Classification of contemporary animal life into aquatic and terrestrial fauna is as old as the science of biology itself. Modern transitional breathers use multiple structures for respiration. Biologically, the taxon constitutes a highly provocative group, representing a modern prototype of the animals that pioneered the transformation of water to air-breathing and the transition from water to land. Bimodal-breating fish rely to varying extents on air and water for their oxygen needs. Factors such as degree of development of the accessory respiratory organs and availability of oxygen in water cause the fish to switch from breathing water to air. The more terrestrial species are obligate air-breathers while the more aquatic ones are facultative air-breathers. Among the actinopterygian fishes, only the polypterids have developed definite lungs. Obligate air-breathers, both Lepidosiren and Protopterus have a better carbon dioxide buffering capacity of blood than the Western Australian lungfish, Neoceratodus forsteri, itself a facultative airbreather.