Air Breathing Organs
Very few air-breathing fish use their gills to extract air from the atmosphere, and indeed having a large gill area can impede survival on land, or in heavily de-oxygenated water. This is because as well as diffusing in through the gills, oxygen may also diffuse out of them. In fish that inhabit very poorly oxygenated water, this would be a distinct disadvantage, as much of the oxygen in their blood may be lost. Depending on the extent to which they rely on air-breathing, many such fish have lost much of the gill area, to reduce this problem. In such species, the excretion of ammonia may also be affected, with a greater proportion of it being converted to urea. This is particularly well developed in those fish that spend time out of the water, where ammonia excretion would be impossible. To prevent levels building up and becoming toxic, they convert it to relatively harmless urea which can be stored for excretion later on. Air breathing organs (ABOs) are varied in their structure and position. Wherever they are, they usually consist of a highly vascularised 'respiratory epithelium', which is specially developed for the absorption of oxygen from air.
The most primitive forms of ABO include the lungs of Lungfish and Bichirs, and modified gas bladders, as seen in Garfish and Butterfly Fish. Other species have modified areas of the stomach, intestine, buccal and pharyngeal cavities, or even specific organs such as the 'labyrinth' in Gouramis, and the 'arborescent' organ in Walking Catfish.
Air-breathing in tropical freshwater fishes
http://www.fbas.co.uk/ABOS.html