^532 Natural History of Molluscous Aiiimals : — 



has been asserted, that those of snails at least are in every 

 respect insensible to light, for the creatures creep and climb 

 as correctly in the dark as by daylight ; they do not at any 

 time perceive obstacles, placed of purpose in their way, until 

 they touch them ; and when deprived of the organs, they 

 crawl on as unconcernedly, and guide themselves as safely, as 

 they did previously to the mutilation. On these grounds some 

 naturalists of eminence deny that the little black points, deno- 

 minated eyes by the vulgar and the learned, are ocular bodies, 

 and find in them nothing more than the organs of an exquisite 

 sense of touch.* 



Organs of touch they may be in snails (Helix, Limax, &c.), 

 in which they are elevated on movable and filiform tubes 

 capable of being directed to all points ; but organs of touch 

 they surely cannot be in the greater number of the class, on 

 which they are placed, as it were purposely, to be removed 

 from the contact of external objects. Such, for example, is 

 their position in the whelks (jSuccinum) and rock-shells 

 ( Murex), in the Lymnae^ae, Turbonidae, and others. Seeing, 

 too, how ampfy the MoUusca are otherwise furnished with 

 tactic organs, additional ones might be deemed superfluous ; 

 and, notwithstanding the facts opposed to it, I am firmly of 

 opinion that what have been called the eyes have been pro- 

 perly designated organs susceptible not merely of the impres- 

 sions of light, but capable of distinguishing objects, and perhaps 

 colours. They are placed on the anterior part of the body, 

 as the eyes of every other animal are ; their size and num- 

 ber are constant in individuals of the same species ; they bear 

 a very exact resemblance to the eyes of many insects, and to 

 the stemmata of others, which are believed to be eyes ; and the 

 snail, when confined, makes unequivocal attempts to turn that 

 part of the body which is furnished with them to the light, f 

 I have occasionally, on a summer's dewy evening, when the 

 animals were on the alert, made experiments on our common 

 slugs and snails ; and I am satisfied, as Lister appears to 

 have been J, that they do perceive obstacles placed in their 

 way, diverging from them when within from one to three 

 inches. They rarely touch the opposing substance, but 

 often they alter their course so slightly as to pass it by in 

 freedom with a shortening of the tentaculum on the near 

 side, while sometimes the track is changed entirely. Nay, I 

 have seen, or imagined I have seen, in more than one instance. 



* Gaspard, in Zool. Journ. i. 179. 



f Miiller Verm. Hist., i. praef. 3. et 4. 



j Lister's Exercit. Anat. de Cochleis, p. 10. 1694. 12mo. 



