140 OYSTERS, AND ALL ABOUT THEM. 



more or less closed in front, or only open in that part for 

 the passage of the foot. In the latter case, the mantle on 

 one or either side is folded, so as to form a single or 

 double tube. This takes place usually on the posterior 

 side, where the shell is broadest ; but in a few instances 

 (as in the Kelliidse) the incurrent or alimentary tube is 

 placed on the anterior side, and the current or anal tube 

 on the posterior side. The tubes are of various lengths, 

 and when they do not project beyond the edges of the 

 shell, they are called "sessile" The excretory organ is 

 always situated on the upper part of the posterior side. 

 That by which the animal takes in its nourishment, and 

 which supplies the gills with aerated water, is usually on 

 the same side, but below the other opening. The excre- 

 tory opening or tube is the smaller of the two. 



REPRODUCTIVE SYSTEM OF THE LAMELLIBRANCHIATA. 



It has always been an unsettled question whether any 

 (or which of them) have separate sexes. Love*n is positive 

 that such is the case in the Modiola, the Embryogeny of 

 which he has investigated with his usual care. And Sars 

 assures us that AXINUS is unquestionably also dioecious. 



