38 LAND AND FRESH-WATER SHELLS. 



tory aperture. A nerve supplies it in Liiimcea 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 Liniax ciiiereo-niger, Succinea 

 amphibia, Bulimus detritus and Bulinius decollates, 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 

 Paliidina 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 otoUths, 

 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-oesophageal 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, 



