LIFE HISTOEY. 85 



redise may be obtained from specimens oi Limncsa truncatula, 

 kept in the same vessel. 



The main features in the life history are as follows. 



A. The First Generation is produced sexually, but is itself 

 asexual. 



1. The eggs, laid in large numbers in the bile-ducts of the 



sheep or other infected mammal, pass with the bile 

 into the intestine, and so escape from the body. 

 They are ovoid bodies 0"005 inch long and O'OOS inch 

 broad, enclosed in smooth brownish chitinous shells. 

 Within each shell are a single ovum or germ-cell 

 and a large number of yolk-cells. After the escape 

 of the egg from the body of the host segmentation 

 occurs, an embryo develops, and a circular operculum 

 at one end of the shell opens to allow it to escape. 



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



broad anterior end ; the whole surface is covered 

 with long cilia, enabling the larva to swim rapidly. 

 Two eye-spots ^are present, and two flame-cells, 

 probably excretory. The ectoderm is a single layer 

 of flattened cells, usually arranged in five transverse 



Figs. 15 to 19.— Fasciola hepatica. Five stages in the life, history. 



(After Thomas.) 

 KiG. 15. — The free-swimming larva. 

 Fio. 16. — A sporocyst, containing developing redise. 

 Fig. 17. — A young redia. The shaded area represents the d^gestive sac. 



Fig. 18. — An adult redia, containing one daughter-redia, two cercariffl 

 approaching maturity, and germs in various stages. The shaded 

 area represents the digestive sac. 



Fig. 19. — A free cercaria. 



The following letters have the same signification in the five figures ; — O, 

 nearly ripe cercarise. 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, ethbryo in optical section ; gastrula stage. W, pharynx 

 of redia. O, digestive sac. OE, oesophagus of cercaria. P, lips of redia. 

 Q, collar of redia. R, processes of redia serving as feet. S, embryos 

 between the morula and gastrula stages. T, trabeoulse crossing body-cavity 

 of redia. TJ, cells in redia supposed to be glandular. V, birth-opening, 

 by which cercarise and daughter-redise 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. 



D 2 



