 The skins of cephalopods such as the octopus contain complex units , each consisting of a chromatophore with surrounding muscle and nerve cells . The cephalopod chromatophore has all its pigment grains in a small elastic sac , which can be stretched or allowed to relax under the control of the brain to vary its opacity . By controlling chromatophores of different colours , cephalopods can rapidly change their skin patterns and colours . 
