Flatworms
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Worms
'Worms' has no specific scientific meaning, but in common speech it covers the groups of animals that have no legs. This includes species in several unrelated Phyla or major divisions of animal life. Classification is complicated, and frequently revised or updated, and identification to species-level is usually very difficult.
Annelids or segmented worms - as the name suggests, these species have their body divided into segments. Many are hermaphrodite, with both male and female sex organs. It includes earthworms, blackworms, leeches and the marine ragworms.
Nematodes have adapted to nearly every ecosystem in the planet and are abundant, but despite this are rarely recorded. Their bodies are not segmented. The Phylum includes internal parasites of vertebrates ('roundworms') and invertebrates, animal pathogens, plant gall-causers and plant pathogens ('eelworms'), but also many species that live in soil, the sea and fresh waters. They are found from the poles to the tropics, and from mountain-tops to the deep ocean floor.
Platyhelminthes or flatworms are simple organisms that lack specialised circulatory or respiratory systems. Unlike other 'worms' their digestive system has only one opening. Oxygen and nutrients pass into their flattened bodies by diffusion. The Phylum include many parasites of vertebrates, such as tapeworms and flukes, but also free-living flatworms that are found in the soil, leaf litter and water.
Flatworms
Flatworms are Platyhelminths, and are free-living in water or shaded humid terrestrial environments.
Terminology is confusing. They are usually classified as 'Turbellaria' to separate them from the parasitic species, but this is now considered incorrect following DNA studies; however the term is still used in many publications. The larger freshwater flatworms are also referred to as 'triclads' or 'planarians', in the order Tricladida - which have a triple-branched intestine.
Identification resources: aquatic species
Reynoldson, T.B. & Young, J.O. (2000) A Key to the Freshwater Triclads of Britain and Ireland, with Notes on their Ecology. Freshwater Biological Association.
Geoplanidae
family
