X THE COLOURS OF FISHES 213 



impression that fishes are dull-coloured as compared 

 with birds is due. Sexual dimorphism of colour is 

 common both in those from temperate and from 

 tropical seas, and so-called protective coloration is 

 not infrequent. 



The foregoing remarks, it should be noticed, 

 refer almost exclusively to the modern dominant 

 bony fishes or Teleosteans. Sharks and rays, as 

 typical of the older Elasmobranchs, are dull in 

 colour without the silvery sheen of modern fishes ; 

 and although they may display a certain amount of 

 beauty of marking, the colours are relatively dull 

 and sombre. 



The colours of fishes are due to the structure of 

 the dermis or to colouring- matter contained in it. 

 The colouring -matter is in part at least deposited 

 in contractile pigment cells — chromatophores — the 

 result being that many fishes, like Amphibians, are 

 capable of a considerable amount of colour change, 

 varying according to a familiar observation with the 

 colour of the ground upon which they lie. The 

 mechanism is set in motion through the eye, the 

 phenomenon not being observable in blind fishes. 

 Mr. Cunningham (1893, b) states thatjn the case of 

 the floun der" (Pleuronectes) d eficiency of oxygen or 

 alarm causes t he chroma to phore s_to_c ontract. and Lso 

 d imimsE es thejntensity _of the_c olou r ; exclusion of 

 light causes them to expand and so deepens the 

 colour. Probably, as in the case of the chameleon, 

 the colour depends in part upon the psychological 

 state of the individual. Direct mechanical stimula- 

 tion also causes the pigment cells to contract. 



