Precambrian Worms (also known as Fog Worms) are Giant Slithering Worms from Primeval. They are seen in 2.2 and 5.5 of Primeval.
In Episode 2.2, hundreds came Threw an Anomaly from the Precambrian to the present. At the beginning they fort it was a fire because Fog came Threw the Anomaly be for the worms a janitor went to check it out and was eaten alive by a worm. The arc found out when Leek told the team about a potential creature sighting so the team went to the Anomaly site to try and get trapped people out Nick Cutter and Stephen Hart opened a door and quickly realized that the fog was from Precambrian they heard a scream so they rushed to help a fire fighter who said there's something in the fog Stephen Threw a trolley at one of the worms and nick killed one with the fire fighters axe they ran out of the fog but one got the fire fighters leg as nick was about to cut the worms head of with an axe it started choking which means they can't breath outside the Fog so they must have come from the Precambrian Era where there is no oxygen not long after the fire fighter got out of the building but nick and stephen still had to get the other trapped people out so they ran to a lift and opened it but it turns out the lift had worms in it one worm knocked the axe out of cutters hand then cutter told stephen to retreat back to the stairs but Stephen got cut off and had to run another way instead stephen makes it to a lift shaft after he distracts the worms with his wristwatch's alarm. Stephen later came into a office where he had to stand on the tables and desks to stay out of reach from the worms then he fell from the desk into the fog two worms attacked but he was able to kill them both Meanwhile Jenny Lewis made it out of a lift and was attacked by a worm cutter chopped the worms head off with a sword he found on a desk then seconds later a worm caught cutter off guard and tried to eat his leg then jenny chopped that worms head off the two trapped civilians had the door of there office open by a worm and one of the trapped civilians killed the worm with a laptop then the team found the two civilians and got them out by using leaf blowers to clear the fog while the team left the building Connor temple was attacked by a worm and was be swallowed head first but cutter decapitated the worm before it killed Connor then the team got the two civilians out and tried to come up with a plain to get fog out and save Stephen. Connor temple and Abby Maitland used the ventilation shafts to reach the sever room and turn up the temperature to force the fog out of the building which they both succeed then Abby was ambushed by a worm but abby beat it to death with karate chops meanwhile cutter and jenny found Stephen as the fog was leaving the building but because of the heat the worms exploded releasing their parasitic young and they tried to burrow into Nicks,Stephens and jennys skin to find a new host cutter knew he had so bring the temperature down to kill them so he used a lighter on the sprinklers and the water killed the worms offspring.
In Episode 5.5 when Convergence started in 2011 an Anomaly with the fog and the worms opened in an infant school Abby Maitland and Hilary Becker ran in there with guns and gas marks and jumped from table to table shooting all the worms and locking the Anomaly then when Convergence was over the Anomaly presumably closed.