AMCEBA. 65 



have an explanation of many points in the physiology of 



man: 



1. The amoeba moves ; it has the power of contraction or 

 contractility, by means of which it accomplishes 

 motion and locomotion. 



2. It feels ; when any disturbance, such as contact with 

 a foreign body, is brought to bear on an amoeba at 

 rest, it moves ; this is not because it is pushed or 

 pulled, but is due to its own activity, the contraction 

 of its protoplasm. Any living matter which, when 

 acted on by a stimulus, is excited to activity, is said 

 to be irritable (sensitive) in the sense of "susceptible 

 to impression from without." 



3. But the amoeba does not always wait to be thus stim- 

 ulated by something external ; it moves " of its own 

 accord," as we would say; such action is called auto- 

 matic; the amoeba is an individual capable of spon- 

 taneous and independent activity. 



4. The protoplasm of which the body of the amoeba is 

 composed is constantly undergoing chemical change ; 

 this change is of two kinds: one a wearing out, de- 

 pending on the degree of activity, the other a building 

 up, to make good the loss. The food is changed so 

 that it is ready to become part of the body ; this 

 process i3 digestion. The building of this material 

 into the body is known by the name assimilation. 

 The waste products resulting from the breaking up of 

 the old protoplasm are called excretions, and are 

 thrown off into the same surrounding medium from 

 which food is taken. All these processes of taking 

 food, digesting and assimilating it, the decomposition 



