Animals in this phylum were first described in 1900, but it was not until 60 years later that whole specimens were obtained. The early descriptions did not include the opisthosoma (see right). To date about 80 species have been described, all coming from depths of over 200 m. The worms range in length from 5 - 300 cm, and all are less than 3 cm in diameter. They live in a self-secreted chitinous tube the length of their body. They appear to have a mutualistic association with chemoautotrophic bacteria which live in their body, and may also absorb some nutrients through their hollow, anterior tentacles. About half-way down their trunk they have two rings of chaetae which anchor to the tube wall. They may share a common ancestor in the Annelida, but there is no fossil record.