134 THE BIOLOGY OF AN ANIMAL. 



Respiratory System. Respiration or breathing is a twofold 

 operation, consisting of the taking in of free oxygen and the 

 giving off of carbon dioxide, by gaseous diffusion through the 

 surface of the body. Strictly speaking, this free oxygen must 

 be regarded as food, while carbon dioxide is to be regarded as 

 one of the excretions. Hence respiration is tributary both to 

 alimentation and excretion ; but since many animals possess 

 special mechanisms to carry on respiration, it is convenient and 

 customary to treat of it as a distinct process effected by a distinct 

 respiratory system. 



Respiration in this sense is essentially an exchange of gases 

 between the blood and the air, carried on through a delicate 

 membrane lying between them. The earthworm represents 

 the simplest conditions possible, since the exchange takes place 

 all over the body, precisely as in a plant. Its moist and delicate 

 walls are everywhere traversed by a fine network of blood-vessels 

 lying just beneath the surface. The oxygen of the air, either in 

 the atmosphere or dissolved in water, readily diffuses into the 

 blood at all points, and carbon dioxide makes its exit in the 

 reverse direction. Freed of carbon dioxide and enriched with 

 oxygen, the blood is then carried away by the circulation to the 

 inner parts, where it gives up its oxygen to the tissues and 

 becomes once more laden with carbon dioxide. 



It is interesting to study the various devices by which this function is 

 performed in different animals. In the earthworm the whole outer sur- 

 face is respiratory, and no special respiratory organs exist. In other ani- 

 mals these arise simply by the differentiation of certain regions of the gen- 

 eral surface, which then carry on the gaseous exchange for the whole or- 

 ganism. In many aquatic animals such regions bear filaments or flat 

 plates or feathery processes known as gills or bnuichicc., which are 

 bathed by the water containingfdissolved air, though in many such ani- 

 mals respiration takes place to 'some extent over the general surface as 

 well. In insects the respiratory surface is confined to narrow tubes 

 (trachea) which grow into the body from the surface and branch through 

 every part, but must nevertheless be regarded as an infolded part of the 

 outer surface. In man and other air-breathing vertebrates the respiratory 

 surface is mainly confined to the lungs, which are simply localized in fold- 

 ings of the outer surface specially adapted to effect a rapid exchange of 

 gases bet ween the blood and the air. 



Moreover, it is easy to see why special regions of the outer surface have 

 in higher animals been set aside for respiration. 



It is essential to rapid diffusion that the respiratory surf ace should be 



