FOOD AND AIR TURNED INTO FLESH AND ENERGY 8i 



only respiratory surface. Such animals have only sluggish 

 and weak motions however. Much increase in size and 

 activity make certain demands on the surface of the body 

 which unfit it for respiration. The hard covering of in- 

 sects, crabs, and other animals necessary in connection 

 with locomotion and for protection from injuries illustrate 

 this. Again, while in a minute form like Amoeba, the 

 slight increase of surface attained by its protruded pro- 

 cesses answers the increased respiratory needs, the sur- 

 face 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. There- 

 fore the larger animals must have special respiratory sur- 

 faces with special respiratory apparatus to move the air 

 or water over these surfaces externally, and special cir- 

 culatory 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- 

 nally; thus gills are formed. The other is by the exten- 

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

 as the tracheje in insects, or of sacs, as the lungs in the 

 vertebrates. Water-breathers have gills and air-breathers 

 have trachea; or lungs. 



In crayfishes or crabs the gills have the form of feather- 

 like projections from one of the upper leg joints, and 

 extend up into a cavity formed by a projection of the car- 

 apace over the sides. It is interesting to note that these 

 animals have a paddle for bailing the water out of the gill 

 cavity, and that by it a constant current is kept flowing 

 over the gills. Fishes breathe by means of gills, of which 

 they have four pairs (fig. 45). These are placed on the 

 sides of the head and consist of minute projections of the 

 skin appearing as a fine red fringe. They are supported 

 by bony or cartilaginous arches. The heart lies close to 



