Ribbon worms range in length from 1mm to very long ones indeed. Some species can reach 30m! Those we have seen range from short ones 10-15cm long, longer ones 30-40cm long and some more than 1m long. Ribbon worms are NOT segmented worms. The body of a ribbon worm is rather flattened. Although it appears smooth, the body is covered with microscopic hairs (cilia). Ribbon worms may have zero to 80 'eyes' (light-dectecting sensors). Some ribbon worms produces mucus through which they move.
Most ribbon worms are voracious predators, often specialising in a particular prey although some will eat a wide variety of prey. Ribbon worm prey include other worms, crustaceans and molluscs.
To capture its prey, the ribbon worm has a unique eversible proboscis at the front end of the body. This is a hollow, muscular structure that can shoot out with explosive force and is prehensile (can be used to grip) and retractable (can be pulled back). The proboscis is usually wound around the prey which is then hauled back toward the worm's mouth. Sticky mucus is secreted to help grip the prey.
In one group of ribbon worms, the proboscis is armed with a piercing stylet that can inject a potent paralysing toxin. Such a worm releases the prey after injecting it, and waits for the prey to be paralysed before moving in to feed on it. If the prey is worm-shaped, it may be swallowed whole. For other awkwardly shaped prey, the worm inserts part of its digestive system into the prey and sucks up the victim's juices. The proboscis may also be used to burrow or to drag itself along the surface.
Being soft and very long, ribbon worms appear defenceless. Studies, however, suggest that they harbour bacteria that produce powerful neurotoxins. These may make the worms toxic to eat. Indeed, many ribbon worms are brightly coloured, perhaps serving to warn of their distasteful nature.