Traditionally, the Osteichthyes is considered a class, recognised on having a swim bladder, only three pairs of gill arches, hidden behind a bony operculum and a predominately bony skeleton.[9] Under this classification systems, the Osteichthyes are paraphyletic with regard to land vertebrates as the common ancestor of all Osteichthyes includes tetrapods amongst its descendants. The largest subclass, the Actinopterygii (ray-finned fish) are monophyletic, but the inclusion of the smaller sub-class Sarcopterygii, Osteichthyes is paraphyletic.