156 MARINE AND FRESHWATER FISHES 



FAMILY IV. FLAT FISHES (Pleuronectidce). 



Body flattened, strongly compressed, naked or clothed 

 with scales, one of the sides coloured, the other normally 

 colourless ; both eyes, in adult fish, located on the superior 

 or coloured surface, the bones of the two sides of the head 

 unequally developed, those forming the lower surface almost 

 rudimentary ; a single long dorsal and anal fin ; branchi- 

 ostegal rays six to eight in number ; air-bladder absent. 



The Flat fishes, in common with the Cod fishes and 

 Herring tribe, rank among those forms which from an 

 economic view are of the highest utility to man, and con- 

 stituting as they do one of the most important subjects of 

 our fishing industries, their more elaborate description 

 may be appropriately left to the handbooks- devoted to our 

 Food-fishes and Sea Fisheries. The remarkable structural 

 peculiarity which distinguishes the Pleuronectidae from all 

 other fish, i.e. the unsymmetrical development of the head, 

 and the location of the two eyes upon one side of this 

 region, it is singular to relate, does not represent the con- 

 dition in which as young fish they first leave the egg. At 

 such an early period they are bilaterally symmetrical, with an 

 eye on either side like all ordinary fish, but from the acquired 

 habit of lying constantly on one side, the eye-socket on the 

 side directed towards the ground becomes gradually oblite- 

 rated, and the eye itself, pushing its way over the top of the 

 head, ultimately takes its place near the other eye on that 

 side, which is popularly known as the upper surface. Simul- 

 taneously with this migration of the visual organ, the pig- 

 mentary substances which give to the adult fish its charac- 

 teristic tint become developed only on that side, the so-called 

 upper surface, which is exposed to the light, the opposite or 

 underneath one remaining colourless. In the majority of our 



