GASTEROPODA. 305 



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 Biiccinum or Whelk, low down 

 on the tube ; in the Purpura^ beneath 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 internuntiate chords. 

 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 carti- 

 laginous hardness. A coloured pigment is not unfrequently found 

 occupying a position analogous to that of the arachnoid, between 

 the dense outer membrane and the ganglions. In the Limnceus 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 analogous to those 

 which have received determinate names in the lamellibranchiate 

 Mollusca. 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 thebivalves, and 

 the two divisions are wider apart, in consequence of the great breadth 

 of the foot. In those Gasteropods 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 than usually minute or 

 wanting, the cephalic ganglions are always larger than in the 

 Acephala, and more decidedly superior in position. When separate, 

 they are united by a thicker communicating chord, and are larger in 

 proportion to the nerves given off from them. With the cephalic 

 ganglions, likewise, we find connected the labial and pharyngeal 

 ganglions, in which, perhaps, may reside the olfactory sense ; there 

 is good evidence, at least, that snails scent their food. The anterior 

 of the aggregated ganglions, which form the suboesophageal mass 

 in most Gasteropods, are in immediate connection with the acoustic 



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