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- 



FiG. ^ 



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, Scomberomoriis (Fig. 3), 

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



