The delicate shell of the female paper nautilus, Argonauta argo, is shown in this x-ray. The paper nautilus is a cephalopod, a type of mollusc related to the octopus and squid, and lives in warm tropical seas. The male of the species is dwarfed, typically 1 or 2 centimeters long. The female, by contrast, can exceed 30 centimeters. Only she can secrete the thin, papery shell that gives the species its name. The shell protects her developing embryos, of which there may be 50,000 at any one time.