60 THE ANIMALS AND MAN 



Protozoa, however, there is a definite mouth-place, as in 

 Paramoecium, where the food-particles are gathered to- 

 gether in a little ball by the cilia, and then pushed through 

 the body-wall. The body of the fresh-water hydra (see 

 Chapter XIII), incloses a digestive cavity, the mouth being 

 but an opening to this. In the higher animals we find 

 mouths arranged for cutting, filing, sucking, crushing, 

 gnawing, grinding, chiseling, piercing, sawing; in fact 

 almost every device one could think of for working in wood, 

 bone, shell, flesh, liquid, soft and hard material of many 

 forms. 



To understand the process of digestion some knowledge 

 of the nature of food substances is necessary. In con- 

 sidering the production of energy and making of body 

 material we saw that the same substances provided for both. 

 In fact whatever the form of food, animal or plant, the 

 elementary substances are the same, being conveniently 

 classified into two great groups, organic and inorganic 

 substances. 



Inorganic food substances are water and certain minerals 

 of which common salt is one. Organic food substances 

 are of three kinds or groups. The first group, called the 

 proteids, of which the white of egg is an example, forms 

 a large part of the tissues of animals; the second group 

 is made up of the fats and oils; the third, known as the 

 carbohydrates, consists of the starches and sugars. 



Digestion consists in changing all these substances into 

 soluble form so that they can be absorbed into the body, 

 circulate with the blood, if there be any, and then pass 

 into the living cells for their use. This change is ac- 

 complished by certain liquids called digestive fluids. The 

 digestive apparatus varies like other parts of the animal 

 organism, being very simple in some forms and very com- 

 plex in others. In Amoeba the food-particles are retained 

 in spaces in the cell until they are digested. So in other 



