THE ALIMENTARY CANAL. 



81 



valve, but is not so clearly distinguished from the gullet, 

 so that regurgitation is easy. 43 Indeed, it is common for 



FIG. 46. Anatomy of a Lamellibranch (Mactra) : a, shell ; h, mantle ; e, tentacles, or 

 lips ; d, mouth ; e, nerves ; /, muscles ; g, anterior, mid n, posterior ganglion ; h, 

 liver; i, heart: k, stomach ; I, intestine passing through the heart; w, kidney o, 

 anal end of the intestine ; p, exhaleiit, aud q, iuhalent respiratory tubes, or si- 

 phoiis ; r, gills ; 8, foot. 



