ostracod

from WordNet (r) 3.0 (2006)

ostracod
    n 1: tiny marine and freshwater crustaceans with a shrimp-like
         body enclosed in a bivalve shell [syn: {seed shrimp},
         {mussel shrimp}, {ostracod}]

from The Collaborative International Dictionary of English v.0.48

ostracod \ostracod\ n.
   A member of the {Ostracoda}, an order of tiny marine and
   freshwater crustaceans with a shrimplike body enclosed in a
   bivalve shell.

   Syn: seed shrimp, mussel shrimp.
        [WordNet 1.5]