ASSIMILATION 



51 



which proved best suited to their needs, and they could 

 not now exchange diets. Grain-eating animals have 

 teeth fitted for grinding and a well-developed system 



Fig. 36.— Diagram to illustrate the development of the alimentary canal in the 

 advance from lower to higher forms. 



for digesting plant tissues. Flesh-eating animals have 

 teeth fitted for tearing flesh and crushing bone, and a 

 less highly developed system for the digestion of flesh. 

 Animals that eat a mixed diet have both kinds of teeth 

 and a digestive system which in size and structure lies 

 between the two extremes. 



The Five Food Substances. — Although diets vary, we 

 find that the foods that animals eat differ only super- 

 ficially. ~No matter what an animal eats there are con- 

 tained in it only the five food substances: water, salts, 

 carbohydrates, hydrocarbons, and proteids. These all 

 animals must have. 



Water. — Water is contained to a greater or less ex- 

 tent in everything that we eat. It is possibly the most 

 important of the food substances, for not only does it 

 act as a food but also as a solvent of other food sub- 

 stances. These can not enter the cells unless they are 

 dissolved, for only liquids or gases can pass through the 



