152 ANIMALS 



in the vicinity of the mouth. For amoeba no such distinction 

 is possible, and we can, therefore, only speak of a chemical 

 sense. Amoeba has been observed to engulf particles of sand 

 and other inorganic substances which could not serve as food, 

 but in spite of this fact there is much evidence to show that there 

 is a chemical sense. Ordinarily the animal distinguishes food 

 particles from others and has even been observed to follow a 

 moving protozoan, which it also captured and devoured. Many 

 observations lead to the general conclusion that the protozoa 

 generally are sensitive to the chemical condition of the sur- 

 rounding medium. They are attracted to or repelled from the 

 source whence such substances are diffusing through the water. 

 Strong stimuli produce decided responses, such as a contraction 

 of the amoeba into a spherical mass. 



354. Amoeba is also sensitive to mechanical stimuli, such as a 

 touch or a jar. By this means it ''feels" the presence of a 

 foreign object to which it may adhere. If the stimulus is 

 irritating the response may take the form of a secretion of 

 slime, the withdrawal of the pseudopodium or the complete 

 contraction of the amoeba into a spherical mass. 



355. Amoeba is also sensitive to strong light though not as 

 much so as some other colorless protozoa. Usually, when 

 other things are equal, protozoa may be observed to seek a 

 point where the light is neither excessively strong nor weak. 



356. To changes in temperature amoeba is also sensitive. 

 With increase in temperature it becomes more active, until at 

 about 35° C. it contracts and remains motionless. With a 

 falling temperature activity is also lowered until at a little 

 above 0° C. it ceases entirely, often without contraction. 

 When possible amoeba will move out of a region of extremely 

 high or low temperature to one more nearly normal. 



357. Though amoeba is sensitive to these various stimuli, 

 yet it has no sense organs. There is no differentiation of organs 

 for the reception of the different stimuK, nor yet for the re- 



