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THE POPULAR SCIENCE MONTHLY 



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maining germ redia? continue to grow. The sporocysts also multiply 

 by simple division, thus causing still greater increase in the number 

 of parasites in the snail. A young redia is shown in Fig. 8, with the 

 contents forming into the third generation of the liver-fluke. A full- 

 grown redia is seen in Fig. 9, much 

 magnified. Some of the germs of 

 the cercaria, as they are termed, 

 may be seen within the redia. Each 

 of these germs develops into a tad- 

 pole-like animal, with a slender 

 tail. A redia may produce a score 

 of the cercaria?, which escape from 

 the parent through a special open- 

 ing and then wriggle their way 

 out of the snail. The free-swim- 

 ming life of the " tadpole " does 

 not last long, and after coming to 

 rest it draws its body into a small 

 sphere, and exudes a gummy sub- 

 stance, which protects it from in- 

 jury. These encysted cercaria? are 

 destined to find entrance to the 

 liver of the sheep and then de- 

 velop into the full-grown fluke. 

 Fig. 10 shows a free cercaria as 

 seen swimming in water, and in 

 Fig. 11 is seen a portion of grass 

 upon -which three cysts are fast- 

 ened. The cysts remain attached 

 to the herbage of the pasture, and 

 are swallowed by the sheep in 

 feeding upon the ground. If the 

 cysts are not picked up by the 

 sheep within a few weeks, the 

 young flukes which they contain will perish. It has been determined 

 that two hundred or more cercaria? may descend from a single fluke- 

 egg, and, if the rediae give rise to a generation of daughter-redia?, a 

 single egg may produce more than a thousand cercaria 1 . " Not only 

 does the race of the liver-fluke multiply and increase abundantly in the 

 sheep by producing myriads of eggs, but there is a further and great 

 increase of the forms within the snail. If only the greatest degree of 

 ordinary increase were reached, a single fluke might give rise to more 

 than a hundred million descendants in the next generation of liver- 

 flukes proper inhabiting the sheep. But, fortunately for farmers, the 

 chances are enormously against any such disastrous increase." 



Professor Thomas has determined that at least six weeks elapse, 



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Fig. 8. 



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Fig. 9. 



