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most ancient annelid group consists of the free-living polychaetes that evolved in the Cambrian period, being plentiful in the Burgess Shale about 500million years ago. Oligochaetes evolved from polychaetes and the leeches branched off from the oligochaetes. Leech fossils are known from the middle Permian period around 266million years ago, there is an unpublished study about a possible leech from the Virgilian

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