Class: Cephalopoda Collection Highlights | Updated 7 years ago Hapalochlaena sp. Photo by Clay Bryce, image copyright WA Museum Cephalopods, whose name means "head-footed”, have a well-developed brain and also complex and very efficient eyes. Cephalopods are considered the most intelligent invertebrates and are very social. Around the mouth is a ring of eight arms and, in some groups, a pair of tentacles. The mantle cavity is muscular and, from it, water can be ejected through a funnel for “jet propulsion” – where they can move up to 50 meters. There are two main groups of cephalopods, one with an internal or absent shell (such as octopus, squid and cuttlefish), and the other group has an external shell (such as the nautilus). The sexes of cephalopod species are separate. Mollusc (Malacology) Section View the discussion thread.