Chapter 5: Platyhelminthes

BRUSA FRANCISCO; LEAL-ZANCHET ANA MARIA; NOREÑA CAROLINA; DAMBORENEA MARÍA CRISTINA

Vol. V: Keys to Neotropical and Antarctic Fauna. Thorp and Covich?s Freshwater Invertebrates

Academic Press, Elsevier

Año: 2020; p. 101 - 120

The phylum Platyhelminthes comprises a group of obligate parasitic organisms (Monogenea, Digenea, Aspidogastrea, and Cestoda), grouped in Neodermata, and the paraphyletic ?Turbellaria?. The latter are generally free-living, but symbiotic and parasitic flatworms are not rare. ?Turbellaria? includes an amazing variety of forms, but with a similar structural plan. All have the following characteristics: bilateral symmetry; organs embedded in a solid cellular matrix (the parenchyma); a sac-like gut without an anus; a nervous system with an anterior ?brain? and lateral nerve chords; and internal fluids regulated by protonephridia. Most are cross-fertilizing hermaphrodites, with morphologically diverse structures like sclerotic parts of the male copulatory organ that forms stylets of different shape and complexity, and with endolecithal or ectolecithal eggs. Variation also can be found in the location and shape of the pharynx along the main body-axis.