THE BEHAVIOR OF AMCEBA 



II 



J-^ 



a 



13 

 12 



10 



\ 



jy^^4^ 13 d o"^ si^^i ^^ animal responds by 



contracting the part affected and 

 moving in some other direction. 

 Reactions to Light. — Light has a peculiar 

 effect on Amoeba. In general its functions 

 seem better performed in the dark ; strong 

 light interferes with them seriously. Rhum- 

 bler (1898) observed that if Amoebae are 

 suddenly subjected to light while busy feed- 

 ing on Oscillaria filaments, they cease to 

 feed, and even give out the partly ingested filaments. 

 Harrington and Learning (1900) found that ordinary 

 white light thrown on a moving Amoeba causes it to 

 come to rest at once. Blue light acts in the same way, 

 while in red light the movements are as free as in dark- 

 ness. Other colors have intermediate effects. Engelmann 

 (1879) found that sudden illumination causes an extended 

 Pelomyxa (which is merely a very large Amoeba) to con- 

 tract suddenly. It is well known that exposure to strong 

 light is destructive to most lower organisms. 



In correspondence with the fact that light interferes 

 with its activities, we find that Amoeba moves away from 

 a source of strong light. If the sun is allowed to shine 

 on it from one side, it moves, as Davenport (1897) shows, 

 in the opposite direction. It thus moves in a general 

 way in the same direction as the rays of 

 light (Fig. 16). It is a peculiar fact that 

 experiments so far have not shown a 

 negative reaction to occur when light is 

 thrown from directly above or below on 

 one side or end of an Amoeba. The fact 

 that the whole body contracts when illumi- 

 nated, as shown by the work of Engel- 

 mann (1879) on Pelomyxa, would lead us 

 to expect that when a portion of the body 

 is illuminated, this would contract, pro- 

 ducing thus a negative reaction. But this 

 has not been demonstrated. The experi- 

 mental difficulties are great, and this may 

 account for the lack of positive results. 

 If future work substantiates the fact that 

 light falling obliquely on one side causes 



Fig. 16. — Reaction of Ameeba 

 to light, after Davenport (1897). 

 The Amoeba was first moving in lie 

 direction indicated by the arrow x. 

 Light coming from the direction 

 shown by the arrow a was then 

 thrown upon it. It changed its 

 course, occupying successively the 

 position!! I, n, 3, 4. The direction of 

 the light was successively changed as 

 indicated by the arrows h, c, d; the 

 numbers 5-14 show the successive 

 positions occupied by the animal. It 

 will be observed that in every case as 

 soon as the direction of the light is 

 changed, the Amoeba changes its 

 course in a corresponding way, so as 

 to retreat steadily from the source of 

 light. 



