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.

Therefore, it contains the majority of species in the phylum Platyhelminthes, excluding the Catenulida, and the Macrostomorpha.

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.