348 Animal Biology 



coordinated in order that it may perform the numerous activities essential 

 for its life. This is accomplished by the properties of its living protoplasm 

 without the benefit of any specialized sensory or nervous equipments. 

 Amoeba responds to a number of types of stimuli (en\ironmental fac- 

 tors), and its reactions to them are called its responses. Such activities 

 as changes in shape, formation of pseudopodia, locomotion, capture of 

 foods, etc., constitute its behavior. These changes may be due to ex- 

 ternal, as well as internal, factors. Movement toward a stimulus is called 

 a positive reaction; movement away from a stimulus, a negative reaction. 

 These two reactions may be influenced by the quantity and quality of 

 the particular stimulus. The following reactions are typical: 



1. Contact (thigmotropism or thigmotaxis) (thig -mot' ro pizm; thig- 

 mo-tak'sis) (Gr. thigema, touch; trope, turn; taxis, arrangement) : At 

 first the Amoeba may cease locomotion and then move away. 



FISSION 1" STAGE 



NUCLEAR DIVSION 

 COMPLETED 



FlSSiCN COMPLETED 



FISSION 2"»STAGE 



Fig. 162. — Reproduction of Amoeba by binary fission. Note how the nucleus 

 divides and how the cells eventually separate to form two Amoebae. (From 

 Parker and Clarke: Introduction to Animal Biology, The C. V. Mosby Co.) 



2. Chemicals (chemotropism ov chemotaxis) (ke -mot' ro pizm; kem o- 

 tak' sis) (Gr. chymos, juice) : The reaction is negative to such chemicals 

 as sodium chloride, cane sugar, and acetic acid, but it is positive to cer- 

 tain foods and other chemicals, depending upon the concentration, etc. 



3. Temperature {thermotaxis or thermotropism) (ther mo -tak' sis; 

 ther -mot' ro pizm) (Gr. therme, heat): The reactions vary with the 

 temperature, but movements stop if the temperature is decreased suffi- 

 ciently. Response is negative to higher temperatures. 



4. Light [phototaxis or phototropism) (fo to -tak' sis) (fo-tot'ro- 

 pizm) (Gr. phos, light). The response is negative to strong light but 

 may be positive to weak light. 



Amoeba may also be affected by such stimuli as gravity, electrical 

 currents, water currents, etc. 



