The ultrastructure of male copulatory organs having a stylet has been studied in some genera of the Proseriata. Within the Monocelididae there was a variety of stylet-like hard structures. The stylet in Monocelis fusca was a differentiation of the basement membrane of the epithelium lining a penis-like muscular papilla. The penis papilla in Ectocotylaconsisted of circular muscles surrounded by a thickened basement membrane and an epithelium. Archilopsis sp. and Archilina sp. with a duplex copulatory bulb, had a stylet within a spiny cirrus. The stylet structure in Archilina sp. was composed of four long spines which were derivatives of the basement membrane. InEctocotyla multitesticulata and Dupliminona corsicana, the accessory prostatoid organ was provided with a hook-shaped stylet that was differentiated in the basement membrane and of which the material was continuous with the fibrous matrix between the muscles of the prostatic bulb. The stylet and needles in the Archimonocelis species were intracellular differentiations. The copulatory organ in Carenscoilia biforamen consisted of a tubiform stylet and four needles, all of which were also intracellular specializations. I consider copulatory hard structures in the Turbellaria to be taxonomically significant in terms of structure, differentiation, and location (whether subcellular, in the basement membrane, or intracellular).
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