A simple marine worm whose natural habitat is in soft mud bottoms at a depth of around 60 m (200 ft) off the coasts of Sweden and Norway. Xenoturbella was first discovered in 1915 by the Swedish biologist Sixten Bock, but was not described until 30 years later by Einar Westblad. Due to its simplicity of form, Westblad initially classified Xenoturbella as a primitive turbellarian, a class of ciliated nonparasitic flatworms, and named it accordingly (Xenoturbella = strange flatworm). However, recent analyses using deoxyribonucleic acid (DNA) have found Xenoturbella to be a primitive member of the deuterostomes, the group of animals that includes the vertebrates.