220 COLLEGE ZOOLOGY 



duced when the pharyngeal cavity is enlarged by the contrac- 

 tion of the muscles which extend from the pharynx to the body- 

 wall. 



In the pharynx, the food receives a secretion from the pharyn- 

 geal glands; it then passes through the oesophagus to the crop, 

 where it is stored temporarily. In the meantime the secretion 

 from the calciferous glands in the oesophageal walls is added, 

 neutralizing the acids. The gizzard is a grinding organ; in it 

 the food is broken up into minute fragments by being squeezed 

 and rolled about. Solid particles, such as grains of sand, which 

 are frequently swallowed, probably aid in this grinding process. 

 The food then passes on to the intestine, where most of the diges- 

 tion and absorption takes place. 



Digestion in the earthworm is very similar to that of higher 

 animals. The digestive fluids act upon proteids, carbohydrates, 

 and fats; in them are special chemical compounds, called fer- 

 ments or enzymes, which break up complex molecules without 

 themselves becoming changed chemically. The three most im- 

 portant enzymes are: (i) trypsin, which dissolves pro teid; (2) dias- 

 tase, which breaks up molecules of carbohydrates; and (3) steap- 

 sin, which acts upon fats. These three enzymes are probably 

 present in the digestive fluids of the earthworm. The proteids 

 are changed into peptones, the carbohydrates into a sugar com- 

 pound, and the fats are divided into glycerine and fatty acids. 



The food is now ready for absorption. This is accomplished 

 through the wall of the intestine by a process known as osmosis, 

 assisted by an ameboid activity of some of the epithelial cells. 

 ^"Osmosis is the passage of a liquid through a membrane. Upon 

 reaching the blood, the absorbed food is carried to various parts 

 of the body. Absorbed food also makes its way into the ccelomic 

 cavity andjs^ carried directly to those tissues bathed by the 

 ^ccelomic fluid. In 'one-celled animals, and in such METAZOA 

 as Hydra, PTanaria, and Ascaris, no circulatory system is neces- 

 sary, since the food either is digested within the cells or comes 

 into direct contact with them; but in large, complex animals a 



