<P> Pulmonates are mostly land snails and slugs that have developed a simple lung from the mantle cavity . An externally located opening called the pneumostome allows air to be taken into the mantle cavity lung . </P> <P> The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs, as outpocketings of the esophagus, that allowed early fish to gulp air under oxygen - poor conditions . These outpocketings first arose in the bony fish . In most of the ray - finned fish the sacs evolved into closed off gas bladders, while a number of carp, trout, herring, catfish, and eels have retained the physostome condition with the sack being open to the esophagus . In more basal bony fish, such as the gar, bichir, bowfin and the lobe - finned fish, the bladders have evolved to primarily function as lungs . The lobe - finned fish gave rise to the land - based tetrapods . Thus, the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). </P>

How many tertiary bronchi are typically present in each lung