It describes a life form instead of a taxonomic group. As individuals of the phylum Chordata, fish share sure elements with other vertebrates. These points are gill slits at some point in the life cycle, a notochord, or skeletal assisting rod, a dorsal hollow nerve cord, and a tail. Living fishes constitute some five categories, which are as diverse from one an alternate as are the four classes of conventional air respiration animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes Agnatha have gills in pouches and absence limb girdles. Extant agnathans are the lampreys and the hagfishes.