290 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



those upon the arms and pinnules remain at the stage of development represented 

 by the first ones to be formed upon the arm bases. 



Invariably in the young of the comatulids the five lappets about the mouth 

 contain each a large plate primarily more or less oval or rounded in shape, but 

 with the edges abruptly upturned in such a way that the major portion presents 

 a triangular outline (the oral) ; invariably in the recent stalked crinoids. and in 

 a large proportion of the mature comatulids, the lappets along the ambulacra! 

 grooves contain similar plates. 



Since the ambulacral lappets appear to be clearly homologous with, or rather 

 reduplications of, the five original lappets about the mouth, the covering plates 

 which they contain must be, to some extent at least, reduplications of the orals 

 which in size, shape, and structure they resemble. 



While it appears reasonable to consider the covering plates as at least analogous 

 to larval orals, which they resemble even in their manner of closure, no such close 

 comparison is possible between the side plates and the basals; and yet the two have 

 certain features in common which can not be overlooked. 



At the time the orals are at the stage represented by the covering plates in 

 those comatulids in which they are most developed, the basal beneath each bears 

 the same relation to the ring systems about the mouth as the side plates do to the 

 brachial extensions of these ring systems ; furthermore, the lower ends of the 

 basals are bent inward over the topmost columnal in just the same way that the 

 lower borders of the side plates are bent inward over the ventral surface of the 

 pinnulars. The basals in the comatulids eventually pass inward and transform 

 into the rosette, protecting the dorsal nerve cords; the side plates develop strong 

 inward extensions, and the distal borders, as we have seen, may pass entirely 

 within the fleshy portion of the pinnule. The orals are much less variable than 

 the basals, and similarly the covering plates are less variable than the side plates. 



The interest in the preceding discussion lies not so much in the homologies, or 

 analogies, involved as in the possible bearing on the origin of the basals and orals 

 themselves. 



It is clear that the adambulacral structures are reduplications of the five 

 perioral lappets of the larva ; it is not improbable, therefore, that the calcification 

 of the ambulacral lappets and the formation of the basals and orals followed the 

 same path. The calcification of the basals and orals must be complete in order to 

 furnish a support for the larval calyx; the ambulacral lappets are supported by 

 the brachials or pinnulars, and there is no such necessity for further support. 

 Thus the calcification of the ambulacral lappets may occur in any form between 

 the most primitive and the most perfected, while only the perfected type is possible 

 in the case of the basals and orals. 



A reasonably complete sequence can be traced in the calcification of the lappets 

 from an integument containing multitudes of minute spicules, suggesting those of 

 such ascidians as Didzmnum, through enlarged single spicules, which may reach a 

 very large size, to plates. 



If the similarity between the adambulacral plates and the basals and orals is 

 more than superficial, the available facts would suggest the following inferences: 



