DIGESTION 



199 



tube which is composed largely of muscle fibres. The muscles 

 by peristaltic contraction force the contents of the canal slowly 

 backward. They also regulate the size of the canal as the 

 volume of the contents may demand. 



Fig. 108. — Wall of the intestine in a small aquatic Annelid, Chaitogaster. 

 There are only two thin layers of cells, one (o) which forms the linJlig of the 

 body cavity and (b) the intestinal epithelium. 



458. The separation of the digestive processes advances a 

 step farther in Arthropods and the digestive tract is divided 

 into well-defined regions. In the crayfish there are, in the 

 immediate region of the mouth, six pairs of segmental appen- 

 dages which are modified for grasping and tearing up the food. 



Fig. 109. — A section through one of the folds of the intestinal epithelium of 

 nereis, showing a few of the glandular cells. The inner ends of the greatly 

 elongated cells are filled with a granular secretion. The accumulation of the 

 secreted substance in the ends of the cells causes them to swell and hence throws 

 the surface into folds. 



From the mouth a short oesophagus leads into a large muscular 

 stomach, which consists of two divisions. The first is Hncd 

 with chitin and is provided with a mechanism consisting of 

 several chitinous hooks or teeth and a set of muscles for oper- 

 ating them. By means of this the food is still further broken 

 up. A pair of large digestive glands lying in the body cavity 



