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. 1 
Figs. 1 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- 
—— 
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