Echinoderms are radially symmetrical. This means they basic body plan is like spokes on a wheel. Each arm contains the water-- filled vascular system which connects to tube feet on the bottom of each arm. These tube feet work like suction cups and are for locomotion as well as used as sensory organs, for feeding and for respiration. Inside each arm are digestive glands and gonads. In the center is a mouth through which their stomach can be everted. The arms enable to put pressure on bivalves' shells which are their prey, forcing them to open the shells so that the echinoderm can eat the soft bodied bivalve. They have sinuses and open channels in the body cavity with flagellated cells that create a current and help in circulation. There is no specialized excretory system.