Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 312 PLATYHELMINTHES. 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 (Catenula) 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 lie wanting, for, according to Metschnikoff, in Prosto
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 312 PLATYHELMINTHES. 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 (Catenula) 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 lie wanting, for, according to Metschnikoff, in Prostomum linectre the male generative organs are sometimes developed, while the female's remain rudimentary; and sometimes, on the other hand, the reverse holds. In Acmostomum dicecwm 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 vesi- culse 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- riurn (yolk gland) and ovary are united; the ova being produced at the upper blind end of the ovary, and the yolk at the In the marine Dendrocoela, 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 Schizostoiuum 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- FIG. 249.—Generative apparatus of Mesosto- iiiinii E/
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