What must an organism be to be characterized as a chordate?

To be characterized as a chordate, an organism must possess, at some stage of its development, a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These features are typically present during the embryonic stage, although some may not persist into adulthood. Chordates include a diverse range of animals, including vertebrates like mammals, birds, reptiles, amphibians, and fish, as well as some invertebrate groups. The presence of these anatomical features sets chordates apart from other animal phyla.