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1. The efficiency of gas exchange over the lung and gill surfaces of Protopterus has been investigated. 2. Animals confined in water or in air showed an increased respiratory frequency in the remaining medium, indicating that both routes were important in the total gas exchange. 3. Direct measurement of the oxygen and carbon dioxide tensions of pulmonary air and inspired and expired branchial water showed gas exchange ratios (R) of 0.2 for the lung and 5.0 for the gills approximately, demonstrating that more oxygen was consumed via the lungs and more carbon dioxide excreted via the gills. 4. Oxygen consumption and carbon dioxide production were measured directly in a respirometer in which respiratory air and water streams could be kept separate except during lung ventilation. At least 90% of the animals' oxygen consumption occurred in the lung, while 60 % of the carbon dioxide excreted passed via the aquatic route. 5. The results are discussed with reference to the animals' adaptation to its environment and with reference to the evolution of the terrestrial vertebrates.

The Relative Efficiency of Gaseous Exchange Across the Lungs and Gills of an African Lungfish Protopterus Aethiopicus