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. The pressure of the fluid and action of the surrounding circular, longitudinal, or helical muscles are used to change an organism's shape and produce movement, such as burrowing or swimming. They alternately contract and expand their body segments along their length. Some examples are soft-bodied animals such as sea anemones and earthworms. Hydrostatic skeletons have a role in the locomotion of echinoderms (starfish and sea urchins), cnidarians (jellyfish), annelids (earthworms), nematodes, and other invertebrates. They have some similarities to muscular hydrostats.
University of Massachusetts researcher Diane Kelly documents a non-helical hydrostatic skeleton structure as the functional basis of the mammalian penis which must function similarly to a rigid element in use. Helically reinforced hydrostatic skeleton structure is typical for flexible structures as in soft-bodied animals.