Members of the subphylum Urochordata (1250 species) lack a backbone but at some stage of their lives possess the three features that distinguish the Phylum Chordata from all other phyla: a notochord; a dorsal, hollow nerve cord; pharyngeal clefts. There are three classes:

Class Ascidiacea: adults are sessile, covered by a complex secreted tunic, hence their popular name of ‘tunicate’; only the microscopic, tadpole-like larval stage possesses distinct chordate features.

Members of the Classes Larvacea and Thaliacea are pelagic and specialised to live in the plankton.

Classes Thaliacea & Larvacea Class Ascidiacea