  When the method of feeding employed by the Snail is compared with that of the Butterfly or the common Fly, a very striking difference in the construction of the mouth-parts will be noticed. The common snail (fig. d), for example, feeds by the constant licking, or rather rasping, motion of a very wonderful tongue—a tongue which, stretched out, appears to be longer than the whole body! Yet only a small portion of this curious organ is in use at one time. 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. 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. The horny jaws, to which reference has been made, are generally not conspicuous; but in the Cuttle-fish and Octopuses they are of huge size, and have been aptly compared to the beak of a parrot. But we must return to this subject again on another occasion, for it is one of quite unusual interest. |
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