What is the body cavity of an Echinoderm like?

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.