This creature is nneither the blue whale, nor a lion’s mane jellyfish but the humble bootlace worm, Lineus longissimus, one of the most remarkable marine invertebrates of them all. View image in fullscreen When threatened, the bootlace worm discharges a poisonous mucus. When threatened, the blackish-brown bootlace worm discharges a poisonous mucus which may remind you of the smell of sewage if you pick up a beached worm. The bootlace worm is one of many little-known nemerteans – ribbon or proboscis worms. Vote bootlace worm!