I find that I brought home a pair, both under 4 inches long, from the White Rock beach, in the first week of October, last year. Digging through the sand, then, I found a worm that stretched out to 7 inches long.
It's been almost seven months since then. Those three have turned into eight, at least twice the size of the biggest one, back then, plus a few more 4-inch specimens.
I had narrowed the species down to Nereis, possibly vellixosa, which grows to about 6 inches. I'll have to revise that. I think, now, that they are Neries vixens, recently renamed Alitta virens, or possibly A. brandti, which is also common and virtually indistinguishable from vixens. Both of these, the Giant piling sea-nymph and the Giant clam worm, grow to at least a foot, like mine.
One thing that confused me is that the parapodia on these big worms are different from those on the smaller ones. Those are like pencil points with hair on the end; these are triangular paddles, with no hair visible.
Last October's worms, with hairy pencil points. The smaller ones I have now are like these.
A website from Wales reassured me that I was on the right track; their A. vixens, which they call King Ragworms, have paddles, although near the head end, the hairy points are still visible. (Look at the middle of the second worm for the paddles.)
I found the explanation in Kozloff. He writes, about N. vexillosa,
"... the sexually mature phase of this species is rather unlike the phase typical of mussel beds and bay habitats. The fleshy parapodia become expanded into paddlelike structures for swimming, and periodically during the summer the ripe males and females swarm at night near the surface. ... The worms do not survive long after their nocturnal orgy, during which they simply spew out their eggs or sperm through openings that develop in the body wall."The Giants swarm, too. Kozloff says it's an "exciting spectacle.I can imagine; think of yesterday's video, but with dozens of worms in the open water, thrashing about in the moonlight.
So I've sent my critters out into the big world, just in time to !!PARTY!! I'll have to keep an eye on the few that I kept at home.
I found a couple of interesting head shots of these worms; interesting, if not exactly cute. PZ picked up one from an underwater image competition, back in 2006. The original site is no longer available, but the photos are still on Pharyngula. The third one down is Nereis.
And Ugly Overload has two face shots, from Alexander Semenov. This second link goes to his Flickr page; it's worth clicking over there to see the rest of his macro photos. Amazing and wonderful!