24 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



of the pinnule is more lateral, then with the shortening of the brachial, the pinnule 

 being of normal size, a tendency toward greater shortening of the antipinnular side 

 may make itself felt, and a certain inclination to dichostichality or biseriality appears 

 in the arms. Among recent forms this is strongly pronounced in certain Comasteridae, 

 in which there may be observed a thickening of the arm bases and at the same time a 

 relative shortening of the brachials. This tendency toward biseriality is very evident, 

 for example, in Comatula pectinata, but is best developed in Comatulella brachiolata. 

 Gislen said it might be imagined that the thickening of the arms has been stimu- 

 lated by two causes. First, in these types the gonads are developed in the arms; but 

 this circumstance can scarcely play any very large part since in various other recent 

 crinoids (Metacrinus, Notocrinus, and Isometra) gonads in the same position do not 

 induce any thickening of the arms. Second, one may refer to the role that the creep- 

 ing motions typical of the Comasteridae undoubtedly play. 



THE ACTIVE FLEXING POWER OF THE LIGAMENTS 



In the interbrachial articulations the transverse ridge serves as a hinge or fulcrum. 

 On each side of this ridge are ligaments that are about equally powerful the inter- 

 articular ligaments and the dorsal ligament. Gisle"n remarked that if the dorsal liga- 

 ment were antagonistic to the muscles alone it would be difficult to understand of 

 what service the interarticular ligaments could be. Possibly it might be imagined 

 that with the relaxation of the muscles they serve to counteract a too hasty flexion of 

 the arms dorsally, which might involve the risk of breakage. It might also be sup- 

 posed that with the ventral flexion of the arms the interarticular ligaments become com- 

 pressed so that when the muscles are relaxed they would supplement the action of the 

 dorsal ligament. Both these suppositions are confuted, however, by the fact that 

 when the muscles are cut through no very strong dorsal bending of the arms occurs. 

 A little dorsal flexion often appears which, however, is soon effaced. Another circum- 

 stance speaking in favor of the ligament's capacity for motion is the fact that the 

 two ligament bundles of the synarthries may sometimes be rather unequally contracted, 

 thus giving the arm or arm pair an oblique position. From this Gisle"n believes that 

 the ligaments play a certain active part in the flexions of the arm. 



Gislen said that the arms of stalked crinoids seem to move quite slowly in com- 

 parison with those of recent comatulids according to Bock's observations on living 

 Metacrinus and his own observations on Rhizocrinus. It should be noticed, too, that 

 the ligaments between the proximal cirrus segments in the pentacrinites are very 

 strongly developed so that even a slight contraction gives a powerful effect in the 

 distal part of the very long cirrus. 



DEVELOPMENT OF THE LIGAMENTARY ELEMENTS AMONG CREEPING TYPES 



The Oligophreata in general and the comasterids in particular have only incon- 

 siderable muscular attachments in comparison with their ligamentary interarticular 

 connections. In certain comasterids the proximal portion cf the arms is greatly 

 thickened, and this thickening, so far as regards the calcareous ossicle itself, is caused 

 by the specially strong development of the ligamentary connections. 



Gislen pointed out that for a swimming type, like Antedon, a very important con- 

 dition is that the structure of the arms should be light and slender. For creeping 



