This polychaet is probably Typhloscolex sp. It has paddle-like dorsal and ventral cirri and as a holoplanktonic organism it can swimm very well. (Holoplanktonic animals spend their entire life cycle in the plankton, whereas meroplanktonic organisms are living in the plankton only a part of their live as eggs and or/as larvae before they settle on the bottom).
The polychaet was 1000 µm long. I photographed it without any eyepiece (therefore the illumination is not very regular).
Images made through a microscope. All subject types.