INTRODUCTION. 73 



was owing to the distention of theii air-bladder that 

 fishes floated after death, they should not, as they 

 usually do, turn belly upwards under these circum- 

 stances the air-bladder being above their centre of 

 gravity but should present themselves in their 

 ordinary posture. This circumstance seems to be a 

 sufficient proof, that the gasses which occasion the 

 floating of fishes after death are formed principally 

 in the organs contained in the belly, which are, in 

 all animals, among the first to putrefy ; and some 

 fishes, such as the Diodons and Tetrodons, or por- 

 cupine fishes, employ sometimes the device of 

 swallowing air when they wish to inflate their body, 

 and thus to raise their bristles in self-defence ; 

 which air, passing into their stomach, renders the 

 belly, in spite of their air-bladder, which runs 

 along the spine, the lightest part of their body, and 

 they always assume, accordingly, the posture of a 

 dead fish as it floats upon the water. 



But by whatever immediate means the air-bladder 

 of fishes is either expanded or contracted, there can- 

 not be any reasonable doubt that it is by means of 

 changes in the volume of this organ, and, conse- 

 quently, of the whole body of the animal, that such 

 fishes as are possessed of it are enabled to rise and 

 sink in the water with little or no muscular effort 

 In proof of this it is sufficient to observe, that in 

 these fishes the power of rising in the water is quite 

 lost if the air-bladder be perforated, or otherwise 

 incapacitated for retaining air; and that they are 

 equally incapable of sinking in this fluid if the 



