84 THE NERVOUS SYSTEM OF MOLLUSKS. 



istics of the principal nerve-centres is the fact of the exist- 

 ence of a very large optic ganglion (fig. 29, 2) on each 

 side, in connection with an extremely well-developed eye. 

 Each optic lobe, according to Lockhart Clarke, is "as 

 large as the rest of the cephalic ganglia on both sides 

 taken together." From each of these lobes an optic 

 peduncle passes inward to join a supra- cesophageal gan- 

 glionic mass, wdiich bears on its surface a large bilobed 

 ganglion (1), thought by Clarke to be homologous w^ith 

 the cerebral lobes of fishes. It is connected, by means 

 of two short cords, with a much smaller bilobed ganglion, 

 known as the pharyngeal (7). This latter ganglion re- 

 ceives nerves from what are presumed to be the organs of 

 taste and smell, and gives off nerves to the tongue and 

 powerful parrot-like jaws with which the creature is pro- 

 vided. 



The supra-oesophageal mass is connected by cords, at 

 the sides of the ossopbagus, with a very large ganglion 

 lying beneath it (4), which is i:)artially divided into an 

 anterior and a posterior division. The anterior division — ■ 

 regarded by Huxley as in part homologous with the pedal 

 gangha of lower Mollusks — is in relation by means of 

 large nerves (e) with the feet and tentacles. A com- 

 missure also unites it with the pharyngeal ganglion, so 

 that the tentacles and arms are thus able to be brought 

 into correlated action with the jaws. The posterior 

 portion of the sub-oesophageal mass receives nerves from, 

 and also gives off nerves (14) to, the branchia) and other 

 viscera, as well as to the muscular mantle (is, 13). 



The ' auditory saccules ' and their nerves are connected 

 with this great branchio-pallial ganglion. These organs 

 are lodged in the substance of the cartilaginous framework 

 {p p) investing the nerve-ganglia — a structure which seems 

 to answer to a rudimentary skull or cranium. 



