A now-obsolete taxonomic group name for animals that lack a true body cavity or coelom, including flatworms and similar organisms.
From Greek 'a-' (without), 'koile' (cavity), and '-mata' (plural suffix). This was a major classification in 19th and early-20th-century zoology before scientists better understood animal evolution and relationships.
The term 'acoelomata' was huge in old biology textbooks—scientists thought animals fit neatly into 'no body cavity' or 'has body cavity' groups, but genetics later showed this wasn't the best way to organize animal families!
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