Oligostraca is a superclass of crustaceans. It consists of the following classes:

Class Mystacocarida: Minute crustaceans (0.5 to 1 mm in length) restricted to interstitial marine sediments. Locomotion depends completely on the presence of dorsal and ventral substrates.

Class Ostracoda (seed shrimp): Small planktonic, demersal and benthic crustaceans with a cosmopolitan aquatic distribution in both freshwater and marine environments, and a few in damp terrestrial habitats. Often called seed shrimps because their body is enclosed within a small and bivalved (with one exception) carapace, which makes them look like seed.

Class Ichthyostraca:

Class †Skaracarida: Long-bodied, marine, likely benthic detritivores. Only contains the genus Skara. Extinct, fossils known from the Cambrian.