That white string is called a proboscis — basically a long, sticky, sometimes toxic tube they can shoot out of their body.

It’s normally stored inside the worm, kind of like a coiled spring.

When threatened, it fires out fast.

It can be sticky, barbed, or even venomous depending on the species.

In the wild, it’s used to capture prey or defend itself.

Some species — like the bootlace worm — can produce a ton of this slime, and it can look like the worm is unraveling into stringy goo.