Which phylum includes sea urchins and starfish?

The phylum that includes sea urchins and starfish is Echinodermata. This classification is based on specific shared characteristics of these organisms, such as their unique body structure, which typically includes a radial symmetry (especially evident in adult forms), a calcareous endoskeleton made of ossicles, and a water vascular system that aids in locomotion and feeding. Echinoderms are exclusively marine animals and are known for their ability to regenerate lost body parts, a feature that is notable in species like starfish.

In contrast, Chordata includes animals with a notochord at some stage of their development, such as vertebrates, which is not applicable to sea urchins and starfish. Prokaryota encompasses organisms like bacteria that lack a true nucleus, which distinctly categorizes them away from the complex structures seen in echinoderms. Mollusca is another distinct phylum that includes animals such as snails and octopuses, which do not share the defining characteristics of echinoderms. Therefore, Echinodermata is the only correct choice for the classification of sea urchins and starfish.