50 



A HISTORY OF FISHES 



closes up or disappears altogether. It nearly always opens into 

 the pharynx by a small aperture in the roof, but in the Lung- 

 fishes {Dipneusti) and Bichirs (Polypterus) y in which the air- 

 bladder is a true breathing organ, the opening takes the form 

 of a small sHt, the glottis, with well-defined lips, situated in the 



STURGEON 



AND MANY OTHER 



bony-fishcs. 



GAR -PIKE 

 ANQ BOW-FIN. 



CHARACIN. 



AUSTRALIAN 



LUNG -FISH. 



BICHIR. 



AFRICAN if S.AM£/?lC^M 



LUNO-FISH. 



REPTILES, 



BIRDS, 



MAMMALS. 



Fig. 22. AIR-BLADDER AND LUNG. 



A series of diagrams showing the relation of the air-bladder or lung to the gullet 

 in different fishes, as seen in cross-section (left-hand column) and from the side. 



(After Dean.) 



floor of the gullet (Fig. 22). The lungs of higher vertebrates 

 arise in exactly the same way, as an outgrowth from the gullet, 

 and the glottis occupies the same position as in the Lung-fishes. 

 Furthermore, whereas in the generaHty of fishes the air-bladder 

 is a simple sac filled with a mixture of gases, in the Bow-fin 

 {Amia)^SLnd Gar Pike (Lepidosteus) , two other air-breathing 



