Animal studies

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CHAPTER

I. — Conditions of animal life . ^11. —Principles of classification 4III. — The simplest animals or protozoa ^IV. — The slightly complex animals or sponges *V. — The coslenterates ....

VI. — The worms . . .

Chapter I

1. Divisions of the subject.— Biology is the science which treats of living things in all their relations. It is subdivided into Zoology, the science which deals with animals, and Botany, which is concerned with plants. The field covered by each of these branches is very extensive. Within the scope of zoology are included all subjects bearing on the form and structure of animals, on their development, and on their activities, including the consideration of their habits and the wider problems of their distribution and their relations to one another.

These various subjects are often conveniently grouped under three heads : Morphology, which treats of the form and structure or the anatomy of organisms ; Physiology, which considers their activities ; and Ecology, which includes their relations one to another and to their surroundings. All the phases of plant or animal existence may be considered under one or another of these three divisions.

2. 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 feeding 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 animals. 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 power of spontaneous locomotion, and all plants have some power of motion, as especially 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 substances, especially of carbon which is taken from the carbonic-acid gas in the atmosphere, and of water. But some green-leaved plants feed also in part on organic food. Such are the pitcher-plants and sun-dews, and Venus-flytraps, 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 characteristics constant in all typical plants. In addition, the fixed life of plants, and their general use of inorganic sub-

UUNmTlOJNS OF ANIMAL LIFE 3

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.

3. 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 distinctions. But when we try to define living organic matter, 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 characteristics commonly ascribed to organisms, as peculiar to them, are not so. The possession of organs, or the composition of the body of distinct parts, each with a distinct function, 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 oxidation or burning of this fuel in the engine is wholly comparable 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 inorganic 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 themselves ; and adaptation, the power of responding to external conditions in a way useful to the organism. Through adaptation 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.

4. Primary conditions of animal life.— 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 special 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.

5. Food. — All the higher plant?, 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 matter maybe 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 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, greatest 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 occasional gorging, succeeded by a long period of abstinence. Many of the lower animals can go without food for surprisingly 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 conception of food, then special mention must be made of the necessity of water as one of the primary conditions of animal 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.

6. 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 changes which take place in the body of every living animal, a supply of oxygen is required. This oxygen is derived directly or indirectly from the air. The atmosphere of the earth is composed of 79.02 parts of nitrogen (including argon), .03 parts of carbonic acid, and 20.95 parts of oxygen. Thus all the animals which live on land are enveloped 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 atmosphere, 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.

7. Temperature, pressure, and other conditions. — Some physiologists include among the primary or essential general conditions of animal life such conditions as favorable temperature and favorable pressure. It is known from observation 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 animals 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.

for some time and then gradually thawed them out unhurt. 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 temperature of —20° C. killed the fish. Frogs lived after being kept at a temperature of —28° C, centipedes at —50° C, and certain snails endured a temperature of —120° C. without dying. At the other extreme, instances are known of animals living in water (hot springs or water gradually heated with the organisms in it) of a temperature as high as 50° C. Experiments with Amcehce 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 pressure or weight of the atmosphere on the surface 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 condition. For this reason they do not notice it. The animals that live in water, however, sustain a much greater pressure, 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 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 suddenly 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 destroyed 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 cousequences may ensue, and if too high an altitude is reached death occurs. This death may be in part due to the difficulty in breathing in sufficient oxygen to maintain life,. but it is probably chiefly due to the disturbances caused by the removal of the pressure to which the body is accustomed and is structurally adapted to withstand. All living animals are accustomed to live under a certain pressure, and there are evidently limits of maximum or minimum pressure beyond which no animal at present existing can go and remain alive.

But in the case both of temperature and pressure conditions 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 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 other physical conditions, without which existence or life of any kind would be impossible 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.

8. Cells. — If we examine very carefully the different parts of some highly developed animal under the high powers of the microscope we find that they are composed of a multitude of small structures which bear the same relations to the various organs that bricks or stones do to a wall ; and if the investigation were continued it would be found that every organism is composed of one or more of these lesser elements which bear the name of cells. In size they vary exceedingly, and their shapes are most diverse, but, despite these differences, it will be seen that all exhibit a certain general resemblance one to the other.

9. Shape of cells. — In many of the simpler organisms the component cells are jelly-like masses of a more or less spherical form, but as we ascend the scale of life the condition of affairs becomes much more complex. In the muscles the cells are long and slender (Fig. 1, D) ; those forming the nerves and conveying sensations to and from all parts of the body, like an extensive telegraph system, are excessively delicate and thread-like ; in the skin, and lining many cavities of the body, where the cells are united into extensive sheets, they range in shape from high and columnar to flat and scale-like forms (Fig. 1, E, E, G). The cells of the blood present another type (Fig. 1, B) ; and so we might pass in review other parts of the body, and continue our studies with other groups of animals, always finding new forms dependent upon the part they play in the organism.

10. Size of cells. — Also in the matter of size the greatest variations exist. Some of the smallest cells measure less than one micromillimeter ( 8 gSoo of an inch) in diameter. Over five hundred million such bodies could be readily stowed away into a hollow sphere the size of the letter beginning this sentence. In a drop of human blood of the same size, between four and five million blood-cells or corpuscles float. And from this extreme all sizes exist up to those with a diameter of 2.5 or 5 cm. (one or two inches), as in the case of the hen's or ostrich's egg. On the average a cell will measure between .025 to .031 mm. (^ and rta- of an inch) in diameter, a speck probably invisible to the unaided eye. While the size and external appearance of a cell are seen to be most variable, the internal structures are found to show a striking resemblance throughout. All are constructed upon essentially the same plan. Differences in form and size are superficial, and in passing to a more careful study of one cell we gain a knowledge of the important features of all.

11. A typical cell.— An egg-cell (Fig. 1, A) or some simple one from the liver or skin may be chosen as a good representative of a typical cell. To the naked eye it is barely visible as a minute speck ; but under the microscope the appearance is that of so much white of egg, an almost transparent jelly-like mass bearing upon its outer surface a thin structureless membrane that serves to preserve its general shape and also to protect the delicate cell material within. The comparison of the latter substance to egg albumen can be carried no further than the simple physical appearance, for albumen belongs to that great class of substances which are said to be non-living or dead, while the cell material or protoplasm, as it is termed, is a living substance. We know of no case where life exists apart from protoplasm, and for this reason the latter is frequently termed the physical basis of life.

In addition to the features already described, the protoplasm of every perfect cell is modified upon the interior to form a well-defined spherical mass known as the nucleus. Other structures are known to occur in the typical cell. Experiment shows that the nucleus and cell protoplasm are absolutely indispensable, whatever their size and shape, and therefore we are at present justified in defining the cell as a small mass of protoplasm enclosing a nucleus.

12. Structure of protoplasm. — When seen under a glass of moderate power protoplasm gives no indication of any definite structure, and even with the highest magnification it presents appearances which are not clearly understood. According to the commonly accepted view, it consists of two portions, one, the firmer, forming an excessively delicate

Fig. 1. — Different types of cells composing the body of a highly developed animal. A, cell ; f, food materials ; n, nucleus, B, blood-cell. C, nerve-cell with small part of its fiber. D, muscle fiber. E, cells lining the body cavity. F, lining of the windpipe. G, section through the skin. Highly magnified.

meshwork (Fig. 1, A) enclosing in its cavities the second more fluid part. Therefore, when highly magnified, the appearance would he essentially like a sponge fully saturated with water ; but it should be remembered that in the protoplasm the sponge work, and possibly the fluid part, is living, and that both are transparent.

There are reasons for thinking that the structure and the composition of protoplasm may change somewhat under certain circumstances. It certainly is not everywhere alike, for that of one animal must differ from that of another, and different parts, such as the liver and brain, of the same form must be unlike. These differences, however, are minor when compared to the resemblances, for, as we shall see, this living substance, wherever it exists, carries on the processes of waste, repair, growth, sensation, contraction, and the reproduction of its kind.

13. Animal functions. — Animals in general lead active, busy lives, collecting food, avoiding enemies, and producing and caring for their young. While the activities of all animals are directed to their own preservation and to the multiplication of their kind, these processes are carried on in the most diverse ways. The manner in which an organ or an organism is made, and the method by which it does its work, are mutually dependent one on the other. As there is an enormous number of species of animals, each differently constructed, there is, accordingly, a very great variety of habits. As we shall see, the lower forms are remarkably simple in their construction, and their mode of existence is correspondingly simple. In the higher t}*pes a much greater complexity exists, and their activities are more varied and are characterized by a high degree of elaboration. In every case, the animal, whether high or low, is fitted for some particular haunt, where it may perform its work in its own special way and may lead a successful life of its own characteristic type.

Chapter II Principles Of Classification

14. Classification. — It is plain that natural relations of dome sort exist among living organisms. A dog is more like a' cat than it is like a sheep. A dog is more like a sheep than either is like a butterfly. The very existence of such terms as animals and plants, insects and fishes, implies various grades of relationship. Classification is the process of reducing our knowledge of these grades of likeness and unlikeness to a system. By bringing together those which are alike, and separating those which are unlike, we find that these rest on fixed and inevitable laws. Classification is thus defined as " the rational, lawful dis' position of observed facts."

15. Homology. — All rational classification of plants or animals concerns itself with homologies. Homology means fundamental identity of structure, as distinguished from analogy, which means incidental resemblance in form or function. Thus the arm of a man is homologous with the fore leg of a dog, because in either we can trace throughout deep-seated resemblances or homologies with the other. In every bone, muscle, vein, or nerve the one corresponds closely with the other. The " limb " of a tree, the " arm " of a starfish, or the fore leg of a grasshopper shows no such correspondence. In a natural classification, or one founded on fact, those organisms showing closest homologies are placed together. An artificial classification is one based on analogies. Such a classification would place together a cricket, a frog, and a kangaroo, because they all jump ; or a bird, a bat, and a butterfly, because they all have wings and can fly, although, the different kinds of wings are made in very unlike fashion.

16. Natural classification based on homology. — The closest homologies are shown by those animals which have sprung from a common stock. The basis of natural classification, which is an expression of the ancestry of blood relationship of animals, is therefore homology. So far as we know, the actual presence of homologies among animals implies their common descent from some stock possessing the same characters. The close resemblance or homology among the different races of men indicates that all men originally

came from one stock.

As homology implies blood-relationship, so, on the other hand, common descent implies homology, the similar parts being derived from a common ancestral stock. It is sometimes said that the inside of an animal tells what it is, the outside where it has been.

In the internal structure, ancestral traits are perpetuated with little change through long periods. The external characters, having more to do with surroundings, are much more rapidly altered in response to demands of the environment.

A perfect classification would indicate the line of descent of each member of the series, those now living

Fig. 2. — Wings showing homology and analogy. a, fly ; b, bird ; c, bat.

PRINCIPLES OF CLASSIFICATION having sprung in natural sequence, by slow processes of change, from creatures of earlier geological periods. It is said that in classification we have " three ancestral documents " : Morphology, Embryology, and Paleontology. In Morphology we compare one form with another, thus

a. Elephant. b. Coney.

'Ill

Fig. 3. — Homology of digits of four odd-toed mammals, showing gradual reduction in number and cousolidation of bones above.— After Komanes.

tracing resemblances and differences. In Embryology we trace the development of individuals from the egg, thus finding clues in heredity that will enable us to trace the development of the race. In Paleontology we study the extinct forms directly, thus often finding evidence as to the origin of forms now existing.

17. Scientific names. — Each of the different kinds of animal or plant is called a species. There is no better definition of species. Thus the red squirrel is a kind or species of squirrel, the gray squirrel is another, the fox squirrel a third. The black squirrel of the East is not a species, because black squirrels and gray squirrels are sometimes found in the same nest, born from the same parents.

A genus is a group of closely related species — one or more — separated from other genera by tangible structural characters. Thus all the squirrels named above constitute

±>ftlNCIPLES OF CLASSIFICATION 17

a single genus. Other squirrel-like animals, as the chipmunk, the flying squirrel, the prairie gopher, or the prairie dog, belong to as many different genera.

In the binomial system, invented by Linnaeus and applied by him to animals in 1758, the scientific name of an animal consists of two words — the name of its genus and species taken together. The name of the genus comes first. It is a noun, in Latin form, though usually of Greek derivation — thus Sciurus, the squirrel, in Greek meaning shadow-tail. The name of the species is an adjective in meaning, placed after the noun and agreeing with it. Thus Sciurus liudsonicus is the name of the red squirrel, Sciurus carolinensis of the Eastern gray squirrel, and Sciurus ludovicianus of the fox squirrel; Sciuropterus volans is the flying squirrel, Tamias striatus the Eastern chipmunk, and Sperraopliilus franklini one of the prairie gophers. The specific name is usually a descriptive adjective — often the name of a locality, sometimes the name of a man. The authority usually written after the name of an animal is that of the one who gave it its. specific name — thus Sciurus liudsonicus Erxleben, which means that Erxleben first called it liudsonicus. Usually the name of the authority is that of the discoverer of the species. When several names are given to the same animal they are called synonyms. The earliest of these names is the right name. All the rest are wrong.

18. Families of animals. — A group of related genera is called a family. The name of a family is derived from that of its principal genus, with the termination idm. Thus all the squirrel-like animals belong to the family of Sciuridm. All the sorts of mice to the Muridce, from the principal genus, Mus, the mouse. The rabbits are LeporidcB, from Lepus, the rabbit, and the beavers Castoridce, from Castor, the beaver.

19. Higher groups of animals. — In the higher groups we first trace out the different plans of structure. There is

Fig 5—Three species of jack-rabbits, differing in size, color, and markings but believed to be derived from a common stock. The differences have arisen through isolation and adaptation. The upper figure shows the head and fore legs of the black jack-rabbit (Lepus insularis), of Espiritu Santo Island, Gulf of California ; the lower right-hand figure, the Arizona jack-rabbit {Lepus alleni) specimen from Port Lowell, Arizona ; and the lower left-hand figure is the San Pedro Martir jack-rabbit (Lepus martirensis), from San Pedro Martir Baia California.