1892.] The Origin of Lungs. 977 
bladder, while every other species possesses one. A like singu- 
lar variation occurs in the case of related genera. In the genus 
Sebastes the air-bladder is very large, yet in a closely related 
genus it is scarcely the size of a pea. 
Its variations in form are equally marked. Ordinarily it is 
a simple sac, with smooth interior. Yet in some instances it 
has acellular interior, and in others it is divided by trans- 
verse partitions into from two to four sections. In other cases 
it is divided by a longitudinal partition into two lateral 
halves. In some families there is a remarkable development 
of lateral appendages, of varying character. In others the 
internal sacculation becomes so great that the bladder 
resembles the batrachian lung, and evidently does duty in the 
breathing of air. 
The bladder itself is composed of two layers of membrane, 
the outer one being usually provided with muscular fibres, 
while the inner one is abundantly supplied with blood vessels. 
These take the form of capillary plexuses, or what are known 
as retia mirabilia, whose purpose may be to secrete the gas 
with which the bladder is filled. This gas differs in char- 
acter in different fishes. In fresh-water fishes it is nearly pure 
nitrogen, the percentage of oxygen being small. In marine 
fishes, on the contrary, oxygen is in excess. This is particu- 
larly the case in the deep-swimmers, in some of which the 
bladder has been found to contain as much as eighty-seven 
per cent. of oxygen. 
These gaseous contents, if, as seems probable, obtained 
through secretion by the blood vessels, are not always so 
obtained, for in many fishes an arrangement exists by which 
air may directly enter the bladder from without. This is 
what is known asthe pneumatic duct, a tubular connection 
between the cesophagus and the air-bladder, not unlike that 
which supplies the lungs of air-breathers. This duct presents 
the same remarkable variability which we have observed in 
other characteristics of the air-bladder. Its point of con- 
nection with the alimentary tract varies, being usually in the 
esophagus, but in some fishes as far back as the stomach. In 
the Ganoid order of fishes the duct is a short one, and always 
