Entomostraca
Entomostraca is a historical subclass of Crustacea, no longer in technical use. It was originally considered one of the two major lineages of crustaceans (the other being the class Malacostraca), combining all other classes – Branchiopoda, Cephalocarida, Ostracoda and Maxillopoda. The Ostracoda have the body enclosed in a bivalve shell-covering, and are normally unsegmented. The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the post-oral appendages normally branchial. The Copepoda normally have a segmented body, not enclosed in a bivalved shell-covering, fewer than twelve segments, the limbs not branchial. Under the heading Crustacea, the Entomostraca have already been distinguished not only from the barnacles, but also from the Malacostraca.
References
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press