page_title: Aquatic locomotion
section_title: Invertebrates
hierarchical_section_title: Aquatic locomotion / Invertebrates
caption_attribution_description: English: Daphnia pulex, a species waiting in the wings to achieve “Model” Status.
context_page_description: 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, molluscs, reptiles, birds, and mammals.
context_section_description: Among the radiata, jellyfish and their kin, the main form of swimming is to flex their cup shaped bodies. All jellyfish are free-swimming, although many of these spend most of their time swimming passively.  Passive swimming is akin to gliding;  the organism floats, using currents where it can, and does not exert any energy into controlling its position or motion.  Active swimming, in contrast, involves the expenditure of energy to travel to a desired location.
  In bilateria, there are many methods of swimming. The arrow worms (chaetognatha) undulate their finned bodies, not unlike fish. Nematodes swim by undulating their fin-less bodies. Some Arthropod groups can swim - including many crustaceans. Most crustaceans, such as shrimp, will usually swim by paddling with special swimming legs (pleopods). Swimming crabs swim with modified walking legs (pereiopods). Daphnia, a crustacean, swims by beating its antennae instead.
  There are also a number of forms of swimming molluscs.  Many free-swimming sea slugs, such as sea angels, flap fin-like structures. Some shelled molluscs, such as scallops can briefly swim by clapping their two shells open and closed. The molluscs most evolved for swimming are the cephalopods.
  Among the Deuterostomia, there are a number of swimmers as well. Feather stars can swim by undulating their many arms . Salps move by pumping waters through their gelatinous bodies. The deuterostomes most evolved for swimming are found among the vertebrates, notably the fish.
caption: Daphnia swims by beating its antennae
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