274 



A YEAR IN SCIENCE 



Not all one celled animals are as simple in structure 

 as the ameba. Neither are the processes in these ani- 

 mals carried on in such a simple fashion. In many 

 protozoa the body is fixed in form, and is moved by 

 hair-like projections, cilia, which cover 

 the body. There is often a permanent 

 mouth through which food enters the 

 body, and definite canals through which 

 waste products leave it. 



Complex animals. We already know 

 that by far the greater part of the 

 animal world is made up of forms much 

 larger and more complex than the 

 ameba. Most animals consist of collec- 

 tions of cells living together. In some 

 animals, such as the sponge, there is 

 very little difference in the cells which 

 comprise it. In other forms, like the 

 mammals, there are innumerable cells of which there 

 are many different kinds. 



As we look higher in the animal scale we find that 

 certain parts of the animal are set apart to do certain 

 work and only that work. In a community of people, 

 there are some men who do manual labor ; others who are 

 skilled mechanics; some who are shopkeepers; and still 

 others who are professional men. Just so, wherever an 

 animal is composed of many cells, there is division of 

 labor. Some cells are fitted to do one kind of work, 

 others to do another kind. As a result of this division 



Fig. 132. Para- 

 moecium, a one 

 celled animal, 

 but not so prim- 

 itive in struc- 

 ture as the 

 ameba. 



