160 COLLEGE ZOOLOGY 



double eye-spot on the dorsal surface near the anterior end, a 

 pair of excretory organs, the nephridia, and a number of centrally 

 placed germ-cells. It swims about until it encounters a certain 

 fresh-water snail, Lymnoea truncatula of Europe, or probably 

 Lymncea humilis in this country. If no snail is found within 

 eight hours, the larva dies. 



• When a snail is reached, the larva forces its anterior papilla 

 (Fig. 106, a) into its tissue, and by a whirling motion bores its way 

 into the soft parts of the body. Here in about two weeks it 

 changes into a sac-like sporocyst (Fig. 106, b). Each germ-cell 

 within the sporocyst, after passing through blastula and gastrula 

 stages, develops into a second kind of larva, called a redia (Fig. 

 106, b R; c). The redia? soon break through the wall of the 

 sporocyst and enter the tissue of the snail. Here, by means of 

 germ-cells (Fig. 106, c, K) within their bodies, they usually give 

 rise to one or more generations of daughter redia (Fig. 106, c, R), 

 after which they produce a third kind of larva known as a cer- 

 caria (Fig. 106, c, C). The cercariae (Fig. 106, d) leave the body 

 of the snail, swim about in the water for a time, and then encyst 

 on a leaf or blade of grass. If the leaf or grass is eaten by a 

 sheep, the cercariae escape from their cyst wall and make their 

 way from the sheep's alimentary canal to the bile ducts, where 

 they develop into mature flukes in about six weeks. 



It will be seen from the above description that the life-history 

 of the liver-fluke is complicated by the interpolation of several 

 generations which develop from unfertilized germ-cells; 



(i) The fertilized egg produces a ciliated larva, the miracidium 

 (Fig. 106, a) ; 



(2) The miracidium changes to a sporocyst within which 

 rediae are developed from unfertilized germ-cells (Fig. 106, b); 



(3) The rediae produce other rediae from unfertilized germ- 

 cells (Fig. 106, c); 



(4) The rediae finally give rise to cercariae from unfertilized 

 germ-cells (Fig. 106, d) ; and 



(5) The cercariae develop into mature flukes (Fig. 105). 



