Locomotion in Echinodermata
Locomotion in echinoderms is accomplished through a unique canalicular system which is termed as the water-vascular system. This system is characteristic of echinoderms, in most of that it is quite well developed. It consists of a series of canals derived from the coelom. The canals consist of a fluid. Sea water has free access to the system via a perforated sieve plate called the madreporite on the aboral side of the animal. The fluid is much more like seawater but has a high content of potassium ions. Some proteins and several types of amoebocytes float in it. The structural organisation of the water-vascular system is chiefly the same in all echinoderms, though deviations from the basic plan occur. To study the components of the system, we might take the water-vascular system of the star-fish, as typical example. Its main component applicable from the point of view of locomotion consists of a pentagonal circumoral vessel that is termed as the ambulacral ring canal. From the ring canal are given off five radial vessels. Every radial canal extends up to the tip of the arm. It gives off a large number of side vessels along its course. Each side vessel leads into a tube-foot or podium, which is a hollow conical or cylindrical process with an ampulla and a terminal sucker. A Valve is present at the junction of lateral vessel with the tube-foot.