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. [Illustration: FIG. 166.--_Patellidæ_ 1. _Patella vulgata._ 2. _P. pellucida._ 3. _P. athletica._ 4. _Acmæa testudinalis_] The Common Limpet (_Patella vulgata_) is found on all our rocky coasts between the tide-marks, often at such a level that it is left exposed to the air for eight or nine hours at a time. The apex of the shell of this species is nearly central, and the exterior is sometimes nearly smooth, but more commonly relieved by radiating ribs. Although the shell itself is not a particularly pretty object, it is often rendered very beautiful and interesting by the various animal and vegetable organisms that settle on it. Those shells that are left dry for hours together are commonly adorned with clusters of small acorn barnacles, while the limpets that have found a home in a rock pool and are perpetually covered with water, often resemble little moving gardens in which grow beautiful tufts of corallines or other weeds, as well as polyzoa and other animal forms. It appears that limpets are not great travellers, the appearance of the rock from which they have been removed being such as to point to a very long period of rest. Those on hard rocks are generally situated on a smooth surface just the size of the shell and generally worn slightly below the surrounding level by the constant friction of the shell; while others that have settled on very rugged spots have their cones adapted to the irregular surface. It has been suggested that the animals make occasional short excursions from their chosen spot, but return again to it; and whether or not this is the case, it is evident that they frequently keep to one small spot for a considerable length of time.