Comprising animals with a hollow, dorsal nerve cord and at some stage in their development a cartilaginous rod, the notochord. Furthermore, chordates are characterized by the presence of gill slits and a postanal tail at some stage in their life cycle. The notochord, which runs dorsal to the gut in the early embryo, is replaced by the vertebral column during the development of vertebrates.
Chordates have a segmented body which fact usually is obvious only in early embryonic stages and which in the adult stage is reflected by the backbone of repeated vertebrae. Chordates comprise what we commonly know as vertebrate animals (mammals, birds, amphibians, reptiles, and fish), but include also a small number of invertebrate organisms (Tunicata and Cephalochordata). There are about 45.000 species of chordates.