The development of /Balanus/ from the larval stage, as also the anatomy of the adult, is similar to that of /Lepas/. When covered with water and unmolested, there may be seen over a bed of barnacles thousands of tiny fringed feet waving to and fro. The motions look like gestures; they are perfectly regular and rapid, numbering eighty to a hundred a minute. The shell covering formed by barnacles on piles of wharves and bridges is said to be a protective agent; otherwise the barnacles seem to have no economic value in nature, as, unlike other animals, they do not serve, except in very small measure, as food to other classes. The tautog and perhaps some other fishes feed partly upon them. They are sometimes an obstacle to oyster-culture, as they fasten upon the objects intended for oyster embryos, and, growing faster than the latter, soon crowd them off. One species, /Coronula diadema/, fastens to the skin of whales. It attains the size of two inches in diameter. The shell is half an inch thick and full of cavities, into which the skin of the whale is drawn, giving the barnacle a secure hold. (Plate LIX.) ^B. balanoides^ (Stimp.), ^B. ovularis^ (Gould), the rock-barnacle. This is perhaps the most conspicuous of the barnacles. It inhabits the whole northern Atlantic coast, and is so abundant that it not only whitens the rocks with a complete incrustation of shells, but the animals are so crowded that many of them lose their normal shapes and become greatly elongated. When the rocks are covered with water they seem alive, on account of the thousands of waving tentacles. This species also incrusts woodwork between tide-marks. The shell is small, white, and variable in shape; sometimes its height is less than the diameter of its base; again the height is several times greater and the summit broader than the base. In its early stages the valves are smooth, but later the base is scalloped by four or five grooves. The summit of the plates is even and blunt. The aperture is diamond-shaped. Two valves of the operculum are pointed at the tips; two are blunt, making a deep notch in the summit. These valves are the distinctive feature by which to recognize this species, which varies so much in outward form; the species is also distinguished by its membranous base, which does not form a solid plate like that of other species.