Histological and corrosion-casting techniques have been used to provide a comparison of lung structure between a terrestrial load (Bufo marinus) and an aquatic one (Xenopus laevis). The pulmonary microvascular anatomy is distinctly different in these two species and both differ from classical descriptions of other anuran lungs. 2. In X. laevis large peribronchial vessels form a number of direct connections between the pulmonary artery and vein at the base of the lung. As a result of their location these vessels could easily function as respiratory bypasses or shunts. No similar connections were found in B. marinus lungs, nor have they been previously described for any other anuran amphibians. Possible respiratory implications of their presence in X. laevis are discussed. 3. The patterns of major vessel distribution are markedly different in the two species. Despite these differences the appearance of the respiratory capillary beds and their patterns of supply and drainage are surprisingly similar, suggesting that common design problems have been encountered in the evolution of both lung types. 4. The lungs of X. laevis have considerably less septal and lung wall smooth muscle than is the case in B. marinus. However, Xenopus lungs are generally thicker-walled and of more solid construction. The differences are discussed in terms of the widely different habitats of the two species.
