312 



FLATYHELMINTHES . 



project here and there freely into the vessels. As a rule, several 

 openings occur on the main trunk of this excretory apparatus. 



Reproduction may take place asexually by transverse fission, e.g., 

 Derostomea (Catenida) and Microstomea (fig. 248). With the 

 exception of the Microstomea, the Turbellaria are hermaphrodite; 

 but steps intermediate between the hermaphrodite and the dioecious 

 condition seem by no means to be wanting, for, according to 

 MetschnikofF, in Prostomum lineare the male generative organs 

 are sometimes developed, while the female remain rudimentary ; 



and sometimes, on the other 

 hand, the reverse holds. In 

 Acmostomum dioecum also the 

 sexes are separate. In the her- 

 maphrodite forms the male sexual 

 organs consist of testes, which 

 mostly lie as paired tubes at the 

 sides of the body, also of yesi- 

 cula3 seminales, and of a protru- 

 sible copulatory organ beset with 

 hooks. The female organs con- 

 sist of ovaries, yolk glands 

 (vitellarium), a receptaculum 

 seminis, a vagina, and a uterus 

 (fig. 249). The male copula- 

 tory organ and the vagina open 

 as a rule by a common orifice 

 upon the ventral surface. Some- 

 times, as in the Rhabdoccele 

 genus Macrostomum, the vitella- 

 rium (yolk gland) nnd ovary 

 are united; the ova being 

 produced at the upper blind end 

 of the ovary, and the yolk at the 

 lower end of the same gland. In the marine Dendroccela, on the 

 other hand, the vitellarium is generally absent. After fertilization, 

 a hard, usually reddish-brown shell begins to be formed round the 

 ovum. In such cases, the hard-shelled eggs are laid ; but among the 

 Rhabdoccela, in Schizostomum and certain Mesostomea (M. Ehren- 

 bergii), transparent eggs furnished with thin, colourless capsules, 

 and undergoing development in the body of the parent, are often 

 produced. According to Schneider, the production of these thin* 



249. Generative apparatus of Mesotto- 

 mum Ehrenbergil (combined from Graff 

 and Schneider). S, Pharynx ; Go, sexual 

 openings ; Ov, ovary ; Ut, uterus, with 

 winter eggs ; Do, yolk gland ; Dg, duct 

 of yolk gland ; T, testis ;-. Vd, vas deferens ; 

 P, penis ; jR#, receptaculum seminis. 



