* ORGANS OF SENSE. It 



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 stalks covers them. Externally they are enveloped by a firm 

 laminated membrane [sclerotica) which becomes thinned out 

 anteriorly to a cornea. Internally the sclerotica is covered by 

 a pigment contained in polygonal cells, the choroidea^ which 

 extends forward 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, j^ellow, red and 

 green ; often several colors appear in separate rings behind each 

 other, numerous instances of which are figured by Quoy and 

 Gaimard in the Voyage de I'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 concen- 

 tric layers. The posterior part of the eyeball is occupied by the 

 so-called vitreous or glassy body which Keferstein regards as a 

 retina. 



The eyes of the land snails or Helices (vi, 12, 13, 74), are 

 placed at the extremity of their tentacles or feelers, and can be 

 retracted entirely within the head by invagination, a process 

 which may be likened to drawing in the finger of a glove (vi, 14). 

 The transparency^ of the horns of a snail is siich that invagination 

 may be readily perceived, bj^ touching them and watching the 

 black eye-bulb descend through the constantly shortening stalk, 

 until concealed under the skin of the head. This contraction 

 takes place in a manner which completely changes the relative 

 positions of the crystalline lens and ocular bulb or special 

 retractor muscle of the former, causing it to descend in advance 

 of the latter, which is, morever, drawn to one side and partly 

 upset in position in descending. Janthina and most of the 

 pteropods, the large Auriculee, Natica, Sigaretus, BuUia, Chiton, 

 Lepeta, etc., are blind, and in Doris the e^^es are covered with a 

 very thick integument, so that vision must be limited to a vague 

 perception of light. In certain genera and species of pulmon- 

 ates, or land snails confined in habit to grottoes and caverns the 

 eyes, are very imperfect. Even a species of Helix, a genus 

 usually so well provided with vision occurs in the grottoes of 

 Carniola with imperfect organs. Certain gastropods exhibit a 

 character of inferioritj^ in the multiplication and diffusion of 

 their visual organs, which appears to ally them to the lamelli- 

 branchiates : for like them, the ocelli of Trochus, Margarita, the 



