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In light of the relationship of lungfish to the origin of tetrapods, information on the respiratory biology of lungfish can give insight into the functional morphological and physiological prerequisites for the conquest of land by the first tetrapods. Stereological methods were employed in order to determine the respiratory surface area and thickness of the water-blood barrier or air-blood of the gills, lungs, and skin, respectively, of the South American lungfish Lepidosiren paradoxa. The morphometric diffusing capacity was then determined by multiplying by the appropriate Krogh diffusion constants (K). Our results indicate a total diffusing capacity of all respiratory organs of 0.11 mL min(-1) mmHg(-1) kg(-1), which is more than twice the value of the physiological diffusion capacity (approximately 0.04 mL min(-1) mmHg(-1) kg(-1)). Of this, 99.15% lies in the lungs, 0.85% in the skin, and only 0.0013% in the gills. Since K for CO(2) is 20-25 times greater than for O(2), diffusing capacity of CO(2) through the skin is potentially important. That of the gills, however, is negligible, raising the question as to their function. Our results indicate that the morphological prerequisites for terrestrial survival with regard to supporting aerobic metabolism already existed in the lungfish.

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The oxygen uptake ( i'Ol). breathing frequency (.f,), breath volume (V,,), gill ventilation ( bG) and oxygen extraction ( O h ) from the ventilatory current of four groups of 0rrochronii.s tiilvricirs during graded hypoxia were measured under the following acclimation temperatures: 20. 25. 30 and 35 C. The critical oxygen tensions (P,Oz). determined from VO, v. PO, of inspired water at each experimental temperature were. respectively. 19. I f 3.1. 18.0k4.9. 29.7 1 4 . I and 30.2 t 0.6 m m H g The.lk remained nearly constant during the reductions of 0, at all the temperatures studied. b' G increased discretely from normoxic levels until the P,O, was reached. below which it assumed extremely high values (17-fold higher or more). The increases observed in C' G resulted. at all the acclimation temperatures, in an elevation in V, rather than in,f,. The extraction of O1 dccreased gradually from normoxia until the P,O1 was reached. below which an abrupt reduction ofewtraction was recorded. except at 35 C when fish showcd a gradual reduction in extraction just below the tension of 80 mmHg.

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