The ray-finned fishes Actinopterygii are the largest assemblage of all vertebrate groups, with over 29,000 species known. Together with the Sarcopterygii, the lobe-finned fishes and tetrapods, they are the bony fishes or osteichthyes. Ray-finned fishes are so named due to the membranes of skin (fins) on the body, which are supported by stiff spines (rays). In some species, the spines are capable of injecting toxic venom, which is used as a valuable form of defence against any potential predators that try to eat them (or humans that try and handle them). Lobe-finned fishes have the unique ability among fishes, there are able to breathe air using a single lung during extended dry periods or if water quality is low.