Protozoa. 



289 



The amoeba shows that it has a sense of touch, for it 

 frequently avoids solid objects with which it comes into 

 contact, passing to one side. It has the characteristic 

 termed irritability. This characteristic does not involve 

 any special degree of sensitiveness, but simply the power 

 to receive impressions through contact with external objects. 

 If stimulated by slight electric shocks, the amoeba may 

 be made to withdraw its pseudopods and remain quiet 

 in a spherical form. It is said to have contracted, and 

 is said to be endowed with the property of contractility, 

 but it is simpler merely to say that it has changed 

 its form. 



But the amceba does not always wait to be stimu- 

 lated from without to make it move. It sometimes 

 appears to move " of its own accord," as we say. That 

 is, it is automatic, in the sense of self-moving. 



, mm 



FIG. 170. AMCEBA DIVIDING. 



,,, 



If amoebae are well nourished, they are likely to mul- 

 tiply. They do this by simple division. An amoeba 

 becomes constricted in the middle. The constriction 

 increases until the individual is divided into two. 



Encysted Amoeba. Sometimes an amoeba, when sub- 

 jected to drouth, or perhaps other unfavorable conditions, 

 forms a tough outer wall, and remains in a quiet condi- 

 tion. The tough covering is called a "cyst," and the 

 amoeba is said to be "encysted." It may thus remain 



