. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 318 PLATTnELMIXTIIES. D Dr. cirrus sac, which encloses the protrnsible terminal part (cirrus) 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). T
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 318 PLATTnELMIXTIIES. D Dr. cirrus sac, which encloses the protrnsible terminal part (cirrus) 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 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. Subsequently 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 Trematodes lay their eggs; only a few are viviparous. The just-hatched young either possess (in most Polystomea] 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-. FIG. 25-i.—Distomum hepaticum (after Sommer). O, M
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