LTFE-HISTORY. 43 



in the cavity of the sporocyst. Each of these morulae 

 becomes flattened on one side, and then acquires 

 a rudimentary gut, and body-cavity. The larvae 

 increase in size, elongate, and develop into rediae. 

 The rediae so formed force their way out of the 

 sporocyst and become free : the wound in the sporo- 

 cyst heals, and other rediae are formed in the same 

 way within it. 

 The free rediee wander about in the snail, increasing in 

 size and being especially abundant in the liver. The 

 adult redia (fig. 20) is a cylindrical body about - 06 

 inch long, with a collar -like ridge running round it 

 near the anterior end, and with a pair of blunt pro- 

 cesses projecting from the hinder part of the ventral 

 surface, which aid in locomotion. The body-wall 

 resembles that of the sporocyst in structure, but is 

 more muscular, and has definite excretory canals 

 which commence as funnels with ' flame-shaped ' 

 bunches of cilia. 



The alimentary tract is a comparatively short- 

 blind sac, with walls one cell thick : the mouth is 

 at the anterior end, and behind it the wall of the sac 

 is thickened to form a strong muscular pharynx. 



Figs. 17 to 20. Fasciola hepatica. Five stages in the life-history. 

 (After Thomas.) 



Fig. 17. The free-swimming larva or ' miracidium.' 



Fig. 18. A sporocyst, containing developing rediae. 



Fig. 1.9. A young redia. The shaded area represents the digestive sac. 



Fig. 20. An adult redia, containing one daughter-redia, two cercariae 

 approaching maturity, and germs in various stages. The shaded 

 area represents the digestive sac. 



Fig. 21. A free cercaria. 



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

 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, embryo in optical section; gastrula stage. N, 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 stages. T, trabeculae crossing body-cavity of redia. u, cells 

 in redia supposed to be glandular. V, birth-opening, by which cercarisB 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. 



