The Echinoid Directory

In true compound ambulacral plates the elements are fused together to form a single compound plate with a V-shaped perradial margin. When the test is dissociated, compound ambulacral plates do not disaggregate into their individual elements. Semi-compound plates, by contrast, retain the appearance of simple plates, each element having its own V-shaped perradial suture. However, two or more of these elements are overgrown by a tubercle externally and thus fused together. Pseudocompound plates differ in having only one of the elements greatly enlarged and supporting all the tuberculation, the other elements being reduced to tiny plates, little larger than the pore-pair. These plates readily dissociate and are not fused.

Plates which have two ambulacral pore-pairs to each primary tubercle are termed bigeminate, and those with three pore-pairs to a compound plate are termed trigeminate. Where there are more than three pore-pairs to a compound plate, the plates are said to be polygeminate. Pore-pairs may be arranged in a single line (uniserial), as a double band (biserial) or triple band (triserial), or may be arranged into arcs that are more or less oblique to the vertical suture.