190 SENSES — SIGHT. 



jects, and perhaps colours. They are placed on the anterior 

 part of the body, as the eyes of every other animal are ; 

 their size and number 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 

 uneqviivocal attempts to turn that part of the body which 

 is furnished with them to the light.* 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 tou.ch 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 some- 

 times the track is changed entirely. Nay, I have seen, 

 or imagined I have seen, in more than one instance, a snail 

 follow, with apparent eagerness, the purple-coloured flower 

 of a thistle held near its tentacula, and gradually with- 

 drawn. J Adanson very readily distinguished the lens and 

 iris in the eyes of the Cypraeae, whose sight, he asserts, is 

 pretty acute — " assez fin ; " § and Swammerdam affirms that 

 the Littorina littorea draws itself suddenly within the shell 

 when anything is suddenly presented to its eyes ; " so that," 

 he adds, " I may venture to aflirm from hence, that this 

 is the only species of snails that I know wherein any mani- 

 fest signs of sight appear." || Further, such mollusca as 

 have oculiferous tentacula do not use them in touching 

 objects ; for, as Mr. Guilding has properly observed, they 

 carry them usually erect ; and the inferior ones, with the 

 lobes of the cheeks, are principally used as tactors. 



I grant to you that these arguments are not decisive of 

 the question, and that one drawn from the anatomical struc- 

 ture of the organs would be of superior convincement ; and 

 that argument is now, thanks to the dexterity of modem 

 anatomists, entirely in my favour. In the Cypraeadae and 

 the allied families, the structure of the eye is said to be by no 



* Muller, Verm. Hist. i. prref. 3. et 4. 



t Lister's Excrcit. Anat. dc Cochleis, p. 10. 1694, 12mo. 



% " Experiments are said to have been recently made, both by Lenchs and 

 by Steifensand, in wliich a snail was repeatedly observed to avoid a small 

 object presented near the tentaculum ; thus affording evidence of its possess- 

 ing this sense." — Roget's Bridgw. Treat, ii. 482, 



§ Senegal, p. 71. 



II Book of Nature, p. 81. 



