A note to ophie's previous writeup. Current theory is that unlike sharks, which first appeared in the oceans, these fishes (commonly called bony fishes) first developed in fresh water and then moved to salt water. The swim bladder, common to almost all bony fishes, originated as a lung to aid in respiration in swamps and other oxygen-poor environments. That capability has been lost in most fishes and now the swim bladder is mainly used to control buoyancy.
Class Osteichthyes is a problem to taxonomists because it's too diverse to conveniently define. Due to its age and diversity (over 21,000 species have been recognized), there are exceptions to every "defining" characteristic. (Members of Osteichthyes do have gill covers, also called opercula, but I don't consider this a defining characteristic as they also appear in all non-bony fishes.)
Most of the web sites that I checked list the characteristics of Osteichthyes that ophie listed. If your zoology instructor does this and you want to cause trouble, here are some exceptions: