Littlewood et al. (1999) (citation) and Noren & Jondelius (2002) (citation) present rDNA-sequence data showing a sister-group position of Haplopharynx rostratus and the Lecithoepitheliata, with moderate support. However, Rieger (2001) (citation) also considered such molecular-sequence data (from Littlewood et al. 1999) (citation) and rejected the notion that Haplopharynx rostratus would fall outside of the Macrostomorpha in light of the numerous morphological characters shared by the macrostomorphs and their evolution. That is, a split of the Macrostomida and Haplopharyngida is not valid, and a sister-group relation between Haplopharyngida and Lecithoepitheliata is unlikely. The only morphological characters supporting the Haplopharyngida/Lecithoepitheliata relationship are those of the penial structures and the follicular surrounding of accessory cells (Rieger, 2001).
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