One of the most surprising and significant advances in metazoan phylogenetics in the last few years has been our changing perception of the phylogenetic position of two groups of morphologically simple flatworm-like organisms: the Acoela and the Nemertodermatida (Figures 1 and 3). Acoels and nemertodermatids are small ciliated worms that were previously considered to be primitive members of the phylum Platyhelminthes. Chiefly based on similarities of their epidermal cilia, the two groups were united in the clade Acoelomorpha. In common with platyhelminths, acoelomorphs have a blind-ending gut that may or may not have a lumen. However, unlike most platyhelminths, acoelo-morphs lack a continuous epidermal basement membrane, as well as protonephridia, and their nervous system is only weakly differentiated in the form of a nerve net with a weakly differentiated brain and several longitudinal cords. The brain in acoels has been described as commissural, rather than gang-lionic, and it has been argued that a true ganglionic brain with a central neuropil evolved after the acoe-lomorphs diverged from the remaining bilaterians. The name Nephrozoa has been proposed for the clade of all nonacoelomorph bilaterians, based on the widespread presence of nephridia in these taxa (Jondelius et al., 2002).
Currently, molecular evidence in the form of nucleotide sequences of SSU, LSU, and myosin II heavy chain, as well as amino acid sequences from several mitochondrial genes support the position of the acoelomorphs as the basal-most extant bilater-ians. In addition, possession of a complement of Hox/ParaHox genes intermediate between those found in cnidarians and the remaining bilaterians appears to buttress the basal position of the acoelo-morphs. However, molecular evidence has not yet provided unequivocal evidence for the monophyly of Acoelomorpha, and the acoels and nemertoder-matids may be distinct taxa, branching off subsequent to each other at the base of the Bilateria.
The acoelomorphs can therefore serve the important function as a bridge to help close the architectural gap between bilaterians and the more basal metazoans. As the most basal-living bilater-ians, the morphology and life cycle of the acoelomorphs is especially important for informing the primitive conditions for the Bilateria as a whole. Their lack of segmentation, coeloms, excretory organs, their relatively simple nervous systems, the absence of distinct larval stages in their life cycles, and their apparently limited set of Hox/ParaHox genes strengthen the inference that the most recent common ancestor of the Bilateria was a relatively simple, compact, direct-developing organism, with a sac-like gut (BaguM and Riutort, 2004).
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