CHAPTER X 



THE PROTOZOA AS PARASITES OF MAN 



The interest which the study of the Protozoa has for man- 

 kind is not merely theoretical, in virtue of the remarkable 

 peculiarities of their organisation, but is very near and 

 practical, by reason of the fact that a number of them live 

 in the bodies of men, and that there they sometimes cause 

 very serious diseases. In this chapter we shall study briefly 

 examples, drawn from all the four classes of the group, 

 which are parasitic in man. In so doing, our attention 

 must be given both to facts which, directly or indirectly, are 

 of medical importance, and to others, especially those con- 

 cerning life history and the nucleoplasm, which have wider 

 biological significance. 



The several kinds of Entamoeba differ from Amoeba only 

 in that they have no contractile vacuole. 1 

 They have one or two large blunt pseudopodia, 

 chiefly composed of ectoplasm, and they are all parasites, 

 usually in the alimentary canal of one of the backboned or, 

 as they are called, " vertebrate " animals. E. coli lives in 

 the upper part of the large intestine of man, feeding upon 

 the bacteria which infest that region, and also upon the 

 remains of the food of its host, which are probably of 

 little value. It is harmless, and possibly sometimes even 

 beneficial by keeping down the bacteria. Its life-history 

 differs considerably from that of Amceba proteus. In the 

 intestine it reproduces by both binary and multiple fission 

 into eight little amxbulce, and, as some of the in- 

 dividuals are being passed down the gut and cast out 

 with the faeces, certain of them undergo another process. 



1 A contractile vacuole has been found in one organism which has 

 been classed with the Entamceba. 



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