584 



LECTURE XXIII. 



inferior ganglion (c) meets its fellow 215 

 below the oesophagus : the posterior 

 ganglion (c?), in like manner, joins 

 that of the opposite side and forms 

 a second and posterior (Dcsophageal 

 ring. The nerves given off imme- 

 diately from the super-oesophageal 

 mass supply the muscular and other 

 parts of the mouth, and have small 

 pharyngeal ganglions developed upon 

 them. The anterior cesophageal ring 

 gives off principally the nerves to the 

 tentacula {f,f\ and the two median 

 ones {g) are connected with a gan- 

 glion {h\ which supplies the tenta- 

 cula of the inferior labial processes 

 and the lamellated organs on that 

 part of the oral sheath. The ten- 

 tacular nerves are continued, like 

 those of the arms in the hio^her 

 Cephalopods, along the middle of the 

 tentacle, attached by loose cellular 

 tissue to the vessels of the part. The 

 posterior collar gives off numerous 

 nerves {m) of a flattened form, which 

 supply the muscles of the shell. The 

 respiratory nerves form a small ganglion {q) at the base of each pair of 

 gills, from which branches are sent to those organs, to the heart, and 

 to the appendages of the veins. A plexus of more delicate visceral 

 nerves (r) is continued backward along the interspace of the branchial 

 nerves, and the chief branches are connected with a small ganglion 

 situated between the cardiac and pyloric orifices of the stomach. The 

 posterior suboesophageal nervous mass combines the homologues of 

 both the branchial and pedial ganglions in the inferior Mollusca : the 

 anterior ring answers to the ganglia in the higher Cephalopods, 

 called " pes anseriaus " by Cuvier : the ophthalmic tentacula, which 

 derive their nerves (?^, 7i) close to the origin of the optic ganglion, 

 may be considered as homologous to the four tentacula in the Aplysia. 

 The hollow plicated process beneath the eye, which Valenciennes 

 regards as the olfactory organ, likewise receives its nerves from the 

 extremity of the super-oesophageal chord. Three small nervous fila- 

 ments, described by the same author as passing from the extremity 

 of the super-ocsophngeal chord to the adjoining part of the cephalic 



Nervous System. Nautilus. 



