Acoels have historically been placed within the
Platyhelminthes, but recent molecular phylogenetic
analyses now suggest that they probably represent a
distinct phylum, the Acoela, that falls well outside of the
Platyhelminthes (see [5] and references therein).
Specifically, acoels are now thought to represent the most
basal lineage within the Bilateria, being the sister group
to all other bilaterian animals (Figure 1a). That a similar
neoblast system is found in both Platyhelminthes and
Acoela has important implications for the evolution of