Siboglinidae is the only group of polychaetes to completely lack a mouth, anus and digestive lumen. While there is a transient mouth and anus present during early development, this becomes occluded quite quickly. The endoderm lining the gut expands and blocks the gut cavity. The endoderm becomes filled with chemo-autotrophic bacteria that enter the body while the mouth or anus is still open.Siboglinids live in areas where there is a high level of reduced-sulphur or methane in the water or sediment around their tubes. This is usually in deeper water sof the sea and the most famous are those thatlive near hydrothermal vents. There is now considerable evidence that most, if not all, of their nutritional requirements are derived from the bacteria in the body, which can make up 15% of the animal's total body weight. Large taxa, such as Riftia seen below, live around deep-sea hydrothermal vents from which heated water, sulfide and carbon dioxide are ejected. The palps of siboglinids project from the tube and are richly supplied with blood vessels that serve for the respiratory purposes of the worm, but also to transport the sulphide and carbon dioxide that are needed by the symbiotic bacteria.

To see a short Quicktime video of Riftia click here. Note the downloand is ~ 2mB.