The primitive fishes of the class Agnatha lack jaws and the paired pelvic and pectoral fins characteristic of more advanced fishes. This largely extinct class includes two living groups, the bloodsucking lampreys and the scavenging hagfishes. Fishes of the extinct class Placodermi were the first vertebrates to develop jaws and paired fins. These fish had bony skeletons and were covered with bony armor. A branch of this group probably gave rise to the two main modern classes of fish, the cartilaginous fish and the bony fish.
The cartilaginous fish (sharks, rays, and chimaeras) are distinguished from the bony fish by their cartilage skeletons, by the absence of either a swim bladder or lungs, by the construction of their tail fins, and by the absence in most of a gill covering, or operculum. The skin of members of this group is covered with imbedded toothlike structures called denticles, giving it a rough, sandpapery quality. Sharks are almost exclusively marine in distribution.
The bony fishes are distinguished from other living fishes by their bone skeletons and by the presence of either a swim bladder (which functions as a float) or, in a few fishes, lungs. The bony fishes are divided into two subclasses, the fleshy-finned fish and the ray-finned fish. The latter group includes over 95% of all living fish species.
The earliest bony fishes were fleshy-finned. They evolved during a period of widespread drought and stagnation and gave rise to the amphibians (the first terrestrial vertebrates) on the one hand and to the ray-finned fish on the other. The only surviving fleshy-finned fishes are the lungfishes and one species of coelacanth (see lobefin). These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs.
Ray-finned fishes, now predominant in both fresh and marine waters, represent an advanced adaptation of the bony fishes to strictly aquatic conditions; they are the most highly successful and diverse of the fishes. In nearly all of these fishes the lung has evolved into a hydrostatic organ, the swim bladder. The fins in this group consist of a web of skin supported by horny rays. Each ray is moved by a set of muscles, giving the fin great flexibility. Most ray-finned fish have overlapping scales made of very thin layers of bone. Their skeletal structure is light but strong and most have excellent vision.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.