Ascidiacea (sea squirts; subphylum Urochordata) A class of sea squirts, most of which inhabit shallow waters where they attach themselves to such structures as rocks, pilings, the bottoms of ships, and coral reefs. Deep-water species inhabit soft substrates and are known to occur at depths exceeding 2000 m. Most ascidians are inconspicuous and are often covered by smaller sessile animals. Halocynthia pyriformes, of the Atlantic coast of N. America, is known as the ‘sea peach’ because it resembles the fruit in size, shape, and colour. The class contains four orders: Aplousobranchiata; Aspiraculata; Phlebobranchiata; and Stolidobranchiata.

sea squirts See ASCIDIACEA; UROCHORDATA.