i44 The 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. 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 



