WORMS. 243 



developed within one sporocyst. Germinal cells are 

 also present in the redia, and they develop into little 

 heaps of cells from which, while the weather is warm, 

 new redise are developed, that live in the lung cavity 

 of the snail, like their parents. When the weather 

 becomes colder, " cercarice " (Fig. 142, /) are developed 

 from the germinal cells, some fifteen to twenty of 

 them within a single redia (Fig. 142, e). The cercaria 

 has a long tail, by means of which it can propel itself 

 in the water, after leaving the mother redia and the 

 snail. It possesses two suckers, and a forked intestine 

 without anus. After the cercarise have moved about 

 in the water for some time they fix themselves, by 

 means of their suckers, to various plants growing in 

 the water. The tail being now superfluous is lost, 

 the cercaria becomes spherical, and contracts some- 

 what within an investment which it secretes (Fig. 

 142, g). When, later on, the pools in which the 

 cercariee live dry up, the plants which were before 

 submerged are now on dry land, and may be devoured 

 by sheep, together with the encysted cercarige found 

 upon them. The waU of the cyst is dissolved in the 

 intestine of the sheep, and the contained cercaria 

 gradually becomes a young liver-fluke, which at first 

 is oval and very small (Fig. 142, h), while, later on, its 

 anterior third becomes the triangular projection at 

 the front end of the adult fluke, and its posterior two- 

 thirds grow very considerably. Meanwhile the young 

 liver-fluke has travelled from the intestine of the 

 sheep into its bile-ducts. It appears that the fluke 

 is completely grown within a few weeks after its 

 introduction into the body of the sheep. Infection 

 almost always takes place in summer or autumn. 

 The life history of the liver-fluke exemplifies change 

 of host and metagenesis (p. 16). From the egg of 

 a sexual worm a free-swimming larva is developed, 

 which becomes an asexual sporocyst within the snail. 

 This sporocyst produces asexual redice, and from 



