The essentials of the taxonomy and cytology of Radiolaria at the order level are summarized from approximately 110 papers. Living Radiolaria comprise representatives of the following orders: Acantharia, Collodaria, Spumellaria, cyrtid Nassellaria, spyrid Nassellaria and Taxopodia (=Sticholonche). This analysis is based on the most recent molecular biological and fossil data. Phaeodaria, which used to belong to the Radiolaria, belong to the Cercozoa now. Heliozoa are closer to the Cercozoa than to the Alveolata or Radiolaria. A molecular phylogenetic analysis indicates that Polycystina (including Collodaria, Spumellaria and Nassellaria) should not be treated as a monophyletic group.
“Polycystine” Radiolaria are characterized by the presence of axopodia, a capsular wall, and a fusule. The endoplasm consists of the Golgi bodies, mitochondria, and other organelles, whereas the ectoplasm is an alveolated reticulum with food, digestive, and perialgal vacuoles, suggesting zonal specialization. The Acantharia are characterized by the presence of a periplasmic cortex with myoneme, acting as a motile contractile plasmalemma, rather than a capsular wall. Taxopodia have thick axopodia and a thick nuclear wall instead of a capsular wall. Characteristic protoplasmic structures such as an intracapsular axopodial system and nucleus are found in “polycystine” Radiolaria, but these structures do not seem to reflect phylogenetic relationships.