RESPIRATION 



31 



of oxygen, a special membrane is set apart for this 

 function. This means (1) that the oxygen comes into 

 the body at a definite point, (2) that this oxygen must be 

 transferred from this point to every cell in the body. 

 The blood is the carrier. It comes to the tissues laden 

 with oxygen. In the tissues the pressure of oxygen is 

 low and the pressure of carbon dioxide is great. Oxygen 

 therefore passes into the tissues and carbon dioxide 

 passes into the blood. When the blood comes again in 



Fig. 17.— The passage of oxygen and carbon dioxide between the blood and the 

 lymph in the tissues. 



communication with air it gives up this carbon dioxide 

 and receives a fresh supply of oxygen (Fig. 17). 



Respiratory Organs in Aquatic Animals. — In aquatic 

 forms such as lobsters, oysters, and fish the membrane 

 that has been specialized for breathing is located on the 

 surface of the gills (Fig. 18). The gills lie on each side 

 of the body so near the surface that water can continually 

 bathe them. They consist usually of a feathery out- 

 growth, or of two double flaps, covered with a very thin 

 membrane. 



