Cephalopods are fast-moving, voracious predators, and can change colour with breath-taking rapidity. They range from the giant squid, the world's largest marine invertebrate, to species of only 2 cm in length. Inhabitants of most seas of the world, they are found from the surface to great depths. Most cephalopods have short lives yet their efficiency in capturing and consuming prey ensures rapid growth. These animals possess highly-developed nervous systems, large brains, elaborate senses, complex behaviour and are capable of learning. Many of these features are described and illustrated with line drawings and photomicrographs.