PROTOZOA 137 



vacuole, which pulsates regularly and empties its accumu- 

 lated wastes through several pores opening through the 

 ectosarc. 



Self -protection. The amoeba is so small that it has few 

 enemies . It could no t run away very successfully if pursued, 

 for it moves very slowly by projecting little soft finger-like 

 pseudopods on which it walks or flows along. Sometimes 

 one amceba will chase another and eat it. 



The worst dangers for amcebas, however, are the changes 

 in physical conditions which may come with the drying up 

 of the water in which they live, or with the changing seasons. 

 If such unfavorable changes do not come too suddenly an 

 amoeba secretes a resistant covering about its body and 

 remains in a quiescent state, biding its time until conditions 

 are again favorable. Such a resting amceba is said to be 

 encysted, and is called an amceba cyst. 



The amceba lives from day to day so as to escape ordi- 

 nary dangers as far as possible. It is wholly without any- 

 thing which may be interpreted as an eye, yet is sensitive to 

 light. This sensitiveness is of use to the amceba in enabling 

 it to avoid the direct rays of the sun, which would be injuri- 

 ous, and allows it always to creep away from the light into 

 dark crevices where it finds food and shelter. The amceba 

 is therefore protected from harm because it seldom comes 

 out into the open. 



Race Preservation. For an amoeba, the formation of 

 new individuals is an easy matter. One individual has only 

 to undergo cell-division to produce two, and if each of the 

 " offspring" produced in this way continues to divide at in- 

 tervals, the final total will be enormous. When an amoeba 

 divides, the body forms two cells of equal size and hence 

 the process is known as binary fission. Such fission may 

 continue to be the only means of multiplication for several 

 hundreds of generations, but if continued too long the last 

 generations lack vitality and the stock finally dies out. An 

 amceba, therefore, does not pass through its life cycle as an 

 individual. Starting with a "young" individual it divides 



