32 THE LIVEK-FLUKE 



number of yolk-cells. The germ-cell segments 

 before the egg is laid, but no further development 

 occurs till after its escape from the body of the 

 host. Afterwards an embryo is formed, and a cir- 

 cular operculum at one end of the shell opens to 

 allow it to escape. 



2. The free embryo is conical, with a short papilla at its 



broad anterior end ; the whole surface is covered 

 with long cilia enabling the embryo to swim rapidly. 

 Two eye-spots are present, and two ciliated funnels, 

 probably excretory. The ectoderm is a single layer 

 of flattened cells, usually arranged in five transverse 

 bands, within which is a mass of granular cells. 

 When this embryo meets with Limncea truncatula, 

 a small freshwater snail, the head papilla becomes 

 elongated and by means of it the embryo bores its 

 way into the snail. The free life of the embryo is 

 usually of about eight hours' duration. 



3. Development of the sporocyst. Within the snail, usually 



in its pulmonary chamber, the ectoderm cells of the 

 embryo swell and lose their cilia ; the embryo grows 

 rapidly, and in two or three w r eeks becomes an elon- 

 gated sac, *6 mm. long. This sac, the sporocyst, 



FIGS. 8 to 12. Fasciola hepatica. Five stages in the life-history. (After 



Thomas.) 



FIG. 8. The free-swimming embryo. 

 JT IG> 9, A sporocyst, containing developing redias. 



FIG. 10. A young redia. The shaded area represents the digestive sac. 

 FIG. 11. An adult redia, containing one daughter-redia, two cercarise 



approaching maturity, and germs in various stages. The shaded 



area represents the digestive sac. 

 FIG. 12. A free cercaria. 



The following letters have the same signification in the five figures : C, 

 nearly ripe oercarise. CC, cystogenous cells of cercaria. DR,, daughter-redia. 

 DT, limbs of the digestive tract. F, head-papilla. H, eye-spots. H', the 

 same degenerating. K', germinal cell. L, epaulet-like cells of first row of 

 ectoderm cells. M, embryo in optical section ; gastrula stage. N, pharynx 

 of redia. O, digestive sa'c. OE, O3sophagus. P, lips of redia. Q, collar. 

 R, processes serving as rudimentary feet. S, embryos between the morula 

 and gastrula stages. T, trabeculse crossing body-cavity of redia. U, 

 cells supposed to be glandular. V, birth-opening, by which cercarite and 

 daughter-redia3 escape from the redia. "W, morula, still retained in the body-wall. 

 "W, larger morula. Y, oral sucker of cercaria. Y' ventral sucker. Z, 

 pharynx of cercaria. 



