COLORATION 227 



a motor impulse is transmitted to the muscle fibres controlling 

 the chromatophores. The whole process is purely reflex, and 

 occupies only the fraction of a second. 



Some fishes exhibit a condition known as xanthochroism, 

 sometimes occurring in the wild state, but generally brought 

 about by the artificial conditions associated with domestication. 

 In such individuals the black or brown pigment is entirely 

 wanting and the whole body has a golden coloration, or, if the 

 orange and red are also undeveloped, a uniformly silvery hue. 

 The familiar Gold-fish {Carassius auratiis) is a native of Eastern 

 Asia, and in its natural habitat has the greenish and brownish 

 colours of the other Cyprinids, only the domesticated varieties 

 exhibiting the golden or silver liveries. Individuals that have 

 escaped from artificial ponds and regained the rivers often 

 revert to the original coloration. Golden Tench {Tinea), 

 Golden Orfe {Idus), and Golden Trout (Salmo) are other 

 well-known varieties produced by fish culturists, but Trout of 

 this type are sometimes found in the wild state, and golden- 

 coloured Eels (Anguilla) are by no means rare. 



The relation between the incidence of light and the distribu- 

 tion of pigment in the skin has already been mentioned in 

 connection with obliterative shading, and it is interesting to 

 find that in the African Cat-fish {Synodontis) , with the remarkable 

 habit of swimming with the belly upwards {cf. p. 30), the normal 

 coloration is reversed, the lower parts being dark-brown or 

 black and the back a pale silvery grey (Fig. 11). It has been 

 stated that the Shark-sucker or Remora (Echeneis) exhibits a 

 similar reversal of coloration when attached to a shark by the 

 sucker on top of its head, but this requires confirmation. It 

 has been suggested that the absence of dark pigment on the 

 lower surface of a fish has no relation to concealment, but is due 

 to the fact that these parts are shaded from the light. There is 

 certainly some evidence in support of the view that the action 

 of light may be directly related to the production of pigment, 

 and fishes living in caves and wells, where light is absent, are 

 almost invariably colourless {cf. p. 232), while bottom-living 

 forms are generally unpigmented on the lower side. Professor 

 Cunningham carried out a series of experiments in the laboratory 

 at Plymouth, in which the lower sides of Flounders were 

 illuminated by a special arrangement of mirrors at the bottom 

 of the tank. He found that chromatophores were slowly 

 developed on the lower surface, which is normally quite colour- 

 less, and in some individuals exposed to these conditions for 



