Aquatic locomotion
![The flagellum of a Gram-negative bacteria is rotated by a molecular motor at its base](/Images/godic/202412/21/Flagellum_base_diagram_en.svg4148.png")
![Shrimp paddle with special swimming legs (pleopods)](/Images/godic/202412/21/Penaeus_diagram_pleopods4148.png")
![Daphnia swims by beating its antennae](/Images/godic/202412/21/Daphnia_pulex4149.png")
![Octopuses swim headfirst, with arms trailing behind](/Images/godic/202412/21/Octopus34149.jpg")
Aquatic locomotion is biologically propelled motion through a liquid medium. The simplest propulsive systems are composed of cilia and flagella. Swimming has evolved a number of times in a range of organisms including arthropods, fish and molluscs.