LUNG FISH

The Australian lungfish (Neoceratodus forsteri) is considered the most primitive extant species of lungfish, with the African and South American species more derived and more closely related.
As their name suggests, members of this group possess 1 lung (Ceratodontiformes) or 2 lungs (Lepidosireniformes) which they use to breathe atmospheric oxygen.  When young, lungfishes possess external gills, which are lost (in most species) as the fish develops.  Members of Lepidosireniformes are capable of estivation, a state of dormancy and low metabolism during periods of desiccation.

 As lungfish develop from juveniles to adults, their teeth fuse together to form tooth plates which they use to chew their food (all lungfishes are omnivorous).
 All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition. Basal ("primitive") lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish groups, including all modern species, show a significant reduction in the marginal bones and a cartilaginous braincase. The bones of the skull roof in primitive lungfish are covered in a mineralized tissue called cosmine, but in post-Devonian lungfishes, the skull roof lies beneath the skin and the cosmine covering is lost.

 All modern lungfish show significant reductions and fusions of the bones of the skull roof, and the specific bones of the skull roof show no homology to the skull roof bones of ray-finned fishes or tetrapods. During the breeding season, the South American lungfish develops a pair of feathery appendages that are actually highly modified pelvic fins. These fins are thought to improve gas exchange around the fish's eggs in its nest.

lungfish have a highly specialized respiratory system. They have a distinct feature that their lungs are connected to the larynx and pharynx without a trachea. While other species of fish can breathe air using modified, vascularized gas bladders,^[5] these bladders are usually simple sacs, devoid of complex internal structure. In contrast, the lungs of lungfish are subdivided into numerous smaller air sacs, maximizing the surface area available for gas exchange. Most extant lungfish species have two lungs, with the exception of the Australian lungfish, which only has one. The lungs of lungfish are homologous to the lungs of tetrapods. As in tetrapods and bichirs, the lungs extend from the ventral surface of the esophagus and gut.

Lungfish are omnivorous, feeding on fish, insects, crustaceans, worms, mollusks, amphibians and plant matter. They have an intestinal spiral valve rather than a true stomach.^[8]
African and South American lungfish are capable of surviving seasonal drying out of their habitats by burrowing into mud and estivating throughout the dry season. Changes inallow it to slow its metabolism to as little as 1/60th of the normal metabolic rate, and protein waste is converted from ammonia to less-toxic urea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water). 
Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. It has been proposed^{[by whom?]} both that burrowing is plesiomorphic for lungfish, and that gnathorhizids are directly ancestral to modern Lepidosireniformes, but the similarity possibly is simply due to convergent or parallel evolution.
Lungfish can be extremely long-lived. A Queensland lungfish at the Shedd Aquarium in Chicago was part of the permanent live collection from 1933 to 2017, when it was euthanized following a decline in health consistent with old age.