552 LECTURE xxn. 



seen folded up within the body. In the figure, a a indicates the 

 origins of the retractor muscles of the foot from the columella ; b, the 

 right superior tentacle fully protruded ; c, the left superior tentacle 

 partially retracted ; d, the left inferior tentacle extended, and o, the 

 right inferior tentacle fully retracted and concealed within the body ; 

 f, the nerve supplying the superior tentacle elongated by its exten- 

 sion ; g, the retractor muscle of the same tentacle arising from the 

 common retractor muscle of the foot, and inserted into the extremity 

 of the tube ; h, the nerve of the opposite side thrown into folds ; i, 

 the retractor muscle of the same tentacle contracted ; k, the aperture 

 through which the nerve and retractor muscle enter the tentacle d ; 

 Z, the brain ; m, the sub-oesophageal ganglion ; ?^, the eye. In the 

 CyclostomidcE the tentacles are contractile, but not invertible. 



The anterior is the normal or constant pair of tentacles ; the pos- 

 terior pair, which supports the eyes in the snail, is reduced to two 

 short processes, which extend from behind the basis of the anterior 

 tentacula in the Turbo, and which form slight projections from the 

 outer side of the base of those tentacula themselves in the Paludina 

 {Jig. 204.), and in most Pectinibranchiata. In the Aplysia, however, 

 which has four tentacula, the ocelli are sessile, and situated in ad- 

 vance of the bases of the posterior pair. 



The eyes never exceed two in number in the Gastropods : they 

 are always very small in comparison with the bulk of the body ; they 

 present their largest relative size in the Pectinibranchiata. In the 

 preparation * from a large species of Murex, there may be readily 

 discerned the sclerotic tunic with its anterior orifice, the expansion 

 of the optic nerve posteriorly between the fibrous and the pigmental 

 tunic, and the large spherical crystalline lens, covered anteriorly by 

 the transparent corneal integument; between which and the lens 

 there is a very small interspace for the aqueous humour and the 

 pupillary circular opening left by the pigmental layer or choroid. 

 M. Lespes has summed up the results of an extensive series of 

 researches on the eyes of the Gastropods, by the following definitions 

 of the leading types of their structure : — a lenticular crystalline lens; 

 the vitreous humour fluid, non-adherent {Helix) : a lenticular crys- 

 talline body ; the vitreous humour thick and united to the lens : the 

 crystalline lens thick and slightly convex ; the vitreous humour 

 viscous and slightly adhering thereto. 



The existence of the sense of hearing in the Gastropods was 

 inferred by Dr. Grant, long before the organ was detected : he justly 

 concluded that the sounds emitted by the Tritonia arborescens under 



* No. 1628. 



