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The evolution of the protostome coelom is an adaptation to burrowing as is segmentation in the annelids. The burrowing ancestral polychaete resembled in body-form recent terrestrial oligochaetes. Recent polychaete families originated in an adaptive radiation following the migration of the ancestral annelids into the flocculent detrital layers in shallow marine muds and from there onto hard bottoms and other specialized environments.
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Kristian Fauchald (1974) studied this question.
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