Echinoderms use tube-like feet that protrude through their bodies to move, contracting and expanding these muscular structures by taking in and expelling water through an organ called an ampulla. This muscular action is very slow and unfolds over time as the ampulla expands and contracts depending on levels of water, making echinoderms like starfish and sea urchins slow to move.
Water from the ampulla finds its way into the tube-like foot muscle, which is hollow, when flexion deflates the ampulla. When the tube foot moves, the water is forcibly ejected back into the ampulla so that the process of movement through fluid distribution can begin again.
Tube feet function as suction cups, creating vacuums where their cup-like ends touch ground and allowing the echinoderm to push itself forward before disengaging. Echinoderms have hundreds of tube feet and can use them all in concert to direct themselves across the sea floor or even to navigate small seabed obstacles like rocks and debris.
The echinoderm's endoskeleton provides the rigid frame needed for the tube foot system to be effective. This is because the tube foot system generates a huge amount of resistance and must be able to push back against something unyielding in order to move the echinoderm.