STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY 15 



factorily the work for which the locomotive engine is 

 intended. An important difference between the locomo- 

 tive engine and the toad's body is that one is a lifeless 

 machine and the other a living machine. But there is a 

 real similarity between the two in that both are composed 

 of special parts, each part performing a special kind of 

 work or function, and all the parts and functions so fitted 

 together as to form a complex machine which successfully 

 accomplishes the work for which it is intended. And this 

 similarity is one which should help make plain the funda- 

 mental fact of animal structure and physiology, namely, 

 the division of the body into numerous parts or organs, 

 and the division of the total work of living into various 

 processes which are the special work or functions of the 

 various organs. 



The essential functions or life-processes. The toad 

 has a great many different special parts in its body. Its 

 body is very complex. It performs a great many differ- 

 ent functions, that is, does a great many different things 

 in its living. And the structure and life of most of the 

 other animals with which we are familiar are similarly 

 complex: a fish, or a rabbit, or a bird has a body com- 

 posed of many different parts, and is capable of doing 

 many different things. Are all animals similarly complex 

 in structure, and capable of doing such a great variety of 

 things ? We shall find that the answer to this question 

 is No. There are many animals in which the body is 

 composed of but a few parts, and whose life includes the 

 performance of fewer functions or processes than in the 

 case of the toad. There are many animals which have 

 no eyes nor ears nor other organs of special sense. 

 There are animals without legs or other special organs of 

 locomotion ; some animals have no blood and hence no 

 heart nor arteries and veins. But in the life of every 

 animal there are certain processes which must be per- 



