550 LECTURE XXII. 



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

 brain. 



The position of the cerebral ganglions varies according to the 

 degree of extensibility of the mouth and oesophagus. Thus, in the 

 Helix, they are placed above the mouth ; in Carocolla, at the com- 

 mencement of the oesophagus ; in the Buccinum or Whelk, near the 

 end of the tube ; in the Purpura, beyond the stomach. 



As a general rule, we find that the superior ganglions give off 

 tentacular, ocular, and oral nerves, whilst the inferior masses are the 

 centres of the muscular, respiratory, and visceral intern untiate chords. 

 The latter have, in addition to the infero-pharyngeal centres, also a 

 posterior splanchnic ganglion, or pair of ganglions, from which a 

 plexus of nerves proceeds, in many Gastropods ( C'arinaria, Doridium, 

 Pleurobranchus). They are situated beneath the folds of the intes- 

 tine, communicate with the sub-cesophageal ring and the branchial 

 ganglion, and supply the intestinal canal, liver, and genital glands. 

 In the spiral pectinibranchiate univalves, where the branchiae and 

 their nerves are twisted to the left side, it is the left branchia which 

 is atrophied, while the right one is of large size. The nerves are 

 similarly affected, the left one being filamentary ; whilst the right is 

 a large chord, and has the accessory branchial ganglion developed 

 upon it. The principal oesophageal ganglionic circle is surrounded 

 by a thick membrane, which, in the large Tritons, assumes almost a 

 cartilaginous hardness. A coloured pigment is not unfrequently 

 found occupying a position analogous to that of the arachnoid, be- 

 tween the dense outer membrane and the ganglions. In the lAmncBus 

 and in the Planorbis this pigment gives to the ganglions their orange 

 or roseate hue. 



Amongst all this diversity in the number, size, and position of the 

 nervous masses, certain ganglia are obviously homologous with those 

 which have received determinate names in the lamellibranchiate 

 Mollusks.* The branchial ganglions receive impressions from, and 

 transmit them to, the gills : they communicate also with the brain, 

 and through that centre associate the gills with all other parts of the 

 body. The pedial ganglion is more commonly divided than in the 

 bivalves, and the two divisions are wider apart, in consequence of 

 the great breadth of the foot. In those Gastropods which possess a 

 naked muscular mantle, we find a pallial ganglion associated with a 

 pedial one, as in the Aplysia. The cephalic ganglions assume the 

 character of optic lobes concurrently with the constancy and better 

 development of the eyes ; even when the organs of vision are more 



* CCCLVI. p. 488. 



