See text Trepaxonemata (from trepa-, spiral + axoneme) is a subclass of the Platyhelminthes or flatworms.

[1] It includes all parasitic flatworms (clade Neodermata) and several free-living species that were previously grouped in the now obsolete class Turbellaria.

This central core appears as a spiral when seen in longitudinal sections in transmission electron microscopy.

However, the cilia in cells other than spermatozoa in species of Trepaxonemata have the classical 9+2 structure, for example in the epidermis of in protonephridia.

Current classification of Trepaxonemata based on several morphological and molecular studies:[3]