130 PROCEEDINGS OF THE ACADEMY OF [1885. 



as the gills shrink and the lungs increase, come more frequently 

 to the surface for air." 1 



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

 Of these the Australian lung fish (Ceratodus), has but a single 

 air-bladder, bul (his is provided with breathing pouches that pos- 

 sess a symmetrical lateral arrangement. It has no pulmonary 

 artery, l>ul receives branches from the Arteria cceliaca. It is 

 supposed that this fish ordinarily breathes with the gills, but uses 

 its lungs when the water has become thick and muddy, or is 

 charged with gases from decomposing organic matter. Finally 

 Lepidosiren 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. 



The facts here cited certainly seem to lead to the conclusion 

 that the ait-bladder Avas originally developed as an air-breathing 

 Organ, and only became adapted to other purposes when it had 

 become no longer of value in this direction. We may find evi- 

 dence in favor of this conclusion in the condition of the fishes 

 which still use it as a breathing organ. With them the gill is the 

 ordinary breathing apparatus. The lung is not called into use 

 except when the water becomes foul or unaerated. It is a sup- 

 plementary organ, which could be easily dispensed with if the 

 fish should gain the habit of swimming in search of better aerated 

 water. It is impossible to imagine that the air-bladder developed 

 into a lung under the force of such a minor necessity as this. It 

 is very much more probable that it was once an important breath- 

 ing organ with these fishes, and has retained its functional value 

 from its occasional use, but has become of minor importance, and 

 has been largely superseded by the gill. • 



If now we ask, what were the conditions of life under which 

 this organ was developed, and what were the later conditions 

 which rendered it in great measure or entirely useless, some 

 definite answer may be given. The question takes us back to the 

 Devonian and Silurian geological periods, during which it is 

 probable that its original development took place. In thiserathe 

 sens were thronged with fishes of two distinct orders, the Elas- 

 mobranchs and the Ganoids, the former without, the latter with. 



For other instances of the same character, sec Scinpcr's "Animal 

 Note 75. 



