PTEKOPODA AND GASTROPODA. 



549 



Ijorder is the single visceral ganglion, from which proceed the nerves 

 forming the plexuses about the viscera, which are in communication 

 with the pharyngeal or anterior ganglia of this stomatogastric system 

 of nerves.* 



In the Paludina vivipara the super-cesophageal ganglions (Jig. 204. 

 u, «) are distinct, and connected together by a transverse commis- 

 sural filament. The sub-oesophageal mass sends its principal branches 

 to the foot ; but one nerve comes off from the right side, crosses the 

 ossophagus, and expands into a small ganglion x, which distributes 

 its filaments to the retractor muscle that attaches the animal to its 

 shell. 



In the slug and snail {fig. 201.) the principal centres of the ner- 

 vous system are a super-oesopha- 201 

 geal (/) and a sub-oesophageal {m) 

 mass ; but the complex character 

 of the latter and larger mass is 

 indicated by the triple nervous 

 chord, which completes on each 

 side the collar round the alimen- 

 tary tube. From the inferior mass 

 the nerves radiate to the muscular 

 foot, the soft and susceptible in- 

 tegument, and the circulating and 

 respiratory organs. The upper 

 ganglion receives the large nerves 

 of the tentacles {ef) and ocelli 

 («) ; it also communicates on each 

 side by two minute filaments, pro- 

 ceeding from its posterior and 

 outer angles, with a small pair of 

 stomatogastric ganglions situated on the side of the oesophagus. 



In the Aplysia the sub-oesophageal ganglionic mass is divided into 

 two parts, which are joined together by a transverse chord, and 

 brought into communication with the cerebral ganglions by ascend- 

 ing and converging chords. The cerebral ganglions are blended 

 together above the oesophagus, and assume the position of a true 

 brain. They supply nerves to the tentacles, and give off anteriorly 

 two chords, which turn forwards to join below the mouth, where 

 they form a second oesophageal collar upon which the pharyngeal 

 ganglions are developed. The branchial ganglion is situated towards 

 the posterior part of the body ; the connecting chords of this ganglion 



* CCCXLVI. 



N N 3 



Structure of the tentacles in the Garden-Snair 

 {Helix nemaralis). 



