DERMAPHIIODITISM. 



99 



eggs or of the developing embryos (uterus). Their terminal section 

 presents differentiations subserving fertilization (receptaculmn 

 seminis, vagina, copulatory pouch, external generative organs). The 

 efferent ducts of the testis, the vasa deferentia, likewise frequently 

 give rise to reservoirs (vesicular seminales) and receive glands (pros- 

 tate), the secretion of which mixes with the sperm fluid or surrounds 

 aggregations of the spermatozoa with a firm sheath (spermatophors). 



The terminal section of the vas deferens becomes exceedingly 

 muscular, and gives rise to a ductus ejaculatorius, which, as a rule, 

 is accompanied by an external organ of copulation to facilitate the 

 conveyance of the semen into the female generative organs. The 

 generative organs present 

 either a radial (Coeienterata, 

 Echinodermata) or a bilate- 

 rally symmetrical arrangement 

 (fig. 91), a contrast which is 

 visible in the typical arrange- 

 ment of all the systems of 

 organs. 



The simplest and most 

 primitive condition of the 

 generative organs is the her- 

 maphrodite. Ova and sper- 

 matozoa are produced in the 

 body of one and the same 

 individual, which thus unites 

 in itself all the conditions 

 necessary for the preservation 

 of the species, and alone 

 represents the species. Instances of hermaphroditism are found in 

 every group of the animal kingdom. But they are especially nume- 

 rous in the lower groups, and also in animals in which the movements 

 are slow (Land-snails, Flat- worms, Hirudinea, Oligochoeta), or which 

 live singly (Cestoda, Trematoda), or in attached animals which are 

 without power of changing their position (Cirripedia, Tunicata, 

 Bryozoa, Oysters). The hermaphrodite arrangement of the gene- 

 rative organs presents great variation, which, to a certain extent, 

 forms a gradual series tending towards the separation of the sexes. 



In the simplest cases, the points of origin of the two kinds of 

 generative products lie close to one another, so that the spermatozoa 

 and ova meet directly in the parent body (Ctenophora, Chrysaora). 



U 



FIG. 92. Sexual organs of a Pteropod (Cymbulia) 

 (after Gegenbaur.) a, Zd, hermaphrodite gland 

 vith common duct ; Kg, receptaculum seminis ; 

 V, uterus, b, Acinus of the hermaphrodite gland 

 of the same. 0, ova ; S, spermatozoa. 



