A class of crustaceans. Formerly, four extant orders were recognized: Anostraca, Notostraca, Conchostraca, and Cladocera, the first two of which are well defined. The Conchostraca consist of two groups which, although superficially similar, differ in so many fundamental features that they have been placed in separate orders, Laevicaudata and Spinicaudata. The Cladocera are in fact a heterogeneous assemblage of organisms that have now been split into four orders: Anomopoda, Ctenopoda, Onychopoda, and Haplopoda. Although the name Cladocera has to be abandoned as a taxonomic unit, it will doubtless continue to be used as a convenient descriptive term for members of these four orders. There are also two extinct orders, Lipostraca and Kazacharthra. Branchiopods have a long history. Both the Lipostraca and Spinicaudata were differentiated by Devonian times, and their ancestors doubtless originated even earlier.