Thus in a highly developed edrioasteroid, such as Edrioaster itself (fig.
Turning to fossil Asteroidea, we find the earlier ophiurids scarcely distinguishable from the asterids, while in the alternation of the ambulacrals, which undoubtedly correspond to the flooring-plates of Edrioaster, both groups approach the Pelmatozoan type.
No other way out for them is apparent, and it is clear that Edrioaster was not permanently and solidly fixed to the sea-floor.
We suppose some such form as Edrioaster, which appears to have lived near the shore, to have been repeatedly overturned by waves.
If this be correct, the only change from Edrioaster, as regards the ambulacra, was that in Palaeodiscus the covering-plates could no longer open, but closed permanently over the whole groove, while the podia issued through slits between them.
The genital organs in both Asteroidea and Echinoidea would retain the interradial position they first assumed in Edrioaster; and in Echinoidea their primitive temporary openings to the exterior were converted into definite pores, correlated with five interradially placed plates at the aboral pole.
All Palaeozoic. The structure and importance of Edrioaster have been discussed above (figs.
On the thinly plated under surface of well-preserved specimens of Edrioaster are seen five interradial swellings (fig.
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