The tentacles or feelers are usually retractile, and, when retracted, are turned outside-in. Each one is provided with a muscle that runs from the body internally to the tip; and, by the contraction of this muscle the tentacle is involuted just in the same way as the finger of a glove could be by pulling a string attached to the tip inside. In addition to these tentacles, and the eyes and mouth previously mentioned, the head is furnished with ear-sacs, which are little cavities, filled with fluid containing solid particles, with nerve filaments distributed in the walls.
On the floor of the mouth there is a ribbon, supported on a base of gristle, and covered with numerous minute teeth arranged regularly in rows. The gristle is moved backwards and forwards by means of muscles in such a manner that this ‘lingual ribbon’ acts like a rasp, and is employed in scraping or tearing away portions of the substance on which the animal is feeding. By this action the teeth are gradually worn away in front, but this is of no consequence, for the lingual ribbon is always growing forwards, the worn material being replaced by new growth behind.
Fig. 158.—A portion of the Lingual Ribbon of the Whelk, magnified; and a single row of Teeth on a much larger Scale
b, medial teeth; a and c, lateral teeth
The arrangement and form of the teeth are characteristic and important; and since they afford one of the means by which we may trace the natural affinities of similar species, they will be frequently referred to when dealing with the principles of classification. For this reason the student should be prepared to examine the lingual ribbons of molluscs with the aid of a compound microscope as occasion requires. As a rule the ribbon is easily stripped away from the floor of the mouth; and, if placed in a drop of water and covered with a cover-glass, the teeth are readily observed. Until a little experience has been gained the observations may be confined to some of the larger species, in which the ribbon is both large and easily obtained. In the common whelk, for example, it often measures more than an inch in length.
It is difficult to understand how the univalve mollusc manages to glide along so rapidly and gracefully on its expanded foot when we observe it from above, but the difficulty is cleared away when we see it creeping on the side of a glass aquarium, or when we place it on a sheet of glass and observe its movements from the other side. We then see that the foot is in complete contact with the glass, and that a steady but rapid undulatory movement is produced by the successive expansions and contractions of the disc, brought about, of course, by the action of muscular fibres.
A few of the univalves are viviparous—that is, they produce their young alive; but the majority lay eggs. The eggs are often enclosed in horny cases, some of which may be commonly seen washed up on the beach, or attached to rocks and weeds between the tide-marks. The larvæ are always enclosed in a shell, though they are sometimes wholly or partially concealed by the mantle. The shell is usually closed by an operculum; but as the animal advances in age the shell sometimes disappears altogether, or is reduced to a mere shelly plate, as is the case with the land and marine slugs and sea lemons. The young of the water-breathers always swim about freely by means of a pair of ciliated lobes or fins, but these remain only for a brief period, after which the animal settles to the bottom for a more or less sedentary existence.