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The Order Teuthida contains the squids and, like all other cephalopods, they use jet propulsion for locomotion. However, they have evolved a number of features that increase the efficiency of this system. During normal swimming, the mantle cavity is expanded by elastic recoil of stretched collagen fibres extending through the mantle wall; these are then contracted rhythmically forcing water out through the hyponome. This differs from fast escape swimming, when circular muscles are used to hyperinflate the mantle cavity, increasing its volume and thus the amount of water that can be expelled. This allows some species of squid to reach speeds of 40km/h, the fastest of any marine invertebrate. The squid's body is highly adapted to this mode of locomotion; it is long and tapered posteriorly, with a pair of lateral fins that can be used as rudders, for stabilisation and even propulsion at low speeds.

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