240 ENTO-PAEASITES AND HYGIP:NE, 



man, but is not so dangerous here. The liver-fluke has 

 its seat in the liver, the gall-bladder and the gall-ducts, 

 in which it produces many disastrous changes; the func- 

 tions of the liver being thus impeded, the animal becomes 

 weak, and when the liver is completely destroyed death 

 comes as the inevitable result. There is no remedy for 

 this disease, for if the worms are once in the body the 

 fatal end cannot be averted, because we possess no way 

 to reach them, neither by means of medicines nor with 

 the surgeon's knife. It is therefore necessary to guard 

 against the introduction of the germs, but the life-history 

 of the early stages ot the liver-fluke being entirely un- 

 known, nobody could say what was to be done. As soon 

 as untiring researches stated the different crooked ways 

 of development, the farmer and ranchman could protect 

 his sheep and thus himself against damage. 



This first necessity was therefore to find the inter- 

 mediate host which housed the first stage of embryonal 

 development. After many fruitless experiments and in- 

 vestigations a little snail, limnsea minuta, was found to 

 be the first intermediate host. It has a very wide exten- 

 sion, living especially in shallow ditches and ponds and is 

 scattered in spring when rain and the melting of snow 

 swell the waters far over the adjoining meadows. In 

 May, when these localities are not yet dry, the cercaiise 

 emigrate and encyst on plants of all kinds. Here they 

 wait as long as their surroundings remain moist, and are 

 finally swallowed again by the sheep into whose liver 

 they wander. Thus we know how sheep may be in- 

 fected, and it is not hard to guard ourselves against pos- 

 sible harm: keep the sheep from wet places, use the 

 grass dry only, since cercarige cannot live for any length 

 of time when dried up. 



In similar way, somewhat simplified, the development 

 of the cestods take place. The grown tape- worm has a 

 rather sharply defined head with four sucking-cups and 



ITS 



