The class Cephalopoda includes the largest and the most intelligent molluscs. The cephalopods include octopus, squid, cuttlefish, and chambered nautilus. All are exclusively marine

Around 1,000 known species.

The word cephalopod means "head footed", and the animals are so named because the arms surround the mouth. They have a well developed head and they have their foot divided into tentacles with suckers to catch prey. They are free swimming predator

Other characteristic is that they have no external shell with the exception of one species: Nautilus, that live in Pacific Ocean and Indian Ocean. This species is very similar to other species of cephalopods that lived millions of years ago.

 

The most of cephalopods has a internal shell called pen.

Use jet propulsion to move by forcing water out their excurrent siphon

Many living cephalopods possess an ink sac capable of ejecting a dark fluid to confound predators.

Cuttlefish have the remarkable ability to change color and color patterns to better blend in with the surrounding seafloor.

They have separate sexes with internal fertilization

Cephalopods are classified according to their number of tentacles:

Orden Octopoda: Eight arms or tentacles (octopus)
Orden Decapoda: Ten tentacles (squid)