have also developed jet propulsion, passing water through the gills to supplement fin-driven motion. In some dragonfly larvae, jet propulsion is achieved by the expulsion of water from a specialised cavity through the anus. Given the small size of the organism, a great speed is achieved. Scallops and cardiids, siphonophores, tunicates (such as salps), and some jellyfish also employ jet propulsion. The most efficient jet-propelled organisms are the salps, which use an order of magnitude less energy (per kilogram per metre) than squid. See also Jet aircraft References Energy conversion Aquatic locomotion Propulsion