10/3/2023 0 Comments Characteristics of flat wormsThey are elongate forms that occur in freshwater and marine habitats. Order Catenulida has a simple pharynx and sac-like gut the mesenchyme is poorly differentiated ova are enotlecithal.The characteristics of each order is the following: There are more than 4,500 described species within the Acoela and Turbellaria combined however, many species have yet to be discovered and described. The ten recognized orders constitute the remaining members of the class Turbellaria. The Acoela and Nemertodermatida are considered a separate, distinct taxon. It also has been postulated that the acoelomate condition (no body cavity) of the other platyhelminths may be secondarily derived from more advanced protostomes. They may be the closest relatives to the acoeloid ancestor that gave rise to bilateral metazoa. 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. 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
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