Cephalopods belong to the phylum Mollusca, an invertebrate group characterized by a shell, a muscular foot, internal gills, and a mantle that encloses body parts. Ancient cephalopods (represented by fossils) are ammonites and nautiloids; modern cephalopods are nautiluses, octopuses, and decapods, which are divided into squid and cuttlefish. In modern representatives of the group, the muscular foot has transformed into arms and/or tentacles. Nautiluses have more than ninety arms, octopuses eight arms, and squid and cuttlefish eight arms and two tentacles. The external shell that characterized early cephalopods is present only in nautiluses. It has been totally lost in octopuses; however, squid have an internal structure embedded in their mantle wall, and cuttlefish have a semi-bony chambered structure called the cuttlebone. These details and more can be found in the classic guide to mollusks written by C. M. Yonge and T. E. Thompson, Living Marine Molluscs; its last chapter deals with cephalopods.
Cephalopods are found only in marine environments. Nautiluses live only in the southwest Pacific Ocean. Octopuses, which are bottom dwellers for the most part (though a few have transitioned to open water), are found worldwide. Squid and cuttlefish are distributed across the open water of all oceans.
Although cephalopods live and hide in the water, humans have long known about them. The accuracy of knowledge about them, however, has varied; Aristotle thought octopuses were dumb. Cephalopods have made their way into fiction, nonfiction, and film as well. Herman Melville devoted an entire chapter of Moby-Dick to them. Recent work on octopuses—including Sy Montgomery’s nonfiction bestseller Soul of an Octopus, Shelby van Pelt’s novel Remarkably Bright Creatures, and the television documentary My Octopus Teacher—have brought these creatures into living rooms.
