1 68 MINUTE MARVELS OF NATURE 



same time undergoing certain changes essential 

 to their perfect development. After completing 

 this period, they leave the snail and take to the 

 water again ; and if it should happen that no 

 sheep, in the act of drinking, offers them hospi- 

 tality, they patiently encyst themselves on stems 

 of crrass, &c., erowingr out of the water, in the 

 hope, apparently, that they will ultimately be 

 eaten by a sheep. If this good fortune favours 

 them, they finally complete their history in the 

 bile ducts and liver tubes of the unfortunate 

 animal. 



One of the most recent and remarkable in- 

 stances of parasites that require more than one 

 host, which science has revealed, is that of the 

 malarial parasite. It has been shown that this 

 organism lives in one of the tiny blood-corpuscles 

 of man ; but here it on'y vegetatively increases 

 its numbers. To breed and develop so that it 

 can be conveyed to other human victims, it has 

 first to be taken from the human blood by a 

 particular species of mosquito — which insect I 

 shall speak of and illustrate in the next chapter 

 — then continuing its development in the blood- 

 corpuscles ot this insect. The perfect or malarial 

 spores are eventually conveyed to man again 

 when bitten by the mosquito. 



On learning that a simple parasitic organism 



