ID worm-like, not bristle worm

[dendro982]

What could it be - in the rock it looks like closed button polyp, on the glass it moves, like worm. Both ends seems to be active. Good or bad, what should I do with it?

Thanks.

 

[venwu225]

http://en.wikipedia.org/wiki/Sipuncula

Peanut worm.

 

[LeslieH]

Sipunculida, genus Aspidosiphon or something related. It's a detritivore and beneficial.