Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha

Xenoturbella bocki, a marine animal with a simple body plan, has recently been suggested to be sister group to the Acoelomorpha, together forming the new phylum Xenacoelomorpha. The phylogenetic position of the phylum is still under debate, either as an early branching bilaterian or as a sister group to the Ambulacraria (hemichordates and echinoderms) within the deuterostomes. Although development has been described for several species of Acoelomorpha, little is known about the life cycle of Xenoturbella. Here we report the embryonic stages of Xenoturbella, and show that it is a direct developer without a feeding larval stage. This mode of development is similar to that of the acoelomorphs, supporting the newly proposed phylum Xenacoelomorpha and suggesting that the last common ancestor of the phylum might have been a direct developer. Xenoturbella is a simple marine worm recently suggested to be either a deuterostome or an early branching bilaterian. Nakano et al. report the first observations of naturally spawned eggs and embryos from Xenoturbella, and uncover new insights into the evolutionary history of metazoan development.