272 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The individual ossicles of the cirri are formed as a result of the segmentation and 

 solidification, and simultaneous division, of a primitive uniform spicular calcareous 

 mvestment of the cirri. 



The ossicles of the cirri are therefore precisely similar to the pinnulars beyond 

 the second in their origin, and quite different from the primary plates of the calyx 

 as well as from the brachials. 



Thus it is that the length of the cirrus segments is strictly inversely proportionate 

 to the amount of motion to be accommodated between them, a correlation which 

 is not observable in the series of brachials where, on the contrary, the most motion 

 is permitted between the longest (distalmost) ossicles. 



Morphologically the first two segments of the pinnules are merely atrophied 

 brachials, wliilo the remaining portion of the pinnules, including the third and 

 succeeding segments, is merely a tentacular process exactly comparable to the 

 cirri, but carr\nng ambulacral structures on its ventral side. 



Each brachial originates as, and is fundamentally, an axillary; one of the 

 two derivatives from this axillary, after the formation of two ossicles, which are 

 united to each other just as are the paired ossicles of the division series, abruptly 

 ceases its development, while the other continues to increase in size, its basal 

 segments attaiiiing the same diameter as the brachial upon which its rests. The 

 atropliied branch from the original axillary stage of the growing brachial serves as 

 the base from which there extends outward a long tentacular structure wnth no 

 phylogenetic history, which forms witliin itself a series of skeletal braces as necessity 

 itKjuires, and which is in every way exactly comparable to a cirrus, which also is a 

 long tentacular structure with no phylogenetic history forming witliin itself a 

 series of skeletal braces as necessity requires, excepting only that it bears ambulacral 

 structures along its ventral surface. 



Since pinnules beyond the second segment are merely elongated tentacular 

 processes in which a skeleton is formed as needed, and cirri are also elongate ten- 

 tacular processes in which a skeleton is formed as needed, it necessarily follows 

 that the skeleton of the two sets of organs will be essentiaUy identical, differing 

 only in such modification as will enable the pinnule to carry ambulacral organs on 

 its ventral side; and further, that if for any reason the pinnules are not supphcd 

 with ambulacral organs on their ventral side the difference between the cirri and 

 the jjinnulcs beyond the second segment will almost or entirely disajjpoar. 



The fundamental identity in structure between the cirri and the jnnnules 

 beyond the second segment is best illustrated by well-developed specimens of 

 ComatuleUa bracMoTata. In this species all the arras bear ungrooved jjinnules in 

 equal numbers. In the proximal portion of the arms the pinnules on either side 

 typically alternate, grooved and ungrooved; further out there are two grooved 

 pinnules between adjacent ungrooved pinnules, and toward the arm tips all of 

 the pinnules are grooved. There is a very groat difl'orenco in the structure of the 

 groovo<l and ungrooved punmlos, which is well shown in the earlier i)ortion of the 

 arm where the two types alternate regularly. The grooved pinimlos, after the 

 first two segments, which are rather large, are slender, delicate, and very flexible; the 

 ungrooved pinnules have slightly larger basal segments than the grooved and taper 



