LIFE HISTORY OF LIVER FLUKE. 169 



conditions of parasitism, and almost essential to the con- 

 tinuance of species whose life cycles are full of risks. Out- 

 side of the host, but still within the egg case, the embryo 

 develops for two or three weeks, and eventually escapes at 

 one end of the shell. Those which are not deposited in or 

 beside pools of water must die. The free embryo is conical 

 in form, covered with cilia, provided with two eye spots, 

 and actively locomotor. By means of its cilia it swims 

 actively in the water for some hours, but its sole chance of 

 life depends on its meeting a small amphibious water-snail 

 (Lymncea truncatula), into which it bores its way. In 

 an epidemic among horses and cattle in the Hawaiian 

 Islands, the host was L. cahuensis (Lutz). Within the 

 snail, e.g., in the pulmonary chamber, the embyro becomes 

 passive, loses its cilia, increases in size, and becomes a 

 sporocyst. Sometimes this sporocyst divides transversely 

 (Fig. 54, 4). 



Within the sporocyst certain cells behave like partheno- 

 genetic ova. Each segments into a ball of cells or morula, 

 which is invaginated into a gastrula, and grows into another 

 form of larva the redia. These rediae burst out of the 

 sporocyst, and migrate into the liver or some other organ, 

 killing the snail if they are very numerous. Indeed the 

 death of the snail is probably necessary for the escape of 

 the final larvae. Each redia is a cylindrical organism with 

 a short alimentary canal (Fig. 54, 6). 



Like the sporocysts, the redise give rise internally to more 

 embryos, of which some are simply rediae over again, while 

 the last set are quite different, long tailed cercarice, with 

 two suckers and a forked food canal. These emerge from 

 the rediae, 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 rediae are developed, 

 and these may similarly give rise to other rediae. 



Eventually, within the rediae the tailed cercariae are formed, 



