AMCEBA 137 



formed, often at the opposite end of the body. The effect of 

 an electrical stimulus is clearly marked. If an Amoeba is 

 placed in a drop of water between the electrodes of an induc- 

 tion battery, and a single weak shock is passed, it may be 

 seen to withdraw its pseudopodia and contract itself into an 

 irregularly shaped lump, gradually recovering its form and 

 resuming its pseudopodial movement after the effects of the 

 shock have passed away. But if subjected to a constant 

 current the effects are different. At the make of the current 

 the Amoeba contracts in the same way as under the influence 

 of a single shock, but immediately recovers, emits pseudopodia 

 in the direction of the kathode, and continues to move in this 

 direction as long as the current passes. If the current is 

 reversed so are the movements of the Amoeba. On the break 

 of the current, the animal contracts as before, and after a 

 short time resumes its normal movements. 



There is also evidence that Amoebae are sensitive to light, 

 though experiments in this direction have been so far uncon- 

 vincing. But Pelomyxa palustris has been observed to avoid 

 the light, creeping under the mud at the bottom of the vessel 

 in which it was confined during the day, and crawling out on 

 the sides of the vessel during the night. In these diurnal 

 migrations the animalcule traversed a distance of 20 cm. 

 during the twenty-four hours. On the introduction of water- 

 weeds into the vessel the Pelomyxse were seen to crawl up 

 them during the night, and on being suddenly exposed to 

 bright sunlight they contracted themselves into balls and 

 dropped off the weed to the bottom of the vessel, where they 

 hid themselves under the mud. But after the water-weed had 

 been in the vessel for some time the Pelomyxae no longer left 

 the mud but stayed buried in it both by day and night. It 

 would seem that, before the introduction of the water-plant, 

 the water became deficient in oxygen, and that the -deficiency 

 was greatest in the mud at the bottom of the vessel. During 

 the day the animalcule's aversion to light proved stronger 

 than its intolerance of an insufficient supply of oxygen, so it 

 remained buried in the mud, but during the night left its 

 hiding-place in search of oxygen. When the whole contents 

 of the vessel became appropriately oxygenated the migration- 

 ceased. 



Thus Amoeba, simple as its constitution is, exhibits the 



