Sub-Phylum Tunicata (Urochordata)

Abstract

Three main divisions of this ancient group have been recognised: (1) the sessile Ascidiacea, (2) the Thaliacea, and (3) the Appendicularia. Each has an organisation which, when taken as a whole, is ‘significantly unique in itself’ (Berrill). The tunicates are marine. The Ascidiacea are all sessile, whilst the other two groups are pelagic. They are especially interesting for several reasons. They undergo a remarkable series of changes in the course of their life-history. Some exhibit an alternation of generations quite as marked as that existing among so many invertebrates. In most tunicates there is a retrogressive metamorphosis almost, if not quite as striking as that which has been described among the parasitic copepods or the Cirripedia. The sessile form and ciliary feeding mechanism link the adult tunicates with the pterobranchs, Rhabdopleura and Cephalodiscus. Multiplication by budding, so common in the lower groups of invertebrates, but exceptional or absent in the higher, is of very general occurrence in the Tunicata.