PATELLA. 229 



are set near 'the tentacle bases and are swollen to form 

 the well-marked cerebral ganglia. These are connected 

 by the cerebral commissure (forming the upper side of 

 the quadrilateral) which runs across very far forward 

 just beneath the dorsal outer lip. 



The right and left sides of the quadrilateral (continuous 

 above with the respective cerebral ganglia) have become 

 double by separation of the connectives going to the 

 pleural from those going to the pedal ganglia. 



The lower side of the quadrilateral is a very short pedal 

 commissure thickening at either end into the commence- 

 ment of the great pedal nerve cord, which 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. From the ganglionic centres a number of 

 nerves are given off, as follows (see fig. 23) : — 



(a) From each cerebral ganglion : 



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

 nective, and the cerebro-pedal and cerebro-pleural 

 connectives. 



(2) The branching tentacular nerve. 



(3) The fine optic nerve supplying the eye which is 

 placed on the posterior side of the base of the tentacle. 



(4) Fine nerves (3 in number) going to the snout. 



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

 the cerebro-pedal connective, but which diverges from this 

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

 the dorsal body wall. 



(0) A nerve which is at first fused with the cerebro- 

 pleural connective but soon becomes distinct from it and 

 supplies the otocyst which lies at the base of the pleural 

 ganglion against the pleuro-pedal connective. 



