1892.] The Origin of Lungs. 983 
opening is partly closed by a fold of the mucous membrane. 
Another family, the Labyrinthici, of similar habits, possesses 
a more developed breathing organ. This is a cavity formed 
by the walls of the pharynx, in which are thin laminæ or 
plates, which undoubtedly perform an oxygenating function. 
The most interesting member of this family is Anabas scandens, 
the Climbing Perch. In this fish, which not only leaves the 
water, but climbs trees, the air-breathing organ is greatly 
developed. The Labyrinthici moreover have usually large 
air-bladders. As regards the occasional breathing of air by 
fishes, even in species which do not leave the water, it is quite 
common, particularly among fresh-water species. Cuvier 
remarks that air is perhaps necessary to every kind of fish; 
and that, particularly when the atmosphere is warm, most of 
our lacustrine species sport on the surface for no other 
purpose. 
It is not difficult to draw a hypothetical plan of the develop- 
ment of the air-bladder as a breathing organ. In the two 
families of fishes just mentioned, whose air-bladders indicate 
that they once possessed the air-breathing function and have 
lost it, we perceive the process of formation of an air-breathing 
organ beginning over again, under stress of similar circum- 
stances. The larval development of the air-bladder points 
significantly in the same direction. In fact, we have strong 
reason to believe that air-breathing in fishes was originally 
performed, as it probably often is now, by the unchanged walls 
of the esophagus. Then these walls expanded inwardly, form- 
ing a simple cavity, partly closed by a fold of membrane, like 
' that of the Ophiocepbalidw. A step further reduced this 
membraneous fold to a narrow opening, leading to an inner 
pouch. As the air-breathing function developed, the opening 
became a tube, and the pouch a simple lung, with compressing 
muscles and capillary vessels. By a continuation of the pro- 
cess the smooth-walled pouch became sacculated, its surface 
being increased by folding into breathing cells. Finally a 
longitudinal constriction divided it into two lateral pouches, 
such as we find in the lung of the Dipnoi. This brings us to 
the verge of the lung of the Batrachia, which is but a step in 
