Page:Natural History, Mollusca.djvu/184

 arranged lengthwise. But probably few are aware how very elegant a creature it is when tenanted by its living inhabitant, and crawling at ease in clear water. The foot, on which it glides with a slow but smooth motion over the surface of the rock on which it habitually dwells, or, if you please, on the bottom of the saucer of sea-water in which you are examining it, is a broad expansion spreading out to twice the superficies of the base of the shell. Above this is the fleshy mantle, which is so turned up as closely to invest the shell, conforming to its shape, and even fitting into the grooves between the ridges. This mantle can be protruded at the will of the animal, so far that the two sides meet along the top of the shell, and completely cover it; or can be completely retracted within the wrinkled lips beneath; and it is capable of all gradations of extension between these limits. From the front of the shell protrudes the head, armed with two straight and lengthened tentacles, answering in function and appearance to the upper part of horns in a snail; except that the little black points which constitute the visual organs are not in this case placed at the tips, but on a little prominence on the outside of the base of each tentacle. Above and between these, which diverge at a considerable angle, projects the proboscis, a rather thick, fleshy tube, formed by a flat lamina, with its edges bent round so as to meet along the under side. The interior of this proboscis is lined with delicate cilia, by whose constant vibrations a current of water is drawn into the tube, and poured over the surface of the gills for the purpose of respiration. This current may be readily perceived by any one who will take the trouble to watch with a