They have flat, soft bodies that are less than 2.4 cm long. Their bodies are not segmented like earthworms or leeches. Their most unusual feature is a long thread-like appendage sticking out from their front ends which is called the 'proboscis'. The proboscis can be extended, or pushed out, as much as two or three times the length of the worm's body. With this proboscis, the worm can feel about for food and enemies.
Ribbon worms feed on small animals and algae.
Little is known about how they produce young. For some reason, not yet known, ribbon worms are found in larger numbers in the autumn than during the spring and summer, when they are rarely found. They provide food for larger animals, especially in the autumn.