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Cephalopod molluscs are a branch of related marine invertebrates. They include octopus, squid, cuttlefish, nautilus and the extinct ammonites.

"Cephalopod" comes from the Greek Κεφαλόποδ (kephalópod), meaning roughly "head-foot." Univalve molluscs include a muscular "foot"; in gastropods such as snails, the foot is found on the bottom, near the stomach, whereas in cephalopods, it is part of the head.

Some cephalopods, such as octopus, are widely considered to be among the most intelligent marine invertebrates. Octopus and cuttlefish are capable of camouflaging themselves to match many complex environments, a behavior which aids them in predation. Octopus are also capable of planning ahead and performing feats of dexterity such as opening jars. Some marine biologists claim that other cephalopods, such as Humboldt squid, may be capable of communication.

The giant axons of squid were used by early biophysicists to uncover the manner in which nerve signals propagate along nerve cells. Since the axons have a very large diameter—the biggest known of any invertebrate—they were an ideal model system to use in studying this phenomenon.