REJD GURNARD. 43 



cation. That one use of these air-bladders to the fishes pos- 

 sessing them is to enable them to alter their specific gravity 

 with reference to that of the fluid they inhabit, seems almost 

 certain. We see the Gold-fishes in our ornamental vases 

 ascend and descend in the water without making any visible 

 external muscular effort. In this respect their action is to be 

 understood and explained by the well-known hydrostatic toy 

 of the philosophical instrument makers, in which a small 

 glass-balloon, or other figure, confined in a column of water 

 has its weight, by the introduction of a small quantity of air, 

 so nicely balanced in reference to the specific gravity of the 

 water, that it is made to ascend or descend according to the 

 degree of pressure made by the finger on the elastic cover of 

 the top. 



In other respects, however, the function is quite as anoma- 

 lous and uncertain as the quality of its contained gas. Our 

 two Red Mullets have no swimming-bladder, yet they appear 

 in the water to possess all the powers of the Indian or Ame- 

 rican species, which are well provided with them. The two 

 British species of Mackerel, hereafter to be described, both 

 swim near the surface of the water with the same apparent 

 swiftness and ease : one has a swimming-bladder, the other 

 none. Of our two species of Orthagoriscus, which, as far as 

 the habits of such rare fishes are known, appear to possess the 

 same powers, one has a swimming-bladder, the other not. 



" The swimming-bladder of fishes," says Dr. Roget in his 

 excellent Bridgewater Treatise, " is regarded by many of 

 the German naturalists as having some relations with the re- 

 spiratory function, and as being the rudiment of the pulmo- 

 nary cavity of land animals ; the passage of communication 

 with the oesophagus being conceived to represent the trachea." 



Hervey long ago observed " that the air in birds passed 

 into cells beyond the substance of the lungs ; thus showing a 

 resemblance to the cellular lungs in Reptiles, and the air- 



