VII 



MOLLUSOA—THE ALIMENTARY GANAL 



191 



The stomach is not infrequently a lateral outgrowth of the mesen- 

 teric wall, so that the aperture (cardia) leading into it from the oeso- 

 phagus and that leading out of it into the small intestine (pylorus) 

 are more or less near one another. A sort of connection between 

 these apertures may arise, a ciliated furrow or channel bounded by 

 longitudinal folds running between them, and in some cases continued 

 into the adjoining sections of the alimentary canal. 



In the Cephalopoda, the duct of the digestive gland (the so-called 

 hepatic or gall duct) does not open direct into the stomach, but into a 

 ccecal outgrowth of the stomach, the spiral eoeeum. 



In very many LameUibranchia there is a diverticulum of the 

 stomach which contains within its lumen a rod-shaped gelatinous cuti- 

 cular formation, called the crystalline stylet. Similar structures occur 

 in the Prosobranchia, and especially in the Rhipidoglossa and Toxoglossa. 



In many Opisthoh'anchia, the inner wall of the stomach carries 

 variously-arranged cuticular teeth, dental plates, jaw plates, etc., which 

 serve for triturating the food. In such cases the muscular wall of the 

 stomach is strongly developed. 



The stomach is succeeded by a narrower tubular section of the 

 mid-gut, called the small intestine (intestinum), which usually forms 

 coils or loops ; these are more numerous in herbivorous or detri- 

 tivorous than in carnivorous Molluscs. 



The stomach, small intestine, and digestive gland, together with 

 part of the sexual organs, compose the whole or by far the largest 

 portion of the visceral dome, where this is present. 



1. Amphlneura. 



The ChitonidiE show the typical division of the mid-gut into stomach, digestive 

 gland, and small intestine. The stomach 

 lies far forward, and has a wide outgrowth 

 on one side, which is, functionally, a reser- 

 voir of secreted matter. The cardia and 

 the pylorus lie near one another. The 

 digestive gland is paired ; the larger liver 

 to the right has four apertures, while the 

 smaller one to the left has only one prin- 

 cipal aperture into the stomach. The 

 small intestine is more than four times as 

 long as the body, and it forms many loops 

 which are constant in their arrangement. 

 Chiton feeds on small or even microscopic 

 algfe. 



Unlike the Chitonidce, the Solenogasti-es 

 show no separation of the mid - gut into 

 stomach and small intestine. The mid-gut 

 runs straight through the body, the greater 

 part of which it fills. The glandular lateral 

 coeca found in Neomenia, Proneomeiiia, 

 etc., and called hepatic diverticula, are caused by the projection into the lumen from 



Fig. 163.— Part of a liorlzontal median 

 section througli Proneomeuia Sluiteri. 

 Septa of the first, second, third, and fourth 

 order are seen projecting from the right and 

 left into the lumen of the mid-gut. In the 

 baelcground is the dorsal wall of the gut, with 

 the groove which outs into the hermaphrodite 

 gland (of. Fig. 68, p. 42). 



