Late Cambrian
Weeks Shale Formation
House Range UT, USA

133mm if uncurled worm on 100mm slab.
We think of worms as members of the same group but they are too diverse for that especially when fossil forms are examined.  Many are only distantly-related to each other.  Some belong to separate phyla.

This worm lived at a time before there were fish with jaws and before nautiloids and eurypterids rose to power but there were still a lot of animals that ate worms.

Click the bottom picture to see a video of what living polychaete worms do.