The central disc contains all the viscera and
the arms are simply greatly enlarged vertebrae, which are homologous
to the fused ambulacral plates of asteroids. Their structure is
often very well preserved in their fossil
Special articulating hinges allow the extreme flexibility of the arms.
The vertebrae are surrounded by a sheath of plates, with the strong muscles move the arms housed in the space between the two.
|Figure, after Clarkson 2001, shows the adoral surface and illustrates the internal structure of the arms, the enlarged vertebrae.|
Figure, after Clarkson 2001, shows the cross section of the arm with vertebrae and radial water vessel behind.
The bottom part of the figure shows the lateral view.
This ancient group had a pentagonal body shape
and the arms were only just beginning to differentiate.
The skeleton consisted of five biserial rows of plates forming these proto-arms, which had lateral rod-like 'virgalia' projecting from them, filing up the space between the arms.
The arm structure resembles the pinnae of a biserial inverted crinoids in some respects, and some workers believe this is evidence that somasteroids were derived from crinoids.
continue on to the fossil record.
or go back to the start of internal morphology
Author: Elizabeth Sweet
Last updated: 22nd November 2005
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