MOLLUSCA—THE ALIMENTARY CANAL 



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 ajDertures 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 

 coecal outgrowth of the stomach, the spiral eoeeum. 



In very many LameUibranchia there is a diverticulum of the 

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

 cular formation, called the crystalline stylet. Similar structures occur 

 in the Frosobranchia, and es^jecially in the lihipidoglossa and Toxoglossa. 



In many Opisthobranchia, 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 Avith 

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

 portion of the visceral dome, where this is present. 



1. Amphineura. 



The Chitonidre show the typical division of the mid-giit 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 anotlier. The 

 digestive gland is paired ; the larger liver 

 to the right has four apertures, while tlie 

 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 microsco})ic 

 algse. Fio. i(i3.— Part of a horizontal median 



Unlike the Chitonidce, the Solenogastres section through Proneomenia Sluiteri. 

 show no separation of the mid -gut into ^«f^ °^ ^^'^ first, second third and fourth 

 , . mi • 1 order are seen projecting from the right and 



Stomach and small mtestnie. The mid-giit jgft jn^o the lumen of the mid-gut. In the 

 runs straight through the body, the greater background is the dorsal wall of the gut, with 

 [lart of which it tills. The glandular lateral the groove which cuts into the hermaphrodite 

 cceca found in Neomenia, Proneoniema, gl^"*^ ('^f- ''S- 53, p. 4l). 

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



