Page:Natural History, Mollusca.djvu/87

 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, in which they are placed as minute shining black points at the tips of the upper tentacles. 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 Strombi, 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 beautiful." 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 Fusus despectus, and more especially some of the larger Strombi, 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 cutting 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