General Organization of Recent Cephalopods. Extant cephalopods with the exception of Nautilus have an internal shell covered by muscular mantle. Only Nautilus is able to withdraw into its shell and close the aperture with a hood. The internal shells of Sepia and Spirula are mineralized and have chambers that function in buoyancy control (Denton & Gilpin-Brown, 1961, 1971; Denton et al., 1961, 1967) while other teuthids and Vampyroteuthis have an unmineralized internal skeleton that acts as an elastic support to the soft body.