DIFFERENT KINDS OF RESPONSE 147 



If the mass is nearer one edge of the disk this edge usually 

 begins to sink, while at the same time the tentacles between 

 the edge and the waste object collapse and practically 

 efface themselves. Thus a smooth, sloping surface is pro- 

 duced and the waste mass slides' off the disk. If this does 

 not occur at once, after a little time the region lying behind 

 the mass (between it and the center of the disk) begins to 

 swell, producing a high, rounded elevation, with tentacles 

 plump and swollen. The waste mass is now on a steep 

 slope, and is bound soon to slide down and over the edge. 

 Sometimes by a continuation of this process the entire disk 

 comes to take a strongly inclined position, with the side 

 bearing the debris below. Often one portion of the edge of 

 the disk after another is lowered hi this way, till all the w r aste 

 matter has been removed. The disk then resumes its 

 horizontal position, with nearly flat or slightly concave 

 surface." If the edge near which the foreign body is placed 

 cannot be lowered the part external to the body may be 

 raised while the surface toward the opposite edge is depressed 

 so that the object may be rolled off the disk in another 

 direction. 



In Planaria, according to Pearl, repeated mechanical 

 stimulation of the anterior end causes the worm to turn 

 farther away at each succeeding stimulus, without at first 

 causing any movements of locomotion. After a time the 

 worm jerks back vigorously, bends the body strongly to one 

 side and then extends usually toward the source of stimulus. 

 Stronger stimuli soon alter the general physiological condi- 

 tion of the organism; "the animal becomes 'stirred up' 

 generally, moves about with increased rapidity, its sensitive- 

 ness to stimuli becomes diminished, and it will give only the 

 negative response to stimulation of the anterior end. . . . 

 One may get totally different appearances from an individual 



