Swim bladder
Swim bladders are evolutionarily closely related (i.e., homologous) to lungs. Traditional wisdom has it that the first lungs (simple sacs connected to the gut) allowed the fish to gulp air in oxygen-poor conditions. They evolved into the lungs of today's terrestrial vertebrates and some fish (lungfish, gar, bichir) and also into the swim bladders of the ray-finned fishes.