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 fol- 

 licles on either side of the enteric canal; 

 they have no direct connection with one 

 another, and being arranged regularly in 

 pairs along the body are evidence of meta- 

 merism. 



§ 151. 



In the Nematodes hermaphroditism is 

 a rare exception. As a rule the sexes are 

 separate. Both kinds of organs consist of 

 tubes embedded in the coelom and open- 

 ing on to the surface. The organs of the 

 female system are very generally paired. 

 This is less common in the case of the male organs. The double 

 opening observed in some few cases also speaks to the primitively 



Fig. 88. Female generative 

 organs of Ascaris lnmbri- 

 coides. ov Ovaries, do Ovi- 

 duct, u Uterus, v Vagina. 



