Polychaetes are worms with a distinct head region and segmented 'trunk' which is lined with bristles called chetae. Although primarily marine, some do inhabit freshwater environments. Although the body of worms don't fossilize particularly well, they are well represented by their burrows, tubes, and sediment traces throughout the fossil record since the Cambrian Period (+500 million years ago).
Howell, B. F., 1962. Worms in Moore, R. C., ed., Treatise on Invertebrate Paleontology, Part W, Miscellanea, Volume 1. The University of Kansas and Geological Society of America. 259 pp.