One tiny creature that is often asked about is the Peanut Worm. It can extend out several inches as it seeks food late at night. Once it detects any light at all, it will quickly retract within itself (like laundered socks turned inside out) until it is deep within its protective nook. Once fully out of sight, the hole it bored out will be perfectly round. I've seen many of these over the years, but have never captured a good picture. In this image, most of it has already withdrawn into the rockwork. For a sense of size, note the closed up zoanthid polyps in the upper left corner. Reef-safe.
On a towel, out of water, fully retracted.
Next I put it in the tiny Calcium test vial from my Salifert kit since it was handy, and shot a few pictures.
Normally, I've only seen the striped section in my rockwork, and when light hit it, it quickly retracted. This one didn't seem to mind the light at all, and I got to see what looks like a flowery polyp poke out every so often. It is obviously the mouth, but I've never seen it before. Neat!
Here are a couple more pictures of the peanut worm without the feathery tip, the way I've always seen them.
I put him back in my tank, to encounter it again in the future.