14 BRITISH FRESH-WATER FISHES 



The mention of the Flat-fishes brings under notice the 

 solitary arid remarkable exception to the power of fish to 

 balance themselves in the water, forming as it does 

 in the the exclusive characteristic of the whole of one of 

 the two divisions of the Anacanthini y or Spineless 

 Fishes, an order containing such important food fishes as the 

 cod, ling, halibut, turbot, sole, etc. The Pleuronectidte, or 

 Side-swimmers, commonly termed Flat-fishes, undergo a series 

 of metamorphoses unique, so far as is known, in animated 

 nature, and certainly among vertebrate animals. For some 

 time after leaving the egg they remain perfectly symmetrical, 

 swimming, like other fishes, back uppermost, and carrying an 

 eye on each side of the head. But as they grow older they 

 lose their balance, turn over on one side, and, being un- 

 provided with an air-bladder, sink to the bottom, which they 

 can only leave henceforward by active swimming. The side 

 next the ground becomes pearly white, like the bellies of most 

 vertical swimmers, the upper or exposed side assuming tints 

 closely assimilating in colour with the surrounding sand, rocks, 

 mud, or weeds. Lastly, the eye on the side which falls under- 

 most pushes itself either round or through the head, so as to 

 take up a new and useful position on the " business " side of 

 the face. Owing to the mouth retaining the position it received 

 at first, this contortion of the eyes imparts a most grotesque 

 expression to the countenance. 



It is no matter of accident which side happens to fall 

 undermost. In the flounder, the only species which can be 

 said to be a regular sojourner in English rivers, the fish 

 reclines normally on its left side, though there are occasional 

 exceptions to this rule. The turbot, brill, and some others 

 recline on their right side ; but there occur also abnormal 

 specimens of right-handed flounders and left-handed brill. 



On the whole, having regard to the enormous inroads 

 made upon the numbers of the various kinds of flat-fishes by 

 trawls and other expedients, and to the numbers in which the 



