AMPHIBIOUS VERTEBRATES 



453 



Among recent Ganoid Fishes the Gar-Pike {Lepidosteus) and 

 Bowfin [Amia), which live in the fresh waters of North America, 

 both possess a swim-bladder which is better adapted to act as 

 a lung than is the case in forms so far considered, since its lining 

 is raised into complex folds which give a large surface through 

 which exchange of gases can go on. The Bowfin has been 

 observed to come from time to time to the surface for the purpose 

 of swallowing air. The swim-bladders so far described are un- 

 paired structures which grow out from the side or upper surface 

 of the gullet, facts which have been cited as opposed to the 

 view that lungs have evolved from organs of the sort, for lungs 

 are paired and grow from the under side of the gullet. The 

 first point is of little importance, for the lungs are represented 

 by an unpaired outgrowth when they first appear, this later on 

 dividing into two branches. It may be noted here that both 

 in the Gar- Pike and Bowfin the air-bladder is narrow in the 

 middle and broad at the sides, giving a sort of incipient double- 

 ness. Neither objection applies when we come to examine the 

 air-bladders of the archaic African ganoids, the Bichir {PolypteriLs) 

 and Reed-Fish {Calamoichthys), for in both of them the swim- 

 bladder is double, and grows out from the under side of the 

 gullet. The lining of this organ being smooth in these fishes, 

 the breathing work done by it is probably less in amount than 

 in the two ganoids first mentioned. It is worth while noting 

 that Bichir and Reed-Fish belong to a distinct and much more 

 ancient group than other living forms to which the term ganoid 

 is commonly applied. Fishes of similar kind were exceedingly 

 abundant in some of the older geological periods. It is hence 

 extremely probable that in the first instance the swim-bladder of 

 fish-like forms grew out from the under side of the gullet, but 

 in most cases its point of origin has gradually shifted round to 

 the upper side as a matter of convenience, since this organ is 

 usually placed immediately below the backbone (and above the 

 digestive tube), a position most suitable for the proper performance 

 of its duties in regard to the maintenance of equilibrium. 



Lung- Fishes (Dipnoi). — The contention that lungs have 

 evolved from swim-bladders receives its strongest support from 

 the structure and habits of the remarkable Lung-Fishes, of which 

 the three existing forms, native to the rivers of Australia, Africa, 

 and South America, are the last survivors of an exceedingly an- 



