sea urchin

from WordNet (r) 3.0 (2006)

sea urchin
    n 1: shallow-water echinoderms having soft bodies enclosed in
         thin spiny globular shells

from The Collaborative International Dictionary of English v.0.48

Sea urchin \Sea" ur"chin\ (Zool.)
   Any one of numerous species of echinoderms of the order
   {Echinoidea}.

   Note: When living they are covered with movable spines which
         are often long and sharp.
         [1913 Webster]