Hydrostatic skeleton

A hydrostatic skeleton or hydroskeleton is a structure found in many 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.