Members of the Branchiopoda are unified by the presence of gills on many of the animals' appendages, including some of the mouthparts.  In the groups where the carapace prevents the use of the trunk limbs for swimming the antennae are used for locomotion, as they are in the nauplius. Male fairy shrimp have an enlarged pair of antennae with which they grasp the female during mating, while the bottom-feeding Notostraca, the antennae are reduced to vestiges.  The trunk limbs are beaten in a metachronal rhythm, causing a flow of water along the midline of the animal, from which it derives oxygen an food.