Page:Natural History, Mollusca.djvu/96

 humid stones. The eggs are globular, yellowish, and quite diaphanous: they are hatched about the fifteenth day, and the animals reach their full size about the end of the second year. They do not hybernate.

The Fresh-water Snails were scattered by Linnæus and his followers among various marine and terrestrial genera, on account of the diversity which is found in the shape and appearance of their shells. Since more attention has been paid by conchologists to the structure of the animal inhabiting a given shell, the close similarity which subsists between them has prompted their union into one family, and that one of the most natural of all those into which the Mollusca are divided. They are distinguished by the following characters:—

The animals have a lengthened foot, a spiral body, a short, broad muzzle, two large tentacles, triangular and compressed, or awl-shaped, with the eyes near their outer bases. The tongue is furnished with rows of hooked teeth. The mantle, which is ample, has a thin edge, and is protected by a shell of exceedingly variable form, being spiral, turreted, discoid, or simply conical. Those which are spiral are sometimes regular, and sometimes reversed. The colour is generally pale brown, uniform in hue, and the surface is closed with a hard olive skin, technically called the periostraca, or that which is around the shell. They are destitute of an operculum.

The habits of these Mollusca are as identical as