After Parker, 1982 and “The CAS Echinoderm webpage”, 1998

<==o Echinoidea (sea urchins; merisiilit)
   |--o Perischoechinoidea
   |  `--o Cidariida [Cidaroida]
   |     |-- Cidaridae
   |     `-- Psychocidaridae
   `--o Euechinoidea ()? [paraphyletic?]
      |-- Gnathostomata [non Gnathostomata (jawed vertebrates) (ICZN rules; article 1b4)]
      `-- Atelostomata

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