Fig. 165.—Shells of Dentalium
In these animals, too, the head is imperfectly formed, without eyes or tentacles. The foot is conical and pointed, with two symmetrical side lobes; and the gills, also two in number, are symmetrically disposed. The margin of the mouth is fringed, and the animal is attached to the shell near the posterior end.
The Dentaliadæ are carnivorous, subsisting on minute molluscs, foraminifera, &c., and generally live on sandy or muddy bottoms, in which they sometimes bury themselves.
Our next family includes the familiar Limpets, and is designated Patellidæ on account of the resemblance of the conical shell to a little dish. In these the apex of the cone is not central, but situated more or less towards the anterior; and the muscular impression within is shaped like a horseshoe, with its open end turned to the front.
Unlike the members of the preceding families, the limpets have a well-formed head furnished with both eyes and tentacles, the former situated at the bases of the latter. They have a horny upper jaw, and the tongue, which is very long, is supplied with numerous hooked teeth. The foot is a very large disc, as large as the shell, and the gills consist either of one or two branched plumes, or of a series of lamellæ almost or entirely surrounding the animal between the shell and the margin of the mantle.
The reader has probably experienced the difficulty of detaching a limpet from its hold on the rocks. The tenacity of the grip is not due to the mere adhesive power of the foot itself, but to atmospheric pressure, the effect of which is complete on account of the total exclusion of air from under the disc of the foot; and when we remember that this pressure amounts to fifteen pounds on every square inch of surface, we can readily understand the force required to raise a large limpet from its position.
