The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

Echinoidea
    n 1: sea urchins and sand dollars [syn: Echinoidea, class
         Echinoidea]