v PHYLUM PLATYHELMINTHES 265 



often a bursa copulatrix, " shell-glands," vitelline or yolk-glands, 

 and cement-glands. In most, though not in all, there is a single 

 or paired germarium (ovary), in which the ova are formed, and a 

 set of vitellaria or vitelline glands, producing material which sur- 

 rounds each of the mature impregnated ova before it becomes 

 enclosed in its shell. In some, on the other hand, ova and vitelline 

 matter are formed in the same organ the germ-vitellarium. The 

 shell-glands were so named because they were usually supposed to 

 secrete the chitinoid substance of the egg-shells ; but the share 

 which they take in this process, if they take any, is uncertain, and 

 they are better called accessory female glands. The cement-glands 

 secrete a viscid material for causing the eggs to adhere together, 

 enclosing them in a cocoon or fastening them to some foreign body. 

 The oviduct is the passage by which the ova reach the exterior 

 from the ovary ; but an enlarged part of this passage, into which 

 ducts of the accessory glands open, is distinguishable as the ootype, 

 while a terminal part, leading to the female aperture, may be modi- 

 fied as a vagina. In some cases (Heterocotylean Trematodes) there is 

 a vagina or a pair of vaginaB in the shape of a passage, or a pair of 

 passages, distinct from the oviduct and opening independently on 

 the exterior. A uterus in the form of an enlarged part of the 

 oviduct or of an outgrowth from the latter or from the atrium is 

 very usually developed for the reception of the completed eggs. 

 A special sac or bursa copulatrix, lined with spines, acts as the 

 female copulatory organ. A sac, the receptaculum, opening into 

 the oviduct or into the atrium (Figs. 204, 205, r.v.), may serve as a 

 reservoir for the semen received in copulation or for the vitelline 

 matter or yolk, or for surplus reproductive material. Male and 

 female ducts sometimes have separate and independent openings ; 

 but very usually there is a common chamber or genital atrium 

 into which both lead, opening on the exterior by a single 

 aperture. 



In the Poly clad Turbellaria (Fig. 215) 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 double or 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 two elongated uteri, in which numerous large eggs 

 containing abundant yolk collect. The ducts of the uteri open 

 into a median egg-duct or vagina, with which the ducts of the 

 accessory glands communicate, and in which the eggs receive 

 their chitinoid investment, derived in this case from the yolk- 

 material of the substance of the ova. The vagina leads to the 

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

 by a muscular sheath. 



