FLOUNDERS AND PORCUPINE-FISHES ^5 



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 fro'm the blind to the upper 



Fig. 183. The winter flounder, Pseudopleuronectes americanus (Walb.), Gill. 

 X \. (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 



