THE MOVEMENTS AND REACTIONS OF AMCEBA. l8l 



REACTIONS TO STIMULI. 



Of particular importance for the understanding of the behavior of 

 organisms are those reactions which determine the direction of locomo- 

 tion. Experiments show that the stimuli to such reactions must, in a 

 slow-moving organism like Amoeba, affect only one side of the body, 

 or at least affect different parts of the body differently. Owing to the 

 minute size of Amoeba, it is difficult to apply stimuli in such a way as 

 to fulfill this condition. Heat or cold, or a chemical in solution, for 

 examples, when applied to one side are likely 

 to extend to the other side, and far beyond, 

 before the slow reaction of Amoeba has taken 

 place ; the reaction when it occurs is then to a 

 general, and not to a local stimulation. For 

 this reason the reactions of Amoeba to such 



general stimulation are much better known than those to stimuli locally 

 applied. I have devoted myself to the reactions to localized stimuli, 

 and have succeeded in overcoming the experimental difficulties for a 

 number of different classes of agents, though not for all. 



In examining the reactions to stimuli, it will be necessary to keep in 

 mind the method of locomotion (set forth briefly on p. 169 ; diagram 

 in Fig. 58, p. 170) . The factors to which special attention must be paid 

 are : (i) the sending out (or rolling over, as perhaps it would be better 

 to say) of waves of the ectosarc on one side, determining the anterior 

 end in the locomotion ; (2) the attachment to the substratum ; (3) the 

 contraction of parts of the body. 



The reactions were studied chiefly in Amoeba proteus and A. angu- 

 lata; where other species were used, they are specifically mentioned. 



REACTIONS TO MECHANICAL STIMULI. 



The reaction to mechanical stimuli may be either positive or negative. 



POSITIVE REACTION. 



An Amoeba floating in the water frequently takes a starlike form, 

 with many long pseudopodia projecting in all directions. If one of 

 these pseudopodia comes in contact with a solid object or the surface 

 film (which may always be considered a solid for these purposes) , the 

 portion in contact flattens out, attaches itself to the object, and its proto- 

 plasm begins to flow out in a sheet over the latter. The other pseudo- 

 podia are now slowly withdrawn and the entire animal spreads out on 

 the solid, moving usually in the direction inaugurated by the first 

 pseudopodium which came in contact. Often in passing to the surface 

 of the solid there are a number of rapid jerking movements, due to 



* FIG. 64. A. verrucosa constricting off an injured region, after Penard (1902). 



