176 A HISTORY OF FISHES 



directions, and habitually swimming rapidly from one level 

 to another, the possession of an air-bladder would be of little 

 advantage, if not an actual hindrance to the fish, and this 

 organ has been lost by these fishes in the course of evolution. 

 Nor do those forms living on the sea bottom require a hydro- 

 static float, and no air-bladder is found in the Flat-fishes, 

 Lump-suckers, many Blennies, etc., nor is it present in those 

 forms which spend their lives in the comparatively shallow 

 water of mountain streams. 



