188:").] NATURAL SCIENCES OF PHILADELPHIA. 129 



antique vertebrates we have existing representatives in the 

 Ganoids and the Elasmobranchs, and it is of interest to find that 

 in these modern survivals of the ancient fish life, the Elasmo- 

 branchs are entirely destitute of air-bladders, both in the mature 

 and the larval stage, while all Ganoids possess an air-bladder, 

 which retains a fully developed pneumatic duct in the mature 

 stage. And in the suborder of Dipnoi, the air-bladder is func- 

 tional^ active as a lung. It is well-known that counterparts of 

 the modern Dipnoi existed in the Devonian age, and it is highly 

 probable that they breathed air then as they do now. In fact, 

 we have some warrant for the belief that the antique fishes were 

 divided into two orders, as clearly b} T their breathing habits as 

 by other characteristics, the Elasmobranchs breathing b}' gills 

 only, while the Ganoids had developed a supplementary organ 

 for an occasional breathing of the air. 



If we compare the air-bladder with the lungs of the higher 

 vertebrates, we find that its general condition in the Ganoids is 

 that of a single cavity, with an effective duct opening into the 

 dorsal side of the oesophagus. But there is an exception to 

 this in the Dipnoi, and in Polypterus. In these, the duct con- 

 nects with the ventral side of the oesophagus, as in the lungs of 

 higher animals. Wilder shows that there is a series of forms, 

 mostly Ganoids, leading from Amia and Lepidosteus, with the 

 pneumatic duct entering the throat on the dorsal side, to Lepi- 

 dosi?-en, in which it enters on the ventral side, as in lung- 

 breathing animals. 



In all the fishes just named the air-bladder functions as a lung. 

 In Polyterus it has lateral divisions, and is probably used in air 

 breathing, while in the Dipnoi it becomes a functional lung. In 

 Lepidosteus, the American Gar-Pike, the air-bladder becomes 

 cellular and lung-like. This fish keeps near the surface, and may 

 be seen to emit air-bubbles. It apparently takes in a fresh supply. 

 The American Bow Fin or mud-fish (Amia) has a bladder of the 

 same lung-like character, and it has been seen by Wilder to come 

 to the surface, open its jaws widel} T , and apparently swallow a 

 large quantity of air. Wilder remarks that " so far as the experi- 

 ments go it seems probable that, "with both Amia and Lepidosteus, 

 there occurs an inhalation as well as exhalation of air at pretty 

 regular intervals, the whole process resembling that of the 

 Menobranchus and other salamanders, and the tadpoles, which, 



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