Ostracod

Ostracod, member of a large crustacean subclass (Ostracoda), sometimes also called mussel shrimps, consisting of about 2,000 species of marine and freshwater forms. Ostracods are tiny, with the largest about 25 mm (1 in) long but most reaching only about 4 mm (0.16 in). The body is shrimp-like and is encased within two hinged shells, with legs, antennae, and mouthparts protruding. Some ostracods feed on minute plant and animal matter, some are scavengers, and a number of species are parasitic. Many reproduce through parthenogenesis. Shells of extinct ostracods are useful in dating rock strata.