Platyhelminths, or flatworms, include both free-living and parasitic species. The phylum Platyhelminthes is divided into two monophyletic clades, the Catenulida (with just around 100 known species; Larsson and Jondelius 2008) and the Rhabditophora, which includes nearly all known platyhelminths (Timothy et al. 2004; Willems et al. 2006; Larsson and Jondelius 2008).