RECORDS OF W.A. MUSEUM. 
[iig 
portion of the pinnules, including the third and succeeding segments 
is merely a tentacular body process, exactly comparable to the 
cirri, but carrying 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 in the division series, abruptly ceases its 
development, while the other continues to increase in size, ils 
basal segment attaining the same diameter as the brachial upon 
which it 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 elongate 
tentacular processes in which a skeleton is formed as needed, and 
cirri are also elongate tentacular 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. 
In this connection it is most instructive to see that in this 
specimen the ungrooved pinnules have approached so closely to the 
cirri in structure that they have taken upon themselves the 
performance of exactly the same functions. 
