THE LIVING ORGANISM 217 



where some of their useful work has already 

 been mentioned. An amoeba, as a parasite, 

 is also responsible for a form of disease known 

 as tropical dysentery. 



An amoeboid cell is a microscopic mass of 

 semifluid consistency. As in the majority 

 of cases of living cells, about three-fourths 

 to four-fifths of the material of the mass is 

 water, and in this water are contained colloids, 

 consisting of proteins, fats, carbohydrates 

 and other bodies, so as to make a thin mobile 

 jelly-like mass, which also contains certain 

 crystalloids. At rest, it possesses a roughly 

 globular shape, but it is rarely at rest, and is 

 more usually slowly changing its shape by 

 means of slow flowing movements called out 

 by the chemical stimulus of nutrient matter 

 dissolved in the fluid in which it is floating. 

 Such dissolved excitants usually emanate 

 from particles suspended in the fluid, such 

 as debris, or bacteria, and if any such particles 

 near to an amoeba which is being observed 

 under a microscope be carefully watched, it 

 will often be noticed that the amoeba sends 

 processes out towards them, and finally 

 flows round them and takes them into its 

 mass. 



This does not always happen, the determin- 

 ing factor is whether the particle possesses 



