176 



BEHAVIOR OF THE LOWER ORGANISMS 



thoroughly. This may last for some time, then the animal may leave 

 the debris and swim about again. Other heaps of debris or the sur- 

 faces of solids are explored in the same way. Finally, after ten or twenty 

 minutes or more, one of these is selected for the formation of a new tube. 

 It may be seen that as the Stentor moves about a viscid mucus is se- 

 creted over the surface of the body. To this mucus particles of debris 

 stick and are trailed behind the swimming animal. In a certain region, 

 perhaps between two masses of debris, the animal stops and begins to 

 move backward and forward with an oscillatory motion, through a dis- 

 tance about two-thirds its contracted length. This movement, in pre- 

 cisely the same place, is kept up for about two minutes, while the mucus 



from the surface is rapidly secreted. 

 The movement compacts this mucus 

 into a short tube or sheath, — the tube 

 in which the Stentor is to live. The 

 process is represented in Fig. 112. 

 Next the tip of the foot is pressed 

 against the debris at the bottom of the 

 tube. There it adheres by means of 

 Fig. 112. — Oscillating movement of nne pseudopodia sent out from the in- 



Stentor, by which it forms a new tube. , . , T , „ 



i- 2 , alternating positions, o, the secreted ternal protoplasm. Now the Stentor 

 mucus; b, masses of debris. extends to full length, and we find it 



in the usual attached condition, with the lower half of the body 

 surrounded by a transparent tube of mucus. The Stentor has thus 

 moved away from the place where it was subjected to the mass of car- 

 mine particles, and has established itself in another situation. 



The behavior just described shows clearly that the same individual 

 does not react always in the same way to the same stimulus. The 

 stimulus and the other external conditions remaining the same, the 

 organism responds by a series of reactions becoming of more and 

 more pronounced character, until by one of them it rids itself of the 

 stimulation. Under the conditions described — when a dense cloud 

 of carmine is added to the water — the changes in the behavior may be 

 summed up as follows : — 



(1) No reaction at first: the organism continues its normal activi- 

 ties for a short time. 



(2) Then a slight reaction by turning into a new position, — a seem- 

 ing attempt to keep up the normal activities and yet get rid of the 

 stimulation. 



(3) If this is unsuccessful, we have next a slight interruption of the 

 normal activities, in a momentary reversal of the ciliary current, tending 

 to get rid of the source of stimulation. 



