What is the longest animal by body length?

The bootlace worm is the longest animal by body length. Its scientific name is Lineus longissimus, a marine ribbon worm found around northwestern European coasts.

Most specimens measure about 5–15 metres, while individuals of roughly 30 metres are documented. A famous 1864 specimen reportedly measured more than 55 metres after washing ashore near St Andrews, Scotland, but ribbon worms stretch dramatically, so that measurement is treated cautiously.

The bootlace worm is extraordinarily thin—only a few millimetres wide—and can contract or elongate substantially. That flexibility explains why records are difficult to compare. The blue whale is heavier and has a more reliably measured length, but the worm can exceed it in straightened body length. Its mucus also contains potent neurotoxic compounds that deter predators and have attracted scientific interest as possible biopesticides.