42 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



familiarly as various parts of the body, such as lungs, heart, 

 muscles, eyes, stomach, etc. The life of an animal consists of 

 the performance by it of various processes, such as breathing, 

 getting and digesting food, circulating blood, moving, seeing, 

 etc. These various processes or functions are performed by 

 the various parts or organs of the body. 



The whole body of a many-celled animal is thus really a 

 machine composed of various parts, each part with its special 

 work to do but all depending upon one another and operating 

 to accomplish the work of living. The locomotive engine is a 

 machine similarly composed of various parts, each part with 

 its special work or function, and all the parts depending on one 

 another and so working together as to perform satisfactorily 

 the work for which the locomotive engine is intended. An 

 important difference between the locomotive engine and the 

 animal 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 w r ork 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 fundamental fact of animal structure and physiol- 

 ogy, 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. 



Essential and Accessory Life Processes. A very complex 

 animal, such as a dog, performs a great many different func- 

 tions, that is, does a great many different things in its living. 

 But 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 a dog. There 

 are many animals that have no eyes, nor ears, nor 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 performed, and the 



