Ribbon worms, also called bootlace worms or proboscis worms, derive their common names from their threadlike or ribbonlike form, and from the characteristic reversible proboscis which they use in prey capture or in burrowing. The phylum Nemertea (or Rhynchocoela) includes approximately 900 described species of these worms. Most of them are marine, living in sand or mud, or under shells and rocks; a few are known from freshwater and terrestrial habitats. Many are brightly colored, especially red, orange, and yellow.
The body is either cylindrical or flat, unsegmented, and varies in length from a few centimeters to over 98 ft (30 m). Moreover, it is highly extensible, and can be stretched to many times its normal length. The proboscis is located in a fluid-filled cavity. Increase in pressure of the fluid causes the proboscis to be inverted through an opening situated just above the mouth. Proboscis retraction is effected by means of a retractor muscle. In some species the proboscis is armed with a stylet. A ribbon worm's food consists of segmented worms and small crustaceans which are encountered and captured by trial and error. Whenever the worm is successful in this endeavor, the proboscis coils around the prey organism, and then is retracted to bring the food to the mouth. The digestive tube is straight and non-muscular, and movement of food in it occurs mainly by ciliary action. An anus is present at the posterior end.
In ribbon worms there is no cavity between the body wall and the gut; instead, the space is filled by a spongy tissue called parenchyma. (This "acoelomate" condition is found also in the flatworms.) Sexes are separate. An individual worm has multiple testes or ovaries, each with a separate opening to the outside. Fertilization is external. In most species, the zygote develops into a ciliated, helmet-shaped larval form called pilidium. Most nemerteans also possess remarkable powers of regeneration, which can be an important means of asexual reproduction. Representative genera of ribbon worms include Cerebratulus, Lineus, and Tubulanus.