Larvae of paddle-footed worms (Polychaeta)
The paddle-footed worms are very common marine animals and can be free-living or sedentary. The most distinguishing feature of the worms belonging to this group is the presence of paddle-like appendages with many setae arranged in bundles. Each segment of the body is identical. Some paddle-footed worms have pelagic larvae. The larvae of a few paddle-footed worms are sufficiently distinctive to be recognized, a good example is the mitraria-larvae in the image..
Video shows living Mitraria-larvae.
Video shows movement of another worm