304 LECTURE XXII. 



formed by a double chord, which unite below in a single branchio 

 pedial ganglion; from which the visceral, as well as the branchial and 

 muscular, nerves radiate. 



In the Doris and Onchidium the cerebral, pedial, and branchial 

 ganglions have coalesced into one annular mass, which, however, is 

 chiefly supra-oesophageal in its position, united below by a slender 

 chord passing across the under parts of the oesophagus. Two small 

 nerves are given off, which descend and form two small pharyngeal 

 ganglia, which, according to Cuvier, are united together. In the 

 Doris Solea the quadripartite character of this large mass is however 

 obvious. 



In the Paludina vivipara the supra-oesophageal ganglions (^^. 121. 

 w, 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 

 oesophagus, and expands into a small ganglion Xy which distributes its 

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



In the slug and snail the principal centres of the nervous system 

 are a supra-oesophageal and a sub-cesophageal 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 

 alimentary tube. From the inferior mass the nerves radiate to the 

 muscular foot, the soft and susceptible integument, and the circulating 

 and respiratory organs. The upper ganglion receives the large nerves 

 of the tentacles and ocelli ; it also communicates on each side by two 

 minute filaments, proceeding from its posterior and outer angles, with 

 a small pair of stomato-gastric ganglions situated on the side of the 

 oesophagus. 



The sub-oesophageal mass in the Limnceus stagnalis is of an 

 orange-colour, and consists of seven ganglions united together by a 

 loose cellular tissue. 



In the Aplysia the sub-oesophageal ganglionic mass is divided 

 into two parts, which are joined together by a sub-oesophageal chord, 

 and brought into communication with the cerebral ganglions by ascend- 

 ing and converging chords. The cerebral ganglions are also joined 

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

 brain. They supply nerves to the tentacula, 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 

 join those of the pedial ganglia, but may be traced directly to the 

 brain. 



