16. ANIMAL LIFE 
the body, while the volume or food-using surface of the 
body is increased as the cube of its diameter. The food sup- 
plying can not keep pace with the food using. Hence it is 
absolutely essential that among large animals the food-tak- 
ing surface be increased so that it will remain in the same 
favorable proportion to the mass of the animal as is the 
case among the minute animals, where the simple external 
body surface is sufficient to obtain all the food necessary. 
This increase of surface, without an accompanying increase 
of size of the animal, is accomplished by having the digest- 
ing and assimilating surface inside the body and by having 
it greatly folded. The surface of the alimentary canal is, 
after all, simply a bent-in continuation of the outer surface 
of the body. It is open to the outside of the body by two 
openings, and wholly closed (except by its porosity) to the 
true inside of the body. By the bending and coiling of 
the alimentary canal, and by the repeated folding of its 
inner wall, the alimentary surface is greatly increased. 
The necessity for this increase accounts largely for the 
complexity of the alimentary canal. 
But it is not alone this necessity for increased surface 
that accounts for the great specialization of the alimentary 
canal in such animals as the insects and the vertebrates. 
The structural differences in different portions of the canal, 
resulting in the differentiation of the canal into distinct 
parts, or the differentiation of the whole organ into distinct 
subordinate organs, each with a special work or function to 
perform, are the result of the necessity for the special 
manipulation of the special kinds of foods taken. Animals 
which feed on other animals must have mouth structures 
fit for seizing and rending their prey, and the alimentary 
canal must be specially modified for the digestion of flesh. 
Animals which feed on vegetable substances must have 
special modifications of the alimentary canal quite different 
from those of the carnivores. Some insects, like the mos- 
quito, take only liquid food, the sap of plants, or the blood 
