Cystoidea

The concept of Cystoidea has a complex history, with many emendations from its original conception. Early versions included the homalozoans, eocrinoids, paracrinoids, blastoids, and edrioblastoids. By 1967 the modern usage encompassing only rhombiferans and diploporitans had been established, although questions remained regarding the possible inclusion of blastoids. Despite these removals, speculation continued as to whether cystoids were ancestral to blastoids, crinoids, or echinoids. Work published in 1967 and 1968 questioned whether Cystoidea formed a natural group, and in 1972 the former orders Rhombifera and Diploporita were elevated to class rank and Cystoidea was no longer used as a formal taxon. As an informal group, it encompasses those two former orders (which are no longer thought to be monophyletic either), but not the Blastoidea. ==Description==

Cystoids have a theca featuring many plates, with distinctive pores. These pores are central to the identification of cystoids, and either sit on one plate (in diploporitans) or are shared by adjacent plates (in rhombiferans). These pores have a respiratory function. Some cystoids lost their stems as adults, or possibly do not have stems at all. rhombiferan cystoid Echinosphaerites aurantium (Estonia).It is encrusted by a graptolite (black branches). ==See also==