My favorite quote from this case:
"I don't know what this is, but that picture of the cooked cod with the worm in it is. . . . .disturbing."
Most likely identification: Pseudoterranova decipiens. This organism is usually found in cod, giving it the name "cod worm". It is frequently found in the flesh of fish; most commonly the belly flaps.
Another possibility is the "herring worm": members of Anisakis simplex complex, which are found in a variety of fish, including herring.
If the fish (and the worms) are thoroughly cooked, then ingestion will not cause human disease. However, they cause the diner considerable distress if discovered during a meal (like in this case), and thus, the fish handlers take steps to identify and remove parasites prior to sale.