What it is
A fish that has to breathe air. The African lungfish has gills, but they're too weak to keep it alive, so it swims up and gulps. Cut it off from the air, and it suffocates.
Its trick
When its pool dries up, it digs into the mud and curls up. Its skin pours out slime, which dries into a thin, papery bag with one opening at the mouth. Its heart beats half as fast, and it burns its own muscle for fuel. Scientists kept one sealed in mud for 410 days. When the rain comes back, it swims out.
How old the family is
Lungfish have been around for more than 400 million years, since before the first trees. Their burrows have been found turned to stone, with the fish still inside. They outlived the dinosaurs.