Practice question

Tube feet or podia operate through hydraulic pressure rather than simple muscle shortening. Each unit comprises external podium with suckered disc and internal bulbous ampulla inside coelom, interconnected via lateral canal. Contraction of ampullary muscles pressurizes fluid, everting and elongating podium; longitudinal and retractor muscles of podium contract to withdraw it, creating vacuum adhesion. This hydraulic amplification converts small muscular forces into powerful traction for slow crawling, clinging during wave exposure and opening mollusc shells. Ciliary beating or flagellar action cannot produce comparable force, highlighting evolution of water-vascular hydraulics.