VARIATION OF COLOUR. 183 



black, red, yellow, etc., pigment-cells, or chromatophors, in the 

 skin of the fish ; or by the rapid contraction or expansion 

 of the chromatophors which happen to be developed. The 

 former change is gradual, like every kind of growth or de- 

 velopment; the latter rapid, owing to the great sensitive- 

 ness of the cells, but certainly involuntary. In many bright- 

 shining fishes — as Mackerels, Mullets — the colours appear 

 to be brightest in the time intervening between the capture 

 of the fish and its death : a phenomenon clearly due to the 

 pressure of the convulsively-contracted muscles on the chro- 

 matophors. External irritation readily excites the chroma- 

 tophors to expand — a fact unconsciously utilised by fishermen, 

 who, by scaling the Eed Mullet immediately before its death, 

 produce the desired intensity of the red colour of the skin, 

 without wliich the fish would not be saleable. However, it 

 does not require such strong measures to prove the sensitive- 

 ness of the chromatophors to external irritation, the mere 

 change of darkness into light is sufficient to induce them to 

 contract, the fish appearing paler, and vice versa. In Trout 

 wliich are kept or live in dark places, the black chromatophors 

 are expanded, and, consequently, such specimens are very 

 dark-coloured ; when removed to the light they become paler 

 almost instantaneously. 



Total absence of chromatophors in the skin, or Albi- 

 nism, is very rare among fishes ; much more common is 

 incipient Albinism, in which the dark chromatophors are 

 changed into cells with a more or less intense yellow pig- 

 ment. Fishes in a state of domestication, like the Crucian 

 Carp of China, the Carp, Tench, and the Ide, are particularly 

 subject to this abnormal coloration, and are known as the 

 common Goldfish, the Gold-Tench, and the Gold-Orfe. But 

 it occurs also not rarely in fishes living in a wild state, and 

 has been observed in the Haddock, Flounder, Plaice, Carp, 

 Eoach, and Eel. 



