Echinoderms, Or Echinodermata(Gr. a hedgehog, and skin), the highest class of radiated animals, so named from the spines with which many of the genera are covered. They all have a tough envelope, containing calcareous particles, or a kind of shell of movable pieces, provided with tubercles or spines; the oral or actinal region is beneath, the aboral or abactinal above, the parts radiating from the former and meeting above; along certain of the rows of plates are perforations for the ambulacra or suckers used in locomotion, and between them the spine-bearing plates, when these exist. They have well developed muscles, and a nervous ring around the oesophagus, which sends off branches to the rays, which are generally five or a multiple of five. • Respiration is by means of branchiae, by organs performing other functions, and by water passing into the general cavity of the body. They reproduce by means of eggs, and abound in almost all seas; some of the lower orders were among the earliest created animals, and the class ranges from the Silurian to the present epoch.
They embrace the five orders of crinoids or stone lilies, ophiurans or brittle stars, asterioids or star fishes, echinoids or sea urchins, and holothurians or sea cucumbers, the last being the highest, and seemingly a connecting link between radiates and articulates. There is a remarkable persistency of form in the class from the earliest to the present time, some of the oldest being referred by Goldfuss to existing genera. They are essentially carnivorous.