38 LAND AND FRESH- WATER SHELLS. 



tory aperture. A nerve supplies it in Limncea from the right 

 visceral ganglion : in Planorbis and Physa the nerve comes from 

 the left visceral ganglion. In the Helices the osphradium is 

 apparently absent, but a nerve has been described by Sarasin in 

 Helix personata which arises from the right visceral ganglion, and 

 ends near the pulmonary orifice in a collection of ganglion cells. 

 The same nerve has been found in Limax cinereo-niger, Succinea 

 amphibia, Bulimus detritus and Bulimus decollatus, but the 

 ganglion cells are absent. 



THE AUDITORY ORGANS. Before ever the auditory organs were 

 dissected out the late Dr. Grant surmised their existence, for he 

 could not but think that the sounds emitted by Tritonia arbor- 

 escens under water, were intended to be heard by its fellows. 

 Siebold first discovered the auditory organs or, as they are 

 generally termed, the otocysts. They are a pair of minute white 

 bodies in very close relation with the pedal ganglia, and in 

 Paludina are movable by muscles. Each consists of a connective 

 tissue capsule containing fluid, and a large number of calcareous 

 granules (otoliths] which are in constant motion. These otoliths, 

 disturbed by the sound waves from a sounding body, strike upon 

 the nerve filaments of the auditory nerve, which thus communi- 

 cates a wave of change to the supra-cesophageal ganglion, the 

 result of which is the perception of sound by the animal. 



THE ORGANS OF SIGHT. We have previously considered the 

 relation which the eyes may bear in different species to the dorsal 

 tentacles. The eye consists of a cornea, lens, or vitreous body 

 (Carriere), and a retina composed of pigmented and non-pigmented 

 cells. The lens or vitreous body is structureless, and fills the 

 whole of the cavity of the eye ; the cornea is composed of connec- 

 tive tissue, with a layer of transparent cells on its outer surface 

 The optic nerve is derived from the nerve to the tentacle in the 

 STYLOMMATOPHORA ; in the BASOMMATOPHORA it is an indepen- 

 dent nerve. Snails cannot accommodate for long distances, since 

 they cannot distinguish objects till within a quarter of an inch 

 from the eyes. In Chiton and Vermetus the eyes are absent. 

 In Patella the eye is cup-shaped. The eye can be invaginated 

 into the body by the retractor muscle of the tentacle which is 

 inserted into the columella. 



