CHAPTER VII 

 SOME HARMLESS AND HARMFUL 



THE study of the simplest forms of living animals 

 usually is commenced with that of the common 

 Amoeba, which in many respects is an ideal organism 

 with which to begin the study of a single cell. From 

 a harmless and even beneficial life in pond-water to 

 a parasitic and most destructive existence in the 

 human intestine seems a long step, yet such has 

 occurred in the evolution of the race of amoebae, for 

 to members of the group two dreaded human diseases 

 at least must be ascribed namely, amoebic dysen- 

 tery and liver abscess. 



Amoebae are among the most ubiquitous of 

 organisms. From the Amoeba proteus found among 

 the debris in pond-water it is but a small step to 

 the amoebae living among moss and liverworts that 

 frequent the banks of streams or live in damp places. 

 Vertebrates frequently absorb such amoebae with food 

 or drink, for almost any stagnant pool, particularly 

 in the tropics, will furnish some amoebae, occasionally 

 swarms of them, and certain of these have become 

 habituated to life in the digestive tubes of their 

 hosts. Such a situation provides an abundance of 



142 



