112 Papers from the Department of Marine Biology. 
noticeable in Tropiometra afra Hartlaub and hardly more so in C. pur- 
purea. Itismost marked in Comatella stelligera and in some individuals 
of Comanthus annulatum. Thanks to this viscidity, locomotion is pos- 
sible even on very smooth surfaces, such as that of an enamel-ware 
basin. It is, however, difficult on loose sand. Rock surfaces and tightly 
packed sand are the most satisfactory for permitting the pinnules to 
function successfully. When the pinnules of the extended arms have 
gotten a hold, or perhaps while they are getting hold, three or more 
arms on the opposite side of the body are drawn up in a strongly arched 
position and their widespread tips are closely appressed to the bottom 
very near the disk. Then when the extended arms, having secured 
their hold, begin to contract and pull the animal towards the place of 
attachment, the arched arms begin to straighten out and push the 
animal in the same direction. The result is of course a movement 
more or less marked according to the character of the bottom and the 
size of the comatulid. The total movement may be from one-eighth 
to a quarter of the arm-length. The process is continually repeated 
and may be kept up for hours, with very slight resting periods. 
On the whole, Comatella maculata and adult Comatula purpurea were 
the most active and were rarely at rest in aquaria. A specimen of 
maculata with arms about 75 mm. long traveled approximately 85 mm. 
per minute and continued to go around and around a large basin for 
several hours. It probably traveled about 40 meters per hour, but 
on a natural surface, such as the reef would afford, it would be able 
to do better than that. In comatulids with approximately equal arms, 
there was no evidence of any orientation, one group of arms being 
quite as likely to be in advance as another. Such individuals altered 
the direction in which they traveled simply by changing the groups 
of arms which were to be extended. 
In adult Comatula purpurea, as is well known (the same is true of 
many comatulids not found at Maér), certain arms are much longer 
than others, and the shortest are as a rule opposite the longest. Obser- 
vation on the reef, at the live-car, and at the laboratory showed that 
in this species there is more or less definite orientation. Under ordinary 
conditions the long arms are extended in locomotion and hence are 
anterior, while the short arms do the pushing and hence are postervor. 
The mouth is at the base of the longer arms and hence is at the anterior 
end (or side) of the animal, as one would expect. If locomotion in a 
given direction were prevented, the comatulid never reversed its move- 
ment, using the short arms to pull and the long ones to push, but 
gradually swung itself around until ultimately the long arms were in 
advance. A considerable number of experiments and observations 
showed that the mechanism for orientation was not by any means 
perfectly adjusted, for an individual would often go for some distance 
with the longest arms at one side or only partially in front. Never- 
