echinoid

‖n. pl. [ NL. See Echinus, and -oid. ] (Zoöl.) The class Echinodermata which includes the sea urchins. They have a calcareous shell, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See Spatangoid, Clypeastroid. [ Written also Echinidea, and Echinoida. ] [ 1913 Webster ]