CHAPTER X 



PARASITISM AMONG PROTOZOA 



Adaptation to Environment 



The multitude of substances making up protoplasm are al- 

 ways in a physico-chemical condition of delicate equilibrium. 

 Adjustments are easily made to the ordinary stimuli from the 

 environment, and these adjustments form a part of the daily life 

 of an organism in food-getting, respiration and excretion, avoid- 

 ing enemies, and the like. An unusual environmental stimulus 

 constitutes something of a shock, and adjustment to it depends 

 upon the nature and the intensity of the stimulus. Heat, for 

 example, accelerates all activities and the resultant reactions con- 

 form in a general way with Van't Hoff's law to the effect that 

 a reaction doubles with an increase of ten degrees in tempera- 

 ture. Too great a heat-shock kills, but a succession of smaller 

 shocks builds up a heat-resistance until the organism becomes 

 adapted to the ordinary lethal or fatal dose. Thus William H. 

 Dallinger, an English experimenter, found that flagellates of 

 the genus Dallingeria (Heteromita) , living in a normal tempera- 

 ture of 60° Fahrenheit, were killed by raising the temperature 

 to 78°, but if the temperature was slowly increased, with sufficient 

 time for the organisms to become adapted to the increments, 

 they could be subjected to 78° without harm. He found that 

 some individuals could be trained to live normally in a tempera- 

 ture of 158° Fahrenheit, but it required seven years to bring 

 this about. Similarly, an Amoeba verrucosa living normally in 

 fresh water may be transplanted to salt water and still live, but 

 the transfer involves a marked change in structure — the con- 

 tractile vacuole disappears entirely, much of the protoplasmic 

 water is lost, and the ameba becomes much denser and smaller 

 than it is normally. Various types of poisons — arsenic, strych- 

 nine, mercury, quinine — are lethal to protozoa, but resistance to 

 these poisons may be built up in the organism in the same way 



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