Echinoderms: A hydrostatic structure
The bodies of all members work on a unique hydrostatic principle. The hydrostatic skeleton is closed fluid-filled system that terminates as a series of blind tubes called tube-feet. Each tube foot ends in a sucker. Changing the local pressure within the tube feet allows to be extended and contracted. Extensions and contractions of these tube feet occur as waves down the length of the arms (or ray) and this allows the animal to move itself and to move particulate matter down the arm. The water from this system circulates separately from that in the body cavity. It is drawn through a pore into a canal surrounding the mouth and circulated throughout the body into the myriads of tube feet. When suspended particles of food touches an arm, the tube feet fasten on to it and pass it from one to another until it reaches the groove that runs down the upper surface of the arm to the central mouth. Although stalked, sessile sea-lilies were the most abundant crinoids in the fossil records, the most common form today is the stalkless feather stars.