PREFACE 



tered and usually in the form of isolated researches 

 begun because chance happened to place favorable ma- 

 terial in the hands of the investigators, and carried out 

 with no larger program in view. More is known re- 

 garding Endamooba histolytica than of any other intes- 

 tinal protozoon, largely because this species more fre- 

 quently brings about distressing symptoms and even 

 death. The severe pathogenic effects of this species also 

 make it of particular interest since an excellent oppor- 

 tunity to study the interactions of host and parasite is 

 thus afforded. The other species discussed are of less 

 practical importance because of their apparent harmless- 

 ness or their comparative rarity. Scientifically, how- 

 ever, they are all intensely interesting and protozoologists 

 will not be satisfied until the complete story of their lives 

 in relation to that of their hosts is known. Many of the 

 species of intestinal protozoa have been described within 

 the past decade and their systematic position, morphology 

 and exact habitat are still in doubt. Many other forms 

 have been reported from man, but must await further 

 study before they can definitely be admitted to be "good" 

 species ; and an even larger number have been described 

 as new species that were abnormal forms of species 

 already known, were coprozoic and not real inhabitants 

 of the human intestine, or were free-living species pres- 

 ent in human material because of contamination. 



Much of the work of preparing this volume was done 

 in the laboratory of protozoology at the London School 

 of Hygiene and Tropical Medicine during the spring of 

 1926, where the author was serving as exchange pro- 

 fessor, and in the laboratory of Professor E. Brumpt in 



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