Cephalopods may be divided morphologically into:

ECTOCOCHLIAN FORMS

• have external shells
• encompass the vast majority of
  fossil cephalopods
• for example, Nautilus

Figure 4: A Nautilus pompilius.

ENDOCOCHLIAN FORMS

• have internal shells or other solid internal
  structures, although they include forms without
  solid internal parts; for example, an octopus
• most living cephalopods today are endocochlians whose fossil relatives include the Belemnites