476 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
two colors with diffused and equal light in the two halves, as shown 
by figure 8. 
All the other processes successfully employed in my studies on 
other animals give no definite results in the case under consideration. 
But in the conditions indicated, the Pagurids are very favorable 
subjects for experimentation, as their shell permits them to turn 
aside from all external excitation and they may be placed in a de- 
sired position on the line between the two colored surfaces without 
over exciting them. Placed on this line and left in complete quiet, 
they gradually emerge from their shelter, and at the moment of their 
emergence from their shells they feel the difference in the reaction 
of two colored environments on their eyes and their neuro-muscular 
tonicity. 
This difference manifests itself as soon as the Pagurids begin to 
move, and then they are seen to turn immediately toward the tropic 
color (more strongly positive than the other). 
The color green, is, in the case considered, the most positive; what- 
ever be the spectral color (red, yellow, blue, violet) coupled with the 
green, the Pagurids will turn always toward the green, as is well 
shown in figure 8. They are, then, chlorotropic. If kept a long time 
in the aquarium described, they occupy the green side, never crossing 
(during the day) the fatal limit. But they are at the same time 
phototropic (+), or rather, leucotropic. On the white-black back- 
ground the white will be constantly chosen. This can be represented 
by the formula: 
(1) (—) black — white (+). 
The white background is even more positive than the green, as is 
expressed by the formula: 
(2) (—) green — white (+). 
The tropic value of any other color (green excepted) corresponds 
to its position in the solar spectrum and increases in the spectral 
order toward the violet, according to the formula: 
(3) (—) red — yellow — blue — violet (+). 
Thus, for example, on the red-yellow surface the Pagurids pro- 
ceed toward the yellow; on the yellow-violet surface, toward the 
violet. 
Black is the most negative, thus: 
(4) (—) black — red (+). 
