VII 



MOLLUSGATHE NERVOUS SYSTEM 



141 



ganglia, and enter two ganglia lying side by side ; that to the right represents the 



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, which 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 hirer commissure. The 



parapodia are always innervated from the pedal ganglia. 



The nervous system of the Pteropoda ijf/maosomata, 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. , F '"- 1 -j' ~ 1 J erv us 



System of Notarchus 

 (b) Nudibranchia and AscoglosBa. The nervous system punctatus (after V ays- 



is here characterised by very great concentration of the S j^ re ) i diagrammatic, i, 

 typical Molluscan ganglia, and by a tendency to the forma- Buccal ; 2, cerebral ; 3, 



tion of numerous accessory ganglia (at the bases of the 1'le"! ; 4, pedal ganglia : 

 11-1 c ,i 5, right parietal ganglion ; 



tentacles and rhinophores, and at the roots of their nerves, 6 ' vjsce] ! a] ^nc-iFon 



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 oeso- 

 phagus and parallel with the pedal commissure, some- 

 times even united with it. The fusion of all the 

 ganglia belonging to the peri-cc-sophageal 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 pie tiro - cerebropedal 

 ganglion thus formed shifts towards the dorsal 



FIG. 1-2-2. Nervous System of 

 Janus (after Pelseneer simplified). 

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



middle line close to the similar ganglion of the other 



4, pedal ganglia ; 5, commissure be- si( j e> w {th which it forms a large supra-oesophageal 



tween the two pleura! ganglia, which .. ... c ,, 



ganghonic mass. Its composition out 01 the six 



corresponds with the two pleuro- & 



visceral connectives of other Mol- typical ganglia can, however, be made out by the 

 lusca ; 6, pedal commissure ; 7, grouping of the ganglion cells and the arrangements 

 auditory vesicle ; 8, eye ; 9, ganglion O f the nerve tracts. A nerve leaves this mass on 

 of the rhinophore. ea(jh sid6) the tw() 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. 



