1907] Jennings. — Behavior of the Starfish. 145 



What is the explanation of these facts? Two possible factors 

 suggest themselves. (1) The permanent set of a given indi- 

 vidual might be partly due to habit, resulting from the repetition 

 of certain reaction methods in the past. This possibility will be 

 treated in the next section. (2) The fact that there is a strong 

 preference in all the cases for the use of the rays next the 

 madreporic plate suggests that there are permanent structural 

 or functional peculiarities found in the starfish, which tend to 

 cause the preference for certain rays. The very great pre- 

 ponderance of the use of the rays e and a in our 95 experiments 

 indicates that such peculiarities exist, though they allow a very 

 great latitude for the play of other factors. 



Summary on the Righting Impulse and on the Factors De- 

 termining it. — The foregoing account shows that the number of 

 different factors which may determine the righting impulse is 

 great. The general result may be summed up as follows : When 

 the starfish is placed on its back, this throws the animal as it 

 were into a state of instability, shown in its varied movements, 

 and prepares it to take an impulse toward turning in one direc- 

 tion or another. In this unstable condition any one of a great 

 number of factors may determine the direction of the impulse. 

 In general, whatever factor comes into action first determines 

 the impulse. This factor may be any one of a large number of 

 external conditions, or it may be an internal condition. 



Persistence of the Impulse. — While the direction of the im- 

 pulse is thus easily determined when the animal is in the con- 

 dition of instability, it is not easily changed after the impulse 

 is once established. The same factors that at first determine 

 its direction with the greatest ease are quite without effect after 

 the determination is once made. The impulse persists most 

 obstinately after it is once formed, in spite of changing condi- 

 tions. This is shown in all the experiments. 



Thus a starfish is placed on its back with the light coming 

 from the right. Thereupon the animal shows an immediate 

 impulse to turn to the left (away from the light), on the left- 

 hand rays a - &, we may say. As soon as this impulse is evident, 

 from the inclination of the tube feet toward a - 6, we turn the ves- 

 sel around, so that the light comes from the side a - h. Turning 



