22 ANIMAL LIFE 



eating, breathing, feeling, and multiplying. These pro- 

 cesses are performed among the higher animals by various 

 organs, special parts of the body, each of which is fitted to 

 do some one kind of work, to perform some one of these 

 processes. There is a division or assignment of labor here 

 among different parts of the body. Such a division of 

 labor, and special fitting of different parts of the body for 

 special kinds of work does not exist, or exists only in 

 slightest degree among the simplest animals. The Amceba 

 eats or feels or moves with any part of its body ; all of the 

 body exposed to the air (air held in the water) breathes ; 

 the whole body mass takes part in the process of repro- 

 duction. 



Only very small organisms can live in this simplest way. 

 So all of the Protozoa are minute. When the only part of 

 the body which can absorb oxygen is the simple external 

 surface of a spherical body, the mass of that body must be 

 very small. With any increase in size of the animal the 

 mass of the body increases as the cube of the diameter, 

 while the surface increases only as the square of the diam- 

 eter. Therefore the part of the body (inside) which re- 

 quires to be provided with oxygen increases more rapidly 

 than the part (the outside) which absorbs oxygen. Thus 

 this need of oxygen alone is sufficient to determine the 

 limit of size which can be attained by the spherical or sub- 

 spherical Protozoa. 



That the simplest animals, despite the lack of organs 

 and the primitive way of performing the life processes, live 

 successfully is evident from their existence in such ex- 

 traordinary numbers. They outnumber all other animals. 

 Although serving as food for hosts of ocean animals, the 

 marine Protozoa are the most abundant in individuals of 

 all living animals. The conditions of life in the surface 

 waters of the ocean are easy, and a simple structure and 

 simple method of performance of the life processes are 

 wholly adequate for successful life under these conditions. 



