Why are echinoderms considered as a regressive of animals?

Echinoderms are often considered regressive animals due to their unique evolutionary traits, such as their radial symmetry and a lack of centralized nervous systems, which contrast with the bilateral symmetry and more complex nervous systems seen in many other animal groups. Their larval forms are typically bilaterally symmetrical, but they undergo a metamorphosis that results in the adult form being radially symmetrical, suggesting a simplification in structure. Additionally, their reliance on water vascular systems for movement and feeding further illustrates a departure from more complex systems found in other phyla.