Question

exactly right! roundworms use that characteristic thrashing motion to move through their environment.
clue #9: the final evidence board
you approach the evidence board where someone has posted this final challenge:
\mystery solved... almost! one last piece of evidence remains. of our three suspect phyla, only one has achieved the most advanced body organization. this group has a true body cavity (coelom), segmented body plan, and complex organ systems. they represent a major evolutionary advancement in animal complexity.\
detective challenge: which phylum represents this most advanced level of organization among our three suspects?
hint: think about which group has true segmentation, a real body cavity, and the most complex systems - the one that includes earthworms and marine worms with advanced features!
whats your answer, detective?

Explanation:

Annelids have a true coelom (body cavity), segmented body plan, and complex organ systems. Earthworms (Lumbricus) and marine worms like Nereis are examples of annelids which show these advanced features.

Answer:

Annelida.