Invertebrate Gallery of the Indian Museum. 137 



microscopic particles on which the Bivalve feeds, into 

 the mantle-chamber ; and by the upper one (exbalant, 

 excurrent, or anal siphon) the effete water, charged with 

 excreted waste, leaves the mantle-chamber. Even in 

 those Bivalves in which the mantle-lobes are separate 

 beneath the foot the existence of these two channels — 

 incurrent below, and excurrent above, can be demon- 

 strated, although there are no tubular siphons, but only 

 two slight folds of the posterior edge of the mantle 

 separated by a limited line of cohesion. It is only in 

 a few forms [e.g., the Oysters, etc.) that no definite 

 siphons occur. When the mantle-lobes are separate 

 their thickened edges are often fringed with tentacles, 

 and are sometimes furnished with a row of eyes (see 

 the specimen of Avnissiitni mounted in spirit above Case 

 179). The tentacles are often confined to the posterior 

 edge of the mantle, where they surround the incurrent 

 orifice (see the specimen of the Fresh water Mussel above 

 Case 173). 



As in other typical Molhisca, the intestine, the excretory 

 organs, and the reproductive organs open into the mantle- 

 chamber, which also in the Bivalves sometimes serves as a 

 brood-pouch in which the eggs are hatched. 



The sexes among the Bivalves are sometimes separate, 

 and are sometimes united in the same individual. In the 

 latter case, as in some Oysters, the male and female ele- 

 ments may be matured, not at the same time, but at 

 different seasons. 



The nature of the Lamellibranch shell should be studied 

 in the several preparations in Case 172 A. 



The " valves " are, as already stated, right and left : 

 they are usually, though not always, equal and symmetrical, 

 and are often covered with an outside skin, or epiderm, of 

 varying thickness. 



In each valve we find, typically, a turned-in beak, or 

 " umbo ■", which generally looks forwards and marks the 

 front of the contained animal : the usually short and blunt 

 portion of the shell below the umbo is therefore the front 



