An order of fresh-water branchiopod crustaceans formerly included in the order Conchostraca. The entire body and its appendages are covered by a bivalve carapace up to about 17 mm (0.7 in.) long, hence the name clam shrimp. The carapace is often bilaterally compressed and usually displays several lines of growth. The head cannot be protruded. It usually bears paired sessile eyes, but these are fused in Cyclestheria. Locomotion is by means of large antennae. The mandibles are of the rolling, grinding type.