316 THE ZOOLOGIST. 
the tract; but in others, as the Cod, it becomes shut off from 
it, and forms a closed bladder. This air-bladder has in. such 
Physoklistous Fishes merely the function of a hydrostatic organ ; 
it diminishes the specific gravity of the fish, and aids it in 
supporting itself in the water. In ourselves the lungs are not 
often called upon to perform this hydrostatic function, but we 
find them useful when we try to swim, and without them we 
should indubitably sink to the bottom whenever we fall into 
water beyond our depth. In the Salmon the respiratory 
action of the air-bladder is no doubt unimportant; this for 
two reasons: it is not specially well provided with blood- 
vessels, and the Salmon is not in the habit of swallowing air 
by its mouth. 
What is true of the Salmon is not, however, true of all 
fishes; some have the walls of their air-bladder specially well 
provided with blood-vessels, and some do swallow air by the 
mouth. The American Ganoid-fish, Amia, has been closely 
watched by Prof. Burt G. Wilder, who observed a captive speci- 
men rise to the surface of the water in which it was swimming, 
open its jaws widely, and apparently gulp in a large volume of 
air. In such fishes as the Ceratodus of Australia, the Polypterus 
of West Africa, and the Lepidosiren of South America, which 
are the representatives of the group which ichthyologists call 
Dipnoi, or double-breathers, a special vessel is sent to the air- 
sac. This vessel, curiously enough, comes direct from the gills, 
branches in the wall of the sac, and then makes its way back to 
the heart to have its contents pumped through the body ; such 
blood is, then, doubly aérated. 
Interesting as these stages are, the gradations between air- 
sacs and lungs have not been exhausted; in the Salmon the 
duct of the sac opens on the upper or dorsal side of the digestive 
tract, in Ceratodus it lies to the left of the ventral surface ; 
while in Polypterus, as in higher forms, it opens on the ventral 
middle line. In the Ganoid, Amia, the sac is single; in its 
ally, Lepidosteus (the Gar-pike), it is single externally, but 
divided internally; and in Polypterus it is double. Nor is the 
name sac, as indicating a merely hollow bag, any longer appro- 
priate. Even in the first-mentioned Ganoids the cavity within 
is broken up by bands of intercrossing tissue, which support 
blood-vessels ; the sac, in fine, becomes spongy in texture, and 
ae 
