SELIG HECHT 



231 



the photograph of a living, medium sized individual expanded in sea 

 water. The extent to which the photosensitive siphon may be pro- 

 truded is variable; the animal in the figure represents an average con- 

 dition. On stimulation the siphon is shortened after a clearly defined 

 reaction time. The response is well marked, and involves a movement 

 of the tip of the siphon toward the shell. The amount of this retrac- 

 tion is about 1 cm., often it is more, and it is rarely less than | cm. 

 There is never any doubt about the occurrence of a response, nor of 

 the exact moment when the retraction begins. The reaction time 

 may therefore be measured with considerable accuracy. 



Fig. 1 . From a photograph of a medium sized, living individual of Mya arcnaria, 

 expanded in sea water. The reproduction is a little less than life size. 



The reaction time is not a simple interval. It consists of two dis- 

 tinct periods. The first is the exposure or sensitization period. This 

 is very short, and is the time occupied by the actually necessary ex- 

 posure to light. The bulk of the reaction time is composed of the 

 second phase, the latent period. During this period it is not necessary 

 for the siphon to be illuminated. Thus an animal which has been 

 exposed to a flash of fight of a few hundredths of a second duration 

 will respond in approximately 2 seconds, even though at that moment 

 it is in the dark. 



