FUNCTION AND STRUCTURE 45 
digesting surface is simply the external surface of the body. 
And not alone among the one-celled animals. Many of the 
parasitic worms which live in the bodies of other animals, 
and the larve or “grubs” of many insects which lie in the 
tissues of plants bathed by the sap, have no inner alimen- 
tary canal, but take food through the outer surface of the 
body. But in these cases the food is ready for immediate 
absorption, so that no special treatment of it is necessary, 
hence no complex structures are required. 
Even were no such special treatment of the food neces- 
sary in the case of the larger animals, it would still be im- 
Fie. 37.—Diagram illustrating increase of volume and surface with increase of 
diameter of sphere. 
possible for the simple external surface of the body to serve 
for food absorption, because of the well-known relation 
between the surface and the mass of a solid body. When 
a solid body in the form of a sphere increases in size, its 
mass or volume increases as the cube of the diameter, while 
the surface increases only as the square of the diameter 
(Fig. 37). The external surface of minute animals a few 
millimeters in diameter can take up enough food to supply 
the whole body mass. But among large animals this food- 
getting surface is increased as the square of the diameter of 
