:3is 



PLATTHELMINTHES. 



D 



cirrus sac, which encloses the pvotrusible terminal part (cirras) of the 

 vas deferens. The vas deferens soon divides into two, which lead 

 back to the two large simple or multilobed testes. The supposed 

 third vas deferens, which, according to v. Siebold, runs from one 

 testis to the female sexual apparatus, so as to permit of direct ferti- 

 lization without copulation, has been recognized as a vagina opening 

 to the exterior on the dorsal surface (canal of Laurer). The 



female organs consist of a convo- 

 luted uterus and of the glands 

 concerned in the preparation of the 

 egg, viz., an ovary and two yolk 

 glands. There is sometimes in ad- 

 dition a special shell gland. The 

 true ovary which produces the pri- 

 mary ova is a round body, and is 

 usually placed in front of the testes. 

 The yolk glands which secrete the 

 TT yolk are much ramified tubular 

 glands, and fill the sides of the body 

 (fig. 254). The yolk particles come 

 in contact with the primary ova in 

 the first portion of the uterus, and 

 surround them in greater or less 

 quantities. Subseqiiently each 

 ovum, with its investment of yolk, 

 is surrounded by a strong shell. The 

 ova in their passage along the uterus 

 become packed together, often in 

 great numbers, and undergo the 

 stages of embryonic development 

 in the body of the parent. Most 

 FIG. 2o4,.DistoHiitm krjm/iritm (after Trematodes lay their eggs ; only a 



Sommer). O, Mouth ; D, limb of in- r vivirvivons 



testine; S, sucker; T, testes; Do, ' VlVlpai .1*. 



viteiiarium ; Ou (utems), oviduct ; Dr, The just-hatched young either 



accessory glands. , . 7 



possess (in most roLystomea) the 



form and organization of the parent; or they present the phenomenon 

 of a complicated alternation of generations (heterogamy) connected 

 with a metamorphosis (Distomea). In the first case, the large eggs 

 become attached in the place where the mother lives ; in the last 

 case, the relatively small eggs are deposited in a damp place, usually 

 in the water. After the completion of the segmentation and the em- 



