The shell-fish are called Mollusca, the soft-bodied animals. It will easily be seen that this name was intended to point out the distinction between them and the Arthropoda, as regards the way in which the skin is protected. In the latter, as we have seen, the skin itself is hardened. In the shell-fish, the skin secretes a covering which lies outside it. Just as our skins pass out superfluous moisture to the outside, in the form of perspiration, so the skin of the mollusc continually passes to the outside the solid substances which the body has taken in from the sea-water; and by the continual accumulation of these, the shell is formed. This, at least, is the view taken by modern authorities of the formation of the shell in most instances. The juvenile shell-collector usually begins his knowledge of the classification of the Mollusca, by learning that shells are classified as Univalves and Bivalves. This distinction is useful as a beginning. Univalves, that is to say shells which consist of one piece, are those of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to spell it. Bivalves, or shells which consist of two flaps, are those of the Lamellibranchiata or animals with plate-like gills, such as the mussel or oyster. Let us begin with the former. Everybody knows the snail. The snail proper bears a typical univalve shell: though in its relatives (the slugs), the shell is more or less suppressed. The name, Gasteropoda (stomach-footed animals), is supposed to be descriptive of the way in which a snail crawls. Half getting out of its shell, so to speak, it does its best to lay its body to the ground, and its so-called "foot" is an extensive muscular expansion underlying its body, not just a muscular organ thrust out of the shell, as in some other groups. The shell, the mode of crawling, and the "horns," tipped with eye-specks, and directed, intelligently and inquisitively, towards things of interest--these make up, for most people, the idea of Snail. But the most distinctive feature of the class is a less obvious feature, namely, the structure of the tongue. We may see, on any damp day or dewy evening, the snail working away with its tongue at some tender leaf. Its tongue is practically a file with which it files away the substance of the leaf, the resulting green mash being thus made ready in minute quantities for the snail to swallow. Thus are made the too familiar holes which disfigure the leaves of plants in our garden. When seen under the microscope, the file-like structure of the tongue is visible; indeed, in large tongues, it may, to some extent, be made out with the naked eye. Across the tongue, which is a flat ribbon-like structure, there runs a pattern of small teeth, bilaterally symmetrical, and this pattern is repeated over and over again throughout the whole length of the tongue. It might be thought that snails' tongues, being so much alike in their mode of use, would