sea urchin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various echinoderms of the class Echinoidea, having a soft body enclosed in a rounded, radially symmetrical, calcareous skeleton covered with long spines.

from The Century Dictionary.

• noun An echinoid; any member of the Echinoidea; a sea-egg or seahedgehog.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) Any one of numerous species of echinoderms of the order Echinoidea.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of many marine echinoderms, of the class Echinoidea, commonly found in shallow water, having a complex chewing structure named Aristotle's lantern

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun shallow-water echinoderms having soft bodies enclosed in thin spiny globular shells