THE WEB OF LIFE 309 



the moribund snail, leave the water, wriggle up blades of 

 grass, and encyst themselves, losing their tails in the process. 

 If a sheep pass that way and eat the blade of grass on 

 which the cercaria is encysted, the life- history is continued, 

 and it cannot be continued in any other way ! From the 

 food-canal of the sheep the cercaria, now a young fluke, 

 migrates up the bile-duct to the liver, and there, in the 

 course of a few weeks, becomes mature. In some cases the 

 adult liver-flukes die in the liver after they have repro- 

 duced ; in other cases they migrate out of the liver, are 

 passed down the gut, and die on the ground. It will be 

 noted that in this extraordinary life-history there is point 

 after point at which the process may come to an end. The 

 eggs may light on dry ground ; they may develop in a pool 

 without water- snails ; they may exhaust themselves before 

 they come across the water-snail ; the water-snail con- 

 taining them may be swallowed by a water- wagtail ; the 

 sun may dry up the encysted cercaria ; or it may be that 

 no sheep comes that way to eat the infected grass. The 

 whole life-history is a passage over a Mirza-bridge with an 

 exaggerated number of possibilities of failure. Had it 

 not been for their prolific multiplication, the race of liver- 

 flukes would long since have come to an end. 



Sacculina. The adult parasite which protrudes on 

 the under surface of the abdomen of crabs, is a somewhat 

 bean-shaped sac, consisting very largely of a brood-chamber 

 distended with eggs. The central mass includes a nerve- 

 ganglion, a cement gland which secretes the egg- cases, 

 and the hermaphrodite reproductive organs. There is no 

 trace of digestive or circulatory organs, but the stalk of 

 the parasite is continued into the crab and divides into 

 numerous ' roots ', by which food is absorbed and waste 



