This change is brought about by means of two layers of pigment cells, deeply seated in the skin, and of bluish and yellowish colours.

By suitable muscles these cells can be forced upwards so as to modify the colour of the skin, which, when they are not brought into action, is a dirty white.

These animals are excessively sluggish and defenceless, and the power of changing their colour to that of their immediate surroundings is no doubt of great service to them.

Many of the flatfish are also capable of changing their colour according to the colour of the bottom they rest on; and frogs have a similar power to a limited extent.

Some crustacea also change colour, and the power is much developed in the Chameleon shrimp (Mysis Chamaeleon) which is gray when on sand, but brown or green when among brown or green seaweed.