The deep-sea hydrothermal vent tubeworm Riftia pachyptila (Vestimentifera) lacks a digestive tract entirely. Nutriments are presumably provided to the animal by symbiotic chemoautotrophic bacteria which are harbored in a specialized organ of the animal, the trophosome. These bacteria are proposed to fix carbon dioxide from the ambient seawater using the energy gained from oxidation of sulfide which is emitted by the hydrothermal vents. The significance of the trophosome for the evolution of the pogonophoran and vestimentiferan body plan is discussed.