Linkage of Lives 103 



life are often linked together by being the two hosts 

 of one parasite. Remembering that the sheep and 

 the "little water-snail are the two hosts of the liver- 

 fluke, Dr. Leiper was able during the War to prove 

 that the very formidable human parasite called Bil- 

 harzia spends part of its juvenile life inside fresh- 

 water snails. From these the microscopic larvae pass 

 into the water and swim freely. If an opportunity 

 offers they enter man through cracks and weak spots 

 in the skin. Every third child of the 30,000 born 

 annually in Cairo is infected with Bilharzia, and 

 the parasite is a serious menace to adults as well. 

 Dr. Leiper not only traced the life-history of the 

 worm; he has saved thousands of lives by showing 

 that the dangerous intruders can be kept back by 

 good filters, and that the free-swimming stages die 

 within thirty-six hours in water that is kept drawn. 

 The subtleties of parasitology are often almost 

 incredible. There is a remarkable fluke called Leu- 

 cochloridium that lives in the intestine of singing- 

 birds in European countries. The miscroscopic eggs 

 are voided on the meadow and may be eaten by a small 

 snail (Succinea), in whose stomach they hatch. Each 

 larva gives rise to strange branched forms, with 

 ramifications which extend up to the snail's horns, 

 where they pulsate with great rapidity, sometimes 

 twice a second. The swollen and agitated horns are 

 all the more conspicuous because they have become 

 banded with red and green pigment. They have been, 

 so to speak, painted by the parasite. If a Blackcap 

 or some similar perching songster is attracted by the 



