The Collaborative International Dictionary of English v.0.48:

Ostracoda \Os*trac"o*da\, prop. n. pl. (Zool.)
   An order of Entomostraca possessing shrimplike bodies
   enclosed in hard bivalve shells; called also seed shrimp
   and mussel shrimp. They were formerly referred to as
   Ostracoidea. They are of small size, and swim freely about;
   many are less than 1 mm in length. Microfossils of certain
   extinct orders have been found extending back to the Cambrian
   age.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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