Cnidarians that move do so by flexing weak, gelatinous muscles in the body walls of their bells against a pressurized hydrostatic skeleton. However, many types of cnidarians, such as mature anemones and corals, move little if at all, although juvenile stages may take a more mobile form to disperse individual organisms. Even the more mobile types, such as jellyfish, move only weakly and generally drift as plankton with the current.
Cnidarians are a very ancient group of animals organized only to the tissue level, lacking any true organs. They lack any nervous system to coordinate different parts of their bodies. Thus, they do not swim after prey or away from predators. Some swim in response to light levels and other factors, attempting to stay within an optimal range in the water column. Both swimmers and non-swimmers react on contact with other organisms or objects, but this reaction occurs starting at the stimulus, without relaying the stimulus to any central nervous system.
The hydrostatic skeleton of cnidarians is a basic water-based chamber with branches which their muscles squeeze to create movement. Since it is fluid, it cannot be used in the same ways as the solid endoskeletons and exoskeletons of other animals. Other animals, such as echinoderms and annelids, use similar methods for at least some of their movements.