LIFE-HISTORIES OF TREMATODES. 159 



not only with open water used for washing linen or bathing, but 

 with domestic supplies from wells or ponds used for drinking water. 

 Infection takes place by way of both the mouth and the skin. All 

 water from suspected infected sources should .be allowed to stand 

 for two days before being used, by which time the cercariae are 

 dead. 



Eradication of snails is also desirable. While this is not 

 entirely possible, yet the number of snails can be kept down to a 

 great extent by trimming of banks and by using domestic ducks 

 on open water, for these birds rapidly reduce the snail population, 

 while owing to their domesticated habits they do not fly from pool 

 to pool, bearing snail eggs on their feet as do wild duck. Certain 

 small "millions" fish, such as are used for mosquito control, will 

 also feed on trematode cercariae, and might be employed in dams 

 or bathing pools for the double purpose of mosquito and cercarial 

 destruction. 



Distomes. 



The life-histories of two common distomes, namely, Fasciola 

 hepatica, the European sheep fluke, and F. gigantica, the African 

 cattle and sheep fluke, may be outlined. 



(1) Fasciola hepatica. 



The transmitter of Fasciola hepatica in Europe is the small 

 snail, Limnaea truncatula, as was shown by the magnificent work 

 of Thomas and of Leuckart. I have been able to determine experi- 

 mentally that two transmitters are present in South Africa, 

 namely, the pond snails, Jsidora tropica and Limnaea natalensis. 

 Sexually mature flukes have been obtained by feeding rabbits and 

 laboratory bred sheep with encysted cercariae from these naturally 

 infected molluscs, while laboratory bred snails have been infected 

 direct by exposure to the miracidia from eggs of Fasciola hepatica 

 taken from condemned sheep livers at the Johannesburg Abattoirs. 

 The life-cycle of F. hepatica in either Isidora tropica or Limnaea 

 natalensis is practically the same as that of F . hepatica in Limnaea 

 truncatula. in Europe. 



The ovoid, operculate eggs of F . hepatica measure 130/x to 145/i 

 long by 70/x to 9G> broad. 



The miracidia hatched from the eggs are about 125/x long by 

 25 /x to 3(V broad. They have well marked eyespots. They enter 

 the pulmonary cavity of the snail and penetrate the liver, where 

 they develop into sporocysts varying in my specimens from 500/j, 

 to 720/i in long diameter. Within the sporocyst the rediae develop, 

 and these leave the sporocyst when about 35/a long and then grow 

 considerably, some measured being as much as-8mm. long. There 

 is a well-marked collar and a relatively inconspicuous birth pore. 

 Finally, the rediae produce cercariae, which are rounder than those 

 of F. gigantica, having highly contractile bodies, measuring from 

 250/x to 300/x long and 200/x to 225u broad. The simple tail is 

 about 450/a long. The body has cystogenous granules, but these 

 are not so dense as those of F . gigantica. At encystment a resistant 



