INTRODUCTION 127 



rat us becomes more complete in its details and special 

 arrangements for breathing are developed. We must 

 know the structure and location of these organs in dif- 

 ferent groups in order to understand the conditions 

 under which they do their work. We must know what 

 the animal eats, how it eats, how the food is digested, 

 absorbed, converted into protoplasm, and how the wastes 

 are given off. These things happen in all animals and 

 are governed by the same principles. 



To study any particular animal, then, we study the 

 peculiarities of structure which control its method 

 of moving, its method of assimilation, and its method 

 of reproduction. 



