. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 312 PLATTHELMIN1HES. 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, , Derostomea (Cutenula) and 21 icrostomea (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 Prostomu
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 312 PLATTHELMIN1HES. 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, , Derostomea (Cutenula) and 21 icrostomea (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 dicecum also the sexeo 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 vesi- cular serninales, and of a protru- sible copulatory organ beset with hooks. The female organs con- sist of ovaries, yolk glands (vitellarium), a receptaculnm 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) and 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 Dendrocoula, 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 Rhabdoccola, in Scldzostomum and certain Jhsostomea (M. Hhren- bergii), transparent eggs furnished with thin, colourless capsules, and undergoing development in the body of the parent, arc often produced. According to Schneider, the production of these thin-. 249.—Generative apparatus of Me
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