“The mode of using the tongue can be easily seen in any of the common water-snails, when they are crawling on the glass sides of an aquarium; it may then be observed that from between the fleshy lips a thick mass is protruded, with a motion forwards and upwards, and afterwards withdrawn, these movements being almost continually repeated. The action has the appearance of licking; but when the light falls suitably on the protruded structure, it is seen to be armed with a number of bright points, which are the lingual teeth, so arranged as to give the organ the character and action of a rasp.
“If you proceed to dissection, and open the head of one of these mollusca (say, for instance, a common limpet), you will find the cavity of the mouth almost filled with the thick fleshy mass, the front of which is protruded in the act of feeding; and on its upper surface, extending along the middle line from back to front, is seen the strong membranous band upon which the teeth are set. The mass itself consists of a cartilaginous frame, surrounded by strong muscles; and these structures constitute the whole of the active part of the lingual apparatus....
“But the peculiarity of the toothed membrane, which makes its name of ‘_ribbon_’ so appropriate, is, that there is always a considerable length of it behind the mouth, perfectly formed, and ready to come forward and supply the place of that at the front, which is continually wearing away by use.
“In the limpet this reserve-ribbon is of great length, being nearly twice as long as the body, and the whole of it is exposed to view on simply removing the foot of the animal; nothing, then, can be easier than to extract the tongue of the common limpet. But, unfortunately, what you find in one kind of mollusc is not at all what you find in another. In the Acmæas, for instance, which are very closely related to the limpets, and have shells which cannot be distinguished, the reserve portion of the ribbon has to be dug out from the substance of the liver, in which it is imbedded, that organ being, as it were, stitched completely through by a long loop of it.... It might be thought a comfortable reflection that, at all events, one end of the ribbon can always be found in the mouth; but in many cases this is about the worst place to look for it. Perhaps it may appear strange that in some of the smaller species, with a retractile trunk, a beginner may very likely fail altogether in his attempt to find the mouth; if, however, the skin of the back is removed, commencing just behind the tentacles, there will be very little difficulty in making out the trunk, which either contains the whole of the ribbon, as in the whelk, or the front part of it, as in _Purpura_ and _Murex_, where a free coil is also seen to hang from its hinder extremity.... In the periwinkles the same plan of proceeding, by at once opening the back of the animal, is best; and on doing so, the long ribbon, coiled up like a watch-spring, cannot fail to be found.