The contrast to Annelids the Platyhelminthes show:

Platyhelminthes are acoelomates, meaning they lack a true body cavity (coelom) between their body wall and digestive tract. Instead, their bodies are solid and filled with mesenchyme, a supportive tissue.

On the other hand, Annelids are coelomates, possessing a true coelom, a fluid-filled body cavity that provides space for organs to move independently and allows for the development of more complex body structures and specialized organ systems.