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How do tube feet extend and retract in echinoderms?

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A common comet star and a cushion sea star rest on colorful sponges and seagrass in a vibrant Caribbean underwater scene. — echinoderm Starfishes.

In echinoderms, the extension and retraction of tube feet are managed by the water-vascular system, an internal hydraulic network of fluid-filled canals and reservoirs.12

Extension

Tube feet are extended through the movement of fluid into the podium (the tube foot itself). This process is driven by the contraction of localized muscle areas in the radial canals (as seen in ophiuroids) or, more commonly, by the contraction of ampullae—small, bladder-like sacs connected to the tube feet. When an ampulla contracts, it forces fluid into the tube foot, causing it to elongate. In many species, a valve in the lateral canal closes during this process to ensure the fluid is directed into the podium.123

Retraction

Retraction is achieved through the contraction of the longitudinal muscles located within the walls of the tube feet. As these muscles contract, they pull the tube foot inward. Simultaneously, the fluid is withdrawn from the podium as the ampulla relaxes, allowing the tube foot to swing forward in preparation for another step.23