Eric,
I was talking about the Vermetid (worm) snail. He sucks the web back in when he is disturbed or ready to eat. His home appears to have a flap like top. (think toilet flapper) LOL

I had what looked like the Spaghetti Worm in my other tank, but the body stayed in the sand or rocks, and only the feelers came out at night to catch whatever was on the rocks or floating by. It was neat to watch and never bothered anything in my tank.