Turbellarian taxonomic database
Hooge MD 2003 (citation) - The copulatory organ of the Polycanthiidae is entirely unique among known species of
the Acoela. While members of the genus Pseudaphanostoma in the family Convolutidae have muscular copulatory
organs that also open terminally, their seminal vesicle musculature is not nearly as well developed, and all
members of the genus Pseudaphanostoma have invaginated penes.
The phylogenetic position of the Polycanthiidae within the Acoela is uncertain. In spite of the above-listed
differences between the Polycanthiidae and Pseudaphanostoma, their mutual possession of muscular, terminally
opening copulatory organs and similar body-wall musculature may be indicative of a close phylogenetic