LIFE-HISTOKY 35 



in the cavity of the sporocyst. Each of these be- 

 comes flattened on one side, and then invaginated 

 to form a gastrula : this increases in size, elongates, 

 and develops into a redia. The redise so formed 

 force their way out of the sporocyst and become 

 free : the wound in the sporocyst heals, and other 

 rediae are formed in the same way within it. 

 2. The free rediae wander about in the snail, increasing in 

 size and being especially abundant in the liver. The 

 adult redia (fig. 18) is a cyliadrical body about 1'5 

 mm. 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 ' 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. ' 



Prom the inner surface of the body-wall of the 

 redia, cells are budded off which develop into gastrulas 



Pigs. 15 to 19. — Fasciola hepatica. Five stages in the life-history. 



(After Thomas.) 

 Fig. 15.— The free-swimming embryo. 

 Fig. 16.— a sporocyst, containing developing redise. 

 Fig. 17. — A young redia. The shaded area represents the digestive sac. 

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



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 ia the five figures t—C, 

 nearly ripe oeroarise. CC, oystogenous cells of cercaria. DB, daughter-redia. 

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

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

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

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

 Q, collar of redia. K, processes of redia serving as rudimentary feet. S, 

 embryos between the morula and gastrula stages.' T, trabecule crossing body- 

 cavity of redia. IT, cells in redia supposed to be glandular. "V, birth-opening, 

 by which cercariffi and daughter-rediae escape from the redia. "W, morula, 

 still retained in the body-wall. W, larger morula. T, oral sucker of corcaria. 

 Y', ventral sucker. Z, pharynx of cercaria. 



D 2 ' 



