(June 2013). A hydrostatic skeleton or hydroskeleton is a structure found in many ectothermic organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles.ydrostatic skeletons have a role in the locomotion of echinoderms (starfish and sea urchins), cnidarians (jellyfish), annelids (earthworms), nematodes, and other invertebrates.