The phylogeny of the platyhelminth classes is not clear. Recent morphological and molecular studies have generated numerous hypotheses as to their relation to each other and to other phyla. Traditionally, the class Turbellaria was thought to be the basal ancestor of the parasitic classes (Trematoda, Cestoda, Monogenea) within the 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. 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. 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. They may be the closest relatives to the acoeloid ancestor that gave rise to bilateral metazoa. It also has been postulated that the acoelo-mate condition (no body cavity) of the other platyhelminths may be secondarily derived from more advanced protostomes. The Acoela and Nemertodermatida are considered a separate, distinct taxon.
The ten recognized orders constitute the remaining members of the class Turbellaria. 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 characteristics of each order is the following:
• Order Catenulida has a simple pharynx and sac-like gut; the mesenchyme is poorly differentiated; ova are enotlecithal. They are elongate forms that occur in freshwater and marine habitats.
• Order Haplopharyngida are small worms with a simple pharynx, the proboscis is simple and ventral to the anterior tip of body (reminiscent of nemereans), anal pore is weakly developed; the brain is encapsulated with two ventral-lateral nerve cords; the ovary is simple without accessory organs; the male pore has a circle of hard straight stylets, anterior to female pore; the are free living and marine. They were once considered to be macrostomids. They contain two species.
• Order Lecithoepitheliata's ordinal status is questionable; the pharynx is complex and somewhat variable in the anterior of the body, the gut is simple; the ova are not entolecithal and surrounded by vitelline cells. There are about 30 species.
• Order Macrostomida have a simple pharynx, gut is a simple sac; the posterior end of the body may be broadened into an adhesive disc; there is no asexual zooid formation; the ovaries are often paired, the
eggs entolecithal; the male pore is usually separate. They are mostly small interstitial marine and freshwater forms.
Order Polycladida have ruffled plicate pharynx, the gut is multibranched with diverticula that may be anastomosing; the ovaries are scattered, with en-tolecithal ova; testes are scattered follicular, the male pore is usually anterior to the female pore. They are mostly large free-living marine flatworms that may be brightly colored.
Order Prolecithophora's ordinal status is questionable. They have a plicate or bulbous pharynx, the gut is simple; the ovaries, and testes are follicular or compact, vitellaria is diffuse, eggs are ectolecithal. They are small, free-living or commensal, freshwater and marine forms.
Order Proplicastomata is similar to the Acoela based on a few specimens; they have an elongate plicate pharynx; no statocysts; entolecithal ova. They are free-living marine forms.
Order Proseriata is closely related to Tricladida. They have a cylindrical plicate pharynx, a simple gut, small, compact, paired ovaries at the end of the vitelline duct, the vitellaria is arranged along duct, ectolecithal ova. They are free-living marine forms.
Order Rhabdocoela is a large diverse group with four suborders (Dalyellioida, Typhloplanoida, Kalp-torhynchia, and Temnocephalida). They have a bulbous pharynx, a simple sac-like gut; the mesenchyme is fairly open; protonephridea are paired when present; the anterior brain and ventral nerve trunks are usually with cross connections, no statocyst; testes are compact; ovaries separate or joined with vitellaria, ova ectolecithal, a uterus is sometimes present. They have marine, freshwater, and terrestrial forms, many of which are symbiotic.
Order Tricladida have a cylindrical plicate pharynx posteriorly directed, gut has one anterior and two posterior branches and numerous diverticula; mes-enchyme is thick; no statocyst; male and female cop-ulatory structures are complex, posterior to pharynx; follicular testes; one pair of small ovaries is usually anterior, vitellaria is extensive over most of lateral body, ectolecithal ova. They are usually large, flattened, and sometimes elongate worms with marine, freshwater, and terrestrial forms.
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