258 AN INTRODUCTION TO ZOOLOGY. 



remarked, are usually animals that live on sand, the 

 flat shape being well adapted to prevent their sink- 

 ing in when at rest. Some Flat-fishes have another 

 peculiarity that fits them for their mode of life ; 

 namely, they have the power of changing colour. 

 Although the lower side, turned away from the light, 

 is white, the upper side is full of pigment cells ; and 

 according as these change their shape, thus concen- 

 trating or distributing the colour they contain, the 



Fig. 94. — One of the Flat-fishes, the Cqmmon Sole (Solea FttZgartB). 



animal becomes lighter or darker. Whether the 

 activity of the nerves which govern these pigment 

 cells is always a case of reflex action, or whether they 

 are ever controlled by the will of the fish, is uncer- 

 tain; but the fish changes colour according to the 

 colour of the patch of ground it is resting on, becom- 

 ing dark on a dark patch and light on a light patch, an 

 arrangement which must be an admirable protection 

 against being detected either by animals that want to 



