Specialised Morphology

Ophiuroid specialised anatomy

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 remains.
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.
	Compare the top picture to the photographs to the left to see the greatly enlarged vertebrae. click the photos for enlarged image. photos by author.
	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.

Subclass Somasteroidea

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.

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Author: Elizabeth Sweet
Last updated: 22nd November 2005
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