Protopterus annectens annectens - West African Lungfish
Lungfish are fascinating!
They can grow up to over six feet long, and can either swim like eels, or crawl along the bottom with their thread-like fins. Since they generally live in shallow and oxygen-poor environments like swamps and marshes, the outpouching of the gut that acts as a swim bladder in other fish has actually developed into a lung.
Though they have full gills as hatchlings (and look like fishy tadpoles!), as adults, only the two anterior gill arches remain. The gill arches are too small to provide all of the oxygen that the fish needs, so the fish actually needs to gulp air from the surface to survive. During the beginning of the dry season, lungfish burrow into the bed of drying water and live in their mud nest until the rains come again.
Proceedings of the Zoological Society of London. 1856.