An apical disc in which some of the ocular plates (white) lie between adjacent genital plates (purple) and are in contact with the periproct, and others are separated from the periproct by abutting genital plates
One of a pair of plates that support the tooth in the Aristotle's lantern. The two hemipyramids form a pyramid. Retractor and protractor muscles attatch to these elements
A spine that ends in a cone-shaped hyaline cap. These spines are restricted to certain echinothurioids.
Small perforations usually confined to genital plate 2 which mark the external opening to the water vascular system. These are also known as madrepores.