Ostracoda (mussel shrimps, seed shrimps; phylum Arthropoda, subphylum Crustacea) Class of widely distributed marine and freshwater crustaceans in which the entire body is enclosed in a rounded or elliptical, bivalved carapace, the outer layer of which is impregnated with calcium carbonate. A cluster of transverse adductor muscle fibres closes the valves, the muscles being inserted near the centre of each valve. The trunk is greatly reduced, so that the head accounts for much of the body. Trunk appendages are usually reduced to two pairs and there is no external segmentation of the trunk. By contrast, the head appendages, especially the antennules and antennae, are well developed and are the means of locomotion, even in the remarkable terrestrial species of Mesocypris, which plough through damp humus in the forests of southern Africa and New Zealand. Ostracods show a wide range of feeding types, and the group includes filter feeders, carnivores, herbivores, and scavengers. The sexes are separate, and parthenogenesis is common in freshwater forms. There are more than 2000 living species and the group is known from more than 10 000 fossil species, ranging from the early Cambrian to the present.

Ostracoda (ostracods; subphylum Crustacea) 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.