174 



ANIMAL BIOLOGY 



chalk cliffs of Dover, England, were doubtless formed in 



this way. 



While most of the Protozoa are harmless, there are a few 



forms that have become para- 

 sitic in human beings. We 

 have already discussed the 

 single-celled animal that 

 causes malaria and that is 

 carried from one individual 

 to another by the Anopheles 

 mosquito (39). This parasite 

 resembles an amoeba in form. 

 Another form of Protozoan 

 causes the terrible disease 

 known as the, sleeping sick- 

 ness of tropical Africa. Many 



biologists believe that yellow fever (41) is caused by a 



protozoan that is transmitted by the Stegomyia mosquito. 



Fig. 122. — The shells of one-celled 

 animals as they are found in chalk. 

 (Scott, Geological Survey of Iowa.) 



