From The X-files:

Flukeman (a.k.a Flukey) was born "in a primordial soup of radioactive sewage." Flukeman was found in the Newark sewage system after being dumped from the bilge of a Russian salvage ship.
In The Host, Mulder and Scully try to figure out what exactly is this man/worm (played by Darin Morgan) and stop it from killing workers and reproducing.
At the end, the Flukeman is severed in half, which Mulder and Scully believe to be the end of Flukey, but we see the top half of its body floating in the sewers clearly alive.
See The X-files: Season 2