The longest animal in the world is probably Lineus longissimus, the bootlace worm, in the phylum Nemertea, the ribbon worms. One specimen that washed up on the shore of St. Andrews, Scotland in 1864 measured 180 ft (55 m), almost twice as long as the longest blue whale on record, which measured 98 ft (29.9 m). Bootlace worms are long and thin, with a thickness of only 5-10 mm (0.5 - 1 cm). When handled, they produce large amounts of thick mucus with a faint pungent smell. The animal is brown with lighter longitudinal stripes. If folded up tightly, this record-setting longest animal would occupy a cube only about 17 cm (6.7 in) on a side.
Bootlace worms are free-living creatures which dwell on or in the sea floor, capturing prey with their proboscis. Often found among rocks, seaweeds, barnacle, or mussel beds or buried in sand or gravel substrates, bootlace worms are voracious predators. They wrap themselves around prey, such as crustaceans or annelids, which may be much larger than themselves, and stab it repeatedly with the stylet in their proboscis until the target is dead. If no other food is available, it is said that ribbon worms will consume themselves. They also secrete mucus from their proboscis to trap prey.
Other animals besides the bootlace worm have contended for the title of world's longest animal. One is the siphonophore, which looks and acts like a single organism but is actually a colony of very closely cooperating animals. Siphonophores are cnidarians, like jellyfish, and include the Portuguese Man o' War. Some siphonophores have been measured at 130 ft (39.6 m) in length. Like bootlace worms, they are long and thin, but their front end is led by two swimming bells. Unlike bootlace worms, they are free-swimmers rather than bottom-dwellers. Siphonophores are rarely considered the world's longest animal because they are colonial and the Scotland bootlace worm of 1864 was longer, though most bootlace worms are much shorter.
Other animals of the past which are now extinct may have contended for the title of longest animal, but as most soft-bodied animals never fossilize, the majority are lost forever. One dinosaur, Amphicoelias fragillimus, known only from fossils which have since crumbled to dust, may have been as long as 196 ft (60 m) in length, even longer than the bootlace worm, but this is merely conjectural, as no complete skeleton exists.