What is "ostracoda"

Ostracoidea \Os`tra*coi"de*a\, n. pl. [NL., fr. Gr. 'o`strakon shell of a testacean + -oid.] (Zo["o]l.) An order of Entomostraca possessing hard bivalve shells. They are of small size, and swim freely about. Now usually written Ostracoda. [archaic]

Ostracoda \Os*trac"o*da\, prop. n. pl. (Zo["o]l.) 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.