All vertebrates except fish have lungs, but some molluscs (land snails) have lungs too. Considering the phylogeny of these taxinomic groups, this would mean that lungs evolved from gills two times in evolutionary history, once from aquatic snails and once from fish. But that would make lung-bearing animals a polyphyletic group. How could that be ? Or is it perhaps that the lungs of verterbrates and snails are completely different, but are just described by the same name ?

Hi Alex,

Before we can consider your question we need to ask "are lungs homologous or analagous"?

Homologies are characters that are shared due to common ancestry.

Analogues are characters that are shared due to common functionality.

As it turns out, the lungs of vertebrates and molluscs (and spiders, etc.) share a common function, but there is nothing to suggest that they are shared in any common ancestor of the various lung-bearing organisms. Their very different structure and development is a good indicator that this is the case.

Therefore, lungs are analagous structures and they can't provide any useful information about the relationships in question.

Just to carify the difference in `lungs' between the 2 groups. Vertebrate lungs likely evolved from the swim bladder (an organ used to moderate buoyancy in fish) of eary lobe-finned fishes sometime in the Late Silurian (~425 million years ago)

in lunged molluscs (pulmonates) the lung is adapted from the mantle cavity which isn't present in any vertebrate, hence as Paolo said, they are analogous features.