HOMOLOGIES. 995 
or cup-like projection so characteristic of the Cri- 
noids, — 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. 
Tt does not merge gradually into the arms, as in 
the Star-Fish, but the arms start abruptly from 
its periphery. In these, as in the Crinoids, the 
interambulacral plates are absent, and the inter- 
ambulacral 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, 
er 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, in- 
tertwining with each other, they make a thick 
net-work 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 . 
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