The most advanced class of molluscs, containing the squids, cuttlefishes, octopuses, and the extinct ammonites. Cephalopods have a highly concentrated central nervous system within a protective cartilaginous case. The eye has a well-developed retina and is comparable to that of vertebrates. All cephalopods are predacious carnivores capable of swimming by jet propulsion; they have highly mobile tentacles for catching and holding prey. Octopuses have a large central brain, highly developed vision, and also groups of neurons (‘sub-brains’) in each of their eight tentacles that function semi-autonomously, e.g. when sensing and handling objects. They are now regarded as highly intelligent, with short- and long-term memory and capabilities such as problem-solving, tool use, recognition of individual humans, and play.