Movement of Earthworm

Earthworm have a hydrostatic skeleton 1m

The body wall has both longitudinal and circular muscle which
act antagonistically

The contraction of the circular muscle and relaxation of the
longitudinal muscles cause the segment to extend.

Chaetae are extended to grip the soil / ground

The contraction of the longitudinal muscle and relaxation of
the circular muscle cause the segment to shorten.

Contraction and relaxation of these muscles causes the
transfer of hydrostatic pressure from the anterior to the
posterior.

It causes the body to move to the front. 

The chaetae are retracted to allow movement