766 CLASS XITI. 
another below the cesophagus, 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 cesophagus, or from the two 
lateral ganglia when these, as in Patella, Haliotis, Bulla, &c., are 
only united by a transverse commissure above the cesophagus, 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 cesophagus by one or more nervous filaments (in 
Helix by three). The nervous ring, thus formed, surrounds the 
cesophagus, 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 
cesophagus is much developed, and that which lies beneath it 
feebly ; in the Ctenobranchiata also the first nerve-ganglion that lies 
over the cesophagus 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 cesophagus. [From the central 
nerve-mass, situated below the cesophagus, 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 Limneus, Planorbis, &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!, of which the anterior portion consists 
of small ganglia that lie at the side of the cesophagus, 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’. 
1 Comp. above in the treating of Insects, p. 278. 
2 The nervous system has been described and figured in different Gasteropods, espe- 
cially by Cuvier in his Mémoires pour servir aU Hist. et dV Anat. des Mollusques ; see 
