[Crustacea] The unpaired appendage on the hindmost abdominal somite (see abdomen) which bears the anus.

[Amphipoda] (Abbr. T) a flap attached to the last urosomal segment, above the anus. In Gammaridea it may be divided (= cleft) (Scopelocheiropsis abyssalis 1, Synopia scheelana 3) or undivided (= entire) (Stenopleura atlantica 3, Parandania boecki 2); whereas in Hyperiidea it is always undivided (Hyperia crassa 3, Platyscelus serratulus 4). It may be longer or shorter than the basipodites of uropods III. In hyperiids it is sometimes fused with the urosome.

Alternative form for Telson : T.