406 
R. P. COWLES 
whether there are solid objects such as stones or walls, against 
which they may put their rays. That is, they will not, as a rule, 
remain in the darkened region unless such objects are present. 
EFFECT OF HORIZONTAL DARK SCREENS 
In order to determine the effect of darkened regions with or 
without the stimuli produced by contact with solid walls, a series 
of experiments were made, in which a region of shade was pro- 
duced in the middle of the experimenting dish by supporting a 
piece of black glass in the water horizontally by means of three 
pieces of coral (fig. 8). In this way a shadow was cast on the bot- 
tom of the dish without forming a pocket, such as that in the 
experiments described above. A region of reduced hght intensity 
was produced in the field without introducing any dark vertical 
walls. 
Many trials were made similar to that shown in fig. 8, in 
which the ophiuroid was removed from its resting place a and 
pushed to the position b, close to the shaded region. In no case 
did the specimen pass under the screen, the behavior thus differing 
from the experiments described above, in which there was a dark 
pocket. This difference may have been due to the greater light 
intensity of the field in the former case than in the latter, but I 
believe that the important factor was the absence of vertical 
dark walls, which seem to have an effect, as Jennings (1907) and 
Bohn (1908) have shown. 
After the series of trials just described, another series was tried, 
in which the black screen was suspended in the water in a hori- 
zontal position about two centimeters from the bottom without 
using any pieces of coral for support. Again and again an ophiu- 
roid was tested in the manner shown in fig. 8, and also in fig. 9. 
In the former case the specimen did not move under the shaded 
region, and in the latter, although it passed under it, it never 
remained, but returned to the original brightly-Ughted resting 
place, unless b} chance one of its ra> s touched the hanging screen, 
when it stayed under, although its raj's did not remain in contact 
