Page:The aquarium - an unveiling of the wonders of the deep sea.djvu/61

34 looks like a slender white thread, two inches or more in length, one end of which is attached to the throat, and the other, which is free, you will see coiled in a beautiful spiral within the cavity of the stomach.

By allowing this tiny thread to stretch itself on a plate of glass, which is easily done by putting a drop of water on it first, which then may be drained off and dried, you will find that it is in in reality an excessively delicate ribbon of transparent cartilaginous substance or membrane, on which are set spinous teeth of glassy texture and brilliancy. They are perfectly regular, and arranged in three rows, of which the middle ones are three-pointed, while in each of the outer rows a three-pointed tooth alternates with a larger curved one somewhat boat-like in form. All the teeth project from the surface of the tongue in hooked curves, and all point in the same direction.

The action of this sort of tongue is that of a rasp, the projecting teeth abrading the surface of the plants on which the animal feeds, just as the lion is said to act with the horny papillæ of his tongue on the flesh of his victim. The general structure is common to all the Gasteropod Mollusca, but the varieties in the mode and pattern of the dentation are almost infinite.

The little Top, for example, has the teeth set in eleven longitudinal rows, along the central part of the ribbon, while the edges, which are turned over on each side, are formed into oblique combs;—altogether a very elaborate affair. But even this is exceeded by the tongue of the Livid Top (T. ziziphinus), a larger and handsomer species not rare among the lower