Final answer:
Worms move using a hydrostatic skeleton, where fluid pressure within their coelom aids in peristaltic muscular movements to extend and contract their body, allowing locomotion.
Step-by-step explanation:
A hydrostatic skeleton is a flexible skeleton supported by fluid pressure. In the case of a worm, movement is achieved through the process of peristalsis, a series of muscular contractions. Worms extend their body by contracting longitudinal muscles, which increases the pressure of the fluid in their coelom, pushing the anterior end forward.
They then anchor this part and contract circular muscles to pull the posterior end up. This mode of movement is effective for invertebrate organisms like earthworms and some aquatic animals, which lack hard, jointed skeletons.