HARMLESS AND HARMFUL AMCEB^ 143 



food, both of the indigestible and undigested food- 

 materials of the host and of a rich bacterial flora. 

 These amoebae feed passively on the food, are sluggish, 

 and cause practically no harm to the host harbour- 

 ing them. 



Other Amoebae are not so complaisant. They need 

 protoplasm, not proteid; cell substance, not cell 

 d6bris ; and to satisfy their wants, they penetrate 

 the cell-walls of their host's digestive tract, and 

 within or among the cells of the host they multiply 

 actively. Thus a free-living organism may become 

 first saprophytic, then parasitic, and subsequently 

 pathogenic. 



The fate of the pathogenic Amoebae of man is to be 

 expelled from the bodies of their hosts ultimately in 

 the form of cysts. In the tropics excrement from 

 practically every source is utilized for the manuring 

 of the land, and, consequently, almost every source 

 of water-supply may be contaminated with the cysts 

 of various amoebae. Green vegetables, and especially 

 salads, are most easily exposed to contamination, 

 and it is a fortunate circumstance that under 

 ordinary conditions relatively few amoebae are harm- 

 ful to man. Even fruits, such as strawberries, are 

 not exempt from the suspicion of conveying amoebae 

 to man, and the greatest care is necessary in dealing 

 with either ground-fruits or vegetables grown under 

 tropical conditions. 



Amoebae generally have a very ill-defined shape, as 

 they are capable of protruding numerous portions of 

 their body-substance as pseudopodia, such protru- 

 sions being accompanied by retractions elsewhere, 



