FOOD /IND /41R TURNED INTO FLESH /iND ENERGY 89 



ering the production of energy and making of body mate- 

 rial 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 



Fig. 52. 



-Mouth, with prehensile under lip, of young dragon-fly. 

 Jenkins and Kellogg.) 



(From 



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. 



Now 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 

 accomplished by certain liquids called digestive fluids. 

 The digestive apparatus varies like other parts of the ani- 

 mal organism, being most simple in some forms and very 

 complex in others. In Amoeba the food particles are 

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

 in other Protozoa. The simple digestive cavity of the 

 hydra has been referred to (fig. 54). In the polyps and 

 jelly-fishes (see Chapter X), this cavity is extended, the 

 digestive surface being much increased by partitions, 



