4 Turbellarians are monoecious with the reproductive systems adapted for internal fertilization. And the monogenetic flukes (class Monogenea) are mostly ectoparasites of fishes.

涡虫是雌雄同体的，生殖系统适于体内受精。单起源的吸虫（单殖纲）大多是鱼的外寄生虫。