168 



STEREOTROPIC ORIENTATION 



tirely and drop to the bottom. The least quantity of light which 

 will cause the retraction of the tube feet from a surface may be re- 

 garded as the photic equivalent of stereotropism. 



In making determinations of the photic equivalent of stereotropism 

 the apparatus and procedure were the same as noted above except 

 that the exposures were made on the ventral side of the starfish while 

 it clung to the vertical wall of the dish. It was thus possible to 

 measure the distance of the receptors from the light source with 

 accuracy. Exposures were made on each animal at 5 cm. intervals 

 (Table I). The letters in the table indicate the individual animals. 

 The figures under the letters are the distances in centimeters at 

 which the 260 candle-power light acting for the time interval stated 

 in the first column will just cause the tube feet to be withdrawn from 

 the wall and the ambulacral groove to begin to close. At a distance 



* i represents intensity in candle meters; t, exposure time in seconds. 



of 5 cm. farther away from the light an exposure of the length desig- 

 nated caused no significant response. Each figure is the result of 

 repeated trials. 



While there is a considerable variation in the values when different 

 individuals are compared, each animal yields fairly consistent re- 

 sults. For example, C has higher sensitivity than the others and 

 shows the fact in each of the three exposures. The desirability of 

 more extensive studies is apparent, but the end of the season cut 

 short the progress of the experiments and the data given are sub- 

 mitted for the purpose of illustrating the possibiHties of the method. 



As a result of the measurements made, it can be stated that the 

 average photic equivalent for stereotropism in Asterias is between 

 250 and 350 candle-meter seconds. This is a value 10 to 20 times as 

 great as is required to cause retraction of the tube feet when they are 



