A number of the particularly attractive and large polyclad worms are generally benign tank inhabitants worth considering, however. These free-living tropical reef species typically are generally frilled, and they can use these frilled edges of the body to swim by muscular contraction in an undulating flight that has to be seen to be truly appreciated. Most are predatory scavengers, making a living by capturing the small polychaete worms and tiny crustaceans found in detrital accumulations. Although still a shy group of animals by nature, these larger species can occasionally be seen cruising around the aquarium during the day, or even swimming around the aquarium from time to time. There are only about 900 species of described nemerteans, which range in length from only about 1 cm to several meters when contracted.
Nemerteans generally live on the ocean floor, but some are pelagic, and others are commensal.
These animals are long, flattened worms that superficially resemble some of the flatworms, however, they are different in a number of ways. First, these worms have a complete one-way gut in which food enters the mouth and undigested material is passed out the anus. Second, all members of this phylum are characterized by a highly eversible proboscis . Some species are actually capable of sensing and tracking prey over long distances, whereas others seem to wander aimlessly until they physically contact a prey item. Some nemerteans fire their proboscis down the burrows of potential prey items to capture them and drag them out to be consumed.