A. R. MOORE 167 



which would bring about the reaction in a dark-adapted starfish, 

 the tube feet not being in contact with a surface, was found to have 

 an average value of 10 to 25 candle-meter seconds. 



Illumination of the dorsal surface of the starfish does not cause the 

 retraction of the tube feet nor closure of the ambulacral groove. This 

 shows either that the dorsal surface is insensitive to light or that 

 nervous connection between the sensory cells of the dorsal surface 

 and the tube foot musculature is lacking. Since, as we know from 

 the effects of mechanical stimulation* there is nervous connection 

 between the stereosensitive cells of the dorsal surface and the tube 

 feet, the first hypothesis is probably correct; i.e., the dorsal surface 

 has no light receptor cells. 



The "feelers" of the tips of the rays are relatively insensitive to 

 light since they show no retraction upon illumination. Only tube 

 feet with well developed terminal pads are strongly photosensitive. 

 Illumination of a limited number of tube feet causes reaction only 

 in that area, or at most, in the most sensitive individuals, only in 

 the ray illuminated. The Hght reaction is therefore local in character. 



It was noted in the experiments that tube feet which were not in 

 contact with a surface retracted much more readily in response to 

 illumination than did those which were in contact with a surface. 

 But by the use of more intense light it was found possible to force 

 the retraction of those in contact with a surface. This antagonism 

 between stereotropism and the reaction due to light gives a means 

 of quantitative treatment of stereotropism by the method of indirect 

 measurement. It is therefore only necessary to illuminate the 

 animal with a known quantity of light just sufficient to neutrahze 

 its stereotropism, as shown by the withdrawal of the tube feet from 

 the surface, in order to have a measure of stereotropism in terms of 

 light quantity. Although 10 to 25 candle-meter seconds is sufficient 

 to cause retraction of tube feet which are simply extended in the 

 water without touching a surface, this quantity of light has no appar- 

 ent effect upon the tube feet which are in contact with the glass side of 

 the aquarium. But if exposed to a fight sufficiently powerful, starfish 

 clinging to the side of a glass dish frequently release their hold en- 



< Moore, A. R., /. Gen. Physiol, 1919-20, ii, 319. 



