28 CONCHIFERA. 



back, for the admission of food and water, as well as for 

 the ejection of faecal matter ; while in others it is more 

 or less closed in front, or open only 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 usually takes place on the posterior 

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

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

 placed 011 the anterior side, and the excurrent 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 termed " sessile." The excretory 

 opening is always situate on the upper part of the pos- 

 terior 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 excretory opening or tube is the smaller 

 of the two. With respect to the reproductive system of 

 the Lamellibranchiata, it is by no means settled whether 

 any, or which of them, have separate sexes. Loven is 

 positive that such is the case in Modiolaria, the em- 

 bryogeny of which he has investigated with his usual 

 care; and Sars assures us ih&tAxinus is unquestionably 

 also dio3cious. I have not myself examined the question ; 

 but I would refer my readers to what I have already said 

 in page xxv of the Introduction to the first volume. 



