How Do Xenia Pulse?

I'm sitting here recovering from surgery observing my tank. My Xenia is swaying in the current and happily pulsing away. Back and forth it sways opening and closing its polyps over and over. Its my understanding that nobody knows why it does this. What I want to know is how it does this. Does it have muscles? Does it somehow change the hydrostatic pressure in its polyps to open and close? Its amazing how tightly it can close its polyps when it wants. This happens when it is disturbed by my shrimp. A Google search has revealed nothing about the mechanics of how it does this. I would like to know how something without a brain can do this. Send me a comment if you know the answer or have a theory.