144 The Study of Animal Life part ii 



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. The Functions 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 of. 



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. — All the cells of our bodies are nourished by 

 the stream of fluid food-stuff, 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 activities of the individual cells, — 

 the manner in which they feed themselves. 



The first step in the preparation of the blood is digestion. 

 Most food-stuff is solid and indiffusible ; before it can enter 

 the blood it must be made soluble and diffusible. The 



