108 



ANIMALS AS LIVING ORGANISMS 



Physiological Division of Labor. — If we compare the amoeba 

 and the Paramecium, we find the latter a more complex organism 

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

 of the body ; the Paramecium has a definite gullet. The amoeba 

 may use any part of the body for locomotion; the Paramecium 

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

 the structure of the Paramecium is more complex, we say that it is 

 a '^ higher " animal that the amoeba. 



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. Just as in a community of people there are 

 some men who do rough manual work, others who are skilled work- 

 men, some who are shopkeepers, and still others who are profes- 

 sional men, so among plants and animals, wherever collections of 

 cells live together to form an organism, there is division of labor, 

 some cells being fitted to do one kind of work, while others are 

 fitted to do work of another sort. This is called physiological 

 division of labor. 



Enlarged views and enlarged lengthwise section of the hydra, a very simple ani- 

 mal which shows slight division of labor: 1, hydra extended; 2, contracted; 

 3, diagram of section. 



Division of Labor in Simple Animals. — In the hydra, a tiny 

 simple animal found in fresh-water ponds or streams, the body is 

 like an elastic bag, with an opening at one end. This opening, 



