ioo The Organs of Respiration 



of fishes it exists in an interesting condition. In every modern 

 Dipnoan, Crossopterygian, and Ganoid the air-bladder has an 

 effective pneumatic duct. This in the Ganoids opens into the 

 dorsal side of the oesophagus, but in the Dipnoans and Cros- 

 sopterygians, like the windpipe of lung-breathers, it opens into 

 the ventral side. In the Dipnoans, also survivors from the re- 

 mote past, the duct not only opens ventrally into the oesophagus, 

 but the air-bladder does duty as a lung. Externally it differs 

 in no particular from an air-bladder; but internally it presents 

 a cellular structure which nearly approaches that of the lung of 

 the batrachians. There are three existing representatives of 

 the Dipnoans. One of these, the Australian lung-fish (Neocera- 

 todus) has a single bladder, which, however, is provided with 

 breathing-pouches having a symmetrical lateral arrangement. 

 It has no pulmonary artery, but receives branches from the 

 arteria cceliaca. In the other two forms, Lepidosiren and Protop- 

 terus, the kindred ' mudfishes ' of the Amazon basin and tropi- 

 cal Africa, the bladder or lung is divided into two lateral cham- 

 bers, as in the land animals, and is provided with a separate 

 pulmonary artery. 



" The opinion seems to have been tacitly entertained by 

 physiologists that this employment of the air-bladder by the 

 Dipnoans as a lung is a secondary adaptation, a side issue 

 from its original purpose. It is more likely that this is the 

 original purpose, and that its degeneration is due to the disap- 

 pearance of the necessity of such a function. As regards the 

 gravitative employment of the bladder, the Teleostean fishes, to 

 which this function is confined, are of comparatively modern 

 origin; while the Dipnoans are surviving representatives of a 

 very ancient order of fishes, which flourished in the Devonian 

 age of geology, and in all probability breathed air then as now; 

 and the Crossopterygians and Ganoids, which approach them in 

 this particular, are similarly ancient in origin, and were the 

 ancestors of the Teleosteans. The natural presumption, there- 

 fore, is that the duty which it subserved in the most ancient 

 fishes was its primitive function. 



" The facts of embryology lend strong support to this hypo- 

 thesis. For the air-bladder is found to arise in a manner very 

 similar to the development of the lung. They each begin as an 



