CHAPTER XXXVI 



.ANIMAL RESPIRATION— BACKBONED ANIMALS WHICH 



BREATHE IN AIR 



We have now seen that animals which breathe in water may 

 use for this purpose various surfaces of the body, both external 

 and internal, and that the larger and more complex forms augment 

 the area afforded by the external surface by developing outgrowths 

 known as gills. These vary greatly in shape, and are situated 

 in various places. When they are large and complicated, it is 

 usual to find special arrangements for sheltering them, and also 

 for maintaining a constant flow of water over their surface. 



The higher classes of the Backboned Animals (Vertebrates), 

 i.e. Mammals, Birds, and Reptiles, live mostly on the land, and 

 take in ordinary air for breathing purposes, which, after some of 

 the oxygen has been absorbed from it, is again passed out, heavily 

 charged with two of the waste products of the body, carbonic acid 

 gas and water. The essential nature of the process is precisely 

 the same as in those animals which breathe in water, but these 

 use the oxygen which is dissolved in the surrounding medium. 

 An animal that uses ordinary air for breathing purposes, and 

 which, for brevity's sake, we may call an " air-breathing " animal, 

 relies in most cases upon part of its internal surface, and back- 

 boned forms which do this mostly possess lungs, i.e. hollow out- 

 growths from the under side of the throat-region (pharynx) of 

 the digestive tube, by which the internal breathing surface is aug- 

 mented. These structures are therefore in a way comparable to 

 otHs, which are also a device for increasincr the area over which 

 respiration can take place. 



It is almost if not quite certain that land animals are descended 

 from aquatic forms which breathed the oxygen dissolved in the 

 surrounding water, and in the higher backboned animals the proof 

 of this is unusually clear and particularly striking. If, for example, 

 we examine an embryo chick, taken, say, from an egg upon which 



