CYSTIDS AND CRINOIDS 
297 
older crinoids, the ossicles of the stem, instead of being simple rings, are generally 
composed of five equal parts. In other words, there are five radial sutures or 
joint-surfaces, running the whole length of the stem and dividing each ossicle 
into five parts. These sutures are more con¬ 
spicuous towards the root end of the stem, which 
was of course the first to he formed in each 
individual. Thirdly, examination has shown that 
in some of these stems, especially towards the root 
end, the five portions of each ossicle do not lie 
regularly above the five portions of the under¬ 
lying ossicle, hut alternate with them to a certain 
extent, just in the same way as the circlets of 
plates that make up the cup of a crinoid alternate 
with one another. These facts alone would lead 
us to suppose that the stem was originally com¬ 
posed, like the cup still is, of a series of circlets of 
small plates, five in each circlet, and alternating 
with one another; that the stem was, in fact, 
nothing more than a continuation of the cup, 
with essentially similar structure. Turning to the 
cystids, we may see how this view is confirmed and 
extended. In certain forms, such as Trochocystis, 
that part of the stem next the body consists of a 
double series of alternating plates, which are thin 
and enclose a large hollow. In Aracltnocystis the 
whole stem consists of four or five series of alter¬ 
nating plates. In Dendrocyst is, the plates forming 
the upper part of the stem can only be distinguished 
by their smaller size from those forming the cup; 
below they merge into the normal series of single 
ossicles. Cigara is the name given to a stem 
entirely composed of small irregular plates. We 
may, therefore, conclude that the stem originated 
as a portion of the body of the animal, elongated, 
and gradually becoming more and more regular 
in its structure. The curiously elongate and 
irregularly plated form called Pilocystis may repre¬ 
sent the earliest stage in its evolution, before one 
can even say that a stem is differentiated at all. 
The Stone-Lilies or Crinoids,— 
Class Crinoidea. 
LOFODEN KOOT-CRINOIDS (f liat. size). 
The crinoids differ from the more highly 
developed of the cystids in the greater regularity of their structure—the symmetry 
of which is nearly always governed by the number five,—in the greater development 
