BEHAVIOR OF CCELENTERATA 203 



selves. Thus the mass slides downward off the disk. If this does not 

 occur at once, after a time the region lying behind the mass begins to 

 swell ;^ it often forms in this way a high, rounded elevation. The waste 

 mass is now on a steep slope, and is bound soon to slide over the edge. 

 Sometimes by a continuation of these processes the entire disk comes 

 to take a strongly inclined position, with the side bearing the debris 

 below. ^ Often one portion of the edge after another is lowered succes- 

 sively till all of the waste matter is removed and the disk is thoroughly 

 cleaned. The disk then resumes its horizontal position, with nearly 

 flat or sHghtly concave surface. 



Sometimes the edge bearing the debris cannot be lowered, owing to 

 the fact that it is almost against an elevation in the irregular rock to 

 which the anemone is attached. In this case (after perhaps an attempt 

 to bend this edge downward) the part between this edge and the debris 

 swells and rises, roUing the mass toward the centre, while at the same 

 time the region beyond the debris sinks down. In this way the waste 

 matter is rolled across the disk to the opposite side, and dropped over 

 the edge. The process is slow, often requiring fifteen minutes to half 

 an hour. 



This whole reaction is characterized by great flexibility and vari- 

 ability. The debris sets in operation certain activities; if these do not 

 put an end to the stimulation, other activities are induced, till one is 

 successful. This is an excellent illustration of the general characteris- 

 tics of behavior in the lower organisms. 



6. Locomotor Reactions in Hydra and Sea Anemones 



After contracting in response to stimulation, if the stimulus still 

 continues, Hydra and the sea anemones usually set in operation other 

 activities, having a more radical effect in separating the animal from 

 the source of stimulation. We have examined certain cases of this 

 character in the foregoing section on the rejecting reaction. We shall 

 here consider such reactions as tend to remove the animal, or cause it 

 to take a new position. 



Hydra. — After contracting in response to stimulation. Hydra usu- 

 ally bends over into a new position and soon extends again in a new 

 direction, just as happens in its spontaneous contractions (Fig. 114). 

 This may be repeated many times, the animal occupying successively 

 many different positions. 



In bending thus into a new position in response to a one-sided stim- 

 ulus, does Hydra bend directly away from the source of stimulation? 

 Wagner (1905) and Mast (1903) have answered this question experi- 



