150 ANIMAL LIFE AND HUMAN PROGRESS 



skin. When sexually mature it discharges a very powerful 

 irritant which causes the overlying skin to ulcerate. Through 

 the ulcer the worm slightly protrudes and discharges a milky 

 fluid. Under the microscope this fluid is seen to contain 

 myriads of active young. These young can live in water for 

 about a week, but they do not grow or feed and eventually die. 



In the intestine of the perch, in the upper reaches of the 

 Thames and in nearly all continental rivers, there lives a 

 peculiar, blood-sucking roundworm, Cucullanus elegans, which 

 has no morphological resemblances to the Guinea- worm. The 

 young of these two worms are, however, extraordinarily alike, 

 and differ from the young of other parasitic forms in several 

 remarkable features. 



Comparing these larvae Leuckart arrived at the conclusion 

 that their peculiar structure is a special larval adaptation to 

 assist them in attacking some special intermediary. He 

 succeeded in showing that Cucullanus larvae invade the bodies 

 of small crustaceans of the genus Cyclops and in them undergo 

 a complete metamorphosis, and he maintained that the 

 similarly formed larvae of the Guinea- worm would be found to 

 follow the same course. At his request the Russian traveller 

 Fedschenko undertook to test the hypothesis while visiting 

 Persia. The results of the experiments completely sub- 

 stantiated Leuckart's views. The young Guinea-worms 

 entered and metamorphosed in the bodies of Cyclops. Some 

 years ago I was able, in West Africa, to show that, after com- 

 pleting this metamorphosis, the larvae become quiescent and 

 remain encysted in the body-cavity. When monkeys were 

 fed with these infected Cyclops, the larvae, released by the 

 gastric juice, made their way into the connective tissues, 

 where they were found after several months to have developed 

 into almost full-grown Guinea- worms. 



These facts concerning the life-history of the parasite 

 provided a simple and . efficacious method of avoiding this 

 disease in infested countries. Water has only to be strained 

 through a pocket-handkerchief to be freed from any Cyclops 

 which it contains and which might harbour the infective stage 

 of the Guinea-worm. 



