Flatworms

Platyhelminths

PLATYHELMINTHS: also known as flatworms, a phylum of acoelomate, triploblastic, dorsoventrally flattened, bilaterally symmetrical, unsegmented worms in which the internal organs are well developed
includes >26,500 species, of which around 20,000 are parasitic
originated during the early radiation of Metazoan phyla
most likely originated in a marine environment

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lifecycle in free-living forms is relatively simple
- eggs are laid singly or in batches, and are often attached to an object/surface
- after a period of development, free-swimming larvae or minute worms emerge
parasitic platyhelminths have complex lifecycles, with several larval stages
- simplest lifecycles occur in Monogenea
known to form symbioses with other taxa, e.g., crayfish, algae, and bacteria

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zombie snails are caused by an infection of Leucochloridium, which causes pulsating swelling in the eye stalks of snails which attract the attention of birds
the frog-mutating flatworm (Ribeiroia ondatrae) is a parasite which is believed to be responsible for the recent increases in amphibian limb formations

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Archoophora have endolecithal eggs, and recognisable spiral cleavage
- contains Catenulida, Macrostomorpha, Polycladida, Prorhynchida, and Gnosonesimida
- Catenulida: an order or class of flatworms that are relatively small and free-living, inhabiting freshwater and marine environments, around 100 species described worldwide
Neoophora have ectolecithal eggs, disperse cleavage, and contains most flatworm species
- contains Lecithoepitheliata and Euneoophora
- Euneoophora contains Rhabdocoela, Proseriata, Bothrioplanida, Neodermata, Tricladida, Fecampiida, and Prolecithophora
  - Rhabditophora: subphylum of flatworms, including all parasitic flatworms and most free-living species that were previously grouped in the now obsolete Turbellaria

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