170 AN INTRODUCTION TO ZOOLOGY 



XII. The first pair are pouches pushed out from the alimentary 

 canal and opening directly into the oesophagus. The other two 

 pairs are swellings of the cesophageal wall ; they have a number of 

 small cavities which open directly through the epithelium into 

 the oesophagus in somite XIV. Carbonate of lime is produced 

 by these glands, and poured into the alimentary canal, where it 

 probably neutralizes acid foods (143). 



Nutrition. FOOD. The food of the earthworm consists 

 principally of pieces of leaves and other vegetation, particles of 

 animal matter, and soil. This material is gathered at night. At 

 this time the worms are active; they crawl out into the air, and, 

 with their tails holding fast to the tops of their burrows, explore 

 the neighborhood. 



INGESTION. Food particles are drawn into the buccal cavity 

 by suction produced when the pharyngeal cavity is enlarged. 

 This is accomplished by the contraction of the muscles which 

 extend from the pharynx to the body wall. 



DIGESTION. In the pharynx, the food receives a secretion from 

 the pharyngeal 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 or- 

 gan; in it the food is broken up into minute fragments by being 

 squeezed and rolled about. Solid particles, such as rough pebbles, 

 which are frequently swallowed, probably aid in this grinding 

 process. The food then passes on to the intestine, where most of 

 the digestion 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 

 important enzymes are (i) trypsin, which dissolves proteid, 



(2) diastase, which breaks up molecules of carbohydrates, and 



(3) steapsin, which acts upon fats. These three enzymes are 



