Chordates are members of the phylum Chordata, deuterostome animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. Taxonomically, the phylum includes the subphyla Vertebrata, including mammals, fish, amphibians, reptiles, birds; Tunicata, including salps and sea squirts; and Cephalochordata, comprising the lancelets.
The phylum Hemichordata including the acorn worms has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the echinoderm phylum, including starfish, sea urchins, and sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion.