76 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 



