n. pl. (Zool.) An order of Entomostraca including a large number of species, most of which live in fresh water. They have flattened or leaf like legs, often very numerous, which they use in swimming. Called also Branchiopoda. In some, the body is covered with a bivalve shell (Holostraca); in others, as Apus, by a shield-shaped carapace (Monostraca); in others, like Artemia, there is no carapace, and the body is regularly segmented. Sometimes the group is made to include also the Cladocera.