Cirrate octopods swim by a combination of fin action and medusoid propulsion by the arm/web complex. The fins of cirrate octopods are associated with a unique cartilage-like shell in a shell sac. Whereas Stauroteuthis maneuvers slowly by sculling with the fins, Grimpoteuthis swims primarily using powerful fin strokes. In Stauroteuthis, the mantle is extensively modified. The secondary web in some cirrate species results from a modification of the way the web muscles attach to the arms. The more benthic opisthoteuthids lack this modification. The secondary web enables larger volumes of water to be trapped in the web in some postures. The entrapment of water resulting in a bell-shaped posture in Stauroteuthis could be related to predator defense or to feeding.