Ambulacral Grooves, Tube Feet and the Water Vascular System:
The water vascular system is the major unifying characteristic of the phylum Echinodermata. Water is taken in by the madreporite on top of the seastar and flows into the central ring canal and connecting tubes that run out into each ray. The tube feet (or podia) are extensions of these canals, that penetrate the seastar body wall and skeleton in zones known as the ambulacral grooves (seen here). Every tube foot is attached to a baloon-like structure called an ampula. The muscular ampula works like a squeeze bulb, forcing water into the tube foot, causing it to straighten and extend. The ampula then relaxes and the tube foot contracts, pulling the sea star forward. By coordinating hundreds of tube feet, the sea star can move along while keeping a strong grip on the rock.