COLOUR AND PHOSPHORESCENCE 181 



roundings are the nightly changes of colour which occur with 

 unfailing regularity. Whatever colour the prawns may 

 have by day, they all change to a beautiful transparent 

 blue by night (Plate 66), the alteration in colour taking 

 place quite quickly shortly after nightfall. 



Great numbers of minute " pigment cells " scattered 

 over the surface of the body are responsible for the colour 

 of the iEsop prawn (Plate 68). Each of these cells consists 

 of a central bag running out from which are many fine 

 extensions. Within the central bag region there are three 

 pigments, red, yellow, and blue, and the colour of the 

 animal is controlled by the extent to which these pigments 

 are spread through the branches, thus covering the 

 maximum area, or withdrawn into the centre where they 

 are reduced in area to a minute point and so hardly affect 

 the general colour of the animal. The three colours may be 

 associated in different ways ; thus the red pigment alone 

 occupies the branches when the animal is red, the green 

 colour is produced when the red pigment is withdrawn 

 into the central bag and the yellow and blue pigments 

 together spread into the branches. The nocturnal blue 

 colour is produced when the red and yellow pigments are 

 both withdrawn and only the blue one extends into the 

 branches of the pigment cells. 



What is the mechanism which controls these remarkable 

 and exact changes in colour ? It appears that the pigment 

 cells are influenced by the light which acts either by way 

 of the eyes or directly on the pigment cells. Different 

 coloured light affects the nerves controlling the flow of 

 pigment in different ways, and ensures that the colour 

 scheme produced shall tone with the background of the 

 animal. The supreme importance of these elaborate 

 mechanisms for colour change to these little animals is 

 clear. There can be no doubt that the protection given 



