XIII. METAZOA 



Division of Labor. — If we compare the amoeba and the para- 

 moecium, we find the latter a more complex organism than the 

 former. An amoeba may take in food through any part of the 

 body; the paramoecium has a definite gullet; the amoeba may 

 use any part of the body for locomotion; the paramoecium has 

 definite parts of the cell, the cilia, fitted for this work. Since the 

 structure of the paramoecium is more complex, we say that it is a 

 " higher " animal. 



As we look higher in the scale of life, we invariably find that 

 certain parts of a plant or animal are set apart to do certain work 

 and only that work. This has resulted in what is called division 

 of labor. Just as in a community of people, there are some men 

 who do rough manual work, others who are skilled workmen, 

 some who are shopkeepers, and still others who are professional 

 men, so among plants and 

 animals, wherever collections 

 of cells live together to form 

 an organism, there is division 

 of labor. 



One of the simplest of all colo- 

 nies is a collection of cells called 

 Pandorina. This is a colony of 

 sixteen cells, whether plant or ani- 

 mal is uncertain, which have be- 

 come joined by living together in a 

 mass of jellylike material secreted 

 by the cells. They move by means 

 of cilia, and, as a result of living 

 together, move faster through the 

 water and thus obtain more food 

 than one alone. 



Another form where division of labor is begun is seen in the plant (or 

 animal) called Volvox. This is a hollow sphere of cells, the greater number 



187 



Colony of volvox: R, reproductive cells; 

 C, ciliated cells. (After Kny.) 



