98 THE ZOOLOGIST. 
must be accessory or supplementary to those of other organs 
under certain circumstances. 
If we investigate the origin of this organ in the animal’s 
embryonic state, we find that it originates as an offshoot or bud 
from the stomach or upper portion of the alimentary canal; 
that this offshoot elongates, then enlarges at its terminal 
extremity into what will subsequently be the air-bladder; 
consequently at some period of the fish's existence there is an 
open tube connecting the air-bladder with the alimentary canal, 
into which it opens, usually on its dorsal, rarely on its lateral, 
and occasionally on its inferior or ventral surface. 
In the Dipnoids or highest class of Fishes the air-bladder has 
stiff walls, is distinctly or indistinctly double, lung-like, and 
communicates throughout life by means of a duct and glottis 
with the cesophagus. The duct in Lepidosiren and Protopterus 
opens on the ventral or inferior surface of the alimentary canal, 
as it does in Batrachians and the higher vertebrates. In 
Ceratodus the opening is on the left side, near, but not on, the 
ventral wall of the pharynx. The duct in the forms enumerated 
would evidently be homologous to the trachea or wind-pipe, 
while the functions of the air-bladder are likewise analogous to 
those of lungs, as these fishes employ them under certain con- 
ditions for respiratory purposes, taking in atmospheric air direct. 
The Ganoids afford instances very similar to what obtains 
among the Dipnoids: thus the air-bladder in Amia has a lung- 
like function; its pneumatic tube is short and protected by a 
glottis, but the position where it opens into the alimentary canal 
is dorsal and not ventral, as in the Dipnoids and higher classes. 
Polypterus, however, has its opening on the ventral surface of the 
pharynx: while in Acipenser this organ has no lung-like 
function, it is used merely for hydrostatic purposes, and the 
opening is that of a lower type, being on the dorsal aspect of the 
pharynx, and destitute of any valve. The forms adverted to, 
however, connect the simple air-bladder of the Acipenser, or 
Sturgeon, with the lung-like air-sacs of the Ganoids, and thus 
with the Batrachians and higher vertebrates. 
But these organs are not considered lungs in most fishes by 
many anatomists, for the reason that the air-bladder is supplied 
with blood from the adjacent arteries of the body, not direct from 
the heart, and returns venous blood into the general circulation. 
