196 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The type of oral pinnule seen in Analcidometra, Himerometra, and Antedon, 

 and directly comparable to the Flexibilia arm, is relatively rare, for almost every- 

 where specialization in the form of increased slenderness has occurred, combined in 

 a few types, as in Heliometra, with a considerable addition to the number of com- 

 ponent ossicles. 



In many forms, especially among multibrachiate types, the perfection of the 

 comatulid arm as such has resulted in the loss of the sharp line of demarcation 

 between the proximal and the basal portions of the arms, with the result that two 

 or more pairs of oral pinnules occur, of which the second or third may be more 

 highly developed than the earlier. 



In Antedon (fig. 1040, pi. 12) the oral pinnules appear when only one or two 

 pairs of distal pinnules have formed, and in this genus they remain fairly close to 

 the type of structure characteristic of the Flexibilia arm. In Hathrometra many 

 distal pinnules are present when the oral pinnules first are seen, and in this genus 

 they have assumed certain of the characteristics of distal pinnules (fig. 289, p. 221). 

 It may be that the less the difference between the oral pinnules and the arms of the 

 Flexibilia the earlier do the former appear in the ontogeny. 



The present modification of the oral pinnules into tactile or protective organs 

 does not indicate that the pinnules originally were of this nature. In the phylogeny 

 of the comatulids the functions of the Flexibilia arm as a whole have become segre- 

 gated and localized and distributed in such a way that the distal pinnules are now 

 merely food-collecting organs, the middle pinnules are concerned almost exclu- 

 sively with the development of the sexual products, and the oral pinnules are tactile 

 or protect the disk. 



Division series, and pairing of the ossicles in the ambulacral series. The multi- 

 brachiate comatulids are always 10-armed until a considerable size is reached, when 

 the 10 original arms are cast off at the articulation between the first and second 

 brachials (more rarely between the third and fourth) and from the stumps axillaries 

 surmounted by new arms arise. At the time of the fracture the dividing line be- 

 tween the dorsal and the ventral surface of the visceral mass passes over the distal 

 border of the second brachial ; but before the regeneration of the new arms the 

 visceral mass has so increased in size that the stumps are a considerable distance 

 from this border. The region between the distal edge of what in the primary arms 

 was the second brachial and the new junction of the dorsal and ventral surfaces of 

 the visceral mass is spanned by plates exactly resembling those of the IBr series, 

 which similarly lie entirely in the unmodified dorsal wall of the visceral mass; 

 that is, plates arranged in pairs (or pairs of pairs), of which the more distal ele- 

 ment is axillary, and the two elements are united by a nonmuscular articulation. 



When the junction between the dorsal and ventral surfaces is reached this type 

 of plate formation ceases, and arms are produced which in every essential detail 

 reproduce the arms lost. 



Thus the loss of an arm in the 10-armed young of the multibrachiate types is 

 made good by the reproduction of several similar arms, which are separated from 

 the original first brachial by a new series of ossicles which are reduplications of the 



