54 



AN INTRODUCTION TO ZOOLOGY 



FIG. 18. 



Ameba stimulated 

 glass rod. 



(6) Geotropism = reaction to gravity. 



(7) Chromotropism = reaction to color. 



(8) Rheotropism = reaction to current. 



" Taxis " is often employed instead of " tropism" when the terms 

 read " thigmotaxis," " chemotaxis," etc. If the animal reacts 



by a movement toward the stim- 

 ulus, such as light, it is said to 

 be positively phototropic, photo- 

 tactic, etc.; if away from the 

 stimulus, negatively phototropic 

 or phototactic, etc. Ameba has 

 been found to respond to contact 

 with solids, to chemicals, to heat, 

 to light, to colors, and to elec- 

 tricity. After describing the re- 

 actions of Ameba to each of the 

 above-named stimuli, the general 

 significance of its behavior will be 

 considered. 



Thigmotropism. If a moving 

 Ameba is touched at any point 

 with a solid object, e.g. a glass 

 rod, the part affected contracts 



and moves away. If the anterior edge is stimulated in this way, 

 the part touched stops and contracts while a new pseudopodium 

 is pushed out in some other place and the animal moves away in 

 another direction (Fig. 19). This is clearly a negative reaction, 

 and the animal is said to be negatively thigmotropic. There 

 are, however, certain conditions which call forth positive re- 

 sponses to contact with solids. For example, if the Ameba is 

 floating freely in the water and a pseudopodium comes in contact 

 with the substratum (Fig. 20) the animal moves in the direction 

 of the pseudopodium stimulated until the normal creeping posi- 

 tion has been attained. Contact with food also results in posi- 

 tive reactions, as described on pages 47 and 48. Ameba } there- 



FIG. 19. Ameba moves always 

 from mechanical stimulus. 

 (From Jennings.) 



