Dipnoi is an infraclass (superclass) of bony fishes. It includes ancient and modern fishes with gill and lung respiration. There are 6 species belonging to 2 orders. The only representative of Ceratodiformes, Neoceratodus forsteri, has one "lung" and paired limbs in the form of fleshy blades. Oxygen-deficient, reaches 75 cm in length. Feeds on benthic invertebrates. Occurs in freshwater areas of Australia. The Lepidosireniformes, Lepidosiren paradoxa and 4 Prothortera species have a paired "lung". They live in ponds that have dried up; when they do dry up, they burrow into silt, hibernate and breathe atmospheric air. They are about 2 m long. They feed on grazing invertebrates, fish, and amphibians. Protopters live in fresh waters of tropical Africa, lepidosirens in the central part of South America.
     It is represented by one modern order Ceratodontiformes, widespread in Africa, Australia and South America, as well as by 4 extinct orders. Previously, there was a second modern order, the bipeds or Lepidosireniformes. There are 6 species of modern bipedal lungworms: Australian hornbill, four species of African protopterans, and South American scaleworm. One or two bladders open on the ventral side of the esophagus and function as organs of pulmonary respiration. This allows bivalves to exist in oxygen-depleted bodies of water. Hornbills have one lung, the rest of the bivalves have two. Bivalves and cysteopods evolved from the same ancestor in the Devonian. Of all the fish, the bivalves are the closest relatives of the quadrupeds, as both are descended from the common taxon Choanata. In addition to bivalves, pinnipeds also use the swim bladder as a lung.
     In bipedal lungworms, order Lepidosireniformes, two lungs, connected with esophagus, have pockets and alveoli, raising internal surface. Body elongated, scales fine, buried deep in skin. The paired fins are flagellate. During drought (up to 9 months) completely switch to pulmonary respiration and hibernate. The order includes families Protopteridae and Lepidosirenidae and 4 species of African protopterans and 1 species of South American scales. Protopterans differ in areals, coloration, a number of anatomical features and sizes: Protopterus amphibius is 30 cm long, P. aethiopicus is 2 m long. Feed on invertebrates and fish. They are most active at night. When drought approaches, Protoptera burrow, gnawing out pieces of ground, shredding them with their jaws and ejecting them through their gill covers. The circular passage has a diameter of 5-70 mm and goes vertically downward. At a depth of 50 cm, the passage widens to form a "sleeping" chamber, where the protopter survives a dry period after curling up almost twice. Before hibernating, it closes its hole with a cap of clay and covers itself with a thin cocoon of hardened slime. During hibernation, the protopter loses up to 20% of its body mass; it uses its muscle tissues as a source of life sustaining energy. This energy is spent not only for survival, but also for gonad maturation. With the onset of the rainy season, the protopter prepares for spawning by digging a brood hole in shallow water that has two entrances.
     The brood chamber is located at a depth of 40 cm. It is the male that guards the clutch and takes care of the offspring. At one month of age, larvae 30-35 mm in length leave the nest. Lepidosiren paradoxa lives in central South America. Its body length is 130 cm. It differs from Protoptera in having a more elongated body, more reduced paired fins, fins that are shallower and deeper in the skin, and that it uses up fat during hibernation. Unlike protopterans, which spawn at the bottom of the brood chamber, the scales make a bed of bits of vegetation. Successfully kept in aquariums. The order Hornbill, or Ceratodontiformes, is represented by a single modern species, Neoceratodus forsteri. It lives in the slow, vegetated rivers of northeastern Australia. It is 175 cm long and weighs 10 kg. Their elongated, laterally compressed body is covered with large scales and ends with a diphycercal caudal fin. In contrast to bipinnipeds, they have one lung, more powerful paddle-like paired fins, and a non-bony, cartilaginous skull. In search of food (benthic animals and plants), it crawls on the bottom, relying on its fins. Swims quickly when necessary, bending the body. Every 40-60 minutes he rises to the surface of the water for a portion of air. Exhale and inhale are accompanied by a loud sob. During droughts, when Australian rivers are filled with liquid mud, the hornbill completely switches to lung breathing. However, complete desiccation of a body of water is dangerous to him because he does not hibernate. Breeding is from early spring to late fall. He builds no nests, lays eggs on aquatic vegetation and does not care for them. The eggs are 7 mm in diameter, contain a large quantity of egg yolk and are surrounded by a gelatinous shell. Development of the eggs takes 1.5 weeks. Newborn hornbills have no paired fins; pectoral fins appear after two weeks and ventral fins after 2.5 months.

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