37 



arc set lu'ar llic U'jilacle bases and an* swoIIcmi Io lunu 

 llic well-iuaiked coicbral g-ang"lia. These are coiiiiected 

 by (lie cerebral commissure (forming- the upper side of 

 l]ie (juadrihtteral) wbicli runs across very far forward 

 just beneath the dorsal outer lip. 



Tlie riglit and left sides of the ([uadrilateral (coiilinuous 

 above Avith the res])ective cerebral g-anglia) have become 

 double by se})aratioii of the connectives going- t,() the 

 pleural from those g'oing- to the ])edal g-anglia. 



The lower side of the (juadrilateral is a very short pedal 

 commissure thickening- at either end into the comnuMu-e- 

 nient of the greaf pedal nerve cord, wliich is ganglionic 

 for a considerable portion of its length. The outer side of 

 this anterior end of the pedal nerve cord is continuous with 

 a short stout pleuro-pedal connective, and this latter 

 thickens at the side and a little to the front into a pleural 

 ganglion. Fiom the ganglionic centres a number of 

 nerves are given off, as follows (see fig. 'J-'!) : — 



(0) From each cerebral ganglion : 



(1) The cerebral commissure, the cerebro-hibial con- 

 nective, and the cerebro-pedal and cerebro-pleural 

 connectives. 



(2) The branching tentacular nerve. 



(o) The hne optic nerve sup})l3'ing the eye which is 

 j)laced on the posterior side of the base of the tejitacle. 



(4) Fine nerves (-J in number) going to the snout. 



(5) A nerve which, for some distance, runs along with 

 the cerel)ro-pcdal connective, but whicii diverges from this 

 at about the level of the pleural ganglia, going thence to 

 the dorsal body wall. 



(U) A nerve which is at tirst fused with tiie cerebro- 

 pleural connective but sof)n becomes distinct from it and 

 supplies the otocyst which lies at the base of the pleural 

 ganglion against the pleuro-pedal connective. 



