ganglia. The nerves by which they are connected with the 

 visceral and cerebral ganglia contain no ganglionic cells. From 

 the structural standpoint we would accordingly be justified in 

 considering the circum-pallial nerves as separate ganglia, and 

 the nerves connecting them with the visceral and cerebral gan- 

 glia as connectives. 



The circum-pallial nerves of the two lobes of the mantle are 

 connected with each other anteriorly and posteriorly near the 

 hinge line (fig. 23, cpn) . They are not of constant diameter but 

 suddenly increase or diminish in size so that they have a rather 

 irregular appearance. They lie just inside, that is, toward the 

 median plane of the body, of the large pallial arteries that supply 

 the mantle margin (fig. 26, cpn.) about opposite the line 01 

 attachment of the infolded ridge. From them nerves are sent 

 to the eyes and tentacles, to the infolded ridges and to the pallial 

 muscles. Very likely the pallial muscles are partially supplied 

 from the pallial nerves that come from the visceral ganglia, but 

 of this I am not sure. 



It seems probable that the ganglionic structure of these nerves 

 has been developed to meet the needs of the very complex mar- 

 gins of the mantle. The development of such structure in the 

 immediate region of the sense organs, is an indication of the 

 ease with which such centers may be established when need 

 arises. The branchial nerves are supplied with ganglionic cells 

 throughout their length. These are present not only along the 

 borders of the gills, but from the points vvhere the nerves origi- 

 nate to their extremities. The almost constant activity of the 

 gills no doubt renders such an arrangement desirable. No other 

 nerves or connectives in the body seem to be supplied with 

 ganglion cells. 



The whole nervous system is modfied to meet the special 

 needs of the animal. The cerebral and pedal ganglia are small, 

 corresponding with the slight development of the anterior parts 

 of the body and of the foot. The visceral ganglia are highly 

 developed, corresponding to the excessive development of the 

 parts that are supplied by these ganglia. Accessory centers 

 have also been developed in the margins of the mantle and in 

 the gills. 



46 



