A phylum of benthonic marine worms that is restricted to habitats rich in sulfide (for example, hydrothermal vents and sulfide seeps) and that as adults lack a mouth, gut, and anus, so they are nourished by internal symbionts. All vestimentiferans live in tubes of varying hardness and rigidity. Tube material is secreted by internal glands and is a mixture of chitin and protein.

The phylum has two classes based on the arrangement and orientation of lamellae formed by fusion of branchial filaments: Axonobranchia with one order, Riftiida, and one monogeneric family with one species; and Basibranchia with two orders, Lamellibrachiida and Tevniida.

Vestimentiferans have four regions along their length. The most anterior, obturacular region is provided with a mass of well-vascularized filaments supported by a paired structure, the obturaculum. Normally these branchial filaments protrude from the opening of the tube and act as a gill, allowing for the exchange of dissolved substances between the worm's body and the seawater. The second region, the vestimentum, is quite muscular and serves to maintain the plume of branchial filaments in the open seawater by pressure on the inner tube surface at its opening. The vestimentum is the site of many glands that contribute material for lengthening the tube and thickening it near the opening. The third region, the trunk, is a single segment and constitutes about 75% of the total length of the worm. It is provided with a pair of large longitudinal blood vessels. The remains of the larval gut containing sulfide-oxidizing bacteria and the gonads are also present. The fourth and most posterior region, the opisthosome, is made up of many segments, each segment in the anterior portion bearing a row of small hooks that can be set into the inner surface of the tube. This provides an anchor against which the body of the worm retracts when the longitudinal muscles of the trunk contract during withdrawal into the tube.