The bootlace worm (Lineus longissimus) is a species of ribbon worm and one of the longest known animals, with specimens up to 55 m (180 ft) long being reported. Its mucus is highly toxic. [...] This toxic mucus has been shown to kill crabs and cockroaches, and could have applications as an agricultural insecticide.
Taxonomy
The bootlace worm is in the phylum Nemertea or ribbon worms. It is the most common nemertean found along the coasts of Britain.
Look up bootlace worms. They can grow several times that length.
Lovely.
It's obscenely long, its mucus is highly toxic, and you can find it along the coasts of Britain.
Told you. 8.4m ain't shit.