At home, I dumped the lot into a bowl of seawater, removed the live eelgrass, and left the broken roots for later. When I went back to them, I saw movement in the water. Something tiny, vibrating, attached to all the roots. Here they are:
Skeleton shrimp, Caprellids
I had read about these; I never expected to see any. I find them fascinating, and I'll be looking for another batch next low tide.
There is more info on Caprellids, and clear photos, at the Monterey Bay Aquarium site, on Intertidal MarineInvertebrates of the South Puget Sound, and at RaceRocks; Wikpedia has a brief note. And there's a whole page of photos in the Starfish collection.
I was so tired by the time I was done, what with Picasa scrambling the almost-finished video, so that I had to start from scratch, that I misspelled my own name at the end! And I identified the skeleton shrimp as Caprella equilibra, following Kozloff's diagram, but later I realized they could also be Caprella laeviuscula, which is also common here (Map). (And looks about the same to me.) And I think that the brood chamber is one, made of two leaves. but I'm not going to re-do the whole thing again. Not now, at least.