166 STEREOTROPIC ORIENTATION 



grooves close; the rays bend -ventrally. After several seconds in 

 this position the grooves open and the tube feet are extended. This 

 occurs even if the illumination is continuous. 



For purposes of better control all the experiments with light were 

 made in the dark room. While the retraction is uniformily elicited 

 in the dark-adapted starfish with white light of sufhcient quantity; 

 red light has no such effect. It was therefore practicable to observe 

 the animals at any time by means of red light while white light was 

 admitted by a shutter for any desired length of time. Use was made 

 of the lamp and optical bench described by Hecht.^ The source of 

 white light was a 260 candle-power Mazda lamp. The time of ex- 

 posure was measured with a stop-watch. The exposures were also 

 checked by means of the shutter of a photographic camera. Each 

 starfish was kept in a rectangular glass dish during a series of experi- 

 ments. The test was made by allowing the light to fall perpendicu- 

 larly on the side of the dish on which the animal rested. This pro- 

 cedure resulted in illuminating the ventral sides of one or more rays. 

 Withdrawal of the tube feet and beginning closure of the groove were 

 taken as the end-point of the reaction. In case it was desired to 

 avoid contact on the part of the tube feet, the animal had to be 

 supported vertically in the dish while the exposure was made. The 

 starfish must be kept in the dark for an hour before beginning the 

 experiments and they must not be excited mechanically at the time 

 of the test. It was found that a subliminal exposure to light pre- 

 ceding by a few seconds an otherwise adequate exposure, completely 

 inhibited the reflex. Therefore only one measurement could be 

 made at a time. Accordingly after, each exposure the animals were 

 put into freshly aerated sea water and kept in the dark for 15 minutes 

 before being tested again. 



The shortest reaction time obtainable with a light intensity of 

 26,000 candle-meters intensity was 1.5 seconds. The longest reaction 

 time secured with a weak light was approximately 3 seconds. If 

 the light intensity was so low that an exposure of more than this 

 length of time was necessary to produce the required photochemical 

 eft'ect, no reaction was obtained. The minimum quantity of light 



3 Hecht, S., /. Gen. Physiol, 1919-1920, ii, 229. 



