1. Ax, P., 1963. Relationships and phylogeny of the Turbellaria. In E. C. Dougherty (ed), The lower Metazoa. Comparative biology and phylogeny. University of California Press, Berkeley and Los Angeles: 191?224.
2. Ehlers, U., 1985a. Das phylogenetische System der Plathelminthes. G. Fischer, Stuttgart, N.Y., 317 pp.
3. Ehlers, U., 1985b. Organisation der Statocyste von Retronectes (Catenulida, Plathelminthes). Microfauna Marina 2: 7?22.
4. Ehlers, U. & B. Sopott-Ehlers, 1990. Organization of statocysts in the Otoplanidae (Plathelminthes): an ultrastructural analysis with implications for the phylogeny of the Proseriata. Zoomorphology 109: 309?318.
5. Ferrero, E., 1973. A fine structural analysis of the statocyst in Turbellaria Acoela. Zool. Scr. 2: 5?16.