![]() The Acoela have a primitive nerve net (no brain as in other flatworms), a simple pharynx when present, and a syncytial cellular gut without a cavity, entolecithal ova, and a lack of protonephridia. ![]() Both morphological and molecular studies also suggest that the Turbellaria are paraphyletic and that the orders Acoela and Nemertodermatida should be placed into a separate phylum. However, some researchers believe that the parasitic classes should be separated into a separate phylum (Neodermata) based on their unique tegument, the neodermis that may be adaptive to a parasitic existence. Traditionally, the class Turbellaria was thought to be the basal ancestor of the parasitic classes (Trematoda, Cestoda, Monogenea) within the phylum. Recent morphological and molecular studies have generated numerous hypotheses as to their relation to each other and to other phyla. The phylogeny of the platyhelminth classes is not clear. The monogeneans are external parasites mostly of fish with life cycles consisting of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form.Thumbnail description Mostly free-living flatworms with a cellular epidermis that is usually ciliated the mouth leads to a stomodeal pharynx and incomplete gut Evolution and systematics (credit a: modification of work by Jan Derk credit c: modification of work by “Sahaquiel9102”/Wikimedia Commons credit d: modification of work by CDC) \): Phylum Platyhelminthes is divided into four classes: (a) Bedford’s Flatworm ( Pseudobiceros bedfordi) and the (b) planarian belong to class Turbellaria (c) the Trematoda class includes about 20,000 species, most of which are parasitic (d) class Cestoda includes tapeworms such as this Taenia saginata and the parasitic class Monogenea (not shown).
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