CRYSTALLINE STYLE OF LAMELLIBRANCHS 73- 



acts as a brace for the shield. In Ostrea the smallest of the 

 three lobes is concave on its upper surface, forming a bowl-like 

 depression. 



In Donax, according to Barrois, the gastric shield covers 

 nearly the entire wall of the stomach. At the crypts of the 

 hepatopancreas it dips down into the orifices and lines them. 

 Martesia (fig. 7) exhibits the same type of shield. 



In all cases which I have examined the shield bears at its 

 apex a blunt projection of much heavier nature than the lobes 

 themselves. Under natural conditions the head of the style is 

 directed against this projection, while the food mass around the 

 style is in contact with the lateral lobes of the shield. The 

 significance of this will appear later. 



Nervous and vascular connections of the style sac 



According to Schwanecke ('13), blood is supplied to the style 

 sac and other parts of the alimentary canal through the visceral 

 artery, a large branch which leaves the anterior aorta just ven- 

 trad the stomach, and courses posteriorly to the upper part of 

 the intestine and style sac. Here it divides to form two large 

 trunks, which in turn supply the greater part of the alimentary 

 canal. Most of the blood going to the intestine is carried to the 

 typhlosoles. Here there are no true capillaries but the smaller 

 arteries form large lacunae beneath the epithelium. As a result, 

 an almost continuous haemal cavity i^ formed in close conjunc- 

 tion with the secretory and ciliated cells (fig. 7). 



The venous system is less well defined than the arterial. The 

 blood from the lacunae is collected by several large veins and 

 carried to the sinus venosus. 



The lamellibranchs, according to Pelseneer (Lankester's 

 Treatise), have no differentiated stomatogastric nervous system. 

 Nerve strands to the alimentary canal are given off from the 

 median faces of the two branches of the cerebrovisceral commis- 

 sure. More recently, Splittstoeszer ('13), reviewing the work of 

 Keber and Duvernoy on the nervous system of molluscs, has 

 presented a careful and extensive investigation of the nervous 



