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 

 investment 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, while the remaining portion of the pinnules, including the third and 

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

 cirri, but carrying ambulacral structures on its ventral side. 



Each bracbial 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 attaining the same diameter as the brachial upon which its rests. The 

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

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

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

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

 long tentacular structure with no phylogenetic history forming within 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 essentially 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 supplied 

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

 the pinnules beyond the second segment will almost or entirely disappear. 



The fundamental identity in structure between the cirri and the pinnules 

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

 Comatulella brachiolata. In this species all the arms bear ungrooved pinnules 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 great difference in the structure of the 

 grooved and ungrooved pinnules, which is well shown in the earlier portion of the 

 arm where the two types alternate regularly. The grooved pinnules, 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 



