Hearing, and Smelling 49 



the visceral ganglia. In those mollnsks that possess 

 a distinct head nerves go off from the cerebral 

 ganglion to the tentacles, the eyes, and are connected 

 with a pair of small ganglia (the buccal ganglia) that 

 command all the mouth parts, the pedal ganglia 

 supplies the foot, the visceral or pleural ganglia send 

 nerves to the mantle, the heart, the gills, the stomach, 

 and the other internal organs. 



The eyes of the Mollusca are alike different from 

 those of higher animals, and from one another. The 

 highest development of the molluscan eye is found 

 among the Cuttle-fishes and their allies, but a pair 

 of well-developed eyes is the possession of nearly all 

 the mollusks that are furnished with heads. Yet the 

 presence of a head is not absolutely essential for the 

 acquirement of eyes, for some of the bivalves, such 

 as the Scallops and Ark-shells, have eyes on the 

 fringes of the mantle, and some foreign species of 

 Mail-shells (CJtiton) have been found to have certain 

 parts of their shell-plates closely studded with minute 

 pigmented dots which serve the office of eyes. In 

 the land-snails the eyes are carried at the top of the 

 longer pair of tentacles, a position which allows the 

 snail to see in all directions ; in the water-snails the 

 eyes are at the base of the tentacles. Species that 

 live underground have the eyes more or less aborted. 

 Our own little Needle-shell {Ccecilianella acicida), 

 which lives an entirely subterranean life, has lost 

 them altogether ; so also have several of our marine- 

 snails that are always seeking their food under the 

 sand. Others of similar habit have eyes, but they 

 are covered by thick skin that can allow very little 

 impression of light to reach them. 



