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 that in the case of 
the flounder (Pleuronectes) deficiency of oxygen or 
alarm causes the chromatophores to contract, and so 
diminishes the intensity of the colour ; 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. 
