Colleoid cephalopods have complex organs which they use to change colour rapidly. This is seen in squid, cuttlefish and octopuses. Each chromatophore unit is a single chromatophore cell and muscle, nerve, glial and sheath cells. Inside the chromatophore cell, pigment granules are in an elastic sac. To change colour the animal changes the sac form or size by muscular contraction. This changes its appearance. This differs from the mechanism used in fish, amphibians and reptiles, because the shape of the sac is being changed rather than pigment being moved in the cell. However, a similar effect is achieved.