3i8 THE WORLD OF ANIMAL LIFE 



to become extinct, for the number of young it produces is extra- 

 ordinary. It has been estimated that one female cod will deposit 

 some millions of eggs in a season. 



The whiting and the haddock are close relatives of this fish. 



FLAT-FISH (Family Plueronectid.e) 



The Flounder, the Plaice, and the Sole are among the best- 

 known of what we generally call " flat " fish. 



But instead of calling them " flat '\ it would be more correct to 

 describe them as " narrow " or " thin " fish. For what we call the 

 " back " is really one of the sides, while the lower part of the body 

 is the other side! 



When a flounder is quite young it swims, not in the position in 

 which we are accustomed to see it, but upright in the water; so 

 that the thin edge of its body, instead of the flat surface, is upper- 

 most. And that thin edge, of course, is its back. 



But after a few days a great desire comes upon it to lie down 

 upon its side in the mud at the bottom of the water. This it 

 does; and then a very strange thing happens, for the side which is 

 uppermost becomes dark in colour, while the side upon which it 

 rests remains light. Thus the flounder assumes the appearance 

 in which we are accustomed to see it. 



When once a flounder lies down upon its side it never again 

 swims as it did before, but adopts an entirely different kind of 

 movement. 



If you were to look at one in an aquarium, or vessel for keeping 

 fish, you would at first be unable to see it. Its brown back is so 

 exactly like the mud or sand upon which it rests that only a 

 practised eye can detect it. But when it rises you would see that 

 it waves its whole body in the water, instead of swimming with its 

 tail like other fishes. 



A much more remarkable change than that of colour takes 

 place in its body soon after it lies down. One of its eyes is now 

 rendered useless to it; for, as this eye rests in the mud underneath, 

 the fish cannot of course see with it. 



