REPRODUCTION.—In _Planaria lactea_ the numerous testes (Fig. 17, _te_) are placed both above and below the alimentary canal throughout the greater part of its course. The membrane of each gonad is continued into a minute vas efferens, which unites with those of neighbouring testes. Two vasa deferentia (_v.d_) arise thus on each side, one from the posterior, the other from the anterior testes of the body, and open into the vesiculae seminales (_v.s_), which may be seen in the living animal as tortuous whitish tubes at the sides of the pharynx (Fig. 14, A). These open into the penis (Figs. 14, A; 17, _pe_), a large pyriform organ, the apex of which, when retracted, points forwards, projecting into the penial cavity. When this apical portion is evaginated and turned inside out, it is of considerable length, and is able to pass into the long slender duct of the uterus (_ut_) of another individual. The penial sheath (_ps_) is part of the genital atrium (_gs_), which is developed as a pit from the skin, and invests the end of the genital ducts, the mouth of the pit forming the common genital pore (_go_), through which both male and female genital products are emitted. There are two ovaries (_ov_) placed far forwards, between the third and fourth pairs of intestinal coeca. The oviducts (_ovd_) lie just over the lateral nerves, and have a slightly tortuous course, at each outward bend receiving the duct (_yo_) of a yolk-gland (_yg_), so that ova and yolk are already associated when the oviducts open by a short unpaired tube into the genital atrium. The yolk-glands develop rapidly,[57] and when fully formed are massive glands occupying the spaces between the intestinal branches and the testes which are then aborting. The so-called _uterus_ (_ut_), apparently at first a diverticulum of the genital atrium, expands behind the pharynx into a receptacle lined by long glandular columnar cells, which, however, are not all of the same kind. The uterine duct opens into the atrium just above the aperture of a problematical, eversible, "musculo-glandular organ" (_mgr_).