The Class Cephalopoda contains about  200 species of marine squids, cuttlefishes, octopi and the chambered nautilus. They range in size from a few centimeters to the giant squid over 20 meters in length and weighing over 250 kg, making it the largest invertebrate!

Cephalopods are called the "head-foots" because the  foot is concentrated in the head region, where its edges are drawn out and modified into arms and tentacles equipped with suckers.  Part of foot is also modified to form a funnel (siphon) for expelling water, allowing them to move by "jet propulsion". The funnel is mobile and can be pointed either forward or backward to control direction.  Speed is controlled by the force with which the water is expelled through the siphon.  Except for nautiloids, cephalopods have one pair of gills, which lack cilia.  Instead, water is brought in by radial muscles that expand the mantle wall and expelled by circular muscles that expel the water through the siphon.  A system of valves insures a one-way flow of water over the gills.