|Fifteen-scaled worm, Harmothoe imbricata|
I've counted and counted: I can only find fourteen scales on either side, but the tail end has been damaged, so some may have fallen off.* The scales are like little round, overlapping plates that protect the whole worm, right up to the 4 eyes. They may help in swimming, which these critters do much more gracefully than other worms.
These are free-roaming worms, sometimes crawling, sometimes swimming. I find them on clamshells, on stones, on eelgrass, out hunting. They eat amphipods and anything else small enough to be subdued. The largest I have seen is about an inch long.
*There's a beautiful photo of one of these on Flickr. And this one has the same problem with disappearing plates: I count 15 on one side, 13 on the other.