[Click on the VIMEO link for better quality video] 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 some periodic cycle. 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). Apparently, some star species have an anal aperture at the top side. This is likely what the hole is.
The aperture is lined by a membrane which opens up just below the exterior minute fingers surrounding the hole.
Notice the tiny conical snail shell slightly off centre.
More on sand stars !...
http://www.wildsingapore.com/chekjawa/text/p610.htmhttp://www.wildsingapore.com/chekjawa/text/s603.htmhttp://mangrove.nus.edu.sg/pub/seashore/text/221.htm
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