This class contains the cephalopods, animals commonly known as squid, cuttlefish, octopus, and nautilus. The giant squid is the largest of all mollusks. Most cephalopods are highly adapted for swimming. The body mass is very tall. There is no foot; the lower part of the body wall is drawn out to form a ring of arms, or tentacles, around the head. Among living cephalopods, only the nautilus (subclass Nautiloidea) has a complete external shell; extinct members of the subclass and the extinct ammonites (subclass Ammonoidea) had similar spiral shells. Members of the subclass Coleoidea (the squid, cuttlefish, and octopus), have an internal shell or no shell at all.
All cephalopods are carnivorous and possess a radula and powerful beaks. The nervous system and the sense of vision are highly developed. In most cephalopods the sexes are separate and reproduction requires copulation. Fertilization may occur inside or outside the mantle cavity. Cephalopods are worldwide in distribution and are found in all depths of the ocean. They are an important food staple for many animals, including humans.