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 Ameba 
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, 
