Pogonophorans, the beard worms, are sessile benthic marine
worms that live in fixed upright chitin tubes that they secrete on
sediments, shell, or decaying wood on the ocean floor. This is
the only phylum of free-living animals in which all lack digestive
tracts as adults. The phylum contains about 120 species in
two classes, Class Perviata (perviates) and Class vestimentifera
(Obturata vestimentiferans). Meredith Jones of the Smithsonian
Institution assigns the six vestimentiferan genera, marked with
asterisks in the list of examples above, to a proposed separate
Vestimentifera phylum that includes all 10 or so vestimentiferan
genera, leaving the remaining pogonophorans in Phylum
Pogonophora.
Pogonophorans are probably distributed throughout the
world in cold (2°–4°C), deep ocean waters, in shallow Arctic
and Antarctic seas, as well as in hot (10°–15°C) submarine
vents (hydrothermal vents), where the water contains hydrogen
sulfide and methane. Pogonophorans of these deep, dark communities
do not feed—they derive nutrients and energy from
symbiotic chemoautotrophic bacteria (Phylum B-2). Although
hydrogen sulfide is usually toxic to animals, pogonophorans in
this hot vent habitat use their red extracellular hemoglobin to
carry both hydrogen sulfide—bound so that it cannot absorb
the hemoglobin site that binds oxygen—and oxygen from their
tentacles to the site of bacterial oxidation. Hydrogen sulfide thus
reversibly bound is not toxic to the worm.