LESSON 80.] NERVOUS SYSTEM IN THE GASTEROPODA. 263 



LESSON LXXX. 



NERVOUS SYSTEM IN THE GASTEROPODA. 



1189. The meaning of this word has already been given, and it 

 will be at once apparent that its application is to the slugs and 

 snails, whether inhabitants of the earth, fresh waters, or of the 

 ocean, the whole of which progress upon their belly-foot 



1190. The majority, but not all, of these animals are androgy- 

 nous (aner, a man ; gune, a woman) ; both sexes are combined in 

 the same individual. 



1191. From the great power of locomotion possessed by this 

 class, it is evident that the muscular system has acquired an increase 

 of development as compared with the Lamellibranchiata, and, cor- 

 responding to this, is the increase in development of the nervous 

 system, and the development of a brain. 



1192. Thus we observe, in the lowest and least locomotive 

 Gasteropod^ a tendency in the nervous system to be aggregated at 

 the forepart of the body, the cerebral ganglions rising more to the 

 upper surface of the now well-developed head, and the branchial and 

 pedal ganglions beginning to concentrate themselves about the 

 mouth. 



1193. In the slug and snail the principal centres of the nervous 

 system are a supra-oasophageal and a sub- 03sophageal ganglion, 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 

 gives off the large nerves of the tentacles and ocelli ; it also commu- 

 nicates on each side by two minute filaments proceeding from its 

 posterior and outer angles, with a small pair of sympathetic ganglions 

 situated on the side of the oesophagus. 



1194. In the Bulla lignaria (Fig. 379) there is a small lobed 

 ganglion (a), anterior to the usual cephalic ring (0), which is situ- 

 ated below the bulb of the oesophagus. The cephalic ring (e) sur- 

 rounds the oesophagus, and at its sides are seen two large tri-lobate 

 ganglia (/), which send numerous branches to the surrounding mus- 

 cular parts, and two long branches (h, h) extend backwards from 

 them, along the sides of the abdomen, to two symmetrical ganglia 



