The main structural evidence that the Mollusca, Annelida, Arthropoda, Echinodermata, and Chordata evolved from an ancestor that was not an ancestor to the other animals is that all three groups have all have a coelom .
The coelom is a body cavity, within which internal organs can develop, which is completely lined with tissue of mesodermal origin. This is diagrammed in the following picture:
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