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 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 earliest bony fishes were fleshy-finned. 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.