iv SHIFTS FOR A LIVING 55 



tion and expansion of the pigmented living matter of 

 cells which lie in layers in the under-skin, and are con- 

 trolled by nerves. (See Fig. 11.) 



In a widely different set of animals the cuttle-fishes 

 -the power of rapid colour-change is well illustrated. 

 When a cuttle-fish in a tank is provoked, or when one 

 almost stranded on the beach struggles to free itself, or, 

 most beautifully, when a number swim together and 

 keep time in their locomotor movements, flushes of 

 colour spread over the body. The sight suggests the 

 blushing of higher animals, in which nervous excitement 

 passing from the centre along the peripheral nerves 

 influences the blood-supply in the skin ; but in colour- 

 change the nervous thrills affect the pigment-containing 

 cells or chromatophores, the living matter of which con- 

 tracts or expands in response to stimulus. It must be 

 allowed that the colour-change of cuttle-fish is oftenest 

 an expression of nervous excitement, but in some cases 

 it helps to conceal the animals. 



More interesting to us at present are those cases of 

 colour-change in which animals respond to the hues of 

 their surroundings. This has been observed in some 

 Amphibians, such as tree-frogs ; in many fishes, such as 

 plaice, stickleback, minnow, trout, Gobius ruthensparri, 

 S err anus ; and in not a few crustaceans. The colour 

 of surroundings affects the fishes and frogs through the 



* o ~ 



eyes, for blind plaice, trout, and frogs do not change 

 their tint. The nervous thrill passes from eye to brain, 

 and thence extends, not down the main path of impulse 

 -the spinal cord but down the sympathetic chain. If 

 this be cut, the colour-change does not take place. The 

 sympathetic system is connected with nerves passing 

 from the spinal cord to the skin, and it is along these 

 that the impulse is further transmitted. The result is 

 the contraction or expansion of the pigment in the skin- 

 cells. Though the path by which the nervous influence 

 passes from the eye to the skin is somewhat circuitous, 

 the change is often very rapid. As the resulting resem- 

 blance to surroundings is often precise, there can be no 

 doubt that the peculiai itv sometimes profits its possessors. 



