All the species of Patellidæ, or Limpets, have the power of making cavities with their foot in the surface of the rocks to which they adhere. The cavity exactly corresponds in shape and size with the mouth of the shell, which is sunk and very strongly glued into it, yet the Limpet dissolves the glue with a liquid secretion, roams in quest of food, and returns again to its home: both fluids are secreted by a multitude of glands in the foot, which is the instrument of adhesion. The tongue of the carnivorous Gastropods is a very formidable weapon, used for boring holes in the hardest shells. The round holes in dead shells frequently met with on our coasts show that their inhabitants had fallen a prey to some of these zoophagous Mollusks. The tongue of these predatory Gastropods is a narrow mechanical file, sometimes twice or even three times the length of the whole animal, and when not in use it is curled up near the foot. It is spined in various microscopic patterns according to the genus, and is supported by two firm parts from whence the muscles spring that work the rasp. The Periwinkles have a ribbon-shaped tongue, rough with hooked teeth; the Scalariæ have also predatory tongues, but of all the Gastropod mollusks, the Whelk and its numerous allies are the most predacious. The Purpura or Dog Whelk especially is the most ravenous of mollusks. Its long tongue is armed with hooked and spined teeth, placed three in a row; with this weapon and a proboscis capable of boring, they have been known to exterminate a whole bank of Mussels. The Common Whelk is represented in fig. 174. When in the act of crawling, its head, with two tentacles, is at one extremity, its foot at the other, sometimes used as an organ of prehension; and it has a siphon for carrying water to the gills at the end of the shell. All the families of the naked mollusks or Sea Slugs, furnish beautiful objects for the microscope. The two sexes are united in the same individual, and in their embryonic state they have a shell, which is cast off long before they come to maturity. The gills placed on the naked body are capable of being withdrawn into a cavity in the medial line of the back, and are either plumose, or like the leaf of a plant pinnated again and again, but they vary in form and position in the different genera. [Illustration: Fig. 174. Whelk.] [Illustration: Fig. 175. The Crowned Eolis.] In the group of the Eolidæ, the gills are like leafless trees in most genera, but in the principal genus Eolis, they are long, spindle-shaped, sharp-pointed papillæ, arranged in transverse rows or clusters along the sides of the back, leaving a space between them, as in fig. 175. They are covered with long cilia, whose vibrations send a perpetual current of sea-water along each of them, the respiration is aided by vibrating cilia, scattered almost over the whole body, and the circulation of the blood is very simple. [Illustration: Fig. 176. Tongue-teeth of Eolis coronata.]