78 ELEMENTARY ZOOLOGY. 



water (respiration); the burning (or oxidation) of the 

 substances in the cells whereby heat, motion, and other 

 necessary forms of energy are secured; the getting rid 

 of the waste materials that always result from this oxida- 

 tion (excretion). 



This seems a very complex process, and we have given 

 technical names for the various steps of it; but they are 

 all really parts of one function the preparation and use 

 of food. The organs may be discussed under the following 

 heads: the digestive, the respiratory, the circulatory, and 

 the excretory organs. 



83. The Digestive Tract. This is a cavity in the body 

 into which solids and fluid foods may be taken, in order 

 that they may be absorbed through the wall into the 

 system; for food is not really within the body until it has 

 passed through the wall of the digestive tract, and food 

 cannot be used by the cells until it is capable of passing 

 as a fluid through cell walls. Most commonly the digest- 

 ive tract is similar to that of the grasshopper a tube 

 passing through the body, with two openings (Fig. 5) ; 

 frequently, however, it is merely a "blind" sac, with 

 only a mouth (Figs. 9 and 46) . This tube is always lined 

 with a complete layer of cells, whose duty it is to secrete 

 juices for the changing of the food and to absorb the 

 food from the tract. 



In a general way the anterior part of the tube is intended 

 to prepare the food for absorption (digestion), and the 

 posterior part actually absorbs it, though some of each 

 work is being done throughout the tract. 



Digestion is of two kinds : physical and chemical. Phys- 

 ical digestion consists in the breaking up of the food into 

 small bits as in chewing or dissolving ; the chemical diges- 



