NEEVOUS SYSTEM OE MOLLUSCA, 



349 



tlie cerebral (Fig. 185) ; the visceral ganglia also get to be connected 

 with these latter owing to the abbreviation and complete disap- 

 pearance of the commissures between them (yEolidia). A dorsal 

 plexus of ganglia, which lies on the pharynx, is formed by the more 

 intimate fusion of these ganglia ; in each half of this plexus the 

 various ganglia which compose it can be more or less distinctly 

 made out ; several commissures are given off, which surround the 

 pharynx (Doris, Tritouia), This connection of the ganglia of either 

 side may lead at last to the complete fusion of the ganglionic masses 

 of either side into a single one, and in correlation with this fusion 

 the previously separate commissures may be represented by one 

 only (Tethys), This appears to be a lower stage, but it must not 

 be regarded as an early arrangement, 

 but as the final one in a series of dii¥e- 

 rentiations, exactly similar to what we 

 found to obtain in the Arthropoda. In 

 the same way as the nerves, which 

 pass off from the single nervous mass, 

 indicate that it is made up of separate 

 portions, the circum-oesophageal com- 

 missures prove that the ventral ganglia 

 have become more dorsal in position. 



The nervous system of the Branchi- 

 opneusta is similar in many points to 

 that of the Tectibranchiata, and similar 

 relations can also be made out in the 

 Nephropneusta. Both divisions are 

 characterised by the development of 

 their cerebral ganglia into several por- 

 tions, which can be distinguished as 

 such even from the exterior. 



§ 2G9. 



Fig. 185. Central nervous system 

 of one of the j^ilolidioB (Fiona 

 atlantica) . A Supra-oesoplaagcal 

 mass, formed by the anterior cere- 

 bral, and. posterior visceral or 

 branchial ganglia. B Pedal gan- 

 glia. C Buccal ganglia. D Gastro- 

 QBsophageal ganglia, a Norve to 

 the superior (hinder) tentacles. 

 b Nerve to the inferior tentacles, 

 c Nerve to the generative organs. 

 d Pedal nerves, e Commissure of 

 the pedal ganglia, e' Commissure 

 of the visceral ganglia (after R. 

 Bergh), 



The genetic relations of the nervous 

 system of the Pteropoda to that of the 



Gastropoda may be seen in the Gymnosomata ; the Thecosomata 

 differ in the length of their cerebral commissures. The cerebral 

 ganglia either retain their lateral position or approach the pedal 

 ganglia with which the visceral ganglia are also fused. The central 

 ganglionic mass lies therefore below the pharynx. The arrangement 

 in the Gymnosomata is more primitive; the cerebral ganglia are 

 placed superiorly, and visceral ganglia are evidently present. The 

 pedal ganglia always innervate the fins formed from the foot. In 

 the Gymnosomata the cerebral ganglia give off considerable nerves 

 to the tentacles, at the base of which they form ganglia. The 

 ganglia of either side are connected with one another by commis- 

 sures (Clio). 



The three ganglionic masses already pointed out in the Mollusca 



