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How do hydrostatic skeletons support movement in earthworms and other soft-bodied animals?

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Least tern, Sternula antillarum, flying with wings raised and its yellow-tipped black bill pointed forward. — Locomotion These birds largely reside in coastal areas of North America, although some populations live in the continent's interior along large rivers.

A hydrostatic skeleton supports movement by utilizing a confined, fluid-filled internal space—such as the coelom in earthworms—to act as a firm but flexible base against which muscles can work. Because the fluid volume remains constant, the skeleton allows for the antagonistic action of circular and longitudinal muscles.12

In earthworms, this system facilitates peristaltic locomotion:

Elongation: Contraction of circular muscles narrows the body segment, forcing the fluid to extend the segment forward.324
Shortening and Anchoring: Contraction of longitudinal muscles shortens the segment, which anchors it against the burrow wall or surface via setae.324

By coordinating these waves of contraction along the body, the worm moves forward or burrows. Other soft-bodied animals, such as sea anemones and leeches, use similar principles, where fluid pressure allows muscles to change the body's shape to facilitate crawling, creeping, or burrowing. In these organisms, the hydrostatic skeleton is essential for movement because it provides the necessary resistance for muscular force in the absence of rigid skeletal structures.325416