Cystoidea was defined as a class of extinct paleozoic blastozoan echinoderms established to encompass stalked taxa that were neither crinoids nor blastoids.

It was shown to be polyphyletic in the late 1960s but continues to be used even in recent (as of 2022) literature to discuss both rhombiferans and diploporitans.

By 1967 the modern usage encompassing only rhombiferans and diploporitans had been established, although questions remained regarding the possible inclusion of blastoids.

[2] Despite these removals, speculation continued as to whether cystoids were ancestral to blastoids, crinoids, or echinoids.

As an informal group, it encompasses those two former orders (which are no longer thought to be monophyletic either), but not the Blastoidea.