766 CLASS xui. 



another below the oesophagus, each of which consists of two lateral 

 portions, that either remain distinct or are more or less fused 

 together to form an over- and an under-lying ganglion. From the 

 central nerve-mass situated above the oesophagus, or from the two 

 lateral ganglia when these, as in Patella, Halwtis, Bulla, &c., are 

 only united by a transverse commissure above the oesophagus, arise 

 the nerves of the eyes, of the feelers and of the mouth. On each 

 side this brain-like central organ is united with the central mass 

 lying under the oesophagus by one or more nervous filaments (in 

 Helix by three). The nervous ring, thus formed, surrounds the 

 oesophagus, and is in many short, because the central masses are 

 situated near the fore part of the body and in the neighbourhood 

 of each other ; only in some is this ring very long, as in the Hete- 

 ropoda, because the second central mass is situated backwards. In 

 many nudibranchiates the central nerve-mass placed above the 

 oesophagus is much developed, and that which lies beneath it 

 feebly ; in the Ctenobranchiata also the first nerve-ganglion that lies 

 over the oesophagus is much larger than the second which lies 

 under it. The converse occurs in the Pteropoda, where the upper- 

 most or cerebral nerve-center is ordinarily represented by a trans- 

 verse commissure alone above the oesophagus. From the central 

 nerve-mass, situated below the oesophagus, and consisting of a 

 greater or less number of different ganglia, arise the nerves for the 

 viscera, the respiratory and generative organs. In some several 

 ganglia or nerve-masses remote from each other are present, as in 

 Aplysia, where a distinct ganglion is situated backwards at the origin 

 of the aorta. The nerve-ganglia are often coloured red, as in the 

 genus just referred to, in Limnceus, Planorlis, &c., by a substance 

 situated under the neurilema. In various molluscs a nervous sys- 

 tem corresponding to that of the visceral nerves of articulate ani- 

 mals has also been observed 1 , of which the anterior portion consists 

 of small ganglia that lie at the side of the oesophagus, and are con- 

 nected by nervous filaments with the cerebral ganglion, whilst back- 

 ward, from one or two ganglia, branches are given off to the intes- 

 tinal canal, the liver, and the organs of propagation 2 . 



1 Comp. above in the treating of Insects, p. 278. 



2 The nervous system has been described and figured in different Gasteropoda, espe- 

 cially by CUVIEB in his Mtmoires pour servir a I" Hist, et d I'Anat. des Mollusques ; see 



