COLOURS 57 



chromatophore a separate name may be given to indicate the 

 colour : melanophores are black, erythrophores are red and xantho- 

 phores are yellow. 



The chromatophores are the effectors by means of which a 

 crustacean can change colour. If the pigment extends into all the 

 branches the whole surface of the animal becomes darker. When 

 the pigment contracts into the small central area the colour 

 becomes much lighter, and, in the less heavily calcified species, 

 more transparent. 



These changes can be seen at their simplest in an isopod such as 

 Ligia, which, if placed in the light on a dark background, expands 

 its chromatophores and becomes dark in colour. When placed on 

 a white background the chromatophores contract and the body 

 becomes paler. The complete change takes about two hours. These 

 changes are clearly an aid to concealment. 



If the lighting arrangements of Ligia's world are turned upside 

 down some interesting results are obtained. When lit from below, 

 with a black background above it, Ligia is paler than when lit from 

 above with a black background beneath it. This suggests that Ligia 

 is sensitive to the light coming up from the surface on which it 

 stands, and can adjust its colour accordingly. The obvious organs 

 to examine for such sensitivity are the eyes. By covering different 

 parts of the eyes with an opaque paint it has been found that it is 

 the light reflected from the background and perceived by the lower 

 parts of the eyes which initiates colour change. If the lower parts 

 of the eyes are painted over the isopod becomes dark in colour 

 irrespective of its background. 



It has been suggested that the colour change in Ligia is under 

 the control of two hormones. One causes contraction of the melano- 

 phores and is produced when the lower parts of the eyes are 

 illuminated. The other causes expansion of the melanophores and 

 is produced when the upper parts of the eyes are illuminated. The 

 colour assumed by the isopod will depend on the balance between 

 the two hormones. If this is really so, then maximum paling should 

 occur when the lower parts of the eyes are illuminated, but not the 

 upper parts. In fact it is found that when the upper parts of the 

 eyes are painted over the animals become paler than if the eyes are 

 left unpainted. 



The site of production of the colour-change hormones in Ligia 

 is not accurately known, but it seems certain that a hormone which 

 causes the black chromatophores to contract is produced in the 



