T he muscular system associated with the copulatory organ of the monogenean gill parasite, Calceostoma polyorchis consists of three groups of muscle fibers. The first group forms a sleeve around the longitudinal axis of the copulatory organ that help protrudes the copulatory tube from the male genital opening during copulation. The second group is short, attaches with the base of the copulatory organ and acts in the withdrawal of the copulatory tube inside the body of
the parasite after copulation. The third group connects the terminal ends of both the sclerotized piece and the copulatory tube. It help inserts the anterior region of the copulatory tube inside the vaginal opening and maintains this position during copulation.