58 Invertebrate Zoology. 



The Digestive System. The month, already noted, is 

 provided with an upper and an under lip, two transverse 

 folds that apparently connect the upper and the under 

 palps. (The position of the mouth can be very clearly seen 

 if the clam is carefully removed from its shell and pinned 

 out under water.) The posterior portion of the alimentary 

 tract passing through the ventricle has already been noted 

 as the rectum. Follow the rectum in its further course over 

 the posterior adductor to its final opening into the cloaca. 



To now trace out the course of the entire digestive tube 

 considerable care must be exercised. Carefully pick or 

 cut away, piece by piece, the side of the visceral mass. The 

 short (esophagus will be found to lead upward and back- 

 ward into the stomach. The latter is an irregular sac situ- 

 ated dorsally and anteriorly, and has, opening into it, the 

 ducts from the digestive gland or liver. The liver almost 

 entirely surrounds the stomach. 



The intestine is long and coiled. It begins at the poste- 

 rior end of the stomach, passes to the lower posterior por- 

 tion of the visceral mass, and then curves to the left and 

 forward, soon to again bed itself deeply into the surrounding 

 viscera and take on several convolutions. Finally the intes- 

 tine passes upward, posterior to and nearly parallel with the 

 first section until, at the level of the stomach, it bends 

 sharply backward to pass through the ventricle of the heart. 

 During a portion of its course the lumen of the intestine is 

 partially filled by a fold, the typhlosole. The tract does not 

 lie in a " body-cavity." 



During the dissection of the digestive tract a long gelati- 

 nous cord has doubtless been noted. This is the crystalline 

 style. Its function is not known. 



Make a drawing of the alimentary tract. 



