Nervous Si/stem and Senses. 533 



a snail follow, with apparent eagerness, the purple-coloured 

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

 drawn. Adanson says that the Cyprc^^ce " se servent fort 

 bien de leurs yeux;" and Swammerdam affirms that the 

 Turbo littoreus draws itself suddenly within the shell when 

 any thing is suddenly presented to its eyes ; " so that," he 

 adds, " I may venture to affirm from hence, that this is the 

 only species of snails that I know wherein any manifest signs 

 of sight appear." * Further, such Mollusca as have oculi- 

 ferous 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 structure 

 of the organs would be of superior convincement ; and that 

 argument is, I think, upon the whole, in my favour. In the 

 Cyprce^adce and the allied families, the structure of the eye is 

 said to be by no means obscure; and the giant >Str6mbidae, 

 which inhabit the Caribbean Sea, have eyes more perfect than 

 those of many vertebrated animals. They have, according 

 to Mr. Guilding, a most intelligent and indefatigable natural- 

 ist, a distinct pupil and a double iris, equalling in beauty and 

 correctness of outline those of birds and reptiles ; and he 

 discovers in the organ a vitreous and an aqueous humour, 

 and the black pigment.* Mr. Gray, a naturalist of equal 

 industry and accuracy, tells us that if they who have doubted 

 concerning the nature of these organs " had examined the 

 eyes of the marine carnivorous Mollusca, ^uccinum undatum 

 or Fusus despectus, and more especially some of the larger 

 >Str6mbi, they would have found the eye as fully developed 

 as in the cuttle- fish, showing the cornea and the nearly orbi- 

 cular crystalline lens almost perfectly formed, as may be seen 

 by any person simply cutting the cornea across, and slightly 

 pressing it, when the crystalline lens will protrude." } This 

 evidence seems conclusive ; for if the same parts cannot be 

 demonstrated in the smaller or in the terrestrial Mollusca, 

 it is surely because of the minuteness of the organ and the 

 difficulty of the dissection. But the fact is, that Swammer- 

 dam has described with great minuteness the eye of the com- 

 mon snail, in which he detected "five distinct and visible 

 parts," viz. the uvea, the aqueous, the crystalline, and the 



* Book of Nature, p. 81. 

 t Zool. Journ., iv., 172, 



:f Edin. Journ. NaL Geogr. Sc, i. 52. new series. 

 M M 3 



