Relinquishment of the skin as a respiratory organ was a vital trade-off by amphibious vertebrates in their quest for colonization of the terra firma. The skin, a highly malleable respiratory structure, falls in the latter design category. To varying extents, the skin is used as a respiratory organ by many animals. Many air-breathing fish have few poorly developed scales and well-vascularized skin across which substantial gas exchange can occur. Aquatic nonpiscine vertebrates utilize the skin as a respiratory organ. During normoxic diving, the soft-shelled turtle Trionyx spiniferus asperus and musk turtle Sternotherus odoratus can remain submerged under water for about 100 days while maintaining normal acid-base status. In the aquatic snake Acrochordus javanicus, 33% of carbon dioxide elimination and only 8% of the total oxygen uptake occurs through the skin. Movements that stir the external respiratory medium and subcutaneous perfusion act as the only aids to the diffusive conduction of the skin for oxygen.