CONSIDERED AS A DEGENERATE LUNG. 199 



air-bubbles. The American Bow-Fin or Mad Fish (Amia) has a 

 bladder of the same lung-like character, and has been seen by 

 Wilder to come to the surface, open its jaws widely, and appa- 

 rently swallow a large quantity of air. Wilder remarks that, " so 

 far as experiments go, it seems probable that with both Amia and 

 Lepidosteus, there occurs an inhalation and exhalation of air at 

 pretty regular intervals, the whole process resembling that of 

 Menobranchas and other salamanders." 



The Dipnoi have the air-bladder developed into a true lung. 

 Of these the Australian lung-fish, Ceratodus, has but a single air- 

 bladder, but this is provided with breathing pouches, which have 

 a symmetrical-lateral arrangement. It has no pulmonary artery, 

 but receives branches from the Arteria caliaca. Finally, Lepidos- 

 teus and Protopterus have completely-formed lungs, divided into 

 two lateral chambers^ and provided with a pulmonary artery. 

 Their cellular structure nearly approaches that of the batrachian 

 lung. 



Professor Giinther says that some " fishes breathing air are, 

 even when brought into pure water, obliged to rise to the surface 

 at frequent intervals to take in a quantity of air, and, if kept 

 beneath the surface by means of a gauze net, they perish by suffo- 

 cation." In carrying out researches as to the original condition 

 of any organ, we find it always necessary to begin with low forms. 

 For instance, in studying the history of mammalian dentition, we 

 do not take the cats, whether recent or extinct, as guides ; we 

 study the dentition of the early Eocene mammals, and even peer 

 back into deeper recesses of time to examine the teeth of Jurassic 

 mammals. And the same rule applies to researches as to the 

 development of the brains and limbs of animals. We go as far 

 back and as low down as possible. Therefore, we are only follow- 

 ing sound, scientific precedents, if, when wishing to understand 

 the original function of the air-bladder, we study this organ, not 

 amongst the highly-developed Teleosteans, but amongst the sur- 

 vivals of archaic forms. If in fishes whose prototypes are found 

 in the Devonian age, the air-bladder functions as a lung, we have 

 good reasons for assuming that this was the original function of 

 the organ. To study the air-bladder as it exists in Teleosteans 

 would be Uke studying the development of the fingers in horses 



