RESPIRATION 29 



Free Oxygen. — In breathing, animals use free oxygen 

 as it exists in air; they do not take it from water or 

 from any other compound containing oxygen. Animals 

 that are habitually surrounded by water get their oxygen 

 from air that has entered the water by the process of 

 diffusion. The water brings the air in contact with 

 the membrane, and oxygen enters and carbon dioxide 

 passes off in the manner described above. 



Equilibrium of Nitrogen. — Oxygen and carbon di- 

 oxide are the only gases in air that are concerned in 

 respiration. Any gas that is in the air, however, may 

 enter the body. Air is composed principally of oxygen 

 and nitrogen. Nitrogen as well as oxygen passes 

 through the membrane, but, as living organisms do not 

 as a rule use free nitrogen, it establishes an equilibrium 

 and does not concern itself with the activity of the body 

 except under extraordinary conditions. 



Respiration in Specific Animals. — In considering the 

 respiration of any specific animal it is necessary (1) to 

 locate the moist membrane, (2) to ascertain the means 

 by which air is brought in contact with the membrane. 



Lower Animals. — In one-celled animals the process is 

 simple. It takes place at any point on the body's sur- 

 face, for the animal is merely a mass of protoplasm 

 surrounded by a membrane (Fig. 16). Oxygen is dis- 

 solved in the water in which the animal lives; as it 

 comes in contact with the surface of the animal it enters 

 because its pressure is greater in the water than in the 

 body of the animal. The pressure of oxygen is always 

 low in the animal because as soon as it gets inside it is 

 carried to all parts of the cell and is used in the manu- 



