BEHAVIOR OF CCELENTERATA 207 



lating it repeatedly, it contracted at each stimulation, then bent, in 

 extending, again to the left. But after some fifteen stimulations it 

 turned away, and bent over to the right. Now when stimulated it con- 

 tracted as before, then bent regularly, in extending, over to the right. 

 It seemed to have acquired a new method of behaving, bending to 

 the right instead of to the left. 



Close examination showed that the cause of this phenomenon is as 

 follows : When it contracts in response to stimulation, it does not re- 

 gain a completely symmetrical structure, but remains a little more con- 

 tracted on the side that is concave in extension. In extending anew, 

 this side still remains a little more contracted than the opposite one, so 

 the animal takes a curved form, concave toward the same side as in its 

 previous extension. In other words, the structure conditioning the 

 curved form is not completely lost even when the animal contracts, and 

 it becomes evident again on a new extension. 



Thus in Aiptasia the formation of a stereotyped method of action 

 depends upon very simple conditions. Yet there can hardly be a doubt 

 that the permanent individual peculiarities of form and action found 

 under natural conditions, as mentioned above, have risen in exactly 

 this way. It thus plays the part taken by what is called habit formation 

 in higher animals. 



The facts set forth in the present section show clearly that the cce- 

 lenterates do not always react in the same way to the same external 

 stimulus. Internal conditions of the organism, as determined by past 

 stimuli received, past reactions given, and various other factors, are of 

 equal importance with external conditions in determining behavior. 

 We shall see many further illustrations of this fact in the reactions 

 toward food. 



7. ACCLIMATIZATION TO STIMULI 



Besides the changes in behavior under constant stimuli that we have 

 described in the last section, there are certain others which may perhaps 

 be classed as acclimatization to stimulation. In sea anemones a light 

 stimulus that is not injurious may cause at first a marked reaction, then 

 on repetition produce no reaction at all, or a very slight one. Thus, a 

 drop of water is allowed to fall from a height of 30 cm. on the 

 surface of the water just above the outspread disk of Aiptasia annulata. 

 The animal at once contracts completely. After the animal has 

 expanded, another drop is allowed to fall in the same way. As a rule, 

 there is no response to this or to succeeding drops. Sometimes there 

 is a reaction to the first two or even three drops, but usually reaction 

 ceases after the first one. 



