198 BTJLJ.ETIN 82, UNITED STATES NATIONAL, MUSEUM. 



ossicles preceding the original first brachial. The net result is, no essential change 

 in the free arms beyond the first brachials, but the interpolation of numerous ossicles 

 of a more primitive type between the first brachials and the radials. 



Addition to the plates in the ambulacral series of the comatulids usually takes 

 place at the distal end of the arm— that is, as has been explained, at the oral end of 

 the ambulacral series. But in the multibrachiate forms it is evident that, when 

 opportunity offers, addition to their number also occurs close to the aboral end of 

 the series, as in the other echinoderms. Furthermore, while the distal brachials are 

 entirely different from any plates found in other echinoderm groups, narrow, elon- 

 gate, deep, more or less cylindrical structures uniserially arranged and developing 

 consecutively, each better adapted than the last to the function of supporting the 

 slender body extension, the interpolated plates at the arm base are all of the same 

 type, develop into similar pairs, are always as primitive, broad, short, shallow, and 

 platelike as the first two formed after the radials, and resemble more or less closely 

 the plates of the other echinoderms. 



Stated in other words, the plates of the very highly specialized free arms are in 

 the form of a linear series of more or less cylindrical ossicles, which is without a 

 counterpart in the other echinoderm groups ; but whenever an opportunity is offered 

 for the formation of ambulacral plates on the unmodified body wall the plates are 

 formed in pairs, become broadened and flattened, and in other ways approach more 

 nearly the type in general common to the echinoderms as a class. 



The plates of the division series, therefore, furnish a connecting link between 

 the brachials of the free arms and the ambulacral plates of the other echinoderms, 

 which are formed in a double row in the body wall, differing from the latter chiefly 

 in being arranged in tandem pairs instead of side by side. 



An apparently fundamental attribute of all the ossicles in the ambulacral series 

 of the comatulids, excepting only those of the pinnules, which are truly uniserial 

 structures, is their invariable occurrence in pairs. 



This paired condition is very evident in the basal part of the postradial series, 

 where not only do all the plates up to and including the outermost axillary occur 

 in pairs, the two components of which are united by nonmuscular articulation, 

 but the distal element of each of these pairs also bears on its distal end two 

 exactly similar ossicles side by side. The division series therefore consist of 

 ossicles paired longitudinally, of which the outer carries ossicles paired transversely. 



At the arm tip new elements are always added in pairs. At first the two 

 members of a pair are exactly similar and suggest the two ossicles following an 

 axillary in the division sei'ies, but one of these grows much faster than its fellow, 

 becoming a new brachial, while the other, lagging far behind in its development, 

 becomes the basal segment of a pinnule. Thus every brachial is fundamentally 

 an axillary and bears two arms one of which is of the perfected comatulid type, 

 while the other never develops beyond the Flexibilia type. Sometimes two ossicles 

 appear at the arm tip, one in advance of the other instead of side by side. In this 

 case the two never develop to full size but unite by a close nonmuscular articulation 

 and form a syzygial pair. These syzygial pairs are homologous with the pairs 



