DIRECTION OF LOCOMOTION OF THE STARFISH ' 19 



gradual rotation of the impulse. In seeking an explanation foi 

 this two possible ones suggest themselves, of which one is physi- 

 ological in the strict sense, the other psychological, or based on 

 behavior. According to the first of these, the change in direc- 

 tion may result from fatigue (whether of muscles or controlling 

 centers) which results from activities being continuously directed 

 one way, and as a result they shift to a new position, somewhat 

 as the control of pulsation in a medusa passes from one center 

 to another around the periphery. On the other view, this would 

 probably be a type of modifiability of behavior analagous to 

 that of Stentor and other lower organisms, which, as Jennings 

 ('OG, chapter 10) and others have pointed out, change their mode 

 of reaction when one mode has been tried for a time and proves 

 ineffectual. In this case the impulse in any one direction is 

 ineffectual for the starfish because it fails to 'get anywhere,' 

 and the impulse accordingly shifts to a new position, just as 

 Stentor may bend first one way and then another to avoid an 

 unfavorable stimulus. This 'regular' rotation of the impulse 

 woidd probably not take place under any except uniform con- 

 ditions; if the starfish were to come to an uneven surface or to 

 food, these new stimuli would undoubtedly influence the further 

 direction of the impulse. Like most cases of modifiability of 

 behavior, this behavior of the starfish may be adaptive in nat- 

 ure — at least it would prevent its travelling continually in a 

 straight line.^ Indeed, if we could imagine a starfish on a stretch 

 of level, smooth sea-bottom where directive stimuli were absent, 

 it would, according to these results, tend to swing around in a 

 large circle and come to rest near where it started out. 



5. Relation of arms used in righting to direction of locomotion 



In table 4 are shown the results of a number of tests to deter- 

 mine what relation exists between the arms used in righting 

 when the starfish is placed on its aboral surface and the direction 

 of locomotion previous to and subsequent to the righting reac- 

 tion. The data may be summarized as follows: 



' As has been mentioned (p. G) the straight course is also frequently swerved 

 from somewhat, even though the same ray remains in the lead. 



