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 larve 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 along 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 
