Such being the identity of parts between a Star-Fish and a Sea-Urchin,
let us see now how the Star-Fish may be transformed into the
Pedunculated Crinoid, the earliest representative of its Class, or
into a Comatula, one of the free animals that represent the Crinoids in
our day.
[Illustration: Crinoid with branching crown; oral side turned upward.]
We have seen that in the Sea-Urchins the ab-oral region is very
contracted, the oral region and the parts radiating from it and forming
the sides being the predominant features in the structure; and we
shall find, as we proceed in our comparison, that the different
proportion of these three parts, the oral and ab-oral regions and the
sides, determines the different outlines of the various Orders in this
Class. In the Sea-Urchin the oral region and the sides are predominant,
while the ab-oral region is very small. In the Star-Fish, the oral and
ab-oral regions are brought into equal relations, neither
preponderating over the other, and the sides are compressed, so that,
seen in profile, the outline of the Star-Fish is that of a slightly
convex disk, instead of a sphere, as in the Sea-Urchin. But when we
come to the Crinoids, we find that the great preponderance of the
ab-oral region determines all that peculiarity of form that
distinguishes them from the other Echinoderms, while the oral region is
comparatively insignificant.
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