74 THE STORY OF FISH LIFE. 



known as chromatophores, are at rest, they allow 

 the general ground colour to play a more or less 

 conspicuous part ; but so soon as the proper 

 stimulus is applied, the little strands or cords 

 contract on all sides and pull the bag flat, at 

 the same time, of course, causing its contained 

 colouring matter to be spread out in a thin layer 

 and cover the ground colour beneath. This 

 power of changing colour is of great use, for by 

 this means the animal is enabled to assume the 

 general tone of its immediate surroundings, and 

 so obtain a measure of protection against its 

 enemies. 



^lany animals have this power of changing 

 colour by means of contractile pigment cells or 

 chromatophores. Thus, a species of shrimp 

 (Ati/oida) has been described, which is dark 

 green when among weeds, but changes to a pale 

 brown when resting in dark rocks ; a dark 

 brown form placed in a tank containing numer- 

 ous greenish forms changed at once to this 

 colour. Frogs also change colour according to 

 the nature of the ground on which they rest. 

 The action of these chromatophores is, perhaps, 

 nowhere seen so well as in the Loligo, one of 

 the cuttle-fishes — not a fish, of course, but a 

 mollusc allied to the Nautilus. Here the ex- 

 pansion and contraction of these very active cells 

 goes on with great rapidity. All the blue or all 

 the yellow or all the red-containing cells may be 

 expanded and the others remain at rest, but so 

 quickly do the changes follow one upon another, 

 that a dazzling brilliancy is the result. Some- 

 times the contraction of these cells leaves a 



