Tidepool TIm says,  "Plankton worms look part jellyfish, part centipede, and part sea slug.  They literally walk through the water as they move along. Their parapodia (feet) have two large lobes at their ends - the feet grip the water as they move along like a centipede.   They also have 2 crazy long cirri which are their tentacles - these can be almost as long as the entire worm. Sometimes they also have a long tail. 

Since they feed up in the water column they have a transparent coloration that keeps predators from eating them.  We catch these at night or on dark hazy days when we can see them in the water with polarized glasses.  When they are feeding there can be hundreds in a small area - this may be due to some seawater upwelling a food source that brings them up to the surface. They make remarkable moves in the water as they writhe around hunting their prey of other zooplankton.  In a small aquarium, these are amazing to illuminate with a black light - the colors are unbelievable!”