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 invertebrates – Cephalochordata (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.