MOLLUSCA. LAMELLIBRANCHIA 



201 



margin of the mantle and especially on the ventral siphon. 

 In some forms eyes occur at the ventral margin of the 

 mantle-lobes; they are especially well-developed in the 

 genus Pecten. The heart (h) is placed dorsally, just below 

 the hinge, and is surrounded by a large pericardial cavity; 

 it consists of two auricles, and a ventricle, which, as 

 already mentioned, extends round the intestine. The 

 renal organs consist of a pair of glandular tubes under- 

 neath the pericardium. In almost all cases the sexes are 

 separate, but a few forms are hermaphrodite.] 



u. 



\-aa, 



Fig. 92. Meretrix (Callista) chione, Recent. A, dorsal view of the two 

 valves. B, interior of left valve, x \. 



a, anterior border ; p, posterior ; d, dorsal ; v, ventral ; hi, 

 lunule ; n, umbo ; I, ligament ; aa, anterior adductor impression ; 

 pa, posterior adductor ; pi, pallial line ; s, pallial sinus ; w, x, y, 

 cardinal teeth; z, anterior lateral tooth. 



As already mentioned, the two valves of the shell are 

 placed on the sides of the animal. The margin near the 

 hinge (fig. 92, d) is dorsal, the opposite (v), where the 

 valves open, is ventral ; that near the mouth is anterior (a), 

 that near the anus and siphons posterior (p). In the 

 majority of cases the two valves are equal or almost equal 



