The squids (fig. 110), cuttlefishes, octopi or "devil-fishes," and the three living species of _Nautilus_ constituting the class Cephalopoda are very different from the other molluscs in appearance, and are in fact different in important structural characters. They can move swiftly, have strangely modified organs of prehension, strong biting mouth-parts, and eyes of very complex organization. They are the most highly organized molluscan forms, and their predaceous habits and the great size to which some of them attain have given them distinction among the fierce and dangerous creatures of the sea. They are all strictly marine in habitat, and are all carnivorous. Most of them have no shell, or where the shell is present it is internal in all but a very few forms. The tentacle-like arms or feet surrounding the mouth which occur in all the Cephalopods are provided with sucking organs or suckers, in some cases with a horny toothed rim. These long, powerful, grasping, tentacular feet, with the suckers and five hooks, are very effective means of securing prey, and the pair of strong, sharp, cutting mandibles or beaks are equally effective in tearing to pieces. The eyes of the Cephalopods are almost as highly developed as those of the vertebrates. They are unusually large and staring, and add much to the terrifying appearance of the "devil-fishes." Cephalopods have the power of quickly changing color, because of the presence in the skin of many pigment-cells which can expand so as nearly to touch each other, thus producing a uniform tint over the whole body, or which can contract so as to destroy this uniformity of color. There are several sets of these color-carrying cells or chromatophores, each set of a color different from the others. The purpose of this change of color is protective, the animal being thereby able to make its color so harmonize with that of its immediate surroundings as to become indistinguishable. There are two principal groups of Cephalopods, viz., the Decapods and the Octopods. The Decapods, as their name indicates, have ten feet or arms surrounding the mouth, and in them the body is usually elongate, containing a horny "pen" or calcareous "bone." This group includes the cuttlefishes or sepias, from which are obtained sepia ink and the cuttlefish bone used to feed canary birds. The ink is a secretion which the cuttlefish discharges when attacked to create a cloud in the water and thus escape unperceived. The squids (_Loligo_) commonly used as bait by fishermen belong to the Decapoda. The two extra feet or arms which the Decapods have in addition to the eight possessed by the Octopods, differ from the others in being longer and slenderer and having suckers only on the distal extremities which are expanded into "clubs" (fig. 110).