Class Osteichthyes, the largest class of vertebrates with over 29,000 species, consists of the bony fish. These fish have a skeleton much stiffer than the cartilaginous fish because it is reinforced by calcium salts. Bony fish have excellent smell like cartilaginous fish, but unlike the other class of fish, bony fish also have acute eyesight.

Bony fish also have special adaptations that allow them to remain buoyant. A special organ called a swim bladder housed under the bony skeleton is a gas filled chamber that allows the bony fish to remain floating in the water. Some fish have a connection between this organ and the digestive tract to allow the extraction of oxygen. Another special adaptation is the operculum, a flap on each side of the fish that covers the chambers housing the gills. A bony fish is able to breathe without swimming simply by moving the operculum. Other hallmarks of these fish are paired fins, many teeth, dermal scales in the skin (in most species), and numerous vertebrae. Most bony fish are ray-finned fish, meaning that they have thin, flexible skeletal rays. Lobe-finned fish, the other type of bony fish, have muscular fins supported by bones. Only one species of lobe-finned fish, the coelacanth, still lives. However, it was the lobe-finned fish that made possible the colonization of land.