DIRECTION OF LOCOMOTION OF THE STARFISH 21 



The subsequent locomotion in a direction opposite to the 

 arms which have been used in righting is easier to understand 

 in many cases than why the starfish should use those rays in 

 turning over. But even here there is difficulty in reconciling 

 the direction of the co-ordination in righting with that of the 

 subsequent locomotion. Let us suppose for example a starfish 

 placed on its 'back,' that is, oral side up. Let us suppose, fur- 

 thermore, that it has already succeeded in twisting rays e and 

 a and attaching them to the substratum, and that a co-ordinated 

 impulse toward righting on these arms has become established. 

 The tube feet of all the rays would accordingly be extended in 

 that direction, as indicated in the accompanying diagram (fig. 

 8 a). In some cases these two rays will swing all the others 

 over freely, in which case it would be expected that the impulse 

 would be still towards ae, and that the animal would crawl in 

 that direction. It much more frequently happens, however, 

 that one or more of the other arms become attached before the 

 animal is completely righted. Thus in figure 8 6, e and a are 

 represented as still attached; c has swung over and attached at 

 its tip between e and a; d has twisted and attached before cross- 

 ing over, and b is free. Ray c now becomes the dominating 

 factor in pulling the other arms over. To do this its tube feet 

 must action by pulling toward the tip of the ray, whereas previous 

 to its attachment they were directed towards its base (cf. fig. 

 8 a). Arm b, being free, is how pulled over into position between 

 a and c; but as d is attached it remains crossed over e, even after 

 the starfish is completely righted. It is now, however, swung 

 over, with the help of the movement of the whole animal c-ward, 

 and locomotion now continues in that direction. It will thus 

 be seen that between the time the creature is in the position 

 shown in figure 8 a, and when it is completely over and moving 

 in a general direction opposite to that of the rays on which it 

 turned, there must be a reversal in the direction of tho co-or- 

 dinated impulse of the tube feet. The details of this were not 

 studied as carefully as they should have been owing to the fact 

 that the data on the righting and locomotion were not tabulated 

 until there was no opportunity to continue the experiments. 



