Nemerteans have no rigid skeleton and no segments. Their body wall is a muscular hydrostat wrapped in connective tissue, so a specimen can be drawn out to many times its resting length before it tears. Measuring one is therefore an exercise in deciding how much tension to apply, and different handlers get different answers.
The famous St Andrews specimen was recorded at more than 50 metres after a storm in 1864. Routine live animals are far shorter, commonly 5 to 15 metres, and even that length occupies only a modest coil under a stone. Width is typically under 10 millimetres, so the animal has very little mass for its length.
The worm also carries a chemical defence. Its mucus contains a family of pyridine alkaloids, including anabaseine, which are toxic to crustaceans and have been studied for neurological receptor activity. Whether the record length is a genuine biological maximum or an artefact of stretching a dead animal remains unsettled, which is why the title is usually stated with a qualifier.