CHAPTER III 



Body Covering 



DIVISION of labor among the different organs in the 

 body occurs in all animals except the very low forms. And 

 since all animals have the same essential life processes, they 

 all have organ systems designed for the same purposes. A 

 crab, or even a snail, must have an integument to cover him, 

 eyes to see food and danger with, muscles to move him 

 toward or away from them, a skeleton for the muscles to 

 work on, a stomach to digest his food, a circulating medium 

 to carry the nourishing elements to various parts of the 

 body, a disposal system to get rid of the waste products, a 

 means of reproducing himself, a method for supplying 

 himself with oxygen, and a brain to co-ordinate all these 

 activities. 



Quite an adequate equipment: as far as number of sys- 

 tems is concerned, we and the rest of the mammals have 

 no more. But there is this difference, that the organs which 

 compose the various systems of these invertebrates may be, 

 and usually are, quite different from the organs which com- 

 pose the same systems in mammals. The snail's brain consists 

 of several groups of ganglia, located in his foot, near his 

 mouth, and at the entrance to his stomach, whereas a mon- 

 key's brain is a great mass of nerve-tissue located in his 

 head. A crab's circulatory system consists of a number of 

 tubes leading out from his heart, but instead of joining 

 themselves to returning tubes by a network of delicate capil- 

 laries so that the blood flows through a closed circuit, as in 



i6 



