The notochord is a longitudinal flexible rod of cells that forms the supporting axis of the body in the fetus of the lowest chordates and upper vertebrates.
Urochordates are also known as tunicates because their body is surrounded by a leathery sheath composed of tunicin (cellulose).
Notochord and nerve cord are present only in the nerve phase in urocordates.
The notochord is present only in the larval tail.
The notochord appears in the larval stage in the tail of the larva and disappears in the adult. This is known as retrogressive metamorphosis.