2 FISHES IN GENERAL 



with the utmost accuracy, enabled to rise to the surface or 

 sink readily into deep water. A special organ, the 'air-,' 

 or 'swim-bladder/ has even been acquired by the majority 

 of living fishes, which, whatever may have been its origin 

 or accessory functions (v. p. 21), has certainly to an extraor- 



Figs, i and 2. Movement of fishes, shark and eel. (After MAREY.) 



dinary degree the power of rendering the specific gravity 

 of the fish the same as that of the surrounding water. 



In an example of a swift-swimming fish some of the 

 most striking peculiarities of the aquatic form may be 

 seen. The Spanish mackerel, Scomberomorus (Fig. 3), 

 shows admirably a stout spindle-like outline ; its entire sur- 



