Ostracoda

Ostracoda, an order of Crustacea, belonging to the sub-class Entomostraca (q.v.). The members of the order are ajl small, but a few deep-sea marine species are nearly an inch in length. The body is not divided into segments; the abdomen is rudimentary, and there are seven pairs of appendages, but these are not adapted for swimming. The body is protected by a bivalve shell. The members are all aquatic, and occur in either the sea or freshwater; they swarm in great numbers, and their dead shells often render fresh-water clays easily fissile. The order is widely distributed, and fossil species are known from the Cambrian onwards. Cypris is one of the commonest genera; it is freshwater. Cythere is one of the commonest marine forms. There are seven families.