46 WALTER K. FISHER, 



from the left pleural ganglion. They pursue likewise the same relative 

 directions. But while it is the anterior right pallial that branches 

 near the ganglion, on the left side it is the posterior. The left 

 anterior pallial does not branch till after it has passed into the thick 

 tough tissue of the spindle muscle. Here three branches are given 

 off, which, pursuing different directions, redivide on reaching the 

 pallium, and ultimately join the circumpallial nerve. The left posterior 

 pallial divides into two branches very soon after leaving the ganglion, 

 and both enter the spindle muscle sooner than the corresponding nerve 

 of the right side. The anterior branch presently leaves the other, to 

 which it has been running parallel for a short way, and turning out- 

 ward passes under the left side of the pericardium, and on reaching 

 the spiudle muscle turns backward. Here it receives the 

 posterior left parietal (L. Par) which, like the right, is 

 a terminal branch of the osphradial nerve. The pallial 

 nerve then divides, on reaching the mantle, and joins the circum- 

 pallial. The course of the other branch of the posterior pallial is 

 much like that of the posterior pallial of the right side. As noted 

 above a slender nerve joins the two near their ends (P.P.N"). 



The circumpallial nerve. As has already been indicated 

 the circumpallial nerve (C.P.N) forms a complete circle around the 

 animal, near the mantle edge. It lies on the inner border of what 

 has been called the gland zone, and just outside the circumpallial 

 blood sinus, about midway between the dorsal and ventral surfaces 

 of the pallium. 



The branches of the pallial nerves join the circumpallial from 

 below, passing under the circumpallial sinus. Throughout this nerve 

 ring there are ganglionic cells scattered in the form of frequent nodes, 

 particularly where the large pallial nerves join, and where the more 

 prominent branches to the margin depart. The appearance of the 

 circumpallial with its frequent loops can best be gained from the 

 figure. The nerves which depart to the margin of the mantle vary 

 considerably in size. They anastomose in a very intricate way, and 

 where the more prominent join each other there are tiny ganglionic 

 swellings. The final branches of these nerves reach the mantle border 

 and most of them enter the little sense papillae which are here very 

 numerous. This point can be determined by dissection. Other branches 

 pass downward. 



Another system of branching is present, though not developed to 

 the degree that the nerves to the mantle edge are (Fig. 31). These 



