SPONGES 57 



This is well shown in the higher animals, where certain 

 cells are grouped together for a given work. The diges- 

 tive system contains cells which work to make solutions of 

 the food eaten. These solutions nourish the whole body, 

 not the cells of the digestive tract alone. Certain other 

 cells are modified in such a way for secreting and holding 

 lime that they form bones by which the whole body is 

 benefited. 



Some cells are grouped to form muscles to be used in 

 securing food and in enabling animals to escape from 

 their enemies. Other cells are for the purpose of convey- 

 ing and interpreting impressions, so that the animal may 

 hear the approach of an enemy, or detect the presence of 

 food. It is largely the carrying out of this " division of 

 labor " that tells us the rank of an animal or a plant in 

 biological classification. 



In the business world we know of division of labor. 

 Years ago the cobbler made all the parts of a shoe. In 

 our large shoe factories to-day we find no one man 

 making an entire shoe. One man runs the machine that 

 cuts the leather and does no other part of the work. 

 He may have been a cutter twenty years, and he works 

 rapidly and accurately. Another man runs the machine 

 which sews uppers to the soles. He, too, is a rapid and 

 skillful worker. Other men have their special lines of 

 work to do. In the end they produce more shoes and 

 better shoes than this same number of men could, if they 

 were all cobblers and each finished his product. So in 

 the world of business we find the same plan of division of 

 labor that we are studying in biology. 



42. Sponges. — Sponges are simple metazoa. In them 

 we find division of labor carried out in a more complex 

 way than in gonium and volvox. Simple sponges have 

 a body in the form of a hollow cylinder. Water enters 



