CRYSTALLINE STYLE OF LAMELLIBRANCHS 69 



bend beneath the heart. In some forms (e.g., Mytilus gallo- 

 provincialis, according to List, '01) this diverticulum is pro- 

 longed a short distance posteriorly as a blind sac distinct from 

 the intestine. 



The style sac in certain of the higher lamellibranchs has be- 

 come completely cut off from the intestine, forming thus a dis- 

 tinct caecum. This is entirely independent of the intestine, and 

 opens into the stomach by a separate orifice. Such a condition 

 is found in Martesia (fig. 5). 



As has been pointed out by Matthias ('14), there exist, there- 

 fore, three distinct types of lamellibranchs as regards the posi- 

 tion of the style, with transitional forms making a graded series 

 from one to the other. First, those species in which the intestine 

 itself bears the style; second, those in which the style sac and in- 

 testine form two tubes, incompletely separated by the typhlo- 

 soles, and finally the forms in which the style sac exists as a 

 diverticulum distinct from the intestine proper. 



The epithelium around the opening of the style sac into the 

 stomach is raised into a ring which forms a sort of 'bushing' 

 about the style. When the latter is fully formed it fits this ori- 

 fice so snugly that no particles from the stomach could po-sibly 

 pass between the style and the wall of the sheath, even if the 

 effective stroke of the cilia were in such a direction as to permit it. 



In the forms where the intestine and style sac are incompletely 

 separated by the typhlosoles, the major typhlosole extends an- 

 teriorly into the stomach, forming a sort of tubercle as in Ano- 

 donta, or a long ridge on the ventral wall of the stomach as in 

 Modiolus (fig. 12). Other lamellibranchs examined have shown 

 a somewhat similar prolongation of the major typhlosole. 



The stomach presents the" most diverse modifications in the 

 different species. In general it is an elongated, oval sac, the 

 lateral walls of which are more or less compressed. Various out- 

 pocketings of the epithelium are present, usually in connection 

 with the orifices of the hepatic ducts. The number of these 

 varies, according to Pelseneer, from three in certain of the 

 Nuculidae, to as many as twelve in Mytilus. Three are present 

 in Anodonta (fig. 4). 



