One of the individual members of a compound ascidian which are members of the Ascidiacea: Sea-squirts or sedentary tunicates are regarded as ascidiozooids and live either alone or in colonies. They have a degenerate nervous system, an atrium opening dorsally near the mouth, and numerous gill-slits forming a basket-work. The whole organism is enclosed in a "test" or outer skin made of material resembling cellulose.