What type of organism includes sea stars and sea urchins within the phylum Echinodermata?

The correct answer identifies Echinoderms as the group to which sea stars and sea urchins belong. Echinodermata is a unique phylum known for its characteristic features, including radial symmetry, a calcareous endoskeleton composed of ossicles, and a water vascular system, which aids in locomotion and feeding. This phylum encompasses a diverse range of marine organisms, the most recognizable examples being sea stars and sea urchins.

These organisms share common traits, such as possessing tube feet and ability to regenerate lost parts, which are vital for survival in their marine habitats. They are exclusively marine and play essential roles in their ecosystems, including predator and prey dynamics and contributions to the overall biodiversity of ocean environments.

The other options listed pertain to different phyla, which have distinct characteristics not shared with Echinoderms. For instance, Mollusks are soft-bodied animals, Cnidarians possess stinging cells, and Arthropods have an exoskeleton and jointed limbs, none of which apply to sea stars and sea urchins. This further solidifies the classification of these organisms within the Echinodermata phylum.