Cephalopod, a marine animal. Squids, octopuses, argonauts, nautiluses, and cuttlefish are all cephalopods. Their bodies are more or less cylindrical. The mouth is on the lower surface and has sharp, beaklike jaws, surrounded by flexible, muscular tentacles or arms. The eyes are well-developed. Cephalopods are the largest of the invertebrates (animals without backbones).

Cephalopods make up the class Cephalopoda of the phylum Mollusca. Living cephalopods are classified in the following orders:

Nautilida.

Decapoda.

Vampyromorpha.

Octopoda.