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 dio.xide 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 Postma."" 



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 water. In Asterias a specialized region of the ambulacral svstem and a 

 portion of the madreporite plate have been regarded as respiratory in nature. -'*- 



Polychaetes have evolved some elaborate gill structures, such as parapodia 

 (Nereis), gills (Arcnicola), and branchial tufts ( Dasyhranchns) . Bispara voluti- 



