Source : Webster's Revised Unabridged Dictionary (1913)

Sea urchin \Sea" ur"chin\ (Zo["o]l.)
   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.

Source : WordNet®

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