68 INTRODUCTION. 



animals as foiined essentially upon the same model, 

 and regard tlie air-bladder of fislies, and not their 

 gills, as analogous to the lungs of the superior 

 tribes of animals; the gills of the former, in the 

 mean time, being considered as having no coitcs- 

 ponding organ in the latter, in their mature state, 

 although they are always met with in one stage of 

 their progress tow^ards maturity. The air-bladder 

 of fishes is either a perfectly blind pouch, without 

 any communication with the contiguous organs, or 

 it has an opening into it, called the trachea, either 

 from the gullet or from the stomach. It appears 

 quite blind in many species of the genus Sciama, 

 just alluded to, as w^ell as in the perches. On the 

 contrary, it communicates with either the gullet 

 or the stomach, in the sturgeons, salmon, pike, 

 perch, chub, &c. ; and, in the cod, this com- 

 munication is sometimes effected by two distinct 

 passages. In most fresh water fishes, it is found 

 to be filled with the gas called nitrogen, or azote, 

 which is comparatively light, and is one of the 

 ingredients of common atmospheric air; while in 

 those inhabiting the salt waters, it contains car- 

 bonic acid gas, which is considerably heavier than 

 the former, and is one of the gasses commonly 

 evolved during the progress of combustion and fer- 

 mentation, as well as by the respiration of terrestrial 

 animals. 



The au--bladder is not met with in all fishes. 

 It appears to be wanting among the cartilaginous 

 tribes, in the lampreys and myxines whicli are 



