MONOGRAPH OF THE EXISTING CBINOIDS. 603 



served to set them in motion, and swimming continued for a more or less brief 

 interval, varying greatly with different individuals. It seemed as though the 

 frequent contact with the side of the basin or pail served as a deterrent, and Dr. 

 Clark thinks that they would undoubtedly have gone a greater distance in open 

 water. Nevertheless, it was clear that the individuals observed were " sprinters " 

 and not long-distance swimmers, the movements being more like the flitting of 

 small birds in shrubbery than like long-sustained flight. The gracefulness and 

 beauty of the movements were their most notable feature, but their rapidity and 

 force were also remarkable. 



In the 10-armed specimens swimming was accomplished by using the arms 

 in sets of five alternately, so that when arms 1, 3, 5, 7, and 9 were brought up 

 almost vertically over the disk, arms 2, 4, 6, 8, and 10 struck backward forcibly, 

 with pinnules fully extended, until they nearly met behind the cirri. Only a 

 single stroke was made, but as arms 2, 4, 6, 8, and 10 were relaxed and drawn 

 in and upward over the disk another stroke was made by the alternate five arms. 

 Of course, the movement was much more rapid than a description indicates, but 

 it decreased in rapidity as the comatulid became fatigued. At the start the 

 strokes were at the rate of perhaps 100 a minute, but they rapidly dropped to 

 much less than that and usually ceased altogether in less than a minute. Each 

 stroke appeared to carry the individual about the length of its own arms, so that 

 an individual with arms 50 mm. long started out at the rate of about 5 meters 

 a minute. But the longest distance any specimen was seen to travel continuously 

 was less than 3 meters. 



In the multibrachiate Stephanometridse and Mariametridae the movements 

 were very similar, but exceedingly difficult to analyze satisfactorily. The arms 

 seemed to be used in sets of five in rapid succession. That is, supposing the indi- 

 vidual had 40 arms, the first stroke would be given by arms 1, 9, 17, 25, and 33, 

 followed almost immediately by 2, 10, 18, 26, and 34, then 3, 11, 19, 27, and 35, 

 and so on until the eight sets had been used, when the first set would come into 

 action again. Not so great a part of each arm is used as in the 10-arm species, 

 and the stroke does not carry the arm so far back, but the movements are so rapid 

 and the similarity of the arms to each other is so confusing that it is almost impos- 

 sible to feel sure that one has made no mistake in the analysis of the method of 

 progression. While the swimming is perhaps just as graceful as that of the 

 10-armed species, when contrasted with their movement it is reminiscent of " dog- 

 paddle " swimming as compared with the usual stroke. No satisfactory data were 

 secured regarding the speed of, or distance covered by, the multibrachiate species. 



The creeping movements of the Comasteridae are neither so graceful nor so 

 well coordinated as the swimming movements just discussed. The comasterids 

 usually live in the open spaces among branching corals or on the lower side of 

 large fragments of rock or coral, with their arms extended and floating listlessly 

 in the water. More or less constantly, but irregularly, the arms bend inward 

 toward the mouth, the tips just brushing the disk and the arm bases. While these 

 movements may be connected with feeding, it was not possible to determine what 



