48 THE TROPISMS 



morning even to strong light, but later in the day they be- 

 come negative in light of less intensity. This apparently 

 determines the periodic depth migration of these forms, for 

 they are usually found at the surface of the sea in the morning 

 and in the afternoon in deeper water. If a culture is placed 

 at any time in darkness for some hours, the larvae when 

 first exposed to light are positive, but later become negative, 

 the change taking place more quickly the more intense the 

 light. Similar effects of exposure have been found in other 

 forms. A curious instance of the effect of previous exposure 

 is afforded, however, by the amphipod Orchestia agilis. 

 If specimens are subjected to strong light they are markedly 

 positive and remain so for a long time. If they are suddenly 

 transferred to light of weaker intensity they immediately 

 show a decided negative reaction. This, however, is only 

 temporary; within a few minutes or half an hour they all 

 become positive again. If now they are exposed to still 

 feebler light they again become negative. I have performed 

 this experiment repeatedly and have found that these 

 changes occur in the most decided and striking manner. 



In some cases positive phototaxis may be actually in- 

 creased, up to a certain limit, with the length of the exposure. 

 Animals whose first responses may be hesitating or indefinite 

 get, as it were, "warmed up" to the work, and finally be- 

 come almost violent in their efforts to reach the light. The 

 water scorpion Ranatra after having been kept in the dark 

 for several hours is commonly negative; then it shows 

 spasmodic fits of a positive response which grow longer and 

 stronger until the creature chases wildly after the light 

 and becomes wrought up into the highest pitch of excitement. 

 I have found that fiddler crabs, which when first exposed 

 to artificial light show signs of alarm and run away, gradually 

 become more and more strongly positive and after a time 



