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.

A class of crustaceans; small bivalve animals that live in both salt and fresh water.

(Gr. ostrakon: shell, scale) n. A class of crustaceans that are typical Arthropoda but laterally compressed and enclosed within a bivalved carapace. This pair of calcareous valves is an integral part of the epidermis and is closed by a series of muscles that leave scars (see Muscle Scar) on the valve interiors. The appendages are typically biramous but may be modified for digging, swimming, etc. The group includes herbivores, carnivores, and scavengers, and occurs in most aquatic habitats. Most are small (less than 1 mm long) and more than 10 000 species have been described, occurring from the Cambrian to the present day. The biological classification of recent forms is based on the soft-part anatomy and fossil forms are classified by the nature of the preserved carapaces. Such characters as the nature of the hinge, the pattern of the muscle scars, as well as overall shape and ornamentation are all used in species determination. Ostracods have considerable stratigraphic use, and are also used to demonstrate variations in salinity and fluctuations in the positions of shorelines.