The Fog Worms are a species of giant, fog-dwelling, prehistoric worm that appeared on primeval episode 2.2 and 5.5.
They had features of the priapulid predator worm Ottoia prolifica, i.e. an extensible proboscis with teeth. Their body looks rather like a sea cucumber. In the Cambrian, priapulid worms grew to much bigger size than nowaday though we don't have fossils of any the size shown in the show and they were 1 of the dominant predators.
The worms originated from the Precambrian, and can only breathe in sulphur gases given off in their environment. The worms were blind but were very sensitive to vibrations. They are carnivorous, eating things as large as humans. They reproduce by exploding, with several juveniles coming out of the mess of blood and guts.
Their spit was a black tar like substance which might help locate prey though this is not proven. They had rows of sharp teeth on their tongues. They can't survive in an oxygenated atmosphere and they explode when they come into contact with a mix of oxygen and heat. Their young are inside their bodies and may be parasites, as they made a fleeting attempt to burrow into the team's skin.