The Echinoid Directory

Apical disc

The apical disc, as in other irregular echinoids, is composed of five ocular plates (green) and up to four genital plates (yellow). The ocular plates are pierced by a microscopic perforation that houses the tip of the radial water vessel; ocular plates are always situated at the tip of each ambulacrum. The genital plates are generally larger and are pierced by a much larger circular pore, the gonopore. It is through this opening that the echinoid sheds its gametes into the water column. One of these plates, G2, is larger and perforated by a mass of small holes, the madrepores. This plate is also known as the madreporite. It is the external opening for the internal system of water vessels that include the tube-feet.

In the example shown above plate G2 is elongate and separates both the posterior genital plates G4 and G1, and the posterior ocular plates Oc V and Oc I. This arrangement of apical disc plating is termed ethmolytic. If G2 is smaller and does not separate the posterior genital plates then the disc is said to be ethmophract.

The apical disc plating illustrated above is compact, with all genital and ocular plates in close proximity. Other arrangements are found, with the plates either stretched (holasteroid condition) or with the some of the disc plates anterior and others, separated by interambulacral plates, towards the rear. When the disc is split into two separate parts it is termed disjunct. In primitive forms the posterior oculars abut the periproct and there may be a small, crescent-shaped imperforate plate at the posterior border of the periproct that represents a rudimentary genital plate 5. In these early groups there are often small complemental plates that are incorporated into the disc, in addition to ocular and genital plates. These may have originally been periproctal plates that have become incorporated.

The number of genital plates is variable. Usually four are present, but there can be as few as two in some taxa. Similarly, although it is generally the case that each genital plate is pierced by a gonopore, in some taxa there may be an imperforate genital plate, while occasionally there may be more than one gonopore on a plate. Finally, madrepores are not always confined to genital plate 2, and can sometimes extend over the other anterior plates of the apical disc.

Some variations in disc plating are illustrated below.

Reduced plating
Elongate (holasterid)
Modified Elongate