Hydrostatic skeleton

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A hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded 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 muscles are used to change an organism's shape and produce movement, such as burrowing or swimming. Hydrostatic skeletons have a role in the locomotion of echinoderms (starfish, sea urchins), coelenterates (jellyfish), annelids (earthworms), nematodes, and other invertebrates. They have some similarities to muscular hydrostats.

Sea anemones and earthworms do not have a single bone in their bodies. Instead, they are supported by pressure from a liquid which consists mainly of water in their cells and in spaces between their body. The hydrostatic skeleton allows the earthworm to burrow through the earth.