|Not a polychaete. A gill.|
Example # seventy-umpteen for my list, "Why I Love Blogging". There's always someone with better information, always something new to learn.
In the comments, yesterday, Neil Kelley suggested that the "worm" may actually be a gill. He supplied a link to a diagram I had not seen.
|Ventral (underside) view of a true limpet, with part of the foot cut away to show the gill. (Ignore the two critters at the left.) (Marine Life Information Network, Fig. 18)|
All true limpets have gills. (Wikipedia) They take different forms, sometimes forming a ring around the body, sometimes, as here, one pointed, feathery extension, which may at times be visible behind the foot.
|Another anatomical diagram, this one from the top, so the gill extends to our right. Image from R. Fox, Lander University.|
Other diagrams have not been as clear as these, usually showing the gill squished in alongside the other internal organs.
And I finally found a few photos of limpets with the gills showing. (It's amazing how knowing what to plug into Google turns up what you're looking for!) All of the photos are small, and mostly vague, but here's a clear one. The photo is for sale, so you'll have to click on the link.
|Photo from Seashells of NSW.|
So that's cleared up.
There's still that first worm to identify.
And a bonus: on Google+, Chris Mallory sent me a link to a page about another commensal scale worm in a limpet. The limpet is a keyhole limpet, quite distinct from the true limpets, but here's another instance of the benefit a limpet can receive from hosting a worm:
When Pisaster ochraceous* attacks Diodora aspera** in which this species is living, the worm moves around the pallial groove of the limpet to the side the seastar is attacking from, reaches out, and bites the tube feet or ambulacral area of the seastar. This frequently results in the seastar withdrawing from its attack.*A starfish
** The keyhole limpet