Cephalopods (Cephalopoda, from the Greek kephale – pous head, podos, feet) are the category of marine molluscs that takes in octopus, giant squid and cuttlefish. Cephalopods, which are among the fastest and most intelligent invertebrates, have a body divided into the head, the visceral mass and tentacles. Their bodies display a bilateral symmetry, very well developed head and eyes head, a rounded mouth with a chitinous tip and surrounded by arms and tentacles, and it is on the mouth where the most striking characteristic of the mollusc is found, the radula (a type of file).
They are extremely quick animals that have developed a type of trapezium-type propulsion (jet-propulsion), which is a modification of the feet of other molluscs, which gives them great agility. Their skin contains pigmented cells, called chromatophores, which change colour in order to communicate or to create camouflage; this change in colour is brought about directly by nerve actions. Statocysts are their organs used for balance.