. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 100 AXD JDEVELOPME>'T OF ANIMALS IN GENEKAL. 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 to the ova and spermatozoa (fig. 92), but which, as in HeUx (fig. 93), may partially s^eparate into
. Elementary text-book of zoology, general part and special part: protozoa to insecta. Animals. 100 AXD JDEVELOPME>'T OF ANIMALS IN GENEKAL. 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 to the ova and spermatozoa (fig. 92), but which, as in HeUx (fig. 93), may partially s^eparate into vas deferens and oviduct. In other cases the ovaiies and testes appear as completely separated glands with separate ducts, which may still open into a common cloaca (Cestoda, Trematoda, rhabdocoele Turbellarians, fig. 94), or may possess separate open- ings (Hirudinea, fig. 95). Two hermaphrodite in- dividuals may, and this appeal's to be the rule, mutually fertilise each other at the same time, or cases may occur in such hermaphrodites in which self-fertiUzation is sufficient for the production of off- sjiring. But this original condition of self-fertiliza- tion appears to be tlie ex- ception in almost all hermaphrodites. In those 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 i-udi- mentary, to reach the dioecious condition in which the sexes are completely separated (Disiomumjillicolle and hcematohiimi). Animals in which the sexes are distinct not unfrequently piesent traces of an. Fig. 93.— Sexual organs of the Roman Snail (Helix pomatia). Zd, hermaphrodite gland; Zg, its duct; Ed,
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanimals, bookyear1892
