S6 THE ANIMALS AND MAN 



of those cells. It is, therefore, just as necessary for the 

 carbon dioxide to be removed from the body as for 

 oxygen to be supplied to it. Carbon dioxide, like oxygen, 

 is soluble in water. Blood, which is composed largely of 

 water, can carry off carbon dioxide as well as bring oxygen. 

 Also, since carbon dioxide is made by a combination with 

 oxygen it arises just where it can be carried away by the 

 very means that brings the necessary oxygen. Thus the 

 respiratory, aided by the circulatory apparatus manages 

 both the bringing of a supply of oxygen and the disposal of 

 the carbon dioxide. 



The fundamental fact in the process of respiration is that 

 gases whether free or dissolved in water will readily pass 

 through a thin, moist membrane. Thus, if a closed sac 

 made of thin membrane filled with water in which carbon 

 dioxide is dissolved be immersed in water in which oxygen 

 is dissolved, carbon dioxide will pass out of the sac and 

 oxygen into it until there is the same amount of each outside 

 and inside. If the water outside is constantly replaced all 

 the carbon dioxide will be finally removed. If the oxygen 

 inside the sac is constantly used up and the supply outside 

 is always renewed, oxygen will be constantly going in and 

 carbon dioxide going out. This is just what happens in 

 the living animal. Animals get their oxygen from the air, 

 of which it is a part. The air may be free or dissolved in 

 water. Carbon dioxide is made in the cells of the body. 

 Respiration takes place through the membranes covering 

 all or part of the surface of the body. It requires the con- 

 stant renewal of free air or water containing air on the out- 

 side, and the constant passage of fresh blood on the inside 

 surface of the membrane. This end is attained in a great 

 variety of ways among animals. 



In the simplest forms, the Protozoa, where we have the 

 most primitive means of motion, we find also the simplest 

 means of respiration. The Amoeba, as we have seen, 



