Small perforations usually confined to genital plate 2 which mark the external opening to the water vascular system. These are also known as hydropores.
The genital plate that lies at the apex of interambulacral zone 2 and which is pierced by madrepores. It is usually the largest of the genital plates.
A rounded, ball-like articulation surface forming the top of a tubercle and the bearing surface on which the spine rests. The mamelon may be perforate (with a central pit) or imperforate (as in this picture).
A fasciole band that runs around the ambitus of the test, passing beneath the periproct at the posterior.
Large gaps between plates of the corona that are filled with soft-tissue membranes in life. These are found only in the echinothurioids.
An interambulacral plating arrangement in which the first (primordial) plate (red) is followed by a single sternal plate (2b: green). The second sternal plate (2a: green) is paired with the first episternal plate (3b: blue)
A meridoplacous plastron.
An interambulacral plating arrangement in which the sternal plates (purple) as well as the basicoronal plate (red) are arranged uniserially.
A metaplacous plastron
A system of fine, interconnected pore-space immediately beneath the outer plate surface (present in certain clypeasteroids and best observed in slightly abraided tests).
A microscopic knob-like projection from the plate surface that lacks a distinct mamelon but which supports a fine stalked pedicellaria or tiny spinelet.
A small flange near the base of the spine marking the distalmost limit of muscle attachment onto the spine base.
An apical disc with five ocular plates but only one genital plate (G2). This genital plate dominates.
An apical disc in which all of the ocular plates are in contact with the periproct, and separate adjacent genital plates.