436 FREDERICK KEEBLE. 
to the dim sea-bed beneath deep water, its light-requirements 
would fail to be satisfied, and it would perish. 
“Upness,” then, is a habit, and, like any other habit, it 
TEXT-FIGURE 1.—Phototactism of C. paradoxa: the influence 
of background on phototactic response. The flat, porcelain troughs 
containing the animals are represented (in plan) by the oblongs. 
The bottom of each trough is half white and half black. In the 
diagram the white ground is indicated by the unshaded, the black 
ground by the shaded part of the oblong. The animals are repre- 
sented by dots, and arrows show the direction of the light. 
a. In bright light. 
6. In weak light. 
ce. “ Choice ”’ of black ground in preference to white ground. . 
may manifest itself in opposition to weak stimuli, which, of 
themselves, tend to produce an opposing movement, or it 
