Phylum Platyhelminthes

Abstract

The flatworms of the phylum Platyhelminthes comprise free-living (“Turbellaria”) and obligate parasitic organisms (Monogenea, Digenea, Aspidogastrea, and Cestoda, today grouped in Neodermata). “Turbellaria” includes an amazing variety of forms, but built in a similar way. 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 that are regulated by protonephridia. Most are crossfertilizing 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.