286 G. E. COGHILL 



lated. Immediately following this series of responses the larva 

 lay with its trunk and tail only in the highly illuminated field 

 for five minutes without movement, excepting one small move- 

 ment of the head. In a fainter light, but with illumination 

 sufficiently strong to cast a distinct shadow upon the dark sur- 

 face on which the larva rested, it lay for ten minutes longer 

 motionless. The responses recorded above, therefore, must have 

 been to stimulation of the eyes and not to an action of the light 

 upon the skin; and the irritability of the skin to light in this 

 relatively late stage must be negligible. 



The methods of the above experiment were applied to five 

 embryos of A, punctatum which had almost but not quite reached 

 the swdmming stage, but the results gave no evidence of irrita- 

 bility to light. Four specimens which had reached the early 

 swimming stage, however, gave the following results (the time of 

 application of the stimulus being indicated in seconds, the plus 

 sign indicating a reaction after the time indicated, the minus 

 sign indicating no reaction) : 



Specimen a. 20 +, 20 +, 10 +, 60 -, 30 +, 30 +, 10 +. 



Specimen b. 60 -, 20 +, 120 +, 10 +, 60 +, 10 +, 60 -. 



Specimen c. 60 -, 70 +, 60 -, 60 -, 20 +, 15 +. 



Specimen d. 60 -, 45 +, 50 +, 60 -, 60 -, 60 -, 60 -. 



The specimens were stimulated one after the other so that 

 there was a relatively long interval between successive stimuli 

 applied to the same specimen. The illumination during this 

 period may not have been absolutely constant but it was at all 

 times very brilliant. As embryos of this age only rarely move 

 without some form of external stimulation, it appears that the 

 responses observed in this experiment were due to the stimula- 

 tion of the retina by light, but the threshold of stimulation is 

 exceedingly high. The shortest reaction time was ten seconds, 

 as compared with three seconds in the older larva; whereas one 

 specimen lay in one trial for two minutes before responding and 

 in two other trials gave the shortest reaction time. 



Another set of specimens of the same age were tested in the 

 same manner with the following results: 



