Phylum Acanthocephala:
Acanthocephalans or thorny-headed worms have recently been shown to be closely related to the Rotifera using molecular data. In fact, Garey et al. propose that acanthocephalans should be considered a fourth class of rotifers based on their analysis of the nuclear ribosomal gene complex which placed acanthocephalans and bdelloid rotifers as sister taxa. Morphologically acanthocephalans may have shared derived features with rotifers but the homology of these characters(intracytoplamiclamina, ligamental cords, probosics, and lemnisci is still disputed. In fact, Melone et al. found that an analysis of 60 morphological characters did notsupport a close relationship of bdelloid rotifers and acanthocephalans. There are about 700 described species of these obligate intestinal parasites.
