422 ANIMAL RESPIRATION 



Indeed it must not be rashly assumed that the swim-bladder of 

 one fish is of necessity precisely equivalent to the swim-bladder 

 of any other form. The best evidence to be obtained in support 

 of the view indicated as to the origin of lungs is presented by 

 the remarkable Lung- Fishes (Dipnoi], of which more will sub- 

 sequently be said. These creatures possess not only efficient 

 gills, but also a swim-bladder into which ordinary air is taken 

 for breathing purposes, the purified blood being returned direct 

 to the heart, just as in the case of an ordinary lung. 



Accepting the mode of origin of lungs just outlined, we may 

 next enquire how these organs become more and more complex 

 as we follow them up from Lung- Fishes to Amphibia, and thence 

 to the thorough-going land-groups of Reptiles, Birds, and Fishes. 



LUNGS OF AMPHIBIANS (AMPHIBIA) 



Taking the Common Frog (Rana temporaries) as a good type, 

 we find that its lungs are a pair of simple elastic bags, which can 

 be distended to a considerable size by the taking in of air. The 

 surface presented by their lining is to some extent increased by 

 the presence of a number of ridges, which give a honey-comb-like 

 appearance. The ridges are traversed by delicate capillary blood- 

 vessels, the blood contained in which is only separated from the 

 air in the lungs by the thin lining of those organs and the equally 

 delicate walls of the vessels. Hence diffusion of oxygen into the 

 blood, and of carbonic acid gas out of it, easily takes place. 



We have seen that in the case of gills the maintenance of a 

 stream of pure water over the breathing surface is provided for 

 in a number of ways. Renewal of air in a lung is a matter of 

 equal importance, and the manner in which it is brought about 

 varies in different groups. As regards the Frog, it is of im- 

 portance to notice that in this animal the feeding and breathing 

 tracts are to some extent separated, though the arrangement is 

 not so perfect as in the cases of the Lancelet, Tunicates, and the 

 Acorn-headed Worm (see p. 389), where breathing-water and 

 feeding- water are largely kept apart. It is a general rule among 

 backboned animals where lungs are present, for the air used in 

 breathing to pass through the cavities of the organs of smell, and 

 we see the beginning of this in Lung- Fishes, where these organs 

 not only possess external nostrils (the only nasal openings present 



