On Chiton granosus https://www.inaturalist.org/observations/142092021 Position of worm by the chiton's mouth suggests that it's a kleptoparasite, stealing food from the chiton.
Observations made by the Biodiversity Ilo team from the Florida Museum of Natural History (FLMNH) & the Natural History Museum of Los Angeles (NHMLA). Photos by Jessica Whelpley (FLMNH), Brittany Cummings (FLMNH), Leslie Harris (NHMLA), CC-BY-NC-SA, credit to Biodiversity-Ilo.