Final answer:
Arrow worms have a body cavity supporting a hydrostatic skeleton, developed organ systems with a complete digestive tract, and cephalization with a complex nervous system, setting them apart from simpler worms like flatworms and from segmented organisms like annelids and arthropods.
Step-by-step explanation:
Arrow worms exhibit unique features that distinguish their body regions. They have a body cavity that provides structural support through a hydrostatic skeleton which facilitates muscular movements. Unlike flatworms, arrow worms, which are related to segmented worms, possess a more complex organ system. Arrow worms do not exhibit true segmentation like annelids and arthropods; however, they have a well-defined head region with a brain and sensory organs, indicative of cephalization. Their digestive system is advanced compared to flatworms, with a complete tract including a mouth and anus. The nervous system is characterized by a head ganglion which functions as a brain, with nerve cords extending along the body, much like those found in nematodes.