尾索动物亚门是脊索动物门的一个亚门。本纲动物脊索和背神经管仅存于幼体尾部，成体退化消失。身体表面披有一层棕褐色植物性纤维质的囊包，也称被囊动物（Tunicata）。全部是海洋动物。常见种类有各种海鞘和住囊虫，营自由生活或固着生活。有些种类有世代交替现象。

尾索动物亚门有5个纲：

A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata, which is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. Some tunicates live as solitary individuals, but others replicate by budding and become colonies, each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Most adult tunicates are sessile and are permanently attached to rocks or other hard surfaces on the ocean floor; others, such as salps, doliolids and pyrosomes, swim in the pelagic zone of the sea as adults. Various species are commonly known as sea squirts, sea pork, sea livers, or sea tulips.