An order of freshwater crustaceans (superorder Syncarida) whose members lack a carapace and whose head and thorax are not fused. All the thoracic appendages of bathynellaceans are biramous and similar. The sixth pleomere is fused to the telson, which bears a furca (pair of caudal rami). In addition to uropods, there may be one or two pairs of uniramous pleopods (with various limbs being reduced or absent).  See also: Anaspidacea; Crustacea; Syncarida