112 Papers from the Department of Marine Biology. 



noticeable in Tropiometra afra Hartlaub and hardly more so in C. pur- 

 purea. It is most 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 Maer), 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 posterior. 

 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- 



