LIFE-HISTORY OF THE LIVER-FLUKE. 155 



two suckers and a forked food-canal. These emerge from 

 the redise, wriggle out of the snail, pass into the water, and 

 moor themselves to stems of damp grass. There they lose 

 their tails and become encysted. If the encysted cercaria 

 on the grass stem be eaten by a sheep, it grows, in about 

 six weeks, into the adult sexual fluke. 



To recapitulate, the developing embryo becomes a free- 

 swimming form, which bores into a snail, and changes into 

 a sporocyst. 



From certain cells of the sporocyst redise are developed, 

 and these may similarly give rise to other rediae. 



Eventually, within the redias the tailed cercarise are formed, 

 and these in favouring circumstances grow into the adult flukes. 



The above history has been independently worked out by 

 Leuckart and Thomas. 



It will be noted that the sporocyst is the modified 

 embryo, but that it has the power of giving rise asexually to 

 redise. These develop, however, from special cells of the 

 sporocyst which we may compare to precociously developed 

 parthenogenetic ova. Though the reproduction is asexual, 

 it is not comparable to budding or division. The same power 

 is possessed by the redise, and there are thus several (at least 

 two) asexual generations between the embryo and the adult. 

 Finally, it must be clearly understood that the cercaria is 

 the young fluke. 



The disease of liver-rot in sheep is common and dis- 

 astrous. It has been known to destroy a million sheep in 

 one year in Britain alone. It is especially common after wet 

 seasons, and in damp districts. The preventives suggested 

 are drainage of pastures and dressings of lime and salt ; de- 

 struction of the eggs, the snails, infected manure, and 

 diseased sheep. It is usual to give the infected sheep some 

 salt and a little dry food. 



Classification. 



Trematodes with direct development — Monogenetic. 



e.g., Polystomum integerrimum. This form with many 

 suckers will be found in the bladder of the frog. It 

 attaches itself in its youth to the gills of tadpoles, 

 passes thence through the food-canal to the bladder, 

 where it develops slowly for years. 

 Gyrodactyhts, found on the gills and fins of fresh-water 



