All chordates share four key characteristics: a notochord, hollow nerve cord, pharyngeal slits, and post-anal tail. In vertebrates, the notochord disintegrates and is replaced by a backbone with vertebrae made of cartilage or bone. The evolution of hinged jaws allowed vertebrates to eat a wide variety of prey. Gills function through diffusion, with oxygen diffusing from the higher concentrated water into the lower concentrated blood capillaries. Bony fishes have a stiff calcium-reinforced skeleton while cartilaginous fishes like sharks and rays have a cartilage skeleton that allows smooth movement.