i8 THE COMMERCIAL SEA FISHES OF 



supplemental, under certain conditions, to those of other 

 organs of the body, while it is generally observed that the 

 urinary bladder is largest in those forms in which the air- 

 bladder is absent. 



In the embryo it originates as a bud or offshoot from the 

 upper portion of the alimentary canal, or even from the 

 stomach ; this offshoot next elongates into a blind tube, 

 which enlarges at its terminal extremity into what will 

 eventually form the air-bladder. Consequently at some 

 period of a fish's existence there must of necessity be a 

 tube connecting the air-bladder (where one exists) with the 

 alimentary canal, into which latter it opens usually on its 

 superior or dorsal, rarely on its lateral, and occasionally on 

 its inferior or ventral wall. 



Among the bony fishes the air-bladder is present, either 

 as a closed sac (Physoclisti), in which communication 

 between that organ and the alimentary canal has been cut 

 off, the majority of which are marine ; whereas in such as 

 the communicating duct remains patent (Physostomi), the 

 largest proportion live in fresh water. Also, as a general 

 rule, the air-bladder is more usually present in fresh-water 

 than in marine genera. 



It has long been known that the gas contained in the 

 air-bladder is a mixture of oxygen, azote, and carbonic 

 acid, in variable proportions, in accordance with species, or 

 even with individuals ; while after death this organ is 

 generally found distended with gas, consisting chiefly of 

 nitrogen in the fresh-water forms and oxygen in the 

 marine. It has formed a subject of considerable discussion 

 as to how this gas is generated, but as in those forms in 

 which the air-bladder is a closed sac (Physoclisti\ it is as 

 well seen as in others possessing a pneumatic tube (Physo- 

 stomi\ one cannot resist believing that the gas must be 



