Ohap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 



79 



4-cJ 



However small and nndeveloped the duplex brain of 

 the Limpet may be, this organ exists in an even more 

 rudimentar}' state in its close ally^ the Chiton, which is 

 about the most simply organized of all the Gasteropods, 

 It has neither tentacles nor eyes, and, 

 as a consequence, no distinct supra- 

 cesophageal ganglia are found (fig. 25). 

 There is, in fact, nothing to which the 

 term ' brain ' can be appropriately ap- 

 plied. 



If we turn, however, to the very 

 active Snail, we find the nervous sys- 

 tem existing in a much more deve- 

 loped and concentrated form. There 

 is a large ganglionic mass (fig. 26, I) 

 situated over the oesophagus, each 

 half of which receives a considerable 

 bundle of nerve-fibres (/) from the fig. 25. — Nervous sys- 



,7 , p , . -, 1 • 1 • 'J teni of Chiton marmoratus. 



eye (6) of the same side, which is situ- (Gamer.) v>, Pharyngea 



ated at the tip of the larger tentacle, ganglion (left); b, pedai 



. ^1 1 ni i? ganglion (right) ; c, bran- 



It also receives another bundle 01 cMai ganglion; i, upper 

 nerves (Ji) from the small tentacle on portion of oesophageal ring 



V ^ _ ^ devoid of any distinct 



each side, which has in all probability cerebral ganglia. 

 a tactile function. The ' auditory sac- 

 cules ' are here in their exceptional position — that is in 

 immediate relation with the posterior aspect of the ganglia 

 constituting the brain, though in most other Gasteropods 

 they are, as in bivalve Mollusks, found in connection with 

 the pedal ganglia. There is one group, however — the 

 Heteropoda — in which the ' auditory saccules ' seem to be 

 always in direct relation with the cerebral ganglia, as in 

 Carinaria and Pterotrachea.* 



* See Fig. 

 TransL) 



187, p. 354, Gegeabauer's " Comp. Anat." (Engl. 



