the body, while the volume or food-using surface of the body is increased as the cube of its diameter. The food supplying can not keep pace with the food using. Hence it is absolutely essential that among large animals the food-taking 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 digesting 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 mosquito, take only liquid food, the sap of plants, or the blood

of animals; others, like the weevils, feed on the hard, dry substance of seeds and grains; others, like the grasshoppers 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 performed in simple manner, the body will be simple in structure. 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 peculiar 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.

CHAPTER 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 immature or young condition; it grows (that is, it increases in size), and develops (that is, changes more or less in structure), 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 individualthat 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 organism hours or days or months, perhaps, before its appearance 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 begins 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

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death. The new Amabæ are from their beginning like the full-grown Amaba, except as regards size. And the old Amaba does not die, because its whole body continues to live, although in two parts-the two new Amabæ. 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 reproduction 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 animals (Chapter III) we learned that it is the almost univer

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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; f,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

fusion of two germ cells, a sperm cell from a male individual of the species and an egg cell from a female individual 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―gathered 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 because 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, 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 fertilized egg cell outside of the body, but allow the development 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 anima' issues from the body of the mother, it is said to be born. When the developing animal issues from an egg