REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 157 



the tail, when the lateral stroke of which the response consisted, 

 was at its maximum, and various particles of debris in the prepara- 

 tion. There was no observable difference in the magnitude of the 

 response induced by the reduction of luminous intensity varying 

 from a minimum of approximately 10 per cent to a maximum of 

 practically 100 per cent. Moreover, in numerous observations 

 no difference was discovered in the response produced by a 

 sudden reduction from direct sunlight to very weak illumination 

 and that produced by reduction from weak diffused daylight to 

 the same weak illumination. If there is any difference, it cer- 

 tainly is not proportional to the difference in the reduction in 

 illumination under the two conditions which obtained in the 

 experiment. 



In the experimental observations presented above and in 

 numerous other observations it was very evident that it requires 

 to induce an activating response a certain reduction in the 

 amount of light energy received by the organism. The minimum 

 amount necessary was not accurately ascertained, but it was 

 observed that it is under certain conditions relatively very small. 

 For example, in one experiment a tadpole of Amaroucium con- 

 stellatum exposed in a beam of light of approximately 1000 m.c. 

 responded repeatedly to a shadow produced by passing a finger 

 rapidly through the beam. Thus it required, to induce the acti- 

 vating response, a reduction in energy of probably not over 

 10 m.c. sec. Whether or not the minimum (threshold) amount 

 of reduction in energy is under certain conditions quantitatively 

 constant regardless of luminous intensity in accord with the 

 Bunsen-Roscoe law has not been ascertained. It is, however, 

 clear that there are conditions under which no such quantitative 

 relations hold, for it has been conclusively demonstrated, as the 

 following results show, that the response in question depends 

 upon the time-rate of change in illumination, that no matter 

 how great the reduction in light energy may be the reaction 

 does not follow if the reduction is sufficiently gradual. 



2. In attached specimens of both species it was repeatedly 

 observed that there was no response if the lamp on the track 

 in the dark-room was slowly moved away from the organism. 



