50 WALTER K. FISHER, 



the head cavity, thence passes upward to the floor of the anterior 

 angle of the pericardial cavity. Here, just beneath the surface, a 

 junction is formed with the pleuro-osphradial connective, and usually 

 also, the posterior left parietal (L. Par) departs from this point. From 

 this junction point the main nerve now turns abruptly forward, on 

 the floor of the pericardium, and parallel to the inner edge of the 

 spindle muscle {Ct.N). Sometimes, but not always, this portion of 

 the nerve divides and then rejoins, to form an elliptical loop, as it 

 were. Upon reaching the base of the efferent branchial vein the nerve 

 turns sharply to the right, and at this point there is a ganglion 

 (Ct.G). The ctenidial nerve (Ct.N 1 ) enters the efferent sinus of the 

 gill, and from the ganglion there are also given off two anterior left 

 parietals (L. Par'). The exact details of the ganglion, and the ar- 

 rangement of the two parietals which leave it are subject to con- 

 siderable variation. The figures are fairly typical. On entering the 

 efferent sinus the ctenidial nerve divides into two, one quite small 

 (CLN"), the other more prominent (Ct.N'), which run parallel in the 

 tissue of the outer wall of the vein. Fine nerves are given off at 

 frequent intervals, dorsally and ventrally (ctn). The ventral ones are 

 more easily seen, the dorsal ones being difficult to make out. The 

 former pass along the bases of the gill leaves, and are soon lost. 

 The main ctenidial nerve runs to the tip of the gill. 



The two anterior parietal nerves (L.Par 1 ) start out from the 

 branchial ganglion and soon diverge. The inner one is very delicate 

 and spreads out over the central portion of the hood. The other 

 joins the anteriormost branch of the left anterior pallial, just as the 

 corresponding nerve does on the right side. 



Just posterior to the branchial ganglion there are usually two 

 small nerves which pass outward into the spindle muscle. 



The posterior parietal of the left side {L.Par) leaves the os- 

 phradial nerve at two different points, in different individuals. The 

 most usual point of departure as already described is at the junction 

 of the pleuro-osphradial connective with the osphradial. In others 

 the osphradial ganglion is the point. In both cases, however, the 

 nerve goes outward and backward in the floor of the pericardium, one 

 branch joining the posterior pallial, the other (Nph.N 1 ) following a 

 course exactly like the same branch of the other side. As mentioned 

 above, a direct inter-connection of these two nephridial nerves has not 

 been made out. 



This finishes the principal nerves to the body wall, mantle and foot. 



