172 BULLETIN OF THE BUREAU OF FISHERIES. 



The pedal ganglion lies on the dorsal side of the anterior retractor muscles just in 

 front of the posterior retractor muscles of the foot (fig. 159, PdG). The ganglia are 

 more or less pear shaped with the apex pointing forward and uniting with the cerebro- 

 pedal connective. In most specimens there is a heavy deposit of pigment which gives 

 the ganglia a deep orange-red color. Unlike the other ganglionic centers, the pedal 

 glanglia are fused together into a single mass, but the dual structure is shown by the 

 distinct furrow which runs around the body and separates the stems which project 

 forward to connect with the cerebropedal nerves. Three nerves arise from each ganglion. 



The pedal nerve emerges from the ventral side of the pedal ganglion, passes back- 

 ward over the anterior retractor muscle, penetrates the posterior retractor muscle of 

 the foot, and then continues downward into the foot (fig. 159, PdN). 



The ventral retractor byssus nerve arises from the posterior side of the ganglion and 

 subdivides into several branches that supply the byssus organ and the anterior and 

 posterior retractor muscles in that region (fig. 159, VRBN). 



The dorsal retractor byssus nerve arises from the dorsal surface of the pedal ganglion 

 and runs obliquely backward and upward, subdividing in its course into several branches 

 that go to the posterior retractor muscles above the region supplied by the ventral 

 retractor byssus nerve (fig. 159, DRBN). 



The visceral ganglia are situated on the anterior ventral surface of the posterior 

 adductor muscle just under the epithelium. They lie just inside the line where the 

 gills are suspended (fig. 159, VG), which places them some distance apart. A large 

 visceral commissure (VC) connects them. Each visceral ganglion receives the cerebro- 

 visceral connective at the anterior surface, while laterally and from behind several 

 important nerves are given off. 



The posterior pallial nerve arises from the posterior side of the visceral ganglion and 

 runs backward and slightly outward across the ventral surface of the posterior adductor 

 muscle (fig. 159, PPN). At the posterior ventral surface of the muscle it divides into 

 two branches, one of which penetrates the mantle edge and runs dorsally (PDPN) 

 whereas the other runs obliquely downward and backward some distance across the 

 mantle before it enters the mantle edge (PVPN). 



The posterior dorsal pallial nerve is the branch that arises from the posterior pallial 

 nerve just behind the posterior adductor muscle, penetrates the mantle edge, and 

 continues dorsally and anteriorly, giving off in its course many fine fibers to the sur- 

 rounding tissues. It terminates in the mid-dorsal region in the trunk of the dorsal 

 pallial nerve (fig. 162, PDPN). 



The posterior ventral pallial nerve is the ventral branch of the posterior pallial 

 nerve (figs. 159 and 162, PVPN). It runs downward and slightly backward until it 

 penetrates the inner fold of the mantle edge, where it begins to give off numerous fine 

 side branches that form a network of fibers throughout the inner and middle folds. 

 In its course forward it becomes continuous with the anterior pallial nerve, forming 

 what is commonly called the pallial nerve (fig. 162, PN). 



The sy phonal nerve arises from the posterior ventral pallial nerve, passes backward 

 directly into the mantle edge, and then runs downward to unite again with the posterior 

 ventral pallial nerve where the latter enters the mantle edge (fig. 162, SN). The 

 syphonal nerves take their course through the region where the inner folds of the mantle 

 edge are fused to form the anal syphon. The distribution of the side branches and 



