io AMCEBA 



First of all, as we have already seen, it moves, the move- 

 ment consisting in the slow protrusion and withdrawal of 

 pseudopods. This may be expressed generally by saying 

 that Amoeba is contractile, or that it exhibits contracti- 

 lity. 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. 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 of protoplasm is withdrawn from some other part of the 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 Amoeba that is, that it is one of the actions 

 which the protoplasm is capable of performing. 



A contraction may arise in one 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 automatic. On 

 the other hand, movements may be induced 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 protoplasm, 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 



