Fig. D.—Common Snail.
Fig. E.—Section of Snail's Head (much magnified).
This tongue (fig. [f]) consists of a long flat ribbon, or 'radula,' as it is called, the surface of which is beset by a series of minute teeth, set in rows across the ribbon. The number of these teeth varies greatly in the different species of Mollusca—the group to which the snail belongs. In some there may be as few as sixteen; in others, in some relatives of our garden snail, for example, there may be as many as forty thousand! The working portion of this ribbon is fixed to a sort of tough cushion, which, by means of muscles, can be drawn forward and protruded from the mouth, where it is worked backwards and forwards with a licking or rasping action that effectually scrapes away, in a fine pulp, the edge of the cabbage leaf on which the creature is feeding. The teeth serve, in fact, the same purpose as the horny spines on the tongue of the lion, or, on a small scale, of the cat. You all must have noticed how rough a cat's tongue is when, in a burst of affection, pussy insists on licking your hand. If she went on licking long enough she would wear away the skin. As the snail's teeth wear away in front they are replaced from the reserve store which is kept in a sort of pocket, which lies behind the 'cushion' in the drawing of a section of a snail's head (fig. [e]). It is here that new teeth are being constantly formed, and pushed forward to supply those lost.
Fig. F.—Snail's Tongue (much magnified).
In this drawing, by the way, you will notice a long arrow (a-b): this marks the passage which the food takes from the mouth to the gullet, and thence to the stomach. The head of the arrow points towards the snail's interior.
In many mollusca, the teeth, instead of resembling one another throughout the series, are of different kinds, very large and very small teeth alternating one with another in endless variety.