{39}[Illustration: FIG. 17.—Diagrammatic view of the structure of _Planaria_ (_Dendrocoelum_) _lactea_. × 7. The body has been cut across and a portion removed. In the posterior half the alimentary tract of the left side is removed and the uterus, penis, and muscular organ sliced open horizontally. The nervous system is represented by black, and the yolk-glands by dotted lines. _br_, Brain; _ey_, eye with lens and optic nerve; _go_, external genital aperture for both male and female products; _gs_, genital atrium; _lg_, paired lateral intestinal branch; _ln_, longitudinal nerve; _mg_, unpaired anterior intestine, the branches of which are cut off close to the main stem; _mgr_, eversible "musculo-glandular organ"; _nc_, nerve-cells in the pharynx; _nn_, lateral nerve-twigs; _ns_, nerve-sheath; _ov_, ovary; _ovd_, oviduct; _pe_, the eversible penis, the corrugated inner white portion of which is the apex; _ph_, pharynx; _phs_, pharyngeal sheath; _pr_, "prostate" or granule-gland (represented by dotted lines opening into the penis); _ps_, penial sheath; _te_, testes; _to_, tactile lobe of the head; _ut_, "uterus" opening into the genital atrium just above _mgr_; _vd_, vasa deferentia; _vs_, vesicula seminalis; _yg_, yolk-glands; _yo_, openings of the yolk-ducts into the oviducts.] {40}Fertilisation appears to occur in the uterus, where ova, yolk, and spermatozoa, or (in _P. torva_) spermatophores (Fig. 14, N, _sp_), are found. The formation of the cocoon in _Planaria lactea_ is probably begun in the "uterus," but is undoubtedly completed in the genital atrium. In _P. polychroa_, however, the stalked cocoon is formed wholly in the "uterus." Thus we find two types of cocoons in different species of the genus _Planaria_ associated with two types of reproductive organs (Hallez):— I. Planariae in which the two oviducts open separately into the posterior part of the duct of the uterus. A musculo-glandular organ is absent. The cocoons are spherical and stalked. Examples—_Planaria polychroa_ (Fig. 14, L), _P. albissima_, _P. gonocephala_. II. Planariae in which the two oviducts open by means of an unpaired duct into the genital atrium. A musculo-glandular organ present (_Planaria torva_ (Fig. 14, N), _P. mrazekii_, _P. lactea_, _P. cavatica_), or absent (_P. alpina_, Fig. 14, M). The cocoons are sessile. The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very much from those of _Planaria_. The uterus (greatly reduced in the Land Planarians) lies behind the genital pore, and several ova, together with much milky yolk, are enclosed in a capsule which is formed in the genital atrium.