9/1 DESIGN FOR A BRAIN 



or if it is accompanied by a slight chemical stimulus, as is 

 usually the case with carmine grains, this behaviour lasts 

 but a short time ; then a definite reaction supervenes. The 

 animal bends to one side ... It thus as a rule avoids the 

 cloud of particles, unless the latter is very large. This 

 simple method of reaction turns out to be more effective 

 in getting rid of stimuli of all sorts than might be expected. 

 If the first reaction is not successful, it is usually repeated 

 one or more times . . . 



4 If the repeated turning toward one side does not relieve 

 the animal, so that the particles of carmine continue to come 

 in a dense cloud, another reaction is tried. The ciliary 

 movement is suddenly reversed in direction, so that the 

 particles against the disc and in the pouch are thrown off. 

 The water current is driven away from the disc instead of 

 toward it. This lasts but an instant, then the current is 

 continued in the usual way. If the particles continue to 

 come, the reversal is repeated two or three times in rapid 

 succession. If this fails to relieve the organism, the next 

 reaction — contraction — usually supervenes. 



4 Sometimes the reversal of the current takes place before 

 the turning away described first ; but usually the two 

 reactions are tried in the order we have given. 



4 If the Stentor does not get rid of the stimulation in either 

 of the ways just described, it contracts into its tube. In 

 this way it of course escapes the stimulation completely, 

 but at the expense of suspending its activity and losing all 

 opportunity to obtain food. The animal usually remains 

 in the tube about half a minute, then extends. When its 

 body has reached about two-thirds its original length, the 

 ciliary disc begins to unfold and the cilia to act, causing 

 currents of water to reach the disc, as before. 



4 We have now reached a specially interesting point in 

 the experiment. Suppose that the water currents again 

 bring the carmine grains. The stimulus and all the external 

 conditions are the same as they were at the beginning. 

 Will the Stentor behave as it did at the beginning ? Will 

 it at first not react, then bend to one side, then reverse the 

 current, then contract, passing anew through the whole 

 series of reactions ? Or shall we find that it has become 

 changed by the experiences it has passed through, so that 

 it will now contract again into its tube as soon as stimulated ? 



4 We find the latter to be the case. As soon as the car- 

 mine again reaches its disc, it at once contracts again. This 

 may be repeated many times, as often as the particles come 

 to the disc, for ten or fifteen minutes. Now the animal 

 after each contraction stays a little longer in the tube than 

 it did at first. Finally it ceases to extend, but contracts 



104 



