[Illustration: FIG. 24.—_Polystomum integerrimum_ Fröh., from the bladder of the Frog, and seen from the ventral surface. The alimentary canal is black, the white dots upon it being the yolk-glands, _dvi_, Ductus vitello-intestinalis (probably homologous with the Laurer's canal or "vagina" of Digenea); _eh_, hooks of sucking disc; _int_, intestine; _m_, mouth; _ov_, ovary; _pe_, penis; _ph_, pharynx; _sc_, suckers with an embryonic hook persisting in each; _te_, testes; _ut_, uterus with eggs; _vag_, left vagina; _vd_, vas deferens; _yd_, yolk-duct; _yg_, yolk-glands; ♂ ♀, common genital aperture. (Modified from Zeller.) × 8.] The reproductive organs of the Polystomatidae may be understood from Figs. 24, 27, and 28. At the point of union of the oviduct (Fig. 28, _ovd_), the vitelline ducts (_yd_), and the commencement of the uterus (_ut_), a slender duct is given off which opens into the intestine, and is known as the "vitello-intestinal canal" (Fig. 24, _dvi_; Fig. 28, _gic_). This duct has apparently the same relations as the "canal of Laurer" of Digenea,[73] except only that the latter opens to the exterior directly. In connexion with this vitello-intestinal canal a "vagina" is present, which in _Polystomum_ and most Monogenea is paired (Fig. 24, _vag_), in _Diplozoon_ and in one {58}or two other forms, however, unpaired. The vagina receives the penis of another individual during copulation (Fig. 26), and does not appear to have an homologue in the liver-fluke or other Digenea. [Illustration: FIG. 25.—Eggs of Monogenea. A, Eggs of _Encotylabe pagelli_ v. Ben.-Hesse; B, eggs of _Udonella pollachii_ v. Ben.-Hesse (with young forms just hatching out); C, egg of _Microcotyle labracis_ v. Ben.-Hesse. (After van Beneden and Hesse.) × 50.] LIFE-HISTORIES OF THE POLYSTOMATIDAE.[74]—_Polystomum integerrimum._ After the mutual fertilisation of two individuals, the eggs are laid in the water by the protrusion of the body of the parent through the urinary aperture of the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day for ten days. After about six weeks, the larva (.3 mm. long) hatches out, and swims about freely by means of bands of large ciliated cells (Fig. 26, A); but if it does not meet with a tadpole within twenty-four hours, it dies. Should it, however, encounter one, the larva creeps along it in a looping fashion until it approaches the opercular spout, or opening of the branchial chamber, on the left side; into this it darts suddenly, fixes itself, and throws off its cilia. Here it remains eight or ten weeks, feeding, increasing in size, and forming the suckers from behind forwards. {59}At the time of the tadpole's metamorphosis, the young _Polystomum_ works its way down the pharynx into the oesophagus and along the intestine, till it reaches and enters the opening of the bladder. Three years afterwards it becomes mature.