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mud, but maintain a communication with the atmosphere by 
means of a small aperture. Here in their muddy chamber 
they await the return of the rains, when they emerge from 
the retreat where they have been estivating. Of course in 
this chamber they are unable, owing to the absence of water, 
to employ their gills for respiration, but they are able to use 
their cellular lung, formed by the partition of the air-bladder 
into two chambers, for this purpose. They have a wide duct 
or trachea opening into the cesophagus on its ventral surface, 
where there is a distinct glottis. In short their lung or air- 
bladder is employed in depurating their blood during such time 
as they pass a torpid existence. 
Ceratodus, or the Queensland mud-fish, is possessed of gills 
for respiration, and likewise an air-bladder modified into a 
respiratory organ, to be employed under certain circumstances. 
This latter organ occupies the whole length of the abdominal 
cavity, and is more or less divided into two halves. The right 
half contracts anteriorly, and opens by a short duct, with a 
glottis at its termination, on to the left side of the cesophagus, 
but not quite so far forwards as its ventral wall. The structure 
of this organ is that of a lung, where at times blood is decar- 
bonised, and the oxygenated stream is conveyed by pulmonary 
veins to the atrium of the heart. 
The Ganoms likewise afford instances very analogous to 
what obtains among the Dipnorps. I shall allude to Amia as 
an example of the Heleostei possessing cycloid scales; and 
_ Polypterus with Ganor scales; and subsequently offer a few 
remarks on the Acipenser or sturgeon. 
Amia, a fresh-water fish of the United States, respires 
atmospheric air as Ceratodus is observed to do, as proved by 
Wiper. Its air-bladder is a median sac, extending the entire 
length of the abdomen. Its posterior extremity is rounded, 
while anteriorly it bifurcates with two short horns, between 
which is the connecting communication between it and the 
dorsal surface of the alimentary canal, an opening which is 
guarded by a longitudinal glottis. The interior of this air- 
bladder is cellular, and it is traversed by fine ramifications of 
