Inter-Relations of Living Creatures 607 



larva which leads it to respond to the touch of the only creature 

 which will allow of a continuance of its life-history. It is a racially 

 enregistered responsiveness, but these are only words hiding our 

 lack of understanding. If there is no water-snail in the pool, or if 

 the larva does not find it during the eight hours' limit of its free 

 swimming, it will come to nought. 



Inside the water-snail the larva loses its cilia and eye-spots, 

 and becomes a sporocyst; this gives rise by means of spore-cells 

 (germ-cells requiring no fertilisation) to 5-8 larva; of another type 

 called redise. These give rise in the same way to 8-12 more redia 1 , 

 and these to 12-20 larva? of a third type called cercaria?. These 

 are the young flukes at last, with a bilobed food-canal, the be- 

 ginnings of suckers and reproductive organs, and a locomotor tail. 

 If a water-wagtail were to swallow the infected water-snail at this 

 stage, that would be an end of the parasites as well. This is 

 another chance against success. 



From the moribund snail the tailed larvae wriggle out; they 

 swim in the water, they swarm up blades of grass, they encyst as 

 tiny white spots, losing their tail in the process. If the sun wither 

 the grass the whole story is at an end. The only event that will 

 secure continuance is that a sheep eat the blade of grass bearing 

 the tiny encysted larva. From the food-canal of the sheep the 

 cercaria migrates up the bile-duct to the liver, and in a few weeks 

 becomes mature. In some cases the full-grown flukes die in the 

 sheep's liver after reproduction; in other cases they migrate out 

 of the liver, down the food-canal, and die on the ground. The most 

 important general fact is that the life-history of the liver-fluke 

 shows a succession of risks of complete failure. Had it not been 

 for their prolific multiplication, liver-flukes would have passed 

 off the stage long ago. 



Pearls and Parasites 



It sometimes happens that an irritant body, such as a grain 

 of sand, gets in between the shell of a pearl-oyster (or a fresh- 



