Source : Webster's Revised Unabridged Dictionary (1913)

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}.]