Echinoidea \Ech`i*noi"de*a\, n. pl. [NL. See Echinus, and -oid.]

(Zo["o]l.) The class Echinodermata which includes the sea urchins. They have a calcareous, 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.]

Source: Webster's Revised Unabridged Dictionary (1913)