44 WALTER K. FISHER, 



delicate, and are difficult to make out for any great distance in the 

 muscle, even in well macerated specimens. All these nerves pass 

 nearly to the upper edge of the spindle muscle, where it joins the 

 shell, but in dissection were invariably lost here. 



The median pedal nerves (Ped.N 1 ) are also very slender. There 

 are from fifteen to twenty of them to each cord, and they anastomose 

 with each other and also to a limited extent with those of the op- 

 posite side. They are so delicate that one finds great difficulty in 

 following them in the tough muscle of the body wall. 



Pleural ganglia and mantle nerves (Fig. 30). The pleural 

 ganglia are connected with the pedal ganglia each by a short, thick 

 ganglionic cord. The ganglia are not precisely alike in shape by 

 reason of a slightly different arrangement of the nerves which are 

 given off. Each is produced for some distance dorsally, being, as it 

 were, drawn out by the visceral commissure. In the right this pro- 

 longation is more nearly vertical than in the left, because the visceral 

 loop lies to the right of a sagittal plane. By a comparison of the 

 several figures, the reader will have no difficulty in making out the 

 shape of the ganglia and the origin of the nerves. 



From each ganglion the following sets of nerves arise: mantle 

 nerves, the visceral commissure or loop, several nerves to the ali- 

 mentary canal and viscera, on the left side a nerve connecting the 

 left pleural ganglion with the osphradial or ctenidial nerve, and an 

 unimportant nerve to the body wall. 



Each pleural ganglion gives rise to a pair of prominent nerves 

 to the mantle, the anterior pallial (A.P.N) and posterior (P.P.N) 

 pallial nerves. Those of the right side will be considered first. After 

 leaving the right pleural ganglion they diverge slightly, both however 

 passing outward. Before reaching the upper edge of the spindle 

 muscle the posterior pallial turns backward and soon enters the body 

 wall a short distance below the attachment surface of the spindle 

 muscle. The right anterior pallial, having divided into two branches, 

 turns a little forward and plunges into the shell muscle. These pallial 

 nerves like their fellows of the left side are free from ganglion cells. 

 They are white glistening bands, somewhat flattened and ribbon-like 

 near their origins, but soon round like other nerves, and become 

 constantly smaller. Immediately after entering the tough spindle 

 muscle the anterior branch of the right pallial divides into two parts. 

 At the inner edge of the pallium these two branches diverge, one 

 passing forward in the mantle, the other backward. Each again divides 



