The Organs of Respiration 93 



organ of equilibrium, or perhaps as an organ of hearing) which 

 have no relation to its original purpose. 



The air-bladder is an internal sac possessed by many fishes, 

 but not by all. It lies in the dorsal part of the abdominal 

 cavity above the intestines and below the kidneys. In some 

 cases it is closely adherent to the surrounding tissues. In 

 others it is almost entirely free, lying almost loose in the cavity 

 of the body. In some cases it is enclosed in a bony capsule. 

 In the allies of the carp and catfish, which form the majority 

 of fresh-water fishes, its anterior end is connected through a 

 chain of modified vertebras to the ear. Sometimes its posterior 



FIG. 77. Weberian apparatus and air-bladder of Carp. (From Gunther, after 



Weber.) 



end fits into an enlarged and hollow interhaemal bone. Some- 

 times, again, a mass of muscle lies in front of it or is otherwise 

 attached to it. Sometimes it is divided into two or three parts 

 by crosswise constrictions. Sometimes it is constricted longitudi- 

 nally, and at other times it has attached to it a complication of 

 supplemental tubes of the same character as the air-bladder 

 itself. In still other cases it is divided by many internal parti- 

 tions into a cellular body, similar to the lung of the higher 

 vertebrates, though the cells are coarser and less intricate. 

 This condition is evidently more primitive than that of the 

 empty sac. 



The homology of the air-bladder with the lung is evident. 

 This is often expressed in the phrase that the lung is a developed 

 air-bladder. This is by no means true. To say that the air- 

 bladder is a modified and degenerate lung is much nearer the 



