Ribbon Worm, any of about 900 species of worms that typically live on seacoasts and in the sea. Some species live in freshwater or in moist soil. Ribbon worms range from less than one inch (25 mm) to 85 feet (26 m) in length. Ribbon worms of most species are pale. The ribbon worm is noted for its proboscis (a cordlike organ extending from the head), often several times the length of the body, which it shoots out and wraps around prey. In some, the proboscis ends in a sharp stylet, a bristlelike organ through which the worm injects venom into its prey.

Ribbon worms are the most primitive animals with a circulatory system separate from the digestive system, and with a digestive tract terminating in an anus.

Ribbon worms make up the phylum Nemertina, or Rhynchocoela.