BACKBONED ANIMALS WHICH BREATHE IN AIR 421 



the hen has brooded for three days, it will be quite easy to make 

 out on the side of the neck-region several slit-like openings (vis- 

 ceral clefts) that communicate with the pharynx, and between 

 which are thickened bars (visceral arches). These are undoubtedly 

 equivalent to the gill-clefts and gill-arches of a fish, for they are 

 situated in exactly the same place, and develop in precisely the 

 same way, although the clefts are never of use for breathing 

 purposes, and gill-folds do not grow out from them. In fact they 

 soon close up, leaving no obvious trace that they ever existed. 

 (See also p. 381.) 



THE ORIGIN OF LUNGS 



We have seen that there is considerable ground for the belief 

 that gill-pouches in fishes and the like have probably been evolved 

 from pouches on the side of the pharynx which originally per- 

 formed some other function (see p. 382). And the modification 

 of old organs into structures having a new use is such a frequent 

 occurrence that we may well enquire if the lungs of backboned 

 animals have not been made out of pre-existing structures, as 



Fig. 547. Swim-Bladder of Bichir (Polypterus], reduced and diagrammatic. AP., Aperture on floor of 

 pharynx leading into swim-bladder. 



have the gill-pouches which they supersede. Since Fishes cor- 

 respond in a broad way to the aquatic ancestors of land-verte- 

 brates, it is among them we must seek evidence as regards the 

 evolution of lungs. Many Fishes possess a swim-bladder, con- 

 taining air and developed as an outgrowth from the front part 

 of the digestive tube. Although its primary use is to help in 

 floating and balancing the body, it is known in some instances 

 to assist the breathing, and it seems a likely enough organ to 

 undergo modification into a lung. In the Bichir (Polypterus) of 

 the Nile the swim-bladder is double (fig. 547) and grows out 

 from the under side of the digestive tube, characters which make 

 it more lung-like than that of most other forms, in which it 

 is usually single and grows out of the upper side of the gut. 



