Nemerteans are unsegmented, dorsoventrally flattened predatory marine worms occurring at all ocean depths, freshwater, and terrestrial habitats. Approximately 1300 valid described species of the phylum Nemertea, or ribbonworms, are known worldwide. Current fieldwork suggests that at least several times this number remain to be named or discovered. Nemerteans are unsegmented worms characterized by a unique and remarkable eversible proboscis. Some are very colorful, while others are drab. They range from one millimeter to more than 30 meters long. They can be voracious predators, some are highly specialized while others are more eclectic with diets that favor other worms, crustaceans, and molluscs. They are poorly known to non-specialists because most nemerteans live in concealment, are difficult to collect, and because traditional taxonomy focuses significantly on internal anatomy based on histological study. However, many are common, abundant, and can be key predators, while the phylum itself is important to understanding evolution of early invertebrate body plans.
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