Phylum of unsegmented, non-parasitic worms which are bilaterally symmetrical (see bilateral symmetry) and elongate. Adults are ciliated (see cilium) and anteriorly possess a thin, retractile tube, the proboscis. The mouth and brain are well developed, the coelom is indistinct. 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.