REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 155 



If the response is strong, it results in locomotion; if it is weak, 

 it does not. In older specimens, especially after they are 

 attached and begin to metamorphose, it often consists of but a 

 single stroke of the tail. In younger ones it ordinarily consists of 

 rapid vibration of the tail, which usually results in locomotion. 



The rate of locomotion induced by shadows is not appreciably 

 greater than it is normally, and the period of activity under the 

 two conditions is also essentially the same. That is, if the 

 tadpole starts to swim without external stimulation, it con- 

 tinues to swim at approximately the same rate and for approxi- 

 mately the same length of time as it does when it starts in re- 

 sponse to a reduction in illumination. Moreover, the periods of 

 activity induced by reduction in illumination are largely if not 

 entirely independent of the illumination following the stimula- 

 tion. The former conclusions are based upon numerous obser- 

 vations without actual measurements, the latter upon observa- 

 tions with measurements. The results obtained in two series 

 of measurements are presented in table 1. By referring to this 

 table, it will be seen that there is great variation in the length of 

 the periods of activity in different individuals and in the same 

 individual at different times, but that the average period of 

 activity is probably independent of the illumination following the 

 stimulation which induces activity, it being 11.60 hm.^ in the 

 lower and 13.38 hm. in the higher, which is a fairly close agree- 

 ment, considering the great variation in the periods and the 

 small number of results entering into the compilations. 



These facts seem to show conclusively that the reduction in 

 luminous intensity initiates a process which continues without 

 further stimulation, a process of the ' all-or-none' type. This 

 conclusion is further supported by the fact that there is a thresh- 

 old. No apparent effect is produced by a reduction in light 

 until there has been a given reduction. Then there appears to 

 be a maximum effect regardless of any further change. This 

 is clearly indicated by the results of the following observations : 



1. On September 4th a tadpole of Amaroucium pellucidum 

 attached under a cover-glass was observed under the microscope 



' hm., abbreviation for hundredth-minute. 



