Final answer:
Sea urchins, brittle stars, and sand dollars exhibit radial symmetry as adults, which is a characteristic of echinoderms. They have internal skeletons and, in the case of sea urchins, external spines, but they don't have external shells.
Step-by-step explanation:
The characteristic that is common to sea urchins, brittle stars, and sand dollars is radial symmetry. These organisms are all echinoderms and share this basic body layout, where their bodies are arranged around a central point. Despite their radial symmetry as adults, it's interesting to note that they initially exhibit bilateral symmetry during their larval stages before metamorphosing into their adult forms with radial symmetry. Additionally, echinoderms possess an internal skeleton and, in some cases like sea urchins, external spines. However, they don't have external shells. They also utilize a unique water vascular system with tube feet for slow movement.