266 ZOOLOGY SECT. 



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 : this 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 Poly clad. 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. 216) 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 Rhabdocceles 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 Rhabdocceles (Figs. 214 and 218) there are usually only 

 two compact testes and two vasa deferentia leading to the unpaired 

 male aperture at the extremity of the cirrus. The prostate or 

 granule glands a set of unicellular glands, which secrete round, 

 bright granules destined to mix with the sperms are specially 

 well developed in the Rhabdocceles, and are present in some other 

 Turbellaria and in certain Trematodes. Ovaries (germ-vitellaria) 

 alone occur in some, separate germaria and vitellaria in others ; 

 there are either two germaria or one only. A receptaculum may 

 be present as a swelling or diverticulum of the main female 

 duct, or of the atrium. The terminal part of this duct may form a 

 muscular vagina, or there may be a muscular bursa copulatrix 

 developed from the wall of the atrium. A uterus is present in 

 most cases as an outgrowth from the wall of the atrium. Male 

 and female ducts have a common chamber or genital atrium with a 

 single external opening. 



In the Acwla there are in nearly all cases separate male and 

 female apertures. The two testes are divided into numerous small 



