Comments on this page refer to living Octopoda. for comments on extinct Octopoda see: Teudopseina and Incirrata.

Octopods have rather short, compact bodies and only eight arms; no trace of the missing second arm pair remains even during embryonic development. Many species are benthic (bottom-living) and crawl over the ocean floor with the mouth facing the substratum. Others alternate between a benthic and a pelagic (free-swimming) habitat and some species are completely pelagic. The two suborders of Octopoda are very different in appearance but there is little doubt that it is a natural group as the monophyly of the Octopoda is supported by a large variety of characters. The Cirrata is a group of deep-sea octopods commonly known as the "finned octopods" due to their large, wing-like fins. The Incirrata contain the common (benthic), shallow-water octopods as well as many deep-sea benthic and pelagic species.

Brief diagnosis:

An octopodiform ...

• with one arm pair, presumaably true arms II, absent.
  
• with suckers on proximal halves of arms.