Each plate of the upper circlet supports an arm, and is called a "radial"; the plates of the lower circlet, the "basals," rest on the stem and alternate with those of the upper circlet, i.e.
Some of the thecal plates, infrabasals I B, basals B, and orals 0 are forming around the bodycavities r.pc and l.pc; p is the water-pore.
It is spheroidal, with the mouth and anus at opposite poles; there are five ambulacra, and the ambulacral plates are large, simple and alternating, each being pierced by two podial pores which lie in a small oval depression; the ambulacrals next the mouth form a closed ring of ten plates; the interambulacrals lie in single columns between the ambulacra, and are separated from the mouth-area by the proximal ambulacrals just mentioned, and sometimes by the second set of ambulacrals also; the ambulacra end in the five oculars or terminals, which meet in a ring around the anal area and have no podial pores, but one of them serves as a madreporite; within this ring is a star-shaped area filled with minute irregular plates, none of which can safely be selected as the homologues of the so-called basals or genitals of later forms; within the ring of ambulacrals around the mouth are five somewhat pointed plates, which Jaekel regards as teeth, but which can scarcely be homologous with the interradially placed teeth of later echinoids, since they are radial in position; small spines are present, especially around the podial pores.
At last a quinqueradiate symmetry influenced the plates of the theca, partly through the development of a plate at the end of each groove (terminal), partly through plates at the aboral pole of the theca (basals and infrabasals) arising in response to mechanical pressure, but soon intimately connected with the cords of an aboral nervous system.
These last structures formed a nervous sheath around the axial sinus with its bloodvessels, and became divided into five lobes correlated with the five basals (the "chambered organ") and forming the aboral nerve-centre.
The thecal plates, however irregular in some species, always show defined basals and a distinct plate ("radial") at the end of each ambulacrum; they are in all cases so far affected by pentamerous symmetry that their sutures never cross the ambulacra.
- Blastoidea in which the thecal plates have assumed a definite number and position in 3 circlets, as follows: 3 basals, 2 large and I small; 5 radials, often forkshaped, forming a closed circlet; 5 deltoids, interradial in position, supported on the shoulders or processes of the radials, and often surrounding the peristome with their oral ends.
Interradial, and called "basals (BB)."
Bather (1899), which departs from that of Wachsmuth and Springer mainly in the separation of forms with infrabasals or traces thereof from those in which basals only are present.