Which marine worm can reach lengths of over 50 meters?

The marine worm that can reach lengths of over 50 meters is the bootlace worm, Lineus longissimus. It is a ribbon worm, or nemertean, found around the coasts of parts of northern Europe, including Britain and Scandinavia.

Bootlace worms are extremely thin compared with their length and have flexible bodies that can stretch dramatically. Most reported individuals are much shorter—often around 5 to 15 meters—and confirmed measurements are difficult because the animals elongate when disturbed. A specimen described after washing ashore near St Andrews, Scotland, in 1864 was reported at more than 55 meters.

That record is often cited in discussions of the world’s longest animals, although scientists treat extreme measurements cautiously because nemerteans can be stretched beyond their normal length. The worm captures prey with an eversible proboscis, a tube-like feeding organ, and its mucus contains toxins that help deter predators. Despite its appearance, it is not a tapeworm or an earthworm.