has no red corpuscles. In the blood of mammals the red corpuscles are without nuclei ; while in fish, frogs, turtles, and birds these cells are nucleated. The chief function of this tissue is the transportation of digested food and oxygen to the cells of the body and the removal of waste by- products of metabolism from them. An organ is an arrangement of two or more tissues as a part of the body which performs some specific function or functions. Some organs are made up of all of the different types of tissues just de- scribed. For example the stomach is an organ with an internal cavity. It is covered and lined with epithelium; the wall contains two strong layers of muscular tissue ; blood vessels carrying blood, and lymph spaces bearing lymph, branch through the wall ; nervous tissue reaches all parts of the organ to receive stimuli and distribute impulses; and connective tissue serves to bind all the others in proper relation. A system is an aggregation of organs properly associated and related to perform some general function of life. There are ten different systems usually recognized: a. The Integumentary System is composed of the skin and its out- growths, such as hair, nails, scales, horns, hoofs, and similar struc- tures. Its principal purposes are protection, primarily, with some degree of excretion and respiration, some absorption, and regulation of body temperature. b. The Skeletal System composes the supporting framework of the body. The bony and cartilaginous tissues make up the material of this system. The vertebral column, skull, ribs, sternum, and bones of the limbs are the general parts of the vertebrate skeletoji, and they serve for the support of the body as a whole and for the pro- tection of the internal, vital organs. METAZOAN ORGANIZATION 109 c. The Muscular System consists of muscles, the voluntary, stri- ated group moves skeletal parts and accomplishes locomotion ; the nonstriated, involuntary g-roup is concerned with the movements of the internal organs (viscera), and the cardiac muscle produces the heart action. {JuqularVdn Carotid Artiry Trachea Sabclavian V. Precaval V. Dorsoil Aorta Pulmonary A. ..Left Auricle LeftVentrick Xu.na .Diaphracfm Liver , Liuodenam '- -_ .Stomach Gall Bladder Jro-nswrseColon Dd. polar body^--'' rertili5ed ovum (Zygote) Fig. 47. — Maturation of the germ cells. Oogenesis includes the maturation divisions of the female germ cells or ova, and spermatofjenesis is a similar process of division in the development of mature male germ cells or spermatozoa. Primordial qerm cell J'permafco- ■ gonia --Zbromosome Primary ipermatocybe _ Secondary spermato- cyte /(• jSpermatid Mature spermato- zoon instances each of these cells divides once more. Next, each of these oogonia passes through a growth period without division. During this time the chromosomes in each unite in pairs and fuse together. This fusion is spoken of as synapsis of chromosomes. At the close of this growth each of these cells is called a primary oocyte. Each of these oocytes divides by meiosis, the fused chromosomes dividing as though they were single ones in normal division. This division, therefore, results in cells with half the somatic (diploid) number of chromosomes and is spoken of as the reduction division. The cyto- plasm does not divide equally; nearly all of it goes to one of the METAZOAN ORGANIZATION 1]5 cells in each case. This large cell is called the secondary oocyte and the small one is the first polar body. Each of these cells has four chromosomes. Following this the secondary oocyte divides to form the mature ovum and another polar body. Occasionally the first polar body divides, but none of them have any further significance after cariying away half of the chromosomes. They now degenerate, and their protoplasm is reabsorbed by the surrounding tissue. The series of divisions and changes following the primary oocyte stage constitute the maturatio7i period of the process. The ovum contain- ing the haploid number of chromosomes is now prepared to unite with a mature spermatozoon in fertilization. Spermatogenesis is completed within the tubules of the testis, and, like oogenesis, is a series of mitotic cell divisions. The primordial germ cells divide by mitosis to form spermatogonia, and this process continues just as it does in oogenesis, until the division of the pri- mary spermatocytes which have developed during the growth period. When the primary spermatocytes divide, the division is an equal one and all of the resulting cells are typical secondary spermatocytes with the haploid number of chromosomes. These cells divide to form spermatids. Each spermatid then undergoes a change of shape or transformation to form the mature spermatozoa, each with its half number or, in this ease, four chromosomes. The change from sper- matid to spermatozoa does not involve a cell division but simply rear- rangement. The spermatozoon is a slender, motile cell composed of head, middle piece, and tail. It is now able to swim in fluid and prepared to unite with a mature ovum. The maturation process is very significant for at least two impor- tant reasons. First, during the fusion and subsequent divisions of the cells, there is given opportunity for variation of the genetic com- position. Secondly, the number of chromosomes is reduced to half in each mature germ cell, thereby making it possible for the germ cells to unite without doubling the typical number of chromosomes in each new generation. Each species has a definite and constant number of chromosomes. Fertilization involves the union of a mature ovum and mature spermatozoon to produce a fertilized ovum or zygote. The sper- matozoon SAvims to the q^q and enters it by penetrating the outer membrane which is called the vitelline membrane. For most animals, as soon as one sperm enters an egg, the chemical nature of the vitelline 116 TEXTBOOK OF ZOOLOGY membrane changes and prevents entrance of others. The head of the sperm carries the nucleus and soon takes the form of a rounded male pronucleus inside the cytoplasm of the egg. The egg nucleus is known as the female pronucleus. The male and female pronuclei finally fuse to form the fusion nucleus, and the fertilization is complete. The significance of fertilization is largely centered around two important functions. First, it is the impetus for the development of an embryo from the egg under most normal circumstances; however, partheno- genesis replaces this function in some cases. Secondly, it brings about Fig. 48. — Diagrams showing cleavage in the young embryo of Asterias. 1, Fertilized ess (zygote) ; 2, two-celled embryo following first cleavage division; S, the four-cell stage ; //, the eight-cell stage ; 5, the sixteen-cell stage ; 6, morula stage (solid) ; 7, blastula stage (hollow) ; 8, early gastrula stage (infolding of cell layer at one side) ; 9, later stage of gastrulation. The infolded layer is the en- doderm. (Drawn by T. C. Evans.) the means for inheritance of characteristics from two different lines of ancestry. This union also restores the diploid number of chromosomes. Cleavage is a series of mitotic cell divisions beginning in the zygote immediately following its formation. These divisions occur in rapid order with but very little intervening growth, and the resulting cells adhere to each other in a body. In eggs where the yolk material is scant and evenly distributed, the ensuing cleavage METAZOAN ORGANIZATION 117 divisions extend completely through the zygote, forming nearly equal cells. If the yolk is concentrated in one end of the egg, the divisions of the developing embryo are unequal. During the early divisions all of the cells of the body divide at so nearly the same rate that it appears as if the zygote were being cut with a knife or cleaver into smaller parts. This process provides for the rapid in- crease in the number of cells and growth of the embryo which is necessary before any special parts can be formed. Cleavage will be described more fully in a later chapter under the discussion of the development of the frog. As divisions proceed, a hlastula is formed by the development of a cavity (blastocoele) within the spherical mass of cells, the wall of which is now a single layer. The formation of the blastula, which usually comes at the sixty-four cell stage or later, marks the end of cleavage. The blastula stage of an animal like a starfish or a frog resembles somewhat a hollow rubber ball whose wall is made up of a large number of pieces cemented together. As cell divisions continue in the blastula, a gastrula is finally formed. The blastula does not simply increase in circumference, but there comes a time when the wall on one side pushes in (in- vaginates), finally meeting the wall of cells from the other side. This gradually crowds out the cavity and forms a wall of two layers of cells. The outer layer is known as the ectoderm (outer skin) and represents the portion of the wall of the blastula which has not folded in. The inner layer, or that resulting from the infolding of the wall of the blastula, is called endoderm (inner skin). As divi- sion of cells in this wall proceeds and the infolding continues, the two margins of the infolded part come nearer and nearer each other. This gradually encloses an outside space which is lined by the endoderm and represents the primitive digestive tract or archenteron. This is the beginning of the two primitive germ layers, ectoderm and endoderm. In sponges and coelenterates development stops here. In higher forms, immediately following gastrulation, a third germ layer, the mesoderm (middle skin), is organized from cells usually contributed by one or the other or both of the other germ layers. In some cases it arises as two saclike outgrowths from the endoderm, one on each side in the gastrula. These pouches push into the remains of the blastocoele. In other cases separate cells are shed from ectoderm or endoderm or both, or from an undifferentiated 118 TEXTBOOK OF ZOOLOGY portion to orgajiize as a distinct layer between the other two. The position of the mesoderm is external to the endoderm and internal to the ectoderm. It nearly encircles the endoderm. Sooner or later a space forms within the mesoderm, causing the outer limb of it to join the ectoderm and the inner to join the endoderm. This cavity is the coelom or future body cavity. From each of the germ layers, particular parts of the body are derived. The fate of the germ layers is determined as cell division and development continue. The division proceeds at different rates in different regions and at different times resulting in various infold- ings, outpushings, and extensions which finally bring about the formation of all parts of the mature individual. The ectoderm gives rise to the external surface cells or epidermis of the skin and to the nervous system; the mesoderm furnishes the muscles, skeleton, circulatory system, blood, excretory, and reproductive systems be- sides nearly all connective tissue ; and the endoderm produces the internal linings of the digestive tract, respiratory tract, and such outgrowths as the liver and pancreas. CHAPTEK IX PHYLUM PORIFERA SPONGES The name of this phylum, Porifera (p6 rif'er a), means ''pore- bearers," and this, these animals certainly are. This group is thought to be sort of an aberrant type with peculiar relations, but the group is often considered the simplest and lowest type of Metazoa, notwithstanding the presence of a simple mesoderm which is lacking in Coelenterata. For a long time sponges were thought to be plants, and it was not until 1857, only a little over ninety years ago, that they were fully acknowledged as animals. They are sessile in habit, being fastened to piers, pilings, shells, rocks, etc., for life. There is entire lack of locomotion. Most sponges, bath sponges included, live in the sea. There are only a few small fresh-Avater forms. They have tissues but are without organs. The body is in the form of a hollow sac with many canals piercing the walls and making connection between the internal cavity and the outside. The pores of these canals are essentially mouths. There is only one general exit from the cavity. All sponges have some type of skeletal structure; some possess hard, calcareous, or siliceous spicules, and others have a flexible fiberlike material as a skeleton. The organization of the sponges is a loose one, and the interde- pendence of part upon part is not great. An animal with hundreds of mouths cannot be very highly organized. Some authorities show a rather close comparison between sponges and colonial Protozoa. The sponges possess collar cells or choanocytes which are similar to the cells of the colonial mastigophoran, Proterospongia. There are workers who hold that sponges may have arisen from a common ancestor with the choanoflagellate type of colonial Pro- tozoa. For a time sponges themselves were considered colonial Protozoa. The sponges do not have a distinct enteron or digestive cavity, but digestion is entirely intracellular (within cells). The germ layers are not well-established ; the layer which seems to begin like endoderm develops into the external layer. The so-called ecto- 119 120 TEXTBOOK OF ZOOLOGY derm comes to line the internal cavities and its function is circu- lating the water. The middle layer is very poorly differentiated, being hardly more than a matrix, and is hardly recognizable as the mesoderm of the typical triploblastic animal. If the type can be classified according to germ layers, it might be considered a modi- fied diploblastic (two germ layers) form. Because of these pecu- liarities, some authors have called sponges Mesozoa or Parazoa. Classification Class Calcispongiae. — Single, shallow-water, marine forms, char- acterized by calcareous spicules. There are two orders. Fig. 49. — Glass sponge or Venus's flower basket, Euplectella sp., is probably the most beautiful of the sponges. (Courtesy of General Biological Supply House.) Order Homocoela. — Simplest type, possessing a very thin body wall with pores as perforations in individual cells. The internal cavity is lined with choanocytes. Leucosolenia. PHYLUM PORIFERA 121 Order Heterocoela. — Moderately complex wall. Choanoeytes in radial canals. ScypJia (Grantia). Class Hyalospongiae. — Sponges which possess siliceous spicules with three axes and six rays or a multiple of six. Spicules are white and like spun glass. Often called glass sponges because of this skeleton. Venus's flower 'basket. Class Demospongiae. — Forms which have either nontriaxial sili- ceous spicules or spongin or no skeleton. They have complicated canal systems and are often quite large and brightly colored. A few fresh-water forms are known. Order Tetraxonida. — These are ordinarily attached to the bottom in deep water. Thenea. Order Monaxonida. — Includes shallow-water, marine forms and one family of fresh-water sponges (Spongillidae). There are less than two dozen fresh-water sponges known in this country. Spongilla, Haliclona, or finger sponge, and Cliona, or boring sponge. ^•faiocytes— .•'*•/.;{•• •'I ftatocyiea cong-etfaie •^ '**-••/•* J to jorm gemmates inWrgzd^zrnmuka K"'-'^ Mi- «,'^-*i- Fig. 50. — Spongillaj showing reproduction. (Courtesy General Biological Supply House.) Fresh-Water Sponges In the southwestern part of the United States, at least in central Texas, there are four species of fresh-water sponges: Spongilla fragilis, TrocJwspongilla horrida, Asteromeyenia plumosa, and Ephy- datia crater if ormis. Of the four, Spongilla fragilis seems to be the most abundant in this area. Most of the colonies of this species are irregular in shape, averaging approximately % of an inch in diameter; but there are some as large as 6 inches by 2^ inches. Usually they are not over 1/4 of an inch in height. Most of the colonies are irregular in shape, but some are cushion-shaped and a few are branched. Most of the large colonies of sponges in this region are dark grey or chocolate brown in color and are found 122 TEXTBOOK OF ZOOLOGY on logs either floating in the water or submerged. In some parts of the country there is the idea that sponges require clear water, but in the region referred to they (particularly Trochospongilla horrida and Ephydatia crater if ormis) grow abundantly in muddy ponds and in muddy streams whose turbidity equals 110 parts of solid matter per million. In this region again the growth of the sponge and apparently gemmule formation is a perennial process. The maximum production of gemmules seems to be in the late autumn and throughout the winter, even following periods of low temperature in the spring. These gemmules are ordinarily de- posited in a pavementlike layer on the object to which the sponge is attached, sometimes covering several square inches. The species are usually identified by means of microscopic differences in the gemmule spicules as seen when crushed. Order Ceratosa. — A group of important sponges of which man uses at least a dozen different ones. The representatives of this order have skeletons of spongin and are found in subtropical and tropical marine waters. Euspongia, the bath sponge. Order Myxospongida. — These sponges are entirely devoid of skele- ton. Haliscara. THE SIMPLE SPONGE Scypha coronata* (Ellis and Solander) has been mistakenly called "grantia," the European form, by most textbooks for years. This is a commonly studied representative of the phylum. It is available and is also comparatively simple in structure. It is not as simple, however, as Leucosolenia. Habitat and Behavior This type lives attached to rocks in relatively shallow marine water. The animal is attached by the basal or proximal end; the opposite end is free or distal. A colony may be formed by budding. Water is drawn in through the pores or ostia on the sides of the body, then by way of the canals into the internal cavity. This water is forced up through the cavity and out at the osculum or exit open- ing at the top. The osculum and ostia maj^ be closed and there •A Case of Incorrect Identification American genus is Scypha. M. "W. de Lau- benfels. Pasadena, Calif., Science Vol. 85, No. 2199, Feb. 10, 1937, p. 199. PHYLUM P0R1FEE.A 123 may be contractions of the entire body. These movements are accomplished by individual contractile cells. These reactions may involve the entire body, or they may be local. Laymen and many zoologists think of sponges as sluggish, inactive forms, because they are sessile. On the contrary, these ajiimals work day and night to keep a continuous current of water to supply their vital needs. It is reported that an average sponge will pump approximately forty-five gallons of water through his body in forty-eight hours. Activities and coordination in Scypha and sponges generally are quite limited by lack of a nervous system. Individual cells respond directly to stimuli, and impulses are conducted simply from cell to cell in a primitive fashion. This results in very slow transmission of impulses and is called neuroid transmission. External Anatomy The average length of Scypha is about three-fourths of an inch. It is rather goblet-shaped with the excurrent opening, osculum, at the top. A row of picketlike spicules or spines encircles the osculum, ■ •■ -oiisS^Wy.i.LV;-:^;-^;.,;^..-,, Fig. 51. — Scypha coronata (Grantia), showing habit of life. 124 TEXTBOOK OF ZOOLOGY and other less conspicuous spicules are distributed over the body. The ostia are the incurrent pores through which water is taken into the body. They are quite evenly distributed over the wall. A dermal epithelium covers the outer surface of the animal. Internal Anatomy Internally there is a large central or gastral cavity which is simply a water cavity and is not comparable to a stomach or enteron. In Dctfnal EpiiKctium •1 k«l>M«n FlaftlUitd Oitmbc (Radul Canal) \ 10% 4^ 11... cu.- I ■■■"« DiAfftftm of Akod Sponge S-' . : ■ : S , Caiiral Co.ihcl.um . InkaltAI Canal Charr.bcr Stereogram to illuatntc ample Leucen Sponge Dermal Oatta ^Subdarmal Cavity Ch^lcnf Cnal FUstlUied Cham^r StetcogrKiT) to illustiate Sycon Sponge Diagram L, S, of rSagon (Icucon) type of canal 3truc:ure such «9 occurs in the Demospongiae Fig. 52. — Structure of different types of sponges, shown diagrammaCcally. (Courtesy Pacific Biological Laboratories.) more complex sponges there may be several or even many such cavities, each one opening distally by an osculum. Communicating with and radiating from this cavity is a set of radial canals. They join the cavity through small pores called apopyles and extend nearly to the outer surface of the wall where they end blindly. Lying be- tween these and extending inward from the ostia are the incurrent canals. They connect with the radial canals by rather numerous apertures called prosopyles. This canal system not only serves to PHYLUM PORIFERA 125 carry the water, but it substantially increases the surface area of the animal. This seems to be a definite provision to allow increase in volume by keeping the ratio of surface to volume. In sponges generally, there are three types of canal systems, identi- fied as the ascon, sycon, and rliagon types of which the first is the simplest, the second intermediate, and the third, the most complex. The canal system of Scypha is of the sycon type. The character of the skeleton is a diagnostic feature in the classi- fication of sponges. Some have a skeleton of calcareous spicules, others of siliceous spicules, others of the fibrous spongin, and still others have no skeleton. Spongin of the ordinary bath sponge, which is simply the skeleton of one of these animals, resembles silk chemi- cally. It is formed by some special cells called spongioblasts. The honaxon .raxon Triaxon Hon ax on Jriradiatc Fis 53. — Types of calcareous skeletal spicules found in different sponges. (Drawn by Joanne Moore.) spicules are of several tj^pes with a number of modifications of each. The monaxon type consists of simple straight spines; the triradiate type consists of those that have three rays joining each other in one plane ; the tetraxon type has four rays radiating from a common point in four different planes ; the triaxon type possesses six rays lying in three axes; and the poly axon type has numerous rays. The cells which produce spicules are known as sclerohlasts. The histology of Scypha presents a peculiar arrangement of a number of different types of cells. The outer, dermal layer is com- posed of simple, flat, epithelial cells, contractile cells (myocytes), gland cells wtich secrete the substance for anchorage, as well as the sclerohlasts. A great many of the cells in this layer do not have distinct boundaries, making it a syncytium. In Order Ceratosa, the 126 TEXTBOOK OF ZOOLOGY spongioblasts are located in this layer. The incurrent canals are lined with flat pavement cells. At each prosopyle there is a single large dermal cell, a porocyte, which surrounds the aperture. In the middle layer are found the reproductive cells and some amoeboid wandering cells. The radial canals and the internal wall of the cen- tral cavity have similar histological structure since the former are outpouchings of the latter. The cells here are primarily special shaped ehoanocytes, peculiar to sponges, interspersed with scat- Jpicule — Sckroblast . Dermal cells -flaciei lum collar AirchaeccYbe. Co/lencyte DertnalceU — Choanoo/ie -I (Oslhrceli) Porocyte s^. Ovum '4^~ Fig. 54. — Histology of wall of a simple sponge in longitudinal section. (Redrawn and modified from Lankester, Treatise on Zoology, published by The Macmillan Company, after Minchin.) tered, flat, epithelial cells. Each choanocyte has at its free margin a funnel or collar opening to the central cavity and a flagellum or whip extending from the funnel. The flagella agitate the water and drive the suspended food particles into the funnellike mouths of the collar cell where a food vacuole is formed in the cell in amoeboid fashion. Of course the spicules appear in a histological section. The entire arrangement is quite similar to a large colony of semi- independent cells which do not function as integral parts of a tis- sue as do the cells of higher animals. It has been found that indi- PHYLUM PORIFERA 127 vidual cells can be separated from each other by squeezing some types of sponges through the meshes of a silk cloth. From these living cells, if kept in favorable conditions, a new sponge will reorganize. Metabolism A sponge obtains food from the water which is continually pass- ing by way of ostia, through the canals and central cavity, and out the osculum. Microorganisms and other particles of organic matter are drawn in with the water. The current is produced by the flagella of the choanocytes and contractility of the walls. It is controlled by the contractility of the cells surrounding the ostia. As the current sweeps the potential food particles into the collar cells they are seized and ingested by pseudopodia, according to some authors. At any rate the food particles are taken into the cytoplasm of certain of the cells. Digestion is intracellular (within cells) in the food vacuoles and the process is much the same as has been described in Protozoa. The digested material is assimilated by diffusion from cell to cell. This may be augmented by the amoeboid wandering cells. Respiration is carried on by diffusion through the general surfaces, and the exchange of gases O2 and CO2, is made with the surrounding water. Catabolism, or the union of oxygen with the fuel substance of the cell to liberate energy, goes on in the cells in some degree as long as they are alive. Excretion is largely by general diffusion through the surfaces, per- haps assisted by the wandering cells. Egestion is probably accom- plished much as it is in Amoeba. Reproduction and Life History Scypha is able to reproduce both asexually and sexually. The former may be by budding or by the formation of gemmules. Bud- ding involves the branching of new individuals from the external surface of an old one. These new individuals finally become free from the parent. Sometimes a colony is formed by the buds re- maining attached to the parent. Gemmule formation or internal budding is another type of reproduction, found particularly in fresh-water sponges. Groups of cells become separated from the surrounding deep tissue by limiting membranes, which become in- filtrated with siliceous materials. They are usually formed during adverse conditions and can withstand desiccation and other severe 128 TEXTBOOK OF ZOOLOGY circumstances. In fresh-water forms these gemmules are formed in the middle layer of cells; the parent individual then dies; and the following spring new individuals emerge from the gemmules. Sexual Reproduction occurs here for the first time in our dis- cussions. Sponges are usually hermaphroditic, but the germ cells Sexual I^eprodaction Fig. 55. — Methods of reproduction in Scypha. (Courtesy General Biological Supply House. ) Qranular cells (Dermal epithelium) Segmentation cavity Ragellafced cells {Qastral epithelium) Osculum breaks thru here -Dermal epithelium ■ Gasbral epithelium Gastral cavity (future cloaca) --Dloitopore Typical f ree-5W/mming Awphiblastula Typical /Amphiblaifcu/a at time of attachment Fig. 56. — Diagrammatic sections of Scypha larvae. of the male usually mature before those of the female. The repro- ductive cells are produced in the jellylike middle layer. Fertiliza- tion takes place here and cleavage division progresses. At about the blastula stage the embryos are liberated through the wall of the body as free-swimming, ciliated larvae. These later settle down, become attached, and are modified to form adult, sessile sponges. PHYLUM PORIFERA 129 Economic Relations Many sponges are beneficial to man, and there are a few wliich are detrimental. Oysters and some other Mollusca are injured or destroyed by certain sponges which attach themselves to the mol- lusc's body or by others which bore through its shell and thus kill it. Of positive importance, lesser items include the large flint de- posits from siliceous spicules of some species and those used for ornaments. The chief importance lies in the use of the spongin skeletons of certain groups for bath and surgical sponges. With rapid industrial development sponges have also become useful as a fabric material. The demand has brought about the establishment of sponge farms where they are raised from fragments about an inch square or slips, like plants. Selected sponges of good quality are cut up, and the pieces fastened to hooks or wire on a weighted frame. The "seeded" frame is sunk in the ocean and left for a year or two for the sponges to grow. If everything has gone as expected, the slips will then be marketable. Sponges are usually harvested by diving or hooking between May and October. Dredges which are sometimes used for this purpose are not very satisfactory, because they drag down and kill so many young sponges. Sponges die quickly when taken. They are allowed to rot for a day or two, then beaten and squeezed under water, washed, dried, and sometimes bleached. Next they are trimmed, sorted, and placed on the market. The value of the crop each year is at least $5,000,000. Most of the commercial sponges come from the Mediterrajiean Sea, Red Sea, coasts of Florida, the Bahamas, the West Indies, and Central America. Phylog-enetic Advances of Sponges Wlien Compared With Protozoa This group appears to be somewhat advanced over the Choano- flagellata of the Genus Proterospongia in Class Mastigophora. In sponges the cells are partially organized into layers and are differ- entiated for separate functions. Sexual reproduction is developed here in a simple way. The group has advanced so little that little else can be said. CHAPTER X PHYLUM COELENTERATA The phylum name, Coelenterata (sel eu ter a'ta), means "hollow intestine," and all of the representatives bear this out by possess- ing a single large cavity in the body. There is a single opening to this cavity, and it functions as both mouth and anus. There are two general types of coelenterates ; the polyp form and the jellyfish form. They are all modified gastrulas, have radial symmetry, and possess tentacles with "sting bodies" or nematocysts. Most of the species are marine, but there are a few fresh-water forms. The body wall is composed of two layers of cells, and for that reason they are said to be diploMastic. These two layers are the outer ectoderm and inner endoderm. Most of the representatives do not develop skeletal structures, but coral polyps produce hard, cal- careous cases around themselves. In several species there is the typical alternation of generations of attached and free-living forms. Most coelenterates are attached or very sedentary for at least a part of the life span. The radial symmetry is correlated with an attached habit of life. A good many of the attached forms look much like plants and were so described for a long time. The digestive process is principally extracellular, being accom- plished by enzymes which are secreted by special cells of the endoderm into the internal or gastrov oscular cavity. A limited amount of the digestion, however, takes place within the endoderm cells after par- ticles of partially digested food have been engulfed by these cells. This is called intracellular digestion. Excretion and respiration are carried on by the general surfaces of the body. Asexual reproduc- tion is accomplished by budding and fission. Sexual reproduction, involving production of ova and spermatozoa and their union in fertilization, occurs here too. The group is considered among the simplest of metazoans and shows, in a simple way, typical features of this great division of the animal kingdom. Hydra will be studied in detail, because it is 130 PHYLUM COELENTERATA 131 readily available, easily collected and handled, and is representative of multicellular animals of simple formation. The study of Hydra as a simple metazoan will go far in giving insight into the much more complex make-up of the body and life of man. Classification of the Phylum The phylum is divided into three classes, each with three or four orders. Class Hydrozoa. — These are typical polyp forms, many of which produce medusae forms by budding. The group includes marine, colonial polyps, or hydroids, floating colonial hydroids, such as Portuguese man-of-war, one special gro"up of corals, some smaller jellyfishes, and the fresh-water polyps. Fig. 57. — Structure of Gonionemus. ad. Adhesive pad ; g, gonads ; li, lithocyst ; m, mouth ; mn, manubrium ; n, nematocyst ; ra, radial canal ; re, ring canal ; st, stomach; t, tentacle; ve, velum. (From "White, General Biology.) Order Leptolina — a group which has a sedentary or sessile polyp stage. Such examples as Hydra, Ohelia, Gonionemus, Canipanularia, Tubularia, and Craspedacusta are well-known forms. The first one is a fresh-water polyp form and is the best known of the group. The last one listed is a fresh-water form with a small polyp stage lacking tentacles but with a disclike medusa possessing many tentacles. Hydra, of this order, will be discussed as a general repre- sentative of the phylum, but since Gonionemus and Ohelia are com- mon marine forms, a brief description of them may be included here. 132 • TEXTBOOK OF ZOOLOGY Gonionemus is a small jellyfish form, measuring about a centi- meter across, and is found in the pelagic waters, along our eastern shores. Its shape reminds one somewhat of an umbrella with a fancy fringe but with practically no handle and made of clear cello- phane. The exumhrella is the convex upper, or aboral side while the suhumhrella is the concave, lower, oral side. A short stalklike part, the manuhiHiim hangs down from the center of the subumbrella. At its distal end is the mouth, bordered by four oral lobes. The mouth is the aperture leading into the internal or gastrovascular cavity which has four radial branches or canals. These radial canals join a circumferential or marginal or ring canal. A circular ledge or fold of tissue which extends inward from the margin of the subumbrella and partially encloses this saucer-shaped cavity, is called the velum (craspedon). From a few to more than eighty almost solid tentacles hang down from the margin of the subumbrella. The cell structure of this animal is made up of an outer ectoderm and an inner endo- derm, with a large amount of jellylike mcsoglea between these two genn layers. Wa\y, leaflike folds hanging in the subumbrella and radiating from manubrium to margin are the gonads. A planula-like hydroid form develops from the egg. The animal is able to swim about by drawing water into the partially enclosed cavity of the subumbrella and expelling it through the aperture formed by the velum with enough force to move the animal in the opposite direction. The pressure is developed by contraction of the body. Obelia is a marine, colonial type resembling a branched plant in appearance. The individuals are attached to each other in the colony, and it is fastened to a rock or other substratum by a root- like hydrorhiza. They are distributed in the Atlantic Ocean and Gulf of Mexico out to forty fathoms in depth. The colony begins as a single individual which buds, but they do not separate from the preceding or parent generation. This may continue for several gen- erations. From the hydrorhiza there is an upright stem, the hydro- caidus. This stemlike part gives off lateral branches, hydranths; at the end of each is a mouth and tentacles. These are feeding polyps. Also as branches of the stem, there are the hlastostyles which are modified, nonfeeding polyps capable of producing medusae. The medusa is the third type of individual connected with an Obelia colony. The perisarc, which is composed of chitin, covers the colony. In some parts this is ringed, and it expands at the base of the PHYLUM COELENTERATA 133 Obelia habit Mouth ■? tb/drotheca- Coelenteron ■'' hntoderm tctoderm Qonotheca MedU5Q-bud B\a5bo5t\/ie - Radiol canal ^^ Repiroductive Moubh 7^^-tirf^ Jcatocyjfc— - Tenbacles- Kedusa Obelia Fig. 58. — Obelia, hydrozoan colonial coelenterate, showing: asexual generation, sexual generation (medusa), structure, and habit of life. (Courtesy of General Biological Supply House.) 134 TEXTBOOK OF ZOOLOGY hydranth to form a bowllike case or hydrotheca which supports it. Another modification is the taller, more enclosed case, gonotheca, which nearly encloses the Mastostyle. The blastostyle with this cover- ing is often called the gonangium. Fibrous processes connect the perisarc to the soft, inner parts (coenosarc). The cavity of the hydranth is continuous with that of the hydrocaulus, and is, there- fore, a part of the gastrovascular cavity. Medusa e x J Sperm -from J another J medusa ^__ ..Ferbilucd eqq \*" "^ *;> Cleavacfe ^ cell stage. ^"^ \ Mature, qonancjlcim.^^g BJastula. ^ \arva ^ Position of mature colony Fig. 59. — Life cycle of Ohelia, illustrating polymorphism and metagenesis. Adult hydroid colony with mature gonangium gives rise to sexual medusa which is pro- duced in the gonangium and set free in the water. Germ cells produced by the medusae complete the cycle. Blastula and planula are free-swimming. (Redrawn and modified from Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.) The coenosarc is made up of an outer layer of cells, the ectoderm, just beneath the perisarc, and an inner endoderm layer lining the cavity. The mouth of the hydranth is located in a domelike hypostome at the free end. There are between twenty and thirty solid tentacles at- tached around the basal margin of the hypostome. The hydranth cap- tures and ingests small aquatic organisms as food by the aid of stinging bodies (nematocysts) produced in certain ectoderm cells of the distal portions of the tentacles. The digestion of this food is accomplished PHYLUM COELENTERATA 135 in the internal cavity. With the exception of reproductive proc- esses, a single hydranth of Obelia will be found similar to an entire hydra, to be studied soon. The reproductive cycle is both sexual and asexual, alternating between the sexually produced polyp or hydroid generation and the asexually produced sexual generation, the medusa or jellj'fish form. The medusae arise as buds from the special individuals, blastostyles, escape through the distal pores, and develop to sexual maturity as free-swimming individuals. The sexes of these are separate; some produce eggs, and others, spermatozoa, which are discharged into the water at maturity and unite to form zygotes. Fig. 60. — Diagram of a siphonophore colony (Physophorida) . A. Pneuniatophore ; B, C, swimming bells ; D, protective zooid ; E, sporosac : F. G, dactylozooids ; H, feeding polyps (gastrozooids) ; /, nettling cells. (From Van Cleave, Invertebrate Zoology, published by McGraw-Hill Book Company, after Claus.) The zygote develops into the free-swimming, ciliated planula stage which soon attaches and develops into a polyp from which a new colony arises. After producing a generation of medusae, this colony disintegrates, and after producing germ cells, the medusae die. This process, involving alternation of generation, is described as meta- genesis in Chapter VIII. Obelia presents a very good example of metagenesis as represented in animals. The medusae of this sort are spoken of as hydromedusae to distinguish them from the scyphomedusae or jellyfishes of Class Scyphozoa. 136 TEXTBOOK OF ZOOLOGY Order Trachylina. — This order includes two suborders of hydro- medusae which come from the egg- directly with no polyp stage. Campanella and Liriope are generic examples. Order Hydrocorallina. — This group resembles the corals by produc- ing strong calcareous skeletons. They have extensive, branched hy- drorhiza and powerful nemato cysts (stinging 'bodies). Rudimentary medusalike bodies develop on the coenosarcal canals. Millepora, the staghorn or stinging coral, as it is called, is a good example. Fig. 61. — Physalia, the Portuguese man-of-war a floating colonial coelenterate. (From Hegner, College Zoology, published by The Macmillan Company.) Order Siphonophora. — This is a pelagic order of colonial coelen- terates with extreme polymorphism. A common tube of the coenosarc unites the five kinds of individuals of the colony, and this cavity is continuous from one individual to another. The blind end of the coenosarcal tube is an air-filled, bladderlike float (pneumatopJiore) PHYLUM COELENTERATA 137 with a superior crest. The polyps hang down into the water beneath this float. The types of polyps include : gastrozooids (nutritive or feeding), dactylozooids -with nests of nematocysts and having long tentacles (tactile and protective), gonozooids which are male, repro- ductive zooids, and others which produce ova-bearing medusae. Swim- ming bells (nectocalyces) often occur just below the pneumatophore. Most of the individuals are specialized to such a degree that they care for only limited functions. This specialization and diversity of forms is such that the entire colony appears as a single individual. Physalia, the Portuguese man-of-war, is a typical example. Its sting is quite poisonous ; bathers coming in contact with the trailing ten- tacles, which bear batteries of nematocysts, suffer severe pain. Class Scyphozoa. — The coelenterates belonging here are large jellyfishes having an alternation of generation in which the medusa form is dominant. The scyphomedusa has an eight-notched margin, lacks the velum (therefore acraspedote), and has gonads connected with the endoderm. The polyps have four longitudinal endodermal folds, called taeniolae, which form gastral tentacles or filaments in the medusa. These jellyfish have a complex system of branched radial canals and abundant marginal tentacles as well as oral tentacles. Several of the representatives of this class are thought by some zoologists to exist generation after generation only as medusae, but it may be that the polyp form has not been discovered yet, if it exists. There are records of individuals of this group twelve feet in diameter, and possessing tentacles one hundred feet in length. Order Stauromedusae. — Conical or vase-shaped medusae which usu- ally lack marginal sense bodies (tentaculocysts). The tentacles are distributed perradially and interradially. Lucernaria and Haliclystus are usually cited as examples. Order Peromedusae. — These are cup-shaped, free-swimming forms with four interradial tentaculocysts. The tentacles are adradial and perradial. They occur in the open sea. Pericolpa and Periphylla. Order Cubomedusac. — Forms which have rather cubical shape, four perradial tentaculocysts, interradial tentacles, and are chiefly tropical. Charyhdea is an example. Order Discomedusae. — Scyphozoa whose medusae are dominant, saucer-shaped, and almost transparent. Some of them are more than seven feet in diameter. There are usually eight or more ten- taculocysts perradially and interradially distributed on the margin 138 TEXTBOOK OF ZOOLOGY of the bell. Tentacles are usually present also on the margin of the bell. This is the most numerous and extensively distributed group of Scyphozoa. Aurellia and Stomolophus are common examples. Aurellia* is the typical example, and, like most jellyfishes, is composed largely of water. When they are dried, only a thin film remains. This is a common one and ranges from New England to the Gulf of Mexico. It may reach a foot in diameter. Cut surface of body wall Stomach Qonad ^ Jub-gsnital pit Upper portion of manubrium -^r — Lateral mouth \ Radial canal - -Sub-umbrella space — Grtutor muscle Manubrium (cut surface) Central mouth Oral tentacles Fig. 62.- -Cabbage-head jellyfish, Stomolophus meleagris, a very common form in the Gulf of Mexico. Bisected to show internal structure. The animal has no velum as do the hydromedusae, but there is a square mouth on the subumbrellar side with wing-shaped, liplike oral lohes or arms. A suhgenital pit lies in each quadrant of this side of the animal. The mouth leads through a short passageway into the angular gastrovascular cavity which in turn has four lateral gastric pouches containing the fringelike gonads. There is also a row of small gastric filaments here carrying nematocysts. A large number of branching radial canals extend from the gastrovascular cavity out to the margin of the bell, there joining a circumferential •This spelling is according to Mayer's monograph, proposed by Peronas Le Sueur was so spelled. The generic name flrst PHYLUM COELENTERATA 139 Lonq tentacles -- (in Chryiaoro) Larqe subqenibal pit (as in Chrysaora) TentaculocyJt Admdial canal Perradial canal Jhort5f'mp/e oral - arm (Aurelia) ■ -^^ >-:, -Intenadial canal ■ ;■ . • ■ • . .'iv. - •*. -iy ■ :T*L^5=f'^^^"""V C Admdial canal ■i^-:- ■:■■■ lIfJ&;^-#- vV-i^^:?-----^'"'?^"^' 'S^_]_; V^ _ _ Aon(7 ribboh-like oral ' ^ ) j "^ arm (in Chrysaom) - 'Small 6iibgenibal /^C"""^ p/fc (aj /n Aurelia) ^ '^ Qastric pouch '^ Short tentacles (Aurelia) Fig. 63. Marqinal lobes (as in Chrysaora) -Aurellia and comparative structure of jellyflshes. (Modified from figure in Pacific Biological Laboratories' catalogue.) Stages in the development of the scyphis toma Planula MM larva ^^ Stages in the -development ^of the jbrobila Zygote Sperm from separate adult Fig. 64. — Life cycle of Aurellia aurita, showing staj^es from germ cells to the ephyra stage which precedes the adult condition. 140 TEXTBOOK OF ZOOLOGY canal. The eight tentaculocysts are symmetrically located at eight points ou the margin, each between marginal lappets. The tentaculo- cysts are sense organs of equilibrium. The pigment spot over each is likely sensitive to light. Near it is the olfactory pit. Reproduction involves both sexual and asexual generations. Germ cells are produced by the pinkish gonads in the gastric pouches, and they pass out through the mouth with the water. Fertilization takes place, and the egg develops into a free-swimming plajiula which after attachment becomes a tubelike polyp that reproduces by budding most of the season. Then the polyps form medusae by strohilization, i.e., constrictions are formed around the body making it resemble a stack of saucers ; the upper one periodically frees itself and swims away. The polyp with all of these constrictions is known as a strohila, and the new medusa is called an epliyra. Class Anthozoa. — All animals in this class conform to polyp or- ganization and may be colonial or solitary. They have an ecto- dermal esophagus and longitudinal partitions called septa (mesen- teries) incompletely dividing the gastrovascular cavity. Muscular tissue bands are found in the septa. The mesogloea is quite abun- dant and contains a good many cells that resemble primitive con- nective tissue cells. Many of these animals produce a calcareous external skeleton called coral. Both sexual and asexual reproduc- tion are common. Subclass Zoantharia. — This group has numerous paired septa, typically occurring in multiples of six, and plain tubular tentacles. It includes sea anemones and corals. Order Actinaria. — These anemones are usually solitary polyps ; they have many complete septa and numerous tentacles but no skeleton. Sagartia, Cerianthus, and Bletridmm are common examples. Metridium usually lives attached to rocks or to solid bodies in the water near shore, even in tide pools. They average about three or four inches in height and two or two and a half inches in diam- eter. The free end of the jar-shaped body is covered with tentacles which are provided with nematocysts. The entire body can be expanded and contracted, and it can change its location by "seooch- ing" on its lasal disc (attachM end). The mouth is located in the center of the crown, and food is forced into it and on through the gullet (stomodeum) bj^ action of cilia on the tentacles and part of the lining of the mouth. At each side of the gullet is usually a PHYLUM COELENTERATA 141 ciliated groove, the siphonoglyphe, through which water is constantly carried into the gastrovascular cavity for respiratory purposes. The gastrovascular cavity is divided into radially arranged compart- ments by the primary septa or mesenteries which extend from the wall of the gullet to the inside of the body wall. The primary septa in the axis of and extending parallel with the siphonoglyphes are called directives. At the basal end these cavities are continuous Fig. 65. — Sea anemone, Metridium inarginatum, showing external features. with the main central cavity. Between the primary septa are sec- ondaries which do not quite reach the wall of the gullet, hence their medial ends are free in the cavity. Between these and the pri- maries are some tertiary septa which are still shorter ajid also attached to the inner surface of the body wall. A quarternary set is represented by mere ridges on the inner surface of the wall and is interspersed among the others. There is a band of muscle run- ning vertically on the face of each septum next to the muscle on the adjacent septa of the same rank. Below the gullet the mesentery has secretory filaments which in turn bear long, threadlike acontia. 142 TEXTBOOK OF ZOOLOGY These protrude through pores (cinclides) in the body wall to the outside, and they are supplied with nematocysts and secretory cells. They serve as defensive as well as offensive structures. Asexual reproduction by budding from the margin of the basal disc is practiced by this animal. Occasional longitudinal fission may occur. The gonads develop on the edges of the lower part of the septa to provide for sexual reproduction. The sexes are distinct. Cinclade, with Aconlium protruding SlereogTam of Anthozoan Polyp MeKtilcrie fiUment, Hollcw Tentacle Vf)i.tfsl ^' Siphonogtyph SpSincler Diagrammatic T. S. of Anthozoan Polyp fi\ level A-A EnJoceet, cKamter between two metenteriei of , the ume pair "-^^ Cxaeoel. cKsmber between pair* ^ o( meicnteriei ' ■^ Directive* (Ventral bet of Primary Meccnteriea) Otagrammatic T S. of Anthozoan Polyp at level B-B Fig. 66. — Diagrams to show the structure of the anthozoan, Metridium, (Courtesy of Pacific Biological Laboratories.) Mature ova and spermia are discharged into tlie water of tlie cavity and escape through the mouth to unite in fertilization outside. The development includes cleavage and planula stages, before the new individual attaches and changes form. Order Madreporaria. — The representatives of this order secrete an external limestone skeleton ; most of them are colonial. The indi- viduals of colonies communicate with each other by coenosarcal con- nections. Otherwise they are similar to anemones. Astrangia, Madre- pora, and Oculina are examples. PHYLUM COELENTERATA 143 Astrangia is the common coral polyp, and it is quite similar to a small sea anemone to which calcium carbonate has been added by secretion from the ectoderm cells as well as having budded to form a colony of numerous individuals. Coral poljqDS vary in size from one-sixteenth of an inch to several inches in diameter. In time continually growing colonies of these animals can produce enormous stony barriers (reefs) in the sea. One such reef is over 1,100 miles in length and from ten to twenty-five fathoms deep. Many corals are of beautiful colors. Order Antipathidea. — An order composed of branching colonies whose individuals are joined by a branched tubular axis which is covered by an epidermal layer. Cirripathes and Antipathes are typi- cal examples. srri,.^^.^ •Trv^.- ^»* A "^^^ Fig. 67. — Common coral, Astrangia danae. A, Stone produced by the animals when cleaned ; B, polyps in natural habitat. Subclass Alcyonaria. — The features of this division include eight hollow, feathered tentacles, eight mesenteries, and one siphonoglyphe. Colonial and pol^'morphic forms are not uncommon. Order Alcyonacea. — A colonial group which has calcareous spicules but lacks an axial rod. Body walls of individuals fuse together as one. Alcyoninm is the type example. Organ pipe coral belongs in this order. Order Oorgonacea. — This is another colonial coral which is sessile and has a calcareous axial rod. The colonies are bilaterally sym- metrical. The common sea fan, Gorgonia, as well as the precious Corallium rubrum are well known examples. Order Pennatidacea. — Another colonial form whose body is modi- fied so that one portion is submerged in the substratum. The colony 144 TEXTBOOK OF ZOOLOGY takes a bilateral form, and the individuals are born on a disc or axial stem which is supported by a hard skeleton. There may be dimor- phism of zooids within the colony. Renilla and Pennatula, sea pens and sea feathers, are typical examples. HYDRA Habitat and Behavior Hydra (Chlorohydra) viridissima is likely the most common hydra of the Southwest. It is the small green hydra which is very active and has short tentacles. This species has the green color because of the presence of a unicellular alga, Chlorella vulgaris, in the endoderm cells. The plant uses some of the by-products of metabolism of the hydra, and the hydra benefits by receiving oxygen from the photosynthesis of the alga. This kind of a relationship is called symbiosis. Most of the hj^dras are found in cool fresh water, attached to the surface of plant leaves, smooth sticks, debris, or even the surface film of the water. The brown hydras, such as H. americana, H. carnea, and Pelmatohydra oligactis, are sluggish and have longer tentacles than the green ones.* Hydra is a sedentary kind of animal and may remain stationary for a considerable period of time if living conditions are uniformly good. When the environmental conditions are changing, and the animal is in need of food, it becomes quite active, moving about from place to place. It keeps the tentacles extended, ready to grasp any food which may come into its reach. Nematocysts or sting bodies are discharged when the tentacle comes in contact with potential food, and if it chances to be a small animal, it will likely be paralyzed by the toxin which is injected by the nemato- cysts. The prey is then carried to the mouth and tucked into it by the tentacles. Frequently hydra is able to stretch its body over articles of food which are actually larger than the hydra usually is in normal condition. Hydra will eat only when it is hungry and will not react to food at other times. It is more sensible than many people in this respect. On the other hand, it has been authentically reported that a hungry hydra will perform the characteristic feed- ♦Recent taxonomic information concerning Hydras of the United States may be found in the papers of Libbie H. Hyman, published in tlie Transactions of the American Microscopical Society, Vols. 48, 49, and 50. PHYLUM COELENTERATA 145 ing movements when only beef extract is in solution in tlie water. Thus it responds to a chemical stimulus alone, but it will not respond to a mechanical stimulus only. These animals show response to a number of environmental con- ditions. Any sudden change is likely to bring about a negative response. If the stimulus is of a general nature and of considerable Fig-. 68. — Locomotion in hydra. Successive positions taken when progressing by somersaults. (From Jennings, Behavior of the Loiuer Organisms, published by The Columbia University Press.) intensity, the animal will contract all of the tentacles and the body also. If the stimulus is restricted to one locality and is not too strong, the animal will contract in the affected area, by the with- drawal of one tentacle. The movements of the animal are per- formed by contraction and relaxation of the contractile fibers con- nected with certain of the cells. The activities come in response to internal as well as external stimuli. 146 TEXTBOOK OF ZOOLOGY The common tropisms, which have been described previously, are present in hydras. They respond to light and will find an optimum intensity which varies with the different species. Green hydras react positively to sunlight and withstand moderate temperature; hence they are adapted to the Southwest. They likewise possess an optimum for temperature and prefer relatively cool water. They seem not to become particularly uncomfortable until the tempera- ture gets up to 31° C. ; then they attempt to find lower temperature. As the temperature is lowered on them, they simply become less and less active and finally cease to move as the freezing point is approached. As pointed out previously, both chemotropisms and thigmotropism are concerned in food-taking. Contact stimuli are of considerable significance in a sedentary animal like this. It re- mains attached in contact with some solid body most of the time. Sudden mechanical stimulation like stirring the water or jarring the attachment of the animal will cause it to contract vigorously. Locoynotion is accomplished in at least four ways. Gliding from one point to another by partially releasing the basal disc and slip- ping it to a new location is common. Or the animal may bend over and cling to the substratum by the tentacles, release the basal disc, then draw the body toward this point, where the basal disc is reat- tached. This process is consecutively repeated and is called "loop- ing." Occasionally the animal bends over, holds by the tentacles, then turns a "handspring" or "somersault" to attach the basal disc on the substratum beyond this point. The fourth means by which locomotion is effected is by dropping to the bottom, then secreting a bubble of gas at the basal disc and floating back to the top on that. External Anatomy Hydra is a macroscopic animal, but it is relatively small. Its body is quite contractile, being able to extend from a contracted length of two or three millimeters to a length of eighteen or twenty milli- meters. The column or body is a tubular, cylindrical trunk which ordinarily stands in a vertical position. In some forms the distal (free, oral, or anterior) end of the column is much stouter than the proximal (attached, aboral, or posterior) end, but in H. viridis- sima there is only a slight tapering toward the basal end. Attached around the free end of the column is a circlet of from four to seven fingerlike tentacles, which extends free in the water. Tentacles may PHYLUM COELENTERATA 147 Stretch out to be slender threads five to seven centimeters in length. They are very useful either singly or as a group in capturing and delivering food to the mouth. The mouth is located in the center of the distal end of the column and is surrounded by the tentacles. This conical elevation between the bases of the tentacles in which the mouth is located is called the hypostome. The mouth when closed and viewed from the top looks something like an asterisk. Mouth Tentacle Hypostome Battery of Nematocysts Bud Basal Disc Fig. 69. — Hydra showing external features. From the side it appears simply as an indentation or notch in the conical end of the hypostome. The proximal or attached end termi- nates in a 'basal disc or foot, which secretes an adhesive substance which helps the animal in attaching to objects. From one to several luds are often found on the sides of the trunk, and these occasion- ally bear buds before the first is separated from the original parent. Buds are lateral outgrowths of the column and are found when the animal has favorable living conditions. Budding usually occurs at about the middle of the body in H. viridissima. Occasionally there may be observed rounded projections on the side of the column which 148 TEXTBOOK OF ZOOLOGY are seasonal reproductive organs. Both ovaries (female gonads) and testes (male gonads) may be formed on a single individual, but they are usually seen on separate individuals. If these projections are conical and located nearer the tentacles, they are testes or sper- maries; if they are more nearly knoblike and are located nearer the base, they are ovaries. This animal possesses radial symmetry, but it is arranged with an axis of polarity from basal disc to hypo- stome, which is essentially equivalent to what is called the ventro- dorsal axis of more advanced forms. All of the metazoans have a primary axis. Sedentary and sessile animals very commonly have radial symmetry, while the motile or free-living organisms tend toward bilateral symmetry. Internal Anatomy Another feature of the organization of this animal is the diplo- blastic structure which consists of two layers of cells or the germ layers surrounding an internal space, the gastrov oscular cavity or enteron. These are studied on stained sections. The outer one is the ectoderm, which is thinner and is composed of four types of cells. The most numerous ones are typically cuboidal in shape and serve both as contractile units and as the general external surface of the body; they are appropriately called epitheliomuscular cells. Each of these cells consists of a polyhedral outer or epithelial portion and a basal portion which is drawn into one or two long, slender, fibrils which extend in a direction parallel to the length of the animal. These cells contract to shorten the length of the animal. Interspersed occasionally among these cells are the larger cnidohlasts in which develop the neniato cysts, stinging cells or nettle cells. These are dis- tributed over all the body except the basal disc, but they are much more numerous near the distal part of the column and on the ten- tacles. The nematocysts are usually contained in little raised tubercles in the ectoderm. Each tubercle contains a large barbed one and several of a smaller variety. Four different kinds have been de- scribed. Since the large barbed type is the most conspicuous, it will be described here. In the cnidoblast the nematocyst appears as a sac of fluid within which is inverted a barbed stalk with a coiled thread attached. Projecting out of the superficial surface of the cnidoblast is a triggerlike process called the cnidocil, which when chemically stimulated causes the cnidoblast to discharge the nemato- PHYLUM COELENTERATA 149 cyst. Chemicals, such as weak iodine, acetic acid, or methyl ^een, when added to the water, will bring this about. Contact will not. In this reaction the stalk and thread are everted, probably by de- velopment of pressure. This type of nematocyst produces a hypno- toxin which anesthetizes the animals into which it is discharged. In another form the sac is small, the stalk is barbless, and the thread is elastic ; it becomes coiled around the object against which it is dis- charged, and thus impedes locomotion of the victim. Cnidocil Filament Nemaboo/st Nucleus a._:il1 6orb__. Stalk-- Bag- Remains of Cnidoblasb Barbless nemafcocyifc Fig. 70. — Nematocysts and their function. A, Cnidoblast containing an undis- charged nematocyst, after Schneider ; B, nematocyst everted and extended but still held in the cnidoblast, after Schneider; C. portion of tentacle, after Jennings; D, insect larva attacked by hydra, after Jennings ; E, leg of small aquatic insect with barbless nematocysts on its spines, after Toppe. (Redrawn and modified from Wolcott, Animal Biology, McGraw-Hill Book Company, Inc.) The cnidoblasts are produced by a third type of cell, the interstitial cell, which is small and rounded. These are formative cells about the size of the nuclei of the epitheliomuscular cells and quite densely granular in nature. They crowd in between the other cells, especially near their bases. As a nematocyst is discharged, the entire cnido- blast is replaced by an interstitial cell migrating into position, A damaged or spent cell of the body may be replaced from the inter- stitial cell. Besides these three types, there are the scattered, ir- 150 TEXTBOOK OF ZOOLOGY TEMTACLE — MOUTH •TESTI S GASXRO- VASCUl_AR CAVI TY ECTODERM MESOSLOE A EfslDODERM OVARY BASAU DISC Fig. 71. — Diagrammatic longitudinal section of hydra, showing mature gonads and typical cell layers. (Drawn by Titus C. Evans.) ECTODERM EPITHEl_IO- MUSCUL. AR CELL. I NJTER- STI Tl Al_ CEI_U NEMATOCYST C N I DOBL. AST MESOGUOEA DIC3ESTIVE CEUUS G l_ A IM D CEI_U Fig. 72. — Cross-section through the column of hydra. The central space is the gastrovascular cavity or enteron. (Drawn by Titus C. Evans.) PHYLUM COELENTERATA 151 regular, slender, neuroepithelial cells which are joined into a net by intercellular processes. These cells fit between the others and are either sensory or motor in function, thus receiving external stimuli and also causing contraction of the contractile cells at proper times. Beneath the ectoderm and embedding the bases of the cells is a very thin layer of noncellular substance called mesogloea. It is produced by the cell layers and serves as attachment for them, par- ticularly for the fibrils of the epitheliomuscular cells. In some of the other coelenterates, this layer is exceedingly thick and heavy. The inner, thicker cell layer of the wall is the endoderm which lines the lumen of the gastrovascular cavity. The most conspicuous cells here are the nutritive-muscular cells which are long, vacuolated struc- tures attached to the mesogloea by fibrils which extend in it parallel to the circumference of the animal. By contraction these cells in- crease the length of the animal by reducing its circumference. These cells often possess flagella at the free margin and at times engulf particles of partially digested food like an amoeba. It is seen then, that they serve both as muscles and as digestive cells. Glandular cells are also present in this layer. Being slender, they wedge themselves between the nutritive-muscular cells and secrete what is probably a digestive fluid into the gastrovascular cavity. Neuro- epithelial and interstitial cells are also interspersed among the other cells of this layer. The general morphology of the adult animal is very similar to the gastrula stage of the developing em- bryo of more complex metazoans. Metabolism The food of hydra consists of small insect larvae, minute worms, small bits of organic matter in the water, water fleas, and other small Crustacea. Ingestion of the food has been described already. Upon entering the mouth the morsel of food is moved some distance down in the cavity by successive wavelike contractions of the column progressing from distal to proximal. Such serial contractions are usually called 'peristaltic contractions. Here in the upper half of the enteron digestion takes place. The wall possesses many more of the gland cells in the endoderm, and the food material disintegrates into smaller particles here in this region. The digestion which occurs here is spoken of as intercellular digestion and is brought about by enzymes produced by the secreting cells of the endoderm. The dis- 152 TEXTBOOK OF ZOOLOGY solution of the food by the enzymes is augmented by the churning effect of the contractions of the body. The flagella present on the nutritive-muscular cells create currents of water which also hurry the process. The dissolved material is presumably absorhed by the cells of the endoderm, and by diffusion the nutrient solution reaches the ectoderm cells just outside. Small particles of the partially di- gested substance are engulfed by the free ends of many of the nutritive-muscular cells by virtue of their amoeboid activity. These particles are taken in food vacuoles, and the digestion is completed there just as it is in an amoeba or Paramecium. This illustrates something of the primitive organization of hydra as a metazoan. Fig. 73. — Hydra with body turned inside out in attempting: to Ingest a piece of meat. (From Curtis and Guthrie, Textbook of General Zoology, published by John Wiley and Sons, Inc.) As will be remembered, this process of converting the digested food into an integral part of the protoplasm is known as assimilation. The food is distributed to all parts of the enteron, which extends into the tentacles and buds, by the action of the flagella and by bodily contractions. There is no separate system of transportation or circulation of nutriment. This dissolved material reaches the remote parts of the protoplasm by diffusion through the membranes and protoplasm generally. The gastrovascular cavity has the dual function of digestion and circulation. PHYLUM COELENTERATA 153 Many of the animals used as food have hard skeletal parts that will not digest. These indigestible portions are ejected from the cavity through the mouth by reverse peristalsis, and the process is known as egestion. Eespiration furnishes the necessary exchange of oxygen and carbon dioxide by diffusion through the plasma mem- branes. The dissolved oxygen in the water in which the animal lives is the source of this element. Catabolism or dissimilation takes place in the protoplasm and involves the union of oxygen with the substance of the protoplasm to transform potential energy there to kinetic energy and heat. Accompanying this oxidation there are produced some waste by- products in solution including urea, uric acid, and water which must be expelled from the body. In hydra this excretion is accom- plished by diffusion through the general surface of the body. There is some indication that there may be accumulation of waste prod- ucts in endoderm cells as cytoplasmic granules, which finally escape through the gastrovascular cavity and mouth. It will be noticed that these phases of metabolism are, in general, very similar to the comparable processes in Protozoa and the same similarity will be noticed when they are compared later with the higher forms of animals, because the protoplasmic requirements are the same in all animals. The Nervous System and Nervous Conduction The neuroepithelial cells are distributed among the other cells of the germ layers. There is a greater abundance of them on the hypo- stome, basal disc, and tentacles than along the length of the column. The greatest concentration of these cells is in the hypostome around the mouth, which makes this region in a sense comparable to a primi- tive brain. These cells all over the body are in contact with each other by means of their processes forming what is called a nerve net. When one sensory cell is stimulated, all of the sensory cells seem to be stimulated in some degree. A sufficiently strong stimulus affecting any sensitive point will stimulate the entire body. This is a definite organized type of nervous system but not a very efficient one. Reproduction and Life Cycle Reproduction is both asexual and sexual. Asexual reproduction is accomplished very efiiciently and quite rapidly. This process is essentially reproduction by somatic cell division. Nutrient mate- 154 TEXTBOOK OF ZOOLOGY rial accumulates at some point near the middle of the column. The bud first appears as a slight superficial bulge. The cell division at this point is very rapid, involving considerable activity in inter- stitial cells. This enlargement rapidly increases in size to form a stalk. An extension of the eiateron extends into the bud, which is essentially an outgrowth of the body wall. Tentacles appear as outpushings of ectoderm and endoderm, and in the terminal posi- tion a mouth is developed. After the bud has attained some size, a constriction occurs between it and the parent. This closes the enteron between bud and parent, and the bud finally separates to become a free individual. Embrvo ?6md.l tiydra.- <§exua.l l^production Fig. 74. — Methods of reproduction in hydra. (Courtesy General Biological Supply House.) Sexual Reproduction. — During the summer and fall particularly, hydra reproduces sexually. This involves the production, matura- tion, and union of germ cells. Testes may appear first and ovaries later on the same individual or both gonads may be present at the same time in which case self-fertilization is possible. As a rule, these animals are hermaphroditic or monoecious as suggested before, but it has been reported that individuals of separate sex (dioecious) have been found. The germ cells or gametes develop from inter- stitial cells which accumulate at a certain place between the ectoderm and endoderm, where they multiply by division to form oogonia in PHYLUM COELENTERATA 155 the female gonad and spermatogonia in the male. All phases of maturation (gametogenesis) may be obsei-^^ed in the testis and ovary. The testis produces large numbers of motile spermatozoa, which when mature emerge periodically from an opening in the tip of the testis and are discharged free in the water. In the ovary a single egg develops at the expense of the other oogonia, which are engulfed bodily and used for food. This one cell grows rapidly, and when Fig. 75. — Development of hydra. 1, Fertilized ovum; Z, two-cell stage; S, blastula stage; i gastrula, showing ectocferm (ec) and endoderm (en) ; cc, cleavage cavity (blastocoele) ; m, cyst; p.b., polar bodies. (After Tanreuther, Biological Bulletin, Vol. 14.) mature it fills the ovary. Fertilization is accomplished by the en- trance of spermatozoa through a rupture in the overlying ectoderm and cross-fertilization usually prevails. A single sperm unites with the mature ovum, and this zygote undergoes the total and equal divisions of cleavage here in place. The process continues until a hollow llastula of many cells is formed. Then follows the formation of the gastrula by a shedding of cells into the cavity (blastocoele) from the inside of the original layer of cells. These new cells on the inside become organized as an endoderm layer, while the original 156 TEXTBOOK OF ZOOLOGY outer layer is now known as ectoderm. Further changes involve the secretion of the thin mesogloea which seals the two layers together. In the meanwhile a shell is produced about the outer surface of the embryo, and this encysted body falls from the parent to the bottom. If conditions are favorable for development, it increases in length within the cyst; when it has attained some size it breaks out, after which tentacles and a mouth appear at one end, while the enteron develops within the endoderm. This individual steadily grows and soon attains adult condition. When the zygote is formed in the fall, the embryo does not emerge from the cyst until spring. Regeneration As is the case in many invertebrate and a few vertebrate animals, Hydra is able to replace mutilated parts or an entire animal from a portion of one. Complete animals may be formed from very small pieces (% mm. in diameter) of a hydra. This process is known as regeneration, and while it is not normally a method of reproduction or multiplication, it is of great advantage to the animal. This phe- nomenon was first discovered in animals from studies on Hydra in 1744 by Trembly. Economic Relations of the Phylum The entire group is not worth much in dollars and cents to man directly. A number of different ones are used as food by some of the useful fish. The corals are of importance both positively and negatively. Many of them are valuable as ornaments, while the large coral reefs are very costly to navigation of marine waters. Many corals are quarried for building stone, and in some instances they protect the shore from being washed by the waves. Phylogenetic Advances of Coelenterates (1) Definite organization of diploblastic condition; (2) well- defined gastrovascular cavity with one opening, the mouth; (3) presence of tentacles with (4) nematocysts or sting-bodies ; (5) continuance of sexual reproduction; (6) distinct radial symmetry and, (7) a nerve net. CHAPTER XI PHYLUM CTENOPHORA This is a group of exclusively marine animals, most of which are pelagic (float near the surface). There is a limited number that lives and moves about on the bottom. Ctenophora (te nof '6 ra — comb-bearing), because of their similarity to coelenterates, are often classified as a class in this phylum. There are only twenty-one Amer- ican species representing this phylum, and they are commonly called sea walnuts or comb jellies. Most of them swim by means of eight rows of fused cilia, called swimming plates or combs. These animals are quite clear and transparent, with a faint tint of pink, purple, or blue. They are often phosphorescent. There are two classes in the phylum: (1) Tentac^data, with a pair of tentacles present either in the larva stage or throughout life. Mnemiopsis leidyi is a lumines- cent, transparent form ; Pleurobrachia hachei has long tentacles on a relatively short, oval-shaped body; and Cestus veneris, Venus 's girdle, may be four feet long and only two inches in width, bandlike, transparent, with an iridescence showing violet, blue, and green colors. (2) Nuda, with no tentacles at any stage; Beroe ovata, about 10 to 12 cm. in length, conical in shape, and rather common, is an example. Habitat and Behavior These are primarily surface-living forms with rather wide distri- bution but most abundant in tropical seas. They move about very slowly through the water with the oral end forward and the two long tentacles trailing if tentacles are present. The tentacles have adhesive or glue cells (colloblasts) which produce a secretion, and with these they capture any small organisms making contact with them. Anatomy The size of different individuals of this group ranges from five millimeters to four feet in length, and the shape may be spherical, pear-shaped, ribbonlike or cylindrical. The symmetry is said to be biradial since there are eight rows of radially arranged paddles or plates which are equally distributed on each side of the median 157 158 TEXTBOOK OF ZOOLOGY line. These paddles are tlie locomotor organs. When seen from the side, the paddles resemble a comb. The mouth is in the oral end of the body and leads into the stomachlike stomodeum which is connected with a series of canals running through the body. This stomodeum is lined with ectoderm and leads to an infundibulum or gastTOvascular cavity proper which joins the stomodeum at right 51 iflgitial or StomacS Plane Left CAitrovfticular Cansl Tentaeulftt 'NviVrC^j Sheath ""~-;,.r Tentacle ■ -S A '-i Bilobed Stomodaeum Right CastrovBicular Canal lUdial CanaU ^^ Intcrradial Cant, i^j.^ ;,'• ■ »j^r).iT. 'i — Tranaverse or 'Yvi^^—--''' • Infundibular Plane /.'/ ■ ;■.-• "^ Aboral 5cn«e Orgon ■.■..■•■•;■',:;'•■•'""" — ~ - Plate row with Infundibular Canal underneath Diagram looking down on aboral pole Pleurobrachia bachei Aboral End InfunJibuTum ^ ^ Slomodaeuni Inlerradial Canal Plate Row with ' branch of infundibular canal ayilem undetnealh 'C^ Radial \ Canal o.. Bilobfff Stomo « \\\ _ Rhynchodeum ._ -Ocellus qanqlia .-J^loric caecum .^-Lateral mrve \ ffhynchocoe/ /nfccjfcinc Proboscis vSfcylefc _..Qonad -MetractorM. Anus Fig. 78. Fig. 79. Fig. 78. — A nemertine worm, Lineus socialise with the body coiled. Natural length about 15 cm. (Redrawn and modified from Hegner, College Zoology, published by The Macmillan Company, after Coe.) Fig. 79. — Structure of the nemertine worm, Prostoma rubrum, as it appears when flattened. (Redrawn and modified from Hegner, College Zoology, published by The Macmillan Company, after Coe.) pouches. Both eggs and sperm are discharged from respective in- dividuals through a dorsal pore and fertilization occurs in the sur- rounding water. Following cleavage there is a helmet-shaped larva called pilidium. Cilia develop on the lappets at the lower margins of the body and on a patch at the opposite pole or apical plate. This plate is the principal nerve center of the animal in this stage. The PHYLUM PLATYHELMINTHES 163 adult appears after metamorphosis. In some forms there is a creep- ing larva known as Desor's larva. The vascular system is composed of longitudinal vessels connected by transverse loops. The vascular fluid is usually colorless. The excretory system includes the usual longitudinal tubules and flame cells characteristic of the phylum. Either one pore or several communicate with the exterior. The cen- tral nervous system consists of two ganglia and three longitudinal cords passing through the body. A pair of grooves with cilia along each side of the cephalic portion are sensory and are called cerebral organs. Other tactile organs and eyes are usually developed. Pros- toma, Cerebratulus, Tetrastemma are representatives. Georaqe cavity L'lji^^i^— -Mesenchymal cell Stomach Lctodermal / invagination EsophacjUi .fctodermaf invagination Ventrolateral lobe Fig. 80. — Structure of pilidium larva of the nemertlne worm in partial section. (Redrawn and modifled from Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.) PLANARIA Habitat ajid Behavior This free-living, fresh-water, flatworm thrives beneath the rocks, logs, leaves, algae, or debris at the bottom of shallow spring-fed brooks and pools. They must have pure, clear, cool water. These 164 TEXTBOOK OF ZOOLOGY animals are rather gregarious and when at rest will group together beneath objects where the light is not intense. They respond nega- tively to bright light. They usually feed upon minute plants and animals, dead animal bodies, and living forms, such as small arthro- pods and molluscs. Planaria partially encompasses the food with the body, while the pharynx is protruded to eat it. If tiny scraps of meat are placed in a dish with hungry planarians, they will form a wad of living protoplasm about it. The mouth is located at the middle of the ventral side of the body, and the pharynx is everted through it as a prohoscis which is used to draw food within. It is interesting to watch these animals passing the proboscis about over the surface of fresh meat, apparently sucking up the nourishing fluids from the meat. If very minute quantities of meat juice are liberated in the water at specific points, the planarians are at- tracted to those points. Eye Genital pore Side of head PharjTix sheath Proboscis Fig. 81. — Entire planaria with pharynx extended in position for feeding. (From Hegner, College Zoology, published by The Macmillan Company, after Shipley and McBride.) The locomotion is accomplished in an easy gliding fashion by the action of the beating cilia and muscular contractions of the body. The ability to move along in this way is enhanced by the secretion of slippery mucus which essentially lays a smooth track for the moving animal. It glides over a surface, even the under side of the surface film of water, and adjusts itself easily to any irregularities because of the soft, flexible nature of the body. The ciliary action and muscular contractions are both rhythmic and progress in waves from anterior to posterior. The behavior of this animal is of a reflex or automatic type. The receiving or receptor sensory cell transfers the impulse produced by a stimulus to a ganglion cell or adjustor in the central nervous system which in turn transmits an impulse to an efferent cell carrying it to a muscle or gland. The planarians respond to several tropisms. They possess negative phototropism and thermotropism (as regards high PHYLUM PLATYHELMINTHES 165 temperatures). They react positively to contact (thigmotropism) and water currents (rheotropism). The responses to chemicals are positive in case of food juices and the like; while they are negative to alkalies, acids, strong salts, alcohol, etc. The common species are Plaiiaria niaculata, P. agilis, and P. dorotocephala. External Anatomy The body is elongated, flat, broadly wedge-shaped at the anterior and tapering to a point at the posterior end. It is triplohlastic since the ectoderm, endoderm, and mesoderm are all differentiated and present in a clear-cut fashion for the first time in our studies so far. The symmetry is distinctly hilateral. In Planaria maculata there is considerable pigment in the skin; while in Dendrocoelum lacteum there is much less. On the dorsal side of the anterior region are two pigment bodies called eyespots which are sensitive to light. At each side of the "head" region is a pointed, sensitive, extension of the epidermis in the form of a lappet or "ear," called an auricle. These are sensitive to touch and chemical stimulations but not to sound. The mouth is located in the midventral portion of the body. The pharynx may be protruded through the mouth in the form of a long, trunklike prohoscis which is used in feeding. Posterior to the mouth is a small, constricted, scarlike aperture, the geniial pore. Externally the epidermal cells are soft and the general surface is nearly covered with patches of cilia which are cytoplasmic extensions of these cells. These cilia along with muscular contractions accom- plish locomotion. The average length of fully developed active P. maculata is about three-fourths of an inch. Internal Anatomy The ectoderm covers the outer surfaces of the body and composes the nervous system ; the endoderm lines the intestine and its branches; while the mesoderm constitutes the muscular, excretory, and reproductive systems. The undifferentiated mesoderm lying outside the intestine is composed of a meshwork of large cells and is called mesenchyme or parenchyma. Many of the structures of the animal, which have been observed in none of the forms previously studied, have come into existence with the development of mesoderm. The digestive system is composed of a mouth in the midventral position; a prehensile pharynx held in the pharyngeal chamber or 166 TEXTBOOK OF ZOOLOGY CX PHYLUM PLATYHELMINTHES 167 buccal cavity which it nearly fills; a three-branched enteron or in- testine, which branches immediately from the anterior end of the pharynx into an anterior trunk; and two lateral trunks that turn posteriorly, one along each side of the pharynx, and extend nearly to the posterior end. The pharynx is in the form of a cylindrical fold projecting through the full length of the pharyngeal chamber. It is attached only at its proximal or anterior end and is perfectly free otherwise. When it is extended or protruded through the mouth opening which it fills, it forms a proboscis whose length may be as great as, or greater than, that of the entire body. The trunks of the enteron have many lateral, blind extensions or pockets called diver- ticula which greatly increase the surface exposure of the organ and project among most of the other tissues of the body. The whole arrangement represents a complicated gastrovascular cavity whose wall is endodermal. Cilia Excrelioiy tubule Fig. 83. — Flame cell of planaria. The excretory system is new to our study and is composed of a set of tubules which relate themselves to all parts of the body. There are two principal, longitudinal, coiled tubules, one along each side of the bod}', which receive many small branches and open by minute pores located just posterior to the eyespots, and by several other pores along the length. Each of the numerous smaller branch-tubules has at its blind end a flame cell which is hollow and contains a mass of long cilia that are continually beating in a direction toward the tubule, the movements appearing something like a flickering flame. 168 TEXTBOOK OF ZOOLOGY The cellular walls of the tubules as well as the flame cells arise in the mesoderm. Under strict definition, some authors object to calling this arrangement a system. Another mesoderm organization is the muscular system. It is com- posed of an outer circular layer just under the epidermis; an outer longitudinal layer just medial to the circular layer; oblique bundles of fibers ; and at the medial margin of the mesoderm is another ir- regular, internal, longitudinal layer just medial to a circular layer. By the alternate activity of these layers the animal is capable of great extension and contraction. Another advanced development is the "ladderlike" nervous system which consists of two contiguous lobes of nerve cells just ventral to the eyespots, two ventrolateral longitudinal nerve cords, transverse commissures, branch nerves, and sensory end areas of the epidermis. The double ganglion at the anterior is the central portion of the system. It is known as the cephalic ganglion and gives branches to sensory areas of the head, auricles, etc., besides joining the longitudinal nerve cords. The transverse commissures connect the two longitudinal cords at from 15 to 20 points like the rungs of a ladder. At each point where a transverse commissure meets a longitudinal cord, is a small ganglion composed of a few nerve cell bodies. The branch nerves extend to the surrounding tissue from these points. The reproductive system is fairly well developed in most species except P. dorotocephala which rarely develops sexual organs. Its reproduction is entirely by asexual fission. The sexual reproduction of other planarians is hermaphroditic, which is rather characteristic of sedentary animals. The male organs consist of numerous globular testes located in the parenchyma through most of the length of the body. Vasa efferentia are slender, thin-walled ducts leading from the testes to two larger, longitudinal ducts, the vasa deferentia. These in turn lead posteriorly, enlarge to become seminal vesicles, and converge to form the penis or cirrus, the copulatory organ. This opens into the common cavity called the genital atrium or geiiital cloaca, which opens externally at the genital pore. Some authors describe glands which pour a seminal fluid into the system. The female organs in the same animal consist of two ovaries located well toward the anterior, a tubular oviduct leading posteriorly from each to join the genital atrium at a common point near its posterior end I PHYLUM PLATYHELMINTHES 169 by way of the vagina. There are numerous yolk glands joining each oviduct along its length ; a glandular structure of questionable func- — Auricle CerebraL (jancjlion LorKjitudinaL nerve cord Testis / Vasa efferenbia Lateral nerve Vos l_( deferens Mouth \_^^ lumen of pharynx Seminal vesicle 5eminal receptacle Intestine K-/rax^ Oviduct •H : -— ^ / Pharyngeal chamber Penis Genital pore Fig. 84. — Reproductive system of the planarian worm. Male organs shown on one side only. tion, in the form of a blind tube with an inflated end, is connected with the genital atrium. It has been suggested that the fertilized eggs accumulate and are retained here for a time. The system is 170 TEXTBOOK OF ZOOLOGY notably quite elaborate, and it is found generally that the flatworms have a highly specialized reproductive system. The planarian worms and the representatives of this phylum pos- sess no skeletal system, no respiratory system (breathe through the skin); no coelom or body cavity; and no circulatory system; this function, however, is performed by the branched enteron. It is significant that the reproductive system upon which the continuance of the race depends is highly specialized, this succeeded by the diges- tive system responsible for nourishment of the individual, and this followed by the nervous system which relates the organism to its surroundings. Metabolism The food is principally animal tissue with some plant matter, and ingestion takes place through the proboscis. The food may be par- tially digested by a fluid produced in the pharynx. The principal process of digestion occurs in the cavity of the enteron. Here the process is similar to that of Porifera and Coelenterata, being both in- tercellular and intracellular ; that is, part of the food in the intestinal cavity is digested by secretions from cells in their walls, while other food particles are engulfed by pseudopodia extended from cells lining the cavity and are digested in food vacuoles inside the cells. Absorption and assimilation take place through the plasma mem- branes of adjacent cells. Since the diverticula of this system pene- trate all parts of the body, and the diffusion of materials supplies all other cells, no circulatory system is necessaiy to transport nutriment. There is no anus, so all indigestible material is egested by way of the mouth. Respiration is accomplished through the general surface epithelium, and oxygen is distributed by diffusion through the proto- plasm and fluid-filled spaces of the parenchyma. Catabolism or dis- similation takes place in the cells by union of the oxygen with the organic components of the protoplasm. Excretion or elimination of nitrogenous waste liquids is cared for by the flame cells and system of tubules. The flame cells absorb these wastes from the surrounding tissues and force the fluid into the tubules by the action of the cilia. Reproduction and Life History Sexually the individuals are hermaphroditic. The spermatozoa or male germ cells mature in the testes, then pass through the vasa efferentia and vasa deferentia, to the seminal vesicles where they PHYLUM PLATYHELMINTHES 171 are stored in advance of copulation. Here they become organized into pockets known as spermatophores. The ova mature in the ovaries, pass down the oviducts where yolk cells or nurse cells are added by the yolk glands, through the vagina to the genital atrium, and probably from here to the uterus or seminal receptacle where Young planana batchinq tq(j capsule or cocoon Fig. 85. — Planarian cocoons and the young hatching. Vitelline cells Wandering cells Endodenn Provisional pharynx Ectoderm Wandering " cells Endoderm -Primitive gut Wandering — cells Vitelline ceUs Mouth Fig. 86. — Development of Planaria lactea. 1, E!gg surrounded by yolk; 2, four blastomeres from segmented egg ; S, later stage ; if, still later, after blastomeres have differentiated into ectoderm, endoderm, a provisional pharynx, and wandering cells; 5, cellular differentiation more advanced; 6. embryo becomes flattened and ovoid. (From Hegner, College Zoology, published by The Macmillan Company, after Lankester after Hallez. ) they are thought to be stored. Cross-fertilization is practiced by these animals. Planarians have been observed to copulate with an apparent exchange of spermatozoa in the form of spermatophores. In copula- 172 TEXTBOOK OF ZOOLOGY tion the cirrus or penis is protruded from the genital pore to enter the genital pore and extend into the uterus of the other copulant. In this way spermatozoa may be transferred from each animal to the other. Spermatozoa have been found along the oviduct as far as the anterior portion, so fertilization likely occurs somewhere along this tube. At breeding time zygotes are found in the atrium, and each is surrounded by a large number of yolk cells (nurse cells). Each yolk cell contributes its store of nourishment to the egg cell to which it is attached. From one to several zygotes, surrounded by many thousands of yolk cells, become enclosed in a capsule-like shell Fig. 87. — Fission as it occurs in Planaria dorotocephala. secreted by the genital atrium and known as a cocoon. These are expelled from the atrium and each is attached by a stalk to the under sides of submerged stones or vegetation in the water. In the cocoon the embryo passes through cleavage divisions, blastula stage, gastru- lation and even later stages before the cocoon ruptures and the small wormlike planarians escape into the water. Asexual reproduction by transverse fission occurs quite frequently when the mature animals become slowed down. The individual con- stricts and then divides into anterior and posterior portions each of which forms the missing parts by rapid cell division. The axial i PHYLUM PLATYHELMINTHES 173 orientation of the tissue is retained ; i.e., an anterior portion develops in the position of the original anterior portion, and a posterior por- tion at the original posterior position. This process is not funda- mentally different from budding in Hydra or strobilization in the Scyphomedusae. The retention of the axial orientation during fission has been explained by Dr. Child of Chicago University. The animal pos- sesses a well-defined axial organization in which the "head" por- tion as usual has the highest metabolic activity of the body. Be- ginning at the anterior there is a gradient of decreasing metabolic activity until a level just posterior to the mouth is reached, and here a sudden increase occurs. Posterior to this the decreasing gradient again folloAvs to the posterior tip of the body. The level where the metabolic rate suddenly rises represents the point of fission or the anterior end of the second individual. This seems to indicate a kind of zooid organization in the animal. In larger, older individ- uals there may be other such points of increased metabolism pos- terior to this first one. Such zooids are the result of successive functional isolations of the basal structure accompanying growth in length. This graduation of the rate of metabolism along the principal axis of an axiate animal has been called an axial gradient by Dr. Child. When the animal is young, it is relatively short and the entire body, but particularly the ''head," carries on a high rate of metabolism. The head at this time holds a dominance over the length of the organism. As the animal grows older, it becomes longer, and the entire metabolic rate decreases. This means that the head loses its dominance over the entire length. A new center of dominance and increased metabolism is established just posterior to the point where this "head" dominajice fades out. Regeneration This group shows remarkable powers of replacing lost or muti- lated parts of the body. It can be cut into several pieces, and each piece will replace the missing parts about as the process is carried out in fission. A piece from the middle of the animal will regener- ate a head portion at the anterior margin and a tail portion at the posterior margin. A more complete discussion of this phenomenon will be found in a later chapter on Animal Regeneration. 174 TEXTBOOK OF ZOOLOGY Economic Relations of the Phylum The planarians and other free-living flatworms are of practically no economic importance, but the phylum includes a large number of forms, principally Trematodes and Cestodes, which are parasitic in higher vertebrate animals, including man. Such groups as the intestinal flukes, liver flukes, lung flukes, blood flukes, pork tape- worm, beef tapeworm, margined tapeworm of dog, gid worm, hy- datid worm, common tapeworm of dog, chicken tapeworm, dwarf tapeworm, sheep tapeworm, tapeworm of horse, and fish tapeworm are all serious parasites. They cost many thousands of dollars and much debility each year. A more complete discussion of this topic will be found in the chapter on Animal Parasitism. Phylogenetic Advances of Platyhelminthes (1) Anteroposterior principal axis, (2) bilateral symmetry, (3) a distinct third germ layer, the mesoderm, (4) an excretory system of flame cells, (5) central nervous system extending with the axis of the body, (6) specialized gastrovascular cavity, and (7) perma- nent sexual reproductive organs. CHAPTER XIII PHYLUM NEMATHELMINTHES This group is known as the unsegmented roundworms or thread- worms. Some of the Nemathelmiuthes (nem a thel min'thez, thread- worms) are free-living in soil, fresh water, and salt water; some are found living in plant tissues ; and others live in animal tissues as parasites. The majority of them are microscopic, but a few are macroscopic in size. These worms are long, slender animals whose bodies are more or less cylindrical but tapering toward each end. The range of length is from i/4 mm. to four feet. They differ from the flatworm not only in shape, but also in that the intestine has two openings, there is a dorsal as well as a ventral nerve cord, they are mostly dioecious, and there is a total absence of cilia. They also lack respiratory and circulatory systems, true coelom, and definite locomotor organs. The group is very widely distributed and is deserving of considerable attention. Some of the better known forms are Ascaris (pigworm or eelworm), "horsehair snake," hookworm, pinworm, Trichinella, Filaria, Guinea worm, whipworm, and eye worm. The former will be discussed in some detail in this chapter, and several others will be considered in the chapter on Animal Parasitism. Classification Three classes are usually recognized, although some authors prefer to use only two. The three classes are Nematoda, Gordiacea, and Acanthocephala. Class Nematoda (nem a to'da — threadworm) is a group occupy- ing almost every possible habitat capable of supporting life. There are many free-living, fresh water, marine, and soil-inhabiting spe- cies, and large numbers of parasitic forms living at the expense of other animals and plants. This is a very important class parasiti- cally. In size they range from %o mm. to more than a meter in length. Locomotor organs are found in a few forms, no segmentation is present, and there is no true coelom. Chemical sense organs called amphids are nearly universal, while eyes and tactile organs are com- mon in the free-living forms. The skeletonlike cuticle, common to all, is shed periodically like the molting of arthropods. The nervous 175 176 TEXTBOOK OF ZOOLOGY system is composed of a circumpharyngeal ring from which cords extend posteriorly. It is a sensory-neuro-muscular system. The structure of the free-living forms is generally more complex than that of the parasitic forms. They are adapted to a wide variety of habitats and can withstand many rigors of natural adversity, such as freezing, high temperatures, droughts, and other unfavorable con- ditions. Large numbers of free-living forms have not been named and described. Representatives of this class have an intestine but no proboscis. Order Ascaroidea. — It includes both parasitic and free-living forms. Ascaris (Fig. 90), the common intestinal worm, is the most abundant. Enterohius vermicularis, the human pinworm; Strongy- loides stercoralis, another parasite of man; Ascaridia lineata, the chicken worm, and Toxocara canis of dogs are other familiar examples. Ascaris lunibricoides will be discussed later as a typical example of Nemathelminthes. Order 8trongyloidea. — This is an entirely parasitic group. The males have caudal bursae with rays. The club-shaped esophagus is without a posterior bulb. The hookworms of man, the Strongylus roundworms of horses, and Syngamus trachea which causes gapes in birds by obstructing the windpipe are all common representatives. Order Filaroidea. — This is a completely parasitic order, modified for living in such tissues as lymph, blood, connective tissue, and muscle of chorda te animals, and transmitted by certain insects. Two distinctive characteristics are: (1) lack of bulb on esophagus; (2) lateral paired lips or entire absence of lips. Guinea worm, eye worm, and Filaria are the common humaai parasites. Some species cause elephantiasis (Fig. 390) through occlusion of blood and lymph vessels. This disease results in enormous swelling of the ai^ected parts. These organisms are transmitted by mosquitoes. Several Filaroidea are parasites of horses and dogs. Order Dioctophymoidea. — This is another parasitic group living in the kidneys, body cavity, and alimentary canal of mammals and birds. The genus Dioctophyme includes the largest roundworms, some reaching more than three feet in length. Order Trichinelloidea. — This parasitic group has a peculiar cuticle lining the esophagus, outside of which is a single layer of epithelial cells. The common trichina (Fig. 396) and the whipworm are well- known examples. PHYLUM NEMATHELMINTHES 177 Class Gordiacea (gor di a'she a, a knot). — Superficially these ani- mals resemble the nematodes, but the fundamental structure is quite different, and therefore, it is likely proper to give them the rank of a class. They are free-living as adults but as larvae are parasitic on May flies and other insects. They leave this host and take up abode in a terrestrial form like that of grasshoppers or beetles. After complete development the adult "hair snakes'' escape into Fig. 88. — Hair snake, Gordius, an aquatic roundworm. the water of some stream, puddle, or watering trough. These females again lay eggs in the water in long strings. In the adult worm the intestine is a straight tube, often without a mouth, but opening at the posterior end by an anus. Some have no intestine at all. The outer surface of the body is covered by a cuticle. The body is cylindrical and without lateral lines, excretory organs, or circulatory system. There are four longitudinal spaces or sinuses 178 TEXTBOOK OF ZOOLOGY in the parenchyma; in the adult female the two lateral spaces are lined with peritoneum. The nervous system is composed of a mid- ventral cord with a nerve ring at the anterior end. There are rudimentary eyespots and scattered sensory cells over the body. Fig. 89. — Structure of Acanthocrphala. Internal structure of the genital organs of a young female nematode, Neoechinorhynchus emydis. (From Van Cleave, Invertebrate Zoology^ first edition, second impression, published by McGraw-Hill Book Company, Inc.) Sexually the group is dioecious with the gonads opening into the posterior end of the digestive tract. Fertilization takes place within the body of the female. Gordius aquaticus and Paragordius varius are the common examples of the group. PHYLUM NEMATHELMINTHES 179 Class Acanthocephala (a kan th6 sef 'a la, thorn head) includes a group, known as "spmy-headed worms," which is absolutely para- sitic in its habits. The adults are from a few millimeters to fifty milli- meters in length and have an elongated, flattened body when found in the intestine of a vertebrate but become distended to a cylindrical shape when removed to some solution outside the body. The protru- sible proioscis is a peculiar and characteristic structure located at the anterior end of the body. It bears numerous recurved hooks or spines, and in many species it is capable of receding into a proboscis receptacle or sheath. There is no digestive tract in this parasite, and its food is absorbed through the surface of the body even though it is covered with a cuticle. A single ganglionic mass con- stitutes the central nervous system. The male reproductive organs are the testes and a set of cement glands joining the cirrus which is held in the copulatory bursa at the posterior portion of the body. The bursa is capable of eversion. The sexes are separate, but the female has no permanent gonads. Egg masses develop early and completely rupture to produce a con- siderable number of embryos in the body cavity. Finally the em- bryos are discharged by way of the uterus through the genital pore which is located posteriorly and is the only external aperture of the body. Not only man, but mice, rats, pigs, fish, turtles, and in fact all classes of vertebrates serve as hosts for these animals. ASCARIS, A REPRESENTATIVE ROUNDWORM Habitat and Behavior The animal which is frequently studied as a representative of this phylum is Ascaris lumbricoides which frequents the digestive tract of men and hogs. It is entirely dependent on its host for furnish- ing suitable food and environment. The only time this organism is at the mercy of the elements of nature is during the egg stage when it may remain potent for months or even years if it falls in an environment unsuitable for development. External Anatomy This is one of the largest nematodes, females commonly reaching a length of from eight to fourteen inches and males averaging six 180 TEXTBOOK OP ZOOLOGY to twelve inches. Males are always more slender and have a curled tail instead of the blunt tail of the female. The mouth is guarded by three lips, two in lateroventral positions and one dorsal. These lips have small papillae on their surfaces, two on the dorsal and Fig. 90.— Male and female Ascaris or eel-worm. one on each of the ventral. The shape of the body is generally cylindrical with tapering ends. The smooth surface is marked by four longitudinal lines, two lateral, one dorsal, and one ventral. The genital pore in the female is located on the ventral midline approximate!}^ one-third of the length of the body from the anterior PHYLUM NEMATHELMINTHES 181 extremity. The anus is located near the posterior tip of the body, and in the male the reproductive aperture and two penial setae or spicules are located just within this opening. Internal Anatomy The body wall is composed of the thin, outer, smooth cuticle, the epidermis, whose cells run together, and a thick layer of longitudi- nal muscle fibers, whose medial margins are rather baggy. There /?70uth circumesopfiagea/ aterc/s oyan'es ^ ov/c^ucts I \ pseudoooe/ excretort/ p/)ari/r}jc ^er7ti;a/ /x>re /r?i>€sC>/r?6 i)oc/i/ yya// Ct/7a5 tiactas deferens se/7?//7a/ ves/ch • e/acL'/atory afc/ct psei/cfocoe/ cut/c/e 1 intestine ep/c/er/T}/5\ body ivcr// ^em/'na/ ves/c/e seta/ soi recta/r? pseadocoeJ musc/e ) cutic/e inteet/'ne ej(cretoru cana/ pe/ifcfl_ o//dact setae: oyan/ ner/e oord a/7i/: 3 '/Vi/sc/e at/c/e ep/der/nh Fig. 91. — Internal anatomy of Ascaris Iwmbricoides. A, Diagram of lateral view of dissection of female ; B, cross-section of tlie midregion of the body of female ; C, longitudinal section of posterior portion of male ; D, reproductive system of female; E, reproductive system of male. (From Curtis and Guthrie, Textbook of General Zoology, published by John Wiley and Sons, Inc., modified from Leuckart.) are thickenings of the epidermis in the positions of the longitudinal lines. The excretory tubes follow the lateral lines. The body cavity of this animal is a primitive or false coelom which is lined externally by the mesoderm of the body wall and internally by the endoderm 182 TEXTBOOK OF ZOOLOGY of the intestinal wall. Ordinarily the coelom, when fully developed, is lined both laterally and medially with mesodermic peritoneum. This is the simplest type of animal in which the body cavity or coelom is found. In higher forms the outer coat of the intestine is mesodermic. The alimentary canal is quite straight and simple and lies in the dorsal part of the body cavity. There is no need for great specialization of the digestive system since the food is taken from the digested material in the intestine of the host. A contrac- tile pharynx, which acts as a pump, draws fluid into the long epithelial intestine from which it is absorbed by the other tissues. The narrowed posterior portion is the rectum and leads to the anus fertilization /Tjemi^ranC' Sfye// ze/^ade Fig. 92. — Fertilized ovum, 4., and amoeboid spermatozoa, B, of Ascaris lumbri- coides. (From Curtis and Guthrie, Textbook of General Zoology, published by- John Wiley & Sons, Inc., after Leuckart. ) at the posterior portion of the body. The two laterally located, longitudinal duets open externally by a single pore near the anterior end of the body. There is a nerve ring around the pharynx which gives off a large dorsal longitudinal nerve and a large ventral longitudinal nerve. There are usually four other smaller longitudi- nal nerves and some connectives. In the males the testis is a thread- like structure which is much coiled in the cavity. This tube enlarges posteriorly to become the vas deferens which in turn enlarges still more before reaching the aperture to become the ejaculatory duct. In the female the threadlike ovaries join the coiled oviducts which lead forward and join the two uteri. These tubes join in the vagina, which is a short tube leading to the genital pore. PHYLUM NEMATHELMINTHES 183 Reproduction and the Life Cycle The animals copulate, and at this time the spermatozoa are intro- duced into the vagina of the female to fertilize the mature ova in the oviducts. A mature female may contain as many as 27,000,000 eggs. These eggs pass from the host with the feces. Some workers have reported that each female worm in an infected host may pro- duce a crop of eggs in excess of two thousand per gram of feces. Based on this figure, the daily production is computed to be some- thing like 200,000 eggs. These eggs are so resistant that they can be successfully cultured in 1 to 2 per cent formalin, and they may be stored successfully for four years in a refrigerator. The life history is completed only in case the eggs are swallowed by a sus- ceptible host. They hatch in the small intestine of the host and then go on a ten-day journey by way of the blood stream to the liver, thence to the heart, and thence to the lungs. By burrowing out from here, these larvae make their way to the throat, esophagus, and back to the stomach and intestine. After reaching the intestine, the larval worms, 2 to 3 mm. long, grow to maturity in two to two and one-half months. They likely live a little less than a year in the host. Relations to Man Heavy infestation in man may cause severe hemorrhages and set up pneumonia that is often fatal. Anemia is often the result of such infection; in certain cases the organisms may even tangle in masses and block the intestine until surgical operation is necessary to remove them. The toxic substances from these parasites may bring on coma, convulsions, delirium, nervousness, and other similar symptoms. Drugs like chenopouium, santonin, and hexylresorcinol have been used successfully under physicians' directions as a cure. Effective sanitary disposal of fecal material is the most successful preventive. CHAPTER XIV MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA These groups are rather conveniently considered in the same chapter, because they are more or less isolated, small groups of the unsegmented worm type. MOLLUSCOIDA This is the name of a group composed of two classes, as they are treated here. It is usually considered a phylum name, but many authors prefer to give each of the classes phylum rank. The justi- fication of the latter plan may be questionable. Class Bryozoa (bri 6 zo'a — moss animals) includes a group of co- lonial animals often called Polyzoa, which are similar to colonial hydroids in their manner of growth and forms. It is true that their structure distinguishes them very readily. Nearly all bryozoans are marine, although there are a few fresh water forms. In ex- ternal appearance they resemble certain of the corals and hydroids. It was a long time after their existence was laiown that they were separated from that group. The subclass Ectoprocta includes forms in which the mouth is surrounded by tentacles and the anus is not enclosed in this area. Bugula is an example of a treelike type of this subclass. Another type is one that grows as an incrusting organism. The second subclass Endoprocta is characterized by the circlet of oral tentacles which also encloses the anus. Bugula Bugula is a common marine genus, the individuals of which are associated in a treelike colony that lives attached to some object in the water. These individuals are called zooids of which the soft parts are known as polypide. They are within the primitive coelomic cavity, the wall of which is the zooecium. The presence of retractor muscles make it possible for each zooid to be withdrawn into the vaselike part of the chitinous skeleton. There are some smaller individuals whose shape is similar to that of a bird's head and whose bodies are smaller than the other zooids. These are called avicularia, and they are found on the surface of the colony. Their 184 MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 185 movable jaws seem to serve as grappling hooks Avhich operate to keep the colony free from other small organisms and debris which may be present in the habitat. Yihramdaria (vibracula), which con- Esophaqus. -Jntestine Avicularium / jaws open Muscle to body wa// ' Muscle _run/cu/uj (Mesentery) Ooecium Fig. 93. — Bugula, a marine bryozoan, showing the structure and habit of life of two zooids from a colony. stitute another modified type of zooid, are filamentous, whiplike appendages. They are thought to be variations of the avicularian modification. 186 TEXTBOOK OF ZOOLOGY The mouth of the larger, regular zooid is located at the free end and is surrounded by a tuft of ciliated tentacles. This arrange- ment is known as the lophophore and has the shape of a horseshoe when expanded. Within, the digestive tube is U-shaped and termi- 3^ liaublt K rXUAVO'ELLA, f- PECTIMATELLA Kabi-1- " CKI^TATELLA Fig. 94. — Three forms of fresh-water Bryozoa showing the habit of life for each. (Courtesy of General Biological Supply House.) nates at the anus, which is located just outside the lophophore. The digestive tract is held in place by strands of mesenchymatous tissue extending from the wall of the coelom. Special strands of this tis- sue are termed the funiculus. The body is triploblastic and there- MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 187 fore composed of ectoderm, eudoderm, and mesoderm. The nervous system is centered in a ganglion or mass of nerve cells located in the region of the mouth and from it, nerves extend to the tentacles. Reproduction is accomplished either by budding or sexually. Ovaries and testes make their appearance either in the funiculus or in the lining of the coelom and fertilization occurs in the body cavity. The early development goes on in a modified region of the zooecium, called the broad-pouch or ooecium. When the embryo escapes, it is a free-swimming, ciliated larva which is similar to the trochophore larva found as a developmental stage of certain Annelida and Mollusca. Its form resembles certain adult Rotifera. This larva becomes attached and transforms into a parent indi- vidual, the zooid of which will form a neAv colony by budding. The branching Plumatella, which is supported by a secretion of calcium carbonate, and the slimy Pectinatella, whose skeleton is in the form of a gelatinous mass, are the two forms most frequently found in fresh Avater. These fresh water types may be developed from winter eggs, enclosed in shells, or new individuals may be produced as internal buds. These buds are called statoMasts. They are produced in autumn and may either float on the water or sink to the bottom. They withstand the rigors of winter and are stimu- lated by it. So far as is known this group has little if any eco- nomic value. Class Brachiopoda (brak i op'o da — arm and foot) is a group of marine forms, the individuals of which possess bivalve shells. For this reason they are sometimes confused with the clamlike molluscs. The brachiopods, however, have dorsoventral valves, while the mol- luscan valves are lateral. The shell is secreted by a mantle which lines the valves. The tip of the beaklike valves is penetrated by a foramen which serves as an opening for the peduncle. This fleshy organ makes permanent attachment to some object in the water. Internally, the lophophore is a conspicuous and characteristic struc- ture of this type of animal. This organ is composed of two coiled appendages which bear numerous ciliated tentacles. The cilia pro- duce water currents in the longitudinal groove and carry food particles to the mouth. The digestive tract is U-shaped and is composed of the mouth, lophophore, gullet, stomach, and ventrally directed intestine. This 188 TEXTBOOK OF ZOOLOGY tract ends blindly in many bracliiopods. The entire tube is lined internally with ciliated epithelium. A segmented, true coelom is present, but the septa are a little bit difficult to distinguish. Exten- sions of the coelom enter the arms and mantle of this type of animal. About two pairs of nephridia are connected with the coelom and serve in excretion. The coelomic cavities produce the gonads also. The sexes are distinctly separate and mature germ cells are dis- charged into the coelom, thence to nephridia and outside. Fertili- zation takes place in the water, and a free-swimming, ciliated larva hatches from the egg. Digestive gland Stomach Heart Adductor muscle I ^ophophore / i Dorsal valve ' Dorsal mantle Intestine Nephridium Muscle Mouth Pig. 95. — Diagram of a sagittal section of a brachiopod to show internal organs. (From Hegner, College Zoology, published by The Macmillan Company.) Magellania flavescens and M. lenticularis are commonly studied forms. They are entirely marine and represent an old line of ani- mals. There are relatively few modern forms in existence. The group is of little economic significance. TROCHELMINTHES The rotifers (Rotifera) are common examples of this group known as the Trochelminthes (trok el mm'thez — wheel worm). In early times they were called "wheel animalcules." There is very little difference between the trochophore larva of this group and the adult animal. Gastrotricha constitutes another small division of this group but will not be discussed in detail here. MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 189 Rotifers are plentiful in fresh water, and a few of them inhabit the sea. They are microscopic in size, and they are often associated with Protozoa. They are very resistant to adverse conditions pro- duced by drought and may be distributed in dry form. The body of a rotifer is bilaterally symmetrical and can be di- vided into head, trunk, and foot. It is covered externally by a cuticle. The so-called head is rather largely a troclial disc com- posed of various modifications of two bands of cilia over the anterior end and around the mouth. These cilia are in active motion, often creating two sets of water currents so as to resemble two rotating wheels. They are responsible for obtaining food and for locomo- tion. The mouth is located in an anteroventral position. The trunk tapers toward the posterior and contains numerous organs. At the posterior end is the tail or foot Avhich is forked or toelike in many species. Here, too, in many forms, are located some cement or adhe- sive glands which assist the animal in adhering to most surfaces. The foot as a whole serves in locomotion, pushing the animal along. The internal organs include several systems which lie in the rather extensive body cavity or false coelom. The digestive system begins anteriorly at the mouth which receives other small organisms as food. It is a cavity leading to the pharynx. Inside the pharynx is a mill-like organ or mastax, composed of chitinous jaws, which mas- ticates the particles of food. The movements of these jaws may be observed in certain rotifers when alive. A short tubular esophagus leads to the pouchlike stomach, and extending posteriorly is the smaller cylindrical intestine which leads by way of the cloaca to the anus. Nearly the entire internal surface of the alimentary canal is lined with cilia which aid the movement of the food mate- rial through it. The stomach and intestine are lined internally with endoderm. The excretory system is well developed and consists of a number of flame cells, similar to those of flatworms; and two winding nephridial ducts which lead posteriorly to a contractile bladder. This bladder is pouchlike and empties into the cloaca. (The name cloaca is applied to any cavity which serves as the posterior portion of the alimentary canal and also receives products of the urino- genital system. It opens externally by way of the anus.) The flame cells are distributed in the body wall from the anterior, posteriorly. Some authors believe that the bladder functions also to assist in 190 TEXTBOOK OF ZOOLOGY respiration by collecting the excess water and carbon dioxide. The oxygen is received into the body with water which diffuses through the body wall. A large ganglion, located in a dorsoanterior position and several nerves extending to sense organs and muscles con- - Tactile organ .Brain —Eye Tooth "Mastax Salivary gland Stomach- intestine - Vitellarium -Egg Blasendarm" _ Contractile bladder -Rectum — Anus - Foot-gland, -r\ — Pharynx Salivary gland Salivary gland Flame cell Excretory canal Fig. 96. — A common fresh-water rotifer, Philodina roseola, showing internal structure, a, dorsal view; b, ventral view. (From Hegner, College Zoology, published by The Macmillan Company, after Hickernell.) stitute the nervous system. The body wall is composed of an outer cuticle over a thin layer of ectoderm. Under this layer is the meso- dermal tissue which includes mesenchymatous cells and muscle fibers. MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 191 This group of animals is bisexual, and dimorphism (striking dif- ferences in form of the two sexes) is present. The males are usually much smaller and may even live as a parasite on the female. The males lack a well-developed digestive system and are therefore very short lived. In the female of most species there is one ovary which produces the eggs. Connected with this gonad is a yolk gland or vitellarium. In a few forms there are two ovaries with no distinct yolk gland. Rotifers may be oviparous (lay eggs), ovo viviparous, or even a few are viviparous. The eggs produced during the sum- mer are thin-shelled, of two sizes, and develop parthenogenetically. Large Egg Females Female - / Small Egg / Males Sperm Winter Eggs (Fertilized) Late SuKiner Pass Winter in thick shell Females Females / ^ Eggs'" Eggs / ^ Parthenogenesis Parthenogenesis k Svunner Many Generations Manv Generations f season I "i Females Females i i ^ Large Eggs Small Egg Fig. 97. — Life cycle of the rotifer, Hydatina. The smaller type produce males. The eggs produced during the winter are thick-shelled, produce females only, and require fertili- zation. The eggs when mature, or the young if born alive, are carried by the tubular oviduct to the cloaca and are discharged to the exterior through the anus. The less highly developed males possess a single testis in which spermatozoa are produced. In some there is a peculiar type of copulation during which the special copulatory organ composed of a protrusible cirrus seems to per- forate the body wall of the female. At this time the eggs of the female are fertilized. In oviparous forms, the fertilized eggs are usually carried in the body for a time and then discharged by way 192 TEXTBOOK OF ZOOLOGY of the oviduct. They then lie dormant and inactive in the water for a period before hatching. There is considerable similarity be- tween the adult rotifer and the trochophore larva of some annelids, Spines -4 -Hooks . Brain "Mouth (_ Alimentary canal _ Ventral ganglion — Ovary - Oviduct -Fin Genital -pore ■Anus ■ Vas deferens Seminal — vesicle • Testis Fig. 98. — Sagitta hexaptera, an arrowworm, drawn to show internal organs. (From Hegner, College Zoology, published by The Macmillan Company.) mollusks, Nemertinea and others. This resemblance has prompted the theory that the above groups are rather closely related to the rotifers. MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 193 Class Chaetognatha (ke tog'na tha — horse's mane, jaw). — These small marine worms are often called arrowworms, and they are well adapted to livmg at the surface of the ocean. Horizontal fins sup- port the animal at the surface and also make it possible for it to move about rapidly. The prehensile mouth with its bristles have given the animal the name of "bristle jaws" in addition to other names. The body is divided into three divisions : head, trunk, and tail. These are separated by septa and the coelomic cavity is separated into right and left cavities by a longitudinal mesentery. Internally is a tubelike intestine which extends from the mouth at the anterior, to the anus located near the base of the caudal fin or tail. The nervous system consists largely of a supraesophageal ganglion or brain, ventral ganglion, branch nerves, two eyespots, and other sensory organs. These animals are lacking in circulatory or excretory structure. Each individual is capable of producing both ova and sperma- tozoa, that is, the hermaphroditic condition prevails. The ovaries are located in the posterior portion of the body cavity and the mature ova are carried to the exterior by an oviduct on each side. The testes are located in the cavity of the tail portion. The sper- matozoa are discharged into this cavity and delivered to the ex- terior by a pair of slender vasa deferentia or sperm ducts, which enlarge to become seminal vesicles near the aperture. The fertilized ova become small adults without a typical ciliated larval stage. Sagitta is the best known genus of the group. CHAPTER XV PHYLUM ANNELIDA (By J. Teague Self, University of Oklahoma) The Phylum Annelida (a nel'i da, form of a little ring) comprises an extremely large group of worms characterized by (1) the pres- ence of a coelom surrounded by two layers of muscle, (2) metameres or segments, (3) a ventrally located segmental nervous system, (4) segmented, non jointed, chitinous appendages in most cases, (5) an excretory system composed of nephridia, and (6) a nonchitinous cuticle covering the body. These worms are found in almost every type of free-living habitat where moisture is present. There are many forms which live in the ocean, either swimming freely, bur- rowing in the sand, or living in especially prepared tubes. Fresh- water streams and ponds are inhabited by numerous forms of an- nelids, and moist soil is usually alive with terrestrial earthworms. From this it is evident that the phylum as a group has become adapted to many varied living conditions and comprises one of the large groups of the animal kingdom from the standpoint of num- bers. In the process of adaptation the annelids have become diver- sified in their anatomical features until only a very few characters, such as those mentioned in the beginning of this chapter, are com- mon to the entire phylum. Even then, these distinguishing features are sometimes modified until only an expert can recognize them. The Phylum Annelida may be divided into four classes: Class I. Chaetopoda Order 1. Polychaeta Order 2. Oligochaeta Class II. Archiannelida Class III. Hirudinea Class IV. Gephyrea Order 1. Echiuroidea Order 2. Sipunculoidea Class Chaetopoda (ke top'O da, hair and foot). — This class includes the most commonly known forms of the phylum. There are marine, 194 PHYLUM ANNELIDA 195 fresh-water, and terrestrial forms; and they all possess setae (chaetae), or bristlelike appendages on the body segments. The setae are chitinous and are embedded in pits of the integument. They bear muscle attachments which make them movable and therefore useful in locomotion. The coelom, which surrounds the straight digestive tract, is divided between the segments by partitions known as septae. Fig. 99. — Representative annelids. From left to right, Arenicola cristata, lug worm ; Amphitrite ornata, marine annelid with branching gills ; Hirudo medicinalis , large medicinal leech (upper center) ; Aphrodita ornata, sea mouse (lower center) ; Nereis vii-ens, sand worm or clam worm ; Lumb7-icus terrestris , earthworm or angle- worm. (Courtesy of Denoj-er-Geppert Company.) Typically, each coelomic space possesses a pair of nephridial tubules which communicate with the coelom at one end by means of a ciliated, funnellike opening, the nephrostome. The other end opens to the out- side by means of a nephridiopore. The nephridia remove nitrogenous waste materials from the coelomic cavities and from the blood. 196 TEXTBOOK OF ZOOLOGY The inner body wall of each segment is made up of an inner longi- tudinal layer and an outer circular layer of muscle. Segmental nerves which are derived from segmental nerve ganglia innervate the meta- meres and coordinate the movements of the body. The segmental ganglia communicate with each other through connections extending from one segment to the other. At the anterior end is the brain, which is composed of a suprapharyngeal and a subpharyngeal gan- glion joined together by a pair of commissures. The brain, however, has little to do with the coordination of different parts of interseg- mental and intrasegmental reflexes, so that the stimulation in one segment automatically stimulates the adjoining ones. Reactions which require immediate coordination of the whole body are controlled by three giant nerve fibers which run through the entire length of the nerve chain. The primary function of the suprapharyngeal and subpharyngeal ganglia is to relay sensory impulses. The principal vessels of the circulatory system are a dorsal one, through which the blood moves forward, and a ventral one through which the blood moves posteriorly. These are connected in the an- terior region of the body by a varying number of paired, segmental hearts or connectives. The dorsal vessel exhibits wavelike contrac- tile movements (peristaltic contractions) which force the blood anteriorly. The latter passes through the hearts, which also pulsate, then backward through the ventral vessel to the skin, intestine, and other organs. Hemoglobin is suspended in the blood plasma of some Chaetopoda ; in others, a green pigment known as chlorocru- orin is found ; in still others no known blood pigment occurs. The principal vessels and hearts have valves on their inner surfaces which prevent the blood from flowing in the wrong direction. The class Chaetopoda may be divided into two orders; namely, (1) the Polychaeta and (2) the Oligochaeta. Order Polychaeta. — The polychaetes (majiy bristles) are typically marine Chaetopoda. One of the most widely known forms of this group is Nereis virens or the clamworm, which may be studied as a representative form. It possesses many setae (chaetae) located in fleshy parapodia. In this case the parapodia with their setae con- stitute the segmental appendages. The parapodium is divided into a dorsal notopodium and a ventral neuropodium, and each surrounds a large seta, or aciculum, which serves as a point of attachment for PHYLUM ANNELIDA 197 the parapodial muscles. A dorsal and a ventral cirrus are usually- present. The notopodium and the neuropodium each have a large group of setae. The parapodia are used principally as locomotor and respiratory organs. The head of Nereis seems to have resulted from the fusion and specialization of the anterior segments. It is composed of a prnsto- mium, which bears a pair of tentacles, a pair of palps, and two pairs of eyes. The peristomium constitutes the first segment and bears four paii*s of cirri or tentacles. The pharynx is equipped with muscles by which it can be everted, and a pair of chitinous jaws which protrude Fig. 100. — External anatomy of Nereis virens and parapodium. A, anterior end and posterior end; B^ parapodium (enlarged); 1, palp; 2, terminal tentacle; 5, prostomium ; 4, eye ; 5, lateral tentacles ; 6, peristomium ; 7, segment ; 8, para- podium; 9, anus; 10, anal cirrus; XI, dorsal cirrus; 12, gill plate; 13, setae (chaetae) ; Ui, notopodium; 15, neuropodium; 16, ventral cirrus; 17, aciculum. (Courtesy of General Biological Supply House.) when the pharynx is extended. The jaws serve in capturing small organisms and crushing anything which is to be swallowed. The suc- ceeding segments are all alike except the posterior one which bears a pair of ventral ciVrt extending posteriorly. The digestive tract consists of an essentially straight tube. The mouth opens directly into the muscular protrusible pharynx, which may be everted by use of protractor muscles to form a sort of pro- hoscis. The pharynx leads into the relatively narrow esophagus 198 TEXTBOOK OF ZOOLOGY which extends through about six segments and which has a digestive gland opening into it from both sides. The remainder of the diges- tive tract is a straight intestine which continues to the last segment, where it opens to the outside. Prostomlum Prostomlal tentacles Feristomlal tentacles Parapodia Pharynx CEsophageal glands (Esophagus Intestine Nephrldla Dorsal vessel Ventral vessels Nerye-cord' Fig-. 101. — Internal anatomy of Nereis virens. (From Hegner, College Zoology, published by The Macmillan Company, after Parker and Haswell.) The circulatory system is composed principally of a dorsal and a ventral blood vessel joined in each segment by a pair of connecting vessels. The blood is forced anteriorly through the dorsal vessel and PHYLUM ANNELIDA 199 passes posteriorly through the ventral one. Its movement is effected by wavelike contractions in the walls of the dorsal vessel. It reaches the parapodia and digestive tract through lateral branches of the ventral vessel and is then returned to the dorsal one by parietal branches. Each segment of the body except the peristomium has two nephridia opening directly from the coelom to the outside. The nephridium consists of a ciliated funnel, nephrostome, and a coiled tubule which ends in its external opening, the nephridiopore. The nephridia serve to convey the excretory and reproductive products to the outside. The sexes are separate and there are gonads in all the segments except those in the anterior end of the body. The sex cells arise from the walls of the coelom and when ripe pass to the outside, fertilization taking place in the water. The fertilized egg develops into a trocho- phore larva, which metamorphoses into the adult animal. In the central nervous system there are two suprapharyngeal gan- glia dorsal to the pharynx. These are connected by means of com- missures to the suhpharyngeal ganglion ventral to the pharynx. A nerve chain, composed of segmental ganglia joined by intersegmental connections, extends posteriorly on the ventral side of the body to the anal segment. Lateral nerves from the ventral nerve chain in- nervate the various organs of the worm. Two eyes receive nervous connections from the brain and the animal is apparently able to detect moving objects. Order Oligochaeta. — The best known example of the order Oligo- chaeta is Lumhricus terrestris, the common earthworm, which is used almost universally as a laboratory specimen. Lumbricus is not as common in the Southwest as are other large forms of earthworms, (Dipocardia) but is used here as an example because it is so well known and because its features represent so well those common to the entire order. EARTHWORM The body of Lumbricus terrestris varies from six to fourteen inches in length and gives the appearance of a number of rings joined in a linear arrangement. The rings are the body segments, or meta- meres, and vary in number up to 175. In the adult the number of segments from the anterior end to the posterior end of the clitellum 200 TEXTBOOK OF ZOOLOGY remains constant, while the number posterior to this varies. This is because growth is accomplished by the addition of segments poste- rior to the clitellum. The prostomium is a sort of knoblike lobe at the anterior end, projecting out over the mouth. It is not considered a true meta- mere. The first segment is incomplete due to the opening of the Pros'f'omi'u/n XXVI- xxxrr-i xxxvir- Openjngf ofoviducf ^Open!n0 of "^Mpis deferens ^Sem/'ncfl % groove zm , C//fe//u/7i ^Sefae A'lus Fig. 102. — External anatomy of earthworm, ventral • view, segments in roman numerals. (Prom Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.) mouth through its ventral side. In studying the earthworm it is customary to number the segments with Roman numerals beginning at the anterior end. This simplifies the study because external as well as internal structures are definitely related to specific seg- PHYLUM ANNELIDA 201 ments. The openings of the oviducts through -which the eggs pass to the outside are seen as minute pores, one on each side of segment XIV. The pores of the seminal receptacles occur in pairs, one pair in the groove between segments IX and X, and one between X and XI. The openings of the vasa deferentia (sperm ducts), which convey sperms to the outside, are located, one on each side, in the anterior part of segment XV. In sexually mature worms, segments XXX, XXXI, or XXXII to segment XXXVII are swollen to form the clitellum, a sort of saddle-shaped structure, the function of which is to secrete the cocoon in which eggs are deposited during reproduction. Each segment except the first and last bears four pairs of chitin- ous setae. They are fine, stiff bristles which may be located by passing the hand lightly over the worm. They are moved by protractor and retractor muscles and serve to help the worm move through the soil. A pair of nepliridiopores (the external openings of nephridia) is situated on the posterior ventral side of each seg- ment except the first two or three. The body of the earthworm is covered by a thin, transparent cuticle which is secreted by the epidermal cells just beneath it. It contains numerous minute pores through which secretions of the unicellular glands beneath are poured and through which gaseous exchanges between the blood and moist soil can take place. It serves also as a protection against physical and chemical injury to the animal's body. Internal Anatomy The body of the earthworm, if cut open along the mid-dorsal line, gives the general appearance of a tube within a tube, the digestive tube being the inner one and the body wall the outer one. The space between them is the coelom. The constricted regions dividing the segments on the outside correspond to the positions of the septae which divide the coelom into separate segmental com- partments. These coelomic divisions communicate with each other by means of pores in the septae so that the clear fluid which fills the coelom can circulate freely. The septae are absent between segments I and II and incomplete between segments III and IV, and XVII and XVIII. The walls of the coelom are lined by a thin layer of cells known as peritoneum (mesothelium). 202 TEXTBOOK OF ZOOLOGY Reproductive Organs The earthworm is hermaphroditic, the organs of both sexes being present in every animal. The seminal receptacles, oviducts, and ovaries are female organs, and the testes and seminal vesicles are male organs. The seminal vesicles are three pairs of light-colored bodies located in segments IX, XI, and XII. In sexually mature individ- uals they may extend back through the septae as far as the fifteenth segment. If their contents are examined with a microscope, they Fig. 103. — Diagram showing reproductive system and nervous system in seg- ments VIII to XV of an eartliworm. The seminal vesicles have been cut away in somites X and XI to disclose the testes and sperm funnels, es, egg sac ; nc, nerve cord ; ov, ovary ; sf, seminal funnel ; sm, septum between two somites ; sp, sperm duct (vas deferens) opening in the fifteenth somite; sr, seminal receptacle; sv, seminal vesicle; t, testis; vd, oviduct. (From White, General Biology, published by The C. V. Mosby Company.) are seen to contain the various stages of developing spermatozoa coming from the sperm mother cells. The testes are the two pairs of very minute bodies projecting into the seminal vesicles in seg- ments X and XI and cannot be seen without first removing the dorsal part of the seminal vesicles. The union of the vasa effer- PHYLUM ANNELIDA 203 entia coming from the vesicles on each side forms a single pair of vasa deferentia in segment XII. The seminal receptacles are pairs of small white bodies located in segments IX and X. The ovaries are two minute bodies located one on each side of segment XIII. Digestive System The mouth cavity extends through segments I to III and leads into the bulbous, muscular pharynx which extends through segment V. The pharynx plays the part of a sucker in securing food for Fig. 104. — Diagram of dorsal dissection of an earthworm in region of segments I to XXI. be, buccal cavity ; eg, calciferous glands ; cr, crop ; dv, dorsal blood vessel ; eo, esophagus ; g, gizzard ; n, nephridium ; sb, subpharyngeal ganglion ; st, stomach-intestine ; pe, peristomium ; II-XXI, somites ; pJi, pharynx ; pr, prostomium. (From White, General Biology.) the animal. The esophagus is a straight narrow tube extending from the pharynx through the fourteenth segment. In segments X to XII three pairs of yellow lateral pouches open into it. These are the 204 TEXTBOOK OF ZOOLOGY calciferous glands, the secretions of whicli help to neutralize the acid organic matter taken as food. The esophagus opens into the crop, a larger, thin-walled structure, which extends through segment XVI. This is followed by the muscular gizzard in segments XVII and XVIII. A thin-walled intestine extends to the anus, which opens to the outside through the last segment. The intestine is not a simple tube but has a large fold, the typhlo- sole, protruding into its lumen from the dorsal side giving it more absorptive surface for the assimilation of food. The coelomic side of • 3 xy> *5 -p c o O C .2 <4> O 1 o o .2 -3 o o o to C o '«o O bo O c c XiO^ tiP^Ptn 5 r5 I c o C 4J 1 3 o > £ O >> I- o .<2 X § ^ u ^ ^ 1 1 8 2 (0 D. c g -12 s .'S o •Of o .£ <£: ,^-~ <-^ — ^ -^ o ft) d t c (\) u S ■^7 <3 -Q o "Td cy 00 o o e 05 a. S B Si o ■*-> 03 0) U5 rijriiliiiK'v..,! down upon it and remain attached if the fish is the suitable host for the particular species of clam. The tissues injured due to the attachment of glochidia produce by proliferation new cells which I PHYLUM MOLLUSCA 255 group up around and eventually cover the parasites. Thus a cyst is produced about the glochidium and within this structure the larval clam undergoes metamorphosis. It shortly breaks loose from its host, drops to the stream or pond bed, and leads an independent life. The rapid dissemination of mussels in a river system can be accounted for by the movements of their fish-hosts. Economic Relations of the Phylum Mollusks have been used as food by man from the beginning of civilization. Oysters, clams, scallops, snails, and the arms of cuttle- fish are found in the menus of peoples all over the world. It has been estimated that the oyster industry along the Atlantic Sea- board approximates 40,000,000 dollars annually. Along the Texas coast alone. Federal statistics show that 51,719 barrels of oysters were sold in 1932. Buttons are made from the shells of the large heavy river clams and along the Ohio, Missouri, and Mississippi rivers the button industry amounted to 5,000,000 dollars in 1931. Within some of the clams are found pearls which are formed by some irritating particle, such as a parasite or sand grain that be- comes lodged between the mantle and the shell. Iridescent protec- tive layers of mother-of-pearl are deposited around the foreign par- ticle, thus producing the pearl. The Japanese have been success- ful in artificially stimulating pearl production by planting small objects, such as pieces of mother-of-pearl, between the mantle and shell of pearl-oysters. Pulverized clam shells are also being used as a calcium supple- ment to chicken feed. Shells have also been used as a medium of exchange. The wampum of the eastern coast of North America consisted of strings of cylindrical beads made from brightly colored clam shells. Shells have always been and still are used for orna- mentation. Crushed shells are used in road construction. Some mollusks are injurious to human interests. Among these might be mentioned the marine snail, Urosalpinx cinerea, which drills into and feeds on oysters and other pelecypods; the common shipworm, Teredo navalis, attacks the wood of ships and pilings, making extensive excavations. Certain species of snails serve as the intermediate host of parasitic flatworms or flukes. The liver fluke 256 TEXTBOOK OF ZOOLOGY (Fasciola hepatica) whose intermediate host is the small fresh-water snail, Lymnaea huUnioides, causes the disease, liver rot in livestock, particularly in the sheep of the Southwest. Since shells are easily fossilized they serve as excellent guides to the geologists in determining the type of rock formation and relative age of the strata, CLASSIFICATION Classification of this phylum is based on the nature of the foot, and respiratory organs; shape and structure of the shell; arrange- ment and structure of the nervous and reproductive systems. Class I. Amphineura Includes the Chitons, which are found abundantly on rocks between tide marks along the Atlantic and Pacific Coasts. This class ap- pears to be the most primitive in the phylum, and its members have departed least from the ancestral condition. Bilaterally symmetrical body; tentaculess head, eyes absent; shell, if present, consists of eight overlapping plates. Most species have a flattened foot but other species are slender and wormlike Ischnochiton con- spicuus. Class II. Pelecypoda Includes the bivalve moUusks, such as the oysters, clams, scallops, and cockles. More than ten thousand species have been described, of which approximately four-fifths live in the ocean. Division of the class into orders is based on giU characters. Order 1. Protobranchiata Marine species; gills consist of short, flattened leaflets; dis- tribution along the Atlantic and Pacific Coasts. Order 2. Filibranchiata Marine species; gills composed of long filaments which hang down into the mantle cavity. The edible scallops and the sea mussel, Mytilus, exemplify this order. Order 3. Eulamellibranchiata Fresh-water and marine species; with two platelike gills which hang down into the mantle cavity on each side of the foot. Family 1. Unionidae Fresh-water clams or mussels; shell large or relatively large; valves equal and umbo anterior to center. Family 2u Sphaeriidae Fresh-water species. Shell small; umbo median or pos- terior to middle of shell. PHYLUM MOLLUSCA 257 Order 4. Pseudolamellibranchiata Marine species; gills plaited into vertical folds; shell fre- quently inequivalve. The oyster (Ostrea) and Pecten illus- trate this order. ischnochiton Dental ium Loliqo brevipennis Polypus bimaculatus (Octopus ) Fig. 146. — Repre-sentatives of three classes of mollusks. Class Amphineura, Ischnochiton; Class Scaphopoda, Dentalium ; Class Cephalopoda, Loligo brevipennis (squid) and Polypus bimaculatus (octopus). Class III. Gastropoda Includes the snails and slugs. Approximately fifty-five thousand species have been discovered and described. Shell, if present, uni- valve. 258 TEXTBOOK OP ZOOLOGY Order 1. Prosobranchiata Mostly marine, but fresh-water and land forms are repre- sented. As the name implies, the gills are situated in the mantle cavity anterior to the heart. This order embraces such animals as the limpets, abalones, and periwinkles, all of which live in the sea; also a few fresh- water genera, such as Goniohasis, Campeloma and Pleurococera; Helicina orbicu- lata, a terrestrial southern species which is frequently arboreal in habit, comes under this order. Order 2. Opisthobrmichiata Strictly marine. Gills, when present, are situated jjosterior to the heart; shell, if present, small. Includes the sea slugs. In the sea butterflies (pteropods), the foot may be modified into two fins which are used in swimming. Some of the heavier types have broad cephalic discs, adapted for burrow- ing in the sand. Many are found in coral beds and in sea- weeds, their vivid colors harmonizing with the background. Order 3. Pulmonata Mostly terrestrial and fresh-water snails. Gills are absent, the mantle cavity serves as a pulmonary sac; shell usually present, sometimes rudimentary or absent. Suborder 1. Basommatophora Fresh-water species; eyes located at base of tentacles; external shell present. Includes the families Lym- naeidae, Physidae, Planorbidae and Ancylidae. Suborder 2. Stylommatophora Terrestrial snails and slugs; stalked retractile eyes, and one pair of retractile tentacles; shell in form of elevated or depressed spire, rudimentary and concealed, or absent. Class IV. Scaphopoda Marine. Mantle edges grown together along ventral side forming tube, with a shell of same shape and open at both ends. Commonly known as tooth shells. Approximately 300 kno-u-n living species. (Dentalium.) Class v. Cephalopoda Marine. The most highly organized of the mollusks. A definitely formed head is present which bears a pair of eyes that superficially resemble the eyes of vertebrates. The foot is modified into arms or tentacles. They are carnivorous animals and many of them are used as food by man. (Nautilus, Loligo, Polypus.) Order 1. Tetrabranchiata The chambered nautilus (Nautilus) is a representative of this order. The animal inhabits the last chamber of a flat- tened spiral calcareous shell. As the name Tetrabranchiata implies there are four gills; also four primitive kidneys and I t PHYLUM MOLLUSCA 259 ■ Si ph uncle 1 __ - ' ^ Jepta Fig. 147. — Sectional view of internal structure of Nautilus. Cyrtoceracone Orthoceracone Qyroceracom Goniatlte Ceratite Ammonite Fig. 148. — Evolution of the cephalopods. 260 TEXTBOOK OF ZOOLOGY four auricles; ink sac absent. This suborder reached its peak of development in the Silurian and Devonian periods and is one of the most clear-cut examples of evolutionary development in the invertebrates. During the Ordovician period the cephalopods constituted one of the chief groups of marine animals. Even though at that time cephalopods with coiled shells existed, the predominant ones were the orthocones (those with straight conical shells). This latter group in all probability gave rise to the entire series of coiled shells, culminating in Nautilus. In all nautiloids a series of partitions, termed septa, extend the full length of the shell. The point of union with the septa and sides of the shell may appear as a straight, curved, angulate or highly complex line. This line is called the suture and in fossil shells whose outer shell coating is lost, it stands out rather conspicuously. The suture line is used as a taxonomic character for the group. Order 2. Dibranchiata Octopods and squids are representative types. Shell internal or absent; two gills and two primitive kidneys; ink sac pres- ent; mouth surrounded by 8 to 10 tentacles which are furnished with suckers. This order includes the largest of all moUusks, the giant squid (Architeuthis princepsj which may attain a total length, including arms, of over fifty feet. | The squids and octopods are noted for their ability to change color by the rapid contraction or expansion of chromatophores in their skin. Their juovements are rapid and are produced by expelling water from the mantle cavity through the mus- cular siphon with such force that the animal is jerked back- ward. In the squids, fins along the sides of the body facilitate locomotion. Loligo hreviyennis is the small squid found along the Gulf coast. Wheu taken out of the water it is usually a mottled red or tan. The visceral mass and mantle cavity are enclosed by a thick mus- cular mantle. Beneath the skin along the back is a primitive endo- skeleton in the form of a feather-shaped shell. The squid is preda- tory, feeding on almost any animal it can capture. Within the pharynx are two large jaws moved by powerful muscles. The pharynx connects with an esophagus which in turn terminates in a muscular stomach. Digestive juices from the liver and pancreas are emptied into the stomach, and after the food is partially di- gested, it is passed into a thin-walled cecum where digestion is com- pleted and absorption takes place. Wastes are discharged through the anus which opens near the base of the siphonal fold. The blood PHYLUM MOLLUSCA 261 (. Sucker Hectocotylhed arm. Cartilage Siphon Anus Muscle Esophaaus Rectum Inkiac Anb. aorta 5yitemic heart— - Pen ^ Lt. post cava J Spermabophoric sac rl\ stomach :-il Pen - Skowach- po'uch -Cub edge of body wall .Fir) Fig. 149. — Dissection of squid to sliow internal anatomy. 262 TEXTBOOK OF ZOOLOGY system, which is closed, is composed of arteries, veins, and two branchial hearts. Blood is oxygenated in two feathery gills which project into the mantle cavity. The two light-colored triangular kidneys are situated anterior to the branchial hearts and discharge their contents through small papillae, one located on each side of the intestine. In squids the sexes are separate. The male repro- ductive system is composed of a testis, vas deferens, spermatophoric sac, and penis ; the female system consists of an ovary, oviduct, ovidueal gland, and nidamental gland. -- Cornea Eyelid Iris Lens — Ciliary M. ^ Retina Optic (ganglion Fig. 150.- -Longitudinal section through eye of squid. (Redrawn and modified after Borradaile and Potts by permission of The Macmillan Co.) The nervous system of cephalopods shows a high degree of spe- cialization when compared with the nervous system of other mol- lusks. The "brain" is composed of a close association of ganglia around the esophagus and is protected by a capsule of tough tissue resembling cartilage. Nerves radiate out from the central nerve mass to the various parts of the body ; some of the nerves terminate in large ganglia, such as the stellate ganglia in the mantle. The eyes of the squid are supported by pieces of "cartilage" and are relatively complicated. Statocysts, which are similar but more complicated than those described for the clam, are situated near the brain mass. Ciliated pits which are supposed to be olfactory in function open in the form of a slit just back of each eye. CHAPTER XVIII PHYLUM ARTHROPODA Arthropoda (ar throp'O da, joint foot) is the name of the largest known group of animals. As the name implies, all representatives of the phylum have paired, jointed appendages and a definite tendency toward specialization of them. Their bodies are triplo- blastic, segmented, bilateral, and covered by a chitinous exoskele- ton. The coelom is modified by a marked reduction as a result of specialized vascular spaces. The segmentation or metamerism of the body is expressed in a high degree in this phylum and there is a definite relation of appendages to segments. The segments have undergone greater specialization and greater regional differ- entiation than was the case in annelids. In forms where there is little or no differentiation of segments, the condition is referred to as homonomous, while a highly differentiated condition of segments as found in most arthropods is spoken of as heteronomous. This group has fairly distinct head, thorax, and abdomen. The append- ages on various segments are typically homologous with each other. Some are modified as sense organs, others as mouth parts, others for walking, swimming, and reproduction. The skeleton is entirely exoskeletal, composed of chitin, and fits exactly the shape and contour of the body. Since it is fairly un- yielding to growth, it becomes necessary for the arthropod to shed the skeleton periodically during its growing periods. This molting or ecdysis, as it is called, is quite characteristic of many of the divi- sions of this phylum. The circulatory system is of the ojyen type, since there are large sinuses or spaces surrounding most of the organs instead of a con- tinuous circuit of blood vessels. The nervous system is of a modified ladder type with a ventrally located cord. The digestive system shows specialization in that it is divided into distinct regions as an adaptation to special types of food which require mastication. Classification This phylum is divided into two sections and at least five classes ; some authors recognize as many as eight. The sections are deter- mined according to the means of respiration. . 263 264 TEXTBOOK OF ZOOLOGY Section I. Branchiata (brankia'ta, gill) gill-breathing, aquatic forms for the most part. Class I. Crustacea, craj^fish, crab, pill bug, barnacle, water flea, etc. o o m oi a> > u "a5 S§ 0) o OS >.a £^ P EI) CS O Q) O *M P O ■-< ?>> > m o dB u cS I" OJfl Q > S 1 ■■2 5 ci3!>» C - p-> S.C lU 02 02 -I (DC at, ''i o ^^^^■s:^ «=" Subclass Entomostraca, fairy shrimps, water fleas, and barnacles. Order Branchiopoda, fairy shrimp (Branchipus), water flea (Daphnia) . Order Ostracoda, Cypris. PHYLUM ARTHROPODA — CLASS CRUSTACEA 265 Order Copepoda, cyclops, fish louse (Argulus). Order Cirripedia, goose barnacle (Lepas), rock barnacle (Balanus), Sacculma (Fig. 404). (Some authors prefer to rank Branchiopoda, Ostracoda, Copepoda, and Cirripedia as subclasses, thereby dispensing with Entomostraca.) Subclass Malacostraca, pill bugs, sow bugs, sand fleas, lobsters, craj^fish, and crabs. Order Isopoda, pill bugs and sow bugs. Order Amphipoda, sand fleas and beach fleas. Order Decapoda, crabs, crayfish, lobsters, and shrimps. Section II. Tracheata (tra ke a'ta, rough) both terrestrial and aquatic arthropods which breathe by tracheae, book lungs or book gills. This section is divided into three divisions depending on the primitiveness of the characteristics. Division A. Prototracheata. The primitive form with some arthropod characteristics and certain annelid features, such as nephridia. Class II. Onychophora, Peripatus, the wormlike arthropod. Division B. Antennata. More highly specialized forms with one pair of antennae. Class III. Myriapoda, centipedes and millepedes (thousand legs) having one or two pairs of appendages on each segment. Order Chilopoda, centipedes. Order Diplopoda, millepedes. Class IV. Insecta, beetles, bees, locusts, etc., all with three pairs of thoracic appendages and most of them with wings. Order Thysanura, silver moth. Order Collemhola, springtails. Order Ephemerida, mayflies. Order Odonata, dragonflies and damsel flies. Order Plecoptera, stone flies. Order Emhiidina, embicls. (Texas, California, Florida.) Order Orthoptera, crickets, grasshoppers, roaches. Order Isoptera, termites or "white ants." Order Dermaptera, earwigs. Order Coleoptera, weevils and beetles. Order Strepsiptera, stylopids (parasites in insects), Order Thysanoptera, thrips. 266 . TEXTBOOK OF ZOOLOGY Order Corrodentia, book lice. Order Mallophaga, bird lice. Order Anoplura, body lice ("cooties"), crab louse. Order Hemiptera, true bugs, as squash bug. Order Romoptera, plant lice, scale insects, cicadas. Order Neuroptcra, aphis lions, ant lions. Order Trichoptera, caddis flies. Order Lepidopiera, butterflies and moths. Order Mecoptera, scorpion flies. Order Dipt era, true flies, mosquitoes. Order Siphonaptera, fleas. Order Hymenoptera, wasps, ants, bees. Division C. Arachnoidea (ar ak noi'de a, spiderlike). A group without antennae but Avith tracheae, book lungs or book gills, and four pairs of thoracic appendages. Class V. Arachnida, spider, mite, scorpion, king crab, etc. Order Scorpionida, scorpions. Order Pedipalpi, vinegarroon and tarantula. Order Pseudoscorpionida, book scorpion. Order Phalangida, daddy longlegs or harvestmen. Order Palpigradi, one Texas species. Order Araneida, spiders. Order Acarina, ticks and mites. Order Xiphosura, king crab or horseshoe crab. This summary of the classification of the phylum has been placed early in the chapter in order that the student may realize the mag- nitude of its size and the great variety of animals included. The number of species described under the phylum is approximately one-half million, and there are large numbers still undescribed and unnamed. CRAYFISH OF CLASS CRUSTACEA Since this animal represents a relatively simple type of arthropod and is so generally well known, it serves ideally as a representative species for a more detailed study. The genera Canibarus and Potamdhius or Astacus are commonly found in the streams of North America. The former is distributed east of the Rocky Mountains and the latter on the Pacific slope. PHYLUM ARTHROPODA — CLASS CRUSTACEA 267 Habitat and Behavior For the most part crayfishes (crawfishes, crawclads, fresh-water lobsters) are inhabitants of fresh-water streams and ponds where there is sufficient calcium carbonate in solution for purposes of skeleton formation. These animals may be found moving about on the bottom, or they may be in hiding under some stone or log, or they may be in the mouth of a burrow beneath the water's edge. Some species carry air tunnels vertically from the original hori- zontal burrow to the surface of the earth and deposit mud around the opening of a tunnel. They are much more active at night than during the day. It is possible for them to walk about on the bottom of the stream or pond, moving the body in almost any direction. Their Fig. 152.— Cavibarus clarkii, the swamp crayfish, a very common species in the swamps ol: the Southern States. (Courtesy of Southern Biological Supply Co.) swimming habits are rather peculiar in that they dart backward through the water, as a result of the strong downward stroke of the tail. One stroke of the tail will carry the animal a yard and this is commonly sufficient to avoid the enemy. The daytime is usually spent in hiding under objects or in the mouth of the bur- row. Crayfishes may at times desert their aquatic habitat and go foraging out over swampy land. In some localities certain species build their burrows down to the subterranean water table right out in the fields and become important pests. Sight, touch, and chemoreception are important senses in this animal. The crayfish captures other animals, such as tadpoles, small fish, and aquatic insects, by waiting in hiding and suddenly seizing 268 TEXTBOOK OF ZOOLOGY them. The crayfish is quite well protected, due to its protective color which matches the background, its chitinous skeletal cover- ing, and its pinchers. In spite of this, they are captured by water snakes, alligators, turtles, fish (such as bass and gars), frogs, sala- manders, herons, and raccoons in particular. Many have been ex- terminated by the drainage of swamps, and by their use as food for man. External Structure The chitin-covered body is divided into cephalothorax, abdomen, and appendages. The cephalothorax is a compound division of the body including the thirteen most anterior segments and is divisible into head and thorax. The boundary between these is marked by the oblique cervical groove on each side of the region. The shell- like covering whose lateral edges are free, is known as the carapace. The portion anterior to the cervical groove is the head or cephalic portion, while the portion posterior to the grooves is the thorax. The anterior end of the cephalothorax is drawn out to almost a point, and this portion is called the rostrum. The mouth is located on the ventral side of the head portion and not at the tip of the rostrum where most people look for it. The lateral portions of the carapace are known as hranchial areas or hrancliiostegites, and they cover the gills. Their ventral edges are free. On the ventral side of the thorax between the twelfth and thirteenth segments (about the level of the fourth walking leg) of the female is a cuplike pouch called the annulus or seminal receptacle. It serves in reproduction for the receipt and storage of spermatozoa. The portion posterior to the thorax, which is frequently called "tail" by fishermen, is really the aldomen, and the tail proper is at the posterior end of this. The abdomen is divided into six typical segments and the terminal telson, which has no appendages but is often called the seventh abdominal segment. The anus is found on the ventral side of this part. The skeletal part of the abdominal seg- ment consists of: the dorsally arched tergum; a thin, overhanging lateral plate, the pleuron; and the slender ventral sternum in the form of a narrow bar extending from side to side. A thin arthro- podial membrane extends between successive sterna and allows for movement of the segments upon one another. PHYLUM ARTHROPODA — CLASS CRUSTACEA 269 A2 Endopodite Protopodite.:." External ope'ning of nephtidium. ..3 Protopodite A.l Exopodite Endopodite Mx.l Exopodite Endopodite Mp.l Epipodite Endopodite Protopodite,.,. Mp,3 Eindopodite.., M. ^.Endopodite Endopodite Exopodite Mx,2 •Epipodite Mv.2 Exopodite Protopodite Epipodite- Endopodite Protopodite k Epipodite .ChitinouB thrrada Fig. 153. — Examples of cephalic and thoracic appendages of the crayfish, ventral view. A. 1, Antennule ; A. 2, antenna ; L. i, fourth walking leg ; M, mandible Mp. 1. fir.st maxilliped ; Mp. 2, second maxilliped : Mp. 3, third maxilhped ; Mx. 1, first maxilla; Mx. 2, second maxilla, (From Newman, Outlines of General Zoology, published by The Macmillan Company, after Kerr.) 270 TEXTBOOK OF ZOOLOGY The appendages are paired, with one pair attached to each typi- cal segment. There are nineteen such pairs. They are all de- veloped on the same plan from the typical biramous (two branched) appendage. The five anterior pairs of abdominal appendages are quite typical of the primitive form except for the modification of the first two in connection with reproduction. This group is known as smmmerets or pleopods and all have the fundamental parts con- sisting of a basal protopodite composed of coxopodiie, joining the body and the hasipodite; the exopodite or lateral branch and the endopod'ite or medial branch each have many joints. The first two are much reduced in the female, but in the male the protopodite and endopodite are fused and extended to serve as an organ for transfer of spermatozoa. The posterior pair of swimmerets, at- tached to the sixth abdominal segment, are broadened into fanlike structures for swimming. They are known as uropods and have oval, platelike exopodite and endopodite. The posterior five thoracic appendages are the walking legs or pereiopods. These are uniramous due to the complete reduction of the exopodite. Each is composed of the two joints of the protopodite and five of the endopodite. Join- ing the coxopodiie (first segment of protopodite) is a sheetlike struc- ture which supports a gill and some chitinous threads. The three anterior walking legs possess pinchers or chela which are formed by the terminal segment being set on the side of the second segment. The walking legs are used in locomotion, ofi'ense, and defense. The three anterior segments of the thorax bear three pairs of biramous maxillipeds. The parts are quite typical in most respects. Each has an epipodite joining the basipodite and all except the first bear gills. These appendages are used in getting food to the mouth. | To the segments of the head are attached five pairs of appendages. Just posterior to the mouth and immediately in front of the first maxilliped are two pairs of maxillae, the second of which overlies the first. They are both leaflike and modified. The epipodite and exopodite of the second are fused to form a bladelike hailer or scaphognathite which fits over the gills and by its movement helps circulate the water for respiration. Its endopodite is slender, but the protopodite is broad and foliate. The first maxilla is reduced to a leaflike protopodite and small endopodite. The jawlike man- dible at each side of the mouth is composed of hard protopodite PHYLUM ARTHROPOD A — CLASS CRUSTACEA 271 with teeth ajid a fingerlike endopoclite, which is tucked under the anterior edge of the former. This appendage is used for chewing. In front of these are the antennae which are biramous and are some- times called "feelers." They consist of the protopodite of two parts, a long many-jointed, filamentous endopodite and a relatively short, fan-shaped exopodite. Anterior to these are the antenmiles which are biramous and feelerlike. The exopodite and endopodite are similar in these. The principle of homology is excellently illustrated by the ap- pendages of the crayfish. In general, homologous structures are those which have similar structure and similar origin but may have similar or different functions. By way of contrast, analogous struc- tures are those which, when compared, show different structure and origin but similar function. During early development each of the appendages of the crayfish is similar to all others. Some become modi- fied with development. Other illustrations of homologous structures are the human arm and the bird 's wing. In organisms like crayfish where the appendages of successive segments are homologous to each other, the condition is spoken of as serial homology. Homologous stiiTctures are found in many animal groups and are used in establish- ing relationships. It ha.s been suggested that the parapodia of Nereis represent possible forerunners of crustacean legs. They are both typi- cally biramous and both take about the same position on the body, as well as having a similar segmental distribution. There is also con- siderable similarity in their structure. Internal Structure Beneath the shell-like, chitinous exoskeleton there is a very rep- resentative set of systems. As in most higher animals the segmen- tation is retained in the muscular system, nervous system, and to a degree in the circulatory system. Earlier in the chapter it was pointed out that the coelom is modified as a provision for increased blood sinuses which have occupied much of the space. Respiratory System.^ — Under the branchial areas of the carapace may be found the paired, feathery gills held in the gill cavity or branchial chamber. There are three types of gills present here: pleurohranchiae, attached to the sides of the thorax; podohranchiae, arising from the epipodites of the thoracic appendages; and arthro- 272 TEXTBOOK OF ZOOLOGY branchiae, which arise from the coxopodites of the thoracic append- ages. Several of the segments have lost the pleurobranchiae. The scaphognathite moves in such a way over the external surface of the gills as to move the water in an anterior direction. The water is brought under the free edge of the branchiostegite or branchial area of the carapace and moved forward to be discharged by an anterior aperture. An almost constant stream of water is pumped over the gills to facilitate the exchange of oxygen and carbon dioxide between the blood in the capillaries of the gills and the surrounding water. The aerated blood is then carried to all of the tissues of the body. Carapace removed Hasc/e ^ Ventral thoracic artery Wntral sinus Pericardial sinus Heart Ostium hasck Gonad Intestine Digestive $land Efferent vessel Gill Nerve cord Carapace Fig. 154. — Diagram of cross section throug-h the posterior thoracic region of a crayfish. Arrows indicate flow of blood. The digestive system is in the form of a modified canal and is composed of mouth, esophagus, stomach, and intestine. The mouth opens between the mandibles on the ventral side of the third seg- ment. From this the short, tubular esophagus leads dorsally and joins the ventral side of the stomach almost directly above the mouth. This larger anterior portion of the stomach is the cardiac chamber. Within its wall are a number of hard chitinous bars, known as ossicles, which bear teeth capable of mastication of food when moved over each other by the muscular activity of the wall. This grinding apparatus is known as the gastric mill. Between the cardiac chamber and the posterior or pyloric chamber is an arrange- PHYLUM ARTHROPODA — CLASS CRUSTACEA 273 meut of bristles which serve as a strainer that allows only properly masticated food to pass through. The pyloric chamber is consider- ably smaller and curves downward to continue posteriorly as the tubular intestine which extends almost directly posteriorly through the center of the abdomen to the anus in the last segment. Large digestive glands (hepato-pancreas) lead into the pyloric chamber through hepatic ducts. The secretion of these glands contains diges- tive enzymes. The vascular system consists of a heart, the pumping organ; the arteries, definite vessels; the sinuses, a series of blood spaces; and the Nood which circulates. It consists of the fluid plasma containing 5 6 7 g fl>. ^P^t^Jil'I'ern bunch? tn liberate her efcgs (drawing by C. H. Kennedy) ; 5, nymph ot wesiern "uncii gni'^%L™fcciden^aZis/6f nymph of '■little ^^'^]^:Cvrr^''VZ'%hvistensen 7 adult Of the "big curler." Pteronarcrjs sp. (From Needham and Chnstensen, permission Utah Agricultural Experiment Station.) 320 TEXTBOOK OF ZOOLOGY perniciosus Comst. ; the purple scale, Lepidosaphes becki (Newman) ; and the pine scale, Chionaspis pinifoliae Fitch are important scale insect pests. The Cicadellidae or leaf hoppers are represented by the following insect enemies : the sugar-beet leaf hopper, Eutettix tenellus (Baker) ; Delong's leaf hopper, Empoasca filomenta DeL. ; and the grape leaf hopper, Erythroneura comes (Say). Insects of this order are all plant feeders, and they are very numerous ; over sixteen thou- sand species have been described (Fig. 183). Order Isoptera. — More than five hundred species of termites, often wrongly called white ants, have been named. Termites are white, soft-bodied, mandibulate insects. They feed principally upon wood, and in the tropics they are one of the most destructive insects known. Termites are social in habits, forming large colonies which are used for years and contain as many as five hundred thousand to a million individuals. The Nevada termite Termopsis nevadensis (Hagen) ; and the western termite, Iteticulitermes Jiesperus Banks, are common and destructive. More will be said of these insects under the dis- cussion of social insects, later in this chapter. Order Thysanoptera. — Thrips are very small insects, not more than two to three millimeters in length. They are mostly plant feeders, sucking the juices from the plants. The banded thrip, Aeolothrips fasciatus (L.) and the onion thrip, Thrips idbaci Lindeman are com- mon insect pests. About five hundred species of thrips are known. Order Dermaptera. — -The earwigs are small terrestrial, mandibulate insects with a pair of forcepslike appendages at the tip of the ab- domen. The winged species have a short leathery anterior pair of wings which resemble the elytra of some beetles. The small earwig. Labia minor (L.) ; and the toothed earwig, Spongovostox apiceden- tatus (Caudell) are species commonly found in the western United States. Hemimetabolous Insects With Incomplete Metamorphosis Order Odonata. — The dragonflies and damsel flies are insects with large compound eyes, mandibulate mouth parts, four membranous wings that are finely veined, and a long slender abdomen. The naiads are aquatic and possess a labium which has been highly modi- fied. It can be greatly extended for the catching and holding of PHYLUM ARTHROPODA — CLASS INSECTA 321 prey. The adults are swift flying, brightly colored, predaceous in- sects. Their food consists of mosquitoes, gnats, and many other kinds of flying insects. Much has been written on the dragonflies of the United States. About twenty-eight hundred species have been de- scribed. The order is divided into the suborders, Zygoptera (damsel flies), and Anisoptera (dragonflies). There are two families of damsel flies, the Agrionidae and CaenagTionidae ; also two families of dragonflies, Aeschinidae and Libellulidae. The beautiful ruby spot, Hetaerina americana Fabr; and the stalked-winged, Lestes uncatus Kirby are damsel flies that are widely distributed. The dragonflies Lihellula pulchella Drury and Sympetrum ruMcund^dum are common west of the Mississippi River in the United States (Fig. 184). Order Ephemerida. — The Mayflies are aquatic insects, with man- dibulate naiads, but since the adult stage lasts but a day, the mouth parts are vestigial. The adults have well-developed wings and two or three long abdominal cerci. The life cycle occupies from one to three years. The food of the naiads consists of small aquatic plants and organic matter which is obtained from the rocks and mud on the bottom of streams and along the shores of lakes where they live. They serve as food for larger insects and fishes. The prickleback, Ephemerella grandis Eaton; and the western bunchgill, Siphlurus occidentalis Eaton are common species. Order Plecoptera (Stone flies). — The stone flies are found near streams, flying low over the water. They have mandibulate mouth parts, four wings that are not so thickly netted with veins as are the Odonata, but with longer antennae than the Odouata. They are found on stones along lakes and streams where they pass their naiad stage. They require running water that is well aerated. Their food consists largely of aquatic insects, such as May flies. They are sometimes used as bait for trout. There are four families : the little curler, Pteronarcella hadia Hagen ; and Perla modesta Banks are representative species. HOLOMETABOLOUS INSECTS WiTH COMPLETE METAMORPHOSIS The following are some of the important orders that fall within this group. Order Trichoptera (Caddis flies). — This order includes about eighteen hundred species of "case flies" or "rock rollers," as they 322 TEXTBOOK OF ZOOLOGY are sometimes called. The adults are less than an inch long, with well-developed wings, but with vestigial mouth parts since they prob- ably take no food. The larvae inhabit the bottoms of lakes, ponds, rivers, and creeks, and as a means of protecting their soft bodies they build cases or tubes of small rocks, shells, bits of wood, and plants. The larvae feed upon plant tissue and small animals which they capture in little nets that are placed near the entrance to their case. Pupation takes place in the water. The adults lay their eggs in the water on sticks or stones. About eighteen families are recog- nized. The species Hydropsyche partita Banks and H. scalaris Hagen of the net-making family Hydropsychidae, and Platyphylax designata (Walker) of the family Limnophilidae are common in the western states. Fig. 185.— Larva of net-making caddis worm, Hydropsyche. (From Needham and Cliristensen, permission of Utah Agricultural Experiment Station.) Order Neuroptera (Nerve Winged Insects). — This order contains the doodlebugs, lacewings, snake flies, dobson flies and mantispids. It is probably the most heterogeneous order of insects; all the species, however, have biting mouths and two pairs of net-veined membranous wings. The larvae are both terrestrial and aquatic, and feed mainly upon other insects. There are thirteen families, but probably the families Raphidiidae, snake flies ; Chrysopidae, lacewing flies ; and the Myrmeleonidae, doodlebugs or ant lions contain insects most gener- ally encountered. The lacewing or golden eyes, Chrysopa calif ornica Coquillett, is a beneficial and widespread species. It feeds in the larval stage upon aphids, thrips, scale insects, and psyllids. Order Coleoptera (Beetles). — The beetles are world-wide in their distribution and contain the largest number of species of any order in the animal kingdom. They are adapted for an almost unlimited variety of conditions, living on plants and animals, on land, and in the water. They have biting mouth parts, and the first pair of wings, the elytra, are leathery or hard. They feed on all possible kinds of PHYLUM ARTHROPODA — CLASS INSECTA 323 c. Fig. 186. — Three species of Coleoptera, A, adult Colorado potato beetle, Leptvno- tarsa decimlineata Say ; B, larva or slug of Colorado potato beetle ; C, spotted blister beetle, Epxcauta maculata Say ; D, common blister beetle, Epicmita jmncti- collis Mann. (From Knowlton and Sorenson, permission Utah Agricultural Ex- periment Station.) 324 TEXTBOOK OF ZOOLOGY food. Many species do an enormous amount of damage, while in contrast, some of the most beneficial insects are beetles. In the United States, north of Mexico there are one hundred and nine families and twenty-four thousand species recognized. Over 200,000 species from all parts of the world have been described. Some of the families which contain the most destructive species are the leaf beetles, Chrysomelidae ; the long-horned wood-boring beetles, Cerambycidae ; the click beetles, Elateridae; the June beetles, Scarabaeidae ; the metallic wood-boring beetles, Buprestidae; Fig. 187. — Alfalfa weevil, Phytonomus porticus. Above, larva ; lower left, pupa; lower right adult. (From Knowlton and Sorensen, permission Utah Agri- cultural Experiment Station.) and the weevils, Curculionidae. The following families are, in the main, very beneficial: the tiger beetles, Cicindelidae ; ground beetles, Carabidae ; ladybird beetles, Coccinellidae ; and the carrion beetles, Silphidae. The cotton boll weevil, Anthonomus grandis, and the alfalfa weevil, Phytonomus posticus, have done millions of dollars' worth of damage. Other groups of weevils of which the following are typical do considerable damage : the billbugs, Calendra mormon Chitt. ; Rhynchites hicolor var. cockerelU Pierce ; and Apion pro- dive Lee. (Fig. 188). PHYLUM ARTHROPODA — CLASS INSECTA 325 Order Lepidoptera (The Butterflies and Moths). — In the Lepidop- tera the larvae have biting mouth parts, while the adults have a highly specialized suctorial structure. The antennae are of various shapes and sizes. The two pairs of wings are covered with scales, which are highly colored in many species. Fi^. 188. — Common weevils. 1, the bill-bug, Calendra mormon Chitt ; 2, Apion proclive Lee. ; 5, the rose weevil, Rhynchites bicolor var. cockerlli Pierce. (Draw- ings by Tanner.) This is the second largest order of insects. Approximately ninety- five thousand species are recognized, of which about eight thousand are found in the United States. The order is divided into the sub- orders Rhopalocera, butterflies, and Heterocera, the moths. The larvae or caterpillars are among our most destructive insect pests. They attack the foliage and fruit of the forest, orchard, field, and garden; also, stored food and animal products. 326 TEXTBOOK OF ZOOLOGY Fig. 189. — At left, larva of Capitophorus potentillae (Walker) ; right, straw- berry leaf roller, Ancylis comptana var. fragariae (W. and R.) (From Knowlton and Smith, courtesy of Utah Academy of Sciences, Arts and Letters.) Fig. 190. — Representative of order Lepidoptera. Above, tomato fruitworm (or corn-ear worm) ; below, adult tomato fruitworm moth, Heliothis obsoleta. (From Sorensen and Knowlton, permission Utah Agricultural Experiment Station.) PHYLUM ARTHROPODA — CLASS INSECTA 327 The following are some examples of common species: the mon- arch butterfly, Danails menippe (Hubner), is widely distributed through the United States, parts of Canada, and south into the tropics. This species is typical of the family Danaidae which is one of the nine families of butterflies in this country. Fig. 191. — Insects of the order Lepidoeptera. Above, adult female moth of peach tree borer, Aegeria exitiosa; center, cocoon and empty pupal case ; below, adult male moth of peach borer. (Pi-om Sorensen and Knowlton, permission Utah Agri- cultural Experiment Station.) Some of the most destructive species of this order are among the moths. The Noctuidae (millers) is a large family of injurious species. The corn-ear worm or cotton bollworm, Heliothis ohsoleta (Fabr.), feeds upon many plants, a few of which are tomatoes, corn, the green bolls of cotton, squash, strawberries, cabbage, and at times alfalfa (Fig. 190). The gooseberry fruitworm, Zophodiu grossulariae Riley, is a pest belonging to the snout moths or Pyralididae. The clear- 328 TEXTBOOK OF ZOOLOGY wing moths, Aegeriidae, a rather distinctive family, are represented by the peach-tree worm, Aegeria exitiosa Say, a serious enemy of the peach in most parts of the United States (Fig. 191). The strawberry Fig, 192. — Life history of monarch butterfly. (Prom White, General Biology, pub- lished by The C, V. Mosby Company.) leaf roller, family Eucosmidae, is an imported species from Europe; it feeds on both wild and cultivated strawberries, blackberries, and raspberries and is found in many parts of the United States, PHYLUM ARTHROPODA — CLASS INSECTA 329 Order Siphonaptera (Fleas).— Fleas have strong jumping legs, piercing and sucking mouth parts, laterally flattened bodies, but no wings. They are world-wide in distribution; about four hundred species have been described. All of the species in the adult stage are external parasites on warm-blooded vertebrates. They are pests on cats and dogs and known to be carriers of bubonic plague. © m^^^- Fig. 193. — Life history of the mosquito. 1, mosquito eggs floating m the water (slightly magnified) ; 2, mosquito larva or wiggler ; S, mosquito pupa or tumbler; J,, adult (From Turner, Personal and Community Health, published by The C. V. Mosby Company, after Turner and Collins.) Order Dipt era (Flies and Mosquitoes). — The Diptera may be char- acterized as insects with mouth parts specialized for sucking, in some species for piercing; and with only two wings, the halters or second pair being vestigial structures. / 330 TEXTBOOK OF ZOOLOGY Many of the most useful insects are found in this order. The rob- ber flies, Asilidae; the syrphids, Syrphidae; the bee flies, Bombyli- idae; and the taehinids, Tachinidae, contain many species that are valuable to mankind. On the other hand, the mosquitoes, Culicidae ; Fig. 194. — Adult female sheep tick, Me.lophaous ovinus L,inn. (From Knowlton, Rowe, and Madsen, by permission of the Utah Agricultural Experiment Station.) Fig. 195.— Life history of the housefly, Musca domestica L. A. A, adult; B, ma- ture larva; C, pupa inside puparium ; D, eggs. (From Knowlton, Rowe, and Madsen, by permission of the Utah Agricultural Experiment Station.) the fruit flies, Trypetidae; the houseflies, Muscidae; the botflies, Oestridae; and the sheep tick, Hippoboscidae, damage food and spread disease and suffering. The larvae of some families are called maggots. Some larvae are parasitic, others predacious, or seaven- FHYLUM ARTHROPODA — CLASS INSECTA 331 gers. There are over fifty thousand species of Diptera, ten thou- sand of which are known to occur in the United States. The sub- order Pupipara is a most interesting group, containing the blood- sucking ectoparasites which live upon bats, birds, and mammals. The sheep tick is a fairly common species. Order Hymenoptera (Bees, "Wasps, and Ants). — The Hymenoptera are so named because of their membranous wings; the word hymen means membrane. In the winged species there are two pairs of wings, the second pair being smaller than the first pair. The mouth Fig. 196. — Flies. Above, Chloropisca glabra Meig. Its maggots feed upon beet root aphids. Below, adult western green-headed horsefly, Tabanits phaenops O. S. (From Knowlton, Rowe, and Madsen, by permission of the Utah Agricultural Ex- periment Station.) parts are both biting and sucking, and the females are provided with ovipositors that have become greatly modified. In the ichneu- mon flies, the ovipositor is composed of long slender bristlelike struc- tures, which are used for drilling through the bark of trees and de- positing their eggs upon insect larvae under the bark. The ants, mutillids, and bees use their ovipositors for stinging as well as for depositing eggs. The pigeon horntails bore into trees, causing con- siderable damage. 332 TEXTBOOK OF ZOOLOGY ! Many of the Hymenoptera live as parasites and are of great value in biological control work. The braconids, ichneumon flies, and chal- cid flies are examples of this group of parasites. A number of the Hymenoptera are not beneficial, since they feed upon the leaves of Fig 197. — Above, adult female Simulium vittatum Zett. (From Knowlton. Rowe and Madsen, by permission of the Utah Agricultural Experiment Station.) Below, female big-headed fly, Pipunculus subvirescens Loew. (From Knowlton, courtesy of Utah Academy of Sciences, Arts, and Letters.) our garden, orchard, and forest vegetation. There are many species that are gall makers, attacking a wide variety of plants. Many species are highly developed as far as social organization is con- cerned, thousands of individuals living in a single colony. The PHYLUM ARTHROPODA — CLASS INSECTA 333 ants, honey bees, and social wasps are examples. The Hymenoptera found in this country are divided into three suborders, twenty- eight families and about twelve thousand species. The honey bees and silkworms are the only really domesticated insects. Other Orders Other orders than the ones discussed above are included in the notable treatises on entomology. These are in the main, however, rare and little known insects. Professor Comstock in his An Intro- duction to Entomology, recognizes twenty-five orders : the Zoraptera, insects resembling termites in many respects, and consisting of but Fig-. 198. — The common wasp, or yellow-jacket, Vespula pennsylvamca Saussure. (Prom Sorensen and Knowlton, by permission of the Utah Agricultural Experiment Station.) six known species in a single genus Zorotypus; the Corrodentia, psocids and book lice; the Mallophaga, wingless ectoparasites of birds; the Embiidina, a small group of about seventy species found in the warmer parts of the world, living under stones and in the detritus of the soil; the Anoplura, the true lice, an order consisting of sixty-five species of blood-sucking parasites found on the mam- mals; the Strepsiptera, a group of small twisted- winged insects that live as parasites within the body of other insects ; and the Mecoptera, a group of about forty American species, commonly called scorpion flies, in addition to the eighteen orders discussed above. Brues and Melander in their Classification of Insects recognize thirty-four or- 334 TEXTBOOK OF ZOOLOGY ders; while Imms, the noted English entomologist, has included twenty-three orders in his A General Textbook of Entomology. In this elementary consideration of insect classification we have tried to include information and illustrations which will be of value in interesting the student in the thousands of insects of our environ- ment. Fig. 199. — Hymenoptera. Alfalfa-seed chalcis-fly, Brucophagus funetris How. A, female; B, female antenna; C, male antenna; D, eggs (greatly enlarged) ; E, anterior view of right mandible; F, larva; G, pupa, (enlarged) ; H, worker of the black ant. (From Sorensen and Knowlton, permission Utah Agricultural Experi- ment Station.) SOCIAL LIFE AMONG THE INSECTS The great majority of insects live an individual existence, with- out any cooperation or filial relationship existing between parents and offspring. The processes that have ever been operative have emphasized the importance of the individual in the scheme of prog- PHYLUM ARTHROPODA — CLASS INSECTA 335 ress. Despite this, Wheeler, the great authority on insect societies, pointed out that at least twenty-four different times communism or societies have appeared in the class Arthropoda. He reports that social life occurs in six families of Coleoptera, fifteen families of Hymenoptera, and in the Dermaptera, Embiidina, and Isoptera. Let us look at some of the ways social life has manifested itself. In the beetle family, Scarabaeidae, we find a number of species in which there is a cooperation between the male and female for the perpetuation of their offspring. A common species, Canthon sim- plex var. corvinus Harold, which the writer has ofttimes observed, rolls up small spheres of fresh cow manure, and then excavates be- neath the roll, letting it gradually down into a hole in the ground. The male helps to dig and cover over the sphere of manure upon which the female has deposited an egg. The French naturalist and entomologist, J. H. Fabre, reported many interesting observations re- lating to the preparation of manure pellets for the deposition of eggs by several different scarabaeids. Another beetle, Passalus cornutus, in the family Passalidae, lives in rotten logs. The developing larvae feed upon wood that has been prepared by the adult beetles. The colony is kept together by audible noises made by the mature beetles. The ambrosia beetles of the families Scolytidae and Platypodidae form colonies by making their burrows into the wood of both living and dead trees. Each species of beetle grows a species of fungus which is fed to the developing larvae by the adult beetles. The beetles are probably the least social of all the orders listed. No castes have been developed, and the males take but little part in colony life. In the Hymenoptera are found varying stages of social life. In the solitary wasps, the female digs a burrow in the ground which is provisioned and then an egg is sealed in the cell. No other atten- tion is given to the developing young and the new generation never knows the old. The following excerpt from a study of the nesting habits of Odynerus dorsalis Fabr. made by Mr. Edwin Vest gives a good pic- ture of the activities of this solitary wasp. '' Odynerus dorsalis is a solitary wasp in that each female builds a separate nest, yet there are often several nesting individuals in the same vicinity forming a kind of community. The labor of dig- 336 TEXTBOOK OF ZOOLOGY ging the hole for the nest and gathering the provisions is appar- ently done entirely by the female. At no time was the male seen to engage in any part of this work. After the nesting is begun the females spend the night in the burrows with the head uppermost, while the males roost upon nearby herbs or shrubs. "Their attempts at copulation are very amusing as well as in- teresting. Beginning about one or two o'clock in the afternoon the males become very active. They fly rapidly back and forth over the community usually from six to eight inches above the ground. They often alight on a female as she is working about the nest or returning to the nest with food and knock her to the ground. One female was resting on the ground when a male flew down and alighted on her back as if attempting to copulate; another male attacked with such vigor that the female flew away with still an- other male in pursuit. "The ground where the nests are made is hard, dry, and com- posed principally of clay. In order to penetrate it the female fills a thin pouchlike sac, located within the second segment of the ab- domen, with water and uses this to moisten the ground. With her mandibles she digs the dirt out in small pellets, varying in size from 2 mm. in diameter to 6.8 mm. These pellets are carried a short distance away from the hole. This work is continued until the hole is as deep as desired, the depth varying from 48 to 110 mm. There are usually one or two, rarely three, cells constructed in the tunnel for the deposition of eggs. The bottom of the hole is enlarged slightly into a cell and is made very smooth on the inside. The cell might be lined with a secretion from the body which forms a cementlike protection to the larva during the winter. The average size of the cells is 23 by 14 mm. In general they are ovoid-elliptical in shape. "Each cell is provisioned with from five to twelve Pieridae larvae. The wasp carries these larvae by grasping them with her mandibles just back of the head and supporting them somewhat with her two front legs. Desiring to learn how Odynerus handled the larvae be- fore putting them in the nest, the writer attempted to induce several wasps to pick up worms that were dropped on the ground about the nests. Favorable results were obtained in two cases. When the wasp found the worm she applied her mandibles to various places on the body but spent most of her time biting just back of the head as if PHYLUM ARTHROPODA — CLASS INSECTA 337 trying to cut it off. This is probably a part at least of the process of paralyzing the victim. These paralyzed Pieridae larvae have been kept in the laboratory in bottles for two weeks in warm weather before there began to be any change in their appearance. After that time they began to decompose rapidly. "After the cell is provisioned with the Pieridae larvae the female attaches the egg to the upper part of the cell by a short hairlike process 1.8 mm. in length with the point of attachment to the cell wall concave and about 2 mm. in diameter. Only one egg is de- posited in each cell. The cell is then sealed over by wetting the soil at the surface and then carrying it down to be moulded into an apparently air- and water-tight compartment. In order to observe this process, the writer used a small pocket mirror to reflect the light down into the hole. This did not seem to interfere with the activity of the wasp. "Most of the nests observed in this study consisted of two cells, with single-celled nests ranking second and three-celled nests third in frequency. The writer was not successful in hatching out all the individuals of any three-celled nest dug from the ground but those containing one or two cells were often hatched successfully. Of those individuals successfully reared in the laboratory it was found that in the case of the one-celled nests the individual invariably de- veloped into a female, while with the two-celled nests the larva in the lower cell always developed into a female and the upper in- dividual into a male. No successful observations were made on the three-celled nests. The facts of the case would seem to indicate that the male develops more rapidly than the female, since the egg in the lower cell is laid before that in the upper cell. It was noted that the wasp in the lower cell did not emerge until three days after the top cell had been vacated. The above condition applies primarily to two-celled nests, although it might be equally true of the three-celled types. "It is evident from this study that the eggs laid in July and August hatch and remain in a late larval instar throughout the winter. On August 2 a number of larvae were collected and placed in glass vials. During the warm weather they were kept moistened by placing a few drops of water on blotting paper covering the cells. About the middle of September they were placed in a north room of the writer's home where they were left throughout the 338 TEXTBOOK OF ZOOLOGY winter. Some of the larvae spun their cocoons in the vials while others had already done this before being removed from the ground. The room in which they were kept was cold, the temperature some- times going slightly below the freezing point of water. About the last of May the specimens were removed to the Brigham Young University where they were kept on the writer's desk. The adults emerged fully developed about the middle of July. One female was kept in a breeding cage and fed on a syrup of cane sugar and distilled water. "It is thought that under natural conditions the insects emerge somewhat earlier in the summer than was indicated by the arti- ficially reared specimens since they have been observed to be very active even during the early part of May. It seems evident that these early wasps build their nests in the spring and that their young emerge during the same season. Only the individuals nest- ing in the late summer spend the winter in the larval stage." The social wasps, belonging in the genera Vespula, Polistes, and Polyhia, of the family Vespidae, start new colonies each spring from overwintering queens. After the nests are built and the eggs begin hatching, the queen feeds the larvae until they are completely de- veloped. These workers then come to the aid of the exhausted founder of the colony by taking over the enlarging of the nest and the feeding of the larvae and the queen. The queen's only duty now is to lay eggs. It will be noted that the Vespidae attend their young by gathering food and feeding them; also that in turn the adults may feed upon the saliva of the larvae. Wheeler believes that the exchange of food in many of the social insects, which he chooses to call "trophallaxis," has been the source of the social habit. In the family Bremidae, the bumblebees also start a colony in the spring by overwintering queens seeking out an unoccupied mouse hole or some other suitable hole in the ground. The queen gathers pollen and nectar with which she fills a few cells. She then deposits an egg in each cell and waits for them to hatch and develop into workers. The workers assist in building and feeding the colony. When the winter comes on, the queen, workers, and males die, leav- ing only the females, which developed late in the summer and which hibernate, to carry on the life cycle. All this is very similar to the life habits of the social wasps. PHYLUM ARTHROPODA — CLASS INSECTA 339 In the honey bees, ants, and termites, social life is carried to its highest state of perfection. In these groups the colony is probably perpetuated for hundreds of years. Some ant and termite queens live from ten to fifteen years, building up large colonies consisting of fifty to eighty thousand individuals. Other queens take up the job of continuing the colony. A well-developed caste system, also polymorphism, is found in these social insects. In a swarm of bees there are three kinds of individ- uals, males, females, and workers. The workers are females that are undeveloped sexually. Ants and termites have many different forms of individuals in each species. In a termite colony there are many castes. The principal kinds are perfect males and females, or the royal stock, the fecund pair of the colony; a less fully developed sexual caste, with rudimentary wings; a worker caste, of fairly small, sterile, wingless individuals; a soldier caste, morphologically distinct from other individuals because of their large heads and strong jaws; and finally a caste known as nasuti, which are small individuals with the head produced into a kind of snout. Both males and females are found in the various castes of termites. There is also an interesting symbolic relationship existing between numerous intestinal protozoa and the termites. The wood eaten by the termites is made soluble by the infusoria found in their diges- tive tracts. Ants are world-wide in their distribution ; they are also very numerous as individuals and species, since about four thousand species are known today. Wheeler believed that ants are the most highly developed as well as the dominant group of social insects. The Formicidae have a highly developed caste system and usually the workers and at times the males and queens are polymorphic. Guests There are many species of insects that live in the nests of the social insects; these guests are called myrmecophiles when found with ants, and termitophiles when with the termites. Wheeler re- ports that fully two thousand species of myrmecophiles and one thousand termitophiles have been described. Many of the guests have become so dependent upon living with ants or termites that they are never found outside of the colonies. Aphids and mealy 340 TEXTBOOK OF ZOOLOGY bugs are kept as guests for the droplets of lioneydew which they excrete when stroked by the antennae of the symbiont. Dr. S. A. Forbes has reported most interestingly upon the activitiv'^s of the cornroot aphid, Aphis maidi-radids Forbes and the brown ant, Lasius niger var. americanus Emery. The little ants gather the aphid eggs in October and take care of them during the winter. In the spring before the com commences to grow, the aphids, after hatching, are placed upon the roots of smartweed and some of the grasses. As soon as the corn has started to grow the agamic female aphids are trans- ferred onto the roots. Here many generations are produced par- thenogenetically. Then in later September or October wingless males and females are produced. After mating, eggs are laid, which are gathered and stored for the winter by the ants. The ants are repaid for the care they bestow on the aphids by receiving a honeydew given off by the aphids, which they greedily feed upon. Many of the insect guests are beetles, Histeridae, Staphylinidae, Pselaphidae, and Scarabaeidae. The two histerids, Hetaerms tristri- atus Horn and H. zelus Fall are fairly common in ant nests in the states west of the Rocky Mountains. Several species of Xenodusa, members of the family Staphylinidae, are found in ant hills in the United States and Mexico. A number of species of Batrisodes and Reichenl)achia, pselaphids, and Cremastocheilus angularis LeC. and C. KnocJii LeC, scarabaeids, are found in the colonies of several of the mound ants. Some Diptera are also guests in ant colonies. ECONOMIC RELATIONS Insects attack all kinds of growing crops and plants. The de- struction of plants and their products valuable to man amounts to over a billion dollars annually. This great loss goes on because of the unabated and persistent struggle of the insects to maintain their "place in the sun." Plants are not only eaten and damaged by insects, but many plant diseases are spread by them. Animals and man suffer greatly from the attacks of insects. Many species live as endoparasites or ectoparasites on animals and man, and in so doing also spread disease. Some of the most dreaded diseases known to man are carried by insects. Because of this there has recently developed a new branch of entomology known as "medical entomology." Some of the most notable progress during V PHYLUM ARTHROPODA — CLASS INSECTA 341 the past thirty or forty years has been made in the field of medical entomology. Diseases such as malaria, yellow fever, typhus fever, African sleeping sickness, bubonic plague. Rocky Mountain spotted fever, tularemia, and elephantiasis are now known to be insect borne. Much remains to be done in this new entomological field. After man has produced his crops and harvested them for use, he finds many insects ready to take their toll from these concen- trated products. The "board bill'* of the insect pests of stored foods annually amounts to about four times the cost of all higher institutions of learning in this country. Insects belonging to the orders Coleoptera and Lepidoptera are the main offenders. The pea weevil, bean weevil, granary weevil, and confused flour beetle feed upon and damage practically all kinds of grains and seeds and their products. Much damage is also done to the same products by such species as the Mediterranean flour moth and the Indian meal moth. Practically all pests of stored foods are world-wide in their distribu- tion, which makes it difficult to ship food products long distances or store them for future use without running the hazard of insect damage. Many insects have taken up their abode with man, living upon his upholstered furniture, clothing, furs, and rugs. Great losses are suffered annually by the producers of clothing, as well as in the homes, due to clothes moths. Termites also attack the wooden parts of dwellings, even furniture and books. The tobacco beetles and drugstore beetles live upon tobacco products, home furniture, and many drugs. Useful Insects Fortunately not all insects are our enemies. Many species are allies of man in the struggle against the injurious insects, as well as in many other ways. Everyone knows that honey is produced by the honey bee and silk by the silk moth, but there are many people who do not know that certain insects produce shellac, the pigment cochineal, tannic acid, formic acid, cantharidin or "Spanish fly," inks and dyes, and beeswax. In India a small scale insect, Tachardia lacca Kerr, lives on trees and produces a secretion that forms a layer over the branches. This substance, shellac, is removed by the natives in 342 TEXTBOOK OF ZOOLOGY various ways, millions of pounds being sold throughout the world. Shellac is used for making varnishes and polishes, as an electrical insulating material, in airplane construction, and many other ways. Insects serve as food for many fishes, amphibians, reptiles, birds, and mammals, including man. It is important that insects be recog- nized as playing a major role in this connection. Without the insects the food habits of many of the vertebrates would be entirely changed. Finally, many plants depend upon insects to assist in pollenizing the blossoms. Only as the insect helps in transferring the pollen from plant to plant or from the stamens to the pistil of the same plant is it possible for some fruits, seeds, vegetables, and orna- mental plants to develop. CHAPTER XXII REPRESENTATIVE INSECTS (By Vasco M. Tanner, Brigham Young Universitt) THE LOCUST The locust or grasshopper is one of the most common insects, being known to practically all people, because very few boys and girls grow up without having some experience with a grasshopper. They are widely distributed throughout the world, living on grass and low-growing plants of the fields and open country. In the United States many destructive species are found. As early as 1743 Mr. Smith reported the damaging activities of Melanoplus atlanis in the New England states, and from 1855 to 1877 many outbreaks of grasshoppers were reported in the western United States. Even today the national government is expending large sums annually to keep down the activities of the many destructive species. The grasshopper is a typical insect, and along with the beetles and bees, to be discussed later in this chapter, may serve to illus- trate the general structure of the class Insecta. The insect body is divided into a series of rings, or segments, and the segments are made up of hardened plates. These plates are known as sclerites, and the depression between the plates is called a suture. The hardness of the plates is due to the deposition of a horny substance called ckitin. In many places two or more of these rings have gro^\Ti together, or are fused. Again, in certain regions of the body, parts of the segments may be lost. Eegardless of the amount of varia- tion in this respect, we find that the segments are always grouped into three regions, known as the head, thorax, and abdomen. The head is made up of a number of segments, which are fused together, forming a boxlike structure known as the epicranium. This boxlike piece which surrounds the eyes and forms the basis of attachment for the movable parts of the head extends down the front of the head, between the eyes, to the transverse suture, and down the sides of the head to the base of the mouth parts. The sides of the epicranium below the compound eyes are known as the genae, or cheeks, while the front of the head between the eyes is called the frons. 343 I r C.'p. U i.e. c-.-.w ■A ^' r.-cj. L 1J^V' n^.i. Fig. 200. — 1, The external structure of the grasshopper, Dissosteira spurcata. al.. Hind angle of lateral lobe ; cm., crest of the metazone ; c.p., crest of the prozone ; g., gena ; g.g., genal groove ; I.e., lateral carina of the metazone ; m.p., maxillary palpus ; t.L, transverse incision. 2, Front view of the head of the grass- hopper, Dissosteira spurcata, a.g., Antennal groove; ant., antenna; c.c, lateral carina ; c.e., compound eye ; c.f., central foveola ; e.g., carina of the antennal groove ; cl., clypeus ; c.o., central ocellus ; fas., fastigium of the vertex ; f.e., frontal costa ; g., gena ; la., labrum ; I.e., lateral carina of the fastigium ; l.p., labial palpus ; inan., mandible ; in.e., median carina of the fastigium ; in.p., maxillary palpus ; O.C., ocellus; s.e., sulcation of the frontal costa; t.f., tempora, temporal foveola; ver., vertex. (From Henderson, by permission of the Utah Agricultural Experi- ment Station.) REPRESENTATIVE INSECTS 345 The grasshopper has both compound and simple eyes. The com- pound eyes are situated upon the upper portion of the sides of the head, and are large, oval areas with smooth, highly polished sur- faces. If the eye is examined with a dissecting microscope, the surface will be seen to be made up of a number of hexagonal areas, which are known as facets. The simple eyes or ocelli consist of three small, almost transparent, oval areas. One of the ocelli is situated on the front of the head, just beloAV the margin of the impression which contains the bases of the antennae, and in contact with the upper portion of the compound eye. The antennae or feelers are two threadlike processes situated median to the compound eyes. Each consists of about twenty-six segments. On the front of the head there is a short rectangular piece, called the clypeus, which is attached by its upper edge to the epicranium, and on the lower edge to the labrum. The mouth parts consist of a number of separate parts attached to the ventral region of the epicranium. The first noticeable part is the Idhrum, or upper lip, a flaplike piece attached to the lower edge of the clypeus. The free edge is deeply notched on the median line. Just beneath the labrum are the mandibles, or first pair of jaws. Each mandible consists of a single piece which is notched on the inner grinding surface to form a number of ridges or teeth. A second pair of jaws, the maxillae, may be exposed by the removal of the mandibles. Each maxilla is composed of a number of parts, consisting of the cardo or proximal hinge part of the structure ; the stipes, the lacinia, a sclerite which bears some teeth on its terminal end; the outer lobe or galea; and the maxillary palpus. The caudal part of the mouth parts is the lower lip or laliium, which is composed of the siibmentum which acts as a hinge on the epicranium above; a mentum; labial palpi, and two large outer flaps, the ligulae (Fig. 200). The prothorax is the segment to which the head is attached. It may be divided into two regions, the dorsal part known as the pronotum and the ventral portion known as the sternum. The pronotum is a saddle or bonnetlike piece extending over the dorsal and lateral regions of the prothorax. It is made up of a fusion of four plates, which are indicated by the transverse sutures. The sternum or ven- tral side of the pronotum is also made up of separate plates, or sclerites. The anterior sclerite bears a spine on the median line. 346 TEXTBOOK OF ZOOLOGY aO S-S S « 2i S ^,j2 c o 5 o ■So-- ^t^CLj I !-o '^ I -73 y a> . cq . o3 -^^ .^1 ll fe 0) ,- REPRESENTATIVE INSECTS 347 The next two segments, the mesothorax and metathorax, are made up of sclerites that are intimately associated, and their structure will be discussed together. The mesothorax is joined to the pro- thorax by a membrane which permits of more or less movement. Posteriorly the metathorax is joined immovably with the first ab- dominal segment. The mesothorax and metathorax form a strong, boxlike structure for the support of the wing and leg muscles. Like the prothorax these segments are made up of separate plates, held together by a tough, connecting membrajie. These plates may, how- ever, be divided into three groups: the terguni, or dorsal region; the sternum, or ventral region; and the pleuron, or lateral region. On the dorsal and ventral regions of the body the sutures separat- ing the mesothorax from the metathorax are not very distinct. On the sides of the body, however, there is a very distinct line, or suture, running from the posterior border of the attachment of the second pair of legs toward the dorsal part of the body. This suture divides the mesothorax from the metathorax. The pleura of each of the posterior thoracic segments are again divided by transverse sutures, so that each pleuron consists of two sclerites. A pair of legs arises from the lateral and ventral portions of each of the segments of the thorax. Each leg is composed of five parts. The coxa is the first segment and is attached to the thorax by a tough elastic membrane. The next segment, the trochanter, is a very short piece which is hard to distinguish except in the first pair of legs. The femur is the third and largest segment of the leg, and in the case of the metathoracic leg contains the muscles used in jumping. The fourth seg-ment, the tibia, is slender, but about the same length as the femur. The last division of the leg is the tarsus which is made up of three segments, each movable with the other. The segments bear a series of pads, which terminate on the last one in a large suckerlike disc known as the pulvillus. There are two pairs of wings. The first pair or wing covers, also called tegmina, is attached to the dorsal region of the mesothorax. They are leathery in texture and do not fold fanlike over the abdo- men. They are strengthened by many veins and cross veins. The second pair of wings is attached to the metathorax. They are mem- branous, with many veins to strengthen them, and fold fajilike over the abdomen when not in use. The metathoracic wings are used in flight. 348 TEXTBOOK OF ZOOLOGY The last main division of the insect body is the abdomen. It is composed of eleven segments. The seven anterior segments are similar in both the male and female. In the male the first abdominal segment is made up of a curved dorsal shield, the tergum, which terminates just above the attachment of the third pair of legs. This piece partially" surrounds the tympanic membrane, or ear, which is a large, crescent-shaped area covered with a semitransparent mem- brane. The ventral part of the first segment, the sternum, is not attached to the tergum, owing to the large size of the attachment of the legs. The pleura are entirelj'- absent. The second to the eighth segments are all quite similar, consisting of a dorsal tergum, which extends laterally to near the ventral part of the body, where it joins the sternum. The pleura, or side pieces, noted in connection with the thorax, have been inseparably fused to the tergum. In the ninth and tenth segments the terga are partially fused together, the union of the two being indicated by the presence of a transverse suture. The sterna of these two segments are entirely fused and much modified, forming a broad, platelike piece. The eleventh segment is represented only by the tergum, which forms the termi- nal, dorsal, shield-shaped piece (Fig. 201). The cerci constitute a pair of plates attached to the lateral posterior border of the tenth segment, and extending back, past the end of the eleventh tergum. The podical plates lie directly beneath the cerci and ventral to the eleventh tergum. The anus opens between these plates, and the genital chamber lies directly below them. Attached to the ninth sternum is the subgenital plate which forms the most posterior ventral plate of the body. In the female the eighth segment resembles the other segments, except that the sternum is nearly twice as long, and known as the subgenital plate. The ninth, tenth, and eleventh segments are essen- tially like those of the male, the terga of segments nine and ten being partially fused, and tergum eleven forming the terminal, dorsal shield. The plates called cerci and podical plates are similar to those in the male, except that the podical plates are much more prominent. The ovipositor consists of three pairs of movable plates. The dorsal pair lies just ventral to the eleventh tergum and each plate is long, lance-shaped, and with a hard, pointed tip. The ventral pair arises just dorsal to the eighth sternum and resembles the dorsal REPRESENTATIVE INSECTS 349 pair. When these four pieces are brought together, their points are in contact, forming a sharp organ by means of which the fe- male bores the holes in the ground in which to deposit her eggs. The third set of plates are known as the egg guides. These are much smaller and are located median to the plates of the true ovipositor. There are ten pairs of spiracles, or openings in the respiratory system on the body of the grasshopper. Two pairs of these liplike structures are situated on each side of the thorax on the anterior margin of the pleural plates. The mesothoracic spiracle is con- cealed by the posterior edge of the pronotum. The metathoracic spiracle is located just dorsal to the mesothoracic leg, near the suture separating the two segments. There is another spiracle just dorsal to the attachment of the metathoracic leg, but this belongs to the first abdominal segment. From the second to the eighth abdominal segments there is one pair of spiracles located on the anterior margin of each segment near the union of the sternum and tergum. The spiracles are one of the most useful sets of structures for determining the segmentation of an adult insect body. This is because there are never more than eight pairs of abdominal spiracles present in any fully developed insect. Air passes through the spiracles into the tracheae and is carried to the tissues of the body. This unique system of breathing enables the insect to keep the body tissues well aerated and the carbon dioxide eliminated from the body. The circulator}^ system consists of a single dorsal tube, or heart, which extends along the length of the median dorsal part of the body. In the abdomen of the fully developed insect this vessel is divided into a number of chambers with side valves, which allows the blood to enter but not to escape, except through the vessel toward the head. Due to the pulsating of this portion of the tube, which has been called the heart, the blood is forced to the anterior part of the body where it flows out into the body cavity and slowly returns to the abdominal region. In this process the tissues are supplied with nourishment from the food materials carried in the blood. It will be noted that the circulatory system has practically nothing to do with the carrying of oxygen to the tissues. The digestive system of the grasshopper consists of a practically straight tube extending from the mouth to the anus through the 350 TEXTBOOK OF ZOOLOGY central portion of the body. The food after being ground up by the mouth parts passes into the mouth or pharynx where it is mixed with the salivary mucin and the action of the enzyme, invertase, begins. From the mouth the food is conveyed through the esopha- — \-/A 's-.e./ 5t. -A\.X H'lnt. n -to. -■R;. J.A Fig. 202. — Digestive system of Rhomaelia microptera. A, anus; O, crop; Co., colon ; G.C., gastric caeca ; Int., intestine ; M, mouth ; M.T., Malpighian tubules ; Oe., esophagus; R. rectum; Sal., salivary glands. (From White, General Biology. The C. V. Mosby Co.) REPRESENTATIVE INSECTS 351 gus to the crop and gizzard which are dilatations of the tract filling a great portion of the thorax. The gizzard is muscular and lined with chitinous ridges which strain the coarse particles of food and prevent their entering the next division of the system, the stomach. \— Ab Fig 203 Nervous system of Rhomaelia microptera. Ah., first abdominal ganglion ; C, circumesophageal commissure ; Sp., supraesophageal ganglion ; Su., subesophageal ganglion. (From White, General Biology. Tiie C. V. Mosby Co.) The food is acted upon in the stomach by the secretions of the gas- tric caeca, which are glandular bodies opening into the anterior end of the stomach. They secrete a weak acid which helps in the 352 TEXTBOOK OF ZOOLOGY emulsification of fats and the conversion of albuminoids into pep- tones. Much of the food is absorbed into the hemolymph from the stomach. Between the stomach and the intestines is a pyloric valve which permits the contents of the system to pass in only one direc- tion. In the intestine, which is divided into the ileum, colon, and rectum, absorption of food continues, especially in the ileum. Just back of the stomach many threadlike tubes enter the intestine. These tubes are the excretory organs, known as Malpighian tulules, and perform a similar function to the kidneys of higher animals. The rectum has thick muscular walls with six-surface rectal glands. The feces are expelled from the rectum to the outside of the body through the anus. Fig-. 204. — Anterior aspect of brain (supraesopliageal ganglia) of Rhomaeha microptera. (Magnified.) 1, nerve to paired ocellus; 2, nerve to eye, showing fibers to ommatidia ; S, nerve to antenna; 4 and 5, nerves to mouth parts; 6, nerve to unpaired ocellus; 7, circumesophageal commissure. (From White, General Biology. The C. V. Mosby Co.) The nervous system consists of a series of ganglia or nerve cells connected by a double set of commissures or connecting nerve fibers lying along the ventral body wall. Five ganglia are located in the abdomen. Since there are at least eleven segments in the abdomen of the adult grasshopper, it is apparent that the ganglia of some of the segments have fused together. In the larvae of insects there is usually a ganglion to each segment. Three large, well-developed ganglia are found in the thorax ; the anterior one is connected with the subesophageal ganglia which in turn are connected with the brain or supraesophageal ganglia by nerve fibers which pass on each side of the esophagus. Nerves pass from the brain to the eyes, antennae, and palpi of the head. The subesophageal ganglia supply the mouth parts with nerves. The legs and wings are coordinated in their movements by the thoracic ganglia. In the vertebrates the REPRESENTATIVE INSECTS 353 nervous system is dorsal to the digestive tract, and the foreshadow- ing of this evolutionary change is initiated in the insects by the development in the cephalic region (Figs. 203 and 204). The grasshopper is dioecious; the abdominal structures separat- ing the two sexes are distinctive. The external genital structures have been discussed above. The male organs consist of testes lo- cated dorsal to the intestines. The sperms are borne in ducts which communicate with the penis, which consists of chitinous styles used in copulation with the female. In the female there are two ovaries, Fig. 206. Fig. 205. — Male reproductive organs of Rhomaelia microptera. Te., testes; Y.D-, vas deferens. (From White, General Biology.) Fig. 206. — Female reproductive organs of Rhomaelia microptera. C.S., copula- tory sac; O.T. ovarian tube with eggs; Ov., oviduct; Va., vagina. (From White, General Biology. The C. V. Mosby Co.) which when mature fill the major portion of the abdomen. The oviducts convey the eggs to the vagina, a duct made by the union of the two oviducts, which discharges the eggs through the opening at the base of the egg guide to the outside of the body. The eggs are fertilized by the sperms from the spermatheca, which is dorsal to the vagina and which is connected by means of a sperm duet. The female is able to dig a hole in the ground with the ovipositor 354 TEXTBOOK OP ZOOLOGY and deposit the eggs to the depth of an inch or more. The eggs are covered with a frothy substance which protects them from moisture and, to some extent, from the frost. The eggs are laid in the fall and hatch in the spring of the year. The development of the grasshopper is by gradual metamorphosis. THE JUNE BUG The June bugs or May beetles are members of the family Scara- baeidae, a very large and important family of beetles. More than one hundred and twenty-five species of these beetles have been reported as occurring in the United States and Canada, the majority of them being considered as pests. The larvae or white grubs live underground, destroying the roots of grain, cereal, truck, and gar- den crops, as well as great tracts of pasture and grasslands. The adults live upon the leaves of many kinds of trees and shrubs, often completely defoliating the trees. Because of the general distribu- tion of these beetles, they have been selected as a type to illustrate the characteristics of Coleoptera, the largest order of arthropods. An examination of a specimen of the genus Phyllophaga reveals that there are three body regions: the head, thorax, and abdomen. The rather small, retracted head bears antennae of nine or ten joints and a club composed of three elongate leaflike joints. The antennae are located just beneath the lateral edge of the prominent clypeus. The compound eyes are on the sides of the head near the prothorax. There are no ocelli. The mouth parts are of the biting type, similar to those of the grasshopper. The thorax consists of three segments. The metathorax is fused with the first abdominal segment and with the mesothorax, leaving the prothorax free and movable. Attached to the dorsal portion of the mesothorax are the fore wings that are modified into horny sheaths, or elytra, which cover and protect the back of the thorax and abdomen. The hind wings are membranous and folded under the elytra. The legs are well developed, the prothoracic ones being adapted for digging in the ground. The thorax is provided with yellow setae. The abdomen, which is broadly fused with the metathorax, consists of eight external segments. When the elytra are removed, the spiracles may be seen in the lateral margins of the dorsal surface of the abdomen. The genital organs of both sexes are simple. REPRESENTATIVE INSECTS 355 356 TEXTBOOK OF ZOOLOGY By carefully removing the membranous tergites of the abdomen the heart can be seen to consist of a thin-walled dorsal vessel with paired lateral openings into the body cavity. The blood is forced forward through the heart chambers by the pulsations of the heart walls. There are no arteries and veins, which means that the heart serves mainly as an agitator of the body fluids, helping to distribute the absorbed food to the tissues. The tracheal system is well developed for carrying the air from the spiracles to all parts of the body. There are many changes in the digestive system of the June bug as it passes from the larval stages to the imago. The alimentary tract of the larva consists of a straight tube, except for a bend in the colon. It is much greater in diameter than in the later stages due to the nature of the food, which consists of roots, humus, and some soil. The food passes from the mouth or buccal cavity into the esophagus and then into the crop. At this point there is a valve between the crop or gizzard and the mid-intestines. Two rows of gastric caeca are present on the anterior end of the midintestines. This is a very unique feature, as it is rarely met with in larval stages of other insects. The large saclike stomach or mid-intestine of the larva is transformed into an elongated coiled stomach in the adult, without the two rows of gastric caeca. At the posterior end of the midintestine and in front of the pyloric valve are ten pairs of pyloric caeca. The hind intestine consists of the ileum, colon, and rectum. There are four Malpighian tubules connected to the hind intestine. In the pupal stage the gastric caeca have disap- peared, and the tract is becoming much elongated and coiled. In the adult the excretory organs, the Malpighian tubules, arise in the ileum just posterior to the pyloric valve. They extend into the body and then end blindly at the junction of the colon and rectum. The nervous system consists of a ventral nerve chain, a brain, or supra-esophageal ganglion, a nerve ring which connects the brain and the foremost or infra-esophageal ganglion. There are eight ganglia in the ventral nerve chain, four in the thorax and four in the abdomen. The life histories of the June bugs vary in length from three to four years, depending upon a number of ecological factors. The adult females dig into the ground and deposit from a hundred to two hundred eggs. The larvae are commonly known as ''white REPRESENTATIVE INSECTS 357 grubs." The adults come forth in great numbers in May or June and live from one to two weeks, feeding upon the foliage of many plants. THE HONEY BEE The honey bee belongs to the order Hymenoptera, composed of insects with two pair of membranous wings, well-developed biting or sucking mouth parts, and the females usually with a stinging organ. Many of the Hymenoptera, such as the honey bee, live a social life, developing colonies consisting of three types of bees: a queen, drones, and workers. The worker bee is provided with large compound eyes on the sides of the head and three small ocelli near the median part of the frons. The antennae are attached to the anterior surface of the head. The mouth parts are adapted for both sucking up nectar and chew- ing. The lahrum is attached to the lower edge of the chjpeus. A little organ, the epipharynx, is just below the upper lip. The man- dibles are attached to the ends of the labrum and lie over it. Beneath the mandibles is the proboscis made up of several separate structures : (1) the glossa or long tongue; (2) the laMal palps; and (3) the maxillae, lateral to the labial palps. The maxillae and labial palps are used in sucking the nectar from the flowers. The thorax is divided into the prothorax, mesotlwrax, and meta- thorax. Each segment bears a pair of legs. The wings are borne upon the mesothorax and metathorax. The legs are very well adapted for the work of the hive. The first pair of legs are provided with hairs adapted for various uses. On the tibia are the curved bristles, known as the pollen Irush, and the large spinelike structure, the velum, which is associated with the antenna comb. The metathoracic legs have the tibia modified to form a pollen basket. There are also modified spines and structures on the last pair of legs known as the pecten, auricle, and pollen combs. The modifications found in the legs of the bee are remarkable adaptations for the specialized life of this insect. The abdomen is composed of six external segments consisting of a dorsal tergum and a ventral sternum. At the end of the abdomen is a highly specialized organ, the sting. Associated with the sting are the poison glands, which secrete a substance composed of an acid and an alkali. 358 TEXTBOOK OF ZOOLOGY REPRESENTATIVE INSECTS 359 The digestive tract is well adapted for the specialized life of the bee, that of gathering and feeding upon the nectar of flowers. A study of the digestive sj'stem of the larva, pupa, and adult of the solitary wasp, Odynerus dorsalis, by Mr. Edwin Vest reveals that it is very similar to that of the honey bee, as well as many other Hymen- optera. In Odynerus or the honey bee the digestive tract may be divided into the fore intestines, mid-intestines, and hind intestines, as in the June bug. The divisions of fore intestines are : the mouth or buccal cavity, esophagus; water sac or Jioney stomach; and the pro- ventriculus. The mid-intestine consists of the stomach, while the hind intestine may be divided into the ileum, rectal glands, rectum, and anus. In the larval and early pupal stages the mid-intestine is a thin flat tube, but in the adult it has developed into a convoluted, looped stomach. The number of Malpighian tubules increases from the larval stage to the adult. Only four Malpighian tubules are found in the lar\^a, while there are around one hundred in the adult. There is also a marked change in the length of the esophagus during meta- morphosis. In the adult the esophagus extends from the buccal cavity through the thorax into the first abdominal segment where it enters the water sac, or in the honey bee, the honey stomach. The body of the bee is well filled with tracheae, which are con- nected with two pairs of thoracic spiracles ajid eight pairs on the abdomen. The nervous system is similar to that of the grasshopper. The brain is a ganglion in the head above the esophagus. It is con- nected by a nerve rmg with the subesophageal ganglion, which is in the head but below the esophagus. The two ganglia of the head are connected with two in the thorax and four in the abdomen. The queen bee when fully developed mates with a drone on the virgin flight. By means of the copulatory organ the male transfers a supply of sperms to the seminal receptacle of the queen. Just how the queen is able to regulate the laying of eggs that are fer- tilized by the sperms from the seminal receptacle or those that are not fertilized is not fully known. The fertilized eggs develop into workers and the unfertilized eggs into drones. The life history of the bee, life in the hive, the gathering of nectar and its development into honey for table use, as well as swarming and the rearing of a queen, are fascinating subjects dealt with in the many books devoted exclusively to a study of the honey bee. CHAPTER XXIII PHYLUM CHORDATA Phylum Chordata (kor da' ta, cord) is made up of the group of animals which includes man himself and in general the more con- spicuous, better known animals. Characteristics There is a rather wide range of variation as to form and size in the group. It includes minute sessile forms, small colonial forms, mud-burrowing forms, and on up to the largest and most complex of living animals. All individuals classified in the phylum possess three distinctive characteristics that are most conspicuous in cer- tain primitive forms. The three features clearly distinguish the phylum from all others and bind together individuals which are widely separated in appearance but characterized by certain traits peculiar to this group alone. These three characteristics are: (1) noiochord, a flexible rod extending from anterior to posterior in the longitudinal axis of the body, lying dorsal to the digestive tube and ventral to the nerve cord; (2) pharyngeal clefts or gills, a series of paired slits in the wall of the pharynx and in the body wall of some ; (3) dorsally located tiibular nerve cord, extending the length of the body dorsal to the notochord and other organs. The notochord serves as a stiffening rod and is the foundation axis for the endoskeleton. It is present as such at some time during the life of every cliordate animal. In the adult vertebrate it is replaced by the centra of the vertebrae. The gill clefts are present at some time in the life of all individuals placed in this phylum. Although the gills become modified to form other structures in the adult terrestrial chordates including man, they have had rather typical ones as em- bryos. The pharyngeal clefts or gills provide a more effective mode of respiration for aquatic animals than that used by most non- chordates because the gills are thus interposed directly in the course of the circulation, and the entire blood supply of the body passes through them. The central nervous system is derived from the ectoderm along the middorsal line of the embryo, first as a plate, then as a groove, and finally a tube which results in the spinal 360 PHYLUM CHORDATA 361 cord and brain. In higher forms the anterior end of the tube be- comes expanded and modified to form the brain. The continuous tubular nerve cord is at the apex of the development of centraliza- tion in the nervous system, and allows for an increase in number of nerve cells, increased accessibility, and more intimate association of ganglionic masses to furnish better coordination. These are all advances in both structure and function when compared with other groups. The chordates possess segmentation (metamerism), but it is progressively obscure as one proceeds from simpler to more com- plex forms. There is a tendency toward fusion of metameres and shifting of superficial muscles. The internal skeleton of this group compared with the external one of others studied does not give as great a leverage for the muscles, but it greatly increases the mechani- cal freedom allowed and this is a distinct advantage as well as an advance in structure. Classification There are approximately 40,000 different species in this phylum which is divided into four established subphyla as follows: Hemichorda (hemikor'da, half cord) or sometimes known as En- teropneusta (en ter op nus'ta) includes order Balanoglossida with its four families, ten genera and twenty-eight species, and order Cephalo- discida with its two genera C ephalodiscus and Bhahdopleura. These are all small wormlike animals. Urochorda (u r6 kor'da, tail cord), or Tunicata (tunika'ta) in- cludes the tunicates, all of which are marine and mostly small. Adults show a high degree of degeneration so it is the larvae only that exhibit distinctive characteristics of the phylum. There are three classes: (1) Larvacea, so named because it retains the larva form throughout life. Genus Appendicidaria is an example. (2) Ascidiacea, the sea squirt, either free-swimming or sessile, simple or colonial, may reproduce sexually or by budding. Molgula, Cynthia, and Ascidia are common examples. (3) Thaliacea, free-swimming, pelagic, solitary or colonial forms, usually exhibit alternation of gen- eration. Salpa and Doliolum are the most common examples. Cephalochorda (sef a 16 kor'da, head cord) includes approximately twenty-eight different species of marine, shore-loving, fishlike forms of which Aniphioxus (Branchiostoma lanceolatus) is the most common representative. 362 TEXTBOOK OF ZOOLOGY Yertehrata (ver te bra'ta, jointed) animals with backbone — frog, man. These are the larger, more conspicuous animals and will be discussed at length in later sections of the book. Phylogenetic Advances of Chordata (1) Notochord and endoskeleton, (2) pectoral and pelvic girdles with limbs, (3) development of dorsally located nerve cord with anterior brain, (4) development of five senses, (5) pharyngeal gills and lungs for respiration, (6) voice production, (7) specialization and coordination of muscles. Protochordata (Lower Chordates) Until relatively recent years the two subphyla, Hemichorda and Urochorda were not classified as Chordata; the former was with An- nelida and the latter was independent. With the exception of the value as biological specimens and the use of amphioxus as food by Chinese, this group is of no economic importance. SUBPHYLUM HEMICHORDATA One of the species of Balanoglossus or Dolichoglossus koivalevskii will serve as an example. They are wormlike animals which burrow into the mud and sand along the seashore. They range from 6 to 10 inches in length. Others of the subphylum may be as short as one inch or still others as long as four feet. The three portions of the body are proboscis, a ringlike collar, and a segmented trunk. The proboscis, as well as the collar, is hollow and serves as a water cham- ber. The cavity of the proboscis is filled with water which is dra"wn in and expelled through a proboscis pore or vent located on its dorsal side and just anterior to the collar. Supporting the base of the proboscis is a short skeletal process which is stiff and extends ante- riorly from the roof of the mouth region and assists in burrowing. This process, called the diverticulum, is usually referred to as the rudimentary notochord. However, it is very poorly developed and in a peculiar position. Nevertheless, it has the relationship to the digestive tube which is characteristic in the embryonic development of the notochord for certain higher chordates. The mouth opens on the ventral side just anterior to the collar and leads into the straight alimentary canal which extends to the posterior end of the body PHYLUM CHORDATA 363 and ends in the anus. Like the earthworm, this animal utilizes the mud in which it lives for food, absorbing the organic matter from it as nutriment. Balanoglossus has numerous paired gill slits, located in the lateral walls of the anterior (supposedly pharyngeal) position of the digestive tube. In some of the other representatives the gills are much reduced in numbers or are lacking. Where gills are pres- ent, water is passed through them for respiratory purposes, oxygen being absorbed and carbon dioxide being discharged from the blood here. There is no differentiation of a distinct pharynx. Fig. 209. — External features of DoUchoglossits kowalevskii. Denoyer-Geppert Company. ) (Courtesy of Proboscis coelom Proboscis Pericardium Glomerulus / Collar Heart / Nerve cord Dorsal vessel Trunk Notochord Mouth Ventral vessel Gill slits Alimentary canal Fig. 210. — Diagram of a sagittal section through anterior portion of Dolicho- glossus. (From Hegner, College Zoology, published by The Macmillan Company, after MacBride.) The circulatory system is rather rudimentary. It includes a sinuslike heart which is held in a pericardial sac located in the basal part of the proboscis. A dorsal vessel extends posteriorly from the heart to the posterior end of the trunk. At the collar it is joined by lateral connectives which encircle the body to connect with a ventral vessel extending posteriorly below the intestine. Sinuslike branches of these main vessels supply various parts of the body. The nervous system is composed of a dorsal cord which is tubular in the region of the collar and extends the length of the trunk, a more or less concentrated center of nerve cells in the collar, and a 364 TEXTBOOK OF ZOOLOGY ventral cord rimning longitudinally on the floor of the trunk. The ventral cord certainly is not a chordate characteristic, but the domi- nance and hollow structure of the anterior portion of the dorsal one, represent features which are homologous to the central nervous system of higher chordates. Excretion seems to be accomplished by a mass of vascular tissue (glomerulus?) located in the proboscis just anterior to the heart. The excreted materials are received by the water in the proboscis cavity (coelom) and pass out the pore with the water as it is ex- pelled. These animals are dioecious, with gonads in the form of a genital ridge extending leng-thwise along each side of the anterior portion of the trunk. The mature germ cells escape through the body wall, are fertilized in the water, hatch out and become tornaria Apical plate Mouth Proboscis ccelum Anus Fig. 211. — Tornaria larva of Hemichorda. (From Hegner, College Zoology, pub- lished by The Macmillan Company, after Metchnikoff . ) larvae, which are globular in shape and form a pattern of ciliated bands over the body. In this respect and in habit of life these larvae resemble the larvae of the echinoderms. On this basis a theoretical relationship has been proposed. Until relatively recent times this larva was mistaken for a form of adult nonchordate ani- mal and went under the genus name of Tornaria. Dolichoglossus and its subphylum, though lacking in complete conformity to chordate characteristics, is classified here because of the diverticulum supposedly representing a rudimentary notochord, the gill clefts in the alimentary canal, and the dominance and grooved structure of the dorsal nerve cord. The group includes Cephalodiscus and Rhaldopleura which are colonial forms living in deep sea. PHYLUM CHORDATA 365 SUBPHYLUM UROCHORDA, MOLGULA Subphylum Urocliorda includes a number of common represen- tative marine forms, such as Salpa, Cynthia, Ciona, Clavelma, As- cidia, and Molgula. The latter genus represented by M. manhatt en- sis will be given particular consideration here. This animal is com- monly known as sea lemon, sea peach, or sea squirt. The body of the adult is saclike and averages about one inch in diameter. In this condition it would be an outcast among chordates because as an adult it has no notochord, and no dorsally located, tubular nerve cord. However, it does present pharyngeal gill slits. Incurrent siphon Excurrent i/phon Mantle Tunic Qanqiion Ana5 ^ Genital duct Testis Ovary r - Digestive glands -- Esophagus --Intestim -Stomach — Branchial fold - - End05tyle - /Atrium - - PharynK Fig. 212. — Diagram of Molgula manhattensis from the left side to show the struc- ture with the courses of water and food through the body indicated by arrows. It is saved to the chordates by the presence of all three of the characteristic features in the larval stage. The larva is free-swim- ming and shaped like a tadpole, while the adult is globular and sessile in most of the common forms. Some are brilliantly tinted with color. The adult is covered externally by a cellulose coat or tunic (test), which is secreted by the cells of the underlying mantle. Inside the mantle is the extensive atrial cavity. On the dorsal (unattached) side of the body are two funnellike siphons. The anterior one is the Iranchial siphon (oral funnel, incurrent siphon or mouth) and the other is the atrial siphon (atrial funnel, excurrent siphon, or atrio- pore). When the tunic of Molgula is removed, one may see most of 366 TEXTBOOK OF ZOOLOGY the internal organs through the transparent mantle. Upon viewing this from the left the large saclike pharynx may be seen continuing ventrally and posteriorly from the branchial siphon, finally narrow- ing at its dorsoposterior extremity to become the small tubular esoph- agus which turns sharply downward and anteriorly to become the stomach. The esophagus is partially embedded in a dark-colored digestive gland. The stomach continues anteriorly and upward where it becomes intestine, which turns ventrally on itself in a U-shape. It finally ends with the anus which opens into the atrial cavity shortly below the atrial siphon. A current of water carries food into the digestive system and oxygen for respiratory purposes. The water enters the branchial siphon, passes into the sievelike pharynx, and from here passes through the gill slits or stigmata in its wall into the surrounding atrial cavity, and finally leaves the body by way of the atrial siphon. Oxygen is absorbed by the blood in the walls of the stigmata. The animal's food consists of minute organisms which are entangled in mucus secreted by a glandular groove, the endo- style, which extends from the branchial siphon along the ventral midline of the pharynx to the esophagus. This food mass passes into the esophagus and out through the alimentary canal where digestion and absorption occur. The heart is a contractile tube which pulsates. It lies ventral to the stomach and forces the blood in one direction by a series of contractions and then in the opposite direction by another series. Vessels extend in one direction to the pharynx, primarily, and in the opposite direction to other organs and the body wall. These animals are hermaphroditic or monoecious. Each has two com- pound sets of gonads, one on the left side in the loop of the intestine and the other on the right side of the body. Some of the sessile tunicates, as Molgula, reproduce by budding. There is an oblong, closed excretory sac which may be seen from the right side. The cen- tral nervous system is reduced to a nodulelike ganglion located be- tween the siphons in the dorsal portion. Nerves branch from this to the various parts of the body. The life history of the tunicate is one of interest. Cross- fertilization is the rule; that is, spermatozoa from one individual usually fertilize ova from another ; however, there may be exceptions to this. The fertilization occurs in the water outside the body. The eggs hatch to produce larvae somewhat similar to am- phibian tadpoles which are free-swimming. The larva possesses the typical notochord, gills, and nerve cord of Chordata. For some reason PHYLUM CHORDATA 367 it then settles on the bottom and attaches itself by adhesive papillae located in the anteroventral position. Some authors express it by saying this larva settles on its "chin." It now undergoes regressive changes involving loss of tail, notochord, and posterior portion of nerve cord. The anterior portion of the cord becomes a simple gan- glion. The paired eyes and otocysts (ear structures) also disappear. The dorsal side shortens while the ventral side leug-thens. This places the mouth in a dorsoanterior position, the anus in the dorsoposterior position, and bends the alimentary canal into a U-shape. The num- d.n.c. trhqa. tnt. / ;e. epw. ^'9-3- cie. at.oji Fig. 213. — Metamorphosis of an ascidian lari'a. A, larva ready for fixation. B, an intermediate stage of metamorpliosis. G, completion of metamorphosis. ad~ga., adult ganglion ; at., rudiment of atrium ; at.oj}., atrial opening ; ce.ves., cerebral vesicle ; ci.f., ciliarv funnel ; d.n.c, dorsal nerve cord ; e., eye ; eiric, epi- cardium ; est., endostyle ; fix., fixation papillae ; ga., ganglion ; g.s., gill slits ; h,., heart ; int., intestine ; m., mouth ; ncli., notochord ; st., stomach ; stat., statolith ; trk.ga., trunk ganglion. (From Borradaile and Potts, The Invertebrata, published by The Macmillan Company.) ber of gill slits increases greatly. The atrial cavity is formed by in- foldings from the exterior on each side which surround the pharynx and meet each other. The external opening of this cavity is the atrial siphon. The outer wall of this newly formed cavity is the mantle. Later the tunic is secreted by the mantle to become a protective, cellu- lose covering. This process of metamorphosis has caused an active 368 TEXTBOOK OF ZOOLOGY respectable ehordate to become a lazy, stationary form which is not much more than a water-bag whose level of development has degen- erated almost to that of a sponge. Certain of the sessile forms, which reproduce also by budding, develop colonies with a common tunic. This form is one of the few colonial ehordate animals. In a few instances tunicates reproduce one generatioji sexually, and the next is produced by budding (asexually). This alternation of generation is another retrogressive feature. SUBPHYLUM CEPHALOCHORDA, AMPHIOXUS There are usually listed twenty-eight species in this group which are rather locally distributed over the world. There are four species on American shores : Bra7ichiostoma virginiae, B. floridae, B. lermu- dae, and B. calif orniense. Amphioxus or the lancelet, Branchiostoma lanceolatus, the European form, is an admirable representative of the subphylum and has become classical in its use. However, it is likely that B. virginiae or B. floridae is more commonly studied in the United States. It is a small, fishlike, marine animal whose average adult length is about two or three inches. In its adult form it represents clearly the distinctive characteristics of the phylum in a simple condition. It is a ehordate, possessing only rare essentials. It is usually referred to as a close ancestral relative of Vertebrata. Habitat. — It is found in shore water and on the sandy beaches of the subtropical and tropical portions of the world. These ani- mals are found along our Atlantic Coast as far north as Chesapeake Bay, at certain points in the Gulf of Mexico, and on the southern Pacific Coast. They may be found along the shores of the Mediter- ranean Sea, the Indian Ocean, and along the southern coasts of China. Habits and Behavior. — It burrows rapidly, head first, in the sand by means of a vibratory action of the entire body, but comes to rest with the anterior end exposed to the water. At times, particu- larly at night and during breeding season, the animal leaves the burrow and swims about like a fish by means of lateral strokes of the posterior portion of the body. External Structure. — The body of this animal is shaped like a small lance, the tail being the point. In general, it is similar to Cerebral vesicle Oral cirri Velum Velar tenta- cles Spinal cord Atriopore Intestine Ventral fin Anus Caudal fin Fig. 214. — Diagram of Branchiostoma (Amphioxus) lanceolatus from the right side to show the structure. 370 TEXTBOOK OF ZOOLOGY a small fish, but it does not have a distinct head. The mouth opens on the ventral surface of the anterior portion of the body. It is beneath a rostrumlike projection and is nestled well up in an oral hood which is shaped like an inverted funnel. This hood is fringed with sensory fingerlike oral tentacles. There is a median fin along the dorsal side, continuing around the tail as the caudal fin and anteriorly about one-third of the length of the body as the ventral median fin. There are no clearly defined lateral fins, but a pair of skin structures, the metapleural folds, extending along the anterior Dorsal fin F/n ray Epidermis .^ Spinal nervz \ Nerve cord ^j Nobochord S Myoto;ne muscle \ Myocomma >] Dorsal Aorta ^K— =■■ A^-Epibranchial qroove .Nephridium ^ Atriaicavity _ _ Liver Neurocoek '/^-^ NotochordoL _ '/ sheath Coelom Atrial cavity- Pharynx Gil/rod Gill bars Qonad Ventral aorta. Hypobranchial qroove Endostyle Coelom Metapleural fold S. Fig. 215. — Cross section of Amphioxus thirough the level of the posterior portion of the pharynx. two-thirds of the ventral surface of the body are thought to be their forerunners. The ventral and dorsal fins are supported by small vertical rodlike fin rays. On the ventral side, just posterior to the metapleural folds, is an opening, the atriopore, and beside the ventral margin of the caudal fin is the anus. The segmental divisions of the muscles are apparent on the body wall. There are from fifty-eight to sixty-four of them on each side in B. lanceolatus but sixty-nine in B. calif orniense and they are known as myotomes. The myotomes on the two sides are not paired, but alternate with each other. Adjacent ones are separated by a myocomma or myoseptum. PHYLUM CHORDATA 371 Buccal cirri QUI slit inwall of phorynx Afferent branch- Jal arteries Ventral aorta Dorsal aorta . —Notochord _ -Spinal cord ■^ -Distribution throuqh liver ..HepaticV. . Subintestinal vein . -Atriopore -Vcntro- intestinal V. .Dorso-intestinai A. Anus Caudal vein 1 Caudal artery Fig. 216. — Diagram of the circulatory system of Amphioxus. if 372 TEXTBOOK OF ZOOLOGY Internal Structure and Metabolic Activities. — In small cleared specimens the internal organs are easily observed. The notochord extends the length of the body as a slender rod of vacuolated cells which are filled with fluid to give it turgor or stiffness. Immediately dorsal to this rod is the nerve cord, which also runs the length of the body. It has a small central canal or neurocoele extending length- wise through it and is dilated at the anterior end to form the cerebral vesicle or rudimentary brain. A mass of dark pigment is located at the anterior end which is known as the eyespot. There are smaller pigment bodies distributed along the length of the cord. These are thought to be sensitive to light. The nerve cord gives off nerves to the organs of the body. The two anterior ones are paired, but those behind the cerebral vesicle alternate on the two sides. There are dorsal sensory nerves going to the skin and ventral motor nerves going to the myotomes. There are sensory cells in the skin, oral tentacles, and velar tentacles. The circulatory system does not include a heart, but the blood is moved by the contractions of a ventral aorta, which branches to form the afferent branchial arteries to the gills. Here these vessels branch into capillaries, providing aeration for the blood. These capil- laries converge to form the efferent branchial arteries which lead dor- sally to join the paired dorsal aortae. The dorsal aorta extends pos- teriorly to the tip of the body giving off numerous branches to myo- tomes and internal organs along the way. The posterior direction of the flow of the blood is just opposite to that in the dorsal vessel of the earthworm. The subintestinal vein receives the blood from the in- testine and continues anteriorly to the liver as the hepatic portal vein. The hepatic vein collects from the liver and leads forward as the ventral aorta. The blood in the subintestinal and hepatic portal veins is laden with dissolved nutriment. The blood in these ventral veins flows from posterior toward the anterior (Fig. 216). Digestive System. — A current of water is carried into the mouth by the ciliated bands on the inner surface of the oral hood. These cilia form what is called a wheel organ because of their rotary motion. Surrounding the mouth is a membranous velum to which are attached twelve velar tentacles, which fold across the mouth and serve as a strainer to hold back the coarser particles, as well as being sensory. The mouth leads to the large, barrel-shaped pharynx. The gill slits are clefts in the lateral walls of the pharynx. The number of clefts PHYLUM CHORDATA 373 varies, ranging between fifty and ninety pairs. These open into the atrial cavity which surrounds the pharynx and other visceral organs. In the midline of the roof of the pharynx is an inverted trough, the hyperhrancliial groove, which is ciliated. In the floor of the pharjTix is another ciliated groove, the JiypohrancJiial groove. Its glandular walls, which are capable of secreting mucus, constitute the endostyle. It functions on the same plan here as in tunicates. The strings of mucus entangle the food particles and are moved anteriorly, and then by two peribranchial grooves are carried dorsally to the hyper- branchial groove. The cilia here move the mass back to the intestine. A blind, fingerlike diverticulum of the intestine, the liver or hepatic caecum, extends anteriorly from its connection on the anterior part of the intestine to lie on one side of the pharynx. This organ is a digestive gland and empties a digestive juice containing enzj^mes into the intestine. The intestine extends posteriorly to the anus as a rela- tively straight tube. The food is digested in, and absorbed from, the intestine. Respiratory System and Respiration. — As stated above, the water in passing through the gill slits delivers oxygen to the blood in the capillaries there and absorbs carbon dioxide from it. The water then passes back through the atrial cavity and out through the atriopore. The blood then distributes the oxygen to all tissues of the body. The gill-'bars, which separate the slits, contain the blood vessels, and are supported by chitinous rods. The gills are on the faces of the gill bars and are covered with cilia which help move the water through its course. Excretory System and Excretion. — Ciliated nephridia similar to those of the earthworm lead from the dorsal portion of the coelom to the atrial cavity. The coelomic cavity is reduced in the pharyn- geal region to a narrow space surrounding the dorsal aorta above the pharynx and a narrower one around the ventral aorta below. Between the posterior end of the pharynx and the atriopore, the coelom consists of a narrow space surrounding the intestine with a thin membrane separating it from the atrial cavity. Behind the atriopore it is relatively larger. Reproductive System and Life Cycle. — This animal is dioecious with each mature individual possessing 26 pairs of (31 to 33 pairs in B. calif orniense) nodular gonads embedded in the body wall near the base of the metapleural folds. When the germ cells mature, 374 TEXTBOOK OF ZOOLOGY |; they break through the wall of the gonad into the atrial cavity and pass out through the atriopore with the water. Fertilization occurs in the water. Early summer is the breeding season, and at that time the animals are quite active during the evenings and nights. Fol- lowing fertilization comes a series of cleavage divisions which are total and equal. This is followed by the infolding of one side of the spherical body to form the gastrula and this in turn becomes a free- swimming larva which reaches adult condition without metamor- phosis, only to begin bashfully burying itself in the sand. CHAPTER XXIV THE VERTEBRATE ANIMAL: SUBPHYLUM VERTEBRATA In this group to which man himself belongs are found the dis- tinctive chordate characteristics at some time in the life of the indi- vidual. In terrestrial forms there are certain modifications to pro- duce other structures. Metamerism and bilateral symmetry are universal characteristics among vertebrates. The segmented verte- bral column and other supporting structures form an endoskeleton (internal skeleton) which is the basic support of the body. Paired appendages are usually present at some time in the life of the indi- vidual. The majority have two pairs of fins or limbs in adult con- dition. There is a ventrally located heart which is divided into chambers. The Mood contains hemoglohin hearing red corpuscles and amoeboid white corpuscles. In the vertebrate body is a well-developed coelom, which encloses advanced systems of organs for digestion, ex- cretion, circulation, reproduction, and in terrestrial forms, respiration. Cephalization is developed in all vertebrates and along with this they possess a hollow, five-lobed brain located in the more or less distinct head. The sense organs are in an advanced state of development. The body is divided into head, trunk, and tail. The tail is a posterior prolongation of the body behind the anal opening and is found in some degree in all vertebrates. The nech which is a constricted region between trunk and head is conspicuous in terrestrial forms. The appendages are usually arranged with one pair attached to the anterior, pectoral portion of the trunk and one situated at the posterior, pelvic region. This arrangement is less consistent in the aquatic types where the weight of the body is buoyed up by the water and the limbs are used less for support and locomotion. In different types of vertebrates there are various modifications of pectoral appendages as arms, wings, pectoral fins, forelegs, and flippers. The same is generally true for the pelvic limbs. The body wall is composed of the skin, which usually has char- acteristic tegumentary outgrowths, such as scales, nails, shells, feathers, and hair, as the outer layer, beneath which is the muscular 375 376 I'EXTBOOK OF ZOOLOGY coat and internal to this is the membranous peritoneum. In all vertebrates, except mammals, the coelom consists of only two parts: the pericardial cavity and the general abdominal cavity. In mam- mals it is further divided into pericardial, thoracic, and aldominal cor Fig. 217. — Diagrammatic section of the human skin. Cor, stratum corneum ; D, dermis ; gs, sebaceous gland ; M, Malpighian layer ; niu, muscle ; n, non- meduUated nerve ; nm', nm", nm'", nina, and nmb, myelinated nerve fibers ; P, papilla of hair ; Sc, hair shaft, u, fat tissue ; v and w, external and internal sheaths of hair root; x and y, endings of nonmyelinated nerve fibers. (From Maximow and Bloom, Histology, published by W. B. Saunders Co.) cavities. The first contains the heart, the second the lungs, and the third the organs of the excretory, reproductive, and digestive systems. THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 377 The vertebrate animal is covered by an integument or skin which serves as a protective and sensorj- organ. It also helps in excretion through the sweat glands, mucus glands, and oil glands as well as facilitating temperature regulation in some. Such exoskeletal struc- tures as scales, nails, hoofs, claws, feathers, hairs and enamel of teeth are produced by the skin. The integument is composed of an outer stratified epithelial epidermis which consists of several layers of cells, few nerves, and no blood vessels, and the inner fibrous dermis or corium, which consists of areolar connective tissue, nerves, nerve endings, integumental glands, blood vessels and lymph spaces. The membrane type of bone is developed in the dermis. The maintenance of any living body requires the cooperation of several functions which will attain similar fundamental results wherever in living material they occur. The principal functions performed by the structures in the animal body are: (1) support and protection, (2) movement and locomotion, (3) digestion, (4) respiration, (5) circulation, (6) excretion, (7) reproduction, (8) reception and conduction of stimuli, and (9) internal regulation. These functions merge into one living process which involves the building up of protoplasm, transformation of energy, and repro- duction. During the execution of these activities energy is con- stantly being changed from the potential to the kinetic form. Metabolism. — The collective term metabolism is employed when re- ferring to all of the interactions involved in the living process of pro- toplasm. It includes the processes concerned with conversion of food into protoplasm, release of energy through oxidation, production of heat, movement, elimination of wastes ; or, in other words, these proc- esses are chiefly : Ingestion, digestion, egestion, absorption, transporta- tion, respiration, oxidation, and elimination. The processes concerned with the conversion of food material into protoplasm (building up) constitute the phase of metabolism known as anaholism. Included here are ingestion, digestion, absorption, transportation, and assimila- tion. The oxidation of materials of the protoplasm to liberate energy, and the elimination of wastes incidental to it, is known as cataholism or the "breaking down" phase. Metabolism is one of the fundamental features of all protoplasm, therefore, all physiology, since it is a study of the functions of liv- ing organisms, must be concerned with metabolism. It includes all 378 TEXTBOOK OF ZOOLOGY of the chemical changes and transformations by which energy is supplied for the activities of the protoplasm. The skeleton is quite well developed in the vertebrates and serves them quite efficiently for support, stature, protection, and muscle attachment. It is composed of cartilage entirely in some of the simpler forms and of bone and cartilage in higher types. It is divided into an exosJceleton which is superficial and an inner endoskeleton which includes all of the deeper skeletal parts. The exoskeleton is a rather minor part in vertebrates and consists of nails, claws, scales, hair, feathers, and other outgrowths. The endoskeleton includes the axial and appendicular portions. The first is composed of the skull, vertebral column, ribs, and in some a sternum. The appendicular portion is composed of the anterior and posterior girdles and two pairs of limbs. In their development bones either replace cartilage to be called cartilage I ones or they develop in the connective tissue of the dermis, to be known as membrane hones. The vertebral column is composed of segmental divisions, the vertebrae, and is divided into five regions as follows: cervical vertebrae of the neck, thoracic verte- brae of the chest, lumbar vertebrae of the small of the back, sacral, vertebrae of the hip region, and the caudal vertebrae of the tail region. Bone is a firm, hard tissue consisting of abundant matrix, composed of inorganic salts, and the bone cells which are held in pocketlike lacunae in the matrix. The outer membranous covering of bone is called periosteum. The mineral part of the bone consists chiefly of calcium phosphate and calcium carbonate. They give it firmness and rigidity. The animal matter is composed of the bone cells and cartilage which serve to give the bone life and resilience. A weak acid, such as the acetic acid in vinegar, will dissolve the mineral matter of bone if allowed sufficient time, in which case the bone will lose its rigidity. Caustic solutions will destroy the animal matter and make the bone brittle. The following outline presents a summary of the principal parts of the terrestrial verte- brate skeleton. Divisions of Skeleton of Terrestrial Vertebrate I. Axial Skeleton (a) Skull 1. Cranium 2. Sense capsules 3. Jaw apparatus (Visceral arches) THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 379 (b) Vertebral column 1. Cervical vertebrae (neck) 2. Thoracic vertebrae (chest) 3. Lumbar vertebrae (small of back) 4. Sacral vertebrae (hip) 5. Caudal vertebrae (tail) (c) Thoracic basket 1. Eibs (paired) 2. Sternum (breastbone) II. Appendicular Skeleton (girdles and limbs) (a) Pectoral (anterior) 1. Girdle: scapula, clavicle, procoracoid and coracoid 2. Limb: Humerus (upper arm), radius and ulna (forearm), carpals (wrist), metacarpals (palm), phalanges (bones of digits) (b) Pelvic (posterior) 1. Girdle: ilium, pubis, and ischium 2, Limb: Femur (thigh), patella (knee cap), tibia and fibula (shank), tarsals (ankle), metatarsals (sole), phalanges (bones of toes) In Protozoa there is no very elaborate adaptation toward a skele- ton. The presence of a cuticle in some and the secretion of a hard shell in others seem to be the particular developments related to these special functions in this group. Arcella, Difflugia, the Foram- inifera, and Radiolaria exemplify this adaptation. The skeleton and integumentary structures serve the Metazoa primarily for a support and protection. The corals of the phylum Coelenterata secrete a calcareous or horny skeleton around the ex- ternal surface of the body proper. The sponges, as a rule, each have a calcareous, siliceous (glassy), or horny skeleton extending throughout the body. Such forms as snails, crayfishes, beetles and representatives of their respective phyla secrete a well-developed exoskeleton as an external cover over most of the other tissues of the body. The muscles and other tissues are attached within. There are special cells of the epidermis which function primarily in production of this skeletal material. The echinoderms, including animals like the starfish, possess calcareous skeletal plates which are essentially similar to exokeleton except that they are princi- pally beneath the skin. There is no well-developed endoskeletal structure known in non- chordate animals but the endophragmal structures extending into the thorax of some Crustacea are thought to be the forerunner of the endoskeleton. A number of exoskeletal modifications are used for 380 TEXTBOOK OF ZOOLOGY protection and temperature regulation in most of the groups of ver- tebrates. Such structures as scales, shells, feathers, hair, nails, horns, and even enamel of teeth are of this type. Primitively the notocJiord is the original endoskeletal structure of the chordate group. Around it are developed the basic structures of Ska// Ceri//ca/ vertebrcxe Scapu/a Sternum Thoracic vertebrae Lumbctr vertebrae Sccrum Mefacarpa/s Cranium Orbit Mandib/e C/av/c/e ■Humerus Rib Pe/vis Radius Utna ^Carpa/s Hc^nd Fig. 218.- Tibia Metatarsals -Human skeleton. (From Wolcott, Animal Biology, published by the McGraw-Hill Book Company.) the vertebral column which functions as the principal axial support of all vertebrates. The sternum, girdles, and paired limbs have developed with the terrestrial life of vertebrates and the necessity for locomotion on land. THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 381 The muscular system represents a system of cells highly special- ized in contractility. The muscles are usually attached to the skele- ton or occasionally to other muscles by fibrous cords called tendons. Voluntary muscles are usually connected with the skeleton; those of the visceral organs, e.g., intestine, are involuntary. Cardiac muscle is the highly specialized involuntary muscle which makes up the wall of the heart. Independent power of movement is almost a characteristic of animal life. Contractility as a property of all protoplasm is the fundamental basis for all animal movement. The adult forms of cer- tain animals, such as sponges, corals, oysters, barnacles, and others, are sessile ; however, they all pass through a free, active larval stage. Most of them retain the power to move separate parts in adult condition. Simpler forms of locomotion have already been seen in Protozoa which move from place to place by means of pseudopodia, cilia, or flagella. In ciliary movement the numerous small strands of proto- plasm beat rhythmically with a stroke in one direction, so timed that the beat passes in a wavelike progression from one end of the ciliated area to the other (metachronous rhythm). The stroke of a eilium consists of a vigorous bend in one direction and a very deliberate recovery in the other. In many Protozoa the entire body is covered with cilia while in Metazoa the entire body may be covered where they are used for locomotion; but more often they cover only areas of free surface of epithelium, particularly the linings of passages. Here they serve to move materials along and keep the surface free of foreign material and excess mucus. The development of a high degree of contractility in special cells, such as muscle cells, makes possible muscular movement which is the principal kind in higher animals. A muscular locomotor system con- sists of sets of opposing muscles. In muscular contraction there is a cycle of rapid chemicophysical rearrangement in the cells. Oxida- tion and heat production are involved in the process. Carbohydrates in the form of glucose are oxidized (burned) in the reaction. During the shortening of the muscle there is a hydrolysis or absorption of water by the protein product, creatine-phosphorie acid. By-products of the process include carbon dioxide, lactic acid, urea, creatinine, and phosphoric acid. 382 TEXTBOOK OF ZOOLOGY In animals without a skeleton muscle bands are arranged in both circular and longitudinal directions. The contraction of the circular group tends to lengthen the body, and the shortening of the longi- tudinal strands draws the body along. The pressure exerted on the coelomic fluid is thought to be a factor in bringing about an even extension of the body by this means. In echinoderms with the water vascular system the pressure is exerted on water in a system of tubes which extend to make contact with the surface over which the animal is moving. The Dig"estive System. — The digestive system is typically a straight tube extending through the length of the trunk of primitive vertebrates. In the higher forms there are many outgrowths, such as digestive glands and respiratory organs. The anterior region of the digestive tube is the mouth cavity which contains teeth on the jaws, a tongue, and receives saliva from salivary glands. Following the mouth is the pharynx or throat region which receives the internal nostrils, the Eustachian tubes from the middle ears and opens into the esophagus posteriorly. The esophagus is usually tubular and propels the ''swallows" of food posteriorly by consecutive waves of contrac- tion, a process known as peristalsis. It leads to the saclike stomach, whose walls possess gastric glands for secretion of a digestive fluid containing enzymes (ferments) and weak hydrochloric acid. The peristaltic contractions continue along the wall of the stomach to help digestion by churning and mixing the food with digestive juices. At the posterior end a pyloric valve in the form of a sphincter muscle guards the entrance to the small intestine which follows. This is a convoluted tube in most of the advanced forms of vertebrates and is divided into the anterior duodenum, middle jejunum, and posterior ileum. It is usually longer than the body and therefore it is coiled. Its walls produce digestive enzymes from glands and it receives digestive juices from two other glands : the liver and the pancreas. The small intestine serves both as a digestive organ and as the principal absorptive organ of the body. Its internal lining is provided with numerous fine fingerlike projections which increase the inner surface and enhance absorption. The digested food is taken up by the lymphatic spaces and by blood vessels which are embedded in the wall just outside of the lining epithelium. The liver is the largest organ in the body of most vertebrates. It secretes the bile which is stored in the thin-walled gall Madder, THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 383 which is attached to one of its lobes. The liver also serves to convert carbohydrates to glycogen (animal starch) and store it for future ener^ production. It is also in the liver that protein wastes are converted into urea and uric acid in order that they may be excreted from the blood in the kidneys. The large intestine which is shorter than the small intestine possesses no villi or digestive glands. It receives the fecal matter and delivers it to the anus. In many forms of vertebrates the posterior portion of the large intestine is modified to become a cloaca, which receives also the products from tlie urinary and reproductive organs. The chief function of this entire system is that of dissolving and converting complex food materials into a form which may be absorbed and assimilated by the protoplasm of cells throughout the body. The materials commonly used for foods have large molecules, usu- ally called colloidal in nature. Digestion then must serve to break up these large molecules into smaller ones, thus forming solutions of substances in order that they will readily diffuse through mem- branes. Digestive enzymes are responsible for placing the food materials in solution. So, proteins are converted to soluble amino acids, starches and sugars to maltose and finally glucose, and fats to fatty acids and glycerin. In general, an enzyme is an organic substance which by its pres- ence under certain conditions \vill cause or hasten chemical reaction between other substances without itself being consumed. The en- zymes are formed in the protoplasm of cells and their action is similar to that of a catalyst, since they accelerate chemical action. There are different types of enzymes each capable of producing spe- cific kinds of reactions. There are oxidizing enzymes (oxidases) capable of bringing about oxidation ; reducing enzymes (reductases) which produce reduction in tissues; coagulating enzymes (coagu- lases) which cause clotting or coagulation; and hydrolysing enzymes (hydrolases) act by causing a reaction between a substance and water. Most of the digestive enzymes fall in this latter class. Most enzymes consist of a parent substance or precursor (zymogen) which becomes active only in the presence of a certain other substance, termed acti- vating agent or coenzyme. As an example, the precursor of pep- sin is pepsinogen which becomes activated in the presence of dilute acid. 384 TEXTBOOK OF ZOOLOGY Classes of Digestive Enzymes 1. Diastases or diastatic enzymes — split carbohydrates (a) Ptyalin in saliva (b) Amylase in pancreatic juice (c) Glycogenases — liver and muscles Converts glycogen to glucose 2. Lipase or lipolytic enzyme — splits fats (a) Steapsin in pancreatic juice 3. Inverting enzymes — convert disaccliarids to the less complex monosaccharids (simple sugars) — intestinal juice (a) Maltase (b) Lactase (c) Sucrase (invertase) 4. Proteases or proteolytic enzymes — split complex proteins (a) Pepsin in gastric juice (b) Trypsin in pancreatic juice — functions in small intestine (c) Erepsin in intestinal juice 5. Clotting or coagulating enzyme (a) Eennin in gastric juice In higher Metazoa digestion is accomplished principally extra- cellularly through secretion of enzymes by certain groups of cells. Such systems consist of: (1) an alimentary canal proper; and (2) associated glands which discharge digestive juices into it. The relative length of this canal varies considerably depending on the habitual diet of the organism. In carnivores (flesh-eaters), such as cats and dogs, it is from three to five times as long as the body; while in herbivorous forms (plant-eaters), such as horses and cows, it is over twenty times as long as the body. The length of the human digestive tract is approximately ten times the length of the body. The relative proportion of the internal absorptive surface of the alimentary canal to the external surface of the body is signifi- cant. In carnivorous animals it is about one-half the area of the skin while in herbivorous animals it is about twice the area of the skin. The process of digestion in man is quite well understood, and it is fairly typical and general because of the omnivorous food habits. The action of the several enzymes produced by different glands is a very essential part of the process. The digestion of all organic food materials is brought about by hydrolysis in the same kind of chemical change. In hydrolysis the large molecules of pro- tein, carbohydrate, or fat first combine with water and then split into simpler products. Some foods may require more than one such THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 385 splitting. The splitting of the disaccharide, maltose, will serve as an example of this process: (Malt sugar) (Water) (Glucose) The two molecules of glucose formed are in a form for ready absorption. Gastric Digestion. — The tubular gastric glands located in the mucous layer of the stomach secrete the acid gastric juice which is a solution of 0.2 to 0.5 per cent hydrochloric acid and two important enzymes, pepsin and rennin. The pepsin when present in the acid medium brings about the splitting of complex proteins into inter- mediate proteoses and peptones. Be^inin causes the casein in milk to coagulate. This is the first step in its digestion. It is claimed by some that emulsified fats, such as cream, are partially digested by a gastric lipase. The digesting mass or chyme in the stomach is con- tinually churned and mixed by muscular activity of the walls. "When it becomes saturated (0.4 per cent) with acid and has been reduced to the consistency of soup, it is discharged through the pylorus. Intestinal Digestion. — When the chyme is ejected through the pylorus into the duodenum, the hydrochloric acid stimulates cer- tain cells of the intestinal lining, causing them to secrete into the blood a substance of hormone nature, known as secretin. Upon reaching the pancreas this secretin stimulates it to secrete the diges- tive fluid, pancreatic juice, into the small intestine by way of the pancreatic ducts. There is some evidence that secretin also stimu- lates secretion in the liver. Pancreatic juice is a clear, watery, alkaline solution containing inorganic salts (carbonates, etc.) and three enzymes; the protease, trypsin, the diastase, am.ijlopsin, and the lipase, steapsin. These act respectively on proteins and peptones, starches and sugars, and fats. This protease is in the form of trypsinogen until it reaches the intes- tine and is activated by an intestinal enzyme, enterokinase. Trypsin completes the work begun by the pepsin in that it converts proteoses and peptones into amino acids, but it also digests complex proteins which have escaped the action of pepsin. It acts more rapidly and efSciently than does pepsin. There are nineteen amino acids that are regarded as hmlding stones of the protein molecule. In a com- plex protein like casein, as many as sixteen of these amino acids will 386 TEXTBOOK OF ZOOLOGY be found. The tissues of the animal body must not only have avail- able a wide range of amino acids but must also select in the proper proportion the ones needed to reconstruct their specific protein con- stituency. Amylopsin (amylase) is the pancreatic diastase, and it is able to bring about hydrolysis of carbohydrates in the alkaline medium of the intestine without activation. It produces dextrin and maltose (malt sugar). The pancreatic lipase, steapsin, brings about the split- ting of fats into glycerin (glycerol) and one or more fatty acids, such as stearic acid, oleic acid, butyric acid, etc. The alkaline salts which are introduced by the bile, combine with these fatty acids to form soaps which help in emulsifying the remaining fats, thus mak- ing them more readily split. Intestinal secretions or succus entericus which are produced by glands in the mucous membrane of the small intestine include five enzymes. Enterokinase, which activates trypsinogen to form tryp- sin, has been mentioned already. Erepsin, the intestinal protease, supplements the activity of trypsin by converting proteoses and pep- tones into amino acids. Maltase converts maltose and dextrin into dextrose. Invertase changes sucrose (cane sugar) into dextrose and levulose. Lactase converts milk sugar (lactose) into galactose and dextrose, both simple sugars. The undigested residue passes into the large intestine where prob- ably no enzyme digestion occurs. Certain bacteria (B. coli and others) attack any undigested protein and bring about putrefactive fermentation. Products of this action may be absorbed; some of them are frequently toxic and must be eliminated in either the urine or the feces. Certain other bacteria here feed upon cellulose and may produce some sugar from it. When the chyme reaches the large intestine it is about the consistency of thick cream, but it becomes more and more solid by absorption of water here until finally only concentrated fecal matter remains. Functions of the Liver. — The secretion of the liver is bile and is discharged into the duodenum of the small intestine by way of the common bile duct. This is an alkaline solution which serves to help neutralize the acidity of the chyme as it comes from the stomach. This with the pancreatic juice brings about the emulsification of fats men- tioned above. Cholesterin and two pigment materials are excreted in THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 387 The Digestive Enzymes and Their Functions ENZYME REGION OF DI- GESTIVE TRACT* DIGESTIVE JUICES FOODS AFFECTED SUBSTANCES PRODUCED Ptyalin Mouth Saliva Starch (carbo- hydrates) Maltose Pepsin Stomach Gastric juice from gas- tric glands Proteins in acid medium Proteoses and pep- tones Bennin in mammal Stomach Protein of milk Coagulated to form paracasein Gastric lipase Stomach Emulsifies fats Glycerol and fatty acids None Liver Bile Emulsifies fats None Amylase or amylopsin Produced in pancreas Pancreatic juice pro- duced in pancreas but acting in small intestine Carbohy- drates Maltose Steapsin Produced in pancreas Intestinal Lipins (fats) Glycerol and fatty acids Trypsin Produced in pancreas Proteoses and pep- tones in alkaline medium with enter- okinases Polypeptids Erepsin Small intestine Polypeptids Amino acids Maltase Small intestine Maltose Glucose (dextrose) Lactase in mammals Small intestine Lactose Glucose and galactose Invertase or s^icrase Small intestine Sucrose Glucose and levulose *The esophagus and colon do not secrete any enzymes. the bile. Bile is secreted all of the time, but between meals it is stored in the gall bladder and supplied in quantity at meal time. Besides these digestive and excretory functions the liver serves in another capacity. It is a storehouse for carbohydrates which it converts to glycogen (animal starch) by enzyme action. This sub- stance is also stored in the voluntary muscles. It is easily recon- verted to dextrose for ready oxidation. Most of the protein by- product iirea (and uric acid in some forms) is formed in the liver and carried by the blood to the kidneys for excretion. 388 TEXTBOOK OP ZOOLOGY Absorption and Utilization of Food Materials. — The soluble prod- ucts of digestion are absorbed through the semipermeable epithelial lining of the intestine into the blood of the adjacent capillaries, or in the case of fats into the lacteal IjTnphatics and from here into the sub- clavian vein. The blood supplying the intestine is collected by the hepatic portal vein and delivered to the liver. The two functions of proteins in the body are : to rebuild debili- tated protoplasm; and help supply heat and energy to the body by oxidation. They serve first and best for the purpose mentioned first. Carbohydrates and then fats are more economical and efficient as sources of fuel for production of heat and energy. Oxidation of pro- tein requires the disposal of much more waste products. The com- parative heat production values of the three are as follows : One gram of protein z= 4.100 Calories* One gram of carbohydrate z= 4.100 Calories One gram of fat = 9.305 Calories Some portion of the dextrose is distributed and oxidized directly for immediate energy, but much of it is transformed into glycogen by the enzyme glycogenase in the liver. This may be stored here or in the muscles to be reconverted into dextrose for oxidation by the tissues as needed. Normally there is a constant supply of dextrose (0.1 to 0.15 per cent) in the blood and this level must be maintained. The final oxidation products of carbohydrates in the body are heat, kinetic energy, water, and carbon dioxide. The last two are dis- charged from the body as waste products. Fat is converted to dex- trose and oxidized to produce heat and kinetic energy. It is usually stored as a reserve fuel supply in adipose tissue over the body. Car- bohydrates in excess may be converted to fat, and stored. Vitamins and Their Functions. — Besides proteins, carbohydrates, fats, inorganic salts, and water there is another indispensable class of food material, the vitamins. They are natural substances found in relatively small quantities in a number of different foods. In general, their function is regulatory. They are recognized usually through the abnormal condition brought on by their deficiency. There is little danger of vitamin deficiency for adults living on a balanced and mixed diet. Much of our knowledge concerning the symptoms brought on by lack of different substances has been •A Calorie equals the amount of heat necessary to raise one liter of water one degree centigrade under standard conditions. THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 389 gained by feeding experiments on different kinds of laboratory animals and results applied to human beings. The following outline will give much of the essential information concerning vitamins. The Vitamins and Their Characteristics I. Vitamin A (C20H30O) — antixerophthalmic — fat soluble. (a) Sources: carotene (CjoHBe) a yellow pigment in green plant leaves, carrots, and such plant tissues. Transformation of this pigment into the vitamin which is especially stored in shark, cod, halibut or other fish liver oil, egg yolk, and milk. (b) Functions: Influences efficiency and acuity of vision, important fac- tor in regeneration of visual purple of retina, strengthens and pro- motes hardiness in epithelial tissue. (c) Effects of Deficiency: Xerophthalmia (lack of tear secretion and dry cornea), and "night blindness" in human. "Nutritional" roup in birds. II. Vitamin B* "Complex." 1. Bj or Thiamin (Ci2Hi,ON4S) — Antineuritic. (a) Sources: Germ of wheat and other cereal grains, peanuts, liver, and egg yolk. (b) Functions: Promotes tone in alimentary tract, stimulates appetite, essential for normal growth, essential for carbohydrate metabolism. (c) Effects of deficiency: Beri-beri (neurodigestive disturbance following diet of polished rice), loss of tonus and muscular activity of digestive tract. Cessation of growth. Polyneuritis develops in birds. 2. Riboflavin (C^H^oOoN^). (a) Sources: Eggs, liver, milk, green leaves, yeast. (b) Functions: Necessary for growth, active relation to several enzymes witli intermediate metabolism of food. (c) Effects of deficiency: Irritation and inflammation at corners of mouth in human (cheilosis). "Yellow liver" of dogs. "Curl-toe" paralysis of chickens. Dermatitis of turkeys. 3. Nicotinic Acid (CgHsNOz) — antipellagric. (a) Sources: Meat, liver, egg yolk, green leaves, wheat germ, yeast. (b) Functions: Produces active "coenzymes" (I and II), balances cel- lular function, (c) Effects of deficiency: Pellagra in primates (man and monkeys). Black-tongue in dogs. Swine pellagra. 4. Be or pyridoxine (CgHuOaN). (a) Sources: Milk, liver, cereals, yeast. (b) Functions: Necessary for growth. May influence oxidation of food. (c) Effects of deficiency: Paralysis in chickens. ♦There are still other recently discovered fractions of Vitamin B. whose func- tions are specific. 390 TEXTBOOK OF ZOOLOGY 5. Pantothenic aeid (CgHiTOgN). (a) Sources: Liver, milk, egg yolk, yeast, molasses, peanuts. (b) Functions: Essential for growth. (c) Effects of deficiency: Graying in black rats. Dermatitis in rats and chickens. 6. Biotin (doHieOsNjS). (a) Sources: Egg yolk, yeast, cereal grains, molasses. (b) Functions: Essential for growth. (c) Effects of deficiency: Thickening of skin and dermatitis in clucks and rats. III. Vitamin C or Ascorbic Acid (CoHsOe) — antiscorbutic-water-soluble. (a) Sources: Citrus fruits, tomatoes, turnips (most mammals except pri- mates and guinea pig can synthesize this vitamin). (b) Functions: Maintains structure of capillary walls. (c) Effects of deficiency: Scurvy in human and guinea pig (bleeding in mucous membranes, beneath skin and into joints). IV. Vitamin D (C2SH44O) — antirachitic. (a) Sources: Tuna and cod-fish liver oils. Exposure of skin to ultra- violet radiation. (b) Functions: Eegulation of calcium and phosphorus metabolism. Ke- quired for normal growth and mineralization of bone. (c) Effects of deficiency: Soft, deformed bones in young (rickets). Soft bones (osteomalacia) especially in women of the orient. V. Vitamin E or Tocopherol (CjaHjoO.)- — antisterility. (a) Sources: Wheat germ oil, green leaves, other vegetable fats. (b) Functions: Promotes rapid cell proliferation and differentiation. (c) Effects of deficiency: Sterility in male fowls and rats. Failure of spermatogenesis. Death of rat embryos in uterus. VI. Vitamin K (C3,H4e02) — antihemorrhagic. (a) Sources: Green leaves, alfalfa, also certain bacteria of the "intesti- nal flora." (b) Functions: Influences the production of prothrombin by the liver (prothrombin is necessary for blood clotting). (c) Effects of deficiency: Blood fails to clot. The Respiratory System.— The respiratory system is at least in part an outgrowth of the digestive canal. In most aquatic verte- brates respiration is accomplished by drawing water through gill slits in the wall of the pharynx. Air-breathing, terrestrial forms have developed the trachea (windpipe) and lungs as another out- growth of the pharynx. A certain amount of respiration takes place through the skin. The respiratory process is composed of two phases: exterTial respiration which includes the exchange of the gases, oxygen and carbon dioxide, between the external environ- mental medium and the blood ; and internal respiration which is the THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 391 exchange of the gases between the blood and the protoplasm of the cells over the body. Much of the carbon dioxide given up by the cells becomes carbonic acid and carbonates which may be trans- ported by the plasma (fluid) of the blood. Respiration has been defined as the process involving the ex- change of gases between the protoplasm of an organism and its environment. All living protoplasm must be provided with a means of receiving oxygen and giving up carbon dioxide. In protozoa and simple metazoa, such as sponges, coelenterates, flatworms, round- worms, and even some annelids, this gaseous exchange is made by almost direct diffusion through the cell membranes to the surround- ing medium. This movement of gas through the cell membranes de- pends on the partial pressure of the particular gas on the two sides of the membrane. Gas will flow in the direction toward the least pressure. In the larger and more complex animals where the volume of tissue is such that a more active interchange of gases is required than the general body surface will permit, special organs or modi- fications of the surface must be provided. Also the possibilities of oxygen absorption are greatly increased by the development of respiratory pigments like hemoglobin and hemocyanin, which are carried in a blood vascular system all over the body. These pig- ments readily unite with oxygen to form oxyhemoglobin in the case of the former. Thus the blood is enabled to absorb far more oxygen than an equal quantity of ordinary liquid. When the oxy- gen pressure of the surrounding tissue is sufficiently low, the oxy- hemoglobin releases its oxygen rapidly. Carbon dioxide accumu- lates in excess in the tissues and diffuses from the cells to the lymph, thence to the plasma where much of it combines with sodium as sodium carbonate. Small amounts of CO2 combine with the hemoglobin. The gills of most aquatic forms are richly supplied with a capil- lary supply of blood and then membranous surfaces are directly exposed to surrounding water from which the dissolved oxygen is absorbed. In many aquatic worms the gill filaments are outgrowths of the sides of the body wall. Likewise, the more or less plumelike gills of crayfish are pocketlike outpushings of the body wall. In a number of aquatic insects, worms, fishes, and turtles, the rectum serves as an accessory respiratory organ. 392 TEXTBOOK OF ZOOLOGY Aerial respiration is accomplished in terrestrial animals through special internal surfaces which must be kept moist. In insects a system of branched tubes called tracheae, which open through spiracles along the sides of the body, distribute oxygen to and re- ceive carbon dioxide from all of the cells of the body. In pulmonate snails the "lung*' is simply an invagination of the skin, as are also the tracheae of insects. The real lung is a development found in the terrestrial vertebrate, and it is a specialized surface derived from the anterior or pharyngeal portion of the digestive tube. In higher vertebrates, such as birds and mammals, they are extensively lobed, and made spongy by the innumerable small air sacs which provide the enormous respiratory surface necessary. It has been estimated that if all of these pitlike alveoli of the internal lining of the lungs of the average human being were spread out in an even surface, the area of it would be more than 100 square yards. The mechanism for the accomplishment of breathing in the cat and other mammals by the use of the diaphragm and thoracic wall is described in the chapter on mammals. The muscles which control these actions are automatically stimu- lated through the nervous system to contract when the carbon dioxide level of the blood reaches a certain point. A respiratory center, located in the medulla oblongata, is affected by the carbon dioxide and determines the rate of respiratory movements. There are also nerves from the lungs themselves which extend to this center and contribute to the maintenance of the proper rhythm. Abundance of venous blood stimulates an increase of the respira- tory action. In addition to exchanging gases the lungs also discharge moisture and give off a certain amount of heat. The Circulatory System. — The circulatory system is a closed sys- tem of vessels supplying all parts of the body with blood and a system of spaces, sinuses, and vessels collecting lymph from the vari- ous organs to return it to the blood vessels. The blood circulatory system centers in a contractile heart from which tubular arteries lead out to various organs of the body where they branch into min- ute vessels or capillaries. The capillaries converge as they carry the blood away from the organs to form the veins which carry the blood back to the heart. This is a closed system of vessels. The blood is composed of the clear fluid, plasma, and the Uood corpuscles. The red corpiiscles contain the red pigment matter, hemoglobin, which THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 393 was mentioned in connection with respiration. Due to this sub- stance, the cells have oxygen-carrying power. The white corpuscles or leucocytes are of several varieties and they are amoeboid in char- acter. These cells may make their way among cells of other tissues Veins from upper. part of Bcxjy Lymphatics Thoracic duct - ^uporiop vena cava 'PulmonaP3^ artary Ui^ht aupicla — Infcpiop vena cava- fli'^ht vcntpiclG Lacteal^ — Hepat ic vain. ■ Veins from lowcp papt of Body Lympfiaticj — Arteries to upper* part of Body Pulmonapy vein — Left auricle - Left ventpjclc ArtGplo3 to lowcp par>t of Body Fig. 219. — Diagram of circulation of the blood in a mammal. The oxygenated blood is shown in black ; the venous blood in white. The lymphatics are the black irregular lines. (From Pettibone, Physiological Chemistry, published by The C. V. Mosby Company.) where they engulf bacteria and foreign matter. Upon exposure to air the dissolved fibrinogen in the blood becomes fibrin and forms a clot which is semisolid and blocks flow of blood from most wounds. The remaining' fluid after the blood clots is called serum. Lymph 394 TEXTBOOK OF ZOOLOGY is a fluid similar to plasma which has seeped through the walls of the capillaries in the various organs, and it carries amoeboid white corpuscles. Certain of them are produced in the lymph glands. The spleen is a lymphoid organ in which debilitated red corpuscles are disintegrated and the products placed in the blood. Circulation. — Transportation of materials through the protoplasm of a single cell or a single-celled organism and from cell to cell of the metazoan is a fundamental function among living things. In most Protozoa there is no special arrangement for this function, but the necessary exchange and movement of food materials, waste sub- stances, and gases is accomplished by simple diffusion of materials. In a few forms, however, of which Paramecium is an example, there is a definite course of movement by the endoplasm. This is known as cyclosis, and it serves to circulate the food vacuoles. In double-walled, simple, saccular forms like hydra there is no pro- vision necessary except an exchange of the water in the gastrovascu- lar cavity. In flatworms, such as planaria, the necessity of increased food distribution is cared for by branching of the gastrovascular cavity into diverticula. In sponges the wandering cells assist in transporting materials. A distinct system of tubelike vessels with contractile parts is developed in the annelid worms, as was studied in the earthworm. Here a closed system of vessels forms a complete circuit to carry a circulating medium to all parts of the body. In this group the fluid is known as hemolymph because it bears no red corpuscles. The hemoglobin is borne in the fluid. The vertebrate system is closed, and the blood is circulated by the action of a single heart. The hemoglobin, an iron compound, is carried in the red blood corpuscles. In molluscs and some crustaceans there is a similar respiratory pigment carried in the plasma, which is called hemo- cyanin. Instead of iron, copper is the principal constituent of this pigment. Vertebrate blood is largely water carrjdng dissolved mate- rials and suspended corpuscles. The fluid part is known as plasma. The amount of blood in a mammal is approximately one-twentieth of the body weight, or in the average man a little more than a gallon. The plasma contains enough inorganic salts to taste slightly salty. Its salt content is about equal to that of sea water. When the body is active, the blood is very unequally distributed. One-fourth is always in the heart, large arteries, veins, and lungs. Another fourth is held in the hepatic portal system, the liver and its sinuses; the THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 395 skeletal muscles require another fourth ; and the remaining fourth is distributed through all of the other organs. Human blood contains normally about 5,000,000 red corpuscles (erythrocytes) per cubic millimeter of volume in the male and about 4,500,000 in the female. The average person, weighing 150 pounds, then, would possess ap- proximately 20,000,000,000,000 (20 trillion) of them. Each erythro- cyte is essentially a little capsule filled with hemoglobin which is a compound peculiarly fitted to unite with atmospheric oxygen. When united with oxygen it is known as oxyhemoglobin, which is readily reduced to give up the oxygen to the cells when the blood reaches the tissues. The carbon dioxide given off by the cells is collected principally in the plasma and returned to the lungs. The leucocytes or white corpuscles are quite variable in form and number from 6,000 to 10,000 per cubic millimeter. They are amoe- boid and therefore not confined to the blood vessels. One of their chief functions is the destruction of bacteria and other foreign mate- rial in the tissues. This process is known as phagocytosis. The ac- companying table summarizes essential information concerning blood cells. The plasma of the blood contains a group of substances called antibodies. These have been produced by various tissues of the body upon contact with certain foreign proteins. Since bacteria and patho- genic Protozoa react as foreign protein, they stimulate the body tis- sues to the production of specific protective antibodies and physicians have come to make use of these antibodies in sterile serum for pre- vention and treatment of several diseases. Some of these antigen substances bring about the clumping or agglutination of foreign bac- teria, others dissolve the bacteria, and still others cause them to be precipitated. The chemical nature of these bodies is not yet known. There are individuals known as hemophiliacs or bleeders whose blood will not clot, and any wound is likely to be fatal. The plasma normally contains a soluble protein, called fibrinogen and calcium in solution. Howell 's theory of coagulation of blood holds that there is also an inert substance, antithrombin, which prevents the activation of the prothrombin of the plasma to become thrombin. When blood is shed and exposed to air, the blood cells and platelets produce a sub- stance, cephalin, which, in the presence of calcium, neutralizes the antithrombin, allowing the formation of thrombin. Thrombin reacts with fibrinogen to produce fibrin, the solid fibers of the clot. The rate 396 TEXTBOOK OF ZOOLOGY Average Characteristics of Human Blood Cells KINDS OP CELLS AND AVERAGE NUMBER STRUCTURE PER CUBIC MILLI- COLOR WITH METER OP BLOOD WRIGHT'S STAIN SOURCE FUNCTION Erythrocytes Nonnucleated, circu- Endothelium of Transport oxygen; (red blood cells) lar, biconcave ; capillaries of remain in blood 5,000,000 (males) orange buff ; 7.5 bone marrow vessels 4,500,000 (fe- to 7.7 microns in males) diameter Leucocytes Colorless in life (white cells) 1. Granulocytes Nucleus of lobes Eeticuloend othelial Amoeboid ; can 6,000 to 10,000 joined by thread ; cells outside leave blood ves- stains dark lilac, capillaries of sels and enter cytoplasm pale ; bone marrow tissues blue with gran- Defend against in- ules; 9 to 12 mi- fection crons in diameter a. Neutrophile Granules stain 60 to 70% weakly b. Eosinophile Granules few, eosin 2 to 4% (red) c. Basophile Granules deep blue 0.5 to 1.5% 2. Lymphocytes Nucleus single, Lymphoid tissue. Nonmotile ; related 20 to 30% large, round, deep spleen and to immunity blue ; scant cyto- lymph glands plasm, clear blue 4 to 10 3. Monocytes Nucleus single. Spleen and bone Very motile ; 5 to 10% large, round, deep blue ; much cyto- plasm, muddy blue; 12 to 20 marrow phagocytic Platelets Small, ref ractile, no Bone marrow Provide substance 200,000 to 400,000 nucleus ; dark blue to lilac ; 2 to 3 needed in clotting (Reproduced by peri«is.«ion from Textbook of Zooloay by Storer, copyrighted 1943, by McGraw-Hill Book Co., Inc.) of the heartbeat for an average adult man at rest is about 72 times per minute. The contraction phase of the heartbeat is called the systole and the relaxation phase is the diastole. It has been estimated that an average circuit of the circulation of blood in man can be com- pleted in twenty-three seconds, with about two seconds of this time being spent in capillaries. The Excretory System. — The excretory system of vertebrates con- sists of kidneys, excretory ducts, and often a urinary bladder. The kidneys serve to remove from the blood, waste nitrogen products and excess salts in solution as well as to dispose of excess water. In simpler vertebrates there is a pronephros type of kidney as well THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 397 as a mesonephros. The former is seldom functional, but the latter is the functional organ in vertebrates up to and including the Amphibia, as in frogs and salamanders. The metaiiephros is the higher developed kidney as found in reptiles, birds, and mammals. The ureter is the excretory duct which leads from the metanephric kidney. The life history of these animals as individuals includes successive stages as follows: the pronephros, the sole kidney for a time; followed by the mesonephros which is the dominant func- tional excretory organ when in its glory; and, finally, the develop- ment of the metanephros with retrogression of the others. This is an illustration of the Theory of Recapitulation which says that each individual in its development lives through abbreviated stages of the history of the development of the race. Excretion.^A certain result of the oxidation necessary for metab- olism is the production of end-products which are not only of no further use to the protoplasm but may be a distinct menace to the welfare of the organism because of their toxic effects. The sub- stances are usually dissolved and removed as a waste liquid or occa- sionally as crystals by special parts of the body. In Protozoa this function is performed by general diffusion through the plasma membrane and in many forms by the contractile vacuoles. The quantity of water which passes through the protozoan in twenty- four hours is several times the volume of the animal itself. Among sponges and coelenterates diffusion of liquid wastes through the general surfaces of the body to the surrounding water serves for excretion. In an animal like the flatworm, planaria, excretion is accomplished by a system of canals which begins in numerous capillary-sized tubules whose blind ends are composed of individual cells called flame cells. These flame cells are irregular in shape and each bears a tuft of cilia extending into the end of the tubule. The flickering movement of the cilia in the cell gives the appearance of a flame and moves the accumulated excretion down the tubule. The waste liquid of the surrounding tissues diffuses into this cell. The main excretory ducts open to the surface of the body by excretory pores. This ar- rangement is sometimes called a protonephridial system. The nephridial system is found in Annelida and has been studied in connection with the earthworm. Here a coelomic cavity is present, and a series of segmentaUy arranged pairs of coiled tubes or 398 TEXTBOOK OF ZOOLOGY nephridia extend through the wall to the exterior. The excreted wastes accumulate in the eoelomic cavity and are moved into the nephridia through the ciliated funnellike internal end, known as the nephrostome. This eoelomic fluid is drawn into the canal of the nephridium by the beating of the cilia and is delivered to the outside of the body at the nephridiopore of the next segment. The green glands of crayfish are much more concentrated, although they are modified nephridia. They function as a pair of unit organs, each opening by a duct on the basal segments of the antennae. In mollusks there are both nephridia, known as pericardial glands, and the special cells formed from the eoelomic epithelium. The echino- derms make use of direct diffusion as well as intracellular excretion by which excreted materials are taken up from the eoelomic cavity by the numerous phagocytic, amoeboid cells of the eoelomic fluid. These cells wander out into the cavities of the respiratory organs where they coalesce into large masses, and finally with their enclosed granules are cast out through the membranes of the respiratory papillae. Soluble materials in solution also diffuse through the mem- branous walls of these structures. In the insects excretion is taken care of by the Malpighian tubules, which are considered modified nephridia. They are bunched in the posterior part of the body cavity and discharge excretions into the intestine at its junction with the rectum. Kidneys. — The chief excretory organs of vertebrates are called kidneys, and they are thought by some authors to have developed by modification and condensation from segmentally arranged nephridial tubules. The fact that in vertebrate embryos as well as in lower chordates, even the frog, these tubules open into the coelom as nephrostomes, makes it seem possible that in vertebrates as well as in annelids the coelom was once important in excretion. The essential structures of the kidney for taking waste substances from the blood and delivering it to the exterior of the body are the Malpighian corpuscles, each made up of a glomerulus and a Bow- man* s capsule, and the coiled uriniferous tubules which discharge the excretion through collecting tubules into the ureter at the pelvis of the kidney. This canal leads to the cloaca in most vertebrates below mammals (excepting some fish), or to a urinary bladder in the mammals. THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 399 The wall of each Bowman's capsule is very thin and readily per- mits diffusion of water and dissolved materials from the blood into the cavity of the uriniferous tubule on the opposite side of the mem- brane. The glomerulus carries arterial blood from the afferent ar- terial branch and discharges it into the efferent arterial branch. The latter soon spreads into a capillary network which surrounds the convoluted portions of the uriniferous tubule. Water constitutes the largest volume of materials to be excreted in most animals, ex- cept in some desert forms where water is conserved and the ex- cretion is in crystalline form. Water is eliminated by lungs, skin, alimentary canal, and kidneys. In man the quantity of sweat dis- charged may amount to two or three liters a day. In the dog, which has few sweat glands, the water eliminated by the lungs, through panting, is proportionately greater than in man. The kid- neys are the most important organs in the excretion of water, and the amount they eliminate is inversely proportional to the amount excreted by the skin. Most of the water to be excreted is taken from the blood in Malpighiaji corpuscles. Some of the nitrogenous wastes are excreted in the form of am- monium salts and some free or combined amino acids. However, most of the ammonia which results from protein metabolism is con- verted into urea in the liver and is carried in that form to the kid- neys where it is removed from the set of capillaries ramifying over the convoluted tubules by a process of true secretion. According to this idea, the urine which consists of urea, various salts, other soluble materials, and water is excreted by different parts of the uriniferous tubule. The substances which are excreted by the kidney are not formed there, but are merely removed from the blood by this organ. The Nervous System. — The nervous system in this type of animal is composed of a hrain and spinal cord forming the central nervous system; nerves extending to all parts of the body, ganglia which are groups of nerve cell bodies outside the central nervous system, and the sense organs which serve for receiving stimuli are usually grouped together under the name peripheral nervous system. A por- tion of this latter division, consisting of two longitudinal trunks with ganglia distributed along them, lies parallel to the spinal cord, and constitutes the sympathetic system. Each ganglion has a connec- tion with the adjacent spinal nerve or cranial nerve as the case 400 TEXTBOOK OF ZOOLOGY might be. This system controls the involuntary muscles. The pe- ripheral system includes ten to twelve pairs of cranial nerves from the brain, and ten to thirty-one pairs of spinal nerves in different forms of vertebrates. Each spinal nerve has two roots where it joins the spinal cord. A dorsal root receives fibers from sensory end- ings and therefore conducts impulses toward the cord. This root has a spinal ganglion located on it. The ventral root of each of these Fig'. 220. — Cross section of spinal cord and roots of spinal nerves, sliowing a simple reflex circuit. 1, sensoi-y surface of skin ; 2, afferent nerve fiber with S, its cell of origin, located in the spinal ganglion ; i, cut end of spinal nerve ; 5, efferent nerve fiber ; 6, voluntary muscle ; 7, dorsal root of spinal nerve ; 8, ventral root of spinal nerve ; 9, dendrites of motor nerve cell body in gray matter of the cord. (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, after Morat.) nerves carries fibers extending from the motor cells in the cord to the motor end plates on the voluntary muscle cells. The impulses, therefore, pass from the spinal cord to the muscles over these roots. The reflex arc, which is the simplest kind of a nerve conduction cir- cuit, is set up by the connectives from the sense organ or receptor to the gray matter of the cord and then the return connection from the motor nerve cells over the ventral root to the muscles. In gen- eral, the relationship of parts in regard to function is similar to what has already been seen in the higher nonchordate animals. THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 401 A high development of sense organs for the senses of sight, hear- ing, smell, taste, and touch is characteristic of vertebrates. The organs are receptors and they are stimulated by changes in external environ- mental conditions, such as light, sound waves, chemical changes, and contact. The eye, which is the organ of sight, is a highly developed organ. It is constructed on the plan of a camera with the eyeball forming the light-tight box. The wall of this is composed of an outer fibrous sclera (white of eye) which continues anteriorly as a transparent front, the cornea. Beneath the sclera is a black, pig- mented and vascular layer, the chorioid, which continues anteriorly as the iris, the colored part of the eye. The iris is like a curtain C.Vw crjo.-y\/. Fig. 221. — Diagram of the eyeball; c, cornea; a, aqueous humor; I, lens; v, vitreous humor; sc, sclerotic coat; ch, chorioid coat; r, retina: /, fovea centralis; i, iris; s.L, suspensory ligaments; c.p., ciliary process; cm., ciliary muscles; op.n., optic nerve. (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company.) surrounding a space at the anterior surface of the eye and this space between its medial margins is the pupil. The pupil appears black because there is no light behind it. Behind the pupil is a. transparent lens whose surfaces are curved to bend the rays of light in such a way as to focus them on the sheetlike retina behind. The retina is a lateral extension of the brain and is the sensory part of the eye. It lies as a lining of the inside of the posterior half of the cavity of the eye and is connected directly with the brain by the optic nerve. The general cavity of the eyeball is divided into some chambers. The 402 TEXTBOOK OF ZOOLOGY external or aqueous chamber between the cornea and the lens, with the iris extending into it, is filled with aqueous humor. This cavity is subdivided by the iris. Behind the lens is the large internal or vitreous chamber which is filled with a jellylike vitreous humor. The curvature of the lens can be controlled by the action of the ciliary muscle which encircles its margins. This makes possible an adjust- ment of the eye to near and far objects and particularly so in higher vertebrates. This power is known as accommodation. As people get older they tend to lose this accommodation because of loss of elasticity in the lens. The tension on it due to the attachment of the inside of the eyeball by the ciliary process tends to hold it out Fig. 222. — Diagram of a section through the right ear. B, semicircular canal ; a, external auditory meatus; o, oval window (fenestra ovale) ; P, tympanic cavity containing the three auditory ossicles ; Pt., scala tympani ; r, round window (fenestra rotunda) ; below r is seen the Eustachian tube; 8, cochlea; T, membrana tympani; Yt, scala vestibuli. (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, after Czermak.) in a flattened condition. This focuses the eyes very well on distant objects but does not provide the necessary curvature of the lens to bring near objects in focus. Eyeglasses are used by older people to supply this lost phase of accommodation. A ray of light enters the eye by passing through the cornea, then aqueous humor, pupil, lens, vitreous humor and then to the retina where the sensory cells are stimulated and the impulse carried to the brain by the nerve fibers of the optic nerve. THE VERTEBRATE ANIMAL — SUBPHYI.UM VERTEBRATA 403 The ear structures provide most classes of vertebrates with facilities for two functions: hearing and equilibrium. This organ consists of an external ear, which serves in catching and directing sound waves within, a middle ear or tympanum, containing ossicles, and the inner ear, which contains the sensory cochlea with its organ of Corti for hearing, and the semicircular canals, which are concerned with equi- librium rather than hearing. The latter are common to all verte- brates while the cochlea is limited to Amphibia and higher classes. The external ear is still further limited to reptiles, birds, and mam- mals. The middle ear is a space beneath a tympanic memhrane which separates it from the external auditory canal. In this cavity are three bony ossicles, the malleus, incus, and stapes, which transmit the sound vibrations from the tympanic membrane to the membrane over the fenestra ovalis leading into the internal ear. The mem- branous labyrinth is the name often applied to the chambers of the inner ear. Its ventral chamber is the sacculus connected with the organs of hearing, and the dorsal portion is the utricidus which is related to equilibrium. The two semicircular canals in simpler forms and the three in higher, join the body of the utriculus in as many different planes as there are canals. In the higher forms there are two vertical canals, one anterior and one posterior, with their planes at right angles to each other, and one horizontal canal. At one end of each canal there is a bulblike swelling or ampulla which contains sensory hairs. When the position of the head is moved, the fluid in the canals changes its level and position to stimulate the sensory hairs, thus giving a sense of position. The sound waves which stimulate the sensory cells of hearing enter the external ear and set up vibrations in the tympanic membrane. These are in turn transmitted by the ossicles to the fluid endolymph within the labyrinth. The vibrations of the fluid extend through the cochlea, in which the sensory cells are supported on the organ of Corti stretched across it. These cells are connected with the brain by way of the auditory or eighth cranial nerve (Fig. 222). The sense of smell is centralized in the epithelial lining of the nasal chamber. Special olfactory cells are stimulated by particles of mate- rial from the air dissolving on this membrane and making contact with the sensory cells. The sense of taste is similar except that it is located in sensory cells in taste buds on the tongue, epiglottis, and I 404 TEXTBOOK OF ZOOLOGY lips (and barbels of some vertebrates). The particles come in by way of food and drink and as the material dissolves, it reaches the taste cells. Most of the tactile and pressure sense organs are located just beneath the skin over different parts of the body. A few of the pressure sense organs are found in certain of the internal structures of the body. The lateral line system in fishes is sensory to vibra- tions carried in the water and is quite important to aquatic animals of this type. Nervous Function — Reception and Conduction. — Irritability and conductivity are fundamental functions of all protoplasm, whether it be in the body of an Amoeba or a man. The responsiveness of organisms to change of conditions both externally and internally determines their behavior. Living protoplasm is not only excitable, but it possesses the power to record or store up the effects of previ- ous stimuli. In the final analysis, the perceptions and reactions of man are but expressions of these primitive functions in a more specialized organism. The protozoan organism has only neuromotor apparatus and de- pends largely on the primitive properties of irritability and conduc- tivity to guide its activities. In the simpler Metazoa, such as the co- elenterates, there are scattered nerve cells connected with each other by fibers to form a nerve net. The neuroepithelial or neurovmiscular cells which make up this continuous net through the body are the forerunners of the typical neurone and are called protoneurones by Parker. A protoneurone transmits in every direction while a true neurone transmits in only one. In the net system there is no central exchange and no specific path of conduction. Every part of the receptor surface of such an organism is in physiological continuity with every other part of the body. Next comes the linear type of nervous system in the form of a ladder. It is composed of an organization of neurones into a double chain of ganglia, each cord lying lateral to the digestive tract with transverse connectives and predominant ganglia at the anterior end. Such a system was studied in planaria. In Annelida and Arthropoda the nervous sj'stem is a modified ladder type in which the two longi- tudinal cords of ganglia have fused along most of the midventral line. Toward the anterior end, the cords separate at a paired gan- THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 405 glionic enlargement, the siil) esophageal ganglion, and encircle the alimentary canal to join on the dorsal side as the pair of siipra- esophageal ganglia or ''brain." In Arthropoda the ganglia of the thorax have undergone considerable fusion. In Echinodermata, the starfish for example, the central group of ganglia makes up the cir- cumoral nerve ring around the mouth, and radial branches extend into each arm. Branches from these communicate with the sensory structures of the skin and tube feet. Concentration of the tissue of the nervous system into definite organs is carried farther in vertebrates than in the less highly or- ganized forms. The fact that the central nervous system of verte- brates is dorsally located and hollow has been brought out previously. Even within the group of vertebrates, the nervous system shows a progressive increase in complexity. The highly developed brain of the mammal is the climax of this tendency. The neurones have been referred to before as the units of structure and function of the higher type of nervous system, from worms to man. Each neurone is a nerve cell with processes extending from it, and each of these units must conduct nerve impulses in its normal function. The exact nature of the nerve impulse is still somewhat of a question. It is thought to be transmitted as a metabolic change passing along the nerve fiber (axone or dendrite). This is at least partially a chemical change in which oxygen is necessary and a cer- tain amount of carbon dioxide is produced, but since there is only slight increase in temperature during the change, it seems not to be a typical metabolic oxidation process ; furthermore, the activity seems not to fatigue the nerve fiber. An electrical charge follows the wave of activity along the nerve fiber, but it apparently accompanies the impulse or is a result of it rather than the impulse itself. The speed of electrical transmission has been measured in a number of different animals and nervous transmission is much slower than electrical. At room temperature the sciatic nerve of a frog will transmit a nerve impulse at the rate of about 100 feet per second. Conduction over nonmedullated fibers of invertebrates is much slower than this. On the other hand, measurements of the rate of conduction in man show a velocity of about 400 feet per second. The reflex arc and reflex actions illustrate the simple form of nerv- ous conduction circuit. In its simplest form the reflex arc is com- 406 TEXTBOOK OF ZOOLOGY posed of one motor and one sensory neurone; however, it is usually- more complex. The classical example involves the spinal cord and a spinal nerve. This is known as a reflex of the first level, because it returns the motor impulse over the motor fibers of the same nerves which brought in the sensory impulse. The motor axone carrying the impulse from the motor nerve cell in the gray matter usually j ends in a muscle cell or a gland. There is no protoplasmic union between the axone of the sensory neurone and the dendrite of the motor, for these come in contact only by a synapse which brings them in close proximity. It has been found experimentally that nervous impulses may be conducted in either direction by the fiber but can cross a synapse only from axone to dendrite, thus serving like a valve in a pipeline. Reflex actions may be in the form of motion, as with- drawal from unexpected pain, or shivering or formation of goose flesh, or the contraction of the pupil of the eye with increased light intensity. Still other reflex actions include secretion by glands, breathing, movements of speech, individual actions included in walk- ing, and others. Functions of the Spinal Cord. — This organ serves as a system of reflex centers which control the actions of glands of the trunk, vis- ceral organs, and skeletal muscles. The spinal cord is also a nerv- ous pathway between the brain and numerous organs of the body. It is said that more than half a million neurones join the cord through the dorsal roots of the spinal cord. Functions of the Divisions of the Brain. — Conscious sensations and intelligence are centered in the gray matter or cortex of the cerebrum. This section controls voluntary actions and provides memory associations. The diencephalon serves as a center for spon- taneous actions. The midbrain is one of the centers of coordinated movement which has to do with posture and eye muscles. The cere- bellum is another center of coordinated movement, particularly with reference to equilibrium. The impulses from the muscles, tendons, joints, and semicircular canals of the ear are coordinated so that in a movement or posture the proper muscles may be contracted to the proper extent at the proper time. Below and behind the cerebellum is the medulla oblongata which controls breathing and may be an inhibitor on heart action. It also regulates digestive secretions, movements of digestive organs, and vasomotor activity of the blood THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 407 vessels. As a Avhole, the brain serves as the organ of coninuinication between the sense organs and the body and is the coordinator of the bodily activities. The Reproductive System. — The vertebrate reproductive system shows a fairly high degree of development. The sexes are almost universally separate, with the exception of some cyclostomes. The distinct gonads develop to produce special germ cells. The male gonads are testes, and they produce spermatozoa which are carried from the gonads by the vasa deferentia. The female gonads are ovaries, and they produce ova or eggs. They are carried from the body by oviducts. The males of some classes possess for use in copu- lation certain accessory organs which tend to insure fertilization. The vertebrates which lay eggs are spoken of as being oviparous; in those in which the egg is retained in the body and the embryo develops there, feeding on the yolk of the egg, and is later born alive, the condition is known as ovoviviparous, and in the forms in which the fertilized ovum is retained in the uterus, the embryo be- ing nourished by diffusion of nutriments from the blood of the par- ent, the condition is said to be viviparous, and here too the young are born alive. In vertebrates the possible offspring produced each season by a single individual varies from one to thousands. Reproductive Function. — A living organism is in numerous ways similar to a machine, but reproduction of new units of living mate- rial by existing organisms is hardly comparable to any mechanical processes known in our industries. New organisms all arise from preexisting organisms of the same kind. The process of cell divi- sion is the fundamental basis for all reproduction. For centuries before the invention of the microscope it was commonly believed that living things arose spontaneously from nonliving material, or from the dead bodies of plants and animals. Certain old books carry directions for the artificial generation of mice or bees. Louis Pasteur did as much as anyone to discredit this idea of spontaneous generation. Our present conception is that the protoplasmic sub- stance of the new individual is but a continuation of the specific protoplasm peculiar to an earlier individual or in sexual reproduc- tion to two individuals. Therefore, under ordinary circumstances the structural and physiological complexities which arise through embryonic development must be generally similar to those of the predecessors. 408 TEXTBOOK OF ZOOLOGY In most of the single-celled organisms reproduction may occur by such equal division of the protoplasm (binary fission) that the new individuals cannot be distinguished as parent and offspring. Protozoa may reproduce also by sporulation, by which process the cell forms a protective cyst and by a series of simple divisions (frag- mentation) the internal protoplasm breaks into a number of smaller units. Following this the cyst ruptures and releases these new units as independent individuals. For the most part, reproduction among protozoans is taken to be asexual, but according to a recently pub- lished work by Sonneborn, a distinct sexuality exists in Paramecium. Examples of asexual reproduction by budding and fission have al- ready been pointed out in the studies of reproduction of sponges, hydra, planaria, and even in tunicates. Sexual Reproduction. — In certain of the colonial Protozoa, volvox for example, the colony may reproduce for several generations by asexual division of the individual cells but sooner or later the cells of the colony become specialized into conjugating individuals. In some forms this goes to the extent of certain cells becoming distinct gametes with male and female characteristics. In such forms it is possible to see foreshadowed sexual reproduction as it is known in Metazoa. In the simplest of Metazoa, as in sponges, there are no specially organized gonads for the production of germ cells, but as a rule the germ cells are produced in such organs set apart for this purpose. The ovary produces mature or nearly mature ova and the testis pro- duces mature spermatozoa. Hermaphroditism is the condition in which the same individual produces both ova and spermatozoa. It occurs principally in the simpler Metazoa, a few higher ones, and rarely among normal verte- brates. Previous studies made on the reproduction of hydra have brought out that the gonads are temporary, both being formed by aggregations of formative or interstitial cells between the ectoderm and endoderm. After the seasonal production of germ cells is com- pleted, the gonads disappear. In flatworms and annelid worms the gonads are permanent structures of the mesoderm. Both ovaries and testes are present. Even in these true hermaphrodites cross-fertiliza- tion is insured by copulation or union in such a way that the sper- matozoa of one individual fertilize the ova of another. In certain THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 409 other hermaphroditic forms (as some cyclostomes) the spermatozoa and ova of a particular individual are usually not mature at the same time. Bisexual reproduction is the form of reproduction common to many groups of the higher invertebrates and nearly all vertebrates. Here the sexes are distinct, each with functional gonads abd ac- cessory structures capable of producing only one kind of germ cells. In some of the types of animals already studied individuals of the two sexes have simply deposited the mature germ cells in the same vicinity and at about the same time. Under the sections on re- production in starfish and the bullhead (fish) such a procedure has been described. In animals like the toads and frogs, a special pro- vision is made to bring the individuals of the opposite sexes to- gether in that the male clasps the female and sheds sperm over the eggs as they are expelled from the cloaca. This act is known as amphiplexus. It will be remembered that the first and second pairs of abdominal appendages of the male crayfish are modified for trans- ferring spermatozoa into the seminal receptacle of the female, where they remain until the eggs are laid. This represents a beginning in the development of a copulatory organ. The majority of bisexual or dioecious animals make a still greater provision to insure fertili- zation of the ova by copulation or coitus. At the time of breeding the mature spermatozoa are delivered to the cloaca or vagina of tha female, and the ova are fertilized within the genital tract of the female. In birds and most reptiles after the addition of nutritive and pro- tective coats the egg passes to the outside to develop and hatch (ovip- arous animals) but in all mammals, except monotremes, it is retained within the uterus during the period of embryonic development, and the young are bom as more or less developed individuals (vivipa- rous). In the females of viviparous mammals the posterior portions of the two oviducts are modified into a uterus within which the young are retained and nourished until ready for birth. The internal wall of the uterus and the external embryonic membranes (serosa and allantois-chorion) cooperate to form a placenta through which food, metabolic wastes, and respiratory gases diffuse between parental and embryonic blood. The blood does not pass from parent to embryo 410 TEXTBOOK OF ZOOLOGY or vice versa but tlie necessary materials are allowed to diffuse through the tissue of the placenta in which both systems are dis- tributed. Parthenogenesis. — In some species of invertebrates, sexual re- production may lapse for considerable periods of time, during which period no males are developed. The female produces ova which develop into new individuals like herself without fertilization for a whole season. This is known as parthenogenesis. Usually in the fall of the year males are developed, and fertile eggs, provided with protective hard shells, are produced by the females of this generation to live through the winter. After winter is over such fertile eggs hatch into parthenogenetic females for the next season. This process is common in many smaller Crustacea, aphids, scale insects, some ants, bees, wasps, thrips, a few moths, and rotifers. Artificial partheno- genesis may be induced in many mature eggs by change of osmotic pressure due to change of salt content in the surrounding medium. Fatty acids, saponin, solanin, bile salts, benzol, toluol, chloroform, ether, and alcohol are other substances which will induce it. Electric stimulus, mechanical pricking, and change of temperature are also used. Such methods have produced artificial parthenogenesis in eggs of sea urchins, starfish, molluscs, annelids, moths, and frogs. The immediate cause of the development by an egg thus stimulated is not known. In normal fertilization of an egg by only one spermatozoon, it has been found that the rate of oxidation then increases from 400 to 600 per cent. There are indications that this is also the case in artificial parthenogenesis. This oxidation may be the cause of the development in the ovum. Fertilization, where it occurs, has a dual function: (1) that of stimulating the egg to develop, and (2) that of introducing the properties of the male parent. Classification In most recent classifications this subphylum is divided into seven classes ; however, the second is sometimes found as a subclass under the third. These classes are as follows: Cyclostomata (si klo sto'ma ta, circle and mouth). Round-mouthed fish with only median fins, unsegmented notochord, and jawless. Lampreys and Hagfish. < THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 411 Elasmobranchii (e las mo bran'ki i, metal plate and gills). Pish with jaws, cartilaginous skeleton, persistent notochord, and plaeoid scales. Sharks, Rays, and Chimaeras. Pisces (Pis'es, fishes). True fish with bony skeleton, gill respira- tion, with jaws and paired lateral fins. Catfish, Perch, Bass. Amphibia (am fib'i a, both lives). Cold-blooded, nonscaled aquatic and terrestrial vertebrates with five-fingered, paired appendages. Most of them breathe by gills in the larval stage and by lungs in the adult. Toads, Frogs, and Salamanders. Reptilia (rep til'i a, crawling). Cold-blooded forms which are fundamentally terrestrial, usually possessing a scaly skin and breath- ing by lungs. Turtles, Lizards, Snakes, and Crocodiles. Aves (a'vez, birds) . Warm-blooded, erect forms possessing feathers. The forelimbs have become wings. All birds. Mammalia (mama'lia, mammary or breast). Warm-blooded ver- tebrates with hair and with mammary glands for suckling the young. Cats, Men, Monkeys, Whales, Seals, Bats, etc. CHAPTER XXV CYCLOSTOMATA* Because of the absence of jaws this group is sometimes known as Agnathostomata (ag nath o sto' ma ta). This name is in contrast to Gnathostomata (jaw mouth) which includes all other vertebrates. The mouth of the cyclostomes is round, jawless, and suctorial. There are some exoskeletal teeth located on the roof and floor of the mouth and on the tongue. The body is slender and eel-like in shape. It is covered with a slippery, smooth skin and has only dorsal and ventral median fins. Classification The group is divided into two subclasses (or orders according to some authors) distinguishable by presence or absence of tentacles around the mouth, number of gill slits, and the number of semi- circular canals. These subclasses are Myxinoidea (Hyperotreti) in- cluding the hagfishes; and Petromyzontia (Hyperoartii) including lamprey (or improperly, lamprey eel to some). Myxinoidea or hagfish are all included in one family Myxinidae which is divided into three genera : Myxine of the Atlantic and Pacific Oceans, Bdellostoma and Paramyxine of the Pacific. These each have specific characteristics, but they all agree in having a terminal nostril, four tentacles on each side of the mouth, ability to produce enormous quantities of mucus, and the lack of the oral funnel or sucker. They all possess twelve or more pairs of gills, only one semicircular canal in the inner ear, and a functional pronephros. The development of the hagfish does not include a metamorphosis. They usually live in the mud of the sea bottom except when they are feeding either on the dead body of a fish or attached to a live one. They frequently enter the mouth or gills of fish caught in nets or those dead from natural causes and devour all of the inter- •If the frog or toad Is to be used as the laboratory animal representing the typical vertebrate, and the instructor so desires. Chapters XXV, XXVI, and XXVII may be omitted until after the study of Chapter XXVIII and then assigned if time permits. 412 CYCLOSTOMATA 413 nal organs and flesh. They frequently attack living fish which have been otherwise injured. Subclass Petromyzontia likewise includes only one family, Petro- myzontidae, which follows the type name. There are several genera including Petromyzon, the common Atlantic form, Ichthyomyzon of the lakes and streams and Entosphenus of the Pacific coast. Entosphenus tridentatus trident atus is the northern form and E. tridcntatus ciliatus is the southern form. The lampreys live in both salt and fresh water, and they are quite predaceous, attacking fish of considerable size. The characteristics of the group will be brought out under the discussion of Lamprey as a typical representative. Economic Relations of the Class In a general way lampreys are both beneficial and injurious. They all serve as excellent fish food and fish bait when they are in the larval stage. Brook lampreys are classed as wholly beneficial since they feed on microscopic organisms while larvae and do not feed as adults. Sea lampreys and lake lampreys are both valuable as human food, especially just preceding the spawning season. The sea lamprey, for the two or three years it spends in the ocean, lives at the expense of marine fish. It attaches itself and rasps a hole in the side of a fish about once a month, and through the hole thus formed, it sucks the fish's blood. One will remain to a single fish for about five days, get its fill, and release itself. The fish frequently dies as a result. Since the sea lamprey does not feed after it starts up stream, it does little harm to fresh-water fish except as the newly matured ones are making their trip to sea. The lake lamprey is similar except that it spends its entire life in fresh water. They are very destructive to lake fish since they are predaceous and spend their adult lives in the lakes. THE LAMPREY Habitat All live on or in the muddy bottoms of fresh-water streams dur- ing larval stages. In adult life the sea lamprey goes to the open sea and the lake lamprey goes to the deep water of the lakes. Both return to the fresh-water streams to spawn a few years later. 414 TEXTBOOK OF ZOOLOGY SUCTORIAL MOUTH NASAL OPENING \- EYE -GILL CLEFT V •DORSAL FIN -URINOGENITAL PAPILLA CAUDAL FIN Fig. 223. — Lateral view of the Pacific lamprey, Entosphenus tridentatus. (Drawn by Titus C. Evans.) CYCLOSTOMATA 415 Habits and Behavior The animal is a rather inebriate type of swimmer because it is long and slender and does not possess paired fins. It winds its way through the water and occasionally comes to rest by attaching itself to a rock by means of an oral funnel. External Structure In most respects the Atlantic lamprey, Petromyzo7i marinus, and the Pacific form, Entosphenus trident aUis, correspond quite closely in structure and make excellent representatives for study of the group. The following account will fit them generally. They may reach a length of three feet and three inches. The color is rather variable but might be expressed as being a variegated olive brown. Along the length of the lateral axis of the body are distributed sensory organs. There are two dorsal fins and a caudal fin, but no paired fins. At the anterior end of the animal is the mouth with the luccal funnel extending from it. This funnel is provided with ehitinous teeth used in rasping through the body wall of the host fish. The annular cartilage supports the margin of the funnel and holds it open. Along the margin is a fringe of papillae. The mouth lies at the bottom of the funnel. In the floor of the mouth is a plungerlike tongue supported by a cartilage and bearing teeth. There are seven uncovered gill slits along each side of the anterior portion of the body. In front of the gills on each side of the head is a poorly developed eye. It has no lid, simply being covered with transparent skin. In a middorsal position on the head is located the single nostril which leads into an olfactory chamber, and on ventrally as a pituitary pouch or caecum. The anus is located in the midventral line a short distance anterior to the tail. Immediately behind it is the urinogenital opening at the tip of a papilla. The papilla is larger in the male specimens. Internal Structure The muscular system is quite primitive. It is principally a series of zig-zag myotomes along the length of the body very similar to those in Amphioxus. A large lingual muscle is differentiated for moving the tongue, and several bundles of muscular tissue radiate through the wall of the funnel to expand and contract it. 416 TEXTBOOK OF ZOOLOGY m m. -BUCCAL FUNNEL -TONGUE -NASAL SAC -OESOPHAGUS -PITUITARY POUCH BRAIN PHARYNX NOTOCHORD -INTERNAL GILL PORE -EXTERNAL GILL PORE -GILL POUCH -GILL LAMELLAE ■BRANCHIAL CARTILAGE •PERICARDIAL CARTILAGE -LIVER -MYOTOMES -INTESTINE -KIDNEY -TESTIS -PRONEPHRiC DUCT ANUS -URINOGENITAL APERTURE URINOGENITAL SINUS Fig 224 — Lateral view of dissection of Entosphenus to show principal organs. (Drawn by Titus C. Evans.) CYCLOSTOMATA 417 The skeletal system is cartilaginous, developed around a nonseg- rnented notochord along each side of which are paired cartilages called neural arches. At the anterior end is a skull whose floor and sides are cartilaginous, while the roof is membranous, except for a transverse bar. There are two auditory capsules near the posterior part of the skull. The buccal funnel is supported by the annular cartilage already mentioned and three sets of labial cartilages. The branchial area is supported by the cartilaginous hranchial basket which is composed of a i)air each of dorsal and ventral longitudinal bars, two pairs of sinuous, lateral bars, and nine much-curved, dorso- ventral bars. The anterior one of these is not in contact with a gill aperture. The cartilaginous pericardium joins the branchial basket at the posterior end. The digestive system is not very highly developed because the adult lives entirely on blood and lymph of other fish, obtained by rasping a hole through the body wall and sucking it out. They take a meal about once in three or four weeks. The blood is passed from the mouth down the esophagus which continues into the in- testine at the level of the posterior end of the branchial region. The intestine is slender and almost straight, but it has a slight internal fold which extends spirally through its length. This is called a typhlosole or spiral valve, and it tends to increase the absorp- tive surface. The intestine ends posteriorly at the anus. The liver is found in the anterior part of the body cavity. The circulatory system consists of a heart with two principal chambers, arteries, capillaries, veins, and lymphatic spaces. The posterior and anterior cardinal veins located just lateral to the lower side of the notochord collect blood from the body wall and head region, and empty it into the common cardinal vein which extends ventrally to the sinus venosus. The sinus venosus receives also the single inferior jugular and the hepatic vein from the ventral region. The blood then passes through the sinuauricular valve to the single auricle, thence by the auriculoventricular aperture to the single ven- tricle, thence through hulhus arteriosus to the ventral aorta. Six pairs of afferent branchial arteries carry the blood to the gills where capillaries supply the gill lamellae. The efferent branchial arteries collect this blood, carry it dorsally to join the dorsal aorta which is made up by their convergence. A carotid branch of this artery supplies the brain region, and the main aorta passes posteriorly. 418 TEXTBOOK OF ZOOLOGY giving branches to the viscera and body wall. There is no renal portal system; the caudal vein simply divides, giving one part to each posterior cardinal vein. — DORSAL AORTA 5TH.R. EFFERENT BRANCHIAL A. 4TH. R. AFFERENT BRANCHIAL A. VENTRAL AORTA INFERIOR JUGULAR V. R. JUGULAR VEIN VENTRICLE SINUS VENOSUS ATRIUM HEPATIC V. R. POST CARDINAL V, RENAL A. RENAL V, INTESTINAL V. INTESTINAL A. CAUOAL VEIN CAUDAL ARTERY Fig. 225. — Diagram of oblique ventrolateral view of heart, arteries, and veins of lamprey. Arrows indicate direction of flow of blood. (Drawn by Titus C. Evans. ) The seven pairs of gills and respiratory tube constitute the prin- cipal features of the respiratory system of this animal. When the CYCLOSTOMATA 419 animal is not attached to a host, water may be drawn through the mouth, under the velum, through the respiratory tube, through the paired gills and to the outside through the seven pairs of external apertures. The blood in the gill capillaries is aerated from the oxygen carried in the water as it passes over the gill lamellae. While the lamprey is attached to a host fish, the water is drawn into the respiratory tube through the gill slits and then discharged through them. — OLFACTORY SAC OLFACTORY LOBES PINEAL EYE RIGHT CEREBRAL HEMISPHERE OPTIC NERVE RIGHT GANGLION HABENULAE DIENCEPHALON PITUITARY POUCH OCULO-MOTOR N. MIDDLE CHOROID PLEXUS OPTIC LOBES TRIGEMINUS N, CEREBELLUM FOURTH VENTRICLE AUDITORY SAC AUDITORY NERVE MEDULLA OBLONGATA NOTOCHORD VAGUS N. Fig. 226. — Brain of lamprey. Lateral view ; dorsal view. Evans.) (Drawn by Titus C. The nervous system shows the development of a small, primitive brain, which possesses all five principal divisions of a vertebrate brain. From anterior to posterior it is composed of olfactory lobes, cerebral hemispheres closely fused to preceding, single dicncephalon with its dorsal epiphysis, midbrain with a pair of optic lobes, insig- nificant narrow bandlike cerebellum just behind the optic lobes, and the medulla just posterior to it. This continues directly posteriorly as the flattened spinal cord. The roof of the brain is rather mem- branous, as it is not entirely closed over. The sense organs include 420 TEXTBOOK OF ZOOLOGY the single nasal chamber which is located immediately anterior to the brain. Extending ventrally from the nasal chamber and project- ing beneath the brain to end blindly just above the esophagus is the pituitary pouch. As it passes beneath the diencephalon it makes contact with the infundibulum. The eyes of this animal are not highly developed, and sight is not used extensively by it. The audi- tory organ, which does not include an organ of hearing, is only for equilibrium ; it consists simply of a vestibule and two vertical semi- circular canals. The sense of taste centers in taste buds located in the respiratory tubes between the gill slits and possibly near the inner margin of the buccal funnel. The urinogenital system shows only fair development. The rib- bonlike kidneys lie, one at each side of the notochord and just dorsal P 0 AU HB OA SC N PfJ VN OE MM M LP-. OH OP cc vf a;jc t c VA V A G L F AN CF Fig. 227. — Ammocoetes larva of the lamprey, Entosphentos tridentatus. A, auricle of heart; AN, anus; ANC, anterior end of notochorcl ; AU, ear; BA, bran- chial arteries (afferent) ; G, duct connecting pharynx and thyroid ; CC, cranial cartilage, extending from tip of upper lip to a point slightly anterior to end of notochord, where it divides to form two lateral rods ; CF, caudal fin ; DA, dorsal aorta ; F, folds in intestinal wall, suggesting a possible origin of the spiral valve ; FB, forebrain ; G, gall bladder ; HB, hind brain ; I, intestine ; L, liver ; LP, upper lip, supported by cranial cartilage; M, body muscles (myotomes) ; MB, midbrain; MA'', position in which mesonepliros will develop; N, notochord; O, eye; OE, esorh- agus ; OH, oral hood; OL, olfactory organ; OP, oral papillae; P, pineal body; PC, pericardial cavity ; PN, pronephros, showing pronephric tubules with their ciliated funnels (nephrostomes) ; SC, spinal cord; T, thyroid; UL, under lip; V, ventricle of heart; VA, ventral aorta; VE, velum; VN, hepatic vein. (Courtesy of Albert E. Galigher, Inc.) to the peritoneal lining of the body cavity. A mesonephric duct ex- tends posteriorly along the free edge of each to join the small urino- genital sinus. This is located just posterior to the rectum and opens externally by the urinogenital papilla just behind the anus. The single gonad is rather large and is suspended by a peritoneal fold into the coelom. The sexes are presumably separate, but hermaphroditic con- ditions are occasionally found. Germ cells when mature are dis- charged from the gonad into the body cavity and go by way of two CYCLOSTOMATA 421 genital pores into the iirinogenital sinus, then out through the papilla to the environmental water where fertilization occurs. The life history may be summarized as follows: a. The eggs which contain considerable yolk in the vegetal por- tion and are about one millimeter (%5 inch) in diameter are laid in fresh-water streams, usually between March and June for all kinds of lampreys. The eggs first stick to objects, then fall in the sand. They are fertilized in the water almost immediately after laying. Cleavage follows, in about six hours when the optimum temperature of 22.5° C. prevails. At 20° C, this division requires nine days. b. The adults spawn but once and then die. c. A tadpolelike larval form, ammocoetes or mud lamprey, hatches from the egg and lives from four to five years in the mud along the streams where the eggs are laid. d. At the end of four or five years the ammocoetes undergo meta- morphosis to become adult. They remain under the mud from July or August to February or March while undergoing this transforma- tion to adult condition, e. The sea lamprey then migrates to the ocean and the lake lamprey moves down stream to a large fresh-water lake. They both become parasitic on other fish and continue this existence for from one and one-half to three and one-half years, when they return to fresh-water streams to breed again. CHAPTER XXVI ELASMOBRANCHII* Unlike the cyclostomes, the Elasmobranchs are covered with scales and have two sets of paired fins on the ventrolateral surfaces of the body. In addition to these, there are unpaired or median fins. The gill apertures, except for the first, or spiracle, are slitlike instead of circular, as seen in the lamprey. The gills are supported by gill arches, and the mouth has an upper and lower jaw. The skeleton is entirely cartilaginous, there is a partially persistent notochord, and the exterior is covered and protected by placoid scales. The males have a modification of each pelvic fin known as a clasper which is used as a copulatory organ. The mouth is not right at the anterior end of the body but is ven- tral or subterminal. There is present in the ileum of the small in- testine a spiral valve which increases the internal surface, thus add- ing absorptive area. The Elasmobranchs have no operculum or air bladder. Classification The class is divided into two rather easily distinguished sub- classes. The first group is very common to American shores and the second is rarely seen in our waters. Subclass Selachii. — This group includes the sharks which are cylindrical in shape, possess laterally located gill slits, and are active swimmers; and the rays, which are dorsoventrally flattened, possess ventrally located gills, are less active, and dwell on the bottom of the sea. This subclass is usually divided into two orders. Orders Euselachii and Cyclospondyli. — The sharks make up these orders. The dogfish sharks (Squalus acanthias and others), tiger shark (Galeocerdo arcticus [Faber]), cub shark {Carcharias platydon [Poey]), shovelhead or bonnethead shark {Reniceps tiburo [Linn], and man-eater shark {Carcharodon carcharias [Linn]) are forms com- monly found. The majority of sharks are carnivorous and active, but they rarely attack man unless the person is already wounded. •In collaboration with Miss Mary Fickling. 422 ELASMOBRANCHH 423 The average length of most sharks commonly observed ranges be- tween three and six or eight feet. Their natural food consists prin- cipally of Crustacea, small fish, squids, and refuse. In the Gulf of Mexico and other warm seas the so-called man-eater may occasion- ally reach a length of thirty feet and is sometimes charged with Fig. 228. — Southern sting ray, Dasyatis americana, a common form in the Gulf of Mexico. eating human beings. The shovelnose (bonnethead) and hammer- head sharks are very interesting forms. The shape of the head of each is about the shape ascribed to it by the common name. The former has been considered sufficiently interesting to warrant fur- ther discussion of it as an example of the class. 424 TEXTBOOK OF ZOOLOGY Order Batoidei — Skates and Rays. — This is a group of depressed or dorsoventrally flattened fishes in which the gill slits are located on the broad, flat, ventral side. "These fish lack the anal fin and the caudal is absent or reduced. The saw-fish, Pristis pectinatus is a sharklike ray with a long tooth-bearing rostral process or snout that resembles a double-edged saw. These animals may reach a length of fifteen or twenty feet, with a saw five feet in length. The skates are distributed along our Atlantic shores and are ovip- arous. The eggs are enclosed in dark brown cases or capsules, quadrate in outline and of considerable size. They have hornlike processes extending from each corner. There are about six species of skates, of which Raja erinacea, B. diaphanes, and B. ackleyi are common ones. Fig. 229. — Butterfly ray, a common bottom feeder. The rays are of similar shape, but they bear their young alive and tend to have a smoother skin. The rays are more numerous in the warmer waters. The torpedo ray of family Torpedinidae, has at- tracted considerable attention because of its ability to generate and store electrical energy in the muscles of the bases of the broad pec- toral fins. These electric organs are capable of discharging suffi- cient current to paralyze other animals, ring a doorbell, or light a ELASMOBRANCHn 425 flashlight bulb. The sting ray or stingaree is very common in the Gulf of Mexico. The average width of those usually seen is from eighteen inches to two feet. They have a long, slender, whiplike tail with a strong spine or sting on the dorsal side of its proximal third. Dasyatis sahina and Dasyatis americaiia are two common forms. The butterfly ray, Pteroplatea micrura, is a broad-bodied form with an exceptionally short tail. They, too, are quite common in the water of the Gulf of Mexico. The sting is usually obsolete in this form. It is called butterfly ray because of the manner of flap- ping the lateral expansions about as a butterfly moves its wings in flight. Subclass Holocephali. — This group contains an order with three modern genera. Psychichthys affinis (or Chimaera affinis, as often called) is the only species taken from the waters of the coasts of North America, and then only rarely. Chimaera monstrosa is another species which is found in South American waters. Economic Relations of the Class Many of the smaller sharks, like Squaliis acanthias and Mustelus canis are very destructive to lobsters, crabs, shrimp, squid, and valu- able fish which they use for food. They also damage much fishing gear by tearing through nets. It is estimated that the damage done in this way averages $400,000 in Massachusetts alone. Along the coasts of California and in the Gulf of Mexico both sharks and rays are a nuisance to the seining fisherman. The sting rays or "stingarees" which are armed with the barbed stinging spine on the proximal portion of the tail are generally common in most of the warmer fishing waters. "With a sudden swing of the tail one can inflict an ugly and extremely painful wound. Some people become severely ill as the result of such a sting. Bathers particularly dislike "stingaree" infested beaches as well as those infested with the less common torpedo ray. The skins of certain sharks and skates, which have the sharply pointed, toothlike scales, are used as a polisher of wood and other materials and is called shagreen. Shark skins are now being manu- factured into leather on a commercial basis. Large quantities of oil are extracted from some of the sharks, as the cub shark for ex- ample. This oil is used in currying leather in the tanning industry. 426 TEXTBOOK OF ZOOLOGY Shark liver oil is of high vitamin content and has an important medicinal use. In many countries, particularly of the Orient, these fish are com- monly used as food. It is said that small sharks and skates are offered for sale right along with other fish in the markets of China. They are also salted and dried. In the United States there is an unfounded prejudice against eating these fish, but dogfish are now being canned and sold under the name of "grayfish." The wing- like fins of skates and rays make delicious steaks. Sawfish steaks are quite desirable and the saws are preserved as ornaments. The flesh of sharks and rays is also ground up and used extensively as fertilizer. In some parts of the world the fins of sharks are used in the manufacture of gelatin. A good many dogfish and bonnet- head sharks are sold for purposes of study in zoology laboratories. THE SPINY DOGFISH This shark is the most commonly studied representative of the Elasmobranch group. Squalus acanthias is the scientific name ap- plied to the common form taken along the Atlantic coast and Squalus suckleyi is the name given the similar one of the Pacific coastal waters. The average length of Squalus is between two and one-half and three feet. It is a strong swimmer and is frequently seen as a scavenger in harbors as well as going out to sea for ex- tended periods. It apparently makes a spring migration northward along the coast and a return movement in the fall. Because of the ventral location of the mouth, these fish find it necessary to turn ventral side up to eat morsels of food from the surface of the water. External Features The body is generally spindle-shaped (fusiform) tapering at both head and tail. There are two pairs of fins, the anterior pectoral and the posterior pelvic, or ventral fins. In addition to the paired fins, there are two unpaired, median, dorsal fins, each with a spine at its anterior margin (hence spiny). Male individuals may be distin- guished from females by the fingerlike extensions, or claspers on the pelvic fins. The dorsal and ventral lobes of the caudal fin, or tail, are unequal and based on this, the tail is described as heterocercal. There are six pairs of uncovered gill clefts in the walls of the pharynx. The anterior one, which is dorsally located and greatly ELASMOBRANCHII 427 modified, is called the spiracle. It contains a rudimentary gill struc- ture. The mouth aperture is somewhat the shape of an inverted U, located on the ventral side of the head, and supplied with sharp teeth on the jaws. These teeth are developed by modification of the placoid scales which cover the skin over the body generally. The placoid scales are primitive exoskeletal structures with a hasal plate embedded beneath the skin and a spine projecting on the sur- face. This spine has a pulp cavity, surrounded by dentine, which is covered on its surface by enamel. This structure is considered to be homologous to the vertebrate tooth. The paired nostrils are openings on the ventral side of the snout, anterior to the mouth. The eyes, the lids of which are immovable, are situated on the sides of the head. The cloacal aperture is located between the bases of the pelvic fins. Muscular System The segmental arrangement of myotomes, separated by myo- eommas, is fairly complete along both sides of the body. The principal specializations of independent muscles are found in the form of myotome modification in the region of the mouth gills and paired appendages. The trapezius found above the branchial area; the superficial constrictors extending from the head to beyond the gill slits and assisting in their operation; and the adductor man- dihularis, connected with the lower jaw, are all examples of special developments. Skeletal System The endoskeleton of the sharks is composed of cartilage. It con- sists of axial skeleton (skull and vertebral column) ; visceral skeleton (jaw and gill arches) ; and, appendicular skeleton (pectoral girdle and fins, pelvic girdle and fins). The vertebral column and skull are much more developed than in the cyclostomes. The notochord has become segmented and partially replaced by cartilage. The centrum, which has replaced a considerable portion of the notochord in each vertebra, is deeply concave at each end, and is said to be amphicoelous. Some of the remains of the notochord fills these interstices between vertebrae. The skull is laid on a foundation of the ventral hasal plate. The dorsal side is fairly well enclosed with cartilage. The anterior ex- tension of the skull is the rostrum and the depression in its dorsal 428 TEXTBOOK OF ZOOLOGY side is the anterior fontanelle. The nasal capsules are rounded, car- tilage-encased cavities, one at each side of the base of the rostrum. These capsules house the olfactory sense organ in life. The orbits are laterally located, spherical depressions which normally hold the eyes. Each orbit is guarded anteriorly, dorsally, and posteriorly by slight extensions of the cartilage known as preorhital process, supra- orbital crest, and postorbital process respectively. The orbits are laterally located spherical depressions in which the eyes are set. aosTuuM OLFACTORY CAPSULE EPIPHYSIAL FORAMEN LATERAL LINE FOR^M^^»A ENDOLYMPHATIC DUCT ORblT OPT\C FORAMEN PTERYOO- QU ADR ATE MECKELIAN CARTIlAOE CERATOHYOID HYOMANDl&ULAR CERATO- , &RANCH lALS EPI- BBAMCHIALS CAROTID FORAMEN PHARYNGO-J BRANCH I ALS \. OTIC CAPSULE Fig. 230. — The skull and visceral arches of the dogfish shark, Sqimlus acant.hias. Dorsal view above, ventral view below. Latei'al aspect on the riglit. (From Atwood : Introduction to Vertebrate Zoology, published by The C. V. Mosby Company. ) The visceral skeleton consists of the upper jaw (palatopterygoid or quadratopterygoid cartilages), lower jaw (Meckel's cartilages), hyoid arch (hyomandibular, ceratohyal, basihyal), and five branchial arches (each typical one has pharyngobranchial, epibranchial, cera- tobranchial, hypobranchial, and basibranchial cartilages). ELASMOBR AN C HH 429 NOSTWU OILL CONUS ARTERIOSUS VENTP.\C\.E TESTIS UVER GALL feL^DOER STOMACH B\LE DUCT DORSAL AORTA . COELIAC ARTERY PANCREAS - &PLE.EN MESENTERIC ARTSRV RECTAL GLANO ARTERY RECTAL OLANO SEMINAL VESICLE INTESTINE A60OMIMAU PORE PELVIC FIH CLASPtR Fig. 231. — Ventral view of the visceral anatomy of the dogfish sharlc, Squalus acanthias, male. (From Atwood: Introduction to Vertebrate Zoology, published by The C. V. Mosby Company.) 430 TEXTBOOK OF ZOOLOGY The pectoral girdle is composed of the ventral coracoid bar and the dorsolateral scapular process at each end of it. The fin consists of three flat basal cartilages (propterygimn, mesopterygimn, and metapteryginm), a series of radial cartilaginous rays, and a series of exoskeletal dermal rays. The pelvic girdle is made of one cartilagi- nous bar (ischiopubis) with a fin joining at each end. The basals of this fin are fused into one cartilaginous plate. Fig. 232, A. — Dissection of the valvular portion of the small intestine. A, the bonnet-head shark to show the spiral valve; B, spiral valve of Raia. (B re- produced by permission, from General Zoology by Wieman, copyrighted 1938 by McGraw-Hill Book Co., Inc.) Digestive System Most of the organs of this system and other viscera lie in the pleuroperitoneal portion of the coelomic cavity. Anterior to this the pericardial portion of the coelom contains the heart. The digestive organs are in the form of an alimentary canal with accessory glands. ELASMOBRANCHH 431 The canal begins anteriorly witli the mouth, which leads directly into the pharynx, in whose lateral walls are the gill slits (and spiracle). Following this is the short tubular esophagiis, which leads into the cardiac end of the stomach. This organ, which is somewhat broader than the esophagus, is rather U-shaped. The posterior, or pyloric, portion of the stomach is provided with a sphincter muscle, the pylorus, which controls the passage of food materials into the in- testine. The duodenum is the short anterior portion of the intestine which follows the pyloric portion of the stomach and leads into the valvular portion of the intestine (ileum). This section of the small intestine is of considerably greater diameter than the duodenum and contains internally a spiral valve, which is a spirally arranged in- folding of the mucous lining. This arrangement serves to slow the passage of food and increases the absorption surface. The principal absorption takes place through this part of the intestine. The val- vular portion leads to the short, narrow, large intestine, which emp- ties into the cloaca (Figs. 231 and 232). The liver, pancreas, and rectal gland are accessory glands con- nected with this system. The liver is a large, three-lobed organ with the saclike gall bladder located just dorsal to the junction of the right and middle lobes. The gall bladder stores bile produced by the liver and delivers it to the duodenum through the hile duct. The pancreas is divided into two lobes, an oval ventral and a slender dorsal lobe. Ducts lead from it to the duodenum. The rectal gland is a spindle-shaped gland leading into the large intestine directly. The reddish spleen, which is a lymphoid rather than digestive organ, lies around the greater curvature of the stomach. The digestive tract and adjacent organs in both species are sus- pended from the body wall by mesenteries, which are extensions from the peritoneum, or membranous lining of the coelom. The mesentery supporting the stomach is the mesogaster, the one extending between the spleen and stomach is the gastrosplenic, and the mesorectum sup- ports the large intestine and rectal gland. Circulatory System This centers in the heart, which is located ventrally at about the level of the posterior pair of gills, and consists of two principal chambers and two accessory chambers. The pericardium, a mem- branous extension of the peritoneum, encloses the heart. The two 432 TEXTBOOK OF ZOOLOGY VENTRAL CAROT\0 A AFFERENT E.RANCH\ALS TRUNCUS ARTERIOSUS ATRIUM VENTRICLE DUCT OF CUVIER HEPATIC SINUS CARDINAL SINUS INTERNAL CAROTID A. POSTERIOR CAROTID A. VENTRAL AORTA EFFERENT E.RANCH\ALS ANTERIOR CARDINAL SINUS JUGULAR V. SINUS VENOSUS HEPATIC PORTAL V HEPATIC A GASTRIC A HEPATIC V. LATERAL ABDOMINAL V PANCREATIC A INTESTINAL V INTESTINAL A. RENAL PORTAL V \L1AC V. \LIAC A- CAUDAL A CAUDAL V SUBCLAVIAN V. SUBCLAVIAN A. OVARIAN A. GENITAL V. COELIAC A. OVIDUCAL A. GASTRIC V. DORSAL AORTA POST- CARDINAL V. SPLENIC INTESTINAL V. PANCREATIC V. SPLENIC A. INTESTINAL A. RECTAL GLAND A. RECTAL GLAND V. Fig. 233. — The circulatory system of the spiny dogfish, Squalus acanthias. (From Atwood: Introduction to Vertebrate Zoology, published by The C. V. Mosby Company.) ELASMOBRANCHII 433 Internal carotid Ventral carotid Hyoidcan artery 1st afferent branchial A Ventral aorta Ventricle Subclavian V< Subclavian A. Lateral abdominal V. - Spleen ZQd qui slit Ant. cardinal W. Efferent branch- ial A. Dorsal aorta Duct of Cuvier Coeliac A. LPoit. cardinal Ventral gastric A. Ventral pancreas Qastro-splenic A. Sup. mesenteric A. Kidney A- Gonad Post- mesenteric A Intestine Pelvic fin Uiac A: Inf. mesenteric A. Rectal qiand Cloaca Caudal A. Pig. 234. — Diagram of lateral view of the circulatory system and other organs of bonnet-head shark. (From dissections by Mary Fickling.) 434 TEXTBOOK OF ZOOLOGY main chambers are the single, more dorsal auricle, and the ventral, muscular ventricle. Leading into the auricle is the sinus venosus, which receives blood from the veins of the body. A bulblike en- largement at the base of the ventral aorta, where it leaves the ven- tricle, is the conus arteriosus. The ventral aorta leads anteriorly from the conus and gives off three pairs of afferent branchial arteries which branch to the five pairs of gills. The blood spreads from these by capillaries through the gill lamellae for oxygenation. The pre- trematic and postreniatic branches of the efferent branchial arteries which form four efferent branchials leave the gills and join the dorsal aorta, which is formed by them in the dorsal midline. The hyoidean extends from the ventral portion of the first pretrematic to the spiracle where it spreads in capillaries. The ventral carotid leads for- ward from the spiracle to the internal carotid. Extending anteriorly from the dorsal part of each first efferent branchial is a common carotid which supplies arterial blood to the head and brain. The dorsal aorta extends posteriorly from the junction of the efferent brachial arteries, soon giving off the subclavian arteries to the pectoral fins; coeliac to stomach, liver, and pancreas; gastrosplenic to stomach and spleen; superior mesenteric to the valvular intestine to be- come the posterior mesenteric artery there; renal arteries to the kidneys; the inferior mesenteric to the rectal gland and large intes- tine; and the iliac to the pelvic fins and cloaca. The subclavian artery leaves the aorta more anteriorly, coming off ahead of the pos- terior efferent branchial ; the coeliac is farther back and sends a pan- creaticomesenteric artery above the duodenum, through the ventral pancreas, and along the valvular intestine a gastric to the stomach and the hepatic artery to the liver ; the gastrosplenic and superior mesen- teric arise very near each other. The systemic veins all return venous blood to the sinus venosus, which empties into the auricle by way of the sinu-auricular aperture. Hepatic veins lead directly from the liver to the sinus venosus. The ducts of Cuvier collect blood from the anterior cardinals of the head region and posterior cardinals of the trunk region and empty it into the sinus. A hepatic portal system collects from the stomach, in- testines, pancreas, and spleen and empties into the liver. The two renal portal veins bring blood to the kidneys from the single caudal vein of the tail. This blood spreads through the capillaries of the kidneys and is collected into the postcardinals through the renal ELASMOBRANCnn 435 branches. There is also a system of lymph spaces, which supple- ments the blood circulatory system. Respiratory System The gills in the wall of the pharynx are constantly bathed in water forced through from the mouth. An exchange of carbon dioxide for oxygen is made by the blood as it passes through the capillaries of the gills. This is made possible by diffusion of these gases through the membranes of the gill lamellae. The gills are supported by cartilaginous gill arches. Nervous System The central nervous system consists as usual in vertebrates of the hrain and spinal cord. The brain includes two large olfactory lobes at its anterior, followed by two cerebral hemispheres, a depressed diencephalon, a pair of large optic lobes, a well-developed cerebellum, and behind this the tnedulla oblongata. There is a very slight con- striction between olfactory lobes and cerebrum. The cerebellum is divided into quarters by a longitudinal groove and a transverse groove. It covers a part of the optic lobes as well as the anterior portion of the medulla oblongata. On the lateral walls of the medulla are located the acusticolateral areas, including the earlike auricles. The cavity within the medulla, which opens to the dorsal surface beneath and behind the cerebellum, is the fourth ventricle. There are ten pairs of cranial nerves which are numbered and named from anterior to posterior: I, olfactory; II, optic; III, oculomotor; IV, trochlearis; V, trigeminus; VI, abducens; VII, facial; VIII, audi- tory ; IX, glossopharyngeal ; X, vagus. The olfactory nerves extend from the olfactory lobes ; optic from diencephalon and optic lobes; oculomotor from ventral side of optic lobes or midbrain; trochlear from dorsal side of optic lobes between them and cerebellum; the trigeminus, abducens, facial, and auditory all from the anterior portion of the medulla oblongata; the glossopharyngeal from a more posterior part of the sides of the medulla. These last two supply the gills, lateral line, and certain viscera. The spinal cord is tubular and somewhat flattened. It extends the length of the vertebral column and gives off paired spinal nerves seg- mentally. 436 TEXTBOOK OF ZOOLOGY The sense organs include the eyes, the olfactory organ, internal ear, and the lateral line system. The eyes are in the orbits, one on each side of the cranium. They are quite typical of the vertebrate eye described in the general chapter on phylum Chordata. The olfactory OUPACTORV e>ULB OUFACTORV TRACT CEREBRUM MAX>LUAR.*S OPTIC LO&E OPHTHALMICUS SUPERFICIALIS OPHTHALMICUS PROFUNDUS MANDI&ULARIS TRIGEMINAL FACIAL PALATINUS AUDITORY HYOMANDIBULARIS OLOSSOPHARYNGEAL VAOUS TERMINAL OPTIC TROCHLEAR OCULOMOTOR A60UCENS CORPU& RESTIFORMUS MEDULLA Fig. 235. — Dorsal view of the brain and cranial nerves of the dogfish shark, Sgualus acanthias. (From Atwood : Introduction to Vertebrate Zoology, published by The C. V. Mosby Company.) organ consists of a pair of nasal sacs on the ventral side of the ros- trum which open by nostrils. The nasal chambers are blind sacs and lined with a sensory lamellated olfactory membrane in which the olfactory nerve ends. The internal ears are composed of a vestibule ELASMOBRANCHII 437 and three semicircular canals. An endolymphatic canal leads from the dorsal exterior into the lower part of the vestibule the sacculus. A posterior pouch of the sacculus is the lagena, which is considered the foreranner of the cochlea of higher vertebrates. The ear serves the sense of equilibration in the fish. A canal extends along the side of the body in the lateral line and forward onto the head, lying just beneath the skin. This is the lateral line system. On the head there are some pores with tubes extending beneath the skin to small bulbs called ampullae of Loren- zini. The function of the lateral line system ajid these ampullae is perception of water pressure and vibrations. Endolymphatic duct Anterior semi- circular canal Utriculus Posterior semi- circular canal Horizontal semi- circalar canal Recessus utnculi L^ \^^^^ '-'"?^'^ Sacculus Fig. 236. — Diagram of lateral view of left internal ear of Reniceps tiburo. Urinog-enital System The kidneys are thin, slender organs extending along the dorsal body wall, one on each side of the vertebral column. The posterior, or caudal, portion functions in excretion. There is an accessory meso- nephric duct embedded in each kidney which carries urine to the urinogenital sinus of the male, and the Wolffian duct serves as the urinary duct of the female, emptying into the urinary sinus. A papilla leads from the sinus to the cloaca in both. In the male sper- matozoa are produced in the testes, carried by several vasa efferentia through the mesochorium to the convoluted cranial portion of the Wolffian duct, the epidid^Tnis of each side, which continues poste- riorly as the vas deferens. This tube enlarges to become the semincH vesicle and continues into the inflated sperm sac which is directly con- nected with the urinogenital sinus. The spermatozoa then pass out through the papilla to the cloaca, thence to the outside by way of 438 TEXTBOOK OF ZOOLOGY the anus. During copulation they are transferred to the cloaca of the female by use of the claspers. They swim by their own motility into the uteri and oviducts. Fig. 237. — Urogenital systems of Sgualus acanthias; A, female; B, male. The ova of the female are produced in the ovaries which are located one on each side of the median dorsal line in the anterior portion of the coelom; Each gonad is suspended in a mesentery, the mesovarium. Mature ova rupture from the ovary into the body cavity and enter ELASMOBRANCHII 439 the funnellike ostium of the oviduct, which is held by the falciform mesentery at the anterior end of the peritoneal cavity. As the ova pass down the oviduct, they receive a covering which is secreted bj'' the shell gland in the wall of the duct. Fertilization occurs in the oviduct and the embryo develops in the uterus which is the expanded lower portion of the oviduct. The embryo is nourished by the large yolk mass of the egg. THE BONNETHEAD SHARK, RENICEPS (SPHYRNA) TIBURO COMPARED TO SQUALUS The bonnethead (or shovel-nosed) shark is common in the At- lantic along the coast of the Southern States and in the Gulf of Mexico. It occurs abundantly along the Louisiana and Texas Gulf coast during May and June. It averages about the same size as Squalus. In many respects it is similar to the smooth dogfish, Mustelus canis, and the ground shark, Carcharhinns. Epiphyseal foramen M ,. Anterior fontanel/e m^''"m^ .,..-|-°'^°^torycap5a/e ^^^^ Supraorbital process -Preorbital process - PosbDrbitcil process • Endolymphatic fossa Foramen maqnum' Endolymph i Fig. 238. — Dorsal view of the skull of bonnet-head shark. (From dissections by Mary Fickling.) The peculiar shovel-shaped head with the eyes out on the lateral margins is one of the striking features of Reniceps by which it dif- fers from the others mentioned. In Reniceps there are no spines in front of the dorsal fins and a single anal fin is present on the ventral side between the anus and tail. The spiracle is absent, leaving only the five pairs of gill slits. The other external features are similar to those of Squalus. The skeleton of the skull is shaped considerably different from that of Squalus. This is brought about by lateral extension. Each olfac- 440 TEXTBOOK OF ZOOLOGY toiy capule is extended far to the lateral of the base of the rostrum instead of lying beside it. The orbit with the modified supraorbital crest, preorhital process, and bladelike postorhital process are also Ventral pancreas Cardiac stomach H\dr)ey SmaW'mtzstme - Recta/ qloDd - - Larqe intestine Spleen stomach Mesoneplinc duct -- Cloaca Fig. 239. — Internal anatomy of bonnet-head shark. Reniceps tiburo, from ventral view. (From dissections by Mary Fickllng.) projected at the terminal portion of this arm of cartilage. The pos- terior part of the skull is somewhat narrowed and flattened but otherwise similar to that of Squalus. ELASMOBRANCHII 441 There are a few differences in the digestive systems of the two. In the pharynx there are no spiracles. The stomach of Reniceps is J-shaped instead of U-shaped and the long slender pyloric portion is armlike. The spiral folds of the spiral valve are more telescoped into each other than in Squalus. There are only two lobes in the liver of bonnethead and the gall bladder is nearly embedded in its tissue. In the circulatory system, Reniceps has five afferent branchial arteries branching from the ventral aorta while Squalus has only three, two of which branch. The branching of the coeliac artery is somewhat different in the two animals. The brain has the same general parts as it does in Squalus but they are quite modified. The olfactory lobes are broader and almost completely fused to each other. The cerebrum is a somewhat smaller single lobe just posterior and dorsal to the olfactory lobes. There is no line of demarcation between the hemispheres. The diencepha- lon is entirely hidden from dorsal view by the cerebrum and cere- bellum. The latter is large, irregularly divided into three lobes, and convoluted. It covers not only the diencephalon but also most of the optic lobes (midbrain) and much of the medulla oblongata. The medulla has well-developed acousticolateral areas. The nasal chambers of Reniceps are quite large and kidney-shaped. They contain extensive folds or lamellae of the olfactory membrane. The shape of the testes in male Reniceps is much longer and more slender than in Squalus. In addition to this, there is a long glandu- lar body, the epigonad, which extends from the level of the gonad proper to the region of the cloaca. Copulation in Reniceps probably occurs during May and June in the Gulf of Mexico, at the time when they are so numerous in the shore waters. Fairly mature "pups," ^s the developing young are called, have been found in the uteri of specimens collected off shore in Texas Gulf waters in late August and early September. CHAPTER XXVII PISCES, TRUE FISH This important class includes quite an extensive variety of dif- ferent forms. They are aquatic and possess the usual adaptation of gills for respiration, and paired fins as well as median fins to assist in locomotion. Most forms within the class have scales as an exo- skeletal covering of the skin. The endoskeleton is primarily bony. Pectoral and pelvic girdles are developed to support the paired fins, but the pelvic girdle is usually small. The fins are supported by fairly well-developed fin rays. The majority of families in this class possess a swim bladder. The typical shape of the fish's body is fusiform or spindle-shaped, with all of the original features of stream-lining. The shape assists in dividing the water as the fish moves through it. As the water passes over the thicker part of the body, it rushes in to push forward on the posterior slopes of the spindle form of the body. This is an adaptation for easy production of speed. The sedentary forms of fish usually tend to lose this shape and become flattened or otherwise modified. The shape of the body varies from that of the long slender eel to that of the globe-shaped box-fish and inflated puffers which can float like balloons. The sea horse is one of a group of very peculiarly shaped forms. Still another peculiar adaptation is the flying fish. The fins of fish are found singly in the form of a dorsal median fin, sometimes divided into two ; a single caudal fin over the tail ; a ventral median anal fin in most species; a pair of pelvic or ventral fins which are quite variable in position and in some forms rudimentary, and the paired pectoral fins. These paired fins are supported by bony girdles. The pelvic fins of the perch are located almost immediately ventral to the pectoral fins, while in the bullhead catfish they are just anterior to the anus. In this catfish there is a second dorsal, which is com- posed entirely of skin and is called an adipose fin. The structure of the caudal fin and posterior end of the vertebral column is distinctive and has been classified. The most primitive type of tail is the diphyceroal in which both the cutaneous and osseous parts are equally divided between dorsal and ventral regions. The hetero- cercal tail is asymmetrical and the tip of the vertebral column ex- 442 PISCES, TRUE FISH 443 tends into the dorsal lobe as has already been seen in the dogfish. Still another type of tail is the liomocercal, which is internally un- balanced but externally symmetrical. The original notochord turns Fig-. 240. — Spiny boxflsh, Chilomycterus schoepfli, from Gulf of Mexico and Atlantic Ocean. Fig. 241. — Diagram showing some peculiar bony fish. A, common eel ; -B, sea horse; C, flying fish. (From Krecker, General Zoology, published by Henry Holt & Company, after Jordan.) into the dorsal lobe, but the lobes stroke the water with about equal surface and force. It forces the fish through the water in a hori- zontal plane and is correlated with a terminal mouth. 444 TEXTBOOK OF ZOOLOGY There are three principal types of scales which cover and protect the body of most true fish (a notable exception is the catfish to be described later). These are: ganoid, cycloid, and ctenoid. The first are usually rhombic or oval in shape and are covered by a dentinelike substance called ganoin. Such fish as gar pikes and bowfins possess this type. Cycloid scales are rather disc-shaped with conspicuous concentric lines. They are usually imbricated on the skin, like a shingle roof. The third type is similar to the cycloid except that the free edge of the scale bears some spiny projections or cteni. Fig. 242. — The different types of fish scales. 1, cycloid; 2, ctenoid; 5, ganoid; i, placoid. (From Krecker, General Zoology, published by Henry Holt and Company, after Her twig.) Cycloid scales are found on the carp while the ctenoid are charac- teristic of the perch and sunfishes. The age of many fish can be determined by the distribution of the concentric lines on the scales. The lines formed during nongrowing periods fuse closely together, thus indicating seasonal periods on the scale. The skeleton includes, besides the paired iins and girdles already mentioned, the amphicoelous (concave in both ends) vertebrae and bony cranium, which is complete and independent of the visceral skeleton. This latter portion consists of seven arches, the jaw structures, and five gill arches. The bones of the operculum arise as a part of this division. The digestive tract is in the usual form of a canal with out- growths. Food ranging from vegetation, insect larvae, Crustacea, clams, and snails to small fish and amphibia is utilized. It passes PISCES, TRUE FISH 445 through the toothed mouth, pharynx, esophagus, stomach, duodenum, ileum, and large intestine during the process of digestion. Teeth are located on the jaws, roof of the mouth, and walls of the pharynx, and are used primarily for holding prey. Gastric glands in the wall of the stomach supply some of the digestive juices. Pyloric caeca which join the anterior portion of the duodenum increase the absorptive and digestive surface in many fish. The respiratory system consists of the mouth, gills, and, to some extent, the swim bladder in certain fish. Water is drawn in or inspired through the mouth and forced out or expired through the gill slits. The mouth and pharynx form a water-tight pumplike arrangement with the help of the flaplike oral valves just inside the lips and the IrancJiiostegal memhrane at the margin of the operculum. The exchange of oxygen and carbon dioxide between the blood in the capillaries of the gills and the water occurs as the water passes over the gill lamellae. Oxygen is absorbed by the blood, and carbon dioxide is discharged to the water. The circulation in most fish is, in general, similar to that described for the lamprey, except for certain specializations and phylogenetic developments. The system includes the paired anterior and poste- rior cardinal veins meeting in the duct of Cuvier which joins the sinus venosus, the hepatic portal vein leading to the liver, the hepatic vein from liver to sinus venosus, the two-chambered heart with its accessory sinus venosus and lulhus arteriosus, ventral aorta, bran- chial arteries, dorsal aorta, and the various branches. Excretion is accomplished by a pair of dorsally located meso- nephric kidneys, each of which is connected by a mesonephric duct to a urin 1 1 h .*^ ^) "Ol '^) C cl "p 0 ^\J^ ^ V. ^ ^ ^ -^ ^ L o V. «l) G 'o :3 e « i u o I m a ti to b O d !-• <» V n c S 0) CO > n -Ventral view of brain and cranial ner^^es of Ameiurus natalis Le Sueur. (Drawn by Titus Evans from dissections by Rose Newman.) prominent, due to the large posterior lobes (tubercula acoustica) on each of its anterolateral positions. Dorsally, between these lobes, is a diamond-shaped slit which leads into the cavity of the brain. This is the fourth ventricle. On the ventral side of the diencephalon is the optic chiasma where the optic nerves meet, and behind this are the inferior lobes with the stalklike infundibulum joining the glandu- lar hypophysis. Two peculiarities of the brain of Ameim^us are the large cerebellum and the large posterior lobes. There are ten pairs of cranial nerves emerging from various levels of the brain. Three of these have strictly sensory function, three are strictly motor in function, and four have both sensory and motor function. The bull- head has forty-one pairs of spinal nerves arising segmeutally from 470 TEXTBOOK OF ZOOLOGY the spinal cord. Each has a dorsal raraus, or branch, and a ventral one extending out to certain parts of the body in the region. The sense of taste is highly developed and is centered in the numer- ous and well-developed taste huds which are distributed on the inside and outside of the lips, in the lining of the first three gill slits, on the barbels, and in gTOups over the external surface of the body, even to the tail. The eyes are small and without lids but have fair power of vision as this sense goes in fish. The focal distance is between twelve and eighteen inches, and is better for detecting motion than for recognizing objects. The fish does not have a sense of hearing; Fig. 258. — Eggs of trout with well-developed embryos, and recently hatched fry. A, eggs with embryos; B, fry. (Courtesy of General Biological Supply House.) the ear structures serve in the sense of equilibrium. Ameiurus, perch, and other fish have a well-developed pressure and water-vibration sense centered in the lateral line system. The sense of touch is dis- tributed over the epidermis but is particularly keen in the lips and barbels. Reproduction and the Life History The bullhead, perch, sunfish and many other common fish build nests of one sort or another, lay the eggs in the nest, and guard the nest until the eggs hatch. The details of the reproduction and breeding are not so well known in Ameiurus natalis as they are in PISCES, TRUE FISH 471 A. nehulosus, the brown bullhead. This being the case and since the two are very similar, a brief description will be given for the latter. The observations were made on a pair in an aquarium in AVashing- ton, D. C. They made a nest on July 3 by removing with their mouths more than a gallon of gravel from one end of the tank, leaving the slate bottom bare. On July 5 about two thousand eggs were deposited in four masses. Ninety-five per cent of them hatched in five days with the water at 77° F. The young remained in masses until six days old; then they began to swim. By the end of the seventh day they were swimming actively and most of them collected in a school just beneath the surface, where they remained for two days, afterwards scattering. It is also reported that they ate finely ground liver on the sixth day and had enormous appetites after the eighth day. They were 4 mm. long when hatched and had attained a length of 18 mm. by the fourteenth day. At the age of two months their average length was 50 mm. Both parents assume responsibility in caring for the eggs, keeping them agitated constantly by a gentle fanning motion of the ventral fins. The egg masses are also sucked into the mouth and then blown out with some force. These opera- tions were continued until the fry (newly hatched fish) swam freely. CHAPTER XXVIII CLASS AMPHIBIA (By Ottys Sanders, Southwestern Biological Supply Company) As there are many vertebrate animals which lead an amphibious life, it was natural for Linnaeus to group these together under the class Amphibia. This, of course, was classification based on habits rather than on structure, and as soon as such animals as the seal and crocodile were studied structurally they were removed from the class. Today the name is restricted to a group of vertebrates which we know as frogs, toads, salamanders, and caecilians. They are intermediate between fishes and reptiles. Except in caecilians, they have paired limbs, usually with fingers and toes, and never have paired fins like fishes. They have a moist, naked skin lacking the protective hair of mammals or the feathers of birds. The cae- cilians, none of which has been reported from the United States, are wormlike burrowing creatures of the tropics. They have small scales between their transverse body rings, although these are not usually seen unless a dissection is made. These animals and a few others such as the large South American frog ceratopharys, which has dermal bones or ''scales," are the only ones of the class to have scales. The amphibians are cold-blooded vertebrates, in contrast to the warm-blooded mammals and birds. The frogs, toads, and salamanders usually lay their eggs in water. These develop into tadpoles or larvae breathing with gills before metamorphosing to become adults which breathe with lungs. A few species of frogs and salamanders lay their eggs on land and pass their entire development in the egg. Kicord's frog, Eleuthero- dactylus ricordii, and the slimy salamander, Plethodon glutinosus, are examples of species that lay their eggs on land. These land eggs lack the calcareous shell of reptile and bird eggs. There are other exceptions to the general characteristics of this diverse class. A large group of salamanders, the plethodontids, do not have lungs even as adults, and their respiration takes place in the mouth cavity and through the skin, both of which are richly supplied with blood vessels. 472 CLASS AMPHIBIA 473 Size. — While most modem Amphibia are small creatures, paleon- tological species reached large proportions, as, for example, the Mastodonsaurus, which had a skull 4 feet long and a total length of probably 15 or 20 feet. Among living amphibians, the giant salamander of Japan and China, Megalobatrachus japonicus, grows to a length of 5 feet. In the Southwest, the largest salamanders are Siren lacertina, which attains a length of about 30 inches, and the "hellbender," Cryptobranclnis, which commonly grows to be Fig. 259. — The caecilian, Ichthyophis glutinosus, adult female, guarding her eggs on the left, and a larva showing external gills on the right. Partly after Sarasins. (From Atwood : Introduction to Vertebrate Zoology, published by The C. V. Mosby Company.) about 18 inches long and AmpMuma, the Congo eel. The goliath frog of Africa reaches a body length of nearly a foot, while southern bull- frogs, larger than their northern relatives, may grow to be over 7i/2 inches in body length, with a total length of 16 to 18 inches when the legs are extended. The giant toad or marine toad, Bufo marinus, is the largest of the true toads, and attains a body length of 8% inches. The smallest frog in the United States is the swamp tree frog, Pseu- dacris ocularis, which ranges from North Carolina to southern Florida. Adults measure only % to % of an inch in body length. 474 TEXTBOOK OF ZOOLOGY As far as is known, the length of life of Amphibia ranges from ten to fifty-two years. The larger ones, in general, seem to live longer than the smaller species. Some species of toads may live about thirty years, frogs probably less. Coloration. — Amphibians as a group are very colorful. The bright green tree frog, Eyla cinerea, which makes bell-like calls from the reeds and cattails in the summer months, the small grayish canyon toad, Bufo pnnctatus, with its red warts, the varicolored common tree frog, Hyla versicolor, with its orange groins, are but a few ex- amples of beautiful species. Amphibians possess considerable abil- ity to change color, and many of the tree frogs equal or surpass the chameleon in this respect. y.:..^^^^Ji^ Fig. 260. — Melanophore from Rana temporia. A. pigment distributed m response to light; B, pigment contracted. (Redrawn and modified from Noble, Amphibia of North America published by McGraw-Hill Book Company.) Their different colors are due primarily to various combinations of three kinds of pigment cells in their skin. The black melanophores are branching pigment cells which may contract or expand, and, when these predominate, the skin appears black or brown. Yellow or red results from the action of lipophores contained in spherical cells, and white from the guanophores. Green color results from the reflection of light from guanin granules wherein all the light rays escape absorption except the green. Different arrangements of these pigment cells produce color changes which are initiated by various stimuli, such as light, temperature, moisture, and the chemi- cal composition of the frog's habitat. These color chajiges are CLASS AMPHIBIA 475 directly beneficial to the animal when they help it to resemble more closely its surroundings and thus avoid capture. The Skin. — Amphibians have a soft, moist skin which is kept in that condition primarily by a rich supply of mucous glands. Aquatic Fig. 261. — Spadefoot toad, Scaphiopus coucliii, showing the shape of the pupil of tlie eye. (Photograph by Thos. Mebane Jones.) Fig. 262. — Feet of spadefoot toad, Scaphiopus coucliii, showing the dark-colored, dartlike spades. (Photograph by Thos. Mebane Jones.) and forest-inhabiting frogs and toads have a smoother skin than species which live in drier places. Burrowing frogs and toads, such as the spadefoot toad, Scaphiopus, also have thin, smooth skins, 476 TEXTBOOK Of zoology The skin not only protects the underlying tissues from excessive light but also has other functions. With its pigment it helps to regulate temperature by transformijig light into heat. A most im- portant function is its use as a respiratory organ. As previously mentioned, one large group of salamanders, the plethodontids, lack lungs and use the skin and buccal cavity for respiration. During hibernation, practically all of the respiration of frogs is taken care of through the skin. In Africa there is a frog, with greatly reduced ^ 5^ Pig. 263. — "Hairy frog." (Redrawn and modified from Noble, AmpMiia of North America, publislied by McGraw-Hill Book Company.) lungs, which, in the male sex, has developed a strange aid to respi- ration. It has patches of vascular villosities on the thighs and sides to such an extent that it has been named the ''hairy frog." These villosities help provide sufficient oxygen for its increased metab- olism during the breeding season. Since amphibiajis have moist skins, they are in constant danger of drying out, and therefore seek moist places where they may absorb water through their skins. Most of them are nocturnal in CLASS AMPHIBIA 477 their habits and therefore can be found during the daytime under logs, in crevices or burrows in the earth, or in other situations where they can protect themselves against this constant threat of desiccation. Food and Feeding Habits. — Adult frogs and toads eat animal food, while the tadpoles eat either animal or plant food. The food of the adults consists primarily of living insects, worms, snails, spiders and other small invertebrate animals. Many large frogs and a few smaller ones are cannibalistic. Amphibians depend to a large extent upon their sight in detecting food. While, in gen- eral, frogs and toads will seize a moving object without much ex- amination, the toads quite often stalk their prey and inspect it. If a disagreeable insect, such as a stag beetle with strong mandibles, is swallowed, it can be disgorged because, fortunately, the toad has a wide esophagus. Most of the frogs and toads and many sala- manders utilize their eyeballs in swallowing food. Their eyes can be retracted into the head and by this action they help to push food in the mouth cavity toward the esophagus. Amphibians can go for a long period of time without food. Tad- poles may live for months, and experiments made on axolotls (larvae of the tiger salamander) have demonstrated that they may live for about a year on the food stored in their own tissues. Dur- ing the hibernation season and breeding season most salamanders and frogs do not feed. Enemies of Amphibia. — The enemies of amphibians are many. In their larval or tadpole stages they are a delicate food for giant water bugs, dragonfly nj^mphs, larvae of water beetles, and other aquatic insects. Small crustaceans devour the gills of salamander larvae, and fish appreciate their good flavor. Snakes, turtles, alli- gators, birds, and mammals feed upon the adults and young. Man enjoys the hind legs of frogs, and there is an increasing demand for these as food. Man also destroys amphibians by polluting the streams where they breed, and his automobile kills countless toads and frogs on the highways. Nor are amphibians immune to disease and gross infestation by parasites. Powers of Regeneration. — The power of regenerating lost parts is one way in which Nature aids the group. Young tadpoles may regrow limbs or tails, although adult frogs and toads are appar- ently unable to regenerate lost appendages. The axolotl larva of 478 TEXTBOOK OF ZOOLOGY the tiger salamander, Amhystoma tigrinum, which is found in Texas, New Mexico, Colorado, and elsewhere, has been used extensively in experiments for studying the nature of this regeneration. Means of Defense. — Amphibians have few ways of protecting themselves from their enemies. Their coloration often blends in with their surroundings and camouflages them, and their habit of remaining immobile frequently causes them to be overlooked. Many species practice death feints and some swell up by inflating their lungs, making themselves more difficult to swallow. The mucous glands of frogs and salamanders make them slippery, and, in the case of salamanders particularly, their writhing and twisting movements when captured make them hard to hold. A few salamanders have, in addition, the ability to break off their tails and escape. One of the most protective weapons that amphibians have, how- ever, is the secretion of their poison glands. This is especially effective in the case of toads, many of which have large glands on their shoulders, known as parotoid glands. An animal that has attempted to bite or swallow a toad and felt the effects of the poi- sonous secretion of the parotoids upon the mouth tissues will not soon forget the experience. The largest known toad of the North American continent, Bufo marinus, which ranges from Texas to Patagonia, produces one of the most virulent poisons known among amphibians. There are records of dogs which have been killed by its secretions. Glandular secretions of certain South American toads, Dendrohates, have been used by the Indians of Colombia for poisoning their ari'ows. The secretions of toads are ordinarily quite harmless to man, however, unless they happen to get into his mouth or eyes. Voice. — The amphibians were probably the first vertebrates to de- velop a voice. The calls of modern frogs and toads are very distinc- tive, each species having its own particular call. Most of the croaking is done by the males, and the primary function of these calls seems to be to attract females and other males to the pond or stream. It is during the breeding season that the air resounds at night with their choruses, although certain species may croak at other times. The croaking of frogs and toads is usually done with the mouth and nostrils closed. The air is forced back and forth between the CLASS AMPHIBIA 479 lungs and mouth over the vocal cords, causmg them to vibrate. Vocal sacs, when present usually lying either in the floor or at each corner of the mouth, puff out to make resonating chambers which increase the volume of the call. Bullfrogs quite often call while under water. A few frogs, such as Ascaphus, which lives in the cold mountain streams of Washington and the northwestern United States, have given up their voice and reduced their lungs. Appar- ently voice would not be as useful to this species as to frogs living in quieter places, for its sound would not carry above the noise of the mountain streams. So far as is known, none of the salamanders use voice in attracting mates, and most of them are silent through- out their existence. Breeding and Egg-Laying Habits. — Frogs, toads, and salamanders make periodic migrations to ponds and streams for the purpose of egg-laying. These periods, called the breeding season, usually occur during the spring months or, in tropical climates, during the rainy season. Salamanders often come to the pools much earlier than do the frogs and toads and may also begin their egg-laying earlier. Most amphibians are oviparous, and their eggs are fertilized by the male after they leave the body of the female. Some salamanders and caecilians, however, have the eggs fertilized before they are laid. Among salamanders in many species, the males deposit sper- matophores containing sperm which are picked up by the females and provide internal fertilization. A few species of salamanders such as the fire salamander of Europe, Salamandra salamandra, give birth to living young. While the majority of amphibians lay their eggs in water, and the young pass through tadpole or larval stages, there are many exceptions. The eggs of the Texan cliff frog, Eleutherodactylus latrans, are laid on land, as are the eggs of its relatives in Mexico, and the tadpole stage is passed in the egg. Many salamanders lay their eggs on land. Species in the Southwest, such as Plethodon cinereus, usually lay their eggs in cracks and hollows in logs. The slimy salamander, Plethodon glutinosus, lays its eggs in moist places, often in the walls of caves. Some species of Oriental frogs are reported to lay their eggs in trees high out of the water. There is also reported a South African frog, Arthroleptella lightfooti, which undergoes its entire development on land and cannot swim when placed in water. 480 TEXTBOOK OF ZOOLOGY The marsupial frogs of South America, Gastrotheca, carry their eggs in a dorsal sac or brood pouch which is found in the female. The Amazonian frogs Pipa and Protopipa carry their eggs and tad- poles in individual dermal chambers on the back of the female. In the ease of a small frog (Bhinoderma) in Chile, eggs are carried in the vocal pouch of the male where they metamorphose and hatch as fully formed young. Phyllolates and Dendrobates, two frogs from the northern part of South America and Central America, transport their tadpoles on the back of the male to the stream where they pass the rest of their tadpole stage and metamorphose. In the case of the obstetrical toad of Europe (Alytes olstetricans), the male carries the eggs wrapped around his legs until they hatch. Fig. 264. — Adult Ambystoma tigrinunij tiger salamander. (Photograph by Sanders.) Secondary Sexual Characters. — Secondary sexual characters com- pose those differences, exclusive of the reproductive organs, be- tween males and females of a species. These differences may be both structural and physiological. Familiar secondary sexual char- acters are the nuptial pads of male frogs, their swollen thumbs during the breeding season, and, in the male bullfrog, the size of the tympanum, which is larger than that of the female. These sexual characters may be various. In some salamanders the teeth of the male may elongate; in others, glandular masses at the base of the tail or elsewhere may be present in the males and absent in the females. One of the most bizarre secondary sexual characters is found in an African frog (Petropedetes newtoni). In this frog the male has the columella of the ear pushed through the drum to form a noticeable projection. Hibernation. — ^Amphibia are more or less adapted to their en- vironment; and, when winter comes, bringing low temperatures and CLASS AMPHIBIA 481 a scarcity of food, most of them hibernate. Frogs crawl into the mud in the bottom of ponds or other damp spots, dig into the ground under logs, or crawl into cracks and crevices. Toads burrow into the ground, the depth to which they go depending on the type of soil. They may go as far as 18 inches underground in sandy soil. Salamanders may bury themselves in the mud, under rocks in running streams, in rotting tree stumps or in burrows in the ground. Fig-. 265. — ^Axolotl larva of the tiger salamander, Ambystoma tigrinum. graph by Sanders.) (Photo- '■i'-S*. <«»',•' T "^m^: '*g! *.«».(*'• Fig. 266. — Ambystoma texanum, one of the most common salamanders in Texas. (Photograph by Sanders.) After establishing itself in hibernation quarters the amphibian reduces all vital activities to a minimum. Respiration is carried on entirely through the skin, and the body in its dormant state secures the slight amount of nutriment needed from the food stored in its tissues. In some hot countries during the dry, torrid season amphib- ians aestivate in a protected moist place, reducing their activities until the severest weather is over. 482 TEXTBOOK OF ZOOLOGY Classification There are estimated to be about 1,900 known species of living frogs, toads, and salamanders in the world, and about 60 species Fig. 267. — Typhlomolge rathbuni, the blind cave salamander of Texas. (Photo- grapii by Sanders.) ''^^i.t ' ■ '^M Fig. 268. — Pseudacris streckeri, Strecker's ornate chorus frog. (Photograph by Thos. Mebane Jones.) of caecilians. None of the caecilians have been reported from the United States. In the United States there occur about 79 species of salamanders and about 70 species of frogs and toads.* Many of ♦According to the Check List of North America Amphibia and Reptiles by Stejneger and Barbour, 4th edition. CLASS AMPHIBIA 483 these species are subdivided into several subspecies. The Southwest contains a large proportion of all of these. Some characters used in classifying salamanders are : the presence or absence of gills, either external or internal; color markings; shape and appearance of body; length; number of costal grooves; number of digits ; position of teeth ; presence or absence of a naso- labial groove ; plantar tubercles ; shape of vertebrae ; form of cranial bones and cartilages; presence or absence of lungs ; presence or absence of ypsiloid cartilage. Some characters used in classifjdng adult frogs and toads are : color markings; length of body and of hind limb; shape of head; Fig. 269. — Tree frog, Hyla crucifer. (Photograph by Thos. Mebane Jones.) nature of skin; presence or absence of parotoid glands and their shape ; presence or absence of tympanum ; presence or absence of cranial crests and their shape ; presence or absence of teeth and their situation; the shape of the vertebrae; shape of the sacrum and pectoral girdle ; shape of pupil of the eye ; presence or absence of adhesive discs at the ends of digits. The student interested in classification and identification of species should consult appropriate keys for the various groups of Amphibia. There is appended at the end of the book a list of references deal- ing with this class of animals. 484 TEXTBOOK OF ZOOLOGY A List of Families of the Amphibia in the United States The ranges cited below are not exact but give an idea of the dis- tribution of the genera. Order Caudata (Urodela) (Tailed Amphibians) Suborder Crypto'branchoidea Family Cryptobranchidae Cryptohranchus alleganiensis (1 species). This so-called "hellbender" ranges from the eastern states west to Iowa, south to Louisiana. Suborder Ambystomoidea Family Ambystomidae Amby stoma (13 species). Common species in the Southwest are: the Tiger salamander {A. tigrinum) ; the Texan salamander {A. texanum) ; and the Marbled salamander {A. opacum). Dicamptodon ensatus (1 species). Eegion of San Francisco, Calif. Bhyacotriton olympicus (1 species). Olympic Mountains, Wash. Suborder Salamandroidea Family Salamandridae Triturus (5 species in the United States). The common newt of the South- west is Triturus viridescens louisianensis. The other species represented is T. meridionalis. Family Amphiumidae Amphiuma (2 species). A. tridactylum, the three-toed congo eel, ranges from northern Florida to eastern Texas. Family Plethodontidae Gyrinophilus porphyriticus (1 species). Eastern states west to Kentucky, south to Georgia. Fseudotriton (2 species). Pennsylvania to Louisiana. Eurycea (6 species). Range from New England to Texas. Manculus quadridigitatus (1 species). North Carolina to Texas. This dwarf salamander has only four toes. Stereochilus marginatus (1 species). Dismal Swamp, Virginia to Georgia. Typhlotriton spelaeus (1 species). The blind salamander of the caves of Missouri and Arkansas. Typhlomolge rathbimi (1 species). The blind cave salamander of Texas. Leurognathus marmorata (1 species). North Carolina mountains. Desmognathus (5 species). Southern Canada to the Gulf of Mexico, eastern states westward to Illinois. Most common species in Southwest is D. brimleyorum, Brimley's triton. Plethodon (15 species). Distributed over almost the entire United States. Common in the Southwest is P. glutinosus, the slimy salamander. Hemidactylium scutatum (1 species). Canada to Louisiana. Another four- toed salamander. CLASS AMPHIBIA 485 Flethopsis wrighti (1 species). Oregon. Batrachoseps (2 species). The worm salamander. Both species on the Pacific. Ensatina (3 species). All on the Pacific Coast. Aneides (4 species). On Pacific Coast and in southeastern states. Eydromantes platycephalus (1 species). Yosemite salamander. Suborder Proteida Family Proteidae (with external gills and 2 pairs of limbs) Necturus. According to a recent revision of the genus by Mr, Percy Viosca, of New Orleans, describing two new species from Alabama and two new species from Louisiana, the number of species in the U. S. is increased from three to seven. The common large Necturus from the Great Lakes region is N. maculosus; the species which seems to be the most common in southern states is N. heyeri Viosca, which extends into Texas. Suborder Meantes Family Sirenidae (with external gills, without hind limbs) Siren (2 species). Eastern Virginia to Texas. Both S. lacertina and S. intermedia are found in the Southwest. ' Pseudobranchus striatus (1 species). South Carolina to Florida. Order Salientia (Anura) (Tailless Amphibians) Suborder Amphicoela Family Liopelmidae Ascaphus truei (1 species). Washington and a few other points on the Pacific Coast. Suborder Anomocoela Family Pelobatidae (Spadefoots) Scaphiopus (5 species). One species in the East, one each in Florida and California; four species in the Southwest. These are the spadefoot toads, the pupils of whose eyes are vertical when in daylight. Suborder Procoela Family Bufonidae (Toads) Bufo (17 species). Species of Bufo are distributed over the entire United States. Among common species in the Southwest are B. cognatus, B. com- pactilis, B. debilis, B. insidior, B. marinus, B. fowleri, B. punctatus, B. valliceps, and B. woodhousii. Family Leptodactylidae (Robber Frogs) Leptodactylus labialis (1 species). Found only in Texas. Eleutherodactylus (3 species). One species in Texas (Texas cliff frog), one species in Arizona, one species in Florida. Syrrhophus (2 species). Both species limited to Texas. Family Hylidae (Tree Frogs) Acris gryllus (1 species). The cricket frog, widespread throughout eastern and central United States, including the Southwest. 486 TEXTBOOK OF ZOOLOGY Fseudacris (6 species). Throughout the same regions as Acris. Various subspecies of the swamp cricket frog (P. nigrita) are common in the Southwest. The recently described P. streckeri Wright, ranging through- out Texas, is a very colorful species, and its high-pitched staccato chirp is one of the earliest to be heard at breeding pools in Texas. Hyla (12 species). Various species in all of the United States. They are the most colorful of all the frogs. Common species in the Southwest in- clude: H. arenicolor, H. cvnerea, H. crucifer, H. squirella, H. versicolor. Suiarder Diplasiocoela Family Ranidae (True Frogs) Baria (18 species). Various species occur in all parts of the United States. Common species in the Southwest are: B. sphenocephala, B. pipiens, B. catesbeiana, B. clamitans. Family Brevicipitidae (Narrow-mouthed Toads) Bypopachns cuneus (1 species). In southern Texas. Microhyla (3 species). Ranges from Virginia to Texas, northward to Mis- souri and Indiana. Economic Importance The entire group of Amphibia are of considerable economic value because they feed to such a large extent on insects, thus becoming valuable aids to the farmer in controlling noxious insects. In the flooded rice fields of Louisiana, bullfrogs grow fat eating insects, crayfish, and other small animals. Frogs are used throughout the world as an article of food by man as well as by other animals. In the eastern United States, large quantities of the leopard frog and wood frog are consumed. In the southern states, bullfrog legs have been a favorite food for years. Within recent years businesses have developed which are de- voted to supplying bullfrog legs, and the demands for these from all parts of the country have been so great that it may become necessary to afford some protection to prevent the rapid depletion of these animals. Attempts have been made to operate frog farms and raise a supply. Most of these attempts have been failures be- cause of the high overhead cost. The axolotl is used in Mexico as food; and water dogs, such as our Necturus, are reputed to have a good flavor. Dried frogs and toads have been used in China both as a source of food and for medicinal purposes. It is reported that toad skins have been used in Japan and elsewhere for making a fine type of CLASS AMPHIBIA 487 leather. Dried salamanders have been used as a vermifuge. Adult frogs and salamanders, as well as larval stages, are widely used as laboratory animals. Fig. 270. — Bufo valUceps is a common toad. (Photograph by Thos. Mebane Jones.) NECTURUS MACULOSUS, THE MUD PUPPY Necturus maculosus, the mud puppy or water dog, is a very com- mon example of the salamander division of Amphibia found from the Mississippi basin eastward, and is the one most commonly used for laboratory study. It lives in ponds and streams, spending most of the time in the mud at the bottom, but swimming and crawling about at night. It comes ashore only occasionally. Insect larvae, crayfish, worms, frogs, and occasionally fish comprise much of its food. Fig. 271, A. — Diagram of dissection to show principal organs of Necturus. 1, af- ferent branchial artery, I ; 2, bulbus arteriosus ; S, afferent branchial artery, II ; i, afferent branchial artery. III ; 5, ventricle ; 6, hepatic sinus ; 7, subclavian artery; S, postcaval vein; 0, dorsal aorta; 10, right lung; 11, postcardinal vein; 12, liver; IS, hepatic portal vein; 11,, gastrosplenic vein; 15, pancreas; 16, gall bladder; It, ventral abdominal vein; IS, testis; 19, kidney; SO, renal portal vein; 21, pelvic vein; 22, caudal vein; 23, mesonephric duct; 21/, gill slit; 25, external gills; 26, internal jugular vein; 27, left auricle; 28, external jugular vein; 29, sub- clavian vein; SO, common cardinal vein or duct of Cuvier ; 3/, gastric artery; S2, left lunng ; SS, pulmonary vein ; Sk, stomach ; S5, spleen ; 36, pylorus ; 37, duodenum ; S8, mesenteric vein; S9, ileum; J,0, large intestine; Jil, femoral vein; 1)2, urinary bladder; JiS, vesical vein; H, cloaca. (Courtesy of General Biological Supply House.) CLASS AMPHIBIA 489 The group of vertebrates which Necturus represents is of par- ticular interest because of its transitional position between aquatic and terrestrial forms. Necturus is aquatic and fishlike in its pos- session and use of external gills, although it has only three arches. The body is used like that of a fish in SAvimming ; that is, by lateral strokes of the tail against the water. It is terrestrial in the de- velopment of pectoral and pelvic girdles and limbs for crawling. Also lungs are developed for aerial respiration, although not highly functional. The sixth or last pair of aortic arches of the primitive series gives off a pulmonary artery and still retains the connection to the dorsal aorta. This portion of this arch is known as the duct of Botallus. The heart has become three-chambered; there are now two auricles instead of only one as in fish. The posterior cardinal veins are still present, but their function is partially taken over by the newly developed post cava. Necturus remains in a larval condition throughout its life, be- comes sexually mature, and reproduces without metamorphosis. Such a condition is referred to as neoteny. The retention of external gills is a very marked larval feature. Food and Digestive System This animal is quite inactive and requires relatively little food. It does make use of several aquatic inhabitants including crayfish, other small crustaceans, snails, insect larvae, leeches, some minnows, and occasionally fish eggs for food. The mouth is located in the anterior, terminal position and with a fairly wide gape. Teeth are located on the premaxillae, vomer, and palato-pterygoid bones of the upper jaw, and the dentary and splenial bones of the lower jaw. The tongue is broad and only slightly mov- able. The internal nares enter the mouth cavity as a slit on each side between the two dorsal rows of teeth near their posterior termi- nations. More posteriorly, in the lateral walls of the pharynx are the two pairs of gill slits or pharyngeal clefts. Still more posteriorly there is a very small inconspicuous pharjoigeal prominence with a tiny slit, the glottis. The esophagus leads posteriorly from the pharynx and joins the anterior or cardiac portion of the prominent stomach. The stomach has the typical shape and appearance of this organ in the lower vertebrate groups, possessing only the cardiac por- tion anteriorly and the posterior narrowed pyloric portion. This 490 TEXTBOOK OF ZOOLOGY leads into the anterior section of the small intestine or duodenum, which includes the first S-shaped turn of the tube. The more coiled part of the small intestine following this is the ileum. This empties into the short but somewhat broadened large intestine, which opens into the cloaca, the common receptor of faecal matter from the intes- tine and urinogenital products from the wolffian ducts, urinary blad- der and miillerian ducts. The urinary Madder is a thin-walled sac hanging at the ventral side of the cloaca, whose lumen it joins. The liver is an elongated, dark-colored and somewhat serrated or- gan lying in the ventral portion of the body cavity. The gall bladder is a membranous sac attached to the margin of the liver (usually at the right side). The bladder is connected with the duodenum by a hile duct which is obscured by a mass of pancreatic tissue. The pan- creas is divided into slender lobes and lies in the vicinity of the junc- tion of stomach and duodenum. One lobe extends to the tip of the spleen which lies dorsolateral to the stomach. Another slender lobe extends posteriorly in the mesentery that supports the anterior part of ileum and the mesenteric vein. Both of these lobes join the mass of pancreatic tissue around the bile duct and the pancreatic ducts enter the duodenum at this level. Circulatory System There is a partial conversion from the straight branchial type of circulation of the fish to the pulmonary type of the terrestrial verte- brates, in that the number of functional aortic arches is reduced, pulmonary vessels are added to supply the lungs, and the atrium of the heart is divided into two parts to keep the systematic and aerated, pulmonary blood partially separated. With certain modification of the situation in fish, the system consists of heart, aortic arches, sys- temic and pulmonary arteries, as well as systemic, renal portal, hepatic portal, and pulmonary veins. The heart consists of right and left atria (auricles) and one ventricle with the two usual accessory chambers, the sinus venosus which joins the right atrium and the conus arteriosus which leads from the ventricle to the ventral aorta. The right atrium receives the systemic blood and the left atrium, the pulmonary blood by way of pulmonary veins. Blood in passing through the heart may be traced by entering the sinus venosus from systemic veins, pass by way of sinu-atrial (sinuauricular) valve to right atrium, thence through the atrioventricular valve to the ventricle which it enters simultaneously with aerated blood from left atrium. CLASS AMPHIBIA 491 The blood is expelled from here through the conus arteriosus into the ventral aorta. As will likely be remembered, the theoretical, primitive, and embryonic typical number of aortic arches in vertebrates is six. This number is modified in most adult vertebrates, usually by reduc- tion. Even teleost fish have only four branchial arches. In sala- manders this number may be referred to as four but considerable modification has occurred. The first (anterior), second, and third original arches have been rearranged and combined to form the com- mon external, and internal carotid arteries. The fourth and fifth supply the external gills with the fourth becoming the systemic arches which meet dorsally to form the dorsal aorta. The sixth arch is modified to supply a large pulmonary artery from each side to the respective lung. The portion of the sixth aortic arch which continues on dorsally to join the aorta, from the point where the pulmonary branches off, is known as the duct of BotaUus. The special modification of the veins is centered around the devel- opment of the post cava which is formed posteriorly by the junction of urinogenital veins. The pair of posterior cardinals, which are characteristic of fish, are retained also but are greatly reduced. They usually join the post cava some distance anterior to the kidneys and parallel the aorta to the heart where they enter the ducts of Cuvier, one on each side. Another modification is the pelvic-ventral abdomi- nal complex which connects the renal portal arrangement with the hepatic portal. A pelvic vein branches from the femoral on each side before it joins the renal portal. The two pelvic veins pass ventrally to meet each other at the midventral point of the pelvis and this union forms the ventral abdominal vein which either enters the liver or a branch of the hepatic portal before it enters the liver, thus pro- viding a cut-off in the course of the venous circulation in going anteriorly from the posterior limbs. The lateral veins of sharks form a similar cut-off but enter the duct of Cuvier instead of the hepatic portal system.* Respiratory System and Breathing The respiration may be divided into cutaneous, performed through the wet skin ; branchial through the gills ; and pidmonary through the lungs. There are several parts to the latter arrangement. The small external nares lead by way of passages to the slitlike internal nares which open into the mouth between the posterior ends of the two ♦Helpful Illustrations of the circulatoi-y system of Necturus may be found in Stuart: Anatomy of Necturus maculosus, Denoyer-Geppert Co., Chicago. 492 TEXTBOOK OF ZOOLOGY dorsal rows of teeth. The mouth is made airtight by the shape and fitting of the lips. The general portion of this cavity posterior to the angle of the jaws is the pharynx. Well back in the floor of it, is the tiny slitlike glottis in the midst of a slightly thickened laryngeal prominence, the opening of which would receive only an object the size of the head of a pin. The glottis leads into a recess called the larynx and the two smooth-walled, saclike lungs extend posteriorly from this. These saclike lungs have a fairly abundant vasculariza- tion (blood supply). The air is pumped into and from the lungs by the movements of the floor of the airtight mouth and change of posi- tion of visceral organs within the body cavity. Branchial respiration is accomplished largely by waving the highly vascularized external gills back and forth in the water. The capillary branching of aortic arches 4 and 5 provides most of this blood supply to the gills. The pul- monary artery supplying the lungs is formed by a large branch from aortic arch number 6. Urinogenital System The following organs constitute this composite system : a pair of mesonephric kidneys, a pair of gonads (testes in male, ovaries in female) numerous vasa efferentia from testes, one pair of Wolffian or mesonephric ducts (ducts of Leydig in male), one pair of Miil- lerian ducts or oviducts (in female, only vestigial in male) single cloaca, the urinary bladder and the mesenteries (mesovarium, meso- tubarium, and mesorchium). The kidneys are somewhat elongated and flat but thicker toward the posterior, suspended in the dorsal peritoneum and lying dorsal to the large intestine. The kidney of the female is smaller than that of the male. The Wolffian duct leads from the lateral margin of the kidney in either sex and proceeds directly from the posterior portion of the kidney to make a dorsolateral entrance into the cloaca. Inside the kidney the Malpighian corpuscles, including glomeruli, are connected with the uriniferous tubules, which in turn join the collecting tubules and they lead to the Wolffian duct. After the urine enters the cloaca it collects in the urinary bladder which hangs ventrally and serves as a storage reservoir. Upon becoming filled with urine the bladder contracts and forces the urine back into the cloaca and from here it passes to the exterior by way of the anus. CLASS AMPHIBIA 493 In the male specimen the yellow or brown-colored cylindrical testes are located one in either side of the dorsal part of the body cavity and each suspended by a fold of the dorsal peritoneum, the mesorchium. The vasa efferentia, which are tiny sperm tubules about the size of very fine threads, and the spermatic blood vessels are suspended in this mesentery. The vasa efferentia enter the medial side of the kidney (except at its anterior) and deliver spermatozoa to a longitudinal Bidder's canal just within. This canal is connected with the medial ends of collecting tubules and through them the spermatozoa reach the Wolffian duct, as does the urine. The Wolffian duct carries them to the cloaca. When the spermatozoa reach the cloaca and bladder they clump into bundles called spermatopliores, and are stored until breeding time. A Wolffian duct (mesonephric duct) which serves both for conveying urine as well as spermatozoa is called a duct of Leydig. In the female the pair of ovaries can usually be recognized by the presence of eggs of some stage of development in them. When fully mature each ovary seems to be a large sac full of large yellow ma- ture eggs about the size of small peas. In specimens with immature ovaries the eggs may be about the size of pinheads. Each ovary is suspended from the dorsal peritoneum by a mesentery, the meso- varium. There is a prominent, coiled, white oviduct or Miillerian tube in the body cavity at each side of the other organs whose an- terior end is suspended in the anterior portion of the body cavity and spreads into a wide membranous funnel called the ostium. The mesentery which supports the oviduct is the jnesotuharium. When the ova reach maturity inside the ovary they escape by a rupture in its wall which frees them in the coelomic cavity. Due to the shape of the body cavity and position of visceral organs these eggs move to the anterior part of the cavity and the ciliated mouths of the two ostia receive them one at a time in each. As these ova pass down the MuUerian tube (duct) they are met by spermatozoa, fertil- ization occurs, a mucous substance is added as a cover by the glands in the oviduct. These fertilized cells in a pouchlike posterior part of each oviduct which is called the uterus and after a few accumu- late they are deposited by passing from the body by way of the cloaca and anus. These zygotes (fertilized eggs) are deposited by attachment to the under sides of rocks, logs, etc. in the water in small clutches of from 25 to 90 individuals. The embryonic stages are passed here and the larvae hatch out as tiny fishlike organisms. 494 TEXTBOOK OF ZOOLOGY At about a year of age they are one and one-half or two inches in length with a stripe down the side which gives them a peculiar appearance. The actual breeding and copulation activities (if any) do not seem to be very well understood. There is a prevailing idea that the spermatophores are passed from the male to the female in the autumn and held in the genital tract of the female until the suc- ceeding spring when the eggs mature and pass down the oviducts. The act of transferring spermatophores is described as occurring in shallow water or on the muddy margin of the pond or stream by the male, depositing them here while the female follows and collects them into the cloaca by use of its swollen lips, the papillae there, and the mucus which is secreted by the cloacal glands that lie at the sides of the cloacal aperture.* Skeletal System The skeleton of these animals is classified as a bony skeleton but is not completely ossified and a considerable part of it is cartilage. The axial portion consisting of skull, vertebral column and ribs; and the appendicular portion, consisting of the two girdles with limbs constitute the essential parts of this system. The skull is platybasic (flat and broad) with a marked fusion and loss of primi- tive bones when compared with the teleost fish. The anterior, dorsal surface of it is covered by a single, fused frontal bone, posterior to which, and extending beneath and somewhat lateral to this, is the large pair of ixirietals. At the anterior tip of the frontal are the premaxillae which bears teeth. Just posterior to this and somewhat covered by the frontal is the vomer, which also bears teeth. Both the nasals and maxillae are absent. The braces, at the side of the skull, are the palatopterygoid bones, each of which bears a few teeth; the quadrate cartilage; quadrate hone, which articulates with the lower jaw ; and the squamosal, which appears more dorsally. The otic group is represented only by the prootic, a small, irregular one which lies between the anterior part of the squamosal and the pari- etal, and another small one, the opistliotic, which is at the postero- lateral corner of the skull. The foramen viagnum (large opening) is located at the mid-posterior position and an occipital condyle is located at each side of it for articulation with atlas, the first vertebra. The principal part of the floor of the skull consists of the large flat •Helpful Illustrations of the urinog-enital systems of Necturus may be found In Stuart: Anatomy of Necturus maculosus, Denoyer-Geppert Co., Chicago. CLASS AMPHIBIA 495 parasphenoid. The lower jaw is composed of a pair of each, dentary bones, which bear teeth; splenial bones, which bear the last few teeth; and the angular bones, devoid of teeth and articulating with the quadrate of the skull. Necturus usually has forty-six amphicoelous vertebrae. They articulate with each other by anterior and posterior zygapopJiy.ses as well as the ends of the centra. There is one cervical vertebra, atlas, with which the skull articulates. Posterior to this one are about eighteen thoracolumbar vertebrae each of which bears a pair of short Y-shaped ribs. Each rib has a double head (bicipital) , the dorsal head or tuherculum articulating with the transverse process of the vertebra and the ventral head or capitulum articulating with the side of the centrum. Following the thoracolumbar group is a single sacral vertebra to which the ilium of each side is attached by way of the sacral rib. The remainder of the series, posterior to this point, consists of caudal vertebrae. The pectoral girdle is principally cartilage in structure. The ven- tral portion is formed by a posterior coracoid cartilage in the muscles of the body wall, and an anterior procoracoid. Projecting dorsally and laterally is the third unit of each side, the scapula. The most dorsal, free margin of this is frequently referred to as suprascapula. The recess formed at the junction of scapula with the ventral parts into which the arm articulates is called the glenoid fossa. The skele- ton of the anterior appendage includes the proximal humerus (in the brachium), the radius and idna in the forearm (antebrachium), six carpals in the wrist, four metacarpals in the palm, and four digits each composed of joints or phalanges. The pelvic girdle is likewise largely cartilage, but it is fused in the midventral line. The anterior, ventral part is the puhic plate con- sisting of cartilage, posterior to this is the pair of iscMa which are partly ossified. Extending dorsally on each side is a slender ilium which joins the sacral rib and this in turn the sacrum. In the lateral position where the ilium meets the two ventral parts of the girdle is a concave recess into which the head of the femur of the thigh articu- lates. This recess is called the acetahidum. Distal to the thigh is the shank with two bones, the tibia and the fihula lying parallel to each other. There are six somewhat fused tarsals in the ankle. Distal to this are the four elongated metatarsals and beyond each is the digit, composed of phalanges.* •Illustrations of the skeleton of Necturus may be found in Stuart: Anatomy of Nectwrus maculosus, Denoyer-Geppert Co., Chicago, 496 TEXTBOOK OF ZOOLOGY Muscular System The muscles of the body are divided into segmental myotomes with intervening connective tissue sheets or myosepta. A horizontal septum along the side of the body divides the muscles into a dorsal, epaxial portion and a ventral hypaxial portion. The principal sets of super- Fig. 271, B, — Left lateral view of the muscles of the head and shoulder re- gion of the salamander, Nectiirus maculosis, (From Atwood, Comparative Yerte- brate Dissection, The Blakiston Company.) i— ~ ~ rectus abdominis myotomes '• pectineus I _^ pubofemoralis -■ ■p internus '^ ischiofemoralis— "" rectus exteraus ■-gracilis • *- - - pubotibialis femorofibulariS:- - - ischiocaudalis'' gluteeus maxim us _/— - semimembranosus pyrifonnis— '' flexor commimis - extensor communis Fig. 271, C — The muscles of the hind legs of Necturus maculosus ; ventral view on the left, dorsal view on the right. In the dorsal view tlie ilium has been cut from the pelvic girdle and deflected downward. 1, 2, and 3 are extensors of the foot, and 4 is a flexor. (From Atwood, Comparative Vertebrate Dissection, The Blakiston Company.) CLASS AMPHIBIA 497 ficial muscles or those of the head and gills, body wall, and the appendages. Because of the development of the terrestrial limbs, the latter group is much more complicated in this amphibian than it was in the fishes. For the detailed information concerning the specific muscles the student will depend on the accompanying illus- trations and the laboratory study. The Nervous System and Sense Organs Since this system resembles that of the fish Avhich has been studied already, and is so closely similar to that of the frog, which is de- scribed in the next section of the book, it seems unnecessary to describe it here.* THE BULLFROG Habitat The bullfrog is a solitary animal except during the breeding sea- son. It is strictly aquatic and does not leave the pools as does the leopard frog. It prefers bodies of quiet water where there are both shallows and deeper water, such as lagoons, small lakes, and the cypress ponds of swampy regions. In such a situation, the shore is protected by low willows or other trees, and the shore waters are filled with aquatic plants, pickerel weeds, and floating lily pads. These furnish not only a good hiding place but a good hunting ground for the crayfish, insect larvae, water beetles, snails, and other aquatic organisms which make up the bullfrog's diet. This diet is quite varied and may even include younger frogs. Bullfrogs are found in North America east of the Kockies from Canada to Mexico. They have also been introduced into the western portion of the United States and into various foreign countries. External Structure Bullfrogs obtained in the South and Southwest are usually of two species, Rana catesheiana Shaw, the common bullfrog, or Bana grylio Stejneger, the southern bullfrog. Individuals of the former species attain larger sizes, and the giant bullfrogs of the southern swamps usually are Rana catesheiana. The two species differ not only in size but also in external appearajice, particularly when alive. However, they are essentially the same anatomically, and this chapter is based on a study of Rana catesheiana. •Illustrations of this system mav be found in Stuart: Anatomy of Necturus maculosusj Denoyer-Geppert Co., Chicago. 498 TEXTBOOK OF ZOOLOGY The common bullfrog is ordinarily greenish or olive brown. Un- derparts are mottled with dark spots on a white background, and the upper surfaces may be plain or marked with large dark splotches. The legs are marked with crossbars and other splotches of dark color. Preserved specimens appear brownish gray with the dark mottling lighter in color than on the living specimen. The body of the bullfrog includes the head and trunk. Attached to the trunk on either side anteriorly are the forelegs and posteriorly the hindlegs. The head has two prominent eyes which protrude above its sur- face. These can be drawn back into their orbits and forced some- Fig. 272. — External features of the common bullfrog, Rana catesbeiana. (Courtesy of Southern Biological Supply Company.) what into the mouth cavity. The lower lid of the frog's eye with its attached nictitating membrane is drawn up over the eye, not by independent movement of the eyelid, but as a result of the retraction of the eye into the orbit. The upper eyelid is immovable. Back of each eye is a circular oval area, the tympanum or eardrum. In the females this is about the size of the eye, while in the males it is larger than the eye. A small fold of skin, the tympanic fold, runs from the eye around the posterior margin of the tympanum. The two nostrils or nares are near the anterior part of the head, and each is guarded by a valve. The mouth reaches from one side of CLASS AMPHIBIA 499 the head to the other and has an upper and lower jaw. The anus or vent is at the extreme posterior end of the trunk. The forelimbs are composed of the upper arm, which joins the trunk, the forearm, wrist or carpus, and the hand with its four digits. In the male, particularly during the breeding season, the innermost digit, or thumb, is enlarged, whereas the thumbs of females remain apparently the same size. The digits may have tubercles on them, and their positions in relation to various bones of the hand give rise to specific names for these tubercles. The forelimbs are used not only to help support the body but also as an aid in pushing food into the mouth. The hindlimbs are long and have powerful muscles. Bullfrogs ordinarily leap about three feet but can easily cover a distance of five or six feet. The hindlegs are composed of the thigh, which joins the trunk ; the shank; and the ankle, or tarsus. Following the tarsus is the foot with five digits (toes), which are connected by a web, producing a very efficient swimming organ. The smooth damp skin, which is soft and loosely attached to the body except in the head region, is composed of two layers, an outer epidermis and an inner dermis. The skin is pigmented and very rich in mucous glands, which aid in keeping it moist. Bullfrogs moult or shed the superficial layer of epidermal cells of their skin at varying intervals. Dig^estive System and Digestion The mouth cavity, or buccal cavity continues directly into the phar- ynx with no sharp line of bounjiary between them. The latter narrows toward the esophagus, which is a short gullet leading directly from the pharynx to the stomach. The lining of the esophagus has a number of longitudinal folds and is ciliated. The stomach normally lies on the left side of the body. It is curved, with the convex side toward the bullfrog's left. Its anterior or cardiac end is wide, and the pyloric or posterior end is narrowed and constricted where it joins the small intestine. The duodenum, or anterior part of the small intestine, runs forward almost parallel with the stomach. At the point where the intestine turns back posteriorly the duodenum be- comes the ileum, which composes the remainder of the small intestine and is considerably coiled. The large intestine or rectum is sharply 500 TEXTBOOK OF ZOOLOGY marked off from the small intestine and is wide and short. It passes directly into a muscular part, the cloaca, which terminates in the anus or vent. The buccal cavity has in its roof near the end of the snout two patches of small conical teeth, called vomerine teeth. In addition, the upper jaw has a single series of small conical teeth on its edge known as maxillary teeth. These teeth serve primarily to help hold the cray- fish, insect, or other animal captured for food, and they may help at times in crushing it. The tongue is somewhat leaflike in shape and is deeply notched behind, making it bicornute. Its anterior half is at- tached to the floor of the mouth just back of the tip of the lower jaw. Vomerine teeth Fbor of orbit M/' Isophaqus M,.^^, Vocal 5ac Maxillary teeth . InLcmol nares Sulojs marqinalis _ lustach'ian tuoe Qlotth C Tonque Fig. 273. — Mouth or buccal cavity of the bullfrog. and its posterior end is free. In order to get the tongue out of the mouth the posterior part has to somersault over the attached ante- rior part. The tongue of the bullfrog is somewhat smaller pro- portionally than that of the grass frog, as might be expected, for the latter is more dependent on this organ when it hunts insects inlajid. Taste buds are present on the tongue and palate. Esophagus, stomach, and intestine have an outer longitudinal and an inner circular layer of smooth muscle. The peristaltic contrac- tions of these muscles pass the food through the digestive tract and aid in mixing it with the gastric juice in the stomach. They may also be used to regurgitate a disagreeable substance swallowed by the frog, in which case the stomach turns inside out and protrudes into the mouth cavity. The stomach can be greatly expanded and acts as a reservoir for food which may be available only at irregular intervals and the frog has to take advantage of a food supply when CLASS AMPHIBIA 501 it is present. The mucosa of the intestines has a number of longi- tudinal and transverse folds which produce a great absorptive sur- face through which the digested food can be taken up by the blood stream and transported to different parts of the body. The liver lies on each side of and behind the heart. It is three- lobed, two lobes being on the left and one on the right, connected by narrow bridges of liver tissue. Between the right and left lobes is the gall Madder, which receives an alkaline secretion known as bile from the liver and stores it until needed in the process of digestion. Bile is carried from the gall bladder to the duodenum by the hile duct, which passes through the pancreas on its way. The liver is not primarily a digestive gland, for, while the bile it secretes per- mits the fats to be more easily digested by a lipase from the pan- creas, the bile itself contains no digestive enzymes. Although its function in altering fatty substances is important, of prime impor- tance is its ability to store glycogen and the fat upon which a hiber- nating frog lives. It is also concerned in the formation of urea and in the destruction of red blood corpuscles. The pancreas lies in the loop between the stomach and duodenum. It is a long, whitish, irregularly-lobed gland whose alkaline secre- tion is of considerable importance in digestion, for it contains three digestive enzymes. This secretion is taken from the pancreas by pancreatic ducts which empty into the bile duct that passes through the pancreas before entering the duodenum near its beginning. Intestines, liver, and pancreas are covered with peritoneum. The mesenteries which hold the body organs in position and the internal surface of the body wall likewise are made up of this peritoneal membrane. Digestion. — Since frogs live primarily on insects, crayfish, and other small invertebrate animals, their food is very rich in proteins. Their vomerine and maxillary teeth are too feeble to do more than slightly crush their prey, so digestion begins in the stomach. Here the gastric glands secrete hydrochloric acid and an enzyme, pepsin, which converts the proteins to peptones. Peristaltic contractions of the stomach cause a thorough mixing of the gastric juice with the food and then this partly digested food (chyme) is passed posteriorly into the small intestine. Here, activated by the acid nature of the food, the intestinal glands release into the blood stream a substance, secretin, which on reaching the pancreas causes it to pour forth into the duo- 502 TEXTBOOK OF ZOOLOGY demim its highly alkaline secretion. In addition, this pancreatic juice contains three digestive enzymes : trypsin, which continues the diges- tion begun by pepsin in the stomach, converting proteins to amino acids; an amylase, annjlopsin, which changes starches into sugars; and a lipase, steapsin, which, aided by the bile, causes a splitting of the fats into glycerol and fatty acids. Bile also contributes to the alkaline condition here. The process of digestion is completed in the intestine and the food products are taken up by absorption in its mucosa layer. These foods in solution are taken by the blood stream and lymph vessels to various parts of the body where they are utilized for building tissue or for supplying energy, leaving as by-products urea and carbon dioxide. Sugars that are not used are stored as glycogen in the liver and in voluntary muscles. The liver also serves to store fats and to secrete urea and sugar directly into the blood stream. Food that is not digested passes to the large intestine where it is retained for a time and then passed to the outside through the anus as feces. Other Glands. — Attached by a mesentery to the wall of the intes- tine near the anterior end of the rectum is the spleen. It is a small, reddish, spherical, Ijnnphoid organ, the functions of which are but incompletely known. The destroying of red blood corpuscles is an important duty, as possibly also is the formation in its tissues of lymphocytes, one type of white blood corpuscle. In mammals the spleen is also believed to accumulate iron freed by the metabolism of other tissues. This iron is subsequently used in the formation of hemoglobin. The two thyroid glands are small and lie in front of the glottis under the floor of the mouth. There is one on each side of the hyoid apparatus. The secretion and functions are discussed in the chapter on Internal Regulation. A thymus gland lies under the skin behind the tympanic membrane on each side. It is partly covered with muscle and is small. Further discussion of it will be taken up in the chapter on Internal Regulation. Circulatory System The circulatory^ system comprises the Mood vascular system and the lymphatic system. The two systems are closely interrelated in that they both carry to the tissues of the body nutritive material neces- CLASS AMPHIBIA 503 sary for metabolism aud remove from them to the excretory organs, waste products of body activity. They differ in several respects; the lymph neither contains red blood corpuscles for transporting oxygen nor moves in a continuous closed vascular circuit as does the blood. Other differences will be noted in the discussion. The Blood Vascular System. — The blood moves through a closed system of tubelike vessels of various sizes which distribute it to all parts of the body. The pump is the heart, which, by its con- tractions, forces the blood to flow to the tissues. Since the system is a closed one, the blood eventually returns to the heart. The blood vessels leading away from the heart are the arteries. When these reach the tissues, they break up into very small vessels, the capillaries. The vessels leading back to the heart are the veins. The arteries and veins are connected by the capillaries. Blood is comprised of a clear liquid called the 'plasma, suspended in which are blood corpuscles of three kinds, the red blood corpuscles or erythrocytes, the white blood corpuscles or leucocytes, and the spindle cells or thrombocytes. In addition, the blood may contain dissolved nutritive substances from the digestive system, waste products from tissue repair aud destruction, hormones being trans- ported from organs of one part of the body to another, or foreign substances accidentally introduced. The capillaries are very small vessels, the walls of which are made up of endothelium continued from the linings of arteries and veins. The}^ connect the distal ends of the arteries with the proximal ends of the veins, but in so doing they branch extensively and anastomose to form fine networks in the tissues invaded. Through their thin walls, acting as semipermeable membranes, food products brought by the arterial blood pass into the tissues, oxygen is unloaded from the red blood corpuscles, and carbon dioxide and waste products are taken up to be conducted into the veins. Leucocytes are able to get out of the capillaries, squeezing their way between the cells of the capillary walls, and thus become free in the surrounding tis- sue to engulf bacteria or other harmful objects. The abundance of the capillaries varies with the activity of the organ ; the greater the rate of metabolism the greater their abundance. Examples of such are the various glands and the mucous membrane of the digestive tract, in contrast, a tendon has few capillaries. 504 TEXTBOOK OF ZOOLOGY The arteries are large vessels with elastic walls and carry blood from the heart to the capillary networks in the various organs and tissues of the body. The arteries arise from the conus arteriosus 1 / h Irtternal care ^^ ' ■\uricL:.iari '--■ \ I .5»> ,-j1 _ ■-; ■ / < J \nl3 "iC- r ■r,3SinT.:.r:c^:. r PosX L : — *> m^ /:. ■ , -' ■ :,ta: . ■. .... Dorsal aorta \ Iliur^ ■'. Fig. 274. — X-ray picture of bullfrog with arterial system and a portion of venous system injected. All labels indicate arteries except where otherwise noted. (X-ray courtesy of Dr. Malcolm B. Bowers.) which divides just above the auricles into a right and left truncus arteriosus. Each of these trunks splits into three arches going to CLASS AMPHIBIA 505 each side of the body, the anterior carotid arch, the middle, systemic arch, and the posterior, pulmo cutaneous arch. The Carotid Arch. — Each carotid arch divides into two branches. The more ventral, Ungual artery, or external carotid, passes forward, giving branches to the thyroid, pseudothyroid, muscles of the hyoid External c Auricularis i ral Occipital Internal carotid Cutaneous Carotid qiand Conus arteriosus- Pulmonary. Systsmic arch Lateralis Dorsalis Cozliaco.rnesenteric Brachial Vertc bra I \^Left qastric PANCREAS ].Riijht Cjastric l_Coeliac Anterior mesenteric Splenic (Irinoaenital Epigastric Viae Recto.vesical Sciatic ■■RECTUM! pogfer/or mescnter/c , -'h^^^^^emora I i h Fig. 275. — Arteries of the bullfrog from ventral view. (Drawn by Ruth M. Sanders.) and tongue, and then extends along the edges of the lower jaw. The internal branch is larger and is called the internal carotid. It has at its base a spongy enlargement known as the carotid gland which by its structure serves to steady the pressure of blood passing into the artery. This artery follows the side of the neck to the base 506 TEXTBOOK OF ZOOLOGY of the skull, giving off the palatine artery to the roof of the mouth, the cerebral carotid which enters the skull and supplies the brain, and the ophthalmic artery to the eye. The Systemic Arch. — The systemic arch soon after it leaves the truncus supplies a small laryngeal artery to the larynx and mus- cles of the hyoid. It then curves downward and around the esopha- gus on each side. It gives off an occipitovertehral artery which sends a small artery to the dorsal side of the esophagus, then branches at the spinal cord into the occipital artery, running anteriorly on the dorsal side of the skull to the orbit and tympanum, and the vertebral artery, turning posteriorly along the spinal column. Imme- diately posterior to the occipito vertebral artery the large subclavian artery arises from the systemic arch. It branches to the shoulder and adjacent body wall and enters the arm as the brachial artery. The systemic arches from each side, after curving under the ali- mentary canal, meet near the anterior end of the kidneys and fuse into a single large artery, the dorsal aorta, which extends posteriorly. At or just posterior to this meeting point, there arises from the aorta the large coeliacomesenteric artery which divides into an anterior branch, the coeliac artery, and a posterior branch, the anterior mes- enteric artery. The coeliac artery divides into right and left gastric arteries. The latter runs directly to the dorsal or left side of the stomach, while the former sends off small pancreatic arteries to the pancreas ; a larger hepatic artery to the pancreas, gall bladder, and liver; and continues to the ventral side of the stomach, where it is distributed. The anterior mesenteric artery gives off the splenic (lienal) artery to the spleen and then divides into two parallel ves- sels which send numerous smaller arteries to the small and large intestines. The urinogenital arteries consist of about four to six small much- divided arteries which are given off from the ventral side of the dorsal aorta to right and left, supplying the kidneys, reproductive organs, and fat bodies. A few small lumbar arteries arise either as branches of these or directly from the aorta and go to the body wall on each side. The small posterior mesenteric artery is given off near the posterior end of the aorta, passing to a portion of the rectum and, in the female, to the ovisac. It often anastomoses on the rectum with descending branches of the anterior mesenteric, CLASS AMPHIBIA 507 Near the posterior end of the body cavity the dorsal aorta divides into two iliac arteries going to the hind legs. Each of these gives off (1) an epigastric artery suppljang the bladder and dorsal and ventral body walls of the region, and (2) just below it, a femoral artery passing to the body wall, skin, and proximal muscles of the thigh. As the iliac artery enters the leg, a rectovesicular artery is sent Mandibular Brachial. Sinus venos Cardiac- Hepatic. Cutanecus- !!^lnternal jut^ular External juijular . Subscapular Innominate -Pre cava I Posterior vena cava Soermatic Dorso. lumbar- LIVER LOWER L^FT ;^^0;v^.^\ I.OBC .>-.V ^^^\ Hepatic portal Gastric Splenic /.Mesenteric SPLEEN Renal Renal portal. Vesical External iliac Fe moral - Fig. 276. — Veins of bullfrog from ventral view. (Drawn by Ruth M. Sanders.) off to the rectum, bladder, and skin on the dorsal surface of the thigh. In the upper leg the continuation of the iliac, now called the sciatic, gives off a branch to the right and to the left, supplying the muscles, and then continues down the leg, sending off several branches at the knee. The pulmo cutaneous arch takes blood to the respiratory organs: the lungs, skin, and buccopharyngeal cavity. The pulmocutaneous 508 TEXTBOOK OP ZOOLOGY arch on each side divides into a pulmonary artery to the lungs and a large cutaneous artery, which passes outward to the skin. Impor- tant branches of the cutaneous are: the auricularis, supplying the tympanum and adjacent head region ; the dorsalis, supplying the skin of the back ; and the lateralis, which is distributed to the skin of the side. The Veins. — These vessels usually parallel the arteries that brought blood to the tissues from which the veins are returning it. The walls of the veins are thinner and not as elastic as those of the arteries. Many veins, particularly those of the limbs, have semilunar valves on the internal surface of the wall which open in the direction of flow and prevent the backflow of blood. In returning blood to the heart, the venous system carries some of the blood through the kidneys or through the liver, providing renal or hepatic filters to eliminate urea and other waste products from the blood or to alter it chemically. Pulmonary veins from the lungs carry oxygenated blood, which differs from the type of blood found in the other veins. The venous circulation, therefore, may be divided into four main systems: the systemic, hepatic portal, renal portal, and pulmonary systems. The systemic veins carry the greatest load of blood to the heart. The larger collecting veins of the system consist of two precavals receiving blood from the anterior parts of the body, except the lungs, and a single postcaval or posterior vena cava receiving blood from the posterior parts of the body. The two precavals empty into the anterior end of the sinus venosus of the heart, and the posterior vena cava empties into its posterior end. Each of the two anterior precavals receives blood from three branches: (1) the external jugular bringing blood from the tongue, hyoid, thyroid, pseudothyroid, and floor of the mouth; (2) the in- nominate vein, made up of a fusion of the internal jugular returning blood from the brain and other parts of the head, and the subscapular vein bringing blood from the back of the arm and shoulder; and (3) the subclavian vein, a fusion of the brachial vein, returning blood from the forelimb, and the large musculocutaneous vein, which forms an ellipse down the side of the body and extends up into the head region, returning blood from the skin and outer muscles in these regions. CLASS AMPHIBIA 509 The large posterior vena cava originates between the kidneys and receives blood from each kidney by five or six renal veins, from the gonads by small spermatic or ovarian veins, and from the fat bodies by other small branches. Near the heart the vena cava receives two large hepatic veins from each side of the liver. The Hepatic Portal System. — This system is comprised of two chief veins, the hepatic portal vein and the ventral abdominal vein. These veins, instead of carrying blood directly to the heart, bring it to the liver to pass through a netAvork of sinusoids (modified capillaries). It is returned to the systemic system through hepatic veins that join the postcaval. Veins from the large and small intestines unite to form the mesen- teric vein which is joined as it progresses forward by the splenic vein from the spleen, pancreatic veins from the pancreas, and gastric veins from both sides of the stomach. The vessel resulting from these unions is the hepatic portal vein. It passes through the anterior portion of the pancreas and sends a large branch into the lower left lobe of the liver. At about this point it often receives a final gastric branch which has passed on top of the pancreas to join it. It then continues a short distance to join the abdominal vein just below the heart. The abdominal vein arises as follows: Two large veins, the sciatic and femoral, bring blood from the hindlimbs. The femoral, as it enters the body cavity, gives off the pelvic vein. The pelvic veins from each side of the body join in the middle to form the large ventral abdominal vein. As the abdominal vein runs toward the heart along the median portion of the ventral body wall, it receives vesicular veins from the bladder, parietal veins from the body wall and, at its ante- rior end, a cardiac vein from the heart. In the region of the liver it leaves the body wall, is joined by the hepatic portal vein, and enters the right and upper left lobes of the liver by short branches, dis- charging its blood into sinusoids. The Renal Portal System. — This system, like the hepatic portal system, diverts blood to a purifying organ instead of carrying it directly to the heart. In this ease, the blood is taken to the kidneys. The outer femoral vein and the medial sciatic vein collect blood from the hindlegs. The femoral vein, after giving off the pelvic vein, runs anteriorly and joins the sciatic, to make the renal portal vein. Near the kidney this vein receives the dorsolumbar vein from the body 510 TEXTBOOK OP ZOOLOGY wall and, in the female, several vessels from the ovisacs (uteri). The renal portal vein follows the dorsolateral margin of the kidney, send- ing numerous transverse branches into the organ, where they break up into capillaries. Blood which passes through these capillaries is purified of some of its waste products and then leaves the kidney through the renal veins which empty into and originate the poste- rior vena cava of the systemic system. Pulmonary Veins.— These veins run along the inner walls of each lung, returning the oxygenated blood to the heart. The right and left pulmonary veins unite to form a single vessel which empties into the left auricle on its dorsal side. Other veins which take on oxygen are those coming from the skin and buccopharyngeal cavity. The Heart. — The heart is enclosed in the pericardial cavity, which is lined by a transparent tissue, the pericardium, and is separated from the remainder of the body by the transverse septum. It is the rhythmically contracting organ that circulates the blood. It is coni- cal in shape and in the frog consists of a right and left thin-walled auricle above a single thick-walled ventricle. On the ventral side is a muscular tube, the C07ms arteriosus, described with the arteries. It conducts blood away from the heart. On the dorsal side of the heart is a thin-walled sac, triangular in shape, the sinus venosus, which receives venous blood from the systemic veins. The sinus venosus empties into the right auricle through the sinu- auricular aperture. This aperture has liplike valves on each side to prevent the blood from flowing back into the sinus when the auricle contracts. The smaller left auricle receives oxygenated blood from the pulmonary vein. Valves are not necessary at this opening, for pressure on the auricular w^alls tends to close the small oblique aper- ture when the auricle contracts. Both auricles pass blood into the ventricle through a common open- ing, the auriculoventricular aperture, which is divided by the inter- auricular septum separating the two auricles. This aperture has two large valves on each side and two small valves at each end which regulate the discharge of blood into the ventricle and prevent its backflow. Blood leaves the ventricle and enters the arterial system through the conus arteriosus. The opening into the conus is protected by three pocketlike semilunar valves which open inwardly into the conus when blood is passing out but are tightly closed at other times. The CLASS AMPHIBIA 511 proximal portion of the couus is known as the pylangium, and the distal portion as the synangkim. Running through the length of the pylangium is a longitudinal spiral valve, one edge attached to the dorsal wall of the pylangium and the other edge lying free in the vessel. Upon contraction of the conus this structure is brought into contact with the ventral wall and helps direct the flow of blood into the arches. Near the anterior free end of the spiral valve where it is the widest, there is a pair of small synangial valves which, together Carotid A System.A Pulmocuta- neous f\. R auricle - Spiral valve Conus orte-_ nosus Semilunar vaUz Truncus arteriosus '^-Pulmonary aperburz -Sinu-auricular aper- ture —Loft auricle — Intzrauricular sep- tum --Dmtle inpulmo- cutaneous A. - -/luricufo- ventricular valve - -">:7 Ventricle Fig. 277.- -Heart of frog with the ventral wall removed and bristles shown through the arteries of the truncus arteriosus. with the end of the spiral valve, separate the pylangium from the synangium. Just below these valves is an aperture which leads into the trunk formed by the union of the two pulmocutaneous arteries. The synangial chamber is very short and gives off almost imme- diately two large branches, one to the right and the other to the left. In each of these branches originate the three main trunks or arches of the arterial system. They are formed by two longitudinal septa dividing the vessel into three compartments. All three trunks 512 TEXTBOOK OF ZOOLOGY are therefore enclosed in one large vessel for a short distance before breaking up into three separate vessels. The carotid arch originates from the anterior compartment, the systemic arch from the middle compartment, and the pulmocutaneous arch from the posterior com- partment. Blood enters the anterior and middle compartments from the synangium, but enters the posterior compartment, or pulmocuta- neous arch, from the pylangium. The heart beats in a wavelike peristaltic manner. The sinus venosus contracts first, then the auricles (the right auricle preceding the left by a moment), then the ventricle, and finally the conus. Venous blood from the right auricle enters the right side of the ventricle, and oxygenated blood from the left auricle enters the left side. Muscular ridges of the ventricular wall tend to hold the blood and reduce mixing. Since the heart's contractions are wavelike, the ventricle immediately forces the blood into the conus through the semilunar valve. Venous blood from the right auricle is closest to the conus, and it passes out first, flowing into the closest open- ing offering the least resistance. This is the opening in the pylangium to the pulmonary arch, leading to the lungs. As the contraction of the ventricle comes to an end, forcing out the re- maining oxygenated blood, the pylangial part of the conus contracts, bringing the spiral valve against its ventral wall. This action, together with that of the synangial valves which are anterior to the common opening of the pulmonary arches, completely shuts off the flow of blood into these arches. The blood therefore passes into the synangium and enters the chambers leading to the systemic arteries or the carotid arteries. Since the carotid arteries offer some resistance to blood flow, the blood tends to enter the larger systemic arteries first. As the systemic arteries fill, they offer more resistance to the blood, while resistance in the carotid arteries de- creases due to their emptying into capillaries ; so the last oxygenated blood from the ventricle passes into the carotids and is conveyed to the head region. The heart must beat sufficiently fast and pump a sufficient volume of blood at each stroke to insure an adequate supply of oxygen and food to the body tissues, as well as to remove waste products as they form. The rate of pulsation is influenced greatly by tempera- ture up to a certain maximum rate, for the activity and metabolism of the bullfrog are considerably affected by temperature. Blood CLASS AMPHIBIA 513 pressure is increased by a constriction of the smaller arteries or arterioles. Their muscular walls may contract from stimuli received from the nervous system or from hormones. Blood corpuscles, which are of three kinds, float in the plasma. The erythrocytes are flattened and elliptical, with an oval nucleus in the center. They contain a pigment, hemoglobin, which has the property of absorbing oxygen. The colorless thrombocytes or spindle cells are not as large as the erythrocytes but resemble them except for their tapering ends. When these cells contact certain foreign bodies, they break up, releasing a substance that causes, upon contact with air, the coagulation of certain proteins in the blood plasma in which blood corpuscles become entangled, forming a clot. The insoluble protein strands thus formed are called fibrin (see chapter on The Vertebrate Animal). After the frog has been injured, the formation of a clot prevents indefinite bleeding and makes it possible for the tissues to begin repair. The white blood corpuscles or leucocytes are of three kinds: lym- pJiocytes, monocytes, and granulocytes. Their outline is irregular, due to their amoeboid movement, and the shape of their nuclei varies greatly. They are much less numerous in the blood stream than are the red blood corpuscles and spindle cells. Leucocytes may escape from blood capillaries and engulf bacteria and other harmful substances in the tissues. They are finally returned to the venous system by lymphatic vessels. Worn out corpuscles are re- moved from the blood stream by the liver and spleen. The spleen seems to be the primary organ concerned in supplying new blood corpuscles except for a period in the spring when the bone marrow may produce some. Leucocytes may also increase by fission. Ljnnphatic System. — The lymphatic system of the bullfrog is an open system comprised of a series of large irregular sinuses in vari- ous parts of the body. It collects lymph from the tissues and eventually returns it to the veins. The lymph is a colorless fiuid containing leucocytes but no erythrocytes. It is derived from seep- age of plasma from the capillaries. It bathes all of the cells, col- lects wastes, and distributes food products. In the region of the intestinal tract, lymphatics absorb a considerable amount of fat and are called lacteals. Lymph removes cellular debris and trans- ports leucocytes which engulf harmful material and cleanse the tis- sues of the body. 514 TEXTBOOK OF ZOOLOGY Between the skin and muscle are a series of subcutaneous lymph sacs; other sinuses are in the mesenteries, around the vertebral column, and elsewhere. The peritoneal and pericardial cavities are connected with the lymphatic system. Nephrostomes on the ventral surface of the kidney convey lymph from the peritoneal cavity into the renal veins. Respiratory Organs and Respiration Air enters through the nostrils, passes into a small olfactory chamber and then into the mouth cavity through the internal nares, which open in the roof of the mouth. The mouth is kept tightly closed in breathing. Air is sucked in by lowering the floor of the mouth and is then forced into the lungs by raising the floor, the external nares being closed by valves. This pushes the air through the slitlike glottis immediately behind the tongue in the floor of the mouth, thence into a short larynx which connects with the lungs. The walls of the larynx are reinforced by a framework of cartilage, and the laryngeal chamber supports two horizontal fleshy folds, the vocal cords, which extend across the passageway. When a frog croaks, its mouth and nostrils are kept tightly closed, and the air is forced back and forth between lungs and mouth cavity, causing the vocal cords to vibrate. The sound is amplified in male frogs by the vocal sacs which act as resonating chambers. In the bullfrog the two internal vocal sacs have openings into the floor of the mouth at each corner, and, when inflated, they swell out under the throat and sides of the body in the region of the lungs. Bullfrogs frequently call under water. The two lungs lie dorsal to the heart on each side and dorsal to the liver. They are very elastic sacs with their inner walls raised into a number of ridges, forming chambers which are called alveoli. These chambers are richly supplied with a network of blood vessels for facilitating the oxygenation of the blood. In the bullfrog the lungs are also important as a hydrostatic organ, "While the lungs play the major role in respiration, other factors are of considerable importance. The lining of the mouth of the bull- frog contains a large number of blood vessels and serves for a type of respiration known as huccopharyngeal respiration. With the glottis closed, air is drawn into the mouth cavity and forced out by rhythmi- CLASS AMPHIBIA 515 cal movements of the throat. Oxygen is taken np by blood vessels in the lining of the month by diffusion. The skin of the bullfrog plays a large part in its respiration, and frogs that are not protected from drying out soon die. Gaseous ex- change of carbon dioxide and oxygen can take place through the moist vascular skin, and, since its area is large, it serves effectively as a respiratory organ. This type of respiration is known as cutane- ous respiration. During hibernation, practically all respiration of the bullfrog is of this nature. Even at other times, the skin releases more carbon dioxide than do the lungs. The functions of respira- tion are discussed in the chapter on The Vertebrate Animal. Excretory System and Excretion The two kidneys lie between the parietal peritoneum and dorsal body wall in the posterior region of the body cavity. They are dark red in color, flattened and elongated. They are made up of a very great number of uriniferous tuhules. A mesonephric duct runs from the posterior lateral border of each kidney and empties into the dorsal side of the cloaca. The urinary bladder also opens into the cloaca but does so on its ventral surface, and the ducts do not join the bladder. The bladder is a two-lobed sac with very thin walls which stores the urine collected from the cloaca. When filled, the bladder contracts and forces the urine back through the cloaca and outside through the anus. Embedded in the ventral surface of each kidney is a yellowish red patch, the adrenal gland, which will be dis- cussed in the chapter on Internal Regulators. The waste products resulting from the vital processes of destruc- tion, repair, and growth in the body must be removed if the organism lives. These are taken from the tissues by the blood and more espe- cially by the lymph. We have already mentioned the expulsion of carbon dioxide and water through the skin and lungs. Another prod- uct of protein metabolism is urea. This soluble crystalline substance, formed to a large extent in the liver from the nitrogen of protein metabolism, enters the blood stream and is removed by the kidneys. The kidneys also remove foreign substances from the blood and pass these to the outside through their mesonephric ducts and the cloaca. Frogs and toads excrete considerably more urine per day propor- tionally than does man, although this may vary considerably, for in some forms the bladder may act as a filter for water which is used 516 TEXTBOOK OF ZOOLOGY over and over. It has been estimated that, while man excretes about one-fiftieth of his weight per day, the frog excretes about one-third of its weight. During hibernation and aestivation, however, in com- mon with the slowing down of its other body functions, the kidney function of the frog is practically stopped. Ostium -- Postcaval V Ovaty- Oviduct — L.intest'im — Uterui Fig. 278.- Cloaca 1?- Anus -Urogenital system of the frog from ventral view. Male organs shown on one side, female on the other. The kidney is not only concerned with the elimination of waste products but also has other functions. One of these is the reabsorp- tion by its tubules of useful substances, such as some of the salts and glucose which have filtered out, and their reintroduction to the blood stream. In their food frogs obtain less sodium chloride than do mammals, and this is compensated for in part by a retention of salts from the water taken in, while in mammals water is retained and the salts are eliminated. CLASS AMPHIBIA 517 Another function is in maintaining the concentration of body fluids. Frogs absorb water through their skin at a rather constant rate, varying with the temperature. The kidney in turn expels water at the same rate and thus maintains the proper balance. In addition to its usual function the urinary bladder may be used as a storage reservoir for water during temporary drought. The water may be absorbed from it by other tissues until the proper osmotic equilibrium of the tissues with the blood is produced. The excretory function is further developed in the chapter on The Vertebrate An- imal. Skeletal System The bullfrog has no exoskeleton, its body being covered by smooth skin. The endoskeleton may be considered in two main divi- sions, the axial and appendicular portions. The axial part includes the skull and vertebral column; the appendicular portion consists of the bones of the limbs and their supports, the pectoral and pelvic girdles. Bones are joined to one another by structures made up of connec- tive tissue which allow varying degrees of movement between them. These structures are called joints or articulations. In some cases, as in the skull, the joints are immovable and the bones are separated only by a thin sutural ligament of connective tissue. In other cases, the joints are slightly movable, as in the vertebral column where a plate of dense tissue and cartilage connect the vertebrae. In still other cases the bones are freely movable, as in the limbs, and here the bones are entirely separated, but are held in place by ligaments. The Axial Skeleton. — The skull, which is composed of cartilage, cartilage bones, and membrane bones, forms a case for the brain and capsules for the sense organs. The frog's cranium has considerably more cartilage than do the skulls of higher vertebrates and less than those of lower vertebrates. The cartilage bones are so called because of their origin in cartilage which has subsequently been partly re- placed by ossified tissue, forming bones separated by sutures. These cartilage bones are found at various points on the cartilage box that composes the foundation of the cranium. Cartilage bones are the sphenethmoids, pro-otics, exoccipitals, pterygoids, palatines, and car- tilaginous quadrates. The membrane bones develop from ossifications of membranes which cover the cartilage and cartilage bones. They are thin and may be separated from the others. The membrane bones 518 TEXTBOOK OF ZOOLOGY are the premaxillaries, maxillaries, nasals, frontoparietals, quadrato- jugals, squamosals, parasplienoids, and vomers. The bones enclosing the brain constitute the cranium. On the dorsal surface of the cranium, the two frontoparietals form most of the roof, the pro-otics form the roof of the auditory — PrzmaxlUary Moxi//ary -— -v-%— 5phenethmo(d ^— Frorko-par\eba\ ■J- Pterygoid - - Squamosal -\\— Pro otic I- Squamosal —Quadratojuqal Columella Fig. 279. — Dorsal view of the skull and upper jaw of the bullfrog. - Premaxillary — Palatine fronto-parietal y^- Parasphenoid -Squamosal '^ -Quadrabjugal -Quadrate car- ' $. tilaqe Fig. 280. — Ventral view of the skull and upper jaw of the bullfrog. capsule (inner ear capsule), the sphenethmoids form the posterior wall of the olfactory capsule (nasal chamber), and the two tri- angular nasal bones lie above. On the ventral surface of the era- CLASS AMPHIBIA 519 nium are the slender palatines extending laterally on each side from the anterior end of the sphenethmoid to the upper jaw. The vomers form the floor of the olfactory capsules, and their ventral surfaces bear the vomerine teeth. The parasphenoid forms the floor of the brain case. At the posterior end of the cranium is a large opening, the foramen magnum, through which the spinal cord passes. On each side of this opening are the exoccipital bones. Each bone has a rounded projec- tion at its base, an occipital condyle, which articulates with the ver- tebral column. Dorsal fissure ^ f\ Neural spine OraymatteK. \ | I , Neuralarch Zygapophysis ^^^^^_ ?^\^<^ W^C^ ^/^ y^ Tram'^erx process VJhibzmatten -^^^^"..^^^'^^^^^^^^^^'ssss^livW IXiramaker Dorsal root _ '^:i^£^!!!!!^^^^^::::::i^^ — Pla mater Ventral fissum. ^/^^^^^^^ i- \^ Ventral root I Centrum Fig-. 281. — structure of a single vertebra and cross section of the spinal cord. (Redrawn and modified from Holmes, Biology of the Frog, by permission of The Macmillan Company, after Howe, Atlas of Zootomy.) Visceral Skeleton. — The visceral skeleton is that part of the axial skeleton which consists of the jaws and hyoid apparatus in the adult. The gill arches of the tadpoles are included in this portion. These parts originate in cartilage which is later partially replaced and reinforced by ossifications. The hyoid apparatus is primarily cartilaginous and serves as a support for the base of the tongue and the larynx. According to some authors, the jaws and the hyoid were originally the branched arches supporting the gills, and evi- dence of this is seen when the frog tadpole breathing with gills transforms to the frog breathing without gills. The upper jaw consists of a pair of short premaxillary bones in front, a pair of long maxillae fonning the sides, and a pair of short quadratojugals as the posterior portions. The premaxillae and maxillae each bear a row of small conical teeth. 520 TEXTBOOK OF ZOOLOGY The lower jaw is formed primarily of a cartilaginous rod known as Meckel's cartilage. At the extreme anterior tip of the jaw the rod is ossified to form two small bones, the mentomeckelian bones. It is sub- sequently covered anteriorly by a dentary bone and posteriorly by an angulosplenial bone. The jaws are attached to the cranium by a com- bination of three bones on each side, the squamosal, pterygoid, and palatine, to form a suspensory mechanism. The vertebral column is made up of a series of nine typical verte- brae and a long bone, the urostyle, which includes a fusion of the vertebrae of the tadpole tail. In the neck region, there is one cervical vertebra, the atlas, which articulates with the skull. This is followed by seven trunk vertebrae, then one sacral vertebra whose processes support the pelvic girdle, and finally the urostyle, which contains all of the caudal vertebrae fused into one piece. The basal portion of the typical vertebra is known as the centrum. The centrum is concave in front and convex posteriorly, and there- fore is procoelous except one vertebra which is ampliicoelous in Rana. Attached to the centrum is a bony arch, the neural arch, which ex- tends dorsally from the centrum around the spinal cord. The neural arch has extending from its sides, at the point of union with the centrum, a pair of riblike transverse processes to which muscles are attached. A dorsal projection of the neural arch is the neural spine. In addition, the neural arch has at each its anterior border and pos- terior border a pair of processes known as zygapophyses by which the vertebrae are coupled together, the posterior zygapophyses of one vertebra overlapping the anterior zygapophyses of the succeed- ing one (Fig. 281). This arrangement furnishes a protected canal for the spinal cord and a firm axial support which also allows bending of the body. The spinal nerves emerge between vertebrae through intervertebral foramina protected by the cartilaginous pads between the vertebrae. Appendicular Skeleton. — The anterior portion of the appendicu- lar skeleton is composed of the pectoral girdle, sternum, and bones of the forelimbs. The posterior portion has the pelvic girdle and bones of the hindlimbs. The pectoral girdle and sternum furnish a support and place of at- tachment for the forelimbs and their muscles. They also provide a case to protect the heart, lungs, and other organs in the anterior part of the body. This girdle is not connected to the vertebral column. CLASS AMPHIBIA 521 Each side of dorsal part of the girdle is composed of a large flat bone, the suprascapula, which curves ventrally and joins the scapula, narrowing as it does so. From the ventro-anterior end of the scapula two bones extend to the midventral line of the body and would meet their fellows from the opposite side except that the narrow epicora- coid cartilage intervenes. The anterior of these two bars is the clavicle and the posterior one the coracoid. At the junction of cora- coid and scapula a depression is formed, known as the glenoid fossa, into which the forelimb articulates. The ventral sternum is separated ^j-^y -^ Lpisternum s- — Omosbernum Fig. 282. — Diagram of the ventral view of the pectoral girdle of Rana catesbelana, natural position. into two portions by the pectoral girdle. The anterior portion is com- posed of a bone, the omosternum, to which is attached anteriorly a rounded plate of cartilage, the episternum. The posterior portion is composed of a bone, the sternum proper (mesosternum), and a round- ed cartilage, the xiphisternum, which has a notch at its posterior mar- gin through which the abdominal vein runs as it leaves the body wall. The pelvic girdle furnishes a place of attachment and support for the hindlimbs. Each half of the pelvic girdle is composed of three bones, the ilium, ischium, and piibis. The more slender ilium is at- 522 TEXTBOOK OF ZOOLOGY tached anteriorly to the transverse process of the ninth vertebra, and posteriorly it fuses with the pubis and ischium, forming a dishlike concavity, the acetahulum, which receives the hindlimb. The pubis forms the ventral part of the acetabulum, the ischium the posterior, and the ilium the anterodorsal. The forelimbs join the body by a ball and socket joint at the glenoid cavity in the pectoral girdle. The large bone which makes this articulation is the humerus. The succeeding bone of the forearm is the radio-ulna, a fusion of two originally distinct bones. The wrist, which follows, contains six carpal bones arranged in two rows. Each hand, or manus, contains four metacarpals following the carpals, and distal to these are four complete digits and an exceedingly small rudimentary fifth near the thumb, the prepollex, consisting of only 8^? vertebra Sacral diapophyses cylindrical Uium Urostyle Acetabubm - -Ischium Fig. 283. — Pelvic girdle of Uie bullfrog, dorsal view. a single bone. Each of the four digits, or fingers, extends from a metacarpal bone. This is followed in digits II and III by two phalanges and in digits IV and V by three phalanges. The hindlimhs have essentially the same structure as the fore- limbs. The large bone which joins the girdle at the socketlike acetabulum is known as the femur. This bone articulates with the tihiofibula, which, like the bone of the forearm, is a fusion of two bones. The tarsus or ankle differs from the wrist, being composed of two long bones, the tihiale and fibidare, and two small tarsals. There are also two extremely small bones forming the prehallux, or rudimentary sixth toe. Distal to the tarsals are five long meta- tarsals. Each foot contains five complete digits, each following a metatarsal bone. In digits I and II are two phalanges, in digits III and V three phalanges, and in digit IV four phalanges. CLASS AMPHIBIA 523 Muscular System Muscular tissue controls the movements and positions of various parts of the body of the bullfrog. This it does by contracting, that is, by shortening and thickening its elements. •0 5 3 t) u -a S O O ^ iO -5 c a 3 C <0 to u o ^ « 3 c 'S^ t. ^ e "3 c; t. to o <0 ■+-» 3 <0 3 ts -ts iS < to y) 3 .3 o -+^ -C <0 M- ><-, Q 1 o ■w -Ci Ci 1^ fcfl o 1-c 4> c tB 524 TEXTBOOK OF ZOOLOGY Movements may be under voluntary control, as the skeletal muscles, involved in moving the limbs, in which case the muscle fibers are striated and are known as voluntary muscle. Other movements, such as the heartbeat and the peristaltic movements in the intestines, are not under control of the will. Muscles concerned in these actions are known as involuntary and are usually made up of smooth muscle fibers except in the heart, which contains striated cardiac muscle. Most voluntary muscles are attached to bones at one end or at both by specialized connective tissue bands known as tendons. The end of the muscle which is attached to a relatively fixed and immovable part is called the origin; the end which is attached to the part which moves when the muscle contracts is known as the insertio7i. A typical voluntary muscle is made up of three parts : the tendons attached at its ends; the membrane surrounding the muscle, known as the fascia; and the belly, or fleshy part, of the muscle. The different actions performed by the various skeletal muscles give rise to descriptive names applied to them. Some of these are as follows : Extensor — one that straightens a part, such as extending the foot. Flexor — one that bends a part, such as a joint. Adductor — one that draws the limb toward the median ventral line. Abductor — one that draws the limb away from the median ven- tral line. Levator — one that raises a part, such as the lower jaw. Depressor — one that lowers a part, such as the lower jaw. Rotator — one that rotates one part on another. The 'pectoral muscles cover the chest and ventral portion of the upper body region ; the rectus al)dominis extends along the median ventral region ; the paired ohliquus externus and internus cover most of the sides of the trunk. The muscles of the limbs are numerous. There are some eighteen separate muscles which control various move- ments of the legs. A detailed description of these and other muscles of the frog would be confusing to the elementary student and there- fore is not included. The major muscles of the hind leg are illus- trated in Fig. 284 and can be clearly understood after a careful dis- section in the laboratory. CLASS AMPHIBIA 525 Nervous System The three divisions in which the nervous system of the bullfrog may be considered are: (1) central nervous system, (2) peripheral nervous system, and (3) sympathetic nervous system. o/facb3ry tracb .oifflct»rylobe' Cerebrum Optic nerve Pineal body J)/Gnccphalon i^tic lobe Cerebellum Nedulla oblongata 4Lh ventricle Jjlossophatynqeal .Vacjof nerve J^ spinal nerve 22^ jpinal nerve Trigeminus Facia/, Auditory Thoracic enlargement } Brachial plexus 5piT)at cord Lambar. enlargement ^ 5'^ jpinal mrve .+^ .spinal nerve _ JZalcareoui body 5S! jpinal nerve ,.6t2) ipinal nerve ^ spinal nerve -_a*-bjpinal nerve 5 — spinal nerve jotb jpinal nerve Sciatic plexus Jdatic nerve Fig. 285. — Dorsal view of the nervous system of a frogr. The central nervous system, so called because it comprises the larger number of nerve centers, consists of the brain and spinal cord. The peripheral nervous system consists of (1) the paired cranial and spinal nerves which connect the brain and spinal cord with other FiJumterminale. 526 TEXTBOOK OF ZOOLOGY organs of the body and (2) a large number of small nerve centers, ganglia, distributed throughout the body. The sympathetic nervous system is a part of the peripheral nervous system. It is made up of a large number of small ganglia, two rows of which form the sym- pathetic trunks on each side of the vertebral column and connect with the spinal nerves. The branches of these sympathetic trunks connect with numerous small ganglia throughout the tissues of the body. This system controls and regulates primarily the involuntary move- ments of such organs as the heart, digestive tract, glands, organs of respiration, and walls of blood vessels. Central Nervous System.— The brain is covered with a pigmented membrane known as the pia mater. The brain has three main divi- sions, the forehrain, midbrain, and hindbrain. The forebrain consists of a pair of elongated cerebral hemispheres, separated from each other by a fissure, and two enlargements at the anterior end of the hemi- spheres known as the olfactory lohes. These lobes are fused on the dorsal side but separated by a groove on the ventral side. Immedi- ately behind the forebrain is the diencephalon. On its dorsal sur- face is a vestige of the pineal organ which was more developed in the tadpole. On its ventral surface is the optic chiasma, a crossing of the optic nerves formed by fibers from the right and left sides, each crossing to supply the eye of the opposite side. Just behind the optic chiasma is the inf undibulum, and somewhat behind this is the pituitary body, or hypophysis. The pituitary is of dual origin, developing in part from the diencephalon and in part from the roof of the mouth cavity. The midbrain contains two large rounded optic lobes. The ventral part of the brain below these lobes is the crura cerebri. The hindbrain consists of the cerebellum and the medulla oblon- gata. The cerebellum in the frog is almost rudimentary and consists of a transverse fold of tissue immediately posterior to the optic lobes. The cerebellum is in close connection with the large triang-ular medulla oblongata which constitutes the most posterior part of the brain and is continuous with the spinal cord. Internal Organization. — The central nervous system is hollow. In embryological development the central cavity is large ; but, as maturity is approached, the walls thicken, and the cavity, particu- larly in the spinal cord, is much reduced. In the brain these cavi- ties, known as ventricles, form a continuous channel for the flow of cerebrospinal fluid. The ventricles are connected one with another CLASS AMPHIBIA 527 by openings known as foramina. The cavities are large in four regions: (1) the paired lateral ventricles in the cerebral hemispheres, (2) the single third ventricle in the diencephalon, (3) the paired optic ventricles in the optic lobes, (4) the single large triangular fourth ventricle in the medulla oblongata. Vascular nets of blood vessels in the much-folded pia mater constitute clioroid plexuses that form the roofs of the third and fourth ventricles and extend into the other ventricles somewhat. Most of the cerebrospinal fluid is de- rived from the blood vessels of these plexuses. The spinal cord is continuous with the medulla oblongata ante- riorly, runs posteriorly through the canal formed by the vertebrae, and finally tapers to a narrow filament which ends in the urostyle. It is covered by two membranes, an outer dura mater and an inner pia mater. It is somewhat flattened, and a median fissure occurs on both its dorsal and ventral sides. The central part of the cord comprising its bulk is made up of gray matter consisting primarily of nerve cells. In the center of this gray matter is a small hollow canal, the neurocoele, which communicates with the ventricles of the brain. Surrounding the gray matter is white matter consisting chiefly of nerve fibers. Peripheral Nervous System. — The peripheral nervous system is composed of the cranial, spinal, and sympathetic nerves, the last of which will be considered separately. The cranial nerves arise from the brain, and there are ten pairs of them in the bullfrog. Counting from the olfactory lobes backward, they are as follows: olfactory, optic, ocidomotor, trochlearis, trigem- inus, ahducens, facial, auditory, glossopharyngeal, and the vagus. All of these, with the exception of the tenth or vagus nerve, run to parts of the head. The vagus nerves branch to the heart, lungs, and digestive system. The bullfrog has ten pairs of spinal nerves. Each spinal nerve originates from the gray matter in the spinal cord by a dorsal and a ventral root. These roots pass out of the vertebral column between vertebrae through an opening or intervertebral foramen and unite into a nerve trunk, branches of which extend to the muscles and skin of the body and limbs. The dorsal root is known as the sensory or afferent root and has a ganglion; the ventral root is known as the efferent or motor root and has no ganglion. "Where these roots meet 528 TEXTBOOK OF ZOOLOGY after leaving the spinal cord, they are covered on the ventral side by a large calcareous body, the periganglionic gland, or "gland of Swammerdam. " The first spinal nerve arises between the first and second vertebrae, the second between the second and third vertebrae, and so on until the tenth, which is small and emerges from the urostyle near its an- terior end. These nerves frequently send branches to preceding or succeeding nerves to form plexuses. Two large plexuses in particular are present. Branches from the first and third nerves join with the large second nerve to form the trachial plexus, which supplies nerves to the muscles of the forelimbs and shoulder. Nerves number seven, eight, and nine fuse to form the large sciatic plexus which supplies the sciatic nerve to the hind leg. Sympathetic Nervous System. — From the first sympathetic gan- glion, nerves are given off which form a cardiac plexus on the heart. Another plexus, formed primarily from nerves of the third, fourth, and fifth sympathetic ganglia, is the solar plexus on the dorsal surface of the stomach. In addition, numerous ganglia are scattered through- out the tissues of the body, all being connected by sympathetic nerve fibers and finally communicating with the sympathetic trunks. The cooperation of certain cranial and spinal nerves with the sympa- thetic in relation to the involuntary actions of a number of the vital internal organs is referred to as the autonomic function. The Sense Organs The olfactory sacs, or nasal chambers, are located internal to the external nares. The median portion of the nasal chamber is lined with olfactory epithelium which contains sense cells possessing proto- plasmic processes known as olfactory hairs on their free ends. These olfactory hairs are stimulated by chemical substances present in the air and pass the stimuli received through the olfactory cells to the olfactory nerves. The degree to which the sense of smell is used by amphibians is not known. It is likely, however, that it may cause the frog at times to approach objects and may serve to test the food substances it takes into its mouth. The eyes lie in cavities, or orbits, on the dorsolateral sides of the head. The exposed portion of the eyeball is covered by a transparent membrane, the cornea, which is continuous with the opaque connec- CLASS AMPHIBTA 529 tive tissue sheath covering the remainder of the eyeball and known as the sclera. Attached to the sclera are several muscles which move the eye in various directions. The ms of the bullfrog is colorful, being either golden or reddish bronze, and is clearly visible through the transparent cornea. In its center is an oval opening, the pupil, which can be contracted or expanded by the action of muscle fibers in the iris and, like the shutter of a camera, regulates the amount of light which enters the inner chambers of the eye. The lens lies behind the iris and is flattened on its outer surface. It is enclosed in a membrane and held in place by delicate fibers to the ciliary body. The space between the cornea and lens is filled with a watery trans- parent substance, the aqueous humor. (^^terlorverticafj^ Ampulla -"-■-^ — Sacculus Fig. 286. — The right internal ear of the frog, lateral view. The main cavity of the eye back of the lens is filled with a gelati- nous tissue, the vitreous humor. The walls of this cavity are made up of three layers, the outer sclerotic coat, previously mentioned, then a vascular pigmented chorioid and the innermost layer, the retina. The anterior portion of the chorioid forms the ciliary body, which subsequently is continuous with the iris. The retina contains the photosensitive ceUs of the eye which pass the stimuli received on to the optic ner\-e. These sensitive cells, known as the rods and cones, lie embedded in the tissue so that light has to pass through several layers of nerve fibers, as well as much supporting tissue, before reaching them. The rods and cones com- municate with fine branches of the optic nerve, which enters the eye posteriorly. 530 TEXTBOOK OF ZOOLOGY Sharpness of vision is dependent on both the proper focusing of the lens and the proper amount of light reaching the retina. When the light is too strong, the pupil of the iris contracts and cuts down the volume. The eye of the frog has little if any accommodation or focusing of the lens. It therefore has very imperfect vision. The ear of the bullfrog is covered externally by a membrane, the tympanum. A Eustachian tube runs between the middle ear and mouth cavity. The tympanum has attached to it a bony rod, the columella, the other end of which is joined to a portion of the inner ear. This rod transmits sound vibrations from the tympanum to the inner ear. The inner ear lies in a cavity of the skull known as the auditory capsule. The structures of the inner ear compose a membranous labyrinth which is surrounded by a lymphlike fluid, the perilymph. The labyrinth is formed of a dorsal utriculus concerned with equilib- rium and a ventral sacculus functioning as an auditory organ. The utriculus is connected with three semicircular canals which are placed in planes almost at right angles to one another. Two are vertical canals, and the third, on the outer side of the utriculus, is hori- zontal. The sacculus is irregular, pouchlike, and filled with a fluid, the endolymph. It also contains the nerve endings which receive the stimuli and convey them to the auditory nerve. Sound progresses in the following fashion. The tympanic mem- brane vibrates to sound waves, and these are transported by the columella to the inner ear. These vibrations are taken up by the endolymph of the sacculus and are received by the nerve endings which lead to the auditory nerves. These nerves convey the im- pulse to the brain, subsequently giving rise to auditory sensations. In a similar manner, movements of the endolymph in the utricu- lus affect sensory cells and cause a reaction associated with a sense of position or equilibration. Sound and hearing play an important role in the life of frogs, the calls of the males serving to attract the females and others to the ponds during the breeding season. They are of prime importance in the daily life of the terrestrial toad, who is on the alert when an insect has announced its location by a sound. CLASS AMPHIBIA 531 Reproductive Organs The ovoid testes (Fig. 278) of the male bullfrog are attached to each kidney by a fold of peritoneum. In this fold of peritoneum, running between the testes and kidneys, are several small ducts, the vasa efferentia. These ducts connect with the mesonephric duct through the collecting tubules of the kidney. Spermatic fluid containing the spermatozoa passes from the testes through the vasa efferentia into the kidney, then into the mesonephric duct, which opens into the cloaca, and thence to the outside through the anus. In some species, this duct is slightly expanded prior to its opening into the cloaca to form the seminal vesicle, a reservoir for spermatozoa. This is poorly developed in the bullfrog. The two ovaries of the female bullfrog, when filled with eggs, occupy a large part of the body cavity and consist of folded sacs covered with peritoneum. They originate in about the same posi- tion as do the testes and lie in a fold of the peritoneum ventral to the kidneys. The eggs lie in the outer surface of the ovary and during their growth are surrounded by a network of blood vessels and follicle cells. The two oviducts are greatly convoluted white tubes, one on each side of the body cavity, running from near the base of the lungs to the dorsal wall of the cloaca. Their anterior ends are funnel-shaped and open into the body cavity. Their posterior ends are dilated to form thin-walled ovisacs or uteri which open into the cloaca near the entrance of the mesonephric duct. They are not connected at any point with the ovaries. When the eggs are mature at the breeding season, they break through the walls of the ovary and its peritoneal covering and are free in the body cavity. They make their way to the funnel-shaped opening, or ostium, of the oviduct and, probably by ciliary action or movements of the female, are squeezed into it. The oviducts contain a large number of glands which secrete a clear, jellylike material. As the eggs are forced down the oviduct by ciliary action, they become coated with the gelatinous material, which swells enor- mously when it contacts water. Fertilization in the bullfrog is external, and the spermatozoa of Die male enter the eggs after they have been laid in the water. 532 TEXTBOOK OF ZOOLOGY Attached to the anterior end of the testes of the male frog and to the ovaries of the female are fingerlike projections known as fat bodies. These serve to store a reserve fat supply which the bull- frog may draw on during hibernation or at other times. They are largest before hibernation and smallest after egg lajdng. Recent experiments have also shown that these fat bodies are essential for allowing the normal development of the sex organs and for maintain- ing their health. When they are removed, there is a deterioration of eggs and sperm. Embryology The bullfrog lays its eggs in a large floating mass, forming a sur- face film on the water, usually among brush or plants near the pool's edge. This mass may be from 1 to 2^/2 feet in diameter and may contain ten to twenty thousand eggs. In Texas, bullfrogs may lay their eggs as early as February, though it is more common for them to be laid later in the season. The eggs of the bullfrog are smaller than those of the leopard frog. They hatch in about four or five days, depending on the tem- perature. After hatching, the tadpole normally spends about two years in the water before transforming as a young bullfrog. The tadpole may grow to be four to six inches long, but the average body length of the young bullfrog as it metamorphoses is about 1% to 2 inches. It usually takes about three to four years for this young frog to attain maturity and begin egg laying. The embryology of the bullfrog does not differ materially from that of the leopard frog, and the following account is based, except where otherwise noted, upon the development of the latter. The egg when laid is a single cell. The upper portion of the egg has considerable pigmentation, making it black. This part of the egg is known as the animal hemisphere, and it is thought that the pigmentation serves to absorb and retain heat necessary for development. The lower portion is white and is known as the vegetal hemisphere. The bullfrog egg is surrounded by a layer of transparent jelly, but does not have an inner envelope of jelly, as does the leopard frog egg. This jelly protects the egg and helps it to retain heat. The nucleus of the egg, or germinal vesicle, lies near the animal pole. The boundary of the egg is known as the vitelline membrane. CLASS AMPHIBIA 533 The eggs are fertilized externally by tlie male, who is clasping the female as the eggs are laid and discharges spermatozoa into the water. The first spermatozoon to swim to the Q^g and enter it by piercing the vitelline membrane initiates fertilization. After the sperm has entered, a fertilization membrane is formed which prevents the entrance of additional spermatozoa. Only the head of the sper- matozoon enters, the remainder being discarded. This head, which is composed primarily of the male spermatozoon nucleus, fuses with the nucleus of the e^g to complete fertilization and start development. B/aifomtrts- 2 Ce// VCe// 6 Cell /e Cell 8laslulrej;ut ^oik ^1 VW^V Preorbital crest cS ' ( V^=^^X- - Postorb'ital crest " ""^^-V-- Tympanum J-- Parotoid Fig. 292. — Dorsal view of head region of a toad to show parotoid glands and cranial crests. It may act as a poison or, at any rate, as a powerful repellent when the toad is seized by other animals. A dog that picks up a toad lets it go at once and does not soon forget the experience. The ridges between the eyes and back of them are known as cranial crests. They are made by the bones of the head and are variously named according to their position. Those between the eyes are known as superciliary or supraorbital; the transverse crests back of the eyes and at right angles to the superciliary crests are the postorbital crests. A crest that is absent in Woodhouse's toad but often occurs in other toads is one running from the postorbital crest to the parotoid gland and known as the preparotoid crest. The paro- toid glands are usually in contact with the postorbital crests in "Woodhouse's toad. The toes are about one-third webbed, the webs being fleshy. There are two tubercles on the sole of the foot known as metatarsal tu- CLASS AMPHIBIA 541 bercles. The inner of these is large and has a homy black edge which is used in digging when the toad burrows into the ground. The other metatarsal tubercle is small. The toad, like the frog, has four fingers on each hand and five toes on each foot. The first finger is slightly longer than the second. Internal Structure The internal organs of the toad are similar to those of the frog, and the previous description of these is referred to. Only striking differences will be pointed out. The word ''frog" used subsequently refers to bullfrogs or leopard frogs. Respiratory and Digestive Organs Since the toad is terrestrial and has a thicker epidermis than the frog, it needs to depend to a larger extent on its lungs; these are large and well vascularized, being more spongy than those of the bullfrog or leopard frog. The motith is large and toothless, lacking the maxillary and vo- merine teeth of the frog. Two openings, one on each side of the tongue, are apertures to the single vocal sac. The tongue of the toad is not notched behind like that of the frog. It is thicker and rounded with more of its posterior end free. The surface of the tongue is sticky and holds the captured insect as it is pulled back into the mouth. The liver is two lobed, in contrast to the three- lobed liver of the frog. Urinogenital Organs The mesonephric ducts from the kidneys, which in the frog opened separately into the cloaca, unite in the toad and open into it in a single duet. The urinary bladder in the toad is large and may function also as a reservoir for water to prevent the animal's dry- ing out. It is held in place by sheets of peritoneum and has a sphincter muscle at its mouth which permits its contents to be emptied rapidly. This discharge may lighten the toad and make it easier to escape from its enemies. The testes of the toad are elongated and extend along a good portion of the length of the kidney. At their anterior ends, between them and the fat bodies, occurs an irregularly shaped granular 542 TEXTBOOK OF ZOOLOGY body, Bidder's organ, Avhich is thought to represent a rudimentary ovary. Some experiments on the toad in which the testes were removed have indicated that this body may develop into a func- tional ovary. In male toads there also may be found alongside the mesonephric duct a coiled tube which is the remains of a rudi- mentary oviduct that is nonfunctional. This rudimentary structure is not found in the male bullfrog, although it is encountered in the male leopard frog. Blood Vascular System For studying the blood vascular system, as well as some other systems, the toad is quite superior to the leopard frog. The vessels are of larger caliber, and the broad interior of the body makes the dissection more easily examined. The bullfrog, of course, is superior to either of these. Scapu la Coracoid Epicoracoid Mesosterrium rnuTTi Fig. 293. — Diagram of ventral view of tlie arciferal, pectoral girdle of toad. (Modified from Kellog, Mex. Tailless Amphibia. U. S. National Museum Bulletin No. 160.) Arterial System. — The subclavian artery is larger in proportion to the size of the animal than in the bullfrog. It sends large branches to muscles of the pectoral girdle, forelimb, and to the upper portion of the bulky side muscles. Two or three conspicuous branches of the vertebral artery run diagonally across the dorsal wall of the body cavity to the lateral muscles of the body which they enter. One large femoral branch, instead of two small ones, as in the bullfrog, is given off from the sciatic artery in the upper leg and runs ventrally, branching into the surrounding muscles. Venous System. — The parietal branches of the ventral abdominal vein are relatively larger in the toad and may extend horizontally across the ventral body wall to the large side muscles, The dorso- CLASS AMPHIBIA 543 lumbar vein is often quite large, with branches running the entire leng-th of the body cavity and others coming from the lateral muscles. Skeleton and Muscles The pectoral girdle of the toad is quite different from that of the bullfrog and leopard frog. In the latter specimens, the two halves of the girdle grow together in the midventral line, separated by a cartilage, and the chest is not expansible. This type of pectoral girdle is known as firmisternal. In the toad, however, the epicoracoid cartilages separating the two halves of the girdle overlap in front, and the chest is expansible. This type of pectoral girdle is known as arciferal (Fig. 293). 2 jy? vertebra Sacral diapopbjses dilated Ilium ' Urostyle Ij- Ischium Fig. 294. — Pelvic girdle and urostyle of the common toad, Bufo americamis. As in the bullfrog, the toad has nine typical vertebrae and a tenth which is included in the urostyle. Like the frog's, the vertebrae are procoelous. Unlike that of the frog, however, the single sacral vertebra which precedes the urostyle has its transverse processes (sacral diapophyses) dilated or expanded. In the bullfrog they were more circular. The muscles of the legs, pectoral girdle, and body wall are large and thick. In preserved specimens their origins are clearly out- lined, and they are more easily separated one from another in dis- section than those of the leopard frog. Nervous System and Sense Organs The large calcareous bodies around the spinal ganglia, seen in the bullfrog, are not evident in the toad. While both bullfrogs and 544 TEXTBOOK OF ZOOLOGY toads have poor vision, due to the lack of accommodation in the eye, the toad appears to see better. Toads also retain sound im- pulses longer than do frogs. Sound is of greater importance to the toad, for in his terrestrial hunting ground he is dependent to a large extent on sound in locating insect food. Embryology The eggs pass continuously through the oviduct with the result that they are laid in long strings or tubes of albuminous jelly. The egg mass is laid underwater, and the toad moves around during the egg laying process with the result that the strings may be con- siderably tangled around submerged vegetation. Shallow creeks provide a favorite breeding ground. The length of time taken for the eggs to develop and hatch de- pends upon the temperature. They usually hatch in from two to four days. The tadpoles may transform into young toads in from 30 to 60 days after hatching. They measure % to % of an inch at this time. Toad tadpoles are black. Usually toad tadpoles can be distin- guished from frog tadpoles by the position of the anus. The anus in toad tadpoles is median, while in frog tadpoles it is somewhat on the right side. The spiracle of toad tadpoles is very small and is on the left side. CHAPTER XXIX REPTILIA (By Leo T. Murray and James E. Blaylock) Class Beptilia includes among living- forms, turtles, snakes, lizards, alligators, and similar animals. These are the only living vertebrates which are cold blooded, breathe by lungs, and have a single median occipital condyle. A more complete list of distinguishing charac- teristics has been given by Gadow as follows: "1. The vertebrae are gastroceutrous. "2. The skull articulates with the atlas by one condyle, which is formed mainly by the basioccipital. "3. The mandible consists of many pieces and articulates with the cranium through the quadrate bones. "4. There is an auditory columellar apparatus fitting into the fenestra ovalis. "5. The limbs are of the tetrapodous, pentadactyl type. "6. There is an intracranial hypoglossal nerve. "7. The ribs form a true sternum. "8. The iliosacral connection is postacetabular. "9. The skin is covered (a) with scales, but (b) neither with feathers nor with hairs ; and there is a great paucity of glands. "10. Reptiles are poikilothermos (cold blooded). "11. The red blood corpuscles are nucleated, biconvex, and oval, ' ' 12. The heart is divided into two atria and an imperfectly divided ventricle. It has no conus, but semilunar valves exist at the base of the tripartite aortic trunk. "13. The right and left aortic arches are complete and remain functional. ' ' 14. Respiration is effected by lungs ; functional gills are entirely absent, even during embryonic life. "15. Lateral sense organs are absent. "16. The metanephric kidneys have no nephrostomes. Each kidney has one separate ureter. "17. There is always a tj^ical cloaca. 545 546 TEXTBOOK OF ZOOLOGY "18. The eggs are meroblastic. ' ' 19. Fertilization is internal, 'and is effected, with the single ex- ception of Sphenodon, by means of male copulatory organs. "20. An amnion and an allantois are formed during development. "Numbers 1, 2, 6, 7, 8, 14, 16, 18, and 20 separate the Reptiles from the Amphibia. Numbers 9 (b), 10, 12, and 13 separate them from the Birds and Mammals. Numbers 3, 8, and 11 separate them from the Mammals." The majority of living forms are covered by scales. The turtles have epidermal scutes as an external covering ; and the crocodiles have a tough skin w^ith osseous plates in it. Many living reptiles are capable of very rapid movement ; and the Pterosauria, an ex- tinct order, had wings. Only one lizard and a relatively small number of snakes are poisonous. It is thought that the poison gland is a recent development among reptiles. At present herpetologists place all known reptiles into nineteen groups or orders. Only four of these orders have living representa- tives. This indicates that Reptilia is an old class of vertebrates which is now decadent. FOSSIL REPTILES A study of the fossils of reptiles reveals that during the Triassic, Jurassic, and Early Cretaceous geological periods reptiles were the dominant animals on earth. They occupied and dominated all types of habitats then much as mammals do now. Many of these ancient reptiles were no larger than the common lizards of today, but others were the largest vertebrates that have ever lived. Brontosaurus, "the thunder lizard," was nearly seventy feet long and weighed approximately forty tons. This enormous reptile lived in the shallow waters of ponds and bays and fed on the plants that grew in the mud along the margins of the water. Tyrannosaurus, "the king lizard," reached a length of forty-seven feet and a weight greater than that of any elephant. Since it was entirely carnivorous in its feeding habits, it is easily the most terrible animal that has ever lived on earth. Many variations in form and size have bridged the gap between the largest and the smallest. The five or six known families of flying reptiles varied in expanse of wings from a few feet to as much as twenty-five feet. As a group, they dominated the air for several million years. A few small wormlike burrowing REPTILIA 547 lizards less than two feet in length are known from the strata deposited in the Oligocene time in North America. Though reptiles had become completely adapted to living on land, many of them returned to the water. Most living turtles spend the greater part of their lives in water but must return to land to deposit their eggs. In the heyday of reptiles there were many other aquatic and marine forms showing numerous and diverse adaptations to living in a liquid medium. The ichthyosaurs (fish lizards) were the most per- fectly adapted to aquatic life, being very fishlike in form. CLASSIFICATION OF LIVING REPTILES Many different plans have been proposed for grouping the reptiles. All of these plans have attempted to express the relationships of the various groups to each other; and the relations of representa- tives of a group to other members of the same group. The following arrangement is adapted from Williston, and includes only those groups having living representatives. Class — Reptilia Subclass — Anapsida. Temporal region of skull not perforated. Order — T estiidvnata. A single coracoid; ten dorsal vertebrae, their ribs expanded to meet on the dorsum or a dermal layer of bony plates. Turtles. Suborder — Fleurodira. Neck retracted laterally; pelvis united with the plastron. No representatives of this suborder are found in North America. Suborder — Cryptodira. Neck retracted vertically; pelvis not united with the plastron; carapace with marginal plates. The majority of the living turtles of North America belong to thi? suborder. Suborder — Trionychoidea. Neck retracted vertically; carapace with no marginal plates. The ''soft-shelled" turtles make up this suborder. Subclass — Parapsida. Temporal region of skull with one opening. Order — Squamata. Quadrate freely articulated proximally (streptostylic) or secondarily fixed. Lizards and Snakes. Suborder — Lacertilia (Sauria). Parietals never united to basisphenoid by descending plates; the brain case more or less membranous an- teriorly. The one hundred and seventy-four kinds of lizards known from the United States and Lower California belong to this suborder. 548 TEXTBOOK OF ZOOLOGY Suborder — Ophidia (Serpentes). The braincase enclosed by descending plates from parietals and f rentals; no external limbs present. This suborder includes all snakes. Subclass — Diapsida. Temporal region of the skull with two openings. Order — Bhynchocephalia. Amphicoelous vertebrae, premaxillae with a de- curved beak; pineal eye present. Suborder — Sphenodontia. Has the characters of the order. Sphenodon, the single living genus of this order is famous as being one of the primitive living reptiles. It is found in New Zealand and on pome smaller islands of that region. Order — Crocodilia. Procoelous vertebrae; premaxillae never decurved; no pineal eye present. Crocodiles and Alligators. Suborder — EusucMa. Has the characteristics of the order. The living crocodiles and alligators belong to this suborder. Order Testudinata (Chelonia) Suborder Pleurodira. — Turtles of this group are found in South America, Africa, and Australia. They are known as "side neck" turtles because they do not retract the head and neck under the carapace but lay it along the periphery of the shell. Some members of the group have a pair of bones, the mesoplastra, in the plastron that is not present in other living turtles, though it was common among forms now known only as fossils. The bones of the pelvic girdle are sutured to the plastron as well as to the sacral vertebrae. This is also a characteristic unique among living forms but more common in extinct species. Hence, it is thought that members of this suborder are more primitive in structure than the members of either of the other two suborders having living representatives. Suborder Cryptodira. — There are sixty-one species of turtles in North America north of Mexico or in the oceans that bound the shores of this region. Fifty-seven of this number belong to this suborder. Six families are represented. Family Kinosternidae. — This family includes the turtles commonly known as the "mud turtles,"' "stinkpots," or "musk turtles." They are all small, brown or black turtles, sometimes with white or yellow lines on the head and neck. Many turtles are fre- quently mistaken for members of this family that belong to some other family. In California, where no kinosternid turtle is found, a member of the family Emydidae is called the "mud turtle." All kinosternids possess musk glands that open through pores on REPTILIA 549 the margins of the carapace just anterior and just posterior to the bridge. If a dry musk turtle be disturbed, a drop of yellow liquid can often be seen to appear at each one of the openings of the ducts from the four musk glands. This liquid gives off a disagree- able odor. If it touches hands or clothing, it is very difficult to remove. Family Chelydridae. — The common snapping turtles and the alli- gator snappers are the living members of this family. Both turtles have rows of raised prominences along each side of the carapace; and a row of large bony ossicles along the middorsal region of the tail. The head is large and formidable. The alligator snapper has a pair of wormlike appendages in the mouth, which it is said are used to entice fish within reach of its xiowerful jaws. Both turtles are a plain brown color dorsally and dirty white to black ven- trally. The alligator snapper may attain a weight of ojie hundred and fifty pounds, while the common snapper will seldom exceed forty pounds. Large specimens of either turtle can easily amputate a finger or possibly a hand. Both kinds of snapping turtles possess scent glands very similar to those found in the kinosternids. This family is an excellent example of discontinuous distribution. It was long thought that it was confined to the Western Hemisphere but a genus is also found in New Guinea. Family Emydidae. — Many turtles, diverse in habits and appear- ance, belong in this large family. All of the "hard-shelled" pond and river turtles commonly called "sliders," the painted turtles, the red-bellied turtles, the box turtles and many others fall into this group. While there is great variation among members of the family, they all exhibit certain tendencies as well as fundamental structural similarities. Many species are brightly colored. The painted turtles have bright red colors patterned with dark green and black. Different species of the genus Pseudemys possess vari- ous types of prett}^ colorations. The wood turtle has a somber carapace, a bright plastron with pleasing black spots, and a rich reddish-orange skin on the legs and neck. The males of many species of this family have long straight claws on the toes of the fore feet. Members of this family are found on all the continents except Australia. Emys hlandingii, found in the Great Lakes region, is very similar to Emys orbicularis of Europe. 550 TEXTBOOK OF ZOOLOGY Family Testudinidae. — Only three species of turtles found in the United States belong to this family; namely, the gopher tortoise, Berlandier's tortoise, and Agassiz's tortoise. Members of this family are numerous in Africa, on the Galapagos Islands, and in other widely separated localities. How these three species of turtles come to be in North America is a most interesting problem in animal distribution. All three species found in this country are dark brown or black on the carapace, often lighter ventrally. Young individuals usually show a light area in the center of each dorsal scute. Since they are all dry-land turtles, they lack the streamlined form of water dwellers. The most distinct characteristic of our species is a narrow extension of the plastron into a gular process. Family Cheloniidae. — This family includes the green turtles, the shell turtles, and the loggerhead turtles. All are marine animals, and show modifications for an aquatic existence in the form of the body and in the modification of feet into flippers. Many members of this family may weigh five hundred pounds, but most specimens seen in the markets weigh much less. Family Dermochelidae. — This family contains but one genus with two species. They are commonly known as the leatherback turtles, the trunk turtles, or the harp turtles. Unlike most turtles, they lack the covering of horny scutes, being covered instead with a leathery integument. These are the largest of living turtles. Large individuals may weigh as much as one thousand pounds. Suborder Trionychoidea. — Members of this suborder are found in North America, Africa, Asia, and New Guinea. They are among the most aquatic of all land and fresh-water forms. The only occa- sion on which they leave the water is to deposit eggs. Family Trionychidae. — This is the only family of the suborder having representatives in North America. It is represented by five species and one subspecies, all in the genus Amy da. All are "soft- shelled" turtles covered with a soft rubberlike skin instead of the horny scutes present on most turtles. The color on the dorsal side is olive brown, while the ventral side is white. Any of these species will bite viciously when angered and can inflict painful wounds. It was probably one of these turtles that gave rise to the belief that a turtle would not loose its hold until it heard thunder. They have a habit of retaining their grip on a victim very tenaciously. reptilia 551 Order Squamata Suborder Lacertilia (Sauria) (The Lizards). — This group contains more different kinds of living animals than any other suborder among the reptiles. There are more than 2,500 living species known on earth. Of this number, about 175 species are found in North America, north of Mexico. Kepresentatives of nine families are found among them. Family GekJconidae. — There are about fifty genera, containing some 300 species, in this family. They are found around the world in the tropical and semitropical regions. Seven species are known from the United States and neighboring regions. All of our species are small, seldom attaining a length of six inches; but some tropical forms may be over a foot in length. The colors vary considerably but are often bright, as is usual in noctur- nal animals. The scales of the skin are very minute. This gives the geckos a soft, smooth, appearance unlike that of any of the other lizards. The eyes usually have vertical pupils and are with- out lids, though they are covered by a transparent, cutaneous mem- brane. As in some other lizards, there is nothing obstructing the auditory passage through the head. It is possible to see through this passage. Members of this family are among the few lizards that can make a sound other than hissing. Their characteristic call sounds like the word "gecko." In a great many species the toes are flattened on the end to form adhesive discs. These enable a gecko to walk across the ceiling of a room with ease. In most species the tail is short and thick. Geckos sleep during the day but come forth at nightfall in search of their insect prey, which they capture by means of their short sticky tongues. Family Iguanidae. — This family of lizards has more representa- tives in the United States than any other single family. Of ap- proximately 175 species of lizards known from this country, 90 species and 19 subspecies belong to this family. Representatives are found in all parts of the United States except the most northern portion, as well as throughout Central and South America, and the West Indies. Two genera are found in Madagascar and one in the Fiji Islands. As might be expected in so large a group, great varia- 552 TEXTBOOK OP ZOOLOGY tion in size, form, and coloration occurs. The chameleons (Anolis) change from various shades of brown to light green in response to changes in the intensity of the light. Members of the Central Ameri- can genus Basiliscus are remarkable for erectile middorsal crests. The horned lizards (Phrynosoma) bear conspicuous osseous spines or "horns" on the posterior and lateral borders of the head. All the members of the family, however, have certain structural character- istics in common ; such as, fleshy tongue, and eyes with round pupils and well-developed lids. Femoral pores are usually present on the males. Most of the species found in the United States lay eggs, though some species are known to be ovoviviparous. Various types of habitats have been adopted by different iguanid lizards. The chameleons and many species of the genus Sceloporus are essentially arboreal, while the homed lizards (Phrynosoma) are strictly ter- restrial. Crotaphytus collaris, the collared lizard, is a rather large lizard with a long tail and a heavy body. It is brightly colored and has a yellow collar bordered with black. Its distribution is in the Southwest, from the Plains westward. Certain tropical species are semiaquatic and one species is semi- marine. The majority of species are insectivorous, though three genera are herbivorous. Family Anguidae (Alligator lizards, "glass snake," joint snake, etc.). — This interesting group of lizards is represented in the United States by 10 species. Approximately 40 other species are found in other regions of the world. Most of these other species are native to the tropical regions of the Western Hemisphere, though some are found in Europe, Asia, and Africa. A reduction in the size and strength of the limbs is common in this family. Many species, such as our "glass snake," are entirely legless. Other common charac- teristics are a fold in the skin where the ventral plates join the body wall ; a long, brittle tail ; eye with a lid ; emarginate, protractile tongue; and solid teeth. Many of our species have large auditory openings connected by an unobstructed passage. Our largest species reaches a maximum length of approximately one foot, while a form found on the Balkan peninsula may be three feet long. Our alliga- tor lizards (Gerrhonotus) are ovoviviparous while the "glass snake" (Ophisaurus) lays eggs. All members of this family feed on animal food, such as insects, snails, and small mammals. REPTILIA 553 Family Anniellidae (blindworm, "worm snake," worm lizard). — This family consists of one genus and two species found in southern California. These small, legless, wormlike lizards are burrowing in habit. The ears are concealed and the eyes are covered by translu- Fig. 295. — Collared lizards, Crotaphytus collaris, female and male. This is a beautiful lizard of the Southwest. (Courtesy of Ottys Sanders, Southwestern Biological Supply Company.) cent skin and poorly functional. The tongue is protractile as in the members of the Anguidae. Family Helodermatidae (Gila monsters, beaded lizards). — This family contains one genus and two species. One species, Heloderma suspectum, is found in Arizona, New Mexico, and northern Mexico. 554 TEXTBOOK OF ZOOLOGY The other species, II. horridum, ranges through central and western Mexico to northern Central America. These lizards may reach a length of 2 feet though smaller ones are more commonly seen. The surface of the body is totally unlike that of any other lizard, being covered by beadlike ossicles or tubercles. The most interesting and distinctive structural characteristic of these lizards is their grooved teeth with ducts from poison glands opening at the base of the grooves. No other family of lizards in America is venomous. The color of H. suspectum is black marbled with pale pink, salmon, or flesh. The Mexican species is black with yellow or lemon spots or bars. The short thick tail becomes more slender when the animals fast. The natural food of the animals is not known. They take eggs readily in captivity and thrive on them. Reproduction is oviparous, the eggs being laid in warm, moist sand where they hatch in twenty-eight to thirty days. Family Xantusiidae (night lizards). — This small family contains only three genera with a total of seven species. Five species, all of the genus Xantusiidae, are found in southern California, Lower California, and Arizona. One other genus is found in Central America ; and a third in Cuba. These lizards are seldom over six inches in length. The color changes from dark brown in subdued light to lighter hues in stronger light. The pupils are vertical, and the eyes are without lids. The tongue is only slightly extensible. These little reptiles are strictly nocturnal, hiding by day under fallen Yucca plants or in crevices between boulders. So far as is known they are insectivorous in feeding habits. At least one species is known to be ovoviviparous. Family Teiidae (striped lizards, race runners, sand lizards). — This family contains 40 genera with more than a hundred species. Twenty- two species and subspecies, all belonging to the genus Cnemidophorus, are found in the warmer parts of the United States. All other members of the family are found in South America and the West Indies. The species of Cnemidophorus are long slender, active lizards cap- able of surprising speed in running. The ground color is usually some shade of brown. Lines, bands, or spots of lighter color form various patterns on different species. The tongue is black, forked, and protractile. REPTILIA 555 Our species are found in open, sunny, sandy places. If disturbed they skim over the ground with great rapidity, but if hard pressed they take refuge in burrows. Insects make up the bulk of their food. All the species lay thin-shelled eggs which are deposited in shallow excavations in the sand to be hatched by the heat of the sun. Family Scincidae (skinks, or smooth lizards). — This is one of the largest families of lizards, being composed of over four hundred species arranged in thirty genera. Three genera containing sixteen species are fonnd in the United States. They are more abundant both in number of kinds and in number of individuals in tropical regions, especially in tropical parts of the old world and in the Australian regions. South America has fewer skinks than any other region in the world. All the skinks are relatively small lizards, the largest in this coun- try seldom attaining a length of ten inches. The scales are smooth and usually shining. The color varies with age. The young are darker than the adults and color patterns of lines present on the young often disappear on adults. There is great variation in the development of the limbs. Most of our forms have one or both pairs of legs. The skinks are diurnal, feeding by day and seeking a hiding place at night. Many old world kinds are burrowers in sand, but of all American forms only one Florida species is a burrower. The ma- jority of species are to be found under bark, logs, stones and in other dark, cool places. Some kinds of skinks have been observed to guard the eggs by curling about them. Some old world species are ovoviviparous. Family Amphishaenidae (worm lizards). — This highly modified family is represented in the United States by one genus with one species in Florida and another genus with one species in southern California. Forty species are known from the American tropics and others from northern Africa and the Mediterranean region. These remarkable lizards are all limbless except Bipes hiporus, the two-footed lizard of Lower California, Avhich has the anterior pair of limbs well developed. The skin is without scales and forms numerous rings about the body, suggesting an annelid worm in appearance. The eyes are absent or reduced. There is usually no external ear opening. 556 TEXTBOOK OF ZOOLOGY Both of our species lead a subterranean existence, boring tuimels in which they move backward and forward with equal ease. Suborder Ophidia (Serpentes) (Snakes). — No group of reptiles is of greater natural interest to man than the snakes. Superstitions and stories relating to snakes are as old as written language. Many- religions and cults have used the serpent as a symbol of good or of evil. In many regions of the world today many of the most poisonous of snakes are venerated and protected by the natives. The snake dance of the Hopi Indians of our own country is a well- known example of the symbolic use of snakes. The ophidians are highly modified vertebrates. Their anatomical structure indicates that they have been derived rather recently, geologically speaking, from lizardlike ancestors. Some lizards are totally limbless, while some snakes of the family Boidae have vestiges of the posterior pair of limbs. In the structure of the jaws there is close similarity between some snakes and some lizards. Upon the basis of structure and arrangement of teeth snakes have been arranged into the following four groups: The Aglypha, or those with solid, ungrooved teeth. Our harmless snakes all have this type of dentition. The Opisthoglypha, or those having the posterior maxillary teeth grooved. These snakes are venomous but seldom dangerous to man. The position of the venom conducting teeth makes it difficult for the snake to inflict a wound on man. The lyre snakes (Trimorpho- don) of the Southwest, the black-headed snakes (Tantilla) of the Southern States, and a few other rare snakes belong to this group. The Proteroglypha are those that have the anterior maxillary teeth grooved and often enlarged and elongated. Many of the most dangerous snakes in the world belong to this group. The coral snakes, cobras, and sea serpents have this type of dentition. The Solenoglypha, or those having hollow, hinged fangs in the anterior part of the mouth. The rattlesnakes, copperheads, water moccasins and their relatives make up this group. They are all venomous and dangerous to man. There are approximately 2,300 known species of snakes. Of this number, some 225 species are venomous; but 75 of these poisonous species are so small or rare that there are only 150 to 175 species that man need fear. REPTILIA 557 In the United States there are 234 recognized species and subspecies of snakes. Of this number, 51 species and subspecies are venomous but 13 of these are too small or rare to be considered dangerous to man. Hence, there are 38 kinds in our country that we must avoid. Family Leptotyphlopidae (worm snakes).- — Three species belong- ing to this family are found in southwestern United States. In Mexico, Central America, Asia, and Africa, there are about thirty species. One of these, a Syrian species, is the smallest of all adult snakes. Those found in our own country are small, seldom attain- ing a length of more than a few inches. They are all plain flesh, or various shades of pale pinkish lavender in color. An iridescent, silvery sheen extends over all. The head is blunt and of the same diameter as the neck and body. The small eyes are covered by translucent scales. It is probable that they have very poor powers of vision. The tail is likewise blunt and very short. There is a vestige of a pelvis present in some members of the family. All of the North American species are burrowers, making long tunnels in which they find insect larvae and worms. They seldom come to the surface except when forced out of their burrows by heavy rains. Family Boidae (boas and pythons). — There are sixty to seventy species in this family, some of which are found in all tropical parts of the world. In the United States there are three species, all found in southern California or neighboring desert regions. Our species are all small, but the largest of living snakes are members of the family. There are authentic records of specimens 30 feet long and weighing approximately 300 pounds. There is usually some ex- ternal evidence of vestigial limbs present. Though none are venom- ous, many species have elliptical pupils. All members of the family are constrictors in feeding habits, preferring warm-blooded animals as a rule. The females lay eggs and some species are known to coil about them until the young are hatched. Family Coluhridae. — This is the largest of all the families of snakes, containing 90 per cent of the living species. In the United States more than 100 species of snakes belong to this family. Mem- bers of the family range farther north and south of the equator than those of any other family of snakes. Being so numerous and widespread, it is not surprising that some species should have adopted every available habitat. Hence, the variety in size, form. 558 TEXTBOOK OF ZOOLOGY and color is great. The arboreal species are slender and green. The terrestrial species are heavier in body and varied in color. The subterranean, the semiaquatic, and the aquatic forms all show adaptations to their environments. Most of our Coluberine snakes are nonvenomous, but some are mildly poisonous opisthoglyphs. A majority of the snakes in this family lay eggs but some bring forth the young alive. Family Elapidae (corals, harlequins). — Twenty-nine genera with about 140 species make up this family. All except two genera are found in the Old World only. Africa, Asia, Malay Archipelago, and Australia have representatives of this family. In Australia there are only a few representatives of other families. In the United States there are two genera containing one species each. Together they cover most of the southern half of our country. All the snakes in this family are deadly poisonous proteroglyph serpents. The cobras of Asia and Africa kill thousands of persons every year. This is due partly to superstitions and religious be- liefs that protect these snakes in those regions. The venom of this family of snakes is largely neurotoxic in action; i.e., it acts on the nervous centers. Hence, it usually acts much more quickly than the slower hemolytic and hemotoxic venoms of the pit vipers. Some cobras may attain a length of ten to twelve feet. Men have been known to die in less than an hour after being bitten by such a snake. However, the venom of the coral snakes, the American representa- tives of this family, is more deadly per unit volume. The coral snakes are seldom more than two feet long and are not capable of injecting such large quantities of venom. The coral snakes are beautiful little snakes marked with bril- liant cross bands of red, yellow, and black. There are three harm- less snakes found in the same parts of our country that have the same colors in their patterns. None of them, however, duplicates the sequence of the bands on the coral snakes. In these poisonous snakes the order of the colored bands is red, yellow, and black. In the harmless species the order is red, black, and yellow. The following jingle is a good device for remembering these color schemes: Red and yellow Kill a fellow. Red and black Venom lack. REPTILIA 559 The small, conical head and slender, cylindrical body of the coral snakes fit them for their subterranean life. They seldom come to the surface during the day, but may be found at night crawling about in search of food. They eat other snakes and small lizards. These snakes lay eggs. Family Hijdrophidae.— Members of this family are marine relatives of the Elapidae. Only one species is found in the New World. It occurs off the west coast of Mexico and has been reported as very common in some localities at certain seasons. The females come into shallow coastal waters to give birth to their young. Here the young have some protection from their enemies and access to small fish suitable for food. Adults of Old World species have been sighted one thousand miles from land. All species have the tail flattened for swimming. Family Croialidae (pit vipers).— This family is composed of six genera which contain about eighty species. Members of the family are found over all the temperate and tropical parts of the Western Hemisphere. In the Old World they are found in India, China, and neighboring regions. Three of the six genera in the family have representatives in the United States. In fact, all the dangerously poisonous snakes in this country, except the coral snakes, belong to this family. They are all solenoglyph snakes. There is a prominent pit on each side of the head between the eye and the nostril. The rattlesnakes (Crotahis and Sistrurus) bear rattles on the end of the tail. All members of the family have elliptical pupils. The poison glands and highly developed fangs enable these snakes to capture their food with a minimum of effort on their part. The venom is injected so quickly and so unexpectedly that the prey has little chance to avoid it. Most small animals die very soon after being bitten. The reptile then swallows the carcass at its own pleasure. Rattlesnakes prefer mammals. In regions where these snakes abound wild rats and mice are rare. Water moccasins take frogs and other cold-blooded aquatic animals for food. The cop- perhead appears to enjoy both warm-blooded and cold-blooded animals. All members of this family give birth to living young or lay thin- shelled eggs which hatch in a very short time, usually less than an hour. 560 textbook of zoology Order Rhincocephalia Suborder Sphenodontia (Sphenodon, Tuatara). — The only living representative of this order is Sphenodon punctatum, a lizardlike animal found on a few small islands off the coast of New Zealand. It is often called a "living fossil" because many of its anatomical characters are found in no other living reptile. Some of these characters are old, even in relation to many extinct reptiles. The entire brain is said to be smaller than one of its eyes. Unlike all other living reptiles, it has no external copulatory organ. There are ten separate carpal bones present. This is a primitive number. Many other skeletal features indicate a close relation to reptiles of other geological periods. The adults usually attain a length of about twenty inches. They are dull yellowish or olive brown in color. A middorsal row of spinelike scales extends from the occipital region to the end of the tail. There are other rows of smaller excrescences along the sides. One of the most interesting features of the animal is its pineal eye. This "third eye" is located in the center of the head between the eyes. It is surrounded by a rosette of small scales and covered by a translucent plate. The nerve from this eye is well developed and passes to the brain through a foramen in the cranium. There are a retina and a cornea in the structure of this organ, but the extent of its function as an eye is unknown. The habits of the animal are as unique and interesting as its structure. It lives along the shore in burrows with a small petrel, a shore bird of that region. It is said that the reptile and the bird have special sides in the enlarged chamber at the end of the burrow and neither trespasses on the other's space. The food of these ani- mals consists of insects, spiders, and crustaceans. In captivity they have been known to thrive on a diet of earthworms. They are nocturnal, hiding by day and hunting along the beaches at night. Reproduction is oviparous, about ten eggs being a usual complement. Order Crocodilia Suborder Eiisuchia (crocodiles and alligators). — This order is rep- resented by about twenty living species. They are found along the coastal plains in our Southern States, southward through Mexico, Central America, and in the warmer regions in South America. In REPTILIA 561 the Old World they occur in tropical Africa, Southern Asia, Java, Sumatra, and Northern Australia. In the United States two genera with one species each are found. One, Alligator mississi'p'piensis , is found in all our coastal states from the Kio Grande to the Carolinas. The other, Crocodilus acidus, is found in the southern tip of Florida, ]\Iexico, Central and South America. The general appearance of alligators is well known to everyone. The animals are not covered by scales but are protected by rows of dermal ossicles in the skin. Formerly a crocodile or an alligator might be found that was as long as thirty feet, but it is doubtful whether such an animal can be found today. The females lay eggs in a nest built of sticks and decaying vegeta- tion. The heat generated by decomposing vegetable matter aids in incubating the eggs. It was formerly thought that the growth of alligators was very slow but specimens in captivity have been brought to a length of five and one-half feet in five years. THE HORNED LIZARD The horned lizard is a convenient reptile to study since one or more species is found locally throughout southwestern United States. It is usually found in abundance and is relatively easy to capture. The ''horns" on the head are unique among living reptiles. The general plan of structure is sufficiently generalized to illustrate most of the characteristics typical of reptiles. Habits and Behavior These lizards are diurnal animals, feeding by day and at night bur- rowing into the ground until only their head spines can be seen. Their food consists of small beetles, flies, smooth caterpillars, moths, ants, and other small insects. The animals are solitary, not being found in groups in dens. Each one burrows underground to hibernate during the cooler season of the year. After mating, a female of an oviparous species digs a cup-shaped hole and deposits twenty to twenty-five eggs in it, covering the eggs with several layers of earth. Females of the ovoviviparous species bring forth the young alive. 562 TEXTBOOK OF ZOOLOGY External Structure The horned lizards are typical iguanid lizards, with broad, flat bodies covered with horny, strongly keeled scales and spines. There is at least one row of spines in a marginal fringe at the lateral edges of the belly. Erect, scattered spines of various shapes and sizes are apparent on the animal's back. The body is devoid of a dorsal crest but is covered with small granular scales and with four rows of enlarged, sharply pointed spines. The ventral parts are covered with small, smooth, light-colored, rectangular scales. The body of the animal is divided into three well-defined regions : head, neck, and trunk. The trunk bears two pair of well-developed linibs and tapers posteriorly to a broad, short tail. The head is short and the muzzle descends steeply in profile, but is not separated from the front by a conspicuous angle. It is covered with small scales, and is bordered posteriorly by a row of osseous spines. The most anterior structures are paired, small nostrils. These are small rounded apertures situated one on each side of the snout, a short distance from its anterior end. Just posterior to the nostrils are the eyes. They are situated one on each side of the head about midway between the nostrils and the tympanic area. Each eye is guarded by a short, thick upper eyelid and a thinner lower lid. The lower lid covers most of the eye when that organ is closed. Within the eyelids and attached to the anterior comer of the eye is a thin, transparent nictitating membrane which closes backward over the eye. Above each orbital socket is a bony structure which forms a posterior superciliary angle by being pro- duced into a short postorbital horn. Behind the eye and a little posterior to the mouth is the auditory aperture. A thin tympanic memhrane is stretched over it. Above the auditory aperture and be- tween it and the eye are three temporal horns. They form the post- lateral border of the head. The occipital horns mark the dorsoposte- rior boundary of the head. The gape of the large mouth begins at the snout and extends posteriorly to within a short distance of the auditory aperture. The lower labial scales vary from the small rounded anterior ones to the prominent, acute posterior spinelike scales. The cervical region is thick and stout but well differentiated. A transverse gular fold is present on the ventral surface of the neck. REPTILIA 563 A row of enlarged gular scales parallel the gular fold and a small, single spine may be found on each side at the posterior gular border. Two longitudinal folds are present on each side of the cervical region. These folds descend forward and obliquely downward to the plane of the gular fold. The trunk is depressed and is broadly fusiform in shape. It is flattened ventrally and tapers posteriorly toward the pelvis. The anterior Unibs spring one from each side of the body near the anterior end of the trunk. They are divided into three divisions : proximal or hrachium, middle or antehrachium, and distal or manus. The distal division is terminated by five clawed digits. The first digit is the shortest. It is designated as the thumb or pollex. The posterior limbs arise one from each side of the body near the posterior end of the trunk. They are also divided into three regions : proximal or thigh, middle or shank, and distal or foot. The foot, like the hand, ends with five clawed digits. The first digit, or hallux, is the shortest. The ventral surface of the thigh bears twelve to fifteen femoral pores, whose function is obscure. They are present in adult males only. Between the thighs on the ventral aspect of the body is a slitlike, transverse vent, or anus. It is the common outlet for the digestive and urinogenital systems. The short tail is broad at the base but tapers quickly toward the distal end. In males two genital swellings lie one on each side of the ventral surface of the broadened proximal region of the tail just posterior to the vent. Like most reptiles, these lizards shed their thin epidermal cover- ing periodically. The variable color pattern is brighter for a time after this skin is cast. Digestive System The alimentary canal is composed of the following organs in order : mouth, esophagus, stomach, small intestine, large intestine, rectum, cloaca, anus. Several accessory organs and structures are teeth, tongue, pancreas, and liver. The upper and lower jaws form the anterior boundary of the aper- ture of the mouth, and are each provided with a single row of small, faintly tricuspid teeth. These teeth are not adapted for mastication but for seizing and holding prey. The posterior nares are apertures situated a little behind the end of the snout and separated from one 564 TEXTBOOK OF ZOOLOGY another by a vertical partition supported by the vomerine bones of the skull. The Eustachian pits are paired diverticula from the dorsal wall of the pharynx continuous with the tympanic cavities, which are closed externally by the tympanic memtranes. The mouth cavity ends posteriorly as the pharyngeal cavity which is produced poste- riorly into the horizontal slit which leads to the esophagus. The glottis, or aperture of the trachea, is located centrally on the summit of the laryngeal chamher. The tongue is a fleshy organ lying on the floor of the mouth. It is furnished with many tactile and gustatory cells, and is usuallj^ covered with a sticky secretion which assists it in catching and holding prey. It can be extended from the mouth and is used in capturing insects. The tubular esophagus opens into the mouth cavity by way of a horizontal slit posterior to the pharjmgeal cavity. It is deeply pig- mented anteriorly and lies dorsal to the trachea. The walls are thick and muscular. It opens posteriorly into the cardiac portion of the stomach. The stomach is a cylindrical, muscular organ with relatively thick walls. When empty it is but little greater in diameter than the esophagus or the intestine. Since it lies chiefly on the left side of the body, it is largely concealed by the left lobe of the liver. It is larger at the anterior or cardiac end and constricted at the posterior or pyloric end. The pyloric valve is an annular ridge of muscular tissue which narrows or closes the aperture between the stomach and the small intestine. The inner surface of the walls of the stomach is thrown into longitudinal folds or rugae. Distributed over the entire inner surface are the openings of minute ducts from the numer- ous gastric glands which secrete the gastric juice. This digestive fluid normally begins the digestion of proteins in the food. The intestine forms several coils in the posterior third of the coelom, and is held in place by the mesenteries. As is commonly the case in insectivorous or carnivorous animals the intestine is relatively short. The anterior part of the small intestine is the duodenum. The com- mon bile duct from the liver and the pancreatic duct from the pan- creas join this section of the intestine. There is no definite division between the duodenum and the ileum, the succeeding portion of the intestine. After several turns the ileum passes into the large intestine on the right side of the body. An annular ridge of tissue, the ilea- REPTILIA 565 caecal valve, narrows the aperture between the ileum and the large intestine. The large intestine can be traced from the right side of the body downward and medially to a point where it passes out of sight beneath the pelvis to enter the cloaca. There is a very small Aorta ■Pulmonary artery Liver Qall bladder Duodenum , Pancreas — Ileum— f^^- Large intestine --^^ 0^/ary - -Adipose body Oviduct — Cloaca Fig. 296. — Dissection of a horned lizard to show internal organs, ventral view. blind pouch or cecu7n, if any, at the position of the ileocaecal valve, where the ileum joins the large intestine. Posteriorly the large in- testine becomes the rectum before entering the cloaca which is di- 566 TEXTBOOK OF ZOOLOGY vided into an anterior and posterior chamber. The anus is the pos- terior end of the alimentary canal. The pancreas is an elongated branched body of glandular material located within the first loop of the intestine between the stomach and the duodenum. It secretes an alkaline digestive fluid which is emptied into the duodenum. The liver is a larg'e, bilobed organ divided imperfectly into right and left lobes. It secretes bile, an alkaline digestive fluid. This fluid is stored in the gall Madder until food enters the duodenum. It then passes into the intestine through the common bile duct. Respiratory System A basic feature that distinguishes reptiles and other higher ver- tebrates from amphibians and fish is that they do not breathe by gills during development. The respiratory system of all lizards is typical of air-breathing vertebrates. A tubular trachea conducts air from the mouth to the highly vascularized lungs. This tube is prevented from collapsing by rings of cartilage. A gaseous ex- change is effected between inhaled air and the blood. The oxygen is absorbed and carbon dioxide exhaled. Breathing is effected by the motion of the ribs. Air passes through the nostrils, or external nares, into the olfactory chaml)er. From there it passes through the internal or posterior nares into the mouth cavity to be forced from there through slitlike glottis into the chamber of the larynx. The larynx continues posteriorly as the trachea, which lies in the midventral line of the throat and extends to the lungs. The posterior end of the trachea bifurcates to form two smaller tubes, the right and left hronchi. Each bronchus connects with a corresponding lung a short distance from the apex of the lung. Within the lung each bronqhus divides into secondary bronchi and these in turn give off tertiary branches. The smallest tubules, bronchioles, carry the air into air spaces or alveoli. It is in the alveoli that an exchange of gases takes place. The Circulatory System The circulatory system of reptiles shows distinct advances over the circulatory system of amphibians. Since respiration is carried on exclusively by the lungs, changes which are in keeping with this REPTILIA 567 modification are apparent in the pulmonary circulation. The system consists of a heart, arteries, capillaries, and veins, forming a continu- ous or closed set of vessels throughout the body. The heart lies at the extreme anterior end of the body cavity and is enclosed in a thin- walled sac, the pericardium. The heart consists of a dorsal sinus venosus, a right and a left auricle, located anteriorly, and a posteiior ventricle. The auricles are thin-walled chambers which communicate with the ventricle by means of the auriculoven- tricidar aperture. This aperture is guarded by an auriculoventricular valve which consists of two flaps. The thin vertical inter auricular septum separates the auricles. The ventricle is a thick, cone-shaped, muscular chamber. The base is directed anteriorly and is connected to the auricles at the auriculoventricular aperture. The thick, spongy walls of the ventricle leave only a small cavity within its chamber just below the auriculoventricular aperture. It is completely divided into tw^o chambers by the perforated interventricular septum only w^hen in a state of contraction. Three arterial trunks arise from the ventral surface of the ven- tricle. The pidmonary artery arises from the right portion of the ventricle. Its aperture is separated from that of the right aorta by means of a muscular partition. The pulmonary artery is visible for only a short distance. It origi- nates from the right side of the ventricle, passes anteriorly, then turns dorsal to the heart. It soon divides into right and left trunks supplying the respective lungs. The right aortic arch arises from the left half of the ventricle and at first is dorsal to the other two arterial trunks. Soon the pulmonary artery assumes the dorsal position and the right aortic arch becomes ventral and passes to the right and anteriorly between the auricles. At the anterior end of the heart it forks to form a Y. The branch passing to the left is the left common carotid artery. The right branch forks again. The anterior branch is the right common carotid artery, and the other is the continuation of the right aortic arch, which gives off the two subclavian arteries. The left aortic arch is the most ventral of the three arterial trunks and it also arises from the left side of the ventricle. The right aortic arch is in close relation to the left arch. The left aortic arch then passes laterally and diagonally across the pulmonary artery and the 568 TEXTBOOK OF ZOOLOGY right aortic arch, and continues cephalad on the left side of the inter auricular space. The aortic arches may be traced laterally and are seen to bend around the esophagus and unite with one another on the dorsal side of the esophagus, ventral to the vertebral column, to form the median dorsal aorta. An esophageal artery may branch from the left aorta. Rt. interna] ccroti'd_ Rt. aorta^-.^ps=^ Rfc. Subdavian A Rt aorta Dorsal aorta RthepaticA. Ant. mesenteric A SpermaUc A Renal A A A Kidney_ Femoral A.^fl _ _ ^^ ~^:=;fs:^ Sciatic A-i Cow. ^Lt. common carotid A. External carotid A. internal carotid A. -Connection, 3''-^h.^^arcli _3i ^^.L.aorta Pulmonary A. -.-LpajponaryA. -br^-L. iubclavian A. \ Ventricle Antqastric Coeliac A. Ventral qastric A. ijTis^ .Hepabo-duodenalA. y 1 _5up. mesenteric A. 'n htestinat A. Lumbar A- Adipose A. Fig. 297. — Heart and arterial system of horned lizard, ventral view. The right and left comtnon carotid arteries arise from the right aortic arch at the point where it leaves the left aortic arch. The first branch of each is the external carotid which supplies the lower jaw. Each of the common carotids next gives off an internal carotid artery which supplies the right and left sides of the head. Now the common REPTILIA 569 carotids pass laterally and parallel with the corresponding aortic arches. At the distal end of the common carotids each anastomoses with the corresponding aortic arch. The right and left subclavian arteries arise from the right aortic arch just anterior to its junction with the left aortic arch to form the dorsal aorta. Each passes laterally ; one to each of the forelimbs. The dorsal aorta is a median, unpaired, longitudinal artery lying in the mid-dorsal line ventral to the vertebral column. A little further caudal small gastric arteries pass to the stomach. Near the posterior end of the stomach the large coeliac artery is given off. This soon divides into two trunks. The anterior coeliac artery supplies the digestive glands, stomach, and duodenum. The posterior branch, the ventral gastric supplies the ventral surface of the stomach. The anterior mesenteric is the next unpaired branch of the aorta and it soon divides into the hepatoduodenal to the left side of the liver and the duodenum, and the superior mesenteric to the ileum. The right hepatic artery springs from the dorsal aorta and passes to the right lobe of the liver. The inferior mesenteric arteries include three small vessels arising from the dorsal aorta at the level of the pelvis, which supply the large intestine and rectum. Two pairs of small branches, the genital and renal arteries, supply the gonads and kidneys re- spectively. The right and left iliac arteries branch from the aorta and pass into the respective hindlimbs and bifurcate to become fem- oral and sciatic artery of each limb. Further posteriorly the dorsal aorta continues into the tail as the caudal artery. The veins of the body, with the exception of the pulmonary veins, enter the sinus venosus. This is a thin-walled chamber located dor- sally to the right auricle. It communicates with the right auricle by means of the sinuauricular aperture. This aperture is guarded by the two-lipped sinuauricular valve. The pulmonary veins bring blood from the lungs. They unite into a short common trunk which enters the left auricle on its dorsal wall. The postcaval vein, which receives the wide hepatic vein from the liver, extends from the anterior border of the liver to the sinus venosus. Posteriorly it lies between the gonads, and is formed by the union of a pair of efferent renal veins which lie parallel to the epididymis or oviducts. The postcava also receives a spermatic or ovarian tributary from each gonad. 570 TEXTBOOK OF ZOOLOGY The hepatic portal vein is formed by the union of the gastrosplenic, mesenteric, and the ventral abdominal veins. It passes anteriorly and enters the tissue of the liver. The principal branch of the hepatic portal is continued posteriorly as the abdominal vein. The gastro- splenic is a large vessel and lies within the duodenoJiepatic mesentery, External ju(]u!ar V. ^ Internal juqular V- Sinus venosus J'ulmonary V. _ _^ _ ,^^^y ^.Hepatic V. /- ' ' branches in liver i HepaUc portal V. gastric V. -Ventral flMomi'na I V. J Renal V. I inferior mesentericV ^^ — Femora] V .Sciatic V. — -/.Renal portal V. .Caudal V. Fig. 298. — Venous system of horned lizard, ventral view. parallel to the pancreas. It receives gastric veins from the stomach and small pancreatic veins. It is formed at its distal end by the union of the splenic and mesenteric veins from the spleen and in- testines. Several separate gastric portal veins take blood directly from the stomach to the liver. REPTILIA 571 The renal portal system drains a considerable portion of the body. The venous blood from the tail is carried by means of a caudal vein. It lies ventrally to the caudal vertebrae, and bifurcates at the base of the tail to form the two lateral pelvic veins. Each of these passes across the ventral face of the corresponding kidney to contact the posterior edge of the corpora adiposa. Each receives efferent veins from these bodies, at the anterior border of which the pelvic veins empty into the epigastric or anterior abdominal vein. This vein ex- tends anteriorly along the inner face of the body wall to enter the left lobe of the liver on the posterior face. The renal portal veins receive the pelvic veins near the kidneys. The pelvic veins are formed by the union of the femoral and the sciatic veins from each hindlimb. The right precava is located in the lateral neck region and passes posteriorly to the sinus venosus. It is formed by the union of the internal and external jugulars. For a short distance it is the common jugular. Then it receives the right subclavian vein which returns the blood from the right forelimb. From that point on it is the right precava. The left precava is formed by the union of the left internal jugular and the left subclavian veins. The left external jugular is not present. The right and left precavae converge with the postcava and enter the sinus venosus. The Urinogenital System The reptilian urinogenital system shows decided advances over the types of urinogenital systems found in lower vertebrates. The mesonephric type of kidney found in lower groups is replaced by a metanephric type of kidney in the reptiles. The products of the gonads are conducted through special ducts from these glands and not through the kidney, as in lower animals. Instead of producing hundreds of small eggs in a season, as in certain amphibians and fishes, the horned lizard lays twenty to twenty-five eggs of moder- ate size. The eggs of all vertebrate classes below reptiles are with- out shells. This makes external fertilization possible and necessi- tates an aquatic nesting site. The eggs of reptiles are covered by a tough, thick shell. Fertilization must occur internally before the shell is put on; hence, the males have intromittent copulatory or- gans for introducing spermatic fluid into the females. Such eggs will develop on land and often in rather dry places. The oviducts 572 TEXTBOOK OF ZOOLOGY are much shorter in animals producing only a few ova than in animals producing many. Certain similarities, however, are evident in the urinogenital sys- tem of reptiles and of lower forms. The urinogenital products are voided into the common cloacal chamber as in lower animals. The kidneys of the horned lizard are reddish brown, paired bodies lying on each side of the middorsal line, between the large intestine and the body wall. Technically they are outside of the body cavity since they are dorsal to the peritoneum. Each kidney has a delicate excretory duct, the ureter. In the males this duct opens into the Oviduct — Apertures oviduct and ureter -Intestine J4} Kidney %M Ant. cloaca M^ Cut oviduct '^M' Ureter r Post.Cloaca ^ Anus Fig. 299. — Urinogenital system of female horned lizard from ventral view. vas deferens and a common duct leads to the cloaca. In the females the ureter connects the posterior end of the kidney with the dorsal wall of the cloaca. The male reproductive organs consist of a pair of testes, a pair of eversible hemipenes, and various connecting ducts. The testes are white oval bodies lying in the posterior part of the body cavity, one on each side of the mesorectum. Each testis is attached to the dorsal body wall by a sheet of peritoneum, the mesorchium. The series of ducts connecting each testis with the cloaca includes the rete testis, the epididymis, and the vas deferens in order. The vas deferens passes posteriorly, joins the ureter, and enters the cloacal wall. The paired, eversible hemipenes, or copulatory organs, are vascular REPTILIA 573 sacs embedded in the musculature of the ventral side of the base of the tail. They are elongated, smooth-walled structures which open into the posterior chamber of the cloaca. During copulation these organs protrude through the vent of the cloaca by everting themselves; i.e., turning themselves wrong side out. They are pulled back into place by the contraction of a retractor muscle attached to the distal end of each organ. Often only one hemipenis functions during copulation, but this is not an invariable rule. The female reproductive organs consist of a pair of ovaries and a pair of oviducts. Each ovary is attached to the dorsal body wall in the posterior end of the body cavity by means of a double sheet of peritoneum, the niesovarium. The folded oviducts lie dorsolateral to the ovaries. The anterior end of each oviduct is expanded into a thin-walled ostium; posteriorly, each duct opens into the dorsal wall of the posterior cloacal chamber. The oviducts are held in position by a broad fold of the peritoneum, the hroad ligament. The common cloacal chamber of both sexes receives the products discharged by the alimentary canal, the kidneys, and gonads. The cloaca is divided into two regions, an anterior and a posterior, by an incomplete ridge of mucous membrane. The Nervous System The reptilian nervous system shows certain advances over the nervous system of amphibians. Twelve pairs of cranial nerves are present, as in birds and mammals. The various regions of the brain are better developed than in lower vertebrates ; however, the reptilian brain is still a comparatively simple structure. The nervous system of horned lizards, as in other vertebrates, consists of the central nervous system, the peripheral nervous sys- tem, and the sympathetic nervous system. The central nervous system consists of a 'brain located within the skull, and a spinal cord located within the neural canal. The brain has all the divisions of the typical vertebrate brain. The peripheral nervous system is composed of the twelve pairs of cranial nerves from the brain and sixteen pairs of spinal nerves from the spinal cord. These nerves are arranged segmentally. Typically, there is one pair to each segment. The sympathetic system includes two conspicuous white trunks that extend along each side of the vertebral column and the lateral fibers from these trunks. Each of the sympathetic 574 TEXTBOOK OF ZOOLOGY trunks is connected with all of the spinal nerves on the same side of the body by a number of ganglionic enlargements. The usual sensory and motor functions are carried on by these systems. The autonomic system helps to regulate involuntary reactions. The special sense organs of the head are the eyes and the ears. Each eyelall reposes in an orbital cavity, and is attached to the walls of the cavity by voluntary muscles which serve to move it in all direc- tions. The innervation is from the optic lobes and other parts of the brain by way of the optic nerves. The structure of the eye itself is similar to that of other vertebrates. The ear consists of two principal parts: the middle ear, or tym- panum, and the internal ear, or membranous labyrinth. The cavity is closed outwardly by the tympanic membrane. It communicates with the mouth cavity by means of the Eustachian passage. A small rod of bone and cartilage, the columella, stretches across the cavity from the tympanic membrane and is fixed internally into the mem- brane covering the fenestra ovalis. The internal ear is enclosed by the bones of the auditory region. The Skeletal System The reptilian skeleton retains many features of the amphibian skeleton and forecasts many features found in birds and mammals. The divisions of the skeleton of the horned lizard are the skull, the vertebral column, ribs, sternum, the limb girdles, and limbs. The skull proper is triangular and somewhat pyramidal in form. The base of the pyramid is represented by the posterior aspect of the skull, and the apex is represented by the snout. It is more com- pletely ossified than in amphibians and is composed of a greater number of bones. There are sharp protuberances and excrescences that form the skeletal support of the "horns" of horned lizards. Were these magnified to the size of dinosaurs, they would be equally imposing. The skull articulates posteriorly with the atlas, or first cervical vertebra. The vertebral column consists of several distinct regions and a number of vertebrae. There is a cervical region composed of seven vertebrae which form the skeleton of the neck. A thoracicolumbar region consists of fourteen vertebrae and with the ribs and sternum REPTILIA 575 make up the skeleton of the trunk, A sacral region of two vertebrae gives attachment to the pelvis. A caudal region of fourteen vertebrae supports the tail. txterriQl narc Maxilla 5upra orb'ibal Orbital fossa Parietal £ foramen Parietal j^^^f^ Premoxil/or Nasal Prtal \^ Juqal ik___fronta/ iat. temporal fossa \ Posterio- lateral process Fig. 300. — Dorsal view of the skull of horned lizard. Vomer: Jugal iupna-_ _ orbital BasI--- occipital Internal nare Teeth Palatine Porasphenoid Pbzryqoidi Ba 51* sphenoid Inferior tem- poral fossa ^Occipital condyle Fig. 301. — Ventral view of the skull of horned lizard. The pectoral girdle is composed of bone and cartilage. The two anterior limbs articulate with it. The limb is a rather generalized ex- ample of a typical pentadactylous vertebrate limb. The pelvic girdle 576 TEXTBOOK OF ZOOLOGY is composed of a pair of triradiate bones, or innominate bones. Each innominate bone consists of three separate bones : the ilium, ischium, and pudis. These three bones radiate from a triradiate suture at the acetabulum or hip socket. The pelvic girdle articulates with the sacral vertebrae by means of the ilia. The ischia are the most pos- terior bones of the three. The pubic bones extend anteroventrally and unite at the pubic symphysis. As in the forelimb, the structure of the hindlimb is of the characteristic vertebrate type. Cfav//'c/e__ Bpkorocoid.^. Coracoid fenestra Epbternum Sternum Posterior cornu ■n Supra scapula Scapula -Pre coracoid -Coracoid —Humerus -Vontanelle - -Trochlea -.Olecranon --Radius Ulna Styloid P. --Carpals Metacarpal PholarK^es Fig. 302. — Sternum, pectoral girdle, and disarticulated forelimb of horned lizard, ventral view. Muscular System The reptilian musculature shows few unusual characteristics. In a general way it may be said to be intermediate between that of amphibians, on the one hand, and birds and mammals, on the other. Since the horned lizard is a generalized terrestrial animal, the musculature follows the typical vertebrate plan rather closely. / REPTILIA 577 THE TURTLE The pond turtles of various genera belonging to Family Emydidae have long been used as convenient and useful examples for studies of reptilian characteristics. While there are many variations in details, the main features of Troost's turtle, Pseudemys troostii elegans (Wied), are foundj very little modified, in other members of the family. This is one of the most abundant species of the Mississippi Valley region and is veiy frequently the turtle sold by supply houses for laboratory use. One of the most interesting characteristics of this species (P. troostii elegans) has only recently been discovered. The adult males are frequently melanistic; i.e., have the normal color pattern con- cealed by a superimposed layer of black pigment. The 3'oung males have the same coloration as the females and only gradually take on the melanistic coloring. Hence, all stages from normal coloration to jet black can be found among them. Habits and Behavior These turtles are commonly seen sunning themselves on logs along the margins of ponds and rivers. When disturbed, they fall into the water with a splash and swim hastily for the deeper parts of the body of water. This activity in daylight hours might lead one to suspect that they are entirely diurnal, but turtles are known to feed at night. Hence, it seems that they are not as strictly regu- lated by hours of light and darkness as are most vertebrates. Since all turtles are cold-blooded animals, they hibernate in winter in temperate regions. Turtles are often seen in considerable numbers in a restricted localitj^ but there is no congregating or communal instinct among them. Each one is totally oblivious of the presence of all the others, except during the mating season, when the males seek out the females. Turtles have been observed to mate at all times from spring to autumn but the chief mating period occurs soon after the animal comes out of hibernation in the spring. The eggs are laid soon afterwards in a hole or pit in the earth excavated by the female for that purpose. When the full complement has been deposited, she covers them and returns to the water. Turtles taken in the autumn months are often found to contain a full com- plement of completely shelled eggs. This indicates that the mating that commonly occurs immediately prior to the laying of these eggs 578 TEXTBOOK OF ZOOLOGY does not fertilize the eggs of the year but those of succeeding years. In experimenting upon this phase of the life history of turtles, the United States Fish and Wildlife Service has discovered the surprising fact that a female turtle may lay fertile eggs for as long as four successive years after a single mating. External Structure No vertebrate is so unique in structure as the turtle. No turtle could be mistaken for any other animal. The arched or dome- shaped dorsal covering, the carapace, and the plane floor of this armor, the plastron, set the turtles apart from all living vertebrates. The head and neck can be completely withdrawn into the shelter of this covering; and legs, feet, and tail can be partially protected in this manner. The body of a turtle may be divided into the following regions: head, neck, trunk, and appendages. The head is covered with thin, smooth skin. Various colored lines and patterns -may be present on the skin of the head and neck. Troost's turtle has an oblong red patch on each side of the head and neck unless it has been completely obliterated by black pigment de- posited over it. The neck is relatively long and slender, and re- tractile within the shell by vertical sigmoid flexures. The skeletal elements of the trunk region are completely co-ossified into an im- mobile boxlike structure. This carapace and plastron are covered by scutes of epidermal tissue. These scutes are definite in number and arrangement. Various color patterns are found on or under them. The legs are typical pentadactyl limbs. The toes are united by various degrees of webbing in aquatic forms. The tail is relatively small and short. Digestive System The organs of the digestive tract are mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, cloaca, and anus. Various accessory structures and glands are the cutting edges of the beaklike jaws, the liver and the pancreas. The size and relative length of the various parts of the alimentary canal are correlated with the food habits of the animal. Turtles eating largely vegetable food have intestines many times the length of the body. Those taking REPTILIA 579 chiefly animal matter possess intestines only a little longer than the body. The accessory glands, liver and pancreas, function as in other vertebrates. External jugular vein„ Brachial vein Scapular vein Subclavian vein Thyreo-5capularvein. Right prccavfll vein. Gall bladder- Poslcaval vein SiniiS veno3us Right auricle Pancreas Postcaval vein Kidnej/ Small intestine.. .Trachea .Cervical artery Esophagus ..Lung Axillaryiartery .Left aorta .Thyroid gland ..Subclavian artery .Brachiocephalic apter_y .Pulmonary artery .Left lobe of liver .Ventricle .Stomach .Anterior abdominal vein _Sapepiop mesenteric artery _ Ovary .Dorsal aorta .Oviduct .Renal portal vein _Lar^e intestine .Openini^ of oviduct .Cloaca -tJrinary bladder _Anu5 Fig. 303. — Dissection to show internal organs of tlie turtle, Pseudemys troostii elegans, ventral view. Respiratory System The respiratory system is adapted for breathing air only, re- gardless of the fact that turtles spend the greater part of their lives in water. The organs composing the system are the nostrils, pharynx, trachea, bronchi, and lungs. An indispensable accessory structure is the htjoid apparatus. Since the ribs are made fast in the carapace, breathing cannot be accomplished in the usual vertebrate manner; hence, the hyoid apparatus, located in the gular region, functions as a pump and the turtle seems to swallow air. The lung 580 TEXTBOOK OF ZOOLOGY space is relatively great in some turtles. This enables them to remain submerged for considerable periods. Accessory urinary bladders also serve in aquatic respiration. Marq'inals Scapula Humerus Radius — Entoplasbron Epiplastron Tibia ---rJi Fibula - ■ )(\pHp\aslrov\ Suprapygal Fig. 304. — Skeleton of the turtle, Pseudemys troostii elegans, showing ventral view of carapace and dorsal view of plastron. Circulatory System The circulatory system of the turtle consists of the heart, arteries, connecting capillaries, and veins. The heart is typically reptilian in structure, having two auricles and one ventricle with a perforated partition dividing it. The pul- monary circulation returns blood to the left auricle, but this aerated REPTILIA 581 blood is partially mixed with venous blood in the ventricle before being sent into the aortic arches. Urlnog'enital System The excretory system consists of two kidneys, their ureters, the cloaca, and a bilobed urinary bladder. The urine is voided into the cloaca and stored in the urinary bladder until expelled through the anus. The sexes are separate. The male reproductive system is made up of a pair of testes, a pair of vasa deferentia, and an evertible penis located on the anterior wall of the cloaca. The female genital organs are ovaries, oviducts, and cloaca. The Nervous System This system is typically reptilian (see page 573). In some turtles adaptations in structure of nervous tracts to the eye are known to have occurred, probably in response to feeding habits. That part of the peripheral nervous system which normally innervates the musculature of the costal region is absent. The Skeleton The skeleton of turtles is the most characteristic of their systems Embryological studies have shown that in the early stages the skeleton follows the typical vertebrate plan. Soon, however, the ribs rise above the limb girdles; those above the pectoral girdle bend anteriorly and those over the pelvic girdle bend posteriorly. The process of co-ossification goes on over a period of years and is not altogether completed until maturity is reached. The result is a vertebrate animal with the limb girdles enclosed by the ribs. No other vertebrate follows such a plan. The Muscular System This system shows modifications corresponding to those in the skeleton. The intercostal muscles of the body wall are not present as such. The muscles of the neck are highly developed and adapted to moving the head with great rapidity. The muscles of the legs are little modified, being similar to those of other vertebrates. CHAPTER XXX AVES Class Aves (A' vez), which includes all birds, has several distinc- tive characteristics. The fundamental structural features of this class are quite closely related to those of reptiles. The outstanding characteristics of birds are as summarized in the following state- ments. (1) The skin is covered with feathers which are exoskeletal outgrowths of it. Birds are the only animals with feathers. (2) The jaws are toothless in the adult and are covered with a hard horny beak. (3) All modern birds are bipedal, the forelimbs being modi- fied into wings, or undeveloped. (4) The pelvic girdle is securely anchored to the vertebral column to support the bird adequately on two legs. These limbs serve the animal in locomotion on land and in water as well as for perching and climbing. (5) The caudal ver- tebrae are greatly reduced in number and all except the free an- terior ones are fused into a single bone, the pygostyle. (6) In most birds the digestive system is modified to provide a crop for storage of food, and a muscular portion of the stomach for chewing food. (7) They are warm-blooded vertebrates. Most people associate birds with flight, but there are numerous birds which do not have this power. In fact, several have practically no wings. The wings of the New Zealand kiwi are so rudimentary that they are completely concealed by the body feathers. Many birds are migratory, moving north for the summer season and south for the winter. This is a very aristocratic habit around which the living activities are centered. The migration routes are precisely laid out ; however, birds occupying the same general breed- ing range do not necessarily spend the winter in the same region or vice versa, and they do not necessarily travel the same route in the fall as was traveled in the spring. The birds return to the same breeding range and winter home, season after season, which seems to demonstrate a definite homing sense. An example of extreme migra- tion is the Arctic tern which nests within the Arctic circle and win- ters within the Antarctic circle, a distance of 10,000 or 11,000 miles. 582 AVES 583 This is traversed fall and spring and largely over water. The aver- age migrating bird travels only about 23 miles a day on migration, but this tern makes at least 150 miles a day. Recently the U. S. Army Signal Corps reported that one of its messenger pigeons had flown 600 Fig. 305. — Migration airways of birds in North America. (From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.) miles in less than 14 hours (42 miles per hour) without the aid of a tail wind. Several theories attempting to explain bird migration in- volve such factors as food conditions, temperature, changing length of days, and hormone control. The explanation is still an open question. 584 TEXTBOOK OF ZOOLOGY Classification Birds constitute a relatively uniform group of animals. They are much more similar throughout the class than any other group to be studied. The differences between the most dissimilar birds are no greater than the differences seen among frogs in the order Salientia, in class Amphibia. Such characteristics as size, color, wing develop- ment, type of beak and feet, are the bases for distinguishing the Pig. 306. — African ostrich, Struthio camelus. (From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.) orders of birds. There are four orders of flightless birds which are sometimes placed in a group designated as subclass Ratitae. This group includes such birds as ostriches, rlieas, cassowaries, and kiwis. The sternum has no keel in any of these, the feathers are without barbules, and the wings are either absent or reduced. All other birds would be grouped in subclass Carinatae according to this plan. More than 14,000 species of birds are classified into twenty-five orders. AVES 585 A brief summary of the orders of birds will be given in the follow- ing pages. Some of the orders are divided into numerous families. Struthioniformes (Ostriches). — These are the largest known birds and they are native to Africa, but they are growing in domestication in numerous parts of the United States. The large eggs weighing three or four pounds are laid in communal nests in the sand; they are then incubated by the sun. The wings are very rudimentary, and there are only two toes. Fig. 307. — Kiwi, or apteryx, a wingless bird from New Zealand which is about the size of the domestic fowl and loolts lilce an overgrown chick. (From Krecker, General Zoology, published by Henry Holt and Company, Inc., after Evans.) Rheiformes (Rheas). — This is another form of running bird. They are partially feathered on the neck, have three toes, and inhabit South America. Casuariiformes (Emus and Cassowaries). — These are flightless, running birds with very small wings. The former inhabits Australia and the latter New Guinea. The cassowaries, which are smaller than ostriches and have shorter necks, have a headgear with bright colors on the head and neck. Apterygifornies (Kiwis). — This is the fourth order of running, flightless birds. The feathers of this form are hairlike. The kiwi is about the size of a hen, but its wings are much reduced. It is noc- turnal, and it makes a nest in a hole in the gromid. 586 TEXTBOOK OF ZOOLOGY Crypturiformes (Tinamous). — This is a group of little known quaillike birds of northern South America, Central America and southern Mexico. Sphenisciformes (Penguins). — These are flightless, diving birds. The feathers are almost scalelike, and the wings are modified as "flippers" to be used under water. They dive for fish which can be swallowed under water. They live on barren rocky shores of the Antarctic. Here they nest in colonies, each female laying one or two eggs in depressions among the rocks. Fig. 308. — Penguins. The wings and feet are highly adapted to swimming. (Prom Hegner, College Zoology, published by The Macmillan Company.) Gaviiformes (Loons). — Our common loon is checked with black and white over the back. These birds are also expert divers and swim- mers under water. They have a very peculiar call that sounds like a weird or crazy laugh. Their nesting range is between northern United States and the Arctic circle. The wintering ground is prin- cipally the Gulf Coastal States. Colymh (formes (Grebes). — This is a group of small or medium- sized diving birds with lobed feet. The most common one is the pied- billed gTebe or hell-diver. The legs are far posterior on the body. There are about twenty-five species generally distributed over the world. AVES 587 Procellariifonnes (Albatrosses and Petrels). — The former are birds of the open sea with a wing spread of ten or twelve feet. They will follow ships for days without landing by gliding on nearly motion- less wings. They come to land usually on islands in colonies, to lay their eggs. Laysan Island, far out in the Pacific, is noted for them. The petrels are small and may be found in midocean. They nest in crevices on rocky shores and islands. The members of this order have tubular external nostrils, fully webbed toes, and long narrow wings. Fig. 309. — American bittern, Botaurus lentiginosus, stake driver. Sometimes called shikepoke in common parlance. (From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.) Pelecaniformes (Totipalmate swimmers). — The cormorants are the widely distributed, well-known representatives of the order. They have long necks, long, hooked bills, and webbed feet. Their habits are gregarious, and they dive for fish which can be eaten under water. The oriental peoples take advantage of this ability to catch fish, and train them to retrieve for the master. 588 TEXTBOOK OF ZOOLOGY Ciconiiformes (Long-legged Waders). — This order includes two prominent families : Ardeidae, including bitterns, herons, egrets, and storks, and Phoenicopteridae, including flamingos; of the latter, one species, Phoenicopterus ruber, inhabits the Gulf States. It is a tall bird with long, rosy colored legs and a long, curved neck. The bill is large and peculiarly shaped, with a curved matching of upper and lower parts. It is used for seining food in the form of small animals out of the mud. The tall, conical nests are built of mud on mud flats. Two eggs are laid in the hollowed top. The birds of the first ESIHK1). NCSTl-NG USS THA,V OS! VIIU 1\S AOULT Fig. 317. — Facts concerning English sparrow. (Courtesy Conrad Slide and Pro- jection Company.) Economic Relations Birds constitute one of our very valuable groups of animals be- cause of their several services to man. At the same time there are some forms which have only a negative importance. The meat of several birds has long been used as food for man. The chief groups among the wild birds that are used as food are ducks, geese, quail, doves, grouse, pheasants, and turkeys. Many other birds are abundant enough and palatable enough, but they are too small. The sparrow is such an example. There are others that are abun- dant and easy to secure, but are not particularly palatable; this is AVES 597 true of the flesh-eating, fish-eating:, and particularly carrion-eating forms, such as vultures, eagles, and hawks. During the last few years the common crow has been coming into use as an article of food. Since it lives on an omnivorous diet largely of plant origin, it has a palatable flesh. It certainly is abundant, and it is great sport hunting it. Some restaurants serve it occasionally as a regular part of the menu. The wild ducks and geese have been hunted so extensively that they are no longer a very important source of food ; too, they are necessarily strictly protected at present. The chief domestic birds that are used for food are chickens (domestic fowls), ducks, geese, guinea fowl, pigeons, and peacocks. The domestic chicken is the chief one of these. It likely descended originally from the jungle fowl of India and the East Indian Islands. The original birds were used all through that part of the world as gamecocks. The present game chickens are nearest the jungle fowl of any existing breeds. From this beginning have arisen dozens of valuable breeds of domestic chickens. In thickly populated areas poultry raising is on the increase, not only because the hen is an efficient apparatus for converting grain and garbage into meat and eggs, but also because a large number of chickens may be kept in a relatively small area. Most of the commercial egg production comes from the chicken, which produces more than 2,600,000,000 dozen eggs a year in the United States alone. The turkey is a domesticated form of the native wild turkey of North America. It is a very popular and profitable meat-producing bird. Millions are produced for the markets in the southwestern part of the United States each year. The guinea fowl is the de- scendent of a small turkeylike bird of Africa. It has dark flesh which is prized by some people. The pigeon is used as food prin- cipally while it is young. The young pigeon is known as a "squab," and it can be raised in close quarters, such as back yards, in cities. Besides the production of meat and eggs there are some other im- portant products and services which come from birds. Feathers are used both for ornaments and for pillows. Guano, which is a mixture of the excreta, soil, feathers, unhatched eggs, and dead birds, is rich in nitrogen and is one of the best sources for nitrogen •fertilizer. Hundreds of thousands of tons of it are mined each year and sold. It is estimated that in the areas of the bird roosts the 598 TEXTBOOK OF ZOOLOGY guano accumulates at the rate of about four inches a year or 750 tons per acre per year. Large numbers of different kinds of birds render service of almost inestimable value in destroying insects, rodent pests, and weed seed. The hawks and owls are very im- portant as rat and mice killers. Finches, sparrows, and quails are destroyers of weed seed. Woodpeckers, chickadees, creepers, wrens, bluebirds, robins, thrushes, flycatchers, kingbirds, meadow larks, swallows, warblers, tanagers, vireos, and many others are very im- portant insect feeders. This asset, if computed financially, would run into millions of dollars a year for the country. At the same time the value of the birds as game for sportsmen to hunt cannot be measured wholly in dollars and cents. DOMESTIC CHICKEN Gallus domestica Cbankiva), the domestic fowl, is a convenient ani- mal for study because of its size, its availability, and its universal distribution with man. As previously stated, it is thought to have arisen from the Indian jungle fowl. It is of great interest, not only because of its immense economic value, but because it shows many of the adaptations of this form of animal to its ij^e of life. Although the chicken has partially lost its power to fly, it still retains the features which adapt birds to a life in the air, such as feathers, wings, air sacs, hollow bones, and a rigid skeleton. The principal modification of the chicken from its wild ancestors is the relative increase in weight to give the body a stocky build, failure to de- velop and exercise the short wings, and a great increase in egg production in most breeds. Habits and Behavior This is a diurnal perching bird which spends the time between dusk and daylight sleeping in a squatting position on a perch. During the daylight hours it is an extremely busy animal at hunt- ing food, dusting feathers, running, walking, scratching, and for the females, egg-laying. The flocking habit is fairly well developed, and usually one cock establishes himself as master of a certain flock and all other cocks are more or less subservient to him. They are entirely polygamous and the mating is promiscuous. AVES 599 External Structure The body of the gamecock, the Leghorn, and the Bantam is quite spindle-shaped and graceful, while that of the Plymouth Rock or Rhode Island Red is stocky and almost awkward. The body is divided into three general divisions: head, neck, and trunk. The t-VAtsJE -SHAFT -QUI LU SHAFX BAR B Atsl-TERIOR barbui-e: - posterior BARBUU E --HOOK1_ETS Fig. 318. — A, flight feather showing general features ; B, details of a portion of a feather. head is prolonged at the mouth region by the pointed heak, whose horny covering is derived from the skin and is true horny material. At the base of the beak there is a small fleshy structure which has a naked, waxy appearance. This structure is known as the cere. In the birds of flight, this is larger and serves as a shield to keep the air from blowing directly into the slitlike nostrils and interfering with breathing. One relatively large eye is located on either side m 600 TEXTBOOK OF ZOOLOGY of the head. From the location, the vision is necessarily monofocal. The eyeball is slightly movable and is provided with an upper lid, lower lid, and a semitransparent nictitating membrane which ex- tends across the eye from the inner corner. The ear is lacking in external conchal cartilage, but appears below and behind the eye on each side as the opening to the external auditory meatus. This is usually guarded by a few stiff short feathers. On top of the head is the comh and below the angles of the jaws are the wattles. These are highly vascularized, fleshy modifications of the skin in these regions, and are frequently different in the two sexes. The mobile neck is quite long and joins the dorsoanterior point of the trunk. The trunk is more or less basket-shaped, with the pair of wings folded at the sides. The skin stretching from the body to the wing is the alar membrane. The body stands almost erect on the strong legs. The distal part of the hindlimbs are devoid of feathers but are covered with scales. The toes are provided with nails or claws which originate in the epidermis. The dorsal surface consists of a horny plate which is set in a matrix. The forelimbs and girdle are modified for flight, while the hindlimbs and girdle are modified for bipedal locomotion. The skin which covers the chicken is very thin and does not contain sweat glands, neither does it have a gen- eral distribution of oil glands. The oil is supplied by a tail gland or uropygial gland. The bird squeezes a quantity of oil from this gland into its beak and passes the beak over all feathers one at a time until they are all preened. This treatment renders the feathers practically impervious to water. The feathers are skin developments arising in dermal papillae which are well set in the skin. There are three types of feathers : contour (flight or quill) feathers with a stiff shaft and firm texture; down (plume) feathers with a soft shaft and a free fluffy arrangement of the barbs ; and filoplume feathers with hairlike structure consisting of a slender shaft and a few or more branches. A typical feather is composed of a quill which sets in the feather follicle of the papilla of the skin and continues up into the feather as the shaft. The shaft bears slender barbs which extend obliquely from it to make up the principal surface of the feather. The barbs in turn bear still smaller and more slender projections known as barbules, which hook the barbs together. The barbs in normal position make up the vane, and give the feather a smooth surface. At the end of the quill there is a small AVES 601 opening called the inferior wnhilicus and at the point where the quill emerges from the skin there is another small opening, the superior umbilicus. In many feathers the hyporachis or after-feather arises from this point. The shaft is often called the rachis. The feathers in many parts of the body are developed in rows with long intervals be- tween the rows in which there are no papillae, but these areas are simply covered over by feathers growing in the adjacent areas. The definite feather areas over the body are called pterylae while the inter- vening featherless areas are apteria (apterylae). CEPHALIC PTEIRYLA CERVI CAl_ APTERiUM VE rslTRAU PTERYLA HUME RA1_ PXERYLA SPINAL. PT ALAR PT. L-ATERAl- AP TE RIUM' c'.-vjr'W -J-/ VEN-TRAL. APT. VirtiJ-J.iWT' — FE MORAL. CAUDAl. P CRURAl- P Fig. 319. — Diagram showing feather tracts, or pterylae, and featherless areas, or apterylae (apteria) of a rooster. (Drawn by Titus Evans.) There is a complete but gradual shedding of the feathers or molt in the fall and a partial one in the spring. Of course, new feathers replace the old as fast as they are shed. There may be some color change of the plumage accompanying the molt. Digestive System The alimentary canal consists of the principal organs in order: mouth, pharynx, anterior esophagus, crop, posterior esophagus, pro- ventriculus, gizzard (ventriculus), intestine, large intestine, cloaca, anus. The several accessory organs are beak, tongue, salivary glands, liver, and pancreas. The proventriculus and gizzard constitute the 602 TEXTBOOK OF ZOOLOGY stomach. The chicken feeds largely on vegetal food, particularly grain and plant seeds. This food is ground in the gizzard by the muscular activity of the wall crushing it against the sand and gravel which have been swallowed and lodged there. The walls of the E SOPHAGUS T RACHEA 1_EFT UUNG HEART GALL BLADDER PROVENTRICULUS Gl ZZARD DUODE NUM PANCREAS UARGE IMTESTINE VAS DEFERENS ■URETER C L O A C A Fig. 320. — Visceral organs of male chicken, ventrolateral view. (Drawn by Titus Evans.) proven triculus are glandular and secrete gastric juice which softens the food and begins digestion of the protein. The cro'p, which is a modification of the esophagus, is used as storage space for food as it is ingested. In the pigeon the crop is glandular and secretes "pigeon AVES 603 milk" on which the young are nourished. The small intestine is about 60 inches long and is made up of duodenum first, beyond the stomach, and then at the end of the first turn, the ileum begins. It is coiled considerably and leads to the large intestine. The bile ducts coming from the gall bladder and the liver enter the small intestine about fourteen inches below the stomach. The pancreas, which lies beside the duodenum, also pours its contents into the small intestine. Two blind sacs, each about seven and one-half inches in length, and called caeca, extend forward from their point of origin at the juncture of the small and large intestine. They are usually partly filled with a soft, pasty material. The rectum of the large intestine opens into the saccular cloaca. One portion of this space receives the fecal material from the intestine and another portion receives products from the urinogenital organs. The cloaca opens to the outside by the anus. The ileum and caeca serve as the principal organs in which absorption of digested food by the blood occurs. The digestive functions of the bird are very potent and rapid. This seems to be a necessary compensation for the waste caused by their extensive and energetic motions, and their high state of irritability. Respiratory System The chicken and other birds breathe through the nostrils, nasal chambers, pharynx, superior larynx, trachea, inferior larynx (syrinx), bronchi, bronchial tubes, lungs, and air sacs. Air is brought into the nasal chambers through the slitlike nostrils in the upper jaw. Within the nasal chamber the air is warmed by passing over three scroll-like folds which are the turhinated lamina supported by the turlinated hones. Next it passes to the pharynx through a narrow slit in the hard palate. There is a row of filiform papillae or fingerlike projections marking the junction of mouth and pharynx, and another transverse row of horny ones at the juncture of the roof of the larynx and esophageal margin. There is no epi- glottis but simply the slitlike glottis through the anterior wall of the larynx. This is provided with two lips which can be brought tightly together so that nothing can fall through into the larynx when food is being swallowed. From the boxlike superior larynx the air passes through the tubular trachea to the inferior larynx or syrinx where it bifurcates. The walls of the trachea are supported by cartilaginous 604 TEXTBOOK OF ZOOLOGY rings which in different kinds of birds will vary from 90 to 120, depending on the length of the neck. This inferior larynx is the true voice box in the bird because it is here that the vocal apparatus is located. The two semi-lunar internal tympanic memhranes, one as a fold on each side of the cavity, are caused to vibrate by the air in sound production. In song birds there is a membrane with a second glottis across the lower end of the trachea. The bony bar supporting it is called the pessulus. The air goes next into two hronchi which enter the tissue of the lungs. Within the lung each primary bronchus divides into secondary bronchi and beyond this each of these gives off smaller tertiary bron- chial branches. The further branches are considered a part of the lung. The lungs occupy only about one-seventh of the thoracic space. Bronchioles which are small branches of the tubules carry the air into the air spaces or alveoli of the lungs where most of the respiratory exchange of gases is made with the blood. Birds have, some bladder- like extensions of the bronchial tubes in the form of air sacs which increase the respiratory surface as well as making the body more buoyant in the air or on water. These sacs are arranged in the neck, thorax, and abdomen, and extend into the cavities of bones. The principal ones are : a single anterior thoracic, a pair of cervical sacs along the neck, a pair of posterior diaphragmatic sacs behind the diaphragm, a pair of anterior diaphragmatic sacs, and a pair of abdominal sacs. Circulatory System This system, as a whole, includes the blood vascular system and the lymphatic system. The first consists of heart, arteries, capil- laries, and veins, making a closed system through the body, while the latter is composed of spaces, vessels, and capillaries which empty into the blood vascular veins near the heart. The lymphatic system is somewhat of an open system. The heart of the chicken and most other birds is relatively large and is located near the median line of the thoracic cavity. The double-walled membranous sac, the pericardium, surrounds and holds the heart in place. There are two distinct, thin-walled auricles, and two distinct muscular ventricles. Blood is drawn into the right auricle from the systemic veins ; right and left precava, and the single postcava. This blood needs aeration and passes through the AVES 605 right auriculoventricitlar valve to the rigJit ventricle. Then it is pumped, with the contraction of the heart, through a semilunar valve into the pulmonary artery which divides to supply each lung. The aerated blood from the lungs returns to the left auricle by way of the two pulmonary veins. It passes from the left auricle to the left ventricle through the hicuspid or left auriculoventricular valve. Then ^u. L. CAROTID R. PECTORAL R. BRACHIAL R.BRACHIO- CEPHALIC R. ATRIUM L. BRACHIO- CEPHALI C ^PULMONARY LaoRTIC ARCH L. ATRIUM /-R. V ENTRICLE ENTRICLE COELI AC LUMBAR RENAL ANTERIOR MESENTERIC DORSAL AORTA FEMORAL SCI ATIC I LIAC -POSTER! OR MESENTERIC CAUDAL Fig. 321. — Heart and arteries of domestic chicken from ventral view. (Drawn by Titus Evans.) it is pumped through the semilunar valve in the base of the aorta and out the right aortic arch. Just at the anterior level of the heart this arch passes through the pericardium and gives off the right and left brachiocephalic arteries to the neck, head, and arms ; then it curves to the right and extends posteriorly around the heart as the dorsal aorta. The dorsal aorta supplies esophageal branches to the 606 TEXTBOOK OF ZOOLOGY esophagus, intercostal arteries to the ribs, lunil)ar branches to the small of the back, ovarian or spermatic arteries to the gonads, the coeliac axis to the crop, gizzard, duodenum, spleen, and liver; the anterior mesenteric to the small intestine ; posterior mesenteric to the large intestine; external iliac or crural arteries branching to the pelvis and thigh ; and, the ischiadic continues one to each posterior limb where it branches to all parts. r. jugular r.brachial r. subclavian ventricle Ctipped anteriorly) r, pre cava l.precava r. pectoral ■^R. ATRIUM L. ATRIUM PULMONARY HEPATIC POSTCAVA HEPATIC PORTAL GASTRO- DUODENAL ANTERIOR MESENTERIC COMMON ILIAC POSTERIOR - MESENTERIC FEMORAL RENAL COCCYGEO- MESENTERIC INTERNAL ILIAC CAUDAL Fig. 322. — Veins of cliicken from ventral view. (Drawn by Titus Evans.) Excretory System The urinary apparatus of the bird consists of two metanephric kidneys, from each of which a ureter extends and empties into the cloaca. The kidneys are about 2i/4 inches long, made up of three irregular lobes, and located above the peritoneum in the lumbosacral region. Each is made up of Malpighian bodies, uriniferous tubules. AVES 607 and the surrounding tissue. Urine passes from the collecting tubules in the kidneys to the pelvis of each of the ureters, and through the ureters into the cloaca. From here it passes to the exterior through the anus with the feces. There is no urinary bladder. R. SUBCLAVIAN ARTERY R. CAROTID A. AORTIC ARCH BRACHIO-CEPHALIC A. BRACHIO-CEPHALIC A. LEFT ATRIUM PULMONARY ARTERIES ENTRANCE OF L. SUBCLAVIAN V. ENTR. OF PULMONARIES SEMI-LUNAR VALVES L.ATRIO- VENTRICULAR VALVE RIGHT ATR.-VENTR. VALVE ENTR. POSTCAVAL VEIN RIGHT PRECAVAL VEIN RIGHT ATRIUM INTER-VENTRICULAR WALL RIGHT VENTRICLE LEFT VENTRICLE Fis:. 323. — Horizontal section of heart of chicken to show chambers and valves from ventral view. (Drawn by Titus Evans.) Nervous System The bird has a relatively well-developed brain and nervous sys- tem. In an average chicken the brain weighs about 10 grams. The cerebrum, the anterior division of the brain, shows fair develop- ment but no convolutions, while the olfactory lobes which are acces- sory to it are very small. The optic lobes of the midbrain are well developed and the cerebellum is exceptionallj' developed. The cere- bellum consists principally of a median lobe called the vermis or worm. The ventral part of this division of the brain forms the roof of the fourth ventricle of the medulla oblongata. The sense organs of sight and hearing show improvement when compared with the reptiles. The eye is large, rounded laterally, and rather flattened anteroposteriorly. It has the usual layers of outer sclerotic coat continuous with the coriiea in front, the pigmental 608 TEXTBOOK OF ZOOLOGY chorioid just beneath, and ttie membranous retina lining the inside of the posterior portion. The crystalline lens is of a soft texture, is nearly round, and is enclosed in a capsule. The cornea is of a horny consistency and is transparent. Light thus passes rapidly through it to the posterior part of the eye. There is a comb-shaped membrane stretching from the entrance of the optic nerve at the posterior wall of the eye to the posterior surface of the lens. This structure is called the pecten, and it is supposed that it has something to do either with the nutrition of the eye or with the keen accommodation possessed by the bird. The chicken has a lacrimal duct and sheds tears. - — y^ OLFACTORY NERVES OLFACTORY LOBES CEREBRAL LOBES LEFT OPTIC LOBE PINEAL BODY CEREBELLUM TRIGEMINAL N. -FACIAL N. -AUDI TORY N. -MEDULLA OBLONGATA Fig. 324. — Dorsal view of brain of chicken. (Drawn by Titus Evans.) The organ of hearing is embedded in the skull and consists of the external meatus, tympanic membrane, one ossicle membrane, and the columella which is attached to the tympanic membrane by a cartilage. The inner ear is enclosed in the bony labyrinth and contains the vestibule, three semicircular canals, and the membranous cochlea. The cochlea has arisen from a pocketlike lagena on the vestibule, and is much better perfected in birds than in simpler vertebrates. The Eustachian tube provides a connection between the tympanic cavity AVES 609 and the 'pharynx. This provision allows for equalization of air pres- sure on the two sides of the tjTupanic membrane. The sense of smell and the olfactory apparatus are poorly developed in the chicken. The sense of iaste is centered in the taste buds located on the surface of the tongue and on the dorsal palate. This sense is only fairly developed. Touch is distributed over the skin and in certain feathers. The edge and point of the beak are sen- sitive to touch. Herbst's touch corpuscles are found in all parts of the skin. Skeletal System The skeleton of the bird is remarkable for its rigidity with light weight. Even in the heavy, nonflying chicken, this is noticeable. The strongly keeled sternum serves as the attachment for the strong pectoral muscles which are so important in the flight of most birds. There are short bracing bones between the ribs, called uncinate processes. Many of the bones, even in a chicken, have air cavities in them. These are better developed in the better fliers. Teeth are entirely lacking in the adult. The skull is quite well developed and has a relatively large cranial cavity. The orbit is also large. The cranium consists of three single bones, the occipital, the ethmoid, and the sphenoid. There are also three pairs including the frontals, the parietal, and the temporal. The free quadrate connects the lower jaw with the cranium. The long flexible neck is composed of sixteen cervical vertebrae, the anterior two of which are named the atlas and the epistrophei's (axis). The articulation of the vertebrae is such that the head is well adapted for use in feeding, fighting, and nest building in some birds. The centra of the vertebrae are heterocoelous or saddle-shaped. Following the cervical vertebrae there are five thoracic, thirteen lumbosacral, and seven coccygeal in order. The total number is forty-one. The cervical and coccygeal alone have free movement. The pygostyle at the caudal end supports large tail feathers, which are movable. There are seven pairs of ribs, two of them articulating with cervical vertebrae and five pairs related to thoracic vertebrae. The thoracic ribs join the sternum by sternal ribs. The cervical ribs are floating. The sternum joins the pectoral girdle by a union in front with the coracoid. The coracoid articulates dorsally with the scapula, another girdle bone, and with the humerus of the wing. Besides the scapula or shoulder blade the two clavicles join the upper 610 TEXTBOOK OF ZOOLOGY PRE tv1AXH_l_A -IM ASA.l_ LACRIMAL MAXILLA FRONTAL PARI ETAL OCCI PI XALS ATLAS AXIS QUADRATO JUGAL J MANDIBLE CERVICAL VERTEBRA , HUMERUS RADIUS ULNA I .-.-CARPO- I ] METACARPUS I r-PHALANlGES^' THORACIC VERTEBRAE / SCAPULA / Rl BS ,' SVrslSACRUM PYGOSTYLE-, Fig. 325. — L.ateral view of skeleton of chiiclien. (Drawn by Titus Evans.) AVES 611 ends of the coracoids and fuse with each other ventrally to form the "wishbone" or "collar bone." The wing is a greatly modified front limb, but strictly homologous to the arm. There is a humerus in the upper arm whose head fits in the glenoid cavity. The radius and ulna support the forearm. The carpus or wrist is composed of two bones, scapholunar and cuneiform, followed by two metacarpals. The first digit or pollex has only one phalanx or joint, the second has two phalanges, and the third is located at the end of the metacarpal bone. SARTORIUS GLUTEUS PRIMUS SEMITENDINOSUS GASTROCNEMIUS FLEXOR PERFORATUS INDICIS SECUNDUS PEDIS FLEXOR PERFORATUS MEDIUS SECUNDUS PERONEUS LONGUS Fig. 326. — Lateral view of tlie muscles of the hind leg of the chicken. (Drawn by Titus Evans.) The pelvic girdle is fused quite rigidly to the sacrum. The ilium, ischium, and pulis of each side are closely fused with each other. The pelvic limb fits into the rounded concavity on each side of the girdle. This socket is called the acetabulum. The hindlimb supports the body in nearly an upright position when the bird is walking. The 612 TEXTBOOK OF ZOOLOGY femur is the bone of the thigh which fits close to the body and is covered by the feathers. The section below the knee (shank) is com- posed of the long, larger tibiotarsus and the slender fibula which is more or less free at the proximal end but is bound to the tibiotarsus along its side. The next portion is a single bone, the tarsometatarsus, which has resulted from a fusion of several. Distal to it are attached the four digits, each composed of phalanges. The first toe is directed backward and the other three forward, as an adaptation for perching. In the male chicken there is a horny spur projecting backward from the tarsometatarsus. BICEPS DELTOID EXTENSOR METACARPI RADIALIS LONGUS PRONATOR BREVIS EXTENSOR INDICIS LONGUS EXTENSOR OSSIS METACARPI POLLICIS FLEXOR DIGITORUM FLEXOR CARPI ULNARIS EXTENSOR PROPRIUS POLLICIS FLEXOR BREVIS POLLICIS INTEROSSEOUS PALMARIS Fig. 327. — Ventral view of the muscles of the wing of a chicken. (Drawn by Titus Evans.) Muscular System In the chicken there are 162 voluntary, striated muscles, single or in pairs. These muscles are named in some cases from their loca- tion, others from their attachments, some for their shape and form, others from their use or function, and still others from their direc- tion in the body. They are covered and bound together by the white fibrous connective tissue sheath, called fascia. The muscles of the chest and forelimb are quite well developed as an adaptation to flight. The muscles of the thigh and shank are AVES 613 well developed for bipedal locomotion and perching. The muscles of the long flexible neck are quite intricate. Since this animal does no chewing, the jaw muscles are not strongly developed. The large pectoral muscles which extend from the sternum to the upper arm (Fig. 327) constitute from 15 to 20 per cent of the weight of the body and make up the breast meat of the bird. The muscles of the thigh and shank or ''drumstick" are quite well developed, and the chicken is a strong runner (Fig. 326). Certain muscles and ligaments of the legs are arranged in such a way that the bird is able to cling to the perch due to the down pull of the weight of the body. This makes it possible for the chicken to sleep on the perch without danger of falling off. LEFT TESTIS LEFT VAS DEFERENS CLOACX PAP;'LLA OF VAS D. Fig. 328. — Reproductive organs of male chicken. (Drawn by Titus Evans.) Reproduction and Life History The sexes are distinct but there is no definite pairing for repro- duction; one cock may mate with several hens. The eggs are usually laid in a rounded nest constructed of hay or other soft material. Some birds build no nest at all, Avhile others build very elaborate ones, such as the hanging model of the Baltimore oriole. Copulation is necessary for the transfer of mature spermatozoa from 614 TEXTBOOK OF ZOOLOGY the cloaca of the male to the cloaca of the female. This is accom- plished by the cock covering the back of the hen and bringing the cloacal apertures of the two together. The motile spermatozoa migrate up the uterus and oviduct to meet the mature eggs and unite with them in fertilization. In the cock there is a pair of oval, light-colored testes located near the dorsal body wall anterior to the OVARY FUNNEL OF OV IDUCT GLANDULAR P ORTION SHELL GLAND PO RTl ON CLOACA Fig. 329.— Reproductive organs of female chicken. Notice tlie single left ovary and oviduct. (Drawn by Titus Evans.) kidneys. From each testis the vas deferens, a duct, leads posteriorly to open in the cloaca. Just before joining the cloaca this duct is dilated to form a storage sac, the seminal vesicle. The adult hen has only the left ovary and oviduct, the right having atrophied during embryonic development. The mature ova rupture the wall of the ovary and pass into the oviduct. The AVES 615 glandular wall of the oviduct secretes much of the albumen or ''white" around the egg. In the lower part of the oviduct, the shell membranes are added, and the calcareous shell is deposited in part here and completed in the uterus and vagina. The time re- quired for the egg to pass from the ovary to the exterior at laying averages about twenty-two hours. The average-sized hen's egg weighs about two ounces of which 11 per cent is shell, 32 per cent yolk, and 57 per cent albumen. A dozen or fifteen eggs are usually laid in the same nest by the hen and then incubated with her body temperature by sitting on them almost constantly for twenty-one days. They are kept at a temperature between 98° F. and 100° F., and are turned over each day to avoid internal adhesion of the embryo to shell membranes. At the end of this period they hatch by breaking through the shell with the temporary "egg tooth" on the beak. The chicks are covered with down, have their eyes open, and can run as soon as they are dry. For this reason they are said to be precocial, and since they leave the nest immediately, they belong to a general group called the nidifugae. Another type of bird which requires parental care, such as feeding, is said to be altricial, and since it remains in the nest, it belongs to the group nidicolae. The individual reaches maturity in from six to ten months, depending on the breed. ( CHAPTER XXXI MAMMALIA The class Mammalia (mam'ma'li a, milk-forming) includes many of the animals most familiar to man. Nearly one-third of the total number of them occur in America. Besides our common domestic ani- mals, such familiar forms as rats, mice, ground hogs, bats, foxes, bears, deers, seals, whales, man and many others belong in this group. The skin of them all is provided with sweat glands and sebaceous (oil) glands and is more or less covered with hair. These animals are tj^pieally quadrupeds with five digits on each limb. The females have well-developed mammary glands which secrete milk for the nourishment of the young. There is usually a distinct division of the body into head, neck, trunk, and tail regions. All mammals have a definite temperature regulation and are said to be homoiothermal, or warm-blooded. The body temperatures of different species vary from 77° F. to 104° F. In mammals, as in birds, the heart is com- pletely divided into four principal chambers. The single systemic arch of the aorta is the left one. Kespiration is carried on by lungs and the anterior end of the trachea is modified to form a larynx capable of sound production. In general. Mammalian blood contains nonnucleated, circular red corpuscles ; however, in the camel they are oval in shape. The thoracic cavity of the mammal is separated from the abdominal cavity by a complete diaphragm. Classification The entire class includes approximately 4,000 species of living mammals and 3,500 fossil forms. The class is divided into two sub- classes, several divisions and a number of orders. Subclass Prototheria. — The primitive, egg-laying mammals. This group includes only a single order. Order Monotremata. — The most primitive mammals and the only ones that lay eggs are placed here. They are limited in their dis- tribution to Australia, Tasmania, and New Guinea. The eggs are similar to turtle eggs and are laid either in a pouch on the female 's abdomen, as in the spiny anteater, or in a tunnel in the ground near 616 MAMMALIA 617 water, as in the duckbill. The oviducts in these animals do not unite to form a vagina, but empty directly into the cloaca, which is present in this subclass of mammals only. After hatching, the young are nourished for a time on milk from the mammary glands of the parent. These glands secrete their milk on to the hair of the abdomen and the young either suck or lick it up from here. There are two quite representative animals of this group. One is the duckbill or duck mole, OrnitJiorhynchus anatinus, which is about a foot and a half long; it is covered with hair, has webbed feet, and a peculiar duck bill snout. It feeds on worms which it digs from the mud with its bill. During the daytime it sleeps in its grass-lined tunnel, the entrance of which is under water. The eggs Fig. 330. — The duckbill, Ornithorhynchus anatinus. The "duck mole" of Australia. (From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.) are laid and the young reared in this underground chamber. On the heels of the hind feet of the mole are some spurs which are sup- plied with poison from a gland located in each thigh. A second representative of the order is the spiny anteater, Tachy- glossus aculeatus, which is about one foot long and covered with stiff spines mixed with coarse hair. It has a head and mouth drawn out into a long proboscis with a long, slender tongue which is pro- truded for picking up ants and other insects. This animal lives in a burrow. Subclass Eutheria. — The group includes the true viviparous mam- mals and is divided into two divisions, the marsupials or pouched mammals (Didelphia) and the placental mammals, Monodelphia. 618 TEXTBOOK OP ZOOLOGY photo Pig. 331. — Kangaroo carrying young in the marsupial pouch. MAMMALIA 619 Order Marsupialia. — This group of mammals has no well-developed placenta and is sometimes designated as a separate subclass, called Metatheria. Their shell-less eggs absorb food from the wall of the Icterus. The young are born in a very immature state and make their way to the marsupial pouch where they cling to the teats and are nourished on milk until they can shift for themselves. The group is at its best in Australia and nearby islands. Here is found the true kangaroo with its short, poorly developed forelimbs, powerful hind- limbs and tail, and peculiar upright posture and leaping locomotion. The seven families of the order include not only kangaroos, but also opossums, phalangers, wombats, bandicoots, dasyures, and Caeno- lestes. There are several species in South America. Some of these are no larger than mice or rats and are frequently brought into this country on bunches of bananas. In the United States, the opossum, Didelphis virginiana, is the only representative of the group. It is about the size of the common cat, with a long scaly tail and fur of dirty yellowish white color. Ten or twelve young are born at a time and are carried in the pouch of the mother. The young remain with the mother about two months, often riding as a group on the mother's back during the latter part of this period. The opposum is quite active at night, but it usually sleeps through the day. Order Insectivora. — This group includes the common mole, Scalopus aquaticus, the hairy-tailed mole, Parascalops Ireweri, the star-nosed mole, Condylura cristata, the shrews and short-tailed shrews. They are quite well distributed through North America and Europe, but are absent from Australia and most of South America. The group is largely burrowing and nocturnal in habit. They feed chiefly on insects which they seize with their sharp, projecting incisor teeth. The moles are well adapted to the burrowing habit of life. They have rudimentary eyes, no external ears, short stout forelimbs with strong sharp claws for digging. Their tunnels are just under the surface of the saaidy loam in which they live. They occasionally throw up molehills along the tunnel. The shrews are small, mouselike animals with conical, pointed heads, ratlike feet, small eyes, and external ears. They may live in burrows or on the surface of the ground under logs, rocks, or heavy vegetation. The long-tailed shrew, Sorex personatus, of the North and the East, and the short-tailed shrew, CryptoUs parva, ex- 620 TEXTBOOK OF ZOOLOGY tending into the South and Southwest, are the common forms. Be- cause of their small size and retiring habits, these animals are seldom observed. Order Chiroptera. — Bats are the mammals which have developed the power of flight and are not always distinguished from birds by the layman. The forearm and fingers are extended, and the skin stretches between them as well as to the hind limbs. Most of them are small and nocturnal. They usually have the toes of the hind feet developed for grasping and are able to hang by them, head downward, when at rest. The brown bat, Eptesicus fuscus, is the most common form in the United States. The Mexican free-tailed Fig. 332. — Little brown bat, Myotis lucifugus, in resting position. (From Metcalf, Textbook of Economic Zoology, published by Lea & Febiger.) bat, Tadarida mexicana, extends as far north as central Texas. Carls- bad Caverns near the eastern boundary of New Mexico is famous for its evening bat flight. The bats come out like a cloud of smoke, make a definite flight of about sixty miles along two streams and back to the caverns before daybreak. Here they remain until the next evening. Because of their ability to fly the bats are widely distributed mammals, being found on all of the continents and even on isolated islands. In the East Indies, Australia, Africa, and Asia, there are several different fruit-eating bats. In tropical America there are true and false vampire bats. The true vampires live on the blood of horses, sheep, cattle, and occasionally sleeping human beings. Their MAMMAXiIA 621 It is teeth are well adapted for bringing the blood on the victim, then lapped up from the wound. Order Edentata. — This is a group including the giant anteaters, sloths, and armadillos. The giant anteater is the only one in which the teeth are entirely absent; they are modified in other forms by lack of enamel. The giant anteater, MijrmecopJiaga juhata, reaches a length of six or seven feet. Its long claws are used to open the anthill, and the long prehensile tongue is used for taking up the ants. The sloths live in trees, clinging to the underside of the limbs by the long, clawed feet. The animal is ventral side up. They can even sleep in this position. Their food is principally leaves and buds. They are very slow moving animals and inhabit South and Central America. Fig. 333. — Nine-banded armadillo, Dasypus novemciiictum. The nine-banded armadillo, Dasypus novemcinctum, is the only one of this group inhabiting the United States. It ranges from Argentina to southern New Mexico and northeast to central and east Texas. The head is small, tail elongate, and the body is covered dorsally by bony plates. The ventral parts of the head and body are covered with bristly hair. The nine bands are formed around the trunk by the arrangement of the scutes in that region. These animals dig very rapidly in the ground, and when they are frightened they can roll themselves into a ball as does a pill bug. The normal litter of young is quadruplets produced from a single fertilized ovum (Fig. 334). Order Pholidot a. —This is a group of scaly anteaters found in Africa and eastern Asia. They are well protected by epidermal scales and can roll up like armadillos. They are from one to five feet long. Order Rodentia. — These animals are the gnawers and compose one of the largest orders of mammals. Such forms as pocket gophers, 622 TEXTBOOK OF ZOOLOGY rats, mice, ^-ound squirrels, squirrels, chipmunks, prairie dogs, kan- garoo rats, cotton rats, wood rats, muskrats, woodchucks, porcupines, guinea pigs and beavers are representative. They all have long chisellike incisors but no canine teeth. The prairie dogs {Cynomys ludovicianus) are heavy -bodied, bur- rowing, rodents that live on the plains west of the Mississippi River. Fig. 334. — Identical armadillo quadruplet embryos attached to the placenta. These animals are gregarious, living in "towns" or colonies. The burrows are provided with a mound around the entrance, and they are usually quite deep and fifteen to thirty feet apart. Forty or fifty acres of land may be covered by one town. The ground hog or woodchuck, Marmota monax, is a larger solitary, burrowing animal which hibernates during the winter. It bears about six young in the burrow early in the spring. The ground squirrels are also burrowing animals somewhat similar to the ground hog, but much smaller. They are found from the Mississippi basin MAMMALIA 623 to the West Coast iu shallow burrows in the open fields or low brush. The striped spermophile is a common midwestern form. The chip- munk, Tamias striatus, a small striped squirrel, with a slender non- bushy tail, usually lives on the border of the woods. It feeds :^::\ 0\ %- z^-c '^muo0¥^ ^^'^■■^;^' -;^ ■:-^m. c^^ Fig. 335. — Prairie dog, Gynomvs ludovicianus. Lived in large colonies or "towns" in great abundance over the prairie country at one time. i Fig. 336. — Pocket gopher, Geomys bursar ius. A burrowing rodent with destructive habits. on seeds and nuts which it usually stores for times of need. The squirrels are excellent tree climbers and in fact live in trees most of the time. The red squirrel, Douglass squirrel, gray squirrel, Abert squirrel, fox squirrel, and flying squirrel are the common species. The gray squirrel is more common in the South while the Abert is found in the Rocky Mountain region and the Douglass is the Pacific 624 TEXTBOOK OF ZOOLOGY form of the red squirrel. Most of these squirrels live on nuts, acorns, and seeds primarily. The pocket gopher, of the family Geomyidae, is another burrowing animal which ranges through the South, Southwest, and Midwest. They are provided with strong incisors, large cheek pouches, and large digging claws. They are very able diggers and construct ex- tensive burrows about 12 to 18 inches below the surface. Surplus earth is pushed to the surface in mounds. The cheek pouches are used for gathering tubers, such as potatoes, roots, and seeds, for food. The pocket mice and kangaroo rats are other related forms found in Fig. 337. — Banner-tail kangaroo rat, a common resident of the plains and desert. (Courtesy of Nature Magazine.) the Great Plains region and Southwest. These mice have cheek pouches and long, bush-tipped tails. The kangaroo rats usually live in colonies in sandy land. They build extensive tunnels. These animals are built like small kangaroos with long hind legs and strong tails. The beaver, Castor canadensis, is the largest animal of this order, reaching a length of from three to three and one-half feet. At one time beavers were distributed over most of North America, but now they are reduced to a few in out of the way mountain streams. The beaver has a stout body, strong cutting teeth, webbed hind feet, and a broad, flat naked tail. It is well adapted to aquatic life and builds MAMMALIA 625 dams for its home. The dams are built of trees which are cut with its teeth, floated into position, and chinked with mud. The muskrat. Fiber zibethicus, belongs in a large family with lem- mings, rats, meadow mice, and white-footed mice. It lives in slow streams, ponds, and swamps, and feeds on roots of water plants, fresh-water mussels, dock, corn, and other grain when it is available. It builds houses of stalks, leaves, cattail leaves and mud out in the water of swamps. In ponds it uses a burrow in the bank. The home is lined with cattail down or grass ; here it rears the young and spends the winter. The porcupine, Erethizon dorsatus, is principally a northern ani- mal but is found in the mountains as far south as Virginia and also Fig. 338. — Muskrat, Fiber zibethicus, an important fur bearer. (From Metcalf, Textbook of Economic Zoology, published by Lea and Febiger.) in the mountains of the Southwest in Texas and Arizona. These ani- mals have the hairs of the back modified into spines which normally lie flat, but which can be elevated by muscles when the animal is frightened or angered. The black rat, Battus rattus, the Norway rat, Rattus norvegicus, and the common mouse, Miis musculus, have all been introduced into this country from Europe. The chinchillas, viscachas, and the cavies (guinea pigs) have all been introduced from South America. Order Lagomorpha. — The rabbits and hares constitute a very inter- esting and important group. The jack rabbit is the most common hare of the western plains, mountain region, and Southwest. It is 626 TEXTBOOK OF ZOOLOGY •I Fig. 339, A. — Jack rabbit, Lepus calif oi-nicus. A common prairie siglit. (Courtesy of Nature Magazine.) Fig. 339, B. — Mexican ring-tail cat. (Courtesy of Nature Magazine.) MAMMALIA 627 famous for its long legs, long ears, and speed. All of the hares build nests in heavy grass and bear the young here. The cotton tail rabbits, Sylvikvgus of several species, are generally distributed over the coun- try. They are grizzly gray above and lighter below, with the under- side of the tail cottony white. They dig their own burrows, or borrow burrows from prairie dogs, ground hogs, or badgers. Several litters of blind, helpless young are born in the burrow or in a nest above the ground during the breeding season. In the Gulf States, extending Fig. 340. — Adult golden hamster. (Courtesy Biological Survey House, Chicago.) as far west as central Texas, is a larger semiaquatic relative of the cottontail, the swamp rabbit. It is brownish gray above and is said to have the habit of concealing itself in the water with only the tip of the nose exposed. In the region above the timber line are found the tail- less rock rabbits or pikas. They live among the rocks and look a little like guinea pigs. Order Carnivora. — Most of the mammals of this order are flesh- eating, although a few are omnivorous and one or two are vegetarians. They have the canine teeth well developed and conical premolars as 628 TEXTBOOK OF ZOOLOGY an adaptation to tlieir feeding habits. The suborder Fissipedia in- cludes the terrestrial carnivores with walking feet, while a second suborder Pinnipedia includes the aquatic carnivores with swimming feet. There is a large group of common mammals that belong to the first suborder. The fishers, martens, minks, weasels, ferrets, otters, skunks, and badgers constitute a group of very blood-thirsty killers. The skunks of genus Mephitis are quite common and are well known because of their conspicuous white stripes on black fur background and powerful scent glands. Besides the striped forms there are the smaller, spotted forms. The badger, Taxidea taxus, extends down the plains, and there meets the Mexican variety coming up from the south. It is heavy-set and has short legs with long, sharp, strong claws. It can dig almost as fast as a man with a spade and usually comes out winner in a fight with any dog. It is strictly nocturnal and lives in a burrow. Fig. 341. — The badger, Taxidea taxus. In the family Canidae there are besides the domestic dog, the fox, the coyote, and the wolves. The red fox, Vulpes fulva, is a common form through the North and East, and has been introduced into the Southwest by sportsmen who enjoy fox-hunting. It is a cunning animal and quite difficult to catch or trap. It digs burrows or builds dens in rocky hillsides where the young are raised. The chosen food of the fox includes mice, insects, wild birds, and occasional poultry. Besides the red fox there are the gray fox, the kite fox, and the Arctic fox. The coyote, Cayiis latrans, is somewhat larger than the foxes and is quite crafty. It has managed to keep up its numbers in spite of persistent control measures, such as poisoning, trapping, and hunting. It lives in dens and among the rocks and feeds largely on MAMMALIA 629 rabbits, mice, poultry, game, and small livestock. The wolves are larger than coyotes and frequently hunt in packs. Canis gigas is the timber wolf. In the family Felidae are classified the domestic cat, bobcat (lynx), tiger, leopard, mountain lion, jaguar, and ocelot. Most of the wild representatives of this group have been pushed far back into the more remote parts of the Southwest and the Rocky Mountain area. Felis couguar, the mountain lion (puma, panther, or cougar) ; Felis hernandesii, the jaguar; Felis pardalis, the ocelot; and Lynx rufus, the bobcat or catamount, can all be found in southwestern Texas and New Mexico. The common North American coon or raccoon (Fig. 412) is a plantigrade (walks on entire foot with heel touching the ground) animal about two and one-half feet long and generally dis- tributed east of the Rockies. Procyon lotor is its name, and it is much hunted throughout the South for its fur and its flesh. The bears in- clude the black or brown bear, Euarctos americanus, which is the most widely distributed and most common; the large, grizzly, TJrsus hor- ribilis, of the Rocky Mountains, and the white polar bear, Tlmlarctos maritimus, of the Arctic region. Brown bears and grizzlies are rather omnivorous, feeding on fruit, insects, flesh, honey, and even tourists' lunches. The grizzly bear is more partial to meat than is the brown bear. The polar bear feeds quite largely on fish. There are several carnivores which are considerably modified as an adaptation to a life in the ocean. The body has become fishlike in its shape and specializations. The appendages in particular have be- come swimming organs. Callorhinus alascamis, the fur seal, occurs along the Pacific coast and goes to the Pribilof Islands of the Bering Sea at breeding season. At this time one male or ''bull," depending on his ability to fight other males, will set himself up in charge of from four or five to twenty-five or thirty females. Under present laws, only the unmated males ("bachelors") are allowed to be killed for the furs. Besides the fur seal, there are the California sea lion, Zalophus calif ornianus; the Pacific walrus, Odobenus divergens or 0. obesus; and the Atlantic walrus, Odobenus rosniarus. The walruses have very long canine teeth in the upper jaw and use these for digging mollusks which are used as food. Order Artiodactyla. — This is one of the four orders of hoofed ani- mals and includes those with the even toes. The group includes 630 TEXTBOOK OF ZOOLOGY cattle, camels, llamas, goats, sheep, pigs, alpacas, reindeer, deer, elks, moose, giraffes, bisons, hippopotamuses and gazelles. The deer family, Cervidae, peccary family, Tayassuidae, cattle family, Bovidae, and the pronghorn antelope family, Antilocapridae all have native repre- sentatives in North America. The family Cervidae is the largest in the order. Nearly all males in the deer group have horns which are solid outgrowths of the skull. These are shed each year and a new but larger set grown the next season. The moose, Alces americana is the largest of the group, and it may reach a weight of one thousand pounds. It is found in the mountainous parts of the Northwest. It has large, broadly flattened antlers. The elk, Cervus canadensis, is another large representative which is found only in isolated parts of our western states. In recent years there has been much effort made to conserve the remaining ones, particularly in Wyoming, Montana, and Utah. The antlers of the elk are long, with numerous slender points. The most commonlj^ distributed deer is the white- tailed or Virginia deer, Odocoileus virginianus. It thrives in semi- domestication as well as in the wild. The black-tailed deer, Odo- coileus crooki and the mule deer, Odocoileus hemiomis are both a little larger than the white-tailed. The mule deer is common through the Rocky Mountain states. The black-tailed deer does not range so far north. Both have doubly branching antlers while the white-tailed does not. The caribous, which are native, and the imported European rein- deer are important meat animals of Canada and Alaska. The cari- bou, Rangifer cariltou, is heavy-bodied with stout legs and heavy, irregular antlers in both sexes. The pronghorn antelope, Antilocapra americana (Fig. 410) which once covered the Great Plains and ranged over most of the western states is now limited to a few scat- tered, isolated herds. Much attention is now being given to its resto- ration and fortunately so. It is nearly as large as a small white- tailed deer, has peculiar short, single-branched horns in both sexes, long pointed ears, and a striking white rump patch. It sheds the horns each year. The family Bovidae, of course, includes the domestic cow which is not a native of this continent. The horns of this group are hollow, occur in both sexes, and are permanent. Rocky Mountain sheep, Ovis canadensis, is found in the higher western mountains. The horns of the male are greatly developed and curved. They are the basis for MAMMALIA 631 the common name, Bighorn. It is very sure-footed and can live on mountain ledges. The Rocky Mountain goat, Oreamnos montanus, lives in a similar habitat, but farther north. It has short, dark horns, shaggy wool, and a beard. The musk ox, Ovibos moschatus, is a heavy bodied, hump-shouldered animal with stout, down-curved horns. It lives above the timberline, principally in Canada and Fig. 342. — Bighorn mountain sheep are still to be found in the Rockies. (Courtesy of Nature Magazine.) Alaska. The buffalo. Bison americanus, was extremely abundant all over the Great Plains until the last quarter of the nineteenth century when they were killed out by hunters and crowded out by civilization. They are powerful, heavy-headed, hump-shouldered animals. There are still a few herds, such as the one at Yellowstone National Park, that are kept under semidomesticated conditions. Our domesticated 632 TEXTBOOK OF ZOOLOGY hogs belong in the family Suidae and have come down to us from the European wild boar. In southern Texas and on south to South America is a piglike animal, known as the peccaries, or javelinas, Pecari angulatus. They are gregarious, nocturnal, and feed on roots and nuts primarily. Their heads are large, bodies slender, and tails Fig. 343. — Buffalo cow and calf. A sight which is no longer seen except In special preserves. (Courtesy of Nature Magazine.) short. Hippopotamuses, camels, and giraffes are numerous and impor- tant African members of this order. The llama and alpaca are useful South American forms. Animals, such as camels, cattle, sheep, deer, giraffes, and pronghorn antelopes, chew their cud and are therefore said to be ruminants. MAMMALIA 633 They swallow their food partially chewed and, because of the struc- ture of the stomach, they are able to regurgitate it later for further chewing. For this reason such animals can consume large quantities of bulky food in a short time, then retire to the shade and chew while reclining. The stomach of such an animal is greatly modified by having four divisions. Following the esophagus is the pouchlike rumen at the left, then the small middle reticulum, at the right, is another pouchlike part, the omasum or psalterium, which continues on to the right and posteriorly into the more elongated al) omasum, which in turn joins the duodenum. On the first trip to the stomach, the food passes into the rumen, is stored and moistened. It then goes in small quantities at a time into the reticulum, and this "cud" Psa/fer/u/n Esophagus Aboma^um Hunjerj Fig. 344. — A ruminant stomach. The arrows indicate the direction of tlie move- ment of the food in the formation and regurgitation of a cud. (From Wolcott, A-nxinal Biology, published by McGraw-Hill Book Company.) may pass back to the mouth from here. After it is chewed and re- swallowed it passes through a valve at the entrance to the stomach into the omasum, and on to the abomasum. Order Perissodactyla. — This is the group of odd toed mammals in which the axis of the foot is through the third toe. There are no modern forms which are natives of this country with the possible ex- ception of the horse, and this is very indirect. In the horse, ass, and zebra, the foot is reduced to one hoofed toe. The ass and zebra are African and Asiatic forms. Tapirs are piglike with four toes in front and three behind. They are found in southern Asia, and in Central and South America. The rhinoceros is a large Asiatic and African form. 634 TEXTBOOK OF ZOOLOGY Order Prohoscidea. — This small order includes only two genera of elephants with one species each of the largest terrestrial animals. One species, Elephas indicus, is Asiatic, and the other, Loxodonta africana, lives in the tropical forests of Africa. Both have the nose extended several feet as a muscular trunk or proboscis which is a very handy and useful appendage. The skull is very thick, with air spaces, and the molar teeth are very large, with prominent ridges. Order Sirenia. — This is a very limited order of sea-cows. They are aquatic mammals with a pair of flexible anterior flippers and a strong, rounded tail. The dugongs of the Indian Ocean and Australia, and the Manatees of the Atlantic Ocean represent the group. The Florida manatee, Trichechus latirostris is only rarely found. Order Odontoceti (The Toothed Whales). — All of the whales are mammals which have become adapted to a strictly aquatic life. The body is modified for swimming by the reduction of appendages, the horizontal flattening of the tail and its division into two lobes or "flukes." The head of these animals is large with long jaw bones. The nostrils open by a single aperture from which the breath is spouted when the animal comes to the surface. A thick layer of fat or "blubber' is deposited beneath the skin and this serves to conserve heat in the body. The porpoise, Phocaena pJwcaena, is very common in the Gulf of Mexico and elsewhere. It is about six feet long and rolls around in the water. It catches such fish as mackerel and squeteague for food. The sperm whale, Physeter catodon, is a large whale of about seventy-five feet in length. Such other animals as the beaked whale, dolphin, and narwhal belong in this order. The killer whale, Orcinus orca, is generally distributed. It is about twenty feet long, ferocious and predatory on fish, seals, and even other whales. Order Mystacoceti. — This is the whalebone whale group. Their teeth do not develop beyond the embryonic stage, but they are re- placed by cordlike plates of baleen or whalebone. Whalebone was once an important item of commerce, being used in making whips, stays, and other flexible articles. The largest species in the order and, in fact, the largest of all animals is the sulphur-bottom whale, Sihhaldus musculus. It reaches a length of one hundred feet and lives in the Atlantic and in the Pacific off the coasts of Central America, Mexico, and California. The gray whale, Bhachianectes glaucus, is another Pacific form. The Greenland right whale, MAMMALIA 635 Balaena mysticetus, is a polar inhabitant, each of which yields about 3,000 pounds of whalebone and 300 barrels of oil. The finback and humpback whales also belong to this order. In feeding, all of these whales take large quantities of water into the mouth, pass it out through the whalebone which serves as a sieve, and retain all of the small organisms and particles of organic matter as food. Order Primates. — This is the order which includes the lemurs, monkeys, apes, and man. Because of the large number of primitive characteristics of representatives of this order, some authors place it near the first or middle of the list of orders of mammals instead of at the end. Most of them are tropical, arboreal, and live on nuts, seeds, fruits, insects, and birds. They have a particularly well- developed brain. The thumb and also the great toe in most forms are placed in opposition to the other digits as an adaptation for grasping. Usually only one young is born in a fairly advanced stage, but rather helpless at first and requiring considerable care. The order is usually divided into two suborders, the Lemuroidea and Anthropoidea. The first includes the lemuroids which have the front teeth separated. These are typically small or medium-sized quadrupeds of Madagas- car, with long, bushy tails. The aye-aye is a nocturnal arboreal ani- mal with long ears, large ratlike teeth, bushy tail, and long digits with sharp claws. Its body is about one foot long. The tarsiers are about the size of rats with suckerlike discs on the ends of their fingers and toes. They are also arboreal and nocturnal. The true lemurs have an elongate face and a small cranium. Their hindlimbs are longer than the forelimbs. Most of them live on Madagascar and nearby islands as well as in Africa and Asia. The second suborder, Anthropoidea, includes several families of monkeys and apes. The marmosets make up another family found in South and Cen- tral America. They have a flat nail on the big toe, while the thumb is not opposable to the other digits. The brain case is large, and the space between the nostrils is broad. The Cehidae, or New "World monkeys, have long prehensile tails, broad flat noses, and all digits have nails instead of claws. This group includes the common monkey of hand-organ fame, the spider monkeys, squirrel monkeys, and howlers, all of which are natives of Central America and the northern part of South America. 636 TEXTBOOK OP ZOOLOGY The Old "World monkeys, family Cercopithecidae, have narrow high-ridged noses. Some have 'long tails, others have short tails. Certain of them are almost bipedal. Most of them have heavy cal- louses on the hips which are used as cushions while sitting. Some of -""axuMSO;;^, '^t^mc^imsi^'mti,^^ . / - l... ...-«n«Hnri;j^a&ifcM Fig. 345. — Orangutan holding a glass. (Courtesy of Nature Magazine.) them have cheek pouches. With the exception of the Barbaiy ape, Macaca sylvana, of the Rock of Gibraltar, this entire family is con- fined to Africa and India. The baboon, of genus Cynocephalus, a doglike monkey with a short tail, lives in central Africa. The man- drill, drill, and macaque monkeys are others in this group. MAMMALIA 637 The manlike, or anthropoid, apes are grouped in the family Shniidae which includes the gibbons, Hylolates; the orangutan, Simia satyrus; the chimpanzee, Anthropopitheais troglodytes; and the gorilla. Gorilla gorilla. The gibbons are tailless apes with long arms. They are arboreal and omnivorous. They can accomplish bipedal locomotion. They are only three feet tall and strictly arboreal ; they live on the Malay Peninsula and in the East Indies. The orangutan builds nests in the trees and feeds principally on fruits. It is be- tween four and five feet tall and has an arm spread of seven feet. The chimpanzee is perhaps the most intelligent ape. It is easily tamed and in many respects is more manlike than most of the others. It lives in central and western Africa. The arms are somewhat shorter and the skull rounder and smoother than are those of the gorilla. The gorilla, which is the largest of the group and somewhat more ferocious, is about five and one-half feet tall. It walks on the soles of the feet and on the knuckles of the hands. It has prominent canine teeth and feeds mainly on plants and foliage. Economic Relations Nearly all of the important beasts of burden, such as horses, asses, mules, elephants, camels, llamas, reindeer, oxen, and dogs, are mammals. The history of the origin of the domestication of most of these has been lost to antiquity. Cattle, sheep, hogs, goats, rein- deer, alpaca, and rabbits are the chief meat-producing mammals. A few years ago it was estimated that more than 20,000,000 cattle, 15,000,000 sheep, and 80,000,000 swine are required to supply the meat demand in the United States each year. Cattle and goats are the most important commercial milk-producing animals. The milk of camels, reindeer, and llama is also used in some parts of the world. The leather produced by tanning the hides of meat-producing do- mestic animals particularly is worth millions of dollars. It is used for making shoes, saddles, harness, belts, and for ornamentation. Wool is a very important animal fiber used in the manufacture of fabrics which are resistant to dampness and cold. It is produced principally by sheep and goats in this country. In some parts of the world the alpaca and camel are important wool-producing animals. The skins and furs of many wild mammals, the fur-bearers, are exceedingly important commercially. They were used as clothing and ornamentation even by primitive people. The modern people are 638 TEXTBOOK OF ZOOLOGY demanding more and more furs. The most commonly used and abun- dantly sold furs are in the following order : mole, rabbit, skunk, muskrat, opossum, squirrel, fox, ermine, wallaby, mink, wolf, civet eat, and raccoon. There are several vei*y valuable fur-bearing ani- mals coming very near the point of extinction, and there should be an earnest effort made to restore them. In this group would be in- cluded beaver, fur seal, otter, Russian sable, and chinchilla. A single, choice, silver fox fur may bring a thousand dollars or more, a fisher is valued at about three hundred dollars, the beaver and otter, each at about one hundred ; a wolf is worth about forty as also is the black bear ; the skunk is valued at about six dollars, and the muskrat at about three dollars. Fur farming is being practiced with some species, such as silver fox, mink, muskrat, and rabbits. Rabbit and muskrat farming have the added advantage of producing salable meat. Many of the undomesticated mammals become serious pests at times when the usual balance in nature is disturbed. Rats and mice are very destructive of stored provisions, such as fabrics, clothing, grains, and various foodstuffs. These rodents, along with squirrels, gophers, prairie dogs, and groundhogs which normally live on wild plant tubers, seeds, acorns or nuts, are frequently destructive to grain crops. Field mice, rats, and rabbits sometimes damage young fruit trees by gnawing the tender bark just at the surface of the ground until the tree is girdled, thus causing subsequent death. The burrow- ing forms may be killed by fumigating the burrow with carbon di- sulphide, calcium cyanide, or carbon monoxide from the exhaust of an automobile. Many of the larger carnivorous mammals are quite destructive of young domesticated animals. The wolf, coyote, fox, and, in some parts of the Southwest, the puma, are quite predatory, killing calves, lambs, kids, and poultry. Certain wild mammals are carriers of disease. In the Rocky Mountain region, ground squirrels and other rodents carry Rocky Mountain spotted fever and transmit it to man through the bite of the spotted fever tick. Bubonic plague is carried by rats and other rodents and is transferred to man by the rat flea. The deer and bear are about the only large mammals left that are classified as game animals and hunted for sport. Several others, such as the pronghorn antelope, buffalo, and elk, have been almost I MAMMALIA 639 completely destroyed. With proper protection, however, these species might still be restored iu many parts of the country. Squir- rels and rabbits are the most abundant game mammals of present times. THE CAT, A REPRESENTATIVE MAM3VLAL The common house cat, Felis domestica, is a carnivore which is familiar to everyone. It is so common that it is always available for study. The cat is a quadruped which is well adapted for walking and climbing, at which activities it is very adept. The eyes of the cat are well adapted to sight at night when it is quite active. As a result of domestication the carnivorous diet of the cat has been somewhat modified. The cat is clean in its sanitary habits. External Structure The entire body is covered with a high quality hair or fur which may be one of several colors. On the upper lip and around the eyes are some especially long stout hairs, whiskers or vibrissas. The upper lip is somewhat cleft in the center. In the hare this cleft is extremely prominent, and it is from this that the abnormally cleft lip in man came to be known as ''harelip." The tall, flexible external ear or pinna partially surrounds an ear opening, the external auditory meatus, which leads to the interior of the skull and the tympanic membrane. The large, oblong nostrils are located in the fleshy nose, the end of which is naked. Their large eyes have both upper and lower lids, and each has a sheetlike nictitating membrane which may be drawn over the eye from the medial corner. The trunk is conveniently divided into a thorax or chest, supported by the ribs, and posterior to this the abdomen. Extending posteriorly is the long slender tail. Along the ventral side are four or five pairs of nipples or teats which are prominent in the female but rudimen- tary in the male. They contain the openings of the milk or mammary glands. Ventral to the tail is the perineal region in which is located the anus and external genital organs. The two pairs of limbs extend ventrally from the sides of the trunk. The forelimb is divided into upper arm, forearm, wrist, palm, and five digits with retractile claws at their tips. The hindlimb extends from the hip and is divided into thigh, shank, ankle, arch, and four digits with similar claws. 640 TEXTBOOK OF ZOOLOGY ■■U 1 ti "l-l I w ..rf "3 >>t, a -S o^,ga«:;-M--t.| *ri C r3 *-• ^ a; o ?^ 9 (rf qH — o3 c rT— .'^ 03 3 c5 ' o o C ; w ^ 3 •-'^'^ t:; aP c . &> fi -,3 0 0) « s c ■»-> " , w h-, •""* ■- uj 0) c CC«S ti St-w to .0 t^ !- 05 to ■O1 m to o in tt C 3'C ii^ 2 w 2 rf .^3 3_g----3'3.-SO 5 o oS to irt to 0) a cS m s- 0) o-o o THORACIC NERVES LUMBAR NERVES SACRAL NERVES FILUM TERMINALE Fig. 352. — Dorsal view of nervous system of cat. (Drawn by Titus Evans.) 652 TEXTBOOK OF ZOOLOGY hollowed hilus of the kidney. The slender ureter carries the urine from each kidney to the urinary bladder. From here it is carried to the exterior by the urethra. Reproduction and Life History The ovaries of the female cat are small, yellowish, oval bodies each about the size of a large navy bean and located against the dorsal RIGHT KIDNEY R. URETER -URINARY BLADDER R. VAS DEFERENS INGUINAL CANAL PROSTATE GLAND URETHRA RIGHT BULBO- URETHRAL GLAND PENIS R. TESTIS Fig. 353. — Urinogenital system of male cat. (Drawn by Titus Evans.) body wall just posterior to the kidney. On the surfaces of the ovaries are a number of projections, the graafian follicles, in each of which is a developing egg or ovum. Each of the two oviducts or Fallopiayi tubes has a funnel-shaped end or ostium which fits loosely around the ovary. The oviduct leads into the horn of the uterus. The two uterine horns join in the body of the uterus which continues pos- MAMMALIA 653 teriorly as the vagina. In the ventral wall of the vagina, near the aperture, is a small body of erectile tissue, the clitoris, which corre- sponds to the penis of the male. The pair of testes of the male are contained in a pouch of integ'ument called tiie scrotum, which hangs beneath the anus. Each testis is about one and a half centimeters long by one centimeter thick. The epididymis is a mass of coiled tubules coming from the testis and lying at its dorsal side as a part of the testicle. The vas deferens is the canal leading from this cranially through the ahdominal ring and inguiyial canal into the abdominal cavity, where it enters the urethra on its dorsal side near the prostate gland. During copulation the mature spermatozoa pass into the urethra and are discharged into the vagina of the female by the erected penis. The secretions of the prostate glands and of Cowper's glands just posterior to them, provide a considerable part of the seminal fluid. The cat will first breed at the age of one year. The female will receive the male at certain periods only. Mature ova rupture through the wall of the follicle of the ovary and are normally received by the ostium of the oviduct which covers it. Occasionally an ovum falls into the body cavity. In the oviduct the ova are moved pos- teriorly where, if copulation has occurred, they will meet spermatozoa and be fertilized. Cleavage, or divisions of each zygote follows, and each resulting multicelled embryonic mass moves down to the uterus where it becomes implanted in the wall. Here a placenta is formed from a union of certain embryonic membranes with the internal lining of the uterus. Parental blood carries nourishment and oxygen to this membrane where it diffuses through to the embryonic blood in other vessels in the membrane. The time between copulation and birth of young (gestation period) in the cat is from fifty-five to sixty-five days. From three to six young are usually born in each litter. They are fed from the four nipples of the mammary gland along each side of the ventral surface of the trunk. CHAPTER XXXII ANIMAL ANOMALIES It is rather generally understood among students of biology that no two animals, even of the same species, are exactly counterparts of each other. There is a certain normal range of variation in size and structure as well as in functional efficiency. Any variations beyond these rather fixed limits are classified as malformations or anomalies. A study of such abnormalities is know-n as teratology. Abnormalities may occur at almost any stage in the life of the individual, but a large majority result from abnormalities in the process of develop- ment. Many are due to failure of development and some to over- development. In turn, much of this is due to malregulation resulting from failure of balance in the functioning of the endocrine system. The occurrence of striking malformations in newborn human babies is in approximately the ratio of one to 165. Fortunately, many of the most grotesque anomalies do not reach full development and are bom dead. Anomalies have been classified into several groups on the basis of the following conditions : a. Failure of Development. — The embryonic structure fails to ap- pear or at least fails to develop to a significant degree. Examples of this account for single kidneys or legs where they are usually paired. b. Arrested Development in which the development stops before adult condition is reached, as in the cleft palate, harelip, or dia- phragmatic hernia. c. Overdevelopment.— In such cases growth is exaggerated or the number of parts increased as in gigantism (macrosomia) or in- creased number of digits or accessory mammary nipples. d. Fusion. — The kidneys are sometimes fused together to form a horseshoe kidney. e. Splitting, thus forming extra structures, as in the case of acces- sory spleens or the splitting of the ureter. 654 ANIMAL ANOMALIES 655 f. Displacement of Organs. — Cases are found where organs oc- cupy abnormal positions, as both kidneys on the same side, a finger at the wrist, or the appendix on the left side. Causes of Anomalies. — There are both internal and external agencies which bring about malformations in the individual. Both embryology and pathology contribute to the explanation of the causes of these abnormal conditions. The development may be per- fectly normal and a subsequent disease may be the cause of striking abnormality. On the other hand, certain diseases of the parent will influence the normal development of the fetus. Even twinning by itself is an abnormal process in most animals. It, however, is not usually thought of under the title of anomaly, except when they are Fig. 354. — Grasshoppers at time of diapause, showing some of the abnormahties which very infrequently' occur in their natural development. 1, Normal embryo : 2, embryo with two extra heads and mouth parts; 3, embryo with a lateral twm joined at the abdomen ; J,, almost complete twins back to back ; 5, embryo with a double abdomen. (From Evans, "Effects of Roentgen Radiation on Embryonic Organization and Regulation in Melanoplus differentiaUs [Orthoptera]," Physio- logical Zoology, Vol. X.) physically connected or the individuals are otherwise malformed. The causes of anomalies may be summarized as follows : 1. Internal Causes. a. The germ plasm carries hereditary causes for some, and these characteristics are transmitted as are normal traits. Color blind- ness and hemophilia (bleeder condition) are examples. b. Diseases which cause abnormal growths and conditions, as elephantiasis. (Fig. 389.) c. Unbalancing the chemical regulators (hormones) which are pro- duced by the endocrine glands. (Overactivity of the hypophysis causes gigantism; cretinism, a dwarf condition, results from deficiency physically connected or the individuals are otherwise malformed, in thyroid activity.) 656 TEXTBOOK OF ZOOLOGY d. Fortuitous abnormalities whose causes are not apparent or are influenced by certain variations in other organs. Such anomalies show up most frequently in the circulatory and nervous systems. Venous or nervous supplies to organs are often modified. Another example is the rearrangement brought about by diaphragmatic hernia 2. External Causes. a. Environmental agencies may affect almost any individual whose development occurs outside the body of its parent. Exposure to radium or x-ray radiation, sharp variations in temperature, exces- sive salt content, or contact with toxic substances may all be respon- sible for various degrees of abnormality. The same factors are also effective, if present, in the uterine environment of the placental type of animal. b. Mechanical factors, such as abnormal pressure, blows, and falls, may cause some abnormalities. e. Ahnormal implantation in the wall of the uterus resulting in deficiencies in nourishment and support of the fetus. d. Such diseases as syphilis, which may be transmitted from mother to offspring, are responsible for some types of defects, as impaired vision, e. Developmental inhihition or arrest brought about by deficiencies in metabolism at a time when the rate or efficiency should be high. The work of several embryologists seems to indicate that the pro- duction of twins either as normal individuals or otherwise is re- lated to this condition. Harelip and Cleft Palate These two defects are related and are sometimes found together. The lateral palatine processes may fail to complete growth and unite properly, thus leaving a gap in the roof of the mouth which opens directly into the nasopharynx above. Harelip (cheiloschisis) is a very common defect and is due to the failure of union of the nasal and maxillary processes. There may be double harelip or single harelip. In connection with double hare- lip it sometimes happens that the premaxilla projects beyond the outline of the face to form what is called wolf-snout. Harelips are frequently remedied by a surgical operation early in life. ANIMAL. ANOMALIES 657 Fig. 355. — A case of harelip due to arrested development. Diaphragmatic Hernia (Open Diaphragm) An extreme case of this was found in a cat which was being used for dissection purposes. The animal had lived an apparently normal life and had been killed for laboratory study without showing evi- dence of its abnormality until dissected. From all appearances the diaphragm had not completed its development, but had formed a fringelike projection which reached inward about half an inch from the thoracic wall and extended the entire circumference of the in- side of the thorax. The aperture in its center measured one and three-fourths inches in diameter. Due to this condition the arrangement of several visceral organs was greatly affected. The thorax was somewhat elongated, and the right side of the cavity comprised about two-thirds of the space of the chest. The mediastinum (supporting median mesentery of heart and lungs) had its attachment more than half an inch to the left of the midline. Almost the entire liver was turned forward to occupy the right two-thirds of the chest cavity, and this placed the gall bladder at the level of the junction of the auricle and ventricle of the heart. 658 TEXTBOOK OF ZOOLOGY Approximately half of the spleen, the pyloric portion of the stom- ach, and a large portion of the omentum had been drawn through the aperture in the diaphragm. The right lung was extremely crowded and small. '\ ...^*!'" ^- ^ Might lung ^^Leffc lunq M^'"~ Rt. auricle % Rt.ventricle ^^ Liver # Left lung - Diaphragm cut edge Colon Fig. 356. — Dissection of a fully-grown cat with a congenitally open diaphragm (diaphragmatic hernia), showing many of the abdominal organs everted into the thorax. This anomaly occurs occasionally in human beings, but usually in much less degree. It is not likely that a man would be able to reach maturity and carry on normal activities with such an exten- sive hernia. ANIMAL ANOMALIES 659 Polydactylism (Extra Digits) There are numerous abnormal variations in the number and ar- rangement of digits, ranging from a stumplike structure of no digits through the ''lobster claw" condition of two or three, to as many as two more than normal. The polydactylous condition is rather frequently found in cats. In fact, Wilder once reported that pos- sibly 25 per cent of the cats of the vicinity of Ithaca, N. Y., possessed an extra digit on each foot. The forefeet shown in Fig. 357 are from a living cat which came from a litter of four, two of which showed these complete characteristics on all feet, one of the litter had only an extra toe on one foot, and the fourth was normal. Both hind feet of this animal had five well-developed digits instead of the usual four. This condition is a hereditary one and is brought about by partial duplication of elementary structures. -^'^W ^i:r:-'t ^\|#^ '^lili,"^'^ Fig. 357. — Front feet of a half -grown kitten with six toes. The claws are pro- tracted on the left foot and retracted on the right There were five toes on the hind feet of this cat. Conjoined Twins Instances occur in which individuals of about equal size or of unequal size are fused together. If this occurs at the hips with dorsal sides together, the condition is usually known as Siamese twins. There are a number of different varieties of fusion, includ- ing the head region, chest region, or complete fusion of trunk into a single body with two heads and vice versa. A chicken with two pairs of legs and wings, two backs, and a single head has been dissected and described by Dr. F. L. Fitzpatrick. The feather tracts were double on the neck and trunk. Internal dissection showed the single digestive system to extend between the two necks, follow through the combined body cavity with some modifications, and empty into the cloaca of the right back region. No cloaca was pres- 660 TEXTBOOK OF ZOOLOGY ent in connection with the left back. There was a single heart, two very unequal lungs, two pulmonary arteries and veins to the larger lung, while there was only one common carotid artery, that being the right. Two trachea were present, the right being rather nor- mally developed while the left was rudimentary. The brain was normal, but joining the medulla were two separate and complete Fig. 358. — Conjoined twins of cat showing single head, but double trunk and appendages. spinal cords, one passing to each back region. Most of the internal organs "favored'' the right side, except the lungs, of which the left was much more developed. Dr. P, L. Fitzpatrick reports upon the anatomy of a double pig also. This freak had two tracheae leading to two sets of lungs. The brain consisted of a three-lobed cerebrum, two cerebellums. ANIMAL. ANOMALIES 661 and two medullas. There was a spinal cord in each of the two backs. A third or median eye was present on a ventral stalk beneath the third or transverse lobe of the cerebrum. Two pairs of kidneys, two bladders, four ovaries, double sets of oviducts and uteri, as well as Fig. 359. — Conjoined human twins showing single hips but double trunk and head. (Redrawn and modifled from Arey, Developmental Anatomy, published by W. B. Saunders Co.) a double cloaca, were present. There was a single but modified heart and two aortas leaving it. Externally the animal had two sets of limbs, two tails, two sets of mammae, two anal apertures, and two urinogenital openings. 662 TEXTBOOK OF ZOOLOGY Cerebrum Trachea --Lung of- cavity B Bronchi Lung of '1 I \ : i w' Caudal Fig. 361. — Dorsal aortae and branches in embryo pig which had a single head and two bodies. (After Fitzpatrick.) ANIMAL ANOMALIES . 663 It is suggested that such individuals have developed from a single zygote which underwent an abnormal cleavage. Examples of these conditions are shown as they occur in cats. Similar fusions occur in human beings as is illustrated in the accompanying diagrams. There are also cases of unequal conjoined twins in which a much smaller and less developed individual is fused to the abdomen, head, palate, or sacrum of an otherwise normal individual. Such an indi- vidual is referred to as a parasite on its larger mate. Hermaphroditism There are abnormal cases of sexual development in vertebrates including man, in which the organs of both sexes are combined in one individual. In true cases there are present both ovaries and testes in the one individual, but no cases are known in which both have functional capacity. The external organs are partly male and partly female, and the secondary sexual characteristics, such as voice, mammae, stature, and beard, may be of a mixed nature. Hermaphroditism is the normal condition in the coelenterates, flat- worms, annelid worms, and molluscs. Here both sets of organs are capable of function. Birds and mammals are rarely subject to this condition. A condition spoken of as false herma'plirodiUsm involves the pres- ence of gonads of one sex but the secondary sexual characteristics and external genitalia of the other. In masculine hermaphroditism testes are present but not usually descended into the scrotum, while the external genitalia and secondary characteristics are those of the female. In feminine hermaphroditism ovaries are present, even descended into a scrotum in rare cases, but the clitoris is enlarged and labiae are fused to resemble the penis of the male. In some instances the lips of the slitlike urinogenital aperture on the under side of the penis fail to fuse. There is a very close homology in the organs of the two sexes. The external genitalia are indifferent or sexless until the end of the seventh week of embryonic development in human beings. Then the determination comes, causing normally the modifications of de- velopment to form the organs of one sex or the other. The develop- ment of the external organs of the two sexes is strictly parallel. I I 664 TEXTBOOK OF ZOOLOGY It is likely that this development is controlled by hormone relation- ships, and it is in cases in which this balance is disturbed that hermaphrodites occur. There is still much to be learned concerning the causes of this condition. Cardiac Anomalies Transposition of the heart to the right side of the midline of the body is known as dextrocardia and occurs rarely. It is usually associated with displacement of other visceral organs. There has been a case reported recently in which the autopsy of a 73-year-old farmer disclosed that he had died because of a rup- tured left ventricle of an extra heart. One of the hearts of this man was in the normal position, the other (the one that failed) was located just above the spleen and below the left lung in the lower part of the chest. The hearts were both of about normal propor- tions and both functioned actively. They both joined the aorta. Due to faulty development, an incomplete interventricular septum is occasionally found in the four-chambered hearts. The failure of complete development of the septum between the auricles to close the foramen ovale is a more common anomaly. It has been reported that this occurs in some degree in one case in four for the human being. Actual mixing of auricular blood sufficient to interfere with normal function is much less common, however, because of the over- lapping of the membranous walls which are pressed together by the pressure of contraction. In a small number of human cases the aerated blood and unaerated blood from the two auricles do mix and produce a purplish colored blood and in turn affects the color of the skin. Such a case is known as a ''hlue hahy" and often leads to early death. Abnormalities of Brain and Sense Organs Encephalocoele is due to the protrusion of a sac of the meninges and part of the brain through a defect in the roof of the cranium. An abnormally large brain, which is usually associated with the distention of the cranium by superabundance of cerebral fluid, is known as hydrocephalus, or the size may be macrocephalus. The opposite extreme in which the head and brain are abnonnally small due to failure of development is known as micro cephalus. ANIMAL ANOMALIES 665 There are cases of cleft nose in which the nostrils are in inde- pendent projections. This condition is usually associated with hare- lip and cleft palate. With regard to the eyes there are several possible abnormalities. Cyclopia is a condition in which there is a single median eye like that of the Cyclops instead of the usual paired arrangement. In such cases the nose is usually at the base of the forehead, above the eye, and cylindrical in shape. Failure of complete development of the iris or chorioid, thus leaving a gap or open sector in the margin of the pupil, is known as colohoma. CHAPTER XXXIII THE ENDOCRINE GLANDS AND THEIR FUNCTIONS The great complexity of the structure of organisms, particularly of those animals in the higher ranks of the animal kingdom, makes necessary a means of regulation and coordination of the functions of the organ systems individually and a means of intercommunica- tion between them. This work is cared for in part by the nervous system; but another agency of regulation is present in all higher organisms and many of the lower forms of life, which is of first importance in this respect, one so complex and interwoven with other organs in carrying out its functions that it is not thoroughly understood at the present time. The group of organs doing this work we designate as the endocrine glands. They manufacture and furnish the body with chemical compounds called hormones, a term which refers to compounds formed in the body and capable of encouraging or exciting activity in another part of the body; they are chemical messengers. The endocrine glands pour their hormones directly into the blood stream without the assistance of ducts, and the blood carries them to other parts of the body ; but they are never carried to the outside of the body directly, as in the case of secre- tions from the glands of external secretion, and for this reason the endocrines are often referred to as organs of internal secretion, or ductless glands. The functions of the hormones are numerous, and current research and investigation in the new field of endocrinology are constantly presenting new activities and interrelationships of these secretions. The field of endocrinology is comparatively new, and a complete understanding of the complexities involved is impossible until fur- ther research is completed. The mysterious manner in which the endocrine organs produce their secretions, the hormones, the man- ner in which they choose the raw materials, combine them, and give forth products which are vital to the welfare and happiness and often to the very existence of an organism, is of most intense interest and of the greatest importance. 666 ENDOCRINE GLANDS AND THEIR FUNCTIONS 667 The hormones may be classified, arbitrarily, according to function in three main groups. The first of these groups includes those hor- mones which arouse specific responses in particular organs or in localized parts of the body. A second group is composed of those hormones which affect the general metabolism of the body. The third group is that of the hormones which affect, in particular, the growth and development of the organism. Another general method of classifying substances of this nature terms those substances which serve as excitors or accelerators ''hormones" and those which i7iJiihit or depress activities as ''chalones." Illustrations of each of these will be found in the discussion of the individual organs and their functions. Hormones are found in plants as growth hormones, in the inverte- brate animals, and are best known in vertebrate animals. The follow- ing examples will serve to demonstrate the nature of endocrine func- tion in invertebrates: Earthworms whose testes are completely de- stroyed, do not develop the clitellum, the band-like organ which func- tions during and following copulation. In Bonellia, another annelid worm, there seems to be a relation between sex determination and cer- tain hormones, in that the week-old larvae are indifferent sexually, but those that attach to the proboscis of the female parent become males due to some agent received there. All others develop to become normal females. Among the crabs there is a parasite which attacks and destroys the gonads, and it is found that the parasitized male crab will take on distinctly female characters due to the lack of some humoral agent lost with the destruction of these glands. Too, using radium to destroy the gonads of Asellus, another crustacean, seems to indicate that the development of the brood pouch is controlled by a substance produced in the normal ovaries. Molting and metamorpho- sis both are regulated by hormones in insects. The supraesophageal ganglion seems to produce a substance which initiates pupation activi- ties in moths. The endocrine glands of vertebrates which are best known and most clearly understood at the present time are the thy- roid gland, the parathyroid glands, the suprarenal bodies, the pitui- tary body or hypophysis, the thymus, the gonads, and the pancreas. The Thyroid Gland The thyroid gland is the most familiar of the endocrine organs to the layman. It is a body of two lobes of about the size of wal- nuts, slightly flattened, placed one on each side of the upper part 668 TEXTBOOK OF ZOOLOGY of the trachea just below the larynx, or voice box, the two lobes being connected by a saddle-shaped isthmus. The thyroid is well supplied with blood, receiving in proportion to weight, three and one-half times as much blood as the brain; this permits an easy access of the hormone to all parts of the body. The thyroid is normally not visible externally, but the pathogenic condition of the organ caused by overgrowth, and known as goiter, is familiar to everyone. A. B. C. Fig. 362. — Illustrating- cretinism. A, a cretin, 23 months old and B, the same child after having received thyroid treatment for eleven months. C, an untreated cretin 15 years old. (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, after Osier.) The functions of the thyroid gland, as recognized today, are two : that of control, in conjunction with other endocrine organs, of the growth and development of the body; and, second, a most important role in the regulation of metabolism. Any upset of the normal functioning of the gland or the removal of the gland results in serious physical and mental disorders, if not in death itself. The hormone produced by this organ, designated thyroxine, has been prepared from the fresh thyroid glands removed from various ani- mals and has also been manufactured synthetically. Chemical diag- ENDOCRINE GLANDS AND THEIR FUNCTIONS 669 nosis of the hormone reveals the presence of iodine in its compo- sition, and the amount of iodine available in the body seems to be the determining factor in the degree of control which this hormone exerts on bodily functions. In regions of the world in which iodine is scarce in water and food, goiter is a prevalent disorder, and only in recent years has the understanding of the cause been complete enough to suggest as a remedy and a preventive the introduction of additional iodine into the diet by use of iodized salt. Since the amount of iodine required for normal purposes is almost infinitesimal, a sufficient quantity is supplied by this means. The correction of disorders due to a deficient functioning of the thyroid gland by use of natural thyroxine or by use of the synthetic product is quite com- mon today among human beings. The removal of the thyroid gland in animals which have not ob- tained their complete growth results in delayed or arrested develop- ment. An interesting example of this is found among certain amphibians. The proper functioning of the thyroid gland is essen- tial for the accomplishment of metamorphosis of the frog tadpole into the adult frog. The tadpole lacking sufficient thyroid extract may grow to an unusually large size, but metamorphosis never occurs without the encouragement of this hormone. In the develop- ment of human beings, a deficient supply of thyroxine in early years results in a condition known as cretinism. The growth of the bones does not take place, the entire body is stunted and de- formed, mental development ceases, the facial features are misshapen, growth of hair is scant, and the development of the sexual organs is inco.mplete. Before work on the thyroid gland was undertaken, cretins were often seen in certain parts of Europe and occasionally in America; but the present understanding of the hormone and its use has made possible the prevention and cure of most cases of this nature. Improper functioning of the gland in adult years results often in a condition known as myxedema, in which there is a thickening and drying of the skin, a puffiness of the eyelids and lips, loss of hair due to the condition of the skin, a slowing down of metabolism and heartbeat, a depression of body temperature, the deposition of large quantities of fat, and a final result, in many cases, of im- becility. The administration of thyroxine, especially in the early stages, accomplishes a complete, or at least a temporary, remedy. An overfunctional thyroid gland, in which the condition is known as hyperthyroidism, results in an increased metabolic rate, a loss of 670 TEXTBOOK OP ZOOLOGY body fat, and a condition of hyperirritability of the nervous system. It seems, therefore, that the difference between an overly energetic and a sluggish person, and a lean and an obese person, may often be traced directly to the degree of functioning of the thyroid gland. Hyper- thyroidism is accompanied by increased excretion of calcium. It dif- fers from the calcium upset due to parathyroid disturbance in that in hyperthyroidism its concentration in the blood remains normal. Work on the lower vertebrate groups suggests the probability of an important function of the thyroid gland in determining the hiber- nation periods of certain animals. The thyroid performs additional functions in conjunction with other of the endocrine glands; e.g., the control of sexual activity; but these interrelationships will not be discussed here. Peculiarly, thyroxin, although an accelerator of oxidation in vertebrates, has a depressor effect on cell division and differentiation in such invertebrates as Paramecium, sea urchin, and the hydroid Pennaria. This has not been completely explained. The Parathyroid Glands Connected with the thyroid body are four little glands about the size of small peas, so insignificant in appearance that they were overlooked for many years. The removal of these small bodies along with the thyroid gland in certain operations provoked such startling results, however, as to attract attention to their presence and to evoke considerable interest in their investigation. A complete re- moval of the parathyroids results in unbalancing the blood calcium and in a type of convulsion known as tetany; death is the usual result. A deficient supply of paratJioriJione or parathyrin, the hormone of the parathyroid glands, may be responsible for defective growth of the bones and for deficient formation of enamel and dentine of the teeth. Calcium is needed for both the teeth and the bones and the introduc- tion of either calcium or parathormone into the body is made to sup- plement a deficient supply of calcium due to malfunctioning of the parathyroids. Removal of the parathyroids also brings about a fall in the renal excretion of phosphorus, and the in.jection of parathor- mone causes an immediate rise in the level of renal phosphorus. Parathyroid activity is particularly useful to laying hens where so much calcium is needed in shell formation. There is evidence of a close functional relationship between the pituitary gland and the parathyroid. In dogs the removal of the pituitary causes atrophy of the parathyroids, particularly if the pancreas has been removed also. ENDOCRINE GLANDS AND THEIR FUNCTIONS 671 Conversely it has been noticed that injections of extract from an- terior pituitary raises blood calcium in some species because of in- creased activity of the parathyroids. Too great an activity on the part of the parathyroid glands results in a decalcification of the bones, and an increased content of calcium in the blood and in the excretion of the kidneys. The final result of this softening of the bones may cause serious disfiguration and stunt- ing of the body. Accompanying these results are flabbiness of the muscles, decreased irritability of the nervous system, and other un- favorable conditions. These may be remedied by the removal of a portion of the parathyroids. The Suprarenal Bodies Lying close to and slightly anterior to the kidneys are two small yel- low or reddish masses of tissue, which play a prominent role in the regulation of the body and one of such complexity that much is yet to be learned concerning its method of functioning. These adrenal glands, or suprarenal bodies, are made up of an inner and an outer portion, the medulla and cortex respectively. The former secretes a substance designated as adrenalin (epinephrine or adrenin), which acts upon various organs and raises the level of their functioning. Adrenalin, at times of excitement or emergency, may cause con- striction of blood vessels, increased rate of heartbeat, a greater discharge of glucose from the liver to provide additional energy, erection of hairs, stimulation or inhibition of the various visceral muscles, etc., to provide greater efficiency. The range of control of the medullary portion of the adrenals is thus wide and complex, definitely interrelated with the functions of the sympathetic nervous system, with other endocrine glands, and numerous processes of the body, so that its true importance is difficult to estimate. Adrenalin has been obtained from various animals for use in the treatment of certain disorders, as an anesthesia in minor operations, and to stop small hemorrhages. It has also been successfully employed in the relief of asthma and similar troubles. A hormone kno^vn as cortin has been isolated from the suprarenal cortex, and, while the removal of this portion of the adrenals results in death, the exact functions of the hormone produced therein are not entirely understood. It does relieve the condition known as Addison's disease. There is, without doubt, a close relationship between the cortex and sexual development; and some workers 672 TEXTBOOK OF ZOOLOGY believe that the cortex regulates the normal flow of blood, which would account for the fatal results of its removal. An extract has been secured from the adrenal cortex of cattle in particular and is used for the treatment of conditions resulting from malfunctioning of the cortical portion of the adrenals in other organisms. Some have shown cortin to have capacity for delaying the onset of scurvy in vitamin C deficiency. After complete bilateral removal of the adrenal cortex, the fol- lowing were the average survival periods for these several animals: opossums, six days; guinea pigs, seven days; dogs, ten days; cats, twelve days; while rabbits and rats may live on quite normally, because of the presence of accessory inter-renal tissue. Symptoms of insufficient cortin are loss of appetite with particular distaste for fats, vomiting, diarrhea, muscular twitching, tetanic convulsions, lowering of blood pressure and body temperature, and decline in urine secretion and heart rate. Lack of cortin is said to disturb the salt relation in the blood (particularly sodium chloride and potassium), the water metabolism and redistribution in the body, the metabolism (especially absorption) of intermediate carbohy- drates and fats, and milk production in females (at least in cats). The Pituitary Gland The pituitary gland is the human being is a body weighing about 0.5 Gm., lodged in a depression at the base of the brain. It consists of two principal parts : the anterior lobe and the posterior lobe. These two portions have distinct functions. The vital importance of this body was not realized for many years, but a series of observations has placed it in a position of such importance that it has been referred to as the regulator of the glandular system. The pituitary gland is now known to be the source of a number of hormones, and their functions are so closely connected with those of other endo- crine secretions that they apparently have a part in all hormonal processes of the body. The secretions of the anterior lobe of the pituitary affect the growth and development of the organism in general, the general metabolism of the body, the development of the sex organs, and work with other hormones in controlling additional processes. In- efficiency in the anterior lobe, furnishing the body with either too much or too little of the growth-promoting hormone, phyrone, re- ENDOCRINE GLANDS AND THEIR FUNCTIONS 673 suits in the production of giants or dwarfs. A decreased supply of the hormone in an immature individual, if the condition is not remedied by administration of the hormonal extract, retards the growth of the body and may cause a complete cessation of growth. The dwarfs of the circus furnish examples of this unfortunate con- dition, although not all dwarfism must be thought due to this cause. For example, the cretin described previously is the result of thyroid disorder and is usually a mental dwarf as well as a dwarf in body, while dwarfism resulting from deficient phyrone is accompanied in most cases by a normal mental development. Fig. 363. — A pituitary dwarf at the age of nine and one-half years, compared with a normal child of the same age. (From Zoethout, Textbook of Physiology, after Engelbach.) An overfunctional anterior pituitary results is a marked increase in the growth of bones, although the general development of the individual thus affected may be symmetrical and the physiological processes may be normal in every respect. Cases of this type are commonly seen. One such instance may be cited in which a nine- year-old boy measured six feet and one inch and weighed 178 pounds. His mental condition was normal. Another condition, known as acromegaly, may result from an over- functional anterior lobe of the pituitary during adult years. The 674 TEXTBOOK OF ZOOLOGY facial bones enlarge, particularly in the ridges above the eyes, the nose, and the lower jaw, and the soft tissues of the face undergo an overgrowth resulting in a coarsening of the features. The hands and feet may enlarge, also. The disease may prove fatal if it con- tinues sufficiently long. A second hormone secreted by the anterior lobe has a direct in- fluence on the sex organs. This hormone, prolan, stimulates growth and activity of the gonads, the testes and ovaries, and therefore controls the production of the gonadal hormones, which will be discussed later. The absence of prolan results in an atrophy of the testes and ovaries, and the cessation of the production of the sper- matozoa and the ova; its injection increases the activity of the sex organs. The posterior lobe of the pituitary also produces more than one hormone, although pituitrin is the one concerning which we have the most definite knowledge and which is commonly associated with this portion of the organ. Pituitrin is known to stimulate the mus- cles of the arterial system, increasing or decreasing the blood pres- sure according to the amount of the hormone released in the blood. It is also a stimulant for the musculature of the uterus and the intestinal muscles. It is concerned also with the regulation and disposal of carbo- hydrates in the body. The body is able to use an increased quantity of sugar when the secretion of pituitrin is reduced; and, on the other hand, when the quantity of the hormone is more than normal, the body needs less sugar; carbohydrates not actually needed are stored as fat, resulting often in abnormally fat people, extreme cases of which are seen in the circus. The posterior lobe of the pituitary probably does not affect the development of the bones, but the func- tion just discussed is quite definite. The posterior lobe of the pituitary is concerned also with the regulation of the secretion by the kidneys. A diseased condition, known as diabetes iTisipidus, in which the patient voids large quan- tities of urine, is treated by injection of the postpituitary hormone. It appears that the hormone probably enables tissues to utilize and store larger amounts of water than is possible in its absence. Still another effect of the secretion is found in the case of certain amphibians and reptiles ; that is, its effect on the pigmentation of the skin of these animals. The removal of the pituitary gland of a ENDOCRINE GLANDS AND THEIR FUNCTIONS 675 frog results, among other things, in the bleaching of the animal and the inability of the frog to alter the color scheme of its skin to agree with the surroundings. When in a strong light or on a light background the retinas of the frog's eyes are stimulated by light rays, and some of the impulses reach the pituitary 's posterior lobe, resulting in a suppression of its secretion, and consequently a lightening of the frog's skin. When the light is decreased the pituitary increases its secretion and the frog has a darker pigmenta- tion. These reactions probably do not occur so directly in the higher groups of vertebrates. Kemoval of the pituitary gland tends to cause atrophy of the other endocrine glands. The Thymus Gland The thymus, a small glandular structure located in the chest be- tween the upper part of the sternum and the pericardium, is a tem- porary organ, which normally atrophies in human beings by the time of the onset of puberty. When the gland is too active, a condition is found in children in which an enlargement of the organ results, and breathing is rendered difficult. No distinct hormone has been obtained from this gland. It has been claimed recently that accruing acceleration in the rate of growth and development occurs when successive generations of rats are given daily injections of thymus extract. In third and fourth generations, the rats at twelve days of age compared favor- ably with controls of twenty days. Introduction of thymus extract in young tadpoles causes them to grow rapidly to the size of the adult frog but still retain their tadpole form and appearance. The disappearance of the thymus at the time of puberty permits the differentiation of mature animals and particularly the onset of activity of the sex glands. Some workers claim that the thymus contributes to the orderly and proper development of the bones of the skeleton. The Gonads and Sex Hormones In addition to the usual function of producing germ cells for re- production, the gonads produce hormones which influence the devel- opment of secondary sexual characters and which have a regulatory effect on the reproductive processes and activities. Sex differences are caused in part by various hormones which have a selective action on the male or on the female secondary and accessory sex 676 TEXTBOOK OF ZOOLOGY characteristics. The earlier concept that the male sex produces "male" hormones exclusively and the female produces only "fe- male" hormones is no longer held. For example, extracts have been prepared from the urine of women as well as from men which on injection into capons caused growth of the comb which ordinarily fails to develop as it would in the cock. The sex hormone sub- stances affecting the male are spoken of as androgenic and those affecting the female as estrogenic. The important sex hormones are androsterone, testosterone, tJieelin, and progesterone (progestin). The first two are male hormones, and the others are female. Androsterone is found in male urine and can be crystallized from it. It has a stimulating effect on development of secondary sex char- acteristics and a definite regenerative effect on accessory organs (seminal vesicles, prostate glands, and penis) of castrated male ani- mals. Testosterone is produced in the interstitial tissue of the testis but is absent from the urine. This hormone is several times as effective as androsterone in bringing about regeneration of acces- sory sex organs in castrated males. Recently androgenic hormones, which are potent enough to affect the growth of comb in the capon, have been found in the urine and ovaries of female animals. Theelin (oestrin, estrogen, folliculin, menoform, progynon) has been isolated from liquor folliculi, pregnancy urine, the placenta, and amniotic fluid. This substance causes (1) increased growth of the accessory female organs (uterus, oviducts, etc.), including changes in the glands of their linings and in vascularization; (2) contraction of the smooth muscle of the uterus; (3) initial growth of mammary glands and nipples; and (4) sudden lowering of theelin concentra- tion in blood (suggested as cause for bleeding during menses). The secretion of theelin is influenced by the gonadotropic principle of the anterior pituitarj^ Progesterone (lutein hormone, corporin, luteosterone progestin) is a female hormone produced by the corpus luteum, the yellow body of material which forms in the ruptured Graafian follicle after the escape of the ovum. It produces the following effects: (1) sensitiza- tion of the lining of the uterus so that implantation or attachment of the zygote may take place in case of fertilization; (2) develop- ment of placenta; (3) arrest of rhythmic contractions of the smooth muscle of the uterus; (4) inhibition of ovum production and uterine bleeding. ENDOCRINE GLANDS AND THEIR FUNCTIONS 677 The influence of other hormones of the body is noted in the repro- ductive and sexual processes. The effect of the pituitary has already been mentioned. The thyroid secretion probably plays a role, not clearly understood, in the female reproductive processes, since the thyroid always enlarges at puberty and during pregnancy. Another hormone is thought by some investigators to be formed in the pla- centa during the development of an embryo. The interrelationships of these hormones are involved, and doubt exists in some cases as to their exact functions. The Pancreas This is one of the organs serving a dual purpose in the body; it secretes from a group of its cells, called the islands of Langerhans, a hormone designated as insulin. Experiments have shown the action of insulin to be concerned with the metabolism of carbohydrates and fats. Its presence facilitates the combustion of carbohydrates, regulates the rate of sugar production by the liver, and promotes the storage of sugar as glycogen in the muscles. It therefore de- creases the amount of sugar in the blood. Extracts of insulin are obtained from the pancreas of animals and are used commercially for the treatment of the condition kno^vn as diabetes. This disorder is due to a disturbance of the metabolism of sugars, provoked by a deficiency of insulin. The blood contains too great a percentage of sugar, but this is not turned into needed energy, and much water is excreted by the kidneys in order to elimi- nate the excess sugar. The patient suffers, therefore, from fatigue, excessive hunger, and thirst. The injection of insulin subcutane- ously assists in regulating the condition by restoring the power to transform glucose into glycogen in the muscles. The patient re- gains strength and weight as a result, but the treatment does not perfect a complete cure and additional insulin must be injected at intervals to maintain normal health. Overdoses of insulin result in very serious disturbances, which may be relieved by ingestion of glucose. Among the functions of the hormone, adrenalin, is the accelera- tion of the production of glucose in the blood; it is apparent, there- fore, that the hormones, insulin and adrenalin, are antagonistic in their effect on sugar metabolism, and an upset of the normal pro- duction of these secretions results in metabolic disturbances. 678 o t-t o & I— ( H W w Q O O O !zi TEXTBOOK OF ZOOLOGY -rJ Pi o o rv in o CO nJ O. 03 be o Tl d So-y §^ g O d CO o •r-1 -U O *-■ £ 2 a &p> ^ d -^2 CO ^ tn 1^ S '^

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X 'H 1-3 O Oi o _ c^ o ■- ■ =^' 3 .2 cc -u ■ — ^ ^ c S X ?^ ?^ fli ^ 3 2 a a o.s 2 -2 '-^s as o -s 3 c3 s OJ X o (B -Q 13 c3 ^ n 3 cS 3 Si a O > a) o 3 3 X 3 aj « 3 03 -^ "3 =^ as CO 3.5 ) ■JTJ -^ 3 3 fcC 3 '^'^ -a 2 -3 ^ 2 3 s 'a 3 r^ ^ =^ « ^ -3 O +^ 3 u S 3 3 a> r^ 3 ^H o a '•'"N 3 ,3 s^ OJ cS bx) ^ Ti 3 u Pi 3 ci> C^ ^^^ 3 be 3 3 O O 3 O 'r^ « ^ ^TJ "- « 3 3 ~— ' e<: Jan F^b Fig. 381. — Numbers of insects collected in average catch with 100 sweeps of an insect sweep net in a prairie ravine (Oklahoma) arranged according to orders through the seasons of the year. (Data from Carpenter.) 732 TEXTBOOK OP ZOOLOGY Fig. 382. — Total numbers of insects collected in average catch with 100 sweeps of insect net in prairie ravine (Oklahoma). (Data from Carpenter.) The scale for this graph is one-fourth of that in the preceding figure. c O ■« A Jan. Fih. Alar Apr /^aj/ June JuU' Aaq Sepf-. Oci- Nov Dec. Fig. 383. — Fluctuations in populations of four Protozoa in an artificial lake (Oklahoma). (Data from Bragg.) THE ANIMAL AND ITS ENVIRONMENT 733 Relative Numbers of Ten Most Abundant Insects From Overgrazed and Normal Grassland. (Wichita Mountains Wildlife Refuge.) GENUS common name normal OVERGRAZED Melanoplus (grasshopper) 11.0 220.0 Campylenchia (tree hopper) 0.5 144.0 Mermiria (grasshopper) 14.0 138.0 Scolops (plant hopper) 10.0 120.0 Elleschus (weevil) 0.9 80.0 Poeciloscytus (leaf bug) 1.2 54.0 Agallia (leaf hopper) 8.0 38.0 Harmostes (plant bug) 8.0 17.0 Deltocephalus (leaf hopper) 0.5 3.4 Brucliomorpha (plant hopper) 11.0 0.4 abundance in ten collections of fifty sweeps of the insect net, each. These collections were taken over a period of a month and indicate a very real difference in the insect populations of the two areas. Seasonal Changes It is a matter of common observation that the animals observed in any one place vary greatly from season to season during the year. This is not only true of those animals which, like many birds, migrate southward on the approach of winter and return to north- ern climates for nesting. Some animals hibernate or aestivate, and others spend a part of the year in an inactive stage, such as the egg, or the pupa. The life cycle of an organism must be adjusted to the annual climatic cycle of the climate in which it lives. Some idea of the variations in the number of insects during the year may be gained from a study of the accompanying charts (Figs. 381 and 382) giving the average catch with one hundred sweeps of an insect net at different seasons. A study of the abundance of Protozoa in an artificial lake shows a similar difference in the time of abundance of the various species. Summary This chapter has considered very briefly the distribution of the biotic communities of North America in relation to climate. The phenomena of succession and seasonal fluctuation of populations have been discussed, with examples. Attention has been directed toward the community rather than toAvard the individual organism 734 TEXTBOOK OF ZOOLOGY or the species. Similarly the sum total of physical environment as expressed in climate has been stressed rather than single factors, such as moisture, temperature, light, etc. The animal in nature is subject always to the action of a complex environment and its dis- tribution and reactions are the result of its response to the whole. The student is referred to the chapter on Animal Behavior for a discussion of the response of the individual organism to modifica- tions in single environmental factors. CHAPTER XXXVIII ANIMAL PARASITISM (By Sewell H. Hopkins, Texas A. and M. College) SOCIAL RELATIONS OF ANIMALS It has been explained in the previous chapter that no animal is ever entirely independent of others, since all plants and animals are influenced, directly or indirectly, by all the other organisms in the community. Most animals, however, can and do catch and eat their own food, and such animals are said to be "free-living." But there are thousands of species which depend either completely or partially on others to provide them with a livelihood. The vary- ing degrees of dependence are called commensalism, mutualism, and parasitism. The term symbiosis is applied to all cases of two different kinds of animals living together, and thus includes commensalism, mutualism and parasitism. In commensalism, one animal receives all of the benefit from the association while the other is neither benefited nor harmed. The jackal which follows the tiger and cleans up the carcass of the prey when the tiger has eaten his fill, the small fishes which accompany sharks and feed on the scraps wasted by the shark in feeding, and the oyster crab Avhich lives inside the oyster's shell and feeds on the organisms brought in by the oyster's feeding movements, are examples of commensalism. Mutualism is the kind of symbiosis in which both animals receive benefit from their association. One species of hydra (Hydra viridis) is green in color because a certain species of green alga lives within its cells ; the alga receives protection and some nourishment from its host, while the hydra benefits from the food manufactured by the green plant. A case of mutualism so far developed that the two animals cannot live separately is the relationship between wood-eating termites and their intestinal protozoa. The termite cannot digest the wood which it eats; the protozoa in the termite's intestine break down the wood into a form in which it can be used 735 736 TEXTBOOK OF ZOOLOGY by the host; on the other hand, the protozoa are absolutely de- pendent on the termite for food and the proper environment; neither termite nor protozoan can live without the other partner. The word parasitism in its broad sense applies to all cases in which one animal depends on another to furnish it with food; for instance, ornithologists call cowbirds parasites because they lay their eggs in the nests of other birds and leave the foster-parents to feed and care for the young cowbirds. Most zoologists, however, use the word parasitism only for cases in which the parasite lives in or on the body of its host; for exam^Dle lice live on the bodies of many animals, and tapeworms live in them. Origin of Parasitism How did it happen that some animals became dependent on others to furnish their food, that is, how did parasitism arise? There is a considerable amount of evidence for the belief that all parasites are descendants of free-living ancestors, and that these descendants, in the course of generations, gradually became more and more de- pendent on certain hosts, until in some cases they are now abso- lutely unable to make their own living. For example, certain species of nematodes which are free-living inhabitants of the mud at the bottom of ponds and streams are able to live in the large intestine of a frog if they happen to be swallowed by a frog. Other species, very similar to the mud-dwelling nematodes, have found the intes- tines of frogs such a good habitat that they live nowhere else; in other words, they have become parasites. Some intestinal parasites, in the course of many generations, have lost their locomotor struc- tures or even their digestive organs and yet continue to thrive because there is little or no need for locomotion or digestion when all food is brought to the parasite already digested by the host's intestine. Since such degenerate parasites are unable to secure food elsewhere, they are condemned by their peculiar structure to live as parasites in the intestine of their host. Degrees of Parasitism Free-living animals which sometimes become parasites when they get into another animal (by swallowing, for instance) are called accidental or occasional parasites, as in the case of the mud-dwelling ANIMAL PARASITISM 737 nematodes mentioned above. "Vinegar eels," nematodes in vinegar, sometimes establish themselves as harmless parasites in the human urinary bladder. Facultative parasites are able to live almost equally well as free-living animals or as parasites; many leeches are faculta- tive parasites. Obligate parasites, on the other hand, cannot live without the host. Parasites which are free-living during part of the life cycle, as in the case of the horsehair worms and some ticks and mites, are called temporary parasites, while animals like Acantho- cephala and tapeworms which are parasitic during the entire life cycle are called permanent parasites. The Successful Parasite Like all other ways of living, successful existence as a parasite requires certain modifications or adaptations in structure and func- tion. Parasites which live on the outside of the host's body are called ectoparasites; they must have special organs for attachment in order to maintain their hold on the host; for example, lice have hooklike feet with which they hold on to the skin, hair, or feathers of the host, and ectoparasitic trematodes have either muscular suckers or chitinous hooks for attachment to the outside skin or to the gills of the fishes on which they live. On the other hand, ecto- parasitic insects have no need for wings, so fleas and bedbugs con- tinue to thrive without them. Many ectoparasites, such as fleas, lice, bedbugs, mites, and ticks, also have specially constructed mouth parts for piercing their host 's skin and sucking blood. Endo- parasites, which live inside their hosts, also require special adapta- tions. For maintaining their positions in the intestine or other organs they must have some sort of attachment organ, such as the muscular suckers of trematodes and tapeworms and the hooks of thorny-headed worms. On the other hand, they live in the dark so eyes may be entirely lacking without inconveniencing the endo- parasite; usually all sense organs are either absent or very poorly developed. There is little or no need for rapid locomotion, so most endoparasites have locomotor structures much reduced or even en- tirely lacking. Many endoparasites also have less of a digestive system than their free-living relatives ; parasites in the liver, lungs, blood vessels, etc., usually have some sort of digestive apparatus, but many intestinal parasites, such as tapeworms and thorny-headed worms, have no sign of digestive organs whatever, but depend on 738 TEXTBOOK OF ZOOLOGY the host to furnish them with food already digested and ready for absorption. On the other hand, most endoparasites have their re- productive organs enormously developed, sometimes so much so that 90 per cent of the body is taken up by the reproductive sj^stem. This is in keeping with the general rule that animals whose offspring C. //, D. E. Fig. 384. — Arthropod parasites. A, human itch mite, female, Sarcoptes scabiei, ventral surface; B, ventral surface of male itch mite; C, body louse (cootie), Pediculus humanus corporis; D, head louse, P, humanus capitis; E, crab louse. Phthirius pubis. (From Sutton, Diseases of the Skin, published by The C. V. Mosby Company.) have the least chance to survive usually produce the largest number of offspring. In the ease of a tapeworm, for instance, the chance of any one egg being eaten by the right kind of host, so that it can develop into another tapeworm, is only one in a million, and tape- ANIMAL PARASITISM 739 worms would long ago have become extinct except for the fact that each tapeworm produces manj^ millions of eggs. The peculiar habitat and mode of life of endoparasites also makes necessary peculiar adaptations in the functions or physiology of the parasite, A parasite in the intestine, for instance, must be able to carry on respiration in almost complete absence of oxygen, must secrete substajices to counteract the digestive juices of the host in order to prevent its being digested, must be adapted to a high concen- tration of salts, acids, and other substances in solution in the fluid around it, and if in a warm-blooded animal must be able to live at a relatively high constant temperature. The fact that no host is immortal makes it necessary for a parasite to have some special provision for its offspring to escape to another host, in order to maintain the existence of the species ; this necessity is met by various peculiar adaptations in the life cycle or development, such as the complex succession of larval stages in the endoparasitic trematodes and cestodes. Fig. 385. — Diagram of the tunnel of an itcli mite in human skin. The female animal is depositing eggs. (Reprinted by permission from Introduction to Human farasitologii by Chandler, published by John Wiley and Sons, Inc. Adapted from Riley and Johannsen.) Some parasites are able to carry on parasitic activities without injuring their hosts, Avhile others may weaken or destroy the host. Parasites which injure their hosts are said to be pathogenic (disease- producing), while those which cause no appreciable injury are said to be nonpathogc7iic or commensal. Since most parasites cannot live without their hosts, a parasite which shortens the life of its host destroys its own home and means of livelihood ; nonpathogenic para- sites are more likely to be successful in the long run, and are there- 740 TEXTBOOK OF ZOOLOGY fore more abundant. Some parasitologists consider pathogenic parasites to be imperfect parasites because they are not quite per- fectly adapted for successful parasitic life, while nonpathogenic species are considered perfect parasites. However, no hard and fast line can be drawn between the two. Many parasites which are so perfectly adapted to their customary host that they produce no ill- effects have been found to be strongly pathogenic to other hosts where the adaptation is less perfect; for example, certain trypano- somes which are harmless to the antelopes of Africa, their natural hosts, produce the highly fatal African sleeping sickness when in- jected into men. Means of Infection and Transmission Many different means of transfer from host to host have been developed by the various kinds of parasites. These may be classified as below: A. Passive transmission. 1. In food or in water. 2. By bite of insects. 3. By sexual intercourse. 4. By direct contact. B. Active invasion under own power. By ** passive transmission" is meant the transfer of eggs or larvae from one host to another without any action of their own. For example, the eggs of cestodes and of some nematodes, such as Ascaris, pass out of the host's intestine in the feces; if food of other animals is contaminated by these feces, animals which eat this food will swallow the eggs, which hatch into larval worms within the digestive system of the second host and thus establish a new infection. Sheep may become infected with liver flukes by eating the encysted larvae on grass or swalloAving encysted larvae while drinking water. Trichina larvae encysted in hog meat de- velop into adult worms in the human, intestines if infected pork is eaten raw or improperly cooked. The second means of passive transmission is also widely used. Malaria parasites, the trypanosomes which cause African sleeping sick- ness, and many parasites of domestic and wild animals are carried ANIMAL PARASITISM 741 from infected individuals to new hosts by biting insects wliicli suck up the parasites with the blood and inject it into the new host with their salivary secretions. The third means is used by only a few parasites, most of them Protozoa. The spirochete which causes syphilis, a flagellate pro- tozoan called TricJiomonas vaginalis, and a trj'panosome parasitic in the reproductive organs of horses are examples. Some nematode parasites of insects are also transmitted in this way. The fourth means includes a few cases, such as the acquiring of dog tapeworm by people who kiss dogs and the transmission of pinworm from the hands of infected people who do not have clean habits. Hookworms and the human blood flukes, called schistosomes, are examples of parasites that invade new hosts under their own power. Their larvae are able to penetrate the skin. Parasitism and Host Specificity Since the beginning of the scientific study of parasitism, it has been recognized that different animals have different parasites ; for instance, the parasites found in and on goats are nearly all different from those of man. Some of the early parasitologists leaped to the conclusion that each species had its own peculiar species of parasites found nowhere else, and carried this idea of species specificity so far that they considered presence in different hosts to be sufficient evidence of specific difference of the parasites. Modern knowledge reveals that while some parasites are actually species specific others have a wide range of hosts. Thus the beef tapeworm. Taenia saginata, is found in the adult stage in man only, but the fish tapeworm, Diphyllohothrium latum, seems to.be able to live in nearly all mammals which eat fish. Three main factors determine whether a parasite will infect any given host: (1) opportunity for infection of host, determined by habits or mode of life of parasite and host (malaria parasites may be injected into any land animal by bite of mosquito, but strictly aquatic animals, such as fish, would not be bitten) ; (2) the environ- mental condition of the habitat furnished by the body of the host, involving such factors as body temperature, nature of outside sur- ., 742 TEXTBOOK OF ZOOLOGY face, size, chemical content of internal organs, etc. (intestinal para- sites of birds are seldom found in mammals, which have lower body temperatures, and parasites adapted to the oxygen-rich interior of a frog's lung can find no suitable habitat in a lungless fish) ; and (3) ability of the parasite to adapt itself to the wide range of en- vironmental conditions found in different hosts; thus D. latum, though it finds its optimum conditions in the humaji intestine, is adaptable enough to survive under the very different chemical con- ditions found in the dog, while Taenia saginata is usually unable to survive under these conditions. PARASITES AND THE GROUPS IN THE ANIMAL KINGDOM What kinds of animals are infested by parasites? Not only all phyla and classes, but all known species and probably all individuals of higher forms serve as hosts for some kind of parasite. Even in Fig. 386. — Giardia lamblia, an intestinal flag-ellate. A, face view; B, semlprofile view; G, cyst. (Reprinted by permission from Introduction to Hmnan Parasitology by Chandler, published by John W^iley and Sons, Inc.) the microscopic Protozoa many individuals harbor still smaller protozoans. For example, several species of parasitic Protozoa are found in Amoeba proteus. To which of the main groups of animals do parasites belong ? All animal phyla, except Echinodermata, include some species which ANIMAL PARASITISM 743 live as parasites, but the great majority of parasites belong to one of these four phyla : Protozoa, Platyhelminthes, Nemathelminthes, and Arthropoda. To cerebrospinal fluid cau&'ing steeping sickness and dealh. Transmission by bite of tsetse fjy. ^ Man, Antelope, etc. Xrypanoaomes in human blood causing Trypanosome jever- Transmission by bite of tsetse fly. Tsetse Fly n salivary glands 'or re- infection. -•' 30"- day) Critbidial ^orms in. salivary glands (2, or »3 days later) Forms in miclgut,(V6/ after infective meal). (lewly arrived form in .salivary gland. (I2«'lto,20"'days.) Long slender forms In pnoventriculus. ^about IO*''tol5*''dcry3) Fig. 387. — Life history of Trypanosoma gamhicnse. (Reprinted by permission from Introduction to Human Parasitology by Chandler, published by John Wiley and Sons, Inc.) '\ ,/J ;■! |r. Ml '4 Protozoa. — Of the four classes in this phylum, one, Sporozoa, is entirely parasitic; the other three (Sarcodina, Mastigophora, In- fusoria) also contain a number of parasitic forms. Examples of parasitic Sarcodina are the three common human amoebae, Endor- U S' 744 TEXTBOOK OF ZOOLOGY moeha histolytica (Fig. 391), which invades and destroys the intestinal lining, thns causing amoeljic dysentery; Endamoeba coli, a harmless Fig. 388. — Tsetse fly, Glossina, the transmitting- agent for trypanosoma, which causes African sleeping sickness. (Reprinted by permission from Introduction to Human Parasitology by Chandler, published by John Wiley and Sons, Inc.) Fig. 389. — Balantidmm coli, an infusorian parasite of the intestine. Active form from intestine. c.iK, anterior contractile vacuole; cyt., cytostome ; f.v. food vacuole ; n, nucleus. (Reprinted by permission from Introduction to Human Parasitology by Chandler, published by John Wiley and Sons, Inc.) commensal in the intestine ; and Endamoeha gingivalis, a very common parasite in the human mouth, usually harmless but sometimes ap- parently injurious to the gums. Examples of parasitic Mastigophora 1 ANIMAL PARASITISM 745 are the human intestinal flagellate, Giardia lamhlia (Fig. 386), and the blood-inhabiting trypanosome, Trypanosoma rhodesiense, caus- ative agent of African sleeping sickness which is carried by the Tsetse fly, Glossina (Fig. 388). Examples of parasitic Infusoria are the human intestinal ciliate, Balantidium coli (Fig. 389), the various species of Opalina, and related genera found in the excretory bladder or cloaca of frogs and toads. Of the thousands of species of Sporozoa, all of which are parasitic, probably the best known are the three species of the genus Plasmodium (Fig. 393), which cause human malaria, and Babesia higemina, which produces Texas tick fever of cattle. Platyhelminthes. — This phylum also contains four classes, two of which, Trematoda (flukes) and Cestoda (tapeworms), are all para- sitic, while the other two, Turbellaria and Nemertinea, are mainly freeliving but contain some species which are parasitic on aquatic invertebrates. Among the best known examples of Trematodes are Fasciola hepatica (Figs. 398 and 399), the sheep liver fluke; Clon- orchis sinensis (Fig. 397), the Chinese human liver fluke; and Schis- tosoma Jiaematohium, one of the three species of human blood flukes. Probably the best known tapeworms are Taenia saginata (Fig. 402), the beef tapeworm. Taenia solium, the pork tapeworm and Diphyllo- hothrium latum, the broad fish tapeworm, all three common parasites of the human intestine, and EcJmiococcus granidosus, a dog and wolf tapeworm whose larval stage is the cause of a horrible human disease. Nemathelminthes.— The single class Nematoda includes at least 95 per cent of the species in this phylum ; most of them are free- living, but there are also thousands of parasitic species. Examples of parasitic species are the human hookworms, Necator americanus, the American hookworm, and Ancylostoma duodenale (Figs. 394 and 395), the Old World hookworm; Ascaris lumhricoides (Fig. 90), the large intestinal roundworm of hog and man ; Dracunculus medinensis, the Guinea worm, often over a yard long, which crawls around under the human skin (believed by some to be the "fiery serpent" mentioned in Exodus) ; Trichinella spiralis, which causes the often fatal human disease, trichinosis, when its larvae, encysted in pork, are eaten by man ; and Wuchereria han- crofti, the fiJaria which is injected into the human blood by certain tropical mosquitos and causes elephantiasis, a disease in which the infected limbs may become larger than the body of the victim. 746 TEXTBOOK OF ZOOLOGY Onciiocerca volvulus, transmitted by certain biting flies, is a com- mon cause of blindness in some parts of Mexico. Besides the human nematodes there are thousands of others parasitizing lower animals, both vertebrates and invertebrates. The other two classes of the phylum Nemathelminthes are entirely parasitic; the Acan- thocephala, or thorny-headed worms, are common intestinal para- sites of many vertebrates, including the hog and occasionally man; Fig. 390. — Elephantiasis, some extreme cases. A, of legs and feet; B, of scrotum ; C, varicose groin gland ; D, of scrotum and legs ; E, of mammary glands. (Reprinted by permission from Introduction to Human Parasitology by Chandler, published by John Wiley and .Sons, Inc. A and B sketched from photograps from Castellani and Chalmers; C, D, and E from Manson.) the Gordiacea or horsehair worms (Fig. 88) are parasites of insects until nearly mature; they crawl out of their insect hosts when the latter fall into water, become sexually mature, and lay their eggs. Arthropoda. — All of the classes in this phylum are predominantly free-living, but several classes also include parasitic species. The ANIMAL PARASITISM 747 class Hexapoda or Insecta contains, besides several hundreds of thou- sands of free-living insects, the parasitic fleas, lice, and bedbugs; the class Arachnida, characteristically free-living, contains the para- sitic ticks and mites, and the class Crustacea, though mostly free-liv- ing, includes a number of species parasitic on fishes and other aquatic animals. While most of the parasitic arthropods are ectoparasites, there are also a few endoparasitic species. For example the horse bot, Gastrophilus, which is the larva of a fly, is parasitic in the stom- ach of horses; long wormlike arachnids known as Linguatulids or tongue worms are found in the intestines of some reptiles and mam- mals; and Sacculina (Fig. 404), a crustacean, parasitic on crabs and lobsters, sends rootlike outgro\vths all through the body of its host, although the saclike body remains on the outside. Some Representative Parasites Protozoa. — The very small amoebalike protozoans of the genus Endamoeba are examples of parasites only slightly modified for parasitic life. There are two distinct stages in the life cycle, the Fig. 391. — Amoeba histolytica, one of the important protozoan parasites. It is the causal a.erent of amoebic dysentery. A, Stained vegetative amoeba ; B, cyst with four nuclei ; n, nucleus, showing peripheral chromatin granules and central karj-osome ; r.b.c, ingested red blood corpuscles; chr.b., chromatoid body. (Re- printed by permission from Introduction to Human Parasitology by Chandler, pub- lished by John Wiley and Sons, Inc., after Dobell.) V\ i active form being much like a small amoeba except that the pseudo- podia are shorter and move more slowlj'"; these active forms finally round up and become surrounded by a semirigid, resistant cyst wall. In this encysted condition Endamoeba is passed from the host with the feces or other body excrements. While in the encysted con- 748 TEXTBOOK OF ZOOLOGY dition the parasite divides by binary fission into first two, then four, and finally (in E. coli) eight little amoebae. If the cyst is swallowed by another host, the cyst wall is dissolved and the four or eight young amoebae come out to begin the active stage again. The com- mon Endamoebae of man are E. gingivalis which lives in the mouth and is usually transmitted by kissing, and the two intestinal species E. coli (nonpathogenic) and E. histolytica. The latter species breaks down the cells of the intestinal lining by means of enzymes which it secretes, and then ingests the broken cells in the same way that the common free-living amoebae take in their food. The disease caused by E. histolytica is known as "amoebic dysentery." Infection occurs as the result of eating food or drinking water which has been con- taminated by the feces of infected people, as in the case of the Fig. 392. — Endanioeba coli. A, stained vegetative amoeba; B, cyst witli eight nuclei, n., nucleus, showing coarse peripheral chromatin granules, chromatin granules in "clear zone" between periphery and kai-yosome which is eccentric in position ; chr.b., remnant of chroniatoid body. Numerous food vacuoles in vegeta- tive form. (Reprinted by permission from Introduction to Human Parasitology by Chandler, published by John Wiley and Sons, Inc., after Dobell.) Chicago hotels where contamination of drinking water by water siphoned up from the toilet drains into the water pipes caused a serious outbreak in 1933. The malaria parasites, of which there are three species infecting man (Plasmodium vivax, P. falciparum, and P. malariae, each caus- ing a different form of malaria), are Protozoa belonging to the class Sporozoa, and are very highly modified for parasitic life. The adaptations for parasitism and for transmission from host to host involve a very complex life cycle. The two main phases of the life ANIMAL PARASITISM 749 , iiObf M w rc^ >> ^ S c c=i^ j;^ » c« C cti) o p o- w3 or 0 = ^0,2 2 5.^2 0^ cs to t.c^.2ca "1 .§.2c55j« 0-; o C 3 a-iCQ '^ C C P I 'a gcS"? aP^be'; c S 5 o.d•" .U y OB K^_ 73 u,,^ cu C > c o X O ^ '"^E^feog ^ a a-^-o -M =2S ^ m 750 TEXTBOOK OF ZOOLOGY cycle are the vegetative or schizont stage (merozoites) and tlie sex- ually reproductive or sporont stage. The biology of this parasite has been discussed in the earlier chapter on Protozoa under class Sporozoa and Economic Relations of Protozoa. Nematodes. — Of the thousands of species of parasitic nematodes, space permits mention of only a few which are particularly impor- tant because of their danger to man. Hookworms. — In the United States the most important human nematode parasite, from the public health viewpoint, is the American hookworm, Necator americanus. Although called the "American hookworm" this species probably came originally from Africa and was introduced into America by the negro slaves. The pioneer work on hookworm in the United States was done by Dr. Charles W. Stiles in 1901. Hookworms are slender threadlike nematodes about one- half inch long; the females are tapered to a point at each end, while the slightly smaller males have on the posterior end a fanlike ex- pansion, the copulatory bursa, with curved riblike supports. Both sexes have a large mouth containing hooklike chitinous teetii by means of which they tear holes in the walls of the intestine and start blood flowing from the wounds. A muscular esophagus leading back from the mouth cavity gradually broadens into a large muscular bulb ; by means of rhythmic contractions and expansions of the bulblike esoph- agus blood is drawn into the mouth and forced down into the straight intestine where some of it is digested and the rest passes on through and out of the anus near the posterior end. Because of the large number of worms present there is a serious loss of blood resulting in anemia and lack of energy; in children the growth is stunted or re- tarded by hookwonns, and often there is also a lack of proper mental development. Individuals, very heavily infected during childhood and early youth, may fail to develop sexually. Treatment is fairly easy, hookworms being easily killed by doses of anthelmintics, such as carbon tetrachloride and hexylresorcinol (which are poisonous and should be taken only under doctor's supervision). Each female hookworm produces 9,000 eggs per day; these eggs pass out with the feces of the host; if the infected person defecates on the ground, the eggs hatch and the larvae crawl around in the soil; there they develop into infective larvae which live for several months on the surface of the ground. If bare human skin comes in contact with these microscopic worms they bore through it to the ANIMAL PARASITISM 751 Fig. 394. — Ancylostoma duode- nale, female and male, with head of Necator americanus drawn to same scale. a7i., anus ; b^ bursa ; b.c, buc- cal capsule ; cem.gl., cement gland ; cerv.gl., cervical gland ; cerv.p., cervical papilla ; cl., cloaca ; c.sp., caudal spine ; ex.d., so-called excre- tory' duct; int., intestine; n.ceph.gl., nucleus of cephalic gland ; n.r., nerve ring ; oes., esophagus ; ov., ovaiy ; ovej., ovejector ; sp., spicules ; t., testes ; ut., uterus ; vag., vagina ; V.S.. vesicula seminalis. (After Looss from Chandler, Hookioorm Disease. Reprinted by permission from Introduction to Human Para- sitology by Chandler, published by John Wiley and Sons, Inc.) 752 TEXTBOOK OF ZOOLOGY ,- gn d»l_- Fig. 395.-^Life history of hookworms from egg to infective larva. 1, &^^ of Necator aniericanus at time of leaving host; 2, same of Ancylostoma duodenale; 3 to 7, segmentation and development of embryo in egg ; 8, newly hatched embryo ; 9, same of Strongyloides for comparison (note length of oral cavity and size of genital rudiment, } of cattle and sheep in Louisiana, Arkansas, and Texas, is very similar i in structure and life history. 760 TEXTBOOK OF ZOOLOGY Other important flukes are the human intestinal fluke, Fasciolopsis huskii, which is common among the Chinese, who become infected by eating the cysts on various aquatic food plants; the human lung fluke, Paragonimus westermanii of eastern Asia, where the natives become infected by eating the encysted larvae in fresh-water crabs Fig. 399. — Life history of the liver fluke. Fasciola hepatica. A. adult in liver of sheep ; B, freshly passed egg as it leaves the body of the host ; C, developing em- bryo, ready to hatch in the water ; D^ ciliated miracidium embryo in the water and about to enter the pulmonary chamber of snail ; E, F, sporocyst containing rediae ; G, redia containing daughter rediae ; H, redia of the second generation containing cercaria ; J, same having emerged from snail into water ; K, cercaria encysted on blade of grass ; L, cercaria liberated from cyst after ingestion by sheep ; M, young fluke developing in liver of sheep. (Reprinted by permission from Chandler, Introduction to Human Parasitology, published by John Wiley & Sons Inc.) ANIMAL PARASITISM 761 Fig. 400. — Structure of tapeworm to show different stages of maturity. At lower left. Taenia insiformis, dog- tapeworm, scolex with hooks and sucker discs. At lower right, mature proglottids. Above, scolex and proglottids of three ages. The reproductive organs and pore are shown in most proglottids. (Courtesy or Wards Natural Science Establishment, Inc.) 762 TEXTBOOK OF ZOOLOGY and crayfishes; Gotylophoron cotylophorum, a stomach parasite of cattle in Louisiana, and Dicrocoelium lanccahnn, a common liver fluke of herbivorous mammals in Europe and Asia. Numerous ectoparasitic trematodes, of the Subclass Monogenea, occur on the skin and gills of fish and are often of economic im- portance because they kill goldfish and other aquarium fishes, and also young fishes in state fish hatcheries. 0 \ Fig-. 401. — Development of tapeworm. A, six-hooked embryo ready to become embedded in muscle ; B, cysticercus, or bladder worm as encysted ; C. section through developing scolex in cysticercus; D, later stage; E, scolex everting as it protrudes from bladder; F, extension of scolex from bladder; G, later stage ;^, formation of proglottids. (From Parker and Haswell, Zoology, published by The Macmillan Company, after Jijima and Hatschek.) The Tapeworms (Class Cestoda).— Cestoda differ from Trematoda in the complete lack of a digestive system. In fact, Cestodes never have any sign of a digestive organ at any time during life; they receive their nourishment by absorbing through the surface of their bodies the food already digested for them by the host. Most cestodes also differ from trematodes by having the body divided into a series of segments, one behind the other, each segment having a com- plete set of reproductive organs. This structure characteristic of ANIMAL PARASITISM 763 tapeworms is usually referred to as segmentation of the body, but it is probably more correct to consider a tapeworm as a linear colony, in which the segments are really individuals in various stages of maturity. Taenia solium, the pork tapeworm of man, may be taken as an ex- ample to illustrate the structure and life history of a cestode. The adult tapeworm consists of a scolex or head provided with four mus- cular suckers and a snoutlike rostellum surrounded by a row of Head flead mi /Mature serine nis /Aature Fig. 402. — Common tapeworms, showing different regions of the body. At the left above, scolex of Taenia saginata, beef tapeworm ; left below, proglottids of Monie:sia, sheep tapeworm ; middle, scolex and proglottids of Taenia solium, pork tapeworm ; right, scolex and proglottids of Dipylidium caninum, a dog tapeworm. (Courtesy of General Biological Supply House.) chitinous hooks, which serve as means of attachment to the wall of the human intestine; a narrow unsegmenfed neck behind the scolex, and then a series of several hundred proglottids (the segments) be- coming progressively larger as they get farther from the scolex. The whole chain of proglottids is called the stroMlus. New proglottids are constantly budded off from the neck; consequently, the young- est proglottid is the first one back of the neck and the oldest one is 764 TEXTBOOK OF ZOOLOGY the one at the end of the strobilus farthest from the scolex. The youngest proglottids contain no recognizable structures, except the paired longitudinal nerve cords and longitudinal excretory vessels which run the full length of the strobilus, and a transverse excre- tory vessel in each proglottid. As the proglottids become older and are pushed farther away from the scolex, the reproductive organs begin to develop; each proglottid develops a complete set of both male and female reproductive organs; when these become mature and ready to function, the proglottid is a mature proglottid. Each mature proglottid is capable of copulating with itself, by bending the cirrus down into its own vagina, or it may copulate with another mature proglottid of another tapeworm if one is present close by. After copulation the male organs begin to degenerate, the uterus becomes filled with fertilized eggs and takes up more and more space, then the ovaries, vitellaria, and other female organs degen- erate and leave the uterus to occupy nearly the whole proglottid; in this condition the proglottid, now hardly more than a sack of eggs, is called a gravid proglottid. The gravid proglottid at the extreme end of the strobilus breaks off and passes out of the host's intestine with the feces. It continues to live and crawl slowly through the feces like an independent animal for a few hours, then usually dies, but the embryos within the proglottid remain alive much longer. If gravid proglottids or separate eggs are eaten by a hog, the six-hooked hexacanth larva hatches in the hog's intestine, bores through the intestinal wall, and migrates to other parts of the body where it changes into a bladder-Avorm or cysticercus, which is a saclike larva with an inverted scolex. The cysticerci remain in the flesh of the hog until the pork is eaten by man ; under the influence of human digestive juices the cysticerci become everted so that the scolex is on the outside of the saclike part, then the scolex attaches itself to the wall of the human intestine, proglottids begin to bud off from the neck, and an adult tapeworm is formed within a few weeks. Taenia saginata, the beef tapeworm, has a similar life history, but uses cattle instead of hogs as intermediate hosts. Taenia serrata, a common dog tapeworm often used as a laboratory specimen, is very similar to the two human species in structure and life cycle. ANIMAL PARASITISM 765 Other important cestodes are Hymenolepis nana of man and mice; the broad fish tapeworm, Diphyllodothrium latum, which man gets by eating raw or poorh^ cooked fish ; the peculiar Echinococcus granu- losus, adult in dogs, whose cysticercus stages are dangerous parasites of man (man being the intermediate host in this case) ; and many tapeworms of domestic animals, such as Dipylidium caninum of dogs, Thysanosoma and Moniezia of sheep, goats, and cattle, and majiy others. CHAPTER XXXIX MARINE ZOOLOGY It is true that a good many people never have an opportunity to study the conditions present in the ocean or to observe the animals found in its waters and on its beaches. However, with the improved transportation and awakened interest in the subject, there are more and more students of this fascinating subject. Many of the animal forms seem peculiar and spectacular to those of us who reside in- land. With the modern facilities for travel four hundred miles is not remote from the seashore, and every student majoring in biology in colleges or universities within that distance should be given marine experience, first hand. The life of the ocean is known as lialohios. Marine animals are affected and limited by many factors which in turn influence their distribution. They are affected by the temperature of the water, the height of the tides, the velocity of currents, salinity of the water, its turbidity, light, pressure, oxygen content, and the nature of the bottom and the shore. The occurrence of the proper food greatly affects the range and abundance of any particular species. Rocky shores harbor the choice food of certain species and muddy lagoons supply other groups. The adaptation and adjustment of the marine animal to the salinity of the water is one of the first considerations. The salt content is effective both quantitatively and qualitatively. If an oyster is taken from the ocean and placed in a fresh-water pond or stream, it absorbs water and swells up excessively. On the other hand, a clam taken from fresh water and placed in the ocean loses water and shrinks. In either case the effects will finally be- come lethal. A few animals, such as lamprey, eels, shad, salmon, and even gar pike and mullet are able to make the transfer from marine to fresh water and vice versa. The salinity of the water of the marine habitat is an important and interesting feature to be studied. Along the Texas coast the water is either that of the Gulf of Mexico proper or of the various bays. The analyses* which have been made on the Gulf water out ♦Reported by Mr. J. G. Burr, Texas Game, Fish, and Oyster Commission. 766 MARINE ZOOLOGY 767 from Matagorda and out from Port Aransas show a high degree of salinity. The readings range from 36.6 to 37.1 parts of salt pei* one thousand. The average given for the Atlantic Ocean is be- tween 35 and 36 parts per one thousand. These readings on the Gulf are being checked by further data, but the cause of this dif- ference has not been explained. The salinity of the bays is quite variable, and Galtsoff in 1926 made comparative studies of a number of them. It was found that in bays with a large fresh-water stream entering, as Nueces Baj^ ;T— L/mbrella // -Manubrium 7mm- y/M oral ~^ tentadei ■ Central moufch Fig. 403. -Cabbage-head jellyfish, Stoviolophus meleagris, a very common form in the Gulf of Mexico. the range is from 6.06 parts per thousand in June to 33.06 parts per thousand in September. In Mesquite Bay at Belden Dugout Beacon the range is from 5.03 parts per thousand in June to 18.44 parts per thousand in October. In Aransas Bay, where there is relatively small fresh-water intake, the salinity ranges on the aver- age between 14.79 parts of salt per thousand in June and 25.47 parts per thousand in September. These wide ranges of salinity in the bays create problems for the organisms attempting to live there. It requires a high degree of 768 TEXTBOOK OF ZOOLOGY adaptation to salinity changes to be able to do it. Not only do floods coming" in from streams bring serious disturbances, but the opposite effect may be brought about by storms rolling the waves Fig-. 404. — Sacculina. A common parasite on the crab. It is recognized as an arthropod only by its larval stages because the adult looks like a sponge or fungus. A. and B, larval stages with jointed appendages ; C, crab infested with adult Sac- culina. The figure shows it on one .side only. (From Lull, Organic Evolution^ pub- lished by The Macmillan Company.) in from the sea. There are opportunity and need for much more study of the effects of these phenomena. MARINE ZOOLOGY 769 In general, marine animals are adapted to one of three habits of life : dwelling on the bottom, swimming at various levels, or simply floating at the surface. There are three or four forms of marine animals according to the position they take in the water. (1) Benthos include all nonswimming bottorm-dwelling forms, and they may be vagrant or sessile (stationary). Hydroid and crinoid types exemplify the sessile form and urchin or starfish the vagrant. The benthos may extend from the shore line to the deep sea. (2) Nekton is the name of the entire group of swimming animals that are able to maintain themselves in the water and do not rest on the bottom. The fishes, whales, and porpoises are typical examples. (3) Plankton is the collective name for plants and animals that float in the water. Many of the plankton animals are nearly transparent, and the smaller ones are surprisingly abundant. Because of their trans- parency and size, the casual observer seldom sees this group of animals. The nature of the tissue of the bodies of these animals is largely of gelatinous material and the shape is either that of an umbrella or of a mass with projecting processes. The composition of the tissue is such that it is bulky without great weight. In fact, it is largely water and has a specific gravity slightly greater than water. These adaptations make it possible to float with an easy distribution of weight and very little if any effort on the part of the animal. A large number of Protozoa, jellyfishes, and Entomo- straca (microscopic Crustacea) are typical plankton. (4) Pelagic animals are all of the surface-living forms away from the shore in the open sea. Another expression of the distribution of marine animals is by the following zones. (1) Littoral zone, which is the shallow water along the shore. Here the fluctuations of wave action, temperature, and depth (because of tides) are extreme, and the light intensity is at its maximum. Such aaiimals as certain coelenterates, worms, Crus- tacea, echinoderms, and molluscs are common inhabitants of this zone. It requires a rather sturdy type of body and hardy proto- plasm to Avithstand the hardships of the shore and near-shore. Some clams and oysters, also corals, sea urchins, and starfishes can live out of the water for several hours. The majority of corals live where they can depend on wave action to carry floating objects to 770 TEXTBOOK OF ZOOLOGY them for food. The zone extends from shorelme to a depth of from 50 to 75 feet. (2) Intermediate (sublittoral) zone includes relatively shallow water of from 50 to 500 feet and supports hydroids, sponges, some corals, sea cucumbers, sea urchins, and starfish. There is a grad- ual increase in stability of conditions through this zone. (3) Bathyal (abyssal) zone is in the depths of the ocean ranging from 5,000 to 1. 3. 9. 10. 11. 12. Fig. 405. — A group of typical mollusks occurring along- the Texas coast of the Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Cheatum). 1, Solen viridis. 2, Dosina discus. S, Seniicassis gibba. !i, Oliva sayana. 5, Donax roenieri. 6, Venus campechiensis texana. 7, Crepiduln fornicata. 8, Polinices duplicnta. 9, Thais haeniostoma floridana. 10, Architectonica granulata. 11, Littorina iiTO- rata. is. Modiolus tulipus. 25,000 feet. The pressure increases fifteen pounds per square inch with each thirty feet of increased depth. The conditions become even more constant in this zone. Below 800 feet there is no light and enormous pressure. In the great depths the temperature is nearly constant and is about 2° C, or just above freezing. Most of the animals found in this zone are relatively small and somewhat modified MARINE ZOOLOGY 771 structurally as well as physiologically in order to adjust to the con- ditions. Approximately 150 species are known to exist at 15,000 feet or deeper. There is a tendency toward the development of phosphorescent organs and telescopic eyes among animals inhabiting the bathyal regions. Fig. 406. — Common bivalve and univalve mollusks that occur along the Texas coast of the Gulf of Mexico. (From specimens belonging to Dr. Elmer P. Chea- tum.) 1, Dinocnrdiuni robustum. 3, Busucon perversus. 3, Barnea costata. i, Ostraea virginica. 5, Plagioctenium irradiens. 6, Atrina seminuda. The methods of studying the conditions of the ocean are largely by survey methods, use of light disks, pressure gauges, electric thermocouple temperature recorders, hydrogen ion analysis ap- paratus, oxygen tension analyses, and chemical analysis for salinity. Samples of sea water can be obtained from various depths by use of a sampling tube or cylinder which is lowered while open so that water passes right through it. When the sample is desired, a lead messenger is sent down the cable to trip the trigger and allow the cylinder to snap shut at both ends. The cylinder with the sample 772 TEXTBOOK OF ZOOLOGY Fig. 407. — Peropliora, a common tunicate which rangres from British Columbia to San Diego along the Pacific Coast. Notice siphons, digestive tract (black), and gill slits in wall of pharynx. (Courtesy of Albert E. Galigher, Inc.) PAGURUS LIBINIA Fig. 408. — ^Representative crabs. Vca (fiddler), CalUnectes (blue edible), Pagurus (hermit), Libinia (spider). Abundant on shores of Gulf of Mexico. (Courtesy of General Biological Supply House.) MARINE ZOOLOGY 773 can then be drawn up. Many of the smaller animals will be in- cluded in such samples. Other methods of collecting the animals is by use of metal dredges, dredge nets, shrimp nets, cord mops or tangles, dragnets along shore, townets, and fishing tackle gen- erally. It is necessary to use a spade and seine in the beach sands and some sharp instrument for scraping pilings and rocks. The shore and the drift line offer many opportunities of collecting and observing a wide range of animals with no special apparatus. Studies of distribution and migration of marine animals have been made by tagging large numbers of individuals and compiling rec- ords of the return of these tags. Fig. 409. — -The common pompano, TracMnotus carolinus, a valuable food fish which is taken abundantly along the southern part of our Atlantic Coast and in the Gulf of Mexico. In an effort to give an idea of the typical animal life of the sea- shore and shore waters, two tables of representative animals are included. The first includes many of the marine and shore animals, excluding vertebrates, of the Pacific coast, while the second pro- vides a similar representative group including fish from the shores and waters of the Texas portion of the Gulf of Mexico. The rocky shores of our western coast furnish an abundance of life. To the casual observer standing almost ajiywhere on the Texas beach, it seems almost like a desert at the seashore. The shore fauna is rela- tively sparse because the bare sand and shell fragments are almost sterile of food. Closer observation, however, will reveal much more life than at first thought, as will be indicated by the representative lists which follow. 774 TEXTBOOK OF ZOOLOGY Typical Pacific Shore Animals NAME HABITAT ZONE Porifera Beneira cmeria, purple sponge Plocamia karykinos, red sponge Leucosolenia eleanor, cream colored sponge Leucon/ia heatM, sharp-spined sponge Coelenterata Garveia annulata, hydroid Eudendrvum calif orniaum, hydroid Tubularia marina, solitary hydroid Aglaophenia struthionides, ostrich plume hydroid Gonionemus vertens, hydroid medusa Velella lata, siphonophore Stylatula elongata, sea pen Benilla amiethystina, sea pansy Cribrina xanthogrammica, large green anemone Cribrina elegantissima, verrucose anemone Epiactis prolifera, brown anemone Metridimm, dianthus, anemone Balanophyllia elegans, orange-red coral Ctenophora Pleurobrachia bachei, "cat's eye" comb jelly Bolinopsis microptera, comb jelly Platyhelminthes Planocera califomica, large flatworm Leptoplana acticola, polyclad worm Polychoerus caudatus, red flatworm Nemertina Amphiporus bimaculatus, orange nemertine worm Emplectonema gracile, yellowish green nemertine worm Bryozoa Bulgula califormca, purple Bryozoa Bulgula pacifica, bryozoan Annelida Audounia luxuriosus, hairy gilled worm Nereis vexillosa, mussel worm Nereis brandti, large mussel worm Lwrnbrinereis sp., nereid worm MoUusca Pelecypoda Siliqua paUila, razor clam Tivela stultorum, pismo clam Mytilis edulis, edible clam Mytilis californiamts, mussel Ostraea lurida, western oyster Middle tide pool rocks Middle tide pool rocks Middle tide pool rocks Middle tide pool rocks Lower tide pools Lower tide pools Lower tide pools Lower tide pools Eel grass at water edge Driven ashore by storm Mud flat below tide Sand flats Rocks, lower tide pools Eocks, middle tide Eocks, middle tide pools Low tide pilings Rocks, middle tide pools Cast ashore (Planktonic) Cast ashore (Planktonic) Eocky shore, high tide levels Eocky shore, upper tide pools Middle tide pools Middle tide pools Middle shore Lower tide pools Middle tide pools Between rocks and in mud Cosmopolitan Cosmopolitan Mud and sand flats Sandy beaches Sandy beaches Sandy beaches Sub-tide and half-tide Low tide, rocky bay MARINE ZOOLOGY 775 Typical Pacific Shore Animals — Cont'd NAME HABITAT ZONE Pecten hmdsii, Bcallop Pecten drcidaris, thick scallop Cardita subqundrata, bivalve Schizothaerus Tmttallii, gaper clam Gastropoda Anisodoris noiilis, sea lemon (nudibranch) Dialula sandiegensis, nudibranch PleurophylUdea californica, nudibranch Tethys californica, sea hare Littorina planaxis, gray littorine Littorina scutulata, checkered littorine Littorina sitchana, Sitka periwinkle Littorina rudis, North Pacific white peri- winkle Purpura folitita, a conch Cypraea spadicea, the cowry CoTvus californicus, cone shell Teg-ula funehralis, black turban snail Tegula trunnea, brown turban Norrisia norrisii, smooth turban Tegula ligulata, banded turban Thais lamellosa, wrinkled purple, snail Polinices lewi9ii, moon snail OliveUa biplicata, purple olive snail Urosalpinx cinereus, oyster drill Crepidula adunca, hooked slipper Lottea gigantea, owl limpet Acmnea scaira, scab limpet Acmaea limatula, file limpet Acmaea digitalis, limpet Acmaea scutum patina, plate limpet Acmaea ca^ssis pelt a, shield limpet Acmaea mitra, white cap limpet Acmaea depict a, painted limpet Haliotis nifescens, red abalone Amphineura Mopalia muscosa, moss chiton KathaHna tunicata, black chiton Cryptochiton stelleri, giant chiton Lepidochitona lineata, lined chiton Cephalopoda Polypus iimaculatus, octopus Loligo opalescent, opalescent squid Echinodermata Asteroidea Pisaster ochraceous, five-rayed starfish Pisaster giganteus capitat\is, five-rayed starfish Pisaster hrevispinus, short-spined starfish Evasterias troschelii, small starfish Pycnopodia, helianthoides, large starfish Leptasterias aequalis, six-rayed starfish Patiria mininta, webbed starfish Low tide, rocky bay Low tide, rocky bay Low tide pools Substratum burrow Middle tide pools Eocks, middle tide pools Low tide level, open bay Eocky middle tide pools Eocky shore, high tide level Eocky shore, upper tide pools High tide bay High tide bay Eocky shore, lower tide pools Eocky shore, lower tide pools Eocky shore, lower tide pools Upper tide pools Eocks, middle tide pools Eocky shore, lower tide pools Eocky shore, lower tide pools Eocky shore, bay Low tide level, open bay Low tide level, open bay Mud flats, intertidal Middle tide pools Eocky shore, high intertidal zone Eocky shore, upper tide pools Eocky shore, upper tide pools Eocky shore, high tide level Eocks, middle tide pools Eocks, middle tide pools Eocks, middle tide pools Eocks, lower tide pools Lower tide pools Intertidal rocks Intertidal rocks Eocks at low tide Low tide level Lower tide pools Washed ashore occasionally Lower tide pools under rocks Lower tide pools under rocks Below tide line Below tide line Below tide line Middle tide pool rocks Middle tide on rocks 776 TEXTBOOK OF ZOOLOGY Typical Pacific Shore Animals — Cont'd NAME habitat zone Ophvwroidea Ophioderma panamensis, brittle starfish Ophioploous esmarlci, sand-colored brittle star Amphipholis pugetana, small brittle star- fish Amphiodia ocoidentalis, brittle star OphiophoUs amleata, brittle starfish Echivjoidea Strongylocentrotus franciscamis, large sea urchin Strongylocentrotus purpuratiis, purple sea urchin Lovenia cordiformis, red heart urchin Dendraster exoentricus, sand dollar EchinarachniAis parma, sand dollar Eolotfuu/roid^a Cucumaria miniata, sea cucumber Leptosynapta inhaerens, dirty white sea cucumber Stichopus calif omicws, brown sea cucumber Crustacea Copepoda Tigriopus fulvus, copepod Cirripedia Balarms cariosus, large barnacle Balanus glandula, acorn barnacle Chthamalus fissus, small brown barnacle Balanus rmbilis, large barnacle Mitella polymerias, goose barnacle Balanus tintinnahulum, barnacle AmpMpoda Amphithoe sp., amphipod Melita palmata, amphipod CapreUa scaura, amphipod Orchestia traslciana, beach hopper Orchestoidea calif orniana, large beach hop- per Isopoda Cirolana harfordi, drab isopod Ligyda ocoidentalis, large isopod Stomatopoda Pseudosquilla higelowi, mantis shrimp Becapoda Crangon dentipes, snapping shrimp Betaeus longidactylus, long-fingered shrimp PavAilirus interruptus, spiny lobster Pagurus samuelis, hermit crab Pagurvs hirs^itiiiscuhos, common hermit Pagurus granosimanus, small hermit PagviTUS heringanus, large hermit Pagii/rus hemphilli, hermit crab PetroUsthes cinctipes, porcelain crab Lower tide pools under rocks Middle tide, under rocks Under rocks Low tide Low tide Lower tide pools Lower tide pools Sand flats Sand flats Sand flats Lowest intertidal Middle tide pools Lower tide pools Upper tide pools Eocky bay shore High tide rocks High tide rocks Pilings and boats Intertidal rocks Pilings and boats Middle tide pool rocks Upper tide pools Kelp, sand, or mud Sand above tide Sandy beach among rocks Under rocks, all levels Eocky shore, above tide level Lower tide pools Lower tide pools under rocks Middle tide pools under rocks Lower tide pools Upper tide pools Middle tide pools of bays Upper tide pools of bays Lower tide pools of bays Middle tide pools of bays Middle tide pools of bays MARINE ZOOLOG"?^ 777 :M Typical Pacific Shore Animals — Cont'd NAME HABITAT ZONE Pugettia gracilis, crab Pugettia prodxhcta, kelp crab Cancer oregonensis, round Oregon crab Bandallia omata, round crab Portv/ims zantusii, sand crab Pachygrapsus orassipes, striped shore crab Kemigraspus nudus, shore crab Cancer antennarkis, edible rock crab Ca/ncer productus, crab Lophopanopeus leucomanus, crab Speocarcvrvas calif orniensis, burrowing crab Uca crenulata, common fiddler Araclmoidea Pycnogonv/m stea/rnsi, pycnogonid (sea spider) Chordata Hemichordata DoUchoglossus pusillus, acorn tongue worm UrocJwrdata Styela stvtnpsoni, red tunicate Ealocynthia nmostor, large tunicate Perophora annectens, greenish tunicate Glossophorum plarmm, lobed tunicate Amaroucvwm califomicum, colonial tuni- cate CephaZochordnta Branchiostoma calif omiense, Amphioxus Lower tide Lower tide Lower tide Lower tide Kelp, sand Upper tide Middle tide Lower tide Lower tide Lower tide Mud flats, pools pools of bays pools of bays pools of bays or mud pools pools pools under rocks pools under rocks pools under rocks burrow Mud flats, burrow Lower tide pools Below tide Rocky shores, low tide Rocky shores, low tide Lower tide pools Lower tide pools Lower tide pools Below tide 778 TEXTBOOK OF ZOOLOGY Typical Animals From the Gulf of Mexico — Texas Coast* NAME HABITAT ZONE Porifera Clione celata, yellow-boring sponge Coelenterata Obelia hyalina, branched hydroid Plumularia inermis, branched hydroid Clytia naliformis, simple hydroid Tubularia crocea, matted hydroid Velella mutica, siphonophore (sp. not defi- nitely determined) Physalia pelagica, Portuguese-man-of-war Porpita liwaaecma, (sp. identification not yet confirmed) Daatylometra quinqueoirrha, jellyfish StomolopJms meleagris, cabbage-head jelly- fish Benilla remformis, sea pansy (sp. not defi- nitely determined) Ptilosarcus quadrcmgularis, sea pen (sp. not definitely determined) Stylatula elongata, sea pen (sp. not defi- nitely determined) Leptogorgia carolinensis, yellow gorgonid coral Astrofngia danae, common coral (sp. not definitely determined) Aurellia cmrita, clear jellyfish Sea anemone (Classification not deter- mined) Chiropsalamus sp. Ctenopliora Beroe ovata, sea walnut or comb jelly BoUnopsis vitrea (not definitely deter- mined) Mnemiopsis sp., small sea walnut MoUuscoidea Bugula turrita Zobotryon pellucidum, clear bryozoan (Another bryozoan of family Valkeriidae encrusting sargassum ; classi- fication undetermined) Platyhielinintlies Eustylochus meridianalis, flatworm Annelida Glycera abranchiata Nereis Ivmbata, sandworm Nerme mvrmta Common on oysters Attached to floating sargassum Common on Sargassum Attached to floating sargassum Washed ashore, and taken in shrimp nets Floats, but may be cast up Floats generally, cast up Floats, may be cast ashore Floating in bays and coves Floating in gulf, bays, and coves Sand bottom, shallow water, gulf Bottom, deep water Mud bottom, shallow water Encrusting shells Low tide sands, encrusting small shells Floating in gulf and bays Attached Floating in bays and bayous Floating in bays and bayous In gulf and bays on the bot- tom Among oysters and barnacle? Oyster beds and beaches ♦Appreciation is expressed to Mr. Gordon Gunter, Marine Institute of Science, University of Texas, and to Professor Elmer P. Cheatum, Southern Methodist University, who have assisted in the revision of this section. MARINE ZOOLOGY 779 Typical Animals From the Gulf of Mexico — Texas Coast — Cont'd NAME HABITAT ZONE Euphole grubei Lumhrinereis sp. (possibly young fiori- dana), red sandworm Capitellides teres Paraxiothea (Olymenella) torguata Polydora ciliata, burrowing worm Mollusca Gastrofoda Tethys protea, sea hare FleurophyUidm, nudibranch Crepidula plana, flat slipper shell Crepidula fo-rnicata, arched slipper shell Littorina irrorata, periwinkle Acmaea antiUarum, limpet Sinum per sped ivmn, ear shell EupJeura candata, oyster drill Thnis haemostoma floridana. purple PoUnices duplicata, duplicid moon shell Buscycon pyrum, pear conch Busycon can C3 (p s s o 00 a o M to 814 TEXTBOOK OF ZOOLOGY scarlike corpus albicans. In human beings the usual procedure is for only one ovum to mature in one of the ovaries each four weeks (approximately). At the end of the succeeding period an ovum ma- tures in the other ovary. This alternation proceeds from month to month in the female from puberty to menopause, except during preg- nancy. When the ovum leaves the ruptured follicle of the ovary it is technically in the body cavity, but in practice the funnellike ostium of the oviduct receives it immediately and starts it down the oviduct. It is here in the upper part of the oviduct that maturation of the ovum is completed and fertilization occurs. Spermatozoa, car- ried in semen, are introduced into the vagina of the female genital tract in the act of copulation or coitus. These motile spermatozoa swim up the oviduct and meet the ovum shortly after it enters. Fer- tilization of the ovum by union with a spermatozoon occurs and the zygote continues to move slowly down the oviduct. Cleavage, the next step in development, takes place while the embryo proceeds along the oviduct. The later stages are comi^leted normally in the uterus. This process of successive cell divisoin is modified somewhat when compared with that described for starfish on page 116 and frog on page 533, but the same ultimate purpose of rapidly increasing the cells is accomplished. Cleavage in mammals is complete and nearly equal. The stages of the earlier divisions have been obtained and observed in such mammals as Macacus monkey, rabbit, guinea pig, rat, pig, sheep, and horse. It is thought that cleavage in the human being is similar to these. One of the four cells resulting from the second cleavage division is different from the others. It is the fore- runner of a differentiated group of cells which soon becomes sur- rounded by the other cells (Fig. 422). This enclosed group is then known as the inner cells. The outer layer which surrounds these inner cells is known as the trophohlast. Shortly, small pouches or vesicles filled with watery fluid which is secreted by the newly formed tropho- hlast, appear beneath it. These join to form a common cavity between the trophoblast and the inner cells, except at one pole, where the two groups of cells remain in contact. This cavity goes under the name of blastocyst cavity and the whole structure, which is comparable to a specialized blastula, is called a blastocyst or blastodermic vesicle. The embryo's body will develop from the inner cell mass. The tropho- blast becomes closely associated with the inner lining of the uterus of the mother and soon plays a part in nutrition, respiration, and excretion of the embryo. It sinks into the uterine lining carrying MAMMALIAN DEVELOPMENT 815 the inner cell mass with it. This is known as implantation. The en- tire blastocyst makes a rapid growth and the cavity becomes distended with the lymphlike fluid. Next, a simplified process of gastrulation takes place, in that two successive layers of cells are shed into the blastocyst cavity from the free margin of the inner cell mass. These are layers of endoderm cells. The ends of the layers join to form an elongated enclosure. The inner cells which are joined to the trophoblast become the ectoderm of the body and the trophoblast remains as extraembryonic ectoderm. The cavity enclosed by the endoderm is the archenteron or primitive gut. In coelenterates and sponges development stops with this two-layered or gastrula condition. In higher forms, the mesoderm or third germ layer is formed immediately following gastrulation. At this stage in mammals and most other vertebrates a primitive streak appears along the dorsal midline of the posterior portion of the blastoderm (embryo). When sectioned, the primitive streak appears as a thickened band which is continuous with the ectoderm. The ventral side of this streak pro- duces many cells which organize as a sheet of mesoderm at each side and spread both laterally and caudally between the other two germ layers. The mesoderm continues to spread until it finally encircles the endoderm. The mass of cells thus formed, is soon divided into two layers by the development of a small cavity in it at each side of the archenteron. These cavities grow ventrally in the mesoderm until they meet each other at the ventral side of the archenteron. This cavity is the coelom or body cavity. The layer of mesoderm forming the outer or lateral wall (next to the ectoderm) of the coelom is known as somatic or parietal mesoderm and that on the side next to the endoderm is the splanchnic or visceral mesoderm. Later in de- velopment, when the somatic layer unites with the ectoderm, it forms the somatopleure (body wall). Similarly the splanchnic layer unites with the endoderm to form the splanchnopleure or wall of the ali- mentary canal. Almost coincidental with the early development of the mesoderm, but independent of it, is the formation of the neural plate as a thicken- ing and depression in the ectoderm along its dorsal midline. This begins just above the primitive streak mentioned previously and grows anteriorly. This is the portion from which the nervous system develops. This plate sinks, neural folds develop along its sides form- 816 TEXTBOOK OF ZOOLOGY MAMMALIAN DEVELOPMENT 817 ing a neural groove. These folds meet each other over the groove to form the neural tube (Fig. 423). The mesoderm along each side of the neural plate becomes or- ganized into blocklike thickenings. These are somites, and they are paired opposite each other, marking segmentation in the body. In a chick embryo of thirty-six hours of incubation there are fourteen pairs of somites and in a pig embryo 6 millimeters long there are thirty-two pairs. The younger embryos of different groups of vertebrates are so simi- lar that it is impossible to distinguish them from each other. This is illustrated in Fig. 444 in a later chapter. At a sufficiently early stage, the embryos of the human being, the pig, the rat, the alligator, the salamander, and the fish all appear very similar. The gill slits and segments are conspicuous in all of them. IMuch later the limbs develop from lateral pairs of limb buds in the mesoderm. The hind limbs develop first and the front ones follow. The human body can barely be discerned in embryos of one month, and the embryo must be nearly two months of age before it can be identified definitely as human on the basis of morphological features. Organs and Systems After the three germ layers (ectoderm, endoderm, and mesoderm) have been established in the embryo, the next step is differentiation of these layers each in various ways for the formation of particular organs and systems of organs. The fate of the germ layers has been concisely summarized in the last paragraph in the chapter on meta- zoan organization. Embryonic Membranes. — All three of the terrestrial groups of vertebrates (reptiles, birds, and mammals; i.e., amniota) produce extensive embryonic membranes as a feature of their development which is not found in the aquatic forms. These membranes serve to give the developing embryo added protection, as well as increased facilities for the functions of nutrition, respiration, and excretion. In birds and reptiles the early embryo is flat and the somatopleure extends over the surface of the yolk far beyond the limits of the embryo proper. A fold of this sheet of somatopleure (ectoderm and parietal mesoderm) grows dorsally along each side of the embryo (Fig. 424). These folds finally meet each other above the embryo and fuse together, thus forming an enclosed cavity between the dorsal sur- 818 TEXTBOOK OF ZOOLOGY face of the body and this new membrane. The name of this membrane is amnion and the cavity formed is the amniotic cavity. The super- ficial limb or layer of the original fold of somatopleure over the body is the serosa. In mammals the serosa unites with another embryonic membrane, the allantois, to form the real chorion which is highly vas- cularized and serves as the embryonic portion of the placenta. The allantois is a ventral evagination (outgrowth) from the ventral side H tad Fold ■/^eso6/ashc Somife Cloied fifevra/ 7v6e Open /l/eural Qrooi^e Tail Fold Fig. 423 — Dorsal view of an embryo in the neural tube stage of development. Both ends of the tube are still open. The brain forms at the anterior end later. of the cloaca of the embryo. It extends out through the open, ventral side of the body and spreads between the amnion and serosa until it covers most of the body like a double-walled sac. After develop- ment is complete, its stalk within the body cavity is retained as the urinary bladder. In embryonic birds and reptiles the allantois thus lies quite close to the shell membrane and since it is richly vascu- larized, serves as the respiratory organ. MAMMALIAN DEVELOPMENT 819 In mammals, the allantois spreads between the amnion and serosa in about the same way, and becomes fused with the inner side of the serosa to form the chorion. Many branched processes, chorionic villi, extend from the outer or serosa surface of this membrane and come to fit into corresponding pits in the internal uterine wall of the mother. The yolk sac is another membrane which extends ventrally from the mid-gut and out through the ventral side of the body wall just anterior to the allantois. This structure is present in the fish and amphibia as well as in the amniotes. It is large in the shark for Embryo Ectoderm Ectoderm Parietal Mesoderm Splanchnic Mesoderm Endoderm Yolk Sac Parietal Mesoderm Fig. 424. — ^Diagram showing development of tlie embryonic membranes of a vertebrate. The amnion and serosa (chorion) each consists of a fusion of ectoderm and somatic mesoderm ; while the yolli sac and allantois each consist of a fusion of endoderm and splanchnic mesoderm. The serosa is composed of the two outer layers of the diagram. Notice that the allantois is a double-walled sac from the ventral stalk, and it extends well around the embryo. Amnion ■Amniotic Cai^ity ■Allantois 'Parietal Mesoderm Splanchnic Mesoderm Yolk Sralk Endoderm Vitelline Membrane example. In reptiles and birds it carries the large mass of yolk used for nutriment by the embryo. In mammals it is much reduced be- cause the embryo soon develops a means of nourishment through the placenta. Placenta. — This membranous sac in which the embryo and later the fetus (designation after distinct body form is apparent) is formed from two sources, one embryonic and the other maternal. As suggested in the paragraph above, the chorion of the embryo and the mucous membrane (internal lining) of the uterus unite (actually 820 TEXTBOOK OF ZOOLOGY fuse in some forms) to form this saclike organ, the placenta. The chorion is richly supplied by blood from the fetus while the uterine layer is similarly supplied by maternal blood. Although the em- bryonic and maternal parts of the placenta may be almost interwoven in mammals like the human, there is no actual circulation of maternal blood through the vessels of the fetus. The exchange of materials (nutrition, respiration, excretion) is by diffusion through the tissues here in this intimate relation. As in the case of circulatory connec- Yofk Sac Muscu/ar Layers of Uterus // Basal Plate- of Placenta Umbilical Cord- Amnion Chorion & Decidua- Capsularis Fig. 425. — Sectional view of a fetus in normal position in the uterus, showing also the intimate relations of embryonic membranes and uterine wall. The chorion is the outer embryonic membrane. (Modified after Ahlfeld. ) tions, there are no nerves passing from parent to fetus. The stalklike extension from the abdomen of the fetus to the placenta is the um- hilical cord. It carries the umbilical arteries and vein, extending from the chorion to the fetus; the allantoic stalk; and, a vestigial yolk stalk from the intestine. The Tiavcl of the adult is the scar where the body wall has closed in about the umbilical cord at the point of its severance at birth. The fetus is suspended within the amniotic cavity which is filled with the watery, lymphlike amniotic fluid. CHAPTER XLIII GENETICS AND EUGENICS (By Frank G. Brooks, Cornell College, Iowa) The History of a Great Discovery "Like father like son" is an ancient adage. Since man has been able to think, he has pondered the problem of heredity. Al- though very early he had observed evidence of the inheriting of parental characters by offspring of the various forms of life with which he was familiar and was convinced by these observations that heredity did take place, he has not known the "how" or "why" of it until recently. The fundamental law on which heredity is based was announced by Gregor Johann ]\Iendel in 1866. However, Mendel, an Austrian monk, published his discovery in an obscure journal and it did not receive general recognition until its rediscovery in 1900. Therefore, genetics is really a twentieth century science. Mendel's success in finding the underlying principle of heredity was due, in part, to his choice of an experimental unit. Instead of following the usual trend by considering how a parent conveys his various traits to his offspring, thus making the individual the unit of observation, he chose a definite inheritable character and considered how it was transferred from many parents to all their offspring. In addition to this wise choice of an investigational unit, the patience, mathematical ability, skill as a gardener, and analytical insight of the investigator contributed also to the success of the research. Mendel chose the garden pea as the material for his work. This was a fortunate choice for, although the law that he was to discover underlies practically all inheritance, it is not always as free from complications as it is in the case of those traits of the pea which he investigated. Mendel's Law For one of his projects he planted seeds from stock that had been known to produce nothing but tall plants for many generations. He planted also seeds from stock that had produced nothing but dwarf plants. When the two types of plants were in blossom, he transferred the pollen from the stamens of the one to the stigmas of the other. 821 822 TEXTBOOK OF ZOOLOGY The seeds that resulted from the cross were collected and planted. To the amazement of the investigator they produced nothing but tall peas; peas as tall as the original tall parents. Dwarf ness seemed to be lost and tallness was certainly dominant. A less thorough investi- gator might have called his experiment finished and have proclaimed as a rule, the fact that a trait can disappear when it is crossed with its opposite. But not so with the patient monk. He pollinated these plants with their own pollen and planted another generation. This Fig. 426. — Diagrram to show the result of crossing tall and dwarf peas. The 1:2:1 ratio appears in the F2 generation. time three-fourths of the plants were tall and one-fourth of them were as short as the original dwarf parents. Not being ready even yet to formulate a law, he self -pollinated his plants for several more generations and got results that required his best mathematical skill to interpret. The dwarf peas produced only dwarf peas. Of the tall peas, one-third of them (constituting one-fourth of the whole num- ber) produced tall peas without deviation, but the other two-thirds (half of the whole number) produced stock that repeated the produc- GENETICS AND EUGENICS S23 tion of tall and dwarf peas in the same proportion as did the preced- ing generation. Here, then, was a definite ratio of 1:2:1. Mendel tested his findings with six other traits of the pea and with more than a dozen other kinds of plants, and after verifying his results he was able to announce the following law : When members of a species hav- ing contrasting characters are crossed, all the immediate offspring will show the trait that is dominant, hut if the members of this gen- eration are bred to themselves, one-fourth of the offspring ivill show the dominant trait and breed true for it; one-fourth will show the recessive trait and breed true for it; the remaining two-fourths of the offspring will show the dominant trait, but will reproduce the contrasting characters in the same proportion as did their immediate parents. Derivatives of Mendel's Law From the fundamental law which Mendel discovered, several corol- laries can be drawn, based on factors responsible for the distribu- tional behavior of inheritable characters. These corollaries are : Principle of Dominance. — The determiner for one member of a pair of contrasting characters (pairs of contrasting characters are called allelomorphs) may take precedence over the other member when the two have been brought together in a cross between unlike parents. Which trait is dominant and which is recessive can be de- termined only after a cross has been made, and the determination holds only for the species observed. Thus, taUness may be dominant over dwarfness in one species of plant and recessive in another. In many cases, dominance is not complete, and in some instances it is lacking altogether. These cases will be discussed later in this chapter. Independence of Unit Characters. — The determiner for any given character acts as an independent hereditary unit as it is passed along from generation to generation. In each generation it may have a different set of associates, but its associations in one generation do not affect the company it may keep in the next. Thus tallness in peas may be associated with yellow pod and with wrinkled seed coat in one generation, but it may form a combination with green pod and smooth seed coat in the next. This important principle is responsible for the great variation we find in plants and animals and makes it possible for a breeder to bring about any desired combinations of the traits 824 TEXTBOOK OF ZOOLOGY possessed by the species of plant or animal with which he is working. Thus the modern Shasta daisy has a combination of the three impor- tant characters: size of flower, gloss of petal, and prolific growth, each of which was procured from an original variety having one, but not the other two, of these traits. Principle of Segregation. — Since unit characters are independ- ent, it follows that they can go into various combinations and are free to segregate out again. This was illustrated by dwarf- ness in Mendel's peas which went into a cross with tallness, but segregated out again to form peas that were as consistent for dwarfness as was the original dwarf stock. To illustrate both the principles of unit characters and the related phenome- non of segregation we might use an analogy. If a gallon of white marbles were mixed with a gallon of black marbles, they would make a mass of marbles that would look speckled-gray. But the mixture is not irrevocably gray as would be the case if a gallon each of black and white paint had been mixed. Inheritable traits, like marbles, can be placed in various combinations. These may last for one genera- tion only, after which they can make new combinations with other traits. The Physical Basis With the simple facts in mind of how heredity works, it is desirable to consider the germinal background of the laws governing it. The tiny sperm cells of both plants and animals and the egg cells which, deprived of their food-containing yolk, are about equally small, contain within them something that determines all the traits which the individual resulting from their combination will develop. In fact, the body of the sperm cell and the nucleus of the egg eon- tain little else than a material that carries the combined inheritable traits of one generation over to the next. This important material is called chromatin and during mitosis it becomes arranged in series of hereditary units called genes. Each gene has a definite causal rela- tionship to an inheritable trait. Both the sperm cell and the egg cell contain a complete gene complex, and each, under favorable condi- tions, could produce a complete individual. When the two totipotential gametes are brought together at fertili- zation, the resulting zygote contains two genes for each allelomorph. When the two genes are for the same trait, the resulting individual GENETICS AND EUGENICS 825 is said to be homozygous for that trait and, of course, shows it. When each of the two genes is for a different member of the allelomorph, in which case we use the term heterozygous, one gene takes precedence over the other, but the recessive gene is in no way destroyed ; it sim- ply lies dormant and bides its time. How these genes are arranged into a limited number of "packages" called chromosomes and how genes and chromosomes are distributed in the process of sperm and egg formation was explained in Chapter VIII. Plotting Crosses The Monohybrid Cross. — When the genetic constitution of paren- tal stocks is known, it is possible to plot out the results of various kinds of crosses. The checkerboard is the simplest device for such TT Td •Kl dd g T£ dY Tg dg Fig. 427. Fig. 428. Fig. 427. — The outcome of a monohybrid cross between two heterozygous in- dividuals is according to the ratio 1:2:1, i.e., (TT). (Td), (Td), (dd). Fig. 428. — The checkerboard may be used to determine the possible gene com- binations in the ova. and sperm of a dihybrid cross. computations. The genetic constitution of the various kinds of male gametes are set down along the ordinate and that of the various kinds of female gametes along the abscissa. These values are then copied into the squares horizontally and vertically and their sums give the values of the various kinds of gametes that will result from the cross. In the Fa cross of Mendel's experiment with tall and dwarf peas, half of the male gametes and half of the female gametes contained a gene for taUness (T) and half in each case contained a gene for dwarf ness (d). The outcome, therefore, is shown in Fig. 427. The Polyhybrid Cross. — It is often desirable to know the outcome of a cross in which two or more allelomorphs are considered together as in the case of crossing a taU, green-podded pea (Tg)* with a dwarf, •It is customary to indicate the dominant trait by using a capital letter and the recessive trait by using a lower-case letter. 826 TEXTBOOK OF ZOOLOGY yellow-podded pea (dY). The determination of the possible kinds of gametes may be simplified by first making a small checkerboard for them (Fig. 428). Thus we find that in a dihybrid cross there are four kinds of male gametes and four kinds of female gametes which may be listed as TY, Tg, dY, dg. Now we make a checkerboard of sixteen squares and proceed as we did before. (Fig. 429.) The computation of the dihybrid cross indicates that nine-sixteenths of the progeny will show the two dominant traits because (Fig. 429) we find at least one T and one Y together in that many squares. Three- sixteenths will sJiow the dominant trait of the first allelomorph and the recessive trait of the other. Another three-sixteenths will show the re- Ty Tg dY dg TY TY Tg TY dY TY dg TY TST Tg Tg Tg dY Tg dg Tg dY Tg dY dY dY dg dY TY dg Tg dg dY dg dg dg Fig. Ti Tg dY dg 429. — The outcome of a dihybrid cross between two heterozygous individuals is according to the ratio 9:3:3:1. cessive trait of the first allelomorph and the dominant trait of the sec- ond. One-sixteenth of the offspring will show both recessive traits. It will be noticed, however, that the nine squares showing individuals that will be tall and yellow vary with each other in regard to their entire content. Further examination of the squares will indicate that there are nine different combinations of letters and that in only one case as many as four are exactly similar. Individuals whose genes cause them to look alike are called iihenotypes; those whose genes are exactly alike are called genotypes. A trihybrid cross, such as one between a tall yellow-podded pea with a wrinkled seed-coat (TYw) and a dwarf green-podded pea with a I GENETICS AND EUGENICS 827 smooth seed-coat (dgS) can be plotted by using a checkerboard of sixty-four squares. The ratio of phenotypes of a trihibrid cross is 27:9:9:9:3:3:3:1. The Back Cross. — The checkerboard is also useful in computing the results of a cross between a heterozygous and a homozygous in- dividual as would be the case in which the offspring of a cross between a homozygous dominant and a homozygous recessive individual is bred back to either of the parental stocks ; a procedure often followed in practical animals and plant breeding. When such a cross is made to the homozygous dominant stock the results, in terms of pea traits, are TT, TT, Td, Td. (Fig. 430.) All four will look alike, i.e., they are phenotypes, but there are two pairs of genotypes. T T d d d TT TT Td Td Td Td dd dd Fig. 430. Fig. 431. Fig. 430. — A cross between heterozygous and homozygous tall peas produces a 2:2 ratio. Pig. 431. — A cross between heterozygous tall and homozygous dwarf peas also produces a 2:2 ratio. When the heterozygous stock is bred back to the homozygous reces- sive parental stock, a 2 :2 ratio is also produced. This time the two kinds are not only genetically unlike, but they also appear unlike (Fig. 431). Complications of Mendelian Inheritance If all inheritable characters followed Mendel's law in as simple a fashion as do the pea traits that Mendel first investigated, the science of genetics would be much more easily mastered. Unfortunately this is not the case. Although Mendel's law is found so consistently as the underlying principle of heredity that it can be said to be practically universal in its application, it is often modified and complicated in various ways. Multiple Allelomorphs. — Instead of the allelomorph always consist- ing of the usual two factors, a larger number of alternatives sometimes 828 TEXTBOOK OF ZOOLOGY appears. Dominance in these cases occurs in a graded series, each member, between the extremes, being dominant to the lower members and recessive to the higher members of the series. A simple case of multiple allelomorphs is found in the inheritance of color in rats where there are three factors; namely, ordinary pigmentation, ruby-eyed dilution, and albinism. In a cross between heterozygous parents any two of the three factors may be brought together in the offspring. If ordinary pigmentation is present with either of the other two, it will determine the color, since it is first in the series of dominance. If the other two are the ones present, the color will be ruby-eyed dilution, the second member of the series. A larger series is presented by eye color in the common fruit fly, Drosophila melanog aster, in which case there are eleven members which are, in the order of their dominance : red, apricot, coral, ivory, ecru, buff, tinged, blood, cherry, eosin, and white. Each parent may have any two of these in its germ plasm and transmit either of them to the offspring. Multiple Genes. — Several cases formerly interpreted to be simple blending inheritance not conforming to Mendel's law have been explained by the discovery that there can be more than one allelo- morphic pair concerned with the inheritance of the trait. Thus, in- stead of there being one gene located at some definite place on a chromosome, there are two or more genes variously located. Cases of multiple genes fall into two categories. In the one, each gene inherited as a dominant, produces part of the result, and the effects are cumula- tive. In the other type of case, the inheritance of one dominant gene produces the entire effect, A common example of the cumulative type is the inheritance of skin color in man. It has long been known that a cross between a white person and a pure-blooded negro produces offspring of a medium shade called mulatto, and that a cross between two mulattoes will pro- duce offspring with a range of color varying from intense black to a shade that may allow the person to pass for white. This is ex- plained by the fact that there are two allelomorphic pairs concerned with the inheritance of color, the dominant of either of which will produce a certain amount of pigment per square millimeter of skin surface. The negro of pure skin inheritance is homozygous, having the dominant genes of each pair. Using P and P' to repre- sent the dominant factors of these two allelomorphs and p and p' the i GENETICS AND EUGENICS 829 recessives, the homozygous negro would have PPP'P' while the ho- mozygous white person would have ppp'p' and the mulatto resulting from the first cross would have PpP'p'. The cross between two mulat- toes would be a dihybrid cross which could be plotted by a checker- board essentially similar to that of Fig. 432. The accompanying chart (Fig. 432) shows that there can be four, three, two, one, or no dom- inant factors present, thus accounting for the series known under the following technical terms: ''negro," "chocolate," ''mulatto," "quadroon" and "pass for white." ppt Pp* pP' PP* pp» p?» negro Pp* ppt choc. pP* ppi choc. PP* ppt mulat ppt I'' choc. Pp' mulat pP» mulat PP' quadr ppt pP» choc. Pp» pP* mulat pP' pP* mulat PP* quadr ppt pr>» mulat Pp* K Quadr pP» Quadr PP* PP* white ppl Pp* pp« PP* Fig. 432. — The inheritance of color by children of mulatto parents can be shown by the checkerboard for a dihybrid cross. The other type of case in which multiple genes occur is exemplified by a certain brown-seeded variety of oats in which there are two pairs of genes concerned with the determination of color. Here the pres- ence of the dominant gene of either pair produces the entire effect. This case, too, can be plotted on the dihybrid checkerboard, but the result is a 15 :1 ratio, for fifteen of the sixteen squares would show at least one dominant gene. Crosses between a red and a white variety of wheat in which there are three pairs of genes for color, produce a 63 :1 ratio. This is a trihybrid cross in which each dominant gene can pro- duce the entire effect. Complementary Genes. — In a number of cases color is produced by two allelomorphic pairs of genes, the dominants of which must react on each other to produce the color effect. In the sweet pea, if the dominant factor of the red allelomorph (Rr) is acted on by the 830 TEXTBOOK OF ZOOLOGY dominant factor of the color allelomorph (Cc), then red is produced. If either of these dominant genes is present without the other, white flowers are produced, and if two white-flowered plants of the genetic constitution Rrcc and rrCc are crossed that part of the progeny will be red in which both R and C are inherited. A student familiar with elementary chemistry may understand this better if he thinks of the red factor as being represented by a colorless solution of phenolphthalein and the color factor as a colorless alkaline solution. When both solutions come together, red is produced, but neither solu- tion can produce the color without the other. Supplementary Genes. — But sometimes purple sweet peas appear when two white varieties are crossed or when a red variety is crossed with a white variety. Purple is produced by a third gene which intensifies red if it is already present, but has no effect unless the RC genes are likewise inherited. Thus we find that its effect is to supplement that of the two genes R and C which are complements to each other in the production of red. The accompanying list shows some of the surprising crosses that may be made with sweet peas. Of course the combination of genes shown in the last column is not the only one that will come about in each of the various cases, but it is the one that emphasizes the point desired. Bb represents the in- tensifying allelomorph. White + White = Purple RcB + rCb = RGB Red + White = Purple RCb + RcB = RCB Purple + Red = Red RCB + RCb = RCb Purple + White = Purple RCB + rcb = RCB Purple + White = WJiite RCB + rcb = rcB Purple + White = Red RCB + rcb = RCb I Lack of Dominance.— In several classic cases, some of which are among plants and others among animals, neither factor of an allelo- morph is dominant over the other. In the F^ generation, these cases seem to produce perfect examples of blending inheritance, but the Fa generation exemplifies the 1 :2 :1 ratio so beautifully that these excep- tional cases are often used to explain Mendel's law to beginning stu- dents. The Andalusian breed of chickens includes both black and white individuals. When black fowl are crossed with white fowl, all the off- spring are of a slate color technically known as ' ' blue. ' ' When a blue i GENETICS AND EUGENICS 831 chicken is crossed with another blue chicken, one-fourth the progeny- are black, one-fourth are white and the remaining two-fourths are blue. The blacks and the whites are homozygous for their respective colors while the blues are heterozygous. The inheritance of color in short-horn cattle, and of the color in the four-o'clock flower are also examples of this. In still other cases in which there is not neutrality of dominance, the dominant effect may not be complete. Very often the heterozygous individuals can be picked out from the homozygous dominant ones by casual inspection. Inheritance of Sex In certain lower phyla of animals and in the early embryos of higher forms, including mammals, both male and female repro- ductive systems are present in each individual. Typically in Fig-. 433. — Half of all possible fertilization combinations will be (XY) and half will be (XX). The XY combination will be males and the XX will be females. higher forms, one of these systems is repressed, and we speak of the individual as being either male or female. The deter- mination of whether the male or the female system will develop is an inherited trait. The chromosome that carries the gene of sex deter- mination has come to be designated as the X-chromosome. In typical cases, the female has in each of her germ cells, two of these X-chromo- somes and the male has only one. Therefore, when, in the process of spermatogenesis, the chromosomes match with their homologous chro- mosomes and separate again to give each of the resultant cells one complete set of chromosomes, the X-chromosome cannot pair with an- other like itself, and when the chromosomes are distributed in sets, half the sets will lack an X-chromosome. The sperm cells that do not have an X-chromosome will produce males while those that have such a chromosome will produce females. 832 TEXTBOOK OF ZOOLOGY In other cases that include man and Drosophila, the X-chromosome will pair with a Y-chromosome which has been described as " an empty- case " or a " ghost chromosome. ' ' In cases in which a Y-chromosome is present, we cannot say that the male has one less chromosome than the female, but it practically amounts to that. Linkage Since the number of inherited traits of an animal or plant is very great while the number of chromosomes is comparatively small, it is apparent that a single chromosome must contain many genes. This be- ing so, there is not absolutely free assortment of traits as they are passed from parent to offspring, but rather the offspring must inherit his traits in groups, that are not, ordinarily, broken up. Therefore, when two genes, A and B, are on the same chromosome of one of the parents, if the offspring inherits gene A, he must inherit gene B. In Drosophila we Imow that the gene which determines whether or not the wing will be fringed occurs on the same chromosome as the one that is responsible for the body being or not being black. Now if one of the parents is fringed-winged and black-bodied, the offspring will have to inherit both these traits from that parent if he receives either one of them. This phenomenon is called linkage. Sex Linkage Since there are other genes than those concerned with sex on the X-chromosome, it is to be expected that their inheritance will differ somewhat in the two sexes. This is strikingly illustrated by certain abnormalities of man in cases where the abnormal condition is inher- ited as a recessive; for example, color blindness, blindness resulting from atrophy of the optic nerves (Leber's atrophy), and hemophilia or bleeder's disease, in which great loss of blood occurs even from slight wounds because clotting will not take place. Since the abnormality is a recessive trait produced by a gene on the X-chromosome, its effect can be offset by a dominant for normal in the female, but not in the male, because in the latter case there is not another X-chromosome present. Therefore, the daughter of a color-blind father will not show the defective trait but may pass it to her son. Fig. 434 shows a cross between a color-blind father and a normal mother. Two kinds of male gametes are produced, half of which carry the father's gene for color blindness on the X-chromosome, and half of \ GENETICS AND EUGENICS 833 which have no X-chromosome and, therefore, have no such gene. All female gametes have one X-chromosome bearing a gene for normality. The various possible combinations of the two kinds of sperm with the single type of egg will produce two types of offspring; namely, sons who will be strictly normal in regard to the trait in question and daughters who will inherit the gene, but who will not show the trait because its effect will be overcome by the presence of a dominant gene Fig. 434. — In the cross between a color-blind male and a normal female, tlio results (N, cb), (N, cb), (N), (N) indicate that all daughters, being heterozygous, will not show the defect though they might transmit it to their offspring, while the sons are entirely free from it. Fig. 435. — Results (N, N), (N, cb), (N), (cb). In a cross between a normal male and a female who is heterozygous for color blindness, half of the sons will be normal and half will be color blind. Half the daughters will be homozygous for normal and half will be heterozygous. for normal. Therefore, the daughters will be heterozygous for the trait and might be spoken of as carriers. Fig, 435 shows a cross be- tween such a daughter and a normal male. Inspection of this figure will show that such a cross will produce four types of offspring; namely, normal daughters, "carrier" daugh- ters, normal sons, and color-blind sons. Thus, we see that half the grandsons of a color-blind man will receive the trait through their mothers, and that all daughters and half the granddaughters of the 834 TEXTBOOK OF ZOOLOGY color-blind male will be carriers of the trait, but will have normal color vision. Of course, from the mating of such a person with a color- blind man, all the sons and half the daughters will be color blind, but a young woman who is used to having her father buy pink neckties thinking they are blue, is not likely to marry a man similarly defective. Crossing Over Genetics might be defined as an exact science which has exceptions to many of its rules. A previous paragraph describes the principle of linkage which provided for the inheriting as groups of those traits whose genes occur together on a chromosome. Certain gametogenetic B D D B D B Fig. 436. — Crossing over occurs when chromosomes break apart after synapsis. accidents bring about occasional exceptions to this principle. It will be remembered (page 114) that at synapsis, the chromosomes pair and the homologous chromosomes become loosely attached to each other. At times, the pairing chromosomes will even twist around each other. "When they come to separate in the maturation division, instead of making a complete separation, they sometimes break off at some point so that a part of each will be joined to a reciprocal part of the other, as is shown in Fig. 436. Thus, as a result of crossing over, in- stead of traits A and B, and C and D being associated in the offspring as they were in the respective parents, A will be associated with D, and C with B. GENETICS AND EUGENICS 835 too 0. Q.i ,0.+ \ Q5 \f , ', 1.5 \'P- - 13.t '4.5 . \ 5,3 -■'. '.A9 ',13.1 M6.+ M.± 20. 21. 27.5 27.7 n yellowCB) Hairy winaC'V) Scu+eCHf lefhoil-7 brooiolCw; prune CE) white CE) face+CE) No+ch CB) ^ AbnormalCB; echinus(E) bifid (W) ruby(E) crossveinless (W> club(W) , del+ex(W; cuf(W), singed (.H) tanCB) lozenge CE) { 0. 2. 3.± 6.± 12.± IJ. 14 .± 16. +elegroiph(W) Star (E) aris+cp"' " «'™° «™"P is maintaining itself. Faanily Size in Dysgenic Groups Various studies have shown that larger families occur among peo- ple who have but a poor store of those qualities of intelligence, sta- bility, and physical traits that go to make up racial excellence. Lori- mer and Osborn found in their study of the school children of selected eastern cities that children with the lowest intelligence scores (I.Q. below 60) came from families that averaged nearly six children; that those with medium scores (I.Q.'s 90 to 110) came from families aver- aging less than four; and that the children of superior intelligence (I.Q.'s 140 and over) came from families that averaged less than two and a half children. The writer's study of nearly a thousand improvident families of a type well known to the social workers of southwestern cities revealed GENETICS AND EUGENICS 843 that in this group the number of births occurring in completed families averaged 7.9 and the average number of children born to mothers of all ages was 5.7. The average number of surviving children of the two groups was 6.1 and 4.6 respectively. The significance of these data in comparison with those concerned with the size of the families from III • i i I£CSND Bale without trait Female without trait IT klale and female with trait Sax unknown; 7 inheritance unknown Fig'. 439. — Standard pedigree chart. Charts such as this can be used in tracing a trait tlirough several generations of a family. This particular chart shows the inheritance of a dominant trait starting with a cross between a male who was heterozygous for the trait and a female in whom it was homozygously recessive. which college students come can be realized from the fact that if the reproductive rates in these two groups continue for ten generations, the descendants of one hundred families of the dysgenic group will number more than twenty-eight thousand, while in the same genera- tion, one hundred families of the present-day college-student group will be represented by eleven persons. 844 TEXTBOOK OF ZOOLOGY What Can Be Done? It must be admitted that the present racial situation has been brought about through the advance of humanitarianism and science. The cure for the situation must come through the application of greater humanitarian and scientific measures. The many corrective panaceas that have been suggested must be tested carefully and ap- plied cautiously. Some reforms can be applied at once and are being applied today to some degree ; others must wait until the time for their application is more opportune. Some Eugenic Measures The step that would be most fruitful of racial betterment is general education along eugenic lines. If a sufficient number of thoughtful citizens were informed about racial trends, a eugenic consciousness would be developed that would cause every proposed social or eco- nomic change to be considered from the standpoint of its genetic significance. The marriage and divorce laws of the various states should be uni- fied and rewritten along eugenic lines. They should provide for ad- vance notification of applications for marriage licences as is provided for by the California laws, and sliould contain provision for health examinations as already enforced in a number of states. The latter practice should be extended to include the examination of family histories. Many positive measures have been proposed for granting aid of various kinds to large families of good eugenic stock. At the present time most of these proposals are impractical, but we might look for- ward to wage adjustment to family size starting with government employees, and rental rates of government-owned houses based on a fixed percentage of the family income regardless of the size of the house required. It has been proposed that those who are clearly unworthy of parent- hood should be segregated in colonies of their own sex. The expense of this as well as the probability of many social and other problems that would arise in such a situation challenges the wisdom of such a meas- ure beyond the degree it is now being practiced in our eleemosynary and punitive institutions. Twenty-nine of our states have adopted laws providing for the eugenic sterilization of such persons as those wlio have been committed I I GENETICS AND EUGENICS 845 to sanitaria for mental cases because of an inheritable type of insanity and who are to be returned to their families. A few states provide also for such sterilization of habitual criminals and those who are clearly feebleminded. Eugenic sterilization consists of vasectomy and sal- pingectomy— operations that bring about sterility without interfering with endocrine function or normal sexual reactions. Over twenty-five thousand such sterilizations have been performed under the present laws. It is evident from the data previously discussed that family limita- tion is being practiced by the eugenic group. It is suggested by many who are facing squarely the problems of racial welfare that those who are mentally and in other ways far below the majority of our citizens should have made available to them the means of similarly limiting the sizes of their families. The number of clinics where such measures are made available to persons who will make proper use of them al- ready number in the hundreds. Some of these eugenic measures are questioned on the grounds that they violate human rights. AVe should also be considerate of the rights of the unborn. It is reasonable to say that every child that is to be born into this world has : The right to he horn with a sound mind. The right to he horn with a strong and normal hody. The right to he horn into an environment in which his inherited potentialities will have a fair chance to develop. CHAPTER XLIV ANIMAL BEHAVIOR (By Ina Cox Gardner)* Introduction To most people the study of either human behavior or the behavior of other animals means a study of the mind. This conception came from the early definition of psychology, which meant the science of soul or mind. The term mind is an abstract one, and whether or not any animals including human beings have what we call mind, is a matter of inference. In the past we have sought to explain behavior by the use of such terms as instinct, consciousriess, mind, intelligence, memory, and many others. These abstract terms became supposedly concrete entities and much valuable time was spent in a search for them. Psychologists in the modern period reject these nouns and make use of adjectives such as conscious or mental, which have more precise meaning in the description of behavior. The movement which led to the development of animal psychology into a distinct scientific discipline began with the work of Darwin. This does not mean that the field had been altogether neglected up to that time. Even prehistoric man must have been interested in the behavior of animals that supplied him his daily food. The ancient Greeks observed animal activities intensively with the definite aim of attempting to understand and explain them. From this time onward the problem of the nature of infrahuman behavior has attracted the serious attention of scientists. In fact, this problem has led to almost endless controversy from age to age. Two extreme views have grown out of this controversy. They are antipodal — at one end of the pole animal behavior is interpreted in the best possible light. The higher, nonhuman animals are often re- garded as almost human in intelligence. This view arises from the use, more or less uncritically, of anthropomorphic analogy. The basic inference in such analogy seems to be that if an animal acts like a man it must also feel and reason as a man does. While at the other ♦Acknowledgment is made to Warden, Jenkins, and "Warner, and the Ronald Press of New York for permission to use certain of the materials included in this chapter. 846 ANIMAL BEHAVIOR 847 end of the pole, other animals are rated very low in intelligence. Simple organisms are usually regarded as mere living machines with- out mental life of any sort. The behavior of even the higher, non- human animals is explained as due, in the main, to the operation of blind instincts, although a certain degree of intelligence may be ad- mitted. Both of these views have been rejected by present-day com- parative psychologists. The history of the study of animal behavior may be divided into two periods. The first, known as the anecdotal period, had its origin about the time of Darwin, and the collection of Romanes anecdotes may be taken as a representation of the better classes of anecdotes. Scores of anecdotal collections appeared in which the tendency to humanize the mental powers of the other higher animals was carried to the ridicu- lous. The anecdotes were often taken from unreliable sources and accepted without critical comment. The anecdotal collections were widely read and the popular imagination was deeply stirred. This movement which lasted from about 1859 to 1890 was not altogether without value, in spite of its absurdities and vagaries, since the inter- est aroused was later turned into more scientific channels. A reaction against the humanizing tendency began about 1890 and led into the experimental period. This period was characterized by a more critical use of anthropomorphic analogy which led in time to a rejection of it altogether in favor of a strictly objective position, and the use of more precise observation and the gradual development of carefully controlled experimental methods. The new movement arose from the work of Lubbock, Loeb, Newman, and Lloyd Morgan. Lubbock, the English naturalist, became inter- ested in the behavior of insects through the personal influence of Dar- win. Loeb and Newman, both physiologists of the German school, sought to analyze the behavior of lower organisms along rigidly scien- tific lines. The work of Morgan, an English biologist and philosopher, was restricted mainly to the higher vertebrates. Lubbock was the first of the group to make use of experimental methods, his studies on insect behavior appearing in collection form in 1882. Each of these men, however, was an original and independent worker : Lubbock and Morgan were most directly influential in the spread of the animal laboratory among psychologists after the turn of the century. Morgan was the first to extend experimental methods to the higher vertebrates. As early as 1891, he spoke of the "trial and practice" 848 TEXTBOOK OF ZOOLOGY factor in learning, and in 1893 announced the famous Canon of Mor- gan. This runs as follows : "In no case may we interpret an action as the outcome of the exercise of a higher psychical faculty if it can be interpreted as the outcome of the exercise of one which stands lower in the psychological scale. ' ' This is merely the law of parsimony ap- plied to animal psychology. While it does not rule out the use of anthropomorphic analogy entirely, it did serve to introduce a measure of restraint into such speculation. This canon was very influential among those, who, while objecting to the extreme view of Loeb, were ready to reject the absurd anthropomorphism of the anecdotalists. Two observation methods are used in the study of animal behavior : (1) The genetic method, wherein observations are made upon an ani- mal's behavior as it naturally occurs from the beginning to the end of the life of the animal; (2) The training method, wherein the observa- tions are made upon an animal's behavior during training and after it has been trained in certain habits of action or habits of discrimina- tion among stimuli of different sorts. Every act in an animal's be- havior is a response to one or more stimuli, produced by conditions internal or external to the organism. In the whole phylogenetic series, the internal stimulus is frequently, if not usually, due to the metabolic processes going on constantly within the protoplasm. The restlessness of all organisms is to be ascribed to impulses generated by the meta- bolic processes going on within that organism. An external stimulus is any change in the environment which affects the organism generally, or some special organ or part of it in particular, so that the animal's behavior is modified to some extent at least. No organisms, as a rule, live under conditions that are always the same, i.e., all organisms are subject to the effects of stimuli, and their responses are termed reac- tions. One-celled organisms react to all classes of stimuli that call forth reactions in the many celled organisms. The nervous system and sense organs constitute merely a more delicate, complex, and effective mechanism for the reception of the stimuli to which even the undiffer- entiated protoplasm of Amoeba responds. Such stimuli may be classed as chemical, electrical, thermal (heat), mechanical, or photic (light). The reactions of animals to any and aU stimuli are classed as tropisms, reflexes, instincts, and habits. The first three of these are used to connote the many types of native be- havior, whereas acquired activities are spoken of as habits. In so far as these terms imply a sharp distinction between heredity and en- ANIMAL BEHAVIOR 849 vironment, they are objectionable. They are seldom used in the same sense by different writers in the classification of types of behavior. Tropistic Behavior The concept of tropism has its origin in plant physiology where it was applied to the positive or negative orientation of plant to light, gravity, or other external stimuli. The term was later used by Loeb to cover similar growth responses in lower organisms and simple re- sponses of the reflex type as well. In time, Loeb extended the concept to include practically all reflex and instinctive actions. Loeb used the tropistic theory as a definite explanatory principle. He assumed that symmetrical parts of the body surface of an organism possess identical irritability value. Orientation behavior is due to a forced turning of the organism so that the corresponding sides are equally stimulated. This simple explanation ignores the factor of internal regulation. This has been pointed out by Jennings, Mast, Buddenbrock, and others. Tlie tropistic theory is useful, however, in describing the behavior of Protozoa. Since these forms are composed of but a single cell, any response is likely to involve the whole body. But even in Protozoa it has been asserted that the metabolic or internal activities influence the tropisms. Thus the chemotropic response might be en- tirely different toward a substance that was food to Protozoa, if the animal was hungry rather than satiated. Also it has been found that the reactions of an animal to a strong stimulus may be the op- posite of its reaction to a weak stimulus. Some insects are negatively phototropic to daylight while they are positively phototropic to weak artificial light. And again it has been demonstrated that tropistic re- actions may be modified through experience. Paramecium is a free- swimming organism, and when it finds its progress in one direction hindered by some obstacle, such as a strand of thread, it reverses its course and heads in a new direction. If in the next trial, the obstacle is not avoided, the little animal continues its trials until it does succeed in avoiding the obstacle. The analysis of the behavior of many organisms shows the trial and error method to be a very im- portant factor in their behavior. Many revisions have been made in our concepts of the tropisms during the past few years, but it has not altered the significance of the theory as a means of describing the be- havior of Protozoa and lower vertebrates. Various tropisms are described by the response animals make to these stimuli. Thus we 850 TEXTBOOK OF ZOOLOGY have phototropism, response to light; thermotropism, response to heat ; chemotropism, response to chemicals ; thigmotropism, response to contact; geotropism, response to gravity. Reflex Behavior Tropistic behavior is the dominant type of behavior in the organism without a well-differentiated nervous system. The protozoa are with- out a nervous system, and though a few of them have threadlike struc- tures running through their bodies in such a way as to suggest a cor- relating mechanism of some sort, it seems that, in general, this sensi- tivity and conductivity are properties of the protoplasm itself. In such animals as Hydra and the jellyfish, as well as in all higher ani- mals, we have to do with activities promoted by nervous tissue. In such animals, we no longer call their behavior tropistic but speak of their simplest reactions as reflexes. In principle there is little dif- ference between a tropism and a reflex except in the relative com- plexity of the mechanism involved. The reflex is the dominant type of behavior in the segmented animals which have a simple ganglionic type of nervous system. Also according to Coghill reflexes represent secondary patternings of behavior in the developing vertebrates. The sensitivity and conductivity involved in the production of a reflex are the same qualities of protoplasm as are concerned in tropisms, but they are enhanced by the special differentiation of nervous tissue in these particular directions. The human being, as weU as other animals, exhibits reflexes. They are sometimes called physiological reflexes and are reactions, such as the wink of the eyelids which follows the entrance of a grain of sand into the eye or which follows the sudden approach of an object to the eye. Other reflexes are the knee jerk, contraction of the pupil of the eye (pupillary reflex) when a strong light is flashed into the eye, and the general start of the whole body when a sudden and unexpected noise occurs. These reflexes have two characteristics which are important. First, they are brief in duration, consisting, in general, of a single contrac- tion or relaxation pattern of a group of muscles. Second, they are predictable ; a particular stimulus always, in the normal subject, pro- duces the reaction. If the reaction (such as the knee jerk or the pupil- lary reflex) does not follow the appropriate stimulus (the blow on the I ANIMAL BEHAVIOR 851 tendon or the flash of light) this failure is evidence of an abnormal condition of the reaction. Reflexes terminating in action patterns of smooth muscles and glands also occur. The saliva starts to flow when the odor of food assails the nostrils. It has been conventional to describe reflexes as unconscious reactions, but as a matter of fact there are no such reflexes in the normal human being, and apparently none in other vertebrate animals. Striking a blow on the patellar tendon, for example, does not merely produce a reaction terminating in contraction of the quad- riceps muscle which propels the lower leg forward. Effects are demon- strable in other parts of the body and included in the total response is the perceptional awareness of the blow. Expectant attention of the blow and attention to other stimuli affect the leg movement. The pre- dictability and brevity of the reflex are only relative. The important difference between a reflex and a perceptional reaction is that the re- flex would occur and be still more predictable, even if the animal were so mutilated that the neural patterns, aside from the parts most di- rectly affected, were eliminated. For example, if by accident the spinal cord of a person is broken above the lumbar region, and several months are allowed for the effects of shock to disappear, the knee jerk may be elicited and is more pronounced and more uniform for a given force of blow than before. The person in such a condition does not perceive the blow unless he is allowed to see it or hear it, and he can- not perceive the jerk of the leg except by sight. The discrimination of the lower part of the spinal cord, through which the essential part of the reflex transit occurs from the upper part of the cord and brain, prevents the stimulus from affecting more than the leg muscle. Similar results can be obtained from dogs and cats by experimental severing of the cord at proper levels. In these cases the reflex becomes a real unconscious reaction, which it is not in the normal animal, and it is unconscious because it is restricted to a part of the organism. Chain Reflex Behavior Chain reflex behavior in animals is also often called instinctive be- havior. Watson defines instinct as a series of concatenated reflexes which unfold serially upon proper stimulation. In insects and other animals, we find that reflexes often occur in series ; one directly called out by some appropriate stimulus, itself becomes the cause of another which follows it, and so on through a longer or shorter series. These 852 TEXTBOOK OF ZOOLOGY are often called chain reflexes or instincts. Psychologists are inclined to drop the word instinct, because of the disagreement in definition. The term chain reflex seems to describe better the reaction of animals. A chain reflex, like a simple reflex, is an inherited type of reaction and may appear so complicated and purposeful in character that the inexperienced observer is often led to describe it as intelligent experi- ence or reason. It differs from a simple reflex, furthermore, in that it involves generally the reaction of the animal as a whole rather than that of single parts of organs. In its most complex forms it often de- fies analysis, as in the case of migration of birds, or can be only in- completely analyzed, as in nest-building, care of young, and many others. Habitual Behavior The terms instinct and habit are usually applied to relatively com- plex types of behavior — the one native and the other acquired. Ber- nard has shown that psychologists find it difficult to separate complex behavior into the two categories. This is true because most instinctive acts involve a considerable amount of practice in becoming estab- lished. Practice is the earmark of habit. The capacity to form habits is Dunlap's definition of instinct. Habits are also defined as the modification of response tendencies which show probabilities of cer- tain high degree, that particular responses will occur in particular circiunstances. As we have seen, the animal has a set of native reactions to external stimuli which we caU reflexes. A habit is an acquired response that somehow came to be associated with a stimulus to which it is not ordi- narily attached. Formerly the animal, learning this new association, was said to possess associative memory and this was thought to be indic- ative of consciousness in such animals. Associative memory was ac- credited only to higher animals. Today in many of the lower animals experimentalists find a reaction at first directly called forth by a cer- tain stimulus may later be provoked by another, even an unrelated stimulus. Ahnost every modern student of psychology or physiology knows of Pavlov's experiment with the dog whose mouth watered when a bell was rung as a signal of feeding time. This Pavlov called a conditioned reflex. It has been produced experimentally in a great many kinds of animals. Conditioned reflexes years ago were called associations and were taken to indicate the presence of associative memory. The term associative memory implies a certain subjective ANIMAL BEHAVIOR 853 interpretation, and psychologists today are seeking to leave out all such terms in the study of animal behavior. Most of the behavior of animals can be explained on the basis of tropisms, reflexes, chain reflexes, and conditioned reflexes used in the formation of habits. In most cases the other animals' capacity to form conditioned reflexes and handle themselves in a complex environ- ment is much more limited than in the average man. Whether the higher animals, other than man, can reason is a question that has been much debated. Reasoning is a term upon which there is not unanimous agreement as to meaning. If it merely means problem solving, one would say the individual is able to reason because most higher ani- mals can solve a problem if the problem is suited to the animal. If reasoning involves the use of symbolic processes or abstract ideas, then we are much more in doubt. The cases of so-called intelligent animals such as "Clever Hans" and "The Horses of Elberfield," when examined by comparative psychologists were found to be making use of motor cues, and these had been conditioned by trainers. These so-called intelligent horses could add, subtract, multiply, divide, and extract square root and cube root. They could solve difficult problems, as well as easy ones — but they could not give correct responses when answers were unknown to experimenters and audiences. Reasoning could not be accredited to them. In Hunter's experiments on delayed reaction, the rats and dogs suc- ceeded in going immediately to the lighted box after a period of de- lay only when the body or a part of the body was kept oriented to- ward the lighted box. The monkeys and children were able to succeed when the body was not oriented, thus giving evidence of the use of some other process than bodily orientation. If we hold to the definition that reasoning involves the use of sym- bolic processes, there is little evidence of reasoning in animals, other than human. 1 CHAPTER XLV PALEONTOLOGY (By W. M. Winton, Texas Christian University) Paleontology deals with fossils. One of the most difficult biological concepts to define satisfactorily is the term fossil. Broadly speaking, the word should refer to records left by organisms and not by non- vital forces. Expressions such as "fossil ripple marks," "fossil sand dunes" and the like should never be used. On the other hand, any recognizable trace left by an ancient or- ganism is a fossil. Bones and shells, of course ; but also scales, plates, impressions, burrows, leaf prints, intestinal droppings or coprolites, all are fossils. Among the most interesting fossils are tracks. The poet's "foot- prints on the sands of time" in Triassic valleys of New England, the "thunderbird dance floors" of the Navajo reservation, and the huge tracks around Glenrose, Hamilton, and Hondo, Texas, are all dinosaur tracks. Abundant as are skeletons of Mesozoic dinosaurs, many are known only by their footprints hardened in the shores of ancient seas. Other indirect records may be of considerable biological importance. In the deposits of the Lower Cretaceous division of Texas a few ex- amples of the extinct oyster, Exogyra, have been found which exhibit pearls. These pearls (the best specimen is in the Museum of the Bu- reau of Economic Geology) are of the kind known as body pearls. A quarter of a century of research by the Marquesan commissions of the French Government has shown us that pearls of this kind are caused only by parasitic flatworms. With the Gallic romantic touch evident even in a scientific report, one writer puts it, "our most beautiful pearl, then, is but the sarcophagus of a miserable worm." Pearls in Lower Cretaceous oysters tell us that parasitic flat- worms appeared on the earth at least as early as mid-Mesozoic 854 PALEONTOLOGY 855 times. With this great span of time (150,000,000 years)* one can partly understand how each species of mammal today has one or more species of tapeworms adapted to life in the body of its host only. Some pathogenic bacteria when attacking an animal body leave unmistakable lesions in various parts of the skeleton. The late Dr. Roy L. Moodie, working first at the Baylor Medical School and later at the University of Illinois, was able to date the appearance on the earth of many important disease bacteria. Dr. Moodie studied large collections of skeletons of ancient reptiles and mam- mals and found lesions which the modem pathologist associates with the various bacteria in question. Before leaving the matter of indirect records, one more instance will be mentioned. Some predatory carnivorous dinosaurs left on the vertebrae and other bones of their victims, deep tooth prints and scoriations as the only record which keeps them from oblivion. Are these tooth scratches fossils! The last fifteen years have seen an unprecedented rise in the in- terest in paleontology and in the number of full-time workers in this field. Shortly after World War I, the pressing search for more petroleum brought out strongly the value of fossils in estab- lishing "horizons*' or levels which the oil geologist could use in determining favorable domes and other structures into which to drill. Immediately, workers in what had, hitherto, been one of the most academic subjects were lured from classrooms and museum benches into one of the most competitive and commercial industries. At first attention was given only to the usual larger fossils found at the surface, but as the science of petroleum geology advanced, it became more and more important to know the levels or horizons *How we have come to change such expressions of the earlier geologists as "an- cient," "very ancient" and "extremely ancient" to round figures is a long and fasci- nating story which cannot be taken up here. The students of geochronology or age of the earth after many unsuccessful attempts based on rate of increase of salinity of the sea, change of the ellipticity of the earth's orbit, addition of the elements of the known geological column and others which produced figures which made us human beings, earth's most recent inhabitants, gasp, have now found a dependable check on absolute time. The earlier estimates, and that is all they were, are characterized by only one fea- ture common to all and that is a large figure for the age of the earth. As large as these figures were, they have been replaced by others which are truly staggering, and the new figures are believed accurate. At least different workers get closely parallel results. The unvarying time clock of a disintegrating radio-active mineral and its use in determining the absolute length of time since the beginning of the various geologic periods are discussed fully in any new book on historical geology or perhaps more entertainingly in Wells' Science of Life or Jeans' Through Space and Time 856 TEXTBOOK OF ZOOLOGY which were successively penetrated by the drill. Only microscopic specimens persisted, as the drill crushed the others to unrecogniz- able bits. Then came the remarkable rise of the young science of micropaleontology, a field in which women have distinguished them- selves and proved themselves equal to or better than men. A o D A 0 0 A D O A a""A'n'"A"0 "A'0'0"' first M titled. o A D A 0 0 S econi MetKoA Fig. 440. — Diagram to show the vertical distribution of fossils. AjD.OjA.H.O are characters, each indicating a species which has a thousand feet or more definite vertical range. The middle of the figure illustrates the first method described In the text, and the right portion represents the second method. Some changes have been made in the classic methods of the science of paleontology, although in the main the newer methods are identifiable as merely refinements of the older. Men have long known that some species lived yn the earth much longer than others. In the case of the ordinary marine species, the I I PALEONTOLOGY 857 relative length of the life of the race in question is shown by its vertical extent in the sediments. A short-lived species left its fos- sils in a very few feet of sediments ; others are recorded in hundreds of vertical feet ; others in literally thousands ; and a few species (of Globigerina) have lived uninterruptedly from the Cretaceous period to the present, representing many miles of vertical extent in the sediments. "Whether dealing with larger fossils or with microscopic ones, certain features are desirable when doing geologic work. An ideal fossil species: (1) had a short species life; (2) should have left abundant fossils, since a species which sharply marks off a narrow part of the column is valueless if it is very difficult to find; (3) should have had wide distribution, leaving its record over a large area ; (4) should be readily recognizable, requiring a minimum of time-consuming measures, such as the counting of plates and spines or measurements of different features. The only fossils which completely satisfy the above-mentioned requirements are the ammonites (Fig. 148). The various species were short-lived; in some cases the fossil specimens are actually thicker than the sediments which were laid down during the life span of the species. Most ammonite species occurred in large num- bers, literally dominating the seas. Most ammonites had a wide distribution, sometimes world-wide. Often the chambered shells, comparable to those of nautili, acted as airtight bulkheads after the death of the animals, and the shells floated to great distances. Thus they dated sediments of the same time, whether the animals lived in the waters above any particular locality or not. Finally within a region, most of the ammonite species are characteristic and easily identified. But ammonites are valuable only when working with rocks ex- posed at the surface and only in a limited part of the geologic column. Efforts have been made, naturally, to use less perfect fossils, and methods have been developed to overcome difficulties when the species concerned lack one or more aspects of the ideal set forth above. Since too great a vertical range is the most serious difficulty en- countered in establishing precise levels in the strata, a brief sum- 858 TEXTBOOK OF ZOOLOGY mary will be given of the particular methods which apply to this problem. Two of the methods are here described diagrammatically. First, is the device of bracketing or use of associations, the term being used here with much less exactness than in ecology. Several long-lived species are represented by the symbols (triangles, squares, circles) , Each is a species which has a thousand feet or more of ver- tical range, and the ranges are known. In a well sample the appear- ance of no single one of these species would be of much use to the paleontologist. Certain associations might indicate limited horizons upon which the petroleum geologist could carry his structures from well to well. Using this method of bracketing on the larger fossils in the Lower Cretaceous period of North and West Texas during the past twenty years the writer has been able to locate for engineers, where highways should run to have the greatest volume of the cuts in soft marls. He has also, on many occasions, been able to foretell for builders the exact depth to which foundations would have to be carried for large buildings, dams, and other masonry structures. The greatest development of this method has been in the work with microfossils. As long ago as 1922, three young women,* work- ing in the laboratories of oil companies in Houston, astounded a large audience of petroleum geologists by demonstrating beyond question that they could locate horizons as narrow as twenty feet. This was done in the soft Tertiary sediments of Southeast Texas, where geologists, up to that time, had been satisfied with divisions running from five hundred to a thousand feet. The second methodf is based on the fact that many marine or- ganisms suddenly disappear from the seas which they have long occupied. This extinction is caused by a change in food supply, a change in temperature or other factor, or by the appearance of new enemies. A rapid extinction of a species, in terms of the record in the sediments, may be said to be almost instantaneous. In other words, the end of a species is more likely to be startlingly abrupt than is its beginning. This is true, notwithstanding the fact that with favorable conditions a species can populate an area of land or a portion of the sea in a very short time. An everyday example •Misses Hedwlg Knicker and Alva Ellisor and Mrs. P. L. Applin. tFlrst published by Mr. N. L. Thomas, paleontologist for the Pure Oil Company, ftltliough not claiming- the method as original. PALEONTOLOGY 859 of the rapid disappearance of a species from the face of the earth is the tragic case of the passenger pigeon. Working on this basis, the paleontologist often will use the ''top" of an extended horizon as a fairly precise marker. This method is especially favorable for the micropaleontologist working with well samples, as the drill encountei-s the top first. Biologists working exclusively with living organisms or at least living species sometimes are puzzled by the conventions which paleon- tologists have developed. Show a new flower to a botanist and his first effort is to place the specimen in the family to which it belongs. Show an insect to a zoologist and his first thought is to refer it to its proper order. Just as the botanist recognizes families and the zoologist recognizes orders the paleontologist recognizes (or attempts to do so) both of these divisions in his fossils. But the working unit in paleontology is the genus. Undoubtedly, many of the so-called genera of fossils, if we could see the animals restored to life, might actually be families or in some cases might be species. The old question of the biologist of "what is a species?" is even more difficult to answer in the ease of fossils. A convention which is fairly well adopted is to consider the different examples of a num- ber of varying fossils as the same species if the varying characters exhibit gradations between the extremes. Thus, sometimes, quite variable fossils are put in the same species. On the other hand, examples which do not appear superficially to differ very greatly may be placed in separate species because certain differing charac- ters do not intergrade. Some paleontologists consider, also, that vertical range should be taken into consideration. The time factor is an important one in all branches of geology. No one has ever seen a part of any living species of plant or animal change into a new species. Such changes are common in fossils. No observant person denies the customary assumption that the Kaibab squirrel of the north rim of the Grand Canyon descended from an isolated colony of the more familiar Abert squirrels. This change required many thousands of years, yet in a vertical cliff face of forty or fifty feet, exposing ancient sea bottoms, we may see a record of a quarter of a million years of the history of the sea and its life. The paleontologic picture, however, is a badly distorted one, and the imagination must be exercised vigorously to reconstruct the 860 TEXTBOOK OF ZOOLOGY ancient seas and their life. If we examine the life in a modern tropical sea as shown in a Williamson motion picture, it is difficult to realize how few of the objects we are viewing are apt to leave a permanent record. The corals, of course, will survive eternally. The sea anemones will leave no trace unless some individual may die and leave the impression of its body in the mud before the tissues are disinte- grated by bacteria. Most of the gorgeous fishes will leave no rec- ord. As one grows ill and weak it is attacked by its fellows and the leftover scraps if they reach bottom will be snatched up by Crustacea. Only an occasional tooth or vertebra Avill survive. Sometimes in geologic history a great underwater volcanic deto- nation would kill all of the fish in a small area of the sea, laying them down to be buried in the mud. Thus we get aji occasional glimpse of masses of tangled fish skeletons furnishing us a clue to the great numbers of fish which have lived and of which we have no record. Volcanic detonations of this sort were more common in Devonian time than in ajiy period before or since. For a long time geologists incorrectly called the Devonian "The Age of Fishes," although we know now that since this time, there have been many more kinds of fishes and more of a kind. Returning to our under water movie, we may note that the holothurioids will leave only a few microscopic denticles and dermal plates. The marine worms, if they are tube builders, will leave their tubes. As for the clams and pectens, some of them will leave their shells, although most of the shells will be crushed by octopuses and other scavengers and the small bits will be passed repeatedly through the bodies of marine worms in an effort to squeeze the last milli- gram of nutrient material from them. Some of the bivalves will settle in the mud, and even if the shells disintegrate, neat mud casts will be left. In the Southwest mud casts are the most common fossils. The foraminifera, ostracods, diatoms and other minute organisms, which the picture does not show us but which we know are there, will likely leave a record, provided they have hard tests. Many thick deposits of shales exhibit no fossils which may be seen by the unaided eye, but are teeming with microfossils, principally foraminifera. PALEONTOLOGY 861 The chances of the ordinar}- mariiie animal persisting as a fossil are decidedly slim but are infinitely better than those of a land animal. Land animals are most likely to leave a fossil record in some natural trap in which large numbers of individuals became concentrated. "Well-known examples of this phenomenon are the La Brea asphalt pits of California and the sandstones of the Dino- saur National Monument in Utah. In one, asphalt underlay shallow pools of water where Pleistocene mammals came to drink. The larger herbivores often became mired and their cries would attract their fellows and also the predatory carnivores. Many would be- come trapped. The concentration of some kinds of skeletons in these pits is astonishing. The hard Mesozoic sandstone from which so many reptilian remains have been chiseled in the Dinosaur Monu- ment was originally a quicksand which served as a trap. Since most finds of vertebrate fossils are fragmentary, the art of reconstruction has been highly developed. An exaggerated idea has become widespread that a paleontologist needs only a single bone or tooth to reconstruct the entire animal. It is true that some fos- sil species have been named, based on a single tooth or bone. In some cases, more of the skeleton has been found or remains of other individuals of the species have been turned up.* When the famous fossil Archaeopteryx was found in the litho- graphic limestone of Solenhoffen, comparative anatomists and em- bryologists already were agreed that the birds sprang from a branch of the reptiles. The finding of this specimen, exhibiting a true half- way stage and showing both reptilian and avian structures, started another incorrect idea which has persisted to this day. This is the notion commonly held by unscientific persons that paleontologists are constantly searching for ''missing links." Another incorrect idea, spreading unfortunately to scientific circles, concerns the gaps in the geological record. Admittedly, gaps still exist, but they are far fewer and far less significant than they were in the days when Darwin and Wallace commented so vigorously up- on this matter. In fact, the major developments in paleontology have been made since the daj's of Darwin and Wallace, and the findings have been incorporated in the general disciplines of bio- •The methods used in reconstructinsr vertebrate fossils are describecl in Scott's History of Land Mammals of the Western Hemisphere. 862 TEXTBOOK OF ZOOLOGY logical science. Courageous, indeed, is the modern biologist who will utter generalizations which contradict the ever-growing story told by fossils. ■ Some points, of course, may never be settled. Paleontologists, for example, without exception are neo-Darwinists and believe im- plicitly in natural selection as the major cause of the new species constantly appearing on the earth. In this respect, they are con- \ sidered by some biologists as being old fashioned. Certainly paleon- / tologists could profitably inform themselves on some of the newer aspects of genetics, and it is eciually certain that biologists should know more about fossils CHAPTER XL VI PHYLOGENETIC RELATIONS OF ANIMAL GROUPS AND THE THEORY OF EVOLUTION It is likely that no student of modern biology has gone far before he has realized something of a progressive relationship between the various groups of animals in the animal kingdom. Phylogeny refers to the background of what has gone before in producing a race or phylum and incorporates the thought that different species have arisen from common ancestors. The general idea of it, as now ac- cepted by most biologists, is that all living organisms have been derived through normal reproduction and variation by adaptation from simpler, more generalized ancestors. Phylogenetic relation- ships of organisms and the origin of species have long been topics of exceptional interest to biologists. Two questions which have intrigued the thought of many are : first, Avhence came living ma- terial? and second, what has been its course of events since the time of origin? The observations and thought of Charles Darwin gave the first substajitial support to the idea of all modern species originating from preexisting organisms. Results of his careful studies were presented in 1859 in his book The Origin of Species, which is now a classic in the field. There is still lack of agreement among scien- tists concerning the details of origin of life and species, but there seems to be little doubt that species, genera, and even larger groups have progressively developed since life originated on earth. The ideas of continuity, development, and differentiation of living organ- isms are quite generally accepted by those who have studied these problems. The lines of kinship of animal groups are traced only through common ancestry, and it is not ordinarily assumed that direct relationship exists between representatives of modern groups. Sources of evidences of relationships are quite widespread, coming from such studies as zoogeography, comparative anatomy, embry- ology'', physiology, paleontology, and even others. By combination of information gained from such sources much has been learned of the past history of many groups of organisms. Even these available sources give incomplete and sometimes inconclusive evidence of the history of any particular organism. The individuals are recognized 863 864 TEXTBOOK OF ZOOLOGY as being only points on a long line of modifications. The two ends of this progressive line are seldom recognized with any finality. Darwin, and many others since his time, recognized three funda- mental facts that bring about a progressive continuity among living things: heredity, with the tendency for organisms to resemble their parents ; variation, in that no two organisms are exactly alike, hence the resemblance between parent and offspring is not absolute ; and constant 'production of more offspring than can survive. The phylo- genetic relations of animals show all indications of having been brought about by the operation of the above-mentioned factors in an orderly, evolutionary progress. Variation is one of the most obvious facts and consistent phe- nomena in the animal kingdom. As long as differences among in- dividuals of a group are minor or irreguarly distributed the group is apt to be considered a species. However, the species is not rigidly bounded and it must be remembered that continuous variation is in progress within this group as one generation follows another. It is becoming apparent to man that new groups have arisen and are arising within old groups of animals, also that old groups have broken up, some to become new ones and others to become extinct. These new groups were separated by small differences at first but gradually they attained greater and greater divergence from the common form. This is particularly likely where the different groups have become widely separated geographically or exist under distinctly different conditions of life. Frequently, it is only the widely separated extremes of the group which carry on the posterity, the intermediate forms having died out. In this way rather widely separated species may have arisen from common stock, and the lack of intermediate forms may accentuate the wide divergence between them. During the millions of years which life has existed on earth it seems evident that this process of divergence between groups has been in operation until there now exist many definable groups with distinct lines of rela- tionship. Our natural system of classification is based on the rela- tions and differences established for the different animal groups. Comparative studies of numerous animal groups will help to show some of these relationships. Colony Formation in Certain Protozoa Due to incomplete separation of cells following division in Pro- tozoa, colonies are begun. In some instances groups of cells in some I PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 865 such colonies, e.g., volvox, become specialized as gametes for repro- ductive purposes and the other cells remain as somatic cells. Be- cause of the similarity between various mastigophoran colonies and blastula stages in metazoan development, this group has frequently been cited as the predecessors from which Metazoa at some time originated. Development of the Gastrula The primitive gastrula is thought to have had its origin from a spherical colony of Protozoa by a more rapid growth of the cells at one pole which brought about an infolding or invagination to produce a new cavity from the exterior. This development estab- lishes a body composed of two general layers. Modern adult coe- lenterates, like hydra, and others demonstrate precisely this condi- tion today, with the outer ectoderm and the inner endoderm, each composed of cells serving vital general functions. This allows all cells surface exposure either directly on the exterior or to the cavity. By this means larger and more complex organisms were developed and have lived. Trochophore Larva In Platyhelminthes, Annelida, Molluscoidea, Trochelminthes and Mollusca there occur larval forms of the trochophore type. Al- though the surface ciliation and some other superficial features differ somewhat, a direct relationship among these forms is trace- able through the larva. The fundamental morphology of the cteno- phore is similar to that of the trochophore larva, and the Platy- helminthes are generally thought to have been derived from Cteno- phores. The regular arrangement of gonads and the even, orderly distribution of the diverticula from the intestines of the flatworms are interpreted by many to indicate preparation for segmentation as it appears in annelids. Peripatus and the Wormlike Ancestry of Arthropoda Peripatus, the only representative genus of class Onychophora, was considered a segmented worm for a long time because of its shape, its even, conspicuous segmentation, and possession of nephridia for ex- cretion. But later it was discovered that the respiration is carried on by tracheae, and the body cavity serves as a blood space, both of which are typical arthropod features. In addition, the paired 866 TEXTBOOK OF ZOOLOGY legs are jointed, although similar in appearance to paropodia of Anjielida. There are two jointed antennae on the head and some jawlike plates in the mouth. Situated as it is, midway between annelids and arthropods, this form seems to show an immediate transition from the one group to the other. Upon this basis it is usually held that the arthropods have a wormlike ancestry. Interestingly enough, there exists another idea of arthropod an- cestry from the fact that all of the lower forms of Crustacea pass through a characteristic stage knoAvn as the Nauplius. This larva does not correspond very closely to any strictly annelid stage, but with its short body and three pairs of appendages it resembles a modified trochophore larva. The nauplius larva has some features in common with the rotifers which authorities feel may have arthropod tendencies. Echinoderms and Their Larval Relations Although the adult echinoderms possess radial symmetry, that seems not to have been in the immediate phylogenetical background. The larvae of all echinoderms possess bilateral symmetry. These larvae all have definitely arranged bands of cilia over the body. Embryologists have pointed out the fact that these larvae are not directly related to the trochophore. They show more resemblance to the Tornaria larva of Hemichordates than to any of the non- chordate forms. Ancestry of the Vertebrates The establishment of relationships between the chordate and non- chordate animals has been one of the perplexing problems in the study of phylogeny. Different students of this problem have in- vestigated the possible relationships of such nonehordate groups as the flatworms, nemertine worms, annelids, arachnids, insects, and echinoderms. Their investigations have resulted in the formulation of a number of theories putting forth the various ones of the above- mentioned groups as the progenitors of, or claiming common an- cestry with, chordates. Some authors insist that the chordates have arisen from some segmented form ; others conclude from their evidence that this is not necessarily true or essential. All of the theories establish their relationships to the vertebrates through the protochordates, which are represented by Amphioxus, the tunicates, and hemichordates. I PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 867 Eacli of these has been considered as ancestral stock, which has contributed to the origin of vertebrates. It is usually conceded by authorities in this field of study that Amphioxus is a modified an- cestor of the vertebrates, due to the clear-cut and well-defined con- dition of the distinctive characteristics of the chordates and the presence of a midventral endostyle. Next in the line of thought would come the possibilities of ancestors of Amphioxus. The tuni- cates have been given this distinction by some. The adult has lost most of its typical primitive characteristics, but the larva possesses the distinctive characteristics of chordates and also the ventrally located endostyle, very similar to these structures in adult Am- phioxus. It has been suggested that probably the adult tunicate once existed as an animal similar to its larva of today, and that its forebear was not only the ancestor of modern tunicates but also the form from which the Amphioxus group has descended. Hemichordata, which is usually considered the most primitive of chordates, is regarded as a jjossible ancestor of, or as possessing common ancestral stock with, tunicates and Amphioxus. As will be remembered from the previous study of Hemichordata, its repre- sentatives possess gill slits, a support in the base of the proboscis which may be homologous to the notochord, and four longitudinal nerve cords of which the dorsal is the most highly developed. The above protoehordate relations are rather generally conceded, but there is much less agreement concerning their origin, and several theories have arisen of which the following are important. Annelid Theory.— The segmental condition of this group, the re- lationship of the digestive sj^-stem to other circulatory and nervous systems, and presence of the coelom with related nephridia, present a close comparison to what is found in the embryonic development of the vertebrates. It has been suggested by some scholars that by inverting the body of the nonchordate annelid, the fundamental systems are brought to resemble their relative locations in verte- brates. A fibrous cord has been found in some groups of the an- nelids, and this structure is held to take the place of a notochord in function and position. These fibers are found just dorsal to the chain of ganglia in the annelids. Arachnid Theory. — Such forms as scorpions, Limulus, and other arachnids have been favorably compared to vertebrates. By com- parison of these arachnids with the extinct, fishlike ostraeoderms, 868 TEXTBOOK OF ZOOLOGY an elaborate theory of the possible origin of the vertebrates from this ancestry is derived. Echinoderm Theory. — This theory of vertebrate descent goes again to Hemichordata. The developing egg of this animal becomes a larva known as Tornaria, which floats in marine waters, has bi- lateral symmetry, is almost transparent, and possesses bands of cilia used in locomotion. This larva is almost exactly like that of the starfish and other echinoderms which live in the same habitat. The close correspondence of features of these two groups of larvae has suggested the conclusion that these two types of animals have de- scended from a common ancestor which was similar to these larvae. The line of descent of one branch of this stock has presumably passed through Hemichordata, Tunicata, Ampliioxus, and Vertebrata. The nonchordate ancestors are not j^et conclusively determined, but the foregoing theories suggest the thinking and evidence along that line. Within the subphylum Vertebrata the relations are somewhat more evident, but the phylogenetic sequence is rather obscure at some points. Cyclostomes, the simplest vertebrates, are most closely related to Amphioxus, which has been suggested as the protochordate most similar to vertebrates. These very primitive fish have an eel-like body without paired fins and without jaws. They have from seven to fourteen branchial (gill) apertures in different species, and all of them possess skeletons composed of cartilage. The cartilaginous skull is not entirely closed dorsally but resembles a trough with bars over the roof. The anterior end of the nerve cord has expanded to become a brain (Chapter XXVI). Next in order of complexity are the Elasmobranchii, which possess well-developed, paired appendages (fins) and jaws (Chapter XXVI). They also have a cartilaginous skeleton, but the skull is much more complete dorsally. The number of gill arches is reduced to five, but the apertures are uncovered as in cyclostomes. The number of aortic arches has been reduced from the sixty to ninety pairs of Amphioxus or seven pairs of lamprej^ to five pairs. The group of ganoid fishes, which was the dominant Devonian animal, is generally conceded to have Elasmo- branch ancestry. Most ganoids have more or less cartilage along with the bony structure of the skeleton. Their gills are covered with an operculum, and there are only four aortic arches. The bony ganoids are usually thought to be the ancestors of true bony fish. I PHYLOGENETIC RELATIONS OF ANIMAL, GROUPS 869 There is an extinct form of Amphibia, Stegocephalia, which shows relations to the ganoid fishes and for this reason the ganoids are usually named as ancestors of Amphibia. Some authors hold that the lungfishes, which represent an independent branch of the Elas- mobrajich group, are the ancestors of Amphibia because of their ability to breathe air and live out of water. However, the former view of the phjdogenetic relation between fish and amphibia is most commonly held. Amphibia have well-developed bony skeleton with paired appendages for locomotion on land. Lungs have appeared as a means of aerial respiration, and the aortic arches have been reduced to three. Reptiles are supposed to have descended from Stegocephalia also, with most modern reptiles coming by way of Bhynchocephalia which is represented by one living species, Splienodon punctatum. The snakes, lizards, crocodiles, and the extinct dinosaurs have probably branched from this group, while the turtles are thought to have de- scended through Theromorpha, another extinct branch of Stego- cephalia. The dinosaurs are credited with the ancestry of birds by way of a toothed, feathered, extinct form known as Archaeopteryx. It was essentially a flying reptile. The mammals probably descended from the reptilian group Theromorpha by way of our modern mono- tremes which lay eggs, hatch them out, and then suckle the young with milk from mammary glands. The marsupials, such as kan- garoos and opossums, are next in order, and from these it is thought the Placentalia have arisen. "Within this group some authorities hold the view that the Primates, the order including man, have arisen from Insectivora. The apes and monkeys belong to the Pri- mate group, and there has been some misunderstanding among lay- men generally in regard to the possible relationship of man and the apes. Most people have the misconception that this is a linear descent in which the most advanced member of the lower group represents the immediate ancestor of the next higher group. As a matter of fact, the theory is not that the higher monke3''s are in the process of becoming apes and the higher apes becoming men, but that all three of these groups have had origin as different lines from a common primitive form. Recapitulation Theory. — In the early part of the nineteenth cen- tury von Baer observed that the early stages of vertebrate embryos of different classes had a very close resemblance to each other. He 870 TEXTBOOK OP ZOOLOGY did not subscribe, however, to the recapitulation theory when it was formulated later. Haeckel, coming a little later, became con- vinced that the developing embryo lives over again the stages through which its whole race has passed, and he formulated the recapitulation theory or biogenetic law from this idea. In other words, the organism in its individual life tends to recapitulate the different stages through which its ancestors have passed in their racial history. Briefly this same statement is, ontogeny recapitulates phytogeny. The rehearsal of the phylogeny is in rather slurred form in some details, but the basis for the idea is readily seen. In brief, the theory is applied by comparison. Nearly all metazoan organ- isms begin life by the union of the two germ cells to form a single- celled zygote which is the new organism. At this time it is com- parable to the Protozoa. During the ensuing cleavage divisions a colonial form is represented. Following this, when one side infolds to form a gastrula with two germ layers, the embryo is almost identical to the diploblastic coelenterates as represented by Hydra. Following this, the third germ layer forms between the others and results in the triploblastic metazoan. Basis for the Theory of Evolution One who has thoughtfully studied the field of zoology soon realizes many relationships or homologies in structural make-up of certain different groups of animals ; much similarity of embryonic develop- mental processes, and fundamental coincidences in physiological activities in all living material. Too, it is recognized that fauna and flora are not the same in all parts of the earth at the present, and have not been the same in the past as now. These realizations and other similar ones have been based on scattered sources of evi- dence, much of which is indirect. The evidences which have been discovered, however, have led to the formulation of the theory of evolution and its rather general acceptance as a working hypothesis among biologists. There has been confusion regarding this subject through failure to distinguish between the existence of progressive evolutionary changes, the course of these changes, and the cause of such a progressive series of events. The latter point has been the basis of most of the questions concerning the whole idea, and it is the most speculative of the three. These three are separate though related factors. PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 871 In general, scientists are convinced, according to the evidence they have examined, that organic evolution exists in the form of a progressive change, which has generally proceeded from simpler to more complex organization through a long period of time. Although some sections of the course of this series of developmental events cannot be charted as completely as a ship's course at sea, numerous landmarks serve to indicate what has taken place. Biologists are free to admit that the causes of this process are not understood, and treat it strictly as a scientific problem. Most estimates of the time when living organisms first came into existence range between 60,000,000 and 1,200,000,000 years ago, co- incident with Pre-Cambrian deposits. The manner of origin of protoplasm is purely hypothetical. It is suggested that when con- ditions became suitable, as to chemical elements present, tempera- ture, pressure, etc., a relatively simple colloidal, protoplasmic mix- ture arose, having properties of life though perhaps in a very ele- mentary way. It is assumed that all organisms which have lived are descendants of this simple origin. Cellular organization, with division of labor between nucleus and cytoplasm, is taken to be a first step in the development of protoplasm. It is the simplest vital unit now recognized. If this speculation is followed further, it may be supposed that the unicellular organisms of modern times have descended from such early cells without changing from the simple- celled state, but have developed many specializations as individual types, while metazoans have advanced from certain of these simple forms to a more complexly organized cellular condition. From the nature of protoplasm and from the evidence in the geo- logical records, it is usually assumed that orgajiisms first appeared in the water near the shores of the primitive oceans. Presumably the first oceans were boiling hot, and the land at that time was a molten volcanic mass. If this is true, there must have been a long period after the formation of the atmosphere, seas, and land before protoplasm, as we know it, could have existed. After these cooled and became favorable for life the seas are thought to have sup- ported an abundant life before the land became suitable for its existence. Along with these several speculative aspects of the sub- ject there have been offered several forms of evidence to support the existence of an evolutionary progress of development in organisms. 872 TEXTBOOK OF ZOOLOGY MILLIONS OF OCO,- 100 ARS AGE OF MAN o O QUARTERNARY AGE YEARS (0 >^ OF UJ TERTIARY MAMMALS O ■ o o UPPER 1 100- o CRETACEOUS • o o O (E o o N a J LOWER AGE CRETACEOUS OF KEPTIUES o (COMANCMEAN) u a: K bl Q. 2 - Ul JURASSIC 200- O s. I 2 < q: TRIASSIC q;^ AGE o o O PERMIAN ■ 28s PENNSYLVANIAN 300 - U) c OF AMPHIBIANS uJ (UPPER CARBONIFEROUS) 5: 3a U •< m;ssissippian (t 0 - 0 LJ O 113 >- o u (LOWER CARBONIFEROUS) o o o rs O 400 - d o o d n AGE OF FISHES N o Ul < < o fsl o DEVONIAN M > bl -J a '^ s 1 °- o z SILURIAN (.1 " 500, O AGE OF u o 1 ORDOVICIAN o o INVERTEBRATES ce CAMBRIAN 600- MILLrONS - OF ^ KEWEENAWAN YEARS o o o ANIMIKIAN 700- N o M 5 bl a: bl EVOLUTION OF INVERTEBRATES q: Ml ^ HURONIAN ?g3 0 1" >- q: u W 01 I/) 800- O o o 6 o o a. 51 •C LU "1 ALGOMIAN K 0 in (L , 0 ^ u. in 0 SobJ SUOSURrAN o " h < q; CC UJ < 0 < o X JOZ < EVOLUTION I u 5zu hOQ Ul o> s J > o 1000- £ UNICELLULAR LIFE < o E < u 0 N liJ z □ o Z lij z LI •" 5 - a. a. 0 UJ (/) u =; : < 2 S 1 100- I u CK < GRENVILLE (KEEWATIN) (C0UTCH1CHIN(3) 120o" Si o o o o a crt (-' a ^ d) ^ ^ o ^ he o ^ !/J r/l o t. (t1 ;^ 1) (U >^. rt o o !>. o C o (rf o Q. o s o" o o CI o tH c r( +J (/J s o rt g L. o o >H o o >. XJ o T3 O 'C^ (I) UJ d 42 C 3 m CO _o o o PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 873 Geological Evidence.— Paleontology is the study of fossil remains of organisms deposited in the strata of the earth's crust. Shells and other hard parts are mineralized or petrified; in other cases mud impressions, or tracks, or pitch-preserved individuals, such as insects, and a few frozen forms constitute the majority of fossils. It must be kept clearly in mind that the geologist is able to deter- mine quite accurately the sequence of time or chronological suc- cession of the layers of the earth's crust. The geological time scale shows a long period before any life existed, then the appearance of unicellular plants, then unicellular animals, then colonial forms, simple many-celled forms, and then the more complex ones. Such a timetable estimates the relative period of time during each era and shows some fusion and overlapping of certain types of life. Certain types of unicellular forms are continuous through the en- tire scale. The principal facts shown by the fossil record may be summa- rized: (a) The fossil forms are not strictly identical with any living species, and the remains of plants and animals of each geologic stratum are at least specifically different from the forms in any other stratum, but they may belong to the same genus; (b) the oldest strata containing fossils have represented in them most of the simpler forms of nonchordate animals, while the upper strata contain fossils of all groups more nearly like modern forms, includ- ing chordates; (c) in studying these in sequence, there may be observed a gradual progression from simpler and generalized types toward more specialized and complex forms as one proceeds from the older toward the upper or newer strata; (d) only the more gen- eralized varieties have persisted within the groups that, as a whole, have become specialized ; many of the others have long since reached their climax of specialization and have become extinct; (e) many of the dominant groups of organisms have arisen near the close of a period during which great climatic changes were taking place and have enjoyed dominance during the following period because such a group probably arose in response to the conditions; (f) although many nonchordate phyla had reached an advanced stage of devel- opment in the early Cambrian period, where early fossil records occur, many ancestral sequences have been observed, and these have sup- plied information making possible the detailed description of the course of events that has led to the surviving animals of modern 874 TEXTBOOK OF ZOOLOGY times; (g) the developmental clianges of the chordate groups are more completely read in the fossil record, with the history of the mammals in fullest outline, partly because they are relatively recent and partly because the mammalian skeleton is readily fossilized. The most complete pedigree in fossils has probably been worked out for the horse, and a great deal of its material has been located in the southwestern part of the United States. The Rise of the Horse. — Primitive horselike animals are thought probably to have arisen from an extinct group called Condylarthra, which had five toes on each foot and a large part of the sole resting /J Wrist-- Fig. 442. — Positions of the human hand to show the comparative stages of elevation of tlie horse's foot to the tip of the middle toe. (Courtesy of American Museum of Natural History.) on the ground. The first unquestionable horselike form found in America is the small Eohippus which was about one foot tall and the fossils of which came from the rocks of Eocene times. It had the outer four digits complete on the forefoot, but no trace of the thumb, while the hind foot had three complete digits with vestiges or splints of the first and fifth. Following the foxlike Eohippus, later in the Eocene period came the Orohippus with an enlarged central digit in the forefoot and the loss of the splints in the hind foot. Mesohippus, about the size of a large dog with a three-toed . 1 i § I S ^ o w ^Ei::=rxiO 6 CC: 23t> tC^id f- h^ ~ 0.. L, £ i- ^ -II >. m •«-) -a a /'^ C! b ■M o o o to c o rf a o 3 (1) ho m c 3 & S o C 0) o 4-1 o S c. o > -a ID a l'^,) ^r w »i^ df r^ \' V /' t-\ ^i.-* in. iu. 111 c>/rci<. Pig. 444. — Diagram to show three parallel stages of development of several (From Romanes, Darwin and After Darwin, published by the Open Court Publish- PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 881 r >< asexual reproduction. Sclerotic (skle rot'ik), the dense fibrous outer coat of the vertebrate eye. Scolex (sko'leks), knoblike "head" on anterior proglottid of cestode. Scute (skut), a plate on the ventral side of the body of the snake. Sebaceous glands (seba'shus), small subcutaneous glands, usually connected with hair follicles. Sedentary (sed'en ter i), unattached forms which remain in one place. Semen (se'men), fluid which carries the spermatozoa in the males of most animals. Seminal (sem'inal), pertains to spermatozoa. Semipermeable membrane (semipur'meabl), one which permits the passage of solvents through it but not solutes, unless they dissolve in the membrane. Senescence (senes'ens), period of old age and its effects. Septum (sep'tiim), a wall dividing two cavities. Serial homology (serial homol'ftji), presence of structures of similar origin and form in different segments of the same animal. Serosa (sero'sa), membrane covering peritoneal surface of internal organs; one which secretes a watery fluid. Sertoli cells (ser to'le sels), modified, supporting or nurse cells for forming spermatozoa in the testes. Serum (se'rum), the fluid part of the blood which remains after clotting. Sessile (ses'il), attached directly and incapable of locomotion. Seta (se'ta), hairlike spine or bristle, found in animals. Sinus (si'nus), a cavity in a bone or other part, or a dilated vessel or canal. 920 TEXTBOOK OF ZOOLOGY Siphon (si'fon), a canal or passageway, as the waterways in clams or tunicates. Somatic cells (somat'ik), the cells of the body exclusive of the germ cells; body cells. Somites (sS'mits), segments of the body of a segmented animal. Spermatogonium (spur ma to go'nium), a male germ cell during the period of multiplication and growth in maturation process. Spermatocyte (spur'ma to sit), the male germ cell before its maturation is com- pleted. Spermatogenesis (spur ma to jen'e sis), the maturation of the male germ cells. Spermatozoa (spur ma t6 zo'a), mature male germ cells. Sphincter (sfingk'ter), a muscular band surrounding a tube or aperture which, by its contraction, closes the lumen. Spicule (spik'ul), one of numerous needlelike, solid structures found supporting the tissues in the body wall of sponges. Spiracle (spir'ak'l), openings of air tubes in insects, or modified opening of first gill slit of certain fish. Spireme (spi'rem) thread, the coiled bead-like string of chromatin material that appears during the prophase of mitosis. Splanchnic (splangk'nik), has reference to the visceral organs. Spongtn (spun'jin), the skeletal material of a sponge. Sporulation (spor u la'shun), production of spores by division of a protozoan while encysted. Statoblast (stat'6 blast), an encased, asexual winter bud of a bryozoan. Statocysts (stat'6 sists), sense structures assisting in maintaining equilibrium in certain forms. Steapsin (steap'sin), one of the pancreatic enzymes which is capable of chang- ing fats to fatty acids and glycerin. Stereoblastula (ster e 6 blas'tula), a blastula in which all the cells are in close contact and no blastocoele is formed. Stigma (stig'ma), a sensitive pigment spot of Protozoa or the opening of a spiracle of insects. Stomatodeum (sto ma to de'um), the opening of the developing alimentary tract in an embryo. Stratum (stra'tum), a layer of a series. Striated (stri'at ed), type of muscle with more dense areas across the fibers. Strohila (strobi'la), a series of individuals produced by linear budding, as certain Scj-phozoa and tapeworms. Succession (suk sesh'un), the successive occupation of a given area by several species, either hourly, daily, or seasonally. Sustentative tissue (sus'ten ta tiv), binding together or supporting the various parts of the body. Suture (su'tur), to sew together; a line of junction. Symbiosis (simbi 6'sis), the living together of two organisms. Synapsis (sinap'sis), the pairing of the chromosomes in the germ cells at one stage of maturation. Syncjrtium (sin sish'i um), a mass or layer of protoplasm with numerous nuclei but without distinct cell boundaries. Syngamy (sin'gami), union of mature gametes to form a zygote. System (sis'tem), an aggregation of organs to perform some general function of life. Taxis (tak'sis), a tropismal response involving movement of an organism as a whole. Taxonomy (taks on'o mi) (systematic zoology), the classification or orderly arrangement of organisms according to their natural surroundings. Tegimientary (tegumen'tari), referring to the skin. GLOSSARY 921 Telencephaloii (tel en sef a Ion), the anterior division of the vertebrate brain. Telolecithal (tel 6 les'i thai), type of egg with abundant yolk unequally dis- tributed. Telophase (tel'ofaz), the final stage in mitotic divisions. Tentacle (ten'tak'l), flexible, armlike extension of the body of many non- chordates. Terrestrial (ter res'tri al), a land form; living on or in the ground. Testis (tes'tis), male gonad. Thermotropism (ther mot'rfi pTz'm), response of an organism to temperature. Thlgmotropism (thigm6t'r6 piz'm), response of an organism to contact. Thoracic (tho ras'ik), of or pertaining to the thorax or chest. Thorax (tho'raks\ the middle region of the body. Threshold (thresh'old), the minimum strength of stimulus necessary to get a response. Thrombin (throm'bm), the substance of the blood which plays an important part in clotting. Thyroxin (thi rok'sen or -sin), the hormone which is produced by the thyroid body. Tissue (tish'u), an organization of similar cells into a layer or group for the performance of a specific function. Toxin (tok'sin), any poisonous substance. Trachea (tra'ke a),'the windpipe or a tube for conveying air to the lungs; air tubes in insects. Trichocyst (trik'6 sist), saclike structure in the ectosarc of Paramecium. Triploblastic (trip 16 blas'tik), composed of three germ laj-ers. Trivium (triv'ium), the three anterior ambulacra of Echinodermata, collec- tively. Trochophore (trok'ofor), a semispherical type of larva with cilia; found among flatworms, annelids, mollusks, etc. Tropism (tro'piz'm), the movements of an organism in response to a stimulus. Trypanosome (trip'a n6 som), genus of parasitic Protozoa (Mastigophora) in- cluding the causal agent of African sleeping sickness. Trypsin (trip'sin), a pancreatic enzyme which converts proteins to amino acids. Tsetse (tset'se) fly, a species of fly which serves to transmit the causal agent of African sleeping sickness. Tundra (tobn'dra), level plains region of the arctic region. Tympanum (tim'panum), cavity of the middle ear or more generally any organ serving to receive sound waves. Umhilical (um bil'ikal) cord, the cordlike connection between the fetus and the placenta. Umbilicus (um bil'i kus), the navel or the point of attachment of the umbilical cord to the abdomen. Uncinate (iin'sinat), in the shape of a hook. Ungulate (ung'gulat\ hoofed. Unguiculate (unggwik'ulat), having claws. Urea (ure'a), a nitrogenous compound which is produced as a protein by- product in metabolism. Ureter (ure'ter), the duct which conveys urine from the metanephric kidney to the cloaca or bladder. Urethra (ure'thra), the duct which leads from the urinary bladder to the exterior of the bodv. Uropods (u'ropodz), the sixth pair of abdominal appendages of a crustacean. Vacuoles (vak'uolz), small cavities in a cell filled with water, gasses, or oils. Vagina (vaji'na), the cavity between the uterus and the external genital aperture of the female in many animals. 922 TEXTBOOK OF ZOOLOGY Vascular system (vas'kuler), the circulatory system. Vascular tissue (vas'kuler), fluid tissue consisting of cells known as corpuscles in a fluid medium, plasma. Vein (van), the larger blood vessels leading to the heart. Ventral (ven'tral), side away from the back; literally belly; opposite to dorsal. Villus (vil'us), a fingerlike, vascular process of the internal lining of the small intestine. Vitamins (vi'tamins), substances which occur in small amounts in numerous foods and are essential regulatory substances for the animal body. Vitelline (vitel'lin), the outer membrane of an egg. Vitreous (vit'reus), glassy in appearance. Viviparous (vi vip'a rus), the retention and development of the egg in the mother's body and nourishment of the embryo from the blood of the mother. Volant (vo'lant), able to fly. Zoogeography (z6 6 je og'rafi), the study of the geographical distribution of animals. Zoology (zoo'loji) (animal biologj^), the study of the science which treats of animals. Zygote (zi'got), a fertilized egg, or embryo, after fertilization. Zymogen (zi'm6jen), a pre-enzyme; a substance which is produced in a gland cell and becomes an enzyme when it is discharged and activated by some other substance, perhaps another enzyme. INDEX Abdomen, 348, 639 Abducens nerve, 527 Abductor muscle, 524 Abomasum, 633 Abyssal zone, 713, 770 Acanthocephala, class, 175, 178, 179 Acarina, order, 266, 295. 297 Accipiter, 590 cooper i, 589 Accommodation, 402 Acetabulum, 495, 522, 611 Achiridae, family, 452 Aciculum, 19 6 Acidian larva, 367 Acipenser fulvescens, 447 Acipenseridae, family, 446 Acnidosporidia, 73 Acoela, 160 Acontia, 141 Acris gryllus, 485 Acromegaly, 6 73 Actinaria, order, 140 Actinophrys, 67, 69 Acusticolateral areas, 435 Adambulacral plates, 229 Adaptability and regeneration, 695 Adaptation, 727 Addison's disease, 671 Adductor mandibulare, 468 muscles, 250, 254 Adhesive papillae, 367 Adipose fin, 442, 457 Adrenal glands, 671 Adrenalin, 671 Adrenin, 671 Aedes, 80 Aegeria exitiosa, 328 Aegeriidae, family, 328 Aeolis, 241 Aeolothrips fasciatus, 320 Agamodistomum, 759 Agassiz, 43 Agassiz' tortois! , 550 Aglypha, 556 Agnathostomata, 412 Agriculture, 34 Albatrosses, 587 Albulidae, family, 448 Albumin gland, 246 of hen's egg, 615 Aloes americana, moose, 630 Alcyonacea, order, 143 Alcyonaria, subclass, 143 Alcyonium, 143 Alfalfa beetle, 324 Alimentary canal, 109 AUantois, 818 Allelomorph, 823, 824 Allelomorphs, multiple, 827 Allergy, 839 Alligator mississippiensis, 561 Alligators, 560 AUocosa parva, 295 Alopecia, 838 Alpacas, 630 Alpine zone, 714 Alteration of generation, 112 Altricial birds, 615 Alveoli, 566 of lungs, 514 Alytes obstetricans, 480 Amblema costata, 248 Amblyopsidae, family, 451 Ambulacral groove, 218, 224, 227, 228 ossicle, 229, 230 plate, 228 Ambystoma, 484 axolotl larva, 481 texanum, 480 tigrinum, 478, 480 Ambystomoidea, suborder. 484 Ameiuridae, family, 450 Ameiurus, arteries, 461 cranial nerves, 469 internal structure, 458 life history, 470 natalis, 457 skeleton, 466 veins, 462 Ametabolous, 3 08 Amiidae, family, 4 47 Amino acids, 57 Amitosis, 61 Ammocoetes, 420 Ammonite, 259 Amnicola, 245 comalensis, 247 Amnion, 818 Amoeba, 85 assimilation, 88 behavior, 91 digestion, 87 excretion, 88 histolytica, 744 locomotion, 91, 92 metabolism, 86 proteus, 67 reproduction, 89 Amoebic dysentery, 78 Amoebina, 67 Amoebocytes, 233 Amoeboid acivity, 152 Amphiaster, 63 Amphibia, breeding habits. 479 class, 411. 472 classification, 482 coloration, 474 economic importance, 486 enemies of, 477 families of, 484 hibernation, 480 means of defense, 478 regeneration, 477 secondary sexual characters, 480 Amphibian voice, 478 Amphibians, tailless, 485 Amphiblastula, 128, 808 Amphicoela, suborder, 485 Amphicoelous centrum, 444 Amphids, 175 Amphineura, 256 Amphioxus, 361, 368 circulatory, 371. 372 respiratory system, 373 Amphiplexus, 409 Amphipoda, order, 265 Amphipods. 283 Amphisbaenidae, family, 555 Amphitrite, 195 Amphiuina, 473 Amphiumidae, family. 4 84 Ampulla. 222, 229, 230, 231 923 924 INDEX Ampullae of ]L,orenzini, 437 Amyda, 550 Amylopsin, 385, 386, 502 Anabolism, 86, 377 Anabrus simplex, 315 Anacanthini, suborder, 451 Analogy, 271 Anaphase, 62, 63 Anapsida, subclass, 547 Anasa tristis, 318 Anastomosis, 107 Anatomy, gross, 20 Ancestry of Arthropoda, 865 Anchoviella mitchilli, 448 Anchovies, 448 Ancylis comptana, 326 Ancylostoma, hookworm, 745, 751 Andalusian chickens, 830 Androgenic hormone, 676 Androsterone, 676 Anecdotal period, 847 Anglers, 455 Ansuidae, family. 552 Anguillidae, family, 449 Angulosplenial bone, 520 Ani, 591 Animal and its environment, 719 anomalies, 654 behavior, 846 cell, 58 distribution, 711 kingdom, 25 parasitism, 735 Animals of Gulf of Mexico, 778 Anisoptera, suborder, 321 Annelida, 26, 194 phylogenetic advances, 216 theory of evolution, 867 Anniellidae, family, 553 Annuli, 212 Annulus, 268 Anodonta, 249 stewartiana, 248 Anolis, 552 Anomocoela, suborder, 485 Anopheles mosquito, 74. 79 Anoplura, order, 266, 333 Anseriformes, order, 589 Ant lions, 266, 322 Anteater, giant, 621 scaly, 621 spiny, 617 Antebrachium, 495 Antedon. 225 Antelope, 630, 785 Antennae, 269, 271 Antennata, division, 265 Antennules, 269, 271 Anterior cardinal veins, 434 fontanelle, 428 Anthelmentics, 750 Anthonomus grandis, 324 Anthozoa, class, 140 Anthropoid apes, 637 Anthropoidea, suborder, 635 Anthropopithecus troglodytes, 637 Antibodies, 395, 884 Antigen, 395 Antilocarpa americana, antelope, 785 Antipathes, 143 Antipathidea, order, 143 Antithrombin, 395 Antrum, 812 Ants, 266, 331 Anura, order, 485 Aortic arches of vertebrates, 882 Apes, 635, 637 630, Aphelodactyla, 224 Aphid, beet, 317 black cherry, 318 rosy apple, 316 Aphididae, family, 318 Aphids, 318, 340 Aphis lions, 266 maidi-radicis, 340 roseus, 318 Aphrodita, 195 Apical organ, 237 Ap.on proclive, 324 Aplodinotus, 454 Apodes, suborder, 449 Apopyles, 124 Ai-pendicular skeleton of frog, 520 Appendicularia, 361 Appendix, vermiform, 878 Apteria, 601 Apterygiformes, 585 Ap^erygoi-a, subclass, 309 Apteryx, 585 Aqueous humor, 529 Aquila chrysaetos, 590 Arachnid theory of evolution, 867 Arachnida. class, 266, 292 classification, 295 Arachnoid, 650 Arachnoidea, division, 266 Aranea gemma, 296 Arane.da, order, 266, 295 Arbacia, 220, 222 Arcella, 67 Archaeopteryx, 861, 869 Archenteron, 117, 210. 535 Archianneiida, 194, 212 ArchiteuLhis princeps, 260 Arciferal pectoral girdle, toad, 542 Ardeidae, family, 588 Arenicola, 195 Argentines, 449 Argen.inidae, family, 449 Argiopldae, family, 296 Argulus, 264, 265, 284 Ariidae, family, 450 Aristotle, 3 6 Aristotle's lantern, 221 Armadillidium, 282 Armadillo, nine-banded, 621 embryos, 622 Arrowworms, 193 Artemia, 283 Arteries, efferent branchial, 463 of frog, 503 of horned lizard, 568 of toad, 542 Arthrobranchiae, 271 Arthroleptella lighlfooti, 479 Arthropuda, phyiogeneiic advances. 286 phylum. 263, 746 wormlike ancestry, 865 Arthropodial membrane, 268 Arthrorhabdinus, 291 Articular bone, 467 Artiodactyla, order, 629 Ascaphus, 479 truei, 485 Ascaridia lineata, 176 Ascaris, 175, 176, 179 in.einal anatomy, 181 intestinal worm, 745 life cycle, 182, 183 relations to man, 183 Ascaroidea, order, 176 Ascidiacea, 361 Ascon, 124, 125 Ascorbic acid, 390 Asellus, 281, 282, 283 INDEX 925 AsUidae, family, 330 Asphalt pits of La Brea, 861 Aspidiotus perniciosus, 316. Sis Ass, 633 Assimilation, 278 in Euglena, 81 in Hydra, 152 Astacus, 266 Aster, 62, 63 Aster ias, 228 cleavage, 116 Asteroidea, class, 217, 226 Asteromeyenia, 121 AsLhma, 839 Astrangia, 142, 143 Astropecten, 218 Athermidae, family, 452 Atlas, 494 Atom, 697 Atomic weight, 697 Atrial cavity, 365, 370 Atrina seminuda, pinna shell, Mi Atriopore, 370 Auidae, family. 296 Auditory meatus, 639 nerve, 527 Audubon. John J.. 786 Auks, 591 Aurellia, 138 lite cycle, 139 Australian region, 712 Autonomic function, 528 Autotomy, 219, 234, 281 Aves, class, 411, 582 classification, 584 economic relations, 596 Avicularia, 184. 185 Aviculariidae, family, 296 Avocets, 591 Avoiding reaction, 101 Axial gradient, 173 skeleton, 378 Axoloil larva of Ambystoma tigrinum, 481 Axone, 107 Aye-aye, 635 Babesia bigemina, 80 Baboon, 636 Back cross, 827 swimmers, 318 Badgers, 627, 628 Bagre marina, 450 Bailer, 270 Baker's mealy bug, 318 Balaena mysiicetus, right whale, 635 Balance in life, 32 in nature, 31 Balanoglossida, order, 361 Balantidium coll. 70, 71, 744 Balanus, 265, 285 Baleen, 634 Balistidae family, 4o5 Barb, 599, 600 Barbary ape, 636 Barbels, 457 Barbule. 599. 600 Barnacle, 264 acorn, 264 goose, 264 Barnacles, 284 Barnea costata, 771 Barracudas, 453 Barriers to distribution, 717 Basal disc, 140. 146, 147 Basiliscus, 552 Basipodite, 270 Basisphenoid bone, 465 Basket star, 219 Basommatophora, 258 Bass, freshwater, 454 Basses, eea, 454 Bat, brown, 620 Mexican free-tailed, 620 Baihyal zone, 770 Bathymetric distribution, 713 Batoidei, order, 424 Batrachoididae, family, 455 Batrisodes, 340 Bais, vampire, 620 Bdellostoma, 412 Bears, 629, 791 Beaver, 624, 787 Beavers, 622 Beche-de-mer, 235 Bedbugs, 318 Bee hies, 330 Bees, 266, 331 BeeJe, Colorado potato, 3J3 Beetles, 265, 322 ambrosia, 33 5 passalus, 335 Behavior, chain reflex, 851 habitual, 852 reflex, 850 tropistic, 849 Belfrage, G. W., 48 Belonidae, family, 451 Belostomatidae, family, 318 Benthos, 769 Beroe ovata, 157, 159 Bidder's organ, 542 Bighorn, 631 Billbugs, 324 Binary fission, 90 Biogenetic law, 870 Biological effects of radiations, 697 point of view, 17 Biology, 17, 20 Biomes, 724 Bios, 17 . „„„ Biotic communities, 723 formations, 724 Biotin, 390 Bipes biporus, 555 Bipinnaria larva, 23 2, 234 Biramous appendages, 2(0 Bird migrations, 582 B.rds, 411, 582 economic relations, 596 Birth rate, 839 Bison amencanus, buffalo, bii Bisons, 630 Bittern, 587 Bivium, 227, 228 Black bass, 453 widow spider, 293 Blackbirds, 595 'Blasendarm," 190 Blastema, 688 Blastocoel, 210 Blastocoele, 117, 534, 53o, 80o Blastocyst, 814 Blastoderm, 805 Blastodermic vesicle, 814 Blastomeres, 171. 533, 800 Blastopore, 534. 805 Blastostyles, 132 Blastula. 117, 134, 155 earthworm, 210 of frog, 533 . Blatella germanica, 315 Blattidae, family, 315 Blind cave salamander, 482 926 INDEX Blindworm. 553 Blister beetle, 322 Blood corpuscles, 107 matching, S83 white cells, 513 Blubber, 634 Blue baby, 664 crab, 282 darter, 589 Bluebirds, 595 Boar, wild, 632 Bobcat, 629 Bobolinks, 595 Body louse, 738 Boidae, 556, 557 Boll, Jacob, 48 Bombyliidae, family, 330 Bonellia, 214, 215 Bonitos, 454 Bonnethead shark, 422, 439 circulatory system, 433 Book lice, 333 scorpion, 266 Bot fly, 747 Botalius, duct of, 491 Botaurus lentiginosus, 587 Botflies, 330 Bovidae, family of cattle, 630 Bowfln, 447 Bowman's capsule, 398, 650 Box-elder bug, 318 Brachial artery, 506 Brachiolaria larva, 232, 234 Brachiopoda, 27, 187 Brachium, 495, 563 Brachypephus magnus, 302 Brain of frog, 526 ventricles, 526 Branchial arches, 465 Branchiata, 264 Branchiopoda, order, 264, 283 Branchiostegal membrane, 463 Branchiostegites, 268 Branchiostoma, 361, 368 Branchipus, 264 Breeding habits of amphibia, 479 Bremidae, family, 338 Breviciptidae, family, 486 •'Bristle jaws," 193 Brittle star, 218 Bronchi, 566, 604, 649 Bronchioles, 566, 604 Brontosaurus, 546 Brown, Robert, 49 Brownian movement, 55 Brucophagus funebris, 33 4 Bryobia praetiosa, 298 Bryozoa, 27, 184, 185 Bubo virginianus, 593 Buccal cavity of frog, 500 Buccopharyngeal respiration, 514 Buffalo, 631, 632 Buffon, 44 Bufo americanus, 538 marinus, marine toad, 473. 478 punctatus, 474 toad, 485 valliceps, 485, 487 woodhousii, 485, 538 Bufonidae, family, 485 Bugs, 266 Bulgula, 184, 185 Bulimulus dealbatus liquabilis, 239 Bullfrog. 473, 497 Bullhead, internal structure. 458 life history and reproduction. 470 nervous system, 469 skeleton, 466 yellow, 457 Bumblebees, 338 Bunchgill, 321 Buprestidae, family, 324 Bursa, copulatory, 179 Bursaria, 71 Busycon, 248 egg capsules, 247 shell, 771 Butterflies, 266, 325 Butterfly ray, 424, 425 "Buzzard," 588 Byssus, 253 Cabbage-head jellyfish, 138 Caddis flies, 200, 321 Caeca, 603 pyloric. 459 Caecilianella. 237 Caecilians. 472 Calciferous gland. 204 Calcispongie, class, 120 Calendra mormon, 324 Calliuectes, blue crab, 282. 772 Callorhinus alascanus, fur seal. 029 Callus, 691 Calorie, 388 Calyx, 224 Cambarus. 266 clarkii, 207 Camels, 630 Campanella. 130 Campanularia, 131 Campeloma decisum. 247 Campodoidae, family. 311 Canadian zone. 714 Canidae, family. 028 Canis gigas. timber wolf. 029 latrans. coyote. 028 Cauon of Morgan, 848 Cantharidin. 341 Canthon simplex. 335 Capillaries, 392 Capitophorus potentillae. 320 Capitulum. 495 Caprella. 204 Caprimulgiformes, order. ,593 Carabidae, 324 Carangidae. family. 454 Carapace. 208. 578 Carbohydrates, 50 Carbon cycle, 28 Carcharhinus. 439 Carcharias, platydon. 422 Carcharodon carcharias, 422 Carchesium. 72. 73 Cardiac chamber. 272 muscle. 381 plexus, 528 Cardinals, 595 Cardo, 305. 345 Caiibous, 630 Carinatae, 584 Carnivora, order, 027 Carnivores, aquatic, 629 Carotid arch. 505 gland. 505 Carp, 450 leather. 449 lice. 284 scale. 449 Carriers. 833 Carrion beetles. 324 Carunculina texasensis, 248 Case flies. 321 Cassowaries. 585 Castor canadensis, beaver. 024 Casuariformes. order, 585 INDEX 927 Cat, 411. 639 circulatory system, 64S digestive system. 646 excretory system, 650 Mexican ringtail, 626 muscles, 642 nervous system. 650 reproduction. 652 respiratorv svstem, 649 Catabolism, S6. SS, 153, 278, 377 Catamount. 629 Cataphracti, suborder. 454 Catfish, 411, 450 freshwater. 450 sea, 450 Cathartes aura septentrionales, 588 Catostomidae. family, 450 Cats, 629 Cattle, 630 Caudata. order, 484 Caudina. 224 Cave fishes. 451 Caviar, 447, 456 Cavies, 625 Cebidae. family of monkeys. 635 Cecum, 648 Cell division. 61 principle, 49 theory, its influence. 52 Cellular differentiation. 104 organization. 105 Centipedes, 265 Central disc, 217 Centrarchidae, family. 454 Centrolecithal egg. 803 Centrosome, 59, 60 Centrum, amphicoelous. 465 procoelous. 520 Cephalic ganglion, 16S Cephalin, 395 Cephalization. 375 Cephalochorda. subphylum. 361, 368 Cephalodiscida, order. 361 Cephalodiscus, 361, 364 Cephalopoda, 2.58 Cephalopods, evolution of. 259 Cerambycidae. 324 Ceratite. 259 Ceratium. 75 Ceratobranchial cartilage. 428 Ceratodontidae. family. 455 Ceratohyoid bone, 467 Ceratopharys. 472 Ceratosa, order. 122 Cercaria, 756, 758, 759 Cerci, 348 Cercomonas, 66 Cercopithecidae, family of monkeys. 636 Cere 599 CereiDellum, 406, 469. 607 frog. 526 Cerebral commissure. 252 ganglia. 245 ganglion of Helix, 243 hemispheres. 469 vesicle, 372 Cerebratulus, 163 Cerebropleural ganglion, 252 Cerebrum, 406. 607 Cerianthus. 140 Cervical groove, 268 vertebrae, frog, 520 Cervidae, family of deers. 630 Cervus canadensis, elk. 630 Ceryle alcyon. 593 Cestoda, class, Kil. 762 Cestodes, 160. 161. 174 Cestus veneris. 157 Chaetae. 197 Chaetognatha. class. 27. 184. 193 Chaetopoda. 194 Chagas disease. 79 Chain reflex, 851 Chalcis-fly, 334 Chalones. 667 Chambers. Dr. Robert, 61 Chaos diffluens. 85 Chaparral bird, 591 Characters, unit. 823 Charadriifonnes, order, 591 Charybdea. 137 Cheek pouches. 624 Cheiloschisis, 656 Chela, 270 Chelicerae. 292 Chelonia, order, 548 Cheloniidae, family, 550 Chelydridae, family. 549 Chemotropism, 77 Chenopodium, 183 Cherniidae, family. 318 Chevron bones. 641 Chicken, 590, 597. 598 circulatory systein, 604 digestive sjstem. 601 muscles of hind limb, 611 muscular system, 612 reproduction and life history. iil3 respiratory system, 603 skeletal system, 609, 610 Chilodon, 70 Chilomastix, 67 Chilomonas, 66 Chilomycterus schoepfli, 443 Chilopoda, order, 265, 288, 290 Chimaera affinis. 425 monstrosa. 425 Chimaeras, 411 Chimpanzee. 637 Chinchillas, 625 Chipmunk. 622 Chiroptera, order, 620 Chitin, 106, 343 Chitinous exoskeleton. 286 Chloragogen cells, 204 Chlorella vulgaris, 144 Chlorochroa sayi, 318 Chlorohydra. 144 Chloropisca glabra. 331 Choanocytes, 119. 120, 12G Chondrostei. order. 446 Chordata, 27, 360 classification. 361 phvlogenetic advances. 3(12 Chorioid, 401 layer of eye, 529 plexus. 527 Chorion. 818 Chorionic villi. 819 Chromatin. 59. 824 Chromatophore. 82 Chromomeres, 60, 63 Chromonema. 63 Chromonemata. 60. 61 Chromosome map. 835 Chromosomes. 61. 114 Chrysaora. 139 Chrysomelidae. 324 Chrysopa californica. 322 Chrvsopidae, family. 322 Chvme. 385. 501 Cicadas, 266, 318 Cicadellidae, family. 320 Cicindelidae. family. 324 Ciconiiformes, order, 588 Cilia. 94. 102 I Ciliata. 70 928 INDEX Cimex lectularlus. 318 Cimicidae, family, 318 Cinclides, 142 Circulatory or vascular system, 110 Circulatory system, cat. 648 chicken, 604 open type, 2 63. 273 phrynosoma, 566 pisces. 459 toad. 542 turtle. 580 Circumesophageal commissures. 274 Circumoral canal. 221. 231 Circumpliaryngeal connectives. 208 ring. 176 Cirri, 197. 224 Cirripathes, 143 Cirripedia. 284 order, 265 Cirrus. 179 Clam, circulation, 252 digestive system, 250 excretion. 253 fresh water, 248 nervous system and sense organs. 252 reproduction. 253 respiration. 251 Class Infusoria. 93 Sarcodina. 85 Classification of man. 22 Clavicles. 609 Cleavage, 280, 374, 802 in asterias, 116 in cat. 653 disymmetrical. 804 divisions, 374 earthworm. 211 in frog, 533 holoblastic. 802 in mammal, 816. 814 merotalastic. 802 spiral, 804 superficial. 803 Cleft palate, 656 Click beetles. 324 Cliona. 121 Climax community, 728 ClUeUuiii. 199, 200. 209. 214 Clitoris of cat. 653 Cloaca, 613 Clonorchis. 161. 759 sinensis, Chinese liver fluke. 745. 757 Clupea harengus, 448 Clupeidae. family. 448 Clypeaster. 222 Clypeus, 345. 357 Cnemidophorus. 554 Cnidoblast. 148. 149. 150 Cnidocil. 148, 149 Cnidosporidia, 73 Cobras, 558 Coccidia, 73 Coccinellidae, family, 324 Coccygeal vertebrae. 609 Coccyx. 878 Cochineal. 341 Cochiopa texana. 247 Cochlea. 403. 60S. 650 Cockroaches. 313 Cocoon. 171. 214 earthworm. 209 Codfishes. 451 Cod-liver oil. 456 Codonosiga, 60, 75 Coelenterata, classification. 131 economic relations of. 156 phylum. 26. 130 Coelenterates, phylogenetic advances of, 1.56 Coeliac axis. 606 Coeliacomesenteric artery. 506 Coeloblastula. 807 Coelom. 194. 211 development, 536 Coitus. 409. 814 Colaptes auratus, 594 Coleoptera. order. 265. 322 Coleps, 70 Collar cells, 119 Collared lizard, 553 Collembola, order, 265. 311 Colloid. 53 Colloidal state. 53 CoUum. 288 Colotaoma. 665 Colony formation. 864 Color blind inheritance, 833 Coloration in amphibia. 474 Colubridae. 557 Columbiformes. order. 591 Columella, 574, 608 Colymbiformes, order, 586 Comb jellies, 157 Commensalism. 735 Commissure. 199 Conditioned reflex. 852 Conductivity. 54 Condvlarthra. 874 Condylura cristata, star-nosed mole. 619 Conger eels. 449. 473 Congridae. family. 449 Coniferous forest formation. 725 Conjugation. 96. 98 Couocephalus vicinus, 310 Consciousness. 54 Conservation of wildlife. 784 Continuity of germ plasm. 48 Contractile vacuole. 88, 94 Contractility. 54. 381 Conus arteriosus. 490. 510 Coon. 629 Cooties, 206 Coots. 591 Cope, E. D.. 48 Copepod. 284 Copepoda. order, 265 Copperheads, 556. 559 Copulation, 613. 6.i3. 814 earthworm. 209 Copulatory bursa, 179 Coraciiformes, order. 594 Coracoid bar. 430 bone, 465. 495. 521. 609 Coral, 140. 143 snake. 558 Corallium rubrum. 143 Coregonidae, family, 448 Coreidae, family, 318 Corizldae, family. 318 Cormorants. 587 Coin-ear worm. 320 Cornea. 270. 301. 401. 528. 607 Corneageu cells, 276, 277 Corona radiata, 812 Corporin, 076 Corpus albicans, 813 luteum, 812 Corrodentia. order. 266. 333 Corti. organ of. 6.50 Cortin. 671 Corythucha distincta. 318 Cosmic rays, 700 Cotton boll weevil. 324 Cotvlophoron. 702 Cougar, 629 Cow. 630 Cowper's glands. 653 INDEX 929 Coxa. 293. 306. 347 Coxopodite. 270 Coyote. 628 Crabs. 264. 265. 772 Cranial cre.sts. toad. 540 nerves. .527 of bullhead. 469 Cranium, 380 of fish. 465 of frop. 518 Craspedacusta. 131 Crawfishes. 267 Crayfish. 264. 265. 266 appendages. 269. 270 development. 279. 280 di^jestive system. 272 excretory system. 274 habitat and behavior. 267 metabolism. 27S regeneration. 281 reproduction. 278 respiratory system. 271 vascular system. 273 Creatinin. 381. 464 Cremastocheilus annularis. 340 Creosote bu=-h-kangaroo rat biome. 727 Crepidula. 808 Cretinism. 655, 668 Cricket frog acris, 485 Mormon. 312 Crickets. 265 Crinoid. 225 Crinoidea. class, 224 Cristatella. 186 Croaker. 453 Crocodiles. 411. 548. 560 Crocodilia, order. 548. 560 Crocodilus acutus, 561 Crop, 601. 602 Crossing over. 834 Crossopterygii, order. 446 Crotalidae. family, 559 Crotalus, 5.59 Crotaphvtus collaris, 552 Crows, 595 Crura cerebri. 526 Crustacea, class. 264. 266 Cryptobranchidae. family. 484 Cryptobranclioidea. suborder, 484 Cryptobranchus, 473, 484 Cryptodira, suborder. 547. 548 Cryptotis parva, short-tailed shrew. 619 r'—i-ntn'-ifores. order. .586 Crystalline cone, 276 style, 251 Cteni. 444 Ctenoid scale. 444. 464 Ctenophora, phylum, 26, 1.57 Cub shark. 422 Cubomedusae, 137 Cuckoos, .591 Cuculiformes order. 591 Cucumaria, 224 Culicidae, family, .330 Cumulus. 812 Cuneiform, 611 Curculionidae, 324 Curlews, 591 Cutaneous artery of frog. 508 Cuticle. 81. 201 nonchitinous, 194 Cuticula. 308 Cybiidae. family. 453 Cycloid scale. 444 Cvclophylliriea, order, 161 Cyclopia, 665 Cyclops. 264. 265. 281. 283 Cyclosis. 95 Cyclospondyll, order, 422 Cyclostomata, class. 410, 412 economic relations, 413 Cynocephalus, a baboon, 636 Cynoglossidae, family, 452 Cynomys ludovicianus, prairie dog, 622, 623 Cynthia, 361 Cyprinidae, family, 450 Cyprinodontes, 451 Cyprinus carpio, 449 Cypris, 264 Cyrtoceracone, 2.59 Cysticerous, 762. 764 Cytology. 21 Cytopharynx. 81 Cytoplasm, 60 Cytosome. 86 D Dace. 450 Dactylozooids, 135, 137 Daddy longlegs, 266, 299 Damsel flies, 265, 321 Danaidae, family, 327 Danaiis menippe, 327 Daphnia, 264, 281, 283 Dart sac, 246 Darters, 454 Darwin and Evolution, 885 Charles, 44 Dasyatis, 456 americanus, 423 Dasypus novemcinctum, armadillo, 621 Daughter cells, 62, 64 Decapoda, order, 265, 282 Deciduous forest formation, 726 Declaration of indefensibles, 792 Dedifferentiation, 695 Deduction, 18 Deer, 630, 786 Delamination, 810 Dementia praecox, 839 Demospongiae, class, 121 Dendrobates. 480 Dendrocoelum lacteum. 165 Dengue fever. 80 Dental formula. 643 Dentalium. 257. 2.58 Dentarv bone, 495. 520 Dentine, 643 Dermal branchiae, 226. 229, 233 r)ermaptera. ordei". 265. 320 Dermochelidae. family, .550 Desert formation, 727 Desmognathus. 484 Desor's larva, 163 Development, arrested, 654 of nervous system, .536 of sexual reproduction. Ill of starfish. 2.32. 2.34 Developmental stages of several em- brj-os. 880. 881 de Vries. Hugo, 48, 8,87 Dextral, 240 Dextrocardia, 664 Diabetes. 677 insipidus, 674 Diaphragm, 109 Diaphragmatic hernia. 6.57 Diapophysis. .543 Diapsida. subclass. 548 Diaptomus. 283 Diastase. .384 Diastole. .396 Dibranchiata. 260 Dicamptodon en=atus. 484 Dicrocoelium. 762 Didelphia. 617 Didelphis vlrglniana, 61Q 930 INDEX Didinium. 70 Diencephalon, 419, 4fa9 of frog, 526. Differential birth rate, 8^9 Differentiation, 801 intercellular, 104 Difflugia, 67 Digenea, 161 Digestion in frog, 501 intracellular. 130 , . . +-„„, Digestive enzymes and their functions, 387 system, 109 cat. 646 chicken, 601 toad, 541 tract, solitary wasp, .50» Digitigrade gait, 641 Dihybrid cross, 820 Dinocardium, shell. 7 a Dinosaurs, 869 DioctophjTne. 176 Dioctophymoidea, order, 17b Diodontidae, 455 Dioecious, 112 Diphyllobothrium, 161 latum, fish tape, 741, 742 tape, 765 Diplasiocoela, suborder 486 Diploblastic condition, 130 form, 120 Diploid condition, 800 number. 114 Diplopoda, 265, 288, 290 Dipnoi, subclass, 455 Dipsomania, 839 Diptera, order, 266, 329 ^ Dipylidium caninuin, dog tape, 763, .bo Discoblastula, 806 Discocephali, suborder. 4o4 Discomedusae, 137 . ^ Dispersal in distribution a< Dissosteira spurcata, 344^ Distribution of animals, ai Diverticula, 167 Diverticulae. 213 Dobson flies, 322 Dog, 628 Dogfish, freshwater. 447 .shark. 422 ., Dolichoglo'sus, nervous system, obJ kowalevskii, 362 Doliolum. 361 Dolphin. 634 . . ,^ „- 090 Dominance, principle ot, iiz6 Dominant, 45 Doodlebugs, 322 Dorosomidae, family, 448 Dorsal aorta. 605 Dosimeter, 706 Dourine. 80 Doves, 591 mourning, 591 ^ Dracunculus, Guinea worm. (4.j Dragonflies, 265, 319. 321 Drassidae. family, 295 Drassids, 295 Drassus neglectus. 296 Prill. 636 , ^ „„„ Drosophila melanogaster. 82S Drumfishes. 4.54 Dryobates pubescens, 59o Duck mole, 617 Duckbill. 617 Ducks. 589, 788 Duct of Botallus. 491 of Cuvier. 445 Dujardin. 49 Duodenum. 382 Dura mater, 527, 650 Dussumuriidae, family. 448 Dwarfism, 673 Dysgenic. 839 E Eagles, 589 bald, 590 golden. 590 Ear structure. 402 Eardrum, 498 Earthworm. 199 circulatory system. 20o development, 210 internal anatomy, 201 Earwigs, 265 Ecdysis. 263 Echeneidae, family. 4.54 Echinarachnius, 222 Echinaster, 218 Echinococcus, 756 dog tape, 745 Echinoderm theory of evolution. SbS Echinodermata. 26 Echinoderms, larval relations. 8bb Echinoidea, class, 219 Echiuroidea, order, 194, 214 Ecology, 24, 719 ^ ^ , ,. o,a Economic importance of entomology, 6w relations of birds, 596 of mammals, 637 Ectethmoid, 406 Ectoderm, 117, 130 of frog, 535 Ectoparasite. 737 Ectopistes, 786 migratorius. 591 Ectoprocta, 184 Ectopterygoid, 467 Ectosarc, 81. 86 Edentata, order, 621 Eel, 443 Eels, 449 conger, 449 morey. 4.50 Eelworm, 175, 180 Egg. centrolecithal, 803 guides. 349 of hen, 615 "Egg tooth," 615 - Egret, 588 Egretta candidissma, 08*5 Elapidae, family, 558 Elasmobranchii. class, 411, 4_V economic relations. 425 Elateridae. family, 324 Electrons, 697 Electrotropism, 77 Elephantiasis. 745. 746 Elephants. 634 ^ roj. Elephas indicus, elephant. 6u4 Eleutherodactylus latrans. 479 ricordii, 472 Elks, 630 Elops saurus. 447 Elytra. 322. 354 Embiidma. order. 2b.\ 666 Embids. 205 Embryology, 23 comparative, 798 of frog, 532 later stages, 537 of toad, 544 Embryonic membranes 81 1 Empoasca fllamenta, 317. 3^u Emulsoid. 53 Emus. .585 _,_ ^__ Emvdidae, family of turtles. :i49, on Emys blandingii, 549 INDEX 931 Enamel. G43 Encephalocoele. 664 Endamoeba, 747 coli, 78 g-ingivalis, 78 histolytica, 67. 78 Endocrine glands. 666 summary, 678 system, 110 Endodei-m, 117. 130, 155 of frog, 535 Endolimax nana, 78 Endolympii, 403, 503 Endomixis. 97 Endoparasite. 737 Endoplasm, 81 Endopodite, 2G9. 270 Endoprocta. 184 Endosarc. 81, 86 Endoskeleton, 380. 517 Endostyle, 366 Engranlidae, family, 448 Ensiform process. 644 Enterobius vernicularis, 176 Enterocoele formation. 811 Enterokinase. 385 Enteronomas. 67 Entomobrvidae, family, 312 Entomostraca. subclass, 32, 204. 283 Entosphenus. 413 tridentatus. 415 Entozoic animals. 66 Enzvmes, 56, 383 Eohippus, 874 Ephemerella grandis. 321 Ephemerida. order, 26.'5. 321 Ephvdatia, 121 Ephyra, 139. 140 Epibolic gastrulation. 809 Epibranchial cartilage, 428 Epicauta puncticollis, 323 Epicoracoid cartilage, 521 Epicranium, 343 Epidermis. 499 Epididvnii.'--. 438. .572. 653 Epigyiium. 293 Epinephrine. 671 Epiotic. 465 Epipharynx, 357 Epiphragma, 238 Epiphysis, 419. 469 Epipodite. 269. 270 Episternum of frog. 521 Epistropheus. 609 Epistvh"?. 72. 75 Epitheliomuscular cell, 148, 150 Eptesicus fuscus, brown bat. 620 Equatorial plate. 62, 63 Eavius caballus. horse, 876 T^romobates. 298 Erethizon dorsatus, porcupine, 625 Eriophyes pyri, 298 Ervthrocvtes. 108. 395 of fi-og. 503 Erythroneura comes. 320 ziczac. 317 Escocidae. 450 Esophagus. 203 Esox. 451 Estrogenic hormone. 676 Etheostomidae. family, 454 Ethiopian region. 712 Ethmoid bone. 465 Euarctos americanus, brown bear, 629 Eubranchipus, 283 Euconulus chersinus trochulus. 240 Eucosmidae. family. 328 Eudorina, 73 Eugenic grouDS. 841 measures, 844 Eugenics, 821 Euglandina singleyana, 240 Euglena, 67, 81 behavior. 84 reproduction, 83 respiration, 83 Euglenoid movement, 84 Eulamellibranchiata. 256 Euophrys inonadnock, 296 Eupagurus, regeneration, 685 Euplectella. 120 Euplotes. 71 Eupomatus larva. 237 Eurypelma steindachneri. 297 Euselachii, order, 422 Euspongia, 122 I'jusuchia. suborder. 548, 560 Eustachian tube. 608 frog. .530 Eutheria, subcla.ss of mammalia, 617 Eutherodactj'lus. 485 Eventognathi. suborder, 450 Involution, annelid theory. 867 arachnid theory, 867 distributional evidence. 876 echinoderm theory. 868 embryological evidence. 879 geological evidence. 873 morphological evidence. 878 physiological evidence, 883 theory, 863 Excitor hormones. 667 Excretion, starfish. 233 Excretory organs, cat, 650 cliicken. 606 fish. 464 frog, ,515 or urinarj' system. 110 Bxoccipital bones. 463. 641 Exocoetidae. family. 451 Exogyra. 854 Exopodite. 269. 270 Exumbrella. 132 Eye of chicken. 607 of frog. 528 worm. 176 Eyeball, 401 Eyespot, 233 Facets, 345 Facial nerve. 527 Fairy shrimps. 283 Falconiformes. order, .589 Falcons, 589 Fallopian tubes, 652 Family elopidae. 447 size, 841 in dysgenic groups, 842 Fances, 648 Fascia. 612 Fasciola. 101 ^ _^„ hepatica. liver fluke. 743. 758, (u9. 760 Fascioloides. 7.39 Fasciolopsis. 760 Fat bodies of frog, 532 Feathers. 582. 600 Feeblemindedness. 838 Felidae. family. 629 Felis cougar, mountain lion. 629 domestica. house cat. 639 hernandesii, jaguar, 629 pardalis. ocelot. 629 Femoral vein, frog, 509 Femur, 306. 347 of frog. 522 Fenestra ovalis. 574 Ferrisia. 240 excentrica. 238 932 INDEX Fertilization. 614 in bullfrog-. 531 membrane. 533 Fiber zibethicus. muskrat. 625 Fibrogen. 393, 395 Filaria, 175. 176. 745 Filaroidea, order, 176 Fileflshes, 455 Filibranchiata. 256 Filiform papillae, 603 Filoplume. 600 Filum terminale. 651 Firmisternal girdle, 543 Fish, bony. 442 excretory org^ans, 464 flying, 442 lice. 284 motii. 310 respiratory system. 463 skeletal system, 464 typical bony, 457 Fisher. 638 Fisheries and zoology, 34 Fission, 111 Fissipedia, suborder, 62S Flagellum, 81, 82, 126 Flame cells. 160, 167, 189. 190, 397 Flat-fishes. 451 Flatworms, 26 Fleas, 266. 329 Fleming. 62 Flicker. 594 Flies, 266. 329 Flounders. 451 Fluke, human blood. 755 Flukes. 160 Fly, big-headed. 332 Flying fishes. 451 Folliculin. 676 Folsomides decemoculatus. 312 Foot-gland. 190 Foramen magnum, 519 ovale. 664 Foramina. 527 Foraminifera. 68. 78 Formicidae. family, 339 Fossil, 854 reptiles. 546 Fossils, distribution. 856 Fourth ventricle, 435. 469 Fox. 628 Frog, appendicular skeleton, 520 blastula nnd gastrula, 533, 534 blood, .503 brain, 526 circulatory system. 502 cleavage, 533 common tree. 474 digestive system. 499 embryology, 532 excretory .system. 515 feeriing habits. 477 fertilization. 531 hairy. 476 heart. .510 heart beat. 512 internal ear. 529 later development. 537 muscular system. 523 nervous svstem. 523 pelvic girdle. .521 reproductive organs. .531 respiratory system. 514 sen'se organs. 528 skeletfi system. 517 skin. 475 skull. 517 Bplnal cord. .')27 Strecker's. 482 swamp tree. 473 Texan cUff. 478 Frog — Cont'd tic-e. 4SC1 vascular system, 503 veins. 508 vertebral column. 520 visceral skeleton. 519 Frogs. 411. 472 Frons. 343 Frontal bones. 465 Frontonia. 70. 71 Frontoparietal bone, 517 Fruit flies. 330 Fulica americana. 591 Fungiform papillae. 650 Funiculus. 185. 186 Furs, 637 G Gadidae, family, 451 Galea, 305. 345 Galeichthys felis. 450 Galen. 21. 37 Galeocerdo arcticus. 422 Galliformes. order, 590 Gallinules. 591 Gallus domestica. 598 Galvanotropism, 77 Gambusia. 451 Game, big. 794 Gametes. Ill of hydra, 1.54 Gametocytes, 74. 749 Gametogenesis. 113. 799 Gammarus. 32, 264, 283 Ganglia, 399 Ganoid scale. 444 Ganoin, 444 Gapes in chickens. 176 Garfishes. 4.51 Garpike, 447 Gasterosteidae, family. 451 Gasterostomata. order. 161 Gastric digestion. 385 mill. 272 Gastrocnemius. 523 Gastrocoele. 805 Gastrocopta armifera. 240 Gastrophilus, horse hot, 747 Gastropoda, 257 Gastrotheca. 480 Gastrotrioha. ISS Gastrovascular cavity. 130. 14S, 150, 1-58. 167 Gastrozoids. 1,S5. 137 Gastrula. 117. 1.55 development. 865 enrthworm, 210 of frog, .'^.33. .534 starfl=;h, 234 Gastrulation. 805. .808 earthworm. 211 in Tnammal. 815 Gaviiformes. order, .586 Gazelles. 630 G'^bia stpllpta. 239 "Geoko." 551 Geese. .589 Gci«er. S. W.. 48 Gpkknnidae. family, 551 Hel Pt5>tp, .53 G<^mmulp='. 121 Genae. .^43 Gene"?. 61. 824 comn'<^'"nen'^ary. 829 mult''nle. 828 siinniementary. 830 Genetics. 23 qnd eugenics. 821 Ge^iohvo'd mu^'^ie. 468 Geni*f-mph, 394 Hemophilia, 839 Hemophiliacs, 395 Hepatic caeca, 229 portal vein, 434, 509 Heredity, 821 physical basis, 824 Hermaphrodite, 154 Hermaphroditic condition. 112 Hermaphroditism, 40S, 063 Hermit crab, regeneration. 685 Herodias egretta, 588 Heron, 588 Herring, 447, 448 Herrings, round, 448 Hertzian waves, 701 Hetaerina americana, 321 Hetaerius tristriatus, 340 Heterocera, suborder, 325 Heterocercal tail. 426 Heterocoela, order, 121 Heterocoelous vertebrae. 609 Heterodont condition, 643 Heteronomous condition, 203 Heterophyllidea, order. 161 Heterosomata. suborder. 451 Heterotrichida, 71 Heterozygous individual, 825 Hexylresorcinol, 183 Hibernation, 476 in amphibia, 480 Hiodon tergisus. 448 Hiodontidae. 447 Hippiscus, 315 corallipes, 310 Hippoboscidae, 330 Hippocrates, 36 Hippoglossidae, family, 452 Hippopotamuses. 630 Hirudinea. 194. 212 Hirudo. 195, 212 Histogenesis. 801 Histology, 21 History of zoology, 36 Hogs, 632 Holoblastic cleavage, 802 Holocephali, subclass, 425 Holometabolous, 308 Holostei, order, 447 Holothuria. 224 Holothuroidea. class, 222 Holotrichida, 70 Holozoic animals, 66 Homo sapiens, 22 Homocercal tail. 443 Homocoela, order, 120 Homoiothermal condition. 616 Homology. 271. 663 serial. 271 Homonomous condition, 263 Homoptera, order, 266, 316. 318 Homozygous individual. 825 Honey bee, 357 Honeydew, 340 Hoofed animals, 629 Hooke. Robt, 49 Hormones, 666 of non-chordates, 667 plant growth, 667 sex, 675 Horned lizard, 561 arteries. 568 internal structure, 566 reproduction, 571 urinogenital system, .571 veins, 570 Horned "toad," 561 Horntails, pigeon, 331 Horse, 633 Equus caballus, 876 its rise, 874 phylogenetic development, 875 Horsefly, green-headed, 331 Horsehair snake, 175, 177 Horseshoe crab, 266 Host specificity, 741 Housefly, life history, 330 Hudsonian zone, 714 Human blood cells. 396 heredity, 836 Humboldtiana. 242 ciiisosensis. 239 Hummingbirds. 593 Huxley, Thomas H.. IS, 47 Hyaline cartilage, 107 Hyaloplasm, 00 Hyalospongiae, class, 121 Hydatina, life cycle. 191 Hydra, 131, 144 life cycle, 153 locomotion, 145 metabolism, 151 regeneration. 156 viridissima, 144, 146 Hydranths, 132 Hydrobios, 713 Hydrocaulus, 132 Hydrocephalus, 664 Hydrocorallina. 136 Hydrolysis, 384 Hydromedusae, 135 Hydrophidae, family, 559 Hydropsyche, partita, 322 Hydrorhiza, 132 Hydrostatic organ, 464 Hydrotheca, 133 Hydrozoa, class, 131 Hyla, 486 cinerea, 474 crucifer. tree frog, 483 Hylobates, gibbons, 637 Hyman, Libbie H., 144 Hymenolepis, 161 nana, 765 Hymenoptera, order, 266, 331, 357 Hyoid arch, 465 Hypaxial portion of body. 496 Hyperbranchial groove. 373 Hyperoartii. 412 Hyperotreti, 412 Hyperthyroidism, 669 Hypertrophy, 691 Hypnotoxin, 149 Hypobranchial groove, 372 Hypodermis, 308 Hypopharyux. 305 Hypophysis. 469 frog, 526 Hyporachis Hypostome, Hypothesis, Hypotrichida 601 134, 18 71 147 I I if Ichthyomyzon, 413 Ichthyophis glutinosus, the caecilian, 473 Icius similis. 2!)(i Ideal continent, 722 Idotea, 282 Iguanidae, family. 551 Ileocaecal valve, 565 Ileum, 499, 564 Iliac arteries of frog, 507 Ilium. 611 of frog. 521 INDEX 935 Imago, 309 Implantation. 815 Incus, 403 Inclefensibles, 792 Infrared, 699 Infundibulum. 158 Infusoria, 71 class, 70 Ingestion in amoeba, 87 in hydra, 151 Ingression, polar, 810 Inguinal canal, 652. 653 Iniomi, suborder, 450 Inner cell mass, 816 Innominate vein of frog, 508 Insect characteristics, 301 Insecta, class, 265, 300 Insectivora, order, 619 Insects, economic relations. 340 hemimetabolous, 320 holometabolus, 321 nerve winged, 322 representative. 343 shellac scale, 341 social life, 334 Inspiration, 648 Instinct, 846. 852 Insulin, 677 Integument, 377 Integumentary system, lOS Interambulacral plates, 219. 220 Interauricular septum, 510 Intercalary disks, 107 Intercellular digestion. 151 Intercostal muscles. 581 Intermedin. 678 Internal carotid artery, 433 Interradial pouches. 222 Interstitial cell, 149, 150 Intem'ertebral discs. 643 Interzonal fibers. 62 Intracellular digestion, 119 Invertase, 386 Iris. 401 of frog, 529 Irritability, 54 Ischiopubis, 430 Ischium of frog, 521 Ischnochiton, 257 conspicuus. 256 Islands of Langerhans, 110, 677 Isopoda, order. 265 Isoptera, order, 265. 320 Isospondyli, subordei-. 447 Istiophoridae. family, 453 Itch mite, 739 Ixodidae, family, 298 .Tacana spinosa gymnostoma, 591 Jaguar, 629 .lapanese beetle. 31 Japyx hubbardi. 311 .Tavelina. 032 Jays. 595 Jellyfishes. 26 .loint snake. .")52 Jugular veins of frog, 508 Jugulares, suborder. 4.55 Julidae. family. 2S9 Julus. 289 Jumping spiders, 290 June beetle. 324 bug. 354 life history. 350 Jungle fowl, 597 K Kangaroo. 618 rat, 624 Karyolymph, 61 Karyoplasm. 61 Karyosomes, 61 Katydids, 315 Keratosis, 838 Kerona, 71 Kidneys, 398 of cat, 650 Killdeers, 590, 591 Killifishes, 441 King crab, 266 Kingbirds, 595 Kingfisher, 593. 594 Kinosternidae, family, 548 Kissing bug, 79 Kiwi, 582, 585 Labia minor, 320 Labial palps, 357 Labium. 345 La Brea asphalt pits, 861 Labrum, 345, 357 Lacertilia, suborder, 547, 551 Lacewings, 322 Lacinia, 345. 305 Lacrimal bone. 407. 643 Lacrymaria. 71 Lactase, 3S6 Lacteals. 513 Lactogenic hormone, 678 Ladybird beetles, 324 Ladyfishes. 448 Lagena, 437, 60S Lagomorpha, order, 625 Lamarck, 43 Lamprey, 413 larva. 420 life history, 421 Lampreys. 410 Lampsilis. 248 anodontoides. 249. 254 Langerhans. islands of, 677 Larva, hexacanth. 764 Larvacea, 361 Laryngeal chamber. 564 Lasius. 340 Lateral line system. 470 Latrodectus. 293 mactans, 295 Laveran and Manson. 79 Leaf hoppers. 320 roller, strawberry, 328 Leber's atrophy, 832 Leeuwenhoek, 49 Lemuroidea, suborder. 035 Lemurs. 635 Lens. 401 Leopard. 629 I^epas. 265 Lepidoptera, order, 266, 325 Lepidosaphes becki, 320 Lepidosiren paradoxa. 455 Lepidosirenidae, family, 455 Lepidurus. 204 Lepisma saccharina. 310 Lepismidae. family. 310 Lepisosteidae. family. 447 Leplodactylidae, family. 485 Leptinotarsa decimlineata. 323 Leptocoris trivittatus. 318 Leptodea fragilis. 248 T>eptodora. 264 Lepton squamosum, 239 I^eptosynapta. 224 Leptotyphlopldae, family, 557 936 Lepus, 626 Lestes uncatus, 321 Leucocytes. 393 of frog. 503, 513 Leucosolenia. 120 Levator muscle, 524 Leydig, duct of, 493 Libinia, spider crab, 772 Lice, 266. 333 Life, attributes of. 30 history, ameiurus, 470 housefly. 330 lampiey. 421 monarch butterfly. 328 mosquito, 329 zones, 713. 714 Ligula. 305 Ligulae. 345 Limacina australis. 241 Limulus Polyphemus, 299 Lineus socialis, 162 Liuin, 59 net, 61 Linkage, 832 Linneas, 42 Lion, 629 Lionotus, 71 Liopelmidae, family, 485 Lipase. 384 Lipophores. 474 Liriope, 136 Litanseutria obscura, 318 Littoral zone. 713, 769 Liver, function, 386 Lizard, alligator, 552 collared. 552 fishes, 450 horned, 552 Lizards, 411, 551 night, .'554 sand, 554 worm, 555 Llamas. 630 "Lobster claw," 659. 837 Lobsters, 265 Locust. 343 Locustidae. family, 315 Loligo, 2.58 brevipennis, 2.57. 260 Long-horned wood beetles. 324 Loons. 586 Lophiidae, family, 455 Lophophore, 185, 186, 188 Lovebirds, 591 Lower jaw of frog, 520 Loxodonta africana, elephant, 634 Lubber grasshopper, western. 302 Lucernaria. 137 Lucunae. 107 Lumbricus. 195 terrestris. 199 Lungflshes. 455 Lutein, 676 Lntinnidae. family. 454 Lutianus campechanus. 454 Lycosa. 295 Lycosidae, family, 295 Lygaeidae, family, 318 Lygus pratensis, 318 Lymnaea, 242 bulimoides, 256 techella, 238 palustris. 237 stagnalis, 238 Lymph, 108 Lymphatic system, 502, 513 Lymphocytes, 513 Lynx, 629 ^L-yre snake, 556 INDEX M Macaca sylvana, 636 Macaque monkey, 636 Macaws. 591 Machilidae, family, 310 Machilis orbitalis, 310 Mackerels. 453 Spanish. 452 Macroccphalus. 664 Macrogametes. 74. 112 Macromeres. 534 Macroperipatis, 288 Madrepora. 142 Madreporaria. order. 142 Madrepoiic plate. 218 Madreporite, 217, 220, 221. 223. 226, 2L's, 230. 231 Magellania. 188 Maggots, 330 Malacostraca, subclass, 265 Malaria, 74, 79, 748 Malformations, 654 Malleus, 403 Mallopliaga, order, 266, 333 Malpighi, 40, 49 Malpighian bodies, 606 corpuscles, 398, 492 tubules, 110. 286, 352 Malthus' law, 885 Mammalia, 411 class, 616 classification, 616 economic relations, 637 Mammalian development, 812 Mammary gland, 653 glands, 639 Mammogenic hormone, 678 Man, 6^5 Manatee, 634 Mandrill, 636 Man-eater shark, 422 Maniliin, human, 109 Mantidae, family, 318 Mantispids, 322 Mantle, 365 cavity, 250 Manubrium, 131, 138 Manus, 522 Marine Biological Laboratory, 43 zoology, 766 Marmosets, 635 Marmota monax, woodchuck, 622 Marsupia. 253 Marsupial frogs, 480 pouch, 61S, 619 Marsupialia, order, 619 Marsupials, 617 Martins, 595 Mastax, 189 Mastigamoeba, 66 Mastigophora, 66 class, 81 Mastodonsaurus, 473 Mating among defectives, 839 Maturation of germ cells. 113 Maxillae, 270. 357 Maxillipeds, 269, 270 May beetles. 354 Mayflies, 265. 319, 321 Meantes, suborder, 485 Meckel's cartilage, 520 Mecoptera, order, 266, 333 Mediastinum, 657 Medulla oblongata, 435, 607 of flsh. 469 of frog, 526 Medusae, 113, 132. 133. 134 Megalopidae, family, 447 I I INDEX 937 Megalops, 285 Melanerpes erythrocephalus, 395 Melanophores, 474 Melanoplus atlanis, 343 femur-rubrum, 310 Melophagus ovinus, 330 Membranous labyrinth, 403. 530, 574 Memory, associative, 852 Mendel, Gregor J., 45, 821 Mendel's law, 821 Menetus dilatatus, 238 Meninges, 650 Menofoini, 676 Mentomeckelian bone, 520 Mentum, 305, 345 Mergansers, 589 Mermiria bivittata, 310 Meroblastic eggs, 546 Merozoites, 74 Merychippus, 870 Merzoites, 749 Mesenchyme. 165 Mesethmoid, 456, 643 Mesoblast cell. 211). 211 Jlesoderm, 117, 809 of frog, 536 parietal, 819 splanclinic, 815 Mesoglea. 132, 150. 151 Mesohippus, 874 Mesonephric tube. 464 Mesopterygium. 430 Mesorchium. 492, .572 Mesothelium, 201 Mesothorax, 305. 347, 3.57 Mesotubarium. 492 Mesovaria, 812 Mesovarium. 438, 573 Mesozoa, 120 Metabolism. 28. 54. 377 Metacarpals. 522 Metacercaria. 759 Metacrinus. 225 Metagenesis. 113, 135 Metamere, 211 Metameres, 194. 361 Metamerism. 375 Metamorphosis. 308, 801 in acidia, 367 starfish, 232 Metanephros, 397, 545 Metaphase, 62, 63 Metapleural folds. 370 Metapterj'gium, 430 Metargiope trifasciata, 290 Metatarsal bones of frog, 522 tubeicles, toad, 540 Metatheria, 619 Metatliorax, 305. 347. 357 Metazoa characteristics. 103 Metazoan organization, 103 Metridium, 140, 141 Microcephalus, 664 Microgametes. 74 Microhyla, 486 Micromeres. 534 Micronuclel. 98 Micropodiformes, order. 593 Mocropogon undulatus. 453 Microptci'us salmoides, 453 Microsporidia, 73 Midbrain of fish, 469 Miescher's tube, 73 Migration of animals. 716 airwavs. birds. 583 Millepedes. 265 Millepora, 136 Millers, 327 Millipede, 289 Mlmus polyglottos, 594 Mind, 846 Mink, 638 Minnows, 450 mud, 450 Miracidium, 756, 758, 759 Miridae, family, 318 Mirids, 318 Misumena vatia. 295 Mite, human itch. 738 Mites, 266 Mitochondria, 60 Mitosis, 62 Mnemiopsis, 157 Mockingbird, 594, 595 Mole, 619 Molgula, 361 manhattensis, 365 reproduction, 366 Moilusca, 27 classification, 256 economic relations, 255 internal morphology, 243 phylum. 236 Molluscoida. 27. 184 Mollusk shells. 771 Molt. 280. 601 Monacanthidae. family, 455 Monarch butterfly, 326 life histoiy, 328 Monaxon, 124 Monaxonida, order, 121 Moniezia, sheep tape, 763. 765 Monkeys, 635 Monocytes, 513 Monodelphia, 617 Monoecious condition. 112 Monogenea. 161 subclass. 762 Monohybrid cross, 825 Monos.ga, 66 Monotremata, suborder, 616 Mooneyes, 447 Moose, 630 Moray eels. 450 Momion cricket. 312 Morphology. 20 Morula, 807, 809 Mosaic vision, 277, 278 Mosquito fish, 80, 451 life history. 329 Mosquitoes. 266 Mother-of-Pearl. 255 Moths. 266. 325 Motor cells. 400 end plates, 400 nerve root, 527 Mouse, 625 Mouth parts of insects, 301 Mud hen, 591 puppy. 487 Mugil curema, 452 Mugilidae. family, 452 Mulatta, 828 Mliller, Johannes. 43 Miillerian tube, 493 Mullets, 452 Muraenidae, family. 450 Murex tenuispina, 241 Murgantia histrionica, 318 Mus. 625 Musca dome?tica.330 Muscidae. family. 330 Muscle, actions of. 524 attachments, 524 involuntary, 524 Muscles of cat. 642. 644 of fish. 467 of hind limb, chicken, 611 938 INDEX Muscular or contractile tissue, lOG system, 1C9 chicken. 612 frog-, .523 phrynosoma, 576 Musculium ferrissi, 248 Musculocutaneous vein. .508 Musli ox, 631 turtle, 549 Muskrats, 622, 625 Mustelus canis, 425 Mutation theory, 887 Mutations, 836 Mutualism, 735 Mycetozoa, 69 Mylohyoid muscle, 408 Myocomma, 370 Myocytes, 125 Myoflbrils, 106 Myology, 644 Myosepta, 496 Myoseptum, 370 Myotomes, 370, 372, 427, 496, 537 Myriapoda, class, 265, 287. 289 Myrmecophaga jubata, giant anteater, 621 Myrmecophiles, 339 Myrmeleonidae, family, 322 Mysis, 285 Mystacoceti, order of whales, 634 Myxedema, 669 Myxidium, 73 Myxine, 412 Myxinidae, family, 412 Myxinoidea, subclass, 412 Myxobolus, 73 Myxosporidia, 73 Myzus cerasi, 318 N Nabidae, family, 318 Nabis ferus, 318 Nacreous layer, 250 Nagana, 80 Narwhal, 634 Nasal bones, 465 Natural resources, restoration, 794 selection. 885 Naturalists of the frontier. 48 Nauplius larva, 285 Nautilus. 258 Navel, 820 Nearctic region, 712 Necator, 750 hookworm, 745 Nectocalyces, 137 Necturus, 485 circulatory system, 490 digestive system, 489 internal structure, 488 maculosus, 487 muscular system, 496 respiratory system, 491 skeleton, 494 urinogenital system, 492 Neelidae, family, 312 Nekton. 769 Nemathelminthes. 26, 745 phylum, 175 Nematocyst, 134 Nematocysts, 136, 1.3S. 142, 147, 148 Nematoda, class, 175 Nematode, 750 Nematognathi, suborder, 4.50 Nemertina, class, 161 Nemertinea, 27, 192 Neoceratodus fosteri, 455 Neocomatella, 225 Neoechinorhynchus emydis. i7S Neoscona benjamina, 296 Neotropical region, 713 Nephridia, 110, 188. 195, 206. 207. 214, 39S Nephridial ducts, 189 Nephridiopore, 195, 206, 207 Nephridiopores, 201 Nephridium, 203 Nephrostome, 195, 199, 206. 207 Nephrostomes, on frog kidney, 514 Nereis virens, 195, 196 Nerves, cranial, of frog, 529 Nervous function, 404 system, 110 of bullhead (ameiurus), 469 of cat, 650 chicken, 607 development, 536 of frog, 525 phrynosoma. 573 starfish. 233 sympathetic, 528 toad, 543 vertebrates. 399 tissue. 107 Nettling cells, 135 Neural arch, 465 folds, 536 plate, 536, 815 tube, 537, 817, 818 Neurocoele, 372, 527 Neuroepitlielial, 404 cells, 153 Neuron, 107 Neurones, 405 Neuropodiuni, 196. 197 Neuroptera, order, 266, 322 Neutrons, 698 Newt, common (triturus). 484 Nicotinic acid. 389 Nictitating membrane, 562, 600, 639 Nidamental gland, 262 Nidicolae, 615 Nidifugae, 615 Night lizards. 554 Nipples, 653 Nitrogen cycle. 29 Noctiluca, 67 Noctuidae, family, 327 Nodosaria hispida. 69 Nomenclature, binomial system, 22, 43 Notochord, 360, 372 Notonectidae, fnmily, 318 Notopodium, 197 Nuclear membrane, 59, 60 Nuda. 157 Nutritive-muscular cell, 151 Nymph. 319 Nysius ericae, 318 O Oak-deer biome. 726 Obelia, 113, 131. 132 Occipital condyle. 494 Ocean, conditions, 771 Ocelli, 301, 345 Ocelot, 629 Ocular plate, 221 Oculina, 142 Oculomotor nerve, 527 Odobenus, walrus, 629 Odocoileus, deer, 630 Odonata, order. 265. 320 Odontoceti, order of whaler, 634 Odvnerus dorsalis, 335, 359 Oestridae, family, 330 INDEX 939 Oestrin. 676 Oligochaeta, 194, 196. 199 Omasum, 633 Ommatidia. 277, 302 Ommatidium. 276 Omosternum, 521 Onchocerca volvulus. 745. 746 Oniscus. 282 Ontogeny. 113. 285 recapitulates phylogeny. 870 Onychophora, class, 265. 287 Oocyst, 74 Oocyte, secondary, 115 Ooecium, 185, 187 Oogenesis, 113. 114 Oogonia, 114, 799 Ookinete, 74 Opalina, 70, 745 Operculum, 537 Opliichthyidae, family, 449 Ophidia, suborder, 548, 556 Ophioderma, 219 Ophiothrix, 219 Ophisaurus, 552 Ophiura, 219 Opliiuroidea, class, 219 Opisthobranchiata, 258 Opisthoglypha, 556 Opisthotic bone, 494 Opossum. 619 Optic chiasma. 526 ganglion. 262 lobes. 469 Oral groove. 93 hood, 370 lobes, 138 ossicles, 229 Orangutan. 636. 637 Orcinus orca, killer whale. 634 Oreamnos montanus. 631 Oreaster, 218 Organ. 108 of corti. 403, 650 Organ pipe coral, 143 Organic catalysts, 57 Organizers. 694 Organogenesis. 801 Organs and system. 817 OrioTital region. 712 Origin of species, 43, 863 Ornithorhynchus anatinus. 617 Orohippus. 874 Orthoceracone, 259 Orthoptera, order, 265. 313 Orytricha. 71 Osculum. 122 Osmeridae. family, 448 Osphradium, 253 Ospreys, 589 Ossicles. 227. 228 Osteoblasts. 691 Ostia, 122. 124. 273 Ostium. 531, 573. 652 Ostracoda. order, 204, 284 Ostraea virginica, oyster. 771 Ostrich. African. 584. 585 Otocyst. 237, 367 Otter. 638 Otus asio. 593 Ova, 104 Ovarian follicle. 812, 813 Ovaries of cat, 6i>2 Ovary of hen, 614 Ovibas mo.schatus. musk ox. 631 Oviducts. 407 of frog. 531 Oviparous. 407 Ovipositor. 348 Ovis canadensis, mountain sheep, 630 Ovotestis, 243 Ovoviviparous condition, 248. 487 Ovum. 652 Owls, 593 barred or hoot, 593 great horned, 593 prairie dog. 592 screech. 593 Oxyechus vociferus. 590 Oyster drill. 241 extinct. 854 shell, 771 Oysters, 255 ' Pacific shore animals, 774 Paddlefishes, 446 Pagurus, 282 hermit crab. 772 Palaearctic region. 712 Palatine bones. 517 Paleontology, 854 Paleozoology, 24 Pallial line, 249, 250 Palpigradi, order, 266 Palps, 197 labial, 250 Palpus, 305 Pancreas, 490, 677 Panaorma. To Panther. 629 Pantothenic acid, 390 Papillae, filiform, 603 fungiform, 650 vallate, 650 Papula, 226 Parabacillus coloradus, 318 Paragastric canals. 158 Paragonimus, 161, 760 Paragordius, 178 Parakeets, 591 Paramecium, 70,93, 100 locomotion. 102 metabolism. 95 reaction to temperature. 100 reproduction, 96. 98 structure. 93 Paramylum. 83 Paramj-xine. 412 Parapodia. 196 Parapophyses. 465 Parapsida, subclass. .547 Parascalops breweri, hairy-tailed mole, 619 Parasite, accidental, 736 occasional, 736 Parasites, facultative, 737 their transmission, 740 Parasitism, 735 Parasphenoid bone, 465, 495, 518 Parathormone, 670 Parathyrin. 670 Parathyroid glands, 670 Parenchyma, 105 Parietal bone, 465, 494 Parotoid gland, toad, 540 Parrots, 591 Parthenogenesis, 112, 191. 410 Parthenogonidium. 76 Passalidae. family. 335 Passalus cornutus. 335 Passenger pigeon, 591. 786 Passeriformes. order, 595 Pasteur, 47 Patella, 240. 293. 644 larva. 237 940 INDEX Pathology, 23 Paurometabolous, 308 Pavlov. S52 jr'tjaris, ai tifical, 255 Peas, dwarf. 822 Pecari angulatus, javelina. 632 Peccary, 630, 632, 787 Pecten, 357, 608 Pectinatella, 186, 187 Pectine, 298 Pectoral girdle, toad. 542 muscles, 524 Pedal ganglion, 245 Pedicellariae, 218. 226. 229 Pediculati, suborder, 455 Pediculus, louse, 738 Pedigree chart. 843 Pedipalpi, 292 order, 266 Peduncle, 187 Pelagic animals, 769 Pelecaniformes, order. 587 Pelecypoda, 256 Pellicle, 94 Pelmatohydra oligactis, 144 Pelobatidae, family, 485 Pelvic girdle, frog, 521 Pemphigus betae. 318 Penguins. 586 Penial setae. 181 Penis, 246, 261 Pennatula, 144 Pennatulacea, order, 143 Pentacrinus. 225 Pentastomoids. 292 Pentatomidae. family, 318 Pepsinogen, 383 Peranema. 66 Perca flavescens, 454. 457 Perch. 411. 454 internal structure, 460 skull, 467 yellow, 457 Percidae, family, 454 Percomorphi, suborder, 452 Pereiopods. 270 Pericardial cavity, 376 sinus, 272, 274 Pericardium. 567, 604, 649 of frog. 510 Perichondrium, 691 Pericolpa, 137 Periganglionic gland, 528 Perihemal space, 229 Perilymph. 530 Perioral membrane. 227 Periosteum, 378, 644 Periostracum. 249 Peripatus. 265. 287, 865 Periphylla, 137 Periplaneta americana, 315 Periproct. 221 Perisarc, 132 Perissodactvla, order, 633 Peristalsis, 382 Peristaltic contractions. 151, 196 Peristome. 227 Peristomium. 197 Peritoneum. 376 Peritrichida, 71 Perivisceral coelom. 229 space. 274 Perla modesta. 321 Peromedusae, order, 137 Perophora, a tunicate. 772 Pessulus. 604 Petrels. 5S7 Petromvzon. 413 marinus, 415 Petromyzontla, subclass, 412 Petropedetes newtoni, 480 Phacus. 66 Phagocytosis, 395 Phalanges, 611 Phalangida, order, 266, 295. 299 Phalanx. 611 Phalaropes, 591 Pharyngeal clefts, 360 Pharyngobranchial, 428 Phasmidae, family, 318 Pheasants. 590 Phenotype, 826 Phidippus workmanii, 296 Philodina, 190 Philomycus carolinensis, 240 Pliocaena phocaen i, porpoise, 634 Phoenicopteridae, family, 588 Phoenicopturus ruber. 588 Pholcidae, laniily, 2^7 Pholcids. 297 Pholcus phalangoides, 297 Pholidota, order, 621 Phoronidea, 27 Phosphorescent organs, 771 Photoelectron. 707 Photons, 698 Photosynthesis, 28, 29, 700 Phototropism. 77 Phrynosoma, 552 circulatory system. 566 I digestive system, .563 ear, 574 muscular system, 576 nervous system. 573 respiratory system. 566 skeleton. 574 skull. 575 Phthirius pubius. crab louse, 738 Phyllophaga, 354 anxia. 355 Phylogenetic advances, arthropoda. 286 relations of animals, 863 Phylogeny. 23. 285 Physa anatina. 238 eggs, 247 gyrina. 237 humerosa. 238 Physalia, 136, 137 Physeter catodon, sperm whale, 634 Physiology, 22 Physoclisti. 447 Physocyclus, 297 Physostomi. 447 Phytonomus posticus. 324 Pia mater, 526, 650 Piciformes. order. 594 Pickerels. 450 "Pigeon milk," 591, 602 Pigeons, 591 Pigment cells, 474 Pigmentation of skin, 674 Pigs, 630 Pikas, 627 Pikes 450 Pilidium larva. 162. 163 Pill bug, 264. 265 Pine scale. 320 Pineal body of fish, 469 Pinna. 639 Pinnipedia, 628 Pinnules. 224 Pinworm, 176 Pipa. Amazonian frog, 480 Pipefish, 451 Pipunculus subvirescens, 332 Plsaster, 218 ochraceus, 227 I INDEX 941 Pisces, 442 circulatory system, 459 class, 411, 445 economic relations, 455 muscles, 467 Pitocin, 678 Pitressin, 678 Pituitary body of frog, 526 gland, 672 Pituitrin. 674 Placenta, 653, 819 of armadillo, 622 Placoid scales, 422, 427 Placula. 807, 809 . Plagioctenium irradiens, pecten, 7/1 Planaria, 160, 163 metabolism, 170 regeneration, 173 reproductive system, lbs Plankton, 769 Plant lice, 318 Plantigrade gait, 641 Planula, 128, 134 larva, 139, 140 Plasma, 108, 205, 392 of blood, 503 membrane, 59, 60 Plasmagel, 86 Plasmalemma, 86 Plasmasol, 86 Plasmodium, 79, 745 falciparum, 74 life cycle, 749 malaria parasite, 748 malariae, 74 vivax, 74 Plasmosome, 61 Plastron, 578 Platyhelminthes, 26, 745 economic relations, 174 pliylogenetic advances, 174 phylum, 160 Platyphylax designata, d22 Platypodidae, family, 335 Plecoptera, order, 265, 321 Plectognatlii, suborder, 455 Pleopods, 270 Plethodon cinereus, 479 glutinosus, 472 Plethodontidae, family, 484 Plethodontids, 472 Plethopsis wrighti, 485 Pleura, 649 Pleural ganglion, 245 Pleurobrachia bachei, 15 < Pleurobranciiiae, 271 Pleurocera, 245, 258 Pleurodira, suborder, 547 Pleuron, 268. 347 Pleuronectidae, family, 452 Pleuroperitoneal cavity, 430 Pleurum, 306 Pliny, 36 Pliohippus, 876 Plovers, 591 Plumatella, 186, 187 Pneumatic duct, 463 Pneumatophore, 135, 136 Podical plates, 348 Podobranchiae, 271 Podophrya, 72 Poduridae, family, 312 Poeciliidae, family. 451 Poikilothermous condition, 545 Polar body, 115 ingression. 810 "Polian vesicles, m, ^^^ Polistes, 338 Pollen basket. 357 brush, 357 combs, 357 PoUex, 563, 611 Polvaxon spicule, 125 Polybia, 338 Polychaeta, 194, 196 Polycladida, 160 Polydactylism, 659 Polydactyly, 837 Polydesmidae, family, 290 Polydon spathula, 448 Polygordius, 212, 213 Polygyra dorfeuilliana, 240 roemeri, 239 texasiana, 240 Polyhybrid cross, 825 Polynemidae, family, 453 Polyp, 130. 131 Polypide. 184 Polypteridae, family. 44fa Polypterus. 446 Polypus. 258 bimaculatus, 257 Polystomella crispa. 68 Polyzoa. 184 Pompano. 454. 7i3 Populations. 730 Porcellio. 282 Porcupine. 622. 625 fishes. 455 Porifera, 26 phylum, 119 Porocyte, 126 Porospora gigantea, 59 Porpoise, 634 Portuguese man-of-war, idb Postcaval vein of frog, 508 Postorbital crest, toad, o4U Potamobius, 266 Potato psyllid, 318 Prairie chickens, 590 dog, 623 . ^ ^^^ Precocial bird, 615 Prehallux, 522 Prepartoid crest, toad, 540 Prepollex. 522 Prezygapophyses, 641 Prickleback, 321 Primates, order, 635 Primordial germ cell, lid Pristis pectinatus. 424 Proboscidea order 634 Proboscis, 161, 164. 179, 197, 362 Procellariiformes, order. 587 Procyon lotor. coon, hlv, /o5 Progesterone. 676 Progestin. 676 Proglottids, 161. 761. 763 Progynon, 676 Pronotum. 345 Pronuclei, 800 Pronucleus, 116 Pro-otic bone, 494, 517 Prophase, 62, 63 Proptera purpurata, 248 Propterygium, 430 Prorodon, 71 Prosobranchiata. ^o8 Prosopyles. 124 Prosostomata, order, Ibl Prostate gland, 653 Prostoma rubrum, 162 Prostomium. 197 198, 200 212, 21o Protandrous condition, 245 Protease, 384 Proteida, suborder. 48o Proteins, 56 942 INDEX Proteoses, 385 Proteroglypha, 556 Proterospongia, 119 Prothorax, 305, 345, 357 Prothrombin, 395 Protobranchiata, 256 Protogynous condition, 245 Protons, 697 Protopipa, Amazonian frog, 480 Protoplasm, 49 and the cell, 49 characteristics, 53 chemical nature, 56 fundamental activities, 54 physical nature. 55 structure, 53 Protopodite, 270 Protopterus, 455 Prototheria, subclass, 616 Prototracheata, division, 265 Protozoa, 25 classification, 65 colonial, 75 economic relations, 77 phylum of, 65 Protractor muscle. 249 Proventriculus, 601 Prozoea, 285 Psalterium, 633 Pseudacris, 486 ocularis, 473 streckeri, 482 Pseudemys, 549 troostii elegans, 577 Pseudobranchus stria tus, 485 Pseudococcus maritimus, 316, 318 Pseudocoel, 181 Pseudogryphus calif ornianus, 589 Pseudolamellibranchiata, 257 Pseudophyllidea, order, 161 Pseudopodia, 67, 70, 86 Pseudopodiospore, 90 Pseudoscorpionida, order, 266 Psittaciformes, order, 591 Psocids, 333 Psychichthys afflnis, 425 Psyllid, 316 Pteronarcella bodia, 321 Pteroplatea micrura, 425 Pteropods, 258 Pterosauria, 546 Pterotic bones, 466 Pterygoids, 465, 517, 643 Pterygoquadrate, 465 Pterygota, subclass, 309, 313 Pterylae, 601 Pubis, 521 Pulmocutaneous arch, 507 Pulmonary veins of frog, 510 Pulmonata, 258 Pulmonate snails, 238 Pulvillus, 347 Pulvinaria vitis, 318 Puma, 629 Pupa, 330 Pupil of frog eye, 529 Pupipara, 331 Pupoides marginatus, 240 Purkinje, 49 Pycnogonids, 292 Pygidium, 316 Pygostyle, 582, 609 Pylangium, 511 / Pyloric caeca, 228, 229, 459 chamber, 272 Pyralididae, family, 327 Pyrenoid bodies, 81 Pyridoxine, 389 Q Quadrate bone, 465, 467 Quadratojugal bone, 517 Quadratopterygoid, 428 Quadroon, 829 Quadrula forsheyi, 248 Quadrupeds, 616 Quadruplets, 621, 622 Quail, 590, 794 Quantum, 698 R Rabbits, 625 cotton tail, 627 jack, 625, 626 swamp, 627 Raccoon, 629, 788 Race runners, 554 Racemose bodies, 230 Rachis, 601 Radial canals, 124 cleavage, 802 nerve, 229 Radiata, 217 Radiations, 697 Radio waves, 701 Radioactive atom, 698 Radiolaria, 69, 78 Radium radiation, 708 Radula of snail, 243 Rails, 591 "Rain crow," 591 Raja erinacea, 424 Rana catesbeiana, 497 grylio, 497 pipiens, 486 Range of wild animals, 789 Rangifer caribou, 630 Ranidae, family, 486 Rapliidiidae, family, 322 Rat, Banner-tail kangaroo. 624 Ratitae, subclass, 584 Rats, 622, 625 Rattlesnakes, 556, 559 Rattus, 625 Ray, butterfly, 424 Rays, 424 Reason, 853 Recapitulation tlieory, 284, 869 Recessive, 45 Rectal caeca, 229, 233 Rectus abdominis, 524 Red corpuscles, 395 Rediae, 758, 759 Red-legged locust, 310 Reduction division, 114 References, bibliographic, 889 Reflex arc, 405 behavior, 850 circuit, 400 Reflexes, 852 Regeneration, 681 earthworm, 211 in amphibia, 477 in chordata, 688 in hydra, 156 in planaria, 173 in platyhelminthes, 683 of starfish, 234 Regenerative capacity, 681 Reichenbachia, 340 Reindeer, 630 Relations of animals and plants, 27 Remoras, 454 Renal corpuscles, 650 portal system, 509 vein, 434, 491 I INDEX 943 Reniceps, internal anatomy, 440 tiburo, 422, 439 Renilla, 144 Rennin, 385 Reproduction, 54 ^ , oo . and life cycle, starfish, 234 ^ and life history, chicken, bio asexual. 111, 153 in cat, 652 Reproductive function, 4U( system, frog, 531 Reptiles, classification, 54* flying, 546 fossil, 546 Reptilia, 411 class, 545 Resources, nonrenewable, 7 9 4 renewable, 794 Respiration, aerial, 392 Respiratorj' system, 110 chicken, 603 frog, 514 phrynosoma, obb vertebrates, 390 tree, 223 Rlltomtion^'of^ natural resources, 794 Rete testis, 572 Reticulitermes hesperus, 3^0 Reticulum, 633 Retina, 401, 529 - Retinella indentata paucilirata, 24U Retinula, 276 Retortamonas, 67 Retractor muscles, 224, 2^0 Rhabdamina, 69 Rhabdocoelida, 160 Rhabdom, 276, 277 Rhabdopleura, 361, 364 „,,,^i„ ro. Rhachianectes glaucus, gray whale, 634 Rhagon, 124, 125 Rheas, 585 Rheiformes, order, 585 Rheotropism, 77 Rhincocephalia, order, 560 Rhinoderma, frog of Chile, 480 Rhizocrinus, 225 Rhizopoda, 67 ,. ^. .„ , Rhomaelia microptera. digestive system 350 reproductive system, 353 Rhopalocera, suborder, 325 Rhyacotriton olympicus, 484 Rhynchites bicolor, 324 Rhynchocephalia, 869 order, 548 Riboflavin, 389 Ricord's frog, 472 Ring canal, 223, 230 Roaches, 313 Robber flies, 330 Robins. 595 Robler frogs, 40 Rock rollers, 321 ,^ , . „„ Rocky Mountain spotted fever, SO Rodentia, order, 621 Roe of starfish, 235 Roentgen, 698 radiation, 702 Roman scholars, 36 Ross, Major Ronald, 79 Rostellum, 763 Rostrum, 268, 427 Rotifers, 27, 188 Roundworms, 26, 175 Rugae, 564 Rumina decoUata, 238 Ruminants, 632 Sable, Russian, 638 Sacculina, 265, 284, 74 (, 768 Sacculus, 403, 530 Sacrum, 380 Sagartia, 140 Sagebrush formation, t ^t jack rabbit biome, 726 Sagitta hexaptera, 192 Sailfishes, 453 ,^ , , „,„„^ Salamander, blind cave (Typhlomolge), 482 Salamanders, 411, 472 Salamandra salamandra, 4i9 Salamandroidea, suborder, 484 Salientia, order, 485 Salinity of ocean, 767 Salmon, 447, 448 Salmonidae, family, 448 Salpa, 361 . Salticus senicus, 296 Salts, inorganic, 58 Sand cricket, 315 dollar, 222 fleas, 265 Sandpipers, 591 Santonin, 183 Saprophytic, 66 nutrition, 82 Sarcode, 49 Sarcodina, 67 Sarcolemma, 106 „ Sarcoptes scabiei, itch mite, i6b Sarcosporidia, 73 Sartorius, 611 Sauria, 551 Saw-fish, 424 Scale, ctenoid, 444, 464 cycloid, 444 ganoid, 444 insects, 266 San Jose, 316 Scalopus aquaticus, mole, 619 Scaphiopus, 485 couchi, 475 Scaphognathite, 270 Scaphopoda, 258 Scapula, 465, 495 Scarabaeidae, family, 335 Sceloporus, 552 Sceroblasts, 125 Schistocerca, shoshone, 310 ^ Schistosoma, blood fluke, 745, <•>& Schizogony, 73 Schizont, 750 Schizopod, 285 Schleiden, 43. 50 Schneider, 62 Schwann, 43, 50, 51 Sciaenidae, family, 454 Sciatic vein of frog. 509 Scientific method, 18 Scincidae, family, 555 Sclera, 401 of frog eye, 529 Sclerites, 301, 305, 345 Scolex, 161, 761, 762, 763 Scolopendra, 291 Scolytidae, family, 335 Scomberomorus maculatus, 4i)^ Scorpion flies, 266, 333 Scorpionida, order. 266, 295 Scorpions, 266, 298 Screech owl, 593 Scrombridae, family, 453 Scrotum of cat, 653 Scudderia, 315 944 INDEX Scutigera, 290 Scypha, 121, 122 anatomy of, 123 Scyphomedusa, 137 Scyphozoa, class, 137 Sea butterflies, 258 cows, 634 cucumber, 223 eg-gs, 235 horse, 443, 451 lion, 629 robins, 454 slug, 241 walnuts, 26, 157 Seal, 629 Seasonal changes, 733 Sebaceous glands, 616 Secondary sexual characters, amphibia, 480 Secretin, 385, 501 Sedge-musk ox biome, 724 Segregation, 824 Selachii, subclass, 422 Self, J. Teague, 194 Semicircular canals, 437, 529, 608 Semilunar valves, 459, 510 Seminal groove, 200, 209 receptacles, 202 vesicle, 437, 614 frog, 531 vesicles, 202 Sense organs of frog, 528 Septae, 201 Serial homology, 271 Serosa, 818 Serous membrane, 649 Serpentes, 556 suborder, 548 Serranidae, family, 454 Serum, 393 antihuman, 884 Sesamoid bones, 641 Setae, 195, 196, 200, 201 Sex hormones, 675 inheritance of, 831 Sexual reproduction, 408 development of. 111 Shagreen, 425 Shark, nervous system, 436 pilots, 454 skeleton, 427 urinogenital system, 437 visceral skeleton, 428 Shark-liver oil, 456 Sharks, 411, 422 Sheep, 630 Rocky Mountain, 630 Shikepoke, 587, 588 Ship worm, 241 Shore animals of Pacific, 774 Shovelnose shark, 422 Shrews, 619 Shrimps, 265 Siamese twins, 659 Sibbaldus musculus, whale, 634 Silphidae, 324 Silver moth, 265, 310 Silverfish, 310 Silversides, 452 Simia satyrus, orangutan, 637 Simiidae, family, 637 Simulium vittatum, 332 Sinistral, 240 Sinu-atrial valve, 490 Sinus venosus, 459 Siphlurus occidentalis, 321 Siphonaptera, order, 266, 329 Siphonoglyphe, 141, 142. 143 Siphonophora, order, 136 Siphonophore colony, 135 Siphons, 222, 365 excurrent, 250 exhalant, 252 Siphuncle, 259 Sipunculoidea, 27, 194, 215 Siren lacertina, 473, 485 Sirenia, order, 634 Sirenidae, family, 485 Sistrurus, 559 Skates, 424 Skeletal system, 108 chicken, 609 fish, 464 frog, 517 phrynosoma, 574 Skeleton, appendicular, of fish. 465 of Ameiurus, 466 of cat, 640 of chicken, 610 of shark, 427 of toad. 543 of turile, 580, 581 Skin, frog, 475 human, 376 Skinks, 555 Skull, frog, 517 perch, 467 phrynosoma, 575 Skunk, 638 Sleeping sickness, African, 78 "Sliders," 549 Slimy salamander, 479 Sloths, 621 Slug, 323 Smelts, 448 Sminthuridae, family, 312 Snail, 237 egg masses, 247 Snails, 257 pulmonate, 238 terrestrial, 240 Snake eels, 449 Snake, harlequin, 558 pit vipers, 559 Snakes, 411, 548, 556 coral color bands, 558 lyre, 556 worm, 557 Snappers, 454 Snipes, 591 Social life, insects, 334 relations of animals. 735 Sol state, 54 Solaster, 218 Solenogiypha, 556 ^ Soles, 451 Solitary wasp, digestive tract, 358 Solpugida, order, 298 Somatic cells, 104 mesoderm, 53 6 Somatoplasm, 105 Somatopleure, 815, 817 Sonoran zone, lower, 716 Sorex personatus. long-tailed shrew 619 Sound production in mammals. 616 Sow bugs, 265 Spadefoot toad, 475 "Spanish fly." 341 Sparrows, 595 English, 596 Speotyto cunicularia, 592 hypogaea, 593 Spermatid, 114, 115 Spermatogenesis, 113 Spermatogonia, 799 INDEX 945 Spermatophores, 171, 493 Spermatophoric sac, 2 62 Spermatotheca, 246, 353 Spermatozoa, 104, 115, 407 Sphaeiella, 66 Sphaeriidae, 256 Sphenethmoid bone, 518 Sphenisciformes, order, 586 Sphenodon, 546, 548, 869 punctatum, 560 Sphenodontia, suborder, 548 Sphincter, 644 Sphyraenidae, family, 453 Spicules, 121. 125 types of, 125 Spider, digestive system, 293 Spiders, 266, 292 crab, 295 comb-footed, 295 orb- web, 296 Spinal cord, frog, 527 Spiny dogfish, circulatory system, 4d2 Spiracles, 349, 422, 427 Spiral valve, 422 Splanchnic mesoderm, 536 Splanchnopleure, 815 Splenial bones, 495 Sphnts, 874 Sponge, histology of, 126 metabolism, 127 reproduction of, 127 Sponges, 26, 119 diagrams of different types, 124 economic relations, 129 fresh \Yater, 121 phylogeneuc aavances, 129 Spongilla, 121 Spongin, 125 Spongioblasts, 125 Spongioplasm, 60 Spongovostox apicedentatus, 320 Spoonbills, 446 Sporoblasts, 74 Sporocyst, 756, 758, 759 Sporogony, 73 Sporont stage, 750 Sporosac, 135 Sporozoa, 111 class, 72 Sporozoites, 74 Sporulation, 74, 90, 749 Springtails, 265, 312 Spruce-moose biome, 725 Squalus acanthias, 422 internal anatomy, 429 Squamata, order, 547, 551 Squamosal bone, 494, 518 Squamous epithelium, 107 Squash bug, 318 Squid, dissection of, 261 eye of, 262 Squirrels, 623 Stapes, 403 Stapliylinidae, family, 340 Starfish, 226 development, 232, 234 metamorphosis, 232 regeneration, 234 Starling. 595 Statoblasts, 187 „„ „„„ Statocysts, 121, 133, 159. 253, 262, 276 Statolith, 159, 253 Stauromedusae, order, 137 Steapsin. 385, 502 Stegocephalia. 869 Stegomyia. 80 Stem-mother, 112 Stenopelmatus fasciatus, 312 Stenostomvim, 160 Stentor. 7 0. 71 regeneration, 681 Stereoblastula, 807 Sternal artery, 272 Sternebrae, 644 Sternum. 268, 306. 345 Sticklebacks, 451 Stigma, 81 Stigmata, 366 Stimuli, classes, 848 ' Sting, 357 ray, 423, 456 "Stingaree," 425 Stipes, 345 Stomach, ruminant, 633 Stomodeum, 140, 158 Stomolophus, 138 Stone canal, 221, 230, 231 flies, 265, 321 Strawberry leaf roller, 326 Strecker's frog, 482 Strepsiptera, order, 265. 333 Streptostylic condition. 547 Strigiformes, order, 593 Strix varia, 593 Strobilization, 140 Strobilops labyrinthica texasiana, 240 Strobilus, 763 Strobula, 139. 140 Strongylocentrotus. 222 Strongyloidea, order, 176 S.rongyloides stercoralis. 176 Struggle for existence. 885, 886 Struthio camelus, 584 Struihioniformes, order, 585 Sturgeons, 446 Sturnus vulgaris, 595 Stylommatophora, 258 Stylonychia. 71 Stylopids, 265 Subclavian veins of frog, 508 Subesophageal ganglia, 275 Subgenital plates. 348 Sublittoral zone. 713, 770 Submentum, 345 Subpharyngeal ganglion, 199, 208 Subumbrelia, 132 Succession. 727 Succinea avara, 240 Suckers, 450 Suctoria, 70, 72 Suidae, family, 632 Sunfishes, 454 Sunlight, biological effects, 700 Superciliary crest, toad, 540 Supraclavicle, 465 Supracondyloid fossa, 640 Supraoccipital bone, 465 Suprapharyngeal ganglia, 199 ganglion. 208 Suprarenal glands, 671 Suprascapula of frog, 521 Surra, 80 „„. „„„ Survival of the fittest, 88o, 886 Sustentative tissue, 106 Suture, 343, 641 Swans, 589 Sweat glands, 616 Swifts, 593 Swimmerets. 270 Swimming clam, 284 Sycon, 124, 125 Sylvilagus. cotton tail rabbit, bZ7 Symbiont, 340 Symbiosis, 144, 735 946 INDEX Sympathetic nerves of frog, 526 nervous system, 528 system, 399 Sympetrum rubicundulum, 321 Symphysis, pubic, 576 Symplectic bone, 467 Synangium, 511 Synapse, 406 Synapsis, 1U7, 114 Synch^onlidae, family, 450 Synentognathi, suboi'der, 451 Syngamus trachea, 176 Syrinx, 603 Syrphidae, family, 330 Syrphids, 330 Syrrhophus, 485 System. 108 Systemic arch, frog, 506 veins, 508 Systole, 396 Tabanus phaenops, 331 Tachardia lacca, 341 Tachinidae, family, 330 Tachinids, 330 Tachyglossus aculeatus, 617 Tadarida mexicana, 620 Taenia, 161 pisiformis, dog tapeworm, 761 saginata, beef tapeworm, 741, 742 serrata, dog tape, 764 solium, pork tape, 745, 763 Taeniolae, 137 Tailed amphibia, 484 Tantilla, 556 Tapeworm development, 762 764 Tapeworms, 762 Tapirs, 633 Tarantula, 266, 296 Tarpon, 447 Tarsals, 495 Tarsiers, 635 Tarsometa tarsus, 612 Tarsus, 293, 347 Taste buds of flsh, 470 Taxidea taxus, badger, 628 Taxis, 77 Taxonomy, 21 Tayassuidae, family, 630 Tegmina, 347 Teiidae, family, 554 Teleostei, order, 447 Teleostomi, subclass, 446 Telophase, 62, 64 Telosporidia, 73 Telson, 268 Tendons, 106, 524. 644 Ten-pounders, 447 Tentacles, 131, 133, 137, 147, 197 Tentaculata, 157 Tentaculifera, 72 Tentaculocysts, 137 Teratology. 654 Teredo navalis, 241, 255 Tergum, 268, 306, 347 Termites. 265, 320 Termitophiles, 339 Termopsis nevadensis, 320 Terns, 591 Test, 219 Testes. 572 of cat, 653 Testicle, 653 Testosterone, 676 Testudinata, order, 547, 548 Testudinidae, family. 550 Tetany. 670 Tetrabranchiata, 258 Tetragnatha laborlosa, 296 Tetranychus telarius, 298 Tetraphyllidae, order, 161 Tetrastemma, 163 Tetraxonida, order, 121 Tettigonidae, family, 315 Texas fever, 80 Thalarctos maritimus, polar bear, 629 Thaliacea, 361 Theca, 812 Theelin, 676 Thenea. 121 Theory, 18 of evolution, 863 of recapitulation, 397 Theridiidae, family, 295 Thermotropism, 77 Theromorjiha, 869 Thiamin, 389 Thigmotropism, 77 Thomisidae, family, 295 Thoracostel, suborder, 451 Threadflns, 453 Threadworms, 26, 175 Threshold stimulus, 77 Thrips, 265, 320 tabaci. 320 Thrombocytes of frog, 503 Thunnidae, family, 454 Thymallidae, family, 448 Thymallus tricolor, 448 Thymus gland, 675 Thyone, 222, 224 Thyroid gland, 110, 667 Thyrotropic hormone, 678 Tliyroxine, 668 Thysanoptera, order, 265, 320 Thysanosoma, 765 Thysanura, order, 265, 309 Tibia, 293, 306, 347 Tibioflbula of frog 52"' Tibiotarsus, 612 Tick, sheep, 330 Ticks, 266, 298 Tiedemann's bodies, 230, 231, 233 Tiger, 629 beetles, 324 salamander, 484 ambystoma tigrinum, 480 shark, 422 Time scale, geologic, 872 Tmamous, 586 Tingitidae, family, 318 Tissue, 105 epithelial, 105 muscular, 106 nervous, 107 sustentative. 106 vascular, 108 Toad, arteries, 542 Common, 487 digestive system, 541 embryology, 544 marine, 473 muscles, 543 narrow-mouthed, 486 nervous system, 543 obstetrical, 480 spadefoot, 475 skeleton, 543 urinogenital system, 541 vascular system, 542 veins, 542 Toadflshes, 455 Toads, 411, 472, 538 Tocopherol, 390 INDEX 947 Tomato fruitworm, 326 Tongue flsh, 451 "Tornaria" larva, 364 Totipalmate swimmers, 587 Toxocara canis, 176 Toxopneustes, 800 Tracheae, 110, 603 Tracheata, section, 265 Trachelmonas, 66 Trachinotus carolinus, pompano, 773 Trachvlina, order, 136 Tramatodes, 160, 161, 174, 755 Transitional zone, 715 Transmission of parasites, 740 Trapezius muscle, 427 Trematoda, 161 Trepang, 235 Triatoma, 79 Triaxon, 125 Trichechus latirostris, manatee, 634 Tricliina, 176, 745, 753 Trichinella, 175, 176, 745, 753 Trichinelloidea, order, 176 Trichocysts, 94, 102 Trichomonas, 67 Trichoptera, order, 266, 321 Tricladida, 160 Trigeminus nei-ve, 527 Triggerflshes, 455 Triglidae, family, 454 Trihybrid cross, 826 Trimorphodon, 556 Trionychidae, family, 550 Trionychoidea, suborder, 547, 550 Triploblastic embryo, 810 Tripneustes, 222 Triradiate spicule, 125 Triturus, common newt, 484 Trivium, 227, 228 Trochanter, 293, 306, 347 Trochelminthes, 27, 184 Trochlea, 576 Trochlearis nerve, 527 Trochophore larva, 188, 192, 199, 213, 237, 865 Trochospongilla, 121 Trophoblast, 814 Tropical zone, 716 Tropicorbis liebmanni, 238 Tropism, 77, 849 Tropisms in hydra, 146 in planaria, 164 Tropistic behavior, 849 Trout, 448 eggs and embryos, 470 Truncus arteriosus, 504 Trypanorhyncha, order, 161 Trypanosoma, 79, 743 Trypanosomes, 80 Trypetidae. family, 330 Trypsin, 502 Trypsinogen, 385 Tsetse fly, 744 Tuatara, 560 Tube foot, 221 Tuberculum, 495 Tubularia, 131 Tundra formation, 724 Tunic, 365 Tunicata, 361 Tunnies, 454 Turbellaria, class, 160 Turbinated bones, 603 Turkev vulture. 588 Turkeys, 590 wild, 795 Turtle, circulatory system, 580 digestive system, 578 leatherback, 550 loggerhead, 550 mud, 548 painted, 549 pond, 577 respiratory system, 579 skeleton, 580. 581 snapping, 549 Troost's, 577 urinogenital system, 581 Turtles, 411, 545, 548 Twins, conjoined, 659 Tympanic membrane, 403. 562, 604 Tympanum, 498, 530, 574 Typhlomolge rathbuni. 4 82 l^phlosole. 204 Tyrannosaurus, 546 U Uca, 282 fiddler crab, 772 Ultraviolet, 699 Umbilical cord, 820 Umbilicus, inferior, 601 Umbo, 249 Umbridae, family, 450 Uncinate processes, 609 Undulating membrane, 94 Unionidae, 256 Unit characters, 823 Upper Sonoran zone. 715 Ureter. 398, 572, 606 Ui-eters of cat, 650 Urethra, 653 of cat, 650 Urinary bladder, 490. 650 Urine. 650 Uriniferous tubules, 398. 464, 515 Urinogenital system, 541 horned lizard, 571 turtle, 581 Urochoi'da, subphylum, 361, 365 Urodela, order, 484 Uroglena, 67 Uropods, 270 Uropygial gland. 600 Urosalpinx. 241 cinerea. 255 Urostyle, 520 Ursus horribilus. grizzly bear. 629 Uterus of cat, 652 Utriculus, 403, 437 frog ear, 530 Vagina of cat, 653 Vagus nerve, 527 Vallate papillae, 650 Valve, semilunar. 511 van Leeuwenhoek. 41 Vane, 599 Variation, 864 Vas deferens, 572, 613, 653 Vasa efferentia, 531 Vascular system, functions, 110 tissue, 108 Veins of frog, 508 of horned lizard, 570 of toad, 542 Vejovidae, family, 298 Vejovis mexicanus, 298 Ventral abdominal vein, 491, 509 Ventricles of brain, 526 Ventriculus, 601 948 INDEX Venus's comb, 241 flower basket, 120, 121 girdle, 157 Vermetus spiratus, 240, 241 Vermiform appendix, 878 Vermis, 650 Vertebra of frog, 519, 520 Vertebral column, 378 cat, 643 frog, 520 Vertebrata, classification, 410 digestive system, 382 subphylum, 362, 375 Vertebrate circulatory system, 392 excretory system, 396 skeleton, 378 Vesalius, 21, 37 Vespidae, family, 338 Vespula, 338 pennsylvanica, 333 Vestibule, 608 Vestigial structures, 878 Vibracula, 185 Vibracularia, 185 Vibrissae, 639 Villosities, 476 Vinegarroon, 266 Virchow, 64 Viscachas, 625 Viscera, 109 Visceral ganglia, 245 skeleton, fish, 465 frog, 519 Vitamin D, 390 K, 390 Vitamins and their functions, 388 Vitellarium, 190, 191 Vitelline cells, 171 membrane, 115, 532, 819 Vitellophages, 807 Vitrella, 276 Vitrina glacialis, 237 Vocal cords, 649 Voice of amphibia, 478 box of frog, 514 Voluntary muscle, 524 Volvox, a colony, 76, 77 Vomerine teeth, 500 von Baer, K. E., 23, 43, 284 von Mohl, 49 Vorticella, 70, 71 Vulpes fulva, red fox, 628 VuKure, 589 California, 589 W Walking legs, 270 Wallace, Alfred R., 711 Walrus, 629 Wasps, 266, 331 solitary, 335 Water, 58 dog, 487 fiea, 264 moccasins, 556, 559 striders, 318 Water-vascular system, 230, 231 Wattles, 600 Wave radiations chart, 699 Waxwings, 595 Weevils, 265, 324, 325 Welsmann, 47 Whale, killer, 634 sperm, 634 sulphur-bottom, 634 toothed, 634 Whalebone, 634 Wheel animalcules, 188 Whipworm, 176 White corpuscles, 513 Whiteflshes, 448 Wild turkey, 795 Wildlife conservation, 784 Wolff, 798 Wolf-snout, 656 Wolves, 629 Woodchuck, 622 Woodcocks, 591 Woodpeckers, 594 downy, 595 red-headed, 595 Worm lizard, 553 shell, 241 Wrens, 595 Wuchereria bancrofti, fllaria, 745, 746 Xantusiidae, family, 554 X-chromosome, 831 Xenodusa, 340 Xerophthalmia, 389 Xiphisternum of frog, 521 Xiphoid process, 644 Xiphosura, order, 266, 295, 299 X-rays, 703 Xysticus nervosus, 295 Yellow-jacket, 333 Yolk glands, 169, 191 plug, 535 sac, 819 Z Zalophus californlanus, sea lion, 629 Zebra, 633 Zenaidura macroura, 591 Zoantharia, subclass, 140 Zoea. 285 Zona pellucida, 816 Zonitoides arboreus, 240 Zooecium, 184, 187 Zoogeographical regions of Wallace, 712 Zoogeography, 20, 24 Zooid, 135 Zooids, 184 Zoology, 19 history of, 36 subdivisions of, 19 Zophodia grossulariae, 327 Zoraptera, order, 333 Zorotypus, 333 Zygapophyses, 495, 520. 643 Zygodactyly, 837 Zygonectes notatus, 451 Zygoptera, suborder, 321 Zygote, 111, 115, 134, 139, 493, 824 Zymogen, 383 ( ■i li