Final answer:
The three key components of a sea urchin's body plan are the test, the water vascular system, and Aristotle's lantern.
Step-by-step explanation:
The three key components of a typical sea urchin's body plan are the test, the water vascular system, and Aristotle's lantern.
The test is the exoskeleton of the sea urchin, which provides protection and support for the internal organs. It is made up of many small plates that fit together like a jigsaw puzzle.
The water vascular system is unique to echinoderms and is used for locomotion, feeding, and respiration. It consists of a network of water-filled canals and tube feet that are controlled by a water-filled ring canal and radial canals.
Aristotle's lantern is a complex feeding structure located inside the mouth of the sea urchin. It is made up of five tooth-like structures that scrape algae and other food particles off surfaces.