io AMCEBA LKSS. 



that Amoeba is contractile, or that it exhibits contractility. 

 But here it must be borne in mind that contraction does 

 not mean the same thing in biology as in physics. When 

 it is said that a red-hot bar of iron contracts on cooling, 

 what is meant is that there is an actual reduction in 

 volume, the bar becoming smaller in all dimensions. Bui 

 when it is said that an Amoeba contracts, what is meant is 

 that it diminishes in one dimension while increasing in 

 another, no perceptible alteration in volume taking place 

 each time a pseudopod is protruded an equivalent volume 

 of protoplasm is withdrawn from some other part of the 

 body. 



We may say then that contractility is a function of the 

 protoplasm of Amoeba that is, that it is one of the actions 

 which the protoplasm is capable of performing. 



A contraction may arise in one or other of two ways. Ir 

 most cases the movements of an Amoeba take place withou 

 any obvious external cause ; they are what would be callec 

 in the higher animals voluntary movements movement: 

 dictated by the will and not necessarily in response to an> 

 external stimulus. Such movements are called spontamou, 

 or automatic. On the other hand, movements may be in 

 duced in Amoeba by external stimuli, by a sudden shock 

 or by coming into contact with an object suitable for food 

 such movements are the result of irritability of the proto 

 plasm, which is thus both automatic and irritable that is 

 its contractility may be set in action either by internal or b] 

 external stimuli. 



Under certain circumstances an Amoeba temporarily lose; 

 its power of movement, draws in its pseudopods, am 

 becomes a globular mass around which is formed a thick 

 shell-like, coat, called the cyst or cell-wall (Fig. i, D, cy) 

 The composition of this is not known ; it is certainly no 



