v PHYLUM PLATYHELMINTHES 271 



material. Male and female ducts sometimes have separate and 

 independent openings; but very commonly there is a common 

 chamber or genital atrium into which both lead, opening on the 

 exterior by a single aperture. 



In the Polyclad Turbellaria (Fig. 212) the testes are numerous, 

 and there are a corresponding number of fine tubes which 

 combine to form the two vasa deferentia, leading to the male 

 aperture with its penis. The latter is sometimes multiple. The 

 ovaries consist of numerous small rounded masses of cells, and 

 there are no separate yolk-glands. Numerous narrow oviducts 

 lead from the ovaries, and unite to form larger ducts ; these, in 

 turn, open into elongated uteri, in which numerous eggs collect. 

 The uteri open into a median egg-duct, with which the ducts of 

 the shell glands communicate (polype), and in which the eggs 

 receive their chitinoid investment. This leads to the female 

 aperture, a part of it being, in some cases, surrounded by a 

 muscular sheath to form a bursa copulatrix. 



In many cases the egg-duct gives off posteriorly a narrow duct 

 which usually terminates behind in a vesicle known as the accessory 

 sac or receptaculum seminis, which may be double. In a few Polyclads 

 this duct opens on the exterior on the ventral surface some 

 distance behind the main female aperture, in one instance on the 

 dorsal surface. A genito-intestinal canal connecting this duct with 

 one of the intestinal caeca has been found in one Polyclad. In 

 most cases male and female apertures are distinct from one 

 another, the former being situated in front of the latter. But 

 sometimes, though rarely, both lead into a common chamber or 

 atrium with a single opening on the exterior. 



In the Tricladida (Fig. 213) there are also numerous testes, but 

 the fine tubes connecting them with the two vasa deferentia are 

 absent. There are two germaria, situated far forwards, and 

 numerous yolk-glands. Two oviducts, into which the yolk is dis- 

 charged from the yolk-glands by a series of lateral apertures, lead 

 from the ovaries to unite in a median ootype or vagina, receiving 

 the ducts of glands which may secrete the substance of the cocoon. 

 The condition is thus intermediate between that observable in 

 most of the Rhabdocoeles and that which characterises the Poly- 

 clads. Though germaria and vitellaria are separate, they have a 

 common duct, and might be regarded as distinct lobes of one 

 germo-vitellarium. A uterus is present, formed as an outgrowth 

 of the vagina or of the atrium, or as an independent sac or pair of 

 sacs opening independently on the exterior. There may be a 

 receptaculum seminis, and in some there is a duct of communica- 

 tion between this and the intestine (genito-intestinal canal). A 

 common genital atrium with a single external aperture receives 

 the ducts of both sexes. 



In the Rhabdocoeles (Figs. 211 and 215) there are usually only 



