How does a coelom help an earthworm move?

The coelom acts as a hydrostatic skeleton that is essential for an earthworm's movement and burrowing. Because the coelom is a fluid-filled cavity, it provides an incompressible medium against which the earthworm's muscles can work.12

In earthworms, the coelom is partitioned into successive segments, a feature known as metamerism. This partitioning is critical for locomotion: when circular muscles contract, they narrow a segment and force the coelomic fluid to elongate it; when longitudinal muscles contract, they shorten the segment. By coordinating these contractions across different segments, the worm creates waves of peristalsis that allow it to crawl or burrow. Without these partitions, the fluid would simply flow to other parts of the body, providing little force for movement. Essentially, the coelom allows the worm to anchor itself against the substrate by widening certain segments while others elongate to push the body forward.12