6 SOME MINUTE ANIMAL PARASITES 



of grouse and other game-birds and poultry, popularly 

 known as " white diarrhoea." Here the resting forms 

 of the parasite are taken up with food or drink and 

 thus pass into the food canal of the host. When the 

 parasites reach the intestine, the living germs within 

 are set free by the powerful digestive juice softening 

 the outer walls in which they are enclosed. Life as 

 free forms in the cavity of the gut soon would prob- 

 ably be fatal to these young organisms, which are 

 the primary infecting germs, and they at once pro- 

 ceed to bore a way for themselves into the delicate 

 lining of the alimentary canal. Here they grow at 

 the expense of the living substance that harbours 

 them, and as they increase in size, the host cells in 

 which they are contained gradually die. Multiplica- 

 tion of the parasites within the tissue occurs, and 

 finally the gut is reduced almost to a pulp, literally 

 riddled with parasites. The parasitism, then, is 

 within the tissue cells or is intracellular, and is far 

 more effective for the nourishment of the parasite 

 and incidentally for the destruction of the host 

 than the results'of the two preceding casual or con- 

 taminative methods of infection. 



Leaving the casual method, a reference may be 

 made to the annoyance caused by the stab of a gnat 

 or mosquito, and to the effect of the stab. For 

 many years, the malady known as malaria was in- 

 variably said to be caused by swamps, and could be 

 avoided when the habitations of the people were well 

 above the sea-level. It is still true that swampy or 

 marshy conditions favour malaria, but only because 

 the young " grubs," or larvae, of mosquitoes live in 



