BIOLOGY LIBRARY G TWENTIETH CENTURY TEXT-BOOKS EDITED BY A. F. NIGHTINGALE, PH.D., LL.D. SUPERINTENDENT OF SCHOOLS, COOK COUNTY, ILLINOIS TWENTIETH CENTURY TEXT-BOOKS* ZOOLOGY. ANIMAL STUDIES. A one-book course in Zoology for secondary schools. By DAVID STARR JORDAN, President of Leland Stanford Jr. University ; VERNON L. KELLOGG, M.S., Professor of Entomology, Leland Stanford Jr. University; and HAROLD HEATH, Professor of Zoology, Leland Stanford Jr. University. Cloth, $1.25 net. ANIMAL LIFE. A First Book of Zoology. By DAVID STARR JORDAN and VERNON L. KELLOGG. Cloth, $1.20 net. ANIMAL FORMS. An Elementary Text-Book of Zoology. By DAVID STARR JORDAN and HAROLD HEATH. Cloth, $1.10 net. ANIMALS. A Text-Book of Zoology. By JORDAN, KELLOGG, and HEATH. (The two foregoing in one volume.) Cloth, $1.80 net. TEACHER'S MANUALS. ANIMAL STRUCTURES. A Laboratory Manual of Zoology. By D. S. JORDAN and GEORGE C. PRICE, Associate Professor of Zo- ology, Leland Stanford Jr. University. Limp cloth, 50 cents net. D. APPLETON AND COMPANY, NEW YORK. ; * :"' TWENTIETH CENTURY TEXT-BOOKS ANIMAL LIFE A FIRST BOOK OF ZOOLOGY BY DAVID STARR JORDAN, PH. D., LL. D. \N PRESIDENT OF LELAND STANFORD JUNIOR UNIVERSITY AND VERNON L. KELLOGG, M.S. PROFESSOR IN LELAND STANFORD JUNIOR UNIVERSITY NEW YORK D. APPLETON AND COMPANY 1912 COPYRIGHT, 1900 B* D. APPLETON AND COMPANY BIOLOGY LIBRARY G PREFACE THE authors present this book as an elementary ac- count of animal ecology — that is, of the relations of ani- mals to their surroundings and their responsive adaptation to these surroundings. The book takes the observer's point of view, who is especially concerned with the reasons for the varied structure and habits of animals. To understand how naturally and inevitably all animal form, habit, and life are adapted to the varied circumstances and conditions of animal existence should be the motive of the beginner in this fascinating study. The greatest facts of life, except that of life itself, are seen in the marvelously perfect meth- ods which Nature has adopted in the structure and habits of animals. The keen observation of a fact should lead the student to inquire into the significance of that fact. The veriest beginner can be, and ought to be, an independ- ent observer and thinker. In the study of zoology that phase which treats of the why and how of animal form and habit not only absorbs the attention of the most advanced modern scholars of biology, but should also appeal most strongly to the beginner. The beginner and the most enlightened thinker in zoology should each have the same point of view. With this belief in mind the authors have tried to put into simple form the principal facts and approved hypotheses upon which the modern conceptions of animal life are based. It is unnecessary to say that this book depends for its vi ANIMAL LIFE • best use on a basis of personal observational work by the student in laboratory and field. Without independent personal work of the student little can be learned about animals and their life that will remain fixed. But present- day teachers of biology are too well informed to make a discussion of the methods of their work necessary here. As a matter of fact, the methods of the teacher depend so absolutely on his training and individual initiative that it is not worth while for the authors to point out the place of this book in elementary zoological teaching. That the phase of study it attempts to represent should have a place in such teaching is, of course, their firm belief. The obligations of the authors for the use of certain illustrations are acknowledged in proper place. Where no credit is otherwise given, the drawings have been made by Miss Mary H. Wellman or by Mr. James Carter Beard, and the photographs have been made by the authors or under their direction. DAVID STARR JORDAN, VERNON LYMAN KELLOGG. NOTE.— After the pages of the book were cast, it was thought that a transposition of Chapters III and IV would present a more logical arrangement, and teachers are advised to omit in their study scheme Chapter III until Chapter IV is completed. D. S. J. V. L. K. CONTENTS CHAPTER PAGE I. — THE LIFE OF THE SIMPLEST ANIMALS . . . . •: . . 1 The simplest animals, or Protozoa, 1.— The animal cell, 2. — What the primitive cell can do, 5. — Amoeba, 5. — Paramoecium, 9. — Vorticella, 12. — Marine Protozoa, 15.— Globigerinae and Radio- laria, 16. — Antiquity of the Protozoa, 20. — The primitive form, 20. — The primitive but successful life, 21. II. — THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS . . .24 Colonial Protozoa, 24. — Gonium, 25. — Pandorina, 26. — Eudo- rina, 27. — Volvox, 28. — Steps toward complexity, 30. — Individual or colony, 31. — Sponges, 32. — Polycs, corals, and jelly-fishes, 37. — Hydra, 37. — Differentiation of tne body cells, 41. — Medusje.or jelly-fishes, 41.— Corals, 43.— Colonial jelly-fishes, 45.— Increase in the degree of complexity, 48. III. — THE MULTIPLICATION OF ANIMALS AND SEX . . .50 All life from life, 50. — Spontaneous generation, 51. — The simplest method of multiplication, 53. — Slightly complex methods of multiplication, 54. — Differentiation of the reproductive cells, 55. — Sex, or male and female, 57. — The object of sex, 57. — Sex di- morphism, 58. — The number of young, 61. , IV.— -FUNCTION AND- STRUCTURE .63 Organs and functions, 63. — Differentiation of structure, 64. — Anatomy and physiology, 64. — The animal body a machine, 65. — The specialization of organs, 66, — The alimentary canal, 66. — Stable and variable characteristics of an organ, 73. — Stable and variable characteristics of the alimentary canal, 73. — The mutual relation of function and structure, 77. V. — THE LIFE CYCLE . . •...».. », . . . 78 Birth, growth and development, and death, 78. — Life cycle of simplest animals, 78. — The egg, 79. — Embryonic and post-em- bryonic development, 80. — Continuity of development, 83.— De- velopment after the gastrula stage, 84. — Divergence of develop- vii Yiii ANIMAL LIFE CHAPTER PAGE ment, 84. — The laws or general facts of development, 86. — The significance of the facts of development, 89. — Metamorphosis, 90. — Metamorphosis among insects, 90. — Metamorphosis of the toad, 94. — Metamorphosis among other animals, 96. — Duration of life, 101.— Death, 103. VI. — THE PRIMARY CONDITIONS OF ANIMAL LIFE . • , • . 106 Primary conditions and special conditions, 106. — Food, 106. — Oxygen, 107. — Temperature, pressure, and other conditions, 108. — DitFerence between animals and plants, 111. — Living organic matter and inorganic matter, 112. VII. — THE CROWD OF ANIMALS AND THE STRUGGLE FOR EXIST- ENCE . • . 114 The crowd of animals, 114. — The struggle for existence, 116. —Selection by Nature, 117.— Adjustment to surroundings a re- sult of natural selection, 120.— Artificial selection, 120.— Depend- ence of species on species, 121. VIII.— ADAPTATIONS ., .133 Origin of adaptations, 123.— Classification of adaptations, 123. —Adaptations for securing food, 125.— Adaptations for self-de- fense, 128.— Adaptations for rivalry, 135.— Adaptations for the defense of the young, 137.— Adaptations concerned with sur- roundings in life, 143. — Degree of structural change in adapta- tions, 146.— Vestigial organs, 147. IX. — ANIMAL COMMUNITIES AND SOCIAL LIFE .... 149 Man not the only social animal, 149.— The honey-bee, 149.— The ants, 155.— Other communal insects, 158. — Gregariousness and mutual aid, 163.— Division of labor and basis of communal life, 168. — Advantages of communal life, 170. X. — COMMENSALISM AND SYMBIOSIS 172 Association between animals of different species, 172. — Com- mensalisrn, 173. — Symbiosis, 175. XI. — PARASITISM AND DEGENERATION 179 Relation of parasite and host, 179. — Rinds of parasitism, 180. —The simple structure of parasites, 181.— Gregarina, 182.— The tape- worm and other flat- worms, 183. — Trichina and other round- worms, 184.— Saccul'na, 187.— Parasitic insects, 188.— Parasitic vertebrates, 193. — Degeneration through quiescence, 193. — De- generation through other causes, 197. — Immediate causes of de- generation, 198. — Advantages and disadvantages of parasitism and degeneration, 198. — Human degeneration, 200. CONTENTS ix CHAPTER PAG* XII.— PROTECTIVE RESEMBLANCES AND MIMICRY . . . .201 Protective resemblance defined, 201. — General protective or aggressive resemblance, 202.— Special protective resemblance, 207. — Warning colors and terrifying appearances, 212. — Alluring coloration, 216. — Mimicry, 218. — Protective resemblances and mimicry most common among insects, 221. — No volition in mim- icry, 222.— Color : its utility and beauty, 222. XIII. — THE SPECIAL SENSES 224 Importance of the special senses, 224— Difficulty of the study of the special senses, 224. — Special senses of the simplest ani- mals, 225.— The sense of touch, 226.— The sense of taste, 228.— The sense of smell, 229.— The sense of hearing, 232.— Sound-mak- ing, 235 —The sense of sight, 237. XIV. — INSTINCT AND REASON . . . . • . . . . 240 Irritability, 240.— Nerve cells and fibers, 240.— The brain or sensorium, 241.— Reflex action, 241.— Instinct, 242.— Classifica- tion of instincts, 243.— Feeding, 244.— Self-defense, 245.— Play, 247.— Climate, 248.— Environment, 248.— Courtship, 248.— Repro- duction, 249.— Care of the young, 250.— Variability of instincts, 251.— Reason, 251.— Mind, 255. XV. — HOMES AND DOMESTIC HABITS 257 Importance of care of the young, 257. — Care of the young and communal life, 257. — The invertebrates (except spiders and in- sects), 258.— Spiders, 259.— Insects, 262.— The vertebrates, 264. XVI. — GEOGRAPHICAL DISTRIBUTION OF ANIMALS .... 272 Geographical distribution, 272. — Laws of distribution, 274. — Species debarred by barriers, 274.— Species debarred by inability to maintain their ground, 275.— Species altered by adaptation to new conditions, 276.— Effect of barriers, 283.— Relation of species to habitat, 283.— Character of barriers to distribution, 288.— Bar- riers affecting fresh-water animals, 294.— Modes of distribution, 296.— Fauna and faunal areas, 296.— Realms of animal life, 297.— Subordinate realms or provinces, 303.— Faunal areas of the sea, 304. CLASSIFICATION OF ANIMALS . . . . * . 307 GLOSSARY 313 INDEX . . 319 I ANIMAL LIFE CHAPTEK I THE LIFE OP THE SIMPLEST ANIMALS 1. The simplest animals, or Protozoa. — The simplest ani- mals are those whose bodies are simplest in structure and which do the things done by all living animals, such as eating, breathing, moving, feeling, and reproducing in the most primitive way. The body of a horse, made up of various organs and tissues, is complexly formed, and the various organs of the body perform the various kinds of work for which they are fitted in a complex way. The simplest animals are all very small, and almost all live in the water ; some kinds in fresh water and many kinds in the ocean. Some live in damp sand or moss, and still others are parasites in the bodies of other animals. They are not familiarly known to us; we can not see them with the unaided eye, and yet there are thousands of different kinds of them, and they may be found wherever there is water. In a glass of water taken from a stagnant pool there is a host of animals. There may be a few water beetles or water bugs swimming violently about, animals half an inch long, with head and eyes and oar-like legs ; or there may be a little fish, or some tadpoles and wrigglers. These are evidently not the simplest animals. There will be many very small active animals barely visible to the un- aided eyes. These, too, are animals of considerable com- plexity. But if a single drop of the water be placed 2 1 ANIMAL LIFE on a glass slip or in a watch glass and examined with a compound microscope, there will be seen a number of ex- tremely small creatures which swim about in the water-drop by means of fine hairs, or crawl slowly on the surface of the glass. These are among our simplest animals. There are, as already said, many kinds of these " simplest animals," although, perhaps strictly speaking, only one kind can be called simplest. Some of these kinds are spherical in shape, some elliptical or football-shaped, some conical, some flattened. Some have many fine, minute hairs projecting from the surface ; some have a few longer, stronger hairs that lash back and forth in the water, and some have no hairs at all. There are many kinds and they differ in size, shape, body covering, manner of movement, and habic of food-getting. And some are truly simpler than others. But all agree in one thing — which is a very important thing — and that is in being composed in the simplest way possible among animals. 2. The animal cell — The whole body of any one of the simplest animals or Protozoa is composed for the animal's whole lifetime of but a single cell. The bodies of all other animals are composed of many cells. The cell may be called the unit of animal (or plant) structure. The body of a h&se is complexly composed of organs and tissues. Each of these organs and tissues is in turn composed of a large number of these structural units called cells. These cells are of great variety in shape and size and general character. The cells which compose muscular tissue are very different from the cells which compose the brain. And both of these kinds of cells are very different from the simple primitive, undifferentiated kind of cell seen in the body of a protozoan, or in the earliest embryonic stages of a many-celled animal. The animal cell is rarely typically cellular in character — that is, it is rarely in the condition of a tiny sac or box of symmetrical shape. Plant cells are often of this char- THE LIFE OF THE SIMPLEST ANIMALS 3 acter. The primitive animal cell (Fig. 1) consists of a small mass of a viscid, nearly colorless, substance called protoplasm. This protoplasm is differentiated to form two parts or regions of the cell, an inner denser mass called the nucleus, and an outer, clearer, inclosing mass called the cytoplasm. There may be more than one nucleus in a cell. Sometimes the cell is inclosed by a cell wall which may be simply a tougher outer layer of the cytoplasm, or may be a thin membrane secreted by the pro- toplasm. In addition to the proto- plasm, which is the fundamental and essential cell substance, the cell may FIG. i.-Biood ceil of a crab contain certain so-called cell prod- (after HAECKEL). show- , . ., .. , . , ,.„ ing cytoplasm and nucleus ucts, substances produced by the life (the large> inner, neariy processes Of the protoplasm. The circular spot) and gran- ,, . , . . ., ules of various substances cell may thus contain water, oils, ]ying in the cytoplagm. resin, starch grains, pigment gran- ules, or other substances. These substances are held in the protoplasm as liquid drops or solid particles. The protoplasm itself of the cell shows an obvious division into parts, so that certain parts of it, especially parts in the nucleus, have received names. The nucleus usually has a thin protoplasmic membrane surrounding it, which is called the nuclear membrane. There appear to be fine threads or rods in the nucleus which are evidently different from the rest of the nuclear protoplasm. These rods are called chromosomes. The cell is, indeed, not so simple as the words " structural unit " might imply, but science has not yet so well analyzed its parts as to warrant the transfer of the name structural unit to any single part of the cell— that is, to any lesser or simpler part of the animal body than the cell as a whole. The protoplasm, which is the essential substance of the cell and hence of the whole animal body, is a substance 4 ANIMAL LIFE of a very complex chemical and physical constitution. Its chemical structure is so complex that no chemist has yet been able to analyze it, and as the further the attempts at analysis reach the more complex and baffling the substance is found to be, it is not improbable that it may never be analyzed. It is a compound of numerous substances, some of these composing substances being themselves extremely complex. The most important thing we know about the chemical constitution of protoplasm is that there are al- ways present in it certain complex albuminous substances which are never found in inorganic bodies. It is on the presence of these albuminous substances that the power of performing the processes of life depends. Protoplasm is the primitive basic life substance, but it is the presence of these complex albuminous compounds that makes protoplasm the life substance. A student of protoplasm and the funda- mental life processes, Dr. Davenport, has said, "Just as the geologist is forced by the facts to assume a vast but not infinite time for earth building, so the biologist has to recognize an almost unlimited complexity in the constitu- tion of the protoplasm." * * The physical structure of protoplasm has been much studied, but even with the improved microscopes and other instruments neces- sary for the study of minute structure, naturalists are still very fat from understanding the physical constitution of this substance. While the appearance of protoplasm under the microscope is pretty generally agreed on among naturalists, the interpretation of the kind of structure which is indicated by this appearance is not at all well agreed on. Protoplasm appears as «, mesh work composed of fine granules sus- pended in a clearer substance, the spaces of the mesh work being com- posed of a third still clearer substance. Some naturalists believe, from this appearance, that protoplasm is composed of a clear viscous sub- tance, in which are imbedded many fine granules of denser substance, and numerous large globules of a clearer, more liquid substance. Other naturalists believe that the fine spots which appear to be granules are simply cross sections of fine threads of dense protoplasm which lie coiled and tangled in the thinner, clearer protoplasm. And, finally, THE LIFE OF THE SIMPLEST ANIMALS 5 3. What the primitive cell can do. — The body of one of the minute animals in the water-drop is a single cell. The body is not composed of organs of different parts, as in the body of the horse. There is no heart, no stomach ; there are no muscles, no nerves. And yet the protozoan is a liv- ing animal as truly as is the horse, and it breathes and eats and moves and feels and produces young as truly as does the horse. It performs all the processes necessary for the life of an animal. The single cell, the single minute speck of protoplasm, has the power of doing, in a very simple and primitive way, all those things which are necessary for life, and which are done in the case of other animals by the various organs of the body. 4. Amoeba. — The simple and primitive life of these Protozoa can be best understood by the observation of living individuals. In the slime and sediment at the bottom of stagnant pools lives a certain specially interest- ing kind of protozoan, the Amoeba (Fig. 2). Of all the simplest animals this is as simple or primitive as any. The minute viscous particle of protoplasm which forms its body is irregular in outline, and its outline or shape slowly but constantly changes. It may contract into a tiny ball ; it may become almost star-shaped ; it may become elongate or flattened ; short, blunt, finger-like projections called pseudppods extend from the central body mass, and these projections are constantly changing, slowly pushing out or others believe that protoplasm exists as a foam work ; that it is a vis- cous liquid containing many fine globules (the granule-appearing spots) of a liquid of different density and numerous larger globules of a liquid of still other density. It is a foam in which the bubbles are not filled with air, but with liquids of different density. This last theory of the structure of protoplasm is the one accepted by a majority of modern naturalists, although the other theories have numerous believers. But just as with what little we know of the chemical constitution of proto- plasm, the little we know of its physical structure throws almost no light on the remarkable properties of this fundamental life substance. 2 6 ANIMAL LIFE drawing in. The single protoplasmic cell which makes up the body of the Amoeba has no fixed outline ; it is a cell without a wall. The substance of the cell or body is proto- plasm, semiliquid and colorless. The changes in form of the body are the moving of the Amoeba. By close watching it may be seen that the Amoeba changes its position on the glass slip. Although provided with no legs or wings or Fio. 2.— An Amoeba, showing different shapes assumed by it when crawling. —After VERWORN. scales or hooks — that is, with no special organs of locomo- tion— the Amoeba moves. There are no muscles in this tiny "body; muscles are composed of many contractile cells massed together, and the Amoeba is but one cell. But it is a contractile cell ; it can do what the muscles of the com- plex animals do. If one of the finger-like projections of the Amoeba^ or, indeed, if any part of its body comes in contact with some other microscopic animal or plant or some small fragment of a larger form, the soft body of the Amoeba will be seen THE LIFE OF THE SIMPLEST ANIMALS to press against it, and soon the plant or animal or organic particle becomes sunken in the protoplasm of the formless body and entirely inclosed in it (Fig. 3). The absorbed particle soon wholly or partly disappears. This is the manner in which the Amoeba eats. It has no mouth or e u. FIG. 3.— Amoeba eating a microscopic one-celled plant.— After VERWOKN. stomach. Any part of its body mass can take in and digest food. The viscous, membraneless body simply flows about the food and absorbs it. Such of the food particles as can not be digested are thrust out of the body. The Amoeba breathes. Though we can not readily ob- serve this act of respiration, it is true that the Amoeba takes into its body through any part of its surface oxygen from the air which is mixed with water, and it gives off from any part of its body carbonic-acid gas. Although the Amoeba has no lungs or gills or other special organs of respiration, it breathes in oxygen and gives out carbonic-acid_gas, which is just what the horse does with its elaborately developed organs of respiration. If the Amoeba, in moving slowly about, comes into con- tact with a sand grain or other foreign particle not suitable for food, the soft body slowly recoils and flows — for the movement is really a flowing of the thickly fluid protoplasm — so as to leave the sand grain at one side. The Amoeba feels. It shows the effects of stimulation. Its movements can be changed, stopped, or induced by mechanical or chemical stimuli or by changes in temperature. The 8 ANIMAL LIFE Amoeba is irritable ; it possesses irritability, which is sensa- tion in its simplest degree. If food is abundant the Amoeba soon increases in size. The bulk of its body is bound to increase if new substance FIG. 4.—Amceba polypodia in six successive stages of division. The dark, white- margined spot in the interior is the nucleus.— After F. E. SCHULZE. is constantly assimilated and added to it. The Amoeba grows. But there seem to be some fixed limits to the extent of this increase in size. No Amceba becomes large. A remarkable phenomenon always occurs to prevent this. THE LIFE OF THE SIMPLEST ANIMALS 9 An Amoeba which has grown for some time contracts all its finger-like processes, and its body becomes constricted. This constriction or fissure increases inward, so that the body is soon divided fairly in two (Fig. 4). The body, being an animal cell, possesses a nucleus imbedded in the body protoplasm or cytoplasm. When the body begins to divide, the nucleus begins to divide also, and becomes en- tirely divided before the fission of the cytoplasm is com- plete. There are now two Amoeba, each half the size of the original one ; each, indeed, being actually one half of the original one. This splitting of the body of the Amoeba, which is called fission, is the process of reproduction. The original Amoeba is the parent ; the two halves of the parent are the young. Each of the young possesses all of the characteristics and powers of the parent ; each can move, eat, feel, grow, and reproduce by fission. It is very evident that this is so, for any part of the body or the whole body was used in performing these functions, and the young are simply two parts of the parent's body. But if there be any doubt about the matter, observation of the behavior of the young or new Amcebce, will soon remove it. Each puts out pseudopods, moves, ingests food particles, avoids sand grains, contracts if the water is heated, grows, and finally divides in two. 5. Paramoecium. — Another protozoan which is common in stagnant pools and can be readily obtained and observed is Paramoscium (Fig. 5). The body of the Paramoecium is much larger than that of the Amoeba, being nearly one fourth of a millimeter in length, and is of fixed shape. It is elon- gate, elliptical, and flattened, and when examined under the microscope seems to be a very complexly formed little mass. The body of the Paramoecium is indeed less primitive than that of the Amoeba, and yet it is still but a single cell. The protoplasm of the body is very soft within and dense on the outside, and it is covered externally by a thin mem- brane. The body is covered with short fine hairs or cilia, 10 ANIMAL LIFE which are fine processes of the dense protoplasm of the surface. There is on one side an oblique shallow groove that leads to a small, funnel-shaped depression in the body which serves as a primitive sort of mouth or opening for the ingress of food. The Paramcecium swims about in the water by vibrating the cilia which cov- er the body, and brings food to the mouth opening by producing tiny cur- rents in the water by means of the cilia in the oblique groove. The food, which consists of other living Proto- zoa, is taken into the body mass only through the funnel-shaped opening, and that part of it which is undigested is thrust out always through a particular part of the body surface. (The taking in and ejecting of foreign particles can be seen by putting a little powdered carmine in the water.) Within the body there are two nuclei and two so- called pulsating vacuoles. These pul- FiQ.5.—paramaciumau- sating vacuoles (Amoeba has one) seem relia (after VKKWORN). . , . , . , . , At each end there is a to aid m discharging Waste products contractile vacuoie, and from the body. When the Paramoe- in the center is one of • •> • • i , the nuclei cium touches some foreign substance or is otherwise irritated it swims away, and it shoots out from the surface of its body some fine long threads which when at rest are probably coiled up in little sacs on the surface of the body. When the Para- mcecium has taken in enough food and grown so that it has reached the limit of its size, it divides transversely into halves as the Amoeba does. Both nuclei divide first, and then the cytoplasm constricts and divides (Fig. 6). Thus two new Paramoecia are formed. One of them has to de- velop a new mouth opening and groove, so that there is in THE LIFE OF THE SIMPLEST ANIMALS 11 the case of the reproduction of Paramoscium the beginnings of developmental changes during the course of the growth of the young. The young Amcebcs have only to add sub- stance to their bodies, to grow larger, in order to be exactly like their parent. The new Paramcecia attain full size and then divide, each into two. And so on for many generations. But it has been discovered that this simplest kind of reproduction can not go on indefinitely. After a number of generations the Paramcecia, instead of simply dividing in two, come together in pairs, and a part of one of the nuclei of each mem- ber of a pair passes into the body of and fuses with a part FIG. 6.— Paramcecium putorinum dividing. The two nuclei be- come very elongate before di- viding.—After BUTSCHLI. PIG. 7. — Paramcecium caudatum ; two indi- viduals separating after conjugation. of one of the nuclei of the other member of the pair. In the meantime the second nucleus in each Paramoscium has broken up into small pieces and disappeared. The new nucleus composed of parts of the nuclei from two animals divides, giving each animal two nuclei just as it had before this extraordinary process, which is called conjugation, began (Fig. 7). Each Paramcecium, with its nuclei com- posed of parts of the nuclei from two distinct individuals. 12 ANIMAL LIFE now simply divides in two, and a large number of genera- tions by simple fission follow. Paramwcium in the character of its body and in the manner of the performance of its life processes is distinctly less simple than the Amoeba, but its body is composed of a single structural unit, a single cell, and it is truly one of the " simplest animals." 6. Vorticella. — Another interesting and readily found protozoan is Vorticella (Fig. 8). While the Amoeba can crawl and ParamoBcium swim, Vorticella, except when very young, FIG. 8. — VorticeUa microstoma (after STEIN). A, small, free-swimming individuals conjugating with a large, stalked individual ; B, a stalked individual dividing longitudinally ; C, after division is completed one part severs itself from the other, forms a ring of cilia, and swims away. is attached by tiny stems to dead leaves or sticks in the water, and can change its position only to a limited extent. THE LIFE OF THE SIMPLEST ANIMALS 13 The body is pear-shaped or bell-shaped, with a mouth opening at the broad end, and a delicate stem at the narrow end. This stem is either hard and stiff, or is flexible and capable of being suddenly contracted in a close spiral. In the body mass there is one pulsating vacuole and one nucleus. Usually many Vorticellce are found together on a common stalk, thus forming a proto- zoan colony. The life processes of Vorticella are of the simple kind already observed in Amwba and Paramwcium. Vorticella shows, however, some modifications of the process of repro- duction which are interesting. The plane of division of Vorticella is parallel to the long axis of the pear-shaped body, so that when fission is complete there are two Vorti- cellce on a single stalk. One of the two becomes detached, and by means of a circle of fine hairs or cilia which appear around its basal end leads a free swimming life for a short time. Finally it settles down and develops a stalk. Vorti- cella shows two kinds of fission — one the usual division into equal parts, and another division into unequal parts. In this latter kind, called reproduction or multiplication by budding, a small part of the parent body separates, develops a basal circle of cilia, and swims away. The pro- cess of conjugation also takes place among the Vorti- cella, but they are never two equal forms which conju- gate, but always one of the ordinary stalked forms and one of the small free - swimming forms produced by budding. Here, then, in the life of Vorticella, are new modifica- tions of the life processes ; but, after all, these life processes are very simply performed, and the body is like the body of the Amoeba, a single cell. Vorticella is plainly one of " the simplest animals." 7. Gregarina. — A fourth kind of protozoan to which we can profitably give some special attention is Gregarina (Fig. 9), the various species of which live in the alimentary ANIMAL LIFE canal* of crayfishes and centipeds and certain insects. Gregarina is a parasite, living at the expense of the host in whose body it lies. It has no need to swim about quickly, B FIG. 9.— Gregarinidse. A, a Gregarinid (Actinocephalus oligacanthus) from the intes- tine of an insect (after STEIN) ; B and C, spore forming by a Gregarinid (Coc- cidium oviforme) from the liver of a guinea-pig (after LEUCKART) ; D, E, and F, successive stages in the conjugation and spore forming of Gregarina poly- morpha (after KOBLLIKER). and hence has no swimming cilia like Paramcecium and the young Vorticella. It does need to cling to the inner wall of the alimentary canal of its host, and the body of some species is provided with hooks for that purpose. The * Specimens of Gregarina can be abundantly found in the alimen- tary canal of meal worms, the larvae of the black beetle (Tenebrio moli- tor), common in granaries, mills, and brans. "Snip off with small scissors both ends of a larva, seize the protruding (white) intestine with forceps, draw it out, and tease a portion in normal salt solution (water will do) on a slide. Cover, find with the low power (minute, oblong, transparent bodies), and study with any higher objective to suit." — MURBACH. THE LIFE OF THE SIMPLEST ANIMALS 15 food of Gregarina is the liquid food of the host as it exists in the intestine, and which is simply absorbed anywhere through the surface of the body of the parasite. There is no mouth opening nor fixed point of ejection of waste material, nor is there any contractile vacuole in the body. In the method of multiplication or reproduction Gre- garina shows an interesting difference from Amoeba and Paramwcium and Vorticella. When the Gregarina is ready to multiply, its body, which in most species is rather elongate and flattened, contracts into a ball-shaped mass and becomes encysted — that is, becomes inclosed in a tough, membranous coat. This may in turn be covered externally by a jelly-like substance. The nucleus and the protoplasm of the body inside of the coat now divide into many small parts called spores, each spore consisting of a bit of the cytoplasm inclosing a small part of the original nucleus. Later the tough outer wall of the cyst breaks and the spores fall out, each to grow and develop into a new Gre- garina. In some species there are fine ducts or canals leading from the center of the cyst through the wall to the outside, and through these canals the spores issue. Some- times two GregarincB come together before encystation and become inclosed in a common wall, the two thus forming a single cyst. This is a kind of conjugation. In some spe- cies each of the young or new GregarincB coming from the spores immediately divides by fission to form two indi- viduals. 8. Marine Protozoa. — If called upon to name the char- acteristic animals of the ocean, we answer readily with the names of the better-known ocean fishes, like the herring and cod, which we know to live there in enormous numbers ; the seals and sea lions, the whales and porpoises, those fish-like animals which are really more like land animals than like the true fishes ; and the jelly-fishes and corals and star-fishes which abound along the ocean's edge. But in naming only these we should be omitting certain animals which in point 16 ANIMAL LIFE of abundance of individuals vastly outnumber all other animals, and which in point of importance in helping main- tain the complex and varied life of the ocean distinctly out- class all other marine forms. These animals are the marine Protozoa, those of the " simplest animals " which live in the ocean. Although the water at the surface of the ocean appears clear, and on superficial examination devoid of life, yet a drop of this water taken from certain ocean regions exam- ined under the microscope reveals the fact that this water is inhabited by Protozoa. Not only is the water at the very surface of the ocean the home of the simplest animals, but they can be found in all the water from the surface to a great depth beneath it. In a pint of this ocean water from the surface or near it there may be millions of these animals. In the oceans of the world the number of them is inconceivable. Dr. W. K. Brooks says that the " basis of all the life in the modern ocean is found in the micro- organisms of the surface." By micro-organisms he means the one-celled animals and the one-celled plants. For the simplest plants are, like the simplest animals, one- celled. " Modern microscopical research," he says, " has shown that these simple plants, and the Globigerinse and Kadiolaria [kinds of Protozoa] which feed upon them, are so abundant and prolific that they meet all demands and supply the food for all the animals of the ocean." 9. The Globigerinse and Radiolaria.— The Globigerinas (Fig. 10) and Radiolaria (Fig. 11) are among the most in- teresting of all the simplest animals. Their simple one- celled body is surrounded by a microscopic shell, which among the Globigerinae is usually made of lime (calcium carbonate), in the case of Radiolaria of silica. These minute shells present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The shells are usually perforated by many small holes, through which project long, delicate, protoplasmic threads. These THE LIFE OF THE SIMPLEST ANIMALS 17 fine threads interlace when they touch each other, thus forming a sort of protoplasmic network outside of the shell. In some cases there is a complete layer of protoplasm — part of the body protoplasm of the protozoan — surround- PIG. W.—PolystomeUa strigittata, one of the Globigerinte.— After MAX SCHULTZB. ing the cell externally. The Radiolaria, whose shells are made of silica, possess also a perforated membranous sac called the central capsule, which lies imbedded in the protoplasm, dividing it into two portions, one within and 3 18 ANIMAL LIFE one outside of the capsule. In the protoplasm inside of the capsule lies the nucleus or nuclei ; and from the proto- plasm outside of the capsule rise the numerous fine, thread- like pseudopods which project through the apertures in the shell, and enable the animal to swim and to get food. Most of the myriads of the simplest animals which swarm in the surface waters of the ocean belong to a few kinds of these shell-bearing Globigerinae and Radiolaria. Large areas of the bottom of the Atlantic Ocean are cov- ered with a slimy gray mud, often of great thickness, which is called globigerina-ooze, because it is made up chiefly of the microscopic shells of Globigerinae. As death comes to the minute protoplasmic animals their hard shells sink slowly to the bottom, and accumulate in such vast quanti- ties as to form a thick layer on the ocean floor. Nor is it only in present times and in the oceans we know that the Globigerinae have flourished. All over the world there are thick rock strata which are composed chiefly of the fos- silized shells of these simplest animals. Where the strata are made up exclusively of these shells the rock is chalk. Thus are composed the great chalk cliffs of Kent, which gave to England the early name of Albion, and the chalk beds of France and Spain and Greece. The existence of these chalk strata means thai'Vher^ now is land, in earlier geologic times were oceans, and that in the oceans Globi- gerinae lived in countless numbers. Dying, their shells accumulated to form thick layers on the sea bottom. In later geologic ages this sea bottom has been uplifted and is now land, far perhaps from any ocean. The chalk strata of the plains of the United States, like those in Kansas, are more than a thousand miles from the sea, and yet they are mainly composed of the fossilized shells of marine Pro- tozoa. Indeed, we are acquainted with more than twice as many fossil species of Globigerinae as species living at the present time. The ancestors of these Globigerinae, from which the present Globigerinae differ. -but little, can be* THE LIFE OF THE SIMPLEST ANIMALS 19 It is traced far back in the geologic history of the world, an ancient type of animal structure. The Kadiolaria, too, which live abundantly in the pres- ent oceans, especially in the marine waters of the tropical and temperate zones, are found as fossils in the rocks from the time of the coal age on. The siliceous shells of the PIG. ll.—Heliosphcera actinota (after HAECKEL) ; a radiolarian with symmetrical shell. Radiolaria sinking to the sea bottom and accumulating there in great masses form a radiolaria-ooze similar to the globigerinae-ooze ; and just as with the Globigerinae, the remains of the ancient Radiolaria formed thick layers on the floor of the ancient oceans, which have since been up- lifted and now form certain rock strata. That kind of bock called Tripoli, found in Sicily, and the Barbados earth from the island of Barbados, both of which are used 20 ANIMAL LIFE as polishing powder, are composed almost exclusively of the siliceous shells of ancient and long-extinct Radiolaria. 10. Antiquity of the Protozoa. — All the animals of the ocean depend upon the marine Protozoa (and the marine Protophyta, or one-celled plants) for food. Either they prey upon these one-celled organisms directly, or they prey upon animals which do prey on these simplest animals. The great zoologist already quoted says : " The food sup- ply of marine animals consists of a few species of micro- scopic organisms which are inexhaustible and the only source of food for all the inhabitants of the ocean. The supply is primeval as well as inexhaustible, and all the life of the ocean has gradually taken shape in direct depend- ence upon it." That is, the marine simplest animals are the only marine animals which live independently; they /^lone can live or could have lived in earlier ages without jp depending on other animals. They must therefore be the 'oldest of marine animals. By oldest we mean that their kind appeared earliest in the history of the world. As it is certain that marine life is older than terrestrial life — that is, that the first animals lived in the ocean — it is obvious that the marine Protozoa are the most ancient of animals. This is an important and interesting fact. Zoologists try to find out the relationships and the degrees of antiquity or modernness of the various kinds of animals. We have seen that the Protozoa, those animals which have the sim- \ pleat body structure and perform the necessary life pro- cesses in the simplest way, are the oldest, the first animals. L Tfrs is just what we would expect. 11. The primitive form. — We find among the simplest animals a considerable variety of shape and some manifest variation in habit. But the points of resemblance are far more pronounced than the points of difference, and are of fundamental importance. The composition of the body of one cell, as opposed to the many-celled structure of the bodies of all other animals, is the fact to be most distinctly THE LIFE OF THE SIMPLEST ANIMALS 21 emphasized. The shape of this one-celled body varies. With the most primitive or simplest of the " simplest ani- mals," like Amoeba, -for example, there is no " distinction of ends, sides, or surfaces, such as we are familiar with in in the higher animals. Anterior and posterior ends, right and left sides, dorsal and ventral surfaces are terms which have no meaning in reference to an Amoeba, for any part of the animal may go first in locomotion, and when crawl- ing the animal moves along on whatever part of its surface happens to be in contact with foreign bodies." The one shape most often seen among the Protozoa, or most nearly fairly to be called the typical shape, is the spherical or subspherical shape. Why this is so is readily seen. Most of the Protozoa are aquatic and free swim- ming. They live in a medium, the water, which supports or presses on the body equally on all sides, and the body is not forced to assume any particular form by the environ- ment. The body rests suspended in the water with any part of its surface uppermost or any part undermost. As any part of the surface serves equally well in many of the Protozoa for breathing or eating or excreting, it is obvious that the spherical form is the simplest and most conven- ient shape for such a body. It is interesting to note that the spherical form is the common shape of the egg cell of the higher animals. Each one of the higher, multicellular animals begins life (as we shall find it explained in another chapter of this book) as a single cell, the egg cell, and these egg cells are usually spherical in shape. The full significance of this we need not now attempt to under- stand, but it is interesting to note that normally the whole body of the simplest animals is a single spherical cell, and that every one of the higher animals, however complex it may become by growth and development, begins life as a single spherical cell. 12. The primitive but successful life. — Living consists of the performing of certain so-called life processes, such as 3 22 ANIMAL LIFE eating, breathing, feeling, and multiplying. These pro- cesses are performed among the higher animals by various organs, special parts of the body, each of which is fitted to do some one kind of work, to perform some one of these processes. There is a division or assignment of labor here among different parts of the body. Such a division of labor, and special fitting of different parts of the body for special kinds of work does not exist, or exists only in slightest degree among the simplest animals. The Amo&ba eats or feels or moves with any part of its body ; all of the body exposed to the air (air held in the water) breathes ; the whole body mass takes part in the process of repro- duction. Only very small organisms can live in this simplest way. So all of the Protozoa are minute. When the only part of the body which can absorb oxygen is the simple external surface of a spherical body, the mass of that body must be very small. With any increase in size of the animal the mass of the body increases as the cube of the diameter, while the surface increases only as the square of the diam- eter. Therefore the part of the body (inside) which re- quires to be provided with oxygen increases more rapidly than the part (the outside) which absorbs oxygen. Thus this need of oxygen alone is sufficient to determine the limit of size which can be attained by the spherical or sub- spherical Protozoa. That the simplest animals, despite the lack of organs and the primitive way of performing the life processes, live successfully is evident from their existence in such ex- traordinary numbers. They outnumber all other animals. Although serving as food for hosts of ocean animals, the marine Protozoa are the most abundant in individuals of all living animals. The conditions of life in the surface waters of the ocean are easy, and a simple structure and simple method of performance of the life processes are wholly adequate for successful life under these conditions, THE LIFE OF THE SIMPLEST ANIMALS 23 That the character of the body structure of the Protozoa has changed but little since early geologic times is ex- plained by the even, unchanging character of their sur- ,/ roundings. The oceans of former ages have undoubtedly been essentially like the oceans of to-day — not in extent and position, but in their character of place of habitation for animals. The environment is so simple and uniform that there is little demand for diversity of habits and conse- quent diversity of body structure. Where life is easy there is no necessity for complex structure or complicated habits of living. So the simplest animals, unseen by us, and so inferior to us in elaborateness of body structure and habit, swarm in countless hordes in all the oceans and rivers and *akes, and live successfully their simple lives. CHAPTEE II THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 13. Colonial Protozoa. — When one of the simplest animals multiplies by fission, the halves of the one-celled body sepa- rate wholly from each other, move apart, and pursue their lives independently. The original parent cell divides to form two cells, which exist thereafter wholly apart from each other. There are, however, certain simple animals which are classed with the Protozoa, which show an inter- esting and important difference from the great majority of the simplest animals. These are the so-called colony-form- ing or colonial Protozoa. These colonial Protozoa belong to a group of organisms called the * Volvocinae. The simplest of the Volvocinae are single cells, which live wholly independently and are in structure and habit essentially like the other Protozoa we have studied. They have, however, imbedded in the one- celled body a bit of chlorophyll, the green substance which gives the color to green plants and is so important in their physiology. In this respect they differ from the other Protozoa. Among the other Volvocinae, however, a few or many cells live together, forming a small colony — that is, * These colonial organisms, the Volvocinae, are the objects of some contention between botanists and zoologists. The botanists call them plants because they possess a cellulose membrane and green chroma- tophores, and exhibit the metabolism characteristic of most plants ; but most zoologists consider them to be animals belonging to the order Flagellata of the Protozoa. In the latest authoritative text-book of zoo'logy, that of Parker and Haswell (1897), they are so classed. THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 25 there is formed a group of a few or many cells, each cell having the structure of the simpler unicellular forms. These cells are held together in a gelatinous envelope, and the mass is usually spherical in shape. In most of the colonies each of the cells possesses two or three long, pro- toplasmic, whiplash-like hairs, called flagella, and by the lashing -of these flagella in the water the whole group swims about. 14. Gonium. — If, when one of the simplest animals di- vided to form two daughter cells, these two cells did not move apart, but remained side by side and each di- vided to form two more, and each of these divided to form two more, and these eight divided each into two, each cell com- plete and independent but all remaining together in a group — if this pro- cess should take place we should have produced a group or colony of sixteen cells, each cell a complete animal capable of living independently like the other simplest animals, but all holding together B to form a tiny, flat, plate- FIG. 12.— Gonium pectorale (after STEIN). A, like colony. Now, this is *j^^^ ab°ve; B' C°lony 8een precisely what takes place in the case of those colonial Protozoa belonging to the genus Gonium (Fig. 12). When the mother cell of Gonium di- vides, the daughter cells do not swim apart, but remain side by side, and by repeated fission, until there are sixteen cells side by side, the colony is formed. Each cell of the 26 ANIMAL LIFE colony eats and breathes and feels for itself ; each can and does perform all the processes necessary to keep it alive. When ready to multiply, the sixteen cells of the Gonium colony separate, and each cell becomes the ancestor of a new colony. 15. Pandorina. — Another colony usually composed of six- teen cells is Pandorina, but the cells are arranged to form a spherical instead of a plate-like colony (Fig. 13). In Pan- dorina morum the colony consists of sixteen ovoid cells in a spherical jelly-like mass. Each cell has two flagella, and by the lashing of all the flagella the whole colony moves through the water. Food is taken by any of the cells, is assimilated, and the cells increase in size. When Pan- dorina is ready to multiply, each cell divides repeatedly until it has formed sixteen daughter cells. The inclosing gelatinous mass which holds the colony together dissolves, and the daughter colonies be- come free and swim apart. Each colony soon grows to the size of the original colony. This kind of multiplication or reproduction may be continued for several generations. But it does not go on indefinitely. After a number of these gener- ations has been produced by simple division, the cells of a colony divide each into eight instead of sixteen daughter cells. The daughter cells are not all of the same size, but the difference is hardly notice- able. The eight cells resulting from the repeated division of one of the original cells separate and swim about inde- pendently by means of their flagella. If one of these cells comes near a similar free-swimming cell from another PIG. 13.— Pandorina sp. (from Na- ture). The cells composing the colony are beginning to divide to form daughter colonies. THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 27 colony, the two cells conjugate (Fig. 14) — that is, fuse to form a single cell. This new cell formed by the fusion of two, develops a tough enveloping membrane of cellulose and passes into what is called the "resting stage." That is, the cell remains dormant for a shorter or longer time. It may thus tide over a drought or a winter. It may become dry or be frozen, yet when suitable conditions of moisture or tem- perature are again present the outer wall breaks and the pro- toplasm issues as a large free- swimming cell, which soon di- vides into sixteen daughter cells which constitute a new colony. 16. Eudorina.— Another colo- nial protozoan which much re- sembles Pandorina^ but differs from it in one interesting and suggestive thing, is Eudorina. In Eudorina elegans (Fig. 15) the colony is spherical and is composed of sixteen or thirty- two cells. Each of these cells can become the parent of a new colony by simple repeated divi- sion. But this simple mode of reproduction, just as with Pan- dorina, can not persist indefi- nitely. There must be conjuga- tion. But the process of mul- tiplication, which includes conjugation, is different from that process in Pandorina, in that in Eudorina the conju- B FIG. 14. — Pandorina morum (after GOEBEL). Three stages in the conjugation and formation of the resting spore. A, two cells just fused; B, the two cells completely fused, but with flagella still per- sisting ; C, the resting spore. 28 ANIMAL LIFE gating cells are of two distinctly different kinds. When this kind of multiplication is to take place in the case of Eudorina elegans, to choose a common species, some of the cells of a colony divide into sixteen or thirty -two minute elongated cells, each provided with two flagella. These small cells escape FIG. 15.— Eudorina elegans. A, a mature colony (from Nature); B, formation of the two kinds of reproductive cells. from the envelope of the parent cell, remaining for some time united in small bundles. Other cells of the colony do not divide, but increase slightly in size and become spherical in shape. When a bundle of the small cells comes into contact with some of these large spherical cells the bundle breaks up, and conjugation takes place between the small flagellated free-swimming cells and the large non-flagellate spherical cells. Each new cell formed by the fusion of one of the small and one of the large cells develops a cellulose wall and assumes a resting stage. After a time from each of these resting spores a new colony of sixteen or thirty-two cells is formed by direct, repeated division. 17. Volvox. — Another interesting colonial protozoan is Volvox. The large spherical colonies of Volvox globator THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 29 (Fig. 16) are composed of several thousand cells, arranged in a single peripheral layer about the hollow center of the ball. The cells are ovoid, and each is provided with two long flagella which pro- ject out into the water. The lashing of the thousands of the flagella give the ball- like colony a rotary motion. The cells are held together by a jelly-like intercellular substance and are connect- ed with each other by fine protoplasmic threads which extend from the body pro- toplasm of one cell to the cells surrounding it. When the colony is full grown and ready to reproduce itself jj certain cells of the colony ^&r /^/ (f undergo great changes. C " /' *" Some of them increase in size enormously, having re- serve food material stored in them, and they may be called the egg cells of the colony. Eeproduction may now occur by simple divi- sion of one of these great egg cells into many small cells, all held together in a Common envelope. These FIG. 16.— A, Volvox minor, entire colony form a daughter colony which escapes from the mother colony and by growth and further division comes to be a new full-sized colony. Or reproduction may occur in another, more complex, way. Certain cells of the colony B (from Nature). B, C, and D, reproductive cells of Volvox globator. 30 ANIMAL LIFE divide into bundles of very small, slender cells, each of which is provided with flagella. The remaining cells of the colony (that is, those which have not swollen into egg cells or divided into many — sixty-four to one hundred and twenty-eight — minute, flagellate cells) remain unchanged for a while and finally die. They take absolutely no part in reproducing the colony. One of the minute free-swim- ming cells fuses with one of the enormous egg cells, the new cell thus formed being a resting spore. From this resting spore a new colony develops by repeated division. 18. Steps toward complexity. — Within the group of Vol- vocince there are plainly several steps on the way from simplicity of structure to complexity of structure. Gonium, Pandorina, Eudorina, and Volvox form a series proceeding from the simplest animals toward the complex animals. In Gonium the cells composing the colony are all alike in structure, and each one is capable of performing all the processes or functions of life. In Pandorina and Eudorina the cells are at first alike, but there is, as the time for reproduction approaches, a differentiation of structure ; the cells of the colony, all of which take part in the process of reproduction, come to be in certain generations of two kinds — an inactive large kind which may be called the egg cells, and a small, active, free-swimming kind which seeks out and conjugates with, or, we may say, fertilizes the egg cells. In Volvox there is a new differentiation. Only cer- tain particular and relatively few cells take part in repro- ducing the colony; most of the cells have given up the power or function of reproduction. These cells, when the time of multiplication comes, simply support the special reproductive cells. They continue to waft the great colony through the water by lashing their flagella ; they continue to take in food from the outside. The reproductive cells devote themselves wholly to the business of producing new colonies, of perpetuating the species. And this matter of reproduction is less simple than in the other VolvocincB. THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 31 At least there is much more difference between the two kinds of reproductive cells. The egg cells are compara- tively enormous, and they are stored with a mass of food material. The fertilizing cells are very small, but very active and very different from the egg cells. We have in Volvox the beginnings of a distinct division of labor and an accompanying differentiation of structure. Certain cells of the colony do certain things, and are modified in structure to tit them specially for their particular duties. The steps from the simplest structure toward a complex structure are plainly visible. 19. Individual or colony. — Is the Gonium colony, the Pandorina colony, or the Volvox colony a group of several or many distinct organisms, or is it to be considered as a sin- gle organism ? With Gonium, which we may call the sim- plest of these colonial organisms, the colony is composed of a few wholly similar cells or one-celled animals, each fully capable of performing all the life processes, each wholly competent to lead an independent life. In fact, each does, for part of its life, live independently, as we have already described. In the case of Pandorina and Eu- dorina, while all the cells are for most of the lifetimo of the colony alike and each is capable of living independently, at the time of reproduction the cells become of two kinds. A difference of structure is apparent, and for the perpetua- tion of the species the co-operation of these different kinds of cells is necessary. That is, it is impossible for a single one of the members of the colony to reproduce the colony, except for a limited number of generations. With Volvox this giving up of independence on the part of the individual members of the colony is more marked. There is a real in- terdependence among the thousands of cells of the colony. The function of reproduction rests with a few particular cells, and for the perpetuation of the species there is demand- ed a co-operation of two distinct kinds of reproductive cells. The great majority of the cells take no part in reproduc- 32 ANIMAL LIFE tion. They can perform all the other life processes ; they move the colony by lashing the water with their flagella ; they take in food and assimilate it ; they can feel. All the cells of the great colony, too, are intimately connected by means of protoplasmic threads. The protoplasm of one cell can mingle with that of another cell; food can go from cell to cell. The question whether the Volvox colony is a group of distinct organisms or is a single organism made up of cells among which there is a simple but obvi- ous difference in structure and function ; in other words, whether Volvox is a colony of one-celled animals, of Pro- tozoa, or is a multicellular animal, one of the Metazoa (for so all the many-celled animals are called), is a difficult one to decide. Most zoologists class the Volvocinae with the Protozoa — that is, they incline to consider Gonium, Pan- dorina, Volvox, and the other Volvocinae as groups or col- onies of one-celled animals. 20. Sponges. — If the VolvocincB be considered to belong to the Protozoa, the sponges are the simplest of all the many-celled animals. Sponges are not free-swimming ani- mals, except for a short time in their young stage, but are fixed, like plants. They live attached to some solid sub- stance on the sea bottom. They resemble plants, too, in the way in which the body is modified during growth by the environment. If the rock to which the young sponge is attached is rough and uneven, the body of the sponge will grow go as to fit the unevenness ; if the rock surface is smooth, the body of the sponge will be more regular. Thus a sponge may be said to have no fixed shape of body ; indi- viduals of the same species of sponge differ much in form. The typical form of the sponges is that of a short cylinder or vase attached by one end and with the upper free end open (Fig. 17). Many individuals of one kind usually live together in a close group or colony, and they may be so attached to each other as to appear like a branching plant. This branching may be very diffuse, and the branches THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 33 may become so interwoven with each other as to form a very complex group. A sponge is composed of many cells arranged in three layers — that is, the body of a sponge is a cylinder closed at one end whose wall is composed of three layers of cells. The outer layer of cells is called the ectoderm^ and the cells composing it are flat and are all closely attached to each other. The inner layer is called the endo- derm, and its cells are thicker than those of the ectoderm ; they are also closely attached to each other. Sometimes they are provided with flagella like the flagellate Protozoa. The flagella are, however, not for the purpose of locomotion, but for creat- ing currents in the water, which bathes the interior of the open cylin- drical body. The middle layer, called the mesoderm, is composed of numerous separate cells lying in a jelly-like matrix. From these meso- derm cells fine needles or spicules of lime or silica often project out through the ectoderm. These mi- nute sponge spicules are of a great variety of shapes, and they form a sort of skeleton for the support of the soft body mass. All over the outer surface of the body are scat- tered fine openings or pores, which lead through the walls of the body into the inner cavity. This cavity is of course also con- nected with the outside by the large opening at the free or apical end of the body. There is hardly any differentiation of parts among the 4 FIG. 17.— One of the simplest sponges, Calcolynthus pri- migeniw (after HAECKEL). A part of the outer wall is cut away to show the in- side. ANIMAL LIFE sponges. As in the Protozoa, there are no special organs for the performance of special functions. The sponge feeds by creating, with its flagella, water currents which flow in through the many fine pores of the body and out from the inner body cavity through the large opening at the free end of the body. These cur- rents of water bear fine parti- cles of organic matter which are taken up by the cells lining the pores and body cavity, and assimilated. There are no special organs of digestion. Each cell takes up food and digests it. The water cur- rents also bring air to these same cells, and thus the sponge breathes. Although the sponge as a whole can not move, does not possess the power of locomotion, yet the protoplasm of the cells has the power of contracting, just as with the Protozoa, and the pores can be opened or closed by this cellular movement. Practically, thus, the only FIG. i8.-one of the simple sponges, movements the sponge can Prophysema prtmordiale (after HAECKEL). The body is represented make are the movements made as cut in two longitudinally. The by the individual cells. large cells of the inner layer are the ^ .. egg ceils. Jtteproauction is accom- plished by a process of divi- sion, or by a process of conjugation and subsequent division. In its simplest way multiplication takes place by a group of cells separating from the body of the parent sponge, THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 35 becoming inclosed in a common capsular envelope, and by repeated division and consequent increase in number of cells becoming a new sponge. This is reproduction by " budding." The " buds," or small groups of cells which separate from the parent sponge, are called gemmules. Reproduction in the more complex way occurs as follows : Some of the free amoeboid cells of the mesoderm (the mid- dle one of the three layers of the body wall) become en- larged and spherical in form. These are the egg cells. Other mesodermic cells divide into many small cells, which are oval with a long, tapering, tail-like projection. These cells are active, being able to swim by the lashing of the tapering tail. These are the fertilizing cells. The two kinds of reproductive cells may be formed in one sponge ; if so, they are formed at different times. Or one sponge may produce only egg cells, another only fertilizing or, as they are called, sperm cells. Conjugation takes place between a sperm cell and an egg cell. That is, one of the small active sperm cells finds one of the large, spherical, inactive cells and penetrates into the protoplasm of its bqjly. The two cells fuse and form a single cell, which may be called the fertilized or impregnated egg. This fer- tilized egg, remaining in the body mass of the parent sponge, divides repeatedly, the new cells formed by this division remaining together. The young or embryo sponge finally escapes from the body of the parent sponge, and lives for a short time as an active free-swimming animal. Its body consists of an oval mass of cells, of which those on one side are provided with cilia or swimming hairs. The cells of the body continue to divide and to grow, and the body shape gradually changes. The young sponge finally becomes attached to some rock, the body assumes the typi- cal cylindrical shape, an aperture appears at the free end, and small perforations appear on the surface. The sponge becomes full grown. Those of us who do not live in the vicinity of the sea- 36 ANIMAL LIFE shore where sponges are found can not observe the struc- ture and life history of living specimens. There are, how- ever, among the thousand and more kinds of sponges a few kinds that live in fresh water, and these are so widely spread over the earth that examples of them can be found in almost any region. They belong to the genus Spongilla, and thirty or more species or kinds of Spongilla are known. In standing or slowly flowing water, Spongilla grows erect and branching, like a shrub or miniature tree ; in swift water it grows low and spreading, forming a sort of mat over the surface to which it is attached. Reproduction takes place very actively by the process of budding. The budded-off gemmules are spherical in shape, and the cells of each gemmule are inclosed in an envelope composed of siliceous spicules of peculiar shape. These gemmules are formed in the body substance of the parent sponge toward the end of the year, and are set free by the decaying of that part of the body of the parent sponge in which they lie. They sink to the bottom of the pond or brook, and lie there dormant until the following spring. Then they develop rapidly by repeated division of the cells and growth. It is not the purpose here to describe the many and interesting kinds of sponges which inhabit the ocean. The sponge of the bathroom is simply the skeleton of a large sponge or group of sponges. The skeleton here is not composed of lime or silica, but of a tough, horny substance, which is secreted by cells of the mesodermal layer of the body wall of the sponge. This substance is called spongin, and is a substance allied to silk in its chemical composi- tion. All the commercial sponges, the spongin skeletons, belong to one genus — Spongia. These sponges grow espe- cially abundantly in the Mediterranean and Red Seas, and in the Atlantic Ocean off the Florida reefs, and on the shores of the Bahama Islands. The sponges are pulled up by divers, or by means of hooks or dredges. The THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 37 living matter soon dies and decays, leaving the horny skeleton, which when cleaned and trimmed is ready for use. The most beautiful sponges are those with siliceous skeletons. The fine needles or threads of glass, arranged often in delicate and intricate pattern, make these sponges objects of real beauty. 21. Polyps, corals, and jelly-fishes. — The general or typ- ical plan of body structure of those animals which come next in degree of complexity to the sponges can be best understood by imagining the typical cylindrical body of a sponge modified in the following way: The middle one of the three layers of the body wall not to be composed of cells in a gelatinous mass, but to be simply a thin non- cellular membrane ; the body wall to be pierced by no fine openings or pores, so that tthe interior cavity of the body is connected with the outside only by the single large opening at the free end, and this opening to be sur- rounded by a circlet of arm-like processes or tentacles, continuations of the body wall and similarly composed. Such a body structure is the general or fundamental one for all polyps, corals, sea-anemones, and jelly-fishes. The variety in shape and the superficial modifications of this type-plan are many and striking ; but, after all, the type- plan is recognizable throughout the whole of this great group of animals. Perhaps the simplest representative of the group is a tiny polyp which grows abundantly in the fresh-water streams and pools, and can be readily obtained for observation. It is called Hydra. 22. Hydra,— The body of Hydra (Fig. 19), which is very small and appears to the unaided eye as a tiny white or greenish gelatinous particle attached to some submerged stone, bit of wood, or aquatic plant, is a simple cylinder attached by one end to the stone or weed. The other free end is contracted so as to be conical, and it is narrowly open. Around the opening are six or eight small waving 4 38 ANIMAL LIFE tentacles. The wall of the cylinder is composed of an outer and an inner layer of cells and a thin non-cellular membranous layer between them. The tentacles are hol- low and are simple extensions of the body wall. The cells of the outer layer, or ectoderm, are not all alike. Some are smaller than the others and appear to be crowded in FIG. 19.— The fresh-water polyp, Hydra vulgaris. A, in expanded condition, and in contracted condition; B, cross section of body, showing the two layers of cells which make up the body wall. between the bases or inner ends of the larger ones. The inner ends of the large cells are extended as narrow-pointed prolongations directed at right angles with the rest of the cell. These processes are very contractile and are called muscle processes. Each one is simply a continuation of the protoplasm of the cell body, which is especially con- tractile. Some of the smaller ectoderm cells are very THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 39 irregular in shape and possess specially large nuclei. These cells are more irritable or sensitive than the others and are called nerve cells. The ectoderm cells of the base or foot of the Hydra are peculiarly granular, and secrete a sticky substance by which the Hydra holds fast to the stone or weed on which it is found. These cells are called gland cells. Imbedded in many of the larger ectoderm cells, especially those of the tentacles, are small oval sacs, in each of which lies folded or coiled a fine long thread. When the tentacles touch one of the small animals which serve Hydra as food, these fine threads shoot out from their sacs and so poison or sting the prey that it is paralyzed. The tentacles then contract and bend inward, forcing the captured animal into the mouth opening in the center of the circle of tentacles. Through the mouth opening the prey enters the body cavity of Hydra and is digested by the cells lining this cavity. These cells belonging to the inner layer of the body wall or endoderm are mostly large, and each contains one or more contractile vacuoles. From the free ends — the ends which are next to the body cavity — of these cells project pseudopods or fine flagella. These projections are constantly changing : now two or three short, blunt pseudopods are projecting into the body cavity ; now they are withdrawn, and a few fine, long flagella are projected. In addition to these cells there are in the endoderm, especially abundant near the mouth opening and wholly lacking in the tentacles and at the base of the body, many long, narrow, granular cells. They are gland cells which secrete a digestive fluid. The food captured by the tentacles and taken in through the mouth opening disintegrates in the body cavity, or diges- tive cavity, as it may be called. The digestive fluid se- creted by the gland cells of the endoderm acts upon it, so that it becomes broken into small parts. These par- ticles are probably seized by the pseudopods of the other endoderm cells and are taken into the body protoplasm 40 ANIMAL LIFE of these cells. The ectoderm cells do not take food directly, but receive nourishment only through the endo- derm cells. Hydra is not permanently attached. It holds firmly to the submerged stone or weed by means of the sticky secretion from the ectodermal gland cells of its base, but it can loosen itself, and by a slow creeping or gliding move along the surface of the stone to another spot. Even when attached, the form of the body changes ; it extends itself longitudinally, or it contracts into a compact globular mass. The tentacles move about in the water, and are continually contracting or extending. Like Volvox and the sponges, those other slightly com- plex animals we have already considered, Hydra has two methods of multiplication. In the simpler way, there appears on the outer surface of the body a little bud which is composed, at first, of ectoderm cells alone ; but soon it is evident that it is a budding, or outpushing, of the whole body wall, ectoderm, endoderm, and middle membrane. In a few hours the bud has six or eight tiny, blunt tentacles, a mouth opening appears at the free end, and the little Hydra breaks off from the parent body and leads an inde- pendent existence. In the more complex way, two kinds of special reproductive cells are produced by each individual, viz., large, inactive, spherical egg cells, and small, active sperm cells, each with an oval part or head (consisting of the nucleus) and a slender, tapering tail-like part (consist- ing of the cytoplasm). The egg cell lies inclosed in a layer of thin, surrounding cells, which compose a capsule for it. When the egg cell is ready for fertilization this capsule breaks, and one of the active sperm cells finds its way to and fuses with the egg cell. The fertilized egg cell now divides into several cells, which remain together. The outer ones form a hard capsule, and thus protected the embryo falls to the bottom, and after lying dormant for awhile develops into a Hydra. THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 41 23. Differentiation of the body cells. — In Hydra we have the beginnings of complexity of structure carried a step further than in the sponges. The division of labor among the cells composing the body is more pronounced, and the structural modification of the different cells to enable them better to perform their special duties is obvious. Some of the cells of the body specially devote themselves to food- taking ; some specially to the digestion of the food ; some are specially contractile, and on them the movements of the body depend, while others are specially irritable or sensitive, and on them the body depends for knowledge of the contact of prey or enemies. In the lasso cells — those with the stinging threads — there is a very wide departure from the simple primitive type of cells. There is in Hydra a manifest differentiation of the cells into various kinds of cells. The beginnings of distinct tissues and organs are foreshadowed. The individuals of Hydra live, usually, distinct from each other. There is no tree-like colony, as with the sponges. But most of the other polyps do live in this colonial manner. The new polyps which develop as buds from the body of the parent do not separate from the parent, but remain attached by their bases. They, in turn, produce new polyps which remain attached, so that in time a branching, tree-like colony is formed. 24. Medusae or jelly-fishes. — Most of the other polyps differ from Hydra also in producing, in addition to ordi- nary polyp buds, buds which develop into bell-shaped struc- tures called medusae (Fig. 20). These medusae consist of a soft gelatinous bell- or umbrella-shaped body, with a short clapper or stem which has an opening at its free end. From the edge of the bell or umbrella four pairs of tenta- cles project. The medusae usually separate from the parent polyp and live an independent, free-swimming life. These are the beautiful animals commonly known as jelly-fishes. The medusae or jelly-fishes produce special reproductive 42 ANIMAL LIFE cells, a single medusa producing only one kind of such cells — that is, producing either egg cells alone or sperm cells alone. The active sperm cells produced by one medusa find their way to an egg cell producing medusa, and fuse with or fertilize these egg cells. The fertilized egg develops into a small, oval, free-swimming embryo called a planula, which finally attaches itself to a stone or bit of wood or seaweed, and grows to be a simple cylindrical polyp attached at its base and with mouth and tentacles at its free end. This polyp gives rise by budding to new polyps, which remain attached to it, and gradually a new tree-like colony is formed. From this polyp or this colony new medusae bud off, swim away, and finally produce new *-Z£%2££** Polyps- Th™ there is in the lile of the polyps what is called an alterna- tion of generations. There are two kinds of individuals which evidently belong to the same species ' of animal, or, put in another way, one kind of animal has two distinct forms. This appearance of one kind of animal in two forms is called dimorphism. We shall see later, that one kind of animal may appear in more than two forms ; such a condition is called polymorphism. In alternation of gen- erations we have the polyp animal appearing in one genera- tion as a fixed cylindrical polyp, while in the next generation it is a free-swimming, umbrella-shaped medusa or jelly-fish. The polyps which are dimorphic — that is, have a polyp form of individual and a medusa form of individual — show more differentiation in structure than the simple Hydra. This further differentiation is especially apparent in the medusae or jelly-fishes. Here the nerve cells are aggregated in little groups arranged along the edge of the umbrella THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 43 to form distinct sense organs. The muscle processes are better developed, and the digestive cavity is differentiated into central and peripheral portions. In these dimorphic polyps the fixed polyp individuals reproduce by the simple way of budding, while the medusa individuals reproduce by producing special reproductive cells of two kinds, which must fuse to form a cell capable of developing into a new polyp. 25. Corals. — There are many kinds of polyps and jelly- fishes, and they present a great variety of shape and size and general appearance. Many polyps exist only in the true polyp form, never producing medusae. Others have FIG. 21.— A polyp, or sea-anemone (Metridium dianthus). only the medusa form. Some live in colonies, and others are always solitary. The animals we know as corals are polyps which live in enormous colonies, and which exist only in the true polyp form, not producing medusae. They Fro. 22.— Coral island (Nanuku Levu, of the Fiji group). (After a photograph by MAX AGASSIZ.) FIG. 23.— Shore of a coral island, with cocoanut palms. /After a photograph.) THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 45 form a firm skeleton of lime (calcium carbonate), and after their death these skeletons persist, and because of their abundance and close massing form great reefs or banks and islands. Coral islands occur only in the warmer oceans. In the Atlantic they are found along the coasts of southern Florida, Brazil, and the West Indies ; in the Pacific and Indian Oceans there are great coral reefs on the coast of Australia, Madagascar, and elsewhere, and certain large FIG. 24. — Organ-pipe coral. groups of inhabited islands like the Fiji, Society, and Friendly Islands are composed exclusively of coral islands. More than two thousand kinds of living corals are known, and their skeletons offer much variety in structure and appearance. Brain coral, organ-pipe coral (Fig. 24), the well-known red coral from Italy and Sicily, used as jewelry, and the sea pens and sea fans are among the better known and more beautiful kinds of coral skeletons. 26. Colonial jelly-fishes. — While many of the medusae or jelly-fishes are another form of individual of a true fixed polyp, many of the larger and more beautiful jelly-fishes do not exist in any other form. Some of these larger jelly- fishes are several feet in diameter, and when cast up on the beach form a great shapeless mass of soft, jelly-like sub- 46 ANIMAL LIFE stance. The bodies of all jelly-fishes are soft and gelatinous, the body substance containing hardly one per cent of solid matter. It is mostly water. Many jelly-fishes are beauti- fully and strikingly colored, and as they swim slowly about near the surface of the ocean, lazily opening and shutting their iridescent, umbrella-like bodies, they are among the most beautiful of marine organisms. When one of the jelly-fishes is taken from the water, however, it quickly loses its brilliant colors, and dries away to a shapeless, shrivel- ing, sticky mass. Some of the most beautiful of the jelly-fishes belong to a group called the Siphonophora. These jelly-fishes are elongate and tube-like rather than umbrella- or bell-shaped, and they are polymorphic — that is, there are several dif- ferent forms of individuals belonging to a single kind or species. The Siphonophora are all free-swimming, but nevertheless form small colonies. In the Mediterranean Sea and in other southern ocean waters the surface may be covered for great areas by these brilliantly colored jelly-fish colonies, each of which looks, as a celebrated German natu- ralist has said, like a swimming flower cluster whose parts, flowers, stems, and leaves seem to be made of transparent crystal, but which possess the life and soul of an animal. An abundant species of these Siphonophora (Fig. 25) is com- posed of a slender, flexible, floating, central stem several feet long, to which are attached thousands of medusa and polyp individuals representing several different kinds of forms, each kind of individual being specially modified or adapted to perform some one duty. The central stem is a greatly elongated polyp individual, whose upper end is dilated and filled with air to form a float. This individual holds up the whole colony. Grouped around this central stem just below the float are many bell-shaped bodies which alter- nately open and close, and by thus drawing in and expelling water from their cavities impel the whole colony through the water. These bell-shaped structures are attached me- THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 47 dusa individuals, whose business it is to be the locomotive organs for the colony. These medusae are without tentacles, and take no food and produce no young. They have given up the power of performing these other life processes, and devote themselves wholly to the business of locomotion. From the lower end of the central stem rises a host of structures, among which several distinct kinds are readily perceived. One kind is composed of a pear- shaped hollow body open at its free end, and bear- ing a long tentacle which is furnished with numer- ous groups of stinging cells. These are the polyp individuals whose especial business it is to capture and sting prey and to eat it. - These individuals are the food -getters for the colony. Scattered among these stinging, feeding polyps, are numerous smaller individuals with oval, closed body, each bearing a long, slender thread. These threads FIG. 25.— A colonial jelly-fish, Physoph&ra (after HAECKEL). At the top is the float polyp, around its stem the swimming medusae, and below are the feeding, feel- ing, protecting, and reproducing polyps and medusa. 48 ANIMAL LIFE are very sensitive, and the polyps bearing them have for special function that of feeling or being sensible of stimuli from without. They are the sense organs or sense indi- viduals of the colony. Finally, there are two other kinds of structures, or individuals which produce the special reproductive cells for the perpetuation of the species. These are the modified medusa individuals, and one kind, larger than the other, produces the active sperm cells, while the other produces the inactive egg cells. 27. Increase in the degree of complexity. — In the corals, sea-anemones, and jelly-fishes there is plainly much more of a division of labor among the various parts of an indi- vidual and much more modification of these parts— that is, much more structural complexity than among the sponges and Hydra. And these, in their turn, are more complex than are the colonial Protozoa, the Volvocinae. There is a great difference in degree of complexity among the slightly com- plex animals. But the various groups of these animals which we have studied can all be arranged roughly in a series beginning with the least complex among them and ascending to the most complex. And in this series, and in the always accompanying division of labor among the different parts, the gradual increase in complexity is beau- tifully shown. From an animal composed of many structurally simi- lar cells, each cell capable of performing all the life pro- cesses, we pass to an animal composed of cells of a few different kinds, of slight structural diversity. Each kind of cell devotes itself especially to a certain few life pro- cesses or functions. Next we find an animal in which the cells of one kind are specially aggregated to form a single part of the body which is specially devoted to the perform- ance of a single function. This diversity among the cells increases, this aggregation of similar cells to form special parts or organs increases, and the division of labor or assignment of special functions to special organs becomes THE LIFE OP THE SLIGHTLY COMPLEX ANIMALS 49 more and more pronounced. Among the more complex polyps and jelly-fishes the contractile cells form distinct muscle fibers and muscles ; the sensitive cells form dis- tinct nerve cells and nerve fibers which are arranged in a primitive nervous system ; the digestive cavity becomes complex and composed of different portions ; the reproduc- tive cells are formed by special organs, and the distinction between the egg cells and the sperm cells — that is, be- tween the female reproductive elements and the male reproductive elements — becomes more pronounced. We have followed this increase or development of struc- tural and physiological complexity from simplest animals to fairly complex ones. The principle of this development of complexity is evident. It will not be profitable to at- tempt to follow in detail this development among the higher animals. The complex animals are complex be- cause their life processes are performed by special parts of their body, which parts are specially modified so as to perform these processes well. The animals which are more complex than those we have studied differ from these simply in the degree of complexity attained. In order to understand this better we shall not further consider special groups of animals, but special processes or functions, and attempt to see how the modification and increase in complexity of structure goes hand in hand with the increase of elaborate- ness or complexity in the performance of function. CHAPTEE III THE MULTIPLICATION OF ANIMALS AND SEX 28. All life from life. — On the performance of the func- tion of reproduction or multiplication depends the exist- ence or perpetuation of the species. Although an animal may take food and perform all the functions necessary to its own life, it does not fulfill the demands of successful existence unless it reproduces itself. Some individuals of every species must produce offspring or the species becomes extinct. We have seen in our study of the simple animals that the function of reproduction is the first function to become differentiated in the ascent from simplest animals to complex animals. The first division of labor among the cells composing the bodies of the slightly complex animals «,nd the first structural differences among the cells are connected with the performance of the function of repro- duction or multiplication. We are all so familiar with the fact that a kitten comes into the world only through being born, as the off- spring of parents of its kind, that we shall likely not appre- ciate at first the full significance of the statement that all life comes from life ; that all organisms are produced by other organisms. Nor shall we at first appreciate the im- portance of the statement. This is a generalization of modern times. It has always been easy to see that cats and horses and chickens and the other animals we famil- iarly know give birth to young or new animals of their own kind ; or, put conversely, that young or new cats and horses and chickens come into existence only as the off- 50 THE MULTIPLICATION OF ANIMALS AND SEX 51 spring of parents of their kind. And in these latter days of microscopes and mechanical aids to observation it is even easy to see that the smaller animals, the microscopic organ- isms, come into existence only as they are produced by the division of other similar animals, which we may call their parents. But in the days of the earlier naturalists the life of the microscopic organisms, and even that of many of the larger but unfamiliar animals, was shrouded in mystery. And what seem to us ridiculous beliefs were held regarding the origin of new individuals. 29. Spontaneous generation. — The ancients believed that many animals were spontaneously generated. The early naturalists thought that flies arose by spontaneous genera- tion from the decaying matter of dead animals ; from a dead horse come myriads of maggots which change into flesh flies. Frogs and many insects were thought to be generated spontaneously from mud. Eels were thought to arise from the slime rubbed from the skin of fishes. Aris- totle, the Greek philosopher, who was the greatest of the ancient naturalists, expresses these beliefs in his books. It was not until the middle of the seventeenth century- Aristotle lived three hundred and fifty years before the birth of Christ — that these beliefs were attacked and be- gan to be given up. In the beginning of the seventeenth century William Harvey, an English naturalist, declared that every animal comes from an egg, but he said that the egg might " proceed from parents or arise spontaneously or out of putrefaction." In the middle of the same century Eedi proved that the maggots in decaying meat which pro- duce the flesh flies develop from eggs laid on the meat by flies of the same kind. Other zoologists of this time were active in investigating the origin of new individuals. And all their discoveries tended to weaken the belief in the theory of spontaneous generation. Finally, the adherents of this theory were forced to restrict their belief in spontaneous generation to the case 52 ANIMAL LIFE of a few kinds of animals, like parasites and the animalcules of stagnant water. It was maintained that parasites arose spontaneously from the matter of the living animal in which they lay. Many parasites have so complicated and extraordinary a life history that it was only after long and careful study that the truth regarding their origin was dis- covered. But in the case of every parasite whose life his- tory is known the young are offspring of parents, of other individuals of their kind. No case of spontaneous genera- tion among parasites is known. The same is true of the animalcules of stagnant water. If some water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to be swarming with microscopic plants and animals. Any or- ganic liquid, as a broth or a vegetable infusion exposed for a short time, becomes foul through the presence of innumer- able bacteria, infusoria, and other one-celled animals and plants, or rather through the changes produced by their life processes. But it has been certainly proved that these organisms are not spontaneously produced by the water or organic liquid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores (embryo or- ganisms in the resting stage) germinate quickly when they fall into water or some organic liquid, and the rapid suc- cession of generations soon gives rise to the hosts of bacteria and Protozoa which infest all standing water. If all the active organisms and inactive spores in a glass of water are killed by boiling the water, " sterilizing " it, as it is called, and this sterilized water or organic liquid be put into a sterilized glass, and this glass be so well closed that germs or spores can not pass from the air without into the steril- ized liquid, no living animals will ever appear in it. It is now known that flesh will not decay or liquids ferment except through the presence of living animals or plants. To sum up, we may say that we know of no instance of the THE MULTIPLICATION OF ANIMALS AND SEX 53 spontaneous generation of organisms, and that all the ani- mals whose life history we know are produced from other animals of the same kind. " Omne vivum ex vivo" All life from life. FIG. 26.- The multiplication of Amoeba by simple fission. 30. The simplest method of multiplication. — In our study of the simplest and the slightly complex animals we became acquainted with the simplest methods of multiplication and with methods which are more complex. The method 5 54; ANIMAL LIFE of simple fission or splitting — binary fission it is often called, because the division is always in two — by which the body of the parent becomes divided into two equal parts— into halves — is the simplest method of multiplication. This is the usual method of Amoeba (Fig. 26) and of many other of the simplest animals. In this kind of reproduction it is hardly exact to speak of parent and children. The chil- dren, the new Amoebae, are simply the parent cut into halves. The parent persists ; it does not produce off- spring and die. Its whole body continues to live. The new AmcBbce take in and assimilate food and add new mat- ter to the original matter of the parent body ; then each of them divides in two. The grandparent's body is now divided into four parts, one fourth of it forming one half of each of the bodies of the four grandchildren. The pro- cess of assimilation, growth, and subsequent division takes place again, and again, and again. Each time there is given to the new Amceba an ever-lessening part of the actual body substance of the original ancestor. Thus an Amoeba never dies a natural death, or, as has been said, " no Amoeba ever lost an ancestor by death." It may be killed outright, but in that case it leaves no descendants. If it is not killed before it produces new Amoebce it never dies, although it ceases to exist as a single individual. The Amoeba and other simple animals which multiply by direct binary fission may be said to be immortal, snd the "immortality of the Protozoa " is a phrase which you will be sure to meet if you begin to read the writings of the modern philosoph- ical zoologists. 31. Slightly complex methods of multiplication.— Most of the Protozoa multiply or reproduce themselves in two ways — by simple fission and by conjugation. Paramce- cium, for example, reproduces itself for many generations by fission, but a generation finally appears in which a dif- ferent method of reproduction is followed. Two individu- als come together and each exchanges with the other a part THE MULTIPLICATION OF ANIMALS AND SEX 55 of its nucleus. Then the two individuals separate and each divides into two. The result of this conjugation is to give to the new Paramcecia produced by the conjugat- ing individuals a body which contains part of the body substance of two distinct individuals. The new Paramce- cia are not simply halves of a single parent ; they are parts of two parents. If the two conjugating individuals differ at all — and they always do differ, because no two individual animals, although belonging to the same species, are exactly alike — the new individual, made up of parts of each of them, will differ from both. We shall, as we study further, see that Nature seems intent on making every new individual differ slightly from the individual which produces it ; and the method of multiplication or the production of new indi- viduals which Nature has adopted to produce the result is the method which we have seen exhibited in its simplest form among the simplest animals — the method of having two individuals take part in the production of a new one. The further study of multiplication among animals is the study of the development and elaboration of this method. 32. Differentiation of the reproductive cells. — Among the colonial Protozoa the first differentiation of the cells or members composing the colony is the differentiation into two kinds of reproductive cells. Reproduction by simple division, without preceding conjugation, can and does take place, to a certain extent, among all the colonial Protozoa. Indeed, this simple method of multiplication, or some modi- .fication of it, like budding, persists among many of the com- plex animals, as the sponges, the polyps, and even higher and more complex forms. But such a method of single- parent reproduction can not be used alone by a species for many generations, and those animals which possess the power of multiplication in this way always exhibit also the other more complex kind of multiplication, the method of double-parent reproduction. Conjugation takes place be- tween different members of a single colony of one of the 56 ANIMAL LIFE colonial Protozoa, or between members of different colonies of the same species. These conjugating individuals in the simpler kinds of colonies, like Gonium, are similar; in Pandorina they appear to be slightly different, and in Eudo- rina and Volvox the conjugating cells are very different from each other (Figs. 15 and 16). One kind of cell, which is called the egg cell, is large, spherical, and inactive, while the other kind, the sperm cell, is small, with ovoid head and tapering tail, and free-swimming. In the simpler colo- nial Protozoa all the cells of the body take part in repro- duction, but in Volvox only certain cells perform this func- tion, and the other cells of the body die. Or we may say that the body of Volvox dies after it has produced special reproductive cells which shall fulfill the function of multi- plication. Beginning with the more complex Volvocinae, which we may call either the most complex of the one-celled animals or the simplest of the many-celled animals, all the cjpplex animals show this distinct differentiation between ^ie re- productive cells and the cells of the rest of the body. Of course, we find, as soon as we go up at all far in the scale of the animal world, that there is a great deal of differentia- tion among the cells of the body : the cells which have to do with the assimilation of food are of one kind ; those on which depend the motions of the body are of another kind ; those which take oxygen and those which excrete waste matter are of other kinds. But the first of this cell differ- entiation, as we have already often repeated, is that shown by the reproductive cells ; and with the very first of this differentiation between reproductive cells and the other body cells appears a differentiation of the reproductive cells into two kinds; These two kinds, among all animals, are always essentially similar to the two kinds shown by Volvox and the simplest of the many-celled animals — namely, large, inactive, spherical egg cells, and small, active, elon- gate or " tailed " sperm cells. THE MULTIPLICATION OF ANIMALS AND SEX 57 33. Sex, or male and female. — In the slightly complex animals one individual produces both egg cells and sperm cells. But in the Siphonophora, or colonial jelly-fishes, stud- ied in the last chapter, certain members of the colony pro- duce only sperm cells, and certain other members of the colony produce only egg cells. If the Siphonophora be considered an individual organism and not a colony com- posed of many individuals, then, of course, it is like the others of the slightly complex animals in this respect. But as soon as we rise higher in the scale of animal life, as soon as we study the more complex animals, we find that the egg cells and sperm cells are almost always produced by different individuals. Those individuals which produce egg cells are called female, and those which produce sperm cells are called male. There are two sexes. Male and female are terms usually applied only to individuals, but it is evidently fair to call the egg cells the female reproduc- tive cells, and the sperm cells the male reproductive cells. A single individual of the simpler kinds of animals pro- duces both male and female cells. But such an individual can not be said to be either male or female ; it is sexless — that is, sex is something which appears only after a certain degree of structural and physiological differentiation is reached. It is true that even among many of the higher or complex animals certain species are not represented by male and female individuals, any individual of the species being able to produce both male and female cells. But this is the exception. 34. The object of sex.— Among almost all the complex animals it is necessary that there be a conjugation of male and female reproductive cells in order that a new individual may be produced. This necessity first appears, we remem- ber, among very simple animals. This intermixing of body substance from two distinct individuals, and the develop- ment therefrom of the new individual, is a phenomenon which takes place through the whole scale of animal life. 58 ANIMAL LIFE The object of this intermixing is the production of va- riation. Nature demands that the offspring shall differ slightly from its parents. By having the beginnings of its body, the single cell from which the whole body develops, composed of parts of two different individuals, this differ- ence, although slight and nearly imperceptible, is insured. Sex is a provision of Nature which insures variation. 35. Sex dimorphism. — As we have seen, almost every species of animal is represented by two kinds of individuals, males and females. In the case of many animals, espe- Fm. 27.— Bird of paradise, male. cially the simpler ones, these two kinds of individuals do not differ in appearance or in structure apart from the organs concerned with multiplication. But with many animals the sexes can be readily distinguished. The male and female individuals often show marked differences, especially in external structural characters. We can read- THE MULTIPLICATION OF ANIMALS AND SEX 59 ily tell the peacock, with its splendidly ornamental tail feathers, from the unadorned peafowl, or the horned ram from the bleating ewe. There is here, plainly, a dimor- phism— the existence of two kinds of individuals belonging to a single species. This dimorphism is due to sex, and the condition may be called sex dimorphism. Among some animals this sex dimorphism, or difference between the sexes, is carried to extraordinary extremes. This is espe- cially true among polygamous animals, or those in which the males mate with many females, and are forced to fight for their possession. The male bird of paradise, with its gorgeous display of brilliantly colored and fantastically shaped feathers (Fig. 27), seems a wholly different kind of bird from the modest brown female. The male golden and silver pheasants, and allied species with their elaborate plumage, are very unlike the dull-colored females. The great, rough, warlike male fur seal, roaring like a lion, is three times as large as the dainty, soft-furred female, which bleats like a sheep. Among some of the lower animals the differences be- tween male and female are even greater. The males of the common cankerworm moth (Fig. 28) have four wings ; FIG. £8. — Cankerworm moth ; the winged male and wingless female. the females are wingless, and several other insect species show this same difference. Among certain species of white ants the females grow to be five or six inches long, while the males do not exceed half an inch in length. In the 60 ANIMAL LIFE case of some of the parasitic worms which live in the bod- ies of other animals, the male has an extraordinarily de- graded, simple body, much smaller than that of the female and differing greatly from that of the female in structure. In some cases even — as, for example, the worm which causes " gapes " in chickens — the male lives parasiti- cally on the female, being attached to the body of the female for its whole lifetime, and drawing its nourish- ment from her blood (Fig. 29). A condition known as partheno- genesis is found among certain of the complex animals. Although the species is represented by individu- als of both sexes, the female can produce young from eggs which have not been fertilized. For ex- ample, the queen bee lays both fer- tilized and unfertilized eggs. From the fertilized eggs hatch the work- ers, which are rudimentary females, and other queens, which are fully- 29.-The parasitic worm developed females ; from the unfer- (Syngamus trachealis), which r causes the "gapes "in fowls, tilized eggs hatch only males— the The male is attached to the drones. Many generations of plant female, and lives as a para- ,. site on her. lice are produced each year parthe- nogenetically — that is, by unferti- lized females. But there is at least one generation each year produced in the normal way from fertilized eggs. Some of the complex animals are hermaphroditic — that is, a single individual produces both egg cells and sperm cells. The tapeworm and many allied worms show this condition. This is the normal condition for the simplest animals, as we have already learned, but it is an excep- tional condition among the complex animals. THE MULTIPLICATION OF ANIMALS AND SEX 61 36. The number of young. — There is great variation in the number of young produced by different species of animals. Among the animals we know familiarly, as the mammals, which give birth to young alive, and the birds, which lay eggs, it is the general rule that but few young are pro- duced at a time, and the young are born or eggs are. laid only once or perhaps a few times in a year. The robin lays five or six eggs once or twice a year ; a cow may produce a calf each year. Babbits and pigeons are more prolific, each having several broods a year. But when we observe the multiplication .of some of the animals whose habits are not so familiar to us, we find that the production of so few young is the exceptional and not the usual habit. A lob- ster lays ten thousand eggs at a time ; a queen bee lays about five million eggs in her life of four or five years. A female white ant, which after it is full grown does nothing but lie in a cell and lay eggs, produces eighty thousand eggs a day steadily for several months. A large codfish was found on dissection to contain about eight million' If we search for some reason for this great difference in fertility among different animals, we may find a promis- ing clew by attending to the duration of life of animals, and to the amount of care for the young exercised by the parents. We find it to be the general rule that animals which live many years, and which take care of their young, produce but few young ; while animals which live but a short time, and which do not care for their young, are very prolific. The codfish produces its millions of eggs ; thou- sands are eaten by sculpins and other predatory fishes be- fore they are hatched, and other thousands of the defense- less young fish are eaten long before attaining maturity. Of the great number produced by the parent, a few only reach maturity and produce new young. But the eggs of the robin are hatched and protected, and the helpless fledglings are fed and cared for until able to cope with their natural 62 ANIMAL LIFE enemies. In the next year another brood is carefully reared, and so on for the few years of the robin's life. Under normal conditions in any given locality the num- ber of individuals of a certain species of animal remains about the same. The fish which produces tens of thousands of eggs and the bird which reproduces half a dozen eggs a year maintain equally well their numbers. In one case a few survive of many born ; in the other many (relatively) survive of the few born ; in both cases the species is effect- ively maintained. In general, no agency for the perpetua- tion of the species is so effective as that of care for the young. CHAPTER IV FUNCTION AND STRUCTURE 37. Organs and functions. — An animal does certain things which are necessary to life. It eats and digests food, it breathes in air and takes oxygen from it and breathes out carbonic-acid gas ; it feels and has other sensations ; it pro- duces offspring, thus reproducing itself. These things are done by the simplest animals as well as by the complex animals. But while with the simplest animals the whole body (which is but a single cell) takes part in doing each of these things, among the complex animals only a part of the body is concerned with any one of these things. Only a part of the body has to do with the taking in of oxygen. Another part has to do with the digestion of food, and another with the business of locomotion. These parts of the body, as we know, differ from each other, and they differ because they have different things to do. These different parts are called organs of the body, and the things they do are called their functions. The nostrils, tracheae, and lungs are the organs which have for function the pro- cess of respiration. The legs of a cat are the organs which perform for it the function of locomotion. The structure of one of the higher animals is complex because the body is made up of many distinct organs having distinct func- tions. The things done by one of the complex animals are many ; around each of the principal functions or necessary processes, as a center, are grouped many minor accessory functions, all helping to make more successful the accom- 63 64. ANIMAL LIFE plishment of the principal functions. While many of the lower animals have no eyes and no ears, and trust to more primitive means to discover food or avoid enemies, the higher animals have extraordinarily complex organs for seeing and hearing, two functions which are accessory only to such a principal function as food-taking. 38. Differentiation of structure, — We have seen, in our study of the slightly complex animals, how the body be- comes more and more complex in proportion to the degree in which the different life processes are divided or assigned to different parts of it for performance. With the gradu- ally increasing division of labor the body becomes less homogeneous in structure; a differentiation of structure becomes apparent and gradually increases. The extent of the division of labor and the extent of the differentiation of structure, or division of the body into distinct and dif- ferent parts and organs, go hand in hand. An animal in which the division of labor is carried to an extreme is an animal in which complexity of structure is extreme. 39. Anatomy and physiology. — Zoology, or the study of animals, is divided for convenience into several branches or phases. The study of the classification of animals is called systematic zoology; the study of the development of animals from their beginning as a single cell to the time of their birth is called animal embryology ; the study of the structure of animals is called animal anatomy, and the study of the performance of their life processes or functions is called physiology. Because the whole field of zoology is so great, some zoologists limit themselves exclusively to one of these phases of zoological study, and those who do not so definitely limit their study, at least give their special at- tention to a single phase, although all try to keep in touch with the state of knowledge in other phases. In earlier days the study of the anatomy of animals and of their physiology were held to be two very distinct lines of in- vestigation, and the anatomists paid little attention to FUNCTION AND STRUCTURE 65 physiology and the physiologists little to anatomy. But we have seen how inseparably linked are structure and function. The structure of an animal is as it is because of the work it has to do, and the functions of an animal are performed as they are performed because of the special structural condition of the organs which perform them. The study of the anatomy and the study of the physiology of animals can not be separated. To understand aright the structure of an animal it is necessary to know to what use the structure is put ; to understand aright the processes of an animal it is necessary to know the struc- ture on which the performance of the processes depends. 40. The animal body a machine. — The body of an animal may be well compared with some machine like a locomotive engine. Indeed, the animal body is a machine. It is a machine composed of many parts, each part doing some particular kind of work for which a particular kind of structure fits it ; and all the parts are dependent on each other and work together for the accomplishment of the total business of the machine. The locomotive must be provided with fuel, such as coal or wood or other readily combustible substance, the consumption of which furnishes the force or energy of the machine. The animal body must be provided with fuel, which is called food, which furnishes similarly the energy of the animal. Oxygen must be provided for the combustion of the fuel in the locomo- tive and the food in the body. The locomotive is com- posed of special parts : the firebox for the reception and combustion of fuel ; the steam pipes for the carriage of steam ; the wheels for locomotion ; the smoke stack for throwing off of waste. The animal body is similarly com- posed of parts : the alimentary canal for the reception and assimilation of food ; the excretory organs for the throwing off of waste matter ; the arteries and veins for the carriage of the oxygen and food-holding blood ; the legs or wings for locomotion. 66 ANIMAL LIFE The locomotive is an inorganic machine ; the animal is an organic machine. There is a great and real difference between an organism, a living animal, and a locomotive, an inorganic structure. But for a good understanding of the relation between function and structure, and of the com- position of the body of the complex animals, the compari- son of the animal and locomotive is very instructive. 41. The specialization of organs.— The organ for the per- formance of some definite function in one of the higher animals may be very complex. The corresponding organ in one of the lower animals for the performance of the same function may be comparatively simple. For example, the organ for the digestion of food is, in the case of the polyp, a simple cylindrical cavity in the body into which food enters through a large opening at the apical or free end of the body. The digestive organ of a cow is a long coiled tube, comprising many regions of distinct structural and physiological character and altogether extremely com- plicated. An organ in simple or primitive condition is said to be generalized; in complex or highly modified con- dition it is said to be specialized. That is, an organ may be modified and complexly developed to perform its func- tion in a special way, in a way differing in many particu- lars from the way the corresponding organ in some other animal performs the same general function. The speciali- zation of organs, or their modification to perform their functions in special ways, is what makes animal bodies complex, for specialization is almost always in the line of complexity. Later we shall see more clearly how specializa- tion is brought about. For the present we may study one of the more important organs of the animal body for the sake of having concrete examples of some of the gen- eral statements made in this discussion of function and structure. 42. The alimentary canal — The organ which has to do with the taking and digesting of food is called the ali- FUNCTION AND STRUCTURE mentary canal. In some of the higher animals this is a very complex organ. In the cow, one of the cud-chewing mammals or ruminants, it consists of several distinct por- tions, which differ among themselves very much (Fig. 30). First, there is the mouth, or opening for the entrance of the food. The mouth is sup- plied with teeth for tearing off and chewing the food, with a tongue for manipu- lating it, and with taste pa- pillae situated on the tongue and palate for determining the desirability of the food. Into the mouth a peculiar fluid (the saliva) is poured by certain glands, organs ac- cessory to the alimentary canal. The herbage bitten off, mixed with saliva, and rolled by the tongue into a ball, passes back through a narrow tube, the oesophagus, and into a sac called the ru- men, or paunch. Here it lies until the cow ceases for the while to take in food, when it passes back again through the oesophagus and into the mouth for mastica- tion. After being masticated it again passes downward through the oesophagus, and enters this time another sac called the reticulum, lying next to the rumen. From here it passes into another sac-like portion of the alimentary canal called the omasum, where it is strained throng1! numerous leaf-like folds which line the walls of this part of the canal. From here the food passes into a fourth FIG. 30.— Alimentary canal of the ox (after COLIN and MULLER). a, rumen (left hemsiphere) ; b, rumen (right hem- isphere) ; c, insertion of oesophagus ; d, reticulum ; 0, omasum ; f, abomasum ; g, duodenum ; h and i, jejunum and ileum ; j, caecum ; k, colon, with its various convolutions ; I, rectum. 68 ANIMAL LIFE sac-like part of the canal, called the abomasum. Here the process of digestion goes on. The four cacs — rumen, reticulum, omasum, and abomasum — are called stomachs, or they may be considered to be four chambers forming one large stomach. In the abomasum, or digesting stom- ach, digestive fluids are poured from glands lining its walls, and the food becomes converted into a liquid called chyle. The chyle passes from the stomach into a long, narrow, tubular portion of the canal called the intestine. The intestine is very long, and lies coiled in a large mass in the body of the cow. The intestine is divided into distinct regions, which- vary in size and in the character of the inner wall. These parts of the intestine have names, as duodenum, jejunum, ileum, caecum, colon, etc. Part of the intestine is lined inside with fine papillae, which take up the chyle (the digested food) and pass it through the walls of the intestine to other special organs, which pass it on to the blood, with which it becomes mixed and carried by an elaborate system of tubes to all parts of the body. Part of the grass taken into the alimentary canal by the cow can not be digested, and must be got rid of. This passes on into a final posterior part of the intes- tine called the rectum, and leaves the body through the anus or posterior opening of the alimentary canal. The whole canal is more than twenty times as long as the body of the cow ; it is composed of parts of different shape ; its walls are supplied with muscles and blood-vessels ; the inner lining is covered with folds, papillae, and gland cells. It is altogether a highly specialized organ, a structurally com- plex and elaborately functioning organ. Let us now examine the alimentary canal, or organ of digestion, in some of the simpler animals. The Protozoa, or simplest animals, have no special organ at all. When the surface of the body of an Amceba comes into contact with an organic particle which will serve as food, the surface becomes bent in at the point of its con- FUNCTION AND STEUCTURE 69 tact with the food particle, and the body substance simply incloses the food (Fig. 3). Food is taken in by the sur- face. The whole outer surface of the body is the food- taking organ. In the simplest many-celled animals, the sponges, there is no special food-taking and digestive organ. Each of the cells of the body takes in and assimilates food for itself. The sponge is like a great group of Amcebce holding fast to each other, but each looking out for its own necessities. Among the m polyps, however, there is a definite organ of digestion — that is, food is only taken and di- gested by certain parts of the body. The sim- ple polyp's body (Fig. 31) is a cylinder or vase closed at one end and open at the other end, and attached by the closed end to a rock. The opening is usually of less diameter than at s,~ the diameter body, and it rounded by a of tentacles, of FIG. 31. — Obeliasp.iS. simple polyp; vertical sec- tion, highly magnified, rn, mouth opening ; al. *., alimentary sac. — After PARKER and HASWELL. the is sur- number whose function it is to seize the food and convey it to the mouth opening. There are, of course, no teeth, no tongue, none of the various parts which are in or are part of the mouth of the higher animals. The polyp's mouth is simply a hole or opening into the inside of the body. This body cavity, or simplest of all stomachs, is simply the cylindrical or vase-shaped hollow space inclosed by the body wall. This space extends also into the tentacles. There is no other opening, no posterior or anal opening. We can not 6 70 ANIMAL LIFE speak of an oesophagus or intestine in connection with this most primitive of alimentary sacs. The cells which line the sacs show some differentiation ; some are gland cells and secrete digestive fluids; some are amoeboid and are provided with pseudopods or flagella for seizing bits of food. The food caught by the tentacles comes into the ali- mentary sac through the opening or primitive mouth, and .at C . 32.— Diagrammatic sketch of a flat- worm (Planaria), showing the branched alimentary canal, al. c.— After JIJIMA and HATSCHEK. FIG. as.— Sea-cucumber (Holothurian) dissected to show alimentary canal, al. c.— After LEUCKART. what of it is digestible is, by the aid of the gland cells and the amoeboid cells, taken up and assimilated, while the rest of it is carried out by water currents again through the single opening. In the flatworms (Fig. 32) like Planaria (small, thin, flattened worms to be found in the mud at the bottom of fresh-water ponds) the mouth opens into a short, narrow tube which may be called an oesophagus. The oesophagus FUNCTION AND STRUCTURE 71 connects the mouth with the rest of the alimentary canal, which gives out many side branches or diverticula, which are themselves branched, so that the alimentary sac or stomach is a system of ramifying tubes extending from a central main tube to all parts of the body of the worm. There is no anal opening. In the round or thread worms, of which the deadly Trichina is an example, the alimentary canal is a simple straight tube with both anterior or mouth opening and pos- terior or anal opening. In the sea- urchins and sea-cucumbers (Fig. 33) the alimentary canal is a simple tube with two openings, but it is longer than the body between mouth and anus, and so is more or less bent or coiled. In the earthworm the ali- mentary canal (Fig. 34), although a simple straight tube running through the body, plainly shows a differentia- tion into particular regions. Behind the mouth opening the alimentary tube is large and thick - walled and is called the pharynx; behind the pharynx it is narrower and is called the oesophagus. Behind the oesopha- gus it expands to form a rounded, thin-walled chamber called the crop, and just behind this there is another rounded but Verv thick-walled Cham- FH»- 34.-Earthworm dissected J . T -n ji to show alimentary canal, ber called the gizzard. From the ^ c> gizzard back the alimentary canal is about uniform in size, being rather wide and having thick, soft walls. This portion of it is called the intestine. The 72 ANIMAL LIFE posterior part of the intestine, called the rectum, leads to the anal opening. There is some differentiation of the inner surface of the canal. In the great group of mol- lusks, of which the common fresh-water clam or mussel is an example, the alimentary canal (Fig. 35) shows much variation. The microscopic plants, which are the food of the mussel, are taken in through the mouth and pass into a short oesophagus, thence into a wide stomach and there digested. Behind the stomach is a long, much-folded, nar- row intestine which winds about through the fleshy " foot " and finally reaches the surface of the body, and has an anal opening at a point opposite the position of the mouth. Among the insects there is a great range in degree of complexity of the alimentary canal. The digestive organs are, however, in most insects in a condition of high speciali- zation. The mouth opening is provided with well-developed ctl C. FIG. 35.— Pond mussel dissected to show alimentary canal, al. c.— After HATSCHEK and Com. biting and masticating or piercing and sucking mouth parts ; pharynx, oesophagus, stomach, and intestine are always dif- ferentiated and sometimes greatly modified. In the com- mon cockroach, for example (Fig. 36), the mouth has a complicated food-getting apparatus, and the canal, which FUNCTION AND STRUCTURE 73 is much longer than the body of the insect, and hence much bent and coiled, consists of a pharynx, oesophagus, fore-stomach or proventriculus, true digesting stomach or ven- triculus, intestine, and rectum which opens at the posterior tip of the body. The inner lining of the canal shows much differentiation in the different parts of the canal, and there are numerous accessory glands connected with various parts of the canal. Finally, among the highest animals, the vertebrates, we find still more elaborate special- ization of the alimentary canal. As an example the alimentary canal of a cow has already been described in detail. 43. Stable and variable char- acteristics of an organ. — In spite of all this variation in the structure and general character of the alimentary canal, there are certain characteristics which are features of all alimentary canals. In the examination of an organ we must ever distinguish between its so-called constant or stable characteristics and its inconstant or variable charac- teristics. The constant characteristics are the fundamen- tally essential ones of the organ ; the variable ones are the special characteristics which adapt the organ for the pecul- iar habits of the animal possessing it — habits which may differ very much from those of some other animal of similar size, similar distribution, similar abundance. 44. Stable and variable characteristics of the alimentary canal. — A tiger or a lion has an alimentary canal not more die FIG. 36.— Cockroach dissected to show alimentary canal, al. c.— After HAT- SCHEK and Com. 74 ANIMAL LIFE than three or four times the length of its body, while a sheep has an alimentary canal twenty-eight times as long as its body. The tiger is carnivorous; the sheep her- bivorous. Associated with the different food habits of the two animals is a striking difference in the alimentary canals. Animals like the horse or cat, which chew their food before swallowing it, have a slender oesophagus ; ani- mals like snakes which swallow their food whole have a wide oesophagus. Birds, that have no teeth and hence can not masticate or grind their food in their mouths, usu- ally have a special grinding stomach, the gizzard, for this purpose. And so we might cite innumerable examples of these inconstant or variable characteristics of the ali- mentary canal. On the other hand, the alimentary canals of all the many-celled animals except the lowest agree in certain important characteristics. Each alimentary canal has two openings, one for the ingress of food and one for the exit of the indigestible portions of the matter taken in, and the canal itself stretches through the body from mouth to anus as a tube, now narrow, now wide, now suddenly expanding into a sac or giving off lateral diverticula, but always simply a lumen or hollow inclosed by a flexible mus- cular wall. The inner lining of the wall is provided with secreting and absorbing structures. Indeed, we can reduce the essential characters of the alimentary canal to even more simple features. The organ of digestion or assimila- tion of all the many-celled animals is merely a surface with which food is brought into contact, and which has the power of digesting this food by means of digestive secre- tions, and of absorbing the food when digested. This sur- face is small or great in extent, depending upon the amount of food necessary to the life of the animal and the difficulty or readiness with which the food can be digested. This surface might just as well be on the outside of the animal's body as on the inside, if it were convenient. In fact, it is on the outside of some animals. Among the Protozoa the FUNCTION AND STRUCTURE Y5 digesting surface is simply the external surface of the body. And not alone among the one-celled animals. Many of the parasitic worms which live in the bodies of other animals, and the larvae or " grubs " of many insects which lie in the tissues of plants bathed by the sap, have no inner alimen- tary canal, but take food through the outer surface of the body. But in these cases the food is ready for immediate absorption, so that no special treatment of it is necessary, hence no complex structures are required. Even were no such special treatment of the food neces- sary in the case of the larger animals, it would still be im- Fio. 37.— Diagram illustrating increase of volume and surface with increase of diameter of sphere. possible for the simple external surface of the body to serve for food absorption, because of the well-known relation between the surface and the mass of a solid body. When a solid body in the form of a sphere increases in size, its mass or volume increases as the cube of the diameter, while the surface increases only as the square of the diameter (Fig. 37). The external surface of minute animals a few millimeters in diameter can take up enough food to supply the whole body mass. But among large animals this food- getting surface is increased as the square of the diameter of 76 ANIMAL LIFE the body, while the volume or food-using surface of the body is increased as the cube of its diameter. The food sup- plying can not keep pace with the food using. Hence it is absolutely essential that among large animals the food-tak- ing surface be increased so that it will remain in the same favorable proportion to the mass of the animal as is the case among the minute animals, where the simple external body surface is sufficient to obtain all the food necessary. This increase of surface, without an accompanying increase of size of the animal, is accomplished by having the digest- ing and assimilating surface inside the body and by having it greatly folded. The surface of the alimentary canal is, after all, simply a bent-in continuation of the outer surface of the body. It is open to the outside of the body by two openings, and wholly closed (except by its porosity) to the true inside of the body. By the bending and coiling of the alimentary canal, and by the repeated folding of its inner wall, the alimentary surface is greatly increased. The necessity for this increase accounts largely for the complexity of the alimentary canal. But it is not alone this necessity for increased surface that accounts for the great specialization of the alimentary canal in such animals as the insects and the vertebrates. The structural differences in different portions of the canal, resulting in the differentiation of the canal into distinct parts, or the differentiation of the whole organ into distinct subordinate organs, each with a special work or function to perform, are the result of the necessity for the special manipulation of the special kinds of foods taken. Animals which feed on other animals must have mouth structures fit for seizing and rending their prey, and the alimentary canal must be specially modified for the digestion of flesh. Animals which feed on vegetable substances must have special modifications of the alimentary canal quite different from those of the carnivores. Some insects, like the mos- quito, take only liquid food, the sap of plants, or the -blood FUNCTION AND STRUCTURE YY of animals ; others, like the weevils, feed on the hard, dry substance of seeds and grains ; others, like the grasshop- pers and caterpillars, eat green leaves ; and still others eat other insects. The alimentary canal of each of these kinds of insects differs more or less from that of the other kinds. The specialization of the alimentary canal depends then upon the necessity for a large food-digesting and absorbing surface, and on the complex treatment of the food. The character of this specialization in each case depends upon the special kind or quality of food taken by the animal in question. 45. The mutual relation of function and structure. — The structure of an animal depends upon the manner in which the life processes or functions of the animal are performed. If the functions are performed in a complex manner, the structure of the body is complex ; if the functions are per- formed in simple manner, the body will be simple in struc- ture. With the increase in degree of the division of labor among various parts of the body, there is an increase in definiteness and extent of differentiation of structure. Each part or organ of the body becomes more modified and better fitted to perform its own special function. A pecul- iar structural condition of any part of the body, or of the whole body of any animal, is not to be looked on as a freak of Nature, or as a wonder or marvel. Such a structure has a significance which may be sought for. The unusual structural condition is associated with some special habit or manner of performance of a function. Function and structure are always associated in Nature, and should always be associated in our study of Nature. CHAPTEE V THE LIFE CYCLE 46. Birth, growth and development, and death. — Certain phenomena are familiar to us as occurring inevitably in the life of every animal. Each individual is born in an imma- ture or young condition ; it grows (that is, it increases in size), and develops (that is, changes more or less in struc- ture), and dies. These phenomena occur in the succession of birth, growth and development, and death. But before any animal appears to us as an independent individual — that is, outside the body of the mother and outside of an egg (i. e., before birth or hatching, as we are accustomed to call such appearance) — it has already undergone a longer or shorter period of life. It has been a new living organ- ism hours or days or months, perhaps, before its appear- ance to us. This period of life has been passed inside an egg, or as an egg or in the egg stage, as it is variously termed. The life of an animal as a distinct organism be- gins in an egg. And the true life cycle of an organism is its life from egg through birth, growth and development, and maturity to the time it produces new organisms in the condition of eggs. The life cycle is from egg to egg. Birth and growth, two of the phenomena readily apparent to us in the, life of every animal, are two phenomena in the true life cycle. Death is a third inevitable phenomenon in the life of each individual, but it is not a part of the cycle. It is something outside. 47. Life cycle of simplest animals.— The simplest animals have no true egg stage, nor perhaps have they any true 78 THE LIFE CYCLE 79 -death. The new Amwbce are from their beginning like the full-grown Amo&ba, except as regards size. And the old Amoeba does not die, because its whole body continues to live, although in two parts — the two new Amc&bce. The life cycle of the simplest animals includes birth (usually by simple fission of the body of the parent), growth, and some, but usually very little, development, and finally the repro- duction of new individuals, "not by the formation of eggs, but by direct division of the body. 48. The egg. — In our study of the multiplication of ani- mals (Chapter III) we learned that it is the almost univer- FIG. 38. — Eggs of different animals showing variety in external appearance, a, egg of bird ; b, eggs of toad ; c, egg of fish ; d, egg of butterfly ; e, eggs of katydid on leaf ; /,egg-case of skate. sal rule among many-celled animals that each individual begins life as a single cell, which has been produced by the 80 ANIMAL LIFE fusion of two germ cells, a sperm cell from a male indi- vidual of the species and an egg cell from a female indi- vidual of the species. The single cell thus formed is called the fertilized egg cell, and its subsequent development results in the formation of a new individual of the same species with its parents. Now, in the development of this cell into a new animal, food is necessary, and sometimes a certain amount of warmth. So with the fertilized egg cell there is, in the case of all animals that lay eggs, a greater or less amount of food matter — food yolk, it is called — gath- ered about the germ cell, and both germ cell and food yolk are inclosed in a soft or hard wall. Thus is composed the egg as we know it. The hen's egg is as large as it is be- cause of the great amount of food yolk it contains. The egg of a fish as large as a hen is much smaller than the hen's egg j it contains less food yolk. Eggs (Fig. 38) may vary also in their external appearance, because of the dif- ferent kinds of membrane or shells which may inclose and protect them. Thus the frog's eggs are inclosed in a thin membrane and imbedded in a soft, jelly-like substance ; the skate's egg has a tough, dark-brown leathery inclosing wall ; the spiral egg of the bull-head sharks is leathery and colored like the dark-olive seaweeds among which it lies ; and a bird's egg has a hard shell of carbonate of lime. But in each case there is the essential fertilized germ cell ; in this the eggs of hen and fish and butterfly and cray-fish and worm are alike, however much they may differ in size and external appearance. 49. Embryonic and post-embryonic development. — Some animals do not lay eggs, that is they do not deposit the fer- tilized egg cell outside of the body, but allow the develop- ment of the new individual to go on inside the body of the mother for a longer or" shorter period. The mammals and some other animals have this habit. When such an ani- mal issues from the body of the mother, it is said to be born. When the developing animal issues from an egg THE LIFE CYCLE 81 which has been deposited outside the body of the mother, it is said to hatch. The animal at birth or at time of hatch- ing is not yet fully developed. Only part of its development or period of immaturity is passed within the egg or within the body of the mother. That part of its life thus passed within the egg or mother's body is called the embryonic life or embryonic stages of development ; while that period of development or immaturity from the time of birth or hatch- ing until maturity is reached is called the post-embryonic life or post-embryonic stages of development. 50. First stages in development. — The embryonic develop- ment is from the beginning up to a certain point practically identical for all many-celled animals — that is, there are cer- FIG. 39.— First stages in embryonic development of the pond snail (Lymnoeus). a, egg cell ; b, first cleavage ; p, second cleavage ; d, third cleavage ; e, after numer- ous cleavages ; f, blastula (in section) ; g, gastrula, just forming (in section) ; h, gastrula, completed (in section). — After RABL. tain principal or constant characteristics of the beginning development which are present in the development of all many-celled animals. The first stage or phenomenon of development is the simple fission of the germ cell into halves (Fig. 39, V). These two daughter cells next divide so that there are four cells (Fig. 39, c) ; each of these divides, and this division is repeated until a greater or lesser num- 7 S2 ANIMAL LIFE ber (varying with the various species or groups of animals) of cells is produced (Fig. 39, d). The phenomenon of re- peated division of the germ cell, and usually the surround- ing yolk, is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals. The first division of the germ cell produces usually two equal cells, but in some of the later divisions the new cells formed may not he equal. In some animals all the cleavage cells are of equal size ; in some there are two sizes of cells. The germ or embryo animal consists now of a mass of few or many undifferentiated primitive cells lying together and usually forming a sphere (Fig. 39, 0), or perhaps separated and scat- tered through the food yolk of the egg. The next stage of de- velopment is this : the cleavage cells arrange themselves so as to form a hollow sphere or ball, the cells lying side by side to form the outer circumferential wall of this hollow sphere (Fig. 39,/). This is called the Uastula or blastoderm stage of development, and the embryo itself is called the blastula or blastoderm. This stage also is common to -all the many- celled animals. The next stage in embryonic development is formed by the bending inward of a part of the blasto- derm cell layer, as shown in Fig. 39, g. This bending in may produce a small depression or groove ; but whatever the shape or extent of the sunken-in part of the blastoderm, it results in distinguishing the blastoderm layer into two parts, a sunken-in portion called the endollast and the other unmodified portion called the ectoUast. Endo- means " within," and the cells of the endoblast often push so far into the original blastoderm cavity as to come into contact with the cells of the ectoblast and thus obliterate this cavity (Fig. 39, h). This third well-marked stage in the embry- onic development is called the gastrula* stage, and it also * This gastrula stage is not always formed by a bending in or in- vagination of the blastoderm, but in some animals is formed by the splitting off or delamination of cells from a definite limited region of THE HFE CYCLE 83 occurs in the development of all or nearly all many-celled animals. 51. Continuity of development. — In the case of a few of the simple many-celled animals the embryo hatches — that is, issues from the egg at the time of or very soon after reaching the gasi^rula stage. In the higher animals, how- ever, development goes on within the egg or within the body of the mother until the embryo becomes a complex body, composed of many various tissues and organs. Al- most all the development may take place within the egg, FIG. 40. — Honey-bee, a, adult worker ; 6, young or larval worker. so that when the young animal hatches there is necessary little more than a rapid growth and increase of size to make it a fully developed, mature animal. This is the case with the birds : a chicken just hatched has most of the tissues and organs of a full-grown fowl, and is simply a little hen. But in the case of other animals the young hatches from the egg before it has reached such an ad- vanced stage of development ; a young star-fish or young crab or young honey-bee (Fig. 40) just hatched looks very different from its parent. It has yet a great deal of devel- opment to undergo before it reaches the structural condi- tion of a fully developed and fully grown star-fish or crab or bee. Thus the development of some animals is almost the blastoderm. Our knowledge of gastrulation and the gastrula stage is yet far from complete. 84 ANIMAL LIFE wholly embryonic development — that is, development with- in the egg or in the body of the mother — while the devel- opment of other animals is largely post-embryonic or larval development, as it is often called. There is no important difference between embryonic and post-embryonic develop- ment. The development is continue us from egg cell to mature animal, and whether inside or outside of an egg it goes on regularly and uninterruptedly. 52. Development after the gastrula stage. — The cells which compose the embryo in the cleavage stage and blastoderm stage, and even in the gastrula stage, are all similar ; there is little or no differentiation shown among them. But from the gastrula stage on development includes three important things : the gradual differentiation of cells into various kinds to form the various kinds of animal tissues ; the arrangement and grouping of these cells into organs and body parts ; and finally the developing of these organs and body parts into the special condition characteristic of the species of animal to which the developing individual belongs. From the primitive undifferentiated cells of the blastoderm, development leads to the special cell types of muscle tissue, of bone tissue, of nerve tissue ; and from the generalized condition of the embryo in its early stages de- velopment leads to the specialized condition of the body of the adult animal. Development is from the general to the special, as was said years ago by the first great student of development. 53. Divergence of development.— A star-fish, a beetle, a dove, and a horse are all alike in their beginning-— that is, the body of each is composed of a single cell, a single struc- tural unit. And they are all alike, or very much alike, through several stages of development ; the body of each is first a single cell, then a number of similar undifferen- tiated cells, and then a hollow sphere consisting of a single layer of similar undifferentiated cells. But soon in the course of development the embryos begin to differ, and as THE LIFE CYCLE 85 the young animals get further and further along in the course of their development, they become more and more different until each finally reaches its fully developed ma- ture form, showing all the great structural differences be- tween the star-fish and the dove, the beetle and the horse. That is, all animals begin development alike, but gradually diverge from each other during the course of development. There are some extremely interesting and significant things about this divergence to which attention should be given. While all animals are alike structurally* at the beginning of development, so far as we can see, they do not all differ at the time of the first divergence in development. This first divergence is only to be noted between two kinds of animals which belong to different great groups or classes. But two animals of different kinds, both belonging to some one great group, do not show differences until later in their development. This can best be understood by an example. All the butterflies and beetles and grasshoppers and flies belong to the great group of animals called Insecta, or in- sects. There are many different kinds of insects, and these kinds can be arranged in subordinate groups, such as the Diptera, or flies, the Lepidoptera, or butterflies and moths, and so on. But all have certain structural characteristics in common, so that they are comprised in one great group or class — the Insecta. Another great group of animals is known as the Vertebrata, or back-boned animals. The class Vertebrata includes the fishes, the batrachians, the reptiles, the birds, and the mammals, each composing a subordinate group, but all characterized by the possession of a back- * They are alike structurally, when we consider the cell as the unit of animal structure. That the egg cells of different animals may dif- fer in their fine or ultimate structure, seems certain. For each one of these egg cells is destined to become some one kind of animal, and no other ; each is, indeed, an individual in simplest, least developed con- dition of some one kind of animal, and we must believe that difference in kind of animals depends upon difference in structure in the egg itself. 7 86 ANIMAL LIFE .bone, or, more accurately speaking, of a notochord, a back- bone-like structure. Now, an insect and a vertebrate di- verge very soon in their development from each other ; but two insects, such as a beetle and a honey-bee, or any two vertebrates, such as a frog and a pigeon, do not diverge from each other so soon. That is, all vertebrate animals diverge in one direction from the other great groups, but all the members of the great group keep together for some time longer. Then the subordinate groups of the Verte- brata, such as the fishes, the birds, and the others diverge, and still later the different kinds of animals in each of these groups diverge from each other. In the illustration (Fig. 41) on the opposite page will be seen pictures of the embryos of various vertebrate animals shown as they appear at different stages or times in the course of development. The embryos of a fish, a salamander, a tortoise, a bird, and a mammal, representing the five principal groups of the Vertebrata, are shown. In the upper row the embryos are in the earliest of all the stages figured, and they are very much alike. They show no obvious characteristics of fish or bird. Yet there are distinctive characteristics of the great class Vertebrata. Any of these embryos could readily be distinguished from an embryonic insect or worm or sea-urchin. In the second row there is beginning to be manifest a divergence among the different embryos, al- though it would still be a difficult matter to distinguish certainly which was the young fish and which the young salamander, or which the young tortoise and which the young bird. In the bottom row, showing the animals in a later stage of development, the divergence has proceeded so far that it is now plain which is a fish, which batrachian, which reptile, which bird, and which mammal. 54. The laws or general facts of development.— That the course of development of any animal from its beginning to fully developed adult form is fixed and certain is readily seen. Every rabbit develops in the same way ; every grass- HI ffsJi ii H |^\ Sala. 777 an der ftaUtt FIG. 41. — Different vertebrate animal in successive embryonic stages. I, first or earliest of the stages figured ; II, second of the stages ; III, third or latest of the stages.— After HAECKEL. gg ANIMAL LIFE hopper goes through the same developmental changes from single egg cell to the full-grown active hopper as every other grasshopper of the same kind — that is, development takes place according to certain natural laws, the laws of animal development. These laws may be roughly stated as follows : All many-celled animals begin life as a single cell, the fertilized egg cell ; each animal goes through a certain orderly series of developmental changes which, accom- panied by growth, leads the animal to change from single cell to the many-celled, complex form characteristic of the species to which the animal belongs ; this development is from simple to complex structural condition ; the develop- ment is the same for all individuals of one species. While all animals begin development similarly, the course of devel- opment in the different groups soon diverges, the diver- gence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of ani- mals in their fully developed condition, all standing clearly apart from each other. But in tracing back the develop- ment of any kind of animal, we soon come to a point where it very much resembles or becomes apparently identical with some other kind of animal, and going further back we find it resembling other animals in their young condition, and so on until we come to that first stage of development, that trunk stage, where all animals are structurally alike. To be sure, any animal at any stage in its existence differs absolutely from any other kind of animal, in that it can develop into only its own kind of animal. There is some- thing inherent in each developing animal that gives it an identity of its own. Although in its young stages it may be hardly distinguishable from some other kind of animal in similar stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. The young fish and the young salamander in the upper row in Fig. 41 seem very much alike, but one embryo is sure to THE LIFE CYCLE 89 develop into a fish and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. 55. The significance of the facts of development. — The significance of the developmental phenomena is a matter about which naturalists have yet very much to learn. It is believed, however, by practically all naturalists that many of the various stages in the development of an animal cor- respond to or repeat the structural condition of the ani- mal's ancestors. Naturalists believe that all backboned or vertebrate animals are related to each other through being descended from a common ancestor, the first or oldest backboned animal. In fact, it is because all these back- boned animals — the fishes, the batrachians, the reptiles, the birds, and the mammals — have descended from a common ancestor that they all have a backbone. It is believed that the descendants of the first backboned animal have in the course of many generations branched off little by little from the original type until there have, come to exist very real and obvious differences among the backboned animals — differ- ences which among the living backboned animals are familiar to all of us. The course of development of an individual ani- mal is believed by many naturalists to be a very rapid, and evidently much condensed and changed, recapitulation of the history which the species or kind of animal to which the' developing individual belongs has passed through in the course of its descent through a long series of gradually chang- ing ancestors. If this is true, then we can readily under- stand why the fish and the salamander and tortoise and bird and rabbit are all so much alike in their earlier stages of development, and gradually come to differ more and more as they pass through later and later developmental stages. Some naturalists believe that the ontogenetic stages are not as significant in throwing light upon the evolutionary history of the species as just indicated. Some think that 90 ANIMAL LIFE when the earlier stages of one species correspond pretty closely with the early stages of another, we have a good basis for making up our minds about relationship between the two species. But it is certainly not obvious why we should have a similarity among the younger stages of dif- ferent animals and no correspondence among the older stages of more recent animals with the younger stages of more ancient ones. But on the other hand it is certainly true that a too specific application of the broad generaliza- tion that ontogeny repeats phylogeny has led to numerous errors of interpreting genealogic relationship. 56. Metamorphosis. — While a young robin when it hatches from the egg or a young kitten at birth resembles its par- ents, a young star-fish or a young crab or a young butterfly when hatched does not at all resemble its parents. And while the young robin after hatching becomes a fully grown robin simply by growing larger and undergoing compara- tively slight developmental changes, the young star-fish or young butterfly not only grows larger, but undergoes some very striking developmental changes; the body changes very much in appearance. Marked changes in the body of an animal during post-embryonic or larval development constitute what is called metamorphic development, or the animal is said to undergo or to show metamorphosis in its development. Metamorphosis is one of the most interest- ing features in the life history or development of animals, and it can be, at least as far as its external aspects are con- cerned, very readily observed and studied. 57. Metamorphosis among insects. — All the butterflies and moths show metamorphosis in their development. So do many other insects, as the ants, bees, and wasps, and all the flies and beetles. On the other hand, many insects do not show metamorphosis, but, like the birds, are hatched from the egg in a condition plainly resembling the parents. A grasshopper (Fig. 42) is a convenient example of an insect without metamorphosis, or rather, as there are, after all, THE LIFE CYCLE 91 a few easily perceived changes in its post-embryonic devel- opment, of an insect with an " incomplete metamorpho- sis." The eggs of grasshoppers are laid in little packets of several score half an inch below the surface of the ground. When the young grasshopper hatches from the egg it is of course very small, but it is plainly recognizable as a grasshopper. But in one important character it dif- fers from the adult, and that is in its lack of wings. The adult grasshopper has two pairs of wings ; the just hatched young or larval grasshopper has no wings at all. The young grasshopper feeds voraciously and grows rapidly. FIG. 42.— Post-embryonic development (incomplete metamorphosis) of the Rocky Mountain locust (Melanoplus spretus). a, b, c, d, e, and f, successive develop- mental stages from just hatched to adult individual.— After EMERTON. .In a few days it molts, or casts its outer skin (not the true skin, but a thin, firm covering or outer body wall com- posed of a substance called chitin, which is secreted by the cells of the true skin). In this second larval stage there can be seen the rudiments of four wings, in the condition of tiny wing pads on the back of the middle part of the body (the thorax). Soon the chitinous body covering is shed again, and after this molt the wing pads are mark- edly larger than before. Still another molt occurs, with another increase in size of the developing wings, and after a fifth and last molt the wings are fully developed, and 92 ANIMAL LIFE the grasshopper is no longer in a larval or immature condi- tion, but is full grown and adult. For example of complete metamorphosis among insects we may choose a butterfly, the large red-brown butterfly PIG. 43.— Metamorphosis of monarch butterfly (Anosia plexippm). a, egg ; b, larva ; c, pupa ; d, imago or adult. common in the United States and called the monarch or milkweed butterfly (Anosia plexippus). The eggs (Fig. 43, a) of this butterfly are laid on the leaves of various kinds of milkweed (Asclepias). The larval butterfly or butterfly larva or caterpillar (as the first young stage of the butter- THE LIFE CYCLE 93 flies and moths is usually called), which hatches from the egg in three or four days, is a creature bearing little or no resemblance to the beautiful winged adult. The larva is worm-like, and instead of having three pairs of legs like the butterfly it has eight pairs; it has biting jaws in its mouth with which it nips off bits of the green milk- weed leaves, instead of having a long, slender, sucking proboscis for drinking flower nectar as the butterfly has. The body of the crawl- ing worm -like larva (Fig. 43, #) is greenish yellow in color, with broad rings or bands of shining black. It has no wings, of course. It eats voraciously, grows rapidly and molts. But after the molting there is no appearance of rudimentary wings; it is simply a larger worm- like larva. It continues to feed and grow, molt- ing several times, until after the fourth molt it appears no longer as an active, crawling, feed- ing, worm-like larva, but as a quiescent, non-feeding pupa or chrysalis (Fig. 43, c). The immature butterfly is now greatly contracted, and the outer chitinous wall is very thick and firm. It is bright green in color with golden dots. It is fastened by one end to a leaf of the milkweed, where it hangs immovable for from a few days to two weeks. Finally, the chitin wall of the chrysalis splits, and there issues the full-fledged, great, four-winged, red-brown butter- fly (Fig. 43, d). Truly this is a metamorphosis, and a start- FIQ. 44.— Metamorphosis of mosquito (Culex). a, larva ; 6, pupa. ANIMAL LIFE ling one. But we know that development in other animals is a gradual and continuous process, and so it is in the case of the butterfly. The gradual chang- ing is masked by the outer covering of the body in both larva and pupa. It is only at each molting or throwing off of this unchanging, unyield- ing chitin armor that we perceive how far this change has gone. The longest time of concealment is that during the pupal or chrysalis stage, and the results of the changing or develop- ment when finally re- vealed by the split- ting of the pupal •case are hence the most striking. 58. Metamorphosis of the toad. — Metamorphosis is found in the development of numerous other animals, as well as among the insects. Certain cases are familiar to all — the metamorphosis of the frogs and toads (Fig. 46). The eggs of the toad are arranged in long strings or ribbons in a transparent jelly-like substance. These jelly ribbons with the small, black, bead-like eggs in them are wound around the stems of submerged plants or sticks near the shores of the pond. From each egg hatches a tiny, wriggling tad- pole, differing nearly as much from a full-grown toad as a caterpillar differs from a butterfly. The tadpoles feed on FIG. 45. — Larva of a butterfly just changing into pupa (making last larval molt). Photograph from Nature. THE LIFE CYCLE 95 the microscopic plants to be found in the water, and swim easily about by means of the long tail. The very young tadpoles remain underneath the surface of the water all the time, breathing the air which is mixed with water by means of gills. But as they become older and larger they come often to the surface of the water. Lungs are developing inside the body, and the tadpole is beginning to breathe as a land animal, although it still breathes partly by means of gills, that is, as an aquatic animal. Soon it is apparent that although the tadpole is steadily and rapidly growing larger, its tail is growing shorter and smaller instead of larger. At the same time, fore and hind legs bud out and rapidly take FIG. 46.— Metamorphosis of the toad (partly after GASE). At left the strings of eggs, in water the various tadpole or larval stages, and on bank the adult toads. form and become functional. By the time that the tail gets very short, indeed, the young toad is ready to leave the water and live as a land animal. On land the toad lives, as 96 ANIMAL LIFE we know, on insects and snails and worms. The metamor- phosis of the toad is not so striking as that of the butter- fly, but if the tadpole were inclosed in an unchanging opaque body wall while it was losing its tail and getting its legs, and this wall were to be shed after these changes were made, would not the metamorphosis be nearly as extraordi- Fie. 47.— Metamorphosis of sea- urchin. Upper figure the adult, lower figure the pluteus larva. nary as in the case of the butterfly? But in the metamorphosis of the toad we can see the gradual and continuous character of the change. 59. Metamorphosis among other animals. — Many other animals, besides insects and frogs and toads, undergo meta- morphosis. The just-hatched sea-urchin does not resemble a fully developed sea-urchin at all. It is a minute worm- like creature, provided with cilia or vibratile hairs, by -means of which it swims freely about. It changes next into a curi- ous bootjack-shaped body called the pluteus stage (Fig, 47). In the pluteus a skeleton of lime is formed, and the final true sea-urchin body begins to appear inside the pluteus, THE LIFE CYCLE 97 developing and growing by using up the body substance of the pluteus. Star-fishes, which are closely related to sea- urchins, show a simi- lar metamorphosis, except that there is no pluteus stage, the true star-fish-shaped body forming, with- in and at the expense of the first larval stage, the ciliated free-swimming stage. A young crab just issued from the egg (Fig. 48) is a very different appearing creature from the adult or fully devel- oped crab. The body of the crab in its first larval stage is Composed of a short, FlG' ^-Metamorphosis of the crab. globular portion, fur- nished with conspicuous long spines and a relatively long, jointed tail. This is called the zoea stage. The zoe'a changes into a stage called the megalops, which has many characteristics of the adult crab condition, but differs espe- cially from it in the possession of a long, segmented tail, and in having the front half of the body longer than wide. The crab in the megalops stage looks very much like a tiny lobster or shrimp. The tail soon disappears and the body widens, and the final stage is reached. In many families of fishes the changes which take place in the course of the-li^e cycle are almost as great as in the case of the insect or the toad. In the lady-fish (Albula vulpes) the very young (Fig. 49) are ribbon-like in form, 8 the zoe'a b, the megalops ; c, the adult. 98 ANIMAL LIFE with small heads and very loose texture of the tissues, the body substance being jelly-like and transparent. As the fish grows older the body becomes more compact, and therefore rf/7777?f~/T77ff77T:: . '/ • • • .'/.•' •' .•' ••//••/ : l .;' • FIG. 49. — Stages in the post-embryonic development of the lady-fish (Albula vulpes), showing metamorphosis. —After C. H. GILBERT. shorter and slimmer. After, shrinking to the texture of an ordinary fish, its growth in size begins normally, although THE LIFE CYCLE 99 it has steadily increased in actual weight. Many herring, eels, and other soft-bodied fishes pass through stages simi- lar to those seen in the lady-fish. Another type of devel- opment is illustrated in the sword-fish. The young has a bony head, bristling with spines. As it grows older the spines disappear, the skin grows smoother, and, finally, the bones of the upper jaw grow together, forming a prolonged sword, the teeth are lost and the fins become greatly modi- fied. Fig. 50 shows three of these stages of growth. The a FIG. 50.— Three stages in the development of the sword-fish (Xiphias gladiti-s). a, very young ; b, older ; c, adult.— Partly after LUTKBN. flounder or flat-fish (Fig. 51) when full grown lies flat on one side when swimming or when resting in the sand on the bottom of the sea. The eyes are both on the upper side of the body, and the lower side is blind and colorless. When the flounder is hatched it is a transparent fish, broad and flat, swimming vertically in the water, with an eye on each side. As its development (Fig. 52) goes on it rests itself obliquely on the bottom, the eye of the lower side turns upward, and as growth proceeds it passes gradually 100 ANIMAL LIFE around the forehead, its socket moving with it, until both eyes and sockets are transferred by twisting of the skull to PIG. 51.— The wide-eyed flounder (Platophrys lunatus). Adult, showing both eyes on upper side of head. the upper side. In some related forms or soles the small eye passes through the head and not around it, appearing finally in the same socket with the other eye. Thus in almost all the great groups of animals we find certain kinds which show metamorphosis in their post- embryonic development. But metamorphosis is simply development; its striking and extraordinary features are usually due to the fact that the orderly, gradual course of the development is revealed to us only occasionally, with the result of giving the impression that the development is proceeding by leaps and bounds from one strange stage to FIG. 52.— Development of a flounder (after EMERY). The eyes in the young flounder are arranged normally, one on each side of head. another. If metamorphosis is carefully studied it loses its aspect of marvel, although never its great interest. THE LIFE CYCLED 60. Duration of life. — After an animal has completed its development it has but one thing to do to complete its life cycle, and that is the production of offspring. When it has laid eggs or given birth to young, it has insured the beginning of a new life cycle. Does it now die ? Is the business of its life accomplished ? There are many animals which die immediately or very soon after laying eggs. The May-flies — ephemeral insects which issue as winged adults from ponds or lakes in which they have spent from one to three years as aquatic crawl- ing or swimming larvae, flutter about for an evening, mate, drop their packets of fertil- ized eggs into the water, and die before the sunrise — are extreme examples of the nu- merous kinds of animals whose adult life lasts only long enough for mating and egg- laying. But elephants live for two hundred years. Whales probably live longer. A horse lives about thirty years, and so may a cat or toad. A sea- anemone, which was kept in an aquarium, lived sixty-six years. Cray-fishes may live twenty years. A queen bee was kept in captivity for fifteen years. Most birds have long lives — the small song birds from eight to eighteen years, and the great eagles and vultures up to a hundred years or more. On the other hand, among all the thou- sands of species of insects, the individuals of very few in- deed live more than a year ; the adult life of most insects being but a few days or weeks, or at best months. Even among the higher animals, some are very short-lived. In Japan is a small fish (Solaux) which probably lives ** • V *: t .ct : \ ^.v ANIMAL LIFE but a year, ascending the rivers in numbers when young in the spring, the whole mass of individuals dying in the fall after spawning. Naturalists have sought to discover the reason for these extraordinary differences in the duration of life of different animals, and while it can not be said that the reason or reasons are wholly known, yet the probability is strong that the duration of life is closely connected with, or dependent upon, the conditions attending the production of offspring. It is not sufficient, as we have learned from our study of the multiplication of animals (Chapter III), that an adult animal shall produce simply a single new individual of its kind, or even only a few. It must produce many, or if it produces comparatively few it must devote great care to the rearing of these few, if the perpetuation of the species is to be insured. Now, almost all long-lived animals are species which produce but few offspring at a time, and reproduce only at long intervals, while most short-lived ani- mals produce a great many eggs, and these all at one time. Birds are long-lived animals; as we know, most of them lay eggs but once a year, and lay only a few eggs each time. Many of the sea birds which swarm in countless numbers on the rocky ocean islets and great sea cliffs lay only a single egg once each year. And these birds, the guillemots and murres and auks, are especially long-lived. Insects, on the contrary, usually produce many eggs, and all of them in a short time. The May-fly, with its one evening's lifetime, lets fall from its body two packets of eggs and then dies. Thus the shortening of the period of reproduction with the production of a great many offspring seem to be always associated with a short adult lifetime ; while a long period of reproduction with the production of few offspring at a time and care of the offspring are associated with a long adult lifetime. There seems also to be some relation between the size of animals and the length of life. As a general rule, THE LIFE CYCLE 103 large animals are long-lived and small animals have short lives. 61. Death. — At the end comes death. After the animal has completed its life cycle, after it has done its share toward insuring the perpetuation of its species, it dies. It may meet a violent death, may be killed by accident or by ene- mies, before the life cycle is completed. And this is the fate of the vast majority of animals which are born or hatched. Or death may come before the time for birth or hatching. Of the millions of eggs laid by a fish, each egg a new fish in simplest stage of development, how many or rather how few come to maturity, how few complete the cycle of life ! Of death we know the essential meaning. Life ceases and can never be renewed in the body of the dead animal. It is important that we include the words " can never be renewed," for to say simply that " life ceases," that is, that the performance of the life processes or functions ceases, is not really death. It is easy to distinguish in most cases between life and death, between a live animal and a dead one, yet there are cases of apparent death or a semblance of death which are very puzzling. The test of life is usually taken to be the performance of life functions, the assimila- tion of food and excretion of waste, the breathing in of oxy- gen, and breathing out of carbonic-acid gas, movement, feeling, etc. But some animals can actually suspend all of these functions, or at least reduce them to such a mini- mum that they can not be perceived by the strictest exami- nation, and yet not be dead. That is, they can renew again the performance of the life processes. Bears and some other animals, among them many insects, spend the winter in a state of death-like sleep. Perhaps it is but sleep ; and yet hibernating insects can be frozen solid and remain frozen for weeks and months, and still retain the power of actively living again in the following spring. Even more remarkable is the case of certain minute animals called Ro- 104 ANIMAL LIFE tatoria and of others called Tardigrada, or bear-animalcules. These bear-animalcules live in water. If the water dries up, the animalcules dry up too ; they shrivel up into form- less little masses and become desiccated. They are thus simply dried-up bits of organic matter; they are organic dust. Now, if after a long time — years even — one of these organic dust particles, one of these dried-up bear-animal- cules is put into water, a strange thing happens. The body swells and stretches out, the skin becomes smooth instead of all wrinkled and folded, and the legs appear in normal shape. The body is again as it was years before, and after a quarter of an hour to several hours (depending on the length of time the animal has lain dormant and dried) slow movements of the body parts begin, and soon the animal- cule crawls about, begins again its life where it had been interrupted. Various other small animals, such as vinegar eels and certain Protozoa, show similar powers. Certainly here is an interesting problem in life and death. When death comes to one of the animals with which we are familiar, we are accustomed to think of its coming to the whole body at some exact moment of time. As we stand beside a pet which has been fatally injured, we wait until suddenly we say, " It is dead." As a matter of fact, it is difficult to say when death occurs. Long after the heart ceases to beat, other organs of the body are alive — that is, are able to perform their special functions. The muscles can contract for minutes or hours (for a short time in warm-blooded, for a long time in cold-blooded animals) after the animal ceases to breathe and its heart to beat. Even longer live certain cells of the body, especially the amoeboid white blood-corpuscles. These cells, very like the Amoeba in character, live for days after the animal is, as we say, dead. The cells which line the tracheal tube leading to the lungs bear cilia or fine hairs which they wave back and forth. They continue this movement for days after the heart has ceased beating. Among cold- THE LIFE CYCLE 105 blooded animals, like snakes and turtles, complete cessa- tion of life functions comes very slowly, even after the body has been literally cut to pieces. Thus it is essential in defining death to speak of a complete and permanent cessation of the performance of the life processes. A grasshopper (Melanoplus differentialis) killed by disease caused by a parasitic fungus. On golden-rod. CHAPTER VI THE PRIMARY CONDITIONS OF ANIMAL LIFE 62. Primary conditions and special conditions. — Certain primary conditions are necessary for the existence of all animals. We know that fishes can not live very long out of water, and that birds can not live in water. These, however, are special conditions which depend on the spe- cial structure and habits of these two particular kinds of backboned animals. But the necessity of a constant and sufficient supply of air is a necessity common to both ; it is one of the primary conditions of their life. All animals must have air. Similarly both fishes and birds, and all other animals as well, must have food. This is another one of the primary conditions of animal life. That backboned animals must find somehow a supply of salts or compounds of lime to form into bones is a special condition peculiar to these animals. Other animals having shells or teeth composed of carbonate or phosphate of lime are subject to the same special demand, but many animals have no hard parts, and therefore need no lime. 63. Food. — All the higher plants, those that are green (chlorophyll-bearing), can make their living substance out of inorganic matter alone — that is, use inorganic substances as food. But animals can not do this. They must have already formed organic matter for food. This organic mat- ter may be the living or dead tissues of plants, or the living or dead tissues of animals. For the life of animals it is necessary that other organisms live, or have lived. It is this need which primarily distinguishes an animal from a THE PRIMARY CONDITIONS OF ANIMAL LIFE 107 plant. Animals can not exist without plants. The plants furnish all animals with food, either directly or indirectly. The amount of food and the kinds of food required by various kinds of animals are special conditions depending on the size, the degree of activity, the structural character of the body, etc., of the animal in question. Those which do the most need most. Those with warmest blood, great- est activity, and most rapid change of tissues are most dependent on abundance, regularity, and fitness of their food. As we well know, an animal can live for a longer or shorter time without food. Men have fasted for a month, or even two months. Among cold-blooded animals, like the reptiles, the general habit of food taking is that of an occa- sional gorging, succeeded by a long period of abstinence. Many of the lower animals can go without food for surpris- ingly long periods without loss of life. But the continued lack of food results inevitably in death. Any animal may be starved in time. If water be held not to be included in the general con- ception of food, then special mention must be made of the necessity of water as one of the primary conditions of ani- mal life. Protoplasm, the basis of life, is a fluid, although thick and viscous. To be fluid its components must be dissolved or suspended in water. In fact, all the truly living substance in an animal's body contains water. The water necessary for the animal may be derived from the other food, all of which contains water in greater or less quantity, or may be taken apart from the other food, by drinking or by absorption through the skin. Sheep are seldom seen to drink, for they find .almost enough water in their green food. Fur seals never drink, for they absorb the water needed through pores in the skin. 64. Oxygen. — Animals must have air in order to live, but the essential element of the air which they need is its oxygen. For the metabolism of the body, for the chemical 108 ANIMAL LIFE changes which take place in the body of every living ani- mal, a supply of oxygen is required. This oxygen is de- rived directly or indirectly from the air. The atmosphere of the earth is composed of 79.02 parts of nitrogen (includ- ing argon), .03 parts of carbonic acid, and 20.95 parts of oxygen. Thus all the animals which live on land are en- veloped by a substance containing nearly 21 per cent of oxygen. But animals can live in an atmosphere containing much less oxygen. Certain mammals, experimented on, lived without difficulty in an atmosphere containing only 14 per cent of oxygen ; when the oxygen was reduced to 7 per cent serious disturbances were caused in the animal's condition, and death by suffocation ensued when 3 per cent of oxygen was left in the atmosphere. Animals which live in water get their oxygen, not from the water itself (water being composed of hydrogen and oxygen), but from air which is mechanically mixed with the water. Fishes breathe the air which is mixed with or dissolved in the water. This scanty supply therefore constitutes their at- mosphere, for in water from which all air is excluded no animal can breathe. Whatever the habits of life of the animal, whether it lives on the land, in the ground, or in the water, it must have oxygen or die. 6,5. Temperature, pressure, and other conditions. — Some physiologists include among the primary or essential gen- eral conditions of animal life such conditions as favorable temperature and favorable pressure. It is known from ob- servation and experiment that animals die when a too low or a too high temperature prevails. The minimum or maximum of temperature between which limits an animal can live varies much among different kinds of animals. It is familiar knowledge that many kinds of animals can be frozen and yet not be killed. Insects and other small ani- mals may lie frozen through a winter and resume active life again in the spring. An experimenter kept certain fish frozen in blocks of ice at a temperature of —15° C. THE PRIMARY CONDITIONS OF ANIMAL LIFE 109 for some time and then gradually thawed them out un- hurt. Only very hardy kinds adapted to the cold would, however, survive such treatment. There is no doubt that every part of the body, all of the living substance, of these fish was frozen, for specimens at this temperature could be broken and pounded up into fine ice powder. But a tem- perature of —20° C. killed the fish. Frogs lived after being kept at a temperature of —28° C., centipeds at —50° C., and certain snails endured a temperature of — 120° C. without dying. At the other extreme, instances are known of ani- mals living in water (hot springs or water gradually heated with the organisms in it) of a temperature as high as 50° C. Experiments with Amcebce show that these simplest animals contract and cease active motion at 35° C., but are not killed until a temperature of 40° to 45° C. is reached. The little fish called blob or miller's thumb ( Coitus ictalops) has been seen lying boiled in the bottom of the hot springs in the Yellowstone Park ; but it must have entered these springs through streams of a temperature little below the boiling point. The pressure or weight of the atmosphere on the sur- face of the earth is nearly fifteen pounds on each square inch. This pressure is exerted equally in all directions, so that an object on the earth's surface sustains a pressure on each square inch of its surface exposed to the air of fifteen pounds. Thus all animals living on the earth's surface or near it live under this pressure, and know no other condi- tion. For this reason they do not notice it. The animals that live in water, however, sustain a much greater pres- sure, this pressure increasing with the depth. Certain ocean fishes live habitually at great depths, as two to five miles, where the pressure is equivalent to that of many hundred atmospheres. If these fishes are brought to the surface their eyes bulge out fearfully, being pushed out through reduced expansion ; their scales fall off because of the great expansion of the skin, and the stomach is pushed 110 ANIMAL LIFE out from the mouth till it is wrong side out. Indeed, the bodies sometimes burst. Their bodies are accustomed to this great pressure, and when this outside pressure is sud- denly removed the body may be bursted. Sometimes such a fish is raised from its proper level by a struggle with its prey, when both captor and victim may be de- stroyed by the expansion of the body. Some fishes die on being taken out of water through the swelling of the air bladder and the bursting of its blood-vessels. If an animal which lives normally on the surface of the earth is taken up a very high mountain or is carried up in a balloon to a great altitude where the pressure of the atmosphere is much less than it is at the earth's surface, serious conse- quences may ensue, and if too high an altitude is reached death occurs. This death may be in part due to the diffi- culty in breathing in sufficient oxygen to maintain life, but it is probably chiefly due to disturbances caused by the removal of the pressure to which the body is accustomed and is structurally adapted to withstand. A famous bal- loon ascension was made in Paris in 1875 by three men. After the balloon had reached a height of nearly 24,000 feet (almost five miles) the men began to lose conscious- ness. On the sinking of the balloon to about 20,000 feet the men regained consciousness again and threw out bal- last so that the balloon rose to a height of over 25,000 feet. This time all three became wholly unconscious, and on the balloon sinking again only one regained consciousness. The other two died in the foolhardy experiment. All liv- ing animals are accustomed to live under a certain pres- sure, and there are evidently limits of maximum or mini- mum pressure beyond which no animal at present existing can go and remain alive. But in the case both of temperature and pressure con- ditions it is easy to conceive that animals might exist which could live under temperature and pressure conditions not included between the minimum and maximum limits of each THE PRIMARY CONDITIONS OF ANIMAL LIFE as determined by animals so existing. But it is impossible to conceive of animals which could live without oxygen or without organic food. The necessities of oxygen and organic food (and water) are the primary or essential conditions for the existence of any animals. Of course, we might include such conditions, among the primary conditions, as the light and heat of the sun, the action of gravitation, and olher physical conditions without which existence or life of any kind would be im- possible on this earth. But we here consider by " primary conditions of animal life " rather those necessities of living animals as opposed to the necessities of living plants. Neither animals nor plants could exist without the sun, whence they derive directly or indirectly all their energy. 66. Difference between animals and plants. — It is easy to distinguish between the animal and plant when a butterfly is fluttering about a blossoming cherry tree or a cow feed- ing in a field of clover. It is not so easy, if it is, indeed, possible, to say which is plant and which is animal when the simplest plants are compared with the simplest ani- mals. It is almost impossible to so define animals as to distinguish all of them from all plants, or so to define plants as to distinguish all of them from all animals. While most animals have the power of locomotion, some, like the sponges and polyps and barnacles and numerous parasites, are fixed. While most plants are fixed, some of the low aquatic forms have the po sver of spontaneous loco- motion, and all plants have some power of motion, as espe- cially exemplified in the revolution of the apex of the growing stem and root, and the spiral twisting of tendrils, and in the sudden closing of the leaves of the sensitive plant when touched. Among the green or chlorophyll- bearing plants the food consists chiefly of inorganic sub- stances, especially of carbon which is taken from the car- bonic-acid gas in the atmosphere, and of water. But some green-leaved plants feed also in part on organic food. 112 ANIMAL LIFE Such are the pitcher-plants and sun-dews, and Venus-fly- traps, which catch insects and use them for food nutrition. But there are many plants, the fungi, which are not green — that is, which do not possess chlorophyll, the substance on which seems to depend the power to make organic matter out of inorganic substances. These plants feed on organic matter as animals do. The cells of plants (in their young stages, at least) have a wall composed of a peculiar carbohydrate substance called cellulose, and this cellulose was for a long time believed not to occur in the body of animals. But now 'it is known that certain sea-squirts (Tunicata) possess cellulose. It is impossible to find any set of characteristics, or even any one characteristic, which is possessed only by plants or only by animals. But nearly all of the many-celled plants and animals may be easily distinguished by their general characteristics. The power of breaking up carbonic-acid gas into carbon and oxygen and assimilating the carbon thus obtained, the presence of chlorophyll, and the cell walls formed of cellulose, are char- acteristics constant in all typical plants. In addition, the fixed life of plants, and their general use of inorganic sub- stances for food instead of organic, are characteristics readily observed and practically characteristic of many- celled plants. When the thousands of kinds of one-celled organisms are compared, however, it is often a matter of great difficulty or of real impossibility to say whether a given organism should be assigned to the plant kingdom or to the animal kingdom. In general the distinctive characters of plants are grouped around the loss of the power of locomotion and related to or dependent upon it. 67. Living organic matter and inorganic matter. — It would seem to be an easy matter to distinguish an organism — that is, a living animal or plant— from an inorganic substance. It is easy to distinguish a dove or a sunflower from stone, and practically there never is any difficulty in making such dis- tinctions. But when we try to define living organic matter, THE PRIMARY CONDITIONS OF ANIMAL LIFE H3 and to describe those characteristics which are peculiar to it, which absolutely distinguish it from inorganic matter, we meet with some difficulties. At least many of the char- acteristics commonly ascribed to organisms, as peculiar to them, are not so. The possession of organs, or the composi- tion of the body of distinct parts, each with a distinct func- tion, but all working together, and depending on each other, is as true of a steam-engine as of a horse. That the work done by the steam-engine depends upon fuel is true ; but so it is that the work done by the horse depends upon fuel, or food as we call it in the case of the animal. The oxida- tion or burning of this fuel in the engine is wholly compar- able with the oxidation of the food, or the muscle and fat it is turned into, in the horse's body. The composition of the bodies of animals and plants of tiny structural units, the cells, is in many ways comparable with the composition of some rocks of tiny structural units, the crystals. But not to carry such rather quibbling comparisons too far, it may be said that organisms are distinguished from organic substances by the following characteristics : Organization ; the power to make over inorganic substances into organic matter, or the changing of organic matter of one kind, as plant matter, into another kind, as animal matter ; motion, the power of spontaneous movement in response to stimuli ; sensation, the power of being sensible of external stimuli ; reproduction, the power of producing new beings like them- selves ; and adaptation, the power of responding to external conditions in a way useful to the organism. Through adap- tation organisms continue to exist despite the changing of conditions. If the conditions surrounding an inorganic body change, even gradually, the inorganic body does not change to adapt itself to these conditions, but resists them until no longer able to do so, when it loses its identity or integrity. CHAPTEE VII THE CROWD OF ANIMALS AND THE STRUGGLE FOR EXISTENCE 68. The crowd of animals. — All animals feed upon living organisms, or on their dead bodies. Hence each animal throughout its life is busy with the destruction of other organisms, or with their removal after death. If those creatures upon which others feed are to hold their own, there must be enough born or developed to make good the drain upon their numbers. If the plants did not fill up their ranks and make good their losses, the animals that feed on them would perish. If the plant-eating animals were destroyed, the flesh-eating animals would in turn disappear. But, fortunately, there is a vast excess in the process of reproduction. More plants sprout than can find room to grow. More animals are born than can possibly survive. The process of increase among animals is correctly spoken of as multiplication. Each species tends to increase in geometric ratio, but as it multiplies its members it finds the world already crowded with other species doing the same thing. A single pair of any species whatsoever, if not restrained by adverse conditions, would soon increase to such an extent as to fill the whole world with its progeny. An annual plant producing two seeds only would have 1,048,576 descendants at the end of twenty-one years, if each seed sprouted and matured. The ratio of increase is therefore a matter of minor importance. It is the ratio of net increase above loss which determines the fate of a spe- cies. Those species increase in numbers whose gain exceeds 114 THE STRUGGLE FOR EXISTENCE H5 the death rate, and those which " live beyond their means " must sooner or later disappear. One of the most abundant of birds is the fulmar petrel, which lays but one egg yearly. It has but few enemies, and this low rate of increase suf- fices to cover the seas within its range with petrels. It is difficult to realize the inordinate numbers in which each species would exist were it not for the checks produced by the presence of other animals. Certain Protozoa at their normal rate of increase, if none were devoured or destroyed, might fill the entire ocean in about a week. The conger- eel lays, it is said, 15,000,000 eggs. If each egg grew up to maturity and reproduced itself in the same way in less than ten years the sea would be solidly full of conger- eels. If the eggs of a common house-fly should develop, and each of its progeny should find the food and temperature it needed, with no loss and no destruction, the people of a city in which this might happen could not get away soon enough to escape suffocation from a plague of flies. Whenever any in- sect is able to develop a large percentage of the eggs laid, it becomes at once a plague. Thus originate plagues of grass- hoppers, locusts, and caterpillars. But the crowd of life is such that no great danger exists. The scavenger destroys the decaying flesh where the fly would lay its eggs. Minute creatures, insects, bacteria, Protozoa are parasitic within the larva and kill it. Millions of flies perish for want of food. Millions more are destroyed by insectivorous birds, and millions are slain by parasites. The final result is that from year to year the number of flies does not increase. Linnaeus once said that " three flies would devour a dead horse as quickly as a lion." Equally soon would it be de- voured by three bacteria, for the decay of the horse is due to the decomposition of its flesh by these microscopic plants which feed upon it. " Even slow-breeding man," says Dar- win, " has doubled in twenty-five years. At this rate in less than a thousand years there would literally not be standing room for his progeny. The elephant is reckoned the slow- 116 ANIMAL LIFE est breeder of all known animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval, and surviving till a hundred years old. If this be so, after about eight hundred years there would be 19,000,000 elephants alive, descended from the first pair." A few years more of the unchecked multiplication of the elephant and every foot of land on the earth would be covered by them. Yet the number of elephants does not increase. In gen- eral, the numbers of every species of animal in the state of Nature remain about stationary. Under the influence of man most of them slowly diminish. There are about as many squirrels in the forest one year as another, about as many butterflies in the field, about as many frogs in the pond. Wolves, bears, deer, wild ducks, singing birds, fishes, tend to grow fewer and fewer in inhabited regions, because the losses from the hand of man are added to the losses in the state of Nature. It has been shown that at the normal rate in increase of English sparrows, if none were to die save of old age, it would take but twenty years to give one sparrow to every square inch in the State of Indiana. Such an increase is actually impossible, for more than a hundred other species of similar birds are disputing the same territory with the power of increase at a similar rate. There can not be food and space for all. With such conditions a struggle is set up between sparrow and sparrow, between sparrow and other birds, and between sparrow and the conditions of life. Such a conflict is known as the struggle for existence. 69. The struggle for existence.— The struggle for exist- ence is threefold: (a) among individuals of one species, as sparrow and sparrow ; ( #) between individuals of differ- ent species, as sparrow with bluebird or robin ; and (c) with the conditions of life, as the effort of the sparrow to keep warm in winter and to find water in summer. All three forms of this struggle are constantly operative and with THE STRUGGLE FOR EXISTENCE every species. In some regions the one phase may be more destructive, in others another. Where the conditions of life are most easy, as in the tropics, the struggle of species with species, of individual with individual, is the most severe. No living being can escape from any of these three phases of the struggle for existence. For reasons which we shall see later, it is not well that any should escape, for " the sheltered life," the life withdrawn from the stress of effort, brings the tendency to degeneration. Because of the destruction resulting from the struggle for existence, more of every species are born than can possibly find space or food to mature. The majority fail to reach their full growth because, for one reason or an- other, they can not do so. All live who can. Each strives to feed itself, to save its own life, to protect its young. But with all their efforts only a portion of each species succeed. 70. Selection by Nature. — But the destruction in Nature is not indiscriminate. In the long run those least fitted to resist attack are the first to perish. It is the slowest ani- mal which is soonest overtaken by those which feed upon it. It is the weakest which is crowded away from the feed- ing-place by its associates. It is the least adapted which is first destroyed by extremes of heat and cold. Just as a farmer improves his herd of cattle by destroying his weak- est or roughest calves, reserving the strong and fit for par- entage, so, on an inconceivably large scale, the forces of Nature are at work purifying, strengthening, and fitting to their surroundings the various species of animals. This process has been called natural selection, or the survival of the fittest. But by fittest in this sense we mean only best adapted to the surroundings, for this process, like others in Nature, has itself no necessarily moral element. The song- bird becomes through this process more fit for the song-bird life, the hawk becomes more capable of killing and tear- 9 ANIMAL LIFE ing, and the woodpecker better fitted to extract grubs from the tree. In the struggle of species with species one may gain a little one year and another the next, the numbers of each species fluctuating a little with varying circumstances, but after a time, unless disturbed by the hand of man, a point will be reached when the loss will almost exactly balance the increase. This produces a condition of apparent equi- librium. The equilibrium is broken when any individual or group of individuals becomes capable of doing something more than hold its own in the struggle for existence. When the conditions of life become adverse to the exist- ence of a species it has three alternatives, or, better, one of three things happens, namely, migration, adaptation, extinc- tion. The migration of birds and some other animals is a systematic changing of environment when conditions are unfavorable to life. When the snow and ice come, the fur- seal forsakes the islands on which it breeds, and which are its real home, and spends the rest of the year in the open sea, returning at the close of winter. Some other animals migrate irregularly, removing from place to place as condi- tions become severe or undesirable. The Eocky Mountain locusts, which breed on the great plateau along the eastern base of the Rocky Mountains, sometimes increase so rapidly in numbers that they can not find enough food in the scanty vegetation of this region. Then great hosts of them fly high into the air until they meet an air current moving toward the southeast. The locusts are borne by this cur- rent or wind hundreds of miles, until, when they come to the great grain-growing Mississippi Valley, they descend and feed to their hearts' content, and to the dismay of the Nebraska and Kansas farmer. These great forced migra- tions used to occur only too often, but none has taken place since 1878, and it is probable that none will ever occur again. With the settlement of the Rocky Mountain plateau by farmers, food is plenty at home. And the constant fight- THE STRUGGLE FOR EXISTENCE H9 ing of the locusts by the farmers, by plowing up their eggs, and crushing and burning the young hoppers, keeps down their numbers. Another animal of interesting migratory habits is the lemming, a mouse-like animal nearly as large as a rat, which lives in the arctic regions. At intervals varying from five to twenty years the cultivated lands of Norway and Sweden, where the lemming is ordinarily unknown, are overrun by vast numbers of these little animals. They come as an army, steadily and slowly advancing, always in the same direction, and " regardless of all obstacles, swimming across streams and even lakes of several miles in breadth, and committing considerable devastation on their line of march by the quantity of food they consume. In their turn they are pursued and harassed by crowds of beasts and birds of prey, as bears, wolves, foxes, dogs, wild cats, stoats, weasels, eagles, hawks, and owls, and never spared by man ; even the domestic animals not usually predaceous, as cattle, foals, and reindeer, are said to join in the destruction, stamping them to the ground with their feet and even eat- ing their bodies. Numbers also die from disease apparently produced from overcrowding. None ever return by the course by which they came, and the onward march of the survivors never ceases until they reach the sea, into which they plunge, and swimming onward in the same direction as before perish in the waves." One of these great migra- tions lasts for from one to three years. But it always ends in the total destruction of the migrating army. But the migration may be of advantage to the lemmings which re- main in the original breeding grounds, leaving them with enough food, so that, on the whole, the migration^ results in gain to the species. But most animals can not migrate to their betterment. In that case the only alternatives are adaptation or destruc- tion. Some individuals by the possession of slight advan- tageous variations of structure are able to meet the new 120 ANIMAL LIFE demands and survive, the rest die. The survivors produce young similarly advantageously different from the general type, and the adaptation increases with successive genera- tions. 71. Adjustment to surroundings a result of natural selec- tion.— To such causes as these we must ascribe the nice adjustment of each species to its surroundings. If a species or a group of individuals can not adapt itself to its environ- ment, it will be crowded out by others that can do so. The former will disappear entirely from the earth, or else will be limited to surroundings with which it comes into perfect adjustment. A partial adjustment must with time become a complete one, for the individuals not adapted will be exterminated in the struggle for life. In this regard very small variations may lead to great results. A side issue apparently of little consequence may determine the fate of a species. Any advantage, no matter how small, will turn the scale of life in favor of its possessor and his progeny. " Battle within battle," says a famous naturalist, " must be continually recurring, with varying success. Yet in the long run the forces are so nicely balanced that the face of Nature remains for a long time uniform, though assuredly the merest trifle would give the victory to one organic being over another." 72. Artificial selection.— It has been long known that the nature of a herd or race of animals can be materially altered by a conscious selection on the part of man of these indi- viduals which are to become parents. To " weed out " a herd artificially is to improve its blood. To select for re- production the swiftest horses, the best milk cows, the most intelligent dogs, is to raise the standard of the herd or race in each of these respects by the simple action of hered- ity. Artificial selection has been called the "magician's wand," by which the breeder can summon up whatever animal form he will. If the parentage is chosen to a defi- nite end, the process of heredity will develop the form THE STRUGGLE FOR EXISTENCE desired by a force as unchanging as that by which a stream turns a mill. From the wild animals about him man has developed the domestic animals which he finds useful. The dog which man trains to care for his sheep is developed by selection from the most tractable progeny of the wolf which once devoured his flocks. By the process of artificial selec- tion those individuals that are not useful to man or pleas- ing to his fancy have been destroyed, and those which con- tribute to his pleasure or welfare have been preserved and allowed to reproduce their kind. The various fancy breeds of pigeons — the carriers, pouters, tumblers, ruff-necks, and fan-tails — are all the descendants of the wild dove of Eu- rope (Columba livid]. These breeds or races or varieties have been produced by artificial selection. So it is with the various breeds of cattle and of hogs and of horses and dogs. In this artificial selection new variations are more rap- idly produced than in Nature by means of intercrossing different races, and by a more rapid weeding out of un- favorable— that is, of undesirable — variations. The rapid production of variations and the careful preservation of the desirable ones and rigid destruction of undesirable ones are the means by which many races of domestic ani- mals are produced. This is artificial selection. 73. Dependence of species on species.— There was intro- duced into California from Australia, on young orange trees, a few years ago, an insect pest called the cottony cushion scale (Iccrya purchasi). This pest increased in numbers with extraordinary rapidity, and in four or five years threat- ened to destroy completely the great orange orchards of California. Artificial remedies were of little avail. Finally, an entomologist was sent to Australia to find out if this scale insect had not some special natural enemy in its native country. It was found that in Australia a certain species of lady-bird beetle attacked and fed on the cottony 122 ANIMAL LIFE cushion scales and kept them in check. Some of these lady-birds ( Vedalia cardinalis) were brought to California and released in a 3cale-infested orchard. The lady-birds, having plenty of food, thrived and produced many young. Soon the lady-birds were in such numbers that numbers of them could be distributed to other orchards. In two or three years the Vedalias had become so numerous and widely distributed that the cottony cushion scales began to dimmish perceptibly, and soon the pest was nearly wiped out. But with the disappearance of the scales came also a disappearance of the lady-birds, and it was then discovered that the Vedalias fed only on cottony cushion scales and could not live where the scales were not. So now, in order to have a stock of Vedalias on hand in California it is neces- sary to keep protected some colonies of the cottony cushion scale to serve as food. Of course, with the disappearance of the predaceous lady-birds the scale began to increase again in various parts of the State, but with the sending of Vedalias to these localities the scale was again crushed. How close is the interdependence of these two species ! Similar relations can be traced in every group of ani- mals. When the salmon cease to run in the Sacramento Eiver in California the otter which feeds on them takes, it is said, to robbing the poultry-yards ; and the bear, which also feeds on fish, strikes out for other game, taking fruit or chickens or bee-hives, whatever he may find. CHAPTEK VIII ADAPTATIONS 74. Origin of adaptations. — The strife for place in the crowd of animals makes it necessary for each one to adjust itself to the place it holds. As the individual becomes fitted to its condition, so must the species as a whole. The species is therefore made up of individuals that are fitted or may become fitted for the conditions of life. As the stress of existence becomes more severe, the individuals fit to continue the species are chosen more closely. This choice is the automatic work of the conditions of life, but it is none the less effective in its operations, and in the course of centuries it becomes unerring. When conditions change, the perfection of adaptation in a species may be the cause of its extinction. If the need of a special fitness can not be met immediately, the species will disappear. For example, the native sheep of England have developed a long wool fitted to protect them in a cool, damp climate. Such sheep transferred to Cuba died in a short time, leav- ing no descendants. The warm fleece, so useful in Eng- land, rendered them wholly unfit for survival in the tropics. It is one advantage of man, as compared with other forms of life, that so many of his adaptations are external to his structure, and can be cast aside when necessity arises. 75. Classification of adaptations.— The various forms of adaptations may be roughly divided into five classes, as fol- lows : (a) food securing, (#) self-protection, (c) rivalry, (d) defense of young, (e) surroundings. The few examples which are given under each class, 123 124 ANIMAL LIFE some of them striking, some not especially so, are mostly chosen from the vertebrates and from the insects, because these two groups of animals are the groups with which be- ginning students of zoology are likely to be familiar, and the adaptations referred to are therefore most likely to be best appreciated. Quite as good and obvious examples could be selected from any other groups of animals. The student PIG. 54.— The deep-sea angler (Corynolophus reinhardti), which has a dorsal spine modified to be a luminous "fishing-rod and lure," attracting lantern-fishes (Echiostoma and ^Et?wphora). An extraordinary adaptation for securing food. (The angler is drawn after a figure of L^TKEN'S.) will find good practice in trying to discover examples shown by the animals with which he may be familiar. That all or any part of the body structure of any animal can be called with truth an example of adaptation is plain from what we know of how the various organs of the animal body have come to exist. But by giving special attention to such adaptations as are plainly obvious, beginning stu- ADAPTATIONS 125 dents may be put in the way of independent ob- servation along an ex- tremely interesting and attractive line of zoolog- ical study. 76. Adaptations for securing food. — For the purpose of capture of their prey, some carniv- orous animals are pro- vided with strong claws, sharp teeth, hooked beaks, and other struc- tures familiar to us in the lion, tiger, dog, cat, owl, and eagle. Insect- eating mammals have contrivances especially FIG. 55.— The brown pelican, showing gular sac, which it uses in catching and holding fishes that form its food. Fie. 56.— Foot of the bald eagle, show- ing claws for seizing its prey, (CHAPMAN.) adapted for the catching of insects. The ant-eater, for example, has a curious, long sticky tongue which it thrusts forth from its cylindrical snout deep into the recesses of the ant- hill, bringing it out with its sticky surface covered with ants. Animals which feed on nuts are fitted with strong teeth or beaks for crack- ing them. Similar teeth are found in those fishes which feed on crabs, snails, or sea-ur- chins. Those mammals like the horse and cow, that feed on plants, have usually FIG. 57.— Giraffes feeding. ADAPTATIONS 127 broad chisel-like incisor teeth for cutting off the foliage, and teeth of very similar form are developed in the dif- ferent groups of plant- eating fishes. Molar teeth are found when it FIG. 58. — Scorpion, showing the special devel- opment of certain mouth parts (the maxil- lary palpi) as pincer-like organs for grasp- ing prey. At the posterior tip of the body is the poisonous sting. PIG. 59.— Head of mosquito (fe- male), showing the piercing needle-like month parts which compose the "bill." is necessary that the food should be crushed or chewed, and the sharp canine teeth go with a flesh diet. The long neck of the giraffe (Fig. 57) enables it to browse on the foliage of trees. Insects like the leaf- beetles and the grasshop- pers, that feed on the foliage of plants, have a . . . , FIG. 60.— The praying-horse (Mantis) with pair Of jaWS, broad but fore legs developed as grasping organs. 128 ANIMAL LIFE sharply edged, for cutting off bits of leaves and stems. Those which take only liquid food, as the butterflies and sucking-bugs, have their mouth parts modified to form a slender, hollow sucking beak or proboscis, which can be thrust into a flower nectary, or into the green tissue of plants or the flesh of animals, to suck up nectar or plant sap or blood, depending on the special food habits of the in- sect. The honey-bee has a very complicated equipment of mouth parts fitted for tak- ing either solid food like pol- len, or liquid food like the nectar of flowers. The mos- quito has a "bill" (Fig. 59) composed of six sharp, slender needles for piercing and lac- erating the flesh, and a long tubular under lip through which the blood can flow into the mouth. Some predaceous insects, as the praying-horse (Fig. 60), have their fore legs developed into formidable grasping organs for seizing and holding their prey. 77. Adaptation for self-de- fense.— For self-protection, car- Fio. 61.— Acorns put into bark of tree . . * by the Californian woodpecker niVOrOUS animals US6 the Same (Meianerpes formicivorus bairdii). weapons to defend themselves —From photograph, Stanford Uni- . r versity, California, which serve to secure their prey; but these as well as other animals may protect themselves in other fashions. Most of the hoofed animals are provided with horns, struc- ADAPTATIONS 129 FIG. 62.— Section of bark of live oak tree with acorns placed in it by the Californian woodpecker (Melanerpes formicivorus bairdii). — From photograph, Stanford University, California. tures useless in procuring food but often of great effective- ness as weapons of defense. To the category of structures useful for self-defense belong the many peculiarities of col- oration known as "recognition marks." These are marks, 10 130 ANIMAL LIFE not otherwise useful, which, are supposed to enable mem- bers of any one species to recognize their own kind among the mass of animal life. To this category belongs the black tip of the weasel's tail, which re- mains the same whatever the changes in the outer fur. Another example is seen in the white outer feathers of the tail of the meadow-lark as well as in certain sparrows and warblers. The white on the skunk's back and tail serves the same purpose and also as a warning. It is to the skunk's advan- tage not to be hidden, for to be seen in the crowd of animals is to be avoided by them. The songs of birds and the calls of various creatures serve also as recognition marks. Each species knows and heeds its own characteristic song or cry, and it is a source of mutual protection. The fur-seal pup knows its mother's call, even though ten thou- sand other mothers are calling on the rookery. The ways in which animals make themselves disagreeable or dangerous to their captors are almost as varied as the animals them- selves. Besides the teeth, claws, and horns of ordinary attack and defense, we find among the mammals many special structures or contrivances which serve for de- fense through making their possession unpleasant. The scent glands of the skunk and its relatives are noticed above. The porcupine has the bristles in its fur specialized as quills, barbed and detachable. These quills fill the mouth of an attacking fox or wolf, and serve well the pur- pose of defense. The hedgehog of Europe, an animal of different nature, being related rather to the mole than to FIG. 63.-Centiped. The foremost pair of legs is modified to be a pair of seizing and stinging or- gans. An adaptation for self-defense and for securing food. ADAPTATIONS 131 the squirrel, has a similar armature of quills. The armadillo of the tropics has movable shields, and when it withdraws its FIG. 64.— Flying fishes. (The upper one a species of Cypselurus, the lower of Exoca- tus.} These fishes escape from their enemies by leaping into the air and sailing or "flying" long distances. head (which is also defended by a bony shield) it is as well protected as a turtle. FIG. 65.— The horned toad (Phrynosoma blammllei). The spiny covering repels many enemies. Special organs for defense of this nature are rare among birds, but numerous among reptiles. The turtles are all 132 ANIMAL LIFE protected by bony shields, and some of them, the box-tur- tles, may close their shields almost hermetically. The snakes broaden their heads, swell their necks, or show their forked tongues to frighten their enemies. Some of them FIG. 66.— Nokee or poisonous scorpion-fish (Emmydrichthys vulcanus) with poison- ous spines, from Tahiti. are further armed with fangs connected with a venom gland, so that to most animals their bite is deadly. Besides its fangs the rattlesnake has a rattle on the tail made up of a FIG. 67. — Mad torn (Schilbeodes furiosus) with poisoned pectoral spine. succession of bony clappers, modified vertebrae, and scales, by which intruders are warned of their presence. This sharp and insistent buzz is a warning to animals of other species and a recognition signal to those of its own kind. ADAPTATIONS 133 Even the fishes have many modes of self-defense through giving pain or injury to those who would swallow them. The cat-fishes or horned pouts when attacked set immov- ably the sharp spine of the pectoral fin, indicting a jagged wound. Pelicans who have swallowed a cat- fish have been known to die of the wounds inflicted by the fish's spine. In the group of scorpion- fishes and toad-fishes are certain genera in which these spines are provided with poison glands. These may inflict very severe wounds to other fishes, or even to birds or man. One of this group of poison-fishes is the nokee (Emmydrich- thys. Fig. 66). A group of small fresh- water cat-fishes, known as the mad toms (Fig. 67), have also a poison gland attached to the pectoral spine, and its sting is most exasperating, like the sting of a wasp. The sting-rays (Fig. 68) of many species FIG., ea— A sting ray have a strong, jagged spine on the tail, %£%££** covered with slime, and armed with broad saw -like teeth. This inflicts a dangerous wound, not through the presence of specific venom, but from the dan- ger of blood poisoning arising from the slime, and the ragged or unclean cut. Many fishes are defended by a coat of mail or a coat of sharp thorns. The globe-fishes and porcupine-fishes (Fig. 69) are for the most part defended by spines, but their instinct to swallow air gives them an additional safeguard. When one of these fishes is disturbed it rises to the surface, 10 134 ANIMAL LIFE gulps air until its capacious stomach is filled, and then floats belly upward on the surface. It is thus protected from other fishes, though easily taken by man. The torpe- do, electric eel, electric cat-fish, and star-gazer, surprise and stagger their captors by means of electric shocks. In the torpedo or electric ray (Fig. 70), found on the sandy shores of all warm seas, on either side of the head is a large honeycomb-like structure which yields a strong electric shock whenever the live fish is touched. This shock is felt severe- ly if the fish be stabbed with a knife or metallic spear. The electric eel of the rivers of Para- guay and southern Bra- zil is said to give severe shocks to herds of wild horses driven through the streams, and similar accounts are given of the electric cat-fish of the Nile. FIG. 69. — Porcupine-fish (Diodon hystrix), the . , , lower ones swimming normally, the upper Among the insects, one floating belly upward, with inflated the possession of stingS Btomach. — Drawn from specimens from the • m-i Florida Keys. ls not uncommon. The wasps and bees are fa- miliar examples of stinging insects, but many other kinds, less familiar, are similarly protected. All insects have their bodies covered with a coat of armor, composed of a horny substance called chitin. In some cases this chitin- ADAPTATIONS 135 ous coat is very thick and serves to protect them effectu- ally. This is especially true of the beetles. Some insects are inedible (as mentioned in Chapter XII), and are con- spicuously colored so as to be readily recognized by in- sectivorous bird^. The birds, knowing by experience that these insects are ill-tasting, avoid them. Others are ef- fectively concealed from their enemies by their close resemblance in color and marking to their surroundings. These protective resem- blances are discussed in Chapter XII. 78. Adaptation for rivalry. — In questions of attack and defense, the need of meeting animals of their own kind as well as animals of other races must be considered. In struggles of species with those of their own kind, the term rivalry may be applied. Actual warfare is confined mainly to males in the breed- ing season and to polyga- mous animals. Among those in which the male mates with many females, he must struggle with other males for their possession. In all the groups of vertebrates the sexes are about equal in num- bers. Where mating exists, either for the season or for life, this condition does not involve serious struggle or destructive rivalry. Among monogamous birds, or those which pair, the male courts the female of his choice by song and by display FIG. 70.— Torpedo or electric ray (Nar- cine brasiliensis), showing electric cells. ADAPTATIONS 13Y of his bright feathers. The female consents to be chosen by the one which pleases her. It is believed that the hand- somest, most vivacious, and most musical males are the ones most successful in such courtship. With polygamous animals there i? intense rivalry among the males in the mating season, which in almost all species is in the spring. The strongest males survive and reproduce their strength. The most notable adaptation is seen in the superior size of teeth, horns, mane, or spurs. Among the polygamous fur seals (Fig. 71) and sea lions the male is about four times FIG. 72. — A wild duck (Aythya) family. Male, female, and praecocial young. the size of the female. In the polygamous family of deer, buffalo, and the domestic cattle and sheep, the male is larger and more powerfully armed than the female. In the polyg- amous group to which the hen, turkey, and peacock belong the males possess the display of plumage, and the structures adapted for fighting, with the will to use them. 79. Adaptations for the defense of the young. — The pro- tection of the young is the source of many adaptive struc- tures as well as of the instincts by which such structures are 138 ANIMAL LIFE utilized. In general, those animals are highest in develop- ment, with best means of holding their own in the struggle FIG. 73.— The altricial nestlings of the Blue jay (Cyanocitta cristata). for life, that take best care of their young. The homes of animals are elsewhere specially discussed (see Chapter ADAPTATIONS 139 XV), but those instincts which lead to home-building may all be regarded as useful adaptations in preserving the young. Among the lower or more coarsely organized FIG. 74. — kangaroo (Macropiix mtfns) with young in pouch. 140 ANIMAL LIFE birds, such as the chicken, the duck, and the auk, as with the reptiles, the young animal is hatched with well-devel- oped muscular system and sense organs, and is capable of running about, and, to some extent, of feed- ing itself. Birds of this type are known as prcecocial (Fig. 72), while the name altricial (Fig. 73) is ap- plied to the more highly organized forms, such as the thrushes, doves, and song-birds generally. With these the young are hatched in a wholly helpless condition, with in- effective muscles, deficient senses, and dependent wholly upon the parent. The altricial condition de- mands the building of a nest, the establishment of a home, and the FIG. 75. — Egg-case of California , . -• » i ,1 • barn-door skate (RajaUnocu- continued care of one or both of lata) cut open to show young the parents. The very lowest mammals known, the duck-bills (Monotremes) of Australia, lay large eggs in a strong shell like those of a turtle, and guard them with great jealousy. But with almost all mammals the egg is very small and without much food-yolk. The egg begins its development within the body. It is nourished by the blood of the mother, and after birth the young is cherished by her, and fed by milk secreted by specialized glands of the skin. All these features are adaptations tending toward the preservation of the young. In the division of mammals next lowest to the Monotremes — the kangaroo, opossum, etc. — the young are born in a very im- mature state and are at once seized by the mother and inside. (Young issues natu- rally at one end of the case.) FIG. 76.— Egg-case of the cock- roach. ADAPTATIONS 141 thrust into a pouch or fold of skin along the abdomen, where they are kept until they are able to take care of themselves (Fig. 74). This is an interesting and ingenious adaptation, but less specialized and less perfect an adaptation than the conditions found in ordinary mam- mals. Among the insects, the special provisions for the protection and care of the eggs and the young are wide-spread and various. Some of those adaptations which take the special form of nests or "homes" will be described in a later chapter (see Chapter XV). The eggs of the common cockroach are laid in small packets inclosed in a firm wall (Fig. 76). The eggs of the great water-bugs are carried on the back of the male (Fig. 77) ; and the spiders lay their eggs in a silken sac or cocoon, and some of the ground or FIG. 77.— Giant water-bug (Ser- phus). Male carrying egga on its back. FIG. 78. — Cocoon inclosing the pupa of the great Ceanothus moth. Spun of eilk by the larva before pupation. running spiders (Lycosida?) drag this egg-sac, attached to the tip of the abdomen, about with them. The young spiders when hatched live for some days inside this sac, feeding on each other! Many insects have long, sharp, 142 ANIMAL LIFE piercing ovipositors, by means of which the eggs are de- posited in the ground or in the leaves or stems of green plants, or even in the hard wood of tree-trunks. Some of the scale insects se- crete wax from their bodies and form a large, of ten>beautif ul egg-case, attached to and nearly covering the body in which eggs are deposited (Fig. 79). The various gall insects lay their eggs in the soft tissue of plants, and on the hatching of the larvae an abnormal growth of the plant occurs about the young insect, forming an in- closing gall that serves not only to protect the insect within, but to furnish it with an abun- dance of plant-sap, its food. The young insect remains in the gall until it completes its develop- ment and growth, when it gnaws its way out. Such insect galls are especially abun- dant on oak trees (Fig. 80). The care of the eggs and the young of the social insects, as the bees and ants, are de- scribed in Chapter IX. FIG. 79.— The cottony cushion scale insect (Icerya purchasi), from California. The male is winged, the female wingless and with a large waxen egg-sac (e.s.) attached to her body. (The lines at the left of each figure indicate the size of the insects.) ADAPTATIONS 143 80. Adaptations concerned with surroundings in life. — A large part of the life of the animal is a struggle with the environment itself; in this struggle only those that are adapted live and leave descendants fitted like themselves. The fur of mammals fits them to their surroundings. As the fur differs, so may the habits change. Some animals are active in winter ; others, as the bear, hibernate, sleep- ing in caves or hollow trees or in burrows until conditions are favorable for their activity. Most snakes and lizards hibernate in cold weather. In the swamps of Louisiana, FIG. 80.— The giant gall of the white oak (California), made by the gall insect Andri- cus calif ornicus. The gall at the right cut open to show tunnels made by the insects in escaping from the gall.— From photograph. in winter, the bottom may often be seen covered with water snakes lying as inert as dead twigs. Usually, however, hibernation is accompanied by concealment. Some animals in hibernation may be frozen alive without apparent injury. The blackfish of the Alaska swamps, fed to dogs when frozen solid, has been known to revive in the heat of the •dog's stomach and to wriggle out and escape. As animals resist heat and cold by adaptations of structure or habits, so may they resist dryness. Certain fishes hold reservoirs 144 ANIMAL LIFE of water above their gills, by means of which they can breathe during short excursions from the water. Still others (mud-fishes) retain the primitive lung-like structure of the swim-bladder, and are able to breathe air when, in the dry season, the water of the pools is reduced to mud. Another series of adaptations is concerned with the places chosen by animals for their homes. The fishes that live in water have special organs for breathing under water (Fig. 82). Many of the South American mon- keys have the tip of the tail adapted for clinging to limbs of trees or to the bodies of other monkeys of its own kind. The hooked claws of the bat hold on to rocks, the bricks of chimneys, or to the surface of hollow trees where the bat sleeps through the day. The tree-frogs (Fig. 83) or tree-toads have the tips of the toes swollen, forming little pads by which they cling to the bark of trees. Among other adaptations relat- ing to special surroundings or con- ditions of life are the great cheek pouches of the pocket gophers, which carry off the soil dug up by the large shovel-like feet when the gopher excavates its burrow. Those insects which live under- ground, making burrows or tunnels in the soil, have their legs or other parts adapted for dig- ging and burrowing. The mole cricket (Fig. 84) has its legs stout and short, with broad, shovel-like feet. Some water-beetles (Fig. 85) and water-bugs have one or more of the pairs of legs flattened and broad to serve as oars or pad. dies for swimming. The grasshoppers or locusts, who leap,. FIG. 81.— Insect galls on leaf. ADAPTATIONS 145 have their hind legs greatly enlarged and elon- gated, and provided with strong muscles, so as to make of them "leaping legs." The grubs FIG. 82.— Head of rainbow trout (Salmo irideus) with gill cover bent back to show gills, the breathing organs. or larvae of beetles which live as " borers " in tree-trunks have mere rudiments of legs, or none at all (Fig. 86). They have great, strong, biting jaws for cutting away the hard wood. They move" simply by wriggling along in their burrows or tunnels. Insects that live in water either come - up to the surface to breathe or take down air underneath their wings, or in some other way, or have gills for breathing the air which is mixed with the water. These gills are special adap- tive structures which present a great variety of form and appearance. In the young of the May-flies they are deli- cate plate-like flaps projecting from the sides of the body. They are kept in constant motion, gently waving back and 11 FIG. 83.— Tree-toad (Hyla regilla). 146 ANIMAL LIFE forth in the water so as to maintain currents to bring fresh water in contact with them. Young mosquitoes (Fig. 87) do not have gills, but come up to the surface to breathe. The larvae, or wrigglers, breathe through a 'special FIG. 84.— The mole cricket (Gryllotalpa), with fore feet modified for digging. FIG. 85.— A water-beetle (Hydroph- ilus). tube at the posterior tip of the body, while the pupae have a pair of horn-like tubes on the back of the head end of the body. 81. Degree of structural change in adaptations.— While among the higher or vertebrate animals, especially the fishes and reptiles, most remarkable cases of adaptations occur, yet the structural changes are for the most part ex- ternal, never seriously affecting the development of the internal organs other than the skeleton. The organization of these higher animals is much less plastic than among the invertebrates. In general, the higher the type the more persistent and un- changeable are those structures not immediately exposed FIG. 86.— Wood-boring beetle larva (Prionus). ADAPTATIONS 147 to the influence of the struggle for existence. It is thus the outside of an animal that tells where its ancestors have lived. The inside, suffering little change, whatever the surroundings, tells the real nature of the animal. 82. Vestigial organs. — In general, all the peculiarities of animal structure find their explanation in some need of adaptation. When this need ceases, the structure itself tends to disappear or else to serve some other need. In the bodies of most animals there are certain incomplete or rudimentary organs or structures which serve no distinct use- ful purpose. They are structures which, in the ancestors of the ani- mals now possessing them, were fully devel- oped functional organ.;, but which, because of u change in habits or con- ditions of living, are of no further need, and are gradually dying out. Such organs are called vestigial organs. Ex- amples are the disused ear muscles of man, the vermiform appendix in man, .which is the reduced and now useless anterior end of the large intestine. In the lower animals, the thumb or degenerate first finger of the bird with its two or three little quills serves as an example. So also the reduced and elevated hind toe of certain birds, the splint bones or rudimentary side toes of the horse, the rudimentary eyes of blind fishes, the minute barbel or beard of the horned dace or chub, and the rudimentary teeth of the right whales and sword-fish. FIG. 87. — Young stages of the mosquito, a, larva (wriggler) ; b, pupa. 148 ANIMAL LIFE Each of these vestigial organs tells a story of some past adaptation to conditions, one that is no longer needed in the life of the species. They have the same place in the study of animals that silent letters have in the study of words. For example, in our word knight the Jc and gh are no longer sounded ; but our ancestors used them both, as the Germans do to-day in their cognate word 'Knecht. So with the French word temps, which means time, in which both p and s are silent. The Eomans, from whom the French took this word, needed all its letters, for they spelled and pronounced it tempus. In general, every silent letter in every word was once sounded. In like manner, every vestigial structure was once in use and helpful or necessary to the life of the animal which possessed it. Borne of two male deer interlocked while fighting. Permission of G. O. SHIELDS, publisher of Recreation. CHAPTEE IX ANIMAL COMMUNITIES AND SOCIAL LIFE 83. Man not the only social animal— Man is commonly called the social animal, but he is not the only one to which this term may be applied. There are many others which possess a social or communal life. A moment's thought brings to mind the familiar facts of the communal life of the honey-bee and of the ants. And there are many other kinds of animals, not so well known to us, that live in communities or colonies, and live a life which in greater or less degree is communal or social. In this connection we may use the term communal for the life of those ani- mals in which the division of labor is such that the indi- vidual is dependent for its continual existence on the com- munity as a whole. The term social life would refer to a lower degree of mutual aid and mutual dependence. 84. The honey-bee. — Honey-bees live together, as we know, in large communities. We are accustomed to think of honey-bees as the inhabitants of bee-hives, but there were bees before there were hives. The "bee-tree" is familiar to many of us. The bees, in Mature, make their home in the hollow of some dead or decaying tree-trunk, and carry on there all the industries which characterize the busy communities in the hives. A honey-bee com- munity comprises three kinds of individuals (Fig. 88) — namely, a fertile female or queen, numerous males or drones, and many infertile females or workers. These three kinds of individuals diifer in external appearance sufficiently to be readily recognizable. The workers are 11 149 150 ANIMAL LIFE smaller than the queens and drones, and the last two differ in the shape of the abdomen, or hind body, the abdomen of the queen being longer and more slender than that of the FIG. 88.— Honey-bee, a, drone or male ; b, worker or infertile female ; c, queen or fertile female. male or drone. In a single community there is one queen, a few hundred drones, and ten to thirty thousand workers. The number of drones and workers varies at different times of the year, being smallest in winter. Each kind of individual has certain work or business to do for the whole community. The queen lays all the eggs from which new bees are born; that is, she is the mother of the entire community. The drones or males have simply to act as royal consorts ; upon them depends the fertilization of the eggs. The workers undertake all the food-getting, the care of the young bees, the comb-building, the honey-mak- ing— all the industries with which we are more or less familiar that are carried on in the hive. And all the work done by the workers is strictly work for the whole ' community ; in no case does the worker bee work for itself alone ; it works for itself only in so far as it is a member of the community. How varied and elaborately perfected these industries are may be perceived from a brief account of the life his- tory of a bee community. The interior of the hollow in the bee-tree or of the hive is filled with " comb " — that is, with wax molded into hexagonal cells and supports for these cells. The molding of these thousands of symmet- ANIMAL COMMUNITIES AND SOCIAL LIFE rical cells is accomplished by the workers by means of their specially modified trowel-like mandibles or jaws. The wax itself, of which the cells are made, comes from the bodies of the workers in the form of small liquid drops which exude from the skin on the under side of the abdomen or hinder body rings. These droplets run together, harden and become flat- tened, and are removed from the wax plates, as the peculiarly modified parts of the skin which produce the wax are called, by means of the hind legs, which are furnished with scissor-like contrivances for cutting off the wax (Fig. 89). In certain of the cells are stored the pollen and honey, which serve as food for the community. The pollen is gathered by the workers from certain favorite flowers and is carried by them from the flowers to the hive in the "pollen baskets," the slightly concave outer surfaces of one of the segments of the broadened and flattened hind legs. This concave surface is lined on each margin with a row of incurved i*