238 OILMAN A. DREW. 



the structural standpoint we would accordingly be justified in 

 considering the circumpallial nerves as separate ganglia, and the 

 nerves connecting them with the visceral and cerebral ganglia 

 as connectives. 



The circumpallial nerves of the two lobes of the mantle are 

 connected with each other anteriorly and posteriorly near the 

 hinge line (Fig. 8, 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 sup- 

 ply the mantle margin, about opposite the line of attachment of 

 the infolded ridge. From them nerves are sent to the eyes and 

 tentacles, to the infolded ridge 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 ganglia in the immedi- 

 ate 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 through- 

 out their length. These are present not only along the borders 

 of the gills, but from the points where the nerves originate 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 abundantly supplied with 

 ganglion cells. 



The whole nervous system is modified 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. 



It seems that many students of Mollusca hold that the lamelli- 

 branch ganglia have been derived from a gastropod-like type, a 



