From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
sea urchin
n 1: shallow-water echinoderms having soft bodies enclosed in
thin spiny globular shells