CHAPTER IX. 



THE GENERAL ANIMAL FUNCTIONS AND THEIR 

 APPROPRIATE ORGANS. 



78. Animals may be very much simpler than the 

 grasshopper or the crayfish or they may be considerably 

 more complex; but in either case the work which they 

 must do, in order to be successful in life, is practically 

 the same. The differences are chiefly in the way in which 

 the work is done. 



79. What Must an Animal Do? In our study of the 

 locusts and crayfish we have found that these animals 

 must have food, and in getting this they must usually be 

 able to move about and to move definitely. In order to 

 move in such a way as to secure food satisfactorily, they 

 must be sensitive to the presence of food and be able to 

 control their motion. Furthermore, these animals have 

 enemies, and they must have some power of escaping these 

 enemies, who are also seeking food, or be able to protect 

 themselves from them or be destroyed by them. For this 

 reason, as well as for getting food, they must be sensitive 

 and able to move. 



The soft, living parts of the body must be supported 

 and protected from external influences that might injure, 

 as cold, heat, drouth, rough contacts, crushing, and the 

 like. Thus w r e see the value of the hard parts (skeleton) , 

 which are so common among animals. 



As the result of using food these two animals have the 

 power of growth. Growth, however, does not continue 

 72 



