This is the largest class of vertebrates. There are over 29,000 species of bony fish found in freshwater and marine environments around the world. Like all fishes, Osteichthyes are cold-blooded vertebrates that breathe through gills and use fins for swimming. Bony fishes share several distinguishing features: a skeleton of bone, scales, paired fins, one pair of gill openings, jaws, and paired nostrils. Most bony fish are ray-finned fish, meaning that they have thin, flexible skeletal rays. 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.