It is a singular fact,
to which I have before alluded, and which would seem to have especial
reference to the maintenance of the same numeric proportions in all
times, that, while a Class is represented by few types, those types are
wonderfully rich and varied, but in proportion as other expressions of
the same structure are introduced, the first dwindle, and, if they do
not entirely disappear, become at least much less prominent than
before.
[Illustration: Ophiuran; showing one ray from the oral side.]
There remain only two other Orders to be considered, the Ophiurans and
the Holothurians. The Ophiurans approach the Crinoids more nearly than
any other group of Echinoderms, and in our classifications are placed
next above them. In them the ab-oral region, which has such a
remarkable predominance in the Crinoid, has become depressed; it no
longer extends into a stem, nor does it even rise into the calyx-like
or cup-like projection so characteristic of the Crinoids,--though,
when the animal is living, the ab-oral side of the disk is still quite
convex. The disk in the Ophiurans is small in comparison to the length
of the arms, and perfectly circular; it does not merge gradually in the
arms as in the Star-Fish, but the arms start abruptly from its
periphery.
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