Modifahility in Behavior. 45 1 



Piece d was now replaced on the disk. The reaction to the other 

 pieces was suspended, and d was carried to the mouth. Here it 

 came against the middle of the esophageal lobe that was extend- 

 ing toward a^ — in such a way that d could not well be ingested 

 without a rearrangement of the lobes. Thereupon d w^as again 

 carried away from the mouth and once more dropped over the 

 edge of the disk. The other pieces were now successively swal- 

 lowed. Piece d was readily swallowed when given to another 

 specimen. 



The Rejecting Reaction. — After Stoichactis has become satiated, 

 it rejects food, as we have seen. The rejecting reaction presents a 

 number of points of much interest. By this same reaction the 

 disk is kept clean when debris falls upon it. If a mass of waste 

 matter of any sort (as a mass of dead plankton or a quantity of 

 sand) is placed on the disk of Stoichactis, measures are set in 

 operation which result, within ten or fifteen minutes, in removing 

 this material and leaving the disk free. The behavior in bring- 

 ing about this result is complex and the operation may be accom- 

 plished in more than one way. 



The tentacles bearing the debris or the rejected food body 

 collapse, becoming thin and slender, and lying flat against the disk. 

 At the same time the disk surface in this region begins to stretch, 

 separating the collapsed tentacles widely- As a result the waste 

 mass is left on a smooth, exposed surface, the tentacles here having 

 practically disappeared — though under usual conditions they form 

 a close investment almost completely hiding the surface of the 

 disk. Thus the waste mass is fully exposed to the action of waves 

 or currents, and the slightest disturbance in the water washes it 

 oflF. Under natural conditions this must usually result in an 

 immediate removal of the debris. If this does not occur at once, 

 often the region on which the debris is resting begins to swell, 

 and becomes a strongly convex, smooth elevation, thus rendering 

 the washing away of the mass still easier. 



But the process may go much farther. If the debris is not 

 removed in the way just described, new reactions set in. 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 produced and the waste mass slides 

 off the disk. If this does not occur at once, after a little time the 



