Particularly with regard to gas exchange,why do we have respiratory and cardiovascular systems,but flatworms,which are only a few cells thick,don't?

Name: Particularly with regard to gas exchange,why do we have respiratory and cardiovascular systems,but flatworms,which are only a few cells thick,don't?
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The Answer of Particularly with regard to gas exchange,why do we have respiratory and cardiovascular systems,but flatworms,which are only a few cells thick,don't?

Particularly with Regard to Gas Exchange,why Do We Have Respiratory