MOLLUSCA THEIR EYES. 193 



little service to the animal, enabling them at most to distin- 

 guish light from darkness. * Even this limited use, how- 

 ever, may be here important, for as a great number of 

 mollusca are nocturnal in their habits, their eyes may act as 

 sentinels to warn them of the approach of day and of danger, 

 and that it is now time to seek places of retreat and safety. 



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. In the Gas- 

 teropods they are always lateral or dorsal ; sometimes they 

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

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

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

 less elevation, f In some genera, as in Strombusand Natica, 

 the eyes are on the summit of pedicles appropriate to them- 

 selves ; and in the Helices and Limaces they mount near to 

 the apex of the superior tentacula. These are fleshy tubes, 

 which, as you well know, may be drawn completely within 

 the head, and again protruded by a motion similar to the 

 evolution of the finger of a glove. The muscle which with- 

 draws the tube arises from the muscles that draw the snail 

 into its shell, and, running forward, is fixed to the extremity. 

 When, therefore, it contracts, but still more when assisted 

 by the contraction of the great muscle of the body, the apex 



* Swammerdam infers that "the sight of the snail must be very dim. I 

 could not, at least hitherto, observe that the snail sees well the things which 

 are near it, notwithstanding all the attempts I made for this purpose." — p. 48. 

 Of the Nudibranchiata, Hancock and Embleton say, — " The degree of the 

 vision enjoyed by these animals must be slight. They can distinguish light 

 from darkness, and can probably appreciate imperfectly different degrees of 

 light ; and as the eyes are placed under the skin of the head, their perception 

 of objects must be exceedingly faint and indistinct." — Ann. Mag. N. Hist. 

 ser. 2, iii. 196. 



t Mr. J. E. Gray has the following generalisations : — 



1. Mollusca which have the eyes sessile on the outer side of the tentacula, 

 have the sides of the body simple or unfurnished with filaments; and the 

 mantle of the operculum is also simple. Lacuna is an exception, having a 

 small process on each side of the hinder part of the opercular mantle. 



2. Mollusca which have the eyes pedicelled, have the mantle of the oper- 

 culum extended along each side of the body with lateral filaments on its 

 margin, or just beneatli it ; and in an interruption of its front are placed the 

 eye pedicles and the proper tentacula. The front of this mantle is also con- 

 tinued over the head to form a continuous hood, or two or more crests. The 

 exceptions arc Nerita, Neritina, and Ampullaria, which have pediceled eyes, 

 but appear to want the lateral membrane. In the Syn. Brit. Mus. for 1842, 

 Mr. Gray has used these characters to separate the groups of the principal 

 sections. 



o 



