BEHAVIOR OF CCELENTERATA 221 



them. Finding no food, the swallowing movements of the manu- 

 brium do not occur. Thus "motile touch," as Yerkes (1902 a) calls it, 

 causes the food reaction, while the touch of an object that is at rest 

 causes only a negative reaction. This reaction to a moving object 

 shows clearly the adaptation of the behavior to the natural conditions 

 of life. Usually, when something moves quickly along the tentacles of 

 a medusa, this will be a fish or other small animal, well fitted to serve 

 as food. So the medusa reacts to such a moving thing in such a way 

 as to seize it and bear it to its mouth. If the object turns out not to be 

 good for food, as is rarely the case, there is of course no harm done, 

 and it may be rejected. If the medusa comes in contact with an ob- 

 ject that is not moving, this will probably be a stone or plant or other 

 object not fit for food, hence the animal makes no attempt to take it. 

 The behavior is based, at it were, on the probability that any given 

 case will correspond to the usual condition. Movement serves to the 

 medusa as a sign of something living and fit for food, just as it does to 

 hunters among higher animals and even among men. 1 It is a most in- 

 teresting fact that the positive reaction to a moving object is more rapid 

 than to a quiet one, even though the latter is actually food, while the 

 former is not. The reaction time for a moving object was found by 

 Yerkes (1902 a, p. 440) to be about 0.30 to 0.35 seconds, while the 

 reaction time for quiet objects or food is 0.40 to 0.50. This is again 

 directly adapted to usual conditions ; to a moving animal reaction must 

 be rapid, or it is useless. One can hardly do otherwise than hold that 

 this specialized reaction to moving objects, so appropriate to the natural 

 conditions of the animal, is not a primitive reflex, but must have been 

 historically developed in some way, and that it would not occur if it 

 were not in the long run beneficial. 



C. Food Reactions in Sea Anemones 



In sea anemones the .dependence of the reactions toward food and 

 other agents on the physiological state of the animal, particularly as 

 determined by the progress of metabolism, is very striking. 



Finding Food. Sea anemones remain for the most part quiet, with 

 disk and tentacles outspread, depending for food largely on the acci- 

 dental contact of moving organisms with these organs. But there are 



*In this, as in other cases, such expressions as "serves as a sign" of course does not 

 affirm a mental sign, concerning which we have no knowledge in animals outside of the 

 self. It signifies merely that movement does, as a matter of fact, cause a reaction which 

 is appropriate to something usually accompanying the motion, so that the behavior is 

 objectively identical with that due in higher animals and man to a stimulus that serves as 

 a sign. 



