The phylum Pogonophora ('beard bearers') is an exclusively marine group of worm-like invertebrates, the majority of which live in sediments on the continental slopes. Some species also occur in shallower water and may be found in the North Sea.
The threadlike and delicate body of these coelomate beard worms lacks a mouth and gut. A short forepart bears the tentacles and the trunk, which is followed by a segmented posterior end, forms 90% of the body length. The number of tentacles varies from one in the family Siboglinidae, to over 200,000 in some Vestimentiferans. The tentacles may bear pinnules. Behind the tentacle(s) there are often two indistinct grooves, which mark off the mesosoma, from the protosoma and metasoma. The mesosoma usually bears more conspicuous cuticular grooves, forming the bridle. The bridle grooves run round each side oblique, so tend to form V shapes in the mid-line. The 'ventral' V, which points posteriorly, is particularly distinct. Usually, both the bridle and the protosomal grooves are interrupted 'dorsally' by wide gaps. The metasoma or trunk first bears a series of so called metameric papillae, flanking a 'ventral' furrow. There are usually two or three close-set girdles of uncini. The gonads overlap with the metameric region and extend back nearly as far as the girdles. The hind end (opisthoma) is short but many-segmented, with four blunt chaetae per segment.
The sexes are separate, forming free sperms, spermatophores or yolky eggs. It is not known how sperms in either form reach the females, or how fertilization proceeds.
The posterior half of the metasoma is full of chemoautotrophic bacteria held as symbionts. The symbionts fix carbon dioxide into organic compounds that nourish both themselves and the pogonophores. Pogonophores can take sugars and amino acids from the surrounding seawater, but in nutrient-poor habitats, where they are commonly found, the bacterial symbionts sustain the pogonophores.
The tubes are made of chitin and protein. The worm adds to at both ends as it grows, resulting in a series of rings or slightly funnel-shaped pieces. At first glance, the tubes look like detrital plant fibres.