To the same class[2578] also belongs the sea-urchin,[2579] which has spines in place of feet;[2580] its mode of moving along is to roll like a ball, hence it is that these animals are often found with their prickles rubbed off. Those among them which have the longest spines of all, are known by the name of echinometræ,[2581] while at the same time their body is the very smallest. They are not all of them of the same glassy colour; in the vicinity of Torone[2582] they are white,[2583] with very short spines. The eggs[2584] of all of them are bitter, and are five in number; the mouth is situate in the middle of the body, and faces the earth.[2585] It is said[2586] that these creatures foreknow the approach of a storm at sea, and that they take up little stones with which they cover[2587] themselves, and so provide a sort of ballast against their volubility, for they are very unwilling by rolling along to wear away their prickles. As soon as seafaring persons observe this, they at once moor their ship with several anchors. (32.) To the same genus[2588] also belong both land and water[2589] snails, which thrust the body forth from their abode, and extend or contract two horns, as it were. They are without eyes,[2590] and have, therefore, to feel their way, by means of these horns. (33.) Sea-scallops[2591] are considered to belong to the same class, which also conceal themselves during severe frosts and great heats; the onyches,[2592] too, which shine in the dark like fire, and in the mouth even while being eaten. CHAP. 52.—VARIOUS KINDS OF SHELL-FISH.