ANIMAL PHYSIOLOGY 57 



simply relies on its whole external surface for breathing, the 

 thin outside layer of the body acting as a membrane through 

 which the oxygen passes in and the carbon dioxide out. 

 During periods of activity the processes protruding from the 

 body increase the amount of respiratory surface sufficiently 

 to provide for the increased respiration demanded by the 

 activity. In ciliated forms the cilia greatly increase the 

 surface area and respiration is further assisted by the con- 

 stant contact of the moving body with fresher water. Even 

 in more complex animals, the common earthworm and the 

 larvae of some insects, for example, the whole external skin 

 is sometimes the only respiratory surface. However, such 

 animals have only sluggish and weak motions. Much 

 increase in size and activity make certain demands on the 

 surface of the body which unfit it for respiraton. The hard 

 covering of insects, crabs, and other animals necessary in 

 connection with locomotion and for protection from inju- 

 ries illustrates this. Again, while in a minute form like 

 Amoeba, the slight increase of surface attained by its pro- 

 truded processes answers the increased respiratory needs, 

 the surface of a large animal would fall far short of doing 

 so, because, according to a familiar law of physics, the mass 

 or bulk of a body increases as the cube of the diameter while 

 the surface increases only as the square. Therefore the 

 larger animals must have special respiratory surfaces with 

 special respiratory apparatus to move the air or water over 

 these surfaces externally, and special circulatory apparatus 

 to move the blood over them internally. 



Special respiratory surface is provided for in two ways. 

 One is by the extension of a portion of the surface exter- 

 ally; thus gills are formed. The other is by the extension 

 of the surface within the body in the form of tubes, as the 

 tracheae in insects, or of sacs, as the lungs in the vertebrates. 

 Water-breathers have gills and air-breathers have tracheae or 

 lungs. 



