. The study of animal life [microform]. Zoology; Zoologie. T:^^ V;^,>. 144 TAe Study of Animal Life part n The cells of the outer layer are protective, nervous, and muscular; the cells of the inner layer are digestive and muscular. The cells of Hydra are therefore not so many- sided in function as are Amoebae. In animals higher than the simplest worms, a middle layer of cells is always formed which discharges muscular, supporting, and other functions. With advancing complexity of structure the specialisa- tion of certain cells for the performance of certain functions has become more pronoun
. The study of animal life [microform]. Zoology; Zoologie. T:^^ V;^,>. 144 TAe Study of Animal Life part n The cells of the outer layer are protective, nervous, and muscular; the cells of the inner layer are digestive and muscular. The cells of Hydra are therefore not so many- sided in function as are Amoebae. In animals higher than the simplest worms, a middle layer of cells is always formed which discharges muscular, supporting, and other functions. With advancing complexity of structure the specialisa- tion of certain cells for the performance of certain functions has become more pronounced. In the human body the division of labour has reached a state of great perfection; we shall give a slight sketch of its arrangements. 2. Th« Functioiis of the Body.—Our objective life consists of movement, and of feeding to supply the energy for that movement Growth, reproduction, and decay are elsewhere treated o£ Movement.—We move by the contraction of cells massed into tissues called muscles. Contractility is a property of all living matter; in muscle-cells this function is predomi- nant This is all that need be said here of movement; the processes of nutrition we must follow more closely. Nutrition.—AM the cells of our bodies are nourished by the stream of fluid foodstuff, the blood, which flows in a number of vessels called arteries, veins, or capillaries, according to their place in the system. From this stream each cell picks out its food; and into another stream— the lymph stream—moving in separate channels—the lymphatics, which, however, join the blood channels, each cell casts its waste material; just as a single-celled animal takes food from the water in which it lives and casts its waste into it Nutrition must therefore consist of two series of activi- ties. One series will have for its object the preparation of food-matter so that it may enter the blood, and the excretion of waste products out of the blood. The other series will consist of the act
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