Molecular data from Orthonectid worms show they are highly degenerate members of phylum Annelida not phylum Mesozoa

Summary: The Mesozoa are a group of tiny, extremely simple, vermiform endoparasites of various marine animals (Fig. 1). There are two recognised groups within the Mesozoa: the Orthonectida (Fig. 1a,b; with a few hundred cells including a nervous system made up of just 10 cells [1]) and the Dicyemids (Fig. 1c; with at most 42 cells [2]). They are classic ‘Problematica’ [3] - the name Mesozoa suggests an evolutionary position intermediate between Protozoa and Metazoa (animals) [4] and implies their simplicity is a primitive state, but molecular data have shown they are members of Lophotrochozoa within Bilateria [5-8] which would mean they derive from a more complex ancestor. Their precise phylogenetic affinities remain uncertain, however, and ascertaining this is complicated by the very fast evolution observed in genes from both groups, leading to the common systematic error of Long Branch Attraction (LBA) [9]. Here we use mitochondrial and nuclear gene sequence data, and show beyond doubt that both dicyemids and orthonectids are members of the Lophotrochozoa. Carefully addressing the effects of systematic errors due to unequal rates of evolution, we show that the phylum Mesozoa is polyphyletic. While the precise position of dicyemids remains unresolved within Lophotrochozoa, we unequivocally identify orthonectids as members of the phylum Annelida. This result reveals one of the most extreme cases of body plan simplification in the animal kingdom; our finding makes sense of an annelid-like cuticle in orthonectids [1] and suggests the circular muscle cells repeated along their body [10] may be segmental in origin.