FLOUNDERS AND PORCUPINE-FISHES 175 
lar instance of environmental evolution, its flattened body 
adapting itself both in shape and colour to its bottom 
living. Its entire side, —not the ventral region, as in the 
rays,—is flattened to the bottom. The unpaired fins now 
become of especial value ; they increase in size, and their 
undulatory movements enable the fish to swim rapidly yet 
retain its one-sided position ; ventral fins become useless, 
and degenerate. The further. adaptations of the flat fish 
include its pigmentation only on the upper or light-exposed 
side, and the rotation of the eye from the blind to the upper 
Fig. 183.—The winter flounder, Pseudopleuronectes americanus (Walb.), Gill, 
xX}. (After GOODE in U.S. F.C.) North Atlantic. 
side, — in this giving one of the most remarkable cases of 
adaptation known among vertebrates. (Heterosomata.) 
The Porcupine-fish (Fig. 184) may be referred to as 
another singular result of environmental evolution. Its 
globular and inflatable form bespeaks slowness of motion 
and helplessness if exposed to changes of temperature 
or current. Its fins are reduced and feeble, suited, how- 
ever, to its tranquil habitat; its fused jaws, parrot-like, 
show in how special a way its food is best secured. It 
has evolved a protective casing of enormous needle-like 
scales, whose shape parallels that of the derm denticles 
