lo AMCEBA lESS. 



that Amceba 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 cooHng, 

 what is meant is that there is an actual reduction in 

 volume, the bar becoming smaller in all dimensions. But 

 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 of protoplasm is withdrawn from some other part of the 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 Amceba — 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. In 

 most cases the movements of an Amoeba take place without 

 any obvious external cause ; they are what would be called 

 in the higher animals voluntary movements — movements 

 dictated by the will and not necessarily in response to any 

 external stimulus. Such movements are called spontaneous 

 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 by 

 external stimuli. 



Under certain circumstances an Amoeba temporarily loses 

 its power of movement, draws in its pseudopods, and 

 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 not 



