VII 



MOLLUSCATHE NERVOUS SYSTEM 



141 



Fu;. 1-21. Nervous 

 System of Notarchus 

 punctatus (after Vays- 

 siere), diagrammatic. 1, 

 Bnccal ; 2, cerebral ; 3, 

 pleural ; 4, pedal ganglia : 



5, right parietal ganglion ; 



6, visceral ganglion. 



ganglia, and enter two ganglia lying side by side ; that to the right ivpivsents tin- 

 right parietal ganglion, innervating chiefly the gill and osphradium, the nerves 



running to these organs forming a ganglion at the base of 



each ; that to the left is the visceral ganglion. One of the 



nerves which run from the latter forms a genital ganglion at 



the base of the accessory glands connected with the genital 



organs. In other Anaspidce, such as Notarchus (Fig. 121), 



the pleurovisceral connectives are so much shortened that 



the parietal and visceral ganglia lie close to the periceso- 



phageal group of ganglia, Avhich then consists of two cerebral, 



two pedal, and two pleural ganglia, and further, the right 



parietal and the visceral ganglia. The two cerebral ganglia 



are further connected by a thin lower commissure. The 



parapodia are always innervated from the pedal ganglia. 



The nervous system of the Pteropoda yymuosomata, which 



are nearly related to the Anaspidce, corresponds in all essential 



points with the nervous system of the latter, being of the 



same type as that of Notarchus. 



(b) Nudibranchia and Ascoglossa. The nervous system 



is here characterised by very great concentration of the 



typical Molluscan ganglia, and by a tendency to the forma- 

 tion of numerous accessory ganglia (at the bases of the 



tentacles and rhinophores, and at the roots of their nerves, 



in the course of the genital nerves, etc.). The pleural gan- 

 glion has moved close to the cerebral ganglion, and may fuse with it. The pedal 



ganglia have also moved towards the cerebral ganglia 

 so that now the whole cesophageal complex of gan- 

 glia lies almost entirely on the dorsal side of the oeso- 

 phagus. The pedal commissure which runs under 

 the gullet, and is sometimes double, is thus very 

 much lengthened. The pleurovisceral connectives 

 are short, and occasionally enter an unpaired visceral 

 ganglion, which has also been drawn into the ceso- 

 phageal complex. This single ganglion of the visceral 

 connectives may be wanting (Fig. 122) ; in that case 

 the two visceral connectives appear like a commissure 

 between the two pleural ganglia runningunder the ceso- 

 phagus and parallel with the pedal commissure, some- 

 times even united with it. The fusion of all the 

 ganglia belonging to the peri-ctsophageal complex is 

 carried very far in such animals as Tethys, where the 

 pleural and pedal ganglia of each side may fuse with 

 the cerebral ganglion. The pleuro - cerebropedal 

 ganglion thus formed shifts towards the dorsal 

 middle line close to the similar ganglion of the other 

 side, with which it forms a large supra-oesophageal 

 ganglionic mass. Its composition out of the six 

 typical ganglia can, however, be made out by the 

 grouping of the ganglion cells and the arrangements 

 of the nerve tracts. A nerve leaves this mass on 

 each side, the two uniting under the gullet. These 

 form the pedal commissure, which when closely examined is found to be double. A 

 third delicate commissure running under the oesophagus connects the lateral portions. 



FIG. 122. Nervous System of 

 Janus (after Pelseneer simplified). 

 1, Buccal; 2, cerebral; 3, pleural; 

 4, pedal ganglia ; 5, commissure be- 

 tween the two pleural ganglia, which 

 corresponds with the two pleuro- 

 visceral connectives of other Mol- 

 lusca ; 6, pedal commissure ; 7, 

 auditory vesicle ; 8, eye ; 9, ganglion 

 of the rhinophore. 



