The position of the pores near the centre of the ambulacrals in Bothriocidaris need not be regarded as primitive, since other early Palaeozoic genera, not to mention the young of living forms, show that the podia originally passed out between the plates, and were only gradually surrounded by their substance; thus the original structure of the echinoid ambulacra differed from that of the early asteroid in the position of the radial vessels and nerves, which here lie beneath the plates instead of outside them.
All living, and most extinct, Echinoderms show the following features, almost certainly due to an ancestral pelmatozoic stage: - An incomplete radial symmetry, of which five is usually the dominant number, is superimposed on the secondary bilateralism, owing to the outgrowth from the mouth region of one unpaired and two paired ciliated grooves; these have a floor of nervous epithelium, and are accompanied by subjacent radial canals from the water-ring, giving off lateral podia and thus forming ambulacra, and by a perihaemal system of canals apparently growing out from coelomic cavities.
The choir and the long space between the podia were for ministrants, the podia themselves for kneeling worshippers.
The circumoesophageal water-ring may lose its connexion with the exterior medium; the podia (absent only in some exceptional forms) may be locomotor, respiratory or sensory in function, but usually are locomotor tube-feet.
There were arrangements for the brilliant illumination of the choir and its relief, which was sometimes sculptured on both sides and reversible, while the podia were intentionally more obscure.
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.
C, Ventral groove, where lies the water - vessel, from which branches pass through the ossicle,emerg huge podia with a granular or plated skin.