Diagnosis
Carapace up to 8mm long (usually less than 1mm). Ventral margin of carapace straight or concave at least in the oral region. Anterior rostral incisure always absent, anteroventral incisure occasionally present. Valves usually strongly calcified, smooth or ornamented. Adductor muscle scars varied. Six or seven pairs of appendages. Antenna biramous; exopodite often reduced to a single scale or seta, endopodite generally with four podomeres and with stout terminal chelate setae. Maxilla usually with large branchial plate. Fifth limb with or without a branchial plate, either a maxilliped or a walking leg, or transformed (in males) into a clasping organ. Furca narrow, lamelliform bearing a few setae, or reduced, or absent.
Marine, brackish, freshwater and a few terrestrial. Mostly benthic, littoral and sub-littoral, some (particularly in freshwater) are planktonic.