Cephalopods

The cephalopods (class Cephalopoda), predators that are specialized for locomotion, include the octopuses, squids, cuttlefishes, and other fascinating creatures. Cephalopods adapt the molluscan body plan to an active way of life. Nearly all the agile swimmers with a complex nervous system and reduction of the shell.

Vampire Squid

• All 650 living species are marine.
• A cephalopod (the name means ''head-footed'') is like a gastropod with its head pushed down toward the foot.
• The foot is modified into arms and tentacles, usually equipped with suckers that are used to capture prey.
• The large eyes, usually set on the sides of the head, are remarkably like ours. 
• The body, rounded in octopuses and elongated in squids, is protected by a thick and muscular mantle. The mantle forms a mantle cavity behind the head that encloses two or four gills. Water enters through the free edge of the mantle and leaves through the siphon, or funnel, a muscular tube formed by what remains of the foot, which projects under the head. Cephalopods swim by forcing water out of the mantle cavity through the siphon.

Octopuses have eight long arms and lack a shell. They are common bottom dwellers. Including arms, the size varies from 5cm (2 in) in the dwarf octopus (Octopus joubini) to a record of 9m (30 ft) in the Pacific giant octopus.

Pacific Giant Octopus

Squids are better adapted for swimming than are octopuses. They body is covered by the mantle, which has two triangular fins. The arrangement of the internal organs is similar to that of the octopus, except that the squid's body is elongated. Squids can remain motionless in one place or move backward or forward just by changing the direction of the siphon. Eight arms and two tentacles, all with suckers, circle the mouth. The tentacles are long and retractable and have suckers only at the broadened tips. They can be swiftly shot out to catch prey. The shell is reduced to a stiff pen embedded in the upper surface of the mantle.

Hawaiian Bobtail Squid

Cuttlefishes resemble squids in having eight arms and two tentacles, but the body is flattened and has a fin running along the sides. Cuttlefishes, which are not fish at all, have a calcified internal shell that aids in buoyancy. This shell is the ''cuttlebone'' sold as a source of calcium for cage birds.

Cuttlefish

The chambered nautilus has a coiled external shell containing a series of gas-filled chambers that serve as a buoyancy organ. The shell may be up to 25cm (10 in.) in diameter. The body, which occupies the outer, largest chamber, has 60 to 90 short, suckerless tentacles used to capture prey.

Chambered Nautilus