552 ASA A. SCHAEFFER 



Reactions to moving organisms 



When a hungry anieba comes within 100 microns or less of an 

 'anchored' flagellate or a creeping diatom, the first visible re- 

 action toward the moving object will be a more rapid streaming 

 of the protoplasm in the stimulated region, providing the object 

 is straight ahead of the pseudopod; but if the object lies to the 

 side, there occurs a sudden slowing of streaming followed by the 

 projection of a side pseudopod toward the food organism. The 

 pseudopod flows rapidly forward until almost in contact with the 

 prey, then it stops and forks, the two prongs advancing at the 

 same rate and encircling the prey. Meantime, the covering 

 sheet of protoplasm is sent out from the bases and dorsal sides 

 of the prongs. The encircling pseudopods and the covering sheet 

 of protoplasm then fuse all around, and the ameba attaches it- 

 self closely to the substrate. If stimulation is intense, the whole 

 mass of the ameba travels toward the food cup. Usually a few 

 seconds after the complete closure of the food cup the captured 

 organism becomes very active in its movements, and in darting 

 about comes, usually for the first time, into physical contact with 

 the ameba. If the main stream of protoplasm has not thus far 

 been directed toward the captured prey, it is turned in this direc- 

 tion now, especially if the organism is of some size. The ameba 

 usually remains almost motionless for a very variable length of 

 time. In from one to sixty minutes or longer, depending upon 

 the character of the object and upon the condition of the ameba, 

 the captured prej^ becomes quiet, and very soon thereafter the 

 water in the food vacuole begins to disappear. While the water 

 is being absorbed the inside wall of the food cup sends out irreg- 

 ular projections toward the prey, which seem to be rough and 

 granular on their edges. If the prey is an animal not having a 

 hard exoskeleton, such as a spathidium or a chilomonas, it be- 

 comes spherical at about this stage, and is carried along by the 

 protoplasmic stream. Locomotion is usually resumed when the 

 prey ceases its active movements, but there is very great varia- 

 tion in this matter. 



