The phylum of animals characterized by possession of a notochord, which includes the Cephalochordata (lancelets), Urochordata (tunicates), and Craniata (vertebrates and hagfishes). The phylum also may include the extinct Yunnanozoons and Vetulicolians. Members of the first two groups are small and strictly marine. The vertebrates include the fishes, amphibians, reptiles, birds, and mammals; they are usually free-living and are found in many environments from marine to freshwater and in almost all terrestrial biomes. Some workers include the acorn worms, pterobranchs, and graptolites (Hemichordata) as a subphylum within Chordata, but here it is considered as a separate phylum, because recent morphological and molecular data indicate these organisms are closely related to echinoderms and do not possess a notochord. However, the phylogenetic position of these animals is still controversial.