SELIG HECHT 



151 



pulses in this region (Frohlich, 1903). Consequently it was impos- 

 sible to make a finer localization of the sense organs responsible for 

 the general photic reaction. The presence of the sense organs in the 

 definitely circumscribed region of the heural mass, however, is de- 

 cisive and clear cut. In a medium sized individual this structure 

 occupies a volume of about 1 cc. or less. In addition it should be 

 pointed out that this area of sensitivity is colorless and contains no 

 apparent pigment of any kind (cf. Wenrich, 1916, p. 306). For the 

 most part the structures are transparent; the ganghonic mass itself, 

 however, is glistening white. 



///. Analysis of Reaction Time. 



1. The appearance of the retraction reflex as a result of photic 

 stimulation possesses one very important property. This is the pres- 

 ence of a reaction time representing the interval from the beginning 

 of the exposure to light up to the moment when the retraction takes 

 place. This interval may be measured accurately with a stop-watch, 

 first, because it is of rather long duration, and second, because the 

 appearance of the retraction reflex is sudden and clear cut. 



The reaction time of any individual Ciena to a given intensity of 

 light shows almost no variations from a constant quantity. In Table 

 I are given the reaction times of three animals to the same intensity 

 of light taken at different times of the day. When not exposed to the 

 light, the animals were all kept in a dark room. From these figures it 

 will be seen that the reaction time represents a quantity which may 

 be reproduced under a given set of conditions. 



TABLE I. 



Light Intensity, 4,880 Candle Meters. 



