Fig. 144.—Common liver fluke (Fasciola hepatica), natural size. (Stiles, Annual Report, U.S.A. Bureau of Agriculture, 1901.)
Fig. 145.—The common liver fluke (Fasciola hepatica), enlarged to show the anatomical characters. a, Acetabulum; c, cirrus pouch; i, intestine; m, mouth with oral sucker; o, ovary; p, pharyngeal bulb; s, shell gland; t, profusely branched testicles; u, uterus; va, vagina; vg, profusely branched vitellogene gland. (After Stiles, 1894, p. 300.)
Fig. 146.—Egg of the common liver fluke (Fasciola hepatica), examined shortly after it was taken from the liver of a sheep. At one end is seen the lid or operculum, o; near it is the segmenting ovum, e; the rest of the space is occupied by yoke cells which serve as food; all are granular, but only three are thus drawn. × 680. (After Thomas, 1883, p. 281, Fig. 1.)
Fig. 147.—Egg of the common liver fluke containing a ciliated embryo (miracidium) ready to hatch out: d, remains of food; e, cushion of jelly-like substance; f, boring papilla; h, eye-spots; k, germinal cells. × 680. (After Thomas, 1883, p. 283, Fig. 2.)
(b) Eggs (Figs. 143, 146 and 147).—Each egg is composed of the following parts: (1) A true germ cell, which originates in the ovary and is destined to give rise to the future embryo; (2) a number of vitelline or yolk cells, which are formed in a specialised and independent portion (vitellogene gland) of the female glands—instead of developing into embryos the yolk cells form a follicle-like covering for the true germ cell, and play an important rôle in the nutrition of the latter as it undergoes further development; (3) a shell surrounding the germ cell and vitelline cells, and provided at one end with a cap or operculum. The eggs escape from the uterus of the adult through the vulva, are carried to the intestine of the host with the bile, then pass through the intestines with the contents of the latter, and are expelled from the host with the fæcal matter. Many of them become dried, and then undergo no further development; but others are naturally dropped in the water in marshes, or, being dropped on dry ground, they are washed into water by the rain, or are carried to a more favourable position by the feet of animals pasturing or passing through the fields. After a longer or shorter period of incubation, which varies with the temperature, a ciliated embryo (miracidium) is developed. At a temperature of 20° to 26° C. the miracidium may be formed in ten days to three weeks; at a temperature of 16° C. the development takes two to three months; at 38° C. it ceases entirely. Experiments have shown that as long as these eggs remain in the dark the miracidium will not escape from the egg-shell; accordingly it will not escape during the night. When exposed to the light, however, or when suddenly brought into contact with cold water, the organism bursts the cap from the egg-shell, crawls through the opening, and becomes a—
