144 



MOLLUSCA 



attachment, the pallid linCy being readily seen in the cleaned shell. 

 In the hard-shelled clam^ Venus, which has two well-developed 

 siphons, we find a posterior invagination under the adductor muscle 

 called the pallial sinus. It marks the point of attachment of the 

 siphonal muscles. The siphons of fresh water mussels are very 

 poorly developed. 



Fig. 6 1. Model of clam. (Courtesy of Amer. Mus. of Nat. Hist.) 



Digestion. — The mouth has two pairs of labial palps, ciliated 

 externally; the gullet or esophagus is short. A pair of irregular dark 

 brown glands, the liver, surround the large stomach which has in it a 

 slender gelatinous rod, the crystalline style. T. C. Nelson (Biol. 

 Bull., 1925, vol. 49, no. 2, pp. 86-99) gives three views of the func- 

 tions of the crystalline style, (i) It contains amylotic ferments 

 which digest starch. (2) Its mucous or viscid secretion holds the 

 food long enough for proper digestion. (3) It separates the food 

 from foreign particles, acting as a " stirring rod." 



The intestine passes from the posterior end of the stomach to 

 the visceral mass, coils parallel to the first portion, then turns 

 backwards and proceeds as the rectum through the pericardium and 

 above the posterior adductor muscles, finally discharging into the 



