PARASITISM 



357 



were for a long time a source of much perplexity as no 

 means for transmission from animal to animal could be 

 found. It was agreed on all sides that the malarial 

 parasites did not leave the body in the expired air, 

 in the urine, or in the feces; the embryos of Filaria ban- 

 crofti (Filaria sanguinis hominis) were in the blood in 

 large numbers, but could not be traced from the body, 

 their occasional appearance in the urine being undoubt- 

 edly accidental. It was the genius of Sir Patrick Manson 

 that afforded the first clue. Finding that the embryos of 

 Filaria bancrofti appeared in the blood at night, he 

 conjectured that it might be to adapt them to the visits 



FIG. 125. Filaria embryo, alive in the blood. (F. P. Henry.) 



of some nocturnal blood-sucking insect. Working 

 upon this hypothesis, he and Low found that when a 

 common mosquito, Culex pipiens, draws blood containing 

 these worms into its stomach, the embryos shed their 

 hyaline sheaths, bore through the intestinal wall, migrate 

 to the thoracic muscles, and encyst themselves at the 

 base of the proboscis. After a period of rest, the worms, 

 feeling the stimulating effects of warm blood as the 

 mosquito bites again, leave the muscles, enter the pro- 

 boscis, and work their way into the tissue of the newly 

 bitten host. 



The further history is uncertain; presumably the 

 embryos at once take up their habitat in the lymphatic 



