Origins:

The lungs of today's terrestrial vertebrates and the gas bladders of today's fish have evolved from simple sacs (outpocketings) of the esophagus that allowed the organism to gulp air under oxygen-poor conditions. Thus the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the esophagus and in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the higher orders. (This is an instance of correlation between ontogeny and phylogeny.) There are currently no known animals which have both a gas bladder and lungs.