Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 323 



increase rapidly in size, and browse on the digestive gland of the 

 water-snail (known as the intermediate host\ to which congenial 

 spot they have in the meantime migrated. The series of 

 developmental changes is even yet not complete. For within 

 the redise (besides at times daughter redise) embryos of yet 

 another type are produced by a process of cell-division. These are 

 known as cercarice (Fig. 97, G.). Each has a long tail, by means 

 of which it can swim freely in water. It leaves the intermediate 

 host, and after leading a short active life, becomes encysted on 

 blades of grass. The cyst is formed by a special larval organ, 

 and is glistening snowy white. Within the cyst lies the trans- 

 parent embryonic distoma, which has lost its tail in the process 

 of encystment. 



The last chapter in this life-history is that in which the sheep 

 crops the blade of grass on which the parasite lies encysted ; 

 whereupon the cyst is dissolved in the stomach of the host, the 

 little liver-fluke becomes active, passes through the bile-duct into 

 the liver of the sheep, and there, growing rapidly, reaches sexual 

 maturity, and lays its thousands of eggs, from each of which a 

 fresh cycle may take its origin. The sequence of phenomena 

 which constitute such a cycle is known as alternation of genera- 

 tions, or heterogamy, of which there are many modes among the 

 lower members of both the animal and vegetable kingdoms. It 

 is characterised by discontinuity of development. Instead of the 

 embryo growing up continuously into the adult, with only the 

 atrophy of provisional organs, it produces germs from which the 

 adult is developed. Not merely provisional organs but provisional 

 organisms undergo atrophy. In the case of the liver-fluke there 

 are two such provisional organisms, the embryo-sporocyst and 

 the redia. 



We may summarise the life-cycle thus : 



(1) Ovum laid in liver of sheep, passes with bile into intestine, and 

 thence out with the excreta. 



(2) Free Ciliated Embryo in water or on damp earth ; passes into pul- 

 monary cavity of Limnseus truncatulus, and develops into 



(3) Sporocyst, in which secondary embryos are developed, known as 



(4) Eedice, which pass into the digestive glands of Limnseus, and within 

 which, besides daughter rediae, there are developed tertiary embryos, or 



