438 FREDERICK KEEBLE. 
When offered the alternative of white or black ground 
C. paradoxa takes up its position rapidly and permanently 
on the latter (text-fig. 1 ¢). 
(4) Phototactic response.—C. paradoxa is negatively 
phototactic in light of fairly high intensity ; for example, in 
the north light of the laboratory. This reaction is demon- 
strated readily by the usual methods (text-fig. 2). In light 
of low intensity the sign of the reaction changes, and the 
animals become either feebly positively phototactic or apho- 
totactic (text-fig. 2). 
The intensity of light is not, however, the only factor 
which modifies the mode of response of the animal to the. 
directive influence of light. As is the case in so many other 
littoral animals (Gamble and Keeble [1903], Keeble and 
Gamble [1904]) back-ground may also modify phototactism. 
This is readily demonstrated by putting equal numbers of 
animals in each of two troughs the bottoms of which are half 
black and half white. When such vessels are placed in a 
good light, one with the white half toward the source of light, 
the other orientated in the opposite way, the animals exhibit 
their negative phototactism more rapidly if the movement 
involves a passage from white to black than if it requires a 
passage from black to white ground (text-fig. 1a). Directive 
reaction masters back-ground reaction here though the latter | 
produces an effect. When, however, the same experiment is 
carried out in dim light the tables are turned, and back- 
ground reaction dominates directive reaction (text-fig. 1). 
Thus in dim light the animals acecumulate’on the black 
ground no matter whether the black half of the dish is 
toward or away from the source of light. 
Now to apply these facts to account for the zonal distribu- 
tion of C. paradoxa, and to explain the “ebb and flow” 
tidal movements of the animals within the zone. Experiments 
described in Section III prove that C. paradoxa cannot 
flourish in darkness, and that it requires light of a certain 
intensity in order to carry on its nutritive processes. Now 
the zone in which C. paradoxa lives is characterised, speci- 
