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 sulphorous gases. given off in their environment. The worms were blind but were very sensitive to vibrations. They were carnivorous.Their spit was a black tar like substance. 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.
An anomaly opens from the Precambrian to the present, in a new skyscraper in the centre of the city. It has quickly developed a reputation for teething problems in it's electronics, so no one is surprised when smoke is found creeping out from the server room. On inspection, a janitor finds a thick layer of mist hugging the floor and a foul sulphurous smell - but no fire. he also discovers an anomaly inside a room, where the mist is coming from. Then the Worm can be seen slithering by in the mist and the man suddenly loses his footing as something unseen attacks and drags him out of sight screaming as the mist continues to spread.
Meanwhile at the skyscraper, panicking staff flee as the sinister mist oozes down the stairs and lift wells, accompanied by a foul stench of rotting matter, setting off the fire alarm. A fire crew arrive and enter an office waist high in sulphurous fog. They are attacked by the Worms; we see tantalising glimpses of these odd, frightening beasts that seem to live inside the mist. The skyscraper is evacuated but in one glass-walled meeting room a group of executives refuse to leave, as they are in a middle of an important video conference that will decide the future of their company. The man leading the company tells the Japanese buisnessman he is having the video conference with that the alarm is just a drill and continues with the meeting. They cannot yet see the mist enveloping them...
By the time the team arrive the skyscraper is filling with fog and a fireman has been horribly killed. While they explore offices full of abandoned chairs and desks they discover that there are giant carnivorous worms moving inside it. Leaping from desk to desk they only just manage to escape with their lives. Jenny, who has been appointed as operations manager on the ground (reporting directly to Leek and Lester) is given her first taste of her new job and nearly breaks down with terror when she realises what she is dealing with (after Cutter narrowly rescues her from one of the creatures). But she recovers, showing real guts in the process and Cutter cannot help being impressed, even though the two of them find themselves in almost constant opposition. Meanwhile, Cutter tasks Abby and Connor with obtaining equipment to clear fog: after Oliver Leek's tour of the armoury proves of little use, the pair decide to obtain leaf-blowers from a nearby garden centre.
Meanwhile, the executives finally smell the sulphurous gas. One of them leaves to investigate, but much to the horror of his colleagues, a few moments later he is killed by the worms. The team and a firewoman find the executives. While trying to save them and evacuate them from the building, Connor nearly gets eaten by a giant worm. He and the rest of the crew are walking down the stairs when he says, "I think we're all right now." Then, a huge worm comes from the ceiling and swallows his head and Cutter, with a katana that he snagged from a display, cuts the worm in half to save Connor. While washing the worm's saliva off, Connor accidently washes Caroline's phone number off.
The race is on to rescue the executives in the meeting room before the worms get to them first. In the process, Stephen gets separated and gets trapped in a lift shaft. Cutter realises the worms can't survive outside the fog (as Pre-Cambrian creatures, the worms are more used to a sulphurous atmosphere, making oxygen poisonous to them) and Connor and Abby set about turning up the heating in the building. They succeed in doing this but no one is prepared for the effect the heat has on these deadly creatures - they explode, shooting their parasitic young all over the room. Cutter warns everyone not to let them break the skin, and after setting off the building's fire control system, the worms fall to the ground where they die.
When an anomaly to the Precambrian opens up in a classroom during the convergence and the Precambrian fog pours through, a pair of the Worms come through, slithering around the classroom. Abby and Becker are sent in to capture the Worms, but find it difficult due to the fog obscuring them from view. They were presumably killed or returned, as capturing them and returning them to the menagerie or Creature Prison would have been impossible, as they would have died because of the lack of Precambrian fog.