GENERAL ANIMAL FUNCTIONS. 73 



indefinitely, even though the animal continues to use 

 food. After reaching a certain maturity of size, the 

 animals cease to grow and may reproduce new organisms 

 like themselves. Reproduction merely means that some 

 of the materials which might be used for growth are 

 devoted to making new individuals of the same species. 



These activities which have been enumerated for the 

 crayfish and locust are the activities which we shall need 

 to study in all animals. If any animal does not show 

 them, we shall need to know why it does not. 



79. A classification of animal functions. 



1. Nutrition or metabolism; including the use of food 

 in all its steps, in building up the individual or in produc- 

 ing energy. 



2. Protection and the physical support of the soft 

 parts. 



3. Growth. 



4. Reproduction. 



5. Sensation and sensitiveness, including all the powers 

 and activities which enable the organism to adjust itself 

 to its own needs and to the conditions about it. 



6. Movement, including locomotion as well as all the 

 smaller movements of parts of the body. 



80. How are These Functions Performed in the Simplest 

 Animals? The simplest animals are microscopic in size, 

 and are composed of a single mass of living material 

 (a cell; see 51). The bodies of the larger animals are 

 composed of many such cells united in one body. We 

 have already seen that the essential part of one of these 

 cells is the living substance within called protoplasm. 

 It is a semifluid material which ordinarily has the power 



