Critter ID Please

[JOSEPHLB]

Got a 10g setup for seeding of some of my DIY rock. Went to the LFS and purchased three pounds of small rubble. Its been setup for one week now.

I've noticed this little larva like object. It moves around and swells up like it wants to bust open.

http://i68.photobucket.com/albums/i24/JOSEPHLB/P1000489.jpg

[casingbill]

maybe a peanut worm....just not in the rock

[blot60]

Looks like a Peanut worm

[Paintbug]

looks like a peanut to me as well.
http://www.ronshimek.com/Animal%20Groups%202d%20Sipunculans.htm