Phyllopoda, an order of Crustacea belonging to the sub-class which includes the small, simply organised members of the class, and is known as the Entomostraca (q.v.). The characters that distinguish this from the other three orders (viz. Ostracoda, Copepoda and Cirripedia) are that the body is elongated and generally divided into a number of distinct divisions or segments, that there is usually a rounded, shield-shaped or bivalved shell protecting the head and thorax, and several pairs of swimming feet. The order is divided into two sub-orders, known as the Ciadocera, including small forms with a bivalved shell, as e.g. the water flea, or Daphnia, and the Branchiopoda. The latter are larger, have numerous pairs of swimming-feet. Most of the Phyllopods live in fresh or brackish water; thus Daphnia and Aplts are fresh-water, the Arte/uia or the "Brine-shrimps "occur in salt lakes and lagoons. Many fossils have been referred to this order. This is unquestionably correct with the numerous species of Estheria, but with the more interesting fossils from the Skiddaw Slates (Upper Cambrian) such as Hymenocaris, it is more doubtful; these are probably to be referred to the Leptostraca (q.v.).