Chordate

Chordates are the phylum of animals that includes all vertebrates, from fishes to humans, along with two small groups of marine invertebrates: the lancelets and the tunicates, or sea squirts. They are deuterostomes, the same broad branch of the animal tree as sea stars and sea urchins.

Four hallmarks define the group, and each appears at least during embryonic development. The notochord is a flexible rod of tissue running along the back that stiffens the body and gives the phylum its name. Above it lies a hollow nerve cord, which in vertebrates develops into the central nervous system. The pharyngeal slits, a row of gaps in the wall of the throat, filter food or hold gills in aquatic forms; in land vertebrates the same embryonic region is reshaped into parts of the ear, the tonsils and the thymus. A muscular tail extends beyond the anus. Many textbooks add a fifth feature, the endostyle, a groove in the throat that is the forerunner of the thyroid gland.

In adult vertebrates the backbone replaces the notochord, which survives only as the soft centers of the intervertebral discs, and the human tail shrinks to the coccyx. Adult sea squirts lose most of these features, but their swimming larvae show all of them, which is how these sessile animals were recognized as chordates.