A marine animal shows radial symmetry as an adult, bilateral symmetry as a larva, an endoskeleton of calcareous ossicles, and no distinct excretory system. To which phylum does it belong?

Correct answer: C. Echinodermata

Explanation: These are classic identifying features of Echinodermata (e.g., Asterias, starfish): radial symmetry in adults but bilateral in larvae, calcareous endoskeleton, water vascular system, and no specialized excretory organs.