Entomostraca, one of the two sub-classes of the class Crustacea, including small and simply organised forms, in which the number of somites and appendages is very variable, whereas in the Malacostraca (q.v.) the number of somites is always 19. There are four orders of Entomostraca - (1) the Phyllopods, with elongated, segmented bodies, protected by a shell; (2) the Ostracods, small unsegmented forms with bivalved shells; (3) the Copepods, long and segmented but shell-less; and (4) the Cirripedia, including the fixed, imperfectly segmented barnacles and acorn shells. The Entomostraca are mainly marine, but many of the Phyllopods as Daphnia and Apus, of the Ostracods as Cgpris, and of the Copepods as Cyclops, are fresh-water; Estheria, a Phyllopod, is estuarine. The sub-class commences in the Lower Silurian. Reference should be made for further particulars to each of the four orders.