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 to 

 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, rhabdocoele 

 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 

 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, x 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 (Distoinumjillicolle and hcemaiobium). Animals 

 in which the sexes are distinct not ^infrequently 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, vas deferens ; P, protrusible penis ; 

 Fl, flagellum ; Bn, receptaculum seminis ; D, 

 finger-shaped gland ; L, Spiculum amoris ; Go, 

 common genital opening. 



