The mouth of a Gastropod is a proboscis, with fleshy lips, generally armed with horny plates or spines on the jaws. The Snail has a crescent-shaped cutting plate on its jaw, and a soft bifid lip below, but the tongue is the most remarkable microscopic object in this group of Mollusca. In the terrestrial Gastropods, it is short and entirely contained within the nearly globular head. It is tubular behind, but in front it is spread into a nearly flat narrow plate, traversed by numerous rows of minute recurved teeth, or spines set upon flattened plates; in the Garden Snail or Slug each principal tooth has its own plate. Fig. 170 represents a magnified portion of a Snail’s tongue by Dr. Carpenter; the rows at the edge are separated to show the structure. The teeth are set close one to another, and are often very numerous. In the Helix pomatia, a snail found in the middle and southern counties of England, they amount to 21,000, and in the great slug (Arion ater), there are 26,800. This kind of tongue only serves for rasping vegetable food. All the Trochidæ, which are marine mollusks that are supposed by some naturalists to live on fuci, are remarkable for the length and beauty of their narrow spiny tongues. Fig. 171 is a small portion of the tongue or palate of the Trochus zizyphinus, highly magnified; the large teeth of the lateral bands, as well as the small teeth in the centre, have minutely serrated edges. Fig. 172 shows the Trochus granulatus in the act of crawling. [Illustration: Fig. 170. The tongue of Helix aspersa.] [Illustration: Fig. 171. Palate of Trochus zizyphinus.] [Illustration: Fig. 172. Granulated Trochus.] The Limpet lives on sea-weeds. The animal is large in proportion to the size of its conical shell; it has a long leaf-shaped gill under the edge of the mantle. The head has a short proboscis and pointed tentacles, with eyes at their base. The mouth has a horny jaw and a very long tongue, moved by muscles rising from firm objects on each side of it. Fig. 173 represents the tongue beset with recurved hooks, and A shows a portion highly magnified. These recurved teeth are transparent, amber-coloured, and in the Limpet, as in most of the other Gastropods, they are chitinous. The teeth towards the point of the tongue are sufficiently hard to rasp the food; and it is said that when they are worn down, the part of the tongue supporting them falls off, and that the waste is supplied by a progressive growth of the tongue from behind, and a hardening of the teeth in front. The soft reserved portion is coiled in a spiral when not in use. [Illustration: Fig. 173. Tongue of Limpet:—A, portion of surface magnified.]