Respiration and Metabolism 



219 



oxygen in the swim bladder is given up to the tissues, indicating a storage func- 

 tion of the structure operating in times of stress (Fig. 41).'''' When subjected 

 to asphyxiating conditions in the absence of carbon dioxide the tautog and 

 toadHsh use virtually all of the available oxygen in the swim bladder, the scup 



li 



Fig. 40. Lungs of the dipnoan, Protoptems annectens, viewed from the ventral side. 

 The main arterial blood supplv (p.a.) is derived from the pulmonary (sixth) arch. From 

 Parker.^' 



and killifish use about two-thirds. '"'"' With increase of carbon dioxide in the 

 water, the oxygen utilized from the swim bladder is diminished. 



Diffusion Lungs. In the absence of ventilating mechanisms the lungs quite 

 naturally are supplied with air by diffusion. This system imposes limitations 

 on both the size of the animal and the rate of metabolism, but the mechanism 

 is not uncommon, particularly among the arthropods and molluscs (Scorpioni- 



