226 A HISTORY OF FISHES 



kind of pigment they contain, the manner in which they are 

 blended, mixed and distributed in the skin, as well as the 

 iridescence and the reflecting powers of the iridocytes, all play 

 their part in determining the characteristic colour of a fish. 

 The dark bluish-grey colour of the back of a Whiting [Gadus 

 merlangus) is due largely to the abundance of black and yellow 

 chromatophores in this region, but these are much less 

 numerous on the sides and absent altogether on the pale belly. 

 The iridescence and silvery appearance of the sides are due 

 to the iridocytes lying above the scales, combined with the 

 reflecting but non-iridescent layer of similar structures below 

 the scales. The dead white of the abdominal region owes its 

 appearance to the different reflecting power of another deep 

 layer of iridocytes in this region known as the argenteum, and 

 to the absence of chromatophores. 



The colour changes described in the foregoing pages, whether 

 slow, rapid, or instantaneous, are all due, therefore, to the 

 action of the chromatophores. If these be expanded, the 

 contained pigment is diffused and the particular shade of 

 colour thereby intensified: when contracted, they may shrink 

 to mere dots and thus diminish the vividness of the colour 

 involved. This latter process may even change the colour, for 

 yellow chromatophores become orange when contracted, and 

 orange or red appear brown or black. At the same time, the 

 prevailing hues of the body are altered, not only by the expan- 

 sion or contraction of the chromatophores, but also by an 

 increase or decrease in their numbers, or by an alteration in the 

 manner of their distribution in the skin. 



The initial stimulus to colour change is received through the 

 eyes, as is clearly shown by the following facts. A blind Turbot 

 {Rhombus), although living on a light sandy background, 

 remained much darker and more conspicuous than its normal 

 fellows, and dark-coloured Trout {Salmo), observed among 

 light individuals in a chalky stream, likewise proved to be 

 blind. Further, a Flounder [Paralichthys) placed with its head 

 on a background of one colour and its body on another, always 

 assumed the colour of that on which its head was resting. 

 Even when the whole of the body as well as the posterior part 

 of the head was resting on a white background, the entire 

 fish remained dark so long as the front of the head and the eyes 

 were on the black. Thus, the fish sees the surroundings before 

 making any attempt to imitate them, and an appropriate 

 sensory impulse goes from the eyes to the brain, from whence 



