Ribbon worms (nemerteans) have simple nervous and excretory systems similar to those of flatworms. Unlike flatworms, however, they have a closed circulatory system and a complete digestive tract with a mouth at one end and an anus at the other. Small ribbon worms move slowly by beating their cilia. Larger ones employ waves of muscle contraction to move over the surface of sediments or to burrow into them.

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Within the body of nearly all of the 1,200 known species of ribbon worms is a fluid-filled cavity called the rhynchocoel, within which lies a hollow, muscular proboscis. The proboscis, which is the worm’s feeding organ, may extend much of the length of the body. Contraction of the muscles surrounding the rhynchocoel causes the proboscis to evert explosively through an anterior pore (Figure 31.8A). The proboscis may be armed with sharp stylets that pierce prey and discharge paralysis-causing toxins into the wound.

Most ribbon worm species are marine, although there are species that live in fresh water or on land. Most species are less than 20 centimeters long, but individuals of some species reach 20 meters or more. Some genera feature species that are conspicuous and brightly colored (Figure 31.8B). Recent molecular analyses suggest that ribbon worms may be most closely related to the brachiopods and phoronids.