. Elementary text-book of zoology. Zoology. 100 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-vascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common tq the ova and spermatozoa (fig. 92), but which, as in Helix (fig. 93), may partially separate into vas deferens and oviduct. In other cases the ovaries and t
. Elementary text-book of zoology. Zoology. 100 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. The elements of both sexes arise in layers of cells which have a definite position beneath the entodermal lining of the gastro-vascular canals, and can be traced back to growths of the ectoderm. At a higher stage the ovaries and testes are united in one gland, the hermaphrodite gland (Synapta, Pteropoda), provided with a single duct common tq the ova and spermatozoa (fig. 92), but which, as in Helix (fig. 93), may partially separate into vas deferens and oviduct. In other cases the ovaries and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trematoda, rhabdoccele Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appears to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertilization is sufficient for the production of off- spring. But this original condition of self-fertiliza- tion appears to be the ex- ception in almost all hermaphrodites. In those fio'Jtl animals in which the ovary and testis are not com- pletely separated from one another cross-fertilization is rendered necessary, and self-fertilization prevented by the fact that the male and female elements are matured at different times (Snails, Salps). From *this form of complete hermaphroditism the generative organs pass through a stage of incomplete hermaphroditism, in which, though the organs of both sexes are present, one of them is rudi- mentary, to reach the dioecious condition in which the sexes are completely separated (Distomumfillicolle and hcematobiuiu.). Animals in which the sexes are distinct not unfrequently present traces of an. FIG. 93.— Sexual organs of the Roman Snail (Helix pomatia). Zd, hermaphrodite gland; Zg, its duct; Ed, albumen gland; Od, oviduct and seminal groove; Vd, v
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
