Xenacoelomorpha

Xenacoelomorpha is a phylum of simple, bilaterally symmetric marine worms comprising the acoel flatworms, the nemertodermatids, and the deep sea worm Xenoturbella. Its members lack a through gut, an anus, and a centralized excretory or circulatory system. Molecular studies place the group among the earliest diverging lineages of bilaterian animals, though its exact position on the animal family tree remains debated.