Not all fish species rely entirely on their gills. Some, especially when they are young, absorb a large proportion of the oxygen they need through their skin. In a few other species, the air bladder, which most modern fish use as a ballast organ to control their depth, is specialized as an accessory breathing organ or lung. These fish are obligate air breathers and will drown if they cannot breathe air, even in well-oxygenated water. The air bladder, not gills, is the structure that gave rise to the evolution of the lung and follows the same developmental pattern as the lungs of land vertebrates.