Only non-calcifying forms with or without organic body scales and having two equal flagella are retained in Class Prymnesiophyceae. Haptonema is able to coil.
Casper, S.J. von, (1972): Observations on the fine structure of the flagellar apparatus in Chrysomonads. I. - Uroglena americana Calkins. Archiv für Protistenkunde, 114: 65–82.
Hibberd, D.J., (1976): The ultrastructure and taxonomy of the Chrysophyceae and Prymnesiophyceae (Haptophyceae): a survey with some new observations on the ultrastructure of the Chrysophyceae. - Journal of the Linnean Society of London, Botany 72: 55–80.