FIG. 90. 



A series of diagrams illustrating the life cycle in the LIVER FLUKE (Distomum) . 

 After Thomas, Leuckart, and others. A, egg in its case; B, early embryo, still in 

 case; C, free-swimming ciliated embryo; D, same after encysting in tissues of snail 

 (sporocyst) ; E, sporocyst at later stage producing by internal, non-sexual processes 

 new sporocysts, and redia (r~) which break from the sporocyst and lead an inde- 

 pendent life of their own in the tissues of the snail; F, a mature redia producing 

 within itself new generations of rediae, and a new type of larva, cercariee which 

 escape by a birth-pore (b.p.~) and make their way into the water; G, cercaria; H, 

 same after losing its tail and becoming encysted; /, the young fluke in the liver of 

 the sheep, where it becomes sexually mature and produces perhaps 500,000 new 

 eggs, b, brain; b.p., birth pore; c, cercaria; c.m., cell masses, embryos formed non- 

 sexually within sporocysts and redise; e, eye-spots; ex., excretory tubules and pore 

 (only the posterior portion shows); g, gut; m, mouth; ph, pharynx; r, redia; s, 

 suckers; sc, sporocyst; +, stages in which non-sexual reproduction occurs; *, stage at 

 which sexual reproduction occurs. 



Questions on the figures. In which stages are eyespots found? Num- 

 ber and position of the suckers? In which stages found? What is the 

 result of increasing the points at which reproduction occurs in the cycle? 

 Is this a combination of metamorphosis and alternation of generation? 

 Your reasons for your answer? Compare this with the life history of the 

 tape-worm. Note the encysted stage by which it passes from water to its 

 host in each instance. 



,89 



