We have seen that the Sea-Urchins approach most nearly to
the sphere, and that in them the oral region and the sides predominate
so greatly over the ab-oral region that the latter is reduced to a
small area on the summit of the sphere. In order to transform the
Sea-Urchin into a Holothurian, we have only to stretch it out from end
to end till it becomes a cylinder, with the oral region or mouth at
one extremity, and the ab-oral region, which in the Holothurian is
reduced to its minimum, at the other. The zones of the Sea-Urchin now
extend as parallel rows on the Holothurian, running from one end to the
other of the long cylindrical body. On account of their form, some of
them have been taken for Worms, and so classified by naturalists; but
as soon as their true structure was understood, which agrees in every
respect with that of the other Echinoderms, and has no affinity
whatever with the articulated structure of the Worms, they found their
true place in our classifications.
[Illustration: Holothurian.]
The natural attitude of these animals is different from that of the
other Echinoderms: they lie on one side, and move with the oral
opening forward, and this has been one cause of the mistakes as to
their true nature.
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