534 Nahiy^al Histoj-y of Molluscous Animals : — 



vitreous humours, with the arachnoid tunic ; parts which, he 

 affirms, were as ** clear as the sun at noon." He likewise 

 observed that the eyes of the Lymnae^ae were provided each 

 with " its own proper crystalline humour." The accuracy 

 of this description has been denied, and while I admit that 

 recent observations render it doubtful, I am still unwiHing to 

 believe that they have proved it false or erroneous. Few, 

 very few, have been gifted with the eye and hand of a Swam- 

 merdam, and his honesty is beyond impeachment. This good 

 and great man seems to have foreseen that doubts would 

 arise concerning the correctness of his account. " But who 

 can credit this," says he, " for it seems indeed improbable, 

 that on a point not bigger than the nib of a writing pen, 

 such exquisite art, and so many miracles, should be dis- 

 played." 



The eyes are always two in number, generally minute, 

 although perhaps not disproportionably so, black, convex, 

 and glossy, and incapable of any independent movements, 

 but their position relative to the body is often altered by the 

 motion of the parts on which they are placed. Sometimes 

 they are placed on the body behind or between the tentacula, 

 as in Apiysia ; often they are at the base of the tentacula, and 

 not unfrequently in a notch on their sides at a greater or less 

 elevation. In some genera, as in *Str6mbus and Natica, the 

 eyes are on the summit of pedicles appropriate to themselves ; 

 and in the Helices and Ximaces they terminate the superior 

 tentacula. These are fleshy tubes, which, as you well know, 

 may be drawn completely within the head, and again pro- 

 truded by a motion similar to the evolution of the finger of 

 a glove. The muscle which withdraws the tube arises from 

 the muscles that draw the snail into its shell, and, running 

 forward, is fixed to the extremity. When, therefore, it con- 

 tracts, but still more when assisted by the contraction of the 

 great muscle of the body, the apex of the horn is drawn 

 inwards, in a manner which resembles the turning in of a 

 stocking. The annular fibres, which encircle the horn 

 throughout the whole of its length, unfold the internal part 

 by successive contractions, and thus bring back the eye 

 to its external position.* 



The location of these organs has been varied according to 

 the wants and habits of the animal, and to the medium in which 

 it was destined to five ; but neither this medium nor these 

 habits have had the slightest share in producing the vari- 



* Cuvier, Comp. Anat., trans., ii. 444, 



