The Collaborative International Dictionary of English v.0.48:

Spatangoidea \Spat`an*goi"de*a\, n. pl. [NL. See Spatangus,
   and -oid.] (Zool.)
   An order of irregular sea urchins, usually having a more or
   less heart-shaped shell with four or five petal-like
   ambulacra above. The mouth is edentulous and situated
   anteriorly, on the under side.
   [1913 Webster]