Ultrastructure of the copulatory stylet and accessory spines in Haplopharynx quadristimulus (Turbellaria)

The copulatory organ of Haplopharynx quadristimulus Ax, 1971 (Carolina form, Rieger, 1977) consists of a proximal prostatic vesicle and a distal stylet apparatus comprising a central tubular stylet and four to five peripheral accessory spines . By electron microscopy it could be seen that the stylet and spines were intracellular specializations . The copulatory organ can be interpreted as a specialization of an epithelial canal extending from the testes to the body wall . In the complex stylet apparatus, the epithelium was differentiated into six cell types. The stylet, which was formed in a matrix syncytium next to the prostatic vesicle, extended into the lumen of the stylet canal . The interior of the stylet apparatus contained one group of cells that had thick ciliary rootlets and another that had rootlet-like ribbons . The cells that contain the rootlets enveloped bundles of longitudinally arranged muscles. The accessory spines were formed in cells which lay peripheral to the muscle bundles . The spines, stylet, rootlet-like ribbons, and rootlets had similar patterns of periodic cross striations . The similarity in striation patterns suggests that the accessory spines and stylet are composed of modified ciliary rootlets.