A phylum of unsegmented, non-parasitic worms which are bilaterally symmetrical and elongate. Externally they resemble Platyhelminthes (flatworms) but are generally larger and more elongated. Unlike Platyhelminthes they have a mouth and anus which allows ingestion and egestion to occur simultaneously. Adults are ciliated and possess a long, tubular proboscis that can be thrust out from the body by a sudden contraction of the fluid-filled rhynchocoel. The proboscis, which may bear piercing barbs, is used to capture prey and in defence. The mouth and brain are well developed, the rhynchocoel limited in extent. Most are marine, although freshwater and terrestrial forms occur. Most are not hermaphrodites. The first fossil forms occur in the Middle Cambrian Burgess Shales of British Columbia, Canada.
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.