48 LOUIS PASTEUR 



of parasites within the bodies of higher animals. 

 Some Protozoa in fact live within the bodies of 

 other Protozoa, and there are even protozoan para- 

 sites of protozoan parasites. 



One of the primary divisions of this group, the 

 Sporozoa, is composed entirely of forms parasitic 

 in other animals. Hundreds of different species 

 inhabit the alimentary canal and other organs of 

 insects, crustaceans, and worms. It is a protozoan 

 of this group that causes pebrine, the destructive 

 disease of silkworms which was studied by Pasteur. 

 One whole subdivision of the Sporozoa, the Hemo- 

 sporidia, is peculiar in living within the red blood 

 cells of vertebrate animals; examples of this sub- 

 division are furnished by the Plasmodium causing 

 malaria in man and the parasite causing Texas 

 fever in cattle, a highly malignant disease, trans- 

 mitted from one animal to another by the bites of 

 the wood-tick. 



The amoebas causing amoebic dysentery, the 

 flagellates causing various intestinal diseases, the 

 trypanosomes giving rise to the fatal sleeping- 

 sickness in Africa, are a few of the many Protozoa 

 that cause trouble in the human body. The species 

 afflicting the lower animals are much more numer- 

 ous and are found in the bodies of almost all ani- 



