Cephalopods are a group of molluscs that include 800 living species. Members of this group include the octopuses, cuttlefish, squid and nautiluses. Cephalopods are exclusively marine animals. They include the largest, most intelligent and most mobile of all molluscs. Cephalopods have a large, prominent head, tentacles, large complex eyes and exhibit complex behavior. Like most molluscs, the majority of cephalopods have a mantle, a radula and breath using gills.
The most obvious difference among the various cephalopods is the presence or absence of an external shell. Squids, cuttlefish and octopuses do not have an external shell. Instead, they either have an internal shell called a gladius or they lack a shell entirely. The nautilus has an external shell (in fact, it is the only living cephalopod to have an external shell).
The cephalopod eye is a complex structure and rivals the vertebrate eye in its sophistication. The cephalopod is large relative to the size of its body and consists of a pupil, lens, iris and in some groups (such as octopuses) a cornea (squids and cuttlfish lack a cornea). The shape of the pupil varies between the groups (octopuses have a rectangular pupil, cuttlefsih have a U-shaped pupil and squids have a round pupil).
Many cephalopods rely on their acute vision to detect predators as well as locate prey. Their vision is advanced enough to detect differences in the size, shape, brightness and orientation of objects.
Cephalopods move in part by jet propulsion. Part of the mantle of a cephalopod forms a siphon through which water is forced. As the water pressure moves through the siphon, it forces the cephalopod forward and in this way produces jet propulsion. Cephalopods also use their tentacles to move and help maintain their velocity.
Cephalopods have a beak-like structure that they use to feed. They capture their prey using their tentacles and bring it to their mouth where they use their beak to bite chunks off before ingesting their prey. Most cephalopods also have a radula which consists of several rows of teeth.
Cephalopods have pigment-filled cells in their skin called chromatophores that they can expand and contract to expose or hide spots of color. This enables cephalopods to quickly change color to blend in with their surroundings or aid in courtship and other communication. In some cephalopods, chromatophores are bioluminescent and can shine light in such a way as to conceal their shadow from any predators and thereby escape detection.
Cephalopods are unique among molluscs in having a closed circulatory system. Coleoids (squids, cuttlefish and octopuses) have a two gill hearts (that pump blood through the gills) and a third systemic heart (that pumps blood throughout the body).
Cephalopods have a large, centralized brain, well-developed senses and are able to learn complex behavior. Their brain resides within a protective cranium made up of cartilage.
Most cephalopods (except the nautilus and some octopuses) have an ink sac, a muscular bag that holds dark ink (melanin). The ink is expelled into the water where it forms a dark cloud. This enables the cephalopod to obscure themselves and to confuse predators. The inking habits of cephalopods has earned them the common name of "inkfish".