The Echinoid Directory

Apical disc and periproct

The apical disc of a regular echinoid is composed of five ocular plates (labelled I-V) and five genital plates (labelled 1-5). The ocular plates are pierced by a microscopic perforation that houses the tip of the radial water vessel; an ocular plate is situated at the summit of each ambulacrum. The genital plates are generally larger and are pierced by a much larger pore, the gonopore. It is through this opening that the echinoid releases eggs or sperm into the water column. One of these plates, G2, is larger and perforated by a mass of small holes, the madrepores (visible under a microscope). This circle of plates surrounds a flexible plated membrane termed the periproct. At the centre of the periproct is the anus. The outer margin of the periproct may be smooth and oval or subcircular, or (as in the specimen above) angular. Plating of the apical disc may be firmly sutured to the corona (and thus commonly preserved in fossils), or may be imbricate and only loosely bound to the corona (and thus rarely found in fossils).

In the specimen illustrated above the circle of genital plates is continuous and the ocular plates are removed from the periproct, an arrangement that is termed dicyclic. Where all five ocular plates lie in contact with the periproct and separate genital plates, the disc is termed monocyclic. Some taxa have an intermediate condition, where the anterior ocular plates are external but one or both of the posterior ocular plates is insert and interrupts the circlet of genital plates. This condition is termed hemicyclic. In some taxa there is a distinctly enlarged periproctal plate, the suranal plate. This may be firmly integrated into the circlet of genital plates, or may be present only in juveniles.

Important variations in disc plating are illustrated below (ocular plates are green, genital plates yellow and the periproct is in black).




Suranal plate