In these, as in the Crinoids, the interambulacral plates are
absent, and the interambulacral spaces are filled by an encroachment of
the ab-oral region upon them. There is an infinite variety and beauty
both of form and color in these Sea-Stars. The arms frequently measure
many times the diameter of the whole disk, and are so different in
size and ornamentation in the different Species that at first sight
one might take them for animals entirely distinct from each other. In
some the arms are comparatively short and quite simple,--in others
they are very long, and may be either stretched to their full length or
partly contracted to form a variety of graceful curves; in some they
are fringed all along the edges,--in others they are so ramified that
every arm seems like a little bush, as it were, and, intertwining with
each other, they make a thick network all around the animal. In the
geological succession, these Ophiurans follow the Crinoids, being
introduced at about the Carboniferous period, and perhaps earlier.
They have had their representatives in all succeeding times, and are
still very numerous in the present epoch.
To show the correspondence of the Holothurians with the typical formula
of the whole class of Echinoderms, I will return to the Sea-Urchins,
since they are more nearly allied with that Order than with any of the
other groups.
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