GASTEROPODA. 75 
monly placed either at the bases or at the tips of 
one of the pairs of tentacles. Every one is familiar 
with their appearance in the snails and slugs of 
our gardens, i which they are placed as minute 
shining black points at the tips of the upper ten- 
tacles. Many of the marine Gasteropoda, as the 
great Conchs (Strombus) of the tropical seas, have 
eyes well developed and of elaborate structure. 
Mr. Swainson says—“ In the typical Strombz, these 
organs are so much developed that the iris is richly 
coloured, and the eyes of some of the larger species 
have been described to us as particularly beau- 
tiful.”* According to the Rev. Lansdown Guilding, 
a naturalist who has enjoyed the advantage of 
familiarity with these fine Mollusca in their native 
seas, they have 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.+ And Mr. J. E. Gray affirms, that “the 
eyes of the marine carnivorous Mollusca, Buccinum 
undatum, or Husus despectus, and more especially 
some of the larger Strombz, are as fully developed as 
in the cuttle-fish, showing the cornea and the 
nearly orbicular crystalline lens almost perfectly 
formed, as may be seen by any person simply cut- 
ting the cornea across, and slightly pressing it, 
when the crystalline lens will protrude.” { 
Some species of this Class, few as compared with 
the great body, are naked, but the majority are 
protected by a shell, in some cases very thin, 
brittle, and glassy, in others somewhat horny, but 
more generally of a stony texture, and of great 
* Malacology, 136. + Zool. Journ. iv. 172. 
* Edin. Journ. iii. 52. 
