The classification of the 28,000 living fish species varies from one to the next; however, all are part of the Animal Kingdom, the phylum Chordata and the subphylum Vertebrata. Within this subphylum, four different classes contain fish, including the Myxini, Cephalspidmomorphi, the Chondrichthyes and the Osteichthyes. These four classes are divided further into numerous families, genera, species and subspecies.
The Mxyini and Cephalspidomorphi are composed of the primitive hagfish and lampreys, respectively. These fish lack jaws and are thought to be descendants of some of the most ancient fish. The Chondrichthyes class is comprised of the sharks, rays and their allies. These fish lack bony skeletons and instead feature cartilaginous skeletons. Finally, the Osteichthyes are represented by the bony fish. Most common, familiar fish, such as trout, bass and perch, are members of the bony fishes.
While the scientific classification of the large groupings is unlikely to change and is supported by significant amounts of evidence, many aspects of fish taxonomy are destined to change in the near future. Many fish groups are very poorly understood and may only be known from a few specimens. This can make it difficult to place them in the proper place within the fish family tree.