Ciona intestinalis (sometimes known by the common name of vase tunicate) is an ascidian (sea squirt), a tunicate with very soft tunic. Its Latin name literally means "pillar of intestines", referring to the fact that its body is a soft, translucent column-like structure, resembling a mass of intestines sprouting from a rock.

Ciona intestinalis is a large solitary sea squirt which grows up to 15 cm in length. The body is soft, retractile and a pale translucent greenish/yellow, through which the internal organs are visible. Sometimes there are orange bars on the body. There are two openings or siphons which may have yellow margins with orange/red pigment spots.

Ciona in­testi­nalis are usu­ally found in silty con­di­tions in 0-500 me­ters (1640 feet) of water. Most are found near rocky shores and es­tu­ar­ies, where the tide of the ocean meets a river cur­rent. They are often found grow­ing in great num­bers on man­made struc­tures such as piers, pil­ings, and even in ma­rine aquaria (Cole­man 1991).