The Collaborative International Dictionary of English v.0.48:

Holothurioidea \Hol`o*thu`ri*oi"de*a\, n. pl. [NL. See
   Holothure, and -oid.] (Zool.)
   One of the classes of echinoderms.

   Note: They have a more or less elongated body, often
         flattened beneath, and a circle of tentacles, which are
         usually much branched, surrounding the mouth; the skin
         is more or less flexible, and usually contains
         calcareous plates of various characteristic forms,
         sometimes becoming large and scalelike. Most of the
         species have five bands (ambulacra) of sucker-bearing
         feet along the sides; in others these are lacking. In
         one group (Pneumonophora) two branching internal gills
         are developed; in another (Apneumona) these are
         wanting. Called also Holothurida, Holothuridea, and
         Holothuroidea.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Holothuroidea
    n 1: class of echinoderms including the sea cucumbers [syn:
         Holothuroidea, class Holothuroidea]