The Outline of Science 



are also very quick to get a sprinkling of sand over their upturned 

 side, so that only the eyes are left showing. But there is no doubt 

 as to the exactness with which they often adjust themselves to be 

 like a little piece of the substratum on which they lie; they will do 

 this within limits in experimental conditions when they are placed 

 on a quite artificial floor. As these fishes are very palatable and 

 are much sought after by such enemies as cormorants and otters, 

 it is highly probably that their power of self-effacement often 

 saves their life. And it may be effected within a few minutes, in 

 some cases within a minute. 



In these self-effacing flat-fishes we know with some precision 

 what happens. The adjustment of colour and pattern is due to 

 changes in the size, shape, and position of mobile pigment-cells 

 (chromatophores) and the skin. But what makes the pigment- 

 cells change? The fact that a blind flat-fish does not change its 

 colour gives us the first part of the answer. The colour and the 

 pattern of the surroundings must affect the eye. The message 

 travels by the optic nerve to the brain ; from the brain, instead of 

 passing down the spinal cord, the message travels down the chain 

 of sympathetic ganglia. From these it passes along the nerves 

 which comes out of the spinal cord and control the skin. Thus the 

 message reaches the colour-cells in the skin, and before you have 

 carefully read these lines the flat-fish has slipped on its Gyges ring 

 and become invisible. 



The same power of rapid colour-change is seen in cuttlefishes, 

 where it is often an expression of nervous excitement, though it 

 sometimes helps to conceal. It occurs with much sublety in the 

 uEsop prawn, Hippolyte, which may be brown on a brown sea- 

 weed, green on sea-lettuce or sea-grass, red on red seaweed, and 

 so on through an extensive repertory. 



According to the nature of the background, [Professor 

 Gamble writes] so is the mixture of the pigments com- 

 pounded so as to form a close reproduction both of its colour 

 and its pattern. A sweep of the shrimp net detaches a bat- 



