The five teeth, whose sharp tips meet in a point when closed, are triangular prisms, the inner edge is sharp and fit for cutting. Each tooth is planted upon a larger triangular socket, two sides of which are transversely grooved like a file, and as these two sides are in close contact with the sides of the opposite socket, the food when cut by the small teeth is ground down by the sockets, and a salivary secretion finishes the mastication. The sockets of the teeth are connected by ten additional solid pieces, placed two and two between them, which completes the pyramidal apparatus called Aristotle’s lantern; it consists of forty solid calcareous pieces arranged in fives, and moved by forty muscles attached to five calcareous ridges, and five arches near the internal edge of the corona. Five pairs of these muscles when acting together protrude and retract the teeth; when acting separately they draw them to one side or to the other; five pairs separate the five teeth, five pairs shut them, and the remaining five pairs work the bruising machine. The masticated food passes through a short gullet into the stomach, where it is digested, and the indigestible part is carried by an intestine to the vent in the smaller polar circle. The smaller polar circle is formed of ten triangular plates, five are attached to the bands containing the feet holes, and five to the intermediate bands. The last five are perforated, and are the reproductive plates: the other five are also perforated for the discharge of the liquid that moves the tubular feet, and which, after having circulated in the body, is no longer of use. In five of these polar plates there are red specks, the rudiments of eyes, the only organs of sense these creatures seem to possess except that of touch and probably smell. The nervous system is a slender, equal-sided pentagon round the gullet, from the sides of which five nerves are sent to the muscles of the mouth, and others, extending along the ambulacral or feet bands, end in nerve-centres under the eye-specks. [Illustration: Fig. 139. Sucker-plate of Sea-Egg.] [Illustration: Fig. 140. Section of a sucker-plate.] The mechanism for extending and retracting the feet by a liquid, is the same with that in the star-fishes, but the pores which admit the liquid into the feet are double. The tubular feet swell at their extremity into a fleshy sucker, within which there is a thin glassy reticulated rosette (fig. 139), of which fig. 140 is a highly magnified segment. It is perforated in the centre by a large round opening. The sea-urchins can stretch their feet beyond the spines, and by means of the suckers they can attach themselves even to smooth objects, or aided and directed by their spines they roll themselves along with a rotatory motion head downwards.