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opens usually on its superior or dorsal, rarely on its lateral, 
and occasionally on its inferior or ventral wall. 
The air-bladder is present in the sub-classes Drenorps, all the 
members of which are fresh-water; also in the Ganorps, all of 
which are entirely or partially fresh-water forms. It is absent 
in the CHonpRopTERYGI, except in a rudimentary form as a 
diverticulum opening on the dorsal wall of the pharynx in some 
genera. While in the lowly developed PHaryneoprancui and 
the Marsrroprancui it is entirely wanting. 
In the large sub-class Teleostei it is present in the Orders 
Plectognathi, Lophobranchii, and Acanthopteryqu, in the form of 
a closed sac (Physoclisti) the majority of which are marine. 
Whereas in the Physostomi it generally exists as a sac, having a 
connecting tube opening from it into the alimentary canal; 
the largest proportion of these fishes live in fresh water. As a 
general rule the air-bladder is found existing more constantly 
in the fresh-water than in the marine classes. . 
If we briefly. trace out this organ we find it much as follows. 
Among the Dipnors it is distinctly or indistinctly double, 
lung-like, and communicating throughout life, by means of a 
duct and glottis, with the esophagus or pharynx. It is thus in 
the Lepidosiren of the Brazils, in South America; Protopterus of 
tropical Africa; and Ceratodus of Queensland, South Australia. 
All these fishes, for the purposes of respiration, have been 
observed to ascend to the surface of the water, and take in 
atmospheric air direct; a mode of breathing, however, not 
confined solely to these forms. 
In a fish of the Gambia, Lepidosiren annecteus, the air- 
vessel forms a most interesting connecting link between what 
is usually observed in Fishes and Amphibians. During the 
rains the African river referred to overflows its banks, and 
numbers of Lepidosirens are carried away into what become 
vast watery tracts. These inundated places, as the hot season 
occurs, dry up under the rays of a tropical sun, and as a 
consequence all connection with the river first ceases, subse- 
quently the watery tracts become converted into dry plains. 
As the water commences to evaporate the fish burrow into the 
