It is easier to envisage the addition of features to the simple acoel body plan to achieve the various body plans of other bilaterians in the animal kingdom, than to consider a complex organism evolving into other complex forms via intermediate stages.
The potentially pivotal phylogenetic position of acoel worms amongst the animal kingdom has recently elevated the status of Symsagittifera roscoffensis to ‘model organism’.
Evolutionary developmental studies on this and other acoel species, such as Convolutriloba longifissura and Isodiametra pulchra, are now providing a wealth of comparative biological data for what was once a largely neglected group of animals.
Intriguingly, another simple flat worm, Xenoturbella, has been shown to share various molecular characteristics with acoels, and yet Xenoturbella is now widely considered a member of the Deuterostomia, and most closely related to the echinoderms and hemichordates. Like acoels, xenoturbellids were once thought to share their common simplicity with true flatworms.
Genes of the Hox cluster play an important role in determining body plan patterning. Studies characterising Hox genes - genes that encode transcription factors which specify an animal’s anterior-posterior axis and segment identity during embryogenesis - show that acoels have only 3 Hox genes, 1 from each of the anterior, central and posterior groups. Again, this supports acoels as early divergent bilaterians.
The question remains whether acoels still retain the features of primitive bilaterians after so many millions of years. Within the acoels, the phylogenetic position of the Sagittiferidae appears to be highly derived, so many more acoels and particularly the early divergent forms need attention before we can estimate the body plan of the ancestral acoel, which in turn is needed to inform the nature of the earliest bilaterians.
It is clear that the shear abundance of Symsagittifera roscoffensis means this species will continue to play an important role in studies of animal evolutionary biology, as an exemplar of the Acoela.