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Platyhelminthes (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning "worm") [4] is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called flatworms or flat worms.
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic.There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long [3] or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length.
The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in the Nephrozoa.
The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Coelenterates or cnidarians (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into a system of canals.
Digeneans possess a vermiform, unsegmented body-plan and have a solid parenchyma with no body cavity as in all platyhelminths. Anterior sucker of Overstreetia cribbi a zoogonid digenean [1] There are typically two suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum, on the ventral surface ...
The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall.
Modern groups of animals can be grouped by the arrangement of their body structures, so are said to possess different body plans. A body plan, Bauplan (pl. German: Baupläne), or ground plan is a set of morphological features common to many members of a phylum of animals. [1] The vertebrates share one body plan, while invertebrates have many.
Pseudoceros are simultaneous hermaphrodites [9] and reproduce sexually via random hypodermic insemination through the body tissue. [10] These organisms participate in penis fencing, [9] which is a behavior where the flatworms use their extended penises to stab and inseminate the other, while avoiding becoming inseminated themselves.