SEM photo of an isolated half of an anal pyramid of Hypsiclavus kinsleyi. O = outer plates, I = inner plates

The periproct, or anus, of discocystinids is located in the proximal to distal CD interambulacrum usually along the midline. The periproct is very complicated. It is covered by a valvular anal structure, or anal pyramid, composed of two types of plates (inner plates and outer plates). The outer plates are triangular in outline and arranged into a low cone. The lateral sutures between the outer plates lie directly above the inner plates. The inner plates fit into a notch on the inter surface of the outer plates. In very mature specimens the inner plates can be seen from the exterior. Typically the external plates are eight to ten in number and arranged into a very low circular cone that is 2-6 mm in diameter.

SEM photos of outer anal pyramid plates of Hypsiclavus kinsleyi. 1) End on view showing the notches through which the inner plates articulate. 2) Interior view again showing the notches. 3) Exterior view showing the stereom microstructure.