Came across this sand star (amongst the thousands at Cyrene) which had a peculiar vent at its top. It was opening and closing the hole as if on a cycle of sort. It looked like it was passing water through. This vent was not the madreporite as you can see the madreporite a little to the left. (someone correct me if I am wrong here).

There seems to be a membrane which opens up just below the exterior.
Notice the tiny conical snail shell slightly off centre.

More on sand stars !...
wildsingapore.com/chekjawa/text/p610.htm
wildsingapore.com/chekjawa/text/s603.htm
mangrove.nus.edu.sg/pub/seashore/text/221.htm