Tunicata, or UROCHORDATA, a class of remarkable animals, many of which are popularly known as Ascidians (q.v.) or sea-squirts. As is explained in the article on Ascidians, the Tunicates are now regarded as occupying a lowly place among vertebrate or chordate animals. This is proved by the characteristics of the larval forms, and also by three or four genera—e.g. Appendicularia (q.v.)—which retain throughout life the vertebrate characteristics which the great majority lose in the degeneracy of their adult life. The class includes three orders—(1) Larvacea, of which Appendicularia is type; (2) Ascidiacea, or Ascidians; and (3) Thaliacea, free-swimming forms—Salpa (q.v.) and Doliolum.
See W. A. Herdman, 'Tunicata,' Challenger Reports, vi. 1882; xiv. 1886, &c.