36 



SIGHT, HEARING. 



chus, Nerita, Ampullaria, Paludina, etc., the ommatophores are 

 entire!}^ separate from the tentacles. 



The tentacles are sometimes delicately hairy, and these hairs 

 are evidently tactile also. In the same category of tactile organs 

 must be included the lobes, filiform processes, etc., of the mantle 

 margin, as well as the processes which beset the mantle lobes of 

 Cypraea, which lobes are thi'own over the back of the shell so as 

 nearly to cover it. The anterior lobes of the foot existing in 

 many mollusks, as in Buccinum and Harpa for example, may 

 also be regarded as tactile organs. 



Sight. 



We have already seen that the eyes are variously situated upon 

 or branching from the tentacles in most cases ; in others they 

 are sessile or nearly so, upon the head, and situated behind and 

 outside of the tentacular bases. Tentacles and eyes are both 

 wanting in Chiton. 



The eyes are spherical, oval, or conical structures, embedded 

 in the skin of the eye-stalk in such a way that the epithelium of 

 the stalk covers them. Externally' they are enveloped by a firm 

 laminated membrane (sclerotica) which becomes thinned out 

 anteriorly to a cornea. Internallj' the sclerotica is covered by 

 a pigment contained iu polygonal cells, the Choroidea, which 

 extends forwards to the cornea, and since the cornea does not 

 cover the whole of the external side of the eyeball but only its 

 middle, a dark pigment ring is seen at its border, which might 

 be described as an iris, but cannot be considered equivalent to 

 the same structure in higher animals. In Strombus this iris-like 

 ring exhibits strikingly brilliant colors, yellow, red, and green ; 

 often several colors appear in sepai-ate rings behind each other, 

 numerous instances of which are figured by Quoy and Gaimard 

 in the Voyage de L'Astrolabe, and used by them as specific 

 characters. In this eyeball, just behind the cornea, the lens is 

 placed, which is nearly spherical and consists of concentric layers. 

 The posterior part of the eyeball is occupied by the so-called 

 vitreous or glassy body which Keferstein regai'ds as a retina. 



Hearing. 

 Souleyet first detected auditory organs in univalves, and Sie- 

 bold, Krohn, Kolliker, Schmidt, Lucaze-Duthiers and Jhering 



