Larva of a sea squirt showing the basic features of a chordate.

A chordate is any member of the phylum Chordata: an animal with a notochord (a cartilaginous rod that extends the length of the body), a dorsal hollow nerve cord (a fluid-filled tube that runs the length of the body), gill slits or pouches, and a tail at some stage in its life cycle. Chordates include mammals, birds, reptiles, amphibians, and fishes, known collectively as vertebrates, and three subphyla of invertebratesCephalochordata (lancelets), Hemichordata (acorn worms), and Tunicata (sea squirts).


The basic chordate features are shown clearly by the larva of a sea squirt. It has a hollow dorsal nervous system, the tail is supported by a notochord, and though the gut is not well developed (the larva does not feed) it usually has a pair of gill slits.