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Parasitic flatworms do not have a digestive system because they don't digest their food. They just suck it when the upper gut muscles contract, suck it inside their body then gobble them up and then they're done sucking the nutrients from the food people eat.
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Parasitic flatworms do not have digestive system because there is no need for them to digest the food that they get form their host,  the food obtained by these parasite  has been full digested by the host  so when the flatworm sucks the nutrients taken are already in their simplest form and readily usable by the cells of the parasites body
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