Of the pleon segments the anterior 3 form the pleosome and the posterior 3 the urosome. Each pleosome segment bears a pair of biramous swimming legs - pleopods. They are used for swimming and their constant beating produces respiratory current which aerates the gills and in female also ventilates the marsupium. The lower posterior edges of the pleosome segments that extend below the body are called epimeral plates. The shape of the posterodistal angle of epimeral plate 3 has considerable taxonomic value. The three urosome segments are usually similar in size, in several groups the somites may be fused together. Each urosome segment bears biramous uropods, that are used for burrowing, jumping and swimming. The proportions and structure of them, especially the third pair, are variable between families and are useful in taxonomic studies. Telson is a flap attached above the anus to the third urosome segment. It is of primary taxonomic value and may be totally of partially cleft, entire or emarginated, small or elongate, fleshy or laminar, naked or spinose/setose.