The ultrastructure of oocytes in four species of Acoela ( Archaphanostoma agile, Otocelis rubropunctata, Symsagittifera japonica, and Amphiscolops sp.) and that of female copulatory organs of S. japonica and Amphiscolops sp. are described for the first time. According to the similarity of the reproductive apparatus structure, the sister relationships between the families Sagittiferidae and Convolutidae are confirmed. The ultrastructure of oocytes and specific features of oogenesis are proposed to be useful in phylogenetics of Acoela
