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 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]