Name this sand dweller

I found this animal under a rock, partly buried in the sand, When I dug down to get a better look at it, it disappeared, so I brought home the whole handful of sand. Dumped it in a bowl, added water from the bay, and two of them showed up.

Enough background; can you guess what this is?

Stripy tail.

The entire animal stretches to about an inch or an inch and a half, but as often as not, is contorted in weird lumps and twists, to make a half-inch knot of itself.

I'll post the id in a couple of days. And photos.