ABSTRACT
This phylum comprises three major groups which are sometimes elevated to phyla in themselves, the urochordates (ascidians, larvaceans and salps), cephalochordates (lancelets) and vertebrates. Here we follow Barnes (1998) in regarding them as subphyla. There are no cephalochordates known from the Southern Ocean, and amongst the urochordates only the ascidians are benthic.
