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a subclass of the class Crustacea. The head is not fused with the front thoracic segments. There is no mandibular palp, and both the maxilla and mandible are weakly developed. The thoracic limbs are foliate and nonsegmented as a rule; they are used for locomotion, breathing, and bringing food to the mouth. The nerve stems of the ventral nerve cord are widely spaced. There are from four to 11 (rarely as many as 19) thoracic segments. The cephalothoracic carapace is shaped like a shield or a bivalve shell or is entirely absent. The animals live mainly in fresh water. The subclass comprises three orders: Anostraca, Phyllopoda, and Lipostraca (represented only by extinct forms).