KESPIKATION 43 



with a moist membrane. ^Yhen a gas comes in contact 

 with this membrane the freely moving particles of the 

 gas pass through the spaces between the particles of the 

 membrane. 



In one-celled animals (1) The membrane covers the 

 entire body surface. (2) Oxygen is dissolved in the 

 water which continually bathes the surface. (3) This 

 oxygen is carried to all parts of the body by the con- 

 stant circulation of the protoplasm and enters into vari- 

 ous chemical actions that finally result in the formation 

 of carbon dioxide. (4) The carbon dioxide passes off 

 into the water. This simple process has become com- 

 plicated in higher organisms by certain peculiarities of 

 development. 



In many-celled animals the membrane is located in a 

 definite place and the oxygen passes first into the blood. 

 The blood carries it to all parts of the body and brings 

 back carbon dioxide that it receives along its way. 



The passage of oxygen into the blood and the passage 

 of carbon dioxide into the water are independent of each 

 other. In neither case can an equilibrium be established 

 because the incoming oxygen is continually passing from 

 the blood into the cells where it is used, and the outgoing 

 carbon dioxide is continually passing from the cells 

 where it is formed into the blood, so that in the blood the 

 pressure of oxygen is kept continually less and the pres- 

 sure of carbon-dioxide is kept continually greater than 

 in the water outside. 



In higher animals the membrane is far removed from 

 the surface of the body for protection. A complicated 

 mechanism is therefore necessary to bring air in contact 



