A Polyclad Flatworm with a Disarmed Copulatory Apparatus
Stylochoposthia is a genus of polyclad flatworms, and its family-level assignment is controversial. The molecular phylogenetic analysis in this study strongly suggests that Stylochoposthia sp. is closely related to Planocera species and nested within a clade of Planoceridae. The result indicates that an eversible cirrus armed by numerous spines or teeth in the male copulatory apparatus, a diagnostic characteristic of Planoceridae, was lost in the lineage of Stylochoposthia. This finding challenges the morphological definition of Planoceridae and improves our understanding of the evolution of the copulatory apparatus in polyclad flatworms.
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