The ambulacral plating of regular echinoids provides some of the most important characters for their classification. Ambulacral zones are composed of a double series of plates, each plate being perforated by a pore-pair. This pore-pair connects the external tube-foot to the internal radial water vessel. The pore-pairs are placed towards the outer edge of each ambulacral plate, close to the adradial suture. The adradial suture marks the junction between ambulacral and interambulacral plates, while theperradial suture is the central line separating the two columns of ambulacral plates from one another. The upper edge of an ambulacral plate is the adapical suture while the lower is the adoral suture.
In the simplest case each pore-pair occurs on an elongate plate which extends to the perradius. This plate has one or more tubercles, but the tubercles do not overlap adjacent plates. This arrangement, which is shown in the upper left photograph, is termed simple ambulacral plating. Ambulacral plates are said to be unigeminate. However, in the great majority of species ambulacral plates are associated together to form more complex, compound ambulacral plates. In the upper right photograph, there are three pore-pairs to each primary ambulacral tubercle. This is compound ambulacral plating. The precise way in which elements are united to form compound plates is highly variable and very important taxonomically. Some of the commonly encountered types of ambulacral plating styles are illustrated on the next page.
Finally, the pore-pairs themselves may be small and circular in outline, with the two pores separated by a small partition (termed non-conjugate, as in the upper right photograph) or distinctly elongate with the two pores connected by a shallow groove (termed conjugate; as in the upper left photograph).