蛇尾，通称阳遂足，是属于棘皮动物门的蛇尾纲，是种类最多的一个纲，其下包括有220个属和2000个种。

蛇尾的结构与海星相似，但体盘相对较大，腕5个，盘与腕之间有明显交界，而后者腕与盘的交界多不明显，易脱落，但能再生。蛇尾的腕特别细长，甚至比盘的直径长数倍至十数倍，腕的石灰质骨骼成椎骨状，可弯曲，靠椎骨和附生的小骨片关节作垂直的上下运动，或作水平屈曲和缠绕他物。无肛门。腕的腹面有管足二列，但缺乏步带沟，也不具吸盘，这是蛇尾与海星的主要区别。

Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens. They are also known as serpent stars; the New Latin class name Ophiuroidea is derived from the Ancient Greek ὄφις, meaning "serpent".