A hydrostatic skeleton is an exoskeleton that maintains its form due to the pressure of fluid inside the skeleton. The "coelom" is the term for the cavity that holds the fluid; it works in conjunction with nearby muscles to keep the skeleton's shape. Without the fluid's pressure, the body would collapse.
Hydrostatic skeletons, also known as "hydroskeletons," are found in organisms with soft bodies, such as earthworms, spiders, jelly fish, sea anemones and star fish. When the organism dies, the pressure is no longer maintained and the body curls.
The fluid's pressure and the surrounding muscles also work to provide movement in some creatures and permit other organisms to change the shape of their bodies.