COLOUR AND PHOSPHORESCENCE 183 



for the colour. These are of several varieties, some being 

 black and others orange or yellow, while there are also 

 bundles of crystalline needles which act as reflectors. The 

 pigment cells can be increased or decreased, contracting 

 to tiny points or sending out ramifying branches, and on 

 the relative degree to which the different kinds are expanded 

 or contracted depends the colour pattern of their owner. 

 It is perfectly easy to experiment with flat-fish on different 

 coloured backgrounds, and the accurate way with which 

 the fish tone with them is shown in Plate 69. In this case 

 light appears to be the deciding influence in colour change, 

 though it never acts directly on the pigment cells. A 

 blind fish is not able to change colour. The lack of 

 pigment from the under side of the body is apparently 

 due to the absence of illumination on this side under normal 

 conditions, for if the fish are placed in glass bottomed tanks 

 illuminated from beneath, they develop colour on the 

 under, as well as the upper, surface. 



Many marine animals are coloured in such a way as to 

 blend with their surroundings, but do not possess the power 

 of actually changing colour. This is especially true of many 

 of the sea slugs, such as the common sea lemon, Doris, 

 which lives on rocks and is yellow with mottlings of red 

 and other colours, closely resembling the rock with its 

 encrusting sponges, sea squirts, and red coralline seaweed ; 

 an allied sea slug, the red Doris flammea, always lives on 

 red sponges ; there is a small green sea slug named Elysia 

 which always lives on green weed, and this list could be 

 extended almost indefinitely. Many crustaceans are also 

 difficult to detect in their natural surroundings and in 

 this group, it may be remembered, there are many instances 

 of " masking," the animals draping themselves in seaweed, 

 sponge, or hydroids the better to disguise themselves. 



In other cases the often vivid colouring of animals like 



