The Echinoid Directory

The apical disc lies at the summit of the test at the point of origin of the petals. In almost all living sand dollars and their relatives, the disc is dominated by a single large central plate. This is genital plate 2, also known as the madreporite plate. It is densely perforated by minute pores, which are the openings to the water vascular system, and is perforated at its edge by up to five gonopores, positioned interradially at the top of interambulacral zones. Gonopores are the passageways that eggs and sperm are released through. A small ocular plate lies at the summit of each ambulacral column abutting genital plate 2, and this has a smaller perforation. The tip of the radial water vessel emerges through here to form the terminal tube-foot. Discs of this form, with a single genital plate, are termed monobasal.

In a few cassiduloids and in many fossil taxa the apical disc is composed of four genital plates, not one. In these cases there is a single gonopore to each genital plate. However, genital plate 2 usually remains the largest. Such discs are termed tetrabasal.

Petals are defined by lines of elongate pore-pairs which in life are associated with leaf-like tube-feet specialised for gaseous exchange. In sand dollars, petals may be straight-sided or bowed, as above, and may terminate gradually, with pore-pairs decreasing in size and increasing in spacing towards the ambitus, or sharply (as above).