any member of the invertebrate class Ascidiacea (subphylum Urochordata, also called Tunicata), marine animals with some primitive vertebrate features. Sea squirts are sessile (that is, permanently fixed to a surface), potato-shaped organisms found in all seas; they are found from the intertidal zone to the greatest depths. Sea squirts commonly reside on pier pilings, ships' hulls, rocks, large seashells, and the backs of large crabs. Some species live individually; others live in groups or colonies.

The body has an outer protective covering, the tunic. There are two large pores, one to guide water into the body cavity, the other serving as an exit. Water is propelled through the animal by pharyngeal cilia. Food and oxygen are taken from the water current as water passes through gill slits in the pharynx. Near the shore, debris from dead plants and animals constitutes an important part of the diet; in deeper water, plankton (microscopic plants and animals) is a more important food.

Although all adults are simultaneous hermaphrodites (that is, they possess both functional male and female reproductive organs), eggs shed into the water are fertilized by sperm from other individuals. The tadpole-like larvae are free-swimming. The muscular tail contains a notochord (a flexible, rodlike structure common to all vertebrates) and a nerve cord. When the larva finds a place to metamorphose, it attaches itself by a sucker located at the anterior end of the body. Later, the tail, with its notochord and nerve cord, is absorbed and disappears.

Reproduction also occurs by budding: near the base of the sea squirt fingerlike projections (stolons) break off and settle elsewhere to become new individuals.