THE II EU GHKNAKI). 39 



moutli, is proved not only by tlic perfection of the valves 

 of tlic canal, whicli only open outwards, but also by the 

 want of uniformity in the quality of the air itself, and its 

 existence in those swimming-bladders that have no canal 

 of communication. That one use of these air-bladders 

 to the fishes possessing 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 

 anomalous and uncertain as the quality of its contained 

 gas. Our tw^o Red Mullets have no swimming-bladder, 

 yet they appear in the water to possess all the powers of 

 the Indian or American 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, Avhich, 

 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 respira- 



