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VERTEBRATE ANIMALS 155 
eye would not only be useless on the under surface, but 
would run almost certain risk of being injured. It there- 
fore begins to change its position and gradually works 
round to its final place beside the stationary eye. In some 
flat-fish the eye reaches the upper surface by sinking 
through the head. 
Colour. The movement of the eye is not the only 
change undergone by the animal. When young the colour 
on both sides is the same; it is only when it loses the 
power of being able to maintain its upright position that 
the under surface becomes white. This change of colour 
in the adult fish appears to be due to the effects of light 
acting on the eye and thence on the nerves connected with 
the pigment cells in the skin. 
The dark colour of the upper surface is a great pro- 
tection against enemies, rendering the animal almost 
invisible when lying on the soft sand and mud of the 
sea-bed. Those who have waded over sand-banks 
spearing Flat-fish will know how extremely difficult it is 
at times to detect the animals, the colour of the upper 
surface being almost identical with that of its surround- 
ings; and when, as frequently happens, it buries itself in the 
mud or splashes sand over its body, it is quite impossible 
to see it, 
Method of swimming. Most fish propel themselves 
through the water by a side to side or horizontal movement 
of the tail. This the Flounder is unable to do, for its tail is 
carried in a horizontal position. In swimming it moves 
through the water by throwing its body into a number 
of curves, so that the dorsal and anal fins which in most 
fish act as cut-waters, are used by the flounder as a means 
of locomotion. 
NotTe.—Here again it is not intended that classification should 
precede individual study ; as representatives of each division are 
