68 OKHAMANDAL MA I! INK ZOOLOGY REPORT 



In their enormous relative length the crystalline style and its sac attain a 

 developmenl most remarkable a development almost unique, among Lamellibranchs. 

 ( >nly in Anomia do we find the caecum so extremely elongated relatively to the other 

 sections of the alimentary canal. In Anomia we may also note that the inner 

 curvature of the caecum is turned away from the adductor, so that the caecal or distal 

 termination is near the anterior edge of the mantle, whereas in Placuna the curvature 

 of the caecum embraces the adductor and its distal end is situated a short distance 

 from the anus. 



Regarding the function of the crystalline style, I am fully convinced from a study 

 of this organ in the present species and in the pearl oyster (M. vulgarix) 1 that Barrois' 

 view is correct, that the style is a cuticular secretion which serves to invest diatoms, 

 sponge spicules and the like with a viscous coating which will secure the intestinal 

 walls from damage through abrasion. The extreme length of the style in Placuna 

 shows it to be of extreme importance in the economy of this mollusc ; it is to be 

 correlated with the particular habitat favoured muddy bottom and slightly brackish 

 water both conditions which are well known to be most favourable to the growth and 

 multiplication of diatoms, many of which, like Navicula, have sharply pointed siliceous 

 tests that would abrade the delicate wall of the molluscan intestine unless some 

 viscous investing substance were provided to form them into bolus-masses and serve 

 as a lubricant during the passage through the gut. The stomach contents of Placuna 

 bear this out ; there is a predominance of diatom tests over other recognisable 

 organisms a much greater relative abundance than is seen in the case of the stomach 

 contents of the pearl oyster. 



The intestine arises from the lowermost section of the stomach, close to the mouth 

 of the pyloric caecum and somewhat to the right side. It passes for a short distance 

 posteriorly and ventrally, and then, just anterior to the heart, it turns in a dorsal 

 direction, curves forward, and then passes downwards when near and at the level of the 

 biliary chamber of the stomach. This descending limb of the intestine crosses the 

 ascending limb on the left side, thus completing a single simple intestinal coil largely 

 within the substance of the liver. For some little distance, however, the wall of this 

 coil comes close to the surface of the visceral mass, and is obvious to the naked eye 

 as an elongated clear patch. The ventral curve of the intestinal coil lies close to and 

 just anterior to the heart. 



From this spot the intestine, which may now be differentiated as the rectum, 

 passes in a fairly straight course downwards and backwards along the postero-dorsal 

 curve of the adductor, then turns more distinctly in a posterior direction, bends slightly 

 upwards, and opens on a flask-shaped anal papilla surrounded with a widely-everted 

 membranous collar, and situated immediately dorsal to the distal extremities of the 

 gills. On the upper or postero-dorsal aspect of the rectum is a caecal prolongation of 



1 "Ceylon Marine Biological Reports," part ii., p. 78, Colombo, 1906. 



