Page:Popular Science Monthly Volume 23.djvu/764

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THE POPULAR SCIENCE MONTHLY.

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. PSM V23 D764 Young redia forming into liver fluke.jpgFig. 8.Fig. 9. 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 tadpole-like animal, with a slender tail. A redia may produce a score of the cercariæ, which escape from the parent through a special opening and then wriggle their way out of the snail. The free-swimming 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 substance, which protects it from injury. These encysted cercariæ are destined to find entrance to the liver of the sheep and then develop 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 fastened. 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 cercariæ may descend from a single fluke-egg, and, if the rediæ give rise to a generation of daughter-rediæ, a single egg may produce more than a thousand cercariæ. "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,