396 
R. P. COWLES 
Part of this reef is exposed at low tide and part is not. Under 
the light rocks of the unexposed part many ophiuroids live. If 
one of these rocks is turned over, in the majority of cases, several 
specimens are brought to light, and among these one usually 
finds Ophiocoma riisei and Ophiura appressa. 
It is well known that ophiuroids are sensitive to light. Some 
of them (Ophiocoma riisei) seem to be very sensitive, while others, 
such as Ophiolepis ciliata, which Bohn (1908) studied, are not so 
sensitive. In general, however, I think it may be said that ophiu- 
roids react negatively to brightly lighted fields unless some other 
factor changes the reaction. Now let us see what occurs when a 
rock, covering ophiuroids, is overturned on a bright day. As a 
usual thing it will be found that they move in the direction toward 
which the stone has been turned without reference to the position 
of the sun. This does not mean that they are not sensitive to 
brightly lighted regions but that there are some other factors 
which determine the direction of movement. The ophiuroid 
usually has one or more rays touching the under surface of the 
rock or twisted around some projection, and the stimulation pro- 
duced by this contact is retained by these rays so that they move 
forward with these rays in advance in the direction the stone has 
been moved. In other words, they ''show memory of past stim- 
ulation" as Jennings (1907) calls it. The stimulus of contact with 
some raised solid surface, such as the side of a rock, is retained 
for some little time as we shall see in experiments to be described 
later. Even a small stone lying on the bottom often has an effect 
on a moving ophiuroid w^hen a ray touches it ; the contact is very 
apt to cause hesitancy when the creature is moving in a certain 
direction even though the stone affords practically no shadow 
and no shelter. 
The following behavior also shows the importance of contact 
stimuli in the movements of ophiuroids. If a specimen which is 
lying under the shelter of a rock with one or more rays against 
it is drawn out with the hand, keeping the rays in the same general 
position with reference to the shelter, the ophiuroid will usually 
return to about the same place if it is not removed to too great a 
distance. This behavior is apparently not a reaction to the shade 
