priapium

A complex muscular and bony clasping and copulatory organ derived mainly from the pelvic fins, found under the head of male Phallostethoidea with the anus opening on one side and the genital pore on the other. The structure varies among family members. Hook-like ctenactia articulate basally with the aproctal axial which is movably articulated with the proctal axial bone itself, suspended anteriorly by the outer (and sometimes inner) pulvinular bone from the cleithrum and perhaps the urohyal. Other elements are the priapal ribs, the anteplural cartilage (supporting elements along with the cleithrum and the pulvinar), the toxactinium, the infrasulcar and the uncus (forming the claspers along with the ctenactinium), the penial, basipenial, papillary, prepapillary and cristate bones (forming the papillary unit).