Lungfish (Dipnoi) are a group of air-breathing, freshwater fishes that constitute the subclass Dipnoi. They are members of the ancient lineage of Sarcopterygii, which includes tetrapods. The best known lungfish is the African lungfish (Protopterus annectens).
Lungfishes are found in Africa, Australia, South America, and Eurasia. Most species grow to about 1 m (3 ft) in length, but some can reach 2 m (6 ft 7 in). African and South American species generally weigh up to about 15 kg (33 lb), while Australian species can exceed 20 kg (44 lb). All except one species have an elongated body shape with a thickened tail base.
The skin of lungfishes is usually covered with scales, but may also be naked. The mouth is large and terminal, with well-developed jaws bearing sharp teeth. The gills are internal and branchial filaments that project into each airsack. Each airsack has a rich blood supply and is lined with epithelium that helps to extract oxygen from the air breathed into it.
Lungfishes have a two-chambered heart: one atrium and one ventricle. This arrangement is found in all tetrapods, although the chambers are usually much better developed in tetrapods than they are in lungfishes. In addition to their lungs, which they use for breathing air when conditions become too dry for their gills to function properly, most lungfish also have another organ called a swimbladder that helps them to maintain buoyancy underwater.
Swimbladders vary considerably among different fish groups: they may be absent altogether, as in some eels; they may be simple sacs filled with gas; or they may be highly complex organs involved in sound production or even capable of functioning as lungs themselves.
The swimbladder of a typical lungfish is an elongated sac located just below the backbone on either side of the digestive tract. It has several important functions: firstly, by filling with gas it provides buoyancy so that the fish does not sink; secondly, it acts as an acoustic resonator amplifying sounds produced by movements of the pectoral fins; and thirdly it provides extra surface area over which oxygen from atmospheric air can diffuse into the bloodstream via thin walls lined with capillaries . When conditions become too dry for their gills to function properly , many lungfishes estivate by burying themselves in mud at the bottom of ponds where they remain until rains come and fill these bodies of water again