Bony fish have some distinct improvements over the cartilaginous fis h that proceeded them. Not the least of which is the strengthening of the axial and appendicular skeleton by the deposition of calcium which converts cartilage into bone. Although there are a variety of features that attribute to the success of bony fish it is the combination of these factors that has resulted in a highly active, agile organism adapted to the constraints of an aquatic existence. The bony fish make full use of the advantageous characteristics of their environment but have also solved the problems that the aquatic environment creates for organisms that live in it. In doing so they have also set the stage for the vertebrate groups that were to follow.
Animals living in an aquatic environment live in a three dimensional world.
For them food, oxygen, mates and all of the requirements of life can not only be found to the left, right in front or behind them but above and below as well. All organisms in an aquatic environment have to work, swim, to prevent themselves from sinking to the bottom. That is unless, of course, you are a bottom feeder and that's where you want to be. To take advantage of this additional dimension there is an obvious advantage to finding a away to not sink, and the bony fish have accomplished this using the swim bladder . The swim bladder is either connected to the oral opening in which case air can enter it directly, or it has no connection to the outside.