ALIMENTARY SYSTEM. 49 



A. SYSTEMS OF NUTRITIVE ORGANS: THEIK SPECIAL MOR- 

 PHOLOGY AND PHYSIOLOGY. (For 13 see p. 62.) 



Alimentary System (Organs of Alimentation). Earth-worms 

 feed mainly upon leaves or decaying vegetable matter, but 

 will also eagerly devour meat, fat, and other animal sub- 

 stances. They also swallow large quantities of earth from 

 which they extract not only any organic materials that it may 

 contain, but probably also moisture and a small amount of vari- 

 ous salts. The most essential and characteristic part of their 

 food is derived from vegetal or animal matter in the form of 

 various organic compounds, of which the most important are 

 proteids (protoplasm, albumen, etc.), carbohydrates (starch, 

 cellulose), and/ate. These materials are used by the animal in 

 the manufacture of new protoplasm to take the place of that 

 which has been used up. It is, however, impossible for the ani- 

 mal to build these materials directly into the substance of its 

 own body. They must first undergo certain preparatory chemi- 

 cal changes known collectively as digestion and only after the 

 completion of this process can all the food be absorbed into the 

 circulation. For this purpose the food is taken not into the 

 body proper, but into a kind of tubular chemical laboratory 

 called the alimentary canal through which it slowly passes,. 

 being subjected meanwhile to the action of certain chemical sub- 

 stances, or reagents, known as digestive ferments. These sub- 

 stances, which are dissolved in a watery liquid to form the diges- 

 tive fluid, are secreted by the walls of the alimentary tube. 

 Through their action the solid portions are liquefied and the food 

 is rendered capable of absorption into the proper body. 



The alimentary canal is divisible into several differently con- 

 structed portions playing different parts in the process of alimen- 

 tation. Going backwards from the mouth these are as follows : 



1. The pharynx (Fig. 24, ph\ an elongated barrel-shaped 

 pouch extending to about the 6th somite. Its walls are thick 

 and muscular, and from their crelomic surface numerous small 

 muscles radiate on every side to the body- wall. When these 

 muscles contract, the cavity of the pharynx is expanded ; and if 

 the mouth has been previously applied to any solid object, such 

 as a leaf or pebble, the pharynx acts upon it like a suction-pump. 



