130 MOLLUSC A. LAMELLIBRANCHIATA. 



third pair placed beneath the posterior adductor muscle. 

 From these ganglia, nerves are given off to the muscles, 

 gills, etc. Tactile organs are present on the 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 ; it is surrounded by a large pericardial cavity, 

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

 already mentioned, extends round the intestine. From 

 the ventricle, an anterior and a posterior aorta are given 

 off. In nearly all cases the blood is colourless. The renal 

 organs consist of a pair of glandular bodies underneath 

 the ventricle. In almost all forms the sexes are separate, 

 but a few are hermaphrodite. 



As already mentioned the two valves of the shell are 

 placed on the sides of the animal and not dorsally and 

 ventrally as in the brachiopods. The margin near the 

 hinge (fig. 45 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 practically 

 equal, and each valve is generally inequilateral. But 

 in some (e.g. Pectunculus) the shell is equilateral, in 

 others (e.g. Ostrea) it is inequivalve. Each valve may be 

 regarded as a greatly depressed hollow cone, the apices of 

 which form the umbones (u) ; these are sometimes straight 

 (e.g. Pecten), but generally curved towards the anterior 

 margin. In a few genera (e.g. Nucula, Trigonia) they are 

 directed posteriorly. In Diceras the umbones are spiral. 

 There is sometimes, placed in front of the umbone and 

 bounded by a groove, an oval depressed area (lu), half 

 occurring on each valve, this is termed the lunule, and it 





