of Nemertinea, [106], [110];

of Polychaeta, [273]

Otomesostoma, [46], [49]

Otoplana, [50]

Oudemans, on Nemertinea, [108]

Ovary (and Oviduct), of Leptoplana, [11], [14], [16];

of Polyclads, [27];

of Planaria, [38], [39];

of Rhabdocoelida, [47];

of Temnocephala, [54];