[Radiolaria Polycystina] In Nassellaria: the third segment of nassellarian multisegmented shells. shell structure;shell structure 2.

[Polychaeta] Posterior region of the trunk.

[Crustacea general] The trunk somites (tagma) between the thorax and the telson.

[Amphipoda] Comprising 6 segments, 3 segments bearing pleopods corresponding to the pleon sensu stricto, and 3 segments of the urosome (bearing uropods). In hyperiids urosomal segments (urosomites) II and III are fused. In caprellids the abdomen is reduced. Ventrolateral plate-like extensions of the pleon segments are called epimera or epimeral plates. The shape of the free margin of the epimera III and II is important in gammarid taxonomy.

[Euphausiacea] The segmented hindmost part of the body usually consisting of 5-6 somites and telson.