The Balanidæ (fig. 153) are grouped in innumerable multitudes, crowded together on the rocks of the southern and western coasts of England, like brown acorns. They have an obscurely articulated body, enclosed in a membrane, and defended by a multivalve conical shell. The base of the shell is a broad disk fixed to a foreign substance by a cement secreted by the animal. The walls consist of twelve triangular compartments. Six rise upright from the edge of the disk, and end in a point at the open margin of the shell; the other six are inverted and wedged into the interstices. The whole cone thus constructed is divided into from four to eight pieces by expansive seams. The mouth of the cone is closed by a lid formed of four triangular valves, which meet in a point in the centre, and shut in the creature. Six pairs of long, slender, curly feet rise from the throat of the animal, and bend over the prominent mouth, which is placed at the bottom of a kind of funnel, formed by the divergence of these six pairs of thoracic feet. It is furnished with a broad upper lip, two palpi, and three pairs of jaws, of which the outermost are horny and toothed, the innermost soft and fleshy. Each foot is divided into two similar many-jointed branches: the shortest pair is nearest to the mouth, the others increase gradually in length and number of joints to the most distant (fig. 154). Mr. Gosse estimated that, in a specimen he possessed, the whole apparatus included nearly five hundred distinct articulations. Since each joint is moved by its own system of muscles, the perfection of the mechanism may be conceived. But it is as sensitive as flexible, for every separate joint is furnished with a system of spinous hairs, which are no doubt organs of touch, since the whole of the branches are supplied with nerves. These hairs, which extend at somewhat wide angles from the axis of the curling filaments, are barbed, for they have numerous projections, or shoulders, surrounded by whorls of microscopic hairs.[40] [Illustration: Fig. 154. Tentacles or feet of the Balanus.] This beautiful and complicated structure is the fishing apparatus of the animal, which it is continually pushing out and drawing in through the valved lid of the shell. When the whole is thrown out it is widely spread, and the filaments uncurled; then, as they close again, the innumerable hairs meet and form a sieve through which the water escapes, but whatever minute particles it may contain are inextricably entangled, and when the small animals fit for food have been selected, the filaments curl inwards, and carry them to the mouth; there they are seized by the jaws and sent through a short gullet to be digested.