swim bladder

The needless lung becomes a closed sac filled with air, and commonly known as the swim bladder.

Sometimes, as the need of the theory demands, the swim-bladder is claimed as the relic of a discarded lung.

Thus the swim-bladder of the fish became, it is supposed, the lung of the terrestrial animal.

But in reality it has no gills and no swim-bladder; it lays no eggs; it does not own one truly fish-like organ.

It corresponds with the swim bladder of most modern fishes, and appears to have had a common origin with it.