Sea urchin
WordNet
noun
(1) Shallow-water echinoderms having soft bodies enclosed in thin spiny globular shells
WiktionaryText
Etymology
sea + urchin due to the similarity in appearance to the hedgehog.
Noun
- Any of many marine echinoderms, of the class Echinoidea, commonly found in shallow water, having a complex chewing structure named Aristotle's lantern