AMOEBA PROTEUS 51 



probable that the contractile vacuole has other functions than 

 that of urea excretion, but this has never been demonstrated, 

 while the presence of uric acid crystals has been shown in the 

 fluids of the vacuole. After a vacuole has contracted, the new 

 one is formed in the vicinity of the nucleus by union of small and 

 at first unnoticeable vesicles, one or more of which may be left 

 over by the incomplete emptying of the preceding one. This 

 coalescence continues until the new vacuole becomes large 

 enough to be seen with low magnification. As it grows by the 

 continual addition of fluids it increases in bulk and is less easily 

 carried in the moving protoplasm, so that it becomes left behind, 

 so to speak, until it bursts to the outside, usually in that region 

 which for the time being is posterior. 



Irritability. As a spark may cause an explosion so may cer- 

 tain agents in the environment produce sudden and violent 

 reactions on the part of a living organism. Such reactions are 

 due to the local expenditure of energy. In higher animals 

 special " sensory" organs are activated by heat, light, sound, 

 electrical or other agents called stimuli. The energy released as 

 a response to such stimuli is far in excess of the energy repre- 

 sented by the activating agents, and may involve reactions by 

 every part of the organism. 



In Amoeba proteus there are no sense organs but the organism 

 has the property of reacting to every marked change in external 

 conditions in its environment. This property is called irrita- 

 bility, and is analogous to more complicated reactions to stimuli 

 in higher animals. Innumerable kinds of stimuli may produce 

 these reactions but may be classified according to their qualities 

 into a few large groups, such as mechanical, chemical, photic and 

 electrical, all of which indicate changes in the immediate en- 

 vironment of the organism. The responses of organisms to the 

 great variety of stimuli are so diverse that only the most general 

 definition will cover them all. The physiologist Verworn gives 

 such a definition as follows: " Irritability of living substance is 

 its capacity of reacting to changes in its environment by changes 

 in the equilibrium of its matter and its energy" (Lee, Transla- 

 tion, p. 353). 



