This is a marine worm called Sabellaria cementarium. The species name, "cementarium," refers to this worm's habit of cementing together sand grains and pieces of shell to form a tube that it lives in. You might see these tubes attached to rocks and shells in the intertidal and subtidal zones around Bodega Head. (The tubes we found this weekend were ~20-30 mm long.)
The worm has an operculum or a "trap door" to seal the tube when pulled in; the operculum is shiny gold in the photos.
Notice the abundant black speckles and the bright red stripes.
From some angles, iridescent blue lines also appeared. And a beautiful green hue was visible at the base of the tentacles.
The next view shows an entire worm outside of its tube. I photographed it this way to show you how striking this species is, but it might make you feel better to know that they can rebuild their tubes.
Here are two close-ups showing the tentacles that are used for feeding.
The tentacles have cilia (hair-like structures) that move food particles towards the mouth.
I've read a little bit about Sabellaria cementarium during the last couple of days, but I've yet to find a description that mentions these amazing colors. This is quite a worm! Maybe the next time you see a small sandy tube attached to a rock at your feet, you'll imagine the colorful worm inside!
P.S. Sandcastle Worms (Phragmatopoma californica) look similar to Sabellaria, but they have a black operculum rather than a golden operculum — see photos here.