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]

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]