Back in February, I brought home a big, empty thatched acorn barnacle shell. Empty, that is, except for a colony of tiny acorn barnacles and a handful of mussels. And this worm:
I looked for it and quickly found it on the web. However, the one it looked like was an Atlantic and European species, so over the last month, I've been sporadically going back and hunting around. It wasn't until yesterday that I found it mentioned in a book of invertebrates from California to Oregon, as an import from the Eastern seaboard. Good enough; I think it's a Phyllodoce mucosa.
There is an excellent photos here, and a video showing walking style of a different species of Phyllodoce.