. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Respiration and Metabolism 213 importance of skin and lungs in oxygen and carbon dioxide exchange (Fig. 37). Ox\gen uptake by the lungs and carbon elimination by both lungs and skin reach a peak in April and a minimum in December. Oxygen uptake through the skin remains almost constant throughout the vear. Ihus the cutaneously derived ox\'gen, entering by diflusion, can supply two thirds of the oxygen requirement during the winter months under laboratory condi- tions, but only a small fraction of the spr
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Respiration and Metabolism 213 importance of skin and lungs in oxygen and carbon dioxide exchange (Fig. 37). Ox\gen uptake by the lungs and carbon elimination by both lungs and skin reach a peak in April and a minimum in December. Oxygen uptake through the skin remains almost constant throughout the vear. Ihus the cutaneously derived ox\'gen, entering by diflusion, can supply two thirds of the oxygen requirement during the winter months under laboratory condi- tions, but only a small fraction of the spring oxygen demand. Presumably during hibernation all of the oxygen consumed enters through the cutaneous route. Gills. Gills are respiratory appendages, generally well vascularized, and usually ciliated and motile, or located in the current of water flow. They are usually aquatic but may be aerial, and they are sometimes both. The respira-. Fig. 37. Oxygen and carbon dioxide exchange through the skin and lungs of the frog Rana temporaria, throughout the year. After Dolk and ;" tory functions in some cases have been combined or confused with other processes, such as salt absorption in the so-called "anal gills" of Citlex and Chirononnis (see Ch. 2).-"^ Moxement of water o\'er the surface of aquatic gills is mandatory to insure efficient respiratory exchange. The countercurrent principle of operation in fish, the water outside the gill surface and the blood of the adjacent capillaries inside Rowing in opposite directions, provides for rapid oxygen uptake and almost complete saturation as the blood leaves the gill hlaments.^" Dermal branchiae, the so-called papulae, are found in many of the echino- derms supplementing respiratory exchange through the tube feet. 1 he papulae are evaginations of the body wall ("extensions of the coelom"), bringing the gently moving coelomic fluid in close association with the cilia-stirred external sea
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