I didn't really expect anyone to know, not even Carole
Life sized, it looks like a speck of dust in the water...sometimes it's best not to know what are those little bitty things floating in your water
It's a larval worm, a polychaete to be precise. I think they are soooooo cool. The four little ridges at the top of the worm are actually rows of cillia that beat rapidly, like rows of little oars! (poly = many, chaete = bristles) There are about 10,000 species of polychaeta and some of them are spectacular. Here's a link with some cool pictures.
Polychaeta - Encyclopedia of Life My first love is birds, but a close second is marine life