176 THE FRESH- WATER MUSSEL [CH. VI 



for the absorption of digested products. The walls of the 

 stomach are thin and produce a cuticular lining, the 

 fleche tricuspide, which is thought by some authorities 

 to serve as a protection to the secretory cells against 

 hard or sharp food particles. 



The posterior region of the stomach gives off upon 

 its right side a csecal diverticulum, which, however, is 

 not completely separated either from the stomach itself 

 or from the intestine, but appears rather as a fold of 

 the latter. By constriction of the lips of the fold com- 

 munication between the cavity of the caecum and those 

 of the stomach and intestine can probably be closed. In 

 Dreissensia polymorpha, another British fresh-water la- 

 mellibranch, the caecum is entirely separated from the 

 intestine as a distinct tube opening into the stomach. 

 Within the caecum is developed a remarkable structure, 

 the crystalline style. This is a rod-like body, some 

 two or three inches in length, tapering posteriorly, and 

 composed of rather firm transparent gelatinous material, 

 arranged in concentric layers. It is probably a special 

 development of the fleche tricuspide. The precise function 

 of the crystalline style has been variously assigned. By 

 some it is considered to act as a pestle and to serve 

 in mixing the food with the gastric ferments, the style 

 itself becoming at the same time softened and dissolved, 

 furnishing a soft glutinous envelope to any angular 

 particles in the food (Schultze). According to Hazay 1 , 

 from spring to autumn the stomach is filled with a 



1 Malacozool. Blat. in. 1881 ; cf. Krukenberg, Vergl. physiol. Vortrdge, 

 n. 1882, Heidelberg. 



