Final answer:
Segmented worms such as earthworms use hydrostatic pressure in the coelom to stretch and elongate their bodies through a process known as peristalsis.
Step-by-step explanation:
In contrast to roundworms, segmented worms such as earthworms elongate or stretch lengthwise through hydrostatic pressure in the coelom. This is due to muscles surrounding the coelom that contract, changing the shape of the coelom and thus, due to the pressure of the fluid within, producing movement. This process is known as peristalsis, where waves of muscular contractions of the body wall's skeletal muscle alternate to shorten and lengthen the worm's body. As the segmented worms lengthen, they extend their anterior end forward, and as they contract, they pull the posterior portion of their body forward.