cr tet ee ea = en ae eee 
d 
k 
a 
i 
3 
1892.] The Origin of Lungs. 979 
its utility for some secondary purpose, such as an aid in swim- 
ming or in hearing. That its evolution began very long ago 
there can be no question; and the indications are that it 
began long ago to degenerate, through the loss of its primitive 
function. 
What was this primitive function? In attempting to 
answer this question we must first consider the air-bladder in 
relation to the fish tribe as a whole. In one principal order of 
fishes, the Elasmobranchs, the air-bladder does not exist. No 
shark or ray possesses thisorgan. In some few sharks, indeed, 
there is a diverticulum of the pharynx which: may be a 
rudimentary approach to the air-bladder; but this is very 
questionable. The conditions of its occurrence in the main 
body of modern fishes, the Teleostean, we have already con- 
sidered. But in the most ancient existing order of fishes, the 
Ganoids, of which but a few representatives remain, it exists 
in an interesting condition. In every modern Ganoid the 
air-bladder has an effective pneumatic duct, which usually 
opens into the dorsal side of the cesophagus, but in the sub-order 
Polypterus opens, like the wind-pipe of lung-breathers, into 
the ventral side. Finally, in the small sub-order of Dipnoi, 
also a survivor from the remote past, the duct not only opens 
ventrally into the cesophagus, but the air-bladder does duty 
asalung. 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 bactrachians. There 
are three existing representatives of the Dipnoi. One of 
these, the Australian lung-fish (Ceratodus), 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 celiaca. In the 
other two forms, Lepidosiren and Protopterus,—the kindred 
“mud fishes” of the Amazon basin and tropical Africa,—the 
bladder or lung is divided into two lateral chambers as in 
land animals, and is provided with a separate pulmonary 
artery. 
The opinion seems to be tacitly entertained by physiologists 
that this employment of the air-bladder by the Dipnoi as a 
