Most cephalopods have chromatophores, special pigment cells, in their skins, which allow them to change color rapidly. Cephalopods have the most complex brain of any invertebrate. Most species also have excellent eyes that are astonishingly similar to those of vertebrates. Most species have a single pair of gills, a mostly-closed circulatory system, and accessory or branchial hearts. The gills are ventilated by the muscular action of the mantle wall.
Cephalopods are dioecious.
Fertilized eggs are attached to the substrate and are sometimes tended by the female. This diverse group has around 650 living species in 5 Orders. Animal Diversity . Brusca, R. C., and G. J. Brusca. Sponsored in part by the Interagency Education Research Initiative, the Homeland Foundation and the University of Michigan Museum of Zoology .