126 TREMATODES. PHYLUM PL ATYHELMINTHES 



generations of redia usually succeed one another, but eventuaUy 

 they cease to produce daughters of their own kind, and give 

 birth instead to creatures known as cercariae, with a flat, heart- 

 shaped body, two suckers, a forked gut, and a tail. The cercaria, 

 which is about 0.3 mm. long, with a tail of twice that length, 

 emerges from the redia, works its way out of the snail, and swims 

 by means of the tail. Soon it settles upon a wet blade of grass, 

 loses its tail, secretes around itself a cyst by means of special 

 cystogenous cells of the ectoderm, and waits till the grass is 

 eaten by a sheep ; during this interval the larva, which is now 

 known as a metacercaria, undergoes some slight structural 

 changes. The larva may remain ahve inside the cyst for twelve 

 months, but is killed by sunshine and drying. In the gut of the 

 sheep the cyst is digested and the metacercaria pierces the 

 intestinal wall so that it enters the body cavity, and in two or 



I - qeneration 

 2"-'generorion 

 3"^ qe'^e''«.rior\ 



Egg ».Miracidium >.Sporocysr 



I 



— Redia -.„...^^ Sporocyst 



Cercaria—* Metacercaria— ♦Adulr. Redia 



Fig. 89. — A diagram of the life-history of the liver fluke. 



three days bores into the liver. Later it enters the bile ducts and 

 there grows into an adult fluke. When the gonads are fully 

 developed, which is ten to twelve weeks after infection, the worms 

 begin to lay their eggs, and migrate to the duodenum of the host. 

 Flukes may live for eleven years, or may be lost with the faeces, 

 and if the sheep survives till this happens it will usually recover, 

 though, owing to permanent damage to the liver, the recovery is 

 never complete. 



It will be seen that in this life-history we have an example of 

 alternation of generations far more complicated than that of 

 Obelia, and differing from the latter also in that not sexual and 

 truly asexual, but sexual and parthenogenetic generations succeed 

 one another. The former kind of alternation of generations is known 

 as metagenesis, the latter as heterogamy. It should also be noticed 

 that there are three kinds of individual involved in the cycle. 

 The life-history of the liver fluke is shown by a diagram in Fig. 89. 



The internally parasitic flukes usually have a life-history of 

 the same general pattern, with the adult in a vertebrate and the 



