One-Celled and Many-Celled Animals 325 



In the bird and in all higher animals this is not so. Organs 

 are not lacking to aid in carrying on the life processes. They 

 are necessary in all many-celled animals in order to carry on these 

 processes effectively. 



The amoeba secures air through any part of its body ; the bird 

 requires special organs, called lungs, for this purpose. The amceba 

 may take in food through any part of its body; the bird must use 

 its mouth. The amceba may use any part of its body for locomo- 

 tion ; the bird requires legs and wings to aid in moving from place 

 to place. The amceba is simple in structure while the bird is com- 

 plex in structure. 



As we pass from the lower to the higher forms of life there 

 is an increasing complexity of structure, and this is always accom- 

 panied by a greater division of labor. That is, when organs are 

 found in an animal, each organ performs a separate function, or 

 seems to be set apart to do a special kind of work. Perhaps the 

 idea may be made clearer by comparison with a factory where 

 each person does the kind of work he is best fitted to do and 

 thus aids in the making of the finished product. So in all higher 

 animals and plants there is a division of labor among the organs 

 which compose them, some organs being fitted or adapted for one 

 kind of work and others fitted or adapted for another kind of 

 work. 



Unit of Structure and Function. Although both the para- 

 mecium and the amceba are single-celled animals it should be noted 

 that the amceba is the more simple in structure, since it has no 

 definite part through which food is taken. Each performs all the 

 functions necessary to life, and either may be used to illustrate 

 the fact that the cell is the unit of structure and of function in 

 animals of more complex organization. 



More complex animals are composed of organs. The organs 

 are made up of tissues, and each tissue in turn is made up of a 

 collection of cells of the same structure and function. The study 

 of a protozoan shows how a single cell carries on all the vital 

 activities of life. A tissue, for example a muscle, is complex in 

 structure and simple in function, while a cell, an amceba for 

 example, is simple in structure and complex in function. 



