monocyclic crinoid cup

This is a diagram of a monocyclic cup unwrapped and flattened out. The arm bearing plates have notches called "arm facets" from where the arms extend. These plates are known as "radials", the smaller plates above the radials are the "orals", and the lower plates are "basals". The oral plate with the circlet of triangular plates is the anal pyramid. The two shaded plates are part of a series of anal plates supporting the anal pyramid, and are known as the "anal X" (the lightly shaded plate), and the "radianal" (the darker plate).