58 PHYSIOLOGICAL SERIES. 



One of those on the left side is larger than the rest aiil 

 innervates the spadix an accessory sexual organ in the 

 male formed by the modification of certain of the tentacles 

 of the lateral series. The posterior semicircular band is of 

 equal ealihre throughout. From the hinder margin of its 

 lateral parts a series of nerves are given off to the body- 

 muscles, a< well as a pair on either side to the posterior 

 region of the funnel. From its postero- ventral border it 

 gives rise to two large nerves that run backwards upon the 

 ventral surface of the body to the neighbourhood of the 

 gills ; here each gives off two branchial branches and is >aid 

 to then continue onwards to terminate among the viscera 

 (this continuation is not shown). Nerves from the supra- 

 tesophagoal bar supply the olfactory tentacles and pit, the 

 otocyst, and parts of the hood. This centre also gives origin 

 near either end to two strong connectives, that after a 

 -in nous course join a ganglion (pharyngeal ganglion) upon 

 the lateral surface of the buccal mass. The pharyngeal 

 ganglia of opposite -ides are united beneath the gut by two 

 commissures, one of which pa e- anteriorly along the inner 

 border of the lower beak, giving off two large nerves to the 

 ti nes within it. while the other passes transversely between 

 the oesophagus and radula-sae and has upon its course a 

 pair of buccal ganglia from which nerves are distributed to 

 the hueeal ma-- and (esophagus. 



Although the central nervous system shows a certain 

 degree of specialisation in being concentrated in the head, 

 on the whole in a very simple and primitive condition, 

 and remind- one, especially in the hand-like form of its 

 several parts and the diflu.-e distribution of ganglion-cells 

 upon it- -ui-face, of the condition observed in the Amphi- 

 neura. The three nerxoii- hand- of which it is comj.o-ed 

 prol.al.lv represent the typical centre- of the Mollu-ea in a 

 State of great simplification, namely : cerebral (supra- 

 ii-snphageal l,ar) for the innervation of the head region 

 with it- s,.ii-e-orgaiis, pedal (anterior -emieircular band) 

 innervating the tentacle complex and the funnel, and 

 pleuro-\i-ceral (posterior semicircular hand) for the mantle, 



gills, and riaoem 0. C. l.'iur. ,\ ,/. 



<iraham Ken. I'mc. Xool. Sue. IS',1."), p. ll?.'). 



