What is the main characteristic of the Phylum Echinodermata?

The main characteristic of the Phylum Echinodermata is the presence of radial symmetry and a water vascular system. Radial symmetry refers to the body structure being organized around a central axis, which allows echinoderms to interact with their environment from all directions. This trait is particularly evident in adult forms, which typically exhibit five-part symmetry.

The water vascular system is a unique hydraulic system that operates through a network of canals and tube feet. It serves multiple functions, such as locomotion, feeding, and gas exchange, enabling echinoderms to navigate their aquatic environments effectively and grasp food items. This system is a defining feature of the phylum, differentiating them from other marine organisms.

In contrast to the other options, which describe characteristics of unrelated animal groups, the focus on both radial symmetry and the water vascular system accurately encapsulates the defining traits of echinoderms.