True. Annelids are true coelomates, as they have a muscular system lining their body cavity.
Flatworms possess excretory organs known as nephridia. Specifically as annelids, flatworms possess a variant known as metanephridia.
<span>Unidirectional flow in the heart is ensured because the heart contains valves that prevent backflow.
These valves keep the flow of the heart going in one direction (hence the name "unidirectional").</span>