230 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



visceral ganglia. The cerebral, pleural, and pedal ganglia 

 form a nerve ring surrounding the oesophagus. The two 

 cerebral and two pedal ganglia are united by means of cerebral 

 and pedal commissures respectively, the former dorsal and the 

 latter ventral in position. Connectives link up the cerebral 

 and pedal, cerebral and pleural, and pleural and pedal ganglia. 

 The cerebral ganglia are connected with the ventrally placed 

 stomato-gastric ganglia, which are united by a commissure. 

 The visceral nerve cords originate from the pleural ganglia, 

 and along the course of these cords, which are united posteriorly 

 lie the remaining ganglia, viz., two parietal ganglia and a 

 single visceral or abdominal ganglion. In Aplysia all these 

 ganglia, with the exception of the left parietal (the infra- 

 intestinal ganglion of Prosobranchs), are present as distinct 

 ganglia. There is evidence that the left parietal ganglion has 

 fused with the unpaired visceral ganglion. The right parietal 

 (supra-intestinal ganglion of Prosobranchs) and the visceral 

 ganglion form an apparent pair owing to the shortening of the 

 pleuro-visceral connectives along which they lie. 



The regions innervated by these ganglia are similar to 

 those in other Gastropods. The cerebral ganglia are the main 

 sensory centres supplying the tentacles (touch, smell, taste), 

 the eyes and the so-called auditory organ. The stomato- 

 gastric ganglia are the seat of the extensive sympathetic system 

 which ramifies over the surface of the gut. The pedal ganglia 

 are motor centres innervating the foot and its appendages. 

 From the pleural ganglia originate the pleuro-visceral connec- 

 tives (visceral cords), the ganglia along the course of these 

 cords supplying the mantle and its glands, the gill and the 

 osphradium, the genital organs, heart, kidney, etc. 



In a fresh specimen the nerves appear yellowish white 

 in colour, and the ganglia are a bright yellow or orange. The 

 whole of the nervous system, both central and peripheral, 

 is surrounded by a sheath of connective tissue between which 



