Circulation and respiration in lungfishes (dipnoi)

This paper reviews the cardiorespiratory morphology and physiology of the living lungfishes, in the special context of their highly effective use of both air and water for gas exchange. Particular emphasis is placed on describing those features of the circulatory and respiratory systems that distinguish Neoceratodus from the Lepidosirenidae (Protopterus, Lepidosiren), and which, in turn, distinguish lungfishes from other aquatic vertebrates. Morphological and physiological characters that represent the plesiomorphic condition for the living Dipnoi are indicated (e.g., separate atrial chambers, vertical septum in ventricle, pulmonary veins, conal valves, twisting of bulbus cordis), as are those characters that may be shared derived features of the Lepidosirenidae (e.g., paired lungs, reduced anterior gill arches, well-developed spiral valve in conus). Morphological and physiological comparisons and contrasts with tetrapods are made to elucidate systematic relationships of the Dipnoi with other vertebrates.