lateral sections from which emerge six pairs of 

 nerves. In some bivalves the ganglion and the 

 nerves are pigmented, but not in the oyster. 



The pair of cerebral ganglia of the oyster 

 (fig. 253, e.g.) is located at the bases of the labial 

 palps. The position of these small organs is 

 slightly asymmetrical in relation to the palps, 

 with the left ganglion slightly lower than the 

 right one. Because of such asymmetry, only 

 one ganglion is usually seen in the transverse 

 sections of the visceral mass made at the level 

 of the bases of the palps. In large and thin 

 ("poor") oysters the cerebral ganglia may easily 

 be located, but in "fat" specimens they are not 

 usually clearly visible. 



The narrow cerebral ganglion of each side is 

 bent at a sharp angle: this gives it the appearance 

 of a saddle sitting over the basal membrane of the 

 epithelium of the palps (fig. 255). The cerebral 

 commissure emerges from the end of the ganglion 

 (c.com.) and makes an inverted U-shaped loop 

 which passes dorsally over the esophagus and 

 connects with the ganglion of the opposite side. 



The following nerves emerge from the anterior 

 side of the visceral ganglion: The cerebro-visceral 

 connectives rise from the anterior side of the 

 ganglion and soon are buried in the connective 

 tissue of the visceral mass (figs. 253, 254, 255, 

 c.v.c.) ; next to the connectives and slightly dorsal 

 to their roots are the anterior pallial nerves 

 (a.p.n.), which run forward through the kidney, 

 pass across the pericardium wall toward the dorsal 

 part of the body and along their course give off 

 branches extending to the central part of the 

 mantle. Two branchial nerves (figs. 253, 254, 

 br.n.), arise near the roots of the cerebro-visceral 

 connective and run parallel to it for a short 

 distance. These nerves are more easily recogniz- 

 able than the others because they form a convex 

 curve and enter the gill axis on each side of the 

 body accompanied by the efferent blood vessels. 

 The two distinct branches of the lateral pallial 

 nerves originate from the extreme points on the 

 sides of the visceral ganglion. The outer branch 

 (upper in fig. 254) almost immediately enters the 

 mantle, while the inner branch (lower in fig. 254) 

 divides into numerous smaller nerves which estab- 

 lish contact with the circumpalhal nerve (fig. 253, 

 c.p.n.). The posterior pallial nerve (fig. 253, 255, 

 p.p.n.) runs from the ventral end of the ganghon 

 along the ventral side of the adductor muscle as 

 far as the rectum and divides into smaller branches 



com. p. ft. 

 Ikn 



ad. n. 



[.p. n. 



Figure 255. — Diagram of the nervous system of C. vir- 

 ginica seen from the anterior side. ad.n. — adductor 

 muscle nerve; a.p.n. — anterior pallial nerve; br. n. — 

 branchial nerve; e.g. — cerebral ganglion; c. com. — cere- 

 bral commissure; com.p.n. — common pallial nerve; 

 c.v.c. — cerebro-visceral connective; Ib.n. — labial nerve; 

 l.p.n. — lateral pallial nerves; oe. — esophagus; p.o. — 

 pallial organ; p.p.n. — posterior pallial nerve; v.g. — 

 visceral ganglion. 



which penetrate the mantle. The pallial organ 

 (fig. 255, p.o.) is located along the course of this 

 nerve. A pair of adductor nerves arises from the 

 dorsal side of the ganglion and immediately enters 

 the muscle tissue (fig. 255, ad.n.). They are not 

 visible from the ventral side of the muscle. 



284 



FISH AND WILDLIFE SERVICE 



