ID help

[OgreMkV]

What inverts built a tube out of sand?

I've got a piece of live rock that is very low and plate like. Something is building a tube out of sand grains up along the edge (and past now) the edge of the live rock.

The tube is about 2-4mm in diameter and about 1 cm long. I saw something in the tube, but it disappeared very quickly.

I do not believe it is a worm. The tube is completely empty, except for when I caught a glimpse of the critter just now.

 

[Amphiprion]

Could be a number of things and odds have it that it will be a type of worm. Need a pic to be sure, though.

 

[skippy2]

SPIONID WORMS: Good for your tank. They are filter feeders.

 

[Fishieness]

are you at least going to tell us what this critter looked like? Claws? Legs? Arms?
Even though you said it wasnt a worm, that is most likely. My guess is a spionid worm, mainly from size and how common they are.

 

[Amphiprion]

Without picture ID, it could be a chaetopterid, as well. Both are harmless.