Lungs in fish
Lungfish and bichirs have true lungs; African lungfish can survive dry seasons buried in mud.
Also: air breathing, aestivation
Lungs and swim bladders are related. In lungfish and bichirs the sac works as a true lung, and some of them must breathe air or drown. The African lungfishes, such as the marbled lungfish, survive dry seasons by burrowing into mud and sealing themselves in a cocoon of mucus, slowing their body down for months, a state called aestivation. The Australian lungfish has a single lung and gulps air when the water is poor. These fish are among the closest living relatives of the land vertebrates.