FISHES AND FISHINa. 127 



bladder would make them lighter than water, and they 

 would rise ; whilst a contraction of it makes them 

 specifically heavier, and they sink ; and fish have, as is 

 believed, this power of dilation and contraction. I 

 cannot consider this explanation as being wholly 

 correct ; for instance, the shark, which has no air- 

 bladder, yet ascends and descends with the greatest 

 rapidity, by the aid of its powerful tail, and pectoral 

 fins ; and in the mackerel, by its muscular formation, 

 and by the great strength of its tail it does the same; 

 Flat-fish have the power of locomotion by their side 

 fins, which work like a species of Archimedian screw, 

 for they have no air bladder ; the same is observable 

 in the fins of eels, and was particularly evident in the 

 gymnotus electricus, hereafter noticed. 



A glandular substance exists at the upper part of 

 the air-bladder, containing blood of a florid red colour, 

 shewing it to be highly oxygenated, and from this 

 source probably the oxygen gas contained in this 

 bladder is supplied ; according as the vital action of 

 the gills, and the pressure upon the surface of the 

 fish's body is increased by descending to great depths, 

 so the air-bladder contains more oxygen, even to 

 being wholly of that gas ; or, it may be from their 

 power of decomposing water that the oxygen is 

 obtained. 



Some naturalists think that the quality and quantity 



