. Elements of comparative anatomy. Anatomy, Comparative. 184 COMPARATIVE ANATOMY. separated, owing to the development of a vestibule. In most of the marine Planaria they are completely separated, and there is a double genital pore, the male one lying in front of the female. In most of the Trematoda, too, the openings of the genital organs are distinct, although placed close to one another. The same arrangement is seen in the Cestoda. Even in those cases where the bag of the cirrus and the vagina open into a genital pore, this latter is only a flat pit, walled in by the integument. In other cas
. Elements of comparative anatomy. Anatomy, Comparative. 184 COMPARATIVE ANATOMY. separated, owing to the development of a vestibule. In most of the marine Planaria they are completely separated, and there is a double genital pore, the male one lying in front of the female. In most of the Trematoda, too, the openings of the genital organs are distinct, although placed close to one another. The same arrangement is seen in the Cestoda. Even in those cases where the bag of the cirrus and the vagina open into a genital pore, this latter is only a flat pit, walled in by the integument. In other cases the two pores open directly on to the surface, though close together. Or we have the noteworthy provision of a second female opening, with a vaginal duct, as already described. Finally, the two apertures may be still more separated, the male organ opening on the lateral edge, and the female on the surface of the proglottid. The two kinds of organs are sometimes unequally developed in one and the same individual; in the Khabdoccela especially the sexes are separated, the two organs being unequal in different individuals; in one the female, and in another the male organs are most developed, while the organs of the other sex remain rudimentary (Convoluta). These important examples show us how, by the continued atrophy of one organ, dioecious forms are derived from hermaphrodite organisms. The jDrocess here observed in statu nascenti is complete in other Turbellaria. The Microstorneas have the sexes separate, as have also some Planaria) and Trematoda. The generative system is simplified in the Nemertina, which are almost always dioecious. The various divisions of the excretory ducts and of the accessory organs are absent. The testes and ovaries are the only parts which are distinctly recognisable. In some (Pro- rhynchus) these organs occur singly in each individual (Fig. G7, ov), and so call to mind the rhabdoccelous Turbellaria. In others, however, there is a large number of fo
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