The apical disc is composed of five genital and five ocular plates which surround a flexible plated membrane that contains the anus. This flexible membrane is termed the periproct . The arrangement of plates in the apical disc provide important taxonomic characters. The precise arrangement of ambulacral plates on the test is highly variable and is extremely important. More information on this character is given under Ambulacral plating .
All plates bear spines, which attach and articulate at tubercles.
In the specimen illustrated there is a single large primary tubercle on each interambulacral plate, and numerous small tubercles and granules along the outer margins of interambulacral plates and the perradial zone of ambulacral plates. Inside the test is a complex apparatus termed the Aristotle's lantern . This acts as the animal's jaws, and comprises some 50 skeletal elements and 60 muscles. The morphology of the lantern is very important for the higher taxonomy of this group. The only other internal structure of importance is the perignathic girdle .