DIGESTIVE SYSTEM. 109 



F. The Digestive System. 



1. The alimentary canal is a convoluted tube the greater 

 part of which lies in the visceral mass, imbedded in 

 the liver and generative organ, while the terminal 

 portion runs back through the pericardial cavity to 

 the anus. 

 Insert a seeker into the mouth, and, using this as a guide, 

 slit up the oesophagus and stomach with scissors. Slit open 

 in a similar fashion the whole length of the intestine, begin- 

 ning at the rectum and working backwards to the stomach 

 (cffig. 29, p. 104). A vertical longitudinal section of the whole 

 visceral mass is a useful guide to this dissection. 



a. The mouth lies immediately behind the anterior 



adductor, and between the lips formed by the 

 palps : there are no jaws. The food, which con- 

 sists of minute organisms of various kinds, both 

 animal and vegetable, is swept in at the hinder 

 end of the shell by the inhaled stream of water, 

 and carried forwards to the mouth by the action 

 of the cilia covering the palps. 



b. The oesophagus is a short straight tube passing 



almost vertically upwards behind the anterior 

 adductor. 



c. The stomach is a dilated chamber partially sub- 



divided by folding of its walls. A crystalline 

 style is found within it at certain seasons. 



d. The intestine arises by a small orifice from the 



ventral surface of the stomach, and runs back 

 wards and downwards into the visceral mass, 

 where it is closely surrounded by the generative 

 gland. It follows the course shown in fig. 29, 

 and finally passes into the rectum. 



e. The rectum, which is rather wider than the intes- 



tine, runs upwards through the visceral mass to 

 the anterior end of the pericardial cavity, through 

 which it runs horizontally backwards, sur- 

 rounded by the ventricle. Behind the pericardial 



K 



