The octopus and its relatives, the squid and cuttlefish, are in the Phylum Mollusca .
Mollusks are invertebrates whose soft bodies are usually covered with hard shells.
Clams, oysters and snails are familiar mollusks. More specifically, however, the octopus and similar animals are in the Class Cephalopoda . This class includes the squid, cuttlefish, and chambered nautilus, as well as the octopus. The cephalopods are unusual invertebrates because most lack a hard shell. The chambered nautilus is the only type that has a complete shell.